Unusual high fluorescence of a 7,7'-diazaisoindigo derivative: a photophysical study

7,7'-Diazaisoindigos are π-conjugated compounds but with poor luminescence properties. Their poor luminescence is generally attributed to the twisting around the central C7−C7' bond in the excited state which favors non-radiative decay. We have found an unusual high fluorescence quantum yield (ΦF ≈ 15%) in a N,N‑Octyl-7,7′-diazaisoindigo derivative incorporating two triphenylamine (TPA) subunits at 5,5′-positions (called compound 12). There are very few examples of fluorescent 7,7'-diazaisoindigos have been reported and the emission mechanism is generally associated by the hindering of the rotation around the central C7−C7' bond. Unexpectedly, 12 also experiences a twisting around the central C7−C7' bond in the excited state but is highly emissive. The intense fluorescence emission of 12 was attributed to the coexistence of two TPA-7-azaoxindole subunits acting as two fluorophores into a single molecule. The fluorescence emission of 12 is also sensitive to the polarity of the solvent. The intramolecular charge transfer character of the excited state was studied combining spectroscopic and theoretical methods.