Electronic interactions between inorganic nanowires and organic electron acceptors: Drastic changes in optical response and molecular vibration

We have studied electronic interactions between inorganic nanowires and organic molecules and the effects on optical response and molecular vibration, using quasi-one-dimensional organic - inorganic hybrid compounds, methylviologen lead iodide (MVPb2I6) and piperidinium lead iodide (PDPbI3). The materials consist of lead iodide nanowires with a diameter of 0.5 nm and intervening organic molecules, methylviologen (MV2+) and piperidinium ion (PD+), respectively. MV2+ has a very high electron affinity, in contrast to PD+. By comparison of the optical absorption spectra between the compounds, it is shown that a broad absorption band appears in the visible region by varying organic molecules from PD+ to MV2+. The absorption band in MVPb2I6 is assigned to charge-transfer transitions from the lead iodide nanowire to MV2+. In the infrared absorption spectrum of MVPb2I6, a large amount of low-energy shift and a striking increase in oscillator strength were observed for the molecular vibration around the nitrogen atoms in MV2+. These features in MVPb2I6 are owing to electronic interactions between the nanowire and MV2+. Electronic interactions in MVPb2I6 are attributed to charge-transfer interactions, which result from electronic couplings between the valence band (VB) of the nanowire and the lowest unoccupied molecular orbital (LUMO) of MV2+.