Electronic Absorption and Emission Spectra of 9, 10-Dichloroanthracene and 9, 10-Dibromoanthracene Crystals

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Abstract
The electronic absorption spectra of three crystalline modifications of anthracene derivatives have been measured; they are the α and β form of 9, 10-dichloroanthracene and 9, 10-dibromo-anthracene. The assignment has been presented for each absorption band. The charge transfer absorption which is regarded to the transition between two stacked molecules has been observed in the α-form of 9, 10-dichloroanthracene; also a Rydberg type transition which has an out of plane polarization is found in this crystal starting at 290 mμ. It is unusually broad and is regarded as a band to band transition. The emission band has been measured at different temperature; it is found that excimer type emission is most prominent in the β-form of 9, 10-dichloroanthracene, in which intermolecular approach in the excited state will occur with less difficulty. A criterion for strong, medium, and weak interactions in the excited state is presented by comparing the shift, and the shape of the emission band with the intermolecular vibrational spacing. These results show that the charge transfer interaction and excimer formation are greatly dependent on the relative orientation of molecules, the π-electron overlap, and the difficulty of intermolecular approach in the excited state, particularly with its correlation to the steric repulsion of the bulky substituent groups.