On the basis of laser flash photolysis with detection by picosecond and nanosecond emission as well as absorption spectroscopy, a quantitative description is given of all the deactivation channels of the first excited singlet state of phenols ArOH(S1) such as fluorescence, intersystem crossing to the triplet system (ISC), chemical dissociation into radicals, and radiationless internal conversion (IC). For this purpose, various phenols with different alkyl substitution patterns (mono to 3-fold substitution with methyl and /or tert-butyl groups) were studied in solvents of increasing polarity: cyclohexane, n-butyl chloride, tetrahydrofuran, ethanol, methanol, acetonitrile, and water. The fluorescence lifetimes of the phenols were found to range from a few tens of picoseconds up to a few nanoseconds, correlating with fluorescence quantum yields between 10 -1 and 10 -3 , at room temperature. With a probability less than 0.1, the photodissociation of ArOH(S1) was found to be nearly unaffected by changing the type of solvent. As a result of the ISC the triplet yields amount to 0.2-0.3, with smaller values in nonpolar and the larger ones in polar media. As a very marked exception, sterically hindered phenols (2,6-di-tert-butyl substituted) behave quite differently: they exhibit extremely short living fluorescences, show no ISC, and, radiationless, are only deactivated by internal conversion. This complete physical energy dissipation makes the sterically hindered phenols ideal light quenchers.

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α,ω‐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.

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.