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Enhancing Photoinduced Charge Separation through Donor Moiety in Donor-Accepter Organic Semiconductors Tianyang Wang, Chuanwu Zhao, Linli Zhang, Ting Lu, Haiya Sun, Chelsea N. Bridgmohan, Krishanthi Chandima Weerasinghe, Dongzhi Liu, Wenping Hu, Wei Li, Xueqin Zhou, and Lichang Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09352 • Publication Date (Web): 19 Oct 2016 Downloaded from http://pubs.acs.org on October 25, 2016

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The Journal of Physical Chemistry

Enhancing Photoinduced Charge Separation through Donor Moiety in Donor-Accepter Organic Semiconductors Tianyang Wang,†,‡,§ Chuanwu Zhao,†,‡ Linli Zhang,†,‡ Ting Lu,†,‡ Haiya Sun,†,‡ Chelsea N. Bridgmohan,§ Krishanthi C. Weerasinghe,§ 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 and the Materials Technology Center, Southern Illinois University, Carbondale, IL 62901, United States; ∥Tianjin Engineering Research Center of Functional Fine Chemicals, Tianjin, 300072, China, and ⊥School of Science, Tianjin University, Tianjin 300072, China ‡

Abstract Three systems were designed, synthesized, and characterized to understand decay processes of photoinduced charge separation in organic semiconductors that are imperative for efficient solar energy conversion. A styrene based indoline derivative (YD) was used as D (donor moiety), a triazine derivative (TRC) as A1 (the first acceptor), and 9, 10-anthraquinone (AEAQ) as A2 (a second acceptor) in constructing two systems, YD-TRC and YD-TRC-AEAQ. The lifetime of the photoinduced charge separated states in YD-TRC, a D-A1 system, was found to be 215 ns and that in YD-TRC-AEAQ, a D-A1-A2 system, to be 1.14 µs, a fivefold increase with respect to the YD-TRC. These results show that YD is a more effective donor in D-TRC-AEAQ systems at forming long-lived charge separated states than the previously reported MTPA that generated charge separated states with a lifetime of 80 ns in MTPA-TRC and 650 ns in MTPA-TRC-AEAQ). The third system was constructed using a metal-free porphyrin derivative (MHTPP) to form a MHTPP-TRC-AEAQ structure, a D-L(linker)-A system with a charge separation lifetime less than 10 ns. Therefore, the D-A1-A2 architecture is the best at generating long-lived charge separated states and thus is a promising materials design strategy for organic photovoltaics. *

Corresponding authors: Xueqin Zhou, e-mail: [email protected]; Phone: +86-133-899-37396.

Lichang Wang, e-mail: [email protected]. Phone: +1-618-453-6476.

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1. INTRODUCTION In recent years, many researchers have focused on the studies of photoinduced charge transfer processes and investigated their essential role in improving the efficiency of solar cell devices and other artificial photosynthetic systems.1-16 Generation of suitably long-lived charged states is critical in the formation of photocurrents and accelerating electron transfers in fuel synthesis.17-26 Motivated by the photosynthesis occurred in nature through cascades of energy and electron transfer, more supramolecular systems and arrays have been designed and constructed through various combinations of donors and acceptors.27-33 The relatively short-lived charged states in simple organic D-A (donor-acceptor) systems compared to those in nature indicated that a huge improvement is needed in the construction of organic compounds to produce an efficient device. Among the D-A systems are ambipolar organic materials, which consist of both photoexcited electron and hole transfers and can generate more effectively charge separated states.34-38 In contrast to the non-covalent or non-conjugated linkers such as amino acids39 and alkyl chains,6, 40 π-conjugated linkers in a D-π-A structure offer the through-bond electron transfer,41-46 where they likewise enhance the back electron transfer, which often results in faster charge recombination.20-23,

47-49

One

strategy to prevent the back electron transfer is the formation of the cascade energy levels through a donor-acceptor1-acceptor2 (D-A1-A2) system.32, 50-52 Recently, we reported three D-A1-A2 compounds with MTPA (a triphenylamine derivative) as a donor, TRC (a triazine derivative) as A1, and AEAQ (anthraquinone derivative), or two perylene bisimide derivatives (PDI and PBI) as A2 to investigate and understand the kinetics of three MTPA-TRC-A2 systems using an integrated experimental and computational methods.32, 53 A sequential two-step decay path was detected in both MTPA-TRC-PDI and MTPA-TRC-PBI, while a direct one-step decay was observed in MTPA-TRC-AEAQ system. The charge separated states generated in MTPA-TRC-PDI and MTPA-TRC-PBI have a 2


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lifetime of 22 ns and 75 ns, respectively. These charged states relax firstly to the triplet state then in 827 ns and 29.2 µs to the ground state of MTPA-TRC-PDI and MTPA-TRC-PBI, respectively.53 Our previous work showed that the charge separated state lifetime in MTPA-TRC-AEAQ system is extended to 650ns, about 8 times of that (80ns) in the MTPA-TRC, which consists of a simple D-A architecture.32 These illustrate the importance of the second acceptor in D-A1-A2 systems towards generating long-lived charge separation and indicate that it is worthwhile to further investigate additional D-A1-A2 systems. Furthermore, the construction and characterization of more D-A1-A2 systems will allow us to correlate the properties of these materials to their performance in solar cell devices, which are largely unknown, thus hindering the development of these materials for solar cell applications. As such, we set out to explore the effect of donor moiety in a D-A1-A2 architecture on the lifetime of charge separated states. In this work, we attempted to keep the A1-A2 components of our new D-A1-A2 systems the same as our previously reported MTPA-TRC-AEAQ system. Similar to the triphenylamine derivatives,54 metal-free porphyrin,55-57 and indoline58-60 compounds have been found to have excellent electron-donating and hole-migrating abilities and are widely applied as the electronic donors in many D-A systems. Hence, we chose YD (4-(p-tolyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indole) and MHTPP (5-(4-phenyl)-10,15, 20-tris(4-hexylphenyl)porphyrin) as donors in this work. We performed density functional theory (DFT) calculations to obtain the energetics of the modules, YD, TRC, AEAQ, and MHTPP. The DFT results indicated that YD-TRC can form a D-A1 system and YD-TRC-AEAQ can form a D-A1-A2 system. However, the LUMO (Lowest Unoccupied Molecular Orbital) energy of MHTPP is lower than that of TRC module, which indicates that a MHTPP-TRC cannot form a system with MHTPP as donor and TRC as acceptor.

The LUMO of MHTPP is higher than that of AEAQ, which indicates the 3



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MHTPP-TRC-AEAQ will form a D-L(Linker)-A architecture. Furthermore, the HOMO (Highest Occupied Molecular Orbital) energy of YD and MHTPP are both higher than that of AEAQ. This indicates that the newly constructed systems, YD-TRC-AEAQ and MHTPP-TRC-AEAQ, should also be ambipolar, just like MTPA-TRC-AEAQ.32 Structures of these three compounds are provided in Chart 1 together with MTPA-TRC and MTPA-TRC-AEAQ that were previously synthesized and examined.32 YD-TRC, YD-TRC-AEAQ, and MHTPP-TRC-AEAQ are, to the best of our knowledge, reported here for the first time and the synthetic routes are depicted in Scheme S1 and Scheme S2 in the Supporting Information. Furthermore, the photophysical characterizations of these compounds were made using steady state as well as transient UV-Vis and fluorescence spectrometers. The electronic energy levels of these systems were also studied using electrochemical measurements and DFT calculations. The results of these studies are provided in Section 3 with the methods described in Section 2. Finally, the conclusions are drawn in Section 4. N

H N

N

Cl

N N

H N

N

YD-TRC

N

MTPA-TRC

Cl

N

Cl

N N Cl

H N

H N

N N

N

NH

O

Cl

YD-TRC-AEAQ O

N

H N

H N

N N

NH

N

O

Cl (CH 2) 5CH 3

MTPA-TRC-AEAQ

NH

N H N

H3 C(H 2C)5 N

HN

H N

N N

O

N

NH

O

Cl

O (CH 2) 5CH3

MHTPP-TRC-AEAQ

Chart 1. Compounds YD-TRC, MTPA-TRC, YD-TRC-AEAQ, MTPA-TRC-AEAQ and MHTPP-TRC-AEAQ.

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2. EXPERIMENTAL AND COMPUTATIONAL METHODS Synthesis. 4-(4-methylphenyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]-indole-7-carbaldehyde (YDBc),61 Benzyl triphenylphosphonium bromide (BTB),62 nitrobenzyl triphenylphophon ium bromide (NTB),62 1-(2-aminoethylamino)anthraquinone (AEAQ),32 1-((4,6-dichloro1,3,5-triazin-2-ylamino)ethylamino)-9,10-anthraquinone (AEAQt),32 4,4’-dimethyl-4”-(4-(4, 6-dichloro-1,3,5-triazin-2-ylamino)styryl)triphenylamine (MTPA-TRC)32 (Chart S1) and t he triad 4,4’-dimethyl-4”-(4-(4-chloro-6-(2-(9,10-dioxoanthracen-1-ylamino)ethylamino)-1, 3,5-triazin-2-ylamino)styryl)triphenylamine (MTPA-TRC-AEAQ)32 were synthesized acco rding to the literature. We provide in Scheme S1-S2 the synthetic routes of 4-(4-meth ylphenyl)-7-styryl-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indole (YD), 4-(4-methylphenyl)-7 -(4-nitro-styryl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indole (YDn), 4-(4-methylphenyl)-7-( 4-amino-styryl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indole (YDa), 4-(4-methylphenyl)-7-( 4-(4,6-dichloro-1,3,5-triazin-2-ylamino)styryl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indole ( YD-TRC), 4-(4-methylphenyl)-7-(4-(4-chloro-6-ethylamino-1,3,5-triazin-2-ylamino)styryl)1,2,3,3a,4,8b-hexahydrocyclopenta[b]indole (YD-TRC-EA), 4-(4-methylphenyl)-7-(4-(4-chl oro-6-(2-(9,10-dioxoanthracen-1-ylamino)ethylamino)-1,3,5-triazin-2-ylamino)styryl)-1,2,3,3 a,4,8b-hexahydrocyclopenta[b]indole (YD-TRC-AEAQ), 5-(4-nitrophenyl)-10,15,20-tris(4hexylphenyl)porphyrin(MHTPPn), 5-(4-aminophenyl)-10,15,20-tris(4-hexylphenyl)porphyri n (MHTPPa), 5-(4-(3,5-dichlorotriazine)aminophenyl)-10,15,20-tris(4-hexylphenyl)porphyr in (MHTPPt), 5-(4-(3-Chloro-5-(aminoethylamino-anthraquinone)triazine)aminophenyl)-10, 15,20-tris(4-hexylphenyl)porphyrin (MHTPP-TRC-AEAQ). Reagents and solvents used i n this work were all reagent grade and only purified when necessary. All syntheses were done under dry nitrogen at one atmosphere unless specifically indicated. Mass Spectrometry. We obtained the ESI mass spectra using a Thermo Fisher mass spectrometer, LCQ Deca XP MAX and the MALDI-TOF mass spectra using a Bruker 5



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Autoflex tof/tofIII mass spectrometer. NMR Spectrometry. 1H NMR spectra were collected on a VARIAN INOVA 500MHz spectrometer. The experiments were done at room temperature. Cyclic Voltammetry. The electrochemical properties were taken using a BAS 100W electrochemical analyzer with a three-electrode configuration and a glassy carbon electrode as the working electrode, platinum as the auxiliary electrode and Ag/AgNO3 electrode as the reference electrode. The instrument was calibrated with a ferrocene/ferrocenium redox couple as the external standard and a scan rate of 30 mV/s was used. The dichloromethane containing 0.1 mol•L-1 tetra-butylammonium hexafluorophosphate (TBAPF6) was utilized as the medium for the cyclic voltammetric determination. The concentrations of measured compounds were 5×10-3 mol•L-1. Computational Details. Geometry optimizations of all the systems shown in chart 1 were carried out using B3LYP (the three-parameter exchange functional of Becke and correlation functional of Lee, Yang and Parr)63-65 with the 6-31G(d,p) basis set without any symmetry constraints. The polarized continuum model (PCM) framework66 was used to describe the effect of solvent, which was toluene in this work. The self-consistent field (SCF) convergence was set to be 10-8 a.u.. The gradient and energy convergence was set to be 10-4 a.u. and 10-5 a.u., respectively. All the calculations were done using Gaussian09 package.67 Frequency calculations were performed to confirm the optimized geometry to be a minimum. More detailed description of the computational methods can also be found in our previous work.54 UV-Vis Spectroscopy in Solution. We obtained the absorption spectra using the Thermo Spectronic, Helios Gamma spectrometer. Quartz cells have a path length of 1 cm. 6


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Fluorescence Spectroscopy in Solution. The fluorescence spectra were obtained using a Varian CARY ECLIPSE fluorospectrophotometer. Time-Correlated Single Photon Counting (TC-SPC). Excitation of the samples was made using picosecond diode lasers (NanoLED, Horiba Jobin Yvon Instruments) at 366 nm or 457 nm, and the instrument response factor was ~ 1500 ps, indicating the accuracy of the instrument was ~ 150 ps. The laser's pulse energy with ca. 15 pJ was attenuated 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 employed for detecting single photons. The signal first passed through a discriminator (Ortec 9307) and then to a TAC (Ortec 566, 100 ns range used). The electrical trigger signal passed through a discriminator (Tennelec TC454) and then on to the TAC (Ortec 566). The TAC output was recorded by a DAQ-1 MCA computer card using 1024 channels then collected with Horiba Jobin Yvon Data Station 2.5. Measurements were done in reverse mode at 5 MHz and under magic angle polarization. A cut-off filter, GG400 with an excitation at 366 nm or GG 515 with an excitation at 457 nm, was employed to block the stray excitation light. A dilute Ludox solution was used to record the instrument response function for solution measurements without any filter. No monochromator was employed, i.e. all wavelengths transmitted were recorded. The solution with a dye concentration of 5×10-6 mol•L-1 was bubbled using argon for 30 min before any measurements. Nanosecond Transient Absorption Spectroscopy. LP-920 laser flash photolysis Spectrometer (Edinburgh) was used to obtain the nanosecond transient absorption spectra. Excitation at 410 nm and 525nm was directed to the sample with a power of 2.0 mJ per pulse 7



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from a computer-controlled Nd:YAG laser/OPO system from Opotek (Vibrant 355 II) operating at 10 Hz. The laser and analyzing light beam pass perpendicularly through a 1 cm quartz cell. A gated CCD camera (AndoriSTAR) was used to obtain the time-resolved spectra. A Tektronix TDS 3012B oscilloscope and a R928P photomultiplier were used to detect the kinetic traces. The analysis was done using the Edinburgh analytical software (LP920). Before the measurements, the samples in the experiments were bubbled with argon for about 30 min. The sample concentrations were 1×10-5 mol•L-1. Spectroelectrochemical measurements. Absorption spectra were recorded on a PE Lambda 750 UV−Vis−NIR spectrophotometer at room temperature. Spectroelectrochemical measurements were performed in a thin layer cell (optical length = 0.2 cm), in which an ITO glass electrode (

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