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

Abstract

Abstract 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.