Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India.
Chemphyschem. 2009 Dec 7;10(17):2979-94. doi: 10.1002/cphc.200900309.
The dynamics of the excited states of 1-aminofluoren-9-one (1AF) and 1-(N,N-dimethylamino)-fluoren-9-one (1DMAF) are investigated by using steady-state absorption and fluorescence as well as subpicosecond time-resolved absorption spectroscopic techniques. Following photoexcitation of 1AF, which exists in the intramolecular hydrogen-bonded form in aprotic solvents, the excited-state intramolecular proton-transfer reaction is the only relaxation process observed in the excited singlet (S(1)) state. However, in protic solvents, the intramolecular hydrogen bond is disrupted in the excited state and an intermolecular hydrogen bond is formed with the solvent leading to reorganization of the hydrogen-bond network structure of the solvent. The latter takes place in the timescale of the process of solvation dynamics. In the case of 1DMAF, the main relaxation pathway for the locally excited singlet, S(1)(LE), or S(1)(ICT) state is the configurational relaxation, via nearly barrierless twisting of the dimethylamino group to form the twisted intramolecular charge-transfer, S(1)(TICT), state. A crossing between the excited-state and ground-state potential energy curves is responsible for the fast, radiationless deactivation and nonemissive character of the S(1)(TICT) state in polar solvents, both aprotic and protic. However, in viscous but strong hydrogen-bond-donating solvents, such as ethylene glycol and glycerol, crossing between the potential energy surfaces for the ground electronic state and the hydrogen-bonded complex formed between the S(1)(TICT) state and the solvent is possibly avoided and the hydrogen-bonded complex is weakly emissive.
通过稳态吸收和荧光以及亚皮秒时间分辨吸收光谱技术研究了 1-氨基芴-9-酮（1AF）和 1-（N，N-二甲基氨基）-芴-9-酮（1DMAF）的激发态动力学。在非质子溶剂中，1AF 以分子内氢键形式存在，其激发态为单重激发态（S(1)），激发后仅观察到分子内质子转移反应为唯一的弛豫过程。然而，在质子溶剂中，分子内氢键在激发态下被破坏，并与溶剂形成分子间氢键，导致溶剂氢键网络结构的重新排列。后者发生在溶剂化动力学过程的时间尺度内。对于局部激发单重态 S(1)(LE)或 S(1)(ICT)，1DMAF 的主要弛豫途径是构象松弛，通过二甲氨基基团几乎无势垒的扭曲形成扭曲的分子内电荷转移态 S(1)(TICT)。激发态和基态势能曲线之间的交叉是导致极性溶剂中 S(1)(TICT)态快速无辐射失活和非辐射特性的原因，无论是质子性溶剂还是非质子性溶剂。然而，在粘性但强供氢溶剂中，如乙二醇和甘油，S(1)(TICT)态和溶剂之间形成的氢键复合物的基态电子势能面和氢键复合物之间的交叉可能被避免，氢键复合物的荧光强度较弱。
