Effect of medium acidity on the spectral behaviour of some pyrazinyl diolefinic laser dyes

Abstract

The spectral behaviour of the diolefinic dyes, 2,5-distyrylpyrazine (DSP) and 1,4-bis(β-pyrazinyl-2-vinyl) benzene (BPVB) have been studied in different media. The electronic absorption and emission spectra of the two dyes are bathochromically shifted with increase in medium acidity. A solution of each dye in 98% sulphuric acid acquired a deep blue colouration which changes through orange to a pale yellow upon dilution with water. The deep blue colour observed is responsible for the appearance of a new absorption peak at 550 nm, indicating the presence of a charge-transfer species being formed by the protonation of nitrogen heteroatoms of the pyrazinyl rings. The excimeric emission and excitation spectra of DSP and BPVB adsorbed from toluene onto the surface of silica-alumina ( ∼ 13% Al 2O 3) catalyst (activated at 650°C in air) are broad, structurless and substantially bathochromically shifted by ∼ 110 nm compared with the corresponding peaks obtained for dilute solutions containing no catalyst. Poly-DSP undergoes monomerization upon adsorption on the surface of the activated silica-alumina catalyst. The depolymerization process is believed to be via cyclobutane ring rupture.