Molecular-Level Insight of Cu(I) Complexes with the 7,8-Bis(diphenylphosphino)-7,8-dicarba-nido-undecaborate Ligand as a Thermally Activated Delayed Fluorescence Emitter: Luminescent Mechanism and Design Strategy

Abstract

Investigation of the clear structure−property relationship and microscopic mechanism of thermally activated delayed fluorescence (TADF) emitters with high emission quantum yield is a direction worthy of continuous efforts. The instructive theoretical principle of TADF material design is critical and challenging. Here, we carried out theoretical calculation on two experimental Cu(I) complexes with the same 7,8-bis(diphenylphosphino)-7,8-dicarba-nido-undecaborate (dppnc) but different N^N ligands [dmbpy = 6,6′-dimethyl-2,2′-bipyridine (1) or dmp = 2,9dimethyl-1,10-phenanthroline (2)] to briefly elaborate the structure−TADF performance relationship and luminescence mechanism. It was found that enhanced rigidity by the fused benzene ring between two pyridyl units in complex 2 leads to (i) higher allowedness of S 1 → S 0 , (ii) more effective reverse intersystem crossing (RISC), and (iii) better relative stability of the T 1 state, which could be responsible for its excellent TADF behavior. Thus, a strategy of extending π conjugation in the N^N ligand could be deduced to further enhance the quantum yield. We validated it and have succeeded in designing analogue complex 4 by extending π conjugation with an electron-withdrawing pyrazinyl. Benefiting from the smaller energy gap (ΔE ST ) and plunged reorganization energy between the S 1 and T 1 states, the rate of RISC in complex 4 (1.05 × 10 8 s −1 ) increased 2 orders of magnitude relative to that of 2 (5.80 × 10 6 s −1 ), showing more superiority of the TADF behavior through a better balance of RISC, fluorescence, and phosphorescence decay. Meanwhile, the thermally activated temperature of 4 is only 165 K, implying that there is a low-energy barrier. All of these indicate that the designed complex 4 may be a potential TADF candidate.

Impact of ΔEST on Delayed Fluorescence Rate, Lifetime, and Intensity Ratio of Tetrahedral Cu(I) Complexes: Theoretical Simulation in Solution and Solid Phases

“…To keep expeditious ISC and RISC ( k ISC , 10 10 –10 12 s –1 ; k RISC , 10 8 –10 10 s –1 ), the balance of those parameters could be regulated within the colored regions in Figures and S6. In our previous research, introducing the dmdpq (3,6-dimethyl-dipyrido­[3,2-f:2′,3′-h]-quinoxaline) as an N∧N ligand in tetrahedral Cu­(I) complex could lead to a reduced Δ E ST and λ, which is validated to be efficacious to further increase the k ISC and k RISC . Likewise, a similar structural modification could be carried out for complexes 1 and 2 in this work.…”

“…In our previous research, introducing the dmdpq (3,6dimethyl-dipyrido[3,2-f:2′,3′-h]-quinoxaline) as an N∧N ligand in tetrahedral Cu(I) complex could lead to a reduced ΔE ST and λ, which is validated to be efficacious to further increase the k ISC and k RISC . 26 Likewise, a similar structural modification could be carried out for complexes 1 and 2 in this work. Moreover, it is well-known that rapid k ISC and k RISC could expedite the rate of delayed fluorescence, but the quantitative relationship between these rates is blurry.…”

“…In this work, the effects of the surrounding environment (both solution and solid) as well as the molecular structural rigidity, − which the tetrahedral Cu­(I) complexes are susceptible to, were contemplated in detail. Normally, the more rigid a structure is, the smaller the PLQY improvement induced by enhancing the rigidity of the surrounding environment would be.…”

“…To make a comparison between S 0 and S 1 states, the HOMO energy levels are obviously rising (0.56 eV for 1 -solution, 0.38 eV for 1 -solid, 0.45 eV for 2 -solution, and 0.31 eV for 2 -solid) relative to that of S 0 state. These increases can be attributed to the intrinsic metal-to-ligand charge transfer (MLCT), in which the geometries deviate from tetrahedral to planar quadrilateral in the excited state, making stronger effects on the σ* antibonding formed between the metal and ligand in S 1 than that in S 0 state. , In theory, the more it tends to be planar, the higher the HOMO energy increases. There is no doubt that the tetrahedral geometries of the two complexes in the solid phase are maintained better than that in solution.…”

Synthesis and Reactivity of Cyclic Oxonium Derivatives of nido-Carborane: A Review

“…Another important area of use for nido-carborane derivatives as water-solubilizing boron moieties is the design of potential drugs for boron neutron capture therapy of cancer [18][19][20][21][22][23]. In addition, based on nidocarborane derivatives, it is possible to obtain reagents for radioimaging of tumors [24][25][26][27][28], new optical and luminescent materials [29][30][31][32][33][34][35][36][37], ionic liquids [38,39], etc. Thus, the development of convenient approaches to the modification of the nido-carborane cluster remains highly relevant.…”