A Highly Emissive Molecular Material with Tunable Emission Color: Investigations as Phosphor for the Elaboration of a Hybrid White-Light-Emitting Diode

4,4′-([2,2′-Bipyrimidine]-5,5′-diyl)bis[N,N-bis(4-methoxyphenyl)aniline] (1), a molecular compound built by connecting an electron-deficient 2,2′-bipyrimidine core to two electron-rich triarylamine derivatives further enriched with electron-donating methoxy groups, was designed and synthesized for luminescent purpose. The X-ray crystal structure obtained from tetrahydrofuran (THF) and a dichloromethane (DCM)/hexane mixture revealed that 1 could adopt slightly different conformations in the solid state. Indeed, the bipyrimidine core is almost planar when the molecule is recrystallized from DCM/hexane and forms dimers, while a twist angle of 28.20° between the two pyrimidine rings and the formation of infinite π-stacks were found upon recrystallization from THF. Because of the donor–acceptor–donor structure and the resulting charge transfer, the molecule possesses some polarity in the excited state that makes it sensitive to its surroundings. As a consequence, solutions emitting from blue to red could be obtained by solubilizing 1 in solvents of various polarities. The presence of aromatic units is responsible for π–π-stacking interactions, as revealed by X-ray diffraction, leading to aggregation in solution and in the solid state, which also impacts the photophysical properties. A maximized phenomenon of aggregation-enhanced emission was observed in a THF solution containing 75% water. This sensitivity of 1 to the solvents in terms of photophysical properties was also demonstrated on films deposited from different solvents. These studies revealed the appropriate experimental conditions in which 1 can be highly emissive either in solution or in the solid state. Especially, a photoluminescence quantum yield as high as 82.2% was obtained in a film of 1 doped into poly(methyl methacrylate) at a rate of 10 wt %. Investigations on the photostability of 1 were performed under accelerated aging conditions (375 nm, 300 mW cm^–2). Lowering the power resulted in an increase in stability and makes the molecular material an interesting candidate for luminescence purposes.