Edited by Michael L. Klein, Temple University, Philadelphia, PA, and approved February 2, 2018 (received for review November 5, 2017)
Conjugated polymers are promising materials for flexible electronics and photovoltaics. Recent progress in polymer design led to a rise in device efficiency. Tailoring intramolecular interactions is a central design element, which allows fine-tuning of optical and electronic properties. However, prediction and measurement of intrinsic properties of newly synthesized polymers is challenging, as they are often hidden by ensemble effects. Single-molecule spectroscopy allows revelation of the intrinsic changes upon chemical modification, here specifically a variation of the side chains. Surprisingly, a more disordered, bulky side chain leads to a higher order and better conjugation within the electronically active backbone of a single chain. This study gives detailed insights into changes in photophysical properties and suggests new ideas for synthesis.
The backbone conformation of conjugated polymers affects, to a large extent, their optical and electronic properties. The usually flexible substituents provide solubility and influence the packing behavior of conjugated polymers in films or in bad solvents. However, the role of the side chains in determining and potentially controlling the backbone conformation, and thus the optical and electronic properties on the single polymer level, is currently under debate. Here, we investigate directly the impact of the side chains by studying the bulky-substituted poly(3-(2,5-dioctylphenyl)thiophene) (PDOPT) and the common poly(3-hexylthiophene) (P3HT), both with a defined molecular weight and high regioregularity, using low-temperature single-chain photoluminescence (PL) spectroscopy and quantum-classical simulations. Surprisingly, the optical transition energy of PDOPT is significantly (∼2,000 cm−1 or 0.25 eV) red-shifted relative to P3HT despite a higher static and dynamic disorder in the former. We ascribe this red shift to a side-chain induced backbone planarization in PDOPT, supported by temperature-dependent ensemble PL spectroscopy. Our atomistic simulations reveal that the bulkier 2,5-dioctylphenyl side chains of PDOPT adopt a clear secondary helical structural motif and thus protect conjugation, i.e., enforce backbone planarity, whereas, for P3HT, this is not the case. These different degrees of planarity in both thiophenes do not result in different conjugation lengths, which we found to be similar. It is rather the stronger electronic coupling between the repeating units in the more planar PDOPT which gives rise to the observed spectral red shift as well as to a reduced calculated electron−hole polarization.
↵1Present address: Polymerchemie, Technische Universität Chemnitz, 09111 Chemnitz, Germany.
Author contributions: L.S., P.J.R., and R.H. designed research; D.R., L.S., S.P., K.S., and F.P. performed research; S.B., D.S., R.L., M.T., and M.S. contributed new reagents/analytic tools; D.R., L.S., S.P., K.S., and R.H. analyzed data; D.R., L.S., S.P., K.S., J.K., A.K., P.J.R., and R.H. interpreted data; and D.R., L.S., and R.H. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1719303115/-/DCSupplemental.
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