Saikat Mukherjee

Intramolecular proton transfer in the first-excited electronic states of 4-methyl-2,6-diformyl phenol in some hydrocarbon solvents

Abstract Proton translocation in the singlet excited state of 4-methyl-2,6-diformyl phenol (MFOH) results in an interesting fluorescence band at 535 nm. The fluorescence emission and excitation spectra of MFOH are recorded and a time-correlated single-photon counting technique is used to determine the fluorescence lifetimes in some hydrocarbon solvents at room temperature. The stable molecular structure in the ground state of MFOH is an intramolecularly hydrogen bonded closed conformer from which proton transfer takes place in the first excited 1 (ππ * ) state. The Stokes-shifted fluorescence band observed in the yellow—green region is likely to originate from the enol tautomer which is found to exist even at 77 K. From nanosecond measurements and the quantum yields of fluorescence we have determined proton transfer rates. It is suggested that the proton transfer in MFOH is relatively slow in these solvents and MFOH can have only one ground state conformation, except in benzene and toluene.

Proton transfer and anion formation in the ground and excited states of 4-methyl-2,6-diformyl phenol in highly polar aprotic solvents

Abstract From a detailed study of the fluorescence spectra, excitation spectra and time-resolved kinetics, two ground state conformers of 4-methyl-2,6-diformyl phenol (MFOH) in two proton-accepting solvents (dimethylsulphoxide (DMSO) and N,N -dimethylformamide (DMF)) were identified: the 4-methyl-2,6-diformyl phenolate (MFO − ) anion and the hydrogen-bonded closed conformer. The yellow—green and blue emissions at 520 and 460 nm are assigned to MFO − and the hydrogen-bonded open conformer respectively. At 77 K the fluorescence due to the open conformer is markedly suppressed, showing that this conformer is less stable than (MFO − )*. At this low temperature the fluorescence spectrum is shown to be contaminated by phosphorescence. Fluorescence decay times were measured for both species. Experimental evidence is in favour of an equilibrium between two conformers in both the ground and excited states. It is shown that the short- to long-wavelength fluorescence intensity ratio is dependent on the excitation energy.

Excited state intramolecular proton transfer in a new o-hydroxy Schiff base in non polar solvents at room temperature and 77 K

Abstract A new orthohydroxy Schiff base, 7-ethylsalicylidenebenzylamine (ESBA) has been synthesised. The excited state intramolecular proton transfer (ESIPT) processes have been investigated by means of absorption, emission and nanosecond spectroscopy at room temperature and at 77 K in non polar solvents. The ESIPT is evidenced by a large Stokes shifted emission (∼11 000 cm−1) only at 77 K. From fluorescence and excitation spectra it is suggested that at least three different species are present in the excited state at room temperature. Our theoretical calculation at AM1 level confirm the cis-isomer to be the only viable form in the ground state.

Proton transfer and excitation-wavelenght-dependent fluorescence and phosphorescence spectra of 4-methyl-2,6-diformyl phenol: interaction with triethylamine

Abstract The absorption, emission and excitation spectra of 4-methyl-2,6-diformyl phenol (MFOH) in pure acetone and acetonitrile and in the presence of triethylamine (TEA) were recorded at room temperature (298 K) and 77 K. The broad emission band at 460 nm is assigned to the hydrogen-bonded open conformer. It is proposed that interaction of MFOH with TEA results in the formation of a hydrogen-bonded ion pair and solvent-separated ion pair in the ground state and 4-methyl-2,6-diformyl anion (MFO − and ion pair in the excited state. From the nanosecond measurements and quantum yields of fluorescence, we estimated the decay rate constants. At 77 K, deactivation occurs via phosphorescence although fluorescing species also exist. Both fluorescence and phosphorescence are found to be excitation wavelenght dependent.

Conical intersections and diabatic potential energy surfaces for the three lowest electronic singlet states of H3

We calculate the adiabatic Potential Energy Surfaces (PESs) and the Non-Adiabatic Coupling Terms (NACTs) for the three lowest singlet states of H3 (+) in hyperspherical coordinates as functions of hyperangles (θ and ϕ) for a grid of fixed values of hyperradius (1.5 ⩽ ρ ⩽ 20 bohrs) using the MRCI level of methodology employing ab initio quantum chemistry package (MOLPRO). The NACT between the ground and the first excited state translates along the seams on the θ - ϕ space, i.e., there are six Conical Intersections (CIs) at each θ (60° ⩽ θ ⩽ 90°) within the domain, 0 ⩽ ϕ ⩽ 2π. While transforming the adiabatic PESs to the diabatic ones, such surfaces show up six crossings along those seams. Our beyond Born-Oppenheimer approach could incorporate the effect of NACTs accurately and construct single-valued, continuous, smooth, and symmetric diabatic PESs. Since the location of CIs and the spatial amplitudes of NACTs are most prominent around ρ = 10 bohrs, generally only those results are depicted.

Proton transfer reaction of 4-methyl-2,6-dicarbomethoxyphenol in nonpolar and weakly polar solvents

Proton transfer processes in the ground and excited state of 4-methyl-2,6-dicarbomethoxyphenol (CMOH) have been investigated by means of steady-state and nanosecond transient spectroscopy in some nonpolar and weakly polar solvents at room temperature and 77 K. The results obtained for CMOH were compared with that of 4-methyl-2,6-diformylphenol (MFOH) reported previously. Unlike MFOH the fluorescence spectra of CMOH show dual emission. This was explained as due to the presence of both the enol tautomer and intramolecularly hydrogen-bonded closed conformer as the fluorescing species. At 77 K the emission spectra show phosphorescence only in the presence of a base like triethylamine. The fluorescence decay rates of CMOH are relatively slower than that of MFOH and the nonradiative rates are always found higher than the radiative rates. A theoretical calculation at the Austin model 1 level of approximation revealed that the excited-state intramolecular proton transfer barrier in the ground singlet and excited ...

Proton transfer reaction of 4-methyl-2,6-diacetylphenol and an analysis with AM1 potential-energy surfaces

The ground and excited state proton transfer processes of 4-methyl-2,6-diacetylphenol (MAOH) have been studied by means of steady-state absorption, emission and time resolved spectroscopy in different protic and aprotic solvents at room temperature and 77 K. The relative fluorescence quantum yield measurements are made at different excitation wavelength and both in presence and absence of added base. The emission properties of MAOH at 77 K have been examined in relation to those of 4-methyl-2,6-diformylphenol (MFOH). At this temperature, unlike MFOH, MAOH show phosphorescence only in presence of base like triethylamine in all the solvents studied here. From nanosecond measurements and fluorescence quantum yield we have estimated the decay rate constants. The nonradiative decays are always dominant in the decay processes of the excited states. The energetics of the ground- and excited-state proton transfer in MAOH molecule has been studied by the configuration interaction method at AM1 level of approximati...

Photoinduced proton transfer in the excited singlet state of 3,7-dihydroxynaphthoic acid and solvent effect

AbstractThephotophysicsof3;7-dihydroxynaphthoicacid(DHNA)hasbeenstudiedindifferentaproticandproticsolventsbysteady-stateandnanosecondtransientemissionspectroscopy.BoththemonomerandthedimerofDHNAhavebeendetectedinthegroundaswellasinexcitedstateindilutesolution( 310 5 moldm ).AlargeStokesshiftedemission( 6500cm 1 )indicatesthatDHNAundergoesthermodynamicallyfavourableprotontransferintheexcitedstate.Itisproposedthattheaddedbasepromotestheprotontransferonlyinpolarproticsolvents.Themechanismofthestaticprocesseshasbeeninvestigatedbymakingsteady-statefluorescencequenchingmeasurementsandthatofthedynamicprocessesbynanosecondfluorescencelifetimetechniques. 2002ElsevierScienceB.V.Allrightsreserved. 1. IntroductionAlthoughprotontransfer(PT)reactionofsal-icylicacid(SA)wasknownfourdecadesago[1],notmanystudieshavebeendoneonthissystem.Thisisduetothefactthato-hydroxycarboxyliccompounds such as SA dimerize readily even indilute solution and get ionised in polar solvents.Two types of emission are generally observed inthe case of SA and its derivatives. Large Stokesshiftedemission(redband)isgenerallyobservedinhydrocarbonandnonpolarsolvents.Anothertypeof emission with relatively shorter Stokes shift(blueband)isobservedinpolarsolvents.Pantetal.[2,3]suggestedthatthelargerStokesshifted‘red’bandisduetotheexcitedstatePTtogiverisetothetautomer.Ontheotherhand,thenormalformemitstogivetheblueband.However,inthecaseof3;5-t-butylsalicylicacid,Lawetal.[4]showedthatthecarboxylateanionistheonlyprecursortotautomerisation.It has been shown earlier that in the case ofsalicylicacidthemoststableforminthegroundstateisthenormalprimaryformandthatintheexcitedstateisthetautomericform[5].Ithasalsobeensuggestedthat boththeprimary andtauto-meric forms are intramolecularly hydrogen bon-ded.ThetautomericformresultsduetoaPTintheprimaryform.Salicylicacidexistsasadimerinthesolidstateandinnonpolarsolventsathigherconcentration[2,3,6,7].Itexistsasamonomerinpolar solvents and gets protonated and deproto-

Proton transfer reaction of 4-methyl-2,6-dicarbomethoxyphenol in polar aprotic solvents at room temperature and 77 K

Proton transfer processes of 4-methyl-2,6-dicarbomethoxyphenol (CMOH) have been investigated by means of absorption, emission and nanosecond transient spectroscopy in some polar aprotic solvents at room temperature and 77 K. Unlike MFOH, CMOH does not show any phosphorescence spectra by lowering the temperature at 77 K in any of the pure solvents studied here. The fluorescence decay rates of CMOH are relatively faster compared to those of 4-methyl-2,6-diformylphenol (MFOH) and nonradiative decay rates are dominant over the proton transfer processes.

Surface temperature effect on the scattering of D2(v = 0, j = 0)-Cu(111) system

We perform four-dimensional (4D⊗2D) as well as six-dimensional (6D) quantum dynamics on a parametrically time- and temperature-dependent effective Hamiltonian for D(2)(v, j)-Cu(111) system, where such effective potential has been derived through a mean-field approach between molecular degrees of freedom and surface modes with Bose-Einstein probability factor for their initial state distribution. We present the convergence of the theoretically calculated sticking probabilities employing 4D⊗2D quantum dynamics with increasing number of surface atoms as well as layers for rigid surface and the surface at a particular temperature, where the temperature-dependent sticking probabilities appear exclusively dictated by those surface modes directed along the Z-axis. The sticking and state-to-state transition probabilities obtained from 6D quantum dynamics are shown as a function of initial kinetic energy of the diatom at different surface temperature. Theoretically calculated sticking probabilities display the similar trend with the experimentally measured one.