Singlet fission is
a photophysical process that provides a pathway
for more efficient harvesting of solar energy in photovoltaic devices.
The design of singlet fission candidates is non-trivial and requires
careful optimization of two key criteria: (1) correct energetic alignment
and (2) appropriate intermolecular coupling. Meanwhile, this optimization
must not come at the cost of molecular stability or feasibility for
device applications. Cibalackrot is a historic and stable organic
dye which, although it has been suggested to have ideal energetics,
does not undergo singlet fission due to large interchromophore distances,
as suggested by single crystal analysis. Thus, while the energetic
alignment is satisfactory, the molecule does not have the desired
intermolecular coupling. Herein, we improve this characteristic through
molecular engineering with the first synthesis of an aza-cibalackrot
and show, using ultrafast transient spectroscopy, that singlet fission
is successfully “turned on.”