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Triplets with a Twist: Ultrafast Intersystem Crossing in a Series of Electron Acceptor Materials Driven by Conformational Disorder

Authors :
Murali, Sai Shruthi
Gallaher, Joseph K.
Janiseck, Céline
Tay, Elliot J.
Wagner, Isabella
Thorn, Karen E.
Ilina, Aleksandra
Tamming, Ronnie R.
Wang, Jiayu
Sester, Clément
Sutton, Joshua J.
Price, Michael B.
Gordon, Keith C.
Chen, Kai
Zhan, Xiaowei
Hodgkiss, Justin M.
Hume, Paul A.
Source :
Journal of the American Chemical Society; January 2023, Vol. 145 Issue: 1 p732-744, 13p
Publication Year :
2023

Abstract

Control over the populations of singlet and triplet excitons is key to organic semiconductor technologies. In different contexts, triplets can represent an energy loss pathway that must be managed (i.e., solar cells, light-emitting diodes, and lasers) or provide avenues to improve energy conversion (i.e., photon upconversion and multiplication systems). A key consideration in the interplay of singlet and triplet exciton populations in these systems is the rate of intersystem crossing (ISC). In this work, we design, measure, and model a series of new electron acceptor molecules and analyze them using a combination of ultrafast transient absorption and ultrafast broadband photoluminescence spectroscopies. We demonstrate that intramolecular triplet formation occurs within several hundred picoseconds in solution and is accelerated considerably in the solid state. Importantly, ISC occurs with sufficient rapidity to compete with charge formation in modern organic solar cells, implicating triplets in intrinsic exciton loss channels in addition to charge recombination. Density functional theory calculations reveal that ISC occurs in triplet excited states characterized by local deviations from orbital π-symmetry associated with rotationally flexible thiophene rings. In disordered films, structural distortions, therefore, result in significant increases in spin–orbit coupling, enabling rapid ISC. We demonstrate the generality of this proposal in an oligothiophene model system where ISC is symmetry-forbidden and show that conformational disorder introduced by the formation of a solvent glass accelerates ISC, outweighing the lower temperature and increased viscosity. This proposal sheds light on the factors responsible for facile ISC and provides a simple framework for molecular control over spin states.

Details

Language :
English
ISSN :
00027863 and 15205126
Volume :
145
Issue :
1
Database :
Supplemental Index
Journal :
Journal of the American Chemical Society
Publication Type :
Periodical
Accession number :
ejs61479734
Full Text :
https://doi.org/10.1021/jacs.2c12605