Sivaprasad Mitra

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.

Emission spectra of 4-methyl-2,6-diacetylphenol in 3-methylpentane at room temperature and 77 K

Abstract The fluorescence properties of 4-methyl-2,6-diacetylphenol (MAOH) are examined in 3-methylpentane and quantum yield measurements are made at room temperature and 77 K. The fluorescence quantum yield is temperature and excitation wavelength dependent. 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 does not show any phosphorescence in 3-methylpentane. From this observation it is confirmed that phosphorescence appears due to the rotation of the formyl group in MFOH. The fluorescence decay rate of MAOH is relatively faster than that of MFOH and non-radiative decays are dominant in the decay processes of the excited states of both compounds.

Excitation wavelength and solvent dependent emission spectra in weakly polar aprotic solvents at room temperature and 77 K

Abstract Excitation wavelength-dependent emission spectra of 4-methyl-2,6-diformylphenol (MFOH) and o -hydroxybenzaldehyde (OHBA) have been examined both in pure weakly polar aprotic solvents and in the presence of a base at room temperature and 77 K. It is shown that fluorescence quantum yield shape, position of the spectra, and number of conformers are dependent upon the excitation energy and also on the proton-accepting ability of the solvents. Fluorescence spectra cannot be correlated with the solvent dielectric properties. At 77 K, deactivation occurs via phosphorescence only at a particular experimental condition in all the solvents studied here. The decay rates are relatively slower in an environment where the probability of hydrogen bonding interaction is stronger.

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.

Complex formation and photophysical properties of luminol: solvent effects

Abstract The photophysical properties of the lowest excited singlet state of luminol were studied in various protic and aprotic solvents with different dielectric constants using steady state and nanosecond emission spectroscopy at room temperature and 77 K. A red shift is observed in the emission spectra on going from aprotic to protic solvents. We show that this red shift is due to the formation of a relatively long- lived complex in the excited state between luminol and protic solvents. The decay rates correlate well with the solvent shifts. The dependence of the quantum yields of fluorescence on the excitation energy and solvent characteristics was studied.

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...

Study of proton transfer reactions in binary solvent mixtures by steady state and nanosecond spectroscopy

Abstract The proton transfer reaction of 4-methyl-2,6-diformyl phenol (MFOH) in different solvent mixtures has been studied by steady state and nanosecond transient emission spectroscopic technique. The nature of the product formed are shown to be dependent on the dielectric properties and proton accepting ability of the solvent, both in the ground and excited states. Ethanol is found to be more basic (‘higher proton accepting’) than both water and butanol. In alcoholic mixtures, proton transfer reaction can be described by a dielectric continuum model whereas in water/organic solvent mixtures, the molecular aspect of the solvent needs to be considered. Solvation of the dissociated proton is also shown to be very important in the proton transfer reaction. The fluorescence quantum yield, spectral position, decay rate and the nature of the conformer formed are found to vary with solvent composition.

Intramolecular proton transfer in the first excited singlet state of 4-methyl-2,6-diformylphenol: effect of non-polar and weakly polar aprotic solvents

Abstract Steady state and transient emission studies reveal that an ultrafast excited state intramolecular proton transfer (ESIPT) reaction takes place in the first excited singlet state of 4-methyl-2,6-diformylphenol (MFOH). The large Stokes shifted emission orignates from the proton transferred form of the intramolecularly hydrogen-bonded closed conformer of MFOH in the S0 state. From the solvent dependence of non-radiative decay rate constant kfnr, an enol tautomer is suggested for the fluorescing species. The change in luminescence at 77 K from yellow fluorescence to blue phosphorescence in n-hexane and chloroform is attributed due to the formation of open contormer. The T1 state of MFOH in n-hexane is purely 3nπ∗ in character but in chloroform it is 3ππ∗.

Ground and excited state proton transfer of 4-methyl-2,6-diformylphenol in different solvent mixtures

Abstract Proton transfer in the ground and excited states of 4-methyl-2,6-diformylphenol (MFOH) in ethanol/water, ethanol/methanol and ethanol/dioxane solvent mixtures has been studied using steady state and nanosecond transient emission spectroscopy. The lifetime is not affected much in water/alcohol mixtures, although in ethanol/dioxane mixture the lifetime varies considerably with solvent composition. The emission intensity and fluorescence quantum yield varies with the mixed solvent composition. It is proposed that both dissociation and recombination reactions contribute to the decay rates in ethanol/water and ethanol/methanol solvent mixtures. The presence of methanol or water in the mixture quenches the fluorescence in ethanol. To study the photochemical quenching intensity the data are analyzed by the Stern—Volmer equation.