Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India.
J Phys Chem B. 2010 Sep 23;114(37):12129-43. doi: 10.1021/jp1038249.
Dynamics of the excited singlet (S(1)) state of curcumin has been investigated in a wide varieties of solvents using subpicosecond time-resolved fluorescence and absorption spectroscopic techniques. As a consequence of extra stability of the cis-enol conformer due to the presence of an intramolecular hydrogen bond, it is the major form existing in the ground-state and the excited-state processes described here has been attributed to this form. Steady-state absorption and fluorescence spectra suggest significant perturbation of the intramolecular hydrogen bond and the possibility of formation of intermolecular hydrogen-bonded complex with the hydrogen-bonding solvents. Both the time-resolved techniques used here reveal that solvation is the major process contributing to the relaxation dynamics of the S(1) state. Solvation dynamics in protic solvents is multimodal, and the linear correlation between the longest component of the solvation process and the longitudinal relaxation time of the solvent suggests the specific hydrogen-bonding interaction between the solute and the solvent. However, a good correlation between the experimentally determined average solvation time and that predicted by the dielectric continuum model in all kinds of solvents also suggests that the dielectric relaxation of the solvent is also an important contributor to the solvation process. The lifetime of the S(1) state is very short in nonpolar solvents (∼44 ps in 1,4-dioxane) because of efficient nonradiative deactivation of the S(1) state, which is an important consequence of the ultrafast excited-state intramolecular hydrogen transfer (ESIHT) reaction in the six-membered hydrogen-bonded chelate ring of the cis-enol form. However, it has not been possible to monitor the ESIHT reaction in real time because of the symmetrical structure of the molecule with respect to the hydrogen-bonded chelate ring. In polar solvents, dipole-dipole interaction perturbs the intramolecular hydrogen bond leading to the reduced efficiency of the nonradiative deactivation process. However, stretching vibration in the intermolecular hydrogen bonds formed in the hydrogen-bonding (both donating and accepting) solvents induces another efficient channel for the nonradiative relaxation of the S(1) state of curcumin.
使用亚皮秒时间分辨荧光和吸收光谱技术，在多种溶剂中研究了姜黄素激发单线态（S(1)）态的动力学。由于顺式烯醇构象由于分子内氢键的存在而额外稳定，因此它是基态中存在的主要形式，这里描述的激发态过程归因于这种形式。稳态吸收和荧光光谱表明，分子内氢键受到显著干扰，并且与氢键溶剂形成分子间氢键复合物的可能性。这里使用的两种时间分辨技术都表明，溶剂化是导致 S(1)态弛豫动力学的主要过程。质子溶剂中的溶剂化是多模态的，溶剂化过程最长组分与溶剂的纵向弛豫时间之间的线性相关性表明溶质和溶剂之间存在特定的氢键相互作用。然而，在各种溶剂中，实验确定的平均溶剂化时间与介电连续体模型预测的平均溶剂化时间之间的良好相关性也表明溶剂的介电弛豫也是溶剂化过程的重要贡献者。由于 S(1)态的非辐射失活，S(1)态的寿命在非极性溶剂中非常短（在 1,4-二恶烷中约为 44 ps），这是非六元氢键螯合环中超快激发态分子内氢键转移（ESIHT）反应的重要结果顺式烯醇形式。然而，由于分子相对于氢键螯合环的对称结构，不可能实时监测 ESIHT 反应。在极性溶剂中，偶极-偶极相互作用会干扰分子内氢键，从而降低非辐射失活过程的效率。然而，在氢键（供体和受体）溶剂中形成的分子间氢键的伸缩振动会诱导姜黄素 S(1)态非辐射弛豫的另一个有效通道。
