Time-resolved fluorescence and fluorescence anisotropy of calix[4]arene: Elucidation of the excitation energies of various conformers

Abstract

Fluorescence, fluorescence excitation, time-resolved fluorescence spectroscopic, time-dependent fluorescence anisotropy, and time-dependent (TD) DFT studies were carried out to elucidate the excitation energies, fluorescence decay kinetics, and excitation transfer dynamics of calix[4]arene. We found that TD DFT calculations help to analyze the absorption spectra by elucidating the variation of the oscillator strengths with the excitation energies of various conformers involved and to clarify their relative importance. The TD DFT calculations imply that a cone-shaped conformer plays the most important role in the absorption spectrum. Fluorescence in a methanol solvent has a lifetime of 3.11±0.1ns and the decay kinetics is not found to have particular propensity with respect to solvent polarity. When the molecules in a glass matrix at 77K were excited by short laser pulses, fluorescence anisotropy was observed. From the time-dependent fluorescence anisotropy, we derived the energy transfer rate constant and fluorescence anisotropy decay time, respectively: ket=1.7×109s-1,τh=0.59ns.