Resistance to Unwanted Photo-Oxidation of Multi-Acene Molecules

Although long acenes remain a key class of π-conjugated molecules for numerous applications, photoinduced oxidation upon exposure of the acene to light, often through sensitization of ¹O₂, is an important reaction requiring mitigation for most applications. In response to this ongoing challenge, this paper presents a series of four new diarylethynyl-substituted long acenes-three tetracenes and one anthradithiophene-in which the arylene pendants are either benzene, naphthalene, or anthracene. UV/vis and fluorescence spectroscopy reveals that the anthracene-substituted derivatives fluoresce poorly (Φ < 0.01). Although all four long acenes react with ¹O₂ at expected rates when an external photosensitizer is included and show the expected changes in fluorescence to accompany these reactions, the anthracene-substituted derivatives resist direct photoinduced oxidation. Through a combination of mechanistic experiments, we conclude that rapid nonradiative decay of the anthracene-substituted derivatives, perhaps because of inter-arene torsions that emerge in theoretical geometry optimizations, makes these compounds poor photosensitizers for ¹O₂ or other reactive oxygen species. This discovery opens new design possibilities for extended acene structures with improved photochemical stability.