Dual Fluorescence from N6,N6-Dimethyladenosine

The adenosine derivative N6,N6-dimethyladenosine (DMA) shows dual fluorescence in solvents of different polarity. In addition to the “normal” fluorescence at 330 nm, another band is observed at 500 nm. The long wavelength emission dominates in aprotic solvents but is dynamically quenched by protic solvents. Steady-state and lifetime measurements show that the emissions originate from two excited state species; the short wavelength emission is from the directly populated excited state which irreversibly isomerizes into the species responsible for the long wavelength emission. It is conceivable to assign the long wavelength emitting species to a twisted intramolecular charge transfer state (TICT). The fluorescence quantum yield of the short wavelength emission is approximately 4 × 10-4 at room temperature and increases by three orders of magnitude when the temperature is lowered to 80 K in accordance with the behavior of normal nucleic acid bases. In contrast, the long wavelength fluorescence quantum yield is almost temperature independent. The different photophysical processes for DMA are summarized into a kinetic scheme where the temperature quenching of the short wavelength fluorescence is exclusively through isomerization into the long wavelength emitting species. Direct internal conversion to the ground state, commonly believed to be the dominant process for nonradiative deactivation of the DNA bases, makes a negligible contribution for DMA.