Excited state intramolecular proton transfer (ESIPT) is a process in which

photoexcited Photoexcitation is the production of an excited state of a quantum system by photon absorption. The excited state originates from the interaction between a photon and the quantum system. Photons carry energy that is determined by the wavelengths ...

molecules relax their energy through

tautomerization Tautomers () are structural isomers (constitutional isomers) of chemical compounds that readily interconvert. The chemical reaction interconverting the two is called tautomerization. This conversion commonly results from the relocation of a hydr ...

by transfer of protons. Some kinds of molecules could have different minimum-energy tautomers in different

electronic state A quantum mechanical system or particle that is bound—that is, confined spatially—can only take on certain discrete values of energy, called energy levels. This contrasts with classical particles, which can have any amount of energy. The t ...

s, 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 In organic chemistry, alkenols (shortened to enols) are a type of reactive structure or intermediate in organic chemistry that is represented as an alkene ( olefin) with a hydroxyl group attached to one end of the alkene double bond (). The t ...

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 __NOTOC__ Stokes shift is the difference (in energy, wavenumber or frequency units) between positions of the band maxima of the absorption and emission spectra ( fluorescence and Raman being two examples) of the same electronic transition. It ...

than common

fluorescent Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, th ...

molecules, or exhibit dual fluorescence that shorter-wavelength one comes from the original tautomer and longer-wavelength one from proton-transferred tautomer. 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. Rate of ESIPT process may slow down by

deuterium Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two Stable isotope ratio, stable isotopes of hydrogen (the other being Hydrogen atom, protium, or hydrogen-1). The atomic nucleus, nucleus of a deuterium ato ...

substitution of hydrogen that is transferred in ESIPT, because the deuteration increases only mass of the transferred significantly while do not change

electrostatic potential Electrostatics is a branch of physics that studies electric charges at rest (static electricity). Since classical times, it has been known that some materials, such as amber, attract lightweight particles after rubbing. The Greek word for ambe ...

in the molecule substantially. 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.

Application

Based on characteristic that molecules usually have extraordinarily larger

when ESIPT occurs, various applications have been developed using red-shifted fluorescence. Applications include turn-on

photoluminescence Photoluminescence (abbreviated as PL) is light emission from any form of matter after the absorption of photons (electromagnetic radiation). It is one of many forms of luminescence (light emission) and is initiated by photoexcitation (i.e. pho ...

sensor,

photochromic Photochromism is the reversible transformation of a chemical species (photoswitch) between two forms by the absorption of electromagnetic radiation (photoisomerization), where the two forms have different absorption spectra. In plain language, th ...

, non-destructive optical memory, and white-light emitting materials. 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 Ethylene glycol ( IUPAC name: ethane-1,2-diol) is an organic compound (a vicinal diol) with the formula . It is mainly used for two purposes, as a raw material in the manufacture of polyester fibers and for antifreeze formulations. It is an od ...

, it became possible to trap 1,4-Dioxaspiro .5eca-6,8-diene 3783-59-7 In 2020, Amit Singh et ''al''. reported a two-photon responsive p-hydroxyphenacyl ESIPT-based DDS with uncaging ability in the phototherapeutic window by simple tagging with a naphthyl moiety. The designed ESIPT-based DDS showed a two-photon absorption cross-section greater than 20 GM and two-photon uncaging of 10 GM at 700 nm. The ESIPT phenomenon enhanced the rate of the photorelease of the anti-cancer drug with real-time monitoring. Singh, A. K.; Kundu, M.; Roy, S.; Roy, B.; Shah, Sk. S.; Nair, A. V.; Pal, B.; Mondal, M.; Singh, N. D. P. A Two-Photon Responsive Naphthyl Tagged p-Hydroxyphenacyl Based Drug Delivery System: Uncaging of Anti-Cancer Drug in the Phototherapeutic Window with Real-Time Monitoring. Chemical Communications, 2020, 56, 9986–9989. https://doi.org/10.1039/d0cc01903h.

References

{{reflist Enols Photochemistry