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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*)¹ 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.

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*)¹ 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.

The objective of this computational study is to learn about the solution phase, singlet excited-state manifold in conformers of the N-(1-pyrenyl)-1-methyluracil-5-carboxamide nucleoside model (PA-UMe). In particular this report describes how these manifolds differ between solution and gas phases and among 11 PA-UMe conformers. Using a CIS INDO/S SCRF theoretical approach, it models the solvent as a dielectric continuum with a spherical solute cavity. This approach is an improvement over intramolecular electron transfer (ET) computations that neglect solvent effects. The results provide considerable insight into the energetics of intramolecular ET in conformers of PA-UMe [and by extension into conformers of the previously studied N-(1-pyrenyl)-2‘-deoxyuridine-5-carboxamide nucleoside (PA-dU)]. They show that in the solution phase the PA-UMe conformers exhibit a strong correlation (R = 0.9) between the energy of their lowest energy singlet ET state (ET1) and its dipole moment. That is, ET1 states with large solution-phase dipole moments are lower in energy than ET1 states with small dipole moments. This result contrasts with results in the gas phase where there is no correlation between the energy of a conformer's ET1 state and its dipole moment. In solution phase, several other factors become important in determining the energy of ET product states in addition to the redox and Columbic effects dominant in the gas phase. To summarize, the dihedral angle between the C5-carbonyl and the uracil plane directly affects two electronic properties of pyrenyl-dU conformers. One is the ease of reduction of the uracil subunit, and the other is the extent of HOMO and LUMO delocalization onto the atoms that link the pyrenyl and uracil subunits. In the gas phase, only the first of these effects is important in determining the energy of ET products, and this effect is modulated by the Coulombic attraction of the cationic and anionic subunits. However, in the solution phase, the extent of HOMO and LUMO delocalization is very important, because delocalization of these orbitals reduces the dipole moment of their ET products. This work shows that ET1 states with sufficiently large dipole moments (≥24 D) have energies below or equal to the energy of the lowest energy pyrenyl (π,π*) singlet state (P1). Additionally, as long as a trans-PA-UMe conformer has an ET1 state below or near the P1 state, there appears to be a good chance that it will have han electronic spectrum like that of the PA-dU nucleoside in solution. [Cis and trans are defined with respect to the NH and CO groups of the amido bond joining the pyrenyl and uracil subunits.] Note that the absorbance spectrum of PA-dU is nearly the same in tetrahydrofuran, acetonitrile, and methanol solvents and is red-shifted from and significantly broader than that of the pyrenyl ligand model N-acetyl-1-aminopyrene. Finally, the computed electronic spectra of cis-PA-UMe conformers in a polar dielectric continuum (εs = 40) do not agree with that of the PA-dU nucleoside in acetonitrile solution.

Intramolecular quenching of fluorescence from a cationic porphyrin by a covalently attached ferrocene in both solution and DNA is reported. Two ferrocenyl porphyrins have been prepared, tris(4-N-methylpyridiniumyl), mono(phenyl-OCH2CH2ferrocene)porphyrin, P3Fc, and cis-bis(4-N-methylpyridiniumyl), bis(phenyl-OCH2CH2ferrocene)porphyrin. Binding studies for P3Fc indicate that intercalation of the porphyrin moiety into DNA occurs at low ionic strength (10 mM NaCl); outside binding of the complex is favored at increased ionic strength (100 mM NaCl). The outside binding of P3Fc at high ionic strength is attributed to the hydrophobicity of the molecule, which causes it to undergo salt-induced stacking in aqueous solution. The photophysical properties of P3Fc are examined in MeOH, in phosphate buffer, and in the presence of double-stranded DNA. Efficient quenching of the photoexcited singlet state of the porphyrin by electron transfer from the appended ferrocene is observed in MeOH and buffer solutions with average rate constants of ≥1 × 10¹⁰ and 9 × 10⁹ s^-1, respectively. When P3Fc is intercalated into DNA, the average rate of photoinduced intramolecular electron transfer is not appreciably reduced (7 × 10⁹ s^-1), suggesting that electronic coupling between the D−A pair is strong even under conditions where close contact is restricted. Assuming that for this donor−linker−acceptor complex, in which the porphyrin and ferrocene are separated by an −OCH2CH2− spacer, intercalation of the porphyrin into DNA does not significantly reduce the electronic coupling between the donor and acceptor, the observed subnanosecond electron-transfer rates in and out of DNA show that changes in redox potentials or reorganization energy either compensate one another or have negligible kinetic consequences.

Comparison of picosecond kinetic and spectroscopic data for Zn octaethylporphine and Fe(III)Cl octaethylporphine with that for Zn—Fe(III)Cl, a cofacial diporphyrin composed of a Zn porphyrin covalently bound to an Fe(III)Cl porphyrin with two chains of five atoms each, supports the assignment of a light-driven electron transfer (k> 1011s-1) within Zn—Fe(III)Cl to form [Zn+·—Fe(II)]Cl. The kinetics (k≈ 1010s-1) and thermodynamics of the reverse electron transfer are compared to those of a similar electron transfer in bacterial photosynthesis, the reduction of an oxidized bacteriochlorophyll dimer, (BChl)2+·, by Fe(II) cytochrome c.