Synthesis and Photophysical Properties of Polarized 7-Heteroaryl Coumarins

Although 7-amino-, 7-hydroxy-, and 7-methoxycoumarins have been extensively studied for over a century, the analogous coumarins bearing other electron-donating groups at position 7 are virtually unknown. Herein, for the first time, we synthesized and investigated photophysics of coumarins possessing pyrrole, thiophene, or indole at position 7. The fluorescence and other photophysical parameters of these polarized coumarins possessing diverse five-membered aromatic substituents at position 7 are highly dependent on their electron-rich character. Delocalization of the LUMO on the entire π-system results in intense emission of all of these coumarins in solvents of intermediate polarity. In the case of highly electron-rich pyrrole-2-yl and indole-2-yl substituents, the fluorescence is bathochromically shifted, and its intensity decreases in polar solvents. The combined experimental and computational studies enabled us to assign it to the formation of a twisted intramolecular charge transfer (TICT) state in highly polar solvents. On the other hand, the presence of less electron-rich furyl and thienyl substituents resulted in blue emission across the polarity scale. Its intensity, however, decreases in nonpolar solvents. The computational investigation enabled us to assign it to the intersystem crossing (ISC) in nonpolar solvents. The lack of decrease in fluorescence quantum yield for these coumarins in a wide range of solvents of intermediate polarity can be attributed to blocking the ISC channel simultaneously with lack of formation of TICT states.