Tuesday, August 14, 2012

1208.2477 (Seth Olsen et al.)

A two-state model of twisted intramolecular chargetransfer in
monomethine dyes

Seth Olsen, Ross H. McKenzie
We describe a two-state model Hamiltonian that can describes the development of twisted intramolecular charge-transfer behavior in monomethine dyes, both near and far from the cyanine limit. Monomethine dyes are useful as biological probes due to their binding-dependent fluorescence turn-on behavior. The model is a generalized Mulliken-Hush diabatic Hamiltonian wherein the diabatic energies and couplings are coupled to twisting about distinct bonds of the monomethine bridge. We parameterize the Hamiltonian against multireference perturbation theory calculations of the ground and excited states of four distinct oxonol protonation states of a green fluorescent protein chromophore model. The four chromophores illustrate different regimes of detuning from the cyanine limit. The model describes correctly the distinct relationships between twisting and charge-transfer behavior in each case. We expose a deep connection between the existence of twist-dependent polarization and the existence of twisted conical intersections in the model, the presence or absence of which are determined by the relative strength of the diabatic biasing potential and the "exchange" terms describing bond twisting energetics. The model indicates a classification of monomethine dyes into qualitatively distinct regimes where the energetic selection of one twisting channel over the other may or may not be accomplished by the application of a diabatic biasing potential. We find that twisting channel selection can only be achieved in a region near the cyanine limit, and that this region coincides with the region of parameter space for which conical intersections can occur.
View original: http://arxiv.org/abs/1208.2477

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