Department of Chemistry, West Bengal State University, Barasat, Kolkata 700126, India.
Langmuir. 2012 May 8;28(18):7147-59. doi: 10.1021/la300103a. Epub 2012 Apr 27.
The dynamics of the excited-state intramolecular proton-transfer (ESIPT) reaction of 2-(2'-furyl)-3-hydroxychromone (FHC) was studied in micelles by time-resolved fluorescence. The proton-transfer dynamics of FHC was found to be sensitive to the hydration and charge of the micelles, demonstrated through a decrease of the ESIPT rate constant (k(PT)) in the sequence cationic → nonionic → anionic micelles. A remarkably slow ESIPT with a time constant (τ(PT)) of ~100 ps was observed in the anionic sodium dodecyl sulfate and sodium tetradecyl sulfate micelles, whereas it was quite fast (τ(PT) ≈ 15 ps) in the cationic cetyltrimethylammonium bromide and tetradecyltrimethylammonium bromide micelles. In the nonionic micelles of Brij-78, Brij-58, Tween-80, and Tween-20, ESIPT occurred with time constants (τ(PT) ≈ 35-65 ps) intermediate between those of the cationic and anionic micelles. The slower ESIPT dynamics in the anionic micelles than the cationic micelles is attributed to a relatively stronger hydration of the negatively charged headgroups of the former than the positively charged headgroups of the latter, which significantly weakens the intramolecular hydrogen bond of FHC in the Stern layer of the anionic micelles compared to the latter. In addition, electrostatic attraction between the positively charged -N(CH(3))(3)(+) headgroups and the negatively charged 4-carbonyl moiety of FHC effectively screens the intramolecular hydrogen bond from the perturbation of water molecules in the micelle-water interface of the cationic micelles, whereas in the anionic micelles, this screening of the intramolecular hydrogen bond is much less efficient due to an electrostatic repulsion between its negatively charged -OSO(3)(-) headgroups and the 4-carbonyl moiety. As for the nonionic micelles, a moderate level of hydration, and the absence of any charged headgroups, causes an ESIPT dynamics faster than that of the anionic but slower than that of the cationic micelles. Furthermore, the ESIPT rate decreased with a decrease of the hydrophobic chain length of the surfactants due to the stronger hydration of the micelles of shorter chain surfactants than those of longer chain surfactants, arising from a less compact packing of the former surfactants compared to the latter surfactants.
研究了 2-(2'-呋喃基)-3-羟基色酮（FHC）在胶束中的激发态分子内质子转移（ESIPT）反应的动力学。通过观察 ESIPT 速率常数（k（PT））在阳离子→非离子→阴离子胶束中的顺序降低，发现 FHC 的质子转移动力学对胶束的水合作用和电荷敏感。在阴离子十二烷基硫酸钠和十四烷基硫酸钠胶束中观察到非常缓慢的 ESIPT，时间常数（τ（PT））约为 100 ps，而在阳离子十六烷基三甲基溴化铵和十四烷基三甲基溴化铵胶束中则非常快（τ（PT）≈15 ps）。在 Brij-78、Brij-58、Tween-80 和 Tween-20 的非离子胶束中，ESIPT 的时间常数（τ（PT）≈35-65 ps）介于阳离子和阴离子胶束之间。阴离子胶束中 ESIPT 动力学较慢归因于前一种胶束的带负电荷的头基比后一种胶束的带正电荷的头基相对更强的水合作用，这显著削弱了 FHC 在阴离子胶束的斯特恩层中的分子内氢键，而与后者相比。此外，带正电荷的-N(CH(3))(3)(+)头基与 FHC 的带负电荷的 4-羰基部分之间的静电吸引有效地屏蔽了阳离子胶束中胶束-水界面水分子对分子内氢键的干扰，而在阴离子胶束中，由于带负电荷的-OSO(3)(-)头基与 4-羰基部分之间的静电排斥，这种分子内氢键的屏蔽效率要低得多。对于非离子胶束，适度的水合作用和没有任何带电头基导致 ESIPT 动力学比阴离子胶束快，但比阳离子胶束慢。此外，由于较短链表面活性剂的胶束比较长链表面活性剂的胶束具有更强的水合作用，ESIPT 速率随着表面活性剂的疏水链长的减小而降低，这是由于前者表面活性剂的组装不如后者表面活性剂紧凑。
