ic9b00403_si_001.pdf (625.18 kB)
Structure–Emission Property Relationships in Cyclometalated Pt(II) β‑Diketonate Complexes
journal contribution
posted on 2019-04-25, 14:39 authored by Adrian Heil, Christel M. MarianExtending
the ligand π-system of phosphorescent (C∧C*)
or (C∧N) cyclometalated platinum(II) β-diketonate
complexes can lead to large and seemingly abrupt variations of the
photophysical properties such as triplet quantum yields and phosphorescence
lifetimes. Quantum chemical studies using methods including elements
from density functional theory (DFT) and multireference configuration
interaction (MRCI) as well as spin–orbit coupling (SOC) provide
a rationale for these observations. In the Franck–Condon region,
the first excited singlet states (S1) of these complexes
are characterized by mixed metal-to-ligand charge-transfer (MLCT)
and ligand-centered (LC) excitations. With increasing extension of
the effective π-system, the lowest-lying triplet state yields
more and more LC character, thus leading to a decrease of the phosphorescence
rate constant. The ability to undergo efficient intersystem crossing
from S1 to T1 is not diminished as the S1 state largely retains its character. In the N-heterocyclic
carbene (NHC) complexes investigated here, at least two triplet states
are found energetically below the S1 state. Out-of-plane
distortion enhances the probability for nonradiative decay of the
triplet population. In the smaller compounds emitting in the violet
or blue spectral region, the phosphorescent state is separated from
the lowest-lying dark metal-centered (MC) triplet state by a small
barrier only, explaining their experimentally observed low photoluminescence
quantum yields in liquid solution. The semiempirical DFT/MRCI-R2018
Hamiltonian employed in our studies proves well-suited for investigating
the absorption and emission properties of these platinum(II) complexes.
Generally, good agreement is observed between our calculated data
and the experimental findings.