Pines Ehud, Pines Dina, Gajst Oren, Huppert Dan
Department of Chemistry, Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva 84105, Israel.
Raymond and Beverly Sackler Faculty of Exact Sciences, School of Chemistry, Tel Aviv University, Tel Aviv 69978, Israel.
J Chem Phys. 2020 Feb 21;152(7):074205. doi: 10.1063/1.5134760.
Steady-state and time-resolved fluorescence techniques were employed to study the excited-state proton transfer (ESPT) from a reversibly dissociating photoacid, 2-naphthol-6,8-disulfonate (2N68DS). The reaction was carried out in water and in acetonitrile-water solutions. We find by carefully analyzing the geminate recombination dynamics of the photobase-proton pair that follows the ESPT reaction that there are two targets for the proton back-recombination reaction: the original O dissociation site and the SO side group at the 8 position which is closest to the proton OH dissociation site. This observation is corroborated in acetonitrile-water mixtures of χ < 0.14, where a slow intramolecular ESPT occurs on a time scale of about 1 ns between the OH group and the SO group via H-bonding water. The proton-transferred RO fluorescence band in mixtures of χ < 0.14 where only intramolecular ESPT occurs is red shifted by about 2000 cm from the free RO band in neat water. As the water content in the mixture increases above χ = 0.14, the RO fluorescence band shifts noticeably to the blue region. For χ > 0.23 the band resembles the free anion band observed in pure water. Concomitantly, the ESPT rate increases when χ increases because the intermolecular ESPT to the solvent (bulk water) gradually prevails over the much slower intramolecular via the water-bridges ESPT process.
采用稳态和时间分辨荧光技术研究了可逆解离光酸2-萘酚-6,8-二磺酸盐（2N68DS）的激发态质子转移（ESPT）。该反应在水和乙腈-水溶液中进行。通过仔细分析ESPT反应后光碱-质子对的双分子复合动力学，我们发现质子反向复合反应有两个目标：原来的O解离位点和8位最接近质子OH解离位点的SO侧基。在χ<0.14的乙腈-水混合物中得到了证实，其中在OH基团和SO基团之间通过氢键水在约1 ns的时间尺度上发生缓慢的分子内ESPT。在χ<0.14的混合物中仅发生分子内ESPT，质子转移的RO荧光带比纯水中的游离RO带红移约2000 cm。随着混合物中水含量增加到χ>0.14以上，RO荧光带明显向蓝色区域移动。对于χ>0.23，该带类似于在纯水中观察到的游离阴离子带。同时，当χ增加时ESPT速率增加，因为向溶剂（大量水）的分子间ESPT逐渐超过通过水桥的分子内ESPT过程，而后者要慢得多。
