Effect of intramolecular hydrogen bond strength on the ultrafast ESIPT dynamics

Excited state intramolecular proton transfer (ESIPT) process is a very fundamental and extensively explored photochemical process.¹ It belongs to the class of essential and fastest unimolecular processes in nature and has been studied by several ultrafast technologies.² This processes is initiated by the transfer of a proton from a donor to an acceptor group, which are in close vicinity within the molecule and is regulated by the interplay of nuclear and electronic dynamics. ESIPT reaction has garnered tremendous attention of theoretical and experimental researchers alike, ever since its first reported occurrence by Weller in 1956,³ owing to its peculiar opto-electronic properties as well as its close resemblance to the proton transfer reactions in the biological systems.⁴ Despite the considerable volumes of research dedicated, the study of ESIPT process continues to pose significant challenges, chiefly because of the intricate nature of its physical and chemical properties, such as quantum nature, cleavage and formation of hydrogen bond, the change in the properties of excited-state hydrogen bond, nuclear rearrange process etc.⁵



We have theoretically investigated excited state double proton transfer in 1,8-dihydroxynaphthalene-2,7-dicarbaldehyde (DHDA). We strategically opted for a molecule with multiple proton-transfer sites, as a single proton transfer event may not be sufficient to mimic the proton transfer reactions in biological systems. This study reveals non-adiabatic dynamics of S₁-S₂-S₃ (first three singlet excited-state) potential energy surfaces (PESs) in DHDA molecule using the surface-hopping approach. The study also sheds light on the time evolution of the O-H bonds, proton transfer timescales and minimum energy conical intersections along the S₁-S₂-S₃ PESs.



References:

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   3. Weller, A. Berichte der Bunsengesellschaft für physikalische Chemie 1956, 60, 1144-1147
   
   4. Zhao, Jinfeng; Zheng, Yujun Sci. Rep. 2017, 7, 44897
   
   5. Peng, Chia-Yu; Shen, Jiun-Yi; Chen, Yi-Ting; Wu, Pei-Jhen; Hung, Wen-Yi; Hu, Wei-Ping; Chou, Pi-Tai J. Am. Chem. Soc. 2015, 137, 45, 14349-14357