Quantum mechanical/molecular mechanical approach for the simulation of UV–Vis absorption spectra of π-conjugated oligomers

Abstract

It is crucial to understand the light absorption features and electronic structures of conjugated organic materials for their optoelectronic applications. However, neither reliable interpretations of reported ultraviolet–visible (UV–vis) absorption spectra nor correct predictions of new organic materials have been made because of the absence of proper calculation methods. To date, excited-state quantum chemical calculations, such as configuration interaction singles (CIS) and time-dependent density functional theory (TDDFT), have provided such information by overlooking the facts that many conformational isomers of conjugated organic molecules can exist at a given temperature and the resulting distributions of backbone structure can modulate π-conjugated electronic structures. In this study, we introduced a computational method combining the quantum mechanical/molecular mechanical molecular dynamics simulation (QM/MM MD) and the excited-state electronic structure calculation to include various conformers distributed due to the finite temperature and present a near-perfect simulation of experimentally obtained UV–vis absorption spectra. The simulated UV–vis spectrum of 3,6-bis(5-(benzofuran-2-yl)thiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione [DPP(TBFu)2], a π-conjugated organic material for organic photovoltaic cells (OPV), shows excellent agreement with its experimental spectrum, and each absorption band is assigned to the different conformers with characteristic excitation energies. We believe that our new method provides an improved interpretation of the electronic structures as well as the conformational variations of π-conjugated organic materials under ambient environments.