A dark excited state of fluorescent protein chromophores, considered as Brooker dyes

The green fluorescent protein (GFP) chromophore is a dihydroxyarylmethine (oxonol) dye. Resonance and molecular orbital theories predict two low-lying excitations with distinct oscillator strength and polarization in these systems. Using a variational quantum-chemical model that reflects both theories, we estimate the energy of both states in resonant anionic and cationic forms of the chromophore. The energy of the higher, dark state in these forms is in the same range as the bright transition of the non-resonant neutral form, estimated by similar methods. This suggests interesting roles for the dark state in GFP photophysics.