N−H‐Type Excited‐State Proton Transfer in Compounds Possessing a Seven‐Membered‐Ring Intramolecular Hydrogen Bond

Abstract

A series of compounds containing 5-(2-aminobenzylidene)-2,3-dimethyl-3,5-dihydro-4H-imidazol-4-one (o-ABDI) as the core chromophore with a seven-membered-ring N-H-type intramolecular hydrogen bond have been synthesized and characterized. The acidity of the N-H proton and thus the hydrogen-bond strength can be fine-tuned by replacing one of the amino hydrogen atoms by a substituent R, the acidity increasing with increasing electron-withdrawing strength of R, that is, in the order H<COCH3 <COPh<Tosyl<COCF3 . The tosyl and trifluoroacetyl derivatives undergo ultrafast, irreversible excited-state intramolecular proton transfer (ESIPT) that results in proton-transfer emission solely in the red region. Reversible ESIPT, and hence dual emission, involving the normal and proton-transfer tautomers was resolved for the acetyl- and benzyl-substituted counterparts. For o-ABDI, which has the weakest acidity, ESIPT is prohibited due to its highly endergonic reaction. The results clearly demonstrate the harnessing of ESIPT by modifying the proton acidity and hydrogen-bonding strength in a seven-membered-ring intramolecular hydrogen-bonding system. For all the compounds studied, the emission quantum yields are weak (ca. 10(-3) ) in dichloromethane, but strong in the solid form, ranging from 3.2 to 47.4 %.

A density functional theory‐time‐dependent density functional theory investigation of photo‐induced hydrogen bond and proton transfer for 2‐(3,5‐dichloro‐2,6‐dihydroxy‐phenyl)‐benzoxazole‐6‐carboxylicacid

“…The excited‐state proton transfer (ESPT) reaction has been recognized as one of the most fundamental and significant processes in various photochemical and photophysical fields . It cannot be denied that ESPT reaction dynamics have been widely investigated, both experimentally and theoretically, by many groups since the first experimental observation of the excited‐state intramolecular proton transfer (ESIPT) phenomenon by Weller et al In particular, after the discovery of ESIPT phenomenon, many scientists investigated the proton transfer dynamics, vibrational‐resolved spectroscopy, elementary photophysics, and femtosecond ESIPT compounds . In fact, the ESIPT exhibits a variety of potential applications in many fields, such as fluorescence sensors, white light‐emitting materials, biological probes, ultraviolet photostabilizers, organic optoelectronic devices, etc …”

Theoretical Study on the Substituent Effect on the Excited‐State Proton Transfer in the 7‐Azaindole‐Water Derivatives

“…Very recently, Chen et al . confirmed that the stronger H‐bond led to more favorable ESIPT thermally after researching a great deal of N‐H seven‐membered‐ring H‐bonded molecules.…”

The preparation of (4H)-imidazol-4-ones and their application in the total synthesis of natural products

“…Thus far, this reaction appears to be used mainly to produce a range of 5-ethylidene-4-imidazolones. [46][47][48][49] There are two different pathways this reaction can be implemented to produce 5-ethylidene-4-imidazolones; both are shown in Scheme 15. In method A, the more popular method, the imidazol-4-one ring is formed from a terminal azidoimide.…”