Spectroscopic and Excited-State Properties of Tri-9-anthrylborane III:  Crystal and Spectroscopic Polymorphisms

We found that tri-9-anthrylborane (TAB) recrystallized from benzene produced both red cubic-like (R-form) and orange hexagon-like crystals (O-form). In both crystal forms, six TAB molecules are arranged in a honeycomb structure in the ab plane and benzene molecules are incorporated in the honeycomb structure, whose spatial geometry and the total number of benzene rings in the unit cell are different between the two forms:  polymorphs with a different benzene content. In the R-form crystal, furthermore, interlayer stacking between left- and right-handed helical TAB molecules was observed in the ac plane, while each layer composed of stacked TAB molecules along the c axis was separated by benzene molecules in the O-form crystal, giving rise to more dense packing of TAB in the R-form crystal as compared to that in the O-form. Reflecting the crystal structures of the two forms, the charge transfer (CT) absorption and fluorescence spectra of the R-form crystal were shifted to the longer wavelength as compared to those in the O-form (i.e., crystal and spectroscopic polymorphisms) and, therefore, electronic interactions between TAB were stronger in the R-form as compared to those in the O-form. Furthermore, in addition to the main absorption (λ_max^a = 499 nm) and fluorescence peaks (λ_max^f = 570 nm), distinct absorption (λ^a = ∼470 nm) and fluorescence bands (λ^f = ∼600 nm) were observed for the R-form crystal, while the relevant absorption band in the O-form crystal (λ^a = ∼460 nm) or in solution (λ^a = ∼435 nm) was ambiguous. The results were discussed in terms of participation of the higher energy second CT transition in TAB.