Modulating Narrow-Band Phosphorescence of Pt₂Au₄ Cluster Complexes by Differently Positioned Bis(acetylide)-Naphthalene Linkers

To modulate phosphorescent characteristics of Pt 2 Au 4 complexes with differently positioned bis(acetylide)naphthalene as a bridging ligand, four isomeric Pt 2 Au 4 complexes ( 1 – 4 ) linked by 1,4-bis(acetylide)naphthalene ( 1 ), 1,5-bis(acetylide)naphthalene ( 2 ), 2,6-bis(acetylide)naphthalene ( 3 ), and 2,7-bis(acetylide)naphthalene ( 4 ) are elaborately designed and prepared. The Pt 2 Au 4 complexes are strongly phosphorescent with the quantum yields as high as 52% in CH 2 Cl 2 solutions, in strikingly contrast to weak phosphorescence of bis(acetylide)naphthalene-linked Pt 2 precursors. After the formation of Pt 2 Au 4 cluster structures, the increased molecular rigidity dramatically suppresses nonradiative relaxation, which becomes less competitive in contrast to the radiative relaxation of the emitting state, thus resulting in a remarkable enhancement of the quantum yields. Depending on the positions of bis(acetylide) groups attached to naphthalene rings, the phosphorescence of Pt 2 Au 4 isomers in CH 2 Cl 2 solutions follows 1 (640 nm) → 2 (582 nm) → 3 (567 nm) → 4 (529 nm) to show progressive blue shifts. As demonstrated by photophysical and theoretical studies, vibronic-structured narrow-band emission with a full width at half maxima (fwhm) of 707–1073 cm –1 is primarily ascribable to ligand-centered triplet states localized at bis(acetylide)naphthalenes, which is dramatically activated upon the formation of the Pt 2 Au 4 cluster because the energy harvested by two PtAu 2 cluster moieties is effectively transferred to bridging bis(acetylide)naphthalene. Taking advantage of Pt 2 Au 4 complexes 1 – 4 as phosphorescent emitters, solution-processed OLEDs achieve narrow-band electroluminescence with peak external quantum efficiency of 11.0% for red-emitting complex 1 (fwhm = 45 nm or 1058 cm –1 ), 9.4% for orange-emitting complex 2 (fwhm = 33 nm or 960 cm –1 ), 13.7% for yellow-emitting complex 3 (fwhm = 33 nm or 1029 cm –1 ), and 15.2% for green-emitting complex 4 (fwhm = 24 nm or 848 cm –1 ).