Tuning the Excited State of Tetradentate Pd(II) and Pt(II) Complexes through Benzannulated N‐Heteroaromatic Ring and Central Metal

Abstract

A series of tetradentate Pd(II) and Pt(II) complexes containing fused 5/6/6 metallocycles with phenyl N-heteroaromatic benzo[d]imidazole (pbiz), benzo[d]oxazole (pboz) or benzo [d]thiazole (pbthz)-containing ligands was developed. Systematic studies by experiments and theoretical calculations reveal that both the central metal and the benzannulated N-heteroaromatic ring have significant influence on the electrochemical, photophysical and excited-state properties of the Pd(II) and Pt(II) complexes. In identical condition, compared to pbiz-based Pd(II) and Pt(II) complexes, the corresponding metal complexes with pboz and pbthz-containing ligand show significant red-shift emission spectra. Pd(II) complexes exhibit blue-shift emission spectra in comparison with the corresponding Pt(II) complexes. The Pt(II) complexes possess more metal-to-ligand charge transfer ( 3MLCT) character in their T 1 states, which enables the Pt(II) complexes to have much higher quantum efficiencies (Φ PL ) and shorter excited-state lifetimes (τ), resulting in large radiative decay rates (k r ). Especially, Pt(pbiz) has a Φ PL of 94% with a τ of 4.0 µs in 5 wt% doped PMMA film at room temperature. Pt(pbiz)-doped green organic light-emitting diode (OLED) using 26mCPy as host demonstrated a peak external quantum efficiency (EQE) of 21.6% and a maximum brightness (L max ) of 55481 cd/m 2 , which still maintained an EQE of 16.5% and 10.8% at 1 000 and 10 000 cd/m 2 , respectively.

Luminescence and Palladium: The Odd Couple

“…Additionally, the Pd(II) complexes are strongly emissive in various matrices and achieve quantum efficiencies of up to 51%. In line with these results, tetradentate Pd(II) complexes 67 – 69 containing fused 5/6/6 metallocycles ( Figure 20 ) from benzo[d]imidazole Pd(pbiz) 67 , benzo[d]oxazole Pd(pboz) 68 , or benzo[d]thiazole Pd(pbthz) 69 were also obtained [ 85 ].…”

“… Synthesis and chemical structures of the 5/6/6 Pd(II) complexes 57 – 69 with tetradentate ligands. Adapted from [ 84 , 85 ]. …”

8-Phenylquinoline-Based Tetradentate 6/6/6 Platinum(II) Complexes for Near-Infrared Emitters

“…All the 8-phenylquinoline-based Pt­(II) complexes exhibited well-defined reversible redox processes except the quasi-reversible reduction process of PtYL3 (Figure ), indicating the good electrochemical stabilities of the Pt­(II) complexes. In contrast, most of the previously reported tetradentate Pt­(II) and Pd­(II) complexes showed irreversible oxidation processes. ,,− ,, PtYL1 , PtYL2 , and PtYL3 had similar oxidation potentials of 0.20, 0.20, and 0.21 V, respectively; however, their reduction potentials were significantly different, which were −1.97, −2.02, −1.91 eV, respectively (Table ); these results revealed that their oxidation processes were mainly on the electron-rich Pt-carbazole moieties, and the reduction processes were dominantly on the electron-deficient quinoline moieties. The introduction of the electron-donating tBu group into the 6-position of the quinoline made the reduction of PtYL2 to be harder than that of PtYL1 ; in contrast, the phenyl group extended the π-conjugation and enabled PtYL3 easier to be reduced.…”

“…Moreover, PtYL3 also exhibited two distinct molecular conformations with dihedral angles between the terminal quinoline and carbazole rings of ∠A/B = 60.61°and ∠A′/B′ = 55.07°, respectively (Figure 5a and S3); a similar result was also observed in our reported Pt(bp-10) and Pt(bp-11) with fused 6/5/6 metallocycles. 27 DFT calculations [24][25][26][27]29,30 showed that the bond lengths, bond angles, and dihedral angles of the S 0 states of PtYL1, PtYL2, and PtYL3 were in good agreement with those of the single crystal structure of PtYL3; however, the C 1 −Pt−N 3 bond angles of T 1 states of PtYL1, PtYL2, and PtYL3 (162.13, 162.29, and 162.17°) were significantly smaller than those of their S 0 states (165.63, 165.69, and 165.72°) (Tables 1 and S3); this indicated that the carbazole rings were away from the molecular core in T 1 states compared to S 0 states, which was also supported by the increased dihedral angles of their T 1 states (Table 1). The above results together with the high reaction yields of the metallization revealed that the molecular core of this 6/6/6 metallocycle system had a certain degree of flexibility, enabling the ligand to coordinate well with the central Pt(II) ion.…”

“…Near-infrared (NIR) emitters with emission peaks beyond 700 nm have great potential applications in photodynamic therapy, − in vivo bioimaging, − optical signal processing, and night-vision technologies. − However, because the main nonradiative deactivation pathway induces the quenching of NIR emission, which is known as the “energy gap law”, the photoluminescence quantum efficiencies (PLQEs, Φ) of most NIR emitters are low, typically less than 1%, such as phosphorescent d 6 , d 8 , and d 10 transition-metal complexes. − Therefore, the design and development of highly efficient NIR emitters remains a great challenge. In the past decades, Pt­(II) complexes had been demonstrated to act as efficient phosphorescent blue-to-red emitters because of the heavy-atom effect-induced strong spin-orbit coupling, which enabled efficient intersystem crossing (IC) from the lowest singlet (S 1 ) to triplet state (T 1 ) and then radiative decay to the ground state (S 0 ). − Recently, great progress had been made for thermally activated delayed fluorescence (TADF) NIR emitters and Pt­(II)-based NIR emitters through rational molecular design (Figure ). One strategy was to employ planar bidentate or tridentate Pt­(II) complexes, such as Pt­(fprpz) 2 , to form trimers or dimers in solid state, which enabled excimer-based NIR emission. − To obtain monomer NIR emission, extension of the conjugated system was another important strategy, such as the reported Pt­(O^N^N^O)-type complexes containing benzo­[ c ]­[1,2,5]­thiadiazole and triphenylaniline fragments, and also Pt­(II) porphyrin complexes. ,, Moreover, dinuclear Pt­(II) complex Pt 2 (bis-dthpym)­(dpm) 2 employing a multidentate ligand could also realize NIR emission with a high PLQE of 17% .…”