Excited state properties of alpha,omega-diphenylpolyenes: photophysical and photochemical studies of donor-acceptor diarylbutadienes

ABSTRACT

alpha,omega-Diphenylpolyenes have attracted a great deal of attention as models of retinyl polyenes that are related to natural photoreceptors involved in energy and sensory phototransductions. Of particular interest have been the topics of their excited state electronic structure and spectroscopic properties. However, the exact nature of the lowest excited state in terms of their structure and energetics is not clearly known. Examination of the photophysics and photochemistry of donor-acceptor diphenylpolyenes can aid in understanding the excited states and photoprocesses of linear polyenes. In this paper are described the absorption, fluorescence and photoisomerization studies of donor-acceptor diarylbutadienes, namely p-(N,N-dimethylamino)-p'-cyano-1,4-diphenylbuta-1E,3E-diene (1), p-(N,N-dimethylamino)-p'-nitro-1,4-diphenylbuta-1E,3E-diene (2), p-(N,N-dimethylamino)-m'-nitro-1,4-diphenylbuta-1E,3E-diene (3), p-(N,N-dimethylamino)-o'-nitro-1,4-diphenylbuta-1E,3E-diene (4). Absorption properties are affected as expected due to mesomeric stabilization by the substituent; however, solvent polarity does not significantly affect the absorption properties of these dienes. In contrast, a pronounced solvatochromic fluorescence behavior of these dienes in organic solvents is observed. Time-resolved fluorescence is characterized by a single exponential fluorescence decay with generally increasing lifetime in polar solvents. The fluorescence quantum yields are very low, particularly in polar solvents, but do not show any clear trend. Irradiation of 1E,3E- 1-4 in organic solvents yields the corresponding 1E,3Z-isomer due to one-photon-one-bond isomerization of the C=C double bond lying closer to the acceptor group. The photoisomerization also depended on the solvent polarity and on the concentration of diene. The photoisomerization efficiency of dienes 1 and 2 under direct irradiation condition is greater than dienes 3 and 4. In comparison to the efficiency of photoisomerization under direct irradiation condition, the photosensitized isomerization efficiency is much less, particularly for dienes 1 and 2. The results are discussed in terms of the involvement of excited-state intramolecular charge transfer and conformationally relaxed polar excited states in the photoprocesses of linear polyenes.