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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Excited-State Proton Transfer to Solvent from Phenol and Cyanophenols in Water
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Excited-State Proton Transfer to Solvent from Phenol and Cyanophenols in Water

机译：激发态质子从水中的苯酚和氰基酚转移到溶剂中

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摘要

The excited-state proton transfer (ESPT) to solvent from phenol (PhOH) and cyanophenols (CNOHs) in water was studied by means of time-resolved fluorescence and photoacoustic spectroscopy. A characteristic property of PhOH and CNOHs is that the fluorescence quantum yields of the deprotonated forms are remarkably small (≤10~(-3)) and the lifetimes are extremely short (≤30 ps). Time-resolved fluorescence measurements for PhOH, CNOHs, and their methoxy analogues at 298 K indicate that o- and m-cyanophenols (o- and m-CNOH) undergo rapid ESPT to the solvent water with rate constants of 6.6 × 10~(10) and 2.6 × 10~(10) s~(-1), respectively, whereas the fluorescence properties of PhOH and p-CNOH does not exhibit clear evidence of the ESPT reaction. Photoacoustic measurements show that photoexcitation of o- and m-CNOH in water results in negative volume changes, supporting the occurrence of ESPT to produce a geminate ion pair. In contrast, the volume contractions for the PhOH and p-CNOH solutions are negligibly small, which indicates that, in these compounds, the yields of solvent-separated ion pairs resulting from the ESPT are very small. The volume change per absorbed Einstein (ΔV_r) for o-CNOH is obtained to be -5.0 mL Einstein~(-1), which is much smaller than the estimated volume contraction per photoconverted mole (ΔV_R). This suggests that the geminate recombination between the ejected proton and the cyanophenolate anion occurs after rapid deactivation of the excited ion pair. In the temperature range between 275 and 323 K, the proton dissociation rates of o- and m-CNOH in H_2O and D_2O are slower than the solvent relaxation rates evaluated from the Debye dielectric relaxation time, indicating that the overall rate constant is determined mainly by the proton motion along the reaction coordinate.
机译：通过时间分辨荧光和光声光谱法研究了水中苯酚（PhOH）和氰基酚（CNOHs）的激发态质子转移（ESPT）。 PhOH和CNOHs的特征是去质子化形式的荧光量子产率非常小（≤10〜（-3）），寿命极短（≤30ps）。 PhOH，CNOH及其甲氧基类似物在298 K的时间分辨荧光测量结果表明，邻-和间-氰基酚（o-和m-CNOH）对溶剂水进行快速ESPT，速率常数为6.6×10〜（10 ）和2.6×10〜（10）s〜（-1），而PhOH和p-CNOH的荧光性质没有清楚地证明ESPT反应。光声测量表明，水中邻-和--CNOH的光激发导致负体积变化，从而支持了ESPT的产生，从而产生了成对的离子对。相反，PhOH和p-CNOH溶液的体积收缩小到可以忽略不计，这表明在这些化合物中，由ESPT产生的溶剂分离离子对的收率非常小。对于o-CNOH，每吸收的爱因斯坦的体积变化（ΔV_r）为-5.0 mL爱因斯坦〜（-1），远小于每光转换摩尔的估计体积收缩（ΔV_R）。这表明在激发的离子对快速失活之后，所喷射的质子和氰基酚酸根阴离子之间发生了重整。在275至323 K的温度范围内，H_2O和D_2O中邻-和间-CNOH的质子解离速率比根据德拜电介质弛豫时间评估的溶剂弛豫速率慢，这表明总速率常数主要取决于质子沿反应坐标运动。

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