Unravelling the Mechanism of Excited-State Interligand Energy Transfer and the Engineering of Dual Emission in [Ir(CN)(NN)] Complexes.

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(CN)(NN)] (HCN = 2-phenylpyridine (-), 2-(2,4-difluorophenyl)pyridine (-), 1-benzyl-4-phenyl-1,2,3-triazole (-); NN = 1-benzyl-4-(pyrid-2-yl)-1,2,3-triazole (pytz, ), 1-benzyl-4-(pyrimidin-2-yl)-1,2,3-triazole (pymtz, ), 1-benzyl-4-(pyrazin-2-yl)-1,2,3-triazole (pyztz, )) are phosphorescent in room-temperature fluid solutions from triplet metal-to-ligand charge transfer (MLCT) states admixed with either ligand-centered (LC) (, , and ) or ligand-to-ligand charge transfer (LL'CT) character (, , and -). Particularly striking is the observation that pyrimidine-based complex exhibits dual emission from both MLCT/LC and MLCT/LL'CT states. At 77 K, the MLCT/LL'CT component is lost from the photoluminescence spectra of , with emission exclusively arising from its MLCT/LC state, while for switching from MLCT/LL'CT- to MLCT/LC-based emission is observed. Femtosecond transient absorption data reveal distinct spectral signatures characteristic of the population of MLCT/LC states for , , and which persist throughout the 3 ns time frame of the experiment. These MLCT/LC state signatures are apparent in the transient absorption spectra for and immediately following photoexcitation but rapidly evolve to yield spectral profiles characteristic of their MLCT/LL'CT states. Transient data for reveals intermediate behavior: the spectral features of the initially populated MLCT/LC state also undergo rapid evolution, although to a lesser extent than that observed for and , behavior assigned to the equilibration of the MLCT/LC and MLCT/LL'CT states. Density functional theory (DFT) calculations enabled minima to be optimized for both MLCT/LC and MLCT/LL'CT states of - and -. Indeed, two distinct MLCT/LC minima were optimized for , , , and distinguished by upon which of the two CN ligands the excited electron resides. The MLCT/LC and MLCT/LL'CT states for 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 MLCT/LC and MLCT/LL'CT states of convincingly reproducing the observed experimental spectral features. Analysis of the optimized excited-state geometries enable the key structural differences between the MLCT/LC and MLCT/LL'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.