Photophysics and nonlinear absorption of cyclometalated 4,6-diphenyl-2,2′-bipyridyl platinum(II) complexes with different acetylide ligands

The photophysical properties of a series of 4,6-diphenyl-2,2′- bipyridyl platinum(II) complexes bearing different σ-alkynyl ancillary ligands (1a-1k) were systematically investigated. All complexes exhibit strong ¹π,π* absorption bands in the UV region; and broad, structureless charge-transfer band(s) in the visible region, which systematically red-shift(s) when the electron-donating ability of the para substituent on the phenylacetylide ligand increases. All complexes are emissive in solution at room temperature. When excited at the charge-transfer absorption band, the complexes exhibit long-lived orange emission (λ_max: 555 - 601 nm), which is attributed to a triplet metal-to-ligand charge transfer/intraligand charge transfer emission (³MLCT/ ³ILCT). Most of these complexes exhibit broad triplet transient difference absorption in the visible to the near-IR region, with a lifetime comparable to those measured from the decay of the ³MLCT/ ³ILCT emission. The reverse saturable absorption (RSA) of these complexes were demonstrated at 532 nm using nanosecond laser pulses. The degree of RSA follows this trend: 1k ≈ 1a > 1c > 1f ≈ 1i > 1h ≈ 1b > 1e > 1d > 1g, which is mainly determined by the ratio of the triplet excited-state absorption cross section to that of the ground-state and the triplet excited-state quantum yield.