Excited-State Dynamics and Thermally Activated Delayed Fluorescence in the Classic Electron Acceptor Tetracyanoquinodimethane

Abstract

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.

Detection of the Dark States in Thermally Activated Delayed Fluorescence (TADF) Process of Electron Donor‐Acceptor Dyads: Insights from Optical Transient Absorption Spectroscopy

“…On the other hand, femtosecond and nanosecond transient absorption spectra give detail information of both emissive and non-emissive states. [32][33][34][35][36][37][38] Pulsed laser excited time-resolved electron paramagnetic resonance (TREPR) spectroscopy is also an useful method, especially to detect the paramagnetic transient species in TADF, for instance the 3 CT and 3 LE states, as well as the ISC mechanisms. [26,[39][40][41] The topic has been summarized in a recent review article and a few research papers.…”

“…However, these spectral methods (especially the former) only supply direct information about the bright (or emissive) excited states, such as the 1 CT state in electron donor‐acceptor type of TADF molecules, it gives no direct information about the dark (non‐emissive) states involved in the TADF processes, such as the 3 LE and 3 CT states (these states are usually non‐emissive in fluid solution at room temperature). On the other hand, femtosecond and nanosecond transient absorption spectra give detail information of both emissive and non‐emissive states [32–38] . Pulsed laser excited time‐resolved electron paramagnetic resonance (TREPR) spectroscopy is also an useful method, especially to detect the paramagnetic transient species in TADF, for instance the 3 CT and 3 LE states, as well as the ISC mechanisms [26,39–41] .…”

New aspect of photophysics of 7,7,8,8-tetracyanoquinodimethane and its solvated complexes: intra- vs. inter-molecular charge-transfer

“…[16][17][18] They recently carried out transient absorption and uorescence spectroscopic studies and described the shorter uorescence lifetime in higher polar solvent to be due to the faster nonradiative decay to the nonuorescent state whose energy is lowered at higher polar solvent. 19 However, the non-uorescent state was not clearly assigned and they proposed a possibility of the triplet state. Despite these interesting excited-state characteristics in bulk system, gas-phase spectroscopic study of a neutral TCNQ molecule has not been performed until 2019, when we rst reported the S 1 (pp*; B 3u )-S 0 (A g ) electronic spectrum of TCNQ in a cold free jet by means of laser-induced uorescence (LIF) spectroscopy.…”

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

“…Recent studies on photophysics have been performed in the gas phase, solutions, and polymer matrices. [13][14][15][16][17][18][19] We previously reported the fluorescence spectra of TCNQ in a variety of solvents. 15 Blue fluorescence of TCNQ was observed in nonpolar solvents, whereas the fluorescence quantum yield (QY) decreased significantly with an increase in solvent polarity.…”

“…15,17 The excited-state dynamics in various solvents were studied using femtosecond transient absorption (TA) and time-resolved fluorescence measurements. 13,19 The TA dynamics demonstrated that relaxation from an emissive locally excited (LE) state to a nonemissive state occurred within a few picoseconds in polar solvents. 13 Thermally activated delayed fluorescence (TADF) was also observed for carbon tetrachloride.…”