When molecular dimers, crystalline films or molecular aggregates absorb a photon to produce a singlet exciton, spin-allowed singlet fission may produce two triplet excitons that can be used to generate two electron–hole pairs, leading to a predicted ∼50% enhancement in maximum solar cell performance. The singlet fission mechanism is still not well understood. Here we report on the use of time-resolved optical and electron paramagnetic resonance spectroscopy to probe singlet fission in a pentacene dimer linked by a non-conjugated spacer. We observe the key intermediates in the singlet fission process, including the formation and decay of a quintet state that precedes formation of the pentacene triplet excitons. Using these combined data, we develop a single kinetic model that describes the data over seven temporal orders of magnitude both at room and cryogenic temperatures.

The dynamics of photoexcited lead-free perovskite films, CHNHSnI, were studied using broadband transient absorption and time-resolved fluorescence spectroscopy. Similar to its lead analogue CHNHPbI, we show that free carrier (electrons and holes) recombination is also the dominant relaxation pathway in CHNHSnI films. The slow hot carrier relaxation time is 0.5 ps. Long carrier diffusion lengths for electrons (279 ± 88 nm) and holes (193 ± 46 nm) were obtained from fluorescence quenching measurements. We also show that SnF doping in the CHNHSnI film effectively increases the fluorescence lifetime up to 10 times and gives diffusion lengths exceeding 500 nm. These results suggest that the photophysics of CHNHSnI perovskite are as favorable as those of CHNHPbI, demonstrating that it is a promising nontoxic lead-free replacement for lead iodide perovskite-based solar cells.

Silicon-based solar cells are approaching the thermodynamic limit of efficiency (Shockley-Queisser limit). Simultaneously, fossil fuels are strongly linked to climate changes. Consequently, new approaches are necessary to satisfy the world's steadily increasing energy demand. Singlet fission (SF) is a process overcoming the core assumptions that Shockley and Queisser postulated for their calculations: it is predicted to generate two charges per photon rather than only one! Basel et al. provide evidence for a charge-transfer-mediated mechanism of SF in a nonconjugated, rigid pentacene dimer.

The use of multiple chromophores as photosensitizers for catalysts involved in energy-demanding redox reactions is often complicated by electronic interactions between the chromophores. These interchromophore interactions can lead to processes, such as excimer formation and symmetry-breaking charge separation (SB-CS), that compete with efficient electron transfer to or from the catalyst. Here, two dimers of perylene bound either directly or through a xylyl spacer to a xanthene backbone were synthesized to probe the effects of interchromophore electronic coupling on excimer formation and SB-CS using ultrafast transient absorption spectroscopy. Two time constants for excimer formation in the 1-25 ps range were observed in each dimer due to the presence of rotational isomers having different degrees of interchromophore coupling. In highly polar acetonitrile, SB-CS competes with excimer formation in the more weakly coupled isomers followed by charge recombination with τ = 72-85 ps to yield the excimer. The results of this study of perylene molecular dimers can inform the design of chromophore-catalyst systems for solar fuel production that utilize multiple perylene chromophores.

Photodriven charge transfer dynamics are described for an atomic layer deposition-stabilized, organic dye-sensitized photocathode architecture that produces hydrogen.

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Synthetic chemistry enables a bottom-up approach to quantum information science, where atoms can be deterministically positioned in a quantum bit or qubit. Two key requirements to realize quantum technologies are qubit initialization and readout. By imbuing molecular spins with optical initialization and readout mechanisms, analogous to solid-state defects, molecules could be integrated into existing quantum infrastructure. To mimic the electronic structure of optically addressable defect sites, we designed the spin-triplet, V 3+ complex, (C 6 F 5 ) 3 trenVCN t Bu (1). We measured the static spin properties as well as the spin coherence time of 1 demonstrating coherent control of this spin qubit with a 240 GHz electron paramagnetic resonance spectrometer powered by a free electron laser. We found that 1 exhibited narrow, near-infrared photoluminescence (PL) from a spin-singlet excited state. Using variable magnetic field PL spectroscopy, we resolved emission into each of the ground-state spin sublevels, a crucial component for spin-selective optical initialization and readout. This work demonstrates that trigonally symmetric, heteroleptic V 3+ complexes are candidates for optical spin addressability.

We demonstrate that the 10-phenyl-10 H-phenothiazine radical cation (PTZ) has a manifold of excited doublet states accessible using visible and near-infrared light that can serve as super-photooxidants with excited-state potentials is excess of +2.1 V vs SCE to power energy demanding oxidation reactions. Photoexcitation of PTZ in CHCN with a 517 nm laser pulse populates a D electronically excited doublet state that decays first to the unrelaxed lowest electronic excited state, D' (τ < 0.3 ps), followed by relaxation to D (τ = 10.9 ± 0.4 ps), which finally decays to D (τ = 32.3 ± 0.8 ps). D' can also be populated directly using a lower energy 900 nm laser pulse, which results in a longer D'→D relaxation time (τ = 19 ± 2 ps). To probe the oxidative power of PTZ photoexcited doublet states, PTZ was covalently linked to each of three hole acceptors, perylene (Per), 9,10-diphenylanthracene (DPA), and 10-phenyl-9-anthracenecarbonitrile (ACN), which have oxidation potentials of 1.04, 1.27, and 1.6 V vs SCE, respectively. In all three cases, photoexcitation wavelength dependent ultrafast hole transfer occurs from D, D', or D of PTZ to Per, DPA, and ACN. The ability to take advantage of the additional oxidative power provided by the upper excited doublet states of PTZ will enable applications using this chromophore as a super-oxidant for energy-demanding reactions.