Near infra-red luminescence from bis-terpyridyl iridium(III) complexes incorporating electron-rich pendants

Abstract

Iridium(III) bis-terpyridine complexes have been prepared which incorporate aminobenzene or pyrene substituents at the 4′-position of the terpyridine. The homoleptic complexes, [Ir(L 1) 2] 3+ and [Ir(L 2) 2] 3+ {L 1 = 4′-( p-Me 2N-C 6H 4)-2,2′:6′,2″-terpyridine; L 2 = 4′-( p-Ph 2N-C 6H 4)-2,2′:6′,2″-terpyridine} display an intense absorption band in the visible region, which exhibits slight positive solvatochromism, due to an excited state of intra-ligand charge transfer (ILCT) character, in which the aminobenzene unit acts as the electron donor and the metal-bound terpyridyl unit as the acceptor. The complexes are weakly luminescent in the near infra-red ( λ max 750–800 nm); this short-lived, low-energy emission, whose likely origin is an excited state of primarily ILCT character, is in contrast to the long-lived 3π–π* luminescence of previously studied bis-terpyridyl iridium complexes. The corresponding ILCT absorption band in the heteroleptic, pyrene-substituted complex, [Ir(tpy)L 3] 3+ {L 3 = 4′-(1-pyrenyl)-2,2′:6′,2″-terpyridine}, displays negative solvatochromism, owing to the excited state having lower polarity than the ground state. This complex emits weakly at slightly higher energy (710 nm). The switch to ILCT character upon appending the [Ir(tpy) 2] 3+ core with a pyrene substituent may be compared to the influence of the pyrene unit in [PtL 3Cl] +, but contrasts with the triplet–triplet equilibrium that is established in structurally related ruthenium complexes incorporating pyrene pendants. The electrochemical properties of the pyrene-substituted ligand and complex in solution are also described.