Exploring the excited-states of squaraine dyes with TD-DFT, SOS-CIS(D) and ADC(2)

We investigate the electronic excited-states of a large panel of squaraine derivatives with first principles tools taking into account environmental effects, with a focus on 0-0 energies and band shapes. First we show that while TD-DFT yields significantly too large 0-0 energies, correcting the TD-DFT values with ADC(2) provides much more accurate estimates. Second, though the geometric relaxation after photon absorption is limited, this investigation also indicates that using theoretical vertical estimates to approximate the experimental 0-0 energies is not a very appealing approach. In contrast, for experimentally non-fluorescent compounds one can use the measured lambda(max) as an approximation of the 0-0 energy with a minimal loss of accuracy. The proposed computational approach delivers a ca. 0.125 eV accuracy for a large number of compounds and therefore stands as one of the first robust method to predict the optical spectra of this important class of dyes. We also investigate the band shapes of selected derivatives and show that TD-DFT vibronic couplings allow an accurate reproduction of the experimental topologies of both absorption and emission bands. (C) 2016 Published by Elsevier Ltd.