Spectroscopic and Luminescence Studies on Square-Planar Platinum(II) Complexes with Anionic Tridentate 3-Bis(2-pyridylimino)isoindoline Derivatives

Reactions of 1,3-bis(2-pyridylimino)isoindoline (HL1), 1,3-bis(2-pyridylimino)benz(f)isoindoline (HL2), or 5,6-dihydro-2,3-diphenyl-5-(pyridin-2-ylimino)pyrrolo[3,4-b]pyrazin-7-ylidene)pyridin-2-amine (HL3) with Pt(tht)2Cl2 (tht = tetrahydrothiophene) afforded the corresponding Pt(L)Cl complexes. A series of neutral platinum(II) alkynyl complexes Pt(L)(CCR) were prepared by reactions of the precursors Pt(L)Cl with alkynyl ligands through CuI-catalyzed platinum acetylide σ coordination. Crystal structural determination of Pt(L3)Cl (3), Pt(L1)(CCPh) (4), and Pt(L1)(CCC6H4But-4) (6) by X-ray crystallography revealed that the neutral platinum(Π) complexes with monoanionic tridentate L ligands display more perfect square-planar geometry than that in platinum(II) complexes with neutral tridentate 2,2′:6′,2′′-terpyridyl ligands. Both the Pt(L)Cl and Pt(L)(CCC6H4R-4) complexes exhibit low-energy absorption at 400−550 nm, arising primarily from π → π*(L) intraligand (IL) and 5d(Pt) → π*(L) metal-to-ligand charge-transfer (MLCT) transitions as suggested from density functional theory calculations. They display bright-orange to red room-temperature luminescence in fluid dichloromethane solutions with microsecond to submicrosecond ranges of emissive lifetimes and 0.03−3.79% quantum yields, originating mainly from 3IL and 3MLCT excited states. Compared with the emissive state in Pt(L)Cl complexes, substitution of the coordinated Cl with CCC6H4R-4 in Pt(L)(CCC6H4R-4) complexes induces an obviously enhanced contribution from the 3[π(CCC6H4R-4) → π*(L)] ligand-to-ligand charge-transfer (LLCT) triplet state. The photophysical properties can be finely tuned by modifying both the L and alkynyl ligands. The calculated absorption and emission spectra in dichloromethane coincide well with those measured in a fluid dichloromethane solution at ambient temperature.