Self-exchange intermolecular Ru III/II electron transfer, a process commonly referred to as "hole-hopping", is of great interest as it provides a means of charge transport across the surface of nanocrystalline (anatase) TiO 2 mesoporous thin films without the loss of free energy. This process was characterized by cyclic voltammetry and chronoabsorptometry for three homologous Ru diimine compounds of the general form [Ru(LL) 2 (dcbH 2 )](PF 6 ) 2 , where LL is 2,2′-bipyridine (bpy), 4,4′-dimethyl-2,2′-bipyridine (dmb), or 4,4′-di-tert-butyl-2,2′-bipyridine (dtb) and dcbH 2 is 2,2′-bipyridyl-4,4′-dicarboxylic acid. Apparent electron diffusion coefficients, D, abstracted from this data increased with dtb < bpy < dmb. Both techniques were consistent with this trend, despite differences in the magnitude of D between the two methods. Temperature dependent measurements revealed an activation barrier for electron selfexchange of 250 ± 50 meV that was within this error the same for all three diimine compounds, suggesting the total reorganization energy, λ, was also the same. Application of Marcus theory, with the assumption that the 900 ± 100 meV total reorganization energy for self-exchange electron transfer was independent of the Ru compound, revealed that the electronic coupling matrix element, H AB , followed the trend dtb (0.02 meV) < bpy (0.07 meV) < dmb (0.10 meV). The results indicate that insulating side groups placed on redox active molecules can be utilized to tune the electronic coupling and hence selfexchange rate constants without significantly altering the reorganization energy for electron transfer on TiO 2 surfaces.

The discovery of the phenomenon known as aggregation-induced emission (AIE) has opened the door to a variety of brilliant organic solid-state lightemitting materials. While AIE is well established in linear optics, the development of AIE luminogens (AIEgens) with highly efficient nonlinear optical (NLO) effects remains relatively unexplored. Particularly, second-order NLO requires the AIEgens to be organized in a non-centrosymmetric fashion, and such examples are rarely reported. Here, an AIEgen, 2,7-di([1,1′-biphenyl]-4-yl)fluorenone (4-DBpFO), is designed and synthesized by introducing a carbonyl group onto the backbone of p-hexaphenylene. The restricted rotation of the compound upon aggregation results in a dramatic enhancement of the linear optical emission when forming self-assemblies. Furthermore, introducing the carbonyl group drives the formation of hydrogen bonded molecular chains, which are attached by the zigzag CH⋅⋅⋅π interactions in a non-centrosymmetric way. As a result, the dipole of each individual molecule contributes accumulatively to a macroscopic dipole of the formed 4-DBpFO microcrystals. This leads to a highly efficient second harmonic generation with very high laser damage treshold. This AIEgen, whose optical response is greatly enhanced in both linear and nonlinear optical regimes upon the formation of well-defined self-assemblies, has potential applications in next generation photonic circuits.

An aggregation‐induced emission luminogen active in both linear and nonlinear optics has been demonstrated by Y. Feng, Th. Rasing, J. Xu and co‐workers on page 8968. The self‐assembled microstructures demonstrate a linear optical quantum efficiency ten times higher than that of the solution state, and a highly efficient second harmonic generation signal greater than that of industrial inorganic materials including KDP or LiNbO3.