Conjugated polymers are widely used as light-emitters, for example in flat-panel displays. However, many of these polymers, whilst very efficient emitters in solution show significantly weaker luminescence in the solid state. Researchers from Nanjing University of Posts and Telecommunications in China, along with colleagues from Imperial College London in the UK,1 have now developed star-shaped macromolecules exhibiting efficient light emission even in the solid state.

The poor emission efficiency of polymers is due to optical quenching by defects and/or interactions between the polymer chains. To prevent intimate contact between the molecular strands, this group developed new six armed nanostar molecules, which hinder close-packing in the solid state because the strands extend in different spatial directions.

Fig. 1: Optical properties of the starburst macromolecules. Absorbance (magenta solid line), photoluminescence (red dashed dot line) and lasing (blue filled area). The inset shows the schematic diagram of the distributed-feedback laser structure that is used for the laser devices (blue: lasing material coated on an etched synthetic quartz grating with period as shown).

These new solid-molecules emitted strong blue light-emission (Fig. 1), and the low lasing threshold of devices was evidence of the high purity of the materials and reduction of interchain interactions.

Interestingly, contrary to conventional polymers composed of long molecules, star-shaped molecules with moderately short arms exhibited superior solid-state light emission. “This provides valuable information on the rational molecular design and development of high-performance materials for efficient blue-light-emitting applications,” says Wei Huang, leader at Nanjing University of Posts and Telecommunications.

The findings suggest a new design strategy for improved organic lasers that show low losses and therefore enhanced lasing characteristics. This is particularly important towards the realization of electrically pumped organic lasers, which to date have not been realized. “These macromolecular nanostars show great potential towards this aim,” says Huang.

For electrically pumped lasing it will be necessary to optimize all aspects of the laser structures, and to further minimize losses. These are the issue being addressed by the Imperial College London team. “Our demonstration of very favorable optical gain characteristics for these nanostars should stimulate a lot more activity to fine tune their electrical and optical properties,” says Donal Bradley, who lead the work on these aspects. “If successful we might finally be suitably armed to tackle the organic laser diode problem.”