Near-infrared electroluminescence from linear and cyclic porphyrin hexamers

Organic and polymer light-emitting diodes (LEDs) have received much attention in recent years for application in displays and lighting panels. Less attention, however, has been focussed on near-infrared organic LEDs because of difficulties in synthesising low-gap molecular emitters with high efficiencies. This synthetic challenge requires innovative approaches to molecular design, and, in a recent paper published in Nano Letters, LCN researchers from the group of Prof. Cacialli in collaboration with colleagues from the University of Oxford, have achieved efficient near-infrared emission from linear and cyclic porphyrin hexamers (Figure 1).

By linking porphyrin units with meso-butadiyne bridges, conjugation is preserved across the whole molecule and it is this extended conjugation which causes a shift in the emission to longer wavelengths. The study shows, however, that efficient emission can only be achieved if aggregation of the hexamers is suppressed. For the linear hexamer, the researchers demonstrate that this can be done by decorating it with 4-benzylpyridine molecules which coordinate to the zinc centres on each porphyrin unit. This approach yields photoluminescence quantum efficiencies of 8% in both the solid state (λ = 882 nm) and dilute solution. The researchers also synthesised a cyclic hexamer (Figure 1) with a view that the curved π-surface of this molecule would provide steric hindrance to aggregation and also further red-shift the emission (920 nm in photoluminescence).

Light-emitting diodes based on blends of the decorated linear hexamer with a conjugated polymer showed infra-red electroluminescence at 883 nm with 0.1 % external quantum efficiency, whilst lower efficiency emission at 960 nm was observed from the analogue cyclic hexamer device.

This work has been published in Nano Letters (O. Fenwick et al, Nano Letters, DOI: 10.1021/nl2008778).

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Figure 1:  The photoluminescence spectra (left) of the two hexamers plotted with their chemical structures.