Role of Pyridinium Groups and Iodide Ions in Photoelectrochromism in Viologen-Based Ion-Pair Charge-Transfer Complexes: Molecular Orbital Analysis

Quantum chemistry calculations were performed to examine the fundamentals of photoinduced electron transfer (ET) in viologen (V)-based ion-pair charge-transfer complexes and the resulting photoelectrochromism with respect to photoswitching applications. For the purpose, the photoinduced ET from counter anions to viologen or biphenyl (BP) derivatives was modeled, and its relationship with their structures was analyzed. Our results showed that the electron reduction of V²⁺, assuming photoinduced ET, that is, V²⁺ → V⁺ → V⁰, changes the conformational preference from a twisted to a planar structure because of the lowest unoccupied molecular orbital (LUMO) in V²⁺ showing a planar tendency. A similar feature appears in the reduction of neutral BP, that is, BP⁰ → BP^- → BP^2-, leading to a twisted → planar preference change because of the LUMO in BP⁰. The similarity between V²⁺ and BP⁰ can be explained by their similar MO shape and their identical number of electrons. The time-dependent density functional theory (TD-DFT) was applied to predict the absorption spectra of viologen and BP derivatives with iodide ions considered as counter anions. In addition, geometrical optimization using the TD-DFT method was performed for viologen derivatives to stabilize a specific excitation to simulate laser irradiation. Our simulation implies that laser absorption can cause a twisted → planar change accompanied by a weak charge transfer if we ignore the time scale. Moreover, it was found that the iodide ion is necessary for near-infrared (NIR) absorption corresponding to the telecommunication wavelength. This is because NIR absorption is attributed to a narrow energy gap generated by the insertion of orbital levels of iodide ions into the original energy gap of viologen. Contrary to the above-mentioned similarity, BP was found to show totally different features from viologen regarding (1) the magnitude of the polarization between the molecule and counter anions, (2) the laser-induced excitation behavior, and (3) the peak position of NIR absorption. From the MO analysis, the role of pyridinium groups, which cannot be replaced by phenyl groups, was theoretically explained.