posted on 2019-07-31, 19:37authored byPremkumar Gnanasekaran, Yi Yuan, Chun-Sing Lee, Xiuwen Zhou, Alex K.-Y. Jen, Yun Chi
Bis-tridentate Ir(III) metal complexes
bring forth interesting photophysical properties, among which the
orthogonal arranged, planar tridentate chelates could increase the
emission efficiency due to the greater rigidity and, in the meantime,
allow strong interligand stacking that could deteriorate the emission
efficiency. We bypassed this hurdle by design of five bis-tridentate
Ir(III) complexes (1–5), to which
both of their monoanionic ancillary and dianionic chromophoric chelate
were functionalized derivative of 2-pyrazolyl-6-phenylpyridine, i.e.
pzpyphH2 parent chelate. Hence, addition of phenyl substituent
to the pyrazolyl fragment of pzpyphH2 gave rise to the
precursors of monoanionic chelate (A1H–A3H), on which the additional tert-butyl and/or methoxy groups were introduced at the
selected positions for tuning their steric and electronic properties,
while precursors of dianionic chelates was judiciously prepared with
an isoquniolinyl central unit on pziqphH2 in giving the
red-shifted emission (cf. L1H2 and L2H2). Factors
affected their photophysical properties were discussed by theoretical
methods based on DFT and TD-DFT calculation, confirming that the T1 excited state of all investigated Ir(III) complexes shows
a mixed metal-to-ligand charge transfer (MLCT), intraligand charge
transfer (ILCT), ligand-to-ligand charge transfer (LLCT), and ligand-centered
(LC) transition character. In contrast, the poor quantum yield of 3 is due to the facilitation of the nonradiative decay in
comparison to the radiative process. As for potential OLED applications,
Ir(III) complex 2 gives superior performance with max.
efficiencies of 28.17%, 41.25 cd·A–1 and 37.03
lm·W–1, CIEx,y = 0.63, 0.37 at 50 mA cm–2, and
small efficiency roll-off.