1 research outputs found
Highly Luminescent Dinuclear Platinum(II) Complexes Incorporating Bis-Cyclometallating Pyrazine-Based Ligands: A Versatile Approach to Efficient Red Phosphors
No full textA series of luminescent
dinuclear platinum(II) complexes incorporating
diphenylpyrazine-based bridging ligands (L<sup><i>n</i></sup>H<sub>2</sub>) has been prepared. Both 2,5-diphenylpyrazine (L<sup>2</sup>H<sub>2</sub>) and 2,3-diphenylpyrazine (L<sup>3</sup>H<sub>2</sub>) are able to undergo cyclometalation of the two phenyl rings,
with each metal ion binding to the two nitrogen atoms of the central
heterocycle, giving, after treatment with the anion of dipivaloyl
methane (dpm), complexes of formula {Pt(dpm)}<sub>2</sub>L<sup><i>n</i></sup>. These compounds are isomers of the analogous complex
of 4,6-diphenylpyrimidine (L<sup>1</sup>H<sub>2</sub>). Related complexes
of dibenzo(f,h)quinoxaline (L<sup>4</sup>H<sub>2</sub>), 2,3-diphenyl-quinoxaline
(L<sup>5</sup>H<sub>2</sub>), and dibenzo[3,2-a:2′,3′-c]phenazine
(L<sup>6</sup>H<sub>2</sub>) have also been prepared, allowing the
effects of strapping together the phenyl rings (L<sup>4</sup>H<sub>2</sub> and L<sup>6</sup>H<sub>2</sub>) and/or extension of the conjugation
from pyrazine to quinoxaline (L<sup>5</sup>H<sub>2</sub> and L<sup>6</sup>H<sub>2</sub>) to be investigated. In all cases, the corresponding
mononuclear complexes, Pt(dpm)L<sup><i>n</i></sup>H, have
been isolated too. All 12 complexes are phosphorescent in solution
at ambient temperature. Emission spectra of the dinuclear complexes
are consistently red shifted compared to their mononuclear analogues,
as are the lowest energy absorption bands. Electrochemical data and
TD-DFT calculations suggest that this effect arises primarily from
stabilization of the LUMO. Introduction of the second metal ion also
has the effect of substantially increasing the molar absorptivity
and, in most cases, the radiative rate constants. Meanwhile, extension
of conjugation in the heterocycle of L<sup>5</sup>H<sub>2</sub> and
L<sup>6</sup>H<sub>2</sub> and planarization of the aromatic system
favored by interannular bond formation in L<sup>4</sup>H<sub>2</sub> and L<sup>6</sup>H<sub>2</sub> leads to further red shifts of the
absorption and emission spectra to wavelengths that are unusually
long for cyclometalated platinum(II) complexes. The results may offer
a versatile design strategy for tuning and optimizing the optical
properties of d-block metal complexes for contemporary applications
