Fluorene Analogues of Prodan with Superior Fluorescence Brightness and Solvatochromism

In a search for environment sensitive (solvatochromic) dyes with superior properties, we extended the electronic conjugation of one of the best solvatochromic dyes, Prodan, by substituting its naphthalene core with fluorene. The newly synthesized fluorene derivatives bearing strong electron-donor (dialkylamino) and acceptor (carbonyl) groups the 2 and 7 positions showed red-shited adsorption (close to 400 nm), twice as large of a absorption coefficient (43 000 M-1 cm(-1)) and a manifold larger two photon absorption cross section (similar to 400 GM) compared to Prodan. Studies in solvents revealed much stronger fluoresnce solvatochromism of the new dyes, which is connected with their twice as large transition dipole moment (14.0 D). Similarly to Prodan, they exhibit high fluorescence quantum yields, while their photostability is largely improved. Thus substitution of the naphthalene core in Prodan with fluorence resulted in new fluorophores with superior spectroscopic and solvatochromic properties. We expect them to find a variety of applications as environmentally sensitive probes and labels in biology.