Simple Phenazine‐Based Compounds Realizing Superior Multicolored Emission

Abstract

are commonly found in Cu [3][4][5][6] and noble metal (e.g., Ir, Pt) complexes. [7][8][9][10][11][12] Comparing with fluorescence materials, the predominant advantage of phosphorescence materials is their internal quantum efficiency that can approach 100%. [13][14][15] However, high cost is a barrier which cannot be circumvented for commercial manufacture. Besides above two categories of materials, pure organic molecules, for instance, thermally activated delayed fluorescence (TADF) materials, are regarded as promising candidates for organic lightemitting diodes (OLEDs). TADF can harness almost 100% exciton accessing to reverse intersystem crossing (RISC) path, thereby achieving relatively superior performance in devices. [16,17] Significant investigations have been performed to get the high-performance TADF materials in past decades. [14,16,[18][19][20][21][22][23][24][25] Generally, TADF materials should have a considerably small energy difference (ΔE ST < 0.5 eV) between the lowest singlet (S 1 ) and triplet (T 1 ) states. [16] This requires the lowest singlet excited state to have a small orbital overlap integral (S if = 〈Φ i |Φ f 〉) between initial (Φ i ) and final (Φ f ) state orbits, corresponding to the degree of spatial overlap of the orbits, thereby gaining a small exchange integral (2J if = ΔE ST ). [20] Based on this strategy, many researchers tended to modify the subject (acceptor) by integrating more electron donors (Ds), [26][27][28] aiming to utilize the steric hindrance of donors to separate the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs). Some previous works are preferred to use special donors, such as 1,8-dimethylcarbazole, [29][30][31] to make a large dihedral angle between donor and acceptor (A). Regardless of which method was used, to achieve high efficiency performance, most compounds either experienced arduously picking of specific subjects (acceptors) that were tedious to synthesize, [21] or were coupled with more and even different donors. [32,33] Meanwhile, even though the interest in TADF materials is shifting to the operational lifetime, [34] color purity, [23] reverse intersystem crossing rate, and so on, [31,[34][35][36] however, according to recent works reported by Ma [37] and Adachi and co-workers, [24,38] new mechanisms about hybrid local and charge-transfer (HLCT)-type TADF have been proposed. We hope that depending on this work, these new mechanisms could be applied to the conventional TADF configuration, further paving the way for the follow-up studies of the mechanisms of TADF.A new family of phenazine-based compounds is synthesized to achieve multicolored emission from deep blue (418 nm) to red (625 nm). Emitters have unique hybrid local and charge-transfer excited states for S 1 , T 1 , T 2 , and even higher-layer excited states, respectively, while all emitters possess thermally activated delayed fluorescence (TADF) nature. The rigid structure of phenazine and steric hindrance of donors are sufficiently utilized to supp...

Suppression and utilization of Kasha's rule: realizing the transformation from blue to near-infrared emission

“…For the synthesis procedures please refer to the previous work 14 and supporting information. On the one hand, we previously found that the oscillator strength of S 1 approaches zero if the dihedral angle is vertical, which means S 1 may no longer be an ideal emissive state.…”

“…Kasha's rule 1 is extensively observed in luminescence fields, [2][3][4] such as in fluorescent molecular rotors, 3,5-8 bioimaging [9][10][11][12][13] and organic light-emitting diodes (OLEDs), [14][15][16][17] in which the excitons return to the ground state from the lowest singlet S 1 (for phosphorescence this process occurs from T 1 ). To date, most organic luminescent materials follow this rule and their photophysical properties can be precisely simulated using quantum chemical calculations.…”

“…18,19,22,[26][27][28][29][30][31][32][33] Additionally, almost all studies focus on the transition from low to high excited states rather than the other way around, which cannot dialectically expose the nature of Kasha's rule. [5][6][7][8][9]20,21,[23][24][25]34 In a previous work, we reported a series of phenazine-based compounds 14 to explore the connection between the electronic configuration of the excited state and the molecular configuration. The results show that the emission of phenazine-based compounds could undergo a bathochromic-shift with the increase of electron-donating ability of the donor.…”

“…In a previous work, we reported a series of phenazine-based compounds 14 to explore the connection between the electronic configuration of the excited state and the molecular configuration. The results show that the emission of phenazine-based compounds could undergo a bathochromic-shift with the increase of electron-donating ability of the donor.…”

New strategy for developing deep blue TADF materials with narrow emission and CIEy < 0.06 employing host materials as donors

“…Generally, the host materials with carbazole units exhibit high energy level, conforming to the design strategy for blue or deep blue emitters. 23 However, when the acceptor unit is connected with stronger electron-donating donors, the compounds will undergo a serious red-shift due to the intramolecular charge transfer (ICT) effect, 24 which is unfavorable for the design of deep blue D–A type TADF materials. Therefore, introducing host materials with weak electron-donating ability into molecules is effective to limit the red-shift of deep blue TADF emitters.…”