Synthesis and photophysics of a 1-pyrenylmethyl-substituted 2 '-deoxyuridine-5-carboxamide nucleoside: Electron-transfer product lifetimes and energies

This paper presents results of the synthesis and photophysical study of N-( 1-pyrenylmethyl)-2'-deoxyuridine-5-carboxamide (PMA-dU) and its spectroscop ic model N-acetyl-1-aminomethylpyrene (PMA-Ac). The goal in these studies i s to learn about the intrinsic forward and reverse electron-transfer (ET) p rocesses in the PMA-dU nucleoside as a means of developing pyrenyl-dU nucle osides with ET product lifetimes in the 0.5 ns time range. Absorbance and e mission spectra, emission quantum yields, and emission lifetimes are report ed for both compounds in three solvents. The data show that the emission yi eld quenching varies from 75 to 98% in the solvent series THF, MeCN, and Me OH. Pyrenyl (pi,pi*)(1) emission quenching is assigned to ET that forms the pyrene(.)+/dU(.-) product as observed previously in other pyrenyl-dU nucle osides. In contrast to the monoexponential emission decay of PMA-Ac, the em ission of PMA-dU at all wavelengths is multiexponential with 4 lifetimes in THF and nearly always with 3 in McCN and MeOH. The multiexponential decay is likely due to the presence of multiple nucleoside conformers. Importantl y, the emission decay for the nucleoside in the 500-550 nm region is assign ed to relaxation of the pyrene(.+)/dU(.-) ET product. The 0.5-4 ns time ran ge contains over 95% of the emission amplitude in this wavelength region fo r the polar solvents MeCN and MeOH. Thus, the ET product in PMA-dU appears to have the desired long lifetime. Additionally, CIS INDO/S computations of the excited-state properties of 19 conformers of the nucleoside model N-(1 -pyrenylmethyl)-1-methyluracil-5-carboxamide (PMA-U-Me) identify two key fa ctors that control the energy of pyrene(.+)/dU(.-) ET products. One is ease of reduction of the uracil subunit, in turn controlled by variation of the angle between the uracil-C5 carbonyl and the plane of uracil (R = 0.90). T he other is Coulombic attraction between the pyrenyl cation and uracil anio n subunits. The Coulombic and CO/U-Me dihedral angle contributions to the e nergy of the ET1 state are independent of each other and can operate either in or out of phase with respect to varying energy of the ET1 state.