Khodia Saurabh, Maity Surajit
Department of Chemistry, IIT Hyderabad, Kandi, Sangareddy, Telangana, India.
Phys Chem Chem Phys. 2022 May 18;24(19):12043-12051. doi: 10.1039/d2cp01121b.
In this report, we present solvent assisted excited-state proton transfer coupled to the deactivation of a photo-excited 2,2'-pyridylbenzimidazole bound to a single water molecule. Experimentally, the mass-selected 1 : 1 complex was probed using two-colour resonant two-photon ionization (2C-R2PI) and UV-UV hole-burning (HB) spectroscopy in a supersonically jet-cooled molecular beam. Computationally, three structural isomers were identified as the normal, the tautomer and the proton transfer product of the PBI-HO complex in the excited S state using B3LYP-D4/def2-TZVPP and ADC(2) (MP2)/cc-pVDZ levels of theory. The most stable form in the ground state, , the normal form, was identified using the excitation spectrum in the 30 544 to 30 936 cm region. The 2C-R2PI spectrum showed a sudden break-off above the 000 + 392 cm region, even though the Frack-Condon activity of the S ← S transition was measured beyond 000 + 1000 cm in the HB spectrum. The intensity of the bands associated with the excited state intermolecular vibrational modes near the break-off region was found to be drastically decreased, which indicates efficient quantum mechanical tunnelling along the hydrogen transfer coordinate. The sudden disappearance of the intermolecular vibrational modes in the spectrum revealed the existence of a deactivation channel in the PBI-HO complex near 392-450 cm above the 000 transition. The computational investigation predicted that the deactivation of the excited-state occurred the intersection between the S and S states, which was associated with the proton transfer from the HO to the PBI molecule along the O(3)-H(4)→N(5) coordinate. The highest energy structure was identified as the point of intersection between the nπ* (S) and ππ* (S) states. The associated barrier height was experimentally determined to be 392-450 cm, which showed a reasonable agreement with the calculated excited-state proton transfer barrier. Competing reaction channels such as dissociation and tautomerization were found to be highly energetically inaccessible.
在本报告中，我们展示了溶剂辅助的激发态质子转移与结合单个水分子的光激发2,2'-吡啶基苯并咪唑失活的耦合。实验上，在超声喷射冷却的分子束中，使用双色共振双光子电离（2C-R2PI）和紫外-紫外空穴烧蚀（HB）光谱对质量选择的1:1络合物进行了探测。计算上，使用B3LYP-D4/def2-TZVPP和ADC(2)（MP2）/cc-pVDZ理论水平，确定了激发单重态下PBI-H₂O络合物的三种结构异构体为正常形式、互变异构体和质子转移产物。利用30544至30936 cm⁻¹区域的激发光谱确定了基态下最稳定的形式，即正常形式。2C-R2PI光谱在000 + 392 cm⁻¹区域以上出现突然截止，尽管在HB光谱中测量到S₁←S₀跃迁的弗兰克-康登活性超过000 + 1000 cm⁻¹。发现在截止区域附近与激发态分子间振动模式相关的谱带强度急剧下降，这表明沿着氢转移坐标存在有效的量子力学隧穿。光谱中分子间振动模式的突然消失揭示了PBI-H₂O络合物在000跃迁以上392 - 450 cm⁻¹附近存在失活通道。计算研究预测激发态的失活发生在S₁和S₀态的交点处，这与质子沿着O(3)-H(4)→N(5)坐标从H₂O转移到PBI分子有关。能量最高的结构被确定为nπ*（S₁）和ππ*（S₂）态的交点。实验确定相关的势垒高度为392 - 450 cm⁻¹，与计算得到的激发态质子转移势垒显示出合理的一致性。发现解离和互变异构等竞争反应通道在能量上极难达到。
