Anomalous Paramagnetism in Closed-Shell Molecular Semiconductors

Anomalous electron paramagnetic resonance (EPR) signals from formally closed-shell phthalocyanines have been a longstanding mystery. For the past few decades, this illogical observation has remained unexplored because of the belief that it is unique to only the one class of chromophores, namely, phthalocyanines. Here we show that, in fact, a broad structural range of molecular semiconductors, including pentacene, diindenoperylenes, rylene diimides, pyrrolo­[c]­pyrroles, and indenofluorenes, show a strong, clear EPR signal (X-band) in the solid state, which is not present in the solution EPR spectra of the same compounds. Further, magnetic susceptibility measurements confirm that these formally closed-shell molecules are paramagnetic (as bulk powders), even at low temperatures. In some compounds, the intensity of the EPR signal or value of magnetic susceptibility increases after sample purification via physical vapor transport. EPR signal evolution can be directly correlated to the evolution of molecular aggregates. We propose that such anomalous paramagnetism arises from a small concentration of intrinsic radical cations or anions generated through exposure to ambient atmosphere (oxygen, water) and light. The phenomenon described herein notably alters how conjugated molecules/polymers are conceptualized, designed, and processed for nascent magnetoelectronic and magneto-optic applications