Effect of Triplet State on the Lifetime of Charge Separation in

Article pubs.acs.org/JPCC

Effect of Triplet State on the Lifetime of Charge Separation in Ambipolar D‑A1‑A2 Organic Semiconductors Tianyang Wang,†,‡ Krishanthi C. Weerasinghe,§ Haiya Sun,†,‡ Xiaoxia Hu,† Ting Lu,†,‡ Dongzhi Liu,† Wenping Hu,‡,∥ Wei Li,†,‡ Xueqin Zhou,*,†,‡ and Lichang Wang*,†,‡,§ †

School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300072, China Collaborative Innovation Center of Chemical Science and Engineering, Tianjin, 300072, China § Department of Chemistry and Biochemistry, Southern Illinois University Carbondale, Illinois 62901, United States ∥ School of Science, Tianjin University, Tianjin, 300072, China ‡

S Supporting Information *

ABSTRACT: An ambipolar organic semiconductor with styrene based triphenylamine derivative (MTPA) as an electron donor (D), triazine group (TRC) as an electron acceptor (A1), and 9,10anthraquinone (AEAQ) as a second electron acceptor (A2) has shown an 8-fold increase in the lifetime of charge separation with a high performance as solar cell materials with respect to the D-A1 architecture and demonstrated a general D-A1-A2 architecture as a promising materials design strategy for photovoltaics. Here we synthesized and characterized two new D-A1-A2 compounds with perylene bisimide derivatives (PDI and PBI) as A2 using an integrated experimental and computational method to study and compare the kinetics of three MTPA-TRC-A2 systems. A two-step sequential decay pathway was observed in both MTPA-TRC-PDI and MTPA-TRC-PBI but a direct decay pathway in MTPA-TRC-AEAQ. The charge separated state with a lifetime of 22 ns in the PDI system and 75 ns in the PBI system relaxes to the corresponding triplet state followed by the decay to ground state in 827 ns and 29.2 μs, respectively. Thus, a triplet state with a lower energy than the charge separated state shortens the lifetime of the charge separated state but increases the overall lifetime of excited states.

1. INTRODUCTION Photoinduced charge transfer processes have attracted great interest because of their fundamental role in applications such as solar cells and artificial photosynthetic systems.1−14 The efficient use of sunlight requires the photogeneration of suitably long-lived charge separated states that are capable of forming applicable photocurrent or driving the multielectron chemistry of fuel synthesis.15−22 Inspired by the natural photosynthetic process, where cascades of short-range photoinduced energy transfer and multistep electron transfer occur, a large variety of supramolecular systems and arrays have been developed with various donors and acceptors to form a donor−acceptor (D− A) architecture.23−27 Charge separated states in D−A systems are generated mostly through photoinduced electron transfer but can also be created by photoinduced hole transfer when the acceptor moiety is initially photoexcited.28 Investigations were carried out to develop ambipolar organic semiconductors, which allow both photoinduced electron and hole transfers, to form the charge separated states more effectively.29−33 Although great advances have been made in the development of organic solar cells, the relatively short-lived charge separated states of constructed D−A arrays in comparison to those in natural photosynthetic centers suggest a huge developmental potential © XXXX American Chemical Society

toward increasing device efficiency. While the electron donating and accepting abilities in D−A systems, which were determined by the relative electronic energies, define the driving force of an electron transfer,34,35 the distance, spatial orientation, and flexibility between the donor and acceptor moieties can significantly influence the efficiency and rate of photoinduced electron transfer. In contrast to noncovalent linkers such as protein36 and nonconjugated linkers such as alkyl chains,6,37 πconjugated linkers favor the through-bond electron migration from the donor to the acceptor,38−43 whereas they also enhance back electron transfer, therefore usually resulting in a faster charge recombination.18−21,44−46 The back electron transfer may be suppressed by design of cascade energy levels using more accepting modules to form a D-A1-A2 architecture.47−49 To investigate the feasibility of this D-A1-A2 design strategy toward increasing the lifetime of charge separated states of pure organic semiconductors, we synthesized 4,4′-dimethyl-4″-(4-(4chloro-6-(2-(9,10-dioxoanthracen-1-ylamino)ethylamino)1,3,5-triazin-2-ylamino) styryl)triphenylamine (MTPA-TRCAEAQ) that consists of a D-A1-A2 structure and measured its Received: February 8, 2016 Revised: April 23, 2016

A

DOI: 10.1021/acs.jpcc.6b01321 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C photophysical properties and compared its lifetime with that of MTPA-TRC (D-A1). Indeed, the lifetime of the charge separated state in MTPA-TRC-AEAQ is elongated to 650 ns, more than 8 times that of the MTPA-TRC (80 ns). Furthermore, the solar cells constructed using MTPA-TRCAEAQ were found to be more efficient than those using MTPA-TRC.50 These encouraging results indicate the D-A1-A2 architecture is a worthy strategy to be further explored. Introduction of an additional electron accepting module to the system allows efficient electron transfer, and it also increases the complexity of decay pathways for the photoexcited electrons. Therefore, detailed studies of other D-A1-A2systems are much needed to provide the insight into the electron transfer and charge recombination processes in these systems and to establish a firm foundation toward the rational design of D-A1-A2 systems for solar cell applications. As such, we designed two new D-A1-A2 systems and studied their photophysical properties. Specifically, we chose to keep the D-A1 component of our new D-A1-A2 systems the same as our recently studied D-A1-A2 system, i.e., MTPA-TRCAEAQ,50 but vary the second acceptor. To form the new MTPA-TRC-A2 systems, we needed to find the candidates as the second acceptor with the lowest unoccupied molecular orbital (LUMO) energy lower than that of the first acceptor, TRC. Therefore, we performed density functional theory (DFT) calculations on perylene bisimide derivatives and found their LUMO energies are lower than that of the TRC module. Interestingly, the highest occupied molecular orbital (HOMO) energies of these perylene bisimide derivatives are also lower than that of MTPA. These indicate that perylene bisimide derivatives can be used as the second acceptor and the newly constructed MTPA-TRC-A2 systems are also ambipolar, like MTPA-TRC-AEAQ.50 Furthermore, these acceptors can be easily synthesized on the basis of the substitution of chlorine in triazine (TRC), making it possible to investigate the effect of the second acceptor on the photophysical processes and in particular on the lifetime of charge separated states. Structures of the key compounds being synthesized and studied in this work are provided in Chart 1. While the other compounds were synthesized and studied previously,50 MTPATRC-PDI and MTPA-TRC-PBI were first reported here and their synthetic routes are shown in Schemes S1 and S2 of the Supporting Information. Photophysical properties of these compounds were studied using both steady state and transient UV−vis and fluorescence measurements coupled with electrochemical measurements and DFT calculations. The results of these measurements and the corresponding analysis are provided in section 3 following the description of methods in section 2. Finally, the conclusions of this work are summarized in section 4.

Chart 1. Structures of Key Compounds: MTPA, MTPATRC, MTPA-TRC-AEAQ, MTPA-TRC-BA, MTPA-TRCPDI, and MTPA-TRC-PBI

1-ylamino)ethylamino)-1,3,5-triazin-2-ylamino)styryl)triphenylamine (MTPA-TRC-AEAQ)50 were prepared according to the procedures described in the literature. The synthetic pathways of N-(1-hexylheptyl)-N′-(4-(4,6-dichloro-1,3,5triazin)aminophenyl)-perylene-3,4,9,10-tetracarboxylbisimide (PDIt), N-(1-hexylheptyl)-N′-(4-(4,6-dichloro-1,3,5-triazin)aminophenyl)-1,7-di(4-tert-butylphenoxyl)-perylene-3,4,9,10tetracarboxylbisimide (PBIt), 4,4′-dimethyl-4″-(4-(4-chloro-6(N-(1-hexylheptyl)-N′-(4-amino)phenyl-perylene-3,4,9,10-tetracarboxylbisimide)-1,3,5-triazin-2-ylamino)styryl)triphenylamine (MTPA-TRC-PDI), 4,4′-dimethyl-4″-(4-(4chloro-6-(N-(1-hexylheptyl)-N′-(4-amino)phenyl-1,7-di(4-tertbutylphenoxyl)-perylene-3,4,9,10-tetracarboxylbisimide)-1,3,5triazin-2-ylamino)styryl)triphenylamine (MTPA-TRC-PBI), and 4,4′-dimethyl-4″-(4-(4-chloro-6-phenyl-1,3,5-triazin-2ylamino)styryl)triphenylamine (MTPA-TRC-BA) were illustrated in Schemes S1 and S2 of the Supporting Information, and the structures of MTPA-TRC-EA, AEAQ, and AEAQt were shown in Chart S1 of the Supporting Information. All reagents and solvents were reagent grade and further purified by the standard methods when necessary. All synthetic procedures were carried out under an atmosphere of dry nitrogen unless otherwise indicated. 2.2. NMR Spectrometry. 1H NMR spectra were obtained on a VARIAN INOVA 500 MHz spectrometer, and the testing temperature was set to 25 °C. 2.3. Mass Spectrometry. The ESI mass spectra were obtained on a Thermo Fisher LCQ Deca XP MAX mass spectrometer. MALDI-TOF mass spectra were obtained on a Bruker Autoflex tof/tofIII mass spectrometer. 2.4. UV−vis Spectroscopy in Solution. The absorption spectra were taken on a Thermo Spectronic, Helios Gamma

2. EXPERIMENTAL AND COMPUTATIONAL METHODS 2.1. Synthesis. 1,7-Di(4-tert-butylphenoxyl)-perylene3,4,9,10-tetracarboxyldianhydride (PDA),51 N-(1-hexylheptyl)N′-(4-aminophenyl)-perylene-3,4,9,10-tetracarboxylbisimide (PDI),52 4,4′-dimethyl-4″-styryltriphenylamine (MTPA),50 4,4′-dimethyl-4″-(4-(4,6-dichloro-1,3,5-triazin-2-ylamino)styryl)triphenylamine (MTPA-TRC),50 4,4′-dimethyl-4″-(4-(6dichloro-1,3,5-triazin-2-ylamino-4-ethylamino)styryl)triphenylamine (MTPA-TRC-EA),50 1-(2-aminoethylamino)anthraquinone (AEAQ),50 1-((4,6-dichloro-1,3,5-triazin-2ylamino)ethylamino)-9,10-anthraquinone (AEAQt),50 and the triad 4,4′-dimethyl-4″-(4-(4-chloro-6-(2-(9,10-dioxoanthracenB


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The Journal of Physical Chemistry C Table 1. Electrochemical Data Obtained vs Ag/Ag+ in Dichloromethane and Frontier Orbital Energies compounds

E1/22−/3− (V)

E1/2−/2− (V)

−2.15

−1.65 −1.18 −1.15

a

MTPA-TRC MTPA-TRC-AEAQa PDIt PBIt MTPA-TRC-BA MTPA-TRC-PDI MTPA-TRC-PBI

−1.20 −1.17

E1/20/− (V)

E1/2+/0 (V)

−1.19 −1.32 −0.88 −0.95 −1.23 −0.88 −0.94

0.55 0.49 1.52 1.24

E1/22+/+ (V) 0.67

0.50 0.50

1.23

EHOMOb (eV)

ELUMOb (eV)

−5.48 −5.42 −6.45 −6.17

−3.74 −3.61 −4.05 −3.98 −3.70 −4.05 −3.99

−5.43 −5.43

EHOMO−1 (eV)

ELUMO+1d (eV)

−5.66

−2.68 −2.62

−6.31c −6.16e

−2.63 −2.63

The data were taken from ref 50 for comparison purposes. bEHOMO = −E1/2+/0 − 4.93 eV; ELUMO = −E1/20/− − 4.93 eV. cEHOMO(EHOMO−1) = ELUMO − EgopPDI, the optical band gap (EgopPDI = 2.26 eV) was estimated from the onset of the absorption band of PDI module in toluene. dELUMO+1 = EHOMO + EgopMTPA, the optical band gap (EgopMTPA = 2.80 eV) was estimated from the onset of the ICT absorption band of MTPA module in toluene. eEHOMO−1 = −E1/22+/+ − 4.93 eV. a

perpendicularly through a 1 cm quartz cell. The complete timeresolved spectra were obtained using a gated CCD camera (AndoriSTAR); the kinetic traces were detected by a Tektronix TDS 3012B oscilloscope and a R928P photomultiplier and analyzed by Edinburgh analytical software (LP920). The samples used in the flash photolysis experiments were bubbled with argon for 30 min before measurements. The compound concentrations were 1 × 10−5 mol/L. 2.9. Computations. Geometry optimizations of the molecules were carried out using the three-parameter exchange functional of Becke and the correlation functional of Lee, Yang, and Parr (B3LYP)53−55 with the 6-31G(d,p) basis set in toluene without any symmetry constraints. The polarized continuum model (PCM) framework56 was used to describe the solvent effect. The SCF convergence was 10−8 a.u.; the gradient and energy convergence were 10−4 and 10−5 a.u., respectively. All of the calculations were performed using the Gaussian 09 package.57 In order to confirm the optimized geometry as a global minimum, frequency calculations at the same level of theory were performed. A detailed description of the computational methods can also be found in our previous studies.58

spectrometer. Quartz cells with a path length of 1 cm were used to observe absorption in the UV region. 2.5. Fluorescence Spectroscopy in Solution. The fluorescence spectra were recorded on a Varian CARY ECLIPSE fluorospectrophotometer. 2.6. Cyclic Voltammetry. The electrochemical properties were measured using a BAS 100 W electrochemical analyzer utilizing the three-electrode configuration with a glassy carbon electrode as the working electrode, Ag/AgNO3 electrode as the reference electrode, and platinum as the auxiliary electrode. The analyzer was calibrated using a ferrocene/ferrocenium redox couple as the external standard prior to the measurements. The scan rate was set to 30 mV/s. Dichloromethane containing 0.1 mol/L tetra-butylammonium hexafluorophosphate (TBAPF6) was employed as the medium for the cyclic voltammetric determination. The compound concentration was 5 × 10−3 mol/L. 2.7. Time-Correlated Single Photon Counting (TCSPC). Excitation of samples was done with picosecond diode lasers (Horiba JobinYvon Instruments) at 366 nm (1.2 ns pulses) or 457 nm (1.2 ns pulses). The laser pulse energy was ca. 15 pJ and attenuated (often more than an order of magnitude) to the desired count rate of ca. 1% or less of the excitation frequency. A cooled (ca. −40 °C) Hamamatsu MCP photomultiplier R3809U 51 was used for detection of single photons, and the signal passed through a discriminator (Ortec 9307) and into a TAC (Ortec 566, 100 ns range used). The electrical trigger signal from the laser was also passed through a discriminator (Tennelec TC454) and onto the TAC (Ortec 566). The TAC output was read by a DAQ-1 MCA computer card using 1024 channels and collected with HoribaJobinYvon Data Station 2.5. Measurements were made in reverse mode at 5 MHz and under magic angle polarization. A cutoff filter, GG400 (excitation at 366 nm) or GG 515 (excitation at 457 nm), was used to block stray excitation light. A dilute solution of Ludox was used to record the instrument response function without any filter for solution measurements. No monochromator was used; i.e., all wavelengths transmitted by the cutoff filter were collected. The sample concentrations were 5 × 10−6 mol/L, and the solutions were bubbled with argon for 30 min before the measurements. 2.8. Nanosecond Transient Absorption Spectroscopy. Nanosecond transient absorption measurements were performed on a LP-920 laser flash photolysis setup (Edinburgh). Excitation at 420, 525, and 545 nm with a power of 2.0 mJ per pulse from a computer-controlled Nd:YAG laser/OPO system from Opotek (Vibrant 355 II) operating at 10 Hz was directed to the sample. The laser and analyzing light beams pass

3. RESULTS AND DISCUSSION 3.1. Determination of Frontier Orbital Energies for Electron and Hole Transfer in MTPA-TRC-PDI and MTPATRC-PBI. Cyclic voltammetry was used to measure the electrochemical curves of MTPA-TRC-PDI and MTPA-TRCPBI and their monomeric component parts. The electrochemical measurements are shown in Figures S1 and S2 of the Supporting Information. The oxidation and reduction potentials as well as the frontier orbital energies derived from these measurements are summarized in Table 1. For comparison purposes, we start by summarizing the electrochemical measurements for the MTPA-TRC-AEAQ system, though the data were previously reported.50 There were two reversible oxidation waves and three reversible reduction waves (Figure S1 in the Supporting Information). The E1/2+/0 (0.49 V) corresponds to the oxidation potential of the MTPA module (0.50 V), while the E1/20/− and E1/2−/2− (−1.32 and −1.65 V) pertain to the reduction potentials of the AEAQ module (−1.32 and −1.77 V). The E1/22+/+ (0.67 V) is assigned to the oxidation potential of the AEAQ module because the corresponding orbital energy (−5.60 eV) is close to the HOMO energy of AEAQ (−5.66 eV) that was calculated from the optical bandgap (2.05 eV). Similarly, the E1/22−/3− (−2.15 V) is assigned to the reduction potential of the MTPA module for similar orbital energy (−2.78 eV) with the LUMO C


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The Journal of Physical Chemistry C +1 energy of the MTPA module (−2.62 eV) obtained from the optical bandgap. For MTPA-TRC-PDI, the electrochemical curve (Figure S2 of the Supporting Information) shows one reversible oxidation wave at 0.50 V and two reversible reduction waves at −0.88 and −1.20 V. For MTPA-TRC-PBI, it exhibits two reversible oxidation waves (at 0.50 and 1.23 V) and two reversible reduction waves at −0.94 and −1.17 V (Figure S2 of the Supporting Information).The E1/2+/0 in both the MTPA-TRCPDI and MTPA-TRC-PBI systems corresponds to the oxidation potential of the MTPA module. The E1/22+/+ in MTPA-TRC-PBI is assigned to the oxidation potential of the PBI module, which is also shown in compound PBIt at 1.24 V. In the two new triads synthesized here, MTPA-TRC-PDI and MTPA-TRC-PBI, the reduction waves of the TRC module were covered by those of the PDI or PBI module. To measure this orbital energy, we used a strategy developed for the MTPATRC-AEAQ system.50 Here we synthesized MTPA-TRC-BA so that the LUMO energy of the TRC module can be evaluated by the reduction potential of MTPA-TRC-BA. Combining with the computational results shown in Figures S3−S5 of the Supporting Information, the energy diagram of the frontier orbitals that facilitate electron and hole transfer is depicted in Figure 1.

Figure 2. Absorption of MTPA-TRC (blue), PDIt (cyan), the equimolar mixture of MTPA-TRC and PDIt (orange), and MTPATRC-PDI (gray) in toluene (concentration: 5 × 10−6 mol·L−1).

Figure 3. The emission spectrum of the equimolar mixture of MTPA-TRC and PDIt is identical to the sum of the spectra of MTPA-TRC and PDIt. This indicates that the intermolecular

Figure 1. Orbital energy levels of MTPA-TRC-AEAQ (from ref 50), MTPA-TRC-PDI, and MTPA-TRC-PBI determined electrochemically. The red and blue lines indicate the observed electron and hole transfers, respectively.

As shown in Figure 1, the excited electron in the MTPA moiety can migrate to the TRC module followed by electron transfer to the AEAQ (or PDI/PBI) module. On the basis of the experimental observation and computational results, all three triads have a typical D-A1-A2 architecture with cascade energy levels and should lead to the consecutive electron transfers. Additionally, the lower HOMO level of the second acceptor module than that of the MTPA module indicates a possible hole transfer process from the excited AEAQ (or PDI/ PBI) to the MTPA module, thus making them ambipolar. 3.2. Absorption and Fluorescence Spectra of MTPATRC-PDI. The absorption spectra of MTPA-TRC, PDIt, MTPA-TRC-PDI, and the equimolar mixture of MTPA-TRC and PDIt in toluene were obtained and are shown in Figure 2. The spectrum of MTPA-TRC-PDI is nearly identical to the sum of the spectra of MTPA-TRC and PDIt and that of the equimolar mixture of MTPA-TRC and PDIt. This illustrates that the inter- and intramolecular interaction between the MTPA and PDI moiety is negligible. The fluorescence emission spectra of MTPA, MTPA-TRC, PDIt, MTPA-TRC-PDI, and the equimolar mixture of MTPATRC and PDIt in toluene were obtained and are depicted in

Figure 3. Fluorescence emission spectra excited at 390 nm (a) and 491 nm (b) of MTPA (green), MTPA-TRC (blue), PDIt (cyan), the equimolar mixture of MTPA-TRC and PDIt (orange), and MTPATRC-PDI (gray) in toluene (concentration: 5 × 10−6 mol·L−1). D


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The Journal of Physical Chemistry C nonbonding interaction between both modules can be ignored in the excited states as well. Similar single-exponential decay behaviors of the perylene bisimide fluorescence59,60 (Table 2 and Figure S6) further confirm this. Table 2. Emission Lifetimes and Fractions (in Parentheses) of MTPA-TRC, PDIt, the Equimolar Mixture of MTPA-TRC and PDIt, MTPA-TRC-PDI, PBIt, the Equimolar Mixture of MTPA-TRC and PBIt, and MTPA-TRC-PBI in Toluene by Fitting Transient Spectra with Exponential Decay Equations emission lifetime, τ/ns (fraction %) compound

λex = 366 nm, λem = 460 nm

MTPA-TRCa

MTPA-TRC-PDI

1.71 (61.2) 0.30 (38.8) 1.55(51.8) 0.97(48.2)

PBIt MTPA-TRC and PBIt MTPA-TRC-PBI a

λex = 457 nm, λem = 574 nm

1.73(60.3) 0.29(39.7)

PDIt MTPA-TRC and PDIt

λex = 366 nm, λem = 574 nm

1.72(61.0) 0.29(39.0)

4.20

4.21

4.19

4.21

0.33(31.4) 3.24(68.6) 4.30

0.37(30.3) 3.26(69.7) 4.31

4.28

4.29

0.63(55.9) 4.11(44.1)

0.64(57.1) 4.07(42.9)

Data taken from ref 50.

As shown in Figure 3a, the emission intensity of the MTPA module in MTPA-TRC-PDI decreased by 98% compared to the reference compound MTPA when it was excited at 390 nm, where the MTPA module has a strong absorption but the PDI module has a weak absorption. For MTPA-TRC-PDI, its fluorescence spectrum shows a drastic fluorescence quench (∼95%) in comparison with PDIt (Figure 3b) upon excitation at 491 nm, where only the PDI module was excited. This indicates that there is a photoinduced through-bond energy dissipative pathway. The time-resolved emission spectra of MTPA-TRC-PDI at 460 nm (Figure S6 of the Supporting Information) reveal that the fluorescence from the MTPA module follows a biexponential decay process with lifetimes of 0.97 and 1.55 ns (Table 2). The fast component is due to the electron transfer from the MTPA to TRC module coupled with an internal charge transfer (ICT) within the MTPA moiety and followed by an electron transfer to the PDI module. We note that the lifetime of the slow component, which is attributed to the solvation relaxation of MTPA singlets, is clearly shorter than that in MTPA-TRC (Table 2). This is due to the intramolecular photoinduced Forster energy transfer,61,62 where the energy transfer is possible from the excited MTPA to the PDI module as the fluorescence emission spectrum of MTPA partially overlaps with the absorption spectrum of PDI. The fluorescence at 574 nm from the PDI module decays biexponentially with lifetimes of 0.37 and 3.26 ns. Combining with the steady-state fluorescence results, we assigned the fast process as the intramolecular photoinduced hole transfer from the excited PDI to the MTPA and the slow component as the intersystem crossing from singlet to triplet state. Figure 4a shows the transient absorption spectra of MTPATRC-PDI excited at 525 nm. The initial spectrum is mainly due to the 1PDI*; then, it evolves in time to the spectrum with the presence of the charge separated state. Each of the remaining

Figure 4. Nanosecond transient absorption spectra (a) and transient absorption kinetics at 705 nm (b) and 500 nm (c) of MTPA-TRCPDI (1 × 10−5 mol·L−1) in toluene following excitation with 525 nm, 8 ns laser pulses. The red lines in parts b and c are the fitting curves by the single-order exponential decay equation.

three spectra in Figure 4a is composed of ground-state bleaches of PDI at 452, 488, and 524 nm and MTPA at 380 nm as well as the appearance of an absorption at 705 nm due to the formation of PDI−.63 The characteristic absorption feature of 3PDI* appears at 500 nm64 as a sharp peak derived from intersystem crossing of 1 PDI*. As time progresses, the perylene bisimide radical anion at 705 nm disappears and the typical absorption of perylene bisimide triplet states at 500 nm grows. These findings reveal that the disappearance of charge separated states, MTPA•+TRC-PDI•−, is due to the relaxation to triplet states. The kinetic analysis of this process (Figure 4b) gives a time constant of 22 ns for the charge recombination. The transient absorption spectra excited at 420 nm (Figure S7 of the Supporting Information) exhibit the absorption features of PDI radical anion at 705 nm, which was unaffected by the introduction of the oxygen. The lifetime of this charge separated state, MTPA•+-TRC-PDI•‑, under this excitation was also estimated to be 22 ns. In an even longer time scale, the perylene bisimide E


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The Journal of Physical Chemistry C triplet decays to the ground state and the kinetic analysis (Figure 4c) gives a time constant of 827 ns. 3.3. Absorption and Fluorescence Spectra of MTPATRC-PBI. The absorption spectra of MTPA-TRC-PBI, PBIt, and MTPA-TRC along with their equimolar mixture of MTPATRC and PBIt in toluene are provided in Figure 5. Compared

Figure 5. Absorption of MTPA-TRC (blue), PBIt (purple), the equimolar mixture of MTPA-TRC and PBIt (pink), and MTPA-TRCPBI (olive) in toluene (concentration: 5 × 10−6 mol·L−1).

to PDIt, the maximum absorption peak of PBIt becomes broadened and is shifted from 530 nm in PDIt to 545 nm due to the introduction of two electron-donating p-tert-butylphenoxy groups onto the position of 1- or 7- of the PDI module.65 The broadening and loss of detailed vibronic structure in the absorption bands of PBIt shown in Figure 5 are attributed to both the participation of the side groups in the conjugation of the perylene ring and the twisting in molecular conformation of the perylene ring because of the 1,7-disubstitution. The absorption peaks of MTPA-TRC-PBI appear at 545, 511, and 389 nm. The two former peaks are attributed to the PBI module and the last one to the MTPA module as compared with the spectra of the components MTPA-TRC and PBIt. Similar to the MTPA-TRC-PDI system, the inter- and intramolecular interactions between the MTPA and PBI module are negligible because the absorption spectrum of MTPA-TRC-PBI is almost identical to the sum of the spectra of MTPA-TRC and PBIt and that of the equimolar mixture of MTPA-TRC and PBIt. The steady-state and time-resolved fluorescence results of the equimolar mixture of MTPA-TRC and PBIt (Figure 6 and Figure S8 of the Supporting Information) are also identical to the sum of the spectra of MTPA-TRC and PBIt. This indicates that the intermolecular nonbonding interaction between the modules is very weak even in the excited states. We mention that a red shift from 536 nm in PDIt to 577 nm in PBIt was observed. In addition, the singlet lifetime of PBIt (4.31 ns) is longer than that of PDIt. All of these can be explained by the electronic and steric effects of the substituents at the bay position of the perylene ring. Upon excitation at 390 nm (Figure 6a), where the MTPA has a strong absorption whereas the absorption of perylene is weak, the fluorescence of the MTPA module of MTPA-TRC-PBI is almost entirely quenched in comparison with MTPA. The

Figure 6. Fluorescence emission spectra excited at 390 nm (a) and 491 nm (b) of MTPA (green), MTPA-TRC (blue), PBIt (purple), the equimolar mixture of MTPA-TRC and PBIt (pink), and MTPA-TRCPBI (olive) in toluene (concentration: 5 × 10−6 mol·L−1).

transient absorption spectra excited at 420 nm (Figure S9 of the Supporting Information) exhibit the absorption features of PBI radical anion at 730 nm, and the lifetime of the charge separated state, MTPA•+-TRC-PBI•−, was estimated to be 75 ns. When MTPA-TRC-PBI was excited at 491 nm (Figure 6b), where only the PBI module can be excited, the emission intensity from the perylene moiety was quenched by about 90% compared to PBIt. This suggests that there are other photoinduced through-bond energy dissipative pathways for the excited PBI module to decay. The fluorescence at 574 nm from the PBI module decays biexponentially with lifetimes of 0.64 and 4.07 ns (Table 2). The fast process is attributed to the intramolecular photoinduced hole transfer from excited perylene to MTPA and the slow component to the intersystem crossing from singlet to triplet state. The transient absorption spectra excited at 545 nm are shown in Figure 7a. The PBI ground-state bleaches occur at 498 and 538 nm, the MTPA ground-state bleaches occur at 380 nm, the PBI•− absorption occurs at 715 nm, and the absorption of 3PBI* appears at 475 nm.66,67 Similar to the MTPA-TRCPDI system, as time goes by, the perylene bisimide radical anion at 715 nm reduces to an almost disappearance, while the typical absorption of the perylene bisimide triplet state at 475 F


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The Journal of Physical Chemistry C

absorption intensity of the AEAQ module is between those of AEAQ (503 nm) and AEAQt (497 nm). This indicates the existence of certain electronic and steric effects between both modules. However, these effects did not change the nature of various transitions, as evidenced by similar shapes of the corresponding peaks. The emission spectrum of MTPA-TRC-AEAQ is identical to the sum of the spectra of MTPA-TRC and AEAQt (Figure S11 of the Supporting Information). The fluorescence from the mixture still follows a two-order decay equation with similar lifetimes to those of pure MTPA-TRC and AEAQt (Table S1 and Figure S12 of the Supporting Information). This illustrates little changes in the photophysical processes induced by the intermolecular interaction. Accordingly, the fast decay component (0.29 ns) of the fluorescence of MTPA-TRC was assigned as the electron transfer from the MTPA to TRC module, whereas the slow process (1.73 ns) was attributed to the solvation relaxation of the MTPA singlet. The fast component (0.47 ns) in the emission of AEAQt was attributed to the solvation relaxation of anthraquinone and the slow one (4.90 ns) to the singlet−triplet conversion. When MTPA-TRC-AEAQ was excited at 490 nm (Figure S11a of the Supporting Information), the AEAQ module showed mild absorption while little absorption was found for the MTPA module. Therefore, only emissions from the AEAQ module were found for the triad and the fluorescence was quenched by about 46% with respect to bare AEAQ, and the fluorescence at 700 nm from the AEAQ module decays biexponentially with lifetimes of 0.33 and 4.33 ns. The fast component was attributed to the photoinduced hole transfer from the excited AEAQ to MTPA module to form the charge separated state MTPA•+-TRC-AEAQ•− that the lifetime was estimated to be 680 ns.50 The slow component was assigned to the singlet−triplet conversion, also shown in AEAQt. Upon excitation at 380 nm (Figure S11b of the Supporting Information), an obviously fluorescence quench (∼98%) was detected for MTPA-TRC-AEAQ in comparison to MTPA. The time-resolved emission spectra of MTPA-TRC-AEAQ at 460 nm (Figure S8) revealed that the fluorescence from the MTPA module also follows biexponential decay with lifetimes of 1.61 and 0.70 ns (Table S1). The fast decay component is due to the electron transfer from MTPA to TRC followed by electron transfer to the AEAQ module. The slow component, which is attributed to the solvent relaxation of the MTPA singlet, is shorter than that of MTPA-TRC (1.73 ns). This is due to the intramolecular through-bond photoinduced Forster energy transfer from the excited MTPA to AEAQ module, which was further confirmed by the phenomenon that a weak characteristic fluorescence from the AEAQ module emerges concurrently at above 579 nm, whereas there is little absorption and fluorescence for either AEAQt or AEAQ. The lifetime of the charge separated state, MTPA•+-TRC-AEAQ•−, generated through this consecutive electron transfer was estimated to be 650 ns,50 similar to the value obtained by photoinduced hole transfer. All three triads, MTPA-TRC-AEAQ, MTPA-TRC-PDI, and MTPA-TRC-PBI, exhibit cascade energy levels, as shown in Figure 1, allowing a sequential electron transfer from the LUMO of the MTPA module to the LUMO of the TRC module and then to the LUMO of the AEAQ (or PDI/PBI) module. Furthermore, the HOMO level of the AEAQ (PDI/ PBI) module is lower than that of the MTPA module,

Figure 7. Nanosecond transient absorption spectra (a) and transient absorption kinetics at 715 nm (b), 475 nm (c) of MTPA-TRC-PBI (1 × 10−5 mol·L−1) in toluene following excitation with 545 nm, 8 ns laser pulses. The red lines in parts b and c are the fitting curves by the single-order exponential decay equation.

nm grows. Kinetic data monitored at 715 nm (Figure 7b) gives a time constant of 75 ns for the charge recombination process of MTPA•+-TRC-PBI•−, and the further decay to 3PBI* at 475 nm (Figure 7c) appears to be monoexponential as well with a lifetime of 29.2 μs. 3.4. Decay Pathways of Charge Separated States in MTPA-TRC-AEAQ, MTPA-TRC-PDI, and MTPA-TRC-PBI. Before we derive and compare the decay pathways and lifetime of three D-A 1-A 2 systems, we describe here the key spectroscopic and kinetics data of the MTPA-TRC-AEAQ system that were obtained previously but not analyzed in detail. Similar to the MTPA-TRC-PBI and MTPA-TRC-PBI systems, the comparison between the absorption spectra of MTPATRC-AEAQ and the equimolar mixture of MTPA-TRC and AEAQt (Figure S10 of the Supporting Information) indicates the inter- and intramolecular interactions between both modules are very weak. In MTPA-TRC-AEAQ, the wavelength maxima (386 nm) and absorption intensity of the MTPA module are between those of MTPA (376 nm) and MTPATRC (387 nm), while the characteristic band (500 nm) and G


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electron transfer decay to the formation of PBI singlet cannot be obtained experimentally using transient fluorescence spectra due to the near complete fluorescence quenching of the MTPA module. We estimate that this value is about 48%, as it has similar photophysical processes to those of MTPA-TRC-PDI. Therefore, a total of about 77% of MTPA singlets convert into the charge separated state in MTPA-TRC-PBI. Herein, all three values (84% for MTPA-TRC-AEAQ, 68% for MTPA-TRCPDI, and about 77% for MTPA-TRC-PBI) are far higher than that of MTPA-TRC (39.7%). This illustrates that the formation of cascade energy levels shown in MTPA-TRC-AEAQ, MTPATRC-PDI, and MTPA-TRC-PBI is better for creating charge separation despite the decrease in the photoinduced electron transfer rates due to the introduction of the second acceptor. It should be mentioned that three triads have greatly different charge separated state lifetimes. MTPA-TRC-AEAQ shows an elongated charge separated state lifetime, 650 ns, which is about 8 times of that of MTPA-TRC (80 ns), whereas the charge separated state lifetimes of MTPA-TRC-PDI and MTPA-TRCPBI were determined to be 22 and 75 ns, respectively. They are shorter than that of MTPA-TRC. This is due to the presence of the triplet state of the second acceptor. As shown in Figure 8a, the energy of the MTPA-TRC-AEAQ charge separated state (MTPA•+-TRC-AEAQ•−, 1.52 eV) is below that of the AEAQ triplet state (MTPA-TRC-3AEAQ*, 1.89 eV). In contrast, MTPA-TRC-PDI and MTPA-TRC-PBI have a higher energy of the charge separated state (MTPA•+-TRC-PDI•−, 1.24 eV; MTPA•+-TRC-PBI•−, 1.26 eV) than the triplet state (MTPATRC-3PDI*, 1.23 eV; MTPA-TRC-3PBI*, 1.17 eV). Thus, the conversion from the charge separated state to the triplet state is allowed for MTPA-TRC-PDI and MTPA-TRC-PBI, which is forbidden for MTPA-TRC-AEAQ, leading to a faster decaying rate for MTPA•+-TRC-PDI•− and MTPA•+-TRC-PBI•− than MTPA•+-TRC-AEAQ•−. Studies of D−A systems reveal that the lifetime of charge separated states could be affected by the relative height of energy levels between the charge separated states and triplet states.70−76 The difference in energy levels of various states in PBI and PDI systems resulted in the difference in lifetime. The rate constant of photoinduced electron transfer from 1 MTPA* to AEAQ (kET) is larger than the rate constant of energy transfer from 1MTPA* to 1AEAQ* combined with intersystem crossing (ISC) of 1AEAQ* (kEnT + kISC), and the rate constant of photoinduced hole transfer from 1AEAQ* to MTPA (kHT) is also larger than the ISC rate constant of 1 AEAQ* (kISC). The small rate constant of back electron transfer (kBET = 1.5 × 106 s−1) determines the lifetime of the charge separated state, and it was estimated to be about 650 ns. The driving force (−ΔG) of back electron transfer (1.52 eV) is obviously larger than that of photoinduced electron/hole transfer (1.15/0.59 eV, respectively); however, the value of kBET is much smaller than that of kET or kHT. This suggests that the back electron transfer is deep in the Marcus inverted region.75,77 Similar data were found for kET, kEnT, kHT, and kISC of MTPA-TRC-PDI (Figure 8b), but the rate constant in MTPA-TRC-PDI (kBET = 4.5 × 107 s−1) is much larger than that in MTPA-TRC-AEAQ (kBET = 1.5 × 106 s−1). This is due to the energy level of the charge separated state (MTPA•+TRC-PDI•−, 1.24 eV) being slightly higher than that of the triplet state (MTPA-TRC-3PDI*, 1.23 eV), thus providing an efficient conversion from the charge separated state to the triplet state, which was confirmed in the analysis of transient absorption spectra. The back electron transfer in MTPA-TRC-

corresponding to the observed hole transfer from the excited AEAQ (PDI/PBI) module to the MTPA module. The energy diagram and photophysical processes were therefore constructed and are shown in Figure 8 with singlet

Figure 8. Energy diagram and photophysical processes of MTPATRC-AEAQ (a), MTPA-TRC-PDI (b), and MTPA-TRC-PBI (c) upon excitation in toluene.

energies from the fluorescence spectra, AEAQ triplet energy from literature data,68,69 PDI/PBI triplet energy from our computational results, and the energies of charge separated states estimated by the offset of the HOMO and LUMO levels of the corresponding compounds. As shown in Figure 8, for MTPA-TRC-AEAQ, about 22% of excited singlet MTPA decays to the electron transfer pathway, and about 78% converts to AEAQ singlets, of which about 80% undergo hole transfer to form the charge separated states. Therefore, upon excitation of MTPA, about 84% of MTPA singlets convert into the charge separated state. For MTPATRC-PDI, although initially about 48% of MTPA singlets follow an electron transfer decay pathway, only 68% of MTPA singlets in total convert into the charge separated state. For MTPA-TRC-PBI, the ratio of MTPA singlets following H


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that the vast difference in the lifetime of the charge separated state is due to the triplet state. The process from the charge separated state to the triplet state is energetically forbidden for MTPA-TRC-AEAQ and resulted in the direct decay of the charge separated state to the ground state with a lifetime as long as 650 ns. For MTPA-TRC-PDI, the efficient conversion from charge separated state to triplet state benefits the back electron transfer but leads to a relatively short charge separated state lifetime. Furthermore, the back electron transfer rate is another key factor to the charge separated state lifetime even in the Marcus normal region. The shorter lifetime in MTPA-TRC-PDI can be tailored by introducing the electron-donating p-tert-butylphenoxy groups to the bay position of the perylene bisimide moiety. Indeed, the lifetime of the charge separated state MTPA•+-TRC-PBI•− was found to be 75 ns, which is more than 3 times that of MTPA•+TRC-PDI•−. Therefore, tuning the final acceptors to control a lower energy level of charge separated state than that of the triplet state of the second acceptor can generate long-lived charge separated states in the designing of ambipolar organic semiconductors with cascade energy levels. To demonstrate their photovoltaic performances as new multichromophore materials for organic photovoltaics, work on device construction using these materials is in progress.

PDI lies at the Marcus normal region owing to the little driving force (0.01 eV), making the relatively larger value of kBET in MTPA-TRC-PDI than that in MTPA-TRC-AEAQ. Therefore, the lifetime of MTPA•+-TRC-PDI•− is just 22 ns, which is shorter than that of MTPA•+-TRC-AEAQ•−. Experimentally, the energies of excited triplet perylene bisimides were found to be around 1.20 eV higher than the ground state for PDI64,78−80 and PBI.66,67,70 This agrees with our calculated values, 1.23 eV for PDI and 1.17 for PBI (Figures S13 and S14 of the Supporting Information). The introduction of the two electron-donating p-tert-butylphenoxy groups to PDI to form PBI clearly reduced the energy level of singlet excited states of the perylene bisimide module from 2.32 eV in MTPATRC-PDI to 2.15 eV in MTPA-TRC-PBI. The energy of the charge separated state in MTPA-TRC-PBI (1.26 eV) is only a bit higher than that in MTPA-TRC-PDI (1.24 eV). However, kBET (1.3 × 107 s−1) and kT (3.4 × 104 s−1) were affected more than kHT (1.6 × 109 s−1) and kISC (2.4 × 108 s−1) by the p-tertbutylphenoxy group in MTPA-TRC-PBI. kBET reduced by about 1/3 as well as kHT reduced by about 2/3 in MTPA-TRCPBI when compared to those in MTPA-TRC-PDI, respectively. This demonstrates that the back electron transfer rate plays a key role in the charge separated state lifetime even though the process is in the Marcus normal region. Moreover, the larger difference between kBET and kHT is shown in MTPA-TRC-PBI with respect to MTPA-TRC-PDI. This reveals the positive effect of the additional p-tert-butylphenoxy group on the electron-donating and of spatial properties in generating longlived charge separated states. As a result, the lifetime of MTPA•+-TRC-PBI•− is 75 ns, which is longer than 22 ns found in MTPA•+-TRC-PDI•−. Another influence from the additional p-tert-butylphenoxy group is the enormous reduction of kT in MTPA-TRC-PBI (3.4 × 104 s−1), compared with that in MTPA-TRC-PDI (1.2 × 106 s−1). Indeed, the lifetime of the triplet state is prolonged from 827 ns in MTPA-TRC-PDI to 29.3 μs in MTPA-TRC-PBI. Meanwhile the photoinduced intramolecular electron and energy transfer are expected to occur in MTPA-TRC-PBI. However, the current transient fluorescence spectra did not allow us to obtain kET and kEnT.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b01321. Synthetic procedures, electrochemical curves of the compounds, B3LYP/6-31G(d,p) optimized structures with molecular orbitals and the corresponding energies of the compounds, additional absorbance and fluorescence curves of the compounds, and nanosecond transient spectra of additional compounds (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86-133-899-37396. *E-mail: [email protected]. Phone: +01-618-453-6476.

4. CONCLUSIONS To understand and provide insight into the photophysical processes of organic semiconductors with a D-A 1 -A 2 architecture, we designed, synthesized, and characterized two new systems here: MTPA-TRC-PDI and MTPA-TRC-PBI. The photophysical processes of these systems were investigated using steady-state and transient absorption and fluorescence spectra as well as electrochemical measurements and computational calculation. The new systems were shown to have the typical D-A1-A2 chromophoric structure with D, A1, and A2 at MTPA, TRC, and the second acceptor (AEAQ, PDI, or PBI), respectively. This allows a sequential electron transfer from MTPA to TRC and then to the AEAQ (PDI/PBI) module. Furthermore, the lower HOMO level of the second acceptor (AEAQ, PDI, or PBI) than that of the MTPA module allows the hole transfer from the excited AEAQ (or PDI/PBI) to the MTPA module. The efficiency to charge separated states was improved in the three triads with respect to the D-A1 system. Different lifetimes of charge separated states were obtained for MTPA•+-TRC-AEAQ•− (650 ns), MTPA•+-TRC-PDI•− (22 ns), and MTPA•+-TRC-PBI•− (75 ns) due to the relative energy levels between the charge separated state and the triplet state of the second acceptor. The photophysical results showed

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Nature Science Foundation of China (Grant No. 21576195 and 21506151). L.W. acknowledges the support by the Tianjin 1000 talent program for her stay at Tianjin University.

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REFERENCES

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