Semiempirical studies of solvent effects on the intramolecular charge transfer of the fluorescence probe PRODAN

The organic chromophore 6-propanoyl-2-(N,N-dimethylamino)naphthalene (PRODAN) is widely used in biochemistry as an efficient fluorescence probe, due to its significant Stokes shift in polar solvents. The nature of the emissive states has not yet been established despite detailed experimental and theoretical investigations. The strong solvent dependence of PRODAN fluorescence requires a quantum mechanical study with explicit consideration of solvent effects. The excited-state properties of PRODAN have been investigated by means of the self-consistent reaction field (SCRF) method in combination with the semiempirical AM1 Hamiltonian. The geometrically unchanged planar conformation and the rotation of the dinethylamino (N) and the propanoyl (O) groups have been explored. Both geometrical relaxation pathways yield a highly polar twisted intramolecular charge-transfer state, the N-TICT and O-TICT states, respectively. These states are energetically stabilised in a polar environment at perpendicular geometries and become the lowest excited state. Based on the calculations, the experimentally observed red-shifted fluorescence is assigned to an emission from the propanoyl TICT state, for which a significantly reduced rotational barrier is found. Agreement with experimental results in various solvents is excellent.