Excited state intramolecular proton transfer (ESIPT) is a process in which photoexcited molecules relax their energy through

tautomerization In chemistry, 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 reloca ...

by transfer of

proton A proton is a stable subatomic particle, symbol , Hydron (chemistry), H+, or 1H+ with a positive electric charge of +1 ''e'' (elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an e ...

s. 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, enols are a type of functional group or intermediate in organic chemistry containing a group with the formula (R = many substituents). The term ''enol'' is an abbreviation of ''alkenol'', a portmanteau deriving from "-ene ...

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 Fluorescence is one of two kinds of photoluminescence, the emission of light by a substance that has absorbed light or other electromagnetic radiation. When exposed to ultraviolet radiation, many substances will glow (fluoresce) with color ...

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 (hydrogen-2, symbol H or D, also known as heavy hydrogen) is one of two stable isotopes of hydrogen; the other is protium, or hydrogen-1, H. The deuterium nucleus (deuteron) contains one proton and one neutron, whereas the far more c ...

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

electrostatic potential Electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is defined as electric potential energy per unit of electric charge. More precisely, electric potential is the amount of work needed ...

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 Stokes shift 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. phot ...

sensor,

photochromic Photochromism is the reversible change of color upon exposure to light. It is a transformation of a chemical species (photoswitch) between two forms through the absorption of electromagnetic radiation (photoisomerization), where each form has a d ...

, 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 odo ...

, it became possible to trap 1,4-Dioxaspiro .5eca-6,8-diene 3783-59-7

References

{{reflist Enols Photochemistry