Unsaturated oxazolones as nonlinear fluorophores

• Nonlinear optical properties of unsaturated oxazolone derivatives were studied.
• Two-photon excited fluorescence has been observed.
• High two-photon absorption cross-sections (up to 500 GM) were measured.

Unwanted bleaching of two-photon absorption microscopy probes can be reduced by designing more efficient nonlinear fluorophores. Here the linear and nonlinear photophysical properties of unsaturated oxazolones are studied. A set of push–pull oxazolone derivatives have been designed by substituting the 2-phenyl-4-benzylidene-1,3-oxazol-5-one unit in two different positions with a combination of electron donor (dimethylaniline) and/or acceptor (nitro, nitrile and benzimidazole) groups. By engineering this green fluorescent protein analogue core we have prepared molecules with high two-photon absorption cross-sections 200–500 GM and fluorescence quantum yields up to 0.75. The factors determining the overall nonlinear emission efficiency of the new unsaturated oxazolones are discussed in terms of the nature of the substituents.

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