Singlet Fission and Aromaticity

Abstract

Qualitative aromaticity-based arguments are often used to explain singlet fission (SF) properties in polycyclic conjugated systems. It has been shown by Fowler and collaborators that magnetically induced ring currents are associated with transitions between occupied and unoccupied molecular orbitals (MOs). Since SF has to do with relative energies of electronic states, it was hypothesized that induced currents may indicate SF properties. The quantitative aromaticity of linear oligoacene and several doubly boron-doped anthracenes and phenanthrenes, in their closed-shell singlet, open-shell singlet (where applicable), and triplet electronic states, has been studied using nucleus-independent chemical shift (NICS)-XY scan methods. It is shown that quantitative magnetic aromaticity can be used to identify SF compounds, at least for initial screening. Thus, the induced current features that are indicative of singlet fission (SF) ability have been found to be global current and local current at each ring of the systems. This conclusion was verified for very different systemsa tetracyclic, nitrogen-containing quinone (6), diphenyl benzofurane (7), and cibalackrot (8), all reported to be SF systems. Finally, it is predicted that some isomers of the doubly boron-doped linear[4]­phenylene should show SF properties.

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

“…The resulting slightly pink product was sufficiently pure for further reaction (2.0 g, 91%). 1 (12). Caution: 1,1,2,2-tetrachloroethane is a very toxic solvent.…”

“…The isolated solid was purified by column chromatography on silica gel using CH 2 Cl 2 and CH 2 Cl 2 �ethyl acetate 100:1 and 50:1 (v/v) as eluents to yield 7-(pentafluorophenyl)-6H-pyrido[1,2-a:3,4-b′]diindole-6,13(12H)-dione (12), identified by 1 H NMR. Freshly prepared pentafluorophenylacetyl chloride (11, 1.4 g, 5.7 mmol) was dissolved in the anhydrous isomer mixture of xylenes (3 mL), and the resulting solution was added to 7-(pentafluorophenyl)-6H-pyrido[1,2a:3,4-b′]diindole-6,13(12H)-dione (12). The reaction mixture was refluxed (oil bath) in a nitrogen atmosphere for 72 h. The solvents were removed under reduced pressure, and the resulting mixture was purified by column chromatography on silica gel using CH 2 Cl 2 and CH 2 Cl 2 −ethyl acetate 100:1 (v/v) as eluents.…”

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

“…A solution of pentafluorophenylacetyl chloride (11, 2.0 g, 8.2 mmol) in anhydrous isomer mixture of xylenes (20 mL) was added to indigo (8, 211 mg, 0.8 mmol), and the resulting mixture was refluxed (oil bath) in nitrogen atmosphere for 22 h. The solvents were removed under reduced pressure and the residue was triturated with hexane (4 × 25 mL). The isolated solid was purified by column chromatography on silica gel using CH 2 Cl 2 and CH 2 Cl 2 -ethyl acetate 100:1 and 50:1 (v/v) as eluents to yield 7-(pentafluorophenyl)-6H-pyrido[1,2-a:3,4-b']diindole-6,13(12H)-dione (12), identified by 1 H NMR. Freshly prepared pentafluorophenylacetyl chloride (11, 1.4 g, 5.7 mmol) was dissolved in anhydrous isomer mixture of xylenes (3 mL), and the resulting solution was added to 7-(pentafluorophenyl)-6H-pyrido[1,2a:3,4-b']diindole-6,13(12H)-dione (12).…”

“…The isolated solid was purified by column chromatography on silica gel using CH 2 Cl 2 and CH 2 Cl 2 -ethyl acetate 100:1 and 50:1 (v/v) as eluents to yield 7-(pentafluorophenyl)-6H-pyrido[1,2-a:3,4-b']diindole-6,13(12H)-dione (12), identified by 1 H NMR. Freshly prepared pentafluorophenylacetyl chloride (11, 1.4 g, 5.7 mmol) was dissolved in anhydrous isomer mixture of xylenes (3 mL), and the resulting solution was added to 7-(pentafluorophenyl)-6H-pyrido[1,2a:3,4-b']diindole-6,13(12H)-dione (12). The reaction mixture was refluxed (oil bath) in nitrogen atmosphere for 72 h. The solvents were removed under reduced pressure and the resulting mixture was purified by column chromatography on silica gel using CH 2 Cl 2 and CH 2 Cl 2 -ethyl acetate 100:1 (v/v) as eluents.…”

“…The sturdy indigoid industrial dye known as cibalackrot (7,14-diphenyldiindolo[3,2,1-de:3',2',1'ij] [1,5]naphthyridine-6,13-dione, 1), 1,2 and its analogs containing thienyl instead of phenyl groups 3,4 have been of potential interest as organic electronic, 5,6 photonic, 7 and singlet fission 3,8,9,10,11,12 materials. Of particular appeal to us is singlet fission, 13 a process in which a singlet exciton splits into two triplet excitons, promising to overcome the Shockley-Queisser limit 14 of about 1/3 to single-junction solar cell efficiency.…”