Solvent effects on the photophysical and photochemical properties of (E,E,E)-1,6-bis(4-nitrophenyl)hexa-1,3,5-triene

The photophysical and photochemical properties of (E,E,E)-1,6-bis(4-nitroph enyl)hexa-1,3,5-triene [(E,E,E)-1] have been studied in various solvents. T he fluorescence emission maxima of (E,E,E)-1 show an increasing Stokes shif t with increasing solvent polarity. Picosecond time-resolved fluorescence ( TF) and transient absorption (TA) spectra do not show any significant time- dependent shifts in nonpolar solvent whereas, in more polar solvents, large red and blue shifts are observed in the TF and TA spectra, respectively. T he fluorescence quantum yield reaches a maximum in moderately polar solvent s and the quantum yield of intersystem crossing decreases strongly with inc reasing solvent polarity. Z-E-Isomerization of triene double bonds is ineff icient in all solvents. In contrast, the absorption and fluorescence maxima for (E,E,E)-1,6-bis(4-cyanophenyl)hexa-1,3,5-triene [(E,E,E)-2] are practi cally solvent-independent. When the solvent polarity is increased, fluoresc ence quantum yield decreases monotonically and Z-E-isomerization quantum yi eld increases strongly. The results for (E,E,E)-1 can be understood in term s of an additional charge transfer excited state, which is absent for (E,E, E)-2.