Abstract
Quantum dynamics and a spin-vibronic Hamiltonian are used to investigate the intersystem crossing (ISC) mechanism of a narrow organoboron molecular emitter, 5,9-diphenyl-5,9-diaza-13 b-boranaphtho [3,2,1-de]anthracene (DABNA-1). We find a rate of ISC (kISC) in good agreement with experiment and which operates via a second-order spin-vibronic coupling mechanism. The nonadiabatic coupling activating this mechanism occurs between the lowest singlet (S1) state and higher lying singlet states promoting ISC into the T2 state. The large S1-T1 energy gap, combined with the slow ISC arising from small spin-orbit coupling and the rigidity of the molecule is the reason for the slow rISC observed experimentally. The importance of the spin-vibronic mechanism, even for narrow Thermally Activated Delayed Fluorescence (TADF) emitters illustrates the importance of identifying the effect of key vibrational modes and their action, when attempting to design molecular emitters combining narrow TADF with efficient rISC.
Highlights
Organic Light Emitting Diodes (OLEDs) have emerged as an attractive approach for fabricating thinner and more efficient flat panel displays
The geometric and electronic structure of DABNA-1 has recently been described in Ref. [13] and we only briefly review it here in the interest of clarity and context for the quantum dynamics simulations
We have used quantum dynamics to demonstrate that despite adopting a novel design approach, the (r)intersystem crossing (ISC) mechanism in DABNA-1 like other organic emitters used for OLEDs, operates via a second-order spin-vibronic coupling mechanism
Summary
Organic Light Emitting Diodes (OLEDs) have emerged as an attractive approach for fabricating thinner and more efficient flat panel displays. The authors demonstrated that using this approach it was possible to reduce the overlap of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) and the energy gap between the S1 and T1 states to ∼0.2 eV, making triplet harvesting via delayed fluorescence feasible This could be achieved without the need of introducing donor and acceptor groups, and two ultrapure blue emitters, 5,9-diphenyl-5,9-diaza-13 b-boranaphtho [3,2,1-de]anthracene (DABNA-1, Figure 1) and 9-([1,1′-biphenyl]-3yl)-N,N,5,11-tetraphenyl-5,9-dihydro-5,9-diaza-13 b-boranaphtho [3,2,1-de]anthracen-3-amine (DABNA-2), were reported, which when incorporated into an OLED exhibited a maximum EQE of 25% and an emission FWHM of only 28 nm. All of the computational details can be found in the Supporting information
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