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.

A close to planar conformation is established for 2-(4-methoxyphenyl)-4,6-diphenyl-1,3,5-triazine (M-H-P) by crystal structure determination. For 2-(4-hexoxy-2-methoxyphenyl)-4,6-diphenyl-1,3,5-triazine (H-MeO-P), hydrophobic interaction between the long hexoxy groups enforces rotation of the dialkoxyphenyl ring at C6 and the C2 phenyl ring relative to the plane of the triazine ring. Time-resolved fluorescence measurements demonstrate the existence of a short-lived Franck-Condon (FC) and a longer-lived TICT (twisted intramolecular charge transfer) state, emitting at longer wavelengths, for the O-methyl derivative 2-(4-hexoxy-2-methoxyphenyl)-4,6-bis(2,4,6-trimethylphenyl)-1,3,5-triazine (H-MeO-Ms). The decay of the protontransferred (PT) fluorescence for crystalline powders of 2-(2-hydroxyphenyl)-4-(dimethylamino)-6-aryl-1,3,5triazines is shown, by the same method, to be retarded within the series phenyl f tolyl f xylyl, well in line with the gradation of quantum yields determined by steady-state methods. The high quantum yields of the PT fluorescence for triazines with only one 2-hydroxyaryl moiety on the triazine ring are reduced by 2 orders of magnitude each upon introduction of a second and a third aryl group. Most of the 2-hydroxyaryl-1,3,5triazines with additional aryl moieties at C4 and C6 show PT fluorescence, as long as the 2-hydroxyaryl and the triazine ring are more or less coplanar and the intramolecular hydrogen bond remains intact. In some cases, however, this fluorescence is very weak, or can be detected only in copolymers, in mixtures of the stabilizer with the polymer or in crystalline powders.

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.

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