Charge-Transfer Natures of Alkali Halide-Organic Complexes Revealed by Dual-Mode Antisymmetric Stretching Vibrations of Cyano Groups

The antisymmetric stretching vibrational mode (b1uν18) of cyano groups is fruitfully used as a sensitive probe for the structural and electronic characterization of TCNQ-/F4TCNQ-based donor–acceptor (D–A) complexes. However, this probe may fail when encountering a small (<0.1) degree of charge-transfer (DCT) situation. Herein, we evaluate the potential of dual-mode (b1uν18/b2uν32) probes by synthesizing a series of alkali halide-F4TCNQ complexes with a significant red shift absorption (200–2000 nm) by a one-step mechanochemical method. Surprisingly, the b2uν32 mode displays appreciable sensitivity and universality for the small DCT situation and makes up the shortage of the b1uν18 mode. Compared to the monomer, these two modes of complexes exhibit a much broader peak width corresponding to a much faster rate of thermal energy dissipation. This work highlights the potential of dual-mode probes for the characterization of complexes without an ideal crystalline structure and proves the validity of the metal–organic complexation strategy to regulate the near-infrared absorption of organics.