Photoionization and Delayed Fluorescence of Dyes in Rigid Organic Matrices. I. A Proposed Model

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Abstract
The decay modes of the delayed fluorescence from rigid-glass solutions of acridine dyes at 77°K have been studied and compared with those of the transient absorptions which have previously been assigned to the radical ions produced from photoionization. While the decay of the radical-ion absorption is nearly exponential from the beginning, the decay of the emission is highly nonexponential and becomes approximately exponential only in the later stage of decay. The lifetime of the ``exponentially'' decaying component of the emission is identical with that of the ``radical ion.'' Although flash and steady excitations produce considerably different initial decay for the emission, the decay rate of the absorption shows very little or no dependence on the excitation mode. Both the integrated intensity of the delayed fluorescence and the initial absorbance of the radical-ion absorption are directly proportional to the intensity of the monochromatic flash-excitation source. These, as well as other results, are interpreted in terms of a model requiring the formation of solute—solvent charge-transfer states from the excited singlet states of the solute molecules as the primary process in photoionization. It is assumed that charge-transfer states which are unstable with respect to a complete charge separation dissociate and produce radical ions (holes) and trapped electrons, while the more stable ones return slowly to the excited singlet state by thermal activation. The formation of the excited singlets from the charge-transfer states and the hole—electron recombination is proposed to be responsible for the occurrence of the delayed fluorescence.