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 <i>ab</i> 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 <i>ac</i> plane, while each layer composed of stacked TAB molecules along the <i>c</i> 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<sub>max</sub><sup>a</sup> = 499 nm) and fluorescence peaks (λ<sub>max</sub><sup>f</sup> = 570 nm), distinct absorption (λ<sup>a</sup> = ∼470 nm) and fluorescence bands (λ<sup>f</sup> = ∼600 nm) were observed for the R-form crystal, while the relevant absorption band in the O-form crystal (λ<sup>a</sup> = ∼460 nm) or in solution (λ<sup>a</sup> = ∼435 nm) was ambiguous. The results were discussed in terms of participation of the higher energy second CT transition in TAB.