Long-Range Observation of Exciplex Formation and Decay Mediated

Jun 16, 2017 - We report herein unprecedented long-range observation of both formation and decay of the .... King, Shimmon, Totonjian, and McDonagh...
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Long-Range Observation of Exciplex Formation and Decay Mediated by One-Dimensional Bridges Jinseok Baek,† Tomokazu Umeyama,*,† Kati Stranius,‡ Hiroki Yamada,† Nikolai V. Tkachenko,*,‡ and Hiroshi Imahori*,†,§ †

Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan Laboratory of Chemistry and Bioengineering, Tampere University of Technology, P.O. Box 541, FIN-33101 Tampere, Finland § Institute for Integrated Cell-Material Sciences, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan ‡

S Supporting Information *

ABSTRACT: We report herein unprecedented long-range observation of both formation and decay of the exciplex state in donor (D)−bridge (B)−acceptor (A) linked systems. Zinc porphyrins (ZnP) as a donor were tethered to single-walled carbon nanotube (SWNT) as an acceptor through oligo(pphenylene)s (ZnP−phn−SWNT) or oligo(p-xylene)s (ZnP− xyn−1−ph1−SWNT) with systematically varied lengths (n = 1− 5) to address the issue. Exponential dependencies of rate constants for the exciplex formation (kFEX) and decay (kDEX) on the edge-to-edge separation distance between ZnP and SWNT through the bridges were unambiguously derived from timeresolved spectroscopies. Distance dependencies (i.e., attenuation factor, β) of kFEX and kDEX in ZnP−phn−SWNT were found to be considerably small (β = 0.10 for kFEX and 0.12 Å−1 for kDEX) compared to those for charge separation and recombination (0.2−0.8 Å−1) in D−B−A systems with the same oligo(p-phenylene) bridges. The small β values may be associated with the exciplex state with mixed characters of charge-transfer and excited states. In parallel, the substantially nonconjugated bridge of oligo(p-xylene)s exhibited larger attenuation values (β = 0.12 for kFEX and 0.14 Å−1 for kDEX). These results provide deep insight into the unique photodynamics of electronically strongly coupled D−B−A systems involving exciplex.

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charge-separated state.10 Direct energy-wasting decay of the exciplex state to the ground state is undesirable, but it frequently happens without forming the charge-separated state. Although the precise understanding of the intrinsic nature of the exciplex state is crucial for efficient formation of the long-lived charge-separated state,8−16 the information is extremely limited in comparison with the charge-separated state because of lack of suitable model systems and difficulty in discovering such systems. D−bridge (B)−A covalently linked systems have received remarkable interest as model systems for addressing the photoinduced electron-transfer properties between molecular entities in more complex architecture.17−23 The covalent linkage can eliminate the complicated factors arising from diffusion in solutions, generating more well-defined D−A geometry. Studies on the D−B−A linked systems can also provide us with basic information on the rational molecular design for organic optoelectronic devices and solar energy conversion. Therefore, it is of special interest to disclose the intrinsic properties of the exciplex state as well as the chargeseparated state in D−B−A systems, where the length and chemical structure of the bridge are varied systematically. On a

INTRODUCTION Understanding photoinduced charge separation (CS) and charge recombination (CR) processes is one of the most ubiquitous and fundamental subjects in chemistry, physics, and biology.1,2 In particular, extensive efforts have been devoted toward the preparation of electron donor (D)−acceptor (A) assemblies to achieve efficient formation of a long-lived chargeseparated state. Manipulating the completely separated positive and negative charges is beneficial for operating various organic devices such as light-emitting diodes, molecular photovoltaics, and solar fuels.2−7 In a simple CS reaction, the locally excited state of the donor directly transfers an electron to the acceptor in the ground state, or the locally excited state of the acceptor receives an electron from the donor in the ground state, to form the complete charge-separated state (D•+ and A•−). However, when electronic coupling between the donor and the acceptor is sufficiently strong, an exciplex state, which is defined as an excited-state complex between two chemically different molecules involving an excited donor and a neutral acceptor, or vice versa, is rapidly formed.8,9 The exciplex state is known to possess partial positive and negative charges located on a donor and an acceptor, respectively, in the excited state, and is frequently observed in light-emitting diodes as well as in nonpolar environment. Moreover, it sometimes appears as the precursor intermediate followed by formation of the complete © XXXX American Chemical Society

Received: May 10, 2017

A

DOI: 10.1021/acs.jpcc.7b04483 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Scheme 1. Synthesis of ZnP−phn−SWNT and ZnP−xyn−1− ph1−SWNT (n = 1−5)a

basis of a number of studies on D−B−A systems,17−23 it has been well-established that the D−A electronic coupling mediated by bridge decays exponentially with increasing D−A separation distance, and thereby, the rates of the CS and CR are also decreased exponentially. However, the examples of the exciplex formation and decay in D−B−A systems have been limited so far compared to those of the corresponding CS and CR.9,10,15,16,24−26 We have prepared zinc porphyrin (ZnP) donor−fullerene C60 acceptor linked systems with systematically controlled D−A separation distances (RDA), where either the exciplex or CS state or both are formed strongly dependent on the ZnP−C60 separation distance and environment.15,16 Very recently, we also constructed two-dimensional (2D) chemically converted graphenes (CCGs) acceptor covalently modified with ZnP donors through tuned lengths of linear oligo(p-phenylene) bridges (ZnP−phn−CCG, n = 1−5).25 Photoexcitation of the ZnP moieties in ZnP−phn−CCG (n = 1−5) was found to cause exclusive formation of an exciplex state, which rapidly decayed to the ground state without yielding the complete charge-separated state. The notable dependence of the exciplex lifetime as a function of the through-space separation distance between the ZnP and CCG has been observed, indicating that the exciplex decay to the ground state occurred via a through-space mechanism.25 However, the exciplex formation (98%) in all ZnP−xyn−1−ph1−SWNT (n = 1−5). This longrange efficient quenching of 1ZnP* by SWNTs in ZnP−phn− SWNT and ZnP−xyn−1−ph1−SWNT (n = 1−5) is in sharp contrast with ZnP−fullerene covalently linked systems by oligo(p-xylene) bridges with controlled lengths (ZnP−xyn−C60, n = 1−5) that showed the bridge length-dependent quenching ratios, i.e., >99%, 94%, 55%, 26%, and 5% for n = 1−5, respectively.15 This result implies that the interaction between the 1ZnP* and SWNTs is exceedingly stronger than that between 1ZnP* and C60 and thereby retains even in the case of relatively long oligo(p-phenylene) and oligo(p-xylene) bridges. It should also be noted here that fluorescence quenching is more evident in ZnP−phn−SWNT (n = 1−5) than in ZnP− phn−CCG (n = 1−5), e.g., the quenching ratio of ZnP−ph5− CCG is 88%, which is lower than that of ZnP−ph5−SWNT (99%). The emissions from ZnP−phn−CCG (n = 1−5) were mainly attributed to the ZnP moieties linked at the edges and defect sites of CCG.15 Therefore, more defect-free structures of ZnP−phn−SWNT and ZnP−xyn−1−ph1−SWNT (n = 1−5) as well as the more intense interaction between 1ZnP* and SWNTs substantiated the drastic fluorescence quenching (Figures 1b and S12b). This also implies that the present model systems are ideal to evaluate the D−A photodynamics with strong interaction in the excited state over the long range. Time-Resolved Spectroscopies and Photoelectrochemical Properties of ZnP−phn−SWNT. To gain deep insight into the excited-state interaction, we monitored the porphyrin fluorescence decays of ZnP−phn−SWNT (n = 1−5) and ZnP-ref in a picosecond time scale by a photon upconversion technique (Figure 2). The fluorescence decay curve of ZnP-ref was fitted by a single component with a lifetime (τ) over 1 ns (Figure 2f), which is longer than the limit of the measurement system. Therefore, we further employed a timecorrelated single-photon counting (TCSPC) technique to monitor the emission decay of ZnP-ref properly (Figure S13). The emission decay of ZnP-ref exhibits a single component with τ = 2.0 ns, which is in good agreement with the literature.45 The fluorescence decay curves of ZnP−phn− SWNT (n = 1−5) were also fitted based on a single component (Figure 2a−e). The extremely shorter lifetimes (τ = 8.0−39.4 ps, Table 1) of 1ZnP* in ZnP−phn−SWNT (n = 1−5) than that (2.0 ns) in ZnP-ref are consistent with the dramatic fluorescence quenching in the steady-state emission spectra of

The 1D structures of SWNTs were verified by TEM (Figures S7 and S8) and AFM measurements (Figures S9 and S10). Although the ZnP units on SWNT could not be observed by the present TEM measurements, we recently confirmed that the aryl groups on SWNT sidewalls stand almost perpendicularly to the tube axis by a high-resolution TEM technique.46 Because oligo(p-phenylene)s and oligo(p-xylene)s have 1D rigid rodlike structures without bending,47 the bridge lengths can be regarded as the separation distances between the ZnP and SWNT in ZnP−phn−SWNT and ZnP−xyn−1−ph1−SWNT (n = 1−5). Steady-State Optical Properties. Figure 1a displays the steady-state UV−vis absorption spectra of ZnP−phn−SWNT

Figure 1. (a) Steady-state UV−vis absorption and (b) steady-state fluorescence spectra of ZnP−ph1−SWNT (orange), ZnP−ph2− SWNT (purple), ZnP−ph 3 −SWNT (red), ZnP−ph 4 −SWNT (cyan), ZnP−ph5−SWNT (green), SWNT−ph1−Bpin (black), and ZnP-ref (navy, 0.7 μM). The spectra in panel a are shown in parallel for comparison. For the excitations in panel b, the absorbances of the porphyrin moieties were adjusted to be identical at the peak position of the Soret band for comparison.

(n = 1−5), SWNT−ph1−Bpin, and 5,10,15,20-tetrakis(3,5-ditert-butylphenyl)porphyrinatozinc(II) (ZnP-ref) in DMF. The absorption spectra of the ZnP−phn−SWNT (n = 1−5) exhibit an intense Soret band at 430 nm and weak Q-bands at 560 and 600 nm, together with the broad structureless absorption of the SWNT, extending into the NIR region. This provides unambiguous evidence for the attachment of the ZnP units on SWNTs in ZnP−phn−SWNT (n = 1−5). In addition, the distinct absorption peaks of Soret bands allow us to excite the ZnP units of ZnP−phn−SWNT (n = 1−5). Upon excitation of D

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Figure 2. Fluorescence decays of (a) ZnP−ph1−SWNT, (b) ZnP− ph2−SWNT, (c) ZnP−ph3−SWNT, (d) ZnP−ph4−SWNT, (e) ZnP− ph5−SWNT, and (f) ZnP-ref measured in DMF by a photon upconversion technique. The excitation and detection wavelengths are 425 and 610 nm, respectively. The solid lines present decay fittings, and the fluorescence lifetimes (τ) are given in the figures.

Figure 3. (a) Transient absorption decay component spectra of ZnP− ph3−SWNT in DMF obtained with global four components fit of the data. (b) Enlarged spectra of panel a. The excitation wavelength is 430 nm. Lifetimes of respective components are given in the figures.

Table 1. Edge-to-Edge Separation Distance between ZnP and SWNTs (RDA), Fluorescence Lifetimes (τS), Rate Constant for Exciplex Formation (kFEX), Lifetimes of the Fourth Component in TA (τ4), and Rate Constant for Exciplex Decay (kDEX) of ZnP−phn−SWNT (n = 1−5) n 1 2 3 4 5

RDA/Å 5.8 10.1 14.5 18.3 22.6

τS/ps

kFEX/s−1

8.0 13.5 18.5 26.5 39.4

× × × × ×

1.25 7.82 5.36 3.72 2.49

τ4/ps 11

10 1010 1010 1010 1010

236 456 746 1077 1950

decay curves at different wavelengths. The TA decaying component spectra of ZnP−ph3−SWNT obtained from global four-exponential fit of the data and corresponding decay kinetics at 645 and 1050 nm are represented in Figure 3 and Figure S16, respectively. The two short-lived components (τ1 = 0.3 ps and τ2 = 1.2 ps) can be assigned to the SWNT excited state with different decay lifetimes, because the spectrum shapes and lifetimes are similar to those of SWNT−ph1−Bpin without the ZnP moieties (Figure S14e). These results also suggest that the SWNT excited state directly decays to the ground state (vide infra). The second component may stem from the exciton localization on defect sites of the SWNT surface. The covalent functionalization produces sp3 defects in the sp2 lattice of the SWNT sidewall, which can trap a mobile exciton.48−50 The localized excitons trapped at the defect sites decay to the ground state rather slowly (τ2 = 1.2 ps) in comparison with free excitons (τ1 = 0.3 ps). In contrast, the third decay component spectrum that exhibits three negative peaks in the visible region at 553, 598, and 658 nm has a lifetime (τ3) of 19 ps (Figure 3b), which coincides well with the fluorescence lifetime of the ZnP moiety in ZnP−ph3−SWNT (18.5 ps, Figure 2c). This concordance suggests that the third component results from the 1ZnP* decay. The negative peaks at 553 and 598 nm can be assigned to instantly bleached Qbands of ZnP, and another peak at 658 nm can be attributed to the stimulated emission from 1ZnP* (Figure S17).51

kDEX/s−1 4.24 2.19 1.34 9.29 5.13

× × × × ×

109 109 109 108 108

ZnP−phn−SWNT (n = 1−5) (Figure 1b). The lifetime of 1 ZnP* becomes monotonically shorter with decreasing the bridge length (Table 1), reflecting degree of the bridgemediated interaction between 1ZnP* and SWNTs. The fluorescence decay measurement of SWNT−ph1−Bpin in the same conditions showed no detectable signals, supporting that the emissions of ZnP−phn−SWNT (n = 1−5) (Figure 2a−e) arise from the ZnP moieties on SWNT. The excited-state interaction between the ZnP and SWNTs in ZnP−phn−SWNT (n = 1−5) was further studied in detail by time-resolved transient absorption (TA) spectroscopy. The TA measurements were implemented in DMF with the excitation wavelength of 430 nm at which both the SWNT and ZnP in the ZnP−phn−SWNT (n = 1−5) are excited (Figure 3, S14 and S15). Multiexponential global fittings were applied to the TA E

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Judging from the above results together with the preceding assignment associated with the exciplex formation in ZnP− C60,9,10,15,16 ZnP−SWNT,30 and ZnP−CCG25 linked systems, the fourth component in the TA spectrum of ZnP−ph3− SWNT can be reasonably assigned to the exciplex state, that is, the photoexcitation of ZnP−ph3−SWNT causes formation of the exciplex that decays to the ground state without forming the complete charge-separated state. The exciplex state has partially charged D and A units, i.e., Dδ+ and Aδ−, and thus, its absorption spectrum tends to be to some extent similar to that of complete charge-separated state, i.e., D•+ and A•− (an example is shown in Figure S22), but significantly broad.54 The present significant difference in the absorptions of the Aδ− and A•− suggests unique distribution of the partial negative charge on the SWNT surface in the exciplex state relative to that of the partial positive charge on the porphyrin. Importantly, as contrasted with complete CS and EN, the exciplex state formed by the interaction between 1ZnP* and SWNTs does not contribute to the photocurrent generation due to the partial charge-transfer (CT) character. We previously proposed the rapid exciplex formation and the direct decay to the ground state by the photoexcitation of ZnP-linked SWNTs with a short phenylene spacer, i.e., ZnP−ph1−SWNT, based solely on the photoelectrochemical properties.30 One of the general features of the exciplex is red-shifted emission relative to the monomeric species. Considering that the energy level of the exciplex state is located between those of the 1ZnP* (2.1 eV) and the complete charge-separated state (1.3 eV), the exciplex state formed in ZnP−ph3−SWNT with the lifetime of 746 ps might show emission in the region of 1.3−2.1 eV (600−1000 nm). However, no exciplex emission of ZnP−ph3−SWNT was observed in the vis−NIR region, indicating that the exciplex composed of ZnP and SWNT is nonemissive and decays to the ground state via vibronic relaxation pathways. This nonemissive character is the same as those of previously reported other porphyrin-linked nanocarbon systems.10,15,16,30,35 Meanwhile, the TA spectrum of ZnP−ph3−SWNT with the excitation wavelength of 1030 nm, where only SWNTs are excited by the first interband transition of semiconducting SWNTs (S11), shows two decay components with analogous spectral shapes and lifetimes to those of SWNT−ph1−Bpin without the porphyrin moiety (Figure S23). This result supports that the exciplex formation does not take place by the photoexcitation of the SWNT moiety. More noteworthy herein is the result that similar broad fourth components, of which the absorptions are explicitly different from that of the complete charge-separated state, are also reasonably derived from the global fit of the TA data of all ZnP−phn−SWNT (n = 1−5) (Figures 3, S14, and S15), demonstrating exclusive formation of the exciplex state [i.e., (ZnPδ+−phn−SWNTδ−)*], which directly decays to the ground state, irrespective of the oligo(p-phenylene) bridge lengths. The strong interaction between 1ZnP* and SWNTs through the oligo(p-phenylene) bridges, which was also demonstrated by the efficient fluorescence quenching (Figure 1b), allows the bridge-mediated, efficient exciplex formation over the wide range of the separation distance (5.8−22.6 Å) in ZnP−phn− SWNT (n = 1−5). Length Dependence on the Rates for the Exciplex Formation and Decay in ZnP−phn−SWNT. Rate constants for the formation of exciplex state (kFEX) can be determined using the following equation:

Additionally, the fourth component with lifetime (τ4) of 746 ps was derived from the global fitting (Figure 3). Although the shape of the fourth component spectrum is somewhat similar to that of the third component, its broad shape with the relatively long lifetime implies that the fourth component is different from 1ZnP*. Furthermore, the absence of the positive absorption band at 650−900 nm characteristic to the ZnP triplet excited state (3ZnP*) (Figure S18)15,52 can exclude the possibility of the 3ZnP* formation. Note that the electron transfer (ET) from 1ZnP* to the conduction band of SWNTs is energetically favorable, but the other ET processes are unfavorable (Figure S19). Thus, the fourth component might be originated from the complete charge-separated state, i.e., the oxidized ZnP (ZnP•+) and the reduced SWNTs (SWNT•−). To judge whether the CS occurred or not with the photoexcitation of ZnP−ph3−SWNT, the electrochemical spectroscopic measurements of the absorption spectra for the ZnP•+ and SWNT•− moieties were conducted (Figure S20). If the charge-separated state were formed, the fourth component spectrum would coincide with the sum of the spectra for ZnP•+ and SWNT•−. Actually, the fourth component spectrum and the sum spectrum of ZnP•+ and SWNT•− show somewhat similar shapes in the visible region (Figure S20c), but the broad feature, especially the rather flat absorption at 850−1080 nm, is distinctly different from the characteristic absorption of completely reduced SWNT•− (Figure S20c). From the close comparison of the TA spectrum and electrochemical absorption spectra, we can rule out the possibility that the fourth component is assigned to the complete charge-separated state. Meanwhile, we also examined the photoelectrochemical properties of the films of ZnP−ph3−SWNT and SWNT−ph1− Bpin fabricated onto a nanostructured SnO2 electrode on fluorine doped tin oxide (FTO) by the electrophoretic method (denoted as FTO/SnO2/ZnP−ph3−SWNT and FTO/SnO2/ SWNT−ph1−Bpin, respectively) (Figure S21).30,35,53 Wavelength dependences of incident photon-to-current efficiency (IPCE) were measured in the photoelectrochemical devices with the FTO/SnO2/ZnP−ph3−SWNT and FTO/SnO2/ SWNT−ph1−Bpin electrodes (Figure S21c). The IPCE spectrum of the FTO/SnO2/ZnP−ph3−SWNT electrode represents a monotonously downward-sloping curve, whereas a Soret band at 430 nm is evident in the corresponding absorption spectrum (Figure S21a). On the other hand, the IPCE and absorption spectra of the FTO/SnO2/SWNT−ph1− Bpin electrodes are similar, implying that only the excitation of the SWNT moiety leads to the photocurrent generation in the FTO/SnO2/ZnP−ph3−SWNT-based device. This is in marked contrast with analogous photoelectrochemical devices with the FTO/SnO2 electrodes modified with porphyrin−fullerene linked systems that generate the complete charge-separated state and the resultant photocurrent with action spectrum following the porphyrin absorption.53 These results correlate well with no formation of the complete charge-separated state by the photoexcitation of ZnP−ph3−SWNT. In other words, this also supports that the fourth component in the TA spectrum of ZnP−ph3−SWNT cannot be attributed to the complete charge-separated state. In addition, no contribution of the porphyrin absorption to the photocurrent generation can also exclude the possibility of the energy transfer (EN) from 1 ZnP* to SWNTs; if such EN occurred to form the SWNT excited state, the photocurrent response corresponding to the porphyrin absorption would be detected in the FTO/SnO2/ ZnP−ph3−SWNT-based device. F

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formation and decay in ZnP−phn−SWNT (n = 1−5) (0.10 and 0.12 Å−1, respectively) are considerably small as compared to the previously reported ones for CS and CR in a D−B−A system with oligo(p-phenylene) bridges (0.2−0.8 Å−1).22,57,58 Such a rather distance-independent character may be attributed to the unique electronic structure with the partial CT character in the exciplex. The small β values are consistent solely with the formation of the exciplex states, but not the charge-separated states by the photoexcitation of ZnP−phn−SWNT (n = 1−5). It is noteworthy that the exciplex formation in ZnP−phn− SWNT (n = 1−5) irrespective of the bridge length (RDA = 5.8− 22.6 Å, Table 1) is in sharp contrast with the photodynamics of ZnP−C60 linked systems, where either the exciplex (RDA ≤ 2.6 Å) or CS (RDA ≥ 10.9 Å) state or both (RDA = 3.4−6.7 Å) are formed strongly dependent on the D−A interaction and environment.15,16 The strong interaction between 1ZnP* and 1D SWNTs through the oligo(p-phenylene) bridges may allow the exciplex formation over the long separation distance. On the other hand, we recently reported that the photoexcitation of the ZnP moieties in ZnP−phn−CCG (n = 1−5) also led to exclusive formation of an exciplex state due to the strong interaction between 1ZnP* and 2D CCG.25 However, the formation and decay of the exciplex state between the ZnP and CCG do not occur via a through-bridge mechanism, but through-space one, as a consequence of the significantly inclined oligo(p-phenylene) bridges onto the 2D graphene surface. The strong through-space interaction between 1ZnP* and CCG resulted in the long-range exponential dependence of kDEX on the ZnP−CCG through-space distance with a β value of 0.12 Å−1. Although the attenuation factor for ZnP−phn− CCG (n = 1−5) is comparable with the present ZnP−phn− SWNT (n = 1−5) system, the lifetimes of the exciplex states of ZnP−phn−CCG (33−59 ps)25 are significantly shorter than those of ZnP−phn−SWNT (236−1950 ps, Table 1). In addition, as contrasted with the case of ZnP−phn−SWNT (n = 1−5), the kFEX values in ZnP−phn−CCG (n = 1−5) could not be determined because the exciplex formation was too fast to be detected even by the femtosecond TA measurement system with the time resolution of ∼150 fs.25 These differences in the photophysical properties of ZnP−phn−SWNT and ZnP−phn−CCG may stem from the difference in the D−A interaction, i.e., through-bridge and through-space, respectively, as well as that in the electronic structures of 1D SWNTs and 2D CCG. Bridge Conformation Effect on Photodynamics. It is well-known that oligo(p-xylene) bridges exhibit a much lower electronic conjugation of benzene rings than oligo(p-phenylene) bridges because the equilibrium torsion angles between adjacent p-xylene units is substantially greater than that between two neighboring phenyl moieties due to steric hindrance.38 Length variation is viable as is the case of oligo(p-phenylene). Reflecting the less conjugated structure, the β value of the oligo(p-xylene) bridge in D−B−A systems is larger than that of oligo(p-phenylene) one, e.g., 0.8 and 0.2 Å−1, respectively, for the long-range ET in linked systems of phenothiazoline (PTZ)−ruthenium(II) tris(2,2′-bipyridine) complex [Ru(bpy)32+].59 Therefore, we also monitored the porphyrin fluorescence decays and TA of ZnP−xyn−1−ph1− SWNT (n = 1−5) (Figures S24−S26). The TA data of ZnP− xyn−1−ph1−SWNT (n = 1−5) can be fitted by four decay components (Figures S25 and S26) with the spectral shapes analogous to those of ZnP−phn−SWNT (n = 1−5) (Figures 3, S14, and S15), and the lifetimes of the third components are

(1)

where τS is the fluorescence lifetime of the ZnP−phn−SWNT (n = 1−5) and τS,ref is the fluorescence lifetime of the ZnP-ref (2.0 ns), respectively.38,55 As the vibronic decay of the exciplex state does not compete with other deactivation processes, the rate constants for the decay of the exciplex state (kDEX) can be the inverse of the measured exciplex state lifetime (τ4), as expressed by the following equation: kDEX = τ4 −1

(2)

The calculated values of kFEX and kDEX are listed in Table 1. The kDEX values are much smaller than the kFEX ones by factors of 30−40. The attenuation factor β is established by analyzing the dependence of kFEX and kDEX values on the D−A edge-to-edge separation distance (RDA). Namely, RDA is defined by the through-bridge distance between the sp3 carbon of the SWNT surface and the meso-carbon of ZnP that are, respectively, connected with the oligo(p-phenylene) bridge in ZnP−phn− SWNT (n = 1−5). The plots in Figure 4 unambiguously

Figure 4. Semilogarithmic representation of D−A separation distance (RDA) dependence of the rate constant of exciplex formation (kFEX, circles) and decay (kDEX, squares) in ZnP−phn−SWNT (n = 1−5). The solid and dotted lines are the linear regression fits in kFEX and kDEX, respectively, of which slope corresponds to the attenuation factor (β value).

exemplify the exponential dependencies of kFEX and kDEX on RDA with a slope of β, which can be described as k = k0 exp(−βRDA) (k0: kinetic prefactor), as is the cases with the CS and CR rates in D−B−A systems via superexchange tunneling mechanisms.20,22,55 In general, the β values for CS and CR in D−B−A rely upon the electronic structure of the bridge, and those for conjugated bridges (0.2−0.8 Å−1) are smaller than nonconjugated, saturated hydrocarbon ones (0.8−1.0 Å−1), demonstrating that conjugated bridges can mediate long-range CS and CR more efficiently than nonconjugated bridges. The β values for the exciplex formation and decay in ZnP−phn− SWNT (n = 1−5) are calculated to be 0.10 and 0.12 Å−1, respectively (Figure 4). As far as we know, this is the first example of the determination of the β values for both the bridge-mediated exciplex formation and decay. The kDEX value is slightly higher than the kFEX one, which is similar to the relationship between the rates of CR and CS, i.e., the exponential distance dependence increases for CR compared to that for CS in accordance with the superexchange tunneling model.56 Furthermore, the β values for both the exciplex G

DOI: 10.1021/acs.jpcc.7b04483 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C coincided with the fluorescence lifetimes (Figure S24). These results strongly suggest the occurrence of the formation and decay of the exciplex state without forming the CS state by the photoexcitation of ZnP−xyn−1−ph1−SWNT (n = 1−5) as is the case of ZnP−phn−SWNT (n = 1−5). The lifetimes of 1ZnP* (τS) and exciplex (lifetimes of the fourth components, τ4) and the calculated rate constants for the exciplex formation (kFEX) and decay (kDEX) in ZnP−xyn−1−ph1−SWNT (n = 1−5) using the eqs 1 and 2 are listed in Table S1. The exponential dependencies of kFEX and kDEX on RDA in ZnP−xyn−1−ph1− SWNT (n = 1−5) are also demonstrated unambiguously by the single logarithmic plots in Figure 5. The β values for kFEX and

tune the D−A interaction with precision by varying the bridge lengths due to the rigid 1D rodlike structures. Unambiguous exponential dependencies of the rates for both the formation and decay of the exciplex state on the separation distance between ZnP and SWNTs through the oligo(p-phenylene) bridges were attained with significantly small β values compared to the corresponding distance dependence of CS and CR. We also found that less conjugated oligo(p-xylene) bridges mediate the long-range exciplex formation and decay less efficiently than oligo(p-phenylene), verifying the through-bridge mechanisms rather than the through-space ones. This work is the first example of unambiguous bridge length and conformation effects on the rates for the bridge-mediated formation and decay of the exciplex state and, therefore, can provide valuable information on unique photodynamics of electronically strongly coupled D−B−A systems, where exciplex plays an essential role. Such information is especially of crucial importance for the design of photofunctional organic devices using a combination of small π-aromatic molecules with large π-systems including nanocarbon materials.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b04483. Synthetic details for ZnP−xyn−Br (n = 1−4), methods for photoelectrochemical measurements, rate constant, XPS, ATR-FTIR, Raman, TEM, AFM, steady-state UV− vis absorption and emission, fluorescence decay, transient absorption, energy diagram, spectroelectrochemistry, and photoelectrochemistry data (PDF)

Figure 5. Semilogarithmic representation of D−A separation distance (RDA) dependence of the rate constant of exciplex formation (kFEX, circles) and decay (kDEX, squares) in ZnP−xyn−1−ph1−SWNT (n = 1− 5). The solid and dotted lines are the linear regression fits in kFEX and kDEX, respectively, of which slope corresponds to the attenuation factor (β value).

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kDEX (0.12 and 0.14 Å , respectively) in ZnP−xyn−1−ph1− SWNT (n = 1−5) are slightly larger than those in ZnP−phn− SWNT (n = 1−5) (0.10 and 0.12 Å−1, respectively). Although the interaction between 1ZnP* and SWNTs is sufficiently strong to form the exciplex through the oligo(p-xylene) bridges, they mediates the long-range exciplex formation less efficiently than oligo(p-phenylene) due to the poorly conjugated electronic structure. To the best of our knowledge, this is also the first example of the bridge conformation effect on the β values for the exciplex formation and decay. This emergence of the difference in the β values of ZnP−phn−SWNT and ZnP− xyn−1−ph1−SWNT systems confirms the through-bridge mechanisms of the exciplex formation and decay rather than the through-space ones. It should also be noted, however, that the β values for ZnP−xyn−1−ph1−SWNT (n = 1−5) are still much smaller than the previously reported ones (0.5−0.8 Å−1)15,22,59 for CS and CR in D−B−A systems with oligo(pxylene) bridges, highlighting the unique electronic structure of the exciplex state.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: nikolai.tkachenko@tut.fi. *E-mail: [email protected]. ORCID

Tomokazu Umeyama: 0000-0003-4145-5784 Nikolai V. Tkachenko: 0000-0002-8504-2335 Hiroshi Imahori: 0000-0003-3506-5608 Notes

The authors declare no competing financial interest.

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REFERENCES

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CONCLUSION In conclusion, we have successfully characterized the exciplex formation and decay over extremely long-range separation distance in D−A linked systems in which (6,5)-chiralityenriched CoMoCAT SWNTs are covalently functionalized with ZnPs through different lengths of oligo(p-phenylene) bridges (ZnP−phn−SWNT, n = 1−5) or oligo(p-xylene)s (ZnP− xyn−1−ph1−SWNT, n = 1−5). It was possible to systematically H

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