Origin of the π-stacking induced shifts in absorption spectral bands of the green fluorescent protein chromophore

We present the results of computational studies of the shifts in absorption bands in the 4-hydroxybenzylidene-1,2-dimethylimidazolinone (HBDI), a synthetic analogue of the green fluorescent protein chromophore, induced by its pi-stacking interaction with aromatic compounds. The set of model systems composed of HBDI in the anionic form pi-stacked with the substituted derivatives of benzene is considered. Ground state equilibrium geometry parameters are obtained at the MP2/cc-pvdz level of theory and the vertical S-0,S-min-S-1 excitation energies are estimated using the XMCQDPT2/sa2-CASSCF(12/12) approach. It is shown that the bathochromic shifts are obtained for systems with smaller values of the dipole moment variation while the hypsochromic shifts correlate with larger values of this quantity. We also demonstrate that absolute values of the S-0,S-min-S-1 transition energy shift well correlate with the calculated bond interaction energy parameter of pi-stacked complexes.