Excited state intramolecular proton transfer

Excited state intramolecular proton transfer (ESIPT) is a process in which photoexcited molecules relax their energy through tautomerization by transfer of protons. Some kinds of molecules could have different minimum-energy tautomers in different electronic states, and if the molecular structure of minimum-energy tautomer in the excited state is proton-transferred geometry between neighboring atoms, proton transfer in excited state can occur. The tautomerization often takes the form of keto-enol tautomerism.

Characteristic

Since a proton-transferred geometry is usually the minimum-energy tautomer only in the excited state and relatively unstable in the ground state, molecules that have ESIPT character may show extraordinarily larger Stokes shift than common fluorescent molecules, or exhibit dual fluorescence that shorter-wavelength one comes from the original tautomer and longer-wavelength one from proton-transferred tautomer.[1][2] However, there are some exceptional cases where ESIPT molecules have no dual luminescence or significantly red-shifted emission from proton-transferred tautomer, from various reasons.[3][4][5]

Rate of ESIPT process may slow down by deuterium substitution of hydrogen that is transferred in ESIPT, because the deuteration increases only mass of the transferred significantly while do not change electrostatic potential in the molecule substantially.[6] However the amount of rate change may lie in the range of 1~50, depending on the shape and size of potential energy surfaces of the molecule.[7]

Application

Based on characteristic that molecules usually have extraordinarily larger Stokes shift when ESIPT occurs, various applications have been developed using red-shifted fluorescence. Applications include turn-on photoluminescence sensor,[8] photochromic, non-destructive optical memory,[9] and white-light emitting materials.[10]

Because phenol does not form a ketal under normal conditions because it does not tautomerize to any useful extent; however under ESIPT in the presence of an alcohol, e.g. ethylene glycol, it became possible to trap 1,4-Dioxaspiro[4.5]deca-6,8-diene [23783-59-7].

References
  1. Joshi, Hem; Antonov, Liudmil (2021). "Excited-State Intramolecular Proton Transfer: A Short Introductory Review". Molecules. 26 (5): 1475. doi:10.3390/molecules26051475. PMC 7963178.
  2. Zhao, Jianzhang; Ji, Shaomin; Chen, Yinghui; Guo, Huimin; Yang, Pei (2012). "Excited state intramolecular proton transfer (ESIPT): from principal photophysics to the development of new chromophores and applications in fluorescent molecular probes and luminescent materials". Phys. Chem. Chem. Phys. 14 (25): 8803–8817. Bibcode:2012PCCP...14.8803Z. doi:10.1039/C2CP23144A. PMID 22193300.
  3. Yin, Hang; Li, Hui; Xia, Guomin; Ruan, Chengyan; Shi, Ying; Wang, Hongming; Jin, Mingxing; Ding, Dajun (21 January 2016). "A novel non-fluorescent excited state intramolecular proton transfer phenomenon induced by intramolecular hydrogen bonds: an experimental and theoretical investigation". Scientific Reports. 6 (1): 19774. Bibcode:2016NatSR...619774Y. doi:10.1038/srep19774. PMC 4726414. PMID 26790961.
  4. Han, Gi Rim; Hwang, Doyk; Lee, Seunghoon; Lee, Jong Woo; Lim, Eunhak; Heo, Jiyoung; Kim, Seong Keun (20 June 2017). "Shedding new light on an old molecule: quinophthalone displays uncommon N-to-O excited state intramolecular proton transfer (ESIPT) between photobases". Scientific Reports. 7 (1): 3863. Bibcode:2017NatSR...7.3863H. doi:10.1038/s41598-017-04114-9. PMC 5478638. PMID 28634405.
  5. English, D. S.; Zhang, W.; Kraus, G. A.; Petrich, J. W. (April 1997). "Excited-State Photophysics of Hypericin and Its Hexamethoxy Analog: Intramolecular Proton Transfer as a Nonradiative Process in Hypericin". Journal of the American Chemical Society. 119 (13): 2980–2986. doi:10.1021/ja962476h.
  6. Flom, Steven R.; Barbara, Paul F. (October 1985). "Proton transfer and hydrogen bonding in the internal conversion of S1 anthraquinones". The Journal of Physical Chemistry. 89 (21): 4489–4494. doi:10.1021/j100267a017.
  7. Agmon, Noam (January 2005). "Elementary Steps in Excited-State Proton Transfer". The Journal of Physical Chemistry A. 109 (1): 13–35. doi:10.1021/jp047465m. PMID 16839085.
  8. Chen, Wei-Hua; Xing, Yu; Pang, Yi (18 March 2011). "A Highly Selective Pyrophosphate Sensor Based on ESIPT Turn-On in Water". Organic Letters. 13 (6): 1362–1365. doi:10.1021/ol200054w. PMID 21338073.
  9. Lim, Seon-Jeong; Seo, Jangwon; Park, Soo Young (November 2006). "Photochromic Switching of Excited-State Intramolecular Proton-Transfer (ESIPT) Fluorescence: A Unique Route to High-Contrast Memory Switching and Nondestructive Readout". Journal of the American Chemical Society. 128 (45): 14542–14547. doi:10.1021/ja0637604. PMID 17090038.
  10. Park, Sanghyuk; Kwon, Ji Eon; Kim, Se Hun; Seo, Jangwon; Chung, Kyeongwoon; Park, Sun-Young; Jang, Du-Jeon; Medina, Begoña Milián; Gierschner, Johannes; Park, Soo Young (7 October 2009). "A White-Light-Emitting Molecule: Frustrated Energy Transfer between Constituent Emitting Centers". Journal of the American Chemical Society. 131 (39): 14043–14049. doi:10.1021/ja902533f. PMID 19480450.
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