Ligand-Tuned Photophysical Properties and Geometrical Isomerism of Bis-Cyclometalated [Ir(C^N)2(CNXyl)X] (X = Cl, C≡CR) Complexes

Abstract

This study reports the synthesis, characterization, and photophysical and electrochemical properties of several bis-cyclometalated Ir(III) complexes based on the three-ligand molecular framework [Ir(C^N)2(CNXyl)(C≡CR)] (CNXyl = 2,6-dimethylphenyl isocyanide), varying the cyclometalated chromophore [C^N = 2,4-difluorophenylpyridinate (dfppy) 1, 2-phenylbenzothiazolate (pbt) 2, and phenylquinolinate (pq) 3] and the alkynyl ligand [R = Tol a, C6H2(OMe)3b, and pyrene c], obtained from the corresponding [Ir(C^N)2(CNXyl)Cl] (1Cl–3Cl). By synthetic control using dfppy, four diastereomers of the precursor [Ir(dfppy)2(CNXyl)Cl] (N,N-trans -1Cl, N,N-cis -( OC -6–24)/( OC -6–44)-1Cl and C,C-trans -1Cl) were isolated, enabling access to three geometrical isomers (N,N-trans -1a/1b, N,N-cis -( OC -6–32)-1a/1b, and C,C-trans -1a/1b) of [Ir(dfppy)2(CNXyl)(C≡CR)] [R = Tol a, C6H2(OMe)3b]. Complexes of the N,N-trans- series (1a,1b–3a,3b) exhibit blue to orange phosphorescence, depending on the cyclometalated ligand with quantum yields (ϕ) up to 47% in deoxygenated solution and 28.8% in PS films. Experimental and DFT calculations indicate a change in the nature of the low-lying excited state from 3ILCT/3MLCT in the chloride complexes to 3L′LCT(C≡CR→C∧N) with some 3MLCT for the C≡CR derivatives. Geometrical isomerism has little effect on emission energy, but significantly influences efficiency, with better results for N,N-trans isomers. The pyrene-substituted alkynyl complexes N,N-trans -1c–3c exhibit three different emission bands arising from 3L′pyrLCT/3MLCT, 3IL′pyr, and 1IL′pyr, depending of the cyclometalated ligand and media.