When an assembly of two or more molecules absorbs a photon to form a singlet exciton, and the energetics and intermolecular interactions are favourable, the singlet exciton can rapidly and spontaneously produce two triplet excitons by singlet fission. To understand this process is important because it may prove to be technologically significant for enhancing solar-cell performance. Theory strongly suggests that charge-transfer states are involved in singlet fission, but their role has remained an intriguing puzzle and, up until now, no molecular system has provided clear evidence for such a state. Here we describe a terrylenediimide dimer that forms a charge-transfer state in a few picoseconds in polar solvents, and undergoes equally rapid, high-yield singlet fission in nonpolar solvents. These results show that adjusting the charge-transfer-state energy relative to those of the exciton states can serve to either inhibit or promote singlet fission.

Singlet fission (SF) in polycrystalline thin films of four 3,6-bis(thiophen-2-yl)diketopyrrolopyrrole (TDPP) chromophores with methyl (Me), n-hexyl (C6), triethylene glycol (TEG), and 2-ethylhexyl (EH) substituents at the 2,5-positions is found to involve an intermediate excimer-like state. The four different substituents yield four distinct intermolecular packing geometries, resulting in variable intermolecular charge transfer (CT) interactions in the solid. SF from the excimer state of Me, C6, TEG, and EH takes place in τSF = 22, 336, 195, and 1200 ps, respectively, to give triplet yields of 200%, 110%, 110%, and 70%, respectively. The transient spectra of the excimer-like state and its energetic proximity to the lowest excited singlet state in these derivatives suggests that this state may be the multiexciton (1)(T1T1) state that precedes formation of the uncorrelated triplet excitons. The excimer decay rates correlate well with the SF efficiencies and the degree of intermolecular donor-acceptor interactions resulting from π-stacking of the thiophene donor of one molecule with the DPP core acceptor in another molecule as observed in the crystal structures. Such interactions are found to also increase with the SF coupling energies, as calculated for each derivative. These structural and spectroscopic studies afford a better understanding of the electronic interactions that enhance SF in chromophores having strong intra- and intermolecular CT character.

Singlet exciton fission (SF) in organic chromophore assemblies results in the conversion of one singlet exciton (S) into two triplet excitons (T), provided that the overall process is exoergic, i.e., E(S) > 2E(T). We report on SF in thin polycrystalline films of two terrylene-3,4:11,12-bis(dicarboximide) (TDI) derivatives 1 and 2, which crystallize into two distinct π-stacked structures. Femtosecond transient absorption spectroscopy (fsTA) reveals a charge-transfer state preceding a 190% T yield in films of 1, where the π-stacked TDI molecules are rotated by 23° along an axis perpendicular to their π systems. In contrast, when the TDI molecules are slip-stacked along their N-N axes in films of 2, fsTA shows excimer formation, followed by a 50% T yield.

Singlet fission (SF) is being explored as a way to improve the efficiency of organic photovoltaics beyond the Shockley-Queisser limit; however, many aspects of the SF mechanism remain unresolved. The generally accepted mechanisms provide simplified models of SF that equivocate over whether a charge transfer (CT) state is involved in SF. A one-step superexchange model allows the CT state to act as a virtual state, reducing the effect of large Gibbs free energy values from SF rate calculations. Also, extending superexchange to an excimer-mediated process allows for further refinement of the triplet formation model. Application of the superexchange and excimer-mediated models to a variety of rylene and diketopyrrolopyrrole derivatives provides new insights into the role of the CT and excimer states, providing a semiquantitative description of SF that is dictated by the CT state energy.

Campylobacter requires iron for successful colonization of the host. In the last 7 years, a wealth of data has been generated allowing detailed molecular characterization of Campylobacter iron-uptake systems. Several exogenous siderophores have been identified as sources of ferric iron for Campylobacter. Ferri-enterochelin uptake requires both the outer-membrane receptor protein CfrA and the inner-membrane ABC transporter system CeuBCDE. Ferrichrome has been shown to support growth of some Campylobacter jejuni strains and the presence of homologues of Escherichia coli fhuABD genes was proposed; the Cj1658–Cj1663 system appears to be involved in the uptake of ferri-rhodotorulic acid. In addition to siderophores, the importance of host iron sources was highlighted by recent studies demonstrating that C. jejuni can exploit haem compounds and the transferrins using ChuABCDZ and Cj0173c–Cj0178, respectively. An additional putative receptor, Cj0444, present in some, but not all, strains has not yet been characterized. Following diffusion through the outer membrane, inner-membrane transport of ferrous iron can occur via the FeoB protein. While it may be assumed that all systems are not essential, there is growing evidence supporting the need for multiple iron-uptake systems for successful host colonization by Campylobacter. In light of this, comparative molecular characterization of iron systems in all Campylobacter strains is necessary to gain further insight into the pathogenesis of members of this genus.

Nanoparticles (NPs) of the singlet fission chromophore 3,6bis(5-phenylthiophen-2-yl)pyrrolo [3,4-c]pyrrole-1,4(2H,5H)-dione (PhTDPP) having average hydrodynamic diameters of 63−193 nm were prepared by rapidly injecting variable concentrations of PhTDPP solutions in tetrahydrofuran into water. These PhTDPP NPs are stable over months in water and exhibit fluorescence quantum yields ≪1%. Femtosecond transient absorption spectros-copy shows that singlet fission is more rapid in smaller NPs, likely reflecting their greater surface area-to-volume ratio and consequent exposure of more molecules to the high dielectric aqueous environment. These observations suggest that charge transfer states, whose energy is sensitive to the dielectric constant of the surrounding medium, serve as virtual intermediates in PhTDPP NP singlet fission. However, the lifetime of the triplet excitons produced by singlet fission is longest in the larger NPs having greater long-range order, which allows the triplet excitons to diff use further from one another thus slowing triplet−triplet annihilation.

Here we describe the synthesis of two Zr-based benzothiadiazole- and benzoselenadiazole-containing metal-organic frameworks (MOFs) for the selective photocatalytic oxidation of the mustard gas simulant, 2-chloroethyl ethyl sulfide (CEES). The photophysical properties of the linkers and MOFs are characterized by steady-state absorption and emission, time-resolved emission, and ultrafast transient absorption spectroscopy. The benzoselenadiazole-containing MOF shows superior catalytic activity compared to that containing benzothiadiazole with a half-life of 3.5 min for CEES oxidation to nontoxic 2-chloroethyl ethyl sulfoxide (CEESO). Transient absorption spectroscopy performed on the benzoselenadiazole linker reveals the presence of a triplet excited state, which decays with a lifetime of 9.4 μs, resulting in the generation of singlet oxygen for photocatalysis. This study demonstrates the effect of heavy chalcogen substitution within a porous framework for the modulation of photocatalytic activity.

Crystalline selenium (c-Se) is a direct band gap semiconductor and has been developed for detector applications for more than 30 years. While most advances have been made using vacuum deposition processes, it remains a challenge to prepare efficient c-Se devices directly from solution. We demonstrate a simple solution process leading to uniform and high-crystallinity selenium films under ambient conditions. A combination of ethylenediamine (EDA) and hydrazine solvents was found to be effective in dissolving selenium powder and forming highly concentrated solutions. These can be used to infiltrate a mesoporous titanium dioxide layer and form a smooth and pinhole-free capping overlayer. Efficient light-induced charge injection from the crystalline selenium to TiO2 was observed using transient absorption spectroscopy. A small amount of EDA addition in the hydrazine solution was found to improve the film coverage significantly, and on the basis of the finding, we are able to achieve up to 3.52% power conversion efficiency solar cells with a fill factor of 57%. These results provide a method to control the crystalline selenium film and represent significant progress in developing low-cost selenium-based solar cells.