Excited state polarizabilities of conjugated molecules calculated using time dependent density functional theory

In this paper, time-dependent density functional theory (TDDFT) calculation s of excited state polarizabilities of conjugated molecules are presented. The increase in polarizability upon excitation was obtained by evaluating t he dependence of the excitation energy on an applied static electric field. The excitation energy was found to vary quadratically with the field stren gth. The excess polarizabilities obtained for singlet excited states are in reasonable agreement with the experimental results for the shorter oligome rs, particularly if the experimental uncertainties are considered. For long er oligomers the excess polarizability is considerably overestimated, simil ar to DFT calculations of ground state polarizabilities. Excess polarizabil ities of triplet states were found to be smaller than those for the corresp onding singlet state, which agrees with experimental results that are avail able for triplet polarizabilities. Negative polarizabilities are obtained f or the lowest singlet A(g) states of longer oligomers. The polarizability o f the lowest B-u and A(g) excited states of the conjugated molecules studie d here are determined mainly by the interaction between these two states. U pon application of a static electric field a quadratic Stark effect is obse rved in which the lower B-u state has a positive excess polarizability and the upper A(g) state exhibits a decrease in polarizability upon excitation. All results are explained in terms of a sum-over-states description for th e polarizability. (C) 2001 American Institute of Physics.