Photophysical properties of trans-platinum acetylide complexes featuring N-heterocyclic carbene ligands

Abstract

A series of trans-N-heterocyclic carbene (NHC) platinum(II) acetylide complexes of the form (ICy)₂Pt(R)₂ (where ICy = 1,3-bis-(cyclohexyl)imidazol-2-ylidene and R = 1-Ethynyl-4-(phenylethynyl)benzene (PE2), 2-(9,9-Diethyl-9H-fluoren-7-yl)benzo[d]thiazole (BTF), and 9,9-Diethyl-7-ethynyl-N,N-diphenyl-9H-fluoren-2-amine (DPAF), 2a–c respectively), were synthesized via Hagihara reaction of the unprotected aryl-acetylide ligands with trans-(ICy)₂PtCl₂ (1) in 47–73% yield. Precursor 1 was generated in a one-pot synthesis via formation of a silver carbene precursor followed by transmetallation, and was obtained in high yield (95%). The single-crystal X-ray structures of 1, 2a–c were determined and analyzed. The photophysical properties of 2a–c were compared to their respective tributyl phosphine (PBu₃) analogues. The optical properties of the series were studied by UV-Vis spectroscopy, photoluminescence spectroscopy, nanosecond transient absorption spectroscopy, and open aperture nanosecond z-scan. Coupling of the organic chromophores to the platinum center affords efficient intersystem crossing as concluded by the complexes’ low fluorescence quantum yields, efficient phosphorescence and intense T₁ − T_n absorption. Open aperture z-scan with 606 nm, 10 ns laser pulses showed comparable optical attenuation relative to a standard sample of (PBu₃)₂Pt(DPAF)₂ (3c). Pulse limiting was achieved via a dual-mechanism of two-photon absorption (2PA) coupled with excited-state absorption (ESA). TD-DFT Computations were also employed for 2a–c to give greater insight into the nature of the singlet-singlet electronic transitions.