Phenylbenzoxazole-amide-cyclen linkage as a ratiometric fluorescent receptor for Zn(II) in water

A phenylbenzoxazole-amide-cyclen linkage (L) behaves as a ratiometric fluorescent receptor for Zn(2+) in water. The receptor dissolved in water at neutral pH shows fluorescence at 383 nm. The addition of Zn(2+), however, leads to a decrease in this emission, along with an appearance of red-shifted emission at 445 nm. This thus facilitates ratiometric Zn(2+) sensing. Other metal cations do not promote such spectral change. Complexation of L with Zn(2+) involves the coordination with four cyclen nitrogens and amide oxygen. IR and potentiometric analysis revealed that strong coordination of Zn(2+) with amide oxygen leads to a deprotonation of the amide moiety and creates red-shifted fluorescence. Ab initio calculation indicated that the deprotonation of the amide moiety allows rotational motion of the benzoxazole moiety in the excited state and stabilizes the twisted intramolecular charge transfer (TICT) excited state. This results in the creation of red-shifted fluorescence from the TICT excited state.