964 J. Am. Chem. SOC. zyxwvu 1992, 114, 964-966 Photophysical Properties of Some 2-( 2’-Hydroxyaryl)benzotriazoles: Dramatic Effect of an Ortho-Located Bulky tert-Butyl Group Javier C a t a h , * Pilar Perez: Fernando Fabero,? John F. K. Wilshire,* Rosa Ma. Claramunt,s and Jose Elgueros Contribution from Departamento de Quimica Fisica, Facultad de Ciencias, Universidad Autdnoma de Madrid, Cantoblanco, 28049 Madrid, Spain, Division zyxwv of Biomolecular Engineering, CSIRO, 343 Royal Parade, Parkville, Victoria 3052, Australia, and Departamento de Quimica Orgrinica, UNED and CSIC, Ciudad Universitaria, 28040 Madrid, Spain. Received June 6, 1991, Revised Manuscript Received September 23, 1991 Abstract: Photophysical experiments carried out on 3’-tert-butyl and 5’-tert-butyl analogues of Tinuvin P provided evidence concerning the origin of the s+called ‘open” forms. They result from the rotation of the hydroxyl groupfollowed by the rotation of the phenyl group, not the other way round. This mechanism is consistent with the negligible amount of ‘open” forms in the 3’-tert-butyl derivative in a base so strong as DMSO. In this series of photoprotectors, a bulky alkyl substituent in ortho position with regard to the hydroxyl group improves considerably their photostability. The 2-(2’-hydroxypheny1)benzotriazoles are ultraviolet ab- sorbers which, because of their excellent photostability, constitute one of the most important classes of photoprotecting agents. Their best known representative, the 5’-methyl derivative (Tinuvin P, l), exhibits marked photostability in nonpolar solvents, e.g. n- H-0, Me 1 H-0, H-0, ,tBu tBu 2 3 hexane, in which aR = 1.7 X However, its photostability is reduced considerably in polar solvents, e.g., acetonitrile, in which aR = 2.1 X lo”.’ Interestingly, parallel solvent-dependent photophysical behavior has also been observed. Thus, in inert solvents,” e.g. methylcyclohexaneisopentane (1 : l), cyclohexane, and n-heptane, Tinuvin P neither fluoresces (at room temperature) nor phosphoresces (at 77 K). However, in polar solvents (cf. ref 4), e.g., diethyl ether, tetrahydrofuran, dimethylformamide, methanol, ethanol, or dimethyl sulfoxide and in a mixture of ethanol, ether, and ~yridine,~ very weak fluorescence and phos- phorescence were both observed. In the ground state (X-ray determinati~n),~ Tinuvin P is planar and possesses a strong intramolecular hydrogen bond (IMHB) (see structure l), the latter feature being considered to be re- sponsible for its exceptional photophysical How- ever, if the IMHB is broken, e.g. by environmental factors, then the molecule loses its planarity, and rotation about the N(2)-C( 1’) bond occurs.6.8*12-18 If this happens, e.g., in solution, then the IMHB is replaced by an intermolecular bond involving the O H group and the solvent, presumably at a site in the latter more basic than is either N(l) or N(3). There is general agreement6 that nonplanar forms of Tinuvin P are luminiscent. Relevant in this regard is the fact that 2-(2’-methoxyphenyl)benzotriazole (Tinuvin P methyl ether, (structure 1 with the OH group replaced by an *To whom correspondence should be addressed at the Universidad Autdnoma de Madrid. ‘Universidad Autdnoma de Madrid. ‘CSIRO. f UNED and CSIC. 0002-786319211514-964%03.00/0 zyxwvuts 0 OMe group), which is nonplanar in the crystalline state (the torsion angle about the N(2)-C( 1’) bond is 56°)3 and does not possess an IMHB, has been found to be l~minescent.~ It is of course intrinsically desirable that the formation of the luminiscent forms of a photoprotecting agent be avoided. Oth- erwise, not only is the agent itself likely to be degraded by light but also the substrate being protected (photosensitization of chromophores or impurities in the latter can occur). A possible method for minimizing environmental effects was f i s t suggested by Heller,7 who reasoned that the introduction of a large alkyl group ortho to the OH group would shield the IMHB from being broken. Recently, Wilshire et al.19-21 have provided support for (1) Otterstedt, J. E. A. J. Chem. Phys. 1973, 58, 5716. (2) Werner, T. J. Phys. Chem. 1979, 83, 320. (3) Woessner, G.; Goeller, G.; Rieker, J.; Hoier, H.; Stezowski, J. J.; Daltrozzo, E.; Neureiter, M.; Kramer, H. E. A. J. Phys. Chem. 1985, 89, 3269. (4) Ghiggino, K. P.; Scully, A. D.; Leaver, I. H. J. Phys. Chem. 1986, 90, 5089. (5) Woessner, G.; Goeller, G.; Kollat, P.; Stezowski, J. J.; Hauser, M.; Klein, U. K. A.; Kramer, H. E. A. J. Phys. Chem. 1984, 88, 5544. (6) Kramer, H. E. A. In Studies in Organic Chemistry, 40 Photochrom- ism, Molecules and Systems; Durr, H., BouasLaurent, H., Eds.; Elsevier: Amsterdam, 1990. (7) Heller, H. J. Eur. Polym. J. Suppl. 1969, 105. (8) Klopffer, W. Adu. Photochem. 1977, 10, 311. (9) Allen, N. S. Polym. Photochem. 1983, 3, 167. (10) Rabek, J. F. In Mechanism of Photophysical Processes and Photo- chemical Reactions in Polymers; Wiley: New York, 1987. (!1) Catalln, J.; Fabero, F.; Guijarro, M. S.; Claramunt, R. M.; Santa Maria, M. D.; Foces-Foces, M. C.; Cano, F. H.; Elguero, J.; Sastre, R. J. Am. Chem. SOC. 1990, 112, 747; Ibid. 1991, 113, 4046. (12) Leaver, I. H.; Waters, P. J.; Evans, N. A. J. Polym. Sci. Polym. Chem. Ed. 1979, 17, 1531. (13) Evans, N. A. Austr. J. Chem. 1981, 34, 691. (14) Leaver, I. H. J. Polym. Sci. Polym. Chem. Ed. 1982, 20, 2417 and 2429. (15) Huston, A. L.; Scott, G. W. Proc. SOC. Photo-Opt. Instrum. Eng. 1982, 322, 215. (16) Werner, T.; Woessner, G.; Kramer, H. E. A. ACS Symp. Ser. 1980, (17) Bocian, D. F.; Huston, A. L.; Scott, G. W. J. Chem. Phys. 1983, 79, (18) Kim, Y. R.; Yardley, J. T.; Hochstrasser, R. M. Chem. Phys. 1989, (19) Evans, N. A.; Rosevear, J.; Waters, P. J.; Wilshire, J. F. K. Proc. 7th (20) Carr, C. M.; Leaver, I. H.; Wilshire, J. F. K. Proc. 7th Int. Wool No. 151. 5802. 136, 311. In!. Wool Text. Res. Conf. Tokyo 1985, 4, 41. Text. Res. Con$ Tokyo 1985, 4, 50. 1992 American Chemical Society