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A number of 2-(2-hydroxyphenyl)-4,6-diaryl-1,3,5-triazines (HPTs) and TIN P (2-(2-hydroxy-5-methylphenyl)benzotriazole) show phosphorescence in polar solvents at 77 K which increases in intensity with UV-irradiation time until an equilibrium value is reached (phosphorescence evolution). TIN P phosphoresces even at the very beginning of irradiation, in contrast to the HPTs, such as M-OH-P (2-(2-hydroxy-4-methoxyphenyl)-4,6-diphenyl-1,3,5-triazine), which exhibit no such initial phosphorescence provided that they were not recently exposed to UV radiation. The corresponding methoxy derivatives (MPTs) of some HPTs, where the H atom of the intramolecular hydrogen bond (IMHB) is replaced by a methyl group, produce intense phosphorescence independent of irradiation time. Considerable relaxation is found for HPTs after dark periods ≤ 1 h at 77 K resulting in a significantly lower initial phosphorescence intensity upon renewed irradiation. TIN P, in contrast, shows much slower relaxation which becomes significant only at elevated temperatures. Phosphorescence evolution is due to open conformers of the molecules, i.e., with intermolecular rather than intramolecular hydrogen bonds, which are formed in polar solvents under the influence of UV radiation. Relaxation, i.e., re-formation of the IMHB of open-form molecules, is faster for the investigated HPTs than for TIN P.

Both UV absorption and fluorescence maxima of 2-(2-methoxyaryl)-1,3,5-triazines show a marked bathochromic shift with increasing proton concentration. Well-defined isosbestic points establish an equilibrium between protonated and nonprotonated species for the ground state. 1 H and 13 C NMR data unequivocally prove a rapid prototropic equilibrium (> 10 2 /s) between tautomers protonated at N-1, N-3, and N-5, respectively. The NMR data also show a substantial increase in charge transfer, upon protonation, from the phenyl, and even more from the alkoxy-substituted aryl rings into the triazine system already for the ground state. At higher proton concentrations, the twisted intramolecular charge transfer (TICT) fluorescence of the nonprotonated (2-methoxyaryl) triazines is gradually replaced by the much weaker fluorescence of the protonated species, which is shifted to still longer wavelengths. Because the electron-accepting capacity of triazines is enhanced in the excited state, their pK a values increase, upon photoexcitation, by 6.8-9 units; thence, the excited-state energy level of the protonated form (S 1 ′) is calculated to be lower by 37-51 kJ/mol than that of the respective nonprotonated species (S 1 ). Protonation thus leads to static quenching of the fluorescence. Halide ions, in contrast, can act as external electron donors toward triazines only in their highly electron-affine excited state, and so effect merely dynamic fluorescence quenching, with a corresponding reduction in fluorescence quantum yield, for the (2-methoxyaryl) triazines. Photochemical stabilization by protonation, therefore, is more efficient than by electron transfer. For all systems investigated, the excitedstate electron transfer is exergonic and hence may be considered as diffusion-controlled, in accordance with the Rehm-Weller equation.

An in-depth photophysical study is presented for a series of 2-(2-hydroxyphenyl)benzotriazoles (HBzTs); the structural characteristic of all these photostabilizers is their strong intramolecular hydrogen bridge (IMHB). Tinuvin P (TIN P, 11a) and six other HBzTs, with no substituent in the 3'-position ortho to the hydroxy function, show pronounced phosphorescence already in the dark (at 77 K in a polar glass). Upon irradiation, the phosphorescence intensity rises further until an equilibrium value is attained (up to 1.5 fold the dark value). A kinetic model is given which excellently reproduces this phosphorescence evolution: it demonstrates phosphorescence to arise from open conformers where the IMHB has been broken. Phosphorescence excitation spectra match the absorption spectra of the open conformer and also that of the O-methyl homologue 11A which cannot form an IMHB. Fluorescence spectra likewise prove the equilibrium between the closed and open conformer for these HBzTs. In unpolar glasses as well as in the crystalline state, TIN P displays a long-wavelength (red) fluorescence (with an enormous Stokes shift of approximately 10.000 cm-1) which is associated with the excited singlet state of the closed form after proton transfer within the IMHB, S1'(C). In polar matrixes, on the other hand, a blue fluorescence is observed (with a regular Stokes shift) for all those HBzTs which have no 3'-substituent shielding the IMHB against being opened by the polar solvent. This blue fluorescence, just as the characteristic phosphorescence evolution for these compounds, is associated with the open conformer. For HBzTs with an (alkyl) group ortho to the bridging OH group, however, a long-wavelength (red) fluorescence is again observed. The shielding effect of the 3'-substituent shows a fine gradation, cumyl >/= 1,1,3,3-tetramethylbutyl (isooctyl) > t-butyl >/= methyl.

The copolymerization parameters for monomer pairs of the copolymerizable UV absorbers MA-TIN 1 (2-[2-hydroxy-3-tert-butyl-5-(O-[2-hydroxy-3-(2-methylpropenoyloxy)-propyl]-2-carbonyloxyethyl)phenyl]benzotriazole) and MA-TZ 1 (2,4-bis(2,4-dimethylphenyl)-6-[2-hydroxy-4-(2-hydroxy-3-[2-methylpropenoyloxy])propoxyphenyl]-1,3,5-triazine) with styrene and methyl methacrylate were determined. The UV absorbers were present to a higher extent in the copolymers than they are when simply present as mixtures of monomeric UV absorbers in the monomer feed. At higher temperatures, the radiationless deactivation from the excited proton-transferred singlet state becomes more efficient for the UV absorbers physically admixed to the polymer than for the respective polymeric UV absorbers. MA-TZ 1 embedded in poly(methyl methacrylate) shows an equal increase of phosphorescence intensity with UV irradiation time as the decrease of the proton-transferred fluorescence. By combining fluorescence and phosphorescence measurements it becomes possible to estimate the proportion of UV stabilizer molecules with an intermolecular hydrogen-bridge to poly(methyl methacrylate) and which are not suitable for light protection of polymers at room temperature. At low pressure and temperature, the increase of light-induced phosphorescence was delayed. This “phosphorescence induction” phenomenon can be put down to the free volume of polymer matrixes, in which various UV absorbers have been incorporated. The emission spectroscopic results are applicable to products which are customary in trade, as shown by investigations on a clear coat binder system.

The degradation pathways for polymeric materials and coatings under the influence of light and oxygen are presented and the consequent need for light-protecting agents (light stabilisers) is discussed. The mechanisms of action of hindered amine light stabilisers and certain UV-absorbers are explained. The use of light stabilisers in photographic-, automotive-and wood-coatings as well as cosmetic applications is described.

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