Photophysics and rotational diffusion dynamics of large prolate non-polar laser dyes

Abstract

The present work deals with experimental determination of steady state fluorescence anisotropies, fluorescence lifetimes and consequentially the rotational reorientation times of three large, nonpolar laser dyes that are modelled as prolate ellipsoids. Steady state and time resolved fluorescence spectroscopic measurements on Exalite 404 (E404), Exalite 417 (E417) and Exalite 428 (E428) dyes in series of alkane and alcohol solutions are discussed. All the three probes are observed to exhibit significantly shorter reorientation times in series of alcohols as compared to alkanes. The experimental results are explained in light of SED hydrodynamic and quasihydrodynamic theories. It is for the first time DKS theory is found to correctly predict the experimental results in alkanes with precision for all the three probes. Particularly in case of E417 DKS theory correlates exceptionally well with predictions of slip hydrodynamics. Interestingly, the largest probe (E428) being reported for the first time, exhibits slip behaviour in alkanes and alcohols; behaviour in higher alcohols as well as alkanes. This suggests the existence of nonhydrodynamic forces and speculates that the straightforward relation between the probe size and the nature of their behaviour may not be appropriate. Further, the nature and geometries of probes were fully explored and characterized using NBO and DFT studies employing Gaussian09 program. Optimized geometries, energy band gap and HOMO-LUMO levels were determined from DFT exchange-correlation function B3LYP with 6-311g basis set. Inter and intra hyper-conjugative interactions were estimated from NBO analysis. A very strong interactions between π(C19C21) → π⁎(C24C25), π(C1C6) → π⁎(C2C3) and π(C33C34) → π⁎(C28C30) with E(2) energies 21.44, 21.75 and 22.75 kcal/mol were observed for E404, E417 and E428, respectively. Electron occupancies and p-character were also determined for these molecules.