02379nas a2200181   4500008004100000022001400041245013900055210006900194260007100263300001400334490000800348520164000356100001701996700002102013700001602034700001702050856013002067       2014                        eng d  a1520-610600aAtypical energetic and kinetic course of excited-state intramolecular proton transfer (ESIPT) in room-temperature protic ionic liquids0 aAtypical energetic and kinetic course of excitedstate intramolec  a1155 16TH ST, NW, WASHINGTON, DC 20036 USAbAMER CHEMICAL SOCcMAR  a2487-24980 v1183 a<p>The excited-state intramolecular proton-transfer (ESIPT) process in 1,8-dihydroxyanthraquinone (18DHAQ) dye has been investigated in protic ionic liquid (PIL) solvents using photochemical measurements. The results demonstrate noteworthy modulations in both steady-state and time-resolved emission characteristics of excited normal (N*) and tautomeric (T*) forms of the dye. That the emission of T* increases unexpectedly upon increasing solvent viscosity indicates that subsequent to the initial forward ESIPT, there is also a relatively slower back ESIPT process involved for the excited dye. It is inferred that the propensity of this back ESIPT process is determined by the dynamics of the diffusive solvent relaxation, a process that is known to be strongly viscosity-dependent in ionic liquids. Evidence of both forward and back ESIPT for the dye has been obtained from femtosecond fluorescence up-conversion measurements. While an unusually fast forward ESIPT is clearly observed in all of the PILs studied, the uncommon back ESIPT process is distinctly indicated in PIL solvents having lower viscosities, certainly due to reasonably fast diffusive solvent relaxation in these solvents that causes a temporal modulation in the energies of the normal and tautomeric forms within a reasonably short time and thereby brings down the energy of N* compared to that of T*, triggering the back ESIPT process. Observation of solvent-viscosity-dependent back ESIPT is an intriguing finding for the present study as to the best of our knowledge, such a behavior has so far not been reported in the literature for the ESIPT reaction.</p>1 aManna, Arpan1 aSayed, Mhejabeen1 aKumar, Anil1 aPal, Haridas  uhttps://library.ncl.res.in/content/atypical-energetic-and-kinetic-course-excited-state-intramolecular-proton-transfer-esipt-003636nas a2200193   4500008004100000022001400041245012000055210006900175260007100244300001600315490000800331520288200339100002103221700001703242700002103259700001603280700001703296856012903313       2014                        eng d  a1520-610600aObservation of the marcus inverted region for bimolecular photoinduced electron-transfer reactions in viscous media0 aObservation of the marcus inverted region for bimolecular photoi  a1155 16TH ST, NW, WASHINGTON, DC 20036 USAbAMER CHEMICAL SOCcSEP  a10704-107150 v1183 a<p>The general observation of Marcus inverted region (MIR) for bimolecular electron-transfer (ET) reactions in different viscous media, e.g., micelles, reverse micelles, vesicles, ionic liquids, DNA scaffold, etc. has been doubted in some recent publications arguing limitations in SternVolmer (SV) analysis to account for the static and transient stages of quenching in these slow diffusing media. Thus, following a theoretical treatment based on a spherically symmetric diffusion equation coupled with conventional Marcus ET description, it has been suggested that the MIR observed in viscous media arises due to the inadequate consideration of different quenching regimes and also due to the differential excited-state lifetimes of the fluorophores used than a genuine one (J. Am. Chem. Soc. 2012, 134, 11396). However, the overall treatment in this study is severely compromised by setting the minimum solvent reorganization energy (?s) to (lambda) to similar to 0.96 eV while fitting the experimental data, which unambiguously suggests that the inversion in ET rate will never appear in the exergonicity (-Delta G(0)) range of 0.16 to 0.71 eV, as is the case for the studied ET systems. Besides, the applicability of the conventional Marcus ET model (instead of Sumi-Marcus two-dimensional ET model) in such extremely viscous media with exceptionally slow solvent response is highly debatable and perhaps is the main cause of the failure in fitting the experimental data quite satisfactorily. In the present study involving ultrafast ET quenching for coumarin derivatives by dimethylaniline donor in viscous ionic liquid media, we demonstrate clear MIR for the intrinsic ET rates, directly obtained from the ultrafast decay components of 110 ps, a time scale in which diffusion of reactants is negligible and the ET rates are either faster than or, at the most, competitive with the solvent reorganization. The appearance of MIR at ?Delta G(0) similar to-0.5 eV, significantly lower than expected from the lambda(s) value, further substantiate the nonapplicability of conventional ET description but certainly advocate for the applicability of the SumiMarcus two-dimensional ET model in such media. Moreover, no obvious correlation has experimentally been observed between the excited-state lifetimes of the coumarin derivatives and the ET rates for a large number of dyes used in the present study. On the basis of the present results and drawing inferences from reported literatures in viscous media, we conclude that not only is the appearance of MIR very genuine but also the mechanistic model necessary to account the observed facts for the bimolecular ET reactions in a viscous medium is the two-dimensional ET description, which deals with an extremely slow relaxing solvent coordinate and a fast relaxing intramolecular coordinate to describe the ET reactions.</p>1 aKumbhakar, Manoj1 aManna, Arpan1 aSayed, Mhejabeen1 aKumar, Anil1 aPal, Haridas  uhttps://library.ncl.res.in/content/observation-marcus-inverted-region-bimolecular-photoinduced-electron-transfer-reactions-002769nas a2200241   4500008004100000022001400041245018600055210006900241260000900310300001200319490000800331520183400339653000602173653004602179653002902225653004502254653001802299653002902317100001902346700001602365700001702381856012902398       2018                        eng d  a1010-603000aExcited state intramolecular proton transfer in 1,8-Dihydroxy-9,10-anthraquinone dye: Revealing microstructures in [C(n)mIm][NTf2] and [C(n)mIm][BF4] series of ionic liquid solvents0 aExcited state intramolecular proton transfer in 18Dihydroxy910an  cJAN   a111-1210 v3503 a<p>Excited state intramolecular proton transfer (ESIPT) in 18-dihydroxy-9,10-anthraquinone (18DHAQ) dye has been investigated in two series of 1-alkyl-3-methylimidazolium ([C(n)mIm](+)) based ionic liquid (IL) solvents, [C(n)mlm][NTf2] and [C(n)mlm][BF4], with n = 2, 4, 6, 8 and 10, using steady-state (SS) and time resolved (TR) fluorescence studies. In both the IL series, fluorescence intensity for tautomer (T*) form gradually decreases relative to normal (N*) form with increasing n value for [C(n)mlm](+) cations. Observed results suggest microstructure formation and its consequent effect on the ESIPT process of the dye in these IL solvents. Since more extensive microstructures are likely with larger n values of [C(n)mIm](+) cations, the dye is expectedly solubilized more in the polar microdomains of the solvent microstructures, resulting better stabilization for the more dipolar N* state and hence a higher emission intensity from this state. This proposition is clearly supported by ultrafast (sub-picosecond) fluorescence kinetics for both N* and T* states, though sub-nanosecond TR results indicate very similar fluoresce decays for both N* and T*, suggesting an eventual kinetic equilibrium between two states subsequent to the initial ultrafast and fast forward and backward ESIPT processes. Small differences between the results in the [C(n)mIm][NTf2] and [C(n)mIm][BE4] series of ILs are attributed to dissimilar size, shape and basicity of [NTf2]- and [BF4]- anions, responsible for some characteristic changes in the microstructures formed in the respective solvent series. To the best of our knowledge, present study is the only report demonstrating modulations in the ESIPT process through microstructure formations in neat [C(n)mIm](+) based IL solvents. (C) 2017 Elsevier B.V. All rights reserved.</p>10a110a1-Alkyl-3-methylimidazolium ionic liquids10a8-Dihydroxyanthraquinone10aEmission modulations of tautomeric forms10aESIPT process10aMicrostructure formation1 aBeniwal, Vijay1 aKumar, Anil1 aPal, Haridas  uhttps://library.ncl.res.in/content/excited-state-intramolecular-proton-transfer-18-dihydroxy-910-anthraquinone-dye-revealing02418nas a2200169   4500008004100000245018000041210006900221260000800290300001400298490000700312520171600319100001902035700001602054700001702070700003302087856012802120       2018                        eng d00a Excited-state prototropism of 7-hydroxy-4-methylcoumarin in [C(n)mim][BF4] series of ionic liquid-water mixtures: insights on reverse micelle-like water nanocluster formation0 aExcitedstate prototropism of 7hydroxy4methylcoumarin in CnmimBF4  cSEP  a1256-12660 v173 aThis study explores the excited state prototropic behavior of the fluorophore, 7-hydroxy-4-methylcoumarin (7H4MC), in the [C(n)mim][BF4] (n = 2, 4, 6, 8, 10) series of ionic liquid (IL)-water mixtures at low water contents. In pure IL media, 7H4MC exists in the neutral form in both ground and excited states. However, on addition of water to the ILs, the excited neutral form of the dye is gradually converted to the anionic and the tautomeric species, leading to characteristic changes in the emission spectra. The similarity in the spectral features of 7H4MC in the IL-water system with that in a conventional reverse micelle system rather than with organic solvent-water mixtures, suggests that in the presence of water, the ILs are organized into reverse micelle-like structures with the consequent formation of confined water pockets. The results further suggest that formation of water nanoclusters and the ensuing changes in excited state prototropic behavior of the dye, is facilitated by increase in the alkyl chain length of the IL cation. These propositions are supported by time-resolved fluorescence studies. To the best of our knowledge this is the first report on proton transfer reaction in IL-water mixtures at low water contents. Considering that ILs are useful as solvents and surfactants, and IL-water mixtures in particular have applications in chemical extractions and biocatalysis, an understanding of the structural organization and water pool formation in these systems is quite important. The insights obtained from the prototropic transformations of 7H4MC are significant not only for fundamental self-assembly studies, but also for the development of ILs as chemical reaction media.1 aBeniwal, Vijay1 aKumar, Anil1 aPal, Haridas1 aChoudhury, Sharmistha, Dutta  uhttps://library.ncl.res.in/content/excited-state-prototropism-7-hydroxy-4-methylcoumarin-cnmimbf4-series-ionic-liquid-water