Confirmation of the Origins of Panchromatic Spectra in Squaraine Thin Films Targeted for Organic Photovoltaic Devices

Squaraines offer significant potential
for organic photovoltaics
because of their broad absorbance and high extinction coefficients
as well as their expected use as mechanistic probes in such devices.
In this work steps are taken to develop a comprehensive understanding
of the excited-state properties of squaraines on the basis of their
molecular structure and the resulting solid-state packing. Accurate
assignments of the absorption spectral peaks are made on the basis
of an essential states model, expanded to include intermolecular charge
transfer (ICT). A comparison of simulated spectra with spectra for
two symmetric squaraine derivatives confirms that ICT has a major
influence on the optical and electronic properties of squaraine aggregates,
fully accounting for the origin of the strong panchromaticity of these
systems.