Rotational Diffusion of Coumarins in Alcohols: A Dielectric Friction Study
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The rotational dynamics of three structurally similar polar molecules viz., coumarin 440, coumarin 151 and coumarin 450 has been studied in alcohols at room temperature using steady-state fluorescence depolarization method and time correlated single photon counting technique. The observed reorientation times of all the three probes are found to be longer than those predicted by Stokes–Einstein–Debye (SED) hydrodynamic theory with stick boundary condition and a deviation towards super-stick behavior is noted. Dielectric friction theories of Nee–Zwanzig and van der Zwan–Hynes, which treat the solute as a point dipole, overestimate the dielectric friction contribution exhibited by all the three coumarins in alcohols. Results are discussed in the light of theoretical models and the possibility of hydrogen bonding between the amino group of the probe molecules and the hydroxyl group of the alcohols.
KeywordsRotational dynamics Hydrogen bonding Dielectric friction Super-stick Dipole moment
The authors are grateful to Dr. P.K. Gupta, RRCAT, Indore, for providing TCSPC facility. The funding in the form of a Major Research Project by the University Grants Commission (UGC), New Delhi, is gratefully acknowledged. One of the authors (BRG) is thankful to UGC for a fellowship under Faculty Improvement Program and to the management of JSS. College, Dharwad for the support and encouragement. JRM thanks CSIR, New Delhi, for a Senior Research Fellowship under a Major Research Project.
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