Xanthene-anchored salen-based open and closed dinuclear indium complexes: synthesis and photophysical properties

Xanthene-anchored dinuclear salen-based indium complexes with well-defined open and closed topologies were designed to elucidate how dinuclear geometry governs excited-state decay processes in indium luminophores. A rigid yet non-planar xanthene linker enables controlled formation of open (XPOIn and XNOIn) and closed (XPCIn and XNCIn) dinuclear architectures via a one-pot synthetic protocol. Single-crystal X-ray diffraction analyses of XNOIn and XNCIn revealed distinct spatial arrangements of the two salen-indium units, accompanied by different degrees of intramolecular π-π interactions and conformational rigidity. All complexes exhibited strong visible absorption bands originating from salen-centered π-π* transitions with partial charge-transfer (CT) characteristics and displayed yellow fluorescence in both solution and solid states. Notably, the open dinuclear systems consistently exhibited higher PLQYs than their closed analogues, despite the latter exhibiting more stabilised frontier molecular orbitals. Photophysical analysis revealed that the enhanced emission efficiency of the open systems originated from the increased radiative decay rates and suppressed non-radiative relaxation pathways. These results demonstrate that dinuclear topology plays a key role in governing excited-state dynamics in salen-indium systems and provide a useful framework for the rational design of multinuclear indium-based luminophores.