Excited State Intramolecular Proton Transfer in π-Expanded Phenazine-Derived Phenols

Two previously inaccessible analogs of 10-hydroxybenzo[h]-quinoline were prepared via a straightforward strategy comprising the formation of pi-expanded phenazines skeleton followed by C-H acetoxylation at position 10. Two bis-phenols possessing C-2 and D-2 symmetry were obtained in yields of 52% and 15%, respectively. The occurrence of excited state intramolecular proton transfer (ESIPT) was detected in all cases because steady state emission was observed only from the excited keto-tautomer. Additionally, a short-lived, similar to 0.1 ps, emission decay was resolved by the femtosecond up-conversion technique at the blue side of the keto-tautomer emission band, 610 nm, and was attributed to the ESIPT, i.e., conversion from enol to keto tautomer. In comparison with the corresponding 10-hydroxybenzo[h]quinoline emissions, the emission spectrum of the pi-expanded phenazine analogues were weaker but displayed a characteristic bathochromically shift into NIR region. These phenazine analogues constitute one of largest heterocycles for which ESIPT was unambiguously detected.