Enlightening the photochemical behavior of INCN-functionalized donor-acceptor molecules.

Parag Das, Cory T. Kornman, Ion Ghiviriga, Khalil A. Abboud, and Ronald K. Castellano.

University of Florida

2-(3-Oxo-2,3-dihydroinden-1-ylidene)malononitrile (INCN) is a popular electron deficient unit appended to small molecule (SM) non-fullerene acceptor (NFA) materials used in bulk-heterojunction (BHJ) organic photovoltaics (OPVs). Since its introduction in 2015, the INCN unit has been showcased in hundreds of push-pull oligomers and has served an important role in the development and understanding of small molecule-based OPVs. Interestingly, the configuration and conformation of the exocyclic olefin which conjugates INCN to pi-conjugated materials have been largely overlooked. It is anticipated based on our prior work that the stereochemistry of the double bond can significantly affect the optoelectronic properties in solution and consequently the solid-state morphology of thin films of such small oligomeric molecules. To begin testing this hypothesis, we have designed and synthesized a library of small molecules functionalized with INCN that vary in donor unit, conjugation length, steric crowding, and end-group substitution to modulate the electronic character and conformational preferences. The molecules all undergo well-controlled Z/E photoisomerization using different selective irradiation sources, yielding a range of Z/E photostationary state (PSS) distributions. Our studies have shown a well-behaved photoisomerization behavior at nearly all selective wavelengths, however, a photoinduced tandem electrocyclization-sigmatropic reaction is also plausible though restricted to one selective wavelength. Facile photoswitching has been demonstrated over ten cycles for a few model compounds. Complementary DFT calculations will also be discussed that provide additional mechanistic insight into the photochemical behavior of INCN-functionalized molecules. The results clarify two recent literature reports of photoisomerization related photodegradation of INCN-functionalized donor-acceptor molecules using broad spectrum irradiation sources.