Determining the Energy Gap between the S₁ and T₁ States of Thermally Activated Delayed Fluorescence Molecular Systems Using Transient Fluorescence Spectroscopy

Abstract

The energy gap (ΔE S−T ) between the lowest single and triple excited states is a crucial parameter for thermally activated delayed fluorescence (TADF) molecular systems with high quantum yield. However, a reliable experimental approach to precisely determine this value is challenging. Here, we introduce a new, simple, and efficient strategy to accurately obtain the ΔE S−T in TADF systems from timeresolved fluorescence spectroscopy using a recently reported TADF molecule, DMACPDO, as a representative. By introducing an explicit model to describe the corresponding singlet−triplet coupling system, elusive intersystem crossing and reverse intersystem crossing rates can be extracted by fitting the kinetics of the observed fluorescence. The ΔE S−T value can then be determined. Moreover, our modeling accurately explained the opposite trend in fluorescence intensity of DMACPDO with solvent polarity under air-saturated and deoxygenated conditions. Additionally, the validity of this approach has been demonstrated in another well-known TADF molecule, 4CzIPN. We demonstrate how this approach of determining ΔE S−T sheds light on a deeper understanding of energy-loss mechanisms involved in related photoconversion processes.

Highly Efficient Triplet–Triplet Annihilation Upconversion with a Thermally Activated Delayed Fluorescence Molecule as Triplet Photosensitizer

“…According to the fact that the strong negative transient absorption peak in Figure is exactly located at the fluorescence emission wavelength range in each solvent, the band at 500–650 nm can be readily assigned to the stimulated emission of DMACPDO. Moreover, its decay rate is consistent with the reciprocal of the reported delayed fluorescence lifetime of DMACPDO . In addition, the intensity of this negative peak shows a consistent trend with fluorescence intensity, providing further evidence for the spectral assignment of stimulated emission.…”

“…The second wide and positive absorption band at 650–800 nm can be readily assigned to the absorption of triplet DMACPDO. This conclusion can be also confirmed by their decay rates close to the reciprocal of delayed fluorescence lifetime of DMACPDO in the same solvent . On the basis of this consistence, we used the decay lifetime of stimulated emission at 500–600 nm to represent the lifetime of triplet DMACPDO owing to its better signal-to-noise ratio and performed the kinetic measurements of the energy transfer between triplet DMACPDO and annihilator.…”

“…Moreover, as a bimolecular Dexter energy transfer process, the TTET rate is solvent-dependent as the diffusion rate and the triplet lifetime vary with the viscosity of the solvent, and solvent viscosity probably has a considerable influence on the TTA efficiency . Additionally, the Δ E S–T value is significantly varied by solvent polarity, as observed recently in TADF molecules . Therefore, revealing the elusive solvent effect in the overall TTA upconversion process is of great significance for the application of TADF photosensitizers.…”

Enhanced Efficiency of Exciplex Emission from a 9-Phenylfluorene Derivative

“…For a better understanding of the effect of hindrance modification (namely, hindrance increase) in electron acceptors on their exciplex-TADF characteristics, we calculated the kinetic parameters of TAPC:1 and TAPC:2 and estimated their ΔE ST values using the reported method. 47 Figures S21−S26 and Table S1 display the fitting results from the transient fluorescence decays of TAPC:1 and TAPC:2 films. Subsequently, we estimated the photophysical parameters for TAPC:1 and TAPC:2 films at 300 K based on the above fitting results and collected the obtained data in Table S2.…”

“…d S 1 − T 1 . e Estimated through a reported method 47. f The onsets of oxidation and reduction curves in CH 3 CN.…”