Ultraviolet absorbers such as Tinuvin P (2-(2-hydroxy-5-methylphenyl)benzotriazole), 1, achieve their exceptional photostabilities as a result of deactivation of excited singlet states through excited state intramolecular proton transfer (ESIPT). Adding a methyl group to the 6′ position of 2-arylbenzotriazoles reveals an additional excited singlet state deactivation mechanism in this class of molecules which does not require intramolecular hydrogen bonding. Steady state fluorescence and fluorescence lifetime measurements for a series of 6′-methyl-2-arylbenzotriazoles provides compelling evidence for a twisted intramolecular charge transfer (TICT) mechanism of excited singlet state deactivation. Due to the steric requirements of the 6′-methyl group, conformations are favored in which the phenyl and triazole rings are no longer coplanar. In the case of compound 11 (2-(6-methoxy-2,3-dimethylphenyl)-2H-benzotriazole), the presence of a 2′-methoxy group enhances nonplanarity and results in large deactivation rates. Compound 12 (2-(6-methoxy-2,3-dimethylphenyl)-5-(trifluoromethyl)-2H-benzotriazole), which possesses both twist and enhanced donor/acceptor properties, undergoes the most efficient fluorescence quenching for the methoxyarylbenzotriazoles. Compounds with both a 6′-methyl and a hydroxy group on the phenyl ring exhibit diffusion controlled quenching (k q ) 2 × 10 10 M -1 s -1) by DMSO. This quenching appears to result from either partial or complete excited state proton transfer to DMSO, which enhances TICT deactivation of the singlet excited state.

A series of highly purified fatty acids and phospholipids each possessing a polar chain-substituent (hydroxy or keto) at varying locations (carbons 8,10,12, and 16 for the fatty acids and carbons 4, 6, 8,10, and 12 for the phospholipids) on an 18-carbon chain were synthesized. Pressure-area isotherms revealed how these molecules pack in monomolecular films. Most of the fatty acids and phospholipids exhibited pressure-area curves indicative of "looping" conformations where both the polar substituent and polar head group contact the aqueous subphase. As the pressure was increased, the polar substituents were forced out of the aqueous interface, and the chains assumed more vertical conformations. Pressure-area isotherms for the hydroxylated fatty acids showed unusually small molecular areas in the condensed state owing to the presence of hydrogen bonding. A phospholipid disubstituted at the 12 position with a keto group gave a molecular area of only 21.6 A2/molecule at 35 dyn/cm; this is consistent with two vertical chains, one in the water and one in the air. Isotherms reflected a strong dependence on the position of the polar substituent along the chain.

Differential scanning calorimetry has been applied to aqueous suspensions comprised of synthetic phospholipids on whose chains reside substituents of diverse size (methyl, n-butyl, phenyl), polarity (alkyl, keto), number (one or both chains), and location (carbons 4, 6, 8, 10, 12, or 16 of 18-carbon chains). Attention focused primarily on the "main" phase transition in the lipid bilayers resulting from disordering ("melting") of the hydrocarbon tails. The thermotropic behavior revealed, among the more interesting observations, that alkyl groups at the center of the chains are far more disruptive than groups at either end. For example, distearoylphosphatidylcholine (DSPC) has a main transition temperature (Tf of 54.8 °C. Methyl substitution at carbons 4, 10, and 16 of the second chain gives Tc values of 41.5, 5.6, and 38.5 °C, respectively. Similarly, DSPC has a transition enthalpy, , of 10.6 kcal/mol. Methyl substitution at carbons 4, 10, and 16 of the second chain gives values of 8.8, 3.7, and 9.8 kcal/mol, respectively. This remarkable midchain effect is rationalized on a molecular basis and reconciled with earlier work by others showing that mobility is restricted in the first half of the chain after which motional freedom and disorder increase uniformly up to the terminal methyl. Systematic "structure-activity" studies of lipids, such as presented here, are important in testing membrane models and in controlling membrane properties through rational design of the synthetic lipids.dered state (below the transition temperature, Tc) to a more disordered, fluidlike state (above the Tc). Tracings of excess heat capacity vs temperature provide (a) Tc, the temperature at the maximum in the excess heat capacity profile, (b) ^, the total enthalpy associated with the transition, (c) MfvH, the van't Hoff enthalpy, and (d) / ^= CU, the cooperativity unit.Below rc, the "gel" phase has its hydrocarbon chains existing mainly in all-trans configurations. Both chain motion and the number of gauche linkages increase in the "liquid-crystal" phase

No abstract

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.