Flexible Alkylene Bridges as a Tool To Engineer Crystal Distyrylbenzene Structures Enabling Highly Fluorescent Monomeric Emission

Aggregation-Induced Emission in Bridged (E,E)-1,4-Diphenyl-1,3-butadiene Derivatives with Six- and Seven-Membered Rings

Recently, we developed a new aggregation-induced emission (AIE) luminogen (AIEgen), bridged stilbene, by incorporating a propylene group into the C=C bond of the luminescent phenyl stilbene. This bridged structure, featuring a seven-membered ring, induces a significant conformational change, causing the C=C bond to twist in the excited state, thereby enhancing non-radiative decay in solution. In this study, we introduced bridged structures with alkylene groups of varying lengths into (E,E)-1,4-diphenyl-1,3-butadiene (DPB). The variation in the bridged structures of the synthesized DPB derivatives notably influenced the environmental sensitivity of fluorescence. Whereas the compound with two six-membered ring structures exhibited emission in solution and in the polycrystalline state, derivatives with a seven-membered ring exhibited AIE properties. Specifically, BDPB[7,7], featuring two seven-membered ring structures, demonstrated AIE characteristics with solid-state luminescence originating from J-aggregates. However, the fluorescence quantum yield was low in poly(methyl methacrylate) (PMMA) dispersion films, where molecular motion was restricted. These findings open new possibilities for designing unique AIEgens that remain nonluminescent even in highly viscous or confined environments, such as PMMA films.

Synthesis of Side-Chain Liquid Crystalline Polyacrylates with Bridged Stilbene Mesogens

In recent years, π-conjugated liquid crystalline molecules with optoelectronic functionalities have garnered considerable attention, and integrating these molecules into side-chain liquid crystalline polymers (SCLCPs) holds potential for developing devices that are operational near room temperature. However, it is difficult to design SCLCPs with excellent processability because liquid crystalline mesogens are rigid rods, have low solubility in organic solvents, and have a high isotropization temperature. Recently, we developed near-room-temperature π-conjugated nematic liquid crystals based on "bridged stilbene". In this work, we synthesized a polyacrylate SCLCP incorporating a bridged stilbene that exhibited a nematic phase near room temperature and could maintain liquid crystallinity for more than three months. We conducted a thorough phase structure analysis and evaluated the optical properties. The birefringence values of the resulting polymers were higher than those of the corresponding monomers because of the enhanced order parameters due to the polymer effect. In addition, the synthesized polymers inherited mesogen-derived AIE properties, with high quantum yields (Φfl = 0.14-0.35) in the solid state. It is noteworthy that the maximum fluorescence wavelength exhibited a redshift of greater than 27 nm as a consequence of film formation. Thus, several unique characteristics of the SCLCPs are unattainable with small molecular systems.

Pd-Catalysed Direct Arylation of Distyrylbenzene: Strong Dual-state Fluorescence and Electrochromism

Distyrylbenzenes (DSBs) are well-known for their strong multicolour fluorescence. Fluorescence tuning of DSB via further functionalization/arylation, on the other hand, is uncommon. This paper reports a Pd-catalysed direct arylation approach for introducing different aryl groups onto fluorobenzene-containing DSB moiety (7) in high yields (67-72 %). The versatile methodology allows the substitution of neutral [tolyl (1)], electron-deficient [p-formyl benzene (2), p-acetyl benzene (3), p-nitrobenzene (4)] and electron-rich [carbazole (5), triphenylamine (6)] aryl groups. The electron-deficient aryls render mono-substitution, while the electron-rich counterparts promote di-substitution. The compounds (1-6) show blue, green, and yellow fluorescence in both the solution and solid states; the fluorescence quantum yields reach >98 % and the peak maxima span from 425 to 560 nm. The mono-carbazole DSB (5) exhibit white light emission (WLM) in polar solvents (acetone, DMF, CH3CN, DMSO and NMP) with very high fluorescence quantum yields (φf) of 60-80 %. For WLM, such high efficiency (φf) is somewhat uncommon. Moreover, visible-to-NIR reversible electrochromism is demonstrated by the TPA-integrated DSB (6). The colour of 6 changes from pristine light yellow to orange, and the absorption maxima shifts from 372 to 1500 nm when a positive potential of 1.0 V vs Ag/Ag+ is applied. Moreover, the system shows high colouration efficiency in the NIR region with fast switching speeds for colouration and decolouration as fast as 0.98 s and 1.05 s.