Article www.acsnano.org Interaction of Luminescent Defects in Carbon Nanotubes with Covalently Attached Stable Organic Radicals Felix J. Berger, J. Alejandro de Sousa, Shen Zhao, Nicolas F. Zorn, Abdurrahman Ali El Yumin, Aleix Quintana García, Simon Settele, Alexander Högele, Núria Crivillers, and Jana Zaumseil* Cite This: ACS Nano 2021, 15, 5147−5157 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: The functionalization of single-walled carbon nanotubes (SWCNTs) with luminescent sp3 defects has greatly improved their performance in applications such as quantum light sources and bioimaging. Here, we report the covalent functionalization of purified semiconducting SWCNTs with stable organic radicals (perchlorotriphenylmethyl, PTM) carrying a net spin. This model system allows us to use the near-infrared photoluminescence arising from the defect- localized exciton as a highly sensitive probe for the short- range interaction between the PTM radical and the SWCNT. Our results point toward an increased triplet exciton population due to radical-enhanced intersystem crossing, which could provide access to the elusive triplet manifold in SWCNTs. Furthermore, this simple synthetic route to spin-labeled defects could enable magnetic resonance studies complementary to in vivo fluorescence imaging with functionalized SWCNTs and facilitate the scalable fabrication of spintronic devices with magnetically switchable charge transport. KEYWORDS: single-walled carbon nanotube, sp3 defect, stable organic radical, photoluminescence, triplet, magnetic field a function itself, such as sensing14−16 or anchoring of T he controlled covalent modification of single-walled carbon nanotubes (SWCNTs) has boosted their potential for a broad range of applications including quantum light emission,1 bioimaging,2 and sensing.3 The one- dimensional structure of SWCNTs, their large charge carrier biomolecules.17 A highly desirable property for functional groups attached to SWCNTs is a net spin, as carried by unpaired electrons. Previously, mixed-chirality samples of SWCNTs and multi- mobilities,4 and high exciton diffusivity5 make them ideal for walled carbon nanotubes were decorated with open-shell efficient interaction of charges and excitations with lattice transition metal18 or lanthanide19,20 complexes, and charge defects, thus further expanding the nanotubes’ properties. In transport measurements at cryogenic temperatures revealed recent years, the low-density functionalization of semiconduct- spin valve switching behavior without the need for ing SWCNTs with luminescent sp3 defects, sometimes referred ferromagnetic contacts.19,20 Another class of spin-bearing to as organic color centers or quantum defects, has received compounds are stable organic radicals, i.e., charge-neutral particular attention.6−8 The deep optical trap potentials (∼100 molecules with an unpaired electron.21,22 One of the most meV) of these defects created by binding of oxygen,9 aryl,10 or prominent examples is the perchlorinated triphenylmethyl alkyl11 substituents lead to efficient exciton localization. These (PTM) radical, in which a propeller-shaped arrangement of trapped excitons give rise to red-shifted photoluminescence phenyl rings protects the carbon-centered radical, resulting in a (PL) with long lifetimes and even enable room-temperature half-life of ∼100 years.23 The interaction of SWCNTs with the single-photon emission at telecommunication wavelengths.12 As the defect states arise from the perturbation of the SWCNTs’ electronic structure, their properties are dominated Received: December 10, 2020 by the precise binding pattern with respect to the SWCNT Accepted: February 5, 2021 lattice rather than the molecular structure of the substituents.13 Published: February 18, 2021 Hence, the molecular groups that are bound to the nanotube surface are often structurally simple, and only few examples are known, in which the attached group contributes and performs © 2021 The Authors. Published by American Chemical Society https://dx.doi.org/10.1021/acsnano.0c10341 5147 ACS Nano 2021, 15, 5147−5157 ACS Nano www.acsnano.org Article Figure 1. Reaction scheme depicting the functionalization of polymer-wrapped (6,5) SWCNTs with aryl defects bearing radical (PTM, red) or closed-shell (PTMH, ArBr, blue) substituents. Figure 2. (a) Normalized PL spectra recorded for dispersions of pristine and PTM- and PTMH-functionalized (6,5) SWCNTs in toluene. (b) EPR spectra (at room temperature) of these dispersions in addition to a dispersion of (6,5) SWCNTs with a high density of ArBr defects (h.f.) and a solution of the PTM-Dz precursor in tetrahydrofuran. unpaired spin of the radical via a luminescent sp3 defect may for magnetic resonance studies and spintronics, radical result in spin-dependent properties and provide optical means functionalization could improve the accessibility of the elusive for probing them. However, up to now there are no reports of triplet states in carbon nanotubes. stable organic radicals bound to carbon nanotubes or chirality- sorted SWCNTs with any covalent spin label. RESULTS AND DISCUSSION In this work, we functionalize purely semiconducting (6,5) SWCNT Functionalization and Characterization. SWCNTs with aryl defects linked to PTM radicals and use the Highly purified (6,5) SWCNTs (diameter ∼0.76 nm, average PL arising from the defect-localized exciton as a highly length 1.0−1.5 μm) wrapped with the fluorene−bipyridine sensitive probe for the short-range interaction between the copolymer PFO-BPy (see the Methods and Figure 1 for radical and the SWCNT. By comparing them to aryl defects molecular structure)24 serve as a robust model system for sp3 with closed-shell substituents, we observe PL quenching functionalization. As shown in Figure 1, diazonium (Dz) associated with the radical. Based on the PL decay dynamics, chemistry was used to introduce aryl defects bearing charge- we propose partial transfer of the exciton population to triplet neutral substituents with either radical (open-shell) or closed- states via radical-enhanced intersystem crossing as the shell character. Reacting (6,5) SWCNTs with the tailored dominant quenching mechanism, while a smaller portion diazonium salt PTM-Dz25,26 produced aryl defects on the may be attributed to a photoinduced electron transfer process. nanotube lattice that act as covalent linkers to PTM radicals. In addition to providing a simple route to spin-labeled defects To identify the impact of the radical on defect state PL, the 5148 https://dx.doi.org/10.1021/acsnano.0c10341 ACS Nano 2021, 15, 5147−5157 ACS Nano www.acsnano.org Article Figure 3. (a) Absorption and (b) PL spectra of PTM- and ArBr-functionalized (6,5) SWCNTs with similar defect density. (c) PL decay traces of defect emission at 1165 nm. All data were measured on SWCNT dispersions in toluene. closed-shell hydrogen-substituted PTMH-Dz was employed to While the presence of an E11* emission feature clearly create reference defects. Additionally, simple closed-shell indicates covalent functionalization, it is relatively insensitive to bromoaryl defects (ArBr) were produced by reaction with the precise molecular structure of the attached group.13 Hence, the commercial Br-Dz reagent, which exhibited a higher EPR measurements were performed to ascertain that the reactivity than the PTMH-Dz and was therefore used for substituent was indeed the desired PTM-styryl unit and the experiments requiring larger defect densities. Since polymer- radical character was preserved after the reaction. Figure 2b wrapped SWCNTs form stable dispersions only in low-polarity shows that the pristine (6,5) SWCNTs do not display any EPR solvents, the functionalization with Br-Dz was facilitated by signal, as expected for highly purified semiconducting ether crown complexation in a toluene/acetonitrile mixture SWCNTs.29 Likewise, closed-shell PTMH-functionalized following our recently reported protocol.27 In contrast to that, SWCNTs show no EPR intensity at room temperature. To PTM-Dz and PTMH-Dz are well-soluble in tetrahydrofuran make sure that this result was not compromised by the low (THF), which also disperses PFO-BPy-wrapped SWCNTs. density of PTMH defects, a second control experiment was They are therefore ideally suited for the reaction with polymer- performed on (6,5) SWCNTs that were highly functionalized wrapped SWCNTs without the need for any solubilizing with closed-shell ArBr defects, which also did not give rise to agents. Nanotube functionalization was always performed in any EPR signal (see Supporting Information Figure S2 for the dark and completed within 16 h at room temperature. additional characterization). The PTM-functionalized Subsequently, the functionalized SWCNTs were collected by SWCNTs, on the other hand, show the clear EPR signature vacuum filtration, carefully washed with pure solvent to remove of the PTM-styryl unit with a g factor of 2.0024, which is unreacted diazonium salt and byproducts, and finally characteristic for PTM radicals, and a peak splitting due to redispersed in fresh toluene for characterization. hyperfine coupling between the PTM radical and the vinylene To confirm the covalent functionalization of (6,5) SWCNTs proton with appropriate symmetry (for more details on the fine structure see the Supporting Information Figure S3). with PTM radicals, photoluminescence and electron para- Furthermore, the EPR signal of the PTM radical attached to magnetic resonance (EPR) spectra were recorded for toluene the SWCNT is broadened compared to that of the molecular dispersions of the nanotubes at room temperature (Figure 2). precursor PTM-Dz (line widths of 1.4 and 1.1 G, respectively; Here, we focus on a comparison between PTM radical- see Supporting Information Figure S4). This broadening may functionalized SWCNTs and the corresponding closed-shell be attributed to the slow tumbling30 of the micrometer-long PTMH-functionalized SWCNTs as well as pristine SWCNTs. nanotube in dispersion with a rotational correlation time of All (6,5) SWCNT dispersions exhibit photoluminescence at ∼10 ms rad−1,31 which is much longer than that of the 1006 nm originating from mobile excitons recombining on unbound PTM-Dz molecule. Consequently, the line broad- pristine lattice segments (E11 transition). In addition, both sp3 ening further corroborates the successful decoration of functionalized nanotubes show emission at ∼1165 nm, which SWCNTs with stable organic radicals. is characteristic of defect-localized excitons (E11*) and strong These results clearly indicate that the room-temperature evidence for the covalent attachment of molecular groups to EPR response of PTM-tailored SWCNTs originates from an the nanotube lattice (Figure 2a). To analyze the influence of unpaired spin localized on the PTM moiety and not on the the radical substituent on the defect state energy, the optical nanotube itself. In contrast to that, Lohmann et al.32 recently trap depth28 (E11−E11*) was extracted from the PL spectra of reported that aryl-functionalization of SWCNTs with closed- PTM- and PTMH-functionalized SWCNTs (Supporting shell substituents in aqueous dispersions may give rise to an Information Figure S1). The optical trap depth of PTM EPR signal at low temperatures. The authors assigned this defects (172 meV) was slightly larger than that of PTMH signal to a nanotube-centered radical, which might be formed defects (168 meV), which is in agreement with the stronger upon addition of an odd number of sp3 defects to the lattice. electron-withdrawing character of the PTM group.10 However, Since the intensity of this EPR signal decreased strongly with the difference is small, presumably due to the relatively long increasing temperature and became undetectable above 200 distance between the central carbon of the PTM moiety and K,32 such a feature is not expected to appear in our room- the SWCNT lattice. We can conclude that the optical trap temperature EPR measurements. depth should only play a minor role with regard to any Impact of Radical on Defect PL. The unusual differences observed between PTM and PTMH defects. observation of a pronounced absorption band accompanied 5149 https://dx.doi.org/10.1021/acsnano.0c10341 ACS Nano 2021, 15, 5147−5157 ACS Nano www.acsnano.org Article by relatively weak PL from the PTM defects (compare Figure 2 and Supporting Information Figure S2) prompted a more detailed investigation of their PL efficiency in relation to a reference sample of closed-shell ArBr-functionalized SWCNTs with similar defect density, as indicated by the E11*/E11 absorbance ratio (Figure 3a). As shown in Figure 3b, the radical-functionalized defects exhibit a significantly lower E11*/ E11 PL intensity ratio than the closed-shell reference despite having a similar defect density. This picture is complemented by PL lifetime measurements via time-correlated single-photon counting (TCSPC) for the E11* PL from different defect types. The data indicate a substantially shorter decay time for the radical-functionalized defects compared to both closed-shell references (Figure 3c). It should be noted that the emission from all samples was filtered by a grating spectrograph to pass only the intensity from a narrow wavelength band around 1165 nm to the single-photon detector. Thereby, we rule out the wavelength-dependence of defect state lifetime as a possible cause for the observed differences.33 Combined with the reduced defect emission intensity, the shortened PL lifetime of PTM-functionalized SWCNTs points toward a nonradiative Figure 4. (a) PL spectra of PTM-functionalized and (b) ArBr- relaxation pathway associated with the radical in close functionalized (6,5) SWCNTs before and after irradiation. (c) PL proximity to the defect. Notably, the E11* PL decay of the lifetime components extracted from biexponential fits to the decay radical-functionalized SWCNTs can still be described by the traces. (d) EPR spectra recorded for the dispersions used in the PL biexponential model typical for localized exciton decay experiments before and after irradiation. Note that the initial EPR dynamics at room temperature.33,34 signal intensity is relatively low, because dilute dispersions were If the observed E11* PL quenching results from the presence employed for the optical characterization. of the radical substituent on the aryl defect, the in situ conversion of open-shell to closed-shell substituents on the sp3 reduced due to the higher density of exciton trapping sites. defects will be the most direct way to confirm and quantify this The unchanged E11 emission confirms that the interaction effect. For this purpose, we exploited the fact that PTM between the radical and the SWCNT is confined to the sp3 radicals are sensitive to UV and blue light (in the range of their defect. To exclude that these changes are related to absorption maximum around 385 nm). It is well-established photochemical reactions on the SWCNT lattice itself, e.g., that irradiation in this spectral range induces chlorine atom rearrangement of sp3 defects, the experiment was repeated with elimination from the PTM followed by ring closure to the a dispersion of ArBr-functionalized SWCNTs. As shown in perchlorophenylfluorenyl (PPF) radical.35,36 Due to the planar Figure 4b, these defects did not respond to UV irradiation. The structure of the fluorene unit hosting the radical, the PPF minor drop of PL intensity is likely due to slight SWCNT radical is much more susceptible to chemical attack than the aggregation over the course of irradiation, and the normalized PTM radical. Hence, we consider hydrogen atom abstraction spectra in Supporting Information Figure S6 illustrate that the from the solvent and formation of the closed-shell PPFH PL spectral shape does not change at all. Hence, the E11* species as a plausible decomposition pathway, as shown brightening observed in Figure 4a can be attributed schematically in Supporting Information Figure S5. In practice, unambiguously to the photochemical transformation of the degradation of the PPF radical yields a mixture of closed-shell PTM radical to a closed-shell system. products. Although the extremely low defect concentrations The marked increase in E11* emission efficiency can be (estimated to be in the range of nmol/L) did not allow us to understood by considering the prolonged PL lifetime of the determine the molecular structure of any photolysis products, defect-localized exciton following the open-shell to closed-shell we confirmed experimentally that all of them were of closed- conversion (Figure 4c). Upon irradiation, the initially short PL shell nature (see below). As noted in the discussion of optical lifetime of the PTM defects rises to the level of the closed-shell trap depths of PTM versus PTMH defects, minor changes in references (PTMH and ArBr). By comparing EPR spectra of substituent molecular structure far away from the SWCNT do the PTM-functionalized SWCNT dispersion before and after not impact the E11* PL properties significantly;13 thus the irradiation (Figure 4d), we can conclude that the PTM radical PPFH substituent is a good closed-shell reference system for was indeed converted to a closed-shell species. Consequently, the open-shell PTM. the mechanism responsible for E11* PL quenching in PTM- For in situ conversion, a dispersion of PTM-functionalized functionalized SWCNTs must be directly linked to the SWCNTs was exposed to UV light (365 nm, 0.2 W cm−2) for presence of the radical on the substituent of the aryl defect. 40 min and PL spectra as well as PL decay traces were The PL quenching effect can be quantified based on the PL recorded under identical conditions before and after spectral and lifetime data. Table 1 details the extracted lifetime irradiation. As expected, the E11* PL intensity strongly components and amplitudes together with the integrated E11* increased due to light-induced radical degradation, whereas to E11 PL intensities for each defect type. We find that the the E11 intensity remained constant (Figure 4a, note that amplitude-averaged E11* lifetime (τamp‑av) of PTM defects absolute PL intensities are compared). Hence, the creation of increases by a factor of 2.4 upon radical decomposition, which additional sp3 defects as the origin of increased E11* intensity is in excellent agreement with the 2.5-fold enhancement of the can be ruled out. In that case, the E11 intensity would be E11*/E11 PL intensity ratio. 5150 https://dx.doi.org/10.1021/acsnano.0c10341 ACS Nano 2021, 15, 5147−5157 ACS Nano www.acsnano.org Article Table 1. PL Lifetime Components (Long: τl, Short: τs) with from pristine or sp3-functionalized regions of the SWCNT Corresponding Amplitudes (Al and As), Amplitude- (donor D) to the PTM (acceptor A) was evaluated according Averaged Lifetimes (τamp‑av), and Integrated Defect-to-E11 to37 Intensity Ratio PL(E11*/E11) e2 Al τl As τs τamp‑av PL ΔG PET = eEred(D+ /D) − eEred(A /A−) − ΔG00 − defect (%) (ps) (%) (ps) (ps) (E11*/E11) 4πεr ε0d PTM 16 162 84 30 51 2.6 where Ered denotes the reduction potential, ΔG00 is the optical PTM irrad. 30 245 70 68 121 6.5 transition energy, and the electrostatic work term amounts to PTMH 29 239 71 81 127 0.6 0.5 eV for the relative permittivity of toluene (εr = 2.38) and ArBr 31 275 69 95 150 3.7 an ion pair separation d ≈1.2 nm. After PET to the functional group, the electron is back-transferred to the ground state of PL Quenching Mechanism. In the following, we discuss the defect/SWCNT, thereby resulting in nonradiative the possible underlying mechanisms of quenching of the recombination. As indicated by a large negative ΔGPET in defect-localized exciton by the radical substituent on the aryl Table 2, PET is thermodynamically highly favorable38 and defect. Three mechanisms must be considered: resonant could potentially quench both E11 and E11* emission. excitation energy transfer, photoinduced electron transfer, However, since the rate of electron transfer reactions decreases and enhanced intersystem crossing. First, the possibility of exponentially on a length scale of ∼0.1 nm,39 the quenching of energy transfer from the sp3 defect state to the PTM radical mobile E11 excitons can be excluded due to their long average can be ruled out based on the much smaller optical gap of the distance to the radical, in agreement with negligible E11 defect compared to the PTM and negligible spectral overlap brightening in Figure 4a. The defect-localized exciton, on the between the two partners, as shown in Supporting Information other hand, is sufficiently close to the PTM to be quenched via Figure S7. PET, even though the separation of ∼1 nm is likely to limit the Second, in order to assess the potential for photoinduced rate of this process. For the sake of completeness, it is worth electron transfer (PET), the redox potentials of all involved noting that the redox potentials in Table 2 permit ground state species and their optical transition energies must be considered charge transfer from the SWCNT/defect to the PTM as well, (see Table 2). From these data, the Gibbs free energy for PET but such a permanent ion pair formation at the defect site is irreconcilable with the observation of the EPR signature from Table 2. Redox Potentials (Eox, Ered), Optical Transition the PTM defect and the PL lifetime shortening, which Energies (ΔG00), and Gibbs Free Energies for indicates dynamic rather than static quenching. Photoinduced Electron Transfer (ΔGPET) As another potential mechanism, covalently bound radicals species Eox (V)a Ered (V)a ΔG00 (eV)c ΔGPET (eV)d have been shown to substantially speed up intersystem crossing in organic fluorophores.42−45 This effect can be modeled by (6,5) pristine 0.615 −0.420 1.24 −2.17e treating the electronic exchange interaction as a perturbation (6,5) sp3 defectb 0.584 −0.398 1.06 −1.95f that promotes spin flips of the unpaired electron on the radical PTM 1.61 −0.19 a and the photoexcited electron on the chromophore.46 The Potentials from Shiraishi et al.40 and Souto et al.41 measured versus a intersystem crossing rate thus depends sensitively on the Ag/AgCl reference electrode. bData from Shiraishi et al.40 for ArBr degree of wave function overlap between the singly occupied defects were used as an approximation to PTM-substituted aryl defects because of their near-identical optical trap depths. cEnergy of molecular orbital (SOMO) on the radical and the singlet the E11 and E11* PL transition, respectively. dIncluding an exciton. It can reach time scales of ∼1−100 ps for not too electrostatic work term of −0.5 eV. eFor the reaction: SWCNT + distant radicals.42−45 Hence, this mechanism is similarly short- PTM → SWCNT+ + PTM−. fFor the reaction: defect + PTM → ranged to PET and could affect defect-localized excitons close defect+ + PTM−. to the PTM radical. As discussed below, such radical-enhanced intersystem crossing (EISC) would allow triplet states to Figure 5. Model of exciton dynamics in SWCNTs with sp3 defects with (a) closed-shell or (b) open-shell functional groups. The energy level diagrams include the ground state (GS), the defect-localized bright (B) and dark (D) singlet states, and a triplet (T) state with energy gaps ΔBD and ΔBT. The relevant rate constants are indicated as follows: k, radiative and nonradiative recombination; kPET, photoinduced electron transfer (PET); kBD, equilibration between B and D state population; kBT, equilibration between B and T state population via radical- enhanced intersystem crossing (EISC) and thermally activated reverse intersystem crossing (RISC). Note that kBD and kBT are not the transition rates between states, but represent the relaxation rates at which the involved populations approach their equilibrium value. The rate of this relaxation process depends on the elementary transition rates. Gray color marks states with small population or rates that are not competitive. (c) Schematic illustration of PET and EISC as the PL quenching processes in open-shell PTM defects. 5151 https://dx.doi.org/10.1021/acsnano.0c10341 ACS Nano 2021, 15, 5147−5157 ACS Nano www.acsnano.org Article Figure 6. (a) Schematic of confocal magneto-PL measurement in Voigt geometry. (b) Magnetic field-dependent PL spectra of a single PTM- functionalized (6,5) nanotube (angle between tube axis and magnetic field: 60°). Note that minor shifts of the emission wavelength are due to spectral jitter and do not correlate with the applied field. (c) Low-temperature PL decay of the defect signal shown in (b) at zero field. All measurements were performed at 4 K. participate in the fast population redistribution process the short decay component is given by τs−1 = kBD + τl−1. In between bright and dark defect states following exciton contrast to that, we propose that radical functionalization trapping.33,34,47 In this case, the radical-induced PL quenching (Figure 5b) enables fast EISC and thermally activated reverse may be explained by exciton population transfer from the intersystem crossing (RISC) between the B state and an bright singlet to a dark triplet state. energetically low-lying T state. As a result, this T state Both the PET and the EISC mechanisms are likely to occur dominates the population redistribution process among bright for PTM defects based on thermodynamic considerations and and dark states (for details see Supporting Information). previous work on structurally similar molecular systems. To Simultaneously, PET is responsible for enhanced nonradiative further corroborate or exclude the involvement of either recombination, resulting in τl−1 = k + kPET and τs−1 = kBT + τl−1 quenching mechanism, we now discuss the E11* PL decay in for radical-substituted defects. more detail, as it is rich in information and data are available A quantitative analysis of the data in Table 1 further for both radical-functionalized and reference defects (Table 1). supports this picture. Assuming that PET is responsible for Hartmann et al.34 and He et al.33 developed a model for the changes in the decay rates τl−1, a PET rate of τl−1(PTM) − biexponential decay of excitons localized at defects with closed- τl−1(PTM irrad.) = (500 ps)−1 is extracted. This moderately shell substituents at room temperature. Since the E11* PL fast electron transfer rate is rationalized by the trade-off decay of radical-functionalized defects remains biexponential between the large thermodynamic driving force for the process with well-separated long and short time scales, we assume that (ΔGPET = −1.95 eV) and the relatively long distance (∼1.2 the dynamics in such defects are still similar to the existing nm) between the SWCNT surface and the radical center. model but should be modified by introducing PET or EISC. Further, the term (τs−1 − τl−1) yields the population With these assumptions, the observed radical-induced changes redistribution rates kBD = (91 ps)−1 and kBT = (37 ps)−1 for in the long and short decay components can only be explained the closed- and open-shell case, respectively. Note that the by a combination of PET and reversible EISC to a triplet state calculated kBT is in good agreement with EISC rates43,45 for that is energetically below the bright state, as discussed in chromophore−radical distances of 1 nm as present in PTM detail in the Supporting Information. defects. As pointed out by He et al.,33 the normalized Figure 5 illustrates our proposed model for defects with amplitude of the long decay component may be used as a open-shell substituents and compares it with the model that is measure of bright state population in equilibrium with respect generally applied for defects with closed-shell substitu- to its initial population immediately after trapping (Supporting ents.33,34,47 The involved states are the defect-localized bright Information). For radical-functionalized defects Al is 16%, singlet exciton (B) and parity-forbidden dark singlet exciton whereas Al is about 30% across all reference defects, in good (D), as well as a triplet exciton (T), which could be either agreement with previous reports.33 This reduction of Al further mobile or defect-localized. These states are assumed to supports EISC-mediated population transfer from the B to the reversibly exchange population at room temperature as the T state as a contribution to E11* PL quenching. Moreover, energy gaps between them are either smaller (ΔBD ≈ 9 meV as because the equilibrium populations of the B, D, and T states found by Kim et al.47) or not much larger than 25 meV (an should simply reflect the Boltzmann distribution, we can use upper boundary of ΔBT ≲ 38 meV will be estimated below). In the information that Al = 16% together with ΔBD ≈ 9 meV for general, the fast decay component is attributed to the E11* defects in (6,5) SWCNTs47 to obtain a rough estimate of redistribution of exciton population among these bright and ΔBT ≈ 38 meV. So far, we assumed a single triplet state to be dark states, which is followed by the slow recombination of involved, although several triplet states are predicted48 within a trapped excitons via radiative and nonradiative channels. More few 10 meV of the bright E11* singlet. Including a second specifically, as intersystem crossing is slow in the absence of triplet state will further reduce ΔBT, so 38 meV should be the radical (Figure 5a); only the D state accepts population regarded as an upper boundary, which justifies the earlier from the B state with the redistribution rate kBD. Subsequently, assumption of RISC at room temperature. Finally, we note that trapped excitons recombine via multiphonon decay and other within this model about ∼1/3 of the defect PL intensity drop pathways with the cumulative rate k = τl−1, while the ratio of B results from PET, whereas ∼2/3 are a consequence of EISC and D state populations retains its equilibrium value. Since (Supporting Information). It is important to emphasize that recombination can occur at any time after the trapping event, our description of the decay dynamics of radical-functionalized 5152 https://dx.doi.org/10.1021/acsnano.0c10341 ACS Nano 2021, 15, 5147−5157 ACS Nano www.acsnano.org Article defects and the extracted physical parameters only apply as induce a change of the defect PL, as the unpaired electron on long as the underlying model for closed-shell defects33,34 is the PTM is merely a spectator to the recombination of the valid. For example, if the biexponential decay instead resulted defect-localized exciton with zero spin. Conceptually, the from shelving into dark states, similar to the case of E11 situation is analogous to that of defect-trapped trions in excitons,49,50 other quenching mechanisms might match the SWCNTs.55 In both cases, the spin state does not change upon data more closely. optical transition from the excited to the ground state, thus As long lifetimes (>1 μs) are a hallmark of triplet states in resulting in a vanishing spin Zeeman effect. organic materials, we also looked for delayed fluorescence at To gain further insights into the energetic position of the room temperature.51,52 To this end, the PL decay of PTM- triplet state, we measured the PL decay of an individual PTM functionalized SWCNTs was monitored at the E11 or E11* peak defect at 4 K (Figure 6c). Interestingly, we observed a emission wavelength by TCSPC over a time window of 0.3 μs. biexponential decay characterized by an unusually47,56 fast However, in neither case was a component with μs lifetime component (τs ≈ 50 ps) with large amplitude (As ≈ 60%) and detectable. While it is possible that the delayed fluorescence a slow component (τl ≈ 800 ps) typical for trapped exciton signal is simply too weak to be detected in our setup, it is recombination at low temperature. Such a dominant fast important to keep in mind that RISC (from the T to the B process has never been reported for aryl defects with closed- state) is enhanced by the same factor as the forward process. shell substituents at low temperature. Note that multiexciton Thus, their efficient coupling via EISC and RISC should lead effects can be excluded as the origin of the fast decay to simultaneous depopulation of the B and the T state on the component, because the pump fluence (∼2 × 1014 photons time scale of τl (∼100 ps). Hence, the effective triplet state cm−2 pulse−1) was kept within the regime of linear PL lifetime is strongly reduced and delayed fluorescence is no response.57 Since PET is too slow in our system while the longer expected. observed τs matches the room-temperature EISC rate well, we Magneto-photoluminescence Spectroscopy. Given assign the fast decay component to EISC and conclude that the the clear evidence for interaction of the radical with the participating triplet state is energetically below the bright state. trapped exciton, we explored how the paramagnetic PTM However, since RISC is prohibited at 4 K, the origin of τl is not substituent influences the PL of sp3 defects in magnetic fields. obvious. We speculate that the surrounding insulating polymer Low-temperature magneto-PL experiments were performed on matrix and cryogenic temperature could facilitate the trapping individual, PTM-functionalized (6,5) SWCNTs embedded in a of charges on the PTM group, e.g., after a PET event. This polystyrene layer (see Supporting Information Figure S8 for a leads to electrochemical reduction of the radical to the closed- scanning confocal PL image). As illustrated in Figure 6a, the shell anion, in which EISC is switched off. In this case, τs sample was mounted such that PL could be collected while corresponds to the decay time when the defect is in its native applying an in-plane magnetic field (Voigt geometry). The radical state, whereas τl represents the slower decay when the random nanotube orientation results in a tube-specific angle defect is trapped in its spin-paired anion state without EISC. between the tube axis and the magnetic field. In total, eight Nevertheless, further work will be required to fully understand nanotubes were investigated that displayed single-line or well- the PL dynamics of radical-functionalized defects in SWCNTs separated multiline defect PL between 1120 and 1180 nm at cryogenic temperatures. (1.05−1.11 eV). Note that the sidebands that are red-shifted by 2−3 meV from the zero-phonon line are typical of the CONCLUSION cryogenic PL spectra of polymer-wrapped SWCNTs.53,54 Up to magnetic fields of 9 T, none of these defect signals In summary, we have demonstrated the functionalization of displayed a significant shift or splitting within the resolution monochiral semiconducting SWCNTs with stable organic limit of the spectrometer (∼300 μeV) and the characteristic radicals via simple diazonium chemistry. The presence of the spectral jitter (∼2 meV) as exemplified in Figure 6b and more radical leads to partial quenching of defect PL, which is likely generally shown in Supporting Information Figure S9. This due to a combination of a photoinduced electron transfer result is slightly surprising, because Kim et al. recently observed process and population transfer to triplet states enabled by magnetic field-induced fine structure in the spectra of sp3 radical-enhanced intersystem crossing. Consequently, radical- defects.48 Yet, peak splitting only occurred in a minority of functionalized sp3 defects are an interesting platform for future nanotubes, and the authors hypothesized that this was related studies of the elusive triplet exciton manifold in carbon to the requirement of nearly perfect energy level alignment nanotubes employing spin-sensitive methods such as optically between the bright E11* singlet and the band-edge triplet detected magnetic resonance.51,58 Moreover, as pulsed EPR exciton. Within our limited statistics we cannot rule out that a techniques can probe the distances between spin labels on the fraction of nanotubes in our sample would show a similar length scale of a few nanometers,59 such defects could be a effect, but based on the above estimation that ΔBT ≲ 38 meV, model system for investigating clustering, which is suspected to it is likely that there is indeed no triplet state within just a few play an important part in both synthetic60,61 and optical56 meV of the bright state in our PTM defects, which would features of luminescent defects. Beyond that, the encapsulation explain the absence of a peak splitting. It is also worth noting of such radical-tailored SWCNTs in biocompatible surfac- that the triplet state, which is populated via EISC from an tants62 might even enable their use as metal-free contrast energetically close singlet state, usually does not show agents for magnetic resonance imaging63,64 complementary to enhanced phosphorescence in magnetic fields, because the in vivo near-infrared fluorescence imaging.2 Finally, our EISC rate also depends on the energy gap between the functionalization approach should be easily transferable to involved states46 and the gap between the excited triplet and metallic SWCNTs, which show even higher reactivity toward the singlet ground state is too large to result in significant diazonium reagents65 and are commonly used in spin valves. In triplet brightening.42 Beyond that, the spin Zeeman effect that order to maximize the coupling between the radical and the acts on the radical and splits its SOMO level is insufficient to itinerant electrons in such a system, the covalent linker should 5153 https://dx.doi.org/10.1021/acsnano.0c10341 ACS Nano 2021, 15, 5147−5157 ACS Nano www.acsnano.org Article be as short as possible to place the radical close to the module (PicoHarp 300, PicoQuant) and reconvolution-fitted using conducting channel. the SymPhoTime 64 software. The instrument response function (IRF) was determined by the fast, instrument-limited decay of the E11 emission from (6,5) SWCNTs. METHODS Low-Temperature Magneto-spectroscopy. For PL micros- Selective Dispersion of (6,5) SWCNTs. As described pre- copy on the single-defect level, a low concentration of functionalized viously,24 (6,5) SWCNTs were extracted from CoMoCAT raw (6,5) SWCNTs (optical density of 0.005 cm−1 at the E11 transition) material (Chasm Advanced Materials, SG65i-L63, 0.38 g L−1) by was blended into a solution of polystyrene (20 g L−1) in toluene and selective wrapping with poly-[(9,9-dioctylfluorenyl-2,7-diyl)-alt- spin-coated at 2000 rpm onto glass slides coated with 150 nm of gold. (6,6′)-(2,2′-bipyridine)] (PFO-BPy, American Dye Source, Mw = The sample was loaded into a home-built confocal microscope setup 40 kg mol−1, 0.5 g L−1) under shear force mixing (Silverson L2/Air, inside a closed-cycle magneto-cryostat (attoDRY1000, attocube 10 230 rpm, 72 h) in toluene. Aggregates and impurities were systems AG) with a base temperature of 3.2 K and magnetic fields removed by two centrifugation steps (60000g for 45 min each) and of up to 9 T. A customized holder was used to mount the sample in subsequent filtration (poly(tetrafluoroethylene) (PTFE) syringe filter, Voigt geometry with the magnetic field parallel and light propagation 5 μm pore size). orthogonal to the sample surface. A wavelength-tunable Ti:sapphire SWCNT Functionalization. (6,5) SWCNTs were reacted with a laser (Coherent Mira) served as the excitation source and was tuned series of diazonium compounds to introduce sp3 aryl defects with to 995 nm (continuous wave, 6 μW) to be in resonance with the E11 either radical (PTM) or closed-shell substituents (PTMH, Br). Since transition of (6,5) SWCNTs. The PL from individual nanotubes was the employed diazonium salts displayed different solubility character- collected by an aspheric lens (Thorlabs 354330-B, NA = 0.68), istics, the functionalization procedure was adapted to this. The dispersed by a grating spectrograph (Acton SpectraPro SP2500, 300 reactions between (6,5) SWCNTs and the tailored diazonium salts lines mm−1) and recorded by a liquid nitrogen cooled InGaAs line PTM-Dz and PTMH-Dz (see the Supporting Information for the camera (Princeton Instruments OMA V:1024-1.7 LN). For TCSPC, synthesis26 and characterization) were performed in THF, which was nanotubes were excited in pulsed mode (∼2 × 1014 photons cm−2 freshly distilled to remove water and any peroxide contaminations. pulse−1) and the PL was directed to a superconducting single-photon First, the PFO-BPy-wrapped (6,5) SWCNTs were transferred to THF detector (TCOPRS-CCR-SW-85, Scontel). The IRF was measured by passing the toluene dispersion through a PTFE membrane filter on the attenuated laser signal. (Merck Millipore, JVWP, 0.1 μm pore size) to collect the SWCNTs, which were subsequently redispersed in a small volume of THF by ASSOCIATED CONTENT bath sonication. To this dispersion was added a solution of the * sı Supporting Information diazonium salt (PTM-Dz or PTMH-Dz) in THF such that the (6,5) The Supporting Information is available free of charge at SWCNT concentration in the reaction mixture was 0.72 mg L−1 (optical density of 0.4 cm−1 at the E11 transition) and the diazonium https://pubs.acs.org/doi/10.1021/acsnano.0c10341. salt concentration 0.15 mmol L−1. In contrast to that, the reaction Optical trap depth of PTM vs PTMH defects, highly with the commercial 4-bromobenzenediazonium tetrafluoroborate functionalized closed-shell reference, fine structure of (Br-Dz, Sigma-Aldrich) was carried out according to our previously the EPR signal, broadening of the EPR signal, proposed reported procedure27 using 18-crown-6 as a phase transfer agent in an reaction scheme for open-shell to closed-shell con- 80:20 vol % toluene/acetonitrile mixture, but without the addition of potassium acetate. The (6,5) SWCNT concentration was adjusted to version, normalized spectra for irradiation experiment, 0.72 mg L−1 and the Br-Dz concentration to 0.37 mmol L−1 for exclusion of energy transfer due to negligible spectral moderate- or 1.85 mmol L−1 for high-density functionalization. All overlap, model for PL decay dynamics of sp3 defects reactions proceeded in air at room temperature under dark with open-shell substituents, scanning confocal PL map conditions. After 16 h, the functionalized SWCNTs were collected of single nanotubes, magneto-PL spectra of single by vacuum filtration through a PTFE membrane filter (Merck nanotubes, synthesis and characterization of diazonium Millipore, JVWP, 0.1 μm pore size) and washed with either THF (3 × compounds including raw spectral data for PTMH-Dz 7 mL) or acetonitrile (10 mL) followed by toluene (5 mL), (PDF) depending on the reaction medium. For analysis, the functionalized SWCNTs were redispersed in a small volume of toluene by bath sonication for 30 min. AUTHOR INFORMATION EPR Spectroscopy. EPR spectra were recorded at room Corresponding Author temperature on a Bruker ESP 300 E spectrometer provided with a Jana Zaumseil − Institute for Physical Chemistry, Universität T102 rectangular cavity that works with an X band (9.5 GHz). The Heidelberg, 69120 Heidelberg, Germany; Centre for signal-to-noise ratio of spectra was increased by accumulation of scans using the F/Flock accessory to guarantee large field reproducibility. Advanced Materials, Universität Heidelberg, 69120 Precautions to avoid undesirable spectral distortion and line Heidelberg, Germany; orcid.org/0000-0002-2048-217X; broadenings, such as those arising from microwave power saturation Email: zaumseil@uni-heidelberg.de and magnetic field overmodulation, were also taken into account to improve sensitivity. Authors Room-Temperature PL Characterization. PL spectra and Felix J. Berger − Institute for Physical Chemistry, Universität TCSPC were measured for SWCNT dispersions using a home-built Heidelberg, 69120 Heidelberg, Germany; Centre for setup. The samples were excited by the spectrally filtered output of a Advanced Materials, Universität Heidelberg, 69120 picosecond-pulsed supercontinuum laser source (Fianium WhiteLase Heidelberg, Germany SC400) tuned to 700 nm to avoid any photochemical reactions of J. Alejandro de Sousa − Institut de Ciència de Materials de PTM-functionalized SWCNTs. Scattered laser light was blocked by Barcelona (ICMAB-CSIC), 08193 Bellaterra, Spain; appropriate long-pass filters. The sample emission was dispersed by a Laboratorio de Electroquímica, Departamento de Química, grating spectrograph (Acton SpectraPro SP2358, 150 lines mm−1) and detected by a liquid nitrogen cooled InGaAs line camera Facultad de Ciencias, Universidad de los Andes, 5101 (Princeton Instruments OMA V:1024-1.7 LN). For TCSPC, the Mérida, Venezuela; orcid.org/0000-0002-7948-8162 spectrally selected output of the spectrograph was focused onto a Shen Zhao − Faculty of Physics, Munich Quantum Center and gated InGaAs/InP avalanche photodiode (Micro Photon Devices). Center for NanoScience (CeNS), Ludwig-Maximilians- Histograms of photon arrival times were created using a counting Universität München, 80539 München, Germany; Munich 5154 https://dx.doi.org/10.1021/acsnano.0c10341 ACS Nano 2021, 15, 5147−5157 ACS Nano www.acsnano.org Article Center for Quantum Science and Technology (MCQST), (5) Hertel, T.; Himmelein, S.; Ackermann, T.; Stich, D.; Crochet, J. 80799 München, Germany Diffusion Limited Photoluminescence Quantum Yields in 1-D Nicolas F. Zorn − Institute for Physical Chemistry, Universität Semiconductors: Single-Wall Carbon Nanotubes. ACS Nano 2010, Heidelberg, 69120 Heidelberg, Germany; Centre for 4, 7161−7168. (6) Brozena, A. H.; Kim, M.; Powell, L. R.; Wang, Y. Controlling the Advanced Materials, Universität Heidelberg, 69120 Optical Properties of Carbon Nanotubes with Organic Colour-Centre Heidelberg, Germany Quantum Defects. Nat. Rev. Chem. 2019, 3, 375−392. Abdurrahman Ali El Yumin − Institute for Physical (7) Shiraki, T.; Miyauchi, Y.; Matsuda, K.; Nakashima, N. Carbon Chemistry, Universität Heidelberg, 69120 Heidelberg, Nanotube Photoluminescence Modulation by Local Chemical and Germany; Centre for Advanced Materials, Universität Supramolecular Chemical Functionalization. Acc. Chem. Res. 2020, 53, Heidelberg, 69120 Heidelberg, Germany 1846−1859. Aleix Quintana García − Institut de Ciència de Materials de (8) Gifford, B. J.; Kilina, S.; Htoon, H.; Doorn, S. K.; Tretiak, S. Barcelona (ICMAB-CSIC), 08193 Bellaterra, Spain Controlling Defect-State Photophysics in Covalently Functionalized Simon Settele − Institute for Physical Chemistry, Universität Single-Walled Carbon Nanotubes. Acc. Chem. Res. 2020, 53, 1791− Heidelberg, 69120 Heidelberg, Germany 1801. Alexander Högele − Faculty of Physics, Munich Quantum (9) Ghosh, S.; Bachilo, S. M.; Simonette, R. A.; Beckingham, K. M.; Weisman, R. B. Oxygen Doping Modifies Near-Infrared Band Gaps in Center and Center for NanoScience (CeNS), Ludwig- Fluorescent Single-Walled Carbon Nanotubes. Science 2010, 330, Maximilians-Universität München, 80539 München, 1656−1659. Germany; Munich Center for Quantum Science and (10) Piao, Y.; Meany, B.; Powell, L. R.; Valley, N.; Kwon, H.; Schatz, Technology (MCQST), 80799 München, Germany; G. C.; Wang, Y. Brightening of Carbon Nanotube Photoluminescence orcid.org/0000-0002-0178-9117 through the Incorporation of sp3 Defects. Nat. Chem. 2013, 5, 840− Núria Crivillers − Institut de Ciència de Materials de 845. Barcelona (ICMAB-CSIC), 08193 Bellaterra, Spain; (11) Kwon, H.; Furmanchuk, A.; Kim, M.; Meany, B.; Guo, Y.; orcid.org/0000-0001-6538-2482 Schatz, G. C.; Wang, Y. Molecularly Tunable Fluorescent Quantum Defects. J. Am. Chem. Soc. 2016, 138, 6878−6885. Complete contact information is available at: (12) He, X.; Hartmann, N. F.; Ma, X.; Kim, Y.; Ihly, R.; Blackburn, J. https://pubs.acs.org/10.1021/acsnano.0c10341 L.; Gao, W.; Kono, J.; Yomogida, Y.; Hirano, A.; Tanaka, T.; Kataura, H.; Htoon, H.; Doorn, S. K. Tunable Room-Temperature Single- Notes Photon Emission at Telecom Wavelengths from sp3 Defects in The authors declare no competing financial interest. Carbon Nanotubes. Nat. Photonics 2017, 11, 577−582. (13) Gifford, B. J.; Kilina, S.; Htoon, H.; Doorn, S. K.; Tretiak, S. ACKNOWLEDGMENTS Exciton Localization and Optical Emission in Aryl-Functionalized The authors thank Dr. Vega Lloveras for performing electron Carbon Nanotubes. J. Phys. Chem. C 2018, 122, 1828−1838. (14) Kwon, H.; Kim, M.; Meany, B.; Piao, Y.; Powell, L. R.; Wang, paramagnetic resonance measurements. This project has Y. Optical Probing of Local pH and Temperature in Complex Fluids received funding from the European Research Council with Covalently Functionalized, Semiconducting Carbon Nanotubes. (ERC) under the European Union’s Horizon 2020 research J. Phys. Chem. C 2015, 119, 3733−3739. and innovation programme (grant agreement no. 817494 (15) Shiraki, T.; Onitsuka, H.; Shiraishi, T.; Nakashima, N. Near “TRIFECTs”). S.Z. acknowledges funding from the Alexander Infrared Photoluminescence Modulation of Single-Walled Carbon von Humboldt Foundation, and A.H. from the European Nanotubes Based on a Molecular Recognition Approach. Chem. Research Council (ERC) under the grant agreement no. Commun. 2016, 52, 12972−12975. 772195 and the Deutsche Forschungsgemeinschaft (DFG, (16) Onitsuka, H.; Fujigaya, T.; Nakashima, N.; Shiraki, T. Control German Research Foundation) under Germany’s Excellence of the Near Infrared Photoluminescence of Locally Functionalized Strategy EXC-2111-390814868. 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