The steady-state fluorescence properties of five 4-nitro-4'-aminostilbenes (4-nitro-4'-aminostilbene (1); 4-nitro-4'-(dimethylamino)stilbene (2); 4-nitro-4'-(diethylamino)stilbene (3); and 1-(4-nitrophenyl)-2-(2,3-dihydro-vV-R-indol-5-yl)ethylene, R = H (4) and R = CH3 (5)) were studied in various solvents and as a function of temperature. The quantum yield of fluorescence for the trans isomer (4>f), the Stokes shift, and the quantum yield of transcis photoisomerization ( ,_^.) depend markedly on the nature of the solvent. When the polarity is increased, the Stokes shift increases, ,^d ecreases strongly, and at first rises and then also decreases strongly. The five trans isomers show similar effects of solvent polarity and, generally, only small differences for a given solvent. This shows that the reduction in ,_^. in solvents of moderate and higher polarity is not caused by deactivation of the first excited trans singlet state ('t*) via rotation of the dialkylamino group since in 4 and 5 it cannot rotate whereas in 1-3 it can. Activation energies in « -xylene, toluene, and dioxane, obtained from ,_( range: 7-13 kcal/mol) and from measurements (3-4 kcal/mol), differ significantly. It is concluded that the substantial decrease of ,_^o n going from nonpolar to slightly polar solvents is due (i) to relaxation to the trans ground state of a transoid excited state (A*) that is not identical with the solvent-relaxed spectroscopic *t* state and (ii) to the presence of an energy barrier along the pathway of trans cis photoisomerization (following the step*t* -A*), which cannot be overcome in more polar solvents.

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The fluorescence properties of trans-4-cyano-4'-dimethylaminostilbene (trans-1) and a rigid analogue, in which the dimethylaminophenyl group is replaced by an indoline ring, trans-1-(4-cyanophenyl)-2-(5-[2,3-dihydro]N-methylindolyl)ethylene (trans-2) were studied in 20 solvents. The quantum yields of fluorescence (tf> f ) and of trans-»eis photoisomerization (<2>t-c) as well as the fluorescence maximum (v f ) and the bandwidth (zlv f ) were examined. The activation energies (range: 3.5-7.8 kcal/mol) obtained from <2> f and <£ t _ c measurements are nearly the same for 1 and 2 in a given solvent. 0 f , v f and Jv f as a function of solvent polarity and temperature show similar trends for 1 and 2. No dual emission was observed for either of the cyanostilbenes. These findings indicate that a twisted intramolecular charge transfer (TICT) state is not involved in the relaxation process of the first excited trans singlet state ('t*). The main deactivation pathway of 't* competing with fluorescence is trans eis photoisomerization by twisting about the C = C double bond in the first excited singlet state for both.

The radical-induced reduction of 16,17-dihydroxy- (M1O2) and 16,17-dimethoxyviolanthrone (M2O2) in argon-saturated 1-phenylethanol or mixtures of dimethyl sulfoxide and acetonitrile or glycerol triacetate was studied by photochemical methods. The photoproduct of MiO2 (i=1, 2) in the presence of a sensitizer, such as acetone, acetophenone or benzophenone, and a donor, e.g. an alcohol or triethylamine, is the fluorescing dihydroquinone, Mi(OH)2 which undergoes back-oxidation. Flash photolysis reveals ketyl radicals after the pulse and indicates radical formation in the 1 ms range en route to Mi(OH)2, while the putative donor and semiquinone radicals are undetectable intermediates. The concentrations of quinonoid dyes and donors were varied to explore the phenomena and to provide data for a mechanistic interpretation.

The photoreactions of 1-nitro-9,10-anthraquinone (N1) and 2-methyl-1-nitro-9,10-anthraquinone (N2) were studied in benzene and acetonitrile in the presence of 1-phenylethanol. For N2, a short-lived 10 ns transient observed upon flash photolysis is attributed to a triplet state, which can be intercepted by 1-phenylethanol to form a monohydro radical of N2 and a spectroscopically not detectable donor-derived radical. The decay of radicals yields the corresponding nitroso compound and eventually 1-amino-2-methyl-9,10-anthraquinone (A2) as photoproduct. The final reduction step requires participation of the anthraquinone carbonyl groups. The yield of radicals and the quantum yield (Phi(NH(2))) of conversion to A2 are small in inert solvents and increase with the donor concentration, approaching Phi(NH(2)) = 0.2. No triplet state was observed in the flash photolysis of N1, but a N1-derived radical and 1-amino-9,10-anthraquinone (A1) as final products were found. Various mechanistic aspects of complete photoreduction of nitroarenes to aminoarenes are discussed.