We report absorption, fluorescence, and Raman spectroscopy of 7,7,8,8‐tetracyanoquinodimethane (TCNQ) in a variety of solvents. The fluorescence quantum yields (QYs) of linear alkane solutions are similar to one another, but QY is shown to acutely decrease in other solvents with increasing polarities. The slope of the solvatochromic plot of absorption maxima is inverted from negative to positive with an increase in solvent polarity. A significant change in the frequency of carbon‐carbon double bond stretching modes is not observed in Raman spectra of TCNQ in different solvents. The molar absorption coefficient is determined to calculate the oscillator strength of the absorption band. The radiative decay rate constant calculated from the oscillator strength is approximately ten times larger than that elucidated from the fluorescence lifetime and QY. These spectroscopic parameters reveal that the relaxation occurs from a Franck‐Condon excited state to a distinct fluorescence emissive state with a smaller transition dipole moment.
Tetracyanoquinodimethane (TCNQ) is
identified as one of the most
important and classic constituents for the synthesis of organic conductors
and shows an acute response of the fluorescence quantum yield to subtle
changes in the polarity of solvents. Here, we report on characterization
of the excited-state dynamics of TCNQ using time-resolved fluorescence
and femtosecond transient absorption (TA) measurements in various
solvents. Fluorescence decay and TA dynamics reveal that the fluorescence
emissive and nonemissive states reach equilibrium within the fluorescence
lifetime in carbon tetrachloride. Thermally activated delayed fluorescence
of TCNQ is also revealed. The fluorescence in the polar solvents is
quenched by the forward relaxation to the nonemissive state within
a few picoseconds and the subsequent rapid de-excitation of the nonemissive
state within a few tens of picoseconds. The nonemissive state is probably
assigned to the triplet state, and the change in the forward and reverse
intersystem crossing rates can be responsible for the response of
the fluorescence to the polarity of solvents.
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