Brightening otherwise-weak molecular electroluminescence via intermolecular energy transfer

Manipulating the electroluminescence of organic molecules is important for the development of advanced organic light-emitting diodes (OLEDs). Here, we demonstrate a single-molecule sensitization strategy to brighten an otherwise-weak molecular emitter by using the scanning tunneling microscope induced luminescence (STML) technique. We show that while the free-base phthalocyanine (H2Pc) molecule is a bright emitter, the molecule upon double deprotonation ([Pc]2−) exhibits electroluminescence that is suppressed by three orders of magnitude. The extremely weak emission of [Pc]2− is traced to a misalignment of its frontier orbitals with the substrate, which fundamentally shifts the excitation from an efficient carrier-injection mechanism in H2Pc to an inefficient inelastic electron scattering process in [Pc]2−. However, by bringing a zinc-phthalocyanine (ZnPc) molecule close to it to form a donor–acceptor dimer (ZnPc–[Pc]2−), we introduce an intermolecular energy transfer pathway that enhances the luminescence of [Pc]2− by approximately 135-fold. Furthermore, combined with theoretical calculations, the dependence of STML spectra on the intermolecular distances (d) indicates that the energy-transfer mechanism is dominated by Förster resonance energy transfer. Our findings demonstrate a viable strategy for overcoming molecular-level charge-injection limitations and provide actionable guidelines for designing OLED architectures with enhanced luminescence efficiency.