Organic molecules encapsulated in single-walled carbon nanotubes

88594_7itab009.pdf

REVIEW

Organic molecules encapsulated in single-walled

carbon nanotubes

Ana Cadena, Bea Botka* and Katalin Kamara´s

Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, P.O. Box 49, 1525,

Budapest, Hungary

*Correspondence address. Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, P.O. Box 49, 1525, Budapest, Hungary. E-mail:

botka.bea@wigner.hu

ABSTRACT

Hybrid materials based on carbon nanotubes continue to attract considerable interest due to the broad variety of both the

cages outside and the encapsulated species inside. This review focuses on organic molecules as guests in single-walled

carbon nanotube hosts. The majority of results presented here has been attained in recent years by various methods of

optical spectroscopy, complemented by transmission electron microscopy. These spectroscopic methods yield information

on electronic structure, as well as dynamic processes as structural transformations and chemical reactions.

Key Words:single-walled carbon nanotubes; encapsulation; peapods; optical spectroscopy; graphene nanoribbons;

nanotube hybrids.

INTRODUCTION

Carbon nanotubes contain a special form of cavity, a few nano- meters wide and often up to a micrometer long. These cavities naturally present themselves as containers that can be filled with molecular-sized species. The one-dimensional (1D) charac- ter of the container determines the size and shape of what can be encapsulated. The specific interactions between the nano- tube wall and the encapsulated molecules can tune the proper- ties of either component or lead to new functional properties for the hybrid as a whole. They also influence possible chemical reactions inside the tubes. The properties of encapsulated species have been the sub- ject of many reviews [1-6]. These treat the subject from various aspects: structure, reactions, manufacturing possibilities, etc. The present review focuses on single-walled carbon nanotubes (SWCNTs) as hosts, mostly organic molecules as guest species, and optical spectroscopy as the principal method of investiga- tion. We also cover specific intermolecular interactions and reactions inside nanotubes that appeared in the literature since the last review by Minerset al.[5].We first want to define the nomenclature used in this re- view. It is now standard to denote encapsulated systems as A@NT, where A is the encapsulated species (‘guest") and NT the nanotube (‘host"). ‘Hybrid systems", in general, mean an assem- bly of an inorganic and an organic unit; as elemental carbon is an inorganic substance, the nanotubes are the inorganic part in the hybrids we present throughout the paper. Sometimes the terms ‘endohedral doping" or ‘endohedral functionalization" are used as synonyms for encapsulation. (The term ‘endohedral", meaning a hollow structure with an atom or a molecule inside, has been introduced in fullerene chemistry and used for objects or processes inside nanotubes as well.) Strictly speaking, func- tionalization means attaching chemical groups by covalent bonds to the nanotube and doping involves charge transfer be- tween the nanotube and the encapsulated species; throughout the review, we will attempt to adhere to this wording but note that a much more varied nomenclature is used in the literature that has to be assessed with some criticism. The chemistry of carbon nanotubes combines heteroge- neous reactions, where the nanotube behaves as a solid, and Submitted:4 March 2021;Revised:25 April 2021;Accepted:21 May 2021 VCThe Author(s) 2021. Published by Oxford University Press.

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/),

which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.

1 Oxford Open Materials Science, 2021, 1(1): itab009 doi: 10.1093/oxfmat/itab009

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homogeneous processes in a confined space, with the nanotube as container. These are unusual circumstances and special care is required when taking into account all the parameters. We will mention some of the possible pitfalls and good practices where applicable.

FILLING THE SWCNTS

Capillarity-induced filling of carbon nanotubes was first demon- strated by Ajayan and Iijima, who encapsulated molten lead into multi-walled carbon nanotubes (MWCNTs) [7]. Ajayanet al. also succeeded in opening the end caps of the tubes by oxida- tion [8]. Later studies revealed the role of surface tension of the liquid [9]. The main steps of any encapsulation procedure have been firmly established since: opening the tubes and clearing their interior, performing the encapsulation step, and isolate and purify the products. In the following sections, we give a summary of the filling process and the necessary pre- and post- treatments, which are summarized inFig. 1.

Opening the SWCNTs and clearing the interior

Opening the tubes and clearing the interior is the necessary starting step of all successful encapsulations; there should be enough space for the guest molecules to enter the nanotubes.

There can be several obstacles present that prevent theencapsulation: closed ends (typical of as-grown SWCNTs), mol-

ecules inside the nanotube cavity (e.g. water, catalyst particles), and functional groups attached to the terminal carbon atoms. The required cleaning procedure depends on the SWCNT pro- duction method and source. SWCNT end caps experience a greater strain than the walls, being consequently more reactive [8]. Oxidizing the nanotubes using gaseous or liquid oxidizers can successfully remove the caps. Generally, the opening is achieved by a short annealing in air. The temperature and the annealing time should be chosen depending on the diameter of the SWCNTs, because the more reactive smaller tubes require lower temperature. This proce- dure can be combined with acid treatment and deionized water washing to also remove catalytic particles [10,11]. To remove the remaining functional groups blocking the entrances and molecules occupying the inner space, the SWCNTs are slowly annealed in dynamic vacuum. High temperatures are necessary for the removal of the functional groups, but at elevated tem- peratures the nanotubes can also start to close [11,12]. One hour at a peak temperature of 800 ?

C was found to be an efficient

compromise between the two effects [13].

Encapsulation methods

Several encapsulation techniques have been developed over the years to suit guest molecules with different physical and chemi- cal properties. Some important characteristics are the melting and boiling point, solubility, and sublimation and decomposi- tion temperature of the guest species. In all cases, for the encap- sulation to occur, the diameter range of the host tubes should be appropriately selected to accommodate the guest molecules [2,14]. Liquid phase encapsulationis a process that can spontaneously occur via capillarity when open SWCNTs are immersed in liquids with a low enough surface tension. For SWCNTs, this limit is between 130 and 170 mN/M [15]. This is an issue in case of molten metals or inorganic compounds, but for organic com- pounds or water the criterion is always met [16]. This technique is simple to perform by immersing the nanotubes into a liquid form of the guest molecules, and yields very high encapsulation rates. Vapor-phase encapsulationis another direct method, where only the host and the guest take part in the process, therefore, similar to the previous case, close to complete filling can be obtained. SWCNTs and guest molecules are enclosed together in a sealed compartment under vacuum or inert gas, then the temperature is raised slightly above the sublimation tempera- ture of the guest species. This method is widely used for encap- sulating fullerenes, polyaromatic hydrocarbons (PAHs), thiophenes and similar compounds (Table 1). A drawback of the vapor-phase method stems from the elevated temperature where it is performed. For example, while pristine fullerenes are stable up to very high temperatures, functionalized fullerenes, such as azafullerenes [17] would disintegrate at the temperature necessary for vapor-phase filling. By-products are also prone to form around the sublimation temperature. For example, dimers and polymers of the guest molecules were detected in the case of PAH and oligothiophene sublimation [18,19,20](Fig. 2). The presence of these by-products, if unnoticed, can cause further problems upon cleaning the samples and during characteriza- tion. By-products can form inside the nanotubes or on the outer surfaces. Sublimation onto other substrates is often used as a reference to test by-product formation, but it should be consid- ered that the substrate material can alter the reaction pathway Figure 1:Schematic representation of the main steps of the sample preparation. The first three steps: opening the tubes and clearing their interior, performing the encapsulation, and isolation and purification of the products are necessary for any encapsulation. The last two steps are optional: reactions can be per- formed within the host tubes resulting in new encapsulated species and/or the hybrids can be further processed, e.g. the nanotube wall can be functionalized, depending on the application.

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Table 1:Guest molecules and the methods used for their encapsulation Guest molecules Filling method Examples References

Fullerenes Vapor phase C

60
,C 70
,C 78
,C 80
,C 82
,C 84
,C 90
,[24-33] endohedral, exohedral fullerenes [17,34-42]

Solution C

60
, endohedral, exohedral fullerenes [23,43-47]

Supercritical C

60
,C 70
,C 78
,C 84
, endohedral, exohedral, [17,48-53] functionalized fullerenes Metallocenes Vapor phase Ferrocene, cobaltocene, nickelocene [4,54,55]

Solution Ferrocene [56]

PAHs Vapor phase Anthracene, picene, coronene, perylene, [18,57-64] a and derivatives [19,29,65,66]

Supercritical Coronene [19,65]

Thiophenes Vapor phase Oligothiophenes (3T, 4T, 5T, 6T) [63,67,68] a [20,69-73]

Solution 6T [74,75]

Supercritical 6T [20]

Other dyes Vapor phase Porphyrins, phthalocyanine [27,76,77] Solutionb-carotene, methylene violet, DTDCI, [74,78-83] phenazine, DPP2, SQ, DANS Organic solvents Liquid phase Alkanes, alkenes, alcohols, ketones [84,85] amides, aldehydes, nitriles, anthracene and benzene derivativesetc.

Others Vapor phase TDAE, TMTSF, TTF, TCNQ, F

4

TCNQ, [29,86,87]

3,5-dinitrobenzonitrile, dodecanethiol

10,10 0 -dibromo-9,9 0 -bianthryl

Liquid phase N-phenylacetamide, TTF [88,89]

Solution Hexamethylmelamine, M(acac) [88,90-94]

pyridinium dichlorobromate benzyl azide, viologen derivatives

Vapor phase Fe(H

2 Bpz 2 ) 2 (phen),Fe(H 2 Bpz 2 ) 2 (bipy) [95] a

The conditions used for encapsulation can cause by-product formation, see the section ‘Removing the adsorbed molecules".

3T, terthiophene; 4T, quaterthiophene; 5T, quinquethiophene; 6T, sexithiophene; DTDCI, diethylthiadicarbocyanine iodide; DPP2, 3,6-Bis-[2,2

0 ]bithiophenyl-5-yl-

2,5-di-n-oc-tylpyrrolo[3,4-c]pyrrole-1,4-dione; SQ, squarylium III dye (1,3-bis[4-(dimethyl-amino)phenyl]-squaraine); DANS, p,p"-dimethylaminonitrostilbene; TDAE,

tetrakis(dimethylamino)ethylene; TMTSF, tetramethyltetraselenafulvalene; TTF, tetrathiafulvalene; TCNQ, tetracyano-p-quinodimethane; F

4

TCNQ, tetrafluorotetra-

cyano-p-quinodimethane; M(acac), metal acetylacetonate; Fe(H

2Bpz2)2(phen), Fe (dihydrobis(pyrazolyl)borate)2(1,10-phenanthroline); Fe(H2Bpz2)2(bipy), Fe (dihydrobis

(pyrazolyl)borate)2(2,2 0 -bipyridine).

Figure 2:Comparison of coronene@SWCNT synthesized using vapor-phase and solution-phase encapsulation from sc-CO2solution. (a) Raman spectra of the guest

molecule (coronene), its dimerized form (dicoronylene), the hybrids and the reference SWCNT. Sample HT was prepared using vapor-phase encapsulation at 450

?

C. For

sample SC, coronene was encapsulated at 50 ?

C from sc-CO

2

solution. The dimer by-products are detectable in the Raman spectra of sample HT. (b) TEM image, the cor-

responding simulated TEM image and structural diagram of the SC sample showing coronene stacks inside the SWCNT. (c) Photoluminescence emission spectra of cor-

onene and dicoronylene in toluene; and filtrates of a toluene and a sodium dodecylsulphate washing of a reference sample of coronene and dicoronylene adsorbed on

the outside of the SWCNT walls. The reference sample was prepared using SWCNTs that are too narrow to encapsulate the coronene molecules exposed to coronene

vapor at 450 ?

C. Washing in toluene removed the adsorbed coronene, dicoronylene only desorbs when the toluene-washed sample is further sonicated in the surfac-

tant-containing solution. (d) Raman spectra of the SC sample after annealing at various temperatures, showing the appearance of GNRs after annealing at 700

? C and double-walled carbon nanotubes at 1250 ?

C. (a-c) were adapted with permission from Ref. [19], https://doi.org/10.1002/smll.201302613. Copyright (2013) WILEY-VCH

Verlag GmbH & Co. KGa. Spectra presented in (d) (unpublished) were measured on the same samples, but with 785 nm excitation wavelength.

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[19]. Therefore careful choice of reference samples is of utmost importance, which we discuss in the section ‘Removing the adsorbed molecules". Encapsulation from solutionsof the guest molecules can be used for a variety of organic molecules, whose physical or chemical properties preclude the use of the two methods de- scribed previously. The use of wet chemistry for filling carbon nanotubes was first demonstrated with inorganic compounds [21,22]. Encapsulation from organic solutions was introduced by Yudasakaet al.[23] involving molecules that would decom- pose below their boiling point or sublimation temperature. The process, referred to as ‘nanoextraction", consists of immersing previously opened SWCNTs into a solution of the guest mole- cules. The method relies on the guest molecules having a strong affinity to the inner surface of the cavity, while the interaction between both the guest and the solvent and the nanotube and the solvent is weak. Encapsulation from solution has been broadly used to prepare various dye@SWCNT hybrids via reflux- ing SWCNTs in solution of the guest molecules (Table 1). Performing the encapsulation using a solvent as diffusion me- dium can be carried out at low temperature, therefore it has a wide applicability. However, the method has a major drawback: the solvent molecules can also enter the nanotube cavity and get trapped inside, blocking the channels. The result is usually a lower filling ratio compared to the direct encapsulation meth- ods. This problem can be alleviated by application of solvents with high diffusivity and small enough molecular size to leave the SWCNT after carrying the guest molecule. Supercritical flu- ids, having a high diffusivity and near-zero surface tension (gas-like characteristic) and also solvation ability (liquid-like characteristic), are suitable choices as solvents [96,97]. The interactions between the guest molecules and the supercritical fluids can be broadly tuned by changing the temperature and pressure within the supercritical phase. Supercritical CO 2 (sc- CO 2 ) is an ideal candidate, because of its small molecular size and low critical temperature preventing undesired chemical reactions during the encapsulation [48]. Another advantage is that CO 2 , being a gas under ambient conditions, easily escapes from the product when brought back into the normal state.The sc-CO 2 was used to encapsulate various fullerene derivatives without destroying the functional groups, and was also found suitable for encapsulation of PAHs or thiophenes without by- product formation [20,65]. Short bursts of sc-CO 2 can poten- tially remove residual blockages from the nanotube channels, and free the way for guest molecules, which also helps to in- crease filling rates [48]. Supercritical fluids other than sc-CO 2 , specifically methanol, ethanol and toluene, have also been used to encapsulate various materials into SWCNTs [49]. Although fullerene hybrids were produced with a high filling rate, there are disadvantages: the higher transition temperature (>200 ? C) and the fact that these solvents are liquids under ambient con- ditions, making their removal after the reaction more difficult. A much more promising application of these supercritical sol- vents is the encapsulation of metal salts, performing a reduc- tion into metal nanoparticles at the same time.

Removing the adsorbed molecules

After the filling step, molecules are residing not only inside the nanotubes, but also adsorbed on the external walls. Small mole- cules adsorbed at different sites of nanotube bundles can be an- alyzed by temperature-programmed desorption (TPD) [98,99]. These measurements are carried out at ultrahigh vacuum (base pressure 10 -5

Pa) [100] and both adsorbed and encapsulatedmolecules are analyzed by mass spectrometry. The temperature

is varied from 80-90 K up to 400 K, where the encapsulated mol- ecules leave the cavities. This way, the relative binding energies and populations of various sites (interstitial vs. internal) could be determined [101], but the method is an analytical one and cannot be used for preparative purposes; moreover, it is re- stricted to gases and volatile compounds as e.g. alkanes up to n¼9[102]. Prior to further processing and characterization, the non- encapsulated molecules need to be removed. In general, mole- cules encapsulated inside the SWCNTs have a stronger interac- tion with the host tubes than the adsorbed ones. Based on different physical properties, various cleaning methods can be applied. Commonly the hybrids are washed in solvents to re- move the adsorbed residual guest molecules in several cycles. Those nanotubes in the sample that are not completely filled with the guest compound, can encapsulate the solvent mole- cules during the washing or other post-processing steps, which should be taken into account during the characterization [84,

85]. With optimized solvent-molecule interaction and SWCNT

diameter, a product with high filling ratio and no external mole- cules can be obtained. Care has to be taken, however, when the interior serves as a nanoreactor and the products interact differ- ently with the nanotubes and the solvent (see section ‘Nanosized reaction vessels"). Annealing the hybrids under dy- namic vacuum can also be used for removing the adsorbed mol- ecules [103]. In case of both cleaning methods, the exact conditions should be chosen based on the strength of the host- guest (and guest-solvent) interactions. In certain cases, the en- capsulated molecules can be removed from inside the nanotube cavity. Some examples will be discussed in the sections ‘Nanosized reaction vessels" and ‘Outlook: boron nitride nanotubes". Special care is required when the filling is performed at a high temperature, and by-products can be present. These com- pounds can have significantly different chemical and physical properties, which can cause problems during the cleaning steps. They may remain on the outer surface of the nanotubes due to their stronger interaction with the nanotube wall or insolubility in the washing solvent, but can detach upon sonication in surfactant-containing solutions, which are commonly used for preparing dispersions for optical measurements, and thus pro- duce misleading results [19]. To verify the successful encapsulation of the molecules, appropriate reference samples should be prepared. Surface adsorption and by-product formation can ideally be tested on similar diameter, but closed tip nanotubes. Narrow SWCNTs, that are too small to incorporate the molecules, or graphene can also serve as a reference, however, it should be taken into account that the curvature of the surface or the different electronic properties might alter the reaction pathway. Depending on the characterization methods, it can be necessary to repeat all synthesis steps without the guest molecule, to account for spontaneous encapsula- tion of solvent molecules during cleaning and post- processing steps.

Further processing

SWCNT hybrids can be further processed depending on the needs of the target application. The sidewalls of the SWCNTs can be functionalized to achieve solubility, or other functional- ity [59,74]. Sorting procedures to separate semiconducting from metallic tubes or to enrich certain chiralities can still be

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conducted after the molecular encapsulation into the nano- tubes [83,84,104-106]. While sidewall functionalization of the host tubes has to be performed after the encapsulation, other- wise the functional groups could block the entrance sites, sort- ing of the host nanotubes can be done before or after the filling of the tubes. If already sorted nanotubes are to be used for the filling, polymers or surfactants used for the sorting procedure have to be removed from the outer surface of the nanotubes be- fore the encapsulation process. If multiple fractions of the sorted guest@SWCNT material are to be used for the applica- tion, it is generally more practical to fill all the tubes first, and then proceed with the sorting. Processing done in this order ensures more uniform and consistent filling, requiring less time and effort.

CHARACTERIZATION

In this section, we give a summary of characterization techni- ques that can be used to confirm the successful encapsulation of molecules inside SWCNTs. We focus on optical spectroscopy and transmission electron microscopy (TEM). As a common SWCNT sample consists of nanotubes having different diame- ters and chiralities, there are naturally differences in the hybrids formed: not necessarily all nanotubes in the sample are able to encapsulate the guest molecules, the alignment of guest molecules can depend on the host size, and also the host-guest interactions can vary. An appropriate combination of optical spectroscopy techniques can give a wealth of information about the filling process independent of the nature of the guest mole- cules, and this information can be obtained selectively, for each SWCNT chirality. Transmission electron microscopy can iden- tify and visualize the encapsulated guest structures. Besides these, several other characterization methods can be used that more specifically target certain aspects of the host-guest inter- action, such as X-ray photoelectron spectroscopy, X-ray emis- sion spectroscopy, X-ray absorption spectroscopy, ultraviolet photoelectron spectroscopy. These are out of the scope of the current review, but discussed elsewhere in detail [107].

Optical spectroscopy

Raman spectroscopyis widely used to analyze the properties of carbon nanotubes. In a typical, unsorted SWCNT sample a range of nanotube diameters and chiralities are present, therefore there is always a set of SWCNTs that fulfill the resonance condi- tions with the exciting laser, leading to a strong Raman signal intensity. By varying the excitation energy, the different SWCNT chiralities can be selectively probed. However, this also implies that when the molecules encapsulated inside the nano- tubes are to be analyzed, the signal is usually very weak com- pared to the Raman scattering intensity of the host tubes. Therefore, it is typically necessary to choose the excitation en- ergy to be in resonance with the transitions of the encapsulated species as well. An example of this can be seen inFig. 2d. The as-prepared sample contains coronene, the one annealed at 500
? C dicoronylene molecules inside the SWCNTs. However, the vibrational modes of the encapsulated species are not de- tectable using 785 nm excitation wavelength. On the other hand, the coronene oligomer formed at 700 ?

C is in resonance

with this excitation. If the same sample is measured using 532 nm excitation wavelength, as shown in Ref. [19], the dicorony- lene modes also appear in the Raman spectra. The resonance profile of the encapsulated species can be studied by recording

the Raman spectra at various wavelengths [108,109].Based solely on the Raman modes of the guest molecule it is

not always straightforward to tell whether the detected species are adsorbed onto the outer surface of the SWCNTs or encapsu- lated inside them. It is therefore important that the analyzed samples are thoroughly cleaned to remove adsorbed molecules. The two types can be distinguished using reference samples without encapsulated guest species, e.g. a graphite surface or thin nanotubes. Complementary characterization methods, e.g. infrared spectroscopy, can be applied as well. Analysis of the SWCNT Raman modes can confirm whether the filling was suc- cessful. The radial breathing mode (RBM) is a breathing-like ex- pansion and contraction of the nanotubes in the radial direction. The frequency of the RBM is inversely proportional to the nanotube diameter. Encapsulation of molecules inside the SWCNTs induces a counteracting force on this motion, there- fore causes the RBM of the nanotubes to shift [110]. The size of the shift depends on the strength of the steric interaction be- tween the guest molecules and the host tubes. The RBM shift is generally observable in solid, bundled SWCNT samples as well, but bundling causes both the resonance window and the RBM linewidth to broaden [111,112]. With high-resolution Raman spectroscopy on appropriately individualized SWCNT samples, filled and empty nanotubes can be distinguished for each SWCNT chirality. To individualize them, SWCNTs can be dis- persed in surfactant or polymer-containing solutions. The nar- rowest RBM linewidths can be observed on SWCNTs dispersed using bile salts, such as sodium deoxycholate [11,113]. By ana- lyzing the shift of the RBM, the minimum SWCNT diameter to encapsulate the guest molecule can be determined (Fig. 3). Raman spectroscopy is also an ideal tool to monitor the interac- tions between the host tube and the guest molecule, such as charge transfer or mechanical strain induced by the guest mole- cules [29,69,106]. These interactions can also contribute to changes in the RBM positions and intensities. The origin of the shift can be clarified based on changes observed in the other Raman-active nanotube modes, such as the G band, corre- sponding to vibrations along and perpendicular to the tube axis, the disorder-induced D mode and its second harmonic, the 2D (G") mode. The presence of charge transfer can be detected by observing changes in the position and the shape of the G band of the SWCNTs [114,115]. Shifts of the G band modes can also indicate mechanical strain exerted by encapsulated species [106]. Encapsulation, unlike covalent functionalization, does not directly result in increase in the intensity of the disorder- induced D band. Secondary effects, such as excessive strain or change of resonance conditions can alter the position and in- tensity of this band. Isolated hybrids can be studied using micro-Raman spectroscopy [75] or tip-enhanced Raman spec- troscopy [116-118]. For example, Gaufre´set al. filled individual- ized and isolated nanotubes with sexithiophene and monitored the formation of single and pair aggregates inside the confined cavity of the tubes [75]. Infrared spectroscopyhas by far not been as popular in this area as Raman spectroscopy, although the vibrational modes of organic molecules can be detected by both methods. Part of the problem is that the carbon nanotubes themselves show very weak infrared activity [119] and contrary to Raman spectros- copy, there is no resonance mechanism to increase the inten- sity of either the nanotube modes or those of the encapsulated species. The upside of the low intensity of the nanotube modes is that there is less danger of obscuring infrared signatures of guest molecules, which are usually also weak. Infrared meas- urements in general have proven technically difficult and yielded somewhat controversial results. Care has to be taken

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with the choice of method: because of the sensitivity of the tubes to extreme mechanical impact, grinding into KBr pellets is not an option. The high absorptivity requires the use of ex- tremely thin layers either deposited on a substrate [98,120]or as self-supporting films [119]. Alternatively, attenuated total re- flection (ATR) can be measured on thick samples [53,121]. Extensive infrared transmission studies have been performed in connection with TPD experiments at low temperature and low pressure by Yates and coworkers [98]. Examinations of several small molecules yielded a redshift of the strongest molecular vibrations upon encapsulation. This was observed for CF 4 [122,

123], CO

2 [124], NO [125] and water [126]. NO was found to form dimers inside the nanotubes, as in the condensed phase. For en-

capsulated water, a vibrational mode appeared that wasassigned to special H-bonds between the water molecules,

based on sophisticated experiments and density functional the- ory (DFT) calculations [126], in accordance with earlier theoreti- cal predictions [127]. Results by Kazachkinet al.[99], however, differ from the ones described above. They combined TPD with infrared spectroscopy for acetone, diethyl ether and n-heptane, following the infrared spectra after removing the externally adsorbed molecules. They found that while the amount of en- capsulated molecules was about 9 times that of the adsorbed ones, the infrared intensity decreased to 10%. Their quantum chemical simulations explained these findings based on the screening of the transition dipoles by the nanotube walls. Experiments on SWCNTs containing larger, non-volatile mole- cules are even more scarce. Britzet al.[50] report an infrared

Figure 3:Analysis of critical filling diameters using Raman spectroscopy. (aandb) Resonant Raman spectra in the energy range of the RBMs of empty, water@SWCNT

and various guest@SWCNT hybrids. The hybrids were dispersed in sodium deoxycholate containing aqueous solution. Due to the narrow linewidth, individual chiral-

ities are well identifiable, assignment of the RBM peaks is indicated in the figure. (candd) Shifts of the RBM positions of perfluorooctane@SWCNT and 1-

bromohexadecane@SWCNT with respect to the water@SWCNT. The numbers in brackets indicate the chiral index of the host SWCNT. Water@SWCNT is used as a ref-

erence, as the open-ended SWCNTs that are too small to incorporate the desired guest molecule can become filled with water molecules, when nanotube dispersions

are prepared for the Raman measurements. The dashed blue line indicates critical minimal filling diameters obtained from calculation. The solid blueline is the cor-

rected value of the calculated minimal encapsulation diameter by the empirical offset determined in Ref. [13]. For nanotubes larger than the critical filling diameter,

the position of the RBM is shifted compared to the water-filled ones. This shift can be both positive or negative, as it depends on the steric interactionbetween the en-

capsulated molecule and the host tube, which can be both stronger or weaker than in water@SWCNTs. Nanotubes that are too narrow to encapsulate the guest mole-

cules (smaller than the critical diameter) are water-filled, and as expected show similar RBM peak positions as the water-filled reference sample. Reproduced with

permission from Ref. [85], https://pubs.acs.org/doi/10.1021/acsnano.0c08352. Copyright (2020) American Chemical Society.

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spectrum of an encapsulated fullerene derivative where the modes of the side groups are seen but interestingly, those of the C 60
core are not. Pekkeret al.[121] also measured a lack of infra- red activity in C 60
@SWCNT and attributed it to image charges in the nanotube walls, in analogy with Kazachkinet al.[99]. As the above measurements have been conducted under very different conditions (temperature, pressure, filling method, type of nano- tube and guest molecule), many open questions remain that should be explored in the future. Optical absorption spectroscopycan be used in several ways during the encapsulation process. The optical spectrum of the starting nanotube material contains all allowed transitions be- tween the Van Hove singularities in the density of states, both in semiconducting and metallic tubes (transitions S 11 ,S 22
,M 11, ...). The diameter distribution can be estimated using the sim- ple relationship E ii ?1=d, wheredis the diameter [128]. This method gives the fullest overall picture of the composition of a nanotube sample because it is not influenced by resonance effects as Raman spectroscopy and does not exclude metallic tubes as photoluminescence. It can also be used to confirm en- capsulation of the guest molecules. Filling the inner cavity of the nanotubes with the guest molecules results in changes in the width and energy of the absorption peaks [55,84,85,129]. The absorption spectra of the guest molecules can also change due to the altered environment and stacking inside the SWCNTs [13,18,59,73,76,78,81,83]. Covalent sidewall func- tionalization of the host tubes can effectively suppress their contribution to the absorption and Raman spectra, while leav- ing the inside molecules intact [59]. Two-dimensional photoluminescence excitation spectroscopy(2D PLE) is an even more powerful technique than optical absorp- tion to selectively study the interactions between encapsulated molecules and the various host SWCNTs, though it is limited to the semiconducting host tubes. As the energy separation of the optically allowed transitions of the SWCNTs depend on their chiral indices, they appear well separated on the 2D PLE map, and provide information about the different nanotube chiral- ities in the ensemble. 2D PLE maps are measured on individual- ized nanotubes in a well dispersed, dilute solution or isolated SWCNTs in a solid form to avoid interference and quenching due to intertube energy transfer [130-132]. The complementary use of 2D PLE and Raman spectroscopy can provide a tool to dis- tinguish which SWCNTs are filled with the intended guest mol- ecules [13]. The shift of the RBM depends on the steric resistance that the encapsulated molecule exerts on the breath- ing motion of the host tube. It is therefore sensitive to the mo- lecular structures inside the SWCNT. 2D PLE maps provide information about changes in the optical transitions. Generally, the peak positions of the transitions are shifting towards lower energies with increasing dielectric constant of the internal me- dium. Besides, 2D PLE maps can also provide information about energy transfer processes between host SWCNTs and encapsu- lated dye molecules [13,81,83,133]. As described in the earlier sections, individualization of the nanotubes is often desired to better resolve vibrational and elec- tronic transitions of the SWCNTs or to prevent intertube energy transfer that can quench the photoluminescence. Nanotubes can be dispersed using surfactants [113] or polymers [134] and then measured in solution form. If individualized nanotubes are to be measured in a solid form, they can be embedded in a poly- mer matrix [131] or dilutely coated onto a surface [135]. Preparation of dispersions generally involves sonication of the samples to improve the debundling, though in the case of bile

salt surfactants stirring can also be sufficient. It is important tounderstand that both the sonication and the presence of surfac-

tant molecules in the solution may alter the hybrids. Sonication can, in certain cases, result in detachment of the components of the hybrids from each other. If this goes unno- ticed, one might believe to characterize the properties of the hybrids, while the analyte contains separate host and guest spe- cies. For example, the strong luminescence, attributed to the in- ner tubes of double-walled carbon nanotubes (DWCNTs) [136], was later traced back to SWCNTs present in the sample. These thin SWCNTs were detected using separation by density gradi- ent ultracentrifugation and quenching the SWCNT photolumi- nescence by heavy doping and sidewall functionalization, which would have left the inner tubes intact [137]. These find- ings were also confirmed by experiments performed on individ- ual DWCNTs. Though inner tube photoluminescence was proved to exist, the photoluminescence quantum yield was found to be four orders of magnitude lower than that of a free- standing SWCNT. [138] The drastic decrease of inner tube lumi- nescence was attributed to the ultrafast inter-tube exciton transfer. Miyataet al. have demonstrated that it is possible to extract the inner tubes of DWCNTs by sonication [139]. The authors used a sample that has been purified of residual single or MWCNTs with a purity better than 90%. After strong sonica- tion in a surfactant-containing aqueous solution, an increase from 5 to 50% of the SWCNT population in the DWCNT sample was observed. Encapsulated species can also leach from inside the nanotubes, even without sonication, depending on the bal- ance of forces between the molecule and nanotube and mole- cule and solvent. Furthermore, as mentioned earlier, SWCNTs with partial encapsulation can easily get filled with other sol- vents they get in contact with during processing. Adsorbed species that withstood the washing steps, due to reasons mentioned earlier in the section ‘Filling the SWCNT", can also detach when the hybrids are sonicated in a surfactant- containing solution [19](Fig. 2). Even a very small amount of molecules liberated this way can have an unproportionally high contribution to the detected photoluminescence signal, com- pared to the molecules inside the SWCNTs. Optical absorption spectra of the hybrids are often compared to spectra of the en- capsulated molecules dissolved in a solvent or in solid state to account for energy shifts due to different aggregation states. It is worth to consider when choosing reference samples, how the guest molecules would behave in a surfactant-containing solu- tion, which is used to disperse the host nanotubes.

Transmission electron microscopy

High-resolution transmission electron microscopy can be used to visualize the successful encapsulation of molecules in carbon nanotubes. The filling ratio can be also determined by taking micrographs of different regions of the sample, estimating the proportion of filling of each area and averaging them. Imaging of organic molecules using transmission electron microscopy is challenging due to their high sensitivity to electron damage, though the confinement within the SWCNTs can significantly increase the stability of the guest molecules [140]. Fullerenes, ortho-carboranes and their derivatives are more straightfor- ward to identify due to their characteristic shapes [25,141-144]. In some cases, even the attached functional groups can be im- aged [143,145,146]. Alternatively, even if the functional group itself cannot be visualized, the increased interfullerene spacing of functionalized C 60
molecules compared to the bare ones can be observed and give information about the functional groups [52,147,148]. PAHs and their derivatives, or oligothiophenes

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can be identified based on their ordered aggregated states within the SWCNTs [19,57,67,68,70,73,149-151]. Nanoribbons can be distinguished from inner tubes based on moving twisted sections during the observation [59,64,89,109,152-154](Fig. 4). Besides using this technique for estimating the filling ratio of the encapsulated species or to observe the structures formed within the host tubes, the electron beam can also be used to in- duce reactions among the encapsulated species, which is dis- cussed in detail in the section ‘Nanosized reaction vessels".

Hybrid materials with improved properties

Encapsulation of molecules into SWCNTs can protect the guest molecules from the environment, and also serve to tune certain properties of the host and/or the guest. In this section, we show examples for each of these potential application areas. Encapsulation of molecules inside their cavity can tune the electronic and optical properties of the host SWCNTs. Although pristine SWCNTs are mostly susceptible to hole doping, both p- and n-tye doping have been shown to be possible by encapsu- lating organic donor or acceptor molecules [29]. The direction of charge transfer is determined by the ionization energy and the electron affinity of the guest molecules. Typical organic accept- ors are, besides C 60
, tetracyanoquinodimethane (TCNQ) deriva- tives, and donors tetrakis(diethylamino)ethylene (TDAE), tetrathiafulvalene (TTF) and metallocenes [29]. The doping level of the SWCNTs can be finely tuned, and the doping is also air- stable and can withstand further processing steps, a distinct ad- vantage compared to doping by gas or ion intercalation. Recently, Villalvaet al. showed that via encapsulation of iron- based spin crossover (SCO) molecules into SWCNTs, the con- ductance of the host tube can be modified by thermally switch- ing the spin state of the guest SCO molecules [95]. As the pristine SCO crystals are insulators, encapsulation inside the conductive host SWCNTs provides the means of electronically reading out the spin state of the SCO molecules, thus enabling their use in nanoelectronic applications. Furthermore, a hyster- esis was also observed to appear in the thermal switching be- haviour of these SCO molecules due to the confinement, therefore this SCO@SWCNT hybrid can also be utilized as a memory element. In cases when encapsulation does not lead to charge trans- fer, the altered dielectric environment inside the carbon nano- tubes still results in changes of the optical properties of the

SWCNTs. When the filling ratio is high enough,i.e.themolecules are close to each other, their properties can be de-

scribed by those of a continuous medium, characterized by a di- electric constant. This potential for optical property modulation was recently demonstrated in a systematic study by Campoet al. involving encapsulation of over 30 compounds of varying di- electric constants into a broad diameter range of SWCNTs [85]. A monotonic energy reduction of the nanotubes" optical transi- tions was observed with increasing dielectric constant of the encapsulated medium. Filling the SWCNTs with nonpolar or- ganic solvents prior to aqueous dispersion can also be used to exclude spontaneously encapsulated water and thus prevent perturbations to the electronic structure of the SWCNTs [84,85]. The inertness and robustness of carbon nanotubes naturally led to the idea to use them as protective packaging to prevent the degradation of molecules due to interactions with the envi- ronment. For example, it was shown that 1-heptene has re- duced reactivity to atomic hydrogen when encapsulated inside an SWCNT [155]. C 60
in SWCNTs is also protected from oxida- tion up to 800 ? C[156]. Kalba´?cet al.[157] conducted an experi- ment doping C 60
@SWCNT with potassium and exposing it to air. They found that while the doping level of the tubes decreased due to degradation of adsorbed potassium in air, the encapsu- lated potassium fulleride salt remained intact.b-carotene, which degrades easily under ambient conditions, was observed to have a significantly higher stability inside SWCNTs, as it was protected from interaction with radical species and isomeriza- tion was hindered due to the confinement [133]. The lumines- cence of the molecule was nevertheless quenched, due to excitation energy transfer from the chromophore to the host tube. This is in line with Kasha"s rule, which states that fluores- cence generally occurs from the excited state with the lowest energy. The energy separation between the highest occupied and lowest unoccupied molecular orbital (HOMO-LUMO gap) in organic dyes typically falls into the visible range, while the band gap of the nanotubes with a suitable size to accommodate them lies in the near infrared. Excitation energy transfer between the encapsulated dyes and the nanotubes was shown in case ofb- carotene, squarylium dye, perylene and quaterrylene, thio- phenes and porphyrin [18,69,76,78,79,81,83]. Dyes encapsu- lated inside SWCNTs can act as sensitizers, expanding the absorption range of the host SWCNTs, acting as acceptors of the photogenerated electron and/or energy. This type of functional- ization, because of the nature of the encapsulation, provides several advantages compared to covalent or non-covalent at- tachment to the outside of the nanotube wall. It can preserve both the remarkable electronic properties of the SWCNTs and protect the guest molecules from the environment. A few cases were published in the literature that reported a strong PL emis- sion from the encapsulated dyes themselves [57,67,68,104], though other works seemed to contradict these results [19,75,

158]. Some of the discrepancies can stem from the encapsula-

tion method. The outlier results were prepared by vapor-phase filling, and based on the reported Raman spectra by-product for- mation was identified later [19,20,65]. The other reason is po- tentially the disintegration of the components of the hybrids due to intense sonication, as we discussed in the section

‘Characterization".

Confinement inside the SWCNTs defines the preferential alignment of the guest molecules, which can lead to new, func- tional properties. The most important factor defining the struc- ture is the extent of the confinement, the size difference between the guest molecule and the diameter of the host SWCNT.

Cambre´et al. showed that p,p

0 -dimethylaminonitrostilbene (DANS), an asymmetric dye molecule can align in a head-to-tail Figure 4:Twists observed on sulfur-terminated GNRs (S-GNRs) inside SWCNTs synthesized by thermal annealing (a) and electron beam (b). The time series from top to bottom show the ribbons reversibly twist and untwist. The presence of such mobile twisted sections can be used to distinguish ribbons from inner tubes. Reprinted with permission from Ref. [89], https://doi.org/10.1021/ nn300137j. Copyright (2012) American Chemical Society.

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fashion inside SWCNTs of an appropriately chosen diameter range [13]. The DANS@SWCNT hybrid shows an extremely large second-order non-linear optical (NLO) response. It was estimated based on hyper-Rayleigh scattering measurements that the sam- ple contains coherently aligned arrays of almost up to 70 mole- cules inside the SWCNTs. Such an alignment would not be possible in the bulk, since Coulomb interactions would result in a pairwise antiparallel ordering, canceling out the NLO response. Due to the confinement, the alignment of the guest molecules is also thermodinamically stable, unlike forced alignments that are achieved, e.g. by electric poling in a heated polymer base. Van Bezouwet al. linked the broadened absorption of the chromo- phores in squarylium@SWCNT hybrids to contribution of differ- ent aggregate geometries of the guest molecules inside different nanotube chiralities by analyzing the excitation energy transfer bands on 2D PLE maps [83]. By comparing the excitation energy of the dye and the corresponding emission of the SWCNTs, a threshold diameter can be observed, above which not only a sin- gle file of molecules can be accommodated inside the host tube, but multiple geometries can appear (Fig. 5). Besides the size effects, also the balance between the host-guest and guest-guest interactions can play a role in the preferential alignment. The in- teraction between the SWCNT andp-conjugated guest molecules can depend on the electric properties of the host tubes. Koyama et al. observed that perylene molecules prefer to stack with each other inside semiconducting SWCNTs, while inside metallic host tubes they rather stack with the nanotube wall [66]. The authors attributed this difference to the larger polarizability of metallic SWCNTs, resulting in increased attractive interaction between the SWCNT and the perylene molecules. If the nanotube diame- ter is large enough to host various alignment geometries, the en- capsulation kinetics can also alter the structures forming within

the SWCNTs. Gaufre´set al. showed that sexithiophene moleculescan arrange as a single or a double row depending on the concen-

tration of the solution used for filling the SWCNTs [75]. Theoretical calculations indicate that single-row or stacked alignment of DANS molecules can also be achieved depending on the reaction kinetics [159,160].

Nanosized reaction vessels

This topic is one of the most active and fast-developing areas of hybrid nanotube chemistry and distinct subfields are emerging. One can by now distinguish between reactions where the start- ing material resides in the host tube and undergoes transforma- tions induced by external stimuli (‘nano-test tubes") or where only one of the reactants is confined to the tube and both the entry of other reactants and the exit of products is continuous (‘nanoreactors"). Confinement of molecules inside SWCNTs can significantly alter the chemical reactions they can undergo. The most straightforward use of these ‘nano-test tubes" is to grow long, quasi-1D molecules inside, the synthesis of which would other- wise not be feasible or be complicated. As SWCNTs are stable and inert under vacuum up to high temperatures, thermal acti- vation of reactions is possible inside their cavity. Typical examples for this templated growth are the synthe- ses of longp-conjugated polymers [20,72,73] and graphene nanoribbons (GNRs). Narrow GNRs of controlled width and edge topology are of great interest for various electronic applications. They can be synthesized via bottom-up fabrication methods us- ing surface-assisted reactions [161-164] or via templated growth [59,153,165]. GNR growth inside SWCNTs was first reported by Talyzinet al.[153]. The authors observed GNR formation from confined PAHs coronene and perylene using TEM and Raman spectroscopy (Fig. 6a and b). Later it was shown that the vapor- phase encapsulation method they used leads to formation of dimerized by-products that also adsorb onto the outer surface of the SWCNTs [19,61,65]. Botkaet al. showed that the GNR pre- cursor can be successfully encapsulated using sc-CO 2 as a sol- vent, which results in a well-defined starting material, ideal for GNR and inner nanotube growth [19,65](Fig. 2). Low- temperature encapsulation and consecutive annealing are pref- erable over the one-step high-temperature encapsulation, such as the one reported by Fujiharaet al.[58], to avoid the formation of problematic, insoluble larger PAHs that adhere to the outer surface of the SWCNTs. Limet al. followed the temperature dependence of the GNR formation inside the SWCNTs, and reported that polymerization of coronene leads to long,

GNR-like ribbons [59](Fig. 6b). 10,10

0 -dibromo-9,9 0 -bianthryl was shown earlier to be a suitable precursor for the growth of 7-arm- chair GNRs (7-AGNRs) on a gold surface via dehalogenative po- lymerization and dehydrogenation cyclization upon annealing [161]. Recently it was shown that the reaction can be performed inside SWCNTs as well [87](Fig. 6c). Ferrocene was also shown to facilitate growth of nanoribbons inside nanotubes [109,166]. In this case, a broad variety of GNRs can form, depending on the nanotube diameter. Kuzmanyet al. were able to identify

6- and 7-AGNR@SWCNT structures among annealed

ferrocene@SWCNT samples using Raman spectroscopy, but they found indications that 5- and 8-AGNRs can also be present in the sample (Fig. 6d). Besides thermal activation, an electron beam can also be used to induce reactions inside SWCNTs. The electron beam of a transmission electron microscope can act both as the energy source to initiate the reactions and as a medium to observe them. The reaction rates can be controlled by changing the Figure 5:Dependence of the absorption energy of the squarylium dye@SWCNT on the diameter of the host tube, revealed by the analysis of the position of the excitation energy transfer excitation peak as a function of the semiconducting SWCNT diameter. The pink shaded area represents the minimal encapsulation diameter range. The purple shaded area represents the diameter range above which a more complex behaviour is found with the diameter of the SWCNTs in- creasing. The measurement shows that the broadened absorption peak of the dyes inside the SWCNTs originates from the contribution of differently stacked dye structures depending on the confinement. Based on quantum chemical modeling, in the smaller diameter range the dye molecules align in a 1D array inside the SWCNTs, while inside larger tubes variousp-stacked geometries can exist. Reproduced with permission from Ref. [83], https://doi.org/10.1021/acs nano.8b02213. Copyright (2018) American Chemical Society.

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beam dose [149,150]. Ideal candidates for such experiments are molecules that can be easily recognized under the beam, such as ones having a well-defined symmetrical shape and/or strong single-molecule contrast. It is also preferable that the molecules do not enter into reactions with each other inside the SWCNT until exposure to the electron beam [149]. Chamberlainet al.[89] observed the formation of sulfur-terminated GNRs from tetra- thiafulvalene (Fig. 6f) both via thermal annealing and triggered by exposure to an electron beam. These sulfur-terminated GNRs were demonstrated to be more stable under the electron beam than the ones with hydrogen-terminated edges. Perchlorocoronene (PCC) and octathio [8] circulene (OTC) can also be converted into nanoribbons inside SWCNTs upon irradi- ation with the electron beam [149,150](Fig. 6e). The encapsulated GNRs can be transformed into inner car- bon nanotubes by annealing above 1000 ?

C[19,64]. Limet al. sug-

gested that the inner tubes form via connection of the edges of the twisted GNRs [64], in contrast to the fullerene precursor growth, where the deformed molecules open and merge to- gether [141]. These narrow, GNR-based inner tubes can be extracted from the host SWCNTs. This approach can be used to produce nanotubes with diameters not easily reachable by con- ventional processes. The removal of the long, quasi-1D molecules mentioned above has not been achieved yet, therefore the emphasis is on the application of these hybrid systems themselves. On the other hand, procedures have been found to use nanotubes as true ‘nanoreactors" of the flow reactor type, where the reaction takes place in the cavity and products leave, taking advantage of the special features of the nanotube interior [5]. Selectivity is an important requirement in organic chemis- try, meaning that the formation of one of the possible reaction products is preferred. Several theoretical works indicate that the confinement inside the SWCNTs can significantly alter the reaction pathway [167-170]. An example of regioselective con-

trol is the aromatic halogenation performed by Minerset al.[88].Bromination of N-phenylacetamide with pyridinium dichloro-

bromate (PyCl 2

Br) performed in bulk is a non-selective reaction

resulting inpara-andortho-products. Here, as first step, the low- melting reactant N-phenylacetamide was encapsulated from the liquid, the exterior of the filled tubes thoroughly cleaned, then the bromination was performed in aqueous solution by PyCl 2 Br. The nanotube diameter plays an important role during two steps: it has to be wide enough for the reactant to enter, yet narrow enough to prevent the formation of the bulkierortho- product. These constraints led to an optimal diameter of 0.7 nm. In this case, the products could freely diffuse out of the tube and thus close to 100% selectivity could be achieved. Besides the steric factors, the large dipole moment of the inter- mediate can be stabilized by the polarizable tube wall, thus re- ducing the activation energy barrier (Fig. 7). Sophisticated DFT/ molecular mechanics calculations have revealed that the cata- lytic role of HCl formed as an intermediate reaction product, and the van der Waals interaction between the guest molecules and the CNT walls both prefer the formation of thepara-isomer [170]. Another preparative reaction reported by Minerset al.[94], the 1,3-dipolar cycloaddition reaction between aromatic alkynes and benzyl azide, illustrates the role of both steric and

Figure 6:GNRs formed from various precursors. GNRs can be formed with regular and undulating edges from PAHs perylene (a) and coronene (b)[59,153]. Halogenated

precursors are ideal for well-defined GNR growth, which is less affected by the host diameter: 7-armchair GNRs (7-AGNRs) can be grown from 10,10

0 -dibromo-9,9 0 -

bianthryl (c)[87]. Annealing of ferrocene (d) results in a variety of nanoribbons, 7-AGNR is shown here, but 5-8 AGNRs were also observed [109]. 4-zigzag GNR with chlo-

rine-terminated edges grown from PCC (e)[149]. Depending on the host SWCNT diameter, various sulfur-terminated GNRs can be grown from tetrathiafulvalene (f)

[89]. Figure 7:Schematic representation of a halogenation reaction mechanism per- formed inside an SWCNT. The nanotube stabilizes the intermediate"s dipole moment for the preferential formation of thepara-product. Reproduced from Ref. [88], https://doi.org/10.1039/C3CC42414F. Copyright (2013) Royal Society of

Chemistry.

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electronic factors in selectivity. Following the considerations in the halogenation case [88], the optimal diameter was deter- mined as 1 nm. Interestingly, using wider (1.3 and 2 nm) nano- tubes still leads to increased selectivity compared with the free reactants. This effect was attributed to intermediate states where the size is unfavourable for the reaction leading to the minority product. In this study, various other factors influenc- ing the reaction pathway were considered: aromaticity, steric bulk and dipole moment. The results showed that while steric effects still dominate the selectivity, electronic structure also has to be taken into account. It was also observed that the mi- nority product showed a larger affinity to the nanotube interior and was therefore more difficult to remove, further enhancing the selectivity. Nevertheless, for purposes of analyzing the final product (in this case by NMR spectroscopy), it was possible to extract all molecules, indicating the direction of reusable nanoreactors. A further example of the creative use of carbon nanotube hybrids in organic synthesis is the encapsulation of metal nano- particles in SWCNTs to act as catalysts in gas-phase hydrogena- tion reactions. Chamberlainet al.[171] applied a continuous flow reactor with Ru nanoparticles confined in SWCNTs as cata- lyst and supercritical CO 2 as reaction solvent. The sc-CO 2 flow helped the reactants enter the SWCNT interior containing the catalyst nanoparticles. Although this method yielded a slightly lower turnover frequency than porous carbon supports, it in- creased the stability of the Ru nanoparticles significantly. Developments in this direction could open the possibility of large-scale preparative synthesis using SWCNTs.

Outlook: boron nitride nanotubes

Boron nitride, composed of the two neighbors of carbon in the periodic system, can form analogues of many carbon-based compounds, notably those based on six-membered rings. Starting from the benzene analogue, borazine [172] graphene- like hexagonal boron nitride (h-BN) can be constructed [173], and by rolling up an h-BN sheet, boron nitride nanotubes (BNNTs) are formed [174]. The aromatic character is somewhat preserved in the boron nitride rings [175], therefore the struc- ture remains planar, but the B-N bond shows considerable po- larity with electrons accumulated on the nitrogen atoms [176]. This polar character causes strong Coulomb interaction be- tween planes and at the same time decreases the mobility of charge carriers. Therefore, h-BN is an insulator with a large band gap and transparent in the visible range. Due to the strong interlayer interaction, BNNT samples consist mainly of few- walled (up to five) tubes and most samples have a much broader diameter distribution than SWCNTs. Both h-BN [177] and BNNTs have become the focus of interest lately, along with vari- ous heterostructures including CNTs [178]. BNNTs can be filled similarly to CNTs, but with some differ- ences. Their larger diameter is compatible with larger encapsu- lated molecules. The lowerp-electron density of BNNTs leads to weaker secondary bonds with the guests and weaker tendency to form bundles. Finally, their large band gap facilitates both the excitation and luminescence emission of the guest molecules. Suenagaet al.[179] in 1997 demonstrated the possibility of tubular heterostructures consisting of separate carbon and bo- ron nitride layers in an arc discharge experiment. The first en- capsulation reaction involving BNNTs was carried out by

Mickelsonet al.[180] by sublimation of C

60
. TEM images showed various packing patterns depending on the nanotube diameter.

Under prolonged electron beam irradiation, the fullerenemolecules fused into an inner nanotube. Vapor-phase encapsu-

lation of C 60
into BNNTs has been performed by Walkeret al. [181] who also succeeded to remove the fullerene molecules from the interior by sonication in toluene. In these hybrids that contain nanotubes with mean inner diameter 2.5 nm, C 60
mole- cules are ordered in a crystalline-like structure. They even show the well-known orientational phase transition at 250 K, detected by infrared spectroscopy. Annealing C 60
@BNNT to 1200
? C leads to inner carbon nanotube formation. Inner nano- tubes could be also produced from amorphous carbonaceous impurities in the inner cavity of a BNNT [182]. CNT@BNNT hybrids can also be grown by deposition of h-BN onto CNTs by various vapor deposition methods, which have been recently discussed extensively in the review by Joneset al.[178]. From the point of view of this review, the SWCNT-based systems pre- pared by the Maruyama group [183-185] stand out, as they also show the potential to include further layers based on 2D materi- als like molybdenum sulphide [183]. Nakanishiet al.[186] suc- ceeded in growing single-walled BNNTs inside SWCNTs. These achievements pave the way towards the use of such layered cy- lindrical ensembles in nanoelectronics, as nanoscale ‘coaxial cables". Because of the transparency of BNNT walls and the reduced electronic interaction, BNNTs may be even more suited to pro- tect luminescent molecules from quenching by the environ- ment. Such a study has been reported by Allardet al.[187]on dye molecules (sexithiophene and oligothiophene-substituted diketopyrrolopyrroles, DPP) transferred into BNNTs by liquid en- capsulation. Because of the large inner diameter, branching DPP molecules also could be investigated besides linear sexithio- phene. As a consequence of the broad diameter distribution of BNNTs, several aggregation states occur in the samples, show- ing large redshifts in luminescence spectra. The external nano- tubes also provided effective protection against photobleaching and chemical reactions. Photopolymerization of fullerenes inside BNNTs was fol- lowed by Datzet al.[188] on the nanoscale using near-field infra- red microscopy. Ano