Dinuclear Design of a Pt(II) Complex Affording Highly Efficient Red Emission: Photophysical Properties and Application in Solution-Processible OLEDs

Abstract

The light-emitting efficiency of luminescent materials is invariably compromised on moving to the red and near-infrared regions of the spectrum due to the transfer of electronic excited-state energy into vibrations. We describe how this undesirable "energy gap law" can be sidestepped for phosphorescent organometallic emitters through the design of a molecular emitter that incorporates two platinum(II) centers. The dinuclear cyclometallated complex of a substituted 4,6-bis(2-thienyl)pyrimidine emits very brightly in the red region of the spectrum (λ_max = 610 nm, Φ = 0.85 in deoxygenated CH₂Cl₂ at 300 K). The lowest-energy absorption band is extraordinarily intense for a cyclometallated metal complex: at λ = 500 nm, ε = 53 800 M^-1 cm^-1. The very high efficiency of emission achieved can be traced to an unusually high rate constant for the T₁ → S₀ phosphorescence process, allowing it to compete effectively with nonradiative vibrational decay. The high radiative rate constant correlates with an unusually large zero-field splitting of the triplet state, which is estimated to be 40 cm^-1 by means of variable-temperature time-resolved spectroscopy over the range 1.7 < T < 120 K. The compound has been successfully tested as a red phosphor in an organic light-emitting diode prepared by solution processing. The results highlight a potentially attractive way to develop highly efficient red and NIR-emitting devices through the use of multinuclear complexes.