Francis A. S. Chipem

Intramolecular Proton Transfer in 2-(2′-hydroxyphenyl)oxazolo[4,5-b]pyridine: Evidence for Tautomer in the Ground State

The intramolecular proton transfer in a newly synthesized molecule, 2‐(2′‐hydroxyphenyl)oxazolo[4,5‐b]pyridine (HPOP) is studied using UV‐visible absorption, fluorescence emission, fluorescence excitation and time‐resolved fluorescence spectroscopy. In the ground state, the molecule exists as cis‐ and trans‐enol in all the solvents. However, in dioxane, alcohols, acetonitrile, dimethylformamide and dimethylsulfoxide the keto tautomer is also observed in the ground state. Dual fluorescence is observed in HPOP where the large Stoke shifted emission is due to emission from the excited‐state intramolecular proton transfer product, whereas the other emission is the normal emission from enol form. The fluorescence (both normal and tautomer emission) of HPOP is less than those of corresponding benzoxazole and imidazopyridine derivatives. This reveals that the nonradiative decay becomes more efficient upon substitution of electronegative atom on the charge acceptor group. The pH studies substantiate the conclusion that (unlike in its imidazole analog) the third ground state species is the keto tautomer and not the monoanion. The effect of temperature on cis‐enol‐trans‐enol‐keto equilibrium and the nonradiative deactivation from the excited state are also investigated.

Photoisomerization of trans-2-[4'-(dimethylamino)styryl]benzothiazole.

Photoreaction of trans‐2‐[4′‐(dimethylamino)styryl]benzothiazole (t‐DMASBT) under direct irradiation has been investigated in dioxane, chloroform, methanol and glycerol to understand the mechanism of photoisomerization. Contrary to an earlier report, isomerization takes place in all these solvents including glycerol. The results show that restriction on photoisomerization leads to the increase in fluorescence quantum yield in glycerol. The results are consistent with the theoretically simulated potential energy surface reported earlier using time‐dependent density functional theory (TDDFT) calculations. DFT calculations on cis isomers under isolated condition have suggested that cis‐B conformer is more stable than cis‐A conformer due to hydrogen‐bonding interaction. In the ground state, cis‐DMASBT is predominantly present as cis‐B. The fluorescence spectra of the irradiated t‐DMASBT suggested that photoisomerization follows not the adiabatic path as proposed by Saha et al., but the nonadiabatic path.

Comment on “Michael Addition Based Chemodosimeter for Serum Creatinine Detection Using (E)-3-(Pyren-2-yl)-1-(3,4,5-trimethoxyphenyl)prop-2-en-1-one Chalcone”

Recently, a chalcone based fluorescence sensor has been developed for the detection of creatinine by Sundaram et al. ( ACS Sens. 2018, 3, 763-771). Though the efficiency of the sensor under clinical condition is appreciated, the proposed mechanism and the interpretations of the authors raise serious concerns. In the present work, DFT calculations were performed on the system. Based on the calculations and the spectral data reported earlier it is established that the Michael addition is not a photophsyical interaction as proposed by Sundaram et al.; instead, it is a ground state reaction. Several other misinterpretations of the authors including that in the ICT mechanism are properly inferred. Since the formula and the procedure followed for the calculation of the fluorescence quantum yield in ACS Sens. 2018, 3, 763-771 is not appropriate, the correct procedure is briefed with the proper formula.