2-Phenylbenzoxazole derivatives: a family of robust emitters of solid-state fluorescence

Abstract

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.

Mechanofluorochromic Properties of an AIEE‐Active 2‐Phenylbenzoxazole Derivative: More than Meets the Eye?

“…This work confirms our assumption that extending the electron conjugation of the PBO framework with a CH=CR 1 R 2 group inserted on the phenyl ring is enough to get AIEE behaviour, which is accompanied here by a MFC effect. The small dicyanovinyl group can be easily introduced in a large variety of dye molecules, and it would be instructive to know to what extent it can endow them with effective AIE/AIEE and MFC properties.…”

“…Alternatively, it has recently been shown that the electrostatic field induced on a single molecule by the crystalline environment is enough to rationalize AIE/AIEE effect . It is noteworthy that the strong red shift of the emission spectrum observed upon aggregation compares well with that reported for benzoxazole styryl dyes, the π‐electron conjugated system of which is larger than that of compound 1 …”

“…The aim of the present work is to study mechanofluorochromism in the 2‐phenylbenzoxazole (PBO) series. Due to their excellent chemical, thermal and photochemical stability, and unique emission properties, PBO derivatives are widely used as fluorescent dyes in solution and in polymers, and their value for solid‐state fluorescence is now well established . Many of them become AIE fluorogens after enlargement of their π‐conjugated system by a −CH=CR 1 R 2 , −CH=NR or −N=CR 1 R 2 group, or after insertion of a −NH−C(O)−R group on the phenyl ring, R and R 1 being an unsaturated or aromatic moiety.…”

Halogen Atom and Position Dependent Strong Enhancement of Solid‐State Fluorescence and Stimuli Responsive Reversible Fluorescence Switching

“…Thus, all halogen substituted molecules showed halogen interaction in the solid state with opposite molecular arrangement. Rigidification and restriction of intramolecular rotation (RIR) of fluorophores in the solid state via supramolecular interactions plays important role for enhancement of fluorescence by deactivating the non‐radiative process . Comparison of solid‐state structure and fluorescence efficiency suggest that dimer with opposite molecular orientation in para ‐F and meta and para ‐Br showed enhanced fluorescence than 1, meta ‐F and meta ‐Cl.…”

Excitation‐Wavelength‐Dependent Emission and Delayed Fluorescence in a Proton‐Transfer System

“…It includes photoexcitation of am olecule in the enol form, S 0 e !S 1 e ,afast enol-to-keto S 1 e !S 1 k ESIPT (k ESIPT % 10 12 s À1 )a nd, finally,t he S 1 k !S 0 k emission in the keto form followedb yf ast ground state intramolecular protont ransfer (GSIPT), S 0 k !S 0 e . [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] The ESIPT leads to significant spatials eparation of HOMO and LUMO which reduces the singlet-triplet (S 1 -T 1 )e nergy separation, DE ST . [17] Importantly,s mall DE ST values ( 0.2 eV or 19 kJ mol À1 )f acilitatet hermally activated delayedf luorescence (TADF).…”