DFT/MRCI-R2018 study of the photophysics of the zinc(ii) tripyrrindione radical: non-Kasha emission?

Stable radical-based fluorescent emitters are rare, even more so at room temperature. The zinc(ii) tripyrrindione [Zn(TD1)(H2O)] complex has recently been described experimentally by Gautam et al. [Inorg. Chem., 2018, 57, 15240] as a new member of the small family of neutral radical emitters with possible applications in electronics and photonics. Upon excitation at the absorption maximum of 599 nm (2.07 eV), strong fluorescence was observed with a maximum at 644 nm (1.93 eV) at room temperature in tetrahydrofuran solution. The fluorescence energy is higher than several low-intensity absorption bands starting at approximate to 930 nm. Here we present a theoretical investigation into the absorption and fluorescence of this zinc complex by means of the recently developed semi-empirical all-multiplicity DFT/MRCI-R2018 method. The DFT/MRCI method combines density functional theory (DFT) in a closed shell or restricted open-shell Kohn-Sham orbital basis and multireference configuration interaction (MRCI). The R2018 Hamiltonian proves to be well-suited for investigating the properties of the radical-based zinc complex. The calculations reveal that the absorption spectrum is dominated by bright transitions to the D-3, D-6 and D-11 states. The experimentally observed emission band lies at considerably shorter wavelengths than the lowest absorption band of the radical. This precludes the D-1 -> D-0 transition as the origin of the emission. Our calculations indicate a non-Kasha emission, with D-3 as the emissive state. Other ways of explaining the experimentally observed emission, such as ion-pair formation or ligand emission after demetalation, are discussed as well.