Excited state intramolecular proton transfer (ESIPT) from phenol OH (OD) to adjacent “aromatic” carbons in simple biphenyls

This work reports results of further studies on a new class of excited state intramolecular proton transfer (ESIPT), from phenol OH to adjacent aromatic carbon atoms of suitably designed biphenyl systems. For this purpose, a number of 2-phenylphenols 3–6 with methyl and methoxy substituents on the adjacent proton accepting phenyl ring were synthesized. In particular, we were also interested in studying the effect of an acetyl (ketone) substituent on the proton accepting ring (biphenyl 7) and the effect on the photochemistry when the ketone is reduced to alcohol (biphenyl 8). All compounds except for 7 were found to undergo deuterium exchange (Ф_ex = 0.019–0.079) primarily at the 2′-position on photolysis in 1:3 D₂O–CH₃CN. This is consistent with a reaction mechanism involving initial ESIPT from the phenol OH to the 2′-position of the adjacent phenyl ring, to generate a biphenyl quinone methide intermediate which rapidly tautomerizes back to starting material. Biphenyl 8 also undergoes a competing photosolvolysis reaction (overall loss of water). Both photosolvolysis and ESIPT reactions react via isomeric quinone methide intermediates and are best interpreted as arising from an excited singlet state that possesses a large degree of charge transfer character, from the phenol ring to the attached phenyl ring. The failure of 7 to react may be due to two possible reasons: (i) high intersystem crossing rate to a non-polarized triplet excited state and/or (ii) a polarized singlet state that is now much more basic at the carbonyl oxygen. The results are consistent with qualitative examination of calculated HOMOs and LUMOs (AM1).     

2,6,8‐Trisubstituted 3‐Hydroxychromone Derivatives as Fluorophores for Live‐Cell Imaging

We present the synthesis and photophysical characterisation of a series of structurally diverse, fluorescent 2,6,8‐trisubstituted 3‐hydroxychromone derivatives with high fluorescence quantum yields and molar extinction coefficients. Two of these derivatives ( 9 and 10 a ) have been studied as fluorophores for cellular imaging in HeLa cells and show excellent permeability and promising fluorescence properties in a cellular environment. In addition, we have demonstrated by photophysical characterisation of 3‐isobutyroxychromone derivatives that esterification of the 3‐hydroxyl group results in acceptable and useful fluorescence properties.     

Remarkable ESIPT induced NIR emission by a selective colorimetric dibenzimidazolo diimine sensor for acetate

A new dibenzimidazolo diimine sensor (DDS) has been designed and synthesized for selective detection of acetate ion. Significant naked eye recognized color change of DDS solution from light yellow to pink upon addition of only acetate ion is accompanied with near infra red (NIR) emission exploiting excited state intramolecular proton transfer (ESIPT).     

Excited-state intramolecular proton transfer (ESIPT) fluorescence from 3-amidophthalimides displaying RGBY emission in the solid state

Fluorescence properties of phthalimide derivatives (1) incorporating sulfonamide and acetamide functionalities at the 3-position were investigated both in solution and in the solid states to reveal the effects of the amide functionalities on the fluorescence properties. In the solid state, sulfonamides 1a and 1b, respectively, gave off red (λ^F_max 595?nm) and green (λ^F_max 537?nm) emission through an ESIPT process. Acetamides 1c and 1d, respectively displayed blue (λ^F_max 432?nm) and yellow (λ^F_max 560?nm) emission. Through simply modifying the amide functionality, phthalimide 1 displayed multicolor RGBY emission in the solid state.     

The structural transformations and photo-induced processes in salicylidene alkylimines

The detailed comparative investigations of the mechanism of the fluorescent properties, formation and photochromic reaction in the salicylidene alkylimine (SALK) molecules with various N-Alk substituents and in corresponding more rigid model structures without C–Ph ring (A) twist have been carried out by the experimental and quantum-chemical methods.

The crucial role of the A ring twist and the importance of the N-Alk substituent structure in the light-induced processes including the ESIPT, TICT formation, fluorescence with anomalous Stokes shift (ASS) and the formation of the metastable coloured so-called “post-TICT” structures has been shown. The mechanism of these processes is similar to that of the salicylidene aniline (SA) molecule but there are some important distinctions caused mainly by the lack of the l_N–π_Ph electrons interactions and also by the steric ones. In general the decrease of the photochromic efficiency and the increase of the ASS in comparison to the SA molecule are provoked by the SALK structures’ peculiarities.     

A DFT/TD-DFT study of effect of different substituent on ESIPT fluorescence features of 2-(2'-hydroxyphenyl)-4-chloro- methylthiazole derivatives

Based on density functional theory (DFT) and time-dependent density functional theory (TD-DFT), the effects of substituent on the excited-state intramolecular proton transfer (ESIPT) process and photophysical properties of 2-(2'-hydroxyphenyl)-4-chloromethylthiazole (HCT) are studied. The electron-donating group (CH₃, OH) and electron-withdrawing group (CF₃, CHO) are introduced to analyze the changes of intramolecular H-bond, the frontier molecular orbitals, the absorption/fluorescence spectra, and the energy barrier of ESIPT process. The calculation results indicate that electron-donating group strengthens the intramolecular H-bond in the S₁ state, and leads to an easier ESIPT process. The electron-withdrawing group weakens the corresponding H-bond and makes ESIPT process a little harder. Different substituents also affect the photophysical properties of HCT. The electron-withdrawing group (CF₃, CHO) has a little effect on electronic spectra. The electron-donating group (CH₃, OH) red-shifts both the absorption and fluorescence emission peaks of HCT, respectively, which causes the Stokes shift to increase.     

Thiazole-based chemosensor: synthesis and ratiometric fluorescence sensing of zinc

A new ratiometric and selective fluorescent chemosensor (1), based on thiazole for quantification of zinc ions in aqueous ethanol, was synthesized and investigated. The mechanism of fluorescence was based on the cation-induced inhibition of excited-state intramolecular proton transfer (ESIPT).     

Solvent-promoted chemiluminescent decomposition of a bicyclic dioxetane bearing a 4-(benzothiazol-2-yl)-3-hydroxyphenyl moiety

An aprotic polar solvent such as N-methylpyrrolidone (NMP) promoted the thermal decomposition of bicyclic dioxetane bearing a 4-(benzothiazol-2-yl)-3-hydroxyphenyl moiety 1 without the addition of any base. This solvent-promoted decomposition (SPD) gave light as effectively as the base-induced decomposition (BID) in an aprotic polar solvent. SPD caused intramolecular CT-induced chemiluminescence similar to BID, but, in contrast to BID, SPD proceeded through a pathway with a large negative entropy of activation. Dioxetane 1 was also shown to give light due to excited state intramolecular proton transfer (ESIPT) upon heating in p-xylene.     

Cascade reaction-based fluorescent probe for detection of H2S with the assistance of CTAB micelles

We report a turn-on fluorescent probe for H₂S through a cascade reaction using a new trap group 4-(bromomethyl)benzoate, based on excited-state intramolecular proton transfer (ESIPT) sensing mechanism. The probe showed good selectivity and high sensitivity towards H₂S and it was capable of detecting and imaging H₂S in living HeLa cells, indicating its potential biological applications.     

A tetraphenylimidazole-based fluorescent probe for the detection of hydrogen sulfide and its application in living cells

A novel probe based on the fluorescence off–on strategy was prepared to optically detect hydrogen sulfide (H₂S) via an excited state intramolecular proton transfer (ESIPT) mechanism. The probe shows high sensitivity and excellent selectivity to H₂S. It also displays a large Stokes shift (∼140 nm) and a remarkable quantum yield enhancement (Ф = 0.412) after interaction with H₂S. Moreover, the cellular imaging experiment demonstrated that it has potential utility for H₂S sensing in biological sciences.