Unusually Long-Lived Photocharges in Helical Organic

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Unusually Long-Lived Photocharges in Helical Organic Semiconductor Nanostructures Regina Judith Hafner, Liangfei Tian, Jan Cornelius Brauer, Thomas Schmaltz, Andrzej Sienkiewicz, Sandor Balog, Valentin Flauraud, Juergen Brugger, and Holger Frauenrath ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b03165 • Publication Date (Web): 23 Aug 2018 Downloaded from http://pubs.acs.org on August 24, 2018

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Unusually Long-Lived Photocharges in Helical Organic Semiconductor Nanostructures

Regina J. Hafner, 1 Liangfei Tian,1 Jan C. Brauer,1,2 Thomas Schmaltz,1 Andrzej Sienkiewicz,3 Sandor Balog,4 Valentin Flauraud,5 Juergen Brugger,5 Holger Frauenrath1* 1 Ecole Polytechnique Fédérale de Lausanne (EPFL)

Institute of Materials Laboratory of Macromolecular and Organic Materials EPFL – STI – IMX – LMOM, MXG 037, Station 12 1015 Lausanne, Switzerland [email protected]

2 Université de Fribourg

Department of Chemistry

3 Ecole Polytechnique Fédérale de Lausanne (EPFL)

Institute of Physics Laboratory of Physics of Complex Matter

4 Université de Fribourg

Adolphe Merklé Institute

5 Ecole Polytechnique Fédérale de Lausanne (EPFL)

Institute of Microengineering Microsystems Laboratory

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Abstract Photocharge generation and formation of long-lived charge carriers is relevant in photosynthesis, photocatalysis, photovoltaics, and organic electronics. A better understanding of the factors that determine these processes in synthetic polymer semiconductors is crucial, but difficult due to their morphological inhomogeneity. Here, we report the formation of exceptionally long-lived photocharges in one-dimensional organic semiconductor nanostructures. These nanostructures consist of chiral oligopeptide-substituted thienothiophene-based chromophores and exhibit a well-defined helical arrangement of these chromophores at their core. The chromophores give rise to spectroscopic H-aggregates and show strong intermolecular excitonic coupling. We demonstrate that all of these parameters are the prerequisites required for the nanostructures to show the efficient formation of polaron-like photocharges upon irradiation with a low-power white light source. The observed charge carriers in the helical nanowires show an unusually long lifetime on the order of several hours and are formed at high concentrations of up to 3 mol% in the absence of any dedicated electron acceptor. They are observed in solution as well as in film and furthermore relate into light-induced increase of the macroscopic charge transport. By contrast, no such photocharge generation is observed either in non-aggregating reference systems of the same chromophores or in aggregated but non-helical systems that do not form one-dimensional nanostructures. Our results thus demonstrate a clear correlation between helicity and the generation of long-lived photocharges.

Keywords organic semiconductor nanostructures; thiophene-based semiconductors; organic nanowires; helical nanostructures; photocharges; long-lived charge carriers; polarons

TOC Figure

light –

e– +

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The formation of photoexcited states, their separation into stable, long-lived charge carriers, and their transport are critical processes relevant to several important research fields and emerging technologies, including natural photosynthesis,1,2 solar fuel generation,3,4 photoelectrochemical water splitting,5,6 organic photovoltaics,7-9 as well as the general understanding of charge generation and transport in polymer semiconductors.10,11 In biological photosynthesis, photoexcited states are rapidly and quantitatively converted into spatially separated, long-lived charges, by means of carefully designed, cascaded energy levels and tailored electrostatic screening.1,2 While they may not be optimized for overall energy conversion,1,2 their charge separation efficiency has so far remained unmatched by any artificial system, and only very few examples with carefully tailored energy levels have shown high-energy charge-separated states that exhibit lifetimes comparable to, or even larger than, their natural counterparts.12-14 In contrast to such meticulously designed molecular and supramolecular systems, charge generation and transport in simple polymer semiconductors are determined by the complex interplay between molecular structure, supramolecular arrangement, and nanoscale crystalline features.11 Whereas the crystalline domains in polymer semiconductors are considered to be mainly responsible for charge transport, it has recently become clear that the amorphous and disordered domains are strongly involved in the intrinsic conversion of photoexcited states into free polarons.15 In bulk heterojunction materials for organic photovoltaic devices, for instance, the roles of disordered, mixed phases of the electron donor and acceptor components and chemical potential gradients is currently debated.16 Along similar lines, it was recently shown that excitons in neat semicrystalline semiconductors predominantly dissociate near interfaces between ordered and disordered domains.17 The paracrystalline nature of high performance polymer semiconductors such as PBTTT18 may thus be interpreted in the sense that creating nanoscopically confined crystalline and amorphous domains with maximized interfaces is decisive to promote the formation and stabilization of charge carriers.19 P3HT nanofibrils have been introduced as model systems to probe charge transport within the nanoscale crystalline domains of typical polythiophene-based semiconductors that feature a high degree of intrachain order and J-type coupling. By contrast, the amorphous domains rather show H-type excitons localized from disorder effects resulting from low intrachain order.20 We have recently described self-assembled one-dimensional nanostructures that comprise single stacks of H-aggregated π-conjugated molecules at their core and might thus serve as models to probe the role of nanoscopic confinement in the amorphous domains of polymer semiconductors.21 Similar examples of self-assembled, helical, one-dimensional aggregates of organic chromophores are abundant,22,23 and particularly molecules with hydrogen-bonded substituents have very successfully been investigated in this context.24,25 Such ACS Paragon Plus 2 Environment

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one-dimensional systems have already proven to be excellent model systems for fundamental investigations of intermolecular exciton delocalization,26 and some thiophene-based systems have shown macroscopic photoconductivity.25 Recently, Giuseppone et al. reported that certain triarylamine derivatives self-assembled into one-dimensional aggregates upon the photoinduced generation of long-lived charge carriers.27,28 While triarylamines are generally known to form stable radical cations in the presence of chlorinated solvents,29 other examples of long-lived charge carriers in organic semiconductors have so far relied on specifically designed donor-acceptor systems.12,30,31 Here, we provide an example of one-dimensional nanostructures from the thienothiophene derivative 1 (Figure 1A) that has been chosen to resemble a segment of the high performance polymer semiconductor PBTTT. Due to strong hydrogen-bonding of the oligopeptide substituents in the hydrophobic environment of the terminal poly(isobutylene) chains, combined with supramolecular helicity induced by the chirality of the peptide groups, these nanostructures comprise a single stack of chromophores at their core (Figure 1B). We demonstrate that the chromophores form spectroscopic H-aggregates, show strong intermolecular excitonic coupling, and thus provide a well-defined model system for the less ordered domains in polymer semiconductors. Further, we demonstrate that compound 1 shows a facile photoinduced formation of very long-lived polaron-like charge carriers already upon exposure to ambient light. This behavior is neither observed for molecularly disperse solutions of 1 nor for the non-aggregating reference compound 2. By contrast, the achiral compound 3 exhibits this unusual photocharge generation, although it comprises the same chemical functional groups and gives rise to spectroscopic H-aggregates but does not form helical stacks. The one-dimensional nanostructures, that are only observed in the case of compound 1, exhibit macroscopic photoconductivity, which can be linked to the spectroscopically characterized charge carriers.

Results and Discussion Consistent with our previous in-depth investigations of the supramolecular self-assembly of molecules with this kind of molecular architecture and the structural characterization of the resulting one-dimensional nanostructures,21 the dipeptide-substituted chromophore 1 self-assembles into well-defined

one-dimensional

nanostructures

upon

thermal

annealing

of

solutions

in

1,1,2,2-tetrachloroethane (TCE), driven by synergistic parallel b-sheet-like hydrogen bonding of the oligopeptide substituents (solution-phase and solid state IR spectroscopy; Supplementary Figure S1) and π–π stacking of the chromophores. This results in a high degree of internal order along their long axis, an intermolecular distance of 4.7 Å as expected for a parallel b-sheet-like arrangement, and a π–π stacking ACS Paragon Plus 3 Environment

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distance of 3.8 Å (wide-angle X-ray scattering; Supplementary Figure S2). Moreover, the nanostructures exhibit typical lengths in the range of a few micrometers and a uniform morphology with a height of about 3 nm in the dried state that corresponds to the rigid core of the molecules (atomic-force microscopy; Figure 1C), and an overall diameter of 6.6 nm in solid films (small-angle X-ray scattering; Supplementary Figure S2). By analogy with our previous investigations,21 these findings imply the presence of a single stack with a helical arrangement of the chromophores at their core. Therefore, these nanostructures can be regarded as defined supramolecular polymers32 with the π-stacked chromophores as the repeating units and a helical secondary structure. As we will show in what follows, this precise arrangement results in strong electronic coupling between neighbors along the stack.

Figure 1. A) Molecular structures of 1 (n = 2, R = Me), 2 (n = 0) and 3 (n = 2, R = H). B) A schematic representation of the resulting twisted supramolecular polymer from 1 with a single stack of chromophores at their core. C) AFM height image and height profile of nanowires of 1. D) Temperature-dependent UV/vis and CD spectra of 1 in TCE show a hypsochromic shift of the chromophores’ UV/vis absorption and an emerging bisignate CD signal upon going from molecularly disperse solution at 373 K to a solution containing hydrogen-bonded aggregates at room temperature. See Supplementary Figures S1–S3 for details.

Temperature-dependent UV/vis and circular dichroism (CD) spectroscopy demonstrate that the aggregation of 1 in solution is thermally fully reversible and occurs upon cooling of molecularly disperse solutions below 323 K. The UV/vis spectra reveal a significant hypsochromic shift of the main optical absorption from 422 to 385 nm upon aggregation, indicating the formation of spectroscopic H-aggregates


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(Figure 1D; Supplementary Figure S3). The significant blue shift of the absorption maximum along with the almost complete suppression of the 0-0, and 0-1 transitions in the aggregates clearly place the system into the coupling regime as defined by Spano et al.33 (Supplementary Figure S4). Moreover, the optically clear solutions of the aggregates, contrary to molecularly disperse solutions at elevated temperatures, show a significant CD activity that presumably arises from the delocalization of the optical excitation over two or more chromophores along the helical stack.34 In order to further elucidate the influence of the molecular structure on the supramolecular aggregation, we have investigated the two reference compounds 2 and 3. While 2 lacks the dipeptide substituents and therefore should not aggregate in solution at all, 3 has the chiral L-alanine units in the dipeptides replaced with glycine units and thus serves as an achiral control system. According to IR, UV/vis, and CD spectroscopy (Figure 2A; Supplementary Figures S1, S4, S5), compound 2 indeed remains molecularly disperse in solution and in films, whereas compound 3, similar to 1, shows a strong aggregation and an electronic coupling in the strong coupling regime, but gives rise to two-dimensionally layered aggregates instead of one-dimensional nanostructures, due to the absence of molecular chirality and a resulting bias for supramolecular helicity. In what follows, we demonstrate that it is the combination of the strong electronic interactions of the chromophores as well as their helical arrangement in the core of the laterally confined one-dimensional nanostructures of 1 that are the prerequisites for a facile photoinduced formation of unusually long-lived polaron-like charge carriers in the absence of a dedicated electron acceptor. We have observed that the solution-phase UV/vis/NIR spectra of the aggregates of compound 1 in degassed TCE show two additional bands at 803 and 1555 nm (with shoulders at 695 and 1270 nm) with intensities of up to a few percent of the main optical absorption upon simple exposure to white light from a standard LED illumination source (Figure 2B). These bands are significantly sharper than the NIR signatures of charge transfer complexes observed for polythiophenes with oxygen.35 The two bands and their fine structure are in fact characteristic of the optical absorptions of positive polarons, where two additional electronic transitions (SOMO→LUMO and SOMO-1→SOMO) occur (Supplementary Figures S6, S7).36-38 Consistent with this interpretation, these bands can be reversibly quenched by the addition of hydrazine as a reducing agent, and regenerated or enhanced using iodine as an oxidant (Figure 2C). Moreover, the same bands and shoulders are observed upon chemical oxidation with iron trichloride (Supplementary Figure S8). However, they have never previously been observed in steady-state spectroscopy of thiophene-based systems even without the explicit addition of a dedicated electron acceptor either in solution (whether of molecularly disperse or aggregated chromophores) or in bulk. ACS Paragon Plus 5 Environment

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381

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1 2 3

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1270

2000

0 600


Figure 2. A) UV/vis and CD spectra of 1–3 in TCE show a strong hypsochromic shift of the aggregating compounds 1 and 3 compared to the non-aggregating compound 2, and strong bisignate CD signal with a negative Cotton effect in the case of the chiral and aggregated compound 1. B) A closer look into the NIR region of compound 1 reveals two additional peaks at 803 and 1555 nm (with shoulders at 695 and 1270 nm) that can be attributed to the formation of positive polarons. These peaks are not observed either in the non-aggregated reference compound 2 nor in the achiral compound 3 that forms spectroscopic H aggregates similar to compound 1 (* the additional NIR features are artefacts originating from the cuvette or the solvent; see Supplementary Figure S7 for details). C) The absorption bands associated with the positive polaron in the case of compound 1 (in TCE solutions) disappear upon reduction with hydrazine and can be regenerated either by illumination (top) or oxidation with iodine (bottom). D) Temperature-dependent NIR spectroscopy of 1 in TCE reveals that the polaron bands completely disappear upon heating the solution above 350 K, when 1 deaggregates, and slowly recover upon cooling and reaggregation.

Notably, assuming typical extinction coefficients for such polaron bands,36 the observed absorption intensities imply a very high photocharge concentration on the order of a few mol%, which we have independently corroborated by a spin count in electron spin resonance spectroscopy (see below). The polaron bands are also observed in toluene, in carefully degassed solutions, and in solid thin films of the ACS Paragon Plus 6 Environment

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materials (Supplementary Figure S9), excluding oxygen, chlorinated solvents, or inadvertent impurities as the oxidants, given the high concentration and reversibility of charge carrier formation (see below). In fact, the strongest arguments against a participation of an impurity or the presence of an inadvertent oxidant is the fact that the polaron bands completely disappear when the aggregates of compound 1 are thermally disassembled by heating the solution to temperatures above 350 K, and they slowly but fully reversibly reappear upon re-aggregation of 1 by cooling the solution (Figure 2D, Supplementary Figure S10). This process can be repeated many times without noticeable degradation of the chromophores. Moreover, we have never observed these polaron bands in molecularly disperse solutions of the non-aggregating reference compound 2 either in solution or in solid films (Figure 2B; Supplementary Figure S9), although it is based on exactly the same chromophore, comprises the same chemical functional groups,

and

forms

radical

cations

upon

chemical

oxidation

with

iron

trichloride

(Supplementary Figure S8). Even more intriguingly, even the achiral reference compound 3 does not show any polaron formation in solution or in solid films either, although the compound has been prepared following the same chemical reactions. Moreover, compound 3 possesses the same chemical functional groups and molecular architecture as 1 except for the absence of chirality, shows an equally strong aggregation in solution and in solid films (according to IR spectroscopy, Supplementary Figure S1), exhibits similarly strong electronic interactions between neighboring chromophores (according to UV/vis spectroscopy; Figure 2A,B; Supplementary Figures S4–S6), and can also be oxidized with iron trichloride to yield polaron charge carriers (Supplementary Figure S8). This comparison not only proves once more that the observed charge carrier formation cannot simply be due to sacrificial oxidants originating in the solvents or impurities; it also indicates that the molecules do not undergo “self-doping” with a chemical group present in the molecule as the acceptor without being placed in the specific environment of the one-dimensional nanostructures. Indeed, thermally annealed solutions of mixtures of the achiral compound 3 and the chiral derivative 1 in varying proportions exhibit a strong correlation between the formation of helical aggregates (as seen from the intensity of the CD signal) and the photoinduced polaron formation determined from the intensity of the NIR band (Supplementary Figure S11). Once the molar fraction of the chiral compound 1 exceeds 50 mol%, the CD signal intensity exceeded the value expected for a mere co-existence of separate aggregates of 1 and of 3, suggesting that the achiral molecules 3 were incorporated into a predominantly helical arrangement resembling the one of pure 1, i.e., we observed a weak manifestation of a ‘sergeants and soldiers’ effect.39 Beyond this concentration, the aggregates of 3 also exhibited the same behavior in ACS Paragon Plus 7 Environment

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terms of charge carrier formation. Moreover, we note that this behavior is not specific for the 2,5-bis(2,2’-bithiophen-5-yl)thieno[3,2-b]thiophene core of compound 1 but is also observed for a broad variety of other electron-rich π-conjugated chromophores with a similar molecular architecture, for instance, for the chiral quaterthiophene derivative 4 that shows the formation of positive polarons in the aggregated but not in the non-aggregated state, whereas its achiral counterpart 5 does not exhibit any trace of polaron formation even in the aggregated state (Supplementary Figures S12, S13). Our results hence demonstrate that (i) the photocharge formation process is entirely reversible, that (ii) the strong excitonic coupling of the chromophores in their aggregated state is one necessary prerequisite, that (iii) the molecular structure and aggregation alone are not sufficient, but that (iv) the specific arrangement of the molecules into helically twisted supramolecular polymers, either because of the specific electronic interactions that result from the helical arrangement or just as a means to reliably obtain one-dimensionally confined nanostructures, is equally decisive to allow for electron transfer to chemical functional groups (such as, for instance, the amide carbonyl groups) that would otherwise be insufficient as electron acceptors. Illumination of a solution of compound 1 (after thermal annealing and re-aggregation upon cooling, under the exclusion of light to suppress the initial formation of polarons to a minimum) with a standard white light source induces a rapid increase in intensity of the polaron bands in the UV/vis/NIR spectra, reaching saturation after some tens of minutes. The bands decay again (but do not completely disappear) in the dark to reach a steady state after several hours (Figure 3A,B). The relaxation of the polaron bands is polychromatic, in the sense that it is the result of a broad distribution of simultaneous relaxations. Therefore, we have analyzed it with a model-free approach (Figure 3C; Supplementary Figure S14), by which we eliminate the bias resulting from fitting a potentially inaccurate parametric distribution modeling the dispersion of the relaxation times (for a more detailed discussion, see Supplementary Information, Section 3.1). In this way, the mean of the apparent relaxation time (and its standard deviation) for the decay of the charge carriers is determined to be tapp = 7.2 (± 4.9) h. Given the finite time-range of the experimental data, this time-averaged mean of the relaxation time can be regarded as a lower boundary, and its large standard deviation indicates a broad and heavily tailed distribution of relaxation times. The latter is a common feature of luminescence decays and observed when charge generation and decay proceed via mechanistically complex pathways. It also implies that a fraction of charge carriers remains in the nanowires even after very long times and will thus not completely decay within the timeframe of standard lab experiments.


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Figure 3. A) Evolution of the NIR spectra of 1 in TCE solutions upon illumination with a white light source; the polaron absorption band at 1555 nm significantly increases in intensity under illumination (top) and slowly decreases again in the dark (bottom), reaching a level that is stable over days in the dark. B) Plot of the intensity of the 1555 nm band versus time under white light illumination (top) and in the dark after switching off the white light source (bottom). C) Plot of the intensity decay of the 1555 nm band in the dark fitted with a polynomial of high degree (top) and distribution of the residuals (bottom); the coefficients of this polynomial describe the inverse statistical moments of the relaxation rates (for more details, see Supplementary Figure S14 and Supplementary Information, Section 3.1). D) The absorption at 1555 nm of a solution of 1 measured after irradiation with white light (red) and after relaxation in the dark for a day (blue) repeated over 10 cycles shows that the photocharge generation is reversible without any degeneration of the sample.

In any case, the apparent relaxation time is more than four orders of magnitude slower than the typical time constants observed for radical cations formed via the light-induced charge transfer from aggregated oligothiophenes to an external electron acceptor (tetracyanoethylene) in dichloromethane40 or for polarons in covalently linked donor-acceptor systems.41,42 These exceptionally long decay time ACS Paragon Plus 9 Environment

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constants also explain the accumulation of charge carriers up to concentrations that were sufficiently high to let us observe the charge carriers in steady-state spectroscopy. The same processes are also observed in toluene solutions (Supplementary Figure S15), where an even higher mean of the apparent time constant tapp = 13.9 (±8.5) h was observed, as well as in solid films of compound 1, albeit with overall lower polaron concentrations and a smaller mean of the apparent time constant tapp = 46 (±33) min in this case (Supplementary Figure S16). To further probe the reversibility of the light-induced polaron generation and exclude the degradation of 1 by follow-up chemical reactions after the initial excitation, we have exposed solutions of 1 to 10 cycles of 30 min of white light illumination followed by one day of relaxation in the dark (Figure 3D). The intensity of the 1555 nm band oscillates within the same steady states over all cycles, proving that the photocharge generation is fully reversible and compound 1 does not degrade, although the photocharges are formed in concentrations of a few mol% in every illumination cycle. Moreover, the polaron bands can be repeatedly quenched by the addition of hydrazine and regenerated by illumination with white light. Furthermore, doping the irradiated nanowires of compound 1 with a large excess of I2 only leads to a small increase of around 10 % in the polaron concentration before the formation of a new band at 1066 nm was observed that can most likely be attributed to the formation of bipolarons. Electron spin resonance (ESR) spectroscopy of thoroughly degassed solutions of compound 1 in TCE or toluene exhibit a strong signal (in contrast to solutions of 2 or 3 that do not show any ESR signal at all) with a g-factor of 2.0024 and a peak-to-peak ESR linewidth ΔHpp = 2.27 G (Supplementary Figure S17, Supplementary Table S1), very close to the g-factor value of the free electron and well in the range of the g-factor values and line widths typically observed for positive polarons in organic and polymer semiconductors.43-48 A detailed peak analysis of the ESR spectra of solid films of compound 1 indicates a Voigt line shape (Supplementary Figure S18), i.e., a convolution of a Gaussian and a Lorentzian profile. This line shape suggests that the ESR features are broadened through both homogeneous and inhomogeneous broadening mechanisms (most likely originating from the structural inhomogeneity of the supramolecular polymers in terms of length, internal defects, and supramolecular conformational flexibility),48 of which the latter could be one explanation for the polychromatic decay kinetics observed in NIR spectroscopy. The spin density for samples of compound 1 in solution determined from the ESR spectra is about 3 mol% upon irradiation with a white light at a power density of 20 mW/cm2 (≈ 0.2 suns), which is about twice as high as for a non-irradiated sample. This value translates into a one-dimensional charge density of about 6.5 · 105 cm–1 or about one charge carrier every 15 nm along the aggregates. Similar results are ACS Paragon Plus 10 Environment

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obtained for vacuum-dried films of compound 1 in argon atmosphere (Figure 4A), albeit with a slightly lower spin concentration of 0.7 % in the dark, but still one to two orders of magnitude higher than spin concentrations reported for films of undoped quinquethiophene with a density of 9 · 10–4 spins per molecule (0.09 mol%) that only marginally increases upon irradiation49 or in poly(3-alkylthiophene) with a density of 7 · 10–6 spins per repeating unit (0.0007 %) in air.50,51 We note that, although a counter charge must of course inevitably be present in the system, we have not observed any corresponding ESR or NIR signatures. However, the same issue has been reported before and either been attributed to a strong localization of the counter charges45,52 or a superposition of radical cation and anion features.47

1

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Curie χ ~1/T 0

100 200 Temperature / K

0 300

Figure 4. A) ESR spectra of 1 in vacuum-dried films before and after irradiation with white light. B) Plot of the paramagnetic susceptibility χ as measured by temperature-dependent ESR spectroscopy versus temperature T reveal a transition from Curie to Pauli behavior at temperatures above 140–180 K.

Temperature-dependent ESR measurements (10–293 K) performed on films of 1 suggest Curie-type behavior (χ ~ 1/T) at low temperatures, consistent with localized radical cations as spin carriers (Figure 4B). At temperatures above approximately 140–180 K, however, the system gradually starts to deviate from Curie-type behavior towards the temperature-independent Pauli-type behavior (χ ≈ const.), which is a direct evidence for the presence of highly delocalized and interacting spins, as it has been described for strongly doped organic semiconductors such as polyanilines53, poly(3-alkylthiophenes)54, and PBTTT47,52. Moreover, the line width of the ESR signal continuously decreases with increasing temperatures, which is indicative of increasing spin-exchange dynamics. The behavior is well described by Wilson’s model combining Anderson’s motional narrowing and a hopping wave function, which was recently applied to carbon nanotube networks by Rice and coworkers.55 Fitting the ESR line width thus ACS Paragon Plus 11 Environment

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gives an activation energy of DEA = 17 meV (Figure 4B, Supplementary Figure S17). This value corresponds to a critical hopping temperature Thop = 202 K, which is in qualitative agreement with the crossover temperature range where the deviation from a Curie to Pauli behavior was observed. This activation energy is low compared to typical values of 50–200 meV reported for highly ordered polythiophene materials and thus implies that trapped charge carriers in the strongly photodoped nanowires are comparably easily activated for transport to become mobile polarons, at least at temperatures above about 200 K.56 Finally, we demonstrate that the spectroscopically observed photocharges directly translate into electrical transport properties along the nanowires, and macroscopic photoconductivity. To this end, we have fabricated devices by spin-coating nanowires of 1 onto a silicon wafer (with a SiO2 dielectric layer), followed by vapor-deposition of gold electrodes using microstencils (Figure 5A–C). In order to improve the alignment of the nanofibrils in the channel of the devices, we have designed the microstencils with a layout where the individual devices were oriented radially from the center of the wafer. Atomic force microscopy (AFM) images of devices with a 2 µm channel length between the gold electrodes and the corresponding height profiles show a sub-monolayer film of individualized one-dimensional aggregates spanning the channel of the devices with an approximately perpendicular orientation relative to the electrodes and, on average, 820 nanowires per 10 µm (Figure 5D,E; Supplementary Figure S19). Electrical conductivity is only observed in devices with channel lengths of 5 µm or less, which, given that the average length of the nanowires is on the order of a few micrometers, implies that charge transport predominantly occurs within individual nanowires, whereas charge transfer between neighboring nanowires is strongly hindered. Notably, two-contact devices from films of 1 show a conductivity that is significantly enhanced upon illumination with white light (Figure 6A). Field-effect transistor devices fabricated from such films on Si/SiO2 show only a marginal gate effect in their I–V characteristics with very high off-currents (Figure 6B, Supplementary Figure S19), which has often been reported56,57 for strongly doped organic semiconductors and should hence be expected due to the high charge carrier density that, according to our ESR results, almost reaches concentrations similar to those observed in P3HT transistors in the on-state.58 The shape of the I–V curves shows the characteristics of a space charge-limited behavior (Supplementary Figure S19) with an Ohmic region (I ~ V) at low voltages and a regime with I ~ V n and n = 3.1 at higher voltages, where the exponent n > 2 indicates that the trap-free space charge-limited behavior is not reached within the applied voltage range and the device is still operating in the trap-filling regime.59 We therefore assume that a considerable fraction of the


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photocharges mainly fill the traps in the nanowires and thus enhance the conductivity by reducing the trap-density rather than actively contributing to the charge transport themselves.

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AFM

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Figure 5. A) Optical image of a wafer with a series of two-contact devices with 2 and 5 µm channel length, respectively, as well as B) an optical micrograph of a device with a 2 µm channel. C) Schematic illustration of a two-contact device used to investigate the macroscopic transport properties of the organic 1D-aggregates. D) AFM height image of partially aligned aggregates of 1 between the two gold electrodes. A partial alignment orthogonal to the electrodes has been achieved by spin coating and using a circular layout of the microstencils. E) AFM height profiles 1–4 of the sub-monolayer film with an average of 820 individualized one-dimensional aggregates per 10 µm spanning the channel.

As anticipated from the preceding results, significant photoconductivity upon irradiating two-contact devices with white light was observed, with a photocurrent on/off ratio of 3 and an estimated


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conductivity of 6.6 · 10–3 S/cm under illumination with a white light source at a power density of 20 mW/cm2 (Figure 6C). This value represents a lower boundary as it assumes that all nanowires contribute to conduction (i.e., span the whole channel, are in contact with both electrodes and have no defects). The conductivity decreased to the initial value in the dark, in contrast to self-assembled triarylamine system previously reported that also exhibit long-lived charge carriers but showed a continued increase in current after irradiation even when the light source was switched off because their self-assembly was triggered by radical cation formation.27,28 The devices responded to irradiation on the time scale of minutes, which corresponds roughly to the spectroscopically observed stability of the photocharges in solid films of 1 (Supplementary Figure S16). Furthermore, their photoresponse was fully reversible over many on/off cycles, confirming once more the high photostability of our system already

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Figure 6. A) Current-voltage profiles of two-contact devices of 1 in the dark (black) and under white light illumination (blue) for a channel length of 2 µm. B) Output curve obtained from measurements of field-effect transistors fabricated from 1 with the same microstencils as above (100 nm SiO2 dielectric layer) show only a marginal gate effect due to the high intrinsic charge carrier density of the aggregates. C) Photoresponse at a constant voltage of 40 V over time (top), with alternating white light illumination (1 min, red) and decay periods in the dark (5 min, blue), and (bottom) under continuous irradiation until saturation of the photocurrent is reached followed by a decay in the dark (bottom).

Conclusions In conclusion, we have demonstrated that the oligopeptide-substituted thienothiophene-based chromophore 1 gives rise to one-dimensional nanostructures with a precise control over their morphology and internal structure. The helically stacked chromophores at the core of these ACS Paragon Plus 14 Environment

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nanostructures show a strong electronic coupling and undergo a facile photocharge generation that has not previously been observed for thiophene-based semiconductors in solution and in the solid state, as it already occurs at a comparably low photon fluence (daylight or a standard LED light source) and in the absence of a dedicated electron acceptor. We have proven the polaron-like delocalized and mobile nature of these photocharges at temperatures above 140–180 K. Moreover, we have observed that the photocharges exhibit extraordinary life times on the order of several hours in solution and dozens of minutes in solid films. Similarly long-lived charge carriers have only very recently been described by Tolber and coworkers where the long lifetimes were achieved by a fast spatial charge separation due to a complex electron-transfer cascade structure in a co-assembled polyelectrolyte and cationic fullerene derivatives.12 Giuseppone et al.27,28 reported long-lived charge carriers for certain self-assembled triarylamines that are, however, well known29 to form persistent radical cations, in marked difference to the thiophene-based chromophores described here. Cowan and coworkers observed long lived radical anions in aggregates of amino acid-substituted perylene bisimide derivatives.30,60 It is an intriguing observation that the common denominator of these last two examples, our own system, as well as columnar phases of triarylamine-based molecules that showed long-range energy transport,61 is that they all (i) employ amide hydrogen bonding to (ii) achieve a nanoscopic confinement to isolated, helical stacks of chromophores. For our system, we have clearly demonstrated that, while the strong excitonic coupling of the chromophores in their aggregated state is one necessary prerequisite, the helical arrangement of the molecules within the aggregates is equally decisive. Our results suggest that this effect may either be due the helical arrangement on the electronic interactions of the π-stacked chromophores itself, or more likely due to the resulting nanoscopic confinement to a single stack of chromophores shielded by a highly polar shell. For instance, it seems possible that this specific nanoscopic structure renders the hydrogen-bonded peptide groups sufficiently strong electron acceptors, which they would otherwise not. This might explain the extraordinary lifetimes of the photocharges delocalized along the central stack by their spatial separation from the highly localized counter charges in the periphery. The picture that emerges from the combined spectroscopic findings and electrical measurements is that the polarons are delocalized within segments of the nanowire but, at the same time, confined to these segments due to defects or distortions of the packing due to the polarons themselves. The exceptionally long lifetimes result in an accumulation of the photocharges to concentrations as high as several mol% that have allowed us to observe them in steady-state spectroscopy and link them to macroscopic photoconductivity, where the nanostructures show a behavior similar to strongly doped organic semiconductors. Our study, hence, relates the formation of self-assembled organic nanowires from H-aggregated chromophores with ACS Paragon Plus 15 Environment

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the formation of charge carriers typical for organic semiconductors and macroscopic properties and thus provides an insight into the structural and electronic prerequisites for (photo)charge generation in the less ordered domains of polymer semiconductors and photovoltaic materials. Methods A comprehensive account of all experimental details, including Supplementary Figures S1–S19, Supplementary Table S1, experimental procedures, and analytical data for all compounds, is provided in the Supplementary Information. Preparation of Sample Solutions for UV/vis/NIR, IR, CD, ESR Spectroscopy and AFM Imaging. The compounds 1–3 were dissolved in TCE or deuterated TCE (c = 1 mmol/L). The solutions were degassed and placed in an oil bath at 140 °C for 2 h under an argon atmosphere. The solutions were cooled to 120 °C and then kept at that temperature for another 2 h, and finally left to slowly cooling to room temperature overnight. Steady state UV/vis/NIR and CD spectra were recorded at c = 0.1 mmol/L, while time dependent spectroscopy was performed at c = 1 mmol/L. IR, UV/vis/NIR, and CD Spectroscopy. Solution-phase IR spectra (c = 1 mmol/L in TCE) were recorded on a Jasco FTIR 6300 spectrometer using a solution-phase cell with KBr windows and a path length of 0.5 mm. Solution-phase UV/vis/NIR spectra (c = 0.1 or 0.33 mmol/L in TCE or TCE-d2) were recorded on a Jasco V-670 spectrometer using a Hellma quartz cuvette with 1 mm path length. Solution-phase CD spectra (c = 0.1 mmol/L in TCE) were recorded on a Jasco J-815 spectrometer using a Hellma quartz cuvette with 1 mm path length. Both the UV/vis and the CD spectrometers were equipped with a Jasco ETCR-762 temperature controller connected to a Jasco MCB-100 mini circulation bath. The resolution was set to 1 nm for both UV/vis and CD spectroscopy. Spectroscopy-grade solvents were used for all spectroscopic investigations. In Situ UV/vis/NIR Absorption Spectroscopy. A solution of 1 in deuterated tetrachloroethane (TCE-d2, c = 1 mmol/L) or toluene were degassed by three freeze-pump-thaw cycles, shielded from light with aluminum foil, and annealed as described above. A volume of 200 μL was subsequently transferred into a 4 ´ 4 mm quartz fluorescence cuvette (Hellma). The cuvette was placed in a four-way cuvette holder (Ocean optics, cuv-all-uv) to allow for two illumination channels, a white light (100 W xenon short-arc lamp in a Newport Oriel Apex fiber illuminator) and a NIR source. A 1000 nm long-pass colored glass filter was employed at the NIR source to avoid excitation during the decay phase. The spectra were recorded with an Ocean Optics NIR-Quest-512 spectrometer.


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SAXS and WAXS. Measurements were performed with a NanoMax-IQ camera (Rigaku Innovative Technologies, Auburn Hills, USA). The samples were kept in vacuum at room temperature during the measurements. The scattering data were presented as a function of the scattering vector q = 4π/λ sin(θ/2), where θ is the scattering angle and λ is the photon wavelength. ESR Spectroscopy. Solid samples were prepared as thin film deposit on a standard ESR tube and filled with helium for the low temperature ESR measurements. All experiments were performed using a Bruker ESR spectrometer E500 EleXsys Series (Bruker Biospin GmbH) equipped with a Gunn diode-based microwave bridge (model SuperX), a high-Q cylindrical cavity, model Bruker ER 4122 SHQE, and an Oxford Instruments Helium-gas continuous flow cryostat (ESR900) for the low temperature measurements. The typical instrumental settings were: microwave frequency 9.40 GHz, microwave power 2.0 mW, conversion time 40.96 msec, number of points 2048, resulting sweep time 83.89 s, lock-in time constant 5.12 ms. The 100 kHz modulation amplitude was kept at 1.5 G (0.15 mT) to avoid modulation broadening. AFM Imaging. Solutions of 1–3 in TCE were prepared as described above, diluted to a concentration of 0.01 mmol/L, drop-cast onto mica substrates, and gently dried in a flow of nitrogen. The obtained samples were analyzed in tapping mode using a Nanoscope IIIa (Veeco Instruments Inc., Santa Barbara, USA) instrument at room temperature in air. Cantilevers with a resonance frequency on average of f0 = 325 kHz and k = 40 N/m were used. Scan rates between 0.5 and 1 Hz were applied, the image resolution was 512×512 pixels. Stencil Fabrication. Si wafers, prime grade, double side polished were first coated by low-pressure chemical vapor deposition with 500 nm low-stress SixNy. Stencil aperture patterns, pairs of pad apertures separated by a narrow gap-defining bridge, were then defined in positive photoresist (AZ 9260 3 μm) by direct laser writing (Heidelberg instruments VPG-200). These patterns were subsequently transferred to the underlying SixNy layer by reactive ion etching (SPTS-APS) and further etched 10-20 μm deep into the silicon wafer by deep reactive ion etching (DRIE) (Alcatel AMS 200). After patterning, the front side of the wafer was protected by sputter deposition of 1 μm Al. A similar process was carried out on the wafer backside with Si DRIE performed through the wafer and using a lithographic mask intended to release the entire region of the front-side pattern, i.e., not including the central bridge. The Al protection layer intended to preserve vacuum during the etch through procedure was then stripped by wet chemistry and the stencil masks were finally exposed to a brief anisotropic wet etching by potassium hydroxide (60 °C, 40 %) in order to preserve SixNy-only freestanding strings between the two contact-defining large


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apertures. The process ultimately allowed for the fabrication of freestanding strings as narrow as 2 μm and 1 mm long with arbitrary orientations in regards to the Si crystalline planes. Two-Contact Conductivity and Field-Effect Transistor Measurements. The electronic conductivity measurements were performed in two-contact resistance mode, and the field-effect transistors with a bottom-gate, top-contact configuration. A nanowire-solution of 1 (0.3 mmol/L or 1 mmol/L) was spin-coated at 4000 rpm onto a silicon wafer (with a 100 nm SiO2 dielectric layer). The electrical contacts consisted of a 30 nm layer of gold deposited onto the deposited nanowires by thermal evaporation in a vacuum chamber at a pressure of 1 · 10–6 mbar using a micromachined stencil. The obtained gold electrodes had channel lengths of 2 µm and a channel width of 500 µm. The samples were electrically characterized in the dark at a pressure of 1 · 10–6 mbar using a Keithley 4200 data acquisition system with multiple built-in source-measure units (SMU) and triax cables. Acknowledgments Funding from the Swiss National Science Foundation (SNF Grants 200020-121812 and 200020-144417) is gratefully acknowledged. T.S. acknowledges the support from the ‘EPFL Fellows’ fellowship programme co-funded by Marie Skłodowska-Curie (grant agreement no 291771). We would like to thank Eric Bremond, Shanshan Wu, Clémence Corminboeuf (EPFL Switzerland) for preliminary computations and discussions. Author Contributions R.J.H. synthesized 1–3 and performed UV/Vis/NIR, CD, IR spectroscopy, AFM, I/V measurements and conducted ESR measurements in collaboration with A.S. S.B. conducted SAXS and WAXS measurements. V.F. and J.B. furnished the microstencils for device manufacturing. R.J.H., L.T., J.C.B., T.S., S.B. and H.F. were responsible for the data analysis and wrote the manuscript. Supporting Information Available Supplementary Figures S1–S19 and Supplementary Table S1, including additional IR, UV-vis, CD, NIR, and ESR spectroscopic results, AFM images, SAXS and WAXS data, as well as electrical device measurements; comprehensive experimental part including our model-free approach describing the relaxation kinetics, materials and methods, as well as NMR and mass spectra of the final compounds. This material is available free of charge via the Internet at http://pubs.acs.org.


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S.; Jiang, J.-X.; Bonillo, B.; Ren, S.; Ratvijitvech, T.; Guiglion, P.; Zwijnenburg, M. A.; Adams, D. J.; Cooper, A. I. Tunable Organic Photocatalysts for Visible-Light-Driven Hydrogen Evolution. J. Am. Chem. Soc. 2015, 137, 3265–3270. (7) Bredas, J.-L.; Sargent, E. H.; Scholes, G. D. Photovoltaic Concepts Inspired by Coherence Effects in Photosynthetic Systems. Nat. Mater. 2016, 16, 35–44. (8) Vandewal, K.; Albrecht, S.; Hoke, E. T.; Graham, K. R.; Widmer, J.; Douglas, J. D.; Schubert, M.; Mateker, W. R.; Bloking, J. T.; Burkhard, G. F.; Sellinger, A.; Fréchet, J. M. J.; Amassian, A.; Riede, M. K.; McGehee, M. D.; Neher, D.; Salleo, A.; Efficient Charge Generation by Relaxed Charge-Transfer States at Organic Interfaces. Nature Mater. 2014, 13, 63–68. (9) Hedley, G. J.; Ruseckas, A.; Samuel, I. D. W. Light Harvesting for Organic Photovoltaics. Chem. Rev. 2017, 117, 796–837. (10) Reid, O. G.; Pensack, R. D.; Song, Y.; Scholes, G. D.; Rumbles, G. 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