Planarity-induced enhancement of excited-state proton transfer and near-infrared emission in conjugated <i>N</i> , <i>N</i> -dimethylamino cyclic chalcone derivatives: a combined photophysical and theoretical investigation

The excited-state dynamics of chalcone-based chromophores are strongly influenced by molecular conformation, electronic delocalization, and solvation. In this work, we investigate the steady-state and ultrafast photophysics of a newly synthesized indanone-bridged chalcone derivative, (<i>E</i>)-2-(4-(dimethylamino)benzylidene)-7-hydroxy-2,3-dihydro-1<i>H</i>-inden-1-one (DHCF), to assess the role of conformational planarity in excited-state intramolecular proton transfer (ESIPT). Incorporation of the rigid, planar indan framework suppresses torsional motion around the phenyl ring and promotes efficient ESIPT, evidenced by additional red-shifted absorption and emission bands assigned to the tautomeric form. To validate this mechanism, we synthesized the corresponding hydroxyl-free analogue, (<i>E</i>)-2-(4-(dimethylamino)benzylidene)-2,3-dihydro-1<i>H</i>-inden-1-one (DCF). The absorption and fluorescence characteristics of DCF closely match those of the locally excited (LE) state of DHCF. Transient absorption spectroscopy of DHCF reveals tautomer formation on a ∼3 ps timescale, coinciding with the rise of the tautomer's stimulated emission band, which subsequently decays within 48-108 ps depending on solvent. Direct comparison with DCF demonstrates that tautomer formation accelerates the LE relaxation, reducing its lifetime from 156 ps to 55 ps in acetonitrile and from 314 ps to 56 ps in dioxane. Complementary DFT and TD-DFT calculations provide insights into the potential energy surfaces and tautomerization pathways. These findings elucidate mechanistic understanding of structure-property relationships in ESIPT-active systems and provide guiding principles for the rational design of organic fluorophores for photonic and optoelectronic applications.