Evaluating push-pull dye efficiency using TD-DFT and charge transfer indices

The performances of different functionals in the prediction of Charge Transfer excitations (CT) have been assessed, both in terms of quantitative agreement with experimental absorption data and on the basis of a recently developed density based diagnostic index, for a family of 18 recently synthesized push-pull compounds, containing 4-5-dicyannoimidazole (DCI) as an acceptor moiety, six different bridges and three different donor groups. The index used also allows obtaining an estimate of the charge transferred upon excitation (q(CT)) and of the spatial extent associated with a given electronic transition (D-CT). From the computed values of these indices an estimate of the transition energy considering a purely electrostatic model (W-CT) can be computed and compared to that expected for an ideal CT between the donor and the acceptor, thus enabling us to estimate the efficiency of the CT transition for the different push-pull systems.