Thermally activated delayed fluorescence (TADF) is fluorescence arising from a reverse intersystem crossing (RISC) from the lowest triplet (T1) to the singlet excited state (S1), where these states are separated by a small energy gap (ΔEst), followed by a radiative transition to the ground state (S0). Rate constants relating TADF processes in 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) were determined at four different solvent polarities (toluene, dichloromethane, ethanol, and acetonitrile). We revealed that the rate constant of RISC, kRISC, which is the most important factor for TADF, was significantly enhanced by a reduced ΔEst in more polar solvents. The smaller ΔEst was mainly attributable to a stabilization of the S1 state. This stabilization also induced a Stokes shift in fluorescence through a relatively large change of the dipole moment between S1 and S0 states (17 D). Despite of this factor, we observed a negative correlation between ΔEst and efficiency of the delayed fluorescence (φd). This was ascribed to a lower intersystem crossing rate, kISC, and increased nonradiative decay from S1, k(s)nrs, in polar solvents.

The electrochemistry and electrogenerated chemiluminescence (ECL) of four kinds of electron donor-acceptor molecules exhibiting thermally activated delayed fluorescence (TADF) is presented. TADF molecules can harvest light energy from the lowest triplet state by spin up-conversion to the lowest singlet state because of small energy gap between these states. Intense green to red ECL is emitted from the TADF molecules by applying a square-wave voltage. Remarkably, it is shown that the efficiency of ECL from one of the TADF molecule could reach about 50%, which is comparable to its photoluminescence quantum yield.

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A fluorescent photochromic compound, composed of diarylethene, fluorescein and succinimidyl ester units, was developed for the controllable fluorescent labeling of biomolecules based on a small molecule.

The photophysical properties and electrogenerated chemiluminescence (ECL) of three donor-acceptor molecules composed of dicyanobenzene and methyl-, tert-butyl-, and phenyl-substituted carbazolyl groups, 1,2,3,5-tetrakis(3,6-disubstituted-carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN-Me, 4CzIPN-tBu, and 4CzIPN-Ph, respectively) are described. These molecules show delayed fluorescence as a result of thermal spin upconversion from the lowest triplet state to the lowest singlet state at room temperature. The three molecules showed yellow to yellowish-red ECL. Remarkably, the ECL efficiencies of 4CzIPN-tBu in dichloromethane reached almost 40 %. Moreover, stable ECL was emitted from 4CzIPN-tBu and 4CzIPN-Ph. In case of 4CzIPN-Me, the ECL intensity decreased during voltage cycles because of polymerization. Quantum chemical calculations revealed that polymerization was inhibited by the steric hindrance of the bulky tert-butyl and phenyl groups on the carbazolyl moieties and lowered the spin density on the carbazolyl groups through electron conjugation for 4CzIPN-Ph.

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