Low-Dimensional Carbon Allotropes: Ground- and Excited-State Charge Transfer with NIR-Absorbing Heptamethine Cyanine

Abstract

•Heptamethine cyanine-pyrene forms stable supramolecular aggregates with SWCNTs and NG•The electron-donating character of cyanines enables the n-doping of carbon allotropes•Statistical Raman analysis is a powerful technique for investigating doping effects•Transient absorption spectroscopy allows determination of the shift or transfer of charges Doping is one of the methods used for controlling the electronic properties of carbon nanostructures. Chemical modification by non-covalent means allows the type and the concentration of carriers to be fine-tuned without affecting the conjugated carbon sp2 frameworks. Besides doping, functionalization schemes that confer an additional element of control over the nanomaterial properties are particularly promising and, in this sense, carbon nanostructures with photo- and electroactive molecules attract considerable attention for artificial photosynthesis or in photovoltaics. Such design principles have been considered for the construction of the nanomaterials presented here, where single-walled carbon nanotubes and nanographene form stable hybrids with heptamethine cyanine near-infrared dyes. We have studied the excited-state dynamics and established the presence of charge-transfer processes that can set the foundation for developing nanomaterials to be incorporated in next-generation devices. At the focal point of our studies are mutual interactions between an anionic heptamethine cyanine and individualized single-walled carbon nanotubes (SWCNTs) or exfoliated nanographene (NG). Here, we report the distinct near-infrared absorption of the heptamethine cyanine, which assists in visualizing the electronic interactions in the ground state with ease and precision. In statistical Raman assays, we conclude from downshifted 2D- and G-modes for SWCNTs, as well as upshifted 2D- and downshifted G-modes for NG, that these electronic interactions result in stable n-doping of SWCNTs and NG. Key to this shift of charge density is the electron-donating character of the heptamethine cyanine. In the excited state, a complete but metastable transfer of charges is accompanied by a radical-ion-pair state that lives for several nanoseconds. Such a time domain has not yet been realized, for example, in NG to date. At the focal point of our studies are mutual interactions between an anionic heptamethine cyanine and individualized single-walled carbon nanotubes (SWCNTs) or exfoliated nanographene (NG). Here, we report the distinct near-infrared absorption of the heptamethine cyanine, which assists in visualizing the electronic interactions in the ground state with ease and precision. In statistical Raman assays, we conclude from downshifted 2D- and G-modes for SWCNTs, as well as upshifted 2D- and downshifted G-modes for NG, that these electronic interactions result in stable n-doping of SWCNTs and NG. Key to this shift of charge density is the electron-donating character of the heptamethine cyanine. In the excited state, a complete but metastable transfer of charges is accompanied by a radical-ion-pair state that lives for several nanoseconds. Such a time domain has not yet been realized, for example, in NG to date. Low-dimensional carbon allotropes, including fullerenes,1Kroto H.W. Heath J.R. O'Brien S.C. Curl R.F. Smalley R.E. C60: buckminsterfullerene.Nature. 1985; 318: 162-163Crossref Scopus (14004) Google Scholar single-walled carbon nanotubes (SWCNTs),2Iijima S. Helical microtubules of graphitic carbon.Nature. 1991; 354: 56-58Crossref Scopus (39495) Google Scholar, 3Iijima S. Ichihashi T. Single-shell carbon nanotubes of 1-nm diameter.Nature. 1993; 363: 603-605Crossref Scopus (7726) Google Scholar, 4Bethune D.S. Klang C.H. de Vries M.S. Gorman G. Savoy R. Vazquez J. Beyers R. Cobalt-catalysed growth of carbon nanotubes with single-atomic-layer walls.Nature. 1993; 363: 605-607Crossref Scopus (3575) Google Scholar and nanographene (NG),5Geim A.K. Novoselov K.S. The rise of graphene.Nat. Mater. 2007; 6: 183-191Crossref PubMed Scopus (33567) Google Scholar represent interesting nanoscale building blocks because they feature physical and chemical properties that are of tremendous interest for applications in the emerging field of nanotechnology.6Sharon M. Sharon M. Carbon Nano Forms and Applications. McGraw-Hill, 2009Google Scholar, 7Guldi D.M. Martín N. Carbon Nanotubes and Related Structures: Synthesis, Characterization, Functionalization, and Applications. John Wiley, 2010Crossref Scopus (25) Google Scholar, 8Akasaka T. Wudl F. Nagase S. Chemistry of Nanocarbons. John Wiley, 2010Crossref Scopus (0) Google Scholar The most prominent examples for applications are found in the areas of organic solar cells,9Chang D.W. Choi H.J. Filer A. Baek J.B. Graphene in photovoltaic applications: organic photovoltaic cells (OPVs) and dye-sensitized solar cells (DSSCs).J. Mater. Chem. A. 2014; 2: 12136-12149Crossref Google Scholar, 10Martín N. Carbon nanoforms for photovoltaics: myth or reality?.Adv. Energy Mater. 2016; https://doi.org/10.1002/aenm.201601102Crossref Scopus (33) Google Scholar ultrasensitive biosensors,11Zan X. Bai H. Wang C. Zhao F. Duan H. Graphene paper decorated with a 2D array of dendritic platinum nanoparticles for ultrasensitive electrochemical detection of dopamine secreted by live cells.Chem. Eur. J. 2016; 22: 5204-5210Crossref PubMed Scopus (50) Google Scholar, 12Shi J. Li X. Chen Q. Gao K. Song H. Guo S. Li Q. Fang M. Liu W. Liu H. et al.A monocrystal graphene domain biosensor array with differential output for real-time monitoring of glucose and normal saline.Nanoscale. 2015; 7: 7867-7872Crossref PubMed Google Scholar, 13Zhu A.Y. Yi F. Reed J.C. Zhu H. Cubukcu E. Optoelectromechanical multimodal biosensor with graphene active region.Nano Lett. 2014; 14: 5641-5649Crossref PubMed Scopus (56) Google Scholar fuel cells,14Yang L. Wang S. Peng S. Jiang H. Zhang Y. Deng W. Tan Y. Ma M. Xie Q. Facile fabrication of graphene-containing foam as a high-performance anode for microbial fuel cells.Chem. Eur. J. 2015; 21: 10634-10638Crossref PubMed Scopus (31) Google Scholar, 15Choi H.J. Ashok Kumar N. Baek J.B. Graphene supported non-precious metal-macrocycle catalysts for oxygen reduction reaction in fuel cells.Nanoscale. 2015; 7: 6991-6998Crossref PubMed Google Scholar, 16Liu M. Zhang R. Chen W. Graphene-supported nanoelectrocatalysts for fuel cells: synthesis, properties, and applications.Chem. Rev. 2014; 114: 5117-5160Crossref PubMed Scopus (845) Google Scholar conductive electrodes for touch screens,17Bae S. Kim H. Lee Y. Xu X. Park J.S. Zheng Y. Balakrishnan J. Lei T. Ri Kim H. Song Y.I. et al.Roll-to-roll production of 30-inch graphene films for transparent electrodes.Nat. Nanotechnol. 2010; 5: 574-578Crossref PubMed Scopus (6940) Google Scholar and field-effect transistors devices.18Lin Y.M. Dimitrakopoulos C. Jenkins K.A. Farmer D.B. Chiu H.Y. Grill A. Avouris P. 100-GHz transistors from wafer-scale epitaxial graphene.Science. 2010; 327: 662Crossref PubMed Scopus (2238) Google Scholar To date, exploring the full potential of SWCNTs and graphene constitutes one of the biggest challenges in the field. A clear bottleneck is their individualization or exfoliation in combination with their stabilization in dispersion. In recent years, chemical functionalization, that is, covalent and non-covalent chemistry of SWCNTs and NG, has led to remarkable advances in the field.19Dirian K. Herranz M.A. Katsukis G. Malig J. Rodríguez-Pérez L. Romero-Nieto C. Strauss V. Martín N. Guldi D.M. Low dimensional nanocarbons – chemistry and energy/electron transfer reactions.Chem. Sci. 2013; 4: 4335-4353Crossref Scopus (93) Google Scholar For instance, covalent derivatization of SWCNTs or NG with functional entities enables them to be solubilized or dispersed in common solvents.20Mateos-Gil J. Rodriguez-Perez L. Moreno Oliva M. Katsukis G. Romero-Nieto C. Herranz M.A. Guldi D.M. Martin N. Electroactive carbon nanoforms: a comparative study via sequential arylation and click chemistry reactions.Nanoscale. 2015; 7: 1193-1200Crossref PubMed Google Scholar, 21Hof F. Schäfer R.A. Weiss C. Hauke F. Hirsch A. Novel λ3-iodane-based functionalization of synthetic carbon allotropes (SCAs)—common concepts and quantification of the degree of addition.Chem. Eur. J. 2014; 20: 16644-16651Crossref PubMed Scopus (44) Google Scholar The side effects of most covalent reactions are uncontrolled modification of the extended carbon network in general and π-conjugation in particular. Notable also is the arbitrary alteration of the electronic structure, which leads to a complete loss of absorption and fluorescence in the case of SWCNTs. On the contrary, non-covalent functionalization enables the immobilization of functional entities via π-π stacking, hydrophobic, and charge-transfer interactions or any combination of these without any of the unwanted side effects.22Subrahmanyam K.S. Ghosh A. Gomathi A. Govindaraj A. Rao C.N.R. Covalent and non-covalent functionalization and solubilization of graphene.Nanosci. Nanotechnol. Lett. 2009; 1: 28-31Crossref Google Scholar, 23Su Q. Pang S. Alijani V. Li C. Feng X. Müllen K. Composites of graphene with large aromatic molecules.Adv. Mater. 2009; 21: 3191-3195Crossref Scopus (736) Google Scholar, 24Englert J.M. Röhrl J. Schmidt C.D. Graupner R. Hundhausen M. Hauke F. Hirsch A. Soluble graphene: generation of aqueous graphene solutions aided by a perylenebisimide-based bolaamphiphile.Adv. Mater. 2009; 21: 4265-4269Crossref Scopus (194) Google Scholar, 25Xu Y. Zhao L. Bai H. Hong W. Li C. Shi G. Chemically converted graphene induced molecular flattening of 5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphyrin and its application for optical detection of cadmium(II) ions.J. Am. Chem. Soc. 2009; 131: 13490-13497Crossref PubMed Scopus (493) Google Scholar In this context, the role of planar and aromatic building blocks such as porphyrins,26Geng J. Kong B.S. Yang S.B. Jung H.T. Preparation of graphene relying on porphyrin exfoliation of graphite.Chem. Commun. 2010; 46: 5091-5093Crossref PubMed Scopus (149) Google Scholar pyrene derivatives,23Su Q. Pang S. Alijani V. Li C. Feng X. Müllen K. Composites of graphene with large aromatic molecules.Adv. Mater. 2009; 21: 3191-3195Crossref Scopus (736) Google Scholar, 27An X. Simmons T. Shah R. Wolfe C. Lewis K.M. Washington M. Nayak S.K. Talapatra S. Kar S. Stable aqueous dispersions of noncovalently functionalized graphene from graphite and their multifunctional high-performance applications.Nano Lett. 2010; 10: 4295-4301Crossref PubMed Scopus (425) Google Scholar, 28Sampath S. Basuray A.N. Hartlieb K.J. Aytun T. Stupp S.I. Stoddart J.F. Direct exfoliation of graphite to graphene in aqueous media with diazaperopyrenium dications.Adv. Mater. 2013; 25: 2740-2745Crossref PubMed Scopus (87) Google Scholar, 29Zhang F. Chen X. Boulos R.A. Md Yasin F. Lu H. Raston C. Zhang H. Pyrene-conjugated hyaluronan facilitated exfoliation and stabilisation of low dimensional nanomaterials in water.Chem. Commun. 2013; 49: 4845-4847Crossref PubMed Scopus (54) Google Scholar perylene diimides,30Kozhemyakina N.V. Englert J.M. Yang G. Spiecker E. Schmidt C.D. Hauke F. Hirsch A. Non-covalent chemistry of graphene: electronic communication with dendronized perylene bisimides.Adv. Mater. 2010; 22: 5483-5487Crossref PubMed Scopus (123) Google Scholar and conjugated polymers,31Liu Z. Liu Q. Huang Y. Ma Y. Yin S. Zhang X. Sun W. Chen Y. Organic photovoltaic devices based on a novel acceptor material: graphene.Adv. Mater. 2008; 20: 3924-3930Crossref Scopus (810) Google Scholar, 32Castelain M. Salavagione H.J. Gomez R. Segura J.L. Supramolecular assembly of graphene with functionalized poly(fluorene-alt-phenylene): the role of the anthraquinone pendant groups.Chem. Commun. 2011; 47: 7677-7679Crossref PubMed Scopus (23) Google Scholar which all exhibit a strong affinity to interact with SWCNTs and NG, has been particularly beneficial in terms of stability. Non-covalent functionalization assists in blending the intrinsic properties of SWCNTs or NG with those of virtually any other functional entity. Solubility and dispersability, next to electron donation or withdrawal and light harvesting, stand out among the properties.33Malig J. Jux N. Guldi D.M. Toward multifunctional wet chemically functionalized graphene—integration of oligomeric, molecular, and particulate building blocks that reveal photoactivity and redox activity.Acc. Chem. Res. 2013; 46: 53-64Crossref PubMed Scopus (73) Google Scholar, 34Fukuzumi S. Kojima T. Photofunctional nanomaterials composed of multiporphyrins and carbon-based π-electron acceptors.J. Mater. Chem. 2008; 18: 1427-1439Crossref Scopus (309) Google Scholar In the current work, we explored the synthesis of a cyanine-pyrene conjugate and, subsequently, the interactions with low-dimensional carbon allotropes. The aim was to realize stable electron-donor acceptor hybrids in dispersions. On the one hand, pyrene as a π-π stacking anchor is known for its strong affinity to π-conjugated carbon networks.35Herranz M.Á. Ehli C. Campidelli S. Gutiérrez M. Hug G.L. Ohkubo K. Fukuzumi S. Prato M. Martín N. Guldi D.M. Spectroscopic characterization of photolytically generated radical ion pairs in single-wall carbon nanotubes bearing surface-immobilized tetrathiafulvalenes.J. Am. Chem. Soc. 2008; 130: 66-73Crossref PubMed Scopus (118) Google Scholar, 36KC C.B. Das S.K. Ohkubo K. Fukuzumi S. D'Souza F. Ultrafast charge separation in supramolecular tetrapyrrole–graphene hybrids.Chem. Commun. 2012; 48: 11859-11861Crossref PubMed Scopus (47) Google Scholar, 37Roth A. Ragoussi M.E. Wibmer L. Katsukis G. Torre G.D.L. Torres T. Guldi D.M. Electron-accepting phthalocyanine–pyrene conjugates: towards liquid phase exfoliation of graphite and photoactive nanohybrid formation with graphene.Chem. Sci. 2014; 5: 3432-3438Crossref Google Scholar On the other hand, cyanines are a class of ionic light harvester with a carbon skeleton with an odd number of π-conjugated carbon atoms. They exhibit unique absorption properties with high extinction coefficients in the near-infrared (NIR) region and good chemical stability. The absorption maxima of heptamethine cyanines are located at wavelengths of 750 nm and beyond, and their extinction coefficients exceed 105 M−1 cm−1. By virtue of their electron-donating strength, they have emerged as a complement to fullerenes, which are the omnipresent electron acceptors in solar energy conversion schemes.38Villegas C. Krokos E. Bouit P.-A. Delgado J.L. Guldi D.M. Martin N. Efficient light harvesting anionic heptamethine cyanine–[60] and [70]fullerene hybrids.Energy Environ. Sci. 2011; 4: 679-684Crossref Scopus (33) Google Scholar, 39Bouit P.-A. Spänig F. Kuzmanich G. Krokos E. Oelsner C. Garcia-Garibay M.A. Delgado J.L. Martín N. Guldi D.M. Efficient utilization of higher-lying excited states to trigger charge-transfer events.Chem. Eur. J. 2010; 16: 9638-9645Crossref PubMed Scopus (34) Google Scholar Importantly, electronic interactions between SWCNTs or NG and heptamethine cyanine are traceable by Raman spectroscopy, which, in the current work, corroborates the n-doping of the carbon allotropes. In the case of SWCNTs, which are intrinsically p-doped, molecular n-doping is rather scarce. The synthesis of the electron-donating cyanine-pyrene 1 was based on an esterification reaction between previously obtained 4-hydroxybenzyl-cyanine40Nieto C.R. Guilleme J. Villegas C. Delgado J.L. Gonzalez-Rodriguez D. Martin N. Torres T. Guldi D.M. Subphthalocyanine-polymethine cyanine conjugate: an all organic panchromatic light harvester that reveals charge transfer.J. Mater. Chem. 2011; 21: 15914-15918Crossref Scopus (36) Google Scholar and 1-pyrene butyric acid in the presence of N-[3-(diethylamino)propyl]-N′-ethylcarbodiimide hydrochloride (EDC·HCl) and 4-dimethylaminopyridine (DMAP) in 66% yield (Scheme 1). The structure of 1 was confirmed by standard spectroscopic techniques including Fourier transform infrared (FTIR), 1H and 13C nuclear magnetic resonance (NMR), and UV-visible-NIR spectroscopy and high-resolution mass spectrometry (see Experimental Procedures for details). From steady-state absorption spectroscopy, maxima were observed for 1 in methanol between 230 and 350 nm and at 874 nm. The earlier maxima relate to the absorption of the pyrene anchor, whereas the latter is associated with the heptamethine cyanine. As a complement, fluorescence measurements were conducted. Figure S1 shows that, regardless of an exciting pyrene or heptamethine cyanine, the strong NIR fluorescence of heptamethine cyanines evolved. An energy transfer from pyrene activated the heptamethine-cyanine-centered fluorescence. In line with energy transfer, fluorescence quantum yields of 0.05% and 4.5% were obtained for the pyrene and heptamethine cyanine fluorescence, respectively. Turning to electrochemistry, 1 was investigated in methanol. A single reduction was observed at −1.06 V, and three oxidations were seen at −0.03, +0.55, and +0.83 V, all versus Fc/Fc+. With this information at hand, we investigated the spectroelectrochemical oxidation of 1 in methanol with tetrabutylammonium perchlorate (TBAClO4) as supporting electrolyte. We obtained the differential absorption spectra (Figure S2) by applying a potential of 0.45 V versus a silver wire. Intense bleaching of the ground-state absorption at 874 nm of 1 was discernable. In addition, new absorption maxima emerged at 513, 540, 585, 641, and 1,068 nm. Finally, femtosecond transient absorption spectroscopic measurements were performed. The differential absorption spectra of 1 in methanol, which are shown in Figure S3, reflected the absorption characteristics of the singlet excited state given that they developed immediately after excitation at 387 nm. New maxima at 554, 614, and 1,246 nm, new shoulders at 494, 561, and 625 nm, and a new minimum at 893 nm were discernable. The 893 nm minimum was accompanied by a 972 nm shoulder as a superimposition of ground-state bleaching and stimulated fluorescence. Information regarding the deactivation was derived from global fitting. Short-lived and long-lived components of 3.4 and 171 ps, respectively, were obtained (Figure S4). The short-lived component was due to the population of the first singlet excited state, which was most likely formed by vibrational relaxation. After its formation, the first singlet excited state decayed as the long-lived component occurred via recovery of the ground state. Next, protocols for forming hybrids of 1 with either SWCNTs (SWCNT-1) or NG (NG-1) were developed41In the following paragraphs, the term graphene is also used for graphene with few layers for simplification. (see Experimental Procedures for details). The step-by-step formation was monitored by steady-state absorption and by fluorescence spectroscopy. Figure 1 shows that the original absorption features of 1 at 874 nm decreased upon the addition of SWCNTs or NG, and a new absorption feature occurred at 944 nm. Moreover, the fluorescence of 1 was completely quenched upon the final enrichment step (Figure S5). The impact of the hybrid formation on the SWCNT-centered transitions was investigated in titration assays. By means of a stepwise addition of 1 to predispersed SWCNTs in methanol, influences stemming from improved debundling of SWCNTs were excluded. The red shifts of, for instance, the (7,6)-SWCNT-centered transition from 1,169 to 1,175 nm (Figure S6) during the titration testify to mutual interactions between 1 and SWCNTs. In the following paragraphs, the term graphene is also used for graphene with few layers for simplification. To rule out possible decomposition of 1 during SWCNT-1 or NG-1 formation, we disintegrated them into the individual components. Here, we opted for Tween 60, a non-ionic surfactant, to convert SWCNT-1 and NG-1 to SWCNTs, NG, and 1 (Figures S7 and S8) and to monitor the absorption spectra. Thereby, the recovery of the initial absorption spectrum of 1 upon the addition of Tween 60 corroborates the fact that SWCNT-1 and NG-1 are based on non-covalent forces between 1 and SWCNTs and NG, respectively. In addition to absorption spectroscopy, we also compared 1 and NG-1 in electrochemical assays. Upon enrichment of NG in a solution of 1, a shift of the first reduction from −1.07 to −0.88 V was observed for NG-1 (Figure S9). In contrast, only negligible changes were noted for the first oxidation. In other words, the electrochemical gap between the first reduction and the first oxidation was reduced by 0.21 V when going from 1 to NG-1. This decrease in band gap is in line with the 70 nm red shift seen in the absorption spectra and signifies that ground-state interactions mostly affect the lowest unoccupied molecular orbital of 1. Furthermore, the formation of SWCNT-1 and NG-1 was corroborated by thermogravimetric analysis (TGA) under inert conditions. The decomposition of 1 occurred in two steps, namely at 195.5°C and 289.4°C, leading to an overall weight loss of 85%. In contrast, SWCNTs and NG were stable up to 800°C with minor weight losses of 6.82% and 11.73%, respectively (Figures S10 and S11). For SWCNT-1 and NG-1, an additional weight loss was observed, which most likely corresponds to the release of 1 from the surface of SWCNT (Figure S10) or NG (Figure S11). For SWCNT-1, the desorption of 1 caused a weight loss of 11.52%, whereas for NG-1, an additional 14.05% weight loss relative to that of NG was observed. FTIR spectroscopy and X-ray photoelectron spectroscopy (XPS) provided further information regarding the composition of SWCNT-1 and NG-1. Carbonyl and cyano functional groups were easily identified via FTIR spectroscopy by their stretching vibrations at 1,736 and 2,214 cm−1, respectively (Figure S12). Furthermore, evidence for the presence of carbon nanomaterial was gathered: the in-plane vibration band of C=C was recognized at 1,575 cm−1 for SWCNT-1 and at 1,584 cm−1 for NG-1. The aliphatic C–H stretching vibrations at ca. 2,850–2,950 cm−1 and the bending vibrations at ca. 1,250–1,500 cm−1 were also discernible. Insights into the elemental composition of 1, SWCNT-1, and NG-1 are based on the element signals in XPS for C (1s), N (1s), and O (1s) at 284.6, 398.6, and 532.6 eV, respectively (Figure S13). The high-resolution C 1s core-level spectra of SWCNT-1 and NG-1 are interpreted in terms of photoelectrons originating from sp2Iijima S. Helical microtubules of graphitic carbon.Nature. 1991; 354: 56-58Crossref Scopus (39495) Google Scholar carbon atoms, sp3 carbon atoms, and oxidized carbon atoms as part of C–O, C–N, and C=O bonds. In the O 1s spectra, signals at 531.4 and 532.1 eV are assigned to C=O surface groups and C–O groups, respectively. Finally, the high-resolution N 1s core-level spectra analysis shed light on three different contributions: 403.1, 399.7, and 398.8 eV for NG-1 and SWCNT-1 (Figure S13). The first is attributed to quaternary ammonium salts. The second is assigned to cyano groups as an integrative part of the cyanine core. Finally, the third is assigned to cyano groups, which are shifted toward lower binding energies because they are predominantly involved in the stabilization of the negative charge density of the cyanine.42Rodríguez-Pérez L. García R. Herranz M.Á. Martín N. Modified SWCNTs with amphoteric redox and solubilizing properties.Chem. Eur. J. 2014; 20: 7278-7286Crossref PubMed Scopus (14) Google Scholar, 43Tseng T.C. Urban C. Wang Y. Otero R. Tait S.L. Alcamí M. Écija D. Trelka M. Gallego J.M. Lin N. et al.Charge-transfer-induced structural rearrangements at both sides of organic/metal interfaces.Nat. Chem. 2010; 2: 374-379Crossref PubMed Scopus (248) Google Scholar Microscopic insights into SWCNT-1 and NG-1 prepared in methanol solutions and drop casted onto lacey carbon-coated copper grids were obtained by TEM (Figure 1). The enhanced stability of SWCNT-1 and NG-1 dispersions was confirmed by comparison with SWCNT and NG dispersions, which revealed aggregation on a timescale of a few minutes (Figure S14). For SWCNT-1, TEM images pointed to debundling of the SWCNT and to the coexistence of individual SWCNT-1 and smaller bundles of SWCNT-1. Regarding NG-1, exfoliation of graphite was evident by the existence of barely restacked, turbostratic NG as a result of interactions with 1. For reference, poor stabilization of individualized SWCNTs and exfoliated NG by means of the solvent was observed in the form of rebundling or restacking (Figures S15 and S16). Insights into doping effects were gathered from statistical analysis of the measured Raman data. Because of a change in sample preparation, no single-layer graphene or highly debundled SWCNT was expected to be found in these measurements (for preparation details, see Experimental Procedures). We conducted these analyses by measuring about 1,000 spectra of each sample with 532 or 633 nm laser excitation. Cluster analyses of the SWCNT and NG reference systems, as well as SWCNT-1 and NG-1, were applied and compared. The resulting mean spectra for SWCNTs and SWCNT-1, as well as NG and NG-1, are illustrated in Figure 2. In terms of SWCNTs and SWCNT-1, the radial breathing modes (RBM) for (8,7) and (7,6) SWCNTs were detected at 222 and 258.5 cm−1 for SWCNT and at 220 and 258 cm−1 for SWCNT-1. Comparison of the G and 2D modes at 1,591 and 2,607 cm−1, respectively, indicated downshifts for SWCNT-1. In particular, the G mode shifted to 1,588 cm−1, whereas the 2D mode was seen at 2,604 cm−1. In good agreement with recent reports, we ascribe the downshifts in SWCNT-1 relative to SWCNTs to an n-type doping.44Maciel I.O. Anderson N. Pimenta M.A. Hartschuh A. Qian H. Terrones M. Terrones H. Campos-Delgado J. Rao A.M. Novotny L. et al.Electron and phonon renormalization near charged defects in carbon nanotubes.Nat. Mater. 2008; 7: 878-883Crossref PubMed Scopus (241) Google Scholar, 45Voggu R. Rout C.S. Franklin A.D. Fisher T.S. Rao C.N.R. Extraordinary sensitivity of the electronic structure and properties of single-walled carbon nanotubes to molecular charge-transfer.J. Phys. Chem. C. 2008; 112: 13053-13056Crossref Scopus (130) Google Scholar In terms of NG and NG-1, D modes were found in both cases at 1,343 cm−1, whereas subtle shifts evolved for the G and 2D modes. The G modes were observed at 1,576 and 1,572 cm−1 and the 2D modes were observed at 2,680 and 2,695 cm−1 for NG and NG-1, respectively. Our results, namely an upshifted 2D mode in combination with a downshifted G band, also indicate an n-type doping of this carbon allotrope.46Dong X. Fu D. Fang W. Shi Y. Chen P. Li L.-J. Doping single-layer graphene with aromatic molecules.Small. 2009; 5: 1422-1426Crossref PubMed Scopus (527) Google Scholar Insights into the excited-state dynamics of SWCNT-1 and NG-1 were obtained from transient absorption measurements, which were analyzed by global and target analyses. Figure 3 shows deconvoluted transient absorption spectra of SWCNT-1 and NG-1 obtained by target and global analysis and their corresponding concentration-time profiles. At first glance, none of them reveal the characteristic fingerprints of photoexcited 1 (see above). The lack of ground-state bleaching of 1 at 874 nm confirms the absence of any free, non-immobilized 1. Instead, the bleaching is in line with the ground-state absorptions of SWCNT-1 and NG-1 red shifted to 944 and 947 nm, respectively. Figures S19 and S21 show the corresponding evolution-associated spectra after 387 nm excitation of the SWCNT and NG references. In the case of the SWCNT reference, instantaneous formation of minima at 665, 743, 1017, 1170, and 1,312 nm corresponding to the ground-state bleaching of the S22 and S11 transitions is discernible (Figure S17). In line with improved debundling observed on microscopy, these are hypsochromically shifted for SWCNT-1 to 661, 738, 1006, 1156, and 1,302 nm (Figure S20). Global and target analysis gave lifetimes of 0.4, 1.89, and 115 ps for SWCNTs and 0.37, 1.77, and 59 ps as well as 2.40 ns for SWCNT-1 and 3.4 and 175 ps for 1. For both NG and NG-1, the broad and featureless negative transients in the NIR, which were caused by phonon-related bleaching of NG, are discernible (Figure S18). For this bleaching, a lifetime of 1.7 and 1.4 ps was determined for NG and NG-1, respectively. Next to the rather broad bleaching, new distinct features in the form of a minimum at 944 nm and a maximum at 996 nm were seen for NG-1 (Figure S22). However, no such transients were found for 1 or NG. From global analysis, we derived a lifetime of 2.63 ns for the latter transient. The decay and evolution-associated spectra of SWCNT-1 and NG-1 related to the longest lifetimes both have a minimum at 944 nm and a maximum at around 996 nm, which resemble the pattern of oxidized 1. It is slightly superimposed by SWCNT-centered transients in SWCNT-1. Taking the aforementioned into account, we reached the conclusion that photoexciting both SWCNT-1 and NG-1 is the start of an ultrafast charge separation. As such, radical-ion-pair states in which 1 is oxidized and either SWCNTs or NG is reduced are formed. Charge recombination at 2.40 and 2.63 ns is much slower and affords the ground state. In summary, we have synthesized conjugate 1 by linking pyrene as an anchor with a NIR-absorbing anionic heptamethine cyanine as an electron donor to enable π-π stacking onto low-dimensional carbon allotropes and to redistribute the electron density. Complementary investigations were based on microscopic and spectroscopic assays to shed light on (1) individualizing SWCNTs and exfoliating graphite by means of 1 and (2) n-type doping of individualized SWNCTs and exfoliated NG. By means of microscopic analyses, the individualization of SWCNTs and exfoliation of graphite and the stability of the resulting dispersions were confirmed. Spectroscopic analyses of SWCNT-1 and NG-1 corroborated the partial shift of electron density from 1 to SWCNT or NG in the electronically dark state and the full shift of electron density in the electronically excited state. The earlier shift, with characteristics of a charge-transfer state, was stable, whereas the latter was a charge-separated state and lived for nanoseconds. EDC·HCl (0.12 mmol) and DMAP (0.12 mmol) were added to 30 mL of CH2Cl2. The mixture was stirred for 20 min at 0°C (ice-water bath) under Ar. Then, 4-hydroxybenzyl-cyanine40Nieto C.R. Guilleme J. Villegas C. Delgado J.L. Gonzalez-Rodriguez D. Martin N. Torres T. Guldi D.M. Subphthalocyanine-polymethine cyanine conjugate: an all organic panchromatic light harvester that reveals charge transfer.J. Mater. Chem. 2011; 21: 15914-15918Crossref Scopus (36) Google Scholar (0.09 mmol) was added dropwise, and the mixture was stirred for another 30 min. The cooling bath was removed, and the mixture was stirred at room temperature for 24 hr. The reaction mixture was washed with deionized water (4 × 30 mL). The organic layer was dried over MgSO4, filtered, and evaporated. The residue was subjected to column chromatography in CH2Cl2/MeOH 9/1 for further purification. Yield: 66%. 1H NMR (300 MHz, DMSO-d6): δ = 8.40–7.91 (m, 9H, Ar-CH), 7.61 (d, J = 14.0 Hz, 2H, CH), 7.32 (d, J = 8.1 Hz, 2H, Ar-CH), 6.95 (d, J = 8.1 Hz, 2H, Ar-CH), 5.93 (d, J = 14.0 Hz, 2H, CH), 5.02 (s, 2H, CH2), 3.21–3.09 (m, 8H, CH2), 2.63–2.42 (m, 2H, CH2), 2.04–1.97 (m, 6H, CH2), 1.88–1.80 (m, 2H, CH2), 1.61–1.51 (m, 8H, CH2), 1.31 (s, 12H, CH3), 1.26–1.23 (m, 8H, CH2), 1.17 (t, J = 7.0 Hz, 2H, CH2), 0.94 (t, J = 7.0 Hz, 12H, CH3) (Figure S23). 13C NMR (75 MHz THF-d8): δ = 173.41, 137.21, 132.63, 132.17, 131.46, 131.25, 131.18, 129.86, 128.50, 128.42, 128.28, 127.58, 126.78, 126.15, 126.07, 125.90, 125.80, 125.71, 124.44, 116.03, 95.22, 66.23, 59.45, 34.41, 33.59, 28.09, 27.19, 20.74, 14.14 (Figure S24). UV-visible (1,2-dichlorobenzene [oDCB]) λmax: 906 (ɛ = 254,000 L·mol−1·cm−1), 346, 331 nm. FTIR (KBr): 2928, 2208, 1730, 1625, 1504, 1438, 1335, 1254, 1214, 1080, 1038, 1000, 903, 840, 729, 648 cm−1. HRMS (ESI−): m/z calculated for C57H43N6O5− = 891.3300; found = C57H43N6O5− = 891.3297. For the preparation of SWCNT and NG dispersions, pristine graphite or SWCNTs were added to either methanol or anhydrous oDCB and dispersed in a sonication bath for 10 min (sweep mode, 37 kHz, 330 W). After sonication, the resulting dispersions were centrifuged, SWCNTs at 5,000 × g for 10 min and NG at 2,000 rpm for 5 min, and the supernatant was separated from the precipitate. TEM images in oDCB showed that NG was formed in fewer than five layers stacked with smaller flakes on the surface (Figure S16). Lateral dimensions of the nanosheets ranged from 100 to 500 nm. The former dispersions were used for further characterization or hybrid formation. SWCNT-1 and NG-1 were produced via similar procedures. A solution of 1 was prepared, and pristine SWCNTs or graphite was added. The resulting mixtures were dispersed in an ultrasonication bath. Hybrid formation could be done either stepwise, where SWCNTs or graphite flakes were added in small portions to the solution of 1 with intermediate ultrasonication steps until no free 1 was in solution, or by the addition of an excess of 1 to SWCNT or NG dispersions and then only one sonication step. In the latter case, a filtration step was added, whereas for the first option, the samples were centrifuged in the last step for the removal of non-exfoliated graphite flakes or bundled SWCNTs. For avoiding misinterpretations due to differences in debundling or exfoliation in the case of Raman measurements, the sample preparation differed from the aforementioned cycle enrichment. Here, SWCNTs and NG were sonicated in methanol and drop casted onto Si/SiO2 wafers as references. Subsequently, SWCNT- and NG-coated wafers prepared in the same way were treated with a concentrated methanol solution of 1. FTIR (KBr): ν = 2924, 2214, 1736, 1635, 1575, 1456, 1401, 1,055 cm−1. TGA: weight loss and temperature desorption (organic anchoring groups): 11.52%, 650°C. XPS: % atomic: C (284.6 eV) = 93.14, N (398.6 eV) = 1.15, O (532.6 eV) = 5.11. FTIR (KBr): ν = 2934, 2214, 1736, 1628, 1584, 1404, 1384, 1117, 1050, 712, 623 cm−1. TGA: weight loss and temperature desorption (organic anchoring groups): 14.05%, 650°C. XPS: % atomic: C (284.6 eV) = 94.76, N (398.6 eV) = 1.09, O (532.6 eV) = 4.14. Conceptualization, M.A.H., N.M., and D.M.G.; Funding Acquisition, D.M.G. and N.M.; Investigation, A.R., C.S., A.F.-R., M.M., C.V., and L.R.-P.; Formal Analysis, L.R.-P. and M.A.H.; Writing – Review & Editing, all authors. This work was supported by the Deutsche Forschungsgemeinschaft as part of the Excellence Cluster Engineering of Advanced Materials and SFB 953 Synthetic Carbon Allotropes as well as by the Bavarian State Government as part of the Solar Technologies go Hybrid initiative. Financial support from the European Research Council (ERC-320441-Chirallcarbon), the Ministerio de Economía y Competitividad (MINECO) of Spain (projects CTQ2014-52045-R and CTQ2015-71936-REDT) and the CAM (PHOTOCARBON project S2013/MIT-2841) is also acknowledged. C.S. gratefully acknowledges the Fonds der Chemischen Industrie (FCI) for funding. Download .pdf (7.22 MB) Help with pdf files Document S1. Supplemental Experimental Procedures and Figures S1–S24 In the following paragraphs, the term graphene is also used for graphene with few layers for simplification.