Binuclear cyclometalated platinum(II) 4,6-diphenyl-2,2′-bipyridine complexes:: Interesting photoluminescent and optical limiting materials

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(mu-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 x 10- 5 mol/L, which can be attributed to a(3)[d sigma*, pi*] state due to metalmetal interactions. [Pt2L2(mu-dppe)](ClO4)(2) (2) and [Pt2L2(mu-dppp)](ClO4)(2) (3) essentially exhibit no metalmetal interactions between the two platinum centers, and their emission can be ascribed to a (MLCT)-M-3 (metal-to-ligand charge transfer) excited state. The emission lifetime is approximately 200 ns for 1 at 1.2 x 10- 4 mol/L, similar to 1.5 mu s for 2 at 1.4 x 10- 4 mol/L, and similar to 2.0 mu s (68%) and similar to 0.4 mu s (32%) for 3 at 1.3 x 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).