S.B. Banerjee

Electronic emission and absorption spectra and the effect of hydrogen bonding on the ground and excited electronic states of some anilines

Abstract From electronic absorption and emission spectra in solutions it appears that intramolecular hydrogen bonding, strong enough to resist rupture by dioxane, exists in o-chloroaniline in the excited state only. Fluorescence quenching behaviour in the presence of dioxane indicates that intermolecular hydrogen bonding significantly increases intersystem crossing rate in m-chloroaniline only. This and other emission spectral characteristics in this hydrogen bonding solvent at 77 K show that the first excited singlet electronic state S1 of m-chloroaniline is ππ*, whereas the states S1 of aniline, toluidines and p-chloroanilines have some nπ* character. On formation of intermolecular hydrogen bond in dioxane, the corresponding triplet states of the molecules acquire pronounced nπ* character. An examination of phosphorescence decay curves reveals triplet complex formation in m- and p-chloroaniline but there is no evidence of triplet complex in the other aromatic amines studied.

Vibrational spectra and molecular configuration of 2,2'-diphenyl ethyl alcohol and 2,2'-diphenyl ethylamine

Abstract The Raman and IR spectra of 2,2′-diphenyl ethyl alcohol and 2,2′-diphenyl ethylamine have been analyzed assuming the phenyl rings vibrate independently. Complete vibrational assignment show that some ring modes for both the molecules are found to appear in pairs. Possible orientation of the two rings with respect to the tetragonally hybridized carbon atom has been discussed. Two probable cases of Fermi resonance have been observed. The general nature of the ring modes to exhibit a pair of frequencies in some diphenyl-type molecules has been described.

External heavy atom effect on the emission of carbazole

Abstract External heavy atom induced quenching of luminescence emission of carbazole in n-hexane and methylcyclohexane in the presence of chloro- and bromoacetic acids at 300 K and 77 K is described. A straight line relation between fluorescence quenching rate constant and the acid dissociation constant of chloroacetic acids, which is related to electron affinity, is observed. Different photophysical parameters at 77 K of unperturbed and perturbed fluorescent carbazole in ternary solutions have been determined and suitable reaction mechanism for quenching has been proposed. Inference has been drawn about complex formation in the triplet state of carbazole from the biexponential nature of the phosphorescence decay curve.

Shift and quenching of fluorescence of tetrahydronaphthols due to hydrogen bonding

Abstract Fluorescence intensity of 5,6,7,8-tetrahydronaphtols in cyclohexane is enhanced due to hydrogen bonding with ethanol and dioxane. Triethylamine produces large changes in absorption and fluorescence spectra, and at 300 K fluorescence is quenched due to transfer of electronic excitation and short-lived CT complex formation in the fluorescent state. At 77 K there is no quenching as the molecular orientation is restricted. Possible reaction schemes are described.

Electronic absorption spectra of 1,2,3-trimethyl benzene

Abstract The electronic absorption spectra of 1,2,3-trimethyl benzene in the vapour and solid phases and in an isobutyl alcohol rigid glass at 90°K are described in this paper. Some characteristic features of the vibrational structure of the spectra are discussed.

Quenching of excited states of xylenols due to hydrogen bonding with triethylamine in different solvents

Abstract Hydrogen bonding between xylenols in the ground state and triethylamine (TEA) brings about red shift and increase of intensity of the absorption bands of the xylenol molecules. TEA quenches fluorescence of the xylenols at 300 K in varying degrees, possibly through a very short-lived CT species formed by strong hydrogen bonding interaction between excited xylenols and TEA, which leads to large charge transfer. At 77 K, restricted solvent relaxation or orientation impedes formation of such CT complex and consequently impedes the quenching process. Quenching is weak and absent in polar aprotic and alcoholic solvents, respectively. The monoexponential nature of phosphorescence decay of xylenols indicates establishment of very rapid equilibrium between hydrogen-bonded complexes in the triplet state and free molecules.

Steady state and time resolved spectroscopic study of exciton splitting of the electronic state of 2,2-diphenylethylamine

Abstract The first excited singlet state of 2,2-diphenylethylamine (DPEA) shows a small exciton splitting of about 137 cm -1 . The measured lifetimes of the upper and lower excitonic levels are τ 1 and τ 2 , respectively, (τ 1 2 ) and only τ 2 is sensitive to polarity of solvents in which the spectrum of DPEA is measured. Flourescence emission mostly takes place from the lower level with longer lifetime which apparently indicates that the upper level is depopulated by rapid internal conversion. It is suggested that the geometry of DPEA, conformation of the NH 2 group, as also restricted orientation of the group, are not favourable for intramolecular CT interaction. Analysis of kinetic parameters of DPEA in different solvents in the presence of chloroalkanes at 77 K shows that it is lower excitonic level that is involved in CT complex formation with the heavy atom quenchers and that from this level the triplet excitation is enhanced.

Effect of hydrogen bonding on the electronic absorption spectra of xylenols

Abstract This paper reports the electronic absorption spectra of 3,5-, 2,3-, 2,5- and 2,6-xylenol in binary and ternary solution in cyclohexane and dioxane. It is observed that the f -value of transition is higher in the case of bonded molecules than in the case of free molecules. This is attributed to larger interaction between the lone pair electrons of oxygen atom of the hydroxyl group of xylenol and ring π-electrons arising from larger σ-electron density in the oxygen atom on intermolecular OH … O bond formation. The equilibrium constant ( K ), free energy (Δ F ) and energy of hydrogen bond formation (Δ H ) are measured. Effect of steric hindrance on hydrogen bond formation in the molecules is discussed.

Heavy-atom effect on emission from free and hydrogen-bonded complexes of excited 1,2,3,4-tetrahydroquinoline

Abstract At 77 K, 1,2,3,4-tetrahydroquinoline (THQ) forms a sinmple hydrogen bond with ethanol while at 300 K, a hydrogen-bonded ion pair is formed. Both intermolecular intersystem crossing and intramolecular singlet-triplet energy-transfer processes induced by heavy-atom-quencher dihalobenzenes are weak in ethanol at 300 K, and the overall fluorescence quenching is smaller in ethanol than in cyclohexane. In the methylcyclohexane rigid glass at 77 K, a triplet-state exciplex is formed between THQ and dihalobenzenes. In ethanol medium at 77 K, there is no reaction between the THQ-ethanol exciplex and dihalobenzenes involving the triplet state of THQ.

Fluorescence spectra of some xylenols in non-polar and hydrogen-bond acceptor solvents at different temperatures

Abstract The red-shift of the fluorescence bands from the excitation bands of 2,6-, 2,5-and 3,5-xylenol, measured in binary and ternary solutions at 300 K, is mainly due to dipole orientation, the hydrogen bond making only a small contribution In viscous media at 90 K, the fluorescence transition occurs from a partially relaxed equilibrium excited state.