Two-dimensional real-space analysis of optical excitations in acceptor- substituted carotenoids

Absorption spectra of substituted carotenoicls with varying acceptor strength are analyzed using collective electronic normal modes obtained using the time-dependent Hartree-Fock (TDf-IF) technique combined with the INDO/S semiempirical Hamiltonian. Two-dimensional plots of the collective excitations in real space show an off-diagonal size associated with relative motion of electron-hole pairs created upon optical excitation and a diagonal size representing the pair's center of mass motion. By varying the polyene chain length we show that the response of symmetric molecules is controlled by 'bulk' delocalized excitations with coherence size ~12 double bonds, whereas the response of short polar molecules is dominated by a localized 'charge-transfer' excitation created at the acceptor end with coherence and diagonal lengths ~12 and ~17 double bonds, respectively.