Excited State Intramolecular Charge Transfer Suppressed Proton Transfer Process in 4‑(Diethylamino)-2-hydroxybenzaldehyde

In this work, we report intramolecular charge transfer (ICT) suppressed excited state intramolecular proton transfer (ESIPT) process in 4-(diethylamino)-2-hydroxybenzaldehyde (DEAHB). Photophysical properties of DEAHB have been extensively studied in different solvents with varying pH, polarity, and hydrogen bonding capability of the solvent using steady state and time-resolved spectroscopy. To establish the competition between the ICT and ESIPT processes in DEAHB, we have synthesized and studied the photophysical properties of 4-(diethylamino)-2-methoxybenzaldehyde (DEAMB) molecule where only the charge transfer process has been observed. Recently, we have reported simple Schiff base molecules (J. Phys. Chem. A 2012, 116, 10948) formed by condensation of DEAHB and hydrazine (5-(diethylamino)-2-[(4-(diethylamino)­benzylidene)­hydrazonomethyl]­phenol (DDBHP) and N,N′-bis­(4-N,N-(diethylamino)­salisalidene)­hydrazine (DEASH)), where charge transfer is assisted by the proton transfer process. In the present case, the DEAHB molecule shows the reverse phenomenon; i.e., charge transfer is suppressed by the proton transfer process. Comparing the photophysical properties of DEAHB with DEAMB it is also found that ICT process in DEAHB is suppressed by the ESIPT process.

本工作报道了4-(二乙氨基)-2-羟基苯甲醛(DEAHB)中激发态分子内质子转移(ESIPT)过程受分子内电荷转移(ICT)抑制的现象。本研究采用稳态与时间分辨光谱法，在pH、极性及氢键能力各不相同的多种溶剂中，对DEAHB的光物理性质进行了系统研究。为明确DEAHB中ICT与ESIPT过程间的竞争关系，我们合成了4-(二乙氨基)-2-甲氧基苯甲醛(DEAMB)并对其光物理性质开展研究，该分子仅表现出电荷转移过程。近期我们曾报道由DEAHB与肼缩合得到的简单希夫碱(Schiff base)分子（J. Phys. Chem. A 2012, 116, 10948）：5-(二乙氨基)-2-[(4-(二乙氨基)亚苄基)腙甲基]苯酚(DDBHP)与N,N′-双(4-N,N-二乙氨基水杨醛亚基)肼(DEASH)，此类分子的电荷转移过程受质子转移过程辅助。而在本研究中，DEAHB分子则表现出相反的现象：即电荷转移过程受到质子转移过程的抑制。通过对比DEAHB与DEAMB的光物理性质，还可证实DEAHB中的ICT过程确实受到ESIPT过程的抑制。