Charge-transfer state and state mixing in tetracyanoquinodimethane probed using electroabsorption spectroscopy

Area of Science:

• Photophysics and Spectroscopy
• Organic Electronics Materials

Background:

• Tetracyanoquinodimethane (TCNQ) is a key component in organic conductors and a potent electron acceptor.
• TCNQ displays thermally activated delayed fluorescence and exceptionally long fluorescence lifetimes, indicating complex excited-state dynamics.

Purpose of the Study:

• To investigate the photophysical properties of TCNQ by examining the Stark effect on its absorption spectrum.
• To elucidate the role of electronic state mixing in the deactivation pathways of excited TCNQ molecules.

Main Methods:

• Electroabsorption spectroscopy was employed to measure the Stark effect on TCNQ's absorption spectrum.
• The electroabsorption spectrum was computationally simulated by modeling multiple absorption bands corresponding to different electronic states.

Main Results:

• Two distinct electronic states were identified: a locally excited (LE) state with high oscillator strength and zero dipole moment, and an intramolecular charge transfer (ICT) state with a non-zero dipole moment.
• An electric field was observed to enhance the mixing between the LE and ICT states.
• Experimental evidence confirmed that the mixing between the emissive LE state and the nonemissive ICT state significantly influences the deactivation pathway of excited TCNQ.

Conclusions:

• The study establishes the critical role of LE-ICT state mixing in the photophysics of TCNQ.
• The determined dipole moment of the ICT state implies a symmetry-breaking distortion of the TCNQ molecule from its D_2h point group symmetry.