Distortion of the fluorescence spectrum of all-trans-1,6-diphenyl-1,3,5-hexatriene with increasing laser pulse excitation energies

At high laser pulse excitation energies the fluorescence spectrum of all-tr ans-1,6-diphenyl-1,3,5-hexatriene, DPH, undergoes distortion that can be mo deled as the development of a new bread fluorescence, lambda(max) = 497 nm, which builds up biphotonically with increasing excitation laser pulse powe r. This phenomenon is observed upon excitation at either 266 or 355 nm in a variety of organic solvents, provided [DPH] and excitation intensity are s ufficiently high. Four possible sources for the distortion of the fluoresce nce spectrum are investigated: (1) excimer emission arising from DPH radica l cation/anion pairs, (2) bicimer fluorescence from the encounter of two S- 1 states, (3) S-n --> S-1 fluorescence, and (4) self-absorption due to over lap of the fluorescence with transient absorption spectra. The first three possibilities are ruled out. The fourth is viable, revealing an effective t ransient absorption spectrum that agrees well with the known S-n <-- S-1 ab sorption spectrum at long wavelengths.