Long-Lived Triplet Excited States of Bent-Shaped Pentacene Dimers

Mar 1, 2016 - Intramolecular singlet fission (ISF) is a promising photophysical process to construct more efficient light energy conversion systems as...
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Long-Lived Triplet Excited States of Bent-Shaped Pentacene Dimers by Intramolecular Singlet Fission Takao Sakuma,† Hayato Sakai,† Yasuyuki Araki,*,‡ Tadashi Mori,§ Takehiko Wada,‡ Nikolai V. Tkachenko,*,¶ and Taku Hasobe*,† †

Department of Chemistry, Faculty of Science and Technology, Keio University, Yokohama, 223-8522, Japan Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan § Department of Applied Chemistry, Osaka University, 2-1 Yamada-oka, Suita 565-0871, Japan ¶ Department of Chemistry and Bioengineering, Tampere University of Technology, 33720 Tampere, Finland ‡

S Supporting Information *

ABSTRACT: Intramolecular singlet fission (ISF) is a promising photophysical process to construct more efficient light energy conversion systems as one excited singlet state converts into two excited triplet states. Herein we synthesized and evaluated bent-shaped pentacene dimers as a prototype of ISF to reveal intrinsic characters of triplet states (e.g., lifetimes of triplet excited states). In this study, meta-phenylene-bridged TIPS-pentacene dimer (PcD-3Ph) and 2,2′-bipheynyl bridged TIPS-pentacene dimer (PcD-Biph) were newly synthesized as bent-shaped dimers. In the steady-state spectroscopy, absorption and emission bands of these dimers were fully characterized, suggesting the appropriate degree of electronic coupling between pentacene moieties in these dimers. In addition, the electrochemical measurements were also performed to check the electronic interaction between two pentacene moieties. Whereas the successive two oxidation peaks owing to the delocalization were observed in a directly linked-pentacene dimer (PcD) by a single bond, the cyclic voltammograms in PcD-Biph and PcD-3Ph implied the weaker interaction compared to that of p-phenylene-bridged TIPS-pentacene dimer (PcD-4Ph) and PcD. The femtosecond and nanosecond transient absorption spectra clearly revealed the slower ISF process in bent-shaped pentacene dimers (PcD-Biph and PcD-3Ph), more notably, the slower relaxation of the excited triplet states in PcD-Biph and PcD-3Ph. Namely, the quantum yields of triplet states (ΦT) by ISF approximately remain constant (ca. 180−200%) in all dimer systems, whereas the lifetimes of the triplet excited states became much longer (up to 360 ns) in PcD-Biph as compared to PcD-4Ph (15 ns). Additionally, the lifetimes of the corresponding triplet states in PcD-Biph and PcD-3Ph were sufficiently affected by solvent viscosity. In particular, the lifetimes of PcD-Biph triplet state in THF/paraffin (1.0 μs) increased up to approximately three times as compared to that in THF (360 ns), whereas those of PcD-4Ph were quite similar in both solvent.

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constants.20−22 As an experimental evidence, the intermolecular interaction related to the molecular packing is associated with spectral change in electronic transition, revealed by the polarized absorption spectra along the different crystal axis.23−25 Therefore, one of the useful strategies for achieving efficient SF in crystal states is to precisely control the molecular packing and proximity.26−29 This aspect is highly related to the distance and orientation between two neighboring dye units in the case of molecular dimers. On the contrary to the above-mentioned intermolecular SF in solid states, intramolecular SF (ISF) using covalently linked molecular dimers has been recently reported.30−32 ISF has a major potential advantage in terms of precise structural control between two dye molecules, being circumvented to control unpredicted molecular packing behavior in solid state.33,34 The

INTRODUCTION Efficient energy conversion from solar energy to the electric or chemical energy utilizing organic materials is one of our major scientific and technological challenges.1−5 Singlet fission (SF) is a multiexciton generation process whereby one singlet exciton converts into two triplet excitons, enabling us to construct nextgeneration photovoltaic cells to overcome the Shockly−Quisser limit.6−9 In most cases, occurrence of efficient SF has been reported mainly in solid and aggregated states.10−12 The recent research progress in SF has pointed out interplay between excited singlet state (S1) and correlated triplet [1(TT)*] states such as Frenkel exciton (FE) and charge-transfer (CT) state mixing in addition to the energy level matching.13−16 The intermolecular coupling does not only overcomes the thermodynamic disadvantage and unusual exciton migration, but also leads to the efficient and ultrafast SF.17−19 The large coupling in crystalline pentacene, for example, is attributed to the molecular packing and the vertical vibration in pentacene moiety, where the efficient SF occurs with extremely large rate © XXXX American Chemical Society

Received: January 29, 2016 Revised: February 29, 2016

A

DOI: 10.1021/acs.jpca.6b00988 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A Chart 1. Structures of Pentacene Derivatives in This Study

at a commercial facility. NMR spectra were acquired on a JEOL ECX-400, AL-400, or ALPHA-400 spectrometer, using the solvent peak as the reference standard, with chemical shifts given in parts per million. MALDI−TOF mass spectra were recorded on Bruker Ultra flex. The density functional theory (DFT) calculations for optimization of molecular structure were performed with Gaussian 09 program43 at ωB97XD/ DGTZVP level of theory.44,45 Synthesis of PcD-3Ph. TPc-Br (50.0 mg, 0.070 mmol), 1,3-bis(pinacolate)benzene (10.4 mg, 0.032 mmol), and K3PO4 (26.9 mg, 0.13 mmol) were dissolved in toluene/EtOH/H2O (6/1/1, v/v/v) under a nitrogen atmosphere. After addition of PdCl2(dppf)CH2Cl2 (5.2 mg, 6.3 μmol), the mixture solution was stirred at 65 °C for 12 h. Next, the solvent was evaporated, and the residue was dissolved in CHCl3. Then, the organic solution was washed with H2O and saturated NaHCO3 aqueous solution, dried over anhydrous Na2SO4 and evaporated. Finally, the crude was purified by chromatography on silica gel eluting with only hexane, and PcD-3Ph (yield: 25.2 mg, 59%) was obtained. 1H NMR (400 MHz, CDCl3), δ: 9.39 (2H, s), 9.36 (2H, s), 9.32 (4H, s), 8.24 (2H, s), 8.17 (1H, s), 8.12 (2H, d, J = 9.3 Hz), 8.00−7.97 (4H, m), 7.87 (2H, d, J = 7.3 Hz), 7.82 (2H, d, J = 8.8 Hz), 7.73 (1H, dd, J = 7.8 Hz), 7.43 (2H, d, J = 2.9 Hz), 7.42 (2H, d, J = 3.4 Hz), 1.35−1.45 (m, 84H). MALDI−TOF MS: m/z calcd, 1350.77; found, 1352 [M + 2H]2+. Synthesis of PcD-Biph. 2,2′-Dibromobiphenyl (14.4 mg, 0.046 mmol) was dissolved in diethyl ether under a nitrogen atmosphere. After cooling to −78 °C, TMEDA (25.5 mg, 0.10 mmol) was added in the solution. Then, 1.6 M n-BuLi in hexane (0.10 mmol, 2.2 equiv) was added dropwise to the mixture solution and stirred at −78 °C for 1 h. Then, 1.8 M solution of boron triisopropoxide in diethyl ether (10 mL) was added dropwise to the solution and stirred at room temperature for 18 h. After an evaporation of the solvent, the residue, TPcBr (136.4 mg, 0.19 mmol) and K3PO4 (26.3 mg, 0.19 mmol) were dissolved in 1,4-dioxane (40 mL) and H2O (4 mL) under a nitrogen atmosphere. PdCl2(dppf)CH2Cl2 (7.5 mg, 9.2 μmol)

efficient ISF has been recently reported using pentacene dimers linked via a diethynylbenzene spacer in meta- and paraarrangements by Zirzlmeier et al.35 The maximum triplet quantum yield (ΦT) reached 156%, whereas the ΦT values depend on the type of solvents. Sanders and co-worker also reported the kinetic control of ISF in pentacene dimers with different number of linearly p-phenylene spacers to focus on the distance-dependent on the dynamics. The triplet lifetimes of quantitatively generated triplet pair increased up to 270 ns.36 In particular, the fast triplet−triplet recombination occurred in a directly linked-pentacene dimer, which suggested that large electronic coupling exists within the aligned pentacene triplets (planar geometry).37 To develop such molecular systems toward efficient solar energy conversion, high-yield and longlived lifetime of triplet excited states are definitely required. Nevertheless, an important question remains to be elucidated regarding the angular orientation-dependent electronic coupling and ISF in pentacene dimers, although the vibrational motion play a significant role in intermolecular SF in crystalline materials.38,39 The structural distortion between two pentacene molecules in dimeric forms is expected to play an additional role to achieve the appropriate balance with the electronic coupling for the triplet formation and subsequent decay.40−42 Herein, we newly synthesized a couple of bent-shaped TIPSpentacene dimers (PcD-3Ph and PcD-Biph) to reveal the instinct characters of triplet excited states. A linearly extended TIPS-pentacene dimers (PcD-4Ph and PcD) and a previously reported monomer (TPc) were also compared as references.36 The structures employed in this study are shown in Chart 1.

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EXPERIMENTAL SECTION General Information. All reactions involving air- and moisture-sensitive reagents were performed under nitrogen atmosphere in oven-dried or flame-dried glassware. A magnetic stirrer was used in all cases. Thin-layer chromatography (TLC) was carried out using silica gel plates with a fluorescent indicator (Sorbent Technologies) and UV as the detection method. High- and low-resolution mass spectra were collected B

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The Journal of Physical Chemistry A Scheme 1. Synthetic Scheme of PcD-Biph and PcD-3Ph

was added in the solution and stirred at 85 °C for 12 h. After an evaporation of the solvent, the solid was dissolved in CHCl3. Then, the organic solution was washed with H2O and saturated NaHCO3 aqueous solution, dried over anhydrous Na2SO4 and evaporated. Finally, the crude was purified by chromatography on silica gel eluting with only hexane, and PcD-Biph (yield: 29.6 mg, 45%) was obtained. 1H NMR (acetone-d6) δ: 9.26 (s, 2H), 9.09 (s, 2H), 9.02 (s, 2H), 8.43 (s, 2H), 7.94 (d, J = 7.6 Hz, 2H), 7.82 (d, J = 8.3 Hz, 2H), 7.63 (d, J = 7.6 Hz, 2H), 7.51 (dd, J = 7.6, 3.8 Hz, 2H), 7.41−7.40 (m, 8H), 7.23 (d, J = 7.6 Hz, 2H), 6.89 (s, 2H), 6.66 (d, J = 8.3 Hz, 2H), 1.25 (m, 80H), 0.61−0.72 (m, 4H). MALDI−TOF MS: m/z calcd, 1426.80; found, 1429 [M + 2H]2+. Steady-State Spectroscopic and Fluorescence Lifetime Measurements. The measurements of UV/vis absorption spectra were recorded on a UV/vis/NIR spectrophotometer (PerkinElmer, Lambda 750). Fluorescence emission spectra were recorded on a spectrofluorophotometer (PerkinElmer, LS-55 and JASCO, FP-6500). Fluorescence lifetimes were measured on a time-correlated single-photon counting system (Horiba Scientific, FluoroCube) with a laser (DeltaDiode, laser diode head) as an excitation source. A laser operation wavelength, pulse width, and frequency were 404 nm, 50 ps and 1 MHz, respectively. The practical time resolution is 15 ps by deconvolution of an observed trace with the analytical software (DAS6). Transient Absorption Measurements. A pump−probe technique was used to detect the fast ISF process in solution with a resolution of 0.2 ps. The instruments of femtosecond absorption measurement have been described earlier.46 In brief, 100 fs pulses at 800 nm and repetition rate 1 kHz were generated by Libra F laser system (Coherent Inc.) and used to pump optical parametric amplifier Topas C (Light Conversion Ltd.) generating pump pulses at desired wavelength and to feed white continuum generator producing probe pulses. A timeresolved spectrometer (ExiPro, CDP Inc.) was used to collect a series of transient absorption spectra at desired wavelengths. Picosecond transient absorption measurements were carried out with a picosecond transient absorption spectrometer (EOS, Ultrafast Systems) with a laser-diode-pumped Nd:YAG laser (PL2210A, Ekspla). A laser operation wavelength, pulse width,

and frequency were 532 nm, 25 ps, and 1 kHz, respectively. Nanosecond transient absorption measurements were carried out using Unisoku TSP-2000 flash spectrometer. Surelite-I Nd:YAG (Continuum, 4−6 ns fwhm) laser with the second harmonic at 532 nm was employed for the flash photoirradiation. The photodynamics was monitored by continuous exposure to a xenon lamp (150 W) as a probe light and a photomultiplier tube (Hamamatsu R-2949) as a detector. Each sample solution was purged with Ar for at least 15 min prior to the measurement.

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RESULTS AND DISCUSSION Synthesis and Optimized Structures of PcD-3Ph and PcD-Biph. We newly synthesized PcD-3Ph and PcD-Biph where the two TIPS-pentacene moieties were covalently linked with m-phenylene (PcD-3Ph) and 2,2′-biphenyl spacer (PcDBiph) as shown in Chart 1. In addition, the pentacene dimers such as PcD-4Ph and PcD were synthesized as reference compounds where the pentacene moieties were covalently linked with a linear para-phenylene spacer (PcD-4Ph) and without the spacer (PcD) as shown in Chart 1. The linearly expanded pentacene dimers PcD and PcD-4Ph were previously reported by Sanders and co-workers, described as BP0 and BP1, respectively, in their report.36 The preparation of PcD-3Ph and PcD-Biph are shown in Scheme 1, using Suzuki−Miyaura coupling as a key reaction. The synthetic details were shown in the Experimental Section. The density functional theory (DFT) calculations were performed with Gaussian 09 program43 at the ωB97XD/ DGTZVP level of theory for optimization of molecular structure of PcD-Biph and PcD-3Ph.44,45 The ωB97XD functional has been recently applied to theoretical exploration of ISF with other systems of pentacene dimers.47 As schematically displayed in Figure 1, the angle between the long axis of TPc in two TPc moieties were found to be ca. 63° (PcD-Biph, Figure 1A) and ca. 137° (PcD-3Ph, Figure 1B), respectively. The center-to-center distance between two TPc moieties in PcD-3Ph (ca. 16.5 Å) was much larger than that in PcD-Biph (ca. 10.9 Å). Steady-State Spectroscopic Measurements. We first performed steady-state spectroscopic measurements to explore C

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The Journal of Physical Chemistry A

PcD, the absorption spectrum became red-shifted (330 cm−1) compared with TPc (Supporting Information, Figure S2), which is related to the degree of interchromophore interactions. Therefore, these results suggest that the bimolecular interactions in the bent-shaped dimers is relatively weak compared with those of the linearly extended dimers.23,48 The similar fluorescence spectral features of pentacene dimers were observed as compared to TPc (Figure 2B) whereas the emission of these dimers were significantly quenched (Supporting Information, Figure S3). This also suggests efficient SF in these dimers. Electrochemical Measurements. The electrochemical measurement was performed to examine the bimolecular interaction between two pentacene moieties (Figure 3). The two broadened peaks in PcD, which is a directly linked pentacene dimer, were observed at 0.81 and 0.95 V vs SCE.

Figure 1. Optimized structures of (A) PcD-Biph and (B) PcD-3Ph calculated at the ωB97XD/DGTZVP level. The C and Si atoms were represented as gray and yellow, respectively.

basic optical features and chromophore coupling in the pentacene dimers. The steady-state absorption and emission spectra in solution at room temperature are shown in Figure 2. The transition bands of the S0 → S1 with maxima at 650 nm (PcD-Biph and PcD-4Ph) and 649 nm (PcD-3Ph) appeared, which are accompanied by similar emissive bands in all pentacene derivatives. The molar absorption coefficients are 39 000 M−1 cm−1 (PcD-Biph) and 44 000 M−1 cm−1 (PcD3Ph), respectively (Supporting Information, Figure S1). These molar absorption coefficients were also similar to that of PcD4Ph (40 000 M−1 cm−1), which is in good agreement with the previously reported value.36 Considering the molar absorption coefficient of TPc (25 000 M−1 cm−1), these large coefficients in pentacene dimers may imply the weak interaction between the TPc moieties.48 The vibrational bands were also clearly observed with the progression mode of ca. 1400 cm−1 in these pentacene dimers. As the progression mode can be ascribed to the aromatic ring vibrations in the TPc moiety,49 the spectral shapes are not derived by the chromophore interactions. The ratio of the 0−1/0−0 peaks in the transition bands of the S0 → S1 has been employed to evaluate the interchromophores coupling in the pentacene dimers, where the increased value being correlated with the increased degree of the coupling.50 Actually, the ratio (0−1/0−0) of PcD-Biph (0.54) and PcD3Ph (0.53) were slightly lower than that of PcD-4Ph (0.57) but higher than that of TPc (0.50). The ratio of PcD was also determined to be 0.57 (Supporting Information, Figure S2). In

Figure 3. Cyclic voltammograms of (a) PcD-Biph, (b) PcD-3Ph, (c) PcD-4Ph, (d) PcD, and (e) TPc in CH2Cl2 with 0.1 M nBu4NP6 as supporting electrolyte. Scan rate: 100 mVs−1.

This suggests quite strong bimolecular interactions related to the electronic delocalization between the two pentacene moieties.51 Close examination of the voltammogram of PcD3Ph reveals that the split peaks became almost identical at around 0.84 V vs SCE, which is coincident with that of TPc (0.85 V vs SCE), indicating the attenuation of the bimolecular interaction.52 In contrast, in PcD-4Ph, a broadened peak at

Figure 2. (A) Steady-state absorption spectra and (B) emission spectra of (a) PcD-Biph, (b) PcD-3Ph, (c) PcD-4Ph, and (d) TPc monomer. The measurements were performed in THF and excitation wavelength was at 532 nm. The emission quantum yields of PcD-Biph, PcD-3Ph and TPc were determined to be 4.4%, 1.9% and 55%, respectively (Supporting Information, Figure S3). D

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The Journal of Physical Chemistry A

Figure 4. (A) fsTA spectra of PcD-Biph at (a) 1.0, (b) 95, and (c) 5000 ps and (B) the corresponding time profiles of PcD-Biph at (a) 525 and (b) 570 nm. (C) fsTA spectra of PcD-3Ph at (a) 1.0, (b) 95, and (c) 5000 ps and (D) the corresponding time profiles of PcD-3Ph at (a) 525 and (b) 570 nm. The measurements of fsTA were performed in THF and excitation wavelength was 600 nm. The raw data is shown in Supporting Information, Figure S5.

the difference in the bimolecular interaction was also observed in electrochemical measurements. Therefore, the bimolecular interactions in these pentacene dimers are attributed to the charge-transfer and FE mixing configuration rather than only FE exciton coupling.23,33,56 Femtosecond Transient Absorption Measurements. The occurrence of singlet fission in the bent-shaped dimers was carefully investigated by femtosecond transient absorption (fsTA) spectroscopy. The fsTA spectra of PcD-Biph and PcD3Ph after excitation at 600 nm are shown in Figure 4. The spectra of PcD-4Ph are also shown in Figure S4 in Supporting Information. The broad positive bands appeared immediately after excitation in the range 470−580 nm in both PcD-Biph and PcD-3Ph systems (spectrum a in Figure 4A and spectrum a in Figure 4C), which is attributed to the excited singlet state absorption (ESA), namely S1−Sn absorption. The negative band related to the ground state bleaching was also observed at around 650 nm. The broad positive bands in the range 470− 580 nm transformed with time into a well pronounced absorption band at 530 nm (spectrum b in Figure 4A and spectrum b in Figure 4C), which is ascribed to the T1−Tn transition.16,36,57 The generation of the triplet state was obviously populated with fast and slow components (trace a in Figure 4B). A stretched exponential function was used to fit the transient absorption data at earlier and later delay times in PcD-Biph, which is useful to analyze the complicated kinetic processes to transfer a simple form related to the kinetics of relaxation time, in addition to checking the number of exponential function. The well-known stretched exponential function58,59 as fraction of time (t) is represented by eq 2

around 0.72−1.00 V vs SCE appeared to be overlapped with the two oxidative peaks. Thus, the bimolecular interaction in PcD-3Ph would be slightly weaker than that in PcD-4Ph. On the contrary, the two successive peaks appeared in PcD-Biph due to the first and second one-electron oxidation potentials, which is in sharp contrast with the trend of PcD. This indicates that there is no sufficient interaction in PcD-Biph in electrochemical measurements.52,53 The bimolecular interactions may mainly originate from the mixing of FE and CT states between two pentacene moieties. Considering the optimized molecular structures (Figure 1), the relative energy differences in the transition bands (ΔE) were calculated according to the following eq 154 based on the interaction between the two dipoles moments in TPc moieties. ΔE =

2 |μ|2 (cos α + 3 cos2 θ ) |r|3

(1)

Here μ is the transition dipole moments of TPc moiety, r is the center-to-center distance between the corresponding dipoles, α is the angle of the polarization axes and θ is the angle made by the polarization axes of TPc moieties. The energy differences (ΔE) were found to be 49 cm−1 (PcD-Biph) and 75 cm−1 (PcD-3Ph) with the value of μ, r, α, and θ estimated by the optimized molecular structures and the absorption spectra (Supporting Information, Table S1). However, the estimation by this model may provide a poor description for reproduction of absorption spectra because the defined spectral changes in electronic transition could not be obtained unfortunately. In the case when only FE interaction is taken into account in this model,55 the underestimation of ΔE may be due to the contribution of the charge-transfer state.25 As discussed above, E

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The Journal of Physical Chemistry A Table 1. Summary of Kinetic Parameters and Quantum Yields for Triplet Generation ki/109 s−1 PcD-Biph PcD-3Ph PcD-4Ph

390 (22.1%) 57 (47.1%) 1.8 (30.8%) 210 (8.2%) 2.9 (91.8%) 85 (100%)

kSF,app/s−1

τSinglet

1.8 × 109

540 ps (41%) 12 ns (59%)

360 nsb (THF) 1.0 μsb (THF/paraffin)

2.9 × 109

390 ps (88%) 12 ns (12%)

8.5 × 1010

20 psa

170 nsb (THF) 230 nsb (THF/paraffin) 15 nsc (THF) 15 nsc (THF/paraffin)

τTriplet

ΦT/% 176 188 ∼200a

a

Reported value.36 bThe lifetime of the triplet state in PcD-4Ph was estimated by the monoexponential function. cThe lifetimes of the triplet state in PcD-Biph and PcD-3Ph were estimated by the monoexponential function. The lifetimes of the excited singlet state were determined by TCSPC measurements (Supporting Information, Figure S6). The details of the kinetic data evaluated by transient absorption measurements were summarized in Supporting Information, Table S2 and Table S4.

Figure 5. (A) Normalized nsTA spectra and (B) the corresponding time profiles of PcD-Biph at 526 nm in (a) THF/paraffin (1/9, v/v) and (b) THF. (C) Normalized nsTA spectra and (D) the time profiles of PcD-3Ph at 526 nm in (a) THF/paraffin (1/9, v/v) and (b) THF. The measurements of nsTA were measured by excitation source at 532 nm.

⎡ ⎛ t ⎞β⎤ f (t ) = exp⎢ −⎜ ⎟ ⎥ ⎣ ⎝ τrelax ⎠ ⎦

in PcD-4Ph was fitted by monoexponential function accordingly, which is in good agreement with the kinetics model.36 The rate constant for PcD-4Ph was determined to be 8.5 × 1010 s−1 as shown in Table 1. On the contrary, the short-lived component with the lifetime of 4.8 ps was marginally monitored in PcD-3Ph; that is, the trace of PcD-3Ph was fully fitted by a biexponential function with the rate constants of 2.1 × 1011 (8.2%) and 2.9 × 109 (91.8%) s−1. Consequently, the bands of charge-transfer states appears to be close to those of the corresponding triplet state,57,61 the fast components in PcD-Biph and PcD-3Ph indicate the occurrence of the intermediated state, which is associated with the corresponding electronic coupling and configurational change based on the molecular skeleton.15,62 In order to simply compare the rate constants of triplet generation in these three dimeric systems, the apparent rate constants kapp of ISF were estimated by simple form of multiexponential function (Supporting Information, Table S2).

(2)

where τrelax and β are the lifetime of relaxation time and the stretching parameter (0 ≤ β ≤ 1). If the parameter β is equal to 1, the stretched exponential function becomes monoexponential function. The relaxation lifetimes (τrelax) of the generation in PcD-Biph were determined to be 12 ps (β = 0.62) in earlier delay time, whereas 570 ps (β = 1.0) in later delay time. The fast component was represented by nonexponential function. This may be due to the occurrence of the multiple process including the charge-transfer state and/or configurational change related to molecular vibration.38,39,60 In contrast with PcD-Biph, in the case of PcD-4Ph and PcD-3Ph, the relaxation processes were not well approximated by the stretched exponential function because the process is too short for stark identification as compared to that in PcD-Biph (Figure 4B, trace a). The trace of the triplet generation process F

DOI: 10.1021/acs.jpca.6b00988 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A The kapp values of PcD-Biph, PcD-3Ph, and PcD-4Ph were determined to be 1.8 × 109, 2.9 × 109, and 8.5 × 1010 s−1, respectively. Although, unfortunately, the photoluminescence lifetime of PcD-4Ph were not clearly observed due to the shortlived species, the observed values of PcD-Biph and PcD-3Ph were approximately identified with the generation of the triplet state (Supporting Information, Figure S6 and Table S2). The spectroscopic data are summarized in Table 1. Overall, these analyzed data suggest that the electronic coupling have a discriminate role in the kinetical process for ISF rather than the quantum yields because the process of the triplet generation is significantly faster compared to the deactivation of instinct excited singlet state in these systems. Nanosecond transient absorption (nsTA) spectra of PcDBiph and PcD-3Ph were measured in THF to examine the deactivation process of the triplet states (Figure 5 and Supporting Information, Figures S7−S9). The spectra of PcD-4Ph are also shown in Figure S10 in Supporting Information. In this delay-time region, the triplet state generated through ISF was dominantly observed. The T1−Tn absorption and the ground state bleaching appeared from 370 to 550 nm and around 650 nm as seen in fsTA measurements (spectrum c in Figure 4A and spectrum c in Figure 4C). By the kinetic analysis, the lifetimes of the corresponding triplet states in PcD-Biph, PcD-3Ph, and PcD-4Ph were determined to be 360, 170, and 15 ns, respectively. The long-lived species of the triplet states were also observed in PcD-Biph (28 μs) and PcD3Ph (21 μs). Whereas the fast species are ascribed to the correlated triplet pair, the slow species are associated with the individual triplets.6,32 The lifetimes of the individual triplet states were approximately close to that of the triplet state in TPc monomer (29 μs, Supporting Information, Figure S11). These results ensured that the coupling constants between two TPc units in these bent-shaped dimers largely contribute to the long-lived lifetimes of triplet excited states. As discussed above, the excited state characters of PcD-Biph were rather complex. The nsTA spectra were consequently measured in a viscous solvent [THF/Paraffin (1/9, v/v)] to examine the role of solvent viscosity on the triplet lifetimes. The spectra and time profiles at 520 nm of PcD-Biph and PcD3Ph in THF and THF/paraffin (1/9, v/v) are shown in Figure 5. As seen in the traces (Figure 5B and Figure 5D), the lifetimes of PcD-Biph and PcD-3Ph significantly increased up to 1.0 μs and 230 ns, whereas the lifetime of PcD-4Ph remained the same in THF/paraffin (15 ns, Supporting Information, Figure S10). The lifetimes of the corresponding triplet state are briefly summarized in Table 1. The ratio of lifetimes in THF/paraffin to THF (τviscous/τTHF) were calculated to be 2.8 (PcD-Biph) and 1.4 (PcD-3Ph). These results clearly indicate that the bentshaped orientation in pentacene dimeric forms influences significantly the triplet lifetimes and determines the solvent viscosity dependence. This influence is based on the weak electronic coupling and the degree of the flexibility in the excited state in bent-shaped organic spacers.

states between two pentacene moieties in a these pentacene dimers. The degree of the electronic coupling is correlated to the ISF process. Whereas the rate constants for triplet generation decrease in the bent-shaped pentacene dimers, the quantum yields of triplet states (ΦT) by ISF approximately remain constant in all dimer systems. More notably, the lifetimes of the triplet excited states became much longer (up to 360 ns) in PcD-Biph as compared to PcD-4Ph (15 ns). The triplet lifetimes in PcD-Biph (360 ns) has increased up to 1.0 μs with increased viscosity of media (in THF/paraffin). Finally, a design and synthetic strategy with bent structures provides important advantages for construction of light energy conversion systems utilizing ISF.

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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.jpca.6b00988. Synthetic details of pentacene derivatives, steady-state absorption, fluorescence spectra, fsTA spectra, psTA spectra, nsTA spectra, fluorescence lifetime measurements, density functional theory calculation, and detail determination of triplet generation (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (T.H.). *E-mail: nikolai.tkachenko@tut.fi (N.V.T.). *E-mail: [email protected] (Y.A.). Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by Grants-in-Aid for Scientific Research (Nos. 26286017, 26620159, 15H01003 “π-System Figuration”, and 15H01094 “Photosynergetics” to T.H.). This work was performed under the Cooperative Research Program of “Network Joint Research Center for Materials and Devices”.

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REFERENCES

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CONCLUSION In conclusion, we exploited a bent-shaped dimer of pentacene to examine the process of ISF compared with the linearly extended dimers. In steady state spectroscopy, the absorption and emission spectra were successfully characterized in PcDBiph, PcD-3Ph and PcD-4Ph. On the contrary, the slightly different features were observed in electrochemical measurements, suggesting the different degree of the charge-transfer G

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