Ultraviolet Absorbers of the 2-(2-Hydroxyaryl)-1,3,5-triazine Class and Their Methoxy Derivatives:  Fluorescence Spectroscopy and X-ray Structure Analysis

Abstract

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.

Twisted Intramolecular Charge Transfer States in 2-Arylbenzotriazoles: Fluorescence Deactivation via Intramolecular Electron Transfer Rather Than Proton Transfer

“…Similar results were observed for the TICT states of the triazine ethers. 27 However, in that case, the shorter lifetime component corresponded to the locally excited state, whereas the longer lifetime component was assigned to the TICT state. We suggest that in our system, both states are TICT states.…”

“…The concept of intramolecular electron transfer as an alternative mode of action for ultraviolet stabilizers has been suggested, 26 and TICT states have been observed in ether derivatives of hydroxyphenyltriazine UVAs. 26,27 However, for the triazine ethers, TICT states are longer lived than the locally excited states, allowing more time for competitive photoreaction. As a result, the ethers serve as poor UVAs.…”

Optical Design of Porous ZnO/TiO2 Films for Highly Transparent Glasses with Broadband Ultraviolet Protection

“…Representatively, ZnO has the UV absorptivity in both UVB and UVA due to the relatively low band gap energy of 3.31 eV, whereas TiO 2 has strong absorption only at UVB due to the relatively high band gap energy of 3.5 eV. Although organic UV absorbers were also used for UV protection to materials or skin, there is a limitation for application to embed the absorbers due to the low stability of the extensively used polymer matrices upon irradiation with UV light [12,13]. Typically, UV shielding is often required together with high visible-light transparency in applications such as safety glasses, automobiles, and windows [9].…”

Adaptable Ultraviolet Reflecting Polymeric Multilayer Coatings of High Refractive Index Contrast

“…Polymers are usually transparent to part of the UV spectrum, so a commonly employed synthetic strategy to attain UV blocking fi lms is based on embedding inorganic [3][4][5][6][7] or organic [8][9][10] UV absorbers within a polymeric matrix. The main drawback of this approximation is the short term durability of the shielding effect as a result of the photodegradation of either host, guest, or both, caused by the absorption of the same radiation from which protection is sought after.…”