Abstract
Solid-state electroabsorption is demonstrated as a powerful tool for probing the charge transfer (CT) character and state mixing in the low-energy optical transitions of two structurally similar thermally activated delayed fluorescent (TADF) materials with divergent photophysical and device performances. The Liptay model is used to fit differentials of the low-energy absorption bands to the measured electroabsorption spectra, with both emitters showing CT characteristics and large changes in dipole moments upon excitation despite the associated absorption bands appearing to be structured. High electric fields then reveal transfer of oscillator strength to a state close to the CT in the better performing molecule. With supporting TDDFT-TDA and DFT/MRCI calculations, this state showed ππ* characteristics of a local acceptor triplet that strongly mixes with the σπ* of the CT. The emitter with poor TADF performance showed no evidence of such mixing.
Highlights
Solid-state electroabsorption is demonstrated as a powerful tool for probing the charge transfer (CT) character and state mixing in the low-energy optical transitions of two structurally similar thermally activated delayed fluorescent (TADF) materials with divergent photophysical and device performances
The past few years have seen a great increase in the amount of fundamental and applied research that aimed to exploit thermally activated delayed fluorescent (TADF) molecules for high-efficiency organic light-emitting diodes (OLEDs).[1−3] Via the TADF mechanism, very small singlet−triplet energy gaps (ΔEST) and vibrational coupling-driven spin−orbit coupling allow the low-energy nonemissive triplet excited states to be up-converted into emissive singlets by reverse intersystem crossing
The EA measurement is simple and free of complications associated with such excited-state relaxation processes and is the basis for this initial study.[8]. These changes in absorption allow determination of parameters such as the change in the electric dipole moment (Δμ) and polarizability Δα upon excitation caused by the perturbing field, as well as the character of the excited states formed on optical transitions
Summary
Extra degree of complexity to sample production, data collection, and interpretation. The perturbing electric field causes coupling of a dark state to an allowed transition, leading to the transfer of oscillator strength.[29] A derivative-like feature with strong first-order differential character is observed, spanning 2.35 and 2.6 eV and centered around 2.49 eV We assign this non-Liptay new feature to the triplet exciton located on the acceptor that becomes optically allowed through coupling to the CT singlet state and confirmed by the measured phosphorescence spectrum of the DBPHZ core (Figure 3b), from which an onset energy of 2.40 eV is obtained. In POZ-DBPHZ, the transfer of oscillator strength reveals the presence of a second state that is overlapping the CT band and is not found in t-BuCZ-DBPHZ that has pure CT character This new state in POZ-DBPHZ is identified as the lowest-energy ππ* local triplet of the acceptor. Further studies of this CT singlet− LE triplet coupling with quantum chemistry approaches may well yield more details of the molecular characteristics that result in efficient TADF molecules
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