Access to a large stokes shift in functionalized fused coumarin derivatives by increasing the geometry relaxation upon photoexcitation: An experimental and theoretical study

A series of new fluorophores with a fused coumarin framework were prepared. The dyes show red-shifted and enhanced absorption compared to the model compound, 7-hydroxycoumarin. The new coumarin bearing a 4-dimethylaminophenylacetylide group, shows absorption at 389 nm (ɛ = 14,300 M−1 cm−1), compared to the model compound which shows blue shifted absorption bands (ɛ = 11,500 M−1 cm−1 at 320 nm). The emission of the new coumarin bearing a 4-dimethylaminophenylacetylide group is remarkably red-shifted (λem = 555 nm) compared to the model compound (λem = 356 nm). The fluorescence quantum yields of the new coumarins are increased up to ca. 9-fold compared to the model compound. The Stokes shifts (84 nm–166 nm) are also much larger than that of the model compound. TDDFT calculations show that the fused coumarins undergo significant geometry relaxation upon photoexcitation, which is responsible for the large Stokes shift. Population of the triplet excited state was observed for the bromo-functionalized coumarin.

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► A novel fused coumarin fluorophore is described.
► New fluorophores show improved fluorescence properties than 7-hydroxycoumarin.
► DFT calculations explained the large Stokes shift of the new fluorophores.
► The photophysical properties were rationalized by DFT calculations.