Design, synthesis and functionalization of luminescent alkynyl compounds of boron(III) and gold(III) with bidentate and tetradentate ligands : from photophysics to organic light-emitting device application studies

Abstract

A series of spirofluorene-containing bis(alkynyl)borane(III) compounds with different substituents on the pyridine ring and different π-conjugation lengths has been synthesized and characterized. The X-ray crystal structures of selected compounds have been successfully determined. The structureless lower-energy absorption band was assigned as intramolecular charge transfer (ICT) transition. The emission color of these compounds could be easily tuned over the visible spectrum from blue to green. The broad and structureless emission band was derived from the ICT excited state. Besides, these compounds demonstrated a very high photoluminescence quantum yield (PLQY) of up to 0.81 and 0.86 in fluid solution and doped thin film, respectively. Solution-processed organic light-emitting devices (OLEDs) with blue, green and red electroluminescence (EL) have been realized.

The highly emissive nature of these bis(alkynyl)borane(III) compounds have led to the further exploration of the incorporation of the emissive N^C ligands into the gold(III) center, to gain access into the triplet emissive state. A series of donor-acceptor type bis(alkynyl)gold(III) N^C complexes have been synthesized and characterized. One of the X-ray crystal structures of these compounds has been determined. The lower-energy absorption was assigned as the intraligand (IL) transition with mixing of some CT character, or intraligand charge transfer (ILCT) [π(arylamine) → π*(pyridine)] transition. Complexes with only fluorene donor displayed dual emission, derived from the 1IL and 3IL [π → π* (C^N)] excited state. Complexes with arylamine donor showed a structureless emission band, derived from the spin-forbidden ILCT [π(arylamine) → π*(pyridine)] excited state. High PLQYs of 0.41 and 0.81 have also been realized in the respective dichloromethane solution and doped thin film. Solution-processed OLEDs based on one of the complexes exhibited yellow EL with EQEs of up to 4.0 %. Interesting mechanochromic behaviors have also been observed in the arylamine-containing complexes. The tuning of the piezo-luminescence color has also been realized by changing the substituents on the pyridine ring.

In addition, a new class of tetradentate alkynylgold(III) complexes has been synthesized and characterized. Two X-ray crystal structures of these complexes have been determined. The lower-energy absorption band of these complexes was assigned as the metal-perturbed 3IL [π → π*(C^N)] transition with mixing of some CT character, or metal-perturbed 3ILCT [π(arylamine) → π*(phenylpyridine)] transition. All the complexes showed an intense emission with PLQYs of up to 0.60 and 0.78 in dichloromethane solution and doped thin film, respectively, which were at least an order of magnitude higher than those of other structurally related uncyclized tridentate alkynylgold(III) complexes. Complexes with no arylamine donor exhibited a vibrionic-structured emission, derived from the metal-perturbed 3IL [π  π* (C^N^C)] excitation with CT character. Complexes with arylamine displayed a structureless emission band, derived from a metal-perturbed 3ILCT [π(arylamine)  π*(diphenylpyridine)] excited state. Besides, the tetradentate gold(III) complexes exhibited a high Td of over 430 °C, much higher than the related tridentate complexes (Td ≈ 350 °C). OLEDs based on these tetradentate gold(III) complexes exhibited EL color ranging from green to orange-red with EQEs of up to 11.1 %, which outperformed those of devices based on the uncyclized counterparts.