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The ability of a computer to engage a student in a dialog----can be used in a variety of ways to improve the quality of instruction____ Computer-assisted instruction (CAI) refers to those applications of computers in instruction where a student engages in a dialog with a computer program to achieve a well-defined and measurable understanding or skill. A computer system that can accommodate a large number of such interactive programs, all of which are readily accessible to students who wish to use them, can support a significant part of the instructional load in a given course or curriculum. This article attempts to summarize the current status of CAI in chemistry instruction. The authors have a total of four decades of experience with diverse CAI systems in different environments, and we hope that the opinions and facts collected here will be of value to potential users of CAI.

The polarization of the phosphorescence of naphthalene, anthracene, pyrene, and chrysene as guest in the benzophenone lattice have been examined. Considerable out-of-plane intensity has been shown to be present in these transitions. The results have confirmed conclusions from previous experimental observations and have led to certain assignments for the lowest triplet states which are in agreement with theoretical predictions. The triplet—triplet absorption spectra of naphthalene, anthracene, and chrysene have been detected using polarized light and the mixed crystals. All these T—T transitions were polarized in the aromatic molecular planes, in accordance with theory. The phosphorescence of pyrene in benzophenone has been analyzed and tentatively interpreted as triplet—singlet emission of the pyrene dimer.

The polarized absorption and emission of the anthracene—TNB complex crystal are shown to provide evidence for localization of low-energy excitation in this crystal. The absorption and fluorescence polarization ratios at 300°K are very different (5.6:1 and about 15:1, ‖c: ⊥c on 110) the former being close to an oriented gas value. The latter corresponds to a charge-transfer complex more closely aligned with the c axis of the crystal. The motion of the excited molecule is in accord with the behavior of a dipole in an electric field which is nearly along the c axis. Possible ramifications of this effect in photoconductive charge-transfer complexes are discussed. The influence of temperature on the luminescence spectrum of anthracene—TNB crystals is discussed, and apart from some artifacts due to cracking of crystals, the expected increase of emission intensity with decreasing temperature is observed.