Nanosecond timescale optical inhomogeneous broadening of dye molecules in liquids at and near room temperature

Abstract

Time-resolved fluorescence depolarization measurements of the S& electronic absorption band of rhodamine B in glycerol and propylene glycol have been made at different excitation wavelengths. It was found that electronic excitation transport between the rhodamine B chromophores is dispersive in glycerol at room temperature and in propylene glycol below 250 K. This demonstrates inhomogeneous broadening of the dye absorption spectrum which persists for the time scale of the fluorescence lifetime (nanoseconds) or longer. Measurements of time-resolved emission spectra at a number of different excitation wavelengths and temperatures support this conclusion and suggest that the chromophore-solvent microenvironments causing inhomogeneous broadening are the same as those responsible for the dynamic Stokes shift about an excited chromophore.

Modulation of the Photophysical Properties of Pyrene by the Microstructures of Five Poly(alkyl methacrylate)s Over a Broad Temperature Range

“…from the lowest vibronic level of the initial state to the lowest vibronic level of the final state) of a guest in a matrix, whether it be a liquid, crystal, polymer, inorganic cavity, etc ., may undergo spectral broadening . The nature of the broadening depends on the type and intimacy of the host–guest interactions, on the guest mobility within its cybotactic region, and on temperature . Generally, two parameters contribute to spectral broadening:…”

“…The individual spectra of the molecules may differ somewhat due to the distribution of environments offered by the cybotactic regions, and the observed emission spectrum from the ensemble is a convolution of the contributions from molecules in each site type weighted by their relative populations; the more disordered the system is, the more varied will be the range of the cybotactic regions and the broader will be the spectrum of the probe. In the absence of energy transfer processes, a “homogeneous” contribution to spectral broadening results from relaxation of the population of excited‐state molecules. It depends on the lifetimes of the excited states, spectral diffusion and the rates of conformational or rotational relaxation processes . Homogeneous relaxation is very common for guest molecules in solid matrixes at lower temperatures, principally when excitation leads to large changes in charge distribution or molecular shape.…”

“…Molecules sorbed in a polymer or in a frozen solution commonly show inhomogeneous broadening of ~300 cm −1 , ~1–10 cm −1 in a solid crystal and only ~0.03 cm −1 in a single crystal . Inhomogeneous broadening is also more important than homogeneous broadening in solids at low temperatures . Additional homogeneous broadening occurs as a result of contributions associated with the lifetime of the excited state (population relaxation), from the dephasing rate (caused by thermal fluctuations of the environment), and from orientational relaxation processes leading to loss of correlation of the ensemble of transition dipoles .…”

“…Each of these has a different dependence on temperature . The inhomogeneous broadening is only slightly dependent on temperature if the configurational interactions between the guest molecules and the cybotactic regions of the host matrix do not change over periods which are “long” with respect to the excited‐state lifetimes.…”

“…The other processes contributing to line broadening are dynamic and are related to fluctuations of the microenvironments, such as dephasing and orientational relaxations; they are attenuated strongly at very low temperatures. Thus, excited‐state lifetimes become a dominant factor under this condition . At higher temperatures, dephasing is a major contributor to homogeneous broadening .…”