The transcription factor ΔFosB and the brain-enriched protein kinase CaMKIIα (calcium/calmodulin-dependent protein kinase II) are induced in the nucleus accumbens (NAc) by chronic exposure to cocaine or other psychostimulant drugs of abuse, where the two proteins mediate sensitized drug responses. Although ΔFosB and CaMKIIα both regulate AMPA glutamate receptor expression and function in NAc, dendritic spine formation on NAc medium spiny neurons (MSNs), and locomotor sensitization to cocaine, no direct link between these molecules has to date been explored. Here, we demonstrate that ΔFosB is phosphorylated by CaMKIIα at the protein-stabilizing Ser27, and that CaMKII is required for the cocaine-mediated accumulation of ΔFosB in rat NAc. Conversely, we show that ΔFosB is both necessary and sufficient for cocaine induction of CaMKIIα gene expression in vivo, an effect selective for D1-type MSNs in the NAc shell subregion. Furthermore, induction of dendritic spines on NAc MSNs and increased behavioral responsiveness to cocaine after NAc overexpression of ΔFosB are both CaMKII-dependent. Importantly, we demonstrate for the first time induction of ΔFosB and CaMKII in the NAc of human cocaine addicts, suggesting possible targets for future therapeutic intervention. These data establish that ΔFosB and CaMKII engage in a cell type- and brain region-specific positive feed-forward loop as a key mechanism for regulating the brain’s reward circuitry in response to chronic cocaine.

Several recent electron spin resonance studies have observed a quintet multiexciton state during the singlet fission process. Here we provide a general theoretical explanation for the generation of this state by invoking a time-varying exchange coupling between pairs of triplet excitons, and subsequently solving the relevant time-varying spin Hamiltonian for a range of transition times. We simulate experimental ESR spectra and draw qualitative conclusions about the adiabatic/diabatic transition between triplet pair spin states.

Singlet fission is a form of multiple exciton generation, which occurs in organic chromophores when a high-energy singlet exciton separates into two lower energy triplet excitons, each with approximately half the singlet energy. Since this process is spin-allowed, it can proceed on an ultrafast timescale of less than several picoseconds, outcompeting most other loss mechanisms and reaching quantitative yields approaching 200%. Due to this high quantum efficiency, the singlet fission process shows promise as a means of reducing thermalization losses in photovoltaic cells. This would potentially allow for efficiency improvements beyond the thermodynamic limit in a single junction cell. Efforts to incorporate this process into solar photovoltaic cells have spanned a wide range of device structures over the past decade. In this review, we compare and categorize these attempts in order to assess the state of the field and identify the most promising avenues of future research and development.

Singlet fission describes the spin-conserving production of two triplet excitons from one singlet exciton. The existence of a spin-2 (quintet) triplet-pair state as a product of singlet fission is well established in the literature, and control of quintet formation is an important step towards applying singlet fission in photovoltaics and quantum information. However, a definitive mechanism for quintet formation is yet to be established, which makes it difficult to design materials for optimal quintet formation. Here we outline a mechanism in which inter-triplet exchange-coupling fluctuations drive fast and efficient quintet formation. We show that quintet formation is possible even in the strong-exchange regime, in accordance with recent experimental prediction. We evaluate the performance of this quintet formation mechanism in two regimes of conformational freedom, and relate quintet dynamics to material properties of singlet fission molecules.

The quintet triplet-pair state may be generated upon singlet fission and is a critical intermediate that dictates the fate of excitons, which can be exploited for photovoltaics, information technologies, and biomedical imaging. In this report, we demonstrate that continuous-wave and pulsed electron spin resonance techniques such as phase-inverted echo-amplitude detected nutation (PEANUT), which have emerged as the primary tool for identifying the spin pathways in singlet fission, probe fundamentally different triplet-pair species. We directly observe that the generation rate of high-spin triplet pairs is dependent on the molecular orientation with respect to the static magnetic field. Moreover, we demonstrate that this observation can prevent incorrect analysis of continuous-wave electron spin resonance (cw-ESR) measurements and provide insight into the design of materials to target specific pathways that optimize exciton properties for specific applications.