Ultrafast formation of the benzoic acid triplet upon ultraviolet photolysis and its sequential photodissociation in solution.
Ultrafast formation of the benzoic acid triplet upon ultraviolet photolysis and its sequential photodissociation in solution.
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DOI：
10.1063/1.4722084
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发表时间：
2012-05
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影响因子：
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通讯作者：
Chunfan Yang;Hongmei Su;Xuezhong Sun;M. George
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文献类型：
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作者：
Chunfan Yang;Hongmei Su;Xuezhong Sun;M. George
采用飞秒和纳秒时间域的时间分辨红外吸收光谱研究了苯甲酸在乙腈溶液中的光解反应。通过结合超快和纳秒TR-IR测量，激发态和光碎片都已被检测到，并获得了关键的机理见解。我们发现，溶剂的相互作用修改的激发态弛豫途径，从而人口动态，导致不同的光解行为在溶液中从气相中观察到的。向溶剂的振动能量转移有效地耗散了激发能，抑制了光解离，并在延迟的1.33ps开始后以1.325ps的速率将激发的S（2）或S（3）态分子减少到最低的T（1）态。苯甲酸在267 nm激发下的光解主要是T（1）激发态的形成，没有检测到光碎片。用193 nm的高能量进行的TR-IR实验结果表明，在这些实验中，光解离比振动能量转移到溶剂中更快，C-C键断裂成为主要的弛豫途径，其特征是COOH光碎片的显著观察和T（1）激发态的可忽略产率。测量到的T（1）激发态的超快形成支持S（2）/S（1）、S（2）/T（2）和S（1）/T（1）/T（2）的表面交叉的存在，并且高达0.65的T（1）量子产率表明激发态减少对三重态流形的重要性，其是影响单体苯甲酸的光物理和光化学行为的关键因素。
Time-resolved infrared (TR-IR) absorption spectroscopy in both the femtosecond and nanosecond time domain has been applied to examine the photolysis of benzoic acid in acetonitrile solution following either 267 nm or 193 nm excitation. By combining the ultrafast and nanosecond TR-IR measurements, both the excited states and the photofragments have been detected and key mechanistic insights were obtained. We show that the solvent interaction modifies the excited state relaxation pathways and thus the population dynamics, leading to different photolysis behavior in solution from that observed in the gas phase. Vibrational energy transfer to solvents dissipates excitation energy efficiently, suppressing the photodissociation and depopulating the excited S(2) or S(3) state molecules to the lowest T(1) state with a rate of ∼2.5 ps after a delayed onset of ∼3.7 ps. Photolysis of benzoic acid using 267 nm excitation is dominated by the formation of the T(1) excited state and no photofragments could be detected. The results from TR-IR experiments using higher energy of 193 nm indicate that photodissociation proceeds more rapidly than the vibrational energy transfer to solvents and C-C bond fission becomes the dominant relaxation pathway in these experiments as featured by the prominent observation of the COOH photofragments and negligible yield of the T(1) excited state. The measured ultrafast formation of T(1) excited state supports the existence of the surface intersections of S(2)/S(1), S(2)/T(2), and S(1)/T(1)/T(2), and the large T(1) quantum yield of ∼0.65 indicates the importance of the excited state depopulation to triplet manifold as the key factor affecting the photophysical and photochemical behavior of the monomeric benzoic acid.