Real-time observation of interfragment vibration and charge transfer within the TCNQF4 dimer

- TCNQF42− deforms in 100 fs after being produced from a planar TCNQF4−.
- The excited singlet state of TCNQF42− has lifetime of 750 fs.
- The time constant of 5 ps reflects the lifetime of the excited triplet state of TCNQF42−.
- Periodical signal modulation reflects molecular structural vibration.
- The dimer was found to dissociate in 350 fs.

The organic electron acceptors, 7,7,8,8-tetracyanoquinodimethane (TCNQ) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (TCNQF4), are widely used to synthesize multivalent charge-transfer complexes. Their corresponding radical anion complexes have specific electronic, optical, and magnetic characters that have attracted attention in the field of materials development. In the present work, we have performed time-resolved absorption spectroscopy of the (Na+)2(TCNQF4−)2 radical anion complex to elucidate the reaction dynamics ([TCNQF4−]2 → TCNQF4 + TCNQF42−). A visible sub-10 fs laser pulse was used to observe the ultrafast dynamics of the charge transfer and real-time change of the molecular structure during the reaction. Unlike our previous work on the (Na+)2(TCNQ−)2 radical anion complex, the interfragment vibrational mode was clearly observed for (Na+)2(TCNQF4−)2. The difference in the intermolecular interaction is considered to reflect the difference in the π-π interaction and/or the difference in the flexibility of the molecular plane predicted by theoretical calculations.