Dual Fluorescence from N 6 ,N 6 -Dimethyladenosine Bo Albinsson Contribution from the Department of Physical Chemistry, Chalmers UniVersity of Technology, S-412 96 Go ¨ teborg, Sweden ReceiVed February 18, 1997 X Abstract: The adenosine derivative N 6 ,N 6 -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. Introduction Excited state lifetimes of the normal nucleic acid bases are very short at room temperature, which has been attributed to an extremely rapid internal conversion. 1-4 This makes the bases and the polynucleotides almost non-fluorescent with fluores- cence quantum yields of 10 -4 or less. 2 The photophysical properties of the nucleic acid bases are important for a mechanistic understanding of the DNA photochemistry. Yet, no mechanism for the suggested rapid internal conversion has been experimentally verified. The prevailing explanation is the near degeneracy of the lowest 1 nπ* and 1 ππ* states that can lead to an enhancement of the nonradiative decay through vibronic interaction between the states. 5-7 This paper is concerned with the photophysics of the adenosine derivative, N 6 ,N 6 -dimethyladenosine (DMA). Me- thylation at the exocyclic amino group of adenosine causes a small red shift of the absorption spectrum, but the spectroscopi- cal properties are otherwise similar. 8 The lowest absorption band of adenine at 260 nm derives most of its intensity from two close lying π f π* transitions. 9-11 Quantum mechanical calculations place one or more 1 nπ* states in the same energy region as the lowest 1 ππ* state. 12 No direct experimental observation of a low-lying nπ* state in adenine has been presented, 13 but comparison with other purine derivatives makes its presence hidden under the main 260-nm band very plau- sible. 14 For example, from polarized absorption experiments purine was shown to have an nπ* state as its lowest singlet state, 15,16 and the second excited singlet state of 2-aminopurine has also been identified as an nπ* state. 17,18 The photophysical properties of purine and 2-aminopurine are understood by classical state rules; 19 purine has a phosphorescence quantum yield close to 1 in rigid organic glass, and 2-aminopurine has a high fluorescence quantum yield as expected for molecules with 1 nπ* and 1 ππ* states as their lowest singlet excited states, respectively. 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