The Lowest Triplet of Tetracyanoquinodimethane via UV–vis Absorption Spectroscopy with Br-Containing Solvents

Abstract

This study was undertaken to find the previously unknown lowest triplet of the isolated molecule of tetracyanoquinodimethane (TCNQ), which is a widely used organic semiconductor. The problem is topical because the triplet excitation of this compound is involved in some processes which occur in electronic devices incorporating TCNQ and its derivatives, and information on the TCNQ triplet is needed for better understanding of these processes. The lowest triplet of TCNQ was obtained at 1.96 eV using UV-vis absorption spectroscopy with Br-containing solvents. Production of the triplet band with sufficient intensity in the spectra was provided by the capacity of the Br atom to augment the triplet excitation and through using a 100 mm cuvette. The assignment of the corresponding spectral band to the triplet transition was made by observation that this band appeared only in the spectra recorded in Br-containing solvents but not in spectra recorded in other solvents. Additional support for the triplet assignment came from the overall UV-vis absorption spectra of TCNQ recorded in various solvents, using a 10 mm cuvette, in the 1.38-6.5 eV energy range. Singlet transitions of the neutral TCNQ molecule and doublet transitions of the TCNQ negative ion were identified in these overall spectra and were assigned with TD B3LYP/6-31G calculations. Determination of the lowest triplet of TCNQ attained in this work may be useful for theoretical studies and practical applications of this important compound.

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

“…Moreover, the fluorescence lifetime for the coupled state becomes longer than that expected from the f of the absorption band. The value of f of the absorption spectrum in hexane is not very different from the value ( f =1.07) for the S 1 – S 0 transition, which was predicted by a TD‐DFT calculation . The S 1 state was assigned to the state, which corresponds to the FC excited state of TCNQ in hexane.…”

“…[32] Energy levels of several electronically excited states of TCNQ were calculated with a density functional theory (DFT) calculation, and the oscillator strength of the S 1 -S 0 transition was reported to be f = 1.07. [33] Weak photoluminescence of some derivatives of TCNQ was also reported. [34] We have recently investigated the photophysics of neutral TCNQ by solvatochromism and photoluminescence studies in a few solvents.…”

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

“…This lifetime is 49 times longer than the reported value for hexane solution . We estimated the radiative lifetime of the S 1 state by using the following equation and reported oscillator strength ( f =1., …”

“…One of the candidates will be the T 1 state. The T 1 state of TCNQ is observed at 1.96 eV (15800 cm −1 ) in hexane solution with Br‐containing solvent . The slow component of the fluorescence decay curves at higher energy vibronic bands may be attributed to the emission from the T 1 state.…”

“…One possibility is the vibronic coupling (VC) between S 1 and higher electronic state(s). However, a reported oscillator strength ( f ) of the S 1 −S 0 transition is 1.07, and those of higher S n =2∼7 −S 0 transitions are less than 0.1 . Thus, S 1 −S 0 is the strongly allowed transition so that other S n states are not the possible candidates to increase the absorption intensity via VC.…”