Delocalization of positive charge in aromatic liquids studied by subnanosecond near-infrared transient absorption spectroscopy

Abstract

Transient absorption spectra of 1,2,4,5-tetracyanobenzene (TCNB) in aromatic liquids (benzene, toluene, m-xylene, and mesitylene) were measured in subnanosecond time resolution. Upon photoexcitation at 355 nm, exciplexes between TCNB and the aromatic liquids formed quickly, and we observed characteristic absorption bands due to a TCNB radical anion in the visible wavelength range and charge resonance (CR) absorption bands due to aggregate radical cations of the donor molecules in the near-IR wavelength range. The peak positions of the CR bands for the exciplexes differed significantly from those of the dimer radical cations of the corresponding donor molecules, indicating that positive charge was delocalized among more than two donor molecules. The TCNB radical anion decomposed efficiently within the lifetime of the exciplexes, and the positive charge remained in the aromatic liquids after the decomposition. Thus, we could observe CR bands due to positive charge (holes) in the aromatic liquids. From the peak positions of the CR bands, we were able to draw some conclusions regarding the delocalization of positive charge in aromatic liquids.