Choudhury Sharmistha Dutta, Pal Haridas
Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India.
J Phys Chem B. 2009 May 14;113(19):6736-44. doi: 10.1021/jp8111759.
The prototropic behavior of the dye, 7-hydroxy-4-methylcoumarin (7H4MC), has been studied in cationic benzyldimethylhexadecylammonium chloride (BDHC) and nonionic poly(oxyethylene)(tetramethylbutyl)phenyl ether (TritonX-100, TX-100) reverse micelles using ground-state absorption and steady-state and time-resolved fluorescence measurements. The results have been compared with the previous results in the anionic sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles. Although the probe dye, 7H4MC, is indicated to reside in the interfacial region in all of the reverse micelles studied, significant differences have been observed in the evolution of the different prototropic species. In BDHC reverse micelles, the anionic form is favored over the tautomeric form at the higher w0 values, which is contrary to the observation in AOT reverse micelles where both of these forms are almost equally produced. The higher propensity for the formation of the anionic form in BDHC reverse micelles has been explained on the basis of the additional electrostatic stabilization of the anionic species in the cationic BDHC reverse micelles compared to that in the anionic AOT reverse micelles. On the other hand, in TX-100 reverse micelles, the anionic form is not very evident, but interestingly, the tautomer form begins to appear beyond w0=2. The appearance of the tautomeric species apparently coincides with the formation of the water pool in the TX-100 reverse micelles. However, due to the more restricted nature of the water molecules within this reverse micelle (mostly dispersed around the oxyethylene chains), deprotonation of the 7H4MC dye and the consequential stabilization of the anionic form are considerably reduced. The results clearly reveal that the aqueous environment in the vicinity of the probe is quite different for the reverse micelles considered, and these differences largely modulate the prototropic processes of the excited dye.
利用基态吸收、稳态和时间分辨荧光测量方法，研究了染料7-羟基-4-甲基香豆素（7H4MC）在阳离子苄基二甲基十六烷基氯化铵（BDHC）和非离子聚（氧乙烯）（四甲基丁基）苯基醚（TritonX-100，TX-100）反胶束中的质子转移行为。已将结果与先前在阴离子1,4-双（2-乙基己基）磺基琥珀酸钠（AOT）反胶束中的结果进行了比较。尽管探针染料7H4MC在所有研究的反胶束中都表明位于界面区域，但在不同质子转移物种的演变中观察到了显著差异。在BDHC反胶束中，在较高的w0值下，阴离子形式比互变异构形式更受青睐，这与在AOT反胶束中的观察结果相反，在AOT反胶束中这两种形式几乎等量产生。与阴离子AOT反胶束相比，阳离子BDHC反胶束中阴离子物种的额外静电稳定作用解释了BDHC反胶束中阴离子形式形成的更高倾向。另一方面，在TX-100反胶束中，阴离子形式不是很明显，但有趣的是，互变异构形式在w0 = 2以上开始出现。互变异构物种的出现显然与TX-100反胶束中水池的形成相吻合。然而，由于该反胶束中水分子的性质更为受限（大多分散在氧乙烯链周围），7H4MC染料的去质子化以及阴离子形式的相应稳定化大大降低。结果清楚地表明，所考虑的反胶束中探针附近的水环境有很大不同，并且这些差异在很大程度上调节了激发染料的质子转移过程。
