Triplet Emitting C^N^C Cyclometalated Dibenzo[c,h]Acridine Pt(II) Complexes

In a series of Pt(II) complexes [Pt(dba)(L)] containing the very rigid, dianionic, bis-cyclometalating, tridentate C^N^C(2−) heterocyclic ligand dba(2–) (H(2)dba = dibenzo[c,h]acridine), the coligand (ancillary ligand) L = dmso, PPh(3), CNtBu and Me(2)Imd (N,N’-dimethylimidazolydene) was varied in order to improve its luminescence properties. Beginning with the previously reported dmso complex, we synthesized the PPh(3), CNtBu and Me(2)Imd derivatives and characterized them by elemental analysis, (1)H (and (31)P) NMR spectroscopy and MS. Cyclic voltammetry showed partially reversible reduction waves ranging between −1.89 and −2.10 V and increasing along the series Me(2)Imd < dmso ≈ PPh(3) < CNtBu. With irreversible oxidation waves ranging between 0.55 (L = Me(2)Imd) and 1.00 V (dmso), the electrochemical gaps range between 2.65 and 2.91 eV while increasing along the series Me(2)Imd < CNtBu < PPh(3) < dmso. All four complexes show in part vibrationally structured long-wavelength absorption bands peaking at around 530 nm. TD-DFT calculated spectra agree quite well with the experimental spectra, with only a slight redshift. The photoluminescence spectra of all four compounds are very similar. In fluid solution at 298 K, they show broad, only partially structured bands, with maxima at around 590 nm, while in frozen glassy matrices at 77 K, slightly blue-shifted (~580 nm) bands with clear vibronic progressions were found. The photoluminescence quantum yields Φ(L) ranged between 0.04 and 0.24, at 298 K, and between 0.80 and 0.90 at 77 K. The lifetimes τ at 298 K ranged between 60 and 14040 ns in Ar-purged solutions and increased from 17 to 43 µs at 77 K. The TD-DFT calculated emission spectra are in excellent agreement with the experimental findings. In terms of high Φ(L) and long τ, the dmso and PPh(3) complexes outperform the CNtBu and Me(2)Imd derivatives. This is remarkable in view of the higher ligand strength of Me(2)Imd, compared with all other coligands, as concluded from the electrochemical data.