New aspect of photophysics of 7,7,8,8-tetracyanoquinodimethane and its solvated complexes: intra- vs. inter-molecular charge-transfer

Abstract

We performed laser-induced fluorescence (LIF) spectroscopy of 7,7,8,8-tetracyanoquinodimethane (TCNQ) and its solvated complexes with acetonitrile (ACN) and benzene (Bz), under the jet-cooled gas-phase condition. We also carried out fluorescence and its time profile measurements in TCNQ/Bz/hexane solution to compare with the gas-phase results. The LIF excitation spectrum of the S₁ (ππ*)–S₀ electronic transition of TCNQ monomer exhibited unusual vibronic structure with the maximum intensity at ∼3000 cm⁻¹ above the band origin. In addition, the fluorescence lifetime is more than 100 times longer than that in hexane solution with most of the bands showing double exponential decay. The unusual feature of the vibronic bands is intrinsic and not due to the presence of other species, as confirmed by UV-UV hole-burning (HB) spectroscopy. These unusual features of S₁ are thought to be due to the coupling with the S₂ state, where S₂ was revealed to have intramolecular charge-transfer (ICT) character. The S₁–S₀ transition of the TCNQ–ACN complex exhibited sharp vibronic bands which are red-shifted by 120 cm⁻¹ from those of the monomer, indicating van der Waals (vdW) interaction between them; however, the fluorescence lifetime was drastically shortened. In contrast, the TCNQ–Bz complex gave a broad electronic spectrum. The study of the fluorescence and its time profile in TCNQ/Bz/hexane solution clearly shows the formation of the CT complex between TCNQ and Bz. Based on the experimental results and density functional theory (DFT) calculations, we propose that in the TCNQ monomer and TCNQ–ACN complex the S₁ (ππ*) state is coupled to the intramolecular CT state, while the S₁ state of TCNQ in the TCNQ–Bz complex is more strongly coupled to the intermolecular CT state.