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Title: Charge-transfer excitations in low-gap systems under the influence of solvation and conformational disorder: Exploring range-separation tuning

Charge transfer excitations play a prominent role in the fields of molecular electronics and light harvesting. At the same time they have developed a reputation for being hard to predict with time-dependent density functional theory, which is the otherwise predominant method for calculating molecular structure and excitations. Recently, it has been demonstrated that range-separated hybrid functionals, in particular with an “optimally tuned” range separation parameter, describe charge-transfer excitations reliably for different molecules. Many of these studies focused on molecules in vacuum. Here we investigate the influence of solvation on the electronic excitations of thiophene oligomers, i.e., paradigm low gap systems. We take into account bulk solvation using a continuum solvation model and geometrical distortions from molecular dynamics. From our study, three main findings emerge. First, geometrical distortions increase absorption energies by about 0.5 eV for the longer thiophene oligomers. Second, combining optimal tuning of the range separation parameter with a continuum solvation method is not straightforward and has to be approached with great care. Third, optimally tuned range-separated hybrids without a short-range exchange component tend to inherit undesirable characteristics of semi-local functionals: with increasing system size the range separation parameter takes a smaller value, leading to a functional of effectivelymore » more semi-local nature and thus not accurately capturing, e.g., the saturation of the optical gap with increasing system size.« less
Authors:
;  [1]
  1. Theoretical Physics IV, University of Bayreuth, D-95440 Bayreuth (Germany)
Publication Date:
OSTI Identifier:
22419826
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Chemical Physics; Journal Volume: 141; Journal Issue: 8; Other Information: (c) 2014 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; ABSORPTION; DENSITY FUNCTIONAL METHOD; EV RANGE; EXCITATION; HYBRIDIZATION; MOLECULAR DYNAMICS METHOD; MOLECULAR STRUCTURE; MOLECULES; POLYCYCLIC SULFUR HETEROCYCLES; SATURATION; SOLVATION; THIOPHENE; TIME DEPENDENCE