Synthesis and Photophysics of a 1-Pyrenyl Substituted 2‘-Deoxyuridine-5-Carboxamide Nucleoside:  Electron Transfer Products as CIS INDO/S Excited States

Abstract

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.

Fluorescence-Quenching Phenomenon by Photoinduced Electron Transfer between a Fluorescent Dye and a Nucleotide Base

“…charge recombination between a dye and nucleobases. 9,20,[46][47][48] We here evaluated the reversibility of the fluorophore-DNA conjugates listed in Table 3 (Alexa 488, 5-FAM, BODIPY FL, Alexa 532, and 6-JOE) by repeating denature and hybridization steps using temperature control under continuously supplying blue LED with the maximum emission of 470 nm (0.1 mW) to all of the hybridization solution (the concentration of DNA probes and its complementary DNA were 40 and 160 nM, respectively). All of the DNA probes examined here were completely reversible after exposure to the above-mentioned conditions over a period of a few hours.…”

DFT Study of Monochlorinated Pyrene Compounds

“…The molecular design and the organic synthesis of light emitting materials are of great interest in the funda-mental research and commercial applications [4]. Monochlorinated pyrene derivatives have been synthesized by organic chemistry and find applications as organic light emitting diodes (OLEDs) [5].…”

Explicit Solvent DRF INDOs/CIS Computations of Charge Transfer State Energetics in a Pyrenyldeoxyuridine Nucleoside Model

“…The spectroscopy of 5-(N-carboxyl-1-aminopyrenyl)-2′-deoxyuridine (PAdU; see Figure 1) in tetrahydrofuran (THF), methanol (MeOH) and acetonitrile (MeCN) 1 has several remarkable features, the most important being the extensive quenching of the photoexcited 1 (π,π*) state of its pyrenyl chromophore relative to N-acetyl-1-aminopyrene (PAAcsa spectroscopic model compound). Stronger pyrenyl emission quenching in the more polar solvents MeCN and MeOH compared to that seen in the less polar solvent THF is consistent with the proposal that the photophysics is controlled by the formation of intramolecular Py •+ /dU •-charge transfer (CT) excited states.…”

“…Strikingly, the energy of the Py •+ /dU •-CT states of most conformers in most solvent environments is too high to permit CT immediately following light absorption. 1,2 This is true because the solvent configuration immediately following photoexcitation is the same as that of the nucleoside's ground state (Frank Condon principle). For very polar excited states (such as most Py •+ /dU •-CT states) in polar solvents, solvent orientation has a very large effect on excited state energy.…”