A quantum chemical investigation of the solvatochromism of a phthalocyanine within a lipid bilayer: Comparison between continuum and atomistic models

The excitation properties of a charged phthalocyanine dye within a phospholipidic membrane have been investigated using hybrid polarizable quantum/classical models. Two different approaches for the description of the membrane environment were compared, based on continuum and atomistic representations, respectively. In the latter case, a two-step scheme was adopted: quantum mechanics calculations of the electronic transitions in a polarizable medium are combined with a classical molecular dynamics simulation of the dye in the membrane to account for thermal fluctuations. The results indicate that the two models reproduce a qualitatively similar red-shift of the lowest bright transitions. However, the thermal effects on the internal geometry of the dye which are included only in the two-step scheme are shown to be fundamental to properly describe the solvatochromism within the membrane.