FULL PAPER DOI:10.1002/ejic.201402495 Elucidating the Origin of Enhanced Phosphorescence Emission in the Solid State (EPESS) in Cyclometallated Iridium Complexes Ashlee J. Howarth, [a] Raissa Patia, [b] David L. Davies,* [b] Francesco Lelj,* [c] Michael O. Wolf,* [a] and Kuldip Singh [b] Keywords: EPESS / Phosphorescence / Density functional calculations / Pi interactions / Iridium A new mechanism for enhanced phosphorescence emission in the solid state (EPESS) in cyclometallated Ir complexes with the general formula [Ir(C ∧ N) 2 (N ∧ O)] involving distor- tion of the six-membered chelate ring of the ancillary ligand Introduction Organic light-emitting diodes (OLEDs) and light-emit- ting electrochemical cells (LECs) require molecules that emit intensely in the solid state. [1,2] Interactions between molecules in the solid state are known to influence emission behaviour. Typically, molecules that are strongly emissive in dilute solution become less emissive in concentrated solu- tions or in the solid state due to “aggregation caused quenching” (ACQ). [3] In 2001, Tang and co-workers re- ported a series of substituted 2,3,4,5-tetraphenylsiloles in which aggregation caused an enhancement in emission in the solid state compared to dilute solution, a phenomenon they dubbed “aggregation induced emission” (AIE). [4,5] Since this discovery, many organic chromophores exhibiting AIE have been reported. [6] Recently, coordination com- plexes of iridium, [7] platinum [8] and rhenium [9] that show AIE have also been reported. In such complexes, emission often arises from states with triplet character, so AIE in this class of compounds is also called enhanced phosphores- cence emission in the solid state (EPESS). The performance of solid-state molecular devices depends strongly on the molecular assembly of components. As a consequence, un- derstanding and controlling molecular arrangements in the solid state is pertinent to these applications. [10] [a] Department of Chemistry, University of British Columbia Vancouver, BC V6T 1Z1, Canada E-mail: mwolf@chem.ubc.ca http://groups.chem.ubc.ca/wolf/ [b] Department of Chemistry, University of Leicester Leicester, LE1 7RH, UK http://www2.le.ac.uk/departments/chemistry/people/academic- staff/david_l_davies [c] Dipartimento di Scienze, Università della Basilicata 85100 Potenza, Italy Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ejic.201402495. Eur. J. Inorg. Chem. 2014, 3657–3664 © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 3657 is proposed. Photophysical and computational studies show that neither π-stacking nor restricted rotation cause the ob- served EPESS in these complexes and that ligand distortions in the triplet excited state are responsible for EPESS EPESS in metal complexes has been attributed to a vari- ety of factors including restricted intramolecular rotation (RIR), [11] or π-stacking. [7] However, the complexity of the excited state manifolds in these complexes makes unambig- uous determination of the origin of EPESS difficult. Cyclo- metallated iridium complexes are of particular interest due to their high photoluminescence efficiencies and the ability to colour tune their emission. [12] Two different mechanisms have been proposed to drive EPESS in cyclometallated irid- ium complexes. One mechanism involves restricted intramo- lecular rotations of substituents on the bidentate ancillary ligand (N ∧ O [11] or N ∧ N [13] ) and the other involves π-stack- ing of cyclometallating phenylpyridine ligands. [7,14,15] Park et al. proposed that restricted intramolecular rota- tion around the N–aryl bond of salicylimine ligands in the solid state suppresses a non-radiative decay pathway giving rise to EPESS. [11] The solid-state absorption and lumines- cence properties of 1–4 were studied in neat films as well as in various polymer films. [11] Due to the presence of strong solid-state emission in the polymer films of 1–4, it was con- cluded that the solid-state emission does not arise from an excimeric or aggregated state. [11] Instead, a combination of low temperature emission and TD-DFT studies were used to conclude that rotation around the N–aryl bond in solu- tion gives rise to a non-radiative decay pathway causing these complexes to be non-emissive in solution. [11] It was proposed that this pathway is slowed down or shut off in the solid state giving rise to the observed emission. [11] Li et al. have suggested that π-stacking of phenylpyridine ligands lowers the energy of an emissive 3 MLLCT state be- low that of a non-emissive triplet ligand ( 3 L) state, resulting in an increase in emission in the solid state compared to solution. [7] This explanation was first proposed from studies of 5–7, where 5 does not show EPESS in contrast to 6 and 7. [7] The triplet energies of the ancillary ligands ( 3 L) of 5–7