Excited-State Intramolecular Proton Transfer in Salicylidene-α-Hydroxy Carboxylate Derivatives: Direct Detection of the Triplet Excited State of the cis -Keto Tautomer
Excited-State Intramolecular Proton Transfer in Salicylidene-α-Hydroxy Carboxylate Derivatives: Direct Detection of the Triplet Excited State of the cis -Keto Tautomer
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水杨基-α-羟基羧酸衍生物中的激发态分子内质子转移：直接检测顺式-酮互变异构体的三重激发态
DOI：
10.1021/acs.jpca.3c00543
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发表时间：
2023
期刊：
影响因子：
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通讯作者：
Ahmed, Noha
中科院分区：
文献类型：
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作者：
Weragoda, Geethika K.;Abdelaziz, Nayera M.;Govorov, Dmitrii;Merugu, Rajkumar;Patton, Leanna J.;Grabo, Jennifer E.;Ranaweera, R. A.;Ratliff, Anna C.;Mendis, W. Dinindu;Ahmed, Noha
与单重态表面上的超快过程相比，水杨醛苯胺衍生物三重态表面上的激发态分子内氢转移受到的关注要少得多。为了加深对这种三重态反应性的理解，对一系列在苯酚部分（1-3）上具有不同取代基的水杨醛-α-羟基酸盐的光化学性质进行了表征。紫外维斯吸收光谱和磷光光谱研究表明，1 - 3以烯醇和酮的互变异构体形式存在，其中以烯醇形式为主。在乙醇玻璃（77 K）中，1在310 nm处产生一个吸收带，其最大吸收波长为λ max = 405 nm，归属于反式-酮互变异构体（trans-1 K）。相比之下，1 - 3在甲醇或乙腈中的激光闪光光解导致λ max在440-460 nm处的瞬时吸收。通过与TD-DFT计算结果的比较，我们将这一衰减时间为微秒级的瞬变现象归因于cis-酮互变异构体（cis-1 K-3 K）的三重激发态（T1）和trans-1 K-3 K的残余吸收。蒽和2，5-二甲基-2，4-己二烯的猝灭研究进一步支持了顺式-1K的T1 ofcis-1 K的归属。在173-293 K的温度范围内，1的激光闪光光解给出了顺式-1K的T1衰变的活化能垒为6.7 kcal/mol。与此相反，计算的活化势垒迫使顺式-1K进行1，5-H原子移位到reform 1较小，表明T1 ofcis-1 K的系统间穿越是1再生的速率决定步骤。
Excited-state intramolecular hydrogen transfer on the triplet surface of salicylideneaniline derivatives has received much less attention than the corresponding ultrafast process on the singlet surface. To enhance the understanding of this triplet reactivity, the photochemical properties of a series of salicylidene-α-hydroxy acid salts with different substituents on the phenol moiety (1–3) were characterized. UV/vis absorption and phosphorescence measurements in ethanol revealed that1–3exist as both enol and keto tautomers, with the enol form being predominant. Irradiation of1at 310 nm in ethanol glass (77 K) yielded an absorption band with a λmaxat ∼405 nm, which was assigned to thetrans-keto tautomer (trans-1K). In contrast, laser flash photolysis of1–3in methanol or acetonitrile resulted in a transient absorption with λmaxat 440–460 nm. This transient, which decayed on the microsecond timescale and was significantly shorter lived in methanol than in acetonitrile, was assigned to the triplet excited state (T1) of thecis-keto tautomer (cis-1K–3K) and residual absorption oftrans-1K–3Kby comparison with TD-DFT calculations. The assignment of the T1ofcis-1Kwas further supported by quenching studies with anthracene and 2,5-dimethyl-2,4-hexadiene. Laser flash photolysis of1in the temperature range of 173–293 K gave an activation barrier of 6.7 kcal/mol for the decay of the T1ofcis-1K. In contrast, the calculated activation barrier forcis-1Kto undergo a 1,5-H atom shift to reform1was smaller, indicating that intersystem crossing of the T1ofcis-1Kis the rate-determining step in the regeneration of1.