Investigation of the UV/visible absorption spectra of merocyanine dyes using time-dependent density functional theory

The visible spectral properties of a set of merocyanine dyes have been determined experimentally and compared to theoretical estimates calculated using time-dependent density functional theory. Three families of merocyanines have been investigated. The merocyanines of the first class contain the 4-thiazolidinone, 2-thioxo group in resonance with different donor groups. In the second family, they present a common pyrrolo[ 2,3c] pyrazole- 6-( 2H)-acetic acid, the 4-[ 4-( dialkylamino) phenyl]-3,5-dihydro- 3,5-dioxo-2-phenyl group with different substituents on the two phenyl rings, while in the third family, they are constituted of an isoxazolone group linked via a conjugated polyenic segment to an aminophenyl group. Two hybrid exchange-correlation functionals have been used, whereas solvent effects have been included using the integral equation formalism of the polarizable continuum model. Moreover, for one compound, the vibrational structure of the electronic transitions has been determined by calculating the Franck-Condon factors within the Born-Oppenheimer approximation. The calculations show that the dominant transitions present a pi-pi* character. This approach reproduces nicely the variations of excitation energies with the nature of the merocyanine so that least-squares fits can be used to correct for the major systematic errors of the computational scheme and to reach an accuracy of 0.09 eV or better. Part of these systematic errors is shown to originate from the description of the solvent effects, whereas vibronic effects can account for most of the remaining differences.