Water oxidation is a key chemical transformation for the conversion of solar energy into chemical fuels. Our review focuses on recent work on robust earth-abundant heterogeneous catalysts for the oxygen-evolving reaction (OER). We point out that improvements in the performance of OER catalysts will depend critically on the success of work aimed at understanding reaction barriers based on atomic-level mechanisms. We highlight the challenge of obtaining acid-stable OER catalysts, with proposals for elements that could be employed to reach this goal. We suggest that future advances in solar fuels science will be accelerated by the development of new methods for materials synthesis and characterization, along with in-depth investigations of redox mechanisms at catalytic surfaces.

The excited state dynamics in polycrystalline thin films of tetracene are studied using both picosecond fluorescence and femtosecond transient absorption. The solid-state results are compared with those obtained for monomeric tetracene in dilute solution. The room temperature solid-state fluorescence decays are consistent with earlier models that take into account exciton-exciton annihilation and exciton fission but with a reduced delayed fluorescence lifetime, ranging from 20-100 ns as opposed to 2 μs or longer in single crystals. Femtosecond transient absorption measurements on the monomer in solution reveal several excited state absorption features that overlap the ground state bleach and stimulated emission signals. On longer timescales, the initially excited singlet state completely decays due to intersystem crossing, and the triplet state absorption superimposed on the bleach is observed, consistent with earlier flash photolysis experiments. In the solid-state, the transient absorption dynamics are dominated by a negative stimulated emission signal, decaying with a 9.2 ps time constant. The enhanced bleach and stimulated emission signals in the solid are attributed to a superradiant, delocalized S(1) state that rapidly fissions into triplets and can also generate a second superradiant state, most likely a crystal defect, that dominates the picosecond luminescence signal. The enhanced absorption strength of the S(0)→S(1) transition, along with the partially oriented nature of our polycrystalline films, obscures the weaker T(1)→T(N) absorption features. To confirm that triplets are the major species produced by relaxation of the initially excited state, the delayed fluorescence and ground state bleach recovery are compared. Their identical decays are consistent with triplet diffusion and recombination at trapping or defect sites. The results show that complications like exciton delocalization, the presence of luminescent defect sites, and crystallite orientation must be taken into account to fully describe the photophysical behavior of tetracene thin films. The experimental results are consistent with the traditional picture that tetracene's photodynamics are dominated by exciton fission and triplet recombination, but suggest that fission occurs within 10 ps, much more rapidly than previously believed.

Water oxidation activity of nickel–iron layered double hydroxide ([NiFe]-LDH) nanosheet electrocatalysts is a function of interlayer anion basicity.

Bichromophoric molecules can support two spatially separated excited states simultaneously and thus provide novel pathways for electronic state relaxation. Exciton fission, where absorption of a single photon leads to two triplet states, is a potentially useful example of such a pathway. In this paper, a detailed study of exciton fission in three novel phenylene-linked bis(tetracene) molecules is presented. Their spectroscopy is analyzed in terms of a three-state kinetic model in which the singlet excited state can fission into a triplet pair state, which in turn undergoes recombination on a time scale longer than the molecule's radiative lifetime. This model allows us to fit both the prompt and delayed fluorescence decay data quantitatively. The para-phenylene linked bis(tetracene) molecules 1,4-bis(tetracen-5-yl)benzene (1) and 4,4'-bis(tetracen-5-yl)biphenylene (2) show intramolecular exciton fission with yields of approximately 3%, whereas no delayed fluorescence is observed for tetracene or the meta-linked molecule 1,3-bis(tetracen-5-yl)benzene 3. Analysis of the temperature-dependent fluorescence dynamics yields activation energies for fission of (10.0 +/- 0.6) kJ/mol for 1 and (4.1 +/- 0.5) kJ/mol for 2, with Arrhenius prefactors of (1.48 +/- 0.04) x 10(8) s(-1) for 1 and (1.72 +/- 0.02) x 10(7) s(-1) for 2. The observed trends in activation energies are reproduced by ab initio calculations of the independently optimized singlet and triplet energies. The calculations indicate that electronic coupling between the two tetracene units is primarily through-bond, allowing differences in fission rates to be qualitatively explained in terms of the linker structure as well. Our results show that it is important to consider the effects of the linker structure on both energy relaxation and electronic coupling in bichromophoric molecules. This study provides insight into the structural and energetic factors that should be taken into account in the design of exciton fission molecules for possible solar cell applications.