Singlet and Triplet Excitons as Probes for Chemical Reactivity

Abstract

In most polynuclear hydrocarbons such as anthracane and its derivatives e.g. CH, CN, C1, OH compounds, photochemical reactivity is in direct competition with prompt fluorescence from the lowest singlet excited states. Accordingly, as reactivity proceeds the quantum yield of host emission decreases and products (guests) having different luminescence properties are formed. Thus the singlet exciton through its properties such as spectral distribution of emission, diffusion coefficient and lifetime may be considered an “active probe” for following such reactions. The triplet exciton on the other hand is not much involved in the reactivity of these systems and through its analogous properties is an efficient “passive probe” that allows us to study changes in the structure and chemical composition of the host-guest system as reactivity proceeds. The study of “dynamic” host-guest systems where the host: guest ratio is continually changing has its foundation in the vast literature available on similar “static” systems.¹ It does, however, possess the advantage that since the volume change following most of the topochemically controlled reactions is small the molecular “mismatch” in the dynamic systems is of the same order as in isotopically-substituted systems e.g. anthracene-d₁₀ in anthracene-h₁₀ and appreciably less than in several static systems e.g. tetracene in anthracene. This offers significant advantages in energy-transfer studies as indicated by Powell and co-workers.²