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The π-extended coumarins derived with nonhydrolyzable iminophosphoranes as two-photon excited fluorophores.

In this study, a novel synthetic strategy was developed for modifying coumarin fluorophores by using the nonhydrolysis Staudinger reaction. Through the mesomeric effect, a series of novel coumarin-iminophosphorane (IPP) products were formed that have stable resonance contribution from aza-ylide. The N=P formation reaction kinetics obey the conventional Staudinger reaction. The absorption and emission profiles of the coumarin-IPP derivatives can be fine-tuned: an electron-donating group at PPh3 enhances absorption and fluorescence quantum yields, whereas an electron-withdrawing group at C-3 drives the absorption and emission peaks toward blue light wavelengths. Based on the electronic "push and pull" substituents, two-photon adsorption accompanied by anti-Stokes fluorescence is achieved under near-infrared femtosecond laser excitation.