Samant Vaishali, Singh Ajay K, Ramakrishna G, Ghosh Hirendra N, Ghanty Tapan K, Palit Dipak K
Radiation Chemistry & Chemical Dynamics Division, Bhabha Atomic Research Center, Mumbai 400089, India.
J Phys Chem A. 2005 Oct 6;109(39):8693-704. doi: 10.1021/jp050848f.
Steady-state fluorescence and time-resolved absorption measurements in pico- and femtosecond time domain have been used to investigate the dynamics of hydrogen bond in the excited singlet (S(1)) state of fluorenone in alcoholic solvents. A comparison of the features of the steady-state fluorescence spectra of fluorenone in various kinds of media demonstrates that two spectroscopically distinct forms of fluorenone in the S(1) state, namely the non-hydrogen-bonded (or free) molecule as well as the hydrogen-bonded complex, are responsible for the dual-fluorescence behavior of fluorenone in solutions of normal alcoholic solvents at room temperature (298 K). However, in 2,2,2-trifluoroethanol (TFE), a strong hydrogen bond donating solvent, emission from only the hydrogen-bonded complex is observed. Significant differences have also been observed in the temporal evolution of the absorption spectroscopic properties of the S(1) state of fluorenone in protic and aprotic solvents following photoexcitation using 400 nm laser pulses. An ultrafast component representing the solvent-induced vibrational energy relaxation (VER) process has been associated with the dynamics of the S(1) state of fluorenone in all kinds of solvents. However, in protic solvents, in addition to the VER process, further evolution of the spectroscopic and dynamical properties of the S(1) state have been observed because of repositioning of the hydrogen bonds around the carbonyl group. In normal alcohols, two different kinds of hydrogen-bonded complex of the fluorenone-alcohol system with different orientations of the hydrogen bond with respect to the carbonyl group and the molecular plane of fluorenone have been predicted. On the other hand, in TFE, formation of only one kind of hydrogen-bonded complex has been observed. These observations have been supported by theoretical calculations of the geometries of the hydrogen-bonded complexes in the ground and the excited states of fluorenone. Linear correlation between the lifetimes of the equilibration process occurring because of repositioning of the hydrogen bonds and Debye or longitudinal relaxation times of the normal alcoholic solvents establish the fact that, in weakly hydrogen bond donating solvents, the hydrogen bond dynamics can be described as merely a solvation process. Whereas, in TFE, hydrogen bond dynamics is better described by a process of conversion between two distinct excited states, namely, the non-hydrogen-bonded form and the hydrogen-bonded complex.
利用皮秒和飞秒时域的稳态荧光和时间分辨吸收测量来研究芴酮在醇类溶剂中激发单重态（S(1)）的氢键动力学。对芴酮在各种介质中的稳态荧光光谱特征进行比较表明，芴酮在S(1)态存在两种光谱上不同的形式，即非氢键结合（或游离）分子以及氢键结合复合物，它们是室温（298K）下芴酮在普通醇类溶剂溶液中呈现双荧光行为的原因。然而，在强氢键供体溶剂2,2,2-三氟乙醇（TFE）中，仅观察到来自氢键结合复合物的发射。在使用400nm激光脉冲光激发后，芴酮S(1)态在质子性和非质子性溶剂中的吸收光谱性质的时间演化也观察到了显著差异。一个代表溶剂诱导振动能量弛豫（VER）过程的超快成分与芴酮在所有溶剂中的S(1)态动力学相关。然而，在质子性溶剂中，除了VER过程外，由于羰基周围氢键的重新定位，还观察到了S(1)态光谱和动力学性质的进一步演化。在普通醇类中，预测了芴酮-醇体系存在两种不同类型的氢键结合复合物，其氢键相对于芴酮的羰基和分子平面具有不同的取向。另一方面，在TFE中，仅观察到一种氢键结合复合物的形成。这些观察结果得到了芴酮基态和激发态氢键结合复合物几何结构的理论计算的支持。由于氢键重新定位而发生的平衡过程的寿命与普通醇类溶剂的德拜或纵向弛豫时间之间的线性相关性证实了这样一个事实，即在弱氢键供体溶剂中，氢键动力学可以仅仅被描述为一个溶剂化过程。而在TFE中，氢键动力学可以更好地用两种不同激发态之间的转换过程来描述，即非氢键结合形式和氢键结合复合物。
