On the Importance of Ligand-Centered Excited States in the Emission of Cyclometalated Ir(III) Complexes.
Inorg Chem 2021;
60:13222-13232. [PMID:
34492762 PMCID:
PMC8424641 DOI:
10.1021/acs.inorgchem.1c01604]
[Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Abstract
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The photophysical
behavior of the cyclometalating Ir(III) complexes
[Ir(ppy)2(bpy)]+, where Hppy is 2-phenylpyridine
and bpy is 2,2′-bipyridine (complex 1), and [Ir(diFppy)2(dtb-bpy)]+, where diFppy is 2-(2,4-difluorophenyl)pyridine
and dtb-bpy is 4,4′-di-tert-butyl-2,2′-bipyridine
(complex 2), has been theoretically investigated by performing
density functional theory calculations. The two complexes share the
same molecular skeleton, complex 2 being derived from
complex 1 through the addition of fluoro and tert-butyl substituents, but present notable differences
in their photophysical properties. The remarkable difference in their
emission quantum yields (0.196 for complex 1 in dichloromethane
and 0.71 for complex 2 in acetonitrile) has been evaluated
by characterizing both radiative and nonradiative decay paths. It
has emerged that the probability of decaying through the nonradiative
triplet metal-centered state, normally associated with the loss of
the emission quantum yield, does not appear to be the reason behind
the reported substantially different emission efficiency. A more critical
factor appears to be the ability of complex 2 to emit
from both the usual metal-to-ligand charge-transfer state and from
two additional ligand-centered states, as supported by the fact that
the respective minima belong to the potential energy surface of the
lowest triplet T1 state and that their phosphorescence
lifetimes are in the same order of magnitude. In contrast, the emission
of complex 1 can be originated only from the metal-to-ligand
charge-transfer state, being the only emissive T1 minimum.
The results constitute a significant case in which the emission from
ligand-centered states is the key for determining the high emission
quantum yield of a complex.
The
reasons behind the significant increase in the emission
quantum yield of the [Ir(diFppy)2(dtb-bpy)]+ complex with respect to [Ir(ppy)2(bpy)]+ are
rationalized on the basis of DFT, TDDFT, and TDDFT-SOC calculations,
revealing the key role that low-lying LC states can play in the emission
properties of the complex.
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