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Dec 7, 2016 - Department of Chemistry and Bioengineering, Tampere University of Technology, P.O. Box 541, FIN-33101 Tampere, Finland. §. Institute fo...
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Remarkable Dependence of Exciplex Decay Rate on Through-Space Separation Distance between Porphyrin and Chemically Converted Graphene Tomokazu Umeyama,*,† Takuma Hanaoka,† Jinseok Baek,† Tomohiro Higashino,† Fawzi Abou-Chahine,‡ Nikolai V. Tkachenko,*,‡ and Hiroshi Imahori*,†,§ †

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

S Supporting Information *

ABSTRACT: A series of chemically converted graphenes (CCGs) covalently functionalized with multiple zincporphyrins (ZnPs) through tuned lengths of linear oligo-p-phenylene bridges (ZnP-phn-CCG, n = 1−5) were prepared to address the bridge length effect on their photodynamics. Irrespective of the bridge length, photoexcitation of ZnP-phn-CCG led to exclusive formation of an exciplex state, which rapidly decayed to the ground state without yielding the completely charge-separated state. The notable dependence of the exciplex lifetime as a function of separation distance between the porphyrin and CCG has been observed for the first time, supporting the hypothesis that the decay to the ground state is dominated by the through-space interaction rather than the through-bond one. The basic information on the interaction between ZnP and CCG in the excited state will provide us with deeper insight on the intrinsic nature of the exciplex state as a function of donor− acceptor interaction.

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INTRODUCTION

photoactive composites due to the synthetic difficulty of access to the composites. In this study, we dealt with a combination of zincporphyrin with 3,5-di-tert-butylphenyl groups at meso positions (ZnP) and chemically converted graphene (CCG) as an electron D−A pair. Since the oxidative excited-state of the ZnP unit is energetically higher than the conduction band of the functionalized CCG and the reductive excited-state of ZnP is lower than the valence band of the functionalized CCG,18−20 CCG may function as both electron donor and acceptor in combination with ZnP. To date, several studies have focused on the covalent linking of porphyrins to graphenes.21−28 However, they only considered the electron-accepting character of CCG. Furthermore, the porphyrin-graphene linked systems in the previous studies have not been designed to evaluate the precise effect of the separation distance between the porphyrin and graphene on the photodynamics because of the use of flexible bridges (B) or a lack of systematic change in the separation distance.21−28 With these in mind, we prepared D-B-A systems, in which the multiple ZnP molecules are covalently attached to the CCG through a series of oligo-p-phenylene bridges (ZnP-

Atomically layered two-dimensional (2D) nanomaterials such as graphene and transition-metal dichalcogenides (TMDs) have garnered great interest due to their fascinating physical and chemical properties arising from the 2D structure.1−4 They hold excellent electronic conductivity, mechanical properties, and large specific surface area, all of which are suitable for solar energy conversion systems, including artificial photosynthesis and photovoltaic devices.5−10 Up until now, much effort has been devoted to integrating the 2D nanomaterials with various photofunctional organic/inorganic compounds toward highly efficient solar energy conversion systems. To achieve this goal, efficient formation of a long-lived charge-separated (CS) state in such electron donor−acceptor (D−A) composites is a prerequisite. However, the D−A interfaces frequently encounter the rapid decay of the exciplex or CS state to the ground state at an early stage, losing the significant fraction of the ideally formable CS state eventually.11−15 The D−A electronic coupling has been demonstrated as an important parameter controlling the photodynamics precisely, and thus its finetuning is essential to inhibit the undesirable rapid decay to the ground state.16,17 In spite of these situations, there have been no systematic investigations to address the effect of the D−A interaction on the photodynamics of 2D nanomaterials-based © XXXX American Chemical Society

Received: October 12, 2016 Revised: November 28, 2016

A

DOI: 10.1021/acs.jpcc.6b10325 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Scheme 1. Synthesis of ZnP-phn-CCG (n = 1−5)a

a

The incomplete Suzuki coupling reactions resulted in conversion of the rest of the pinacolatoboryl groups into the hydrogen atoms.

phn-CCG, n = 1−5, Scheme 1).29−32 Oligo-p-phenylene bridges were selected because they possess one-dimensional (1D) rigid structures,33 and thus the D−A interaction can be fine-tuned by changing the number of the phenylene rings.

Synthesis. All solvents and chemicals were of reagent grade quality, purchased, and used without further purification. Graphene oxide (GO) were prepared according to the reported procedure.27 Synthetic details of ZnP-phn-Br (n = 1−4) are described in the Supporting Information. 4-Pinacolborylphenyl-Functionalized Chemically Converted Graphene (CCG-ph1-Bpin). First, GO was chemically reduced as follows. Sodium dodecyl sulfate (SDS) (2.5 g) was added to a GO dispersion (7 mg mL−1, 250 mL), and the mixture was stirred for 1 h and then sonicated for 10 min. The suspension was filtered through a 5 cm plug of glass wool. Concentrated NH4OH (3.5 mL, 40 mmol) and 98% N2H4·H2O (0.279 mL, 5.6 mmol) were added to the GO suspension, and the mixture was heated at 95 °C for 1 h with no stirring. Then the reaction mixture was cooled to room temperature. After addition of acetone (750 mL), the mixture was filtered through the Ominipore JGWP membrane with an average pore size of 200 nm. The black precipitate, chemically converted graphene (CCG) was washed with acetone and then dispersed into DMF to obtain a stock dispersion of CCG in DMF (1.4 mg mL−1). Next, functionalization of CCG was conducted as follows. The CCG stock dispersion (33 mL) was diluted with DMF (63 mL), and then 4-aminophenylboronic acid pinacol ester (2.5 g, 11.4 mmol) was added to the diluted dispersion. The mixture was sonicated and degassed by argon bubbling. Isopentyl nitrite (1.53 mL, 11.4 mmol) was added to the mixture, which was stirred for 48 h at 80 °C. After filtration through the Ominipore JVWP membrane with an average pore size of 100 nm, the black precipitate was washed with DMF. Redispersion of the black precipitate to DMF yielded the dispersion of CCG-ph1-Bpin in DMF (0.4 mg mL−1). Note that complete removal of the solvent resulted in nondispersible solid of CCG-ph1-Bpin. Porphyrin−Functionalized Chemically Converted Graphene with an Oligophenylene Bridge (ZnP-phnCCG, n = 1−5). The preparation of ZnP-ph1-CCG was conducted as follows. To the mixture of ZnP-ph0-Br (11 mg, 0.011 mmol), Cs2CO3 (0.80 g, 2.5 mmol), and Pd(PPh3)4 (1.5 mg, 1.3 μmol) in toluene (1 mL) was added CCG-ph1-Bpin in

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EXPERIMENTAL SECTION Instruments. 1H NMR spectra were measured with a JEOL JNM-EX400 NMR spectrometer or JEOL JNM-AL300 NMR spectrometer. Attenuated total reflectance (ATR) FTIR spectra were recorded on a Themo Fisher Scientific Nicolet 6700 FTIR. Resonance Raman spectra were measured using a Horiba Jobin Yvon LabRAM ARAMIS spectrometer equipped with an excitation wavelength of 532 nm (2.33 eV). High-resolution mass spectra (HRMS) were obtained with a Thermo Fisher Scientific EXACTIVE. X-ray photoelectron spectroscopy (XPS) was carried out with a ULVAC-PHI MT-5500 system with Mg Kα. Atomic force microscopy (AFM) analyses were carried out with an Asylum Technology MFP-3D-SA in the AC mode. CCG dispersions in DMF were spin-coated on freshly cleaved mica at 1000 rpm. The cross-section analyses of more than 50 parts were conducted to estimate the average thicknesses. Transmission electron microscope (TEM) images were obtained from a JEOL JEM-2200FS. For sample preparation, CCG dispersions in ethanol were dropped on microgrids (Cu mesh with carbon-supporting film) and dried under vacuum. UV−vis absorption spectra were measured with a PerkinElmer Lambda 900 UV/vis/NIR spectrometer. Steadystate fluorescence spectra were recorded on a HORIBA SPEX Fluoromax-3 spectrofluorometer. A time-correlated single photon counting (TCSPC) method was employed to measure the fluorescence lifetime using a HORIBA NanoLOG-TCSPC. Pump−probe measurements were carried out using the instrument described previously.34 Briefly, base 100 fs pulses were generated by Libra F-1K (Coherent Inc.) which were utilized by OPO (Topas-C, Light Conversion Ltd.) to produce pump pulses at 430 nm and probe white continuum in pump− probe detection system (ExciPro, CDP Inc.). A typical time resolution of the instrument was 150 fs (fwhm). B

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The Journal of Physical Chemistry C DMF (5 mL). The reaction mixture was degassed by three freeze−pump−thaw cycles, refilled with argon, and then stirred for 24 h at 100 °C under an argon atmosphere. After cooling to room temperature, the mixture was diluted with THF (50 mL), sonicated, and filtered through Ominipore JVWP membrane with an average pore size of 100 nm. The product was washed thoroughly with THF, methanol, water, acetone, and DMF to remove the excess amount of porphyrin. The resulting wet solid was dispersed again in DMF, yielding ZnP-ph 1 -CCG dispersion. Dispersions of ZnP-phn-CCG (n = 2−5) in DMF were prepared following the same procedure using ZnP-phn‑1Br (n = 2−5) instead of ZnP-ph0-Br, respectively.

Figure 1a displays the steady-state UV−visible absorption spectra of ZnP-phn-CCG (n = 1−5), CCG-ph1-Bpin, and

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RESULTS AND DISCUSSION A series of ZnP-phn-CCG (n = 1−5) were prepared by a twostep functionalization (Scheme 1)27,35,36 [i.e., attachment of phenylpinacolborane groups onto CCG by the aryl addition reaction and subsequent Suzuki coupling reaction between the porphyrins bearing the oligo-p-phenylene bridge with a bromo group at the terminal (ZnP-phn‑1-Br, n = 1−5) and the preprepared phenylpinacolborane-functionalized CCG (CCGph1-Bpin)]. The detailed synthetic procedure is described in the Experimental Section. The structures of these functionalized CCGs were characterized by X-ray photoelectron spectroscopy (XPS) (Figure S1), Fourier transfer infrared (FTIR), and Raman spectra (Figures S2 and S3), whereas the 2D structures were confirmed by atomic force microscopy (AFM) and transmission electron microscopy (TEM) (Figures S4 and S5). The XPS measurement of CCG-ph1-Bpin detected signals of boron (carbon:boron = 39:1), corresponding to a functionalization ratio of one pinacolatoboryl group in ca. 27 carbon atoms of CCG (Figure S1). To estimate the degree of the porphyrin functionalization by Suzuki couplings on ZnP-phnCCG, the ratio of zinc atoms to carbons was also determined by XPS. The functionalization ratios of ZnP-phn-CCG (n = 1− 5) are comparable; one porphyrin unit is attached per approximately 220−250 carbon atoms of CCG. The conversion ratios by the Suzuki coupling reaction on the CCG surface were 11−12%. Although the aryl addition reaction in the first step may cause paired attachments of phenylpinacolborane groups on CCG, the low yield of the Suzuki coupling reaction in the second step rendered the ZnP groups monomeric.36 After the Suzuki coupling reaction, boron signals totally disappeared (Figure S1), suggesting that the rest of the pinacolatoboryl groups was consumed by incomplete Suzuki coupling reactions. ZnP-ph0-Br and CCG-ph1-Bpin were employed in this study to yield ZnP-ph1-CCG, whereas a porphyrin bearing a pinacolborane group at the meso-position (ZnP-ph0-Bpin) and CCG functionalized with iodophenyl groups (CCG-ph1-I) were used to obtain the same ZnP-ph1-CCG in our previous report.27 Despite the inverse approach, the conversion ratios of the Suzuki coupling reactions were similar (10−11%). The use of a series of ZnP-phn−1-Br (n = 1−5) instead of ZnP-phn−1-Bpin was chosen here in consideration of the synthetic cost. The functionalization ratios of ZnP-phn-CCG (n = 1−5) obtained by the XPS measurements predict the existence of one ZnP group at approximately 3.2 × 3.2 nm2 of one side of the CCG surface. By taking into account the size of the porphyrin (1.8 nm)27 and the porphyrin surface coverage on the CCG, the ZnP molecules are likely to be isolated from each other on the CCG with no interaction between the porphyrin units.36

Figure 1. (a) Steady-state UV−visible absorption and (b) steady-state fluorescence spectra of ZnP-ph1-CCG (orange), ZnP-ph2-CCG (purple), ZnP-ph3-CCG (red), ZnP-ph4-CCG (cyan), ZnP-ph5-CCG (green), CCG-ph1-Bpin (black), and ZnP-ref (navy, 24 μM). The spectra in (a) are shown in parallel for comparison. For the excitations in (b), the absorbance of the porphyrin moiety was adjusted to be identical at the peak position of the Soret band for comparison.

5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrinatozinc(II) (ZnP-ref) in DMF. All the spectra of ZnP-phn-CCG (n = 1−5) are almost identical. The Soret bands of ZnP-phn-CCG (n = 1− 5) are slightly broadened and red-shifted by ca. 2 nm relative to that of ZnP-ref, suggesting weak interaction between the ZnP and CCG in the ground state.27,37,38 Note that the optical bandgap (2.03 eV) of ZnP-ref is much smaller than those of the oligo-p-phenylene moieties with a minimum of ca. 4.1 eV (ca. 300 nm) for penta-p-phenylene.39 Upon excitation of ZnP-phnCCG (n = 1−5) and ZnP-ref at the Soret band, where the absorbance of the porphyrin is adjusted to be identical, the fluorescence intensity from the ZnP moieties on the CCG is decreased significantly in comparison with that of ZnP-ref (Figure 1b). This result implies the occurrence of quenching of singlet excited state of porphyrin (1ZnP*) by interaction with CCG in ZnP-phn-CCG, and thereby the lifetime of 1ZnP* in ZnP-phn-CCG becomes shorter than that in ZnP-ref. It is noteworthy that the quenchings in ZnP-phn-CCG are far more intensive (e.g., 88% quenching in ZnP-ph5-CCG) than those of porphyrin−fullerene-linked systems (ZnP-xyn-C60, n = 1−5) with analogous rigid oligo-p-xylene bridges (e.g., only 5% C

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The Journal of Physical Chemistry C quenching in ZnP-xy5-C60).17 This result suggests the significantly enhanced interaction of the excited ZnP with CCG in ZnP-phn-CCG compared to that with C60 in ZnP-xynC60. We further employed a time-correlated single-photon counting (TCSPC) technique to monitor the porphyrin emission decay of ZnP-ref and ZnP-ph5-CCG that exhibits the weakest quenching in ZnP-phn-CCG (Figure S6, panels a and b). The fluorescence decay of ZnP-ref was analyzed by a single component with a lifetime of 2.0 ns, which is in good agreement with the literature.27 On the other hand, the fluorescence decay curve of ZnP-ph5-CCG excited at 430 nm was fitted by a fast component (τ1 < 200 ps, 32%) and a slow component (τ2 = 1.9 ns, 68%). These results suggest that even if the weakest quenching of 1ZnP* by the CCG moiety is expected in ZnP-ph5-CCG due to the longest oligophenylene bridge, the lifetime of 1ZnP* that interacts with the CCG moiety in ZnP-phn-CCG is short (530 nm with the negative peaks at 560 and 600 nm. The analogy of the spectrum shape and lifetime with those of reference CCGs without the porphyrin moiety (Figure S7) suggests that the short-lived component stems mainly from the CCG excited state. The appearance of the negative peaks at 560 and 600 nm derived from the porphyrin bleaching indicates the excited state of CCG in ZnP-ph3-CCG interacts with the linked ZnP moiety in

Figure 2. TA component spectra of ZnP-ph3-CCG in DMF obtained with global two-component fit of the data. The excitation wavelength is 430 nm. The lifetimes of respective components are given in the figure.

the excited state to some extent to form the mixed excited states possessing character of both the excited states. It should be noted again here that CCG may function as both electron donor and acceptor in combination with ZnP, considering the energy levels of oxidative and reductive excited-states of ZnP (Figure S8).18−20 However, the absence of the characteristic absorption bands for ZnP radical cation (ZnP+•) in 600−700 nm11 or radical anion (ZnP−•) in 700− 750 nm41 rules out the possibility of the formation of complete CS states [i.e., electron transfer (ET) from 1ZnP* to the conduction band of CCG (Figure S8a) or ET from the valence band of CCG to 1ZnP* (Figure S8b)]. Alternatively, the second component might be attributed to 1ZnP*. In such a case, the excited state of ZnP in ZnP-ph3-CCG would be quenched rapidly (τ2 = 40 ps, which is beyond the time resolution of the TCSPC system in Figure S6) by the CCG moiety via energy transfer (EN). To judge whether the EN occurred or not by the photoexcitation of ZnP-ph3-CCG, we investigated the photoelectrochemical property of ZnP-ph3CCG using the thin film fabricated onto a nanostructured SnO2 electrode on FTO by the electrophoretic method.40 The photoelectrochemical device in a three-electrode system with the FTO/SnO2/ZnP-ph3-CCG as a working electrode shows a structureless, downward sloping incident photon-to-current efficiency (IPCE) spectrum without any obvious peaks in the UV−visible region, whereas the absorption spectrum of FTO/ SnO2/ZnP-ph3-CCG reveals a peak around 430 nm derived from the porphyrin Soret band (Figure S9). The shape of the IPCE spectrum of the FTO/SnO2/ZnP-ph3-CCG device is similar to that of the CCG-ph1-Bpin one without the porphyrin moiety (Figure S9c). These results imply that the absorption arising from the porphyrin moieties does not contribute to the photocurrent generation, but only the absorption of CCG generates the photocurrent in the FTO/SnO2/ZnP-ph3-CCG device.27,28,40 If the EN from 1ZnP* to CCG occurred to yield the CCG excited state, the corresponding photocurrent generation would be observed in the device with FTO/ SnO2/ZnP-ph3-CCG. Therefore, the possibility of the EN quenching mechanism, that is, the assignment of the second component in Figure 2 to 1ZnP*, can be excluded. With the results above in mind, we can safely assign the longlived component to an exciplex [i.e., a mixing state of the locally excited states (i.e., 1ZnP* and 1CCG*) and CS state (ZnP+•− D

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The Journal of Physical Chemistry C CCG−•)].42,43 With the photoexcitation of the ZnP moiety in ZnP-ph3-CCG, the orbitals of 1ZnP* participate in a charge transfer (CT) interaction with the attached CCG moiety to form the exciplex exhibiting partial charge character on the chromophores and a large dipole moment reflecting on the degree of charge transfer. The partially positive and negative charges are located on ZnP and CCG, respectively, making the TA signal rather broad and structureless. Similarity with the photodynamics of porphyrin−fullerene and porphyrin−singlewalled carbon nanotube linked systems with a short phenylene bridge reasonably rules out the possibility of the inverse exciplex, where the partially negative and positive charges are on ZnP and CCG, respectively.16,17,28,44 The presence of the negative peaks at 560 and 600 nm, corresponding to the ground-state absorption of ZnP, also supports the contribution of the 1ZnP* state to the exciplex to some extent. It is known that under the illumination, the exciplex state generated in the film prepared by donor−acceptor molecules cannot be extracted as photocurrent in the photoelectrochemical devices due to the short-lived, partial charge transfer character. In contrast, the corresponding completely CS state contributes to the photocurrent significantly.44 Since the porphyrin absorption of the ZnP-ph3-CCG film does not show photocurrent response in the IPCE spectrum of the ZnP-ph3-CCG-based photoelectrochemical device, the exciplex formation is likely to occur (Figure S9). The two-components are also reasonably derived from the global fit of the data for all the other ZnP-phn-CCG (n = 1, 2, 4, and 5), as depicted in Figure S10. These TA results highlight the exclusive formation of the exciplex state between the ZnP and CCG [(ZnPδ+-phn-CCGδ−)*], which decays rapidly (τ2 = 33−59 ps, Table 1) to the ground state without forming the

ship to the linear length of the oligo-p-phenylene bridges (Table 1). We previously conducted systematic investigation on the photophysical properties of porphyrin (D)−bridge−fullerene (A) linked systems with various unconjugated bridges.16,17 The obtained results demonstrate that the separation distance between porphyrin and C60 (i.e., the D− A electronic coupling) is one of the important factors in determining the energy balance between the CS state and the exciplex state. With increasing the electronic coupling, transient species formed by the interaction in the excited states vary from the CS state to the exciplex state.17 In addition, the stronger electronic coupling resulted in the shorter lifetime of the exciplex.16 Therefore, the τ2 values of ZnP-phn-CCG (n = 1−5) imply that the order of the linear bridge length does not match with that of the direct through-space separation distance between the porphyrin and CCG plane in ZnP-phn-CCG (n = 1−5).11 In other words, the through-space D−A electronic coupling may play a key role in the decay of the exciplex state to the ground state (i.e., recombination of spatially separated, partially charged electron−hole pairs), where the electron and hole are located on CCG and ZnP, respectively. To verify this hypothesis, we analyzed the cross-section profiles in the AFM height images (Figure S4 and other samples) for the direct determination of the tilt angle (θ) of the rigid oligo-p-phenylene bridges with respect to the surface normal and the edge-to-edge separation distance (Ree) between the porphyrin and CCG because the height can be determined within an error of ±0.1 nm by AFM. For instance, given that the heights of the bare CCG sheet and the substituted ZnP moiety (ZnP-ph3) are 1.0 and 2.7 nm, respectively,27 the average values of θ and Ree for ZnP-ph3-CCG are 67° and 0.43 nm (Figure 3). Note that Ree is defined as the separation distance between the CCG plane and the meso-carbon of ZnP that is connected with the oligo-p-phenylene bridge. The sheet heights and θ and Ree values for all the ZnP-phn-CCG (n = 1− 5) are listed in Table 1. First, the θ value becomes large with increasing the bridge length (n = 1−3) and then levels off at 66−67° (n ≥ 3). It is known that porphyrin carboxylic acids adsorbed on TiO2 are prone to be tilted more heavily on TiO2 as the bridge between the porphyrin core and the anchoring carboxylic acid group becomes long.11,12 Thus, with increasing the number of n, the linear oligophenylene bridge with the porphyrin moiety would be tilted more significantly on CCG, but steric repulsion between the undermost phenylene group and the CCG surface may hinder leaning more than θ = 66− 67°. As a result, the Ree value becomes large in the order of ZnP-ph1-CCG < ZnP-ph3-CCG < ZnP-ph2-CCG < ZnP-ph4CCG < ZnP-ph5-CCG, which agrees well with the order of the exciplex lifetime.17,42 This demonstrates that the through-space mediated decay of the exciplex state to the ground state is the main process rather than the through-bond one via the oligo-pphenylene bridge. Note that oligo-p-phenylene bridges are known to possess 1D rigid structures without bending,33 ruling out the possibility of the bending effect on the separation distance and the photodynamics. The rate constant (k = τ2−1) of the exciplex decay in ZnP-phn-CCG (n = 1−5) is exponentially dependent on the Ree value. The relationship between k and Ree can be described as k = k0 exp(−βRee) with a kinetic prefactor of k0 and an attenuation factor of β (Figure 4), as is the cases with the charge separation and charge recombination (CR) rates via tunneling mechanisms in D-BA systems.29−32 The steepness of the exponential drop-off of the k value is captured by the attenuation factor β. Generally,

Table 1. Exciplex Lifetimes (τ2), Sheet Heights, Tilt Angles of ZnP-phn Groups (θ), and Edge-to-Edge Separation Distances (Ree) between ZnP and CCG in ZnP-phn-CCG n

τ2 (ps)

1 2 3 4 5

33 42 40 50 59

heighta (nm) 3.4 3.6 3.1 3.6 3.8

± ± ± ± ±

0.1 0.1 0.1 0.1 0.1

θ (deg) 49 55 67 66 67

± ± ± ± ±

2 1 1 1 1

Ree (nm) 0.23 0.45 0.43 0.63 0.77

± ± ± ± ±

0.02 0.03 0.02 0.03 0.04

a

Average values are obtained from the measurements of more than 50 parts.

complete CS state, irrespective of the lengths of the oligo-pphenylene bridges. On the other hand, the component with a long lifetime of 1.9 ns was observed in the fluorescence decay of ZnP-ph5-CCG (Figure S6a), but the long-lived component was not detected in the TA measurement (Figure S10d). These seemingly contradictory results are explained by the fact that the nanosecond component stems from minor deactivation pathways of the excited porphyrin moieties such as the detached free porphyrins and/or the ZnP moieties linked at the edges of CCG, which cannot interact with CCG in the excited state (vide supra). Such minor pathways were not reflected in the TA spectrum of ZnP-ph5-CCG owing to the small ratio, although they emit strongly (Figure S10d). It should be noted here that the τ2 value of the exciplex state for ZnP-phn-CCG (n = 1−5) is elongated in the order of ZnPph1-CCG < ZnP-ph3-CCG < ZnP-ph2-CCG < ZnP-ph4-CCG < ZnP-ph5-CCG, which does not bear a proportionate relationE

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Figure 3. (a) AFM image with section profile of ZnP-ph3-CCG spin-coated on mica. (b) Schematic structure of ZnP-ph3-CCG.

ZnP-phn-CCG irrespective of the bridge length are in sharp contrast with the photodynamics of ZnP-C60 linked systems where either the exciplex (Ree ≤ 2.6 Å) or CS (Ree ≥ 10.9 Å) state or both (Ree = 3.4−6.7 Å) are formed strongly dependent on the D−A interaction and environment.16,17 The unique behavior of the exciplex state in ZnP-phn-CCG (n = 1−5) may result from the rather short separation distance through the space (ca. 2−8 Å) as well as the combination and mutual geometry of the porphyrin and 2D CCG that cannot accelerate the complete charge separation compared to fullerene molecules with small reorganization energies.47

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CONCLUSIONS

In conclusion, we have successfully corroborated the effect of the D−A interaction between the porphyrin and CCG on the photodynamics for the first time. The D−A interaction was systematically tuned using 1D rigid oligo-p-phenylene bridges (ZnP-phn-CCG, n = 1−5). Irrespective of the bridge length, photoexcitation of ZnP-phn-CCG (n = 1−5) resulted in the exclusive formation of exciplex (i.e., electron−hole pair where the electron and the hole are located on CCG and ZnP, respectively), which rapidly decays to the ground state without yielding the CS state. Remarkably, the order of exciplex lifetime (ZnP-ph1-CCG < ZnP-ph3-CCG < ZnP-ph2-CCG < ZnP-ph4CCG < ZnP-ph5-CCG) matches with that of the through-space separation distance rather than the through-bond one through the oligo-p-phenylene bridge. As far as we know, this is the first example of unambiguous distance dependence of the decay rate constant of the exciplex state to the ground state. The notably small β value compared to the corresponding distance dependence of charge recombination from the completely CS state to the ground state exemplifies that formation of the exciplex state should be avoided or minimized toward efficient solar energy conversion. Nevertheless, such fundamental information on exciplex states, which can be addressed solely in the present well-defined D−A system, will provide us with deeper insight on the role of the D−A interaction in photodynamics.

Figure 4. Edge-to-edge separation distance (Ree) dependence of the rate constant (k) of the exciplex decay in semilogarithmic representation. The line is the linear regression fit, of which slope corresponds to the distance decay parameter (β value). The deviation from the linear fit may stem from the variation in the torsion angles between the porphyrin and CCG planes.

the β values for charge separation and CR are dependent on the bridge structure, and those for conjugated bridges (0.2−0.8 Å−1) are smaller than saturated hydrocarbon bridges (0.8−1.0 Å−1), reflecting the weakly distance-dependent molecular-wire regime. The β value for the exciplex decay in ZnP-phn-CCG (n = 1−5) is calculated to be 0.11 Å−1 (Figure 4), and to the best of our knowledge, this is the first example of the determination of the β value for the decay from the exciplex state to the ground state. This β value is much smaller than the previously reported ones (0.46−0.78 Å−1)31,45,46 for charge separation and CR in D-B-A systems with oligo-p-phenylene bridges, emphasizing the major role of the unique electronic structure of the exciplex state in the small β value. On the other hand, the exciplex formation from the 1ZnP* state in ZnP-phn-CCG (n = 1−5) is not detectable in the TA measurements, although some contribution of the 1ZnP* state to the fast decay component is visible (Figures 2 and S10). This precludes the comparison of the formation rate constants of the exciplex state as a function of the bridge lengths. Nevertheless, the rapid, selective formation and decay of the exciplex state in F

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b10325. Synthetic procedures of ZnP-phn-Br, XPS, FTIR, resonant Raman spectra, AFM, TEM, fluorescence decays, transient absorption spectra, energy diagrams, and photoelectrochemical properties of the functionalized CCGs (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Tomohiro Higashino: 0000-0002-9531-8569 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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ACKNOWLEDGMENTS This work was supported by Grant-in-Aid (no. 25220501 to H.I.) and Grant-in-Aid for Scientific Research on Innovative Areas “New Polymeric Materials Based on Element-Blocks (No. 2401)” (15H00737 to T.U.) of The Ministry of Education, Culture, Sports, Science, and Technology, Japan. N.V.T. and F.A.C. acknowledge the Academy of Finland for the financial support (no. 263486). T.U. thanks KIST-BIC for Raman measurements and Dr. Masahiko Tsujimoto (WPIiCeMS, Kyoto University) for TEM measurements.

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REFERENCES

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