Optical Limiting Chromophores. Correlation Effects in Computing Triplet-Triplet Absorption Energies of Organic Molecules

The computation of molecular triplet-triplet absorption wavelengths for a series of organic chromophores potentially useful in reverse saturable absorption and photodynamic therapy has been systematically investigated using the semi-empirical INDO/S model Hamiltonian. The effect of parameters defining the model and of the level of configuration interaction on the computed triplet absorption have been explored in detail. From eight different models which vary in the INDO/S model Hamiltonian parameters and the level of excited configurations used in the CI, a reliable model that estimates the triplet absorption of organic π-conjugated systems has been deduced. The model that employs Zerner's suggested triplet parameters and uses a MRD-CI level of configuration interaction reproduces the experimentally observed triplet absorption to reasonable accuracy. While this model performs exceptionally well in the case of acenes (fused unsubstituted ring systems) and α, ω-diphenyl polyene systems, it performs somewhat less well for organic molecules substituted with polar functional groups. The total RMS deviation in the computed triplet absorption λ_max of 100 organic molecules is 58 nm with a correlation coefficient of 0.85 for the entire set, 56 nm (correlation coefficient = 0.93) in the case of acenes, and 56 nm (correlation coefficient = 0.96) for α, ω-diphenyl polyenes.