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

Abstract

To modulate phosphorescent characteristics of Pt2Au4 complexes with differently positioned bis(acetylide)naphthalene as a bridging ligand, four isomeric Pt2Au4 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 Pt2Au4 complexes are strongly phosphorescent with the quantum yields as high as 52% in CH2Cl2 solutions, in strikingly contrast to weak phosphorescence of bis(acetylide)naphthalene-linked Pt2 precursors. After the formation of Pt2Au4 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 Pt2Au4 isomers in CH2Cl2 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 Pt2Au4 cluster because the energy harvested by two PtAu2 cluster moieties is effectively transferred to bridging bis(acetylide)naphthalene. Taking advantage of Pt2Au4 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).