Knowledge Bank

Hitherto, the only media available for room-temperature studies of phosphorescence and triplet-triplet absorption have been a limited number of low-melting glasses, of which boric acid is perhaps the best known. Boric acid glass suffers from being brittle, and hygroscopic and thick samples readily crack unless carefully annealed. The high temperature ($240\circ C)$ needed for fabrication argues against its use for many compounds sensitive to thermal decomposition. Furthermore, the optical quality leaves much to be desired and the material is not amenable to machining and polishing. In searching for a material with better properties we have incorporated naphthalene, chrysene and several other aromatic compounds into various plastic media. Ordinary linear plastics appear to behave like fluid media in that phosphorescence is absent unless the sample first be cooled to low temperatures. However, samples in which extensive cross-linking has been produced exhibit strong phosphorescence even at room temperature and above. In the case of chrysene it is also possible to observe triplet-triplet absorption visually, as the development of a reddish-purple color under strong illumination. It appears possible to correlate the appearance of phosphorescence with the raising of the second-order glass transition temperature brought about by cross-linking, leading to a three-dimensional cage which effectively immobilises the solute molecule. In non cross-linked plastics, on the other hand, segmental translation and rotation may still occur despite the macroscopic solidity, thus leading to collisional deactivation of the triplet state in much the same way as in fluid media. Several techniques are available for producing cross-linked plastics and by subsequent machining, rods, disks and a variety of other shapes may be readily produced.