National Centre for Ultrafast Processes, Taramani Campus, University of Madras, Chennai 600 113, India.
J Phys Chem A. 2012 Jan 12;116(1):37-45. doi: 10.1021/jp207495r. Epub 2011 Dec 23.
Proflavine (3,6-diaminoacridine) shows fluorescence emission with lifetime, 4.6 ± 0.2 ns, in all the solvents irrespective of the solvent polarity. To understand this unusual photophysical property, investigations were carried out using steady state and time-resolved fluorescence spectroscopy in the pico- and femtosecond time domain. Molecular geometries in the ground and low-lying excited states of proflavine were examined by complete structural optimization using ab initio quantum chemical computations at HF/6-311++G** and CIS/6-311++G** levels. Time dependent density functional theory (TDDFT) calculations were performed to study the excitation energies in the low-lying excited states. The steady state absorption and emission spectral details of proflavine are found to be influenced by solvents. The femtosecond fluorescence decay of the proflavine in all the solvents follows triexponential function with two ultrafast decay components (τ(1) and τ(2)) in addition to the nanosecond component. The ultrafast decay component, τ(1), is attributed to the solvation dynamics of the particular solvent used. The second ultrafast decay component, τ(2), is found to vary from 50 to 215 ps depending upon the solvent. The amplitudes of the ultrafast decay components vary with the wavelength and show time dependent spectral shift in the emission maximum. The observation is interpreted that the time dependent spectral shift is not only due to solvation dynamics but also due to the existence of more than one emitting state of proflavine in the solvent used. Time resolved area normalized emission spectral (TRANES) analysis shows an isoemissive point, indicating the presence of two emitting states in homogeneous solution. Detailed femtosecond fluorescence decay analysis allows us to isolate the two independent emitting components of the close lying singlet states. The CIS and TDDFT calculations also support the existence of the close lying emitting states. The near constant lifetime observed for proflavine in different solvents is suggested to be due to the similar dipole moments of the ground and the evolved emitting singlet state of the dye from the Franck-Condon excited state.
吖啶（3,6-二氨基吖啶）在所有溶剂中均表现出荧光发射寿命为 4.6±0.2ns，与溶剂极性无关。为了理解这种不寻常的光物理性质，使用皮秒和飞秒时间域中的稳态和时间分辨荧光光谱进行了研究。在 HF/6-311++G和 CIS/6-311++G水平上通过完全结构优化，使用从头算量子化学计算研究了吖啶的基态和低激发态的分子几何形状。进行了时间依赖密度泛函理论（TDDFT）计算以研究低激发态的激发能。发现吖啶的稳态吸收和发射光谱细节受溶剂影响。吖啶在所有溶剂中的飞秒荧光衰减都遵循三指数函数，除了纳秒分量外，还有两个超快衰减分量（τ(1)和 τ(2)）。超快衰减分量 τ(1)归因于所用特定溶剂的溶剂化动力学。第二超快衰减分量 τ(2)发现取决于溶剂，其范围从 50 到 215ps。超快衰减分量的幅度随波长而变化，并在发射最大值处显示出随时间变化的光谱位移。观察结果表明，时间相关的光谱位移不仅是由于溶剂化动力学，而且还由于在所用溶剂中存在吖啶的多于一个发射态。时间分辨面积归一化发射光谱（TRANES）分析显示等发射点，表明在均相溶液中存在两个发射态。详细的飞秒荧光衰减分析允许我们分离出紧密相邻的单重态的两个独立发射组件。CIS 和 TDDFT 计算也支持存在紧密相邻的发射态。吖啶在不同溶剂中观察到的近恒定寿命被认为是由于染料的基态和从 Franck-Condon 激发态演化的发射单重态的相似偶极矩。
