Bimanes. 9. Solvent and substituent effects on intramolecular charge-transfer quenching of the fluorescence of syn-1,5-diazabicyclo[3.3.0]octadienediones (syn-9,10-dioxabimanes)

syn-1,5-Diazabicyclo[3.3.0]octadienediones (syn-9,10-dioxabimanes, syn-(R₂,R₁)B) are usually strongly fluorescent. Donor moieties in one or both R₂ groups quench the fluorescence in a manner dependent on the donor strength and solvent polarity. Charge transfer is strongly implicated as the quenching mechanism; no charge-transfer emission is observed. Solvent effects on the fluorescence maxima for all compounds are parallel, but the fluorescence quantum yields (φ_F) for the compounds with donor substituents fall dramatically from 0.7 to values between 0.001 and 0.01 over narrow solvent polarity ranges. The rate constant for electron transfer (quenching), k_et, is estimated from the fluorescence quantum yields and radiative rates, with the substituent effect very well correlated by the Brown σ⁺ substituent constant (ρ value, -6, hydrocarbon solvent). Reduction potentials are used to evaluate bimane acceptor strengths. Fluorescence quenching requires conformational mobility within the molecule and/or in the solvent as shown by the increase in fluorescence observed in glasses or viscous solvents. Added amines quench the fluorescence at diffusion-controlled rates. A substantial difference between the two pK_a values [Δ = 5.5] for the bisprotonated, bis(piperidino) derivative, syn-(C₅H₁₀NH⁺CH₂,CH₃)B, is due to the stability of an intramolecularly hydrogen-bonded monoprotonated form, for which the rather low φ_F (ca. 0.01) implies a structure.