Binuclear Cyclometalated Platinum(II) 4,6-Diphenyl-2,2‘-bipyridine Complexes: Interesting Photoluminescent and Optical Limiting Materials

Abstract

The UV−vis spectra, emission spectra and lifetimes, transient absorption characteristics, and optical limiting performances of three binuclear cyclometalated platinum(II) 4,6-diphenyl-2,2‘-bipyridine complexes with bis(diphenylphosphino)methane (dppm), bis(diphenylphosphino)ethane (dppe), and bis(diphenylphosphino)propane (dppp) bridging ligands have been investigated. All three complexes exhibit concentration-dependent photoluminescence in CH3CN at room temperature and 77 K, and the emission energy is affected by the length of the bridging ligand. [Pt2L2(μ-dppm)](ClO4)2 (1) (L = 4,6-diphenyl-2,2‘-bipyridine) shows a broad, structureless emission band at about 667 nm when the complex concentration is higher than 6.0 × 10-5 mol/L, which can be attributed to a 3[dσ*,π*] state due to metal−metal interactions. [Pt2L2(μ-dppe)](ClO4)2 (2) and [Pt2L2(μ-dppp)](ClO4)2 (3) essentially exhibit no metal−metal interactions between the two platinum centers, and their emission can be ascribed to a 3MLCT (metal-to-ligand charge transfer) excited state. The emission lifetime is approximately 200 ns for 1 at 1.2 × 10-4 mol/L, ∼1.5 μs for 2 at 1.4 × 10-4 mol/L, and ∼2.0 μs (68%) and ∼0.4 μs (32%) for 3 at 1.3 × 10-4 mol/L. All complexes show moderately intense, broad positive transient difference absorption bands from near-UV and extending to near-IR spectral regions. The nonlinear transmission experiment at 532 nm using 4.1 ns laser pulses demonstrates that 2 and 3 exhibit stronger optical limiting for nanosecond laser pulses than SiNc, which is likely associated with their very low ground-state absorption cross sections and relatively long triplet excited-state lifetimes (approximately microseconds).