Freitas Adilson A, Quina Frank H, Maçanita António A L
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa , 1649-004 Lisbon, Portugal.
J Phys Chem A. 2014 Nov 13;118(45):10448-55. doi: 10.1021/jp504189m. Epub 2014 Jun 13.
Synthetic flavylium salts like the 7-hydroxy-4-methylflavylium (HMF) cation have been used as prototypes to study the chemistry and photochemistry of anthocyanins, the major group of water-soluble pigments in the plant kingdom. In this work, a combination of fluorescence upconversion with femtosecond time resolution and time-correlated single photon counting (TCSPC) with picosecond time resolution have been employed to investigate in details the excited-state proton transfer (ESPT) of HMF in water and in binary water/1,4-dioxane mixtures. TCSPC measurements as a function of temperature provide activation parameters for all of the individual rate constants involved in the proton transfer, including those for dissociation and recombination of the geminate excited base-proton pair (A*···H(+)) that can be detected in the water/dioxane mixtures (but not in water). Unlike the other rate constants, the deprotonation rate constant kd shows a non-Arrhenius dependence on temperature in both water and water/dioxane mixtures. At low temperatures kd is close to the dielectric relaxation rate of the solvent with a barrier of ca. 8 kJ mol(-1), suggesting that the solvent reorganization is the rate-limiting step. At higher temperatures (>30 °C) the proton transfer process is nearly barrierless and solvent-dependent. Fluorescence upconversion results in H2O, D2O, and water/dioxane mixtures confirm the two-step model for the ESPT of HMF and provide additional details of the early events prior to the onset of proton transfer, attributed to conformational relaxation and solvent reaccommodation around the initially formed excited state. The results are consistent with DFT calculations that indicate that charge redistribution occurs after rather than prior to the onset of the ESPT process.
像7-羟基-4-甲基黄酮鎓（HMF）阳离子这样的合成黄酮盐已被用作研究花青素化学和光化学的原型，花青素是植物界中主要的水溶性色素类别。在这项工作中，结合了具有飞秒时间分辨率的荧光上转换和具有皮秒时间分辨率的时间相关单光子计数（TCSPC），以详细研究HMF在水和二元水/1,4-二氧六环混合物中的激发态质子转移（ESPT）。作为温度函数的TCSPC测量提供了质子转移中涉及的所有单个速率常数的活化参数，包括在水/二氧六环混合物（但不是在水中）中可以检测到的双生激发碱-质子对（A*···H(+)）的解离和重组的速率常数。与其他速率常数不同，去质子化速率常数kd在水和水/二氧六环混合物中均表现出对温度的非阿累尼乌斯依赖性。在低温下，kd接近溶剂的介电弛豫速率，势垒约为8 kJ mol(-1)，这表明溶剂重组是速率限制步骤。在较高温度（>30°C）下，质子转移过程几乎无势垒且依赖于溶剂。在H2O、D2O和水/二氧六环混合物中的荧光上转换结果证实了HMF的ESPT的两步模型，并提供了质子转移开始之前早期事件的更多细节，这些早期事件归因于围绕最初形成的激发态的构象弛豫和溶剂再适应。结果与DFT计算一致，DFT计算表明电荷重新分布发生在ESPT过程开始之后而非之前。
