Unravelling the Mechanism of Excited-State Interligand Energy Transfer and the Engineering of Dual Emission in [Ir(C∧N)2(N∧N)]+ Complexes

Abstract

Fundamental insights into the mechanism of triplet-excited-state interligand energy transfer dynamics and the origin of dual emission for phosphorescent iridium(III) complexes are presented. The complexes [Ir(C∧N)2(N∧N)]+ (HC∧N = 2-phenylpyridine (1a-c), 2-(2,4-difluorophenyl)pyridine (2a-c), 1-benzyl-4-phenyl-1,2,3-triazole (3a-c); N∧N = 1-benzyl-4-(pyrid-2-yl)-1,2,3-triazole (pytz, a), 1-benzyl-4-(pyrimidin-2-yl)-1,2,3-triazole (pymtz, b), 1-benzyl-4-(pyrazin-2-yl)-1,2,3-triazole (pyztz, c)) are phosphorescent in room-temperature fluid solutions from triplet metal-to-ligand charge transfer (3MLCT) states admixed with either ligand-centered (3LC) (1a, 2a, and 2b) or ligand-to-ligand charge transfer (3LL′CT) character (1c, 2c, and 3a-c). Particularly striking is the observation that pyrimidine-based complex 1b exhibits dual emission from both 3MLCT/3LC and 3MLCT/3LL′CT states. At 77 K, the 3MLCT/3LL′CT component is lost from the photoluminescence spectra of 1b, with emission exclusively arising from its 3MLCT/3LC state, while for 2c switching from 3MLCT/3LL′CT- to 3MLCT/3LC-based emission is observed. Femtosecond transient absorption data reveal distinct spectral signatures characteristic of the population of 3MLCT/3LC states for 1a, 2a, and 2b which persist throughout the 3 ns time frame of the experiment. These 3MLCT/3LC state signatures are apparent in the transient absorption spectra for 1c and 2c immediately following photoexcitation but rapidly evolve to yield spectral profiles characteristic of their 3MLCT/3LL′CT states. Transient data for 1b reveals intermediate behavior: the spectral features of the initially populated 3MLCT/3LC state also undergo rapid evolution, although to a lesser extent than that observed for 1c and 2c, behavior assigned to the equilibration of the 3MLCT/3LC and 3MLCT/3LL′CT states. Density functional theory (DFT) calculations enabled minima to be optimized for both 3MLCT/3LC and 3MLCT/3LL′CT states of 1a-c and 2a-c. Indeed, two distinct 3MLCT/3LC minima were optimized for 1a, 1b, 2a, and 2b distinguished by upon which of the two C∧N ligands the excited electron resides. The 3MLCT/3LC and 3MLCT/3LL′CT states for 1b are very close in energy, in excellent agreement with experimental data demonstrating dual emission. Calculated vibrationally resolved emission spectra (VRES) for the complexes are in excellent agreement with experimental data, with the overlay of spectral maxima arising from emission from the 3MLCT/3LC and 3MLCT/3LL′CT states of 1b convincingly reproducing the observed experimental spectral features. Analysis of the optimized excited-state geometries enable the key structural differences between the 3MLCT/3LC and 3MLCT/3LL′CT states of the complexes to be identified and quantified. The calculation of interconversion pathways between triplet excited states provides for the first time a through-space mechanism for a photoinduced interligand energy transfer process. Furthermore, examination of structural changes between the possible emitting triplet excited states reveals the key bond vibrations that mediate energy transfer between these states. This work therefore provides for the first time detailed mechanistic insights into the fundamental photophysical processes of this important class of complexes.