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

Abstract

The photophysical and photochemical properties of (E,E,E)-1,6-bis(4-nitrophenyl)hexa-1,3,5-triene [(E,E,E)-1] have been studied in various solvents. The fluorescence emission maxima of (E,E,E)-1 show an increasing Stokes shift 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. The fluorescence quantum yield reaches a maximum in moderately polar solvents and the quantum yield of intersystem crossing decreases strongly with increasing solvent polarity. Z–E-Isomerization of triene double bonds is inefficient 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 practically solvent-independent. When the solvent polarity is increased, fluorescence quantum yield decreases monotonically and Z–E-isomerization quantum yield increases strongly. The results for (E,E,E)-1 can be understood in terms of an additional charge transfer excited state, which is absent for (E,E,E)-2.