In this work iminyl σ-radical formation in several one-electron oxidized cytosine analogs including 1-MeC, cidofovir, 2′-deoxycytidine (dCyd), and 2′-deoxycytidine 5′-monophosphate (5′-dCMP) were investigated in homogeneous aqueous (D2O or H2O) glassy solutions at low temperatures employing electron spin resonance (ESR) spectroscopy. Employing density functional theory (DFT) (DFT/B3LYP/6-31G* method), the calculated hyperfine coupling constant (HFCC) values of iminyl σ-radical agree quite well with the experimentally observed ones thus confirming its assignment. ESR and DFT studies show that the cytosine-iminyl σ-radical is a tautomer of the deprotonated cytosine π-cation radical (cytosine π-aminyl radical, C(N4-H)•). Employing 1-MeC samples at various pHs ranging ca. 8 to ca. 11, ESR studies show that the tautomeric equilibrium between C(N4-H)• and the iminyl σ-radical at low temperature is too slow to be established without added base. ESR and DFT studies agree that in the iminyl-σ radical, the unpaired spin is localized to the exocyclic nitrogen (N4) in an in-plane pure p-orbital. This gives rise to an anisotropic nitrogen hyperfine coupling (Azz = 40 G) from N4 and a near isotropic β-nitrogen coupling of 9.7 G from the cytosine ring nitrogen at N3. Iminyl σ-radical should exist in its N3-protonated form as the N3-protonated iminyl σ-radical is stabilized in solution by over 30 kcal/mol (ΔG= −32 kcal/mol) over its conjugate base, the N3-deprotonated form. This is the first observation of an isotropic β-hyperfine ring nitrogen coupling in an N-centered DNA-radical. Our theoretical calculations predict that the cytosine iminyl σ-radical can be formed in dsDNA by a radiation-induced ionization–deprotonation process that is only 10 kcal/mol above the lowest energy path.

The efficiency of bulk heterojunction (BHJ) organic photovoltaic (OPV) devices depends significantly upon absorption of photons and the migration of the photogenerated excited state to the heterojunction interface between the electron donor and electron acceptor. Within anilino-squaraine, molecules known for their successful use in the active layer of OPV devices, electronic aggregation strongly influences the absorption spectrum, energy transfer (EnT), and exciton migration to this heterojunction interface. Therefore, the long-range transition dipole coupling and the relative populations of the associated excited states dictate the general effectiveness of these materials in optoelectronic devices. This work presents subpicosecond transient absorption (TA) data that probe the excited-state photophysics of samples with a continuum of intermolecular separation, from monomers in solution to high-concentration solid solution thin films analogous to OPV active layers. EnT times are calculated for each squaraine concentration, and pump-power dependence provides evidence for significant EnT despite a high preponderance of H-aggregation. Theoretical modeling of essential states supports the interpretation from TA spectra that excited states relax into more tightly packed H-aggregates. This work prompts further questions regarding a far-reaching mechanistic EnT bottleneck for molecular and polymeric BHJ devices.

In this work, we have synthesized 5-thiocyanato-2′-deoxyuridine (SCNdU) along with the C6-deuterated nucleobase 5-thiocyanatouracil (6-D-SCNU) and studied their reactions with radiation-produced electrons. ESR spectra in γ-irradiated nitrogen-saturated frozen homogeneous solutions (7.5 M LiCl in H2O or D2O) of these compounds show that electron-induced S-CN bond cleavage occurs to form a thiyl radical (dU-5-S• or 6-D-U-5-S•) and CN− via the initial π-anion radical (SCNdU•−) intermediate in which the excess electron is on the uracil base. HPLC and LC-MS/MS studies of γ-irradiated N2-saturated aqueous solutions of SCNdU in the presence of sodium formate as a OH-radical scavenger at ambient temperature show the formation of the dU-5S-5S-dU dimer in preference to dU by about 10 to 1 ratio. This shows that both possible routes of electron-induced bond cleavage (dUC5-SCN and S-CN) in SCNdU•− and dU-5-S• formation are preferred for the production of the σ-type uracilyl radical (dU•) by 10 fold. DFT/M06-2x/6-31++G(d,p) calculations employing the polarizable continuum model (PCM) for aqueous solutions show that dU-5-S• and CN− formation was thermodynamically favored by over 15 kcal/mol (ΔG) compared to dU• and SCN− production. The activation barriers for C5-S and S-CN bond cleavage in SCNdU•− amount to 8.7 and 4.0 kcal/mol, respectively, favoring dU-5-S• and CN− formation. These results support the experimental observation of S-CN bond cleavage by electron addition to SCNdU that results in the formation of dU-5-S• and the subsequent dU-5S-5S-dU dimer. This establishes SCNdU as a potential radiosensitizer that could cause intra- and inter-strand crosslinking as well as DNA-protein crosslinking via S-S dimer formation.

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Both the nature of the excited state and the morphology of the active layer combine to greatly influence the efficiency in all-small-molecule organic photovoltaic devices. Hence, squaraines, for which synthesis is simple, reproducible, and low-cost, will be favored active layer materials for commercialization when their higher device efficiencies can be correlated with phase separation, miscibility, and crystallinity. Here, multiple measurement methods are used to provide a self-consistent framework that connects active layer morphology, excited-state populations, and device efficiency. Thin-film thickness is accurately measured with atomic force microscopy, which leads to the extinction coefficient for absorption. Spectroscopy is subsequently used to measure populations of electronic states as squaraines are blended with varying amounts of [6,6]-phenyl C 61 butyric acid methyl ester (PCBM). The populations interchange between such observed states as monomers in dilute mixtures, dimers in concentrated mixtures, and chargetransfer H-aggregates in pure/crystalline regions. The measured efficiency of devices is shown to correlate with these changing populations. A model is presented, which allows for incorporation of DBSQ(OH) 2 molecules within PCBM interstices. The model also accounts for phase separation in as-cast squaraine/PCBM mixtures for concentrations of squaraine above the eutectic point. The increased efficiency achieved through changing excited-state populations informs a process such that devices can be manufactured to have their highest efficiency when their thermodynamically stable state is burnt into the morphology while the device is in use.