On the large apparent Stokes shift of phthalimides

The photophysics of N-methylphthalimide (MP) in solution (cyclohexane, ethanol, acetonitrile, and water) was characterized by steady state as well as time resolved fluorescence and absorption spectroscopy. In all solvents the compound exhibits an unusually large Stokes shift of approximate to 10000 cm(-1). It is attributed to an ultrafast (<100 fs) depletion of the initially excited state, which results in the population of a weakly emitting state. Quantum chemical computations (DFT-MRCI) support this. They identify two energetically low-lying singlet * excitations of different oscillator strength. Whereas the Stokes shift and thereby the ultrafast depletion of the initial excitation are hardly affected by the solvent later processes respond strongly. The fluorescence lifetime varies from approximate to 10 ps (cyclohexane) to approximate to 3 ns (water). This is attributed to a varying energetic accessibility of n* excitations.