Freeze drying photochromic filters

Abstract

A fast-acting photochromic filter--i.e. a filter whose optical density in the visible light range of 4000 to 7500 angstroms is increased when subject to radiation shorter than 6000 angstroms--which is room temperature stable, that is from at least about 20.degree. to 80.degree.C., consists of (1) a solid optically transparent polymeric matrix, (2) essentially free from non-combined or molecular oxygen, and preferably protected from diffusion of atmospheric oxygen, in which is (3) dispersed, in solid solution, at least one light absorbing compound having a polynuclear conjugated condensed aromatic ring structure, whose molecules are capable of assuming a metastable triplet state by a process of intersystem crossing (not necessarily of the moiety which assumes the metastable triplet state) and in the triplet state, because of closer triplet states, has energy absorption bands whicch occur at lower energies (larger wavelengths) than the singlet-singlet absorption for such molecules. A synergic component may also be used as a physically adjacent moiety or molecule in which intersystem crossing occurs more readily than in the light absorbing compound, and which transfers triplet energy to the photochromic moiety. The absorption of singlet-singlet activating energy can be in the synergistic moiety, for single energy transfer, or in the photochromic moiety itself, with transfer of the singlet energy to the synergic moiety, intersystem crossing therein, to triplet energy, and transfer back of the triplet energy to the photochromic moiety, or double energy transfer. Two or more photochromic compounds, with different absorption spectra block 'windows' to give absorption over a desired range. Protective glass layers on each side of a photochromic matrix give physical protection, exclude oxygen, and may also be filters for desired ultraviolet and infrared ranges. Polymerization by ionizing radiation gives freedom from interferrants, such as catalysts residues. Evacuation during processing, or heating in vacuum after formation of the filter aids in removal of uncombined oxygen.