高pH介质中1-氨基萘的双重荧光——氨基的激发态酸式电离

1－萘胺（NA）的基态与激发态电离已有诸多报道，但主要集中于低pH介质中1－萘铵离子的酸碱电离过程，并已在NA的酸性水溶液中观察到了分别对应于萘铵离子和中性萘胺分子的双重荧光^1]。萘胺本身作为“酸”的酸碱电离研究则未见报道。已知苯胺的基态pKa。高达25^2]，这可能是难于观察到NA基态酸式电离的重要原因之一。近年来，由于对认识光合作用原初过程具有重要启示意义，有关诱导激发态质子转移和电子转移偶合的研究备受关注^3-5]，并且已经注意到二者之间的相互促进作用。在激发态NA分子中，由于分子内发生电荷转移(ICT)过程 ^6,7]，氨氮原子上电荷密度下降，氨基的酸性将可能大幅度提高^8]，从而有可能在高pH介质中观察到萘胺的激发态酸碱电离过程。因此，我们设计合成了NA的系列衍生物1－萘胺乙酸（NAA）和1－萘胺二乙酸（NADA），研究了NA，NAA，NADA和1－二甲氨基萘（DMAN）（图1）在高pH水溶液中的荧光光谱，首次观察到了NA和NAA在高pH介质中的双重荧光，揭示了1－萘胺中氨基的激发态酸式电离性质。

Dual Fluorescence from Aqueous 1-Naphthylamine Solutions of High pH Excited-State Acidic Dissociation of Naphthylamine

Dual fluorescence at ca. 447 nm and 545 nm was observed from the aqueous 1-naphthylamine (NA) solutions at pHhigher than 13.6. Similar dual fluorescence was also found with sodium 1-naphthylaminoacetate(NAA), but not with N, N-disubstituted 1-aminonaphthalenes such as sodium 1-naphthylaminodiacetate(NADA) and 1-dimethylaminonaphthalene(DMAN). No change in absorption spectra of NAand NAAwas observed in this pHregion.It was proposed that the dual fluorescence observed with NAand NAAwas due to the excited state dissociation of the primary and secondary amines at high pH. From the dual fluorescence intensity ratio pHtitration curve, the p K_a^*' s of NAand NAAwere estimated to be between 14 and 15 which are much lower than the ground state p K_a. The novel approatch is such a simple, convenient and frequent analysis technique that it can be widely used in detecting the substitutional derivatives of aminonaphthalene.