Abstract
Here, a comprehensive photophysical investigation of a the emitter molecule DPTZ‐DBTO2, showing thermally activated delayed fluorescence (TADF), with near‐orthogonal electron donor (D) and acceptor (A) units is reported. It is shown that DPTZ‐DBTO2 has minimal singlet–triplet energy splitting due to its near‐rigid molecular geometry. However, the electronic coupling between the local triplet (3LE) and the charge transfer states, singlet and triplet, (1CT, 3CT), and the effect of dynamic rocking of the D–A units about the orthogonal geometry are crucial for efficient TADF to be achieved. In solvents with low polarity, the guest emissive singlet 1CT state couples directly to the near‐degenerate 3LE, efficiently harvesting the triplet states by a spin orbit coupling charge transfer mechanism (SOCT). However, in solvents with higher polarity the emissive CT state in DPTZ‐DBTO2 shifts below (the static) 3LE, leading to decreased TADF efficiencies. The relatively large energy difference between the 1CT and 3LE states and the extremely low efficiency of the 1CT to 3CT hyperfine coupling is responsible for the reduction in TADF efficiency. Both the electronic coupling between 1CT and 3LE, and the (dynamic) orientation of the D–A units are thus critical elements that dictate reverse intersystem crossing processes and thus high efficiency in TADF.
Highlights
Several charge transfer (CT) molecules have been reported with triplet-harvesting efficiencies close to 100%,[2,5] leading to significantlyExternal quantum efficiencies (EQEs) as large as 25% have been increased device EQEs
In light of our recently reported results with D–A and D–A–D analogues of DPTZ-DBTO2,[7,8] here we show that while a rigid near-orthogonal D–A–D molecular geometry is crucial to minimize the singlet–triplet energy gap, the efficiency of the steady state and delayed fluorescence (TADF) is controlled by two other factors: i) the electronic coupling between the local triplet, (3LE), and the charge transfer singlet states, (1CT, 3CT) yielding interconversion of singlet and triplet states by the spin orbit charge transfer mechanism, a second order process mediated by vibronic coupling,[9] and ii) the need for the D–A geometry to be dynamically rocking about D–A orthogonality
It is crucial to design the correct CT geometry into the ground state of the emitter, thereby guaranteeing that CT formation always occurs even in a nonpolar rigid matrix or device host, and that an underlying local triplet excited state, nearly degenerate with the emissive 1CT state, exists to facilitate TADF, and that this energy gap is the true barrier to RISC
Summary
Several CT molecules have been reported with triplet-harvesting efficiencies close to 100%,[2,5] leading to significantly. From the rigorous photophysical investigation of the D–A–D molecule DPTZ-DBTO2 (Figure 1a) we elucidate the role of local triplet states and CT states, and the role of the D to A orientation in the TADF mechanism. In light of our recently reported results with D–A and D–A–D analogues of DPTZ-DBTO2,[7,8] here we show that while a rigid near-orthogonal D–A–D molecular geometry is crucial to minimize the singlet–triplet energy gap, the efficiency of TADF is controlled by two other factors: i) the electronic coupling between the local triplet, (3LE), and the charge transfer singlet states, (1CT, 3CT) yielding interconversion of singlet and triplet states by the spin orbit charge transfer mechanism, a second order process mediated by vibronic coupling,[9] and ii) the need for the D–A geometry to be dynamically rocking about D–A orthogonality. This article did not show photophysical evidence of TADF in DPTZ-DBTO2, or device data, which we report here in detail to reveal new insights into the TADF mechanism
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