Singlet Fission in Push–Pull <i>Para</i> ‐Azaquinodimethane Films under the Gaze of Time Resolved Optical and Magnetic Spectroscopy

In this study, a totally green synthetic protocol was employed to obtain four new donor-acceptor-donor para-azaquinodimethane (pAQM) derivatives to uncover the effect of tuning the push-pull character on their photobehaviour in solution as well as in thin film and in particular on their capability of undergoing singlet fission (SF). The bisthiophene and substituted benzene were used as electron-donor groups at opposite sides of the pAQM electron-acceptor core in asymmetric structures (AsOMe and AsNMe₂), whereas different 5-phenyl thiophene moieties were symmetrically linked to the pAQM core in TPh and TPhOMe. The photoinduced excited state dynamics was investigated in a synergic effort by employing both time resolved optical and electron paramagnetic resonance (EPR) spectroscopies, with the aid of TD-DFT calculations. Our femtosecond transient absorption results showed that intermolecular SF is enabled in the solid state aggregates of these compounds: the fastest and most efficient SF was revealed for the structure with the strongest push-pull degree (AsNMe₂). The other asymmetric molecule (AsOMe) interestingly exhibited higher energetic SF-generated triplet excitons (1.3 eV) than literature SF-materials, as demonstrated by the nanosecond transient absorption sensitization experiments, with possible intriguing implications toward silicon matched SF photovoltaics. Time resolved EPR measurements unveiled the spectral signature of the quintet multiexciton intermediate ⁵(TT) in the pAQM thin films for the first time, to the best of our knowledge, for such unconventional SF materials. The high-spin photoinduced ⁵(TT) appears to be a promising candidate for Quantum Information Science and technologies as a viable molecular spin qudit.