Lactim–lactam tautomerism is favoured over enol–keto tautomerism in 2-hydroxy-5-(4-fluorophenyl)nicotinic acid: Experimental and quantum chemical approach

Switching of photoinduced proton transfer from one six-membered hydrogen-bonded ring to other: a molecule of hydrazine and pH sensor

The present study describes photophysical properties of 3-(benzo[d]oxazol-2-yl)-5-bromo-2-hydroxybenzaldehyde (HBOB) and (E)-2-(benzo[d]oxazol-2-yl)-4-bromo-6-(hydrazonomethyl)phenol (HBON) molecules with asymmetric two-way proton transfer sites. The purpose of this study is to know the direction of ESIPT out of the two-way proton transfer pathways in these molecules in both the solid and solution state. The steady state and time-resolved spectral behaviour of HBOB and HBON and a comparison of the spectral features with the two distinct control compounds 2-(benzo[d]oxazol-2-yl)-4-bromophenol (HBO) and (E)-4-bromo-2-(hydrazonomethyl)phenol (HBN) having single 6-membered hydrogen bonded network reveal that HBOB undergoes imine-amine photoisomerisation by proton transfer towards the oxazole side and HBON undergoes towards the hydrazone side with characteristic Stokes' shifted emission. Proton transfer forms with the red shifted emission of these molecules shows fast decay than the locally excited state. In the solid state, extremely high fluorescence intensity was observed, following a similar type of ESIPT pattern. Calculated ground (S0) and excited state (S1) energy barriers for the PT process obtained using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) corroborate the unidirectional excited state intramolecular proton transfer (ESIPT) process for HBOB and HBON, and the theoretical spectral features validate our experimental absorption and emission spectra well. Interestingly, the unique unidirectional ESIPT behaviour of HBOB was utilised to detect hydrazine both in solution and solid phases. On the other hand, HBON was found to be a good fluorescence pH sensor with a ratiometric color change from yellow to green in acidic and basic media.

Dual-Channel Imine-Amine Photoisomerization in a Benzoimidazole and Benzothiazole Coupled System: Photophysics and Applications

A molecule, namely 2-(1H-benzo[d]imidazol-2-yl)-6-(benzo[d]thiazol-2-yl)-4-bromophenol (BIBTB), having a two-way proton transfer unit of thiazole and imidazole moieties was synthesized and characterized by NMR, electrospray ionization mass spectrometry (ESI-MS), and single-crystal diffraction studies. Steady state and time-resolved spectral studies of BIBTB support excited state intramolecular proton transfer (ESIPT), causing imine-amine tautomerization through a two-way 6-membered H-bonded ring, where the N atoms of benzothiazole and the benzoimidazole unit are involved as proton acceptor sites. Interestingly, in a nonpolar and moderately polar solvent, photoisomerization in BIBTB is found to be favored toward the thiazole ring, whereas in a highly polar solvent, it is favored toward the imidazole ring. A spectral comparison of BIBTB with judicially designed molecules 2-(benzo[d]thiazol-2-yl)-4-bromophenol (HBT) and 2-(1H-benzo[d]imidazol-2-yl)-4-bromophenol (BIB) supports these inferences. Theoretical calculation using the Density Functional Theory (DFT) at CAM-B3LYP/6-311+G(d,p) level supports the existence of two low-energy 6-membered hydrogen-bonded planar conformers in the ground state in the gas phase and in solvents of different dielectrics. The potential energy curves (PECs) calculated along the proton transfer (PT) coordinate are found to have a high energy barrier in the ground state and to be barrierless or have a low energy barrier in the excited state for both the forms. The calculated vertical excitation and the emission energy from the relaxed excited and PT states show good correlation with the experimental spectral data. Aggregation of BIBTB in water with red shifted emission was established from X-ray single-crystal structure analysis, solid state emission, and Dynamic Light Scattering (DLS) measurement. The molecule BIBTB also acts as a fluorescence probe for sensing the explosive picric acid in the subnano scale and can be used to determine the proportion of water in dimethyl sulfoxide (DMSO) solvent.

Tautomeric photoswitches: anion-assisted azo/azine-to-hydrazone photochromism

The photoswitching behaviour of isatin 4-nitrophenylhydrazones in presence of anions was investigated.

Photophysical and excited-state intramolecular proton transfer of 2-(1-(3,5-dimethylphenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)phenol: DFT analysis

Fluorescent phenenthrimidazole derivatives have been synthesized and characterized by (1)H, (13)C NMR and mass spectral analyses. Synthesized compounds have been. The solvent effect on the absorption and fluorescence bands has been analyzed and supplement by computational studies. Phenenthrimidazole containing hydroxy group shows a single prominent absorption and emission in polar solvents and dual emission in non-polar solvents due to excited state intramolecular proton transfer (ESIPT). The ESIPT of 2-(1-(3,5-dimethylphenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)phenol was cross validated by the analysis of optimized geometry parameters, potential energy surface (PES), mulliken's charge distribution on the heavy atoms and molecular orbitals of its tautomers. Nonviability of ground state electron transfer was explained by HOMO-LUMO analysis. The intramolecular hydrogen bonding (IMHB) interaction has been explored by the calculation of electron density ρ(r) and the Laplacian Δ(2)ρ(r) at the bond critical point (BCP) using atoms-in-molecule (AIM) method and by calculation of hyper conjugative interaction between N17 lone pair and σ*(O53⋯H54) using natural bond orbital (NBO) analysis.

Synthesis, protonation equilibrium and peculiar thermal decomposition behavior of cyclo-tri-μ-imidotetraphosphate

The synthesis and isolation of the sodium salt ofcyclo-tri-μ-imidotetraphosphate,i.e.Na₄cP₄O₉(NH)₃·H₂O, were achieved by the hydrolysis of Na₄cP₄O₈(NH)₄·2H₂O under very weak acidic conditions,i.e.using 0.2 mol L⁻¹propionic acid and the pH-controlled recrystallization procedure.