Excited-State Intramolecular Proton Transfer in 2-(2′-Hydroxyphenyl)pyrimidines: Synthesis, Optical Properties, and Theoretical Studies

Abstract

The development of fluorescence materials with switched on / off emission has attracted great attention owing to the potential application of these materials in chemical sensing. In this work, the photophysical properties of a series of original 2-(2′-hydroxyphenyl)pyrimidines were thoroughly studied. The compounds were prepared by following well-established and straightforward methodologies and showed very little or null photoluminescence both in solution and in the solid state. This absence of emission can be explained by a fast proton transfer from the OH group to the nitrogen atoms of the pyrimidine ring to yield an excited tautomer that deactivates through a nonradiative pathway. The key role of the OH group in the emission quenching was demonstrated by the preparation of 2′-unsubstituted derivatives, all of which exhibited violet or blue luminescence. Single crystals of some compounds suitable for an X-ray diffraction analysis could be obtained, which permitted us to investigate inter- and intramolecular interactions and molecular packing structures. The protonation of the pyrimidine ring by an addition of trifluoroacetic acid inhibited the excited-state intramolecular proton transfer (ESIPT) process, causing a reversible switch on fluorescence response detectable by the naked eye. This acidochromic behavior allows 2-(2′-hydroxyphenyl)pyrimidines to be used as solid-state acid–base vapor sensors and anticounterfeiting agents. Extensive density functional theory and its time-dependent counterpart calculations at the M06-2 X /6-31+G** level of theory were performed to rationalize all the experimental results and understand the impact of protonation on the different optical transitions.

Dual emission of ESIPT-capable 2-(2-hydroxyphenyl)-4-(1H-pyrazol-1-yl)pyrimidines: interplay of fluorescence and phosphorescence

“…Electronic properties of ESIPT-capable molecules and therefore the ESIPT and GSIPT abilities of such molecules can be tuned through introducing various substituents or heteroatoms, expanding the p-system, [37][38][39][40] binding metal ions, [41][42][43][44][45][46][47][48][49][50][51][52][53][54] protonation/deprotonation, [55][56][57][58][59][60][61][62][63][64] solvent effects, [65][66][67][68][69] etc. The synthetic flexibility of such molecules is based on established synthetic methods of organic chemistry and provides researchers with almost unlimited options for the rational design and modification of various ESIPT-capable scaffolds.…”

The arylvinylpyrimidine scaffold: a tunable platform for luminescent and optical materials

“…The absence of emission for 15 can be explained by an excited-state intramolecular proton transfer (ESIPT) from the OH group to the nitrogen atom of the pyrimidine ring. 40 The protonation of the pyrimidine ring by the addition of TFA inhibits the ESIPT process, causing a reversible switch on fluorescence response that results in the progressive appearance of a green-yellow emission band ( λ em = 550 nm). This acidochromic behavior has also been demonstrated in the solid state by exposure to acid–base vapors.…”

A proton-assisted AIE active luminescent material having tunable emission in solution and solid state for anti-counterfeiting application: smart high contrast on–off–on fluorescence switch

“…19,21 Anti-counterfeiting application Nowadays, it is a very serious global problem of illegal trafficking of counterfeit goods as a huge money-making source for organized crimes. 31,32 Thus, developing powerful anticounterfeiting technologies for security is highly demanding. To prevent counterfeiting, various methods, including holograms, luminescence, two-dimensional codes, watermarks, barcodes, etc., have been achieved and explored.…”