From Highly Fluorescent Donors to Strongly Absorbing Acceptors: The Tunable Properties of Fluorubines

The synthesis and characterization
of three novel fluorubine derivatives
is reported via three to four simple reaction steps with isolatable
intermediates. The functional dyes are characterized by their strong
absorption peaks in the visible region and their high fluorescence
quantum yields. A significant and useful feature is that the properties
can be tuned over a wide range by changing the pH. Transformation
of the dyes into protonated amidinium salts leads to narrower band
gaps and to drastically lower LUMO energies. Further reduction of
the pH results in the doubly protonated species with a high electron-deficiency
and LUMO energies of −4.8 eV, bathochromic shifts, and a strong
intensity increase of up to ε = 120 000 M–1 cm–1.