A. L. Verma

Raman study of vibrational dephasing in hydrogen-bonded binary and ternary complexes of C6H5Cl and methanol

Abstract The shifts and linewidth changes of three vibrational modes of C 6 H 5 Cl were studied in hydrogen-bonded binary mixtures C 6 H 5 Cl+CH 3 OH at various mole fractions of C 6 H 5 Cl, C =0.05, 0.1, 0.2, 0.3, 0.5, 0.7 and 0.9. The study reveals that the phenomena of motional narrowing and diffusion play simultaneous roles in the dephasing of the ring breathing and trigonal bending modes at ∼1000 and ∼1024 cm −1 , respectively. Minima at C =0.7 and 0.2 in the Γ L versus C plot are explained by the formation of hydrogen-bonded dimer, C 6 H 5 Cl⋯HOCH 3 , and trimer, C 6 H 5 Cl⋯(HOCH 3 ) 2 , complexes. The dephasing of a third mode at ∼700 cm −1 is explained using the indirect dephasing model of Fischer–Laubereau.

Dynamics of phase transitions in a liquid crystalline compound (TBDA) probed by Raman spectroscopy

The precise value of linewidths obtained after correcting for the slitwidth effect for five Raman bands associated with the core of a liquid crystalline compound, terepthal-bis-decyl aniline (TBDA), have been studied in the temperature range of 12-472 K. The variation of the linewidth with temperature confirms two already reported phase transitions at 346 and 427 K. However, the phase transition at 427 K is not purely first order as reported earlier, but of mixed order in nature. From the measured linewidths of the N and CDN modes below room temperature, the estimated activation energy for rotation along the -N bond closely matches with the SI -SC phase transition temperature suggesting that the rotation about the -N bond is closely linked with this transition.

Laser Raman Study of Terepthalylidene-bis-p-n-Decylaniline (TBDA)

Abstract The Raman spectra of Terepthalylidene-bis-p-n-declyaniline (TBDA), have been recorded as a function of temperature in the temperature range from 55.0°C (solid) to 199.0°C (isotropic liquid) in the 1100–1200cm−1 and 1520–1650cm−1 regions. The precise values of peak positions, integrated intensities and linewidths of some selected Raman bands have been obtained by deconvoluting the bands. The changes in the molecular alignment and its effect on inter/intra molecular interactions at two particular phase transitions viz., solid-smectic G (K-SG ) and smectic I-Smectic C (SI-Sc ) have been discussed on the basis of variation in the Raman spectral parameters with temperature. From detailed considerations, it is inferred that the increased orientational/vibrational freedom of long alkyl chain on the core is responsible for major spectral changes at the K-SG transition while the changes in the intramolecular interactions of the core part with increased fluidity, is responsible for spectral anomalies at th...

A comparative laser Raman study on TB4A, TB7A and TB10A

The basic structural differences between three thermotropic liquid crystals TB4A, TB7A and TB10A have been interpreted by analysing their Raman spectra. Three spectral regions, 925-1025, 1275-1475 and 1525-1650 cm-1, have been chosen in this context to incorporate their structural dissimilarity. The induced planarity of the backbone, as an effect of increasing chain length, has been studied by observing the changes in the respective Raman spectra of the three compounds. A tentative assignment of all the modes observed in the region 400-1700cm-1 is made in this context. The molecular conformations of the three compounds have been predicted in their solution states, and are compared with the molecular environment that exists in their liquid crystalline states. Structural disorder at the solid-SmG transition is discussed and the changes are incorporated systematically.

Simultaneous formation of mono- and dications of free-base tetraphenylporphin during photo-oxidation probed by resonance Raman and absorption techniques

Abstract We report the simultaneous formation of mono- and dication (known as diacid) radicals during the photo-oxidation of free-base tetraphenylporphin by selective laser excitations using resonance Raman and optical absorption techniques. During the process of photo-oxidation of free-base tetraphenylporphin in carbon tetrachloride, the addition of even a small quantity of tetrahydrofuran leads to the formation of dication along with the monocation radical. A mechanism of simultaneous formation of di- and monocation radicals is proposed.

Estimation of Slit Width from Observed Raman Spectra

A simple technique for estimating the slit width of a spectrometer from the ratio of the observed (Δν1/2)R and (Δν1/4)R of a Raman line profile was developed by calculating a set of corresponding values of Z=(Δν1/4)R/(Δν1/2)R and X=S/(Δν1/2)R based on the exact numerical evaluation of the Voigt profile. This technique was applied to some selected Raman lines and was found to give accurate values of the slit width in almost all cases where the slit distortion is appreciable and the band is isolated. Another technique for calculating the fractional Lorentzian character from the observed Raman spectra is also suggested.

Time-resolved resonance Raman evidence for the exciplex formation of free-base porphyrin with an electron acceptor

Abstract Pump/probe time-resolved resonance Raman experiments (TR 3 ) on free-base tetraphenylporphyrin (H 2 TPP) in benzene and benzene plus CCl 4 mixed solvents were carried out to probe the mechanistic details and transient species involved in the photooxidation of H 2 TPP. We have detected spectral features due to the π-radical cation, and exciplex transient species in the singlet and triplet excited states at different timescales in the presence of CCl 4 under laser irradiation. A mechanism for the photooxidation of porphyrins is discussed.

DYNAMICS OF A LOW TEMPERATURE PHASE TRANSITION IN LIQUID CRYSTAL TB10A PROBED BY RAMAN SPECTROSCOPY

Raman spectra of TB10A were recorded in the regions, 925-1000 and 1140-1220cm-1 from room temperature down to 20 K, during cooling and heating cycles. The subtle changes in the spectral features of the bands at 975 and 1195cm-1 at 47 K were attributed to a hitherto unreported stable-metastable phase transition in TB10A. The dynamics of the new phase transition have been explained in terms of splitting of the non-planar mode at 975cm-1 owing to strong intermolecular interaction due to close molecular packing in the low temperature phase. The spectral anomaly of the 975cm-1 band, in terms of variation of relative intensity with respect to temperature, also shows the hysterisis linked with the process of stable-metastable-stable modification in TB10A.

Photoreduction of Fe(TPP)Cl in CH2Cl2 and DMSO in the presence of and in neat 1,2‐dimethylimidazole without alcohol: evidence for a photoreactive state from resonance raman and optical absorption studies

Coordination of 1,2-dimethylimidazole (1,2-Me2Im) to chloroiron(III)tetraphenylporphyrin [Fe(TPP)Cl] in CH2Cl2, DMSO and neat 1,2-Me2Im without alcohol results in clean reduction at the iron centre on photoexcitation at 441.6 nm. Fe(TPP)Cl also photoreduces in CH2Cl2 in the presence of 2-methylimidazole under anaerobic conditions with 406.7 and 441.6 nm excitation. From detailed studies, it is inferred that a photoreactive state with absorption in the 315–335 nm range in the high-spin complexes of Fe(TPP)Cl formed with axial ligands is responsible for photoreduction.

Pre-resonance Raman studies of some mesogens-TB4A, TB7A and TB10A

Pre-resonance Raman spectra of certain liquid crystalline compounds, TB4A, TB7A and TB10A, are reported. The anomaly observed in the pre-resonance Raman spectra in the three compounds was initially explained by the Albrecht-Hutley Model, but its failure leads us to explain the anomalous intensity enhancement mechanism by invoking an interference effect between a weak dipole-forbidden excited state and a nearby strong electronic level via vibrational modes. The presence of a 21Ag dipole-forbidden excited state in TB4A and TB10A is deduced after an elaborate discussion of their centro-symmetric structure. The structural difference of TB7A in comparison with TB4A and TB10A is also discussed by taking their X-ray data into consideration.