When it comes to developing ultrabright solid-state luminescent materials, bridged crystal structures may hold the key to enabling monomeric emission and access to new crystal systems, new research suggests. In the study, a research team from the Tokyo Institute of Technology prepared ultra-bright fluorescent dyes using distyrylbenzenes (DSB) with flexible alkylene bridges using a new crystal engineering study. The findings are sure to have important implications for the field of photofunctional materials.
Fluorescent solid organic dyes have many applications, from functional nanomaterials and organic light emitting diode (OLED) displays to lasers and bioimaging. These molecules have excellent versatility, adaptable molecular design and excellent processing. Improving the luminescence properties, crystallinity, and emission colors of these solid-state fluorescent dyes is a key area of research in this field, especially for the development of advanced OLEDs. However, developments in this direction are limited by three main factors. First, most fluorescent dyes experience concentration quenching (a decrease in fluorescence when the concentration of a fluorescent molecule exceeds a certain level) in the solid state. Second, the tendency of the dye molecules to aggregate in the solid state and produce fluorescence of different colors due to intermolecular electronic interactions. And three, crystal design strategies that can provide monomeric emission (essentially emission of a single wavelength, ie color) are underdeveloped.
To solve this problem, a research team led by Associate Professor Genichi Konishi of the Tokyo Institute of Technology developed a new crystal design strategy using flexible molecular bridges. A study published in Chemistry — European journal, describes the preparation of highly fluorescent monomeric emissive distyrylbenzenes (DSBs) with controlled electronic properties and luminescence. “A typical approach to crystal design for fluorescent solid dyes is a strategy based on steric hindrance, where we manipulate the bulk of the molecule to cause congestion around reactive atoms and suppress intermolecular interactions. But a frequent drawback of this approach is an increase in the distance between chromophores (fluorescent molecules). Our design strategy successfully avoids this side effect,” explains Associate Professor Konishi.
In this study, the research team prepared a very dense crystal structure called DBDBwith 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 structure. The team introduced an organic functional group called propylene as a bridging molecule between the six-membered rings on either side of the double bonds in the DSB structure. This addition resulted in a new compact crystal structure with suppressed intermolecular interactions and shorter distances between chromophores. “Essentially, the introduction of seven-membered (post-switching) rings into the DSB core created a moderate distortion and steric hindrance in the π-plane of the DSB, which allowed us to control the molecular arrangement without increasing the crystal density,” says the collaborator. Professor Konishi.
The team further investigated the photophysical properties of DBDBand found that the small size of the bridging molecules used in this study favors the emission of monomers in the solid state. They also saw that DBDBs was ultrabright with a high quantum yield and emitted the same colors both in the unaggregated dilute solution and in the solid state.
“The crystal structure of the DSB bridge 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.”