Excited state structures projected onto two dimensions: correlations with luminescent behavior

A simple structural comparison method based on relative changes of internal coordinates among different electronic states is presented. As such, any electronic state with respect to the ground state could be represented as a point on two dimensions. Although this analysis is not based on quantum mechanically “correct” description of molecules based on electron density and thus employs geometries rather than topologies, it might have advantages over overlaying procedures in resolving small differences as well as quantification of similarity/dissimilarity. More importantly correlations of structural similarity with luminescence properties were discussed for popular π-chromophores by studying S0, S1, and T1 states of indole, coumarin, BODIPY, and naphthalene. A qualitative difference on the structural similarity plane in spanning of S0 → S1 and S0 → T1 coordinate deformation vectors for popular fluorophores was identified. This emphasized a well-protected/separated S1 state to be a key feature of an efficient fluorophore. Our calculations on excited states suggest that, a quick screening of structure–luminescence relationships is possible. Structural features of efficient fluorophores outlined in this work can hopefully help rational designing of chromophore cores with desired luminescence properties.