A fast-switching optical filter using the photochromic properties of spirobenzopyranoindoline solutions

Functionalized spirobenzopyranoindolines were synthesized and their photophysical and photochemical properties were investigated from the point of view of their application as a photoactive compound in automatic welding filters. 1′,3′,3′-trimethyl-6-nitrospyro[2H-1-benzopyran-2,2′-indoline] (6-nitroBIPS) was selected and its photochemical and photophysical properties in solution were investigated by means of emission and absorption measurements as well as time-resolved fluorescence spectroscopy measurements and the dipole moment change of 6-nitroBIPS was estimated. The fluorescence spectra of 6-nitroBIPS showed to be shifted to the red with increasing solvent polarity. Based on these results, a model of an optical welding filter with a photochromic liquid layer have been developed and tested according to EN standards.