J. Zechner

WAVELENGTH EFFECTS ON THE PHOTOPROCESSES OF INDOLE AND DERIVATIVES IN SOLUTION

Abstract— The two main primary photoprocesses (electron ejection and H‐atom release) for indole, 5‐methoxyindole and N‐methylindole in various polar and nonpolar solvents were studied as a function of the excitation energy and were correlated with the corresponding fluorescence quantum yields. In hydrocarbon solvents, N–H bond cleavage is the main primary photoprocess from the 1Bb band of the substrates with the exception of N‐methylindole. In alcohols, both processes are of negligible importance. Hydrated electrons (e−aq) are ejected from the relaxed singlet states of all three compounds in aqueous solutions with a similar yield for excitation at 280 and 254 nm (1La and 1Lb states). The yield increases when the excitation is into the 1Bb band. The quantum yields of the two primary processes from the higher excited states are generally lower than the fraction of molecules not converting to the fluorescent state. This is explained by an efficient back reaction in competition with a thermally activated radical release from an intermediate state or radical pair formed from the S2 (1Bb) state. The non‐occurrence of a photoionization energy threshold is discussed.

Photoreactions of derivatives of 2-phenylindan-1,3-dione in polar and non-polar solvents

Abstract Two primary photoreactions were observed for derivatives of 2-phenylindian-1,3-dione in various solvents: (1) cleavage of the hydrogen atom in the α position to both keto groups resulting in the formation of 2-arylindan-1,3-dion-2-yl radicals; (2) photoisomerization to benzylidenephthalides as a result of bond rupture via a Norrish type I mechanism. Both processes are possible with different quantum yields from the diketo form (in apolar solvents), from the enol form (in alcohols) and from the enolate form (in alkaline water).

Photogeneration of radical cations from aqueous methoxylated benzenes

Formation of radical cations resulting from photoinduced electron ejection from methoxylated benzenes in aqueous solutions has been observed.

Photoreactions of 2-Amino-2-Phenylindan-1,3-Dione Derivatives

Abstract The photochemical behaviour of several derivatives of 2-amino-2-phenylindan-1,3-dione, substituted in the amino group was investigated by means of steady state irradiation and flash photolysis. It is shown, that two photoisomerization reactions take place simultaneously: i) photoisomerization to 3-aminosubstituted-benzylidene-phthalides; ii) photoisomerization to 2-substituted-3-phenylisoquinolinones.Both processes result from a bond rupture according to a Norrish type I reaction mechanism.

Mechanism of the Photochemical Isomerization of Indan-Dions to Benzylidenephthalides

The photoinduced isomerization of 15 2-arylindan-1,3-diones and 5 2,6-diaryl-s-hydrindacene- 1,3,5,7-tetrones was studied. Spectroscopic data, quantum yields of final products and a probable processes of these compounds are presented.

Flash photolysis of phenylazopyrazolone derivatives in solution

Abstract Flash photolysis investigations were performed on various derivatives of 3-methyl-1-phenyl-4-phenylazopyrazolone-5 in polar and non-polar media. The transients produced in alcohol—water mixtures at pH 7 possess two absorption bands in the range between 300 and 600 nm, which are different from those obtained at pH 12. These transients are assigned to substituted phenylhydrazyl radicals in their neutral and dissociated forms. They are not observed in dioxane and cyclohexane solutions. In these solvents an isomerization process is found. On addition of hydrogen donors, such as methanol, propanol-2 or N-allylthiourea, to dioxane hydrazyl radicals are also photogenerated. In all cases degradation and formation of stable products were also registered. The kinetic behaviour of the short-lived species is complex. Structures of the transients were discussed.

Electronic spectra and flash photolysis of some derivatives of 2,6-diphenyl-s-hydrindacene-1,3,5,7-tetrone

Abstract The electronic absorption and fluorescence spectra of some derivatives of 2,6-diphenyl-s-hydrindacene-1,3,5,7-tetrone - bisindandione - BID, substituted in p, p-position of the phenyl ring in solvents of different polarity, have been investigated. Several derivatives of BID have been studied by flash photolysis and steady state UV irradiation. The results show: 1/ a rapidly formed (less than 1 ns) stable photoproduct; ii/1 transient, which is not the precursor of this stable photoproduct.

Substituent and solvent effects on the UV-absorption spectra of 2-phenylindan-1,3-dion-2-yl free radicals

Abstract 2-Phenylindan-1,3,-dion-2-yl free radical substituted in the 4′-position were produced by flash photolysis of the corresponding indandiones. Their UV-absorption spectra exhibit a pronounced solvent shift indicating a higher dipole moment in the excited state than in the ground state. Substituent effects were analyzed by means of a linear free energy relationship. The effects of substitution on the energy gap between the ground and excited states of the free radicals and on the curvature of the excited states energy hypersurface are discussed.