Abstract
The luminescence of materials in condensed phases is affected by not only their molecular structures but also their aggregated structures. In this study, we designed new liquid-crystalline luminescent materials based on biphenylacetylene with a bulky trimethylsilyl terminal group and a flexible alkoxy chain. The luminescence properties of the prepared materials were evaluated, with a particular focus on the effects of phase transitions, which cause changes in the aggregated structures. The length of the flexible chain had no effect on the luminescence in solution. However, in crystals, the luminescence spectral shape depended on the chain length because varying the chain length altered the crystal structure. Interestingly, negative thermal quenching of the luminescence from these materials was observed in condensed phases, with the isotropic phase obtained at high temperatures exhibiting a considerable increase in luminescence intensity. This thermal enhancement of the luminescence suggests that the less- or nonemissive aggregates formed in crystals are dissociated in the isotropic phase. These findings can contribute toward the development of new material design concepts for useful luminescent materials at high temperatures.
Highlights
Organic materials that luminesce strongly in the solid state are crucial elements of organic light-emitting diodes [1–3]
In sharp contrast to the photoluminescence behavior in the solution, the luminescence spectral shape in crystals showed a clear dependence on the terminal chain length, meaning that photoluminescence of the present materials dramatically depends on the intermolecular interactions
We discussed the effects of phase transitions, namely, changes in the aggregated structures, on the luminescence behavior of Liquid crystals (LCs) materials based on biphenylacetylene with a bulky trimethylsilyl terminal group and a flexible alkoxy chain
Summary
Organic materials that luminesce strongly in the solid state are crucial elements of organic light-emitting diodes [1–3]. Most luminescent organic molecules exhibit efficient photoluminescence in dilute solution, their luminescence is usually partially or completely quenched by luminophore aggregation in condensed phases (e.g., crystals and solid films). This phenomenon, called aggregation-caused quenching (ACQ), is common in organic molecules with π-electron systems and prevents their practical use [4,5]. Organic materials that exhibit enhanced luminescence through molecular aggregation (aggregation-induced emission (AIE)) were developed, paving the way for the design of efficient solid-state emitters [6–9]. Structural control over molecular aggregates is a key technology for developing organic light-emitting materials and for tuning their luminescence properties (e.g., luminescence intensity and color).
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