Ultrafast Photophysics of Multiple-Resonance Ultrapure Blue Emitters

Abstract

Multiple-resonance thermally activated delayed fluorescence (MR-TADF) emitters are becoming increasingly attractive due to their applications in high-resolution organic light-emitting diode (OLED) display technology. Here, we present an investigation on the photophysics of two MR-TADF emitters (t-DABNA and TBN-TPA) by using quantum chamical calculation and ultrafast transient absorption (TA) spectroscopy. Compared with one-step structural planarization of t-DABNA, TBN-TPA undergoes two-step relaxation in S1 state, i.e., fast twisting of the peripheral group and subsequent restrained planarization of the B-N framework. The efficient twisting motion of the peripheral group largely reduces the energy level of the TBN-TPA system and correspondingly increases the barrier for subsequent planarization, which is favored for the narrowband emission. Our work provides a detailed picture for the excited-state deactivation of peripheral group-modified MR-TADF emitters without a pronounced charge-transfer (CT) characteristic mixed in the lowest-lying fluorescent state, which might be helpful for the future design of narrowband OLED emitters.

Gold(I) Multi‐Resonance Thermally Activated Delayed Fluorescent Emitters for Highly Efficient Ultrapure‐Green Organic Light‐Emitting Diodes

“…Taking (BzIPr)AuBN as a representative example, pulse excitation at 267 nm yielded an initial fs-TA spectrum ( � 1.3 ps; Figure 3a) that gives a broad positive TA around 450 nm and 700 nm due to the excited state absorption (ESA), a prominent negative feature at 480 nm ascribed to ground state bleaching (GSB) of the 1 SRCT band, as well as a negative shoulder band around 500 nm due to S 1 !S 0 stimulated emission (SE). [15] Spectral evolution of (BzIPr)-AuBN mainly occurs in two stages: one in the first 35 ps (Figure 3a) and the other over the remaining 3000 ps (Figure 3b). The early stage involves an indistinct ESA development, which should be associated with the vibrational cooling process from S 1hot to S 1 excited states.…”

Tuning Hybridized Local and Charge-Transfer Mixing for Efficient Hot-Exciton Emission with Improved Color Purity

“…6,40−42 However, on the other hand, the emitting color purity of organic emitters is mainly affected by two issues: 11,43 (1) S 1 /S 0 conformational relaxation, i.e., structural rigidity, which can be generalized by reorganization energy of S 1 → S 0 transition, and (2) the dominated vibrational modes in vibronic coupling, i.e., modes with pronounced Huang-Rhys factors. 44,45 The low structural rigidity of cold exciton emitters usually conflicts with requirements for high color purity emitting. In S 1 /S 0 conformational relaxation are correspondingly favored for acquiring qualified color purity, which presents the dilemma of balancing efficiency and color purity.…”

“…58−60 We focused on the twisting angles θ 1 and θ 2 as shown in Figure 1b, which become less twisted on the S 1 state than the ground state, indicating excited-state planarization which has been reported for organic emitters with LE state dominated emission. 44,61,62 To compare the S 1 /S 0 conformational relaxation of para-and meta-emitters, we calculated the S 1 /S 0 difference of twisting angles which are defined as Δθ i = |θ i (S 1 ) − θ i (S 0 )|, i = 1 or 2 (Table 1 and Figure 1b).…”

“…In most cases, the emitting efficiency of DF emitters is largely determined by the rISC rate. − To pursue 100% IQE, minimized Δ E ST and pronounced S 1 –T 1 spin–orbital coupling (SOC) are required. ,− For conventional DF emitters, the CT separated frontier orbitals leads to minimized Δ E ST ; efficient rISC is facilitated between the lowest-lying T 1 and S 1 states, which is therefore called a “cold exciton” emitter. ,− However, on the other hand, the emitting color purity of organic emitters is mainly affected by two issues: , (1) S 1 /S 0 conformational relaxation, i.e., structural rigidity, which can be generalized by reorganization energy of S 1 → S 0 transition, and (2) the dominated vibrational modes in vibronic coupling, i.e., modes with pronounced Huang-Rhys factors. , The low structural rigidity of cold exciton emitters usually conflicts with requirements for high color purity emitting. In contrast, local excited (LE) states with minimized S 1 /S 0 conformational relaxation are correspondingly favored for acquiring qualified color purity, which presents the dilemma of balancing efficiency and color purity.…”

“…The employed functional M06-2X was selected from 12 widely used hybrid and long-range correlated functionals by comparing their calculated vertical emission energy (S 1 → S 0 ) for several involved hot-exciton emitters with the experimentally reported values (see section S3 of the Supporting Information). The D–A twisting angle has been intensively reported to be critical in excited-state relaxation and emission of DF emitters. − We focused on the twisting angles θ 1 and θ 2 as shown in Figure b, which become less twisted on the S 1 state than the ground state, indicating excited-state planarization which has been reported for organic emitters with LE state dominated emission. ,, To compare the S 1 /S 0 conformational relaxation of para- and meta- emitters, we calculated the S 1 /S 0 difference of twisting angles which are defined as Δθ i = |θ i (S 1 ) – θ i (S 0 )|, i = 1 or 2 (Table and Figure b).…”