2,5-bis[2-(2-phenyl-1,3-oxazol-5-yl)phenyl]-1,3,4-oxadiazole – new sterically hindered high Stokes shift fluorophore sensitive to media viscosity

• New sterically hindered high Stokes shift polyheteroaromatic fluorophore related to the ortho-POPOP family was synthesized.
• Its unsymmetric molecular conformation in the crystalline state was formed owing to intermolecular π–π contacts.
• Excited state planarization of the title molecule was revealed by our quantum-chemical modeling and time-resolved fluorescence spectra.
• Experiments with toluene–polystyrene system allow us to propose the investigated compound as potential color-changing fluorescent viscosity sensor.

New sterically hindered high Stokes shift polyheteroaromatic fluorophore (2,5-bis[2-(2-phenyl-1,3-oxazol-5-yl) phenyl]-1,3,4-oxadiazole, (1) belonging to the ortho-POPOP family was synthesized and its molecular structure was confirmed by X-ray and NMR data. Like any other ortho-POPOPs, the title molecule is substantially non-planar in the ground state owing to repulsion of its oxazole/oxadiazole cycles introduced in ortho-positions of the inner-chain benzene rings. Thus the intramolecular π-conjugation in 1 molecule is weakened in respect to its planar para-substituted analogs. Intermolecular π–π contacts in the crystalline lattice result in asymmetrization of the oxadiazole–phenyl–oxazole subunits of 1, which reflects itself in significantly different angles between the planes of azole and their neighboring benzene rings. Quantum-chemical modeling of 1 revealed for this molecule the ground state symmetrization in fluid media with alignment of the inter-ring angles. The significant excited state planarization of the title molecule, which was predicted by our TD-DFT calculations as well, results in partial restoration of the disturbed ground state conjugation, in decrease of the structurally relaxed excited state energy and finally – in theoretically predicted and experimentally observed abnormally high fluorescence Stokes shifts. The excited state conformational changing of 1 requires high amplitude intramolecular motions, which should be affected by the local media viscosity. This was confirmed in our time-resolved fluorescence experiments, which demonstrate decrease of the excited state structural relaxation rate in alcohols of different viscosity: 2-propanol, ethylene glycol and glycerol. Special experiments in toluene–polystyrene system allow us to recommend the investigated compound as potential fluorescent viscosity sensor.

Figure optionsDownload as PowerPoint slide