Single-component white-color photoluminescence from liquid crystal polymers: Color tuning by a combination of luminescence thermo- and mechanochromism

Abstract Pure organic luminescent materials with high efficiency in the solid state are of great importance for potential applications. Unlike blending two or three dyes, getting white-light emission from a single component is a hot research topic for its advantages in practical applications. Although thermo/mechanochromic materials are a smart class of stimuli-responsive systems, using these materials to manipulate the luminescence spectrum to achieve full-color emission from one component is rarely reported. In this work, we have designed two novel liquid crystal copolymers containing different monomer ratios of ethynylbiphenyl and biphenylcarbonitrile skeletons in the side chains. The polymers are free of rare or precious metals and exhibited thermochromism, mechanochromism, white emission from a single component, and high emission efficiency in the solid state. Ethynylbiphenyl skeleton is the key structure as a luminescent group that emits white light upon heating to the liquid crystal phase. However, controlling the ratio between ethynylbiphenyl and biphenylcarbonitrile side chains is essential to get high emission efficiency along with good purity of the white luminescence. The thermochromism and mechanochromism primarily due to the transition between creating and breaking down π-π interactions with different modes of molecular packing. This transition switches the emission color from blue to white (thermochromism) and vice versa (mechanochromism). The developed polymers showed a smart stimuli-responsive behavior and provided a simple strategy to get single-component white-light luminescent materials in the solid state.