We have investigated the Raman profiles of the (CN) and (C=O) vibrational modes of the nematic liquid crystal ME6N (4-cyanophenyl-4’-hexylbenzoate) in the isotropic phase at different temperatures and used them as probes of the dynamics and structural organisation of this liquid. The vibrational time correlation functions of the (CN) mode, rather adequately interpreted within the assumption of exponential modulation function (the Kubo-Rothschild theory), indicate that the system experiences an intermediate dynamical regime that gets only slightly faster with increasing temperature. However, this theory fails in predicting the non-exponential behavior that the time correlation functions manifest in the long time range (t > 3 ps). For this reason we have additionally approached the interpretation of vibrational correlation functions in terms of the theory formulated by Rothschild and co-workers for locally structured liquids. The application of this theory reveals that the molecular dynamics in this liquid crystal in the isotropic phase is that deriving from a distribution of differently sized clusters, which narrows as the temperature increases. Even at the highest temperature reached in this study (87 °C above the nematic-isotropic transition), the liquid has not yet achieved the structure of the simple liquid and the dynamics has not reached the limit of the single channel process. The vibrational and orientational relaxations occur in very different time scales. The temperature independence of the orientational dynamics in the whole range from 55 °C to 135 °C has been referred to the non-hydrodynamic behavior of the system, arising when local pseudonematic structures persist for times longer than the orientational relaxation. The occurrence of the process of resonant vibrational energy transfer between the C=O groups of adjacent molecules has been revealed in the isotropic phase by a slightly positive Raman non-coincidence effect in the band associated with the (C=O) mode. A qualitative interpretation is tentatively given in terms of partial cancellation of contributions deriving from structures having opposite orientations of their C=O groups.

RELAXATION PROCESSES OF THE LIQUID CRYSTAL ME6N IN THE ISOTROPIC PHASE STUDIED BY RAMAN SCATTERING EXPERIMENTS

GIORGINI, MARIA GRAZIA;ARCIONI, ALBERTO;POLIZZI, CIRO;OTTAVIANI, PAOLO
2004

Abstract

We have investigated the Raman profiles of the (CN) and (C=O) vibrational modes of the nematic liquid crystal ME6N (4-cyanophenyl-4’-hexylbenzoate) in the isotropic phase at different temperatures and used them as probes of the dynamics and structural organisation of this liquid. The vibrational time correlation functions of the (CN) mode, rather adequately interpreted within the assumption of exponential modulation function (the Kubo-Rothschild theory), indicate that the system experiences an intermediate dynamical regime that gets only slightly faster with increasing temperature. However, this theory fails in predicting the non-exponential behavior that the time correlation functions manifest in the long time range (t > 3 ps). For this reason we have additionally approached the interpretation of vibrational correlation functions in terms of the theory formulated by Rothschild and co-workers for locally structured liquids. The application of this theory reveals that the molecular dynamics in this liquid crystal in the isotropic phase is that deriving from a distribution of differently sized clusters, which narrows as the temperature increases. Even at the highest temperature reached in this study (87 °C above the nematic-isotropic transition), the liquid has not yet achieved the structure of the simple liquid and the dynamics has not reached the limit of the single channel process. The vibrational and orientational relaxations occur in very different time scales. The temperature independence of the orientational dynamics in the whole range from 55 °C to 135 °C has been referred to the non-hydrodynamic behavior of the system, arising when local pseudonematic structures persist for times longer than the orientational relaxation. The occurrence of the process of resonant vibrational energy transfer between the C=O groups of adjacent molecules has been revealed in the isotropic phase by a slightly positive Raman non-coincidence effect in the band associated with the (C=O) mode. A qualitative interpretation is tentatively given in terms of partial cancellation of contributions deriving from structures having opposite orientations of their C=O groups.
2004
M. G. Giorgini; A. Arcioni; C. Polizzi; M. Musso; Paolo Ottaviani
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11585/5698
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