Anomalous emission of an ESIPT-capable zinc(II) complex: An interplay of TADF, TICT and anti-Kasha behaviour

Abstract

In contrast to the vast majority of organic ESIPT-emitters (ESIPT – excited state intramolecular proton transfer), a pyrimidine-based ESIPT-type ligand HL can bind metal ions without its deprotonation due to the spatial separation of metal-binding and ESIPT cores. The photophysical and photochemical properties of HL and the corresponding ESIPT-capable zinc(II) complex [Zn(HL)Cl2] were theoretically investigated using single-reference (DFT) and multi-reference (CASSCF, NEVPT2) quantum chemical methods. The ESIPT process in both compounds is barrierless and thermodynamically favorable in the S1 and S2 states. HL and [Zn(HL)Cl2] demonstrate abnormal anti-Kasha fluorescence (S2 → S0) and anti-Kasha phosphorescence (T2 → S0) associated with relatively low S2 → S1 and T2 → S1 internal conversion rates. Energetically achievable S1/S0 and T1/S0 intersections are responsible for the fast non-radiative decay of the S1 and T1 states. Quasi-intersections of the S2 and T2 state potential energy surfaces facilitate thermally activated delayed fluorescence (TADF, T2 → S2 → S0) for both compounds. The molecules of [Zn(HL)Cl2] can be trapped and photoexcited from the local minimum of the S0 state in the tautomeric keto form, which results in an anomalously small apparent Stokes shift of 90 nm. The absence of such minimum on the ground state potential energy surface of HL leads to a considerably larger Stokes shift of 165 nm.