Laser Spectroscopy and Lifetime Measurements of the S₁ State of Tetracyanoquinodimethane (TCNQ) in a Cold Gas‐Phase Free‐Jet

Abstract

The S1 electronic state of 7,7,8,8‐Tetracyanoquinodimethane (TCNQ) has been investigated by laser induced fluorescence (LIF), dispersed fluorescence (DF) spectroscopy, and lifetime measurements under jet‐cooled conditions in the gas‐phase. The LIF spectrum showed a weak origin band at 412.13 nm (24262 cm−1) with prominent progression and combination bands involving vibrations of 327, 1098, and 2430 cm−1. In addition, very strong bands appeared at ∼363.6 nm (3300 cm−1 above the origin). Both the LIF and DF spectra indicate considerable geometric change in the S1 state. The fluorescence lifetime of S1 at zero‐point level was obtained to be 220 ns. This lifetime is 40 times longer than the radiative lifetime estimated from the S1−S0 oscillator strength. Furthermore, the lifetimes of the vibronic bands exhibited drastic energy dependence, indicating a strong mixing with the triplet (T1) or intramolecular charge‐transfer (CT) state. This study is thought to disclose intrinsic nature of TCNQ, which has been well known as a component of organic semiconductors and a versatile p‐type dopant.

Photophysics and Inverted Solvatochromism of 7,7,8,8‐Tetracyanoquinodimethane (TCNQ)

“…The absorption band is assigned to the neutral monomer of TCNQ. [27,35,36] Fluorescence excitation spectra for all solvents match the corresponding absorption spectra. Therefore, the fluorescence bands can also be assigned to the neutral TCNQ monomer.…”

“…The mode is the symmetric C≡N stretching mode, hardly changing the frequency in the different solvents. The normal coordinates for the and modes are depicted in Figure . The mode has a large contribution from ring C=C stretching.…”

“…The absorption maximum for 2,2,4‐trimethylpentane slightly deviates from this trend. The absorption band is assigned to the neutral monomer of TCNQ . Fluorescence excitation spectra for all solvents match the corresponding absorption spectra.…”

“…The normal coordinates for the n 3 and n 4 modes are depicted in Figure 9. [36] The n 3 mode has a large contribution from ring C=C stretching. The n 4 mode is mainly wing C=C stretching.…”

“…In addition, using six different solvents, we concluded that the fluorescence of TCNQ showed an acute response to subtle changes in the polarity of the solvent. Laser‐induced fluorescence (LIF) spectroscopy in a cold gas phase was also reported recently to gain insight into the structure and the electronic excited state of neutral TCNQ . The position of the 0–0 band of the S 1 – S 0 electronic transition was determined to be 412.13 nm (24262 cm −1 ) in the gas phase.…”