Three States Involving Vibronic Resonance is a Key to Enhancing Reverse Intersystem Crossing Dynamics of an Organoboron-Based Ultrapure Blue Emitter

Abstract

The recently developed narrow-band blue-emitting organoboron chromophores based on the multiple-resonance (MR) effect have now become one of the most important components for constructing efficient organic light emitting diodes (OLEDs). While they basically emit through fluorescence, they are also known for showing substantial thermally activated delayed fluorescence (TADF) even with a relatively large singlet–triplet gap (Δ E ST ). Indeed, understanding the reverse intersystem crossing (RISC) dynamics behind this peculiar TADF will allow judicious molecular designs toward achieving better performing OLEDs. Explaining the underlying nonadiabatic spin-flip mechanism, however, has often been equivocal, and how the sufficiently fast RISC takes place even with the sizable Δ E ST and vanishingly small spin–orbit coupling is not well understood. Here, we show that a vibronic resonance, namely the frequency matching condition between the vibration and the electronic energy gap, orchestrates three electronic states together and this effect plays a major role in enhancing RISC in a typical organoboron emitter. Interestingly, the mediating upper electronic state is quite high in energy to an extent that its thermal population is vanishingly small. Through semiclassical quantum dynamics simulations, we further show that the geometry dependent non-Condon coupling to the upper triplet state that oscillates with the frequency Δ E ST / ℏ is the main driving force behind the peculiar resonance enhancement. The existence of an array of vibrational modes with strong vibronic rate enhancements provides the ability to sustain efficient RISC over a range of Δ E ST in defiance of the energy gap law, which can render the MR-emitters peculiar in comparison with more conventional donor–acceptor type emitters. Our investigation may provide a new guide for future blue emitting molecule developments.

Rapid predictions of the colour purity of luminescent organic molecules

“…This is a significant deviation, however the approach used captures only direct ISC. Recently, Kim et al 24 computed both the direct and spin-vibronic ISC rates for 13 , finding the spin-vibronic pathway dominates and consistent with the present calculations, they reported a direct k ISC of 10 2–3 s −1 .…”

“…The resulting non-bonding molecular orbitals (MOs) minimise the vibronic coupling, leading to narrow luminescence. The multi-resonance effect also reduces the exchange interaction making these materials suitable for application in thermally activated delayed fluorescence (TADF) and the success of this approach has led to a significant number of experimental 11,[13][14][15][16][17][18][19][20][21] and theoretical [22][23][24] investigations into these systems, which have achieved high-performing 3rd-generation 11 and Hyperfluorescence OLEDs. 25 Despite the importance of the emission FWHM, there are few computational studies computing and/or predicting the emission FWHM of luminescent organic materials.…”

Ultrafast Triplet–Singlet Exciton Interconversion in Narrowband Blue Organoboron Emitters Doped with Heavy Chalcogens

“…Moreover, all three molecules were found to possess the T 2 state below the S 1 state, rendering another effective spin-flip channel, as reported in some D-A type TADF [19-22, 54, 55] and MR-TADF systems. [39,47,[56][57][58] Likewise, the energetically close-lying upper T 3 state can possibly contribute to the upconversion process via the spin-vibronic mechanism. [21,59] All three molecules showed essentially similar NTO patterns for their S 1 and T 1 -T 3 states but markedly different degrees of electronic contributions from the chalcogen atoms.…”

Emission and Absorption Tuning in TADF B,N‐Doped Heptacenes: Toward Ideal‐Blue Hyperfluorescent OLEDs

“…[15] MR-TADF compounds show TADF due to the alternating pattern of electron density between the ground and excited states that leads to small ΔE ST , combined with upper-triplet crossings from thermally populated T n states back to the emissive S 1 state. [16][17][18][19] The excited states thus possess a distinct short-range charge transfer (SRCT) character, [14,20,21] and MR-TADF compounds are endowed with high singlet radiative decay (k r ) rates of around 10 7 s -1 . [5,20] Despite these advantages, the RISC rates reported for MR-TADF materials typically lag ~100 times slower than those of leading D-A or D-A-D TADF materials.…”