Ramprasad Misra

Spectral Signatures of Intramolecular Charge Transfer Process in β-Enaminones: A Combined Experimental and Theoretical Analysis

In this paper, we present spectroscopic signatures of intramolecular charge transfer (ICT) and effects of solvent on the ICT process in 3-(phenylamino)-2-cyclohexen-1-one (PACO), a member of the well-known molecular family, the beta-enaminones. The dual fluorescence in the steady state emission spectra of the molecule in polar solvents indicates the occurrence of ICT, which is further supported by time-resolved studies, using time correlated single photon counting technique with picosecond resolution. To understand the nature of the charge transfer, pH dependent studies of the probe in water were performed, where a quenching of fluorescence was observed even in the presence of very low concentrations of acids. Solvent induced fluorescence quenching was observed in ethanol and methanol. The ICT process was also investigated by quantum chemical calculations. To understand the role of solvents in the ICT process, we have theoretically studied the macroscopic and microscopic solvation of the probe in water. The absorption spectra of the molecule in the gas phase as well as in water were simulated using time dependent density functional theory with cc-pVTZ basis set and self-consistent reaction field theory that models macroscopic solvation. The possibility of microscopic solvation in water was probed theoretically and the formation of 1:3 molecular clusters by PACO with water molecules has been confirmed. Our findings could have a bearing on pH sensing applications of the probe.

Spectral Response of 4-Methyl-2,6-dicarbomethoxyphenol, an Excited-State Intramolecular Proton-Transfer Probe in Cyclohexane–Ethanol Mixtures: Signatures of Medium Microheterogeneity

In this paper, we explore the role of microscopic heterogeneity of the medium on the spectral response of an excited-state proton-transfer (ESIPT) probe, namely, 4-methyl-2,6-dicarbomethoxyphenol (CMOH) using steady-state and time-resolved emission spectroscopy. The mixtures of two solvents with widely different properties, viz., cyclohexane, a nonpolar, and ethanol, a polar protic solvent, were used as microheterogeneous media for spectroscopic studies. Dual fluorescence (normal and tautomer fluorescence) is observed in the nonpolar solvent (cyclohexane), while only a single peak is observed in the protic solvent, ethanol. The spectral responses of CMOH in the binary mixtures have been found to be dependent on the solvent composition and excitation wavelength. The emission spectral properties of CMOH in the cyclohexane-ethanol mixture have been seen to be superposition of spectral properties in their bulk counterparts, indicating the presence of microscopic heterogeneity in the system. A zwitterionic species of CMOH appears to have been detected in binary solvent mixtures with higher ethanol content only through low-energy excitations. The species is converted into an anionic species as excitation energy increases. Density functional theory calculations indicate that two intramolecularly hydrogen bonded rotamers of CMOH have a small energy difference. The formation of a hydrogen bonded 1:1 molecular cluster of CMOH with ethanol has been investigated in the ground state at the same level of theory. Our findings are expected to shed light on the mechanism of many acid-base reactions occurring in microscopically inhomogeneous media that often mimic many biologically relevant processes.

Nanopools Governing Proton Transfer in Diametrical Ways in the Ground and Excited State

We present here the effects of geometrically constrained environments on the proton transfer reaction of 4-methyl 2,6-diformyl phenol (MFOH) both in the ground and excited states by employing steady-state and time-resolved fluorescence spectroscopy having picosecond and femtosecond resolutions. The nanometer-sized water pools formed in the ternary microemulsion of n-heptane-aerosol OT-water promote reprotonation of the probe. As we go on increasing the water content up to a certain value in the ground state whereas deprotonation is favored in the excited state. The emission intensity has a complex behavior as the water content is changed in the system. The lower fluidity of confined water within the reverse micelle with respect to the normal bulk water alters the related dynamics of the H-bonded network. These observations are rationalized on the basis of altered ionic water activity in the confined surroundings, i.e., on dielectric constant, ionic mobility, pH, and the favorable orientation of dipoles in the medium. Our observations might be helpful to infer about the characteristics of nanoreactors, which often mimic many biological hydrophilic pockets.

Structural and Infra Red Spectroscopic Aspects of Ion-Water Clusters: A Study Based on a Combined Stochastic and Quantum Chemical Approach

AbstractIn the present communication, we explore a strategy based on using a stochastic technique, namely Simulated Annealing (SA), in conjunction with quantum chemical calculations to arrive at good candidate structures for ion-water clusters. We investigate if our approach is equally suited to handle structures in which the encapsulated ion is cationic (Na+) as well as anionic (Cl−). To critically analyse the quality of structure being generated, we perform spectroscopic and thermochemical studies on these systems and compare our results with the available experimental ones. We conclude that our idea is both computationally cost effective as well as quantitatively accurate in providing good quality structures for these systems.Graphical Abstract Structure of (H2O)6Na+ obtained after Simulated Annealing optimization (using an emperical potential) followed by further optimization at MP2 level of theory

Tailoring of spectral response and intramolecular charge transfer in β-enaminones through band gap tuning: synthesis, spectroscopy and quantum chemical studies

In this paper, we investigate the synthetic tailoring of the spectral response and intramolecular charge transfer (ICT) of β-enaminones through bandgap modulation. Two donor/acceptor substituted β-enaminones, namely, 3-(4-methoxyphenylamino)-2-cyclohexen-1-one (OACO) and 3-(4-nitrophenylamino)-2-cyclohexen-1-one (NACO) have been synthesized along with their unsubstituted counterpart, 3-(phenylamino)-2-cyclohexen-1-one (PACO). Steady state as well as time resolved spectroscopic techniques with picosecond resolution are used to record their spectral responses. Substitution of the donor group (–OCH3) mildly enhances the charge transfer from the phenyl ring to the enaminone moiety, while substitution of the acceptor group (–NO2) jeopardizes the charge transfer through mutual electron withdrawing effects of PNA and enaminone moieties. Combined experimental and quantum chemical investigations reveal that the ground state photophysics of OACO and NACO in water are controlled by both microscopic and macroscopic solvation with dominant contribution from the former. Time dependent density functional theory (TDDFT) calculations predict that the HOMO to [LUMO+1] transition gives rise to the absorption spectra of OACO in water, while the absorption by the enaminone moiety of NACO arises as a result of a HOMO to LUMO transition. A crossing between the first (S1) and the second excited (S2) states takes place in the microclusters of PACO, OACO and NACO with water. The intersystem crossing (ISC) has been found to be the major reason for low quantum yields in these molecules. The band gap modulation through waxing and waning of the conjugation strength is expected to throw light on many ICT-driven processes and provides means of tuning the properties depending on it.

Exploring NLO response of 9,10- donor-acceptor substituted Bichromophoric Anthracene Derivatives

Ab-initio finite field SCF calculations with split valence double zeta basis set indicate that 9, 10- donor-acceptor substituted anthracene derivatives have large quadratic hyperpolarizabilities for suitable combinations of donor and acceptor moieties. A difference in the first hyperpolarizabilities (β

Investigation of contrasting hydrogen bonding pattern of 3-(phenylamino)-cyclohexen-1-one with solvents in the ground and excited states

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Linear and non-linear optical response properties of β-enamino ketones

) of isomeric organic chromophores containing the same donor and acceptor groups indicate the possible role of intramolecular charge transfer (ICT) in shaping the NLO response in these π-conjugated molecular chromophores. A correlation is sought to be established among calculated β

Probing microcluster formation between PACO and solvents containing oxygen donor sites mediated by the ‘N–H’ Bond

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Photoinduced tautomerism of 2,6-dicarbomethoxyphenol in DMF–water mixtures: Perturbation from intermolecular processes

on one hand and the donor strength, the strength of the acceptor, donor-acceptor interaction and the donor-acceptor separation, on the other by using Genetic Algorithm (GA) to search through the relevant parameter space. It appears that β