Photophysical properties of anti-inflammatory piroxicam and its Cu(II) complex

Abstract

Piroxicam (PRX), a nonsteroidal anti-inflammatory drug, exhibits a large Stokes-shift emission owing to the excited-state intramolecular proton transfer (ESIPT) from enol-PRX to keto-PRX. In contrast, the Cu-coordinated PRX complex (CuPRX) does not show any emission at 300 K. In the time-resolved transient absorption (TA) spectra, PRX showed two positive TA bands at 430 and 700 nm, corresponding to the S-1-S-n transitions of keto-PRX* and a negative bleaching band at 540 nm due to ESIPT emission. These TA bands decayed with a lifetime of approximately 50 ps. CuPRX showed similar TA features, but no bleaching band was observed for CuPRX and decayed rapidly within 3 ps, indicating a rapid intersystem crossing process owing to the heavy atom effect. Based on the experimental results and theoretical calculations, the fate for excited keto-PRX is discussed in terms of the ESIPT and reverse ESIPT processes.