The derivatives of 2-phenylbenzoxazole (PBO) are popular fluorescent organic dyes for use in solution or after dispersion in an appropriate matrix. Their spectroscopic behavior in the solid state is, unjustly, not so well known. Many of them are strongly emissive as pure solid dyes, due to a favorable crystal packing mode. The PBO fragment lends itself well to relatively simple modifications of its chemical structure, aimed at enlarging the conjugated π-electron system. Many molecules thus designed show aggregation-induced emission (AIE). Furthermore, the derivatives of 2-(2′-hydroxyphenyl)benzoxazole (HBO) are familiar excited-state intramolecular proton transfer (ESIPT) dyes. They are particularly well suited for solid-state sensing. Mechanofluorochromism is also observed in complexes and closely-related compounds. Regarding their self-association properties, the general tendency of many PBO derivatives is to give elongated nano and microparticles. Very small chemical changes are enough to tune the shape and size of these particles. Nanofibers may be obtained by simple preparation methods and are of great value for wave-guiding. For all these reasons, as well as for its robustness and high photo- and thermal stability, the PBO fragment is an attractive building block to access new molecules that will be particularly well adapted for developments in the field of photoluminescent materials.

The arrangement of an ionic fluorophore in the crystalline state was regulated by the presence of various counter-ions and the effect on spectroscopic and self-association properties was studied. To do so, nine salts of berberine (i.e. a fluorescent natural alkaloid) were investigated. Most of them contained organic anions and were prepared using an ion-exchange process. Berberine chloride and hemisulfate were also used for the sake of comparison. The diffuse reflectance and photoluminescence spectra were recorded on powder compounds. All salts were emissive in the solid state and the emission efficiency was increased seven-fold with the nature of the anion. The optical properties were tentatively discussed on the basis of the crystal-packing mode. The possibility of implementing a bottom-up approach to generate microparticles was investigated using the reprecipitation method. Salts that contain the most hydrophobic anions gave a large number of homogeneous, elongated microparticles. This study showed that most of the berberine salts could be used as fluorescent materials, but proper choice of the anion allows using the self-association properties to best advantage.

Two isomeric tricarbonyl rhenium(I) complexes, ReL1 and ReL2, that possess a 2-pyridyl-1,2,n-triazole (pyta) ligand (n = 4 and 3, respectively) connected to a 2-phenylbenzoxazole (PBO) moiety, were synthesized in good yields. The X-ray structures showed that in ReL1 the PBO moiety and the pyta ligand almost form a right angle hindering electron delocalization, while in ReL2 their nearly planar arrangement favors the electron delocalization in the whole organic ligand. Therefore, the nature of the ligand significantly influences the electron distribution in the two complexes, as indicated by the results of TD-DFT calculations. An electrochemical study highlighted that, by comparison with ReL2, the smaller HOMO–LUMO energy gap of ReL1 is in line with its lower first reduction potential. From a spectroscopic viewpoint, both complexes emitted phosphorescence in organic solvents, with distinct color and intensity. They also emitted in the solid state, but only ReL1 showed significant aggregation-induced phosphorescence emission (AIPE). This complete study sheds light on the crucial role of structural isomerism of the triazole group, which has been unsuspected for a long time although it can govern the geometry and electronic properties of rhenium complexes. It is shown for the first time that grafting a non-coordinated π-conjugated fragment on the N(4) atom of a 1,2,4-triazole group can be of high value for the design of efficient light-emitting materials based on rhenium complexes.