Low-lying electronic excitations of a water-soluble BODIPY: from the gas phase to the solvated molecule

We present a description of the optical properties of a water-soluble BODIPY derivative in the gas phase and in water. Comparison with a hydrophobic BODIPY derivative with very similar structure but deprived of the hydrophilic groups, clarifies the effects of functionalization. The changes in solution are studied at different levels of modeling. In particular, we make use of Car–Parrinello molecular dynamics to take thermal motion into account, by generating a set of molecular configurations at about room temperature and embedding them in the polarizable continuum model (PCM) for the solvent. The theoretical scheme is that of time-dependent density functional theory which we apply using different approximations for the exchange-correlation functionals. Changes in the low-lying excitation spectrum (≈2.5–4.5 eV) of the solvated molecule in water relative to gas phase are shown to be due not only to the electrostatic interaction with the solvent but also to the thermal motion that induces a downward energy shift and broadening of the absorption lines. Addition of explicit water molecules to the PCM only affects spectral features at higher energies.