Novel Crystal Engineering Strategy to Design Ultra-Bright Fluorescent Solid Dyes

When it comes to designing ultra-bright semiconductor fluorescent materials, the Bridged crystal designs could hold 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.

Fluorescent solid organic dyes have a range of applications ranging 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 focus quenching (a reduction in fluorescence when the focus 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, 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 the mass of a molecule to cause congestion around reactive atoms and suppress intermolecular interactions. But a frequent drawback of this approach is an increased distance between the 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 called DBDBs. DSB and DBDBs are π-conjugated systems, which means that these organic molecules have alternating single bonds (CC) and double bonds (C=C) in their constructions. The team introduced an organic functional group called propylene as bridging molecules between the 6-membered rings on either side of the double bonds in the DSB construct. 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 DSB’s π pattern, which allowed us to control the molecular arrangement without increasing crystal density,” says Affiliate. Professor Konishi.

They also saw that DBDBs were ultrabright with high quantum efficiency and emitted similar colors both in unaggregated dilute alternative and at the solid state.

“The bridged DSB crystal framework described in our study allows access to new crystal systems”, concludes Associate Professor Konishi.”

Crystal Diode Organic ElectroluminescentSteric HindranceFluorescenceEngineeringMoleculeMonomerCrystal System


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