The engineering of stilbazolium/iodocuprate hybrids with optical/electrical performances by modulating inter-molecular charge transfer among H-aggregated chromophores†
The combination of D–π-A stilbazolium-type chromophores bearing ortho-N-alkyl substituents with iodocuprate resulted in three novel hybrids, these were, [(CMAMP)2(CuI3)(acetone)0.5]n (1), [(HMAMP)(Cu3I4)]n (2), and [(HMAHP)2(Cu5I7)]n (3), which present natural quantum-well architectures with void spaces of quasi-2-D organic layers occupied by iodocuprate anions via C–H⋯I hydrogen bonds. The D–π-A stilbazolium dyes exhibit head-to-tail arranged H-aggregations with face-to-face π⋯π stacking interactions. Theoretical calculations suggest that the charge densities on the benzenes can be strengthened from the electron-withdrawing group (–CN) to the electron-donating group (–OH), and the –C2H5OH substituents on the ortho-N of the pyridines not only help to open up the band gaps, but also provide a steric effect for better face-to-face π⋯π stacking interactions. Therefore, HMAHP+ in 3 possesses the strongest face-to-face π⋯π stacking interactions owing to the presence of a longer –C2H5OH group in the ortho-N position, and consequently, blue-shifted photoluminescence, a stronger photocurrent (0.28 mA), and a higher ON/OFF ratio (1.6 × 104) can be observed in 3. In particular, electrical bistability performances can be observed in ITO/hybrids/poly (methyl methacrylate) (PMMA)/Ag devices, which are explained using Schottky emission, space-charge-limited current effect (SCLC) and Ohmic mechanisms. According to theoretical calculations, the band gap switching from the semi-conductor to the conductor (high resistance state (HRS) to low resistance state (LRS)) after trapping electrons is exclusively dominated by the π bonding and anti-bonding orbitals of the stilbazolium dyes, which indicates a future direction for the design of stilbazolium-containing memory devices.