Oxidation/reduction reactions at metal oxide surfaces are important to emerging solar energy conversion processes, photocatalysis, and geochemical transformations. Here we show that the usual description of these reactions as electron transfers is incomplete. Reduced TiO(2) and ZnO nanoparticles in solution can transfer an electron and a proton to phenoxyl and nitroxyl radicals, indicating that e(-) and H(+) are coupled in this interfacial reaction. These proton-coupled electron transfer (PCET) reactions are rapid and quantitative. The identification of metal oxide surfaces as PCET reagents has implications for the understanding and development of chemical energy technologies, which will rely on e(-)/H(+) coupling.

In order to elucidate the mechanism of singlet fission in thin films of 1,3-diphenylisobenzofuran (1) we have performed ultrafast transient absorption spectroscopy as a function of sample temperature and excitation fluence on polycrystalline thin films composed of two polymorphs. Our earlier investigations revealed that films enriched in a particular polymorph of 1 displayed near 200% efficiency for triplet formation at 77 K, while films composed primarily of a second polymorph had a very low triplet quantum yield. Present data confirm the triplet yield disparities in the two polymorphs and demonstrate the distinct fates of the initially prepared singlets in films of different structure. Singlet fission is inhibited in the more stable polymorph due to rapid excimer formation and trapping. The less stable polymorph undergoes highly efficient singlet fission with a dominant time constant of 10-30 ps and without strong thermal activation. Transient absorption measurements with varying excitation fluence indicate that singlet-singlet annihilation is a primary competitor of singlet fission at higher fluence and that fission from higher-lying states can also contribute to the triplet formation process. Measurements employing different excitation energies and sample temperatures reveal the role that trapping processes play in attenuating the triplet quantum yield to produce the complex temperature dependence of the singlet fission yield. The rate constants for singlet fission itself are essentially temperature independent.

Polycrystalline thin films of 1,3-diphenylisobenzofuran (1) with a morphology referred to here as α exhibit highly efficient singlet fission, producing two triplet states for every absorbed photon at 77 K, and about 1.4 triplet states per absorbed photon at room temperature. However, the triplet yield depends strongly on the specific crystalline form of 1, and for the morphology referred to as β the triplet yields are roughly an order of magnitude smaller. In this study, α, β, and mixed α/β films of 1 are prepared by thermal evaporation and solution drop-casting, and the structural and photophysical differences that may account for the very different triplet quantum yields are explored. The crystallites of 1 in thin films have been identified with two bulk crystal polymorphs grown from solution and structurally characterized. Analysis of absorption spectra of the films reveals a 600 cm–1 blue shift in the onset and a unique spectral profile for the form α crystallites as compared to form β. Intermolecular interactions between columns of slip-stacked molecules are different in the two polymorphs, and this likely gives rise to the much smaller triplet quantum yield for β-1.

Vibrational coherence was observed following excitation into the lowest-energy spin-allowed 4 A 2 / 4 T 2 ligand-field absorption of Cr(acac) 3 . The transient kinetics were fit to a rapidly damped 164 cm À1 oscillatory component, the frequency of which is not associated with the ground state of the molecule. The signal is assigned as an excited-state vibrational coherence; the timescale of the event suggests that this vibrational coherence is retained during the 4 T 2 / 2 E intersystem crossing that immediately follows 4 A 2 / 4 T 2 excitation. DFT calculations indicate that the 164 cm À1 oscillation likely corresponds to a combination of Cr-O bond stretching in the ligand-field excited state as well as large amplitude motion of the ligand backbone. This hypothesis is supported by ultrafast timeresolved absorption measurements on Cr(t-Bu-acac) 3 (where t-Bu-acac is the monoanionic form of 2,2,6,6-tetramethyl-3,5-heptanedione) -an electronically similar but more sterically encumbered molecule -which exhibits a 4 T 2 / 2 E conversion that is more than an order of magnitude slower than that observed for Cr(acac) 3 . These results provide important insights into the nature of the reaction coordinate that underlies ultrafast excited-state evolution in this prototypical coordination complex.

Photo-oxidations of hydrogen-bonded phenols using excited state polyarenes are described, to derive fundamental understanding of multiple-site concerted proton-electron transfer reactions (MS-CPET). Experiments have examined phenol-bases having −CPh2NH2, −Py, and −CH2Py groups ortho to the phenol hydroxyl group and tert-butyl groups in the 4,6-positions for stability (HOAr-NH2, HOAr-Py, and HOAr-CH2Py, respectively; Py = pyridyl; Ph = phenyl). The photo-oxidations proceed by intramolecular proton transfer from the phenol to the pendent base concerted with electron transfer to the excited polyarene. For comparison, 2,4,6-tBu3C6H2OH, a phenol without a pendent base and tert-butyl groups in the 2,4,6-positions, has also been examined. Many of these bimolecular reactions are fast, with rate constants near the diffusion limit. Combining the photochemical kCPET values with those from prior thermal stopped-flow kinetic studies gives datasets for the oxidations of HOAr-NH2 and of HOAr-CH2Py that span over 107 in kCPET and nearly 0.9 eV in driving force (ΔGo′). Plots of log(kCPET) vs. ΔGo′ define a single Marcus parabola in each case, each including both excited state anthracenes and ground state aminium radical cations. These two datasets are thus well described by semi-classical Marcus theory, providing a strong validation of the use of this theory for MS-CPET. The parabolas give λCPET ≅ 1.15–1.2 eV and Hab ≅ 20–30 cm−1. These experiments represent the most direct measurements of Hab for MS-CPET reactions to date. Although rate constants are available only up to the diffusion limit, the parabolas clearly peak well below the adiabatic limit of ca. 6 × 1012 s−1. Thus, this is a very clear demonstration that the reactions are non-adiabatic. The non-adiabatic character slows the reactions by a factor of ~45. Results for the oxidation of HOAr-Py, in which the phenol and base are conjugated, and for oxidation of 2,4,6-tBu3C6H2OH, which lacks a base, show that both have substantially lower λ and larger pre-exponential terms. The implications of these results for MS-CPET reactions are discussed.