Departamento Engenharia Química, Centro de Química Estrutural, IST/UTL, Lisbon 1049-001, Portugal.
J Phys Chem A. 2010 Apr 1;114(12):4188-96. doi: 10.1021/jp100281u.
Three water-insoluble, micelle-anchored flavylium salts, 7-hydroxy-3-octyl-flavylium chloride, 4'-hexyl-7-hydroxyflavylium chloride, and 6-hexyl-7-hydroxy-4-methyl-flavylium chloride, have been employed to probe excited-state prototropic reactions in micellar sodium dodecyl sulfate (SDS). In SDS micelles, the fluorescence decays of these three flavylium salts are tetraexponential functions in the pH range from 1.0 to 4.6 at temperatures from 293 to 318 K. The four components of the decays are assigned to four kinetically coupled excited species in the micelle: specifically, promptly deprotonable (AH(+)) and nonpromptly deprotonable (AH(h)(+)) orientations of the acid in the micelle, the base-proton geminate pair (A*...H(+)), and the free conjugate base (A*). The initial prompt deprotonation to form the geminate pair occurs at essentially the same rate (k(d) approximately 6-7 x 10(10) s(-1)) for all three photoacids. Recombination of the geminate pair is approximately 3-fold faster than the rate of proton escape from the pair (k(rec) approximately 3 x 10(10) s(-1) and k(diss) approximately 1 x 10(10) s(-1)), corresponding to an intrinsic recombination efficiency of the pair of approximately 75%. Finally, the reprotonation of the short-lived free A* (200-350 ps, depending on the photoacid) has two components, only one of which depends on the proton concentration in the intermicellar aqueous phase. Ultrafast transfer of the proton to water and substantial compartmentalization of the photogenerated proton at the micelle surface on the picosecond time scale strongly suggest preferential transfer of the proton to preformed hydrogen-bonded water bridges between the photoacid and the anionic headgroups. This localizes the proton in the vicinity of the excited base much more efficiently than in bulk water, resulting in the predominance of geminate reprotonation at the micelle surface.
三种不溶于水的胶束锚定的查耳酮盐，7-羟基-3-辛基查耳酮氯化物、4'-己基-7-羟基查耳酮氯化物和 6-己基-7-羟基-4-甲基查耳酮氯化物，已被用于探测胶束十二烷基硫酸钠（SDS）中激发态质子迁移反应。在 SDS 胶束中，这三种查耳酮盐的荧光衰减在 293 到 318 K 的温度范围内，在 pH 值为 1.0 到 4.6 之间是四指数函数。衰减的四个分量被分配给胶束中的四个动力学偶联激发态物种：即，酸在胶束中的即刻去质子化（AH(+)）和非即刻去质子化（AH(h)(+)）取向、碱基-质子成对（A*...H(+))和游离共轭碱（A*）。对于所有三种光酸，初始快速质子化形成成对质子对的速率几乎相同（k(d)约为 6-7 x 10(10) s(-1))。成对质子对的复合比质子对从质子对中逃逸的速率快约 3 倍（k(rec)约为 3 x 10(10) s(-1)和 k(diss)约为 1 x 10(10) s(-1)），对应于成对的内在复合效率约为 75%。最后，短寿命游离 A*（200-350 ps，取决于光酸）的再质子化有两个分量，只有一个分量取决于胶束内水相间的质子浓度。质子在皮秒时间尺度上向水的超快转移和光生质子在胶束表面的实质性隔室化强烈表明，质子优先转移到光酸和阴离子头基之间预先形成的氢键桥接的水中。这将质子在附近的位置有效地本地化了激基碱，比在体相水中更有效，从而导致在胶束表面主要发生成对质子化。
