High-pressure effect on one- and two-photon-excited fluorescence of organic molecules in solid polymers

We report the pressure effect on one-and two-photon-excited fluorescence from three organic molecules dissolved in solid polymers. The molecules studied are 4-(p-nitrophenyl)-3,4-dihydropyrazo[c]be morpholine (NDPB), 1-phenyl-3-nitrophenylpyrazoline (PNP), and bis[4-(dimethyl aminophenyl)] methylide ammonium chloride-Auramine O (AO). All these molecules exhibit strong fluorescence when subjected to visible or infrared laser light. We determine the pressure dependencies of fluorescence intensity, as well as energy and lifetime of the emitting state for one-and two-photon excitation. The pressure dependence of the last two parameters reveals that the fluorescence, for all molecules, originates in the same state regardless of the mode of excitation. In contrast to this, the emission intensity may change with pressure differently for one-and two-photon excitation. We introduce a parameter defined as a ratio of the emission intensity following two-photon excitation to the emission intensity following one-photon excitation ((I-2'(p)/I-1'(p)). This parameter, with increasing pressure, shows almost no change for NDPB but a significant decrease for AO and PNP. Thus we postulate that absorption transitions may proceed for one-and two-photon excitation to the same state in NDPB but to different states in AO and PNP. Moreover, for AO and PNP, the two locally excited states for both modes of excitation may relax through different pathways to the same emitting state. In the case of NDPB and PNP, a large Stokes shift indicates that the emitting state has a distinctly different charge distribution than the initially excited state and that this distribution is strongly pressure dependent.