Nanoparticle and surfactant controlled switching between proton transfer and charge transfer reaction coordinates.

The reaction coordinates of a molecular photo-switch 2-(4'-diethylamino-2'-hydroxyphenyl)-1H-imidazo-[4,5-b]pyridine (DHP) was tuned with a nanoparticle and surfactant. DHP undergoes excited state intramolecular proton transfer (ESIPT) and emits normal and tautomer emissions in N,N-dimethylformamide. Silver nanoparticles suppress the ESIPT and induce twisted intramolecular charge transfer (TICT). Further addition of surfactants alters the process. Interestingly, different surfactants cause different effects. Accordingly, the luminescence characteristics are altered. The anionic surfactant sodium dodecyl sulfate (SDS) restores the ESIPT process by completely detaching the molecule from the nanoparticle. The nonionic surfactant Triton X-100 (TX-100), at lower concentration, enhances the TICT emission and the ESIPT process is also observed due to the release of some fluorophore from the nanoparticle complex. But at higher concentration the fluorophores are released completely and the ESIPT process is restored. The cationic surfactant cetyltrimethyl ammonium bromide (CTAB), at lower concentration, simply restores the ESIPT process by releasing the fluorophore. But at higher CTAB concentration, DHP enters the metalparticle-CTAB aggregate and shows enhanced ESIPT.

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.

Diacetoxyiodobenzene assisted C-O bond formation via sequential acylation and deacylation process: synthesis of benzoxazole amides and their mechanistic study by DFT.

An efficient method for the transformation of N-substituted-N'-benzoylthioureas to substituted N-benzoxazol-2-yl-amides using diacetoxyiodobenzene (DIB) is described in this work. The transformation follows the C-O bond formation leading to the benzoxazole derivative, due to oxidative dehydrogenation by DIB, instead of the expected C-S bond formation of the benzothiazole moiety. The C-O bond formation leading to benzoxazole is due to consecutive acylation and deacylation in conjunction with the reduction of two moles of DIB. A plausible mechanism was proposed for the reaction and density functional calculations were also performed to study the reaction mechanism.

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.

Excited state proton transfer of 2-(2'-hydroxyphenyl)benzimidazole and its nitrogen substituted analogues in bovine serum albumin.

The interaction of 2-(2'-hydroxyphenyl)benzimidazole (HPBI) and its nitrogen substituted analogues 2-(2'-hydroxyphenyl)-3H-imidazo[4,5-b]pyridine (HPIP-b) and 2-(2'-hydroxyphenyl)-1H-imidazo-[4,5-c]pyridine (HPIP-c) with BSA was explored. Upon interaction with BSA both normal and tautomer emissions are significantly enhanced. However, the fluorescence ratios of the normal band to the tautomer band of HPBI and HPIP-b decrease, but that of HPIP-c increases. From the tautomer emission, the stoichiometry and association constants were determined. HPBI exists as cis- and trans-enolic and zwitterionic forms, whereas HPIP-b and HPIP-c are present as monoanions in addition to cis- and trans-enols. The study shows that different conformers of all three molecules bind at different binding sites of BSA.

Temperature effect on dual fluorescence of 2-(2'-hydroxyphenyl)benzimidazole and its nitrogen substituted analogues.

The effects of temperature on the dual fluorescence of 2-(2'-hydroxyphenyl)benzimidazole (HPBI) and its nitrogen substituted analogues, viz., 2-(2'-hydroxyphenyl)-3H-imidazo[4,5-b]pyridine (HPIP-b) and 2-(2'-hydroxyphenyl)-1H-imidazo[4,5-c]pyridine (HPIP-c), were investigated in solvents of different polarity and hydrogen bonding capability. Absorption, steady-state, and time-resolved emission spectroscopic techniques were employed for the experimental study. Density functional theoretical calculations were performed to find the relative population of the conformers. The calculations predict that, with increase in temperature, the population of trans-enol increases, while that of cis-enol decreases. At all temperatures, the population ratio of cis-enol to trans-enol increases in the order HPIP-c < HPIP-b < HPBI. Except for HPBI in methanol and ethylene glycol, the fluorescence of both emissions decreases with an increase in temperature and is more pronounced in the tautomer band than in the normal band. The data are analyzed using the Arrhenius and van't Hoff equations. The change in the fluorescence with temperature is governed by (i) the change in the relative population of conformers and (ii) the increase in non-radiative decay from the excited states. The increase in non-radiative decay from the normal emission competes with the increase in the relative population of trans-enol with a rise in temperature.

Enhancing excited state intramolecular proton transfer in 2-(2'-hydroxyphenyl)benzimidazole and its nitrogen-substituted analogues by ß-cyclodextrin: the effect of nitrogen substitution.

Excited state intramolecular proton transfer (ESIPT) in nitrogen-substituted analogues of 2-(2'-hydroxyphenyl)benzimidazole (HPBI), 2-(2'-hydroxyphenyl)-3H-imidazo[4,5-b]pyridine (HPIP-b), and 2-(2'-hydroxyphenyl)-3H-imidazo[4,5-c]pyridine (HPIP-c) have been investigated in a ß-cyclodextrin (ß-CD) nanocavity and compared with that of HPBI. The stoichiometry and the binding constants of the complexes were determined by tautomer emissions. Both pKa and NMR experiments were employed to determine the orientation of the molecules inside of the ß-CD cavity. Huge enhancement in the tautomer emission of HPIP-b and HPIP-c compared to that of HPBI in ß-CD suggests that not only is the ESIPT favored inside of the cavity, but also, the environment reduces the nonradiative decay through the formation of an intramolecular charge-transfer (ICT) state. Unlike HPBI, the tautomer emission to normal emission ratio of HPIP-b increases from 0.9 to 2.6, and that of HPIP-c increases from 4.9 to 7.4 in 15 mM ß-CD. The effect of dimethylsulfoxide (DMSO) on complexation was also investigated for all three guest molecules. In DMSO, HPBI is present in neutral form, but the nitrogen-substituted analogues are present in both neutral and monoanionic forms. However, in DMSO upon encapsulation by ß-CD, all three molecules are present in both neutral and monoanionic forms in the nanocavity. The monoanion is stabilized more inside of the ß-CD cavity. The studies revealed that the ESIPT of nitrogen-substituted analogues is more susceptible to the environment than HPBI, and therefore, they are more promising probes.

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.

The role of hydrogen bonding in excited state intramolecular charge transfer.

Intramolecular charge transfer (ICT) that occurs upon photoexcitation of molecules is a vital process in nature and it has ample applications in chemistry and biology. The ICT process of the excited molecules is affected by several environmental factors including polarity, viscosity and hydrogen bonding. The effect of polarity and viscosity on the ICT processes is well understood. But, despite the fact that hydrogen bonding significantly influences the ICT process, the specific role of hydrogen bonding in the formation and stabilization of the ICT state is not unambiguously established. Some literature reports predicted that the hydrogen bonding of the solvent with a donor promotes the formation of a twisted intramolecular charge transfer (TICT) state. Some other reports stated that it inhibits the formation of the TICT state. Alternatively, it was proposed that the hydrogen bonding of the solvent with an acceptor favors the TICT state. It is also observed that a dynamic equilibrium is established between the free and the hydrogen bonded ICT states. This perspective focuses on the specific role played by hydrogen bonding of the solvent with the donor and the acceptor, and by proton transfer in the ICT process. The utility of such influence in molecular recognition and anion sensing is discussed with a few recent literature examples in the end.

A 2-pyridyl (py) attached phosphine imine [P(Npy)(NHpy)3] and an imido phosphinate ion [P(Npy)2)(NHpy)2]- in its Ag(I) complex.

A new phosphine imine 3, [P(Npy)(NHpy)(3)] (py = 2-pyridyl), was synthesized from the phosphonium salt 1, [P(NHpy)(4)]Cl. Subsequent reaction of 1 or 3 with AgClO(4) lead to an unprecedented penta-nuclear Ag(I) complex 4 stabilized by two [P(Npy)(2)(NHpy)(2)](-) anions [L](-). The packing diagram of 4 shows an interesting channel structure which contains solvated molecules of methanol and toluene. The diimine ligand [L](-), which represents the N-analogue of a phosphinate ion (H(2)PO(4)(-)), was obtained in situ under the mild reaction conditions in the absence of a base.

Role of nitrogen substitution in phenyl ring on excited state intramolecular proton transfer and rotamerism of 2-(2'-hydroxyphenyl)benzimidazole: a theoretical study.

A comparative study of 2-(2'-hydroxy-3'-pyridyl)benzimidazole (2',3'-HPyBI), 2-(3'-hydroxy-4'-pyridyl)benzimidazole (3',4'-HPyBI), 2-(4'-hydroxy-3'-pyridyl)benzimidazole (4',3'-HPyBI), 2-(3'-hydroxy-2'-pyridyl)benzimidazole (3',2'-HPyBI), and 2-(5'-hydroxy-4'-pyrimidinyl)benzimidazole (5',4'-HPymBI) with 2-(2'-hydroxyphenyl)benzimidazole (HPBI) was performed theoretically to evaluate the effect of nitrogen substitution in the phenolic ring on the photophysics and rotamerism of HPBI. Density functional theory (DFT) and configuration interaction singles (CIS) combined with time-dependent DFT were employed for ground and excited state studies, respectively. Different possible molecular forms were considered for each molecule viz., cis-enol, trans-enol, open-enol, and keto forms. The computational results revealed that cis-enol is the most stable form in the ground state for all the molecules except in 2',3'-HPyBI. In 2',3'-HPyBI, K-2 keto is the most stable form. Water molecule assisted interconversions between different forms of 2',3'-HPyBI were examined theoretically. Excitation and emission energies for all the forms have been calculated theoretically and the values are in good agreement with the available experimental data. The calculations show that intramolecular proton transfer (ESIPT) is endothermic in the ground state while it is exothermic in the first excited singlet state (except 5',4'-HPymBI). The barrier for the excited state ESIPT reaction increases with nitrogen substitution. Torsional rotation between the benzimidazole and the pyridinyl∕pyrimidinyl rings in the S(1) state depicts that twisted-keto structures involve charge transfer from the hydroxypyridinyl∕hydoxypyrimidinyl to the benzimidazole ring. However, the formation of twisted-keto is not energetically favored in these systems.

The thiocarbonyl 'S' is softer than thiolate 'S': a catalyst-free one-pot synthesis of isothiocyanates in water.

Treatment of the preformed or the in situ generated aryl/alkyl dithiocarbamates triethylammonium salt (ArNHCSS(-).Et(3)NH(+)) with methyl acrylate in an aqueous medium gave solely arylisothiocyanate (ArNCS), whereas the in situ generated aryl dithiocarbamic acid (ArNHCSS(-).H(+)) yielded exclusively the thia-Michael adduct (ArNHCSSCH(2)CH(2)COOMe). This differential reactivity can be explained by two alternative mechanisms which is dependent both on the nature of the counter cation and on the pH of the reaction medium. Irrespective of the counter cations, the thiocarbonyl sulfur (=S) atom, having large orbital-coefficient, is softer compared to the thiol/thiolate sulfur (-SH/S(-)) in a dithiocarbamate salt and the former adds to the Michael acceptor by a 1,4-addition.

Encapsulation of 2-(4'-N,N-dimethylamino)phenylimidazo[4,5-b]pyridine in beta-cyclodextrin: effect on H-bond-induced intramolecular charge transfer emission.

The effect of beta-cyclodextrin (beta-CD) inclusion complex formation on the hydrogen bond-induced intramolecular charge transfer (ICT) of 2-(4'-N,N-dimethylamino)phenylimidazo[4,5-b]pyridine (DMAPIP-b) has been examined by fluorescence excitation, emission and time-resolved fluorescence techniques. The study reveals that DMAPIP-b forms 1 : 1 inclusion complex with beta-CD. The host-guest complex is formed by partial inclusion of DMAPIP-b, i.e. only the dimethylaminophenyl ring is encapsulated inside the core of the beta-CD nanocavity. The imidazopyridine ring of the guest molecule resides outside CD cavity and forms H-bonds with the water molecules that are present near the rim and in the bulk phase. (1)H NMR studies are used to confirm the inclusion complex. The H-bond of water with the pyridine nitrogen ensures the formation of the ICT state and both normal and ICT emissions are enhanced inside the beta-CD cavity. Fluorescence lifetime measurements suggest that the formation of the ICT state from the locally excited state is irreversible. Dual emission is observed in the presence of beta-CD at pH approximately 3.5, due to emission from monocations formed by the protonation of pyridine nitrogen (MC1) and imidazole nitrogen (MC2).

Comparative theoretical study of rotamerism and excited state intramolecular proton transfer of 2-(2'-hydroxyphenyl)benzimidazole, 2-(2'-hydroxyphenyl)imidazo[4,5-b]pyridine, 2-(2'-hydroxyphenyl)imidazo[4,5-c] pyridine and 8-(2'-hydroxyphenyl)purine.

The effect of nitrogen substitution in the benzene ring of 2-(2'-hydroxyphenyl)benzimidazole (HPBI) on the photophysics and rotamerization were examined theoretically by a comparative study of HPBI with 2-(2'-hydroxyphenyl)imidazo[4,5-b]pyridine (HPIP-b), 2-(2'-hydroxyphenyl)imidazo[4,5-c]pyridine (HPIP-c), and 8-(2'-hydroxyphenyl)purine (HPP). Density functional theory (DFT) was used for ground state calculations. Restricted configuration interaction singles (RCIS) combined time dependent DFT (TDDFT) was used for excited state calculations. The calculations reveal in the ground state all of the molecules have two stable rotameric forms, but their relative population is strongly affected by nitrogen substitution. The excitation and emission bands have been calculated theoretically for the rotamers and tautomers. Fluorescence emission and excitation spectra were recorded for HPBI in dioxane and compared with the theoretical results. Theoretical excitation and emission data are in good agreement with the available experimental data. The potential energy surface simulated for the proton transfer processes reflect that it is not favorable in S(0) state, but it is feasible in S(1) state in all of the molecules. Except in HPIP-b, HPIP-b', and HPP', in all other nitrogen substituted molecules, the energy difference between the keto and enol form along the excited state proton transfer coordinates decreases compared to that in HPBI. The study also reveals that torsional relaxation of tautomer to twisted state competes with radiative transitions and leads to fluorescence quenching. Nitrogen substitution enhances this torsional induced nonradiative process and it follows the order HPBI < HPIP-b < HPIP-c < HPP.