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

作者：Iliashenko Rodion Yu; Borodin Oleg O; Wera Michal; Doroshenko Andrey O^*

来源：Journal of Photochemistry and Photobiology A: Chemistry , 2015, 298: 68-77.

DOI：10.1016/j.jphotochem.2014.10.018

• Oxazole
• 1,3,4-Oxadiazole
• Steric hindrance
• X-ray structural analysis
• TDDFT
• ESSA
• Excited state structural relaxation
• Abnormally high fluorescence Stokes shift
• Time-resolved fluorescence
• Viscosity sensing

New sterically hindered high Stokes shift polyheteroaromatic fluorophore (2,5-bis[2-(2-phenyl-1, 3-oxazol-5-y1) 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 pi-conjugation in 1 molecule is weakened in respect to its planar para-substituted analogs. Intermolecular pi-pi 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 I 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.

• 出版日期2015-2-1