Phenyl-Substituted Cibalackrot Derivatives: Synthesis, Structure, and Solution Photophysics

Three symmetrically and three unsymmetrically substitutedcibalackrot(7,14-diphenyldiindolo-[3,2,1-de:3 ',2 ',1 '-ij]-[1,5]-naphthyridine-6,13-dione, 1) dyes carryingtwo derivatized phenyl rings have been synthesized as candidates formolecular electronics and especially for singlet fission, a processof interest for solar energy conversion. Solution measurements providedsinglet and triplet excitation energies and fluorescence yields andlifetimes; 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 solid 1, in which the formation of a charge-separated state followed byintersystem crossing, complemented with excimer formation, outcompetessinglet fission. Results of calculations by the approximate SIMPLEmethod suggest which ones among the solid derivatives are the bestcandidates for singlet fission, but it appears difficult to changethe crystal packing in a desirable direction. We also describe thepreparation of three specifically deuteriated versions of 1, expected to help sort out the mechanism of fast intersystem crossingin its charge-separated state.

Automated summary

Three symmetrically and three unsymmetrically substituted cibalackrot dyes have been synthesized and studied for their potential use in molecular electronics and singlet fission. Solution measurements and computational analysis revealed the molecular properties to be close to ideal for singlet fission. However, the crystal structures obtained through XRD showed competition from charge separation, intersystem crossing, and excimer formation. Calculations indicated the difficulty of changing the crystal packing in a desirable direction. Preparation of deuteriated versions of the dyes was also discussed to further investigate the mechanism of fast intersystem crossing.