Very Long-Lived Photoinduced Charge-Separated States of

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Very Long-Lived Photoinduced Charge-Separated States of Triphenylamine–Naphthalenediimide Dyads in Polymer Matrices Kenshi Kimoto, Tsubasa Satoh, Munetaka Iwamura, Koichi Nozaki, Takafumi Horikoshi, Shuichi Suzuki, Masatoshi Kozaki, and Keiji Okada J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b07705 • Publication Date (Web): 25 Sep 2016 Downloaded from http://pubs.acs.org on September 26, 2016

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The Journal of Physical Chemistry

Very Long-Lived Photoinduced Charge-Separated States of Triphenylamine–Naphthalenediimide Dyads in Polymer Matrices Kenshi Kimoto†, Tsubasa Satoh†, Munetaka Iwamura†, Koichi Nozaki*†, Takafumi Horikoshi‡, Shuichi Suzuki‡┴, Masatoshi Kozaki‡, and Keiji Okada*‡

†Department of Chemistry, Graduate School of Science and Engineering, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan ‡Department of Chemistry, Graduate School of Science, Osaka City University, Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan,

Photoinduced electron transfer was studied in dyads (dyad1 and dyad2) containing triphenylamine (MTA) and naphthalenediimide (MNDI) linked with oligo(phenylene-ethynylene) dispersed

in

rigid

polymer

matrices

of

polystyrene

(PS),

poly(vinylchloride),

and

poly(methylmethacrylate). Photoexcitation of these dyads yielded long-lived charge-separated (CS) states involving MTA+ and MNDI–. The quantum yields of charge separation in dyad1 and dyad2 were approximately 0.4 and 0.3, respectively, in the polymer matrices. The CS lifetime for dyad2 in PS was longer (400 ms) than those in poly(vinylchloride) (120 ms) and poly(methylmethacrylate) (65 ms) at 298 K. In addition, CS state had a very long lifetime of 5.4 s in glassy toluene at 100 K. Below glass 1

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transition temperatures, polymer side-chain motions with various relaxation rates should affect the charge recombination processes. The energy gap (ΔG) and outer-sphere reorganization energy (λ) in the charge recombination process were estimated using a slow-frequency component for dielectric constants. Using ΔG and λ values, the matrix dependence of the CS lifetimes was successfully rationalized based on Marcus theory, and the charge recombination process in PS with low polarity and high polarizability should be in a deeper inverted region than the other polymer matrices. It also suggested that the rigidity of the polymer effectively suppressed intramolecular motions promoting the charge recombination process.

Photoinduced electron transfer in D-A molecular systems, where D is an electron donor and A is an electron acceptor, is a fundamental process for realizing efficient conversion of light energy into electrical and chemical energies. Photosynthetic reaction centers substantialize the production of long-lived photoinduced charge-separated (CS) states with high quantum yields through a multi-step electron transfer process. Several D-A molecular systems in solution that yield CS states with lifetimes of over 100 µs by photoexcitation have been reported.1–9 In porphyrin-fullerene and 9-mesityl-10-methylacridinium systems, formation of the long-lived CS state was proposed to occur through a charge recombination process in the Marcus inverted region (λ < –ΔG) because of the low reorganization energy of the chromophores.10-14 However, considerably different and conflicting results and explanations have been reported for some systems.15-18 In comparison to systems in solution, D-A systems in polymers have much longer lifetimes ( > 10 s) because of intermolecular electron transfer in the polymer.19, 20 Electron transfer processes in polymers have attracted attention recently because they play important roles in development of organic light-emitting diodes and organic photovoltaic cells. Control 2


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of electron transfer rates in polymers is crucial for improving the efficiency of devices. Generally speaking, while the rates of electron transfer in polar solvents are successfully explained using Marcus theory, those in polymers are not well understood. Most electron transfer reactions in solution proceed as nonadiabatic processes because the rate of solvent orientation is faster than electron transfer. However, polymer motions have various modes and their frequencies are distributed over a wide time range. In particular, the rate of motion of a polymer depends on the temperature, and changes significantly around the glass transition temperature (Tg). Thus, it is not easy to adapt the solution electron transfer model to electron transfer in polymer matrices. Theoretical and experimental investigations on electron transfer rates in linked D-A systems in rigid media have been carried out.21-27 The rate of photoinduced electron transfer in polymer matrices above the Tg was explained based on an electron transfer formula including quantum modes.28,29 When the states involved in electron transfer processes are not in equilibrium, such as in polymers below the Tg, the distribution of polymer motions should be taken into account.30,31 The temperature dependence of polymer motions has been considered for understanding electron transfer rates in polymer matrices.26,32 To examine the polymer motions, which can be faster than the electron transfer rates, high frequency components in polymer relaxation were evaluated from effective dielectric constants of the polymer determined using carbazole-terephthalate cyclophane as a fluorescent polarity probe. The rate of photoinduced electron transfer in D-A dyads containing carbazole-benzenecarboxylate in a polar polymer of cyanoethylated pullulan below Tg was explained using the effective dielectric constant in the same time domain as fluorescence (approximately 30 ns).33-35 However, electron transfer rates in non-polar polymer matrices such as PS, PVC and PMMA have never been studied. In such rigid and non-polar matrices, it is anticipated that both the low dielectric constant and the restraint of intramolecular motion largely retard electron transfer process in the Marcus inverted region.

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Figure 1. Molecular structures of dyads and reference compounds. We have previously studied photoinduced electron transfer in D-A compounds, in which platinum(II) complexes were linked with the electron acceptor (naphthalene diimide, MNDI) and the electron donor (triphenylamine, MTA; or phenothiazine, PTZ) through highly twisted biphenylene or phenylene ethynylene bridges.36 Photoexcitation of these D-A compounds in toluene produced a relatively long-lived CS state with high efficiency (τCS = 1.34 µs, ΦCS = 0.96).37,38 In this work, we studied photoinduced electron transfer processes in MTA-MNDI dyads (dyad1 and dyad2) linked through highly twisted p-phenylene ethynylene bridges (Figure 1). We found, for the first time, considerably long-lived CS states were produced with high quantum yields for the dyads dispersed in non-polar polymer matrices. The dyads also gave surprisingly long-lived CS states in glassy toluene at 100 K. We examined the factors controlling the rates of slow charge recombination for the dyads in polymers to understand the electron transfer process in rigid matrices.

Sample preparations 4


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Syntheses of dyad1, dyad2, and reference compounds were described in the supporting information. Polystyrene (PS, n = 2000), poly(methylmethacrylate) (PMMA), and poly(vinylchloride) (PVC, n = 1100) were purchased from Wako Pure Chemical Industries Ltd. (Osaka, Japan) and purified by reprecipitation using toluene and methanol (PS, PMMA) or tetrahydrofuran and methanol (PVC). Films were prepared by solution casting using a toluene solution containing 20 µg of the dyad and 20 mg of PS or PMMA. For PVC films, tetrahydrofuran was used instead of toluene. The films were dried in air at ambient temperature over 2 days, then for 1 day at 320 K, and finally for 2 h at a temperature 400 K for PS and PMMA, 380 K for PVC in vacuum. The films were approximately 100 µm thick. Measurements Ultraviolet-visible absorption spectra were recorded on a multi-purpose spectrometer (MPS-2000, Shimadzu). The measurement systems of the emission spectra and emission lifetimes are described elsewhere.39 High-precision transient absorption measurements Nanosecond transient absorption spectra were measured using the third harmonic of a Q-switched Nd3+:YAG laser with λ = 355 nm (Surelite I-10, Continuum) as the excitation light source. An intensified Xe arc lamp or highly stabilized high-power infrared light emitting diode with λ = 760 nm (OP1-7605P1, Alpha-One Electronics) was used as the monitor light source. The monitor light passed through the photoexcited sample and was dispersed using a grating monochromator (H-20 visible model, HORIBA Scientific), and then detected using a photomultiplier tube (R3896, Hamamatsu) and an amplifier. The amplifier precisely amplified the weak AC signal using a low-pass filter, an I/V converter, and a variable DC component canceler. The signals were accumulated on a digitizing oscilloscope (HP 54520, Hewlett-Packard or MSO4104, Tektronix). The system was able to measure transient absorbance less than 10–5. The temperature of the sample was controlled using a 5



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cryostat (DN1704, Oxford) and a controller (ICT-4, Oxford). To prevent moisture or oxygen affecting the CS lifetimes, the transient absorption of the sample were measured after it was kept under vacuum for 2 h. The quantum yields of photoinduced charge separation were determined from the molar absorptivities of the radical ions and numbers of photons for the excitation light, which were estimated using a chemical actinometer [Ru(bpy)3]2+ in a 1 mm quartz cuvette using our previously reported procedure.40 Density Functional Theory calculations The molecular orbitals and the internal reorganization energies (λi) for charge recombination in the dyads were calculated with density functional theory using the Gaussian 09 package.41 The structures of the dyads in the ground state and the lowest triplet state were geometrically optimized at the B3LYP/6-31G level with a polarizable continuum model using the solvent parameters of THF. The electronic states of the lowest triplet states in this calculations converged to the CS states for both the dyads, which had large dipole moments (110 D for dyad1 and 145 D for dyad2). The λi values were then obtained as the difference in self-consistent field energy at the B3LYP/6-31G* level between the fully optimized ground state and the ground state at the 3CS state geometry, and were estimated to be 0.37 eV for dyad1 and 0.38 eV for dyad2.

UV/Vis absorption spectra In dyad1 and dyad2, MTA and MNDI moieties were linked with highly twisted phenylene ethynylene bridges. The UV/Vis absorption spectra of dyad1 and dyad2 in toluene exhibited bands attributed to the π-π* transition in MTA at 310 nm, and bands attributed to the π-π* transition in MNDI at 365 nm and 385 nm (Figure 2). The bridge moieties of dyad1 and dyad2 showed π-π* transition 6


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bands at 310 nm. The spectra of dyad1 and dyad2 in the visible region were almost superposed with those of the reference compounds, ref-D and ref-A, indicating weak interaction between donor and acceptor moieties.

Figure 2. UV-Vis Absorption spectra of dyad1 and dyad2 in toluene, ref-D in dichloromethane,

and

ref-A

in

N,N-dimethylformamide at room temperature.

Transient absorption spectra When the dyads in PS were photoexcited with a 355 nm laser pulse, which predominantly produced excited states of the MNDI moiety, transient absorption bands were observed at 470 nm and 760 nm (Figure 3). These absorption bands persisted for several tens of milliseconds (dyad1) or for several seconds (dyad2). The bleaching at 380 nm indicates disappearance of the ground state of the MNDI moiety. The transient absorption spectra of dyad1 and dyad2 showed two peaks corresponding to the MTA radical cation (MTA+) at 760 nm, and to the MNDI radical anion (MNDI–) at 470 nm (Figure S1), indicating a photoinduced CS state was produced in the polymer matrices.

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Figure 3. Transient absorption spectra of PS film containing 0.1 % (mass fraction) dyad1 (a) and dyad2 (b) at 298 K under vacuum and transient absorption spectra of dyad1 at 77 K (c) and dyad2 at 100 K (d) in glassy toluene. The transient absorption spectra observed for dyad1 and dyad2 in the other polymer matrices showed the formation of long-lived CS states (Figure S2 and S3). The transient absorption spectra of the CS states were similar to the superposed spectrum of MNDI– and MTA+. Assuming the molar absorption coefficients of the CS state were the same as those of the superposed spectrum, the quantum yields of the charge separation at 298 K in dyad1 were 0.4 ± 0.1 (PS), 0.4 ± 0.1 (PVC), and 0.5 ± 0.1 (PMMA) and those in dyad2 were 0.3 ± 0.1 (PS), 0.4 ± 0.1 (PVC), and 0.3 ± 0.1 (PMMA). It is worth nothing that long-lived CS states were also observed even in glassy toluene solutions of the dyads at 8


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100 K (Figure 3c). Consequently, photoexcitation of the dyads gave long-lived excited states in rigid media, such as polymers and glassy solvents. Charge recombination process An overlay plot (Figure 4) of the normalized decay profiles of transient absorption signals at 470 nm from MNDI– and 760 from MTA+ showed these species had almost identical decay behavior. This indicates the CS states decay through charge recombination in polymer matrices without reacting with the polymers or impurities.

Figure 4. Overlay plots of normalized decay profiles of transient absorption from the donor radical cation (MTA+, 760 nm) and acceptor radical anion (MNDI–, 480 nm) of dyad1 in PS at 298 K. Effects of the dyad concentration on the decay profile of the CS state In polymer films containing high concentrations of dyads, a CS state formed from intramolecular electron transfer possibly causes electron/hole hopping with dyads in the vicinity (Eq. 1) to yield intermolecular CS states with much longer lifetimes than those of intramolecular CS states. D+ ‒ A‒

+ D‒A

→

D+ ‒ A

9


+ D ‒ A‒

(1)


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Although the aggregates of the dyads in the polymer films was presumed to be negligible because the absorption spectra of dyads in PS (

Very Long-Lived Photoinduced Charge-Separated States of

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