Synthesis and photophysics of a 1-pyrenyl substituted 2 '-deoxyuridine-5-carboxamide nucleoside: Electron transfer products as CISINDO/S excited states

This paper reports results of the synthesis and photophysical study of 5-(N -carboxy-1-aminopyrenyl)-2'-deoxyuridine (PA-dU) and its spectroscopic mode l N-acetyl-1-aminopyrene (PA-Ac). Absorbance and emission spectra, emission quantum yields, and emission lifetimes are reported for both compounds in three solvents. The data show that the emission yield quenching of PA-dU re lative to PA-Ac varies from 95 to 99% in the solvent series THF, MeCN, and MeOH. In contrast to the monoexponential kinetics for (pi,pi*)(1) emission from PA-Ac, the (pi,pi*)(1) emission from the PA-dU nucleoside decays with two lifetimes in THF and three in MeCN and MeOH. The multiexponential emiss ion decays are likely due to the presence of multiple nucleoside conformers . The average (pi,pi*)(1) lifetimes for PA-dU in THF, MeCN, and MeOH are, r espectively, 4.8, 2.7, and 0.55 ns and correspond to lifetime quenching val ues of 58, 81, and 96%, respectively. The lifetime quenching values for PA- dU in THF and MeCN do not agree with the emission yield quenching values in these solvents of, respectively, 95 and 96%. Thus in these two solvents th e emitting states for PA-dU have lower radiative rates than do the emitting states for PA-Ac in the same solvents. However, for PA-dU in MeOH the life time quenching percentages of 96% over the 380-440 nm range and 98% over th e 405-440 nm range are in good agreement with the emission yield quenching value of 99% in this solvent. Additionally, the emission spectrum of PA-dU in MeOH matches the (pi,pi*) emission spectrum of PA-Ac in MeOH, while the emission spectra of PA-dU in THF and MeCN do not match the corresponding sp ectra of PA-Ac. Thus choice of solvent tunes the electronic nature and the radiative rate of the emitting state in PA-dU. Emission quenching in PA-dU is assigned to intramolecular electron transfer (ET), and the 550--600 nm e mission region of PA-dU reflects the relaxation dynamics of the pyrene(.+)/ dU(.-) ET product. Strikingly emission lifetimes in this spectral region in all three solvents are extremely short, less than or equal to 100 ps. CIS INDO/S computations of the excited states for the 5-(N-carboxy-1-aminopyren yl)-1-methyluracil (PA-U-Me) model show that two actors are major contribut ors to the variation in ET product (ET1) energy among the eleven PA-U-Me co nformers identified. The first is the energy of the uracil LUMO (a redox ef fect), and the second is the pyrenyl/U-Me subunit separation distance (a Co ulombic effect). Importantly, the dihedral angle between the bridging carbo nyl and the C5-C4 bond in UM, is strongly correlated with the energy of the uracil LUMO (R = 0.94) and thus with the ET1 energy. Finally, quantitative study of the energy of the ET1 state versus pyrenyl/U-Me separation distan ce shows a smooth x(-1) increase in ET1 energy as the separation distance i ncreases (R = 0.9994). Additionally, the extrapolated ET1 energy at infinit e subunit separation equals the difference between the LUMO and HOMO energi es.