Studies of solvent-solute interactions in the photophysics of laser dyes

Abstract

The fluorescence lifetime of rhodamine B in the normal alcohols (C1-C10) and normal nitriles (C2,C5,C6,C8,C9) has been measured using a picosecond laser system. The lifetime measuring technique is time-correlated single photon counting (TCSPC). Absorption and emission spectra of rhodamine B in the alcohols and nitriles have also been determined, thus allowing calculation of quantum yields, radiative, and nonradiative rates. The rotation of the dye's diethylamino groups is related to the nonradiative rate. A decreasing nonradiative rate corresponds to a greater energy barrier to rotation. The behavior of the nonradiative rates, and thus the rotational energy barrier, is modelled as a function of (1) solvent viscosity and (2) solvent polarity. The polarity-dependent model shows better cortelation with the data. The nitrile data differs from the alcohol data in that the barrier appears to be constant and therefore independent of solvent viscosity and polarity. Hydrogen bonding is used to explain the differences between the alcohols and nitriles.

Rotational relaxation times of two laser dyes (cresyl violet and oxazine-1) in polymer solution (poly(ethylene oxide) and methanol) has been measured using the transient absorption spectroscopy (TAS) method. TAS is a pump-probe technique using a picosecond laser system. The pump beam optically bleaches the sample and the probe beam monitors the transient response. The effect of increasing polymer concentration is seen as an increasing rotational relaxation time. This result is examined with respect to the Debye-Stokes-Einstein (DSE) equation governing viscosity-dependent, rotational reorientational times. The greater increase in the rotational times of cresyl violet is explained on the basis of increased polymer-dye interaaion, specifically hydrogen bonding.