One- and two-photon absorption of fluorescein dianion in water: a study using S-QM/MM methodology and ZINDO method.

One- and two-photon absorption (1PA and 2PA) of fluorescein dianion (FSD) in water were studied using a combined and sequential Quantum Mechanics/Molecular Dynamics methodology. Different sets of 250 statistically relevant (uncorrelated) configurations composed by the solute and several solvent molecules were sampled from the classical simulation. On these configurations, the electronic properties were calculated a posteriori using the Zerner's intermediate neglect of differential overlap (ZINDO) method. The linear and nonlinear absorption of FSD in water were calculated using discrete and explicit solvent models. In the largest case, the relevant configurations are composed by FSD and 47 explicit water molecules embedded in the electrostatic field of all remaining water molecules. Both INDO/CIS and INDO/CISD calculations were performed to study the absorption processes of FSD and the Sum-Over-States (SOS) model was used to describe the 2PA process. A semi-classical method for spectrum simulations was employed to simulate the 1PA and 2PA cross-section spectra of FSD in water. For comparison purposes, in the case of the 2PA process two approaches, the "full expression" and "resonant expression" methods, were employed to simulate the nonlinear spectrum. The last method assumes resonant conditions and on the computation point of view it represents an interesting option to study the 2PA process. The INDO/CI calculations give a satisfactory description of the 1PA spectrum of FSD and properly describe the unusual blue-shift of its first π→π(*) transition in water. In the case of 2PA, the introduction of doubly excited configuration interactions (INDO/CISD) has proven to be essential for an appropriate description of the process at the higher energy spectral region. It was observed that the solvent effects do not drastically change the cross-sections of both processes. The simulated 2PA cross-section spectrum provided by the "full expression" method presents a better definition of the bands which appear along the experimental spectrum than the one provided by the "resonant expression" method. However, both approaches provide similar description for the effect of the solvent environment on the 2PA process of FSD in water.