Knowledge Bank

The unusual visible and ultraviolet reflection spectra obtained for single crystals of the polymethinium dye 1,5-bis-(dimethylamino) pentamethinium perchlorate have been reported $previously.1$ These spectra, as well as those for some heretofore uninvestigated faces, will be interpreted in terms of the detailed crystal structure, which was recently determined using x-ray diffraction techniques. The discussion will include the presentation of the results of calculations based on molecular exciton theory and a consideration of the relationship between the reflection spectra and the other optical properties of these crystals.