Three symmetrically and three unsymmetrically substituted
cibalackrot
(7,14-diphenyldiindolo[3,2,1-de:3′,2′,1′-ij][1,5]naphthyridine-6,13-dione,1) dyes carrying
two derivatized phenyl rings have been synthesized as candidates for
molecular electronics and especially for singlet fission, a process
of interest for solar energy conversion. Solution measurements provided
singlet and triplet excitation energies and fluorescence yields and
lifetimes; conformational properties were analyzed computationally.
The molecular properties are close to ideal for singlet fission. However,
crystal structures, obtained by single-crystal X-ray diffraction (XRD),
are rather similar to those of the polymorphs of solid1, in which the formation of a charge-separated state followed by
intersystem crossing, complemented with excimer formation, outcompetes
singlet fission. Results of calculations by the approximate SIMPLE
method suggest which ones among the solid derivatives are the best
candidates for singlet fission, but it appears difficult to change
the crystal packing in a desirable direction. We also describe the
preparation of three specifically deuteriated versions of1, expected to help sort out the mechanism of fast intersystem crossing
in its charge-separated state.