The exact energies of the lowest singlet and
triplet excited states in organic chromophores are crucial to their performance
in optoelectronic devices. The possibility of utilizing singlet fission to
enhance the performance of photovoltaic devices has resulted in a wide demand
for tuneable, stable organic chromophores with wide S<sub>1</sub> – T<sub>1</sub>
energy gaps (>1 eV). Cibalackrot-type compounds were recently considered to have
favorably positioned excited state energies for singlet fission, and they were
found to have a degree of aromaticity in the lowest triplet excited state (T<sub>1</sub>). This
work reports on a revised and deepened theoretical analysis taking into account the excited state Hückel-aromatic (instead of
Baird-aromatic) as well as diradical characters, with the aim to design new organic
chromophores based on this scaffold in a rational way starting from qualitative
theory. We demonstrate that the substituent strategy can effectively adjust the
spin populations on the chromophore moieties and thereby manipulate the excited
state energy levels. Additionally, the improved understanding of the aromatic
characters enables us to demonstrate a feasible design
strategy to vary the excited state energy
levels by tuning the number and nature of Hückel-aromatic units in the excited
state. Finally, our study elucidates the complications and pitfalls of the
excited state aromaticity and antiaromaticity concepts, highlighting that
quantitative results from quantum chemical calculations of various aromaticity
indices must be linked with qualitative theoretical analysis of the character
of the excited states.
Excited State Character of Cibalackrot-Type Compounds Interpreted in Terms of Hückel-Aromaticity: A Rational for Singlet Fission Chromophore Design
