Dynamics of Solvent and Spectral Relaxation

Abstract

In the preceding chapter we described the effects of solvent and local environment on emission spectra, and how spectral changes could be used to determine the properties of the environment surrounding a fluorophore. We showed that emission spectra could be affected by solvent polarity and specific solvent effects. We also showed that fluorophores could display charge separation and/or conformational changes while in the excited state. During these descriptions we did not consider the rate constants for these processes, but mostly assumed the lifetime of a fluorophore was in equilibrium with its environment prior to emission. More specifically, we assumed the decay rates of the fluo-rophores were slow compared to the rate constants for solvent reorientation, charge separation, or conformational changes in the fluorophore. The assumption of emission from equilibrated states was reasonable because fluid solvents reorient around excited fluorophores in 0.1 to 10 ps, and the decay times are typically 1 ns or longer. There are many situations where the fluorophore can emit prior to or during other dynamic processes. For example, in viscous solvents the rate of solvent relaxation around the fluorophore may be comparable to or slower than the decay rate. In this case the emission occurs during solvent relaxation, and the emission spectrum represents an average of the partially relaxed emission. Under these conditions the emission spectra display time-dependent changes. These time-dependent effects are not observed in the steady-state emission spectra, but can be seen in the time-resolved data or the intensity decays measured at various emission wavelengths. Many fluorophores undergo reactions in the excited state, such as the loss or gain of a proton. Depending on the chemical properties of the fluorophore, its exposure to the solvent, and/or the concentration of proton donors or acceptors in the solution, the excited-state reaction may be occurring during emission. In this case the steady-state spectrum will contain contributions from each form of the fluorophore, assuming that both forms are fluorescent.