Laser spectroscopy and lifetime measurements of the S1 state of tetracyanoquinodimethane (TCNQ) in a cold gas-phase free-jet : Unusually long S1 lifetime and its energy dependence.

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 measurement under jet-cooled condition 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 electron dopant.