Interaction of nonconjugated chromophores in rigid model compounds

Abstract

Spectroscopic techniques and photochemistry were used to evaluate the interaction between an electronically excited chromophore and a different one in its ground state in rigid model compounds. These inflexible compounds are better suited for transfer work than binary mixed solutions of donor and acceptor since both the separation distance and the mutual orientation of the two chromophores are known, and complicating factors such as intermolecular transfer and complex formation are excluded by work at high dilution. Phenanthrene (P) and p-dimethoxybenzene (D) were rigidly connected to an inert spirocyclopropane-norbornane σ-frame. The S1 → S0 and T1 → S0 emissions of the two chromophores are found at different wavelengths and therefore can be detected separately. The emission and excitation spectra of the model compound were compared to those of an equivalent one-to-one mixture of each chromophore rigidly attached to the same inert frame. The dilute mixture shows the fluorescence and phosphorescence of each of the two components. In the model compound, however, the D fluorescence is definitely absent. The mechanism is consistent with efficient singlet energy transfer from the selectively excited D to P combined with partial back transfer from the T2 state of P to the lowest triplet of D.