Knowledge Bank

Near-normal incidence specular reflection spectra in the uv and vis energy regions are reported for single crystals of the anthrancene-1, 2,4,5-tetracyanobenzene (A-TCNB) electron-donor-acceptor complex. These data reveal tow charge-transfer (CT) exciton transitions polarized along the c-crystallographic axis which is the stacking direction of the complex molecules in the crystal. A third CT transition, perpendicular to the former two, is suggested on grounds of distortions of the vibronic progression of the first singlet ($\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}B2u\leftarrow 1Ag$) transition of anthracene. The crystal absorption spectra, derived via the Kramers-Kronig relations, are used to obtain transition energies, integrated intensities, and absolute polarizations. All observed excitations are supported by group-theoretical arguments. The spectra also allow an assignment of the singlet molecular transitions of the electron acceptor TCNB. A-TCNB crystals undergo an orientational phase transition upon lowering of the temperature to below T=206 K. The effect of this polymorphism on the CT excitons is discussed using the low temperature crystal spectra of the complex.