Exploring the decay mechanisms of isoindigo from indolin-2-one-based derivatives: molecular isomerism vs. aromatic rigidity

Abstract

The excited state decay pathways of indigo (IND) and its structural isomers, indirubin (INR) and isoindigo (ISO), have received considerable attention in recent years, with a focus on the molecular mechanisms involving excited state proton transfer (ESPT) and rotational isomerisation. In this work, we aim to deepen the understanding of the decay mechanisms of ISO for which we have synthesized two new structures derived from indolin-2-one. The first structure consists of two indolin-2-one units linked by two double bonds, designated 3,3′-(ethane-1,2-diylidene)bis(indolin-2-one), abbreviated as EBI, while the second structure is a fused aromatic system with a double bond, 6,12-dihydrodibenzo[c,h][2,6]naphthyridine-5,11-dione (abbreviated as DBND). EBI consists of a flexible structure, allowing for different isomeric forms in both the ground and excited states, while DBND is a rigid, polyaromatic molecule. Three geometrical isomers of EBI were identified on the basis of their geometries (E,E′-EBI, Z,E′-EBI and Z,Z′-EBI) and efficiently purified using high-performance liquid chromatography (HPLC). The excited state properties were studied using steady-state absorption and emission spectroscopy and femtosecond transient absorption spectroscopy (fs-TA). Quantum chemical calculations provided insight into the observed spectral features. The flexible structure of EBI in the excited state, regardless of its isomeric form, enables efficient radiationless decay. In contrast, the rigid molecular structure of DBND leads to predominant deactivation by a radiative pathway.