New crystal engineering strategy to design ultra-bright fluorescent solid dyes


When it comes to designing ultra-bright semiconductor fluorescent materials, bridged crystal designs could be the key to enabling monomer emission and accessing new crystal systems, a new study reveals. In the study, a research team from the Tokyo Institute of Technology prepared ultra-bright fluorescent dyes using di-bridged distyrylbenzenes (DSBs) with flexible alkylene bridges, using a new crystal engineering study. The results will certainly have important implications for the field of photofunctional materials.

Fluorescent solid organic dyes have a range of applications from functional nanomaterials and organic light-emitting diode (OLED) displays to lasers and bio-imaging. These molecules have excellent versatility, adaptable molecular designs and excellent processability. Improving the luminescent properties, crystallinities, and emission colors of these solid-state fluorescent dyes is a key area of ​​research in the field, especially for the design of advanced OLEDs. However, developments to this end are limited by three major factors. First, most fluorescent dyes experience concentration quenching (a reduction in fluorescence when the concentration of the fluorescent molecule exceeds a certain level) in the solid state. Second, the tendency of dye molecules to aggregate in the solid state and fluoresce in different colors due to the resulting intermolecular electronic interactions. And third, crystal design strategies that can ensure monomeric emission (essentially, single wavelength, i.e. color) emissions are underdeveloped.

To solve this problem, a research team, led by Associate Professor Gen-ichi Konishi from the Tokyo Institute of Technology, has developed a new crystal design strategy using flexible molecular bridges. The study, published in Chemistry – A European Journal, describes the preparation of highly fluorescent monomeric emissive di-bridged distyrylbenzenes (DSBs) with controlled electronic properties and luminescence. “A typical approach to crystal design for fluorescent solid dyes is the steric hindrance-based strategy, where we manipulate most of a molecule to cause congestion around reactive atoms and suppress intermolecular interactions. But a A frequent drawback of this approach is an increased distance between chromophores (fluorescent molecules).Our design strategy successfully avoids this side effect,” says Associate Professor Konishi.

In this study, the research team prepared a very dense crystal structure called DBDB[7]s. DSB and DBDB[7]s are π-conjugated systems, which means that these organic molecules have alternating single bonds (CC) and double bonds (C=C) in their structures. The team introduced an organic functional group called propylene as bridging molecules between the six-membered rings on either side of the double bonds in the DSB structure. This addition gave rise to a new compact crystal structure with suppressed intermolecular interactions and smaller distances between chromophores. “Essentially, the introduction of seven-membered rings (after bridging) into the DSB core created moderate distortion and steric hindrance in the π-plane of the DSB, which allowed us to control the molecular arrangement without increasing crystal density,” says Associate. Professor Konishi.

The team further investigated the photophysical properties of DBDB[7]s and found that the small size of the bridging molecules used in this study favored monomer emission in the solid state. They also saw that DBDB[7]s was ultrabright with high quantum efficiency and emitted similar colors both in unaggregated dilute solution and in the solid state.

“The bridged DSB crystal structure described in our study provides access to new crystal systems,” concludes Associate Professor Konishi. “Our strategy has far-reaching implications for how we approach the design of photofunctional molecular crystals.”

A bridge to brighter screens indeed!

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