Abstract

A series of highly conjugated, rigid Re(I)-based molecular rectangles {[fac-Re(CO)₃Br]₂(μ-bpy)(μ-L)}₂ (1, L = 1,4-bis(4′-pyridylethynyl)benzene, bpeb; 2, L = 1,4-bis(4′-pyridylethynyl)naphthalene, bpen; 3, L = 1,4-bis(4′-pyridylethynyl)anthracene, bpea; and bpy = 4,4′-bipyridine) containing two different types of pyridyl ligands were synthesized, characterized and their photophysical properties studied. Successful emission color tuning was achieved by incorporating rigid alkynyl ligands into the Re(I) rectangles.  Complexes 1-3 exhibited an intense absorption bands with a high e value at > 340 nm in THF solution, which is attributed to mixed two metal-to-ligand charge transfer dπ(Re) → π*(bpy) and dπ(Re) → π*(alkynyl)) along with ligand-to-ligand  charge transfer (¹LLCT)/ and intraligand charge transfer (¹ILCT) transitions. Compound 1 featured a broad and structureless emission band at 619 nm, which was attributed to the emission of ³MLCT dπ(Re) → π*(bpy) and/or [dπ(Re) → π*(alkynyl)] characteristics with an additional luminescence at 431 nm. Whereas complexes 2 and 3 displayed an intraligand (IL) emission at 445 and 489(sh), 521 nm. These compounds represent a new class of visible light-harvesting materials that exhibit greatly enhanced emission decay lifetimes as a result of intervening ligand triplet states (³LLCT/³ILCT) present on the alkynyl appended naphthalene and anthracene chromophores, as evidenced by transient absorption spectra.