  Prototypical n-alkyl terminated anilino squaraines for photovoltaic
applications show characteristic double-hump absorption features peaking in the
green and deep-red spectral range. These signatures result from coupling of an
intramolecular Frenkel exciton and an intermolecular charge transfer exciton.
Crystalline, textured thin films suitable for polarized spectro-microscopy have
been obtained for compounds with n-hexyl (nHSQ) and n-octyl (nOSQ) terminal
alkyl chains. The here released triclinic crystal structure of nOSQ is similar
to the known nHSQ crystal structure. Consequently, crystallites from both
compounds show equal pronounced linear dichroism with two distinct polarization
directions. The difference in polarization angle between the two absorbance
maxima cannot be derived by spatial considerations from the crystal structure
alone but requires theoretical modeling. Using an essential state model, the
observed polarization behavior was discovered to depend on the relative
contributions of the intramolecular Frenkel exciton and the intermolecular
charge transfer exciton to the total transition dipole moment. For both nHSQ
and nOSQ, the contribution of the charge transfer exciton to the total
transition dipole moment was found to be small compared to the intramolecular
Frenkel exciton. Therefore, the net transition dipole moment is largely
determined by the intramolecular component resulting in a relatively small
mutual difference between the polarization angles. Ultimately, the molecular
alignment within the micro-textured crystallites can be deduced and, with that,
the excited state transitions can be spotted.
