Color‐Tunable Fluorescent Dyes Based on Benzo[ c ]coumarin

Hurtley condensation was shown to be a perfect tool with which to assemble a diversified library of derivatives of benzo[ c ]coumarin. Not only do simple resorcinols and ortho ‐bromobenzoic acids undergo this reaction, but also dihydroxynaphthalenes and 3‐bromothiophene‐2‐carboxylic acid, affording the desired compounds in moderate yields. In the case of naphthalenediols, intriguing regioselectivity is observed, which holds promise for its utilization in the synthesis of previously inaccessible molecules. Further transformation made it possible to obtain benzo[ c ]coumarin bearing amino and nitro groups at various positions, which served as entry points to prepare complex hybrids of 1,4‐benzoxazin‐2‐ones and benzo[ c ]coumarins. The hydroxyl group proved to be the synthetic handle, enabling the synthesis of strongly solvatochromic, soluble analogues. The effect of structural variation on photophysical properties was studied in detail for almost 30 compounds. The relationship between the structure and photophysical properties was thoroughly elucidated by comparison with simple analogues (coumarins, benzoxazinones). All of the obtained compounds exhibit moderate to large Stokes shifts (3300–12500 cm –1 ). The type of π‐expansion of the chromophore strongly influences the overall optical phenomena. Compounds possessing alkyl substituents on the benzo[ c ]coumarin core have much higher fluorescence quantum yields than their analogues bearing amino, fluorine, and other complex substituents. Interestingly, the product possessing a fused coumarin–thiophene skeleton exhibits over twofold higher fluorescence quantum yield than any of its coumarin–benzene analogues.