Luminescence measurements were performed using Perkin Elmer LS-55 luminescence spectrometer (Perkin Elmer,Beaconsfield UK) equipped with a xenon discharge flash lamp.
The operation and data processing were controlled by the fluorescence Data Manager Software. The excitation and emission wavelengths were set at 220 nm and 550 nm respectively. The instrument was operated in a phosphorescence mode with a
gate time of 0.04 ms and a delay time of 0.2 ms used throughout the measurements. Absorption spectra were recorded on a Varian CARY 50 Conc UV visible spectrophotometer
(Connecticut, USA) with 1 cm matched quarts cell.