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Summer Research Fellowship Programme of India's Science Academies

Studies on the Torsion Dynamics of Ultrafast Molecular Rotor by Time-Resolved Spectroscopy

Md. Asif Amin

Department of Chemistry, Balurghat Mahila Mahavidyalaya, Balurghat-733101

Sukhendu Nath and Rajib Ghosh

Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Mumbai-400085

Abstract

Molecular rotors serve as excellent sensors of microviscosity for a wide range of relevant heterogeneous chemical and biological environments. Detailed knowledge of the excited state dynamics of molecular rotors is crucial in order to fully understand the molecular rotor mechanism. This may also provide a useful design input that could help in efficiency optimization of molecular rotor activity. In this project, we have studied the ultrafast dynamics of a well recognized molecular rotor 2-[4(dimethylamino)benzylidene] malononitrile (DMN), using femtosecond-resolved transient infrared spectroscopy. The spectral and temporal evolution of the nitrile stretch frequency of DMN has been monitored in different solvents of varying viscosity and hydrogen-bonding ability. Asymmetric stretch frequency of nitrile, in excited state, exhibits a dynamic red shift in few ps timescales, which is assigned to excited state structural relaxation prior to the decay to hot ground state. The cooling of the hot state to a thermally relaxed ground state could be observed in detail from the spectral evolution of the nitrile stretch band. In essence, the complete photodynamics of DMN rotor from the FC state to ground state via excited state torsion, deactivation to hot ground state and subsequent cooling dynamics, has been revealed.

Keywords: Ultrafast molecular rotors, DMN, excited state dynamics, visible pump-IR probe.

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