Small S-0-S-1 energy gaps for certain twisted conformations of unsymmetric polymethine dyes: quantum chemical treatment and spectroscopic manifestations

The AM1-CI (Austin model 1 configuration interaction) method has been used to calculate energy gaps between the ground state (S-0) and the lowest biradicaloid (BR) excited state as a measure for roughly estimating the probability of reaching an S-0-BR conical intersection by bond twisting of one of the bonds along the polymethine chain of unsymmetric cyanine dyes. Invoking qualitative concepts for interpreting the computational data, a pronounced alternation of successive bonds is shown and explained. The magnitude of the S-0-BR energy gap was found to be governed by the position of the corresponding bond with respect to the polymethine chain ends and by the degree of electronic asymmetry of the chain, i.e. by the difference in electron donor abilities of its end groups. These results are compared to experimental photophysical data for a series of differently bridged asymmetric cyanine dyes and suggest that the magnitude of the energy gap for a given twisted bond can be modulated by bridge-induced perturbations. Nonreactive low-lying minima caused by the accessibility of several twisted excited-state conformers have to be invoked in order to explain the experimentally observed inverse-loose-bolt effect of the nonradiative decay rates and should be taken into account in the treatment of possible pathways of fluorescence quenching and in the design of high-quantum-yield fluorescent dyes.