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The Photophysical and Morphological Implications of Single-Strand Conjugated Polymer Folding in Solution Thomas J. Fauvell, Tianyue Zheng, Nicholas E. Jackson, Mark A. Ratner, Luping Yu, and Lin X. Chen Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b00734 • Publication Date (Web): 08 Apr 2016 Downloaded from http://pubs.acs.org on April 11, 2016
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The Photophysical and Morphological Implications of Single-Strand Conjugated Polymer Folding in Solution Thomas J. Fauvell,†,a,b Tianyue Zheng,†,c Nicholas E. Jackson, a Mark A. Ratner, a Luping Yu,*,c and Lin X. Chen*,a,b a
Department of Chemistry and the Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States
b
Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States th
c
Department of Chemistry and Jams Frank Institute, The University of Chicago, 929 East 57 Street, Chicago, Illinois 60637, United States
ABSTRACT: Organic semiconductors have garnered substantial interest in optoelectronics, but their device performances exhibit strong dependencies on material crystallinity and packing. In an effort to understand the interactions dictating the morphological and photophysical properties of a high-performing photovoltaic polymer, PTB7, a series of short oligomers and low molecular weight polymers of PTB7 were synthesized. Chain-length dependent optical studies of these oligomers demonstrate that PTB7’s low-energy visible absorption is largely due to self-aggregation-induced ordering, rather than in-chain charge transfer, as previously thought. By examining molecular weight and concentration dependent optical properties, supplemented by molecular dynamics simulations, we attribute polymeric PTB7’s unique mid-gap fluorescence and concentration independent absorption spectrum to an interplay between low molecular weight unaggregated strands, and high-molecular weight self-aggregated (folded) strands. Specifically, we propose that the onset of PTB7 self-folding occurs between 7-13 repeat units, but the aggregates characteristic of polymeric PTB7 only develop at lengths of ~30 repeat units. Atomistic molecular dynamics simulations of PTB7 corroborate these conclusions, and a simple relation is proposed which quantifies the free-energy of conjugated polymer folding. This study provides detailed guidance in the design of intra- and inter-chain contributions to the photophysical and morphological properties of polymeric semiconductors.
INTRODUCTION Organic semiconducting polymers have garnered interest for their numerous applications in addressing the increasing demand for affordable, lightweight, thin, flexible, and energy-efficient electronics. While extensive studies have focused on the influence of molecular structure1,2 on both the photophysics and device performance of conjugated polymers, differentiating and controlling inter- and intra-chain contributions to optoelectronic functionality remains an elusive goal for materials scientists.3–5 The photophysics and device performance of these soft materials have been shown to strongly depend on the electronic and geometric structures of conjugated polymers, which often drastically influence the resulting thin film morphologies6–8 and optoelectronic properties.9,10 Currently, detailed correlations between molecular properties, solution aggregation structures, and the ultimate film performances of organic semiconductors are still largely unknown because of intricate and intertwined structural factors in these largely disordered and noncrystalline materials. New materials and methodologies are required to deal with the inherent complexity of soft,
disordered materials.11–15 As it stands, understanding the factors leading to the robust self-assembly of solution aggregates and their effect on various electronic processes in films following deposition is the first step towards the bottom-up design of materials with desirable bulk optoelectronic properties. Using a model system with easily manipulated solution and film properties to study the molecular factors leading to desirable bulk properties could prove crucial in beginning to unravel these connections. While the development of low band-gap, conjugated copolymers was initially aimed at the enhancement of solar photon harvesting, especially in the NIR region, the proliferation of copolymers also induced a number of characteristics that are distinctly different from those of conventional homopolymers. For example, due to its broad, low bandgap absorption16, favorable film morphology7,8, efficient exciton dissociation2,17, and, consequently, high photovoltaic performance, PTB7 (a monofluorinated poly-benzodithiophenethienothiophene) has been an important material in organic photovoltaics (OPVs) research in recent years17–19. Studies have detailed the effects of the intra-chain “push-pull” character of its
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conjugated backbone17 on exciton binding energy in PTB7, but complications from possible inter-chain interactions have been largely elusive. PTB7 also possesses somewhat unique properties as a conjugated copolymer, most notably its almost identical optical absorption spectra in dilute solution and neat film, as well as the observed decrease in photovoltaic power conversion efficiency (PCE) upon annealing in bulk heterojunction (BHJ) devices. Disentangling the effects of molecular structure and morphology will not only help optimize current PTB7 solar cells, but will also lead to more informed design of future polymeric semiconductors. In this report, we aim at untangling intra-chain and inter-chain electronic processes in PTB7 by studying electronic structure evolution as a function of chain length. While computational work on chain-length dependent properties has been carried out on some conjugated polymers, 20–22 none to our knowledge has explicitly examined the critical transition of electronic processes between unfolded and folded polymeric structures in solution. Experimental work examining the length-dependent properties of oligomers has been carried out for several conjugated homopolymers which provided much useful information, 23,24 but such studies have not been utilized to examine oligomer/polymer structures in solution for conjugated copolymers. Here, a series of oligomers with different numbers of repeating units were synthesized using the same alternating electron-donating benzodithiophene (BDT) and electron-accepting fluorinated thienothiophene (TT) moieties as in PTB7, with the sequence (BDT-TT)n-BDT (n=1-3) (Scheme 1). This series of oligomers are useful prototypes for examining photophysical properties as well as conformational changes induced by progressive chain elongation. Using these oligomers as models we utilize a combined experimental and computational approach to quantitatively probe both the photophysical effects of aggregation and the onset chain length at which intrachain aggregation occurs in PTB7 oligomers. We hope to gain a better understanding of the hierarchical assembly of microstructural morphologies in organic electronics including bulk heterojunction organic photovoltaics.
one type of fluorine. However, it is difficult to distinguish the 4- and 6-linkage so as to identify the exact structure of M1 at this stage, although it is possible to predict that the 6-linkage is dominant, as, according to one recent study about the similar PTB7-Th, 100% 6-linkage in the monomer is proposed.26 M2a (n=1) is synthesized from 1a and 2 under the same conditions as M2b (n=1). However, M2a has free terminals that are available to be converted to the tin compound M2c (n=1). Starting from M1, M3 (n=2) and M4 (n=3) are synthesized with 1c and M2c, respectively. Note that in M3 and M4, similar to M1, the fluorine atoms may face different directions, but the structures of them shown in Figure 1 and Scheme 1 only show one direction for clarity. This series of oligomers from M1 to M4 mimics the increase in molecular weight of the PTB7 polymer chain, with the number of repeating units increasing from 1 to 3.
Scheme 1. Synthesis of PTB7 Oligomers Synthesis of Low MW PTB7
METHODS Synthesis of PTB7 Oligomers
Scheme 2. Synthesis of Low Molecular Weight PTB7
Scheme 1 shows the synthetic process for the oligomers. Our previous study reported the synthesis of M2b by reacting 1b with 2 at 2:1 ratio25. Changing the ratio to 1b:2 = 1:1, M1 can then be synthesized. Note that there are two positions on 2 available for coupling, the 4- and the 6-position. However, from the 1HNMR spectrum of M1 (Supporting Information), only two singlet peaks (Ha and Hb) in the aromatic region are observed, indicating that there is only one isomer in the product, either 4-linkage or 6-linkage. This is further supported by the 19FNMR spectrum of M1, which shows one singlet peak, suggesting
In order to fill the gap between oligomeric and polymeric samples, short polymer segments were synthesized. Synthetic details are shown in Scheme 2.Molecular weights (MW) and MW distributions of polymers were determined using GPC with a Waters Associates liquid chromatography equipped with a Waters 510 HPLC pump, a Waters 410 differential refractometer and a Waters 486 tunable absorbance detector. Polystyrene was used as the standard and chloroform as the eluent. Optical properties were measured using a Shimadzu UV-
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2401PC UV-Vis spectrophotometer and Shimadzu RF5301PC spectrofluorophotometer. Scheme 2 describes the synthesis to low molecular weight PTB7. PTB7-A and PTB7-B. 2-ethylhexyl-4,6-dibromo-3fluoro thieno[3,4-b]thiophene-2-carboxylate (A, 47.0 mg, 0.0995 mmol) was weighted into a 25 ml round bottom flask together with 2,6-bis(trimethyltin)-4,8-di(2ethylhexyloxyl) benzo [1,2-b:4,5-b’]dithiophene (B, 73.0 mg, 0.0945 mmol). The Pd(PPh3)4 (5.3 mg) and LiCl (0.1 mg) were added inside the glove box. The flask was vacuumized and purged with argon in three successive cycles. Then anhydrous toluene (2 ml) and DMF (0.5 ml) were injected into the mixture via a syringe. The polymerization was performed at 100oC for 24 h under argon protection. Then, 2-bromothiophene (0.03 mL) was added. After 2 hours, 2-(tributylstannyl) thiophene (0.15 mL) was added and kept overnight. A blue mixture was obtained and suction filtered through Celite to eliminate remaining palladium particles. The raw product was precipitated out in methanol and underwent Soxhlet extraction by methanol, acetone, hexane and chloroform. The final polymers were again precipitated out in methanol and dried in vacuum, yielding PTB7-A (36.4 mg, 50.9%) from the hexanes portion and PTB7-B (26.4 mg, 36.9%) from the chloroform portion. PTB7-A: Mn = 17.6 kDa, ~24 repeat units, Ð = 2.60. PTB7-B: Mn = 29.8 kDa, ~40 repeat units, Ð = 2.25. PTB7-C ~ PTB7-F. Following the same procedure as for PTB7-A and PTB7-B, but with (A, 47.7 mg, 0.101 mmol) and (B, 70.2 mg, 0.0909 mmol), PTB7-C (34.2 mg, 49.7%) from the hexanes portion and PTB7-D (30.2 mg, 43.9%) from the chloroform portion were obtained. PTB7-C: Mn = 9.7 kDa, ~13 repeat units, Ð = 2.67. PTB7-D: Mn = 20.3 kDa, ~28 repeat units, Ð = 2.37. A small amount of PTB7C was applied to a column chromatography on silica gel with hexanes/chloroform=1/1 as eluent to obtain the PTB7-E (Mn = 5.0 kDa, ~7 repeat units, Ð = 2.13) and PTB7-F (Mn = 16.9 kDa, ~23 repeat units, Ð = 2.37). Spectra for Oligomers and Polymeric PTB7 UV-Visible spectra were taken on a Shimadzu UV-3600 Spectrophotometer in chloroform at low (
