Generating function approach to the calculation of spectral band shapes of free-base chlorin including Duschinsky and Herzberg-Teller effects.

A generating function approach to the calculation of spectral band shapes including Duschinsky and Herzberg-Teller effects is proposed and applied to the computation of the free-base chlorin Q absorption bands, using molecular geometries and normal vibrations obtained by density functional theory computations. The results clearly show that non-Condon effects can significantly affect the relative intensities of the weakest Q(y) and, to a lesser extent, Q(x) bands. The proposed approach is extremely powerful and can be used in the cases where the molecular size makes the direct calculation of Franck-Condon integrals by recurrence formulas prohibitive.