This paper reports results of the synthesis and photophysical study of 5-(N-carboxy-1-aminopyrenyl)-2‘-deoxyuridine (PA-dU) and its spectroscopic model 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 relative to PA-Ac varies from 95 to 99% in the solvent series THF, MeCN, and MeOH. In contrast to the monoexponential kinetics for (π,π*) emission from PA-Ac, the (π,π*)1 emission from the PA-dU nucleoside decays with two lifetimes in THF and three in MeCN and MeOH. The multiexponential emission decays are likely due to the presence of multiple nucleoside conformers. The average (π,π*)1 lifetimes for PA-dU in THF, MeCN, and MeOH are, respectively, 4.8, 2.7, and 0.55 ns and correspond to lifetime quenching values 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 the 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 lifetime quenching percentages of 96% over the 380−440 nm range and 98% over the 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 (π,π*) emission spectrum of PA-Ac in MeOH, while the emission spectra of PA-dU in THF and MeCN do not match the corresponding spectra 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 emission 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, ≤100 ps. CIS INDO/S computations of the excited states for the 5-(N-carboxy-1-aminopyrenyl)-1-methyluracil (PA-UMe) model show that two factors are major contributors to the variation in ET product (ET1) energy among the eleven PA-UMe conformers identified. The first is the energy of the uracil LUMO (a redox effect), and the second is the pyrenyl/UMe subunit separation distance (a Coulombic effect). Importantly, the dihedral angle between the bridging carbonyl and the C5−C4 bond in UMe 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/UMe separation distance shows a smooth x -1 increase in ET1 energy as the separation distance increases (R = 0.9994). Additionally, the extrapolated ET1 energy at infinite subunit separation equals the difference between the LUMO and HOMO energies.
This paper presents results of the synthesis and photophysical study of N-(1-pyrenylmethyl)-2‘-deoxyuridine-5-carboxamide (PMA-dU) and its spectroscopic model N-acetyl-1-aminomethylpyrene (PMA-Ac). The goal in these studies is to learn about the intrinsic forward and reverse electron-transfer (ET) processes in the PMA-dU nucleoside as a means of developing pyrenyl-dU nucleosides with ET product lifetimes in the 0.5 ns time range. 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 varies from 75 to 98% in the solvent series THF, MeCN, and MeOH. Pyrenyl (π,π*) emission quenching is assigned to ET that forms the pyrene•+/dU•- product as observed previously in other pyrenyl-dU nucleosides. In contrast to the monoexponential emission decay of PMA-Ac, the emission of PMA-dU at all wavelengths is multiexponential with 4 lifetimes in THF and nearly always with 3 in MeCN and MeOH. The multiexponential decay is likely due to the presence of multiple nucleoside conformers. Importantly, the emission decay for the nucleoside in the 500−550 nm region is assigned to relaxation of the pyrene•+/dU•- ET product. The 0.5−4 ns time range contains over 95% of the emission amplitude in this wavelength region for 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-UMe) identify two key factors 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). The other is Coulombic attraction between the pyrenyl cation and uracil anion subunits. The Coulombic and CO/UMe dihedral angle contributions to the energy 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 ΕΤ1 state.
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