Azole-linked coumarin dyes as fluorescence probes of domain-forming polymers

The binding of 7-aminocoumarins, substituted in the 3-position with heterocyclic benzimidazole or benzothiazole groups by domain-forming polymers in water has been studied. The acrylic polyelectrolyte, poly(methacrylic) acid (PMAA) was used as a solubilizing agent for coumarin dyes 6, 7, and 30 in water. The acid-base properties of these bound coumarin dyes were monitored spectroscopically on titration of aqueous solutions. Alterations in the fluorescence wavelength and intensity, quantum yields, lifetimes, and polarization are consistent with the preferential binding of the dyes in compact hydrophobic domains that form at a pH regime in which the polymer is in its protonated (uncharged) state. In this pH range (<4.0), coumarins 7 and 30 are bound as monocations, whereas coumarin 6 remains in its neutral form. Reduced quantum yields and lifetimes of fluorescence for cationic coumarins can be understood in terms of the imposition of a low-lying electron transfer state, an example of a twisted intramolecular charge transfer (TICT) intermediate. Effects of polymer microenvironment on the rate of TICT state decay (a reverse electron transfer) are observed. Coumarins of the azole type may find use as fluoroprobes of the microenvironments of proteins and other biological macromolecules and as agents for pH sensing.