Linear absorbance of the pheophorbide-a butanediamine dendrimer P4 in solution: Computational studies using a mixed quantum classical methodology

The linear absorbance of a particular chromophore complex P4 dissolved in ethanol is computed. P4 is formed by a butanediamine dendrimer to which four pheophorbide-a molecules are covalently linked. The computations utilize a mixed quantum classical methodology and different approximations are compared. The electronic states of the P4 chromophores which form Frenkel excitons in the excited states are treated quantum mechanically, whereas the intramolecular, intermolecular, as well as solvent coordinates are described classically. The computations use an improved exciton model, where the charge and transition densities of the chromophores are described by atomic partial charges, derived from a fit of the respective ab initio electrostatic potentials. Room temperature molecular dynamics simulations of all nuclear coordinates result in a time-dependent exciton model. It includes modulations of chromophore excitation energies due to charge density coupling between all chromophores as well as between the chromophores and solvent molecules, and, finally, modulations of the interchromophore excitonic couplings. The different approximations to the absorbance agree rather well. In particular, they confirm the reliability of adiabatic excitonic states which energies and oscillator strengths are altered by the overall temporal evolution of P4 conformations. The fluctuations of solute-solvent interactions have a significantly larger effect on the absorbance broadening than the excitonic couplings but cannot completely explain the measured spectrum. The additional account for intrachromophore vibrations overcomes this discrepancy.