Patent GB-1177038-A (GB)
Title: Intensity Modulation of Triplet Decay Emission
IPC: G02F2/02
Filed: 1968-06-26
Abstract: 1,177,038. Luminescence. E. I. DU PONT DE NEMOURS & CO. 26 June, 1968 [26 June, 1967; 26 April, 1968], No. 30559/68. Heading C4S. [Also in Divisions G1, G5 and H1] In a light modulating arrangement the light emitted as a result of the decay of triplet states created in a material is intensity modulated by varying the strength of a magnetic field established in the material. The material preferable comprises a crystalline solid, e.g. anthracene or pyrene, although it may comprise a fluid (see later). A triplet state (exciton) in a crystal may be generated by exciting electromagnetic radiation, e.g. visible light, or electrically by hole and electron injecting electrodes, and comprises an electronically excited state having one unit of spin (arising from two unpaired electrons). The rate of the monomolecular decay process which appears as heat or light (phosphorescence) may be increased by adding impurities, in particular paramagnetic impurities, to the crystal, i.e. " quenching." In some crystals, triplet excitons may disappear by a second bimolecular process termed mutual annihilation in which a pair of excitons meet and combine to yield a single higher energy singlet exciton (zero spin, i.e. all electrons paired). This singlet exciton has a shorter lifetime than the triplet exciton and disappears with emission of light which is termed delayed fluorescence. In fluids the triplets involved are triplet states of molecules rather than excitons. The emitted light from the crystal (or fluid) that is influenced by the magnetic field includes, in the presence of paramagnetic quenching both delayed fluorescence and phosphorescence, and is restricted, in the absence of such quenching, to delayed fluorescence. Examples are given in the Specification of crystals which show delayed fluorescence but not phosphorescence and of crystals which show both phosphorescence and delayed fluorescence. For delayed fluorescence in fluid systems, operable solvents are those in which triplet-triple annihilation leading to delayed fluorescence can take place. The solvents must dissolve the solute to some extent, and be inert to the solute and transparent to exciting and emitted light. Paramagnetic impurities (free radicals) may be introduced into a crystal by cocrystallization with the host material or by diffusion of the impurity into a pure crystal or they may be produced by exposing the crystal to high energy radiation, e.g. X-rays. In operation the crystal may be magnetically coupled to a field set up in a coil or in an iron core, Figs. 3a and 3b (not shown). Alternatively, a permanent magnet, e.g. a magnetic tape, may be magnetically coupled to the crystal, Fig. 4 (not shown). A light modulating arrangement, Fig. 5, may comprise a lamp 16, the light from which is passed via an input filter 18 which removes wavelengths shorter than those necessary to create triplet excitons in the crystal 10, the annihilation luminescence from the crystal being collected by lens 17<SP>1</SP> and directed through output filter 19 which rejects the exciting light passed by the input filter. The intensity of annihilation luminescence light is modulated by the magnetic field of the solenoid 11. Instead of the solenoid a magnetic tape may be passed in close proximity to the crystal, the changes in the magnetic field corresponding to information stored on the tape modulating the intensity of the annihilation luminescence of the crystal, Fig. 6 (not shown). The magnetization of a surface may be determined by means of a film, Fig. 11, comprising crystals 70, e.g. of anthracene, embedded in an inert matrix 71 on a non-magnetic support 72. Detailed embodiments of light modulating apparatus embodying the invention are described with references to Figs. 9 and 10 (not shown), in which it was found that the triplet lifetime was increased when the crystal was subjected to a magnetic field or irradiated with X-rays.