Picosecond time-resolved fluorescence experiments are used to study the dynamics of singlet fission in highly disordered films of rubrene. The fluorescence spectral lineshapes are not temperature-dependent, indicating that intermolecular excitonic effects are absent in these films. The temperature-dependent fluorescence decays in the amorphous films are nonexponential, containing both prompt and delayed components. The kinetics are qualitatively consistent with the presence of singlet fission, but to confirm its presence, we examine the effects of magnetic fields on the fluorescence decay. A quantum-kinetic model is developed to describe how magnetic fields perturb the number of triplet pair product states with singlet character and how this in turn affects the singlet state kinetics. Simulations show that the magnetic field effect is very sensitive to mutual chromophore alignment, and the direction of the effect is consistent with a local ordering for rubrene molecules that participate in fission. From our analysis, the dominant fission rate is 0.5 ns–1, about 10 times slower than that observed in polycrystalline tetracene films, but we still estimate that ∼90% of the initially excited singlets undergo fission. Kinetic modeling of our fluorescence decay data and magnetic field dependence reveals that at the low laser intensities used in this experiment geminate triplet pairs do not interact with each other, and that spin–lattice relaxation between triplet sublevels is not complete on the 100 ns time scale. When both exciton fission and fusion are occurring, dynamic measurements in the presence of a magnetic field can elucidate molecular-level details of both processes.

Conjugated polymers blended with graphene represent a possible approach for making organic bulk heterojunction solar cells. In this paper, the time-resolved fluorescence dynamics of poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) blended with graphene microsheets derived from chemically reduced graphitic oxide are studied. Both polymers exhibit strong quenching and shortened fluorescence lifetimes when mixed with graphene. The fluorescence quenching function takes the form of e −k Q t 1/2 , where k Q is linearly proportional to the weight fraction of graphene in the blend. We consider two physical models to explain the origin of the fluorescence quenching. The first assumes that energy transfer occurs within a three-dimensional space to molecular scale defects within the graphene according to the standard Forster model with an energy transfer rate proportional to the donor−acceptor separation R −6. The second model assumes a quasi-two-dimensional environment where the energy transfer rate between the donor and graphene sheets is proportional to R −4. Using the second model, an estimate of ∼5 nm is obtained for the critical energy transfer radius for energy transfer between P3HT chains and graphene sheets. This value is in reasonable agreement with theory. Differences between the quenching behavior of graphene in MEH-PPV and P3HT blends are also discussed.

X-ray emission spectroscopy (XES) is a powerful element-selective tool to analyze the oxidation states of atoms in complex compounds, determine their electronic configuration, and identify unknown compounds in challenging environments. Until now the low efficiency of wavelength-dispersive X-ray spectrometer technology has limited the use of XES, especially in combination with weaker laboratory X-ray sources. More efficient energy-dispersive detectors have either insufficient energy resolution because of the statistical limits described by Fano or too low counting rates to be of practical use. This paper updates an approach to high-resolution X-ray emission spectroscopy that uses a microcalorimeter detector array of superconducting transition-edge sensors (TESs). TES arrays are discussed and compared with conventional methods, and shown under which circumstances they are superior. It is also shown that a TES array can be integrated into a table-top time-resolved X-ray source and a soft X-ray synchrotron beamline to perform emission spectroscopy with good chemical sensitivity over a very wide range of energies.

The synthesis and photocatalytic properties of a heteropolyoxoniobate, K(10)[Nb(2)O(2)(H(2)O)(2)][SiNb(12)O(40)]·12H(2)O (1), are reported, revealing an important role of Zr(4+) additives in the crystallization. Compound 1 exhibits overall photocatalytic water splitting activity, and its photocatalytic activity is significantly higher than that of Na(10)[Nb(2)O(2)][SiNb(12)O(40)]·xH(2)O (2). Fluorescence lifetime measurements suggest that the enhanced photocatalytic activity of 1 likely results from a larger yield of longer-lived charge trapping states in 1 due to the coordination of one water molecule to the bridging Nb(5+), leading to highly unsymmetrical seven-coordinated Nb(5+) sites.

Synthesis and Photocatalytic Properties of a New Heteropolyoxoniobate Compound: K10[Nb2O2(H2O)2][SiNb12O40]·12H2O. -The title compound is hydrothermally synthesized from a mixture of KOH, Nb2O5, Si(OEt)4, and ZrOCl2 in H2O (autoclave, 220°C, 24 h, 70% yield). K10[Nb2O2(H2O)2][SiNb12O40]·12H2O crystallizes in the tetragonal space group P42/mnm with Z = 2 (single crystal XRD). The structure contains anionic zigzag chains formed by [SiNb 12 O 40 ] 16α-Keggin clusters linked together via bridging [Nb2O2(H2O)2] 6+ units. The title compound exhibits overall photocatalytic water splitting activity which is significantly higher than that of Na10[Nb2O2][SiNb12O40]·xH2O -(ZHANG, Z.; LIN, Q.; KURUNTHU, D.; WU, T.; ZUO, F.; ZHENG, S.-T.; BARDEEN, C. J.; BU, X.; FENG*, P.; J. Am. Chem. Soc. 133 (2011) 18, 6934-6937, http://dx.