Metal-dependent modulation of ESIPT in a Furan-substituted chromone : A comparative DFT/TD-DFT study of Zn²⁺, Ca²⁺, and Cu²⁺ complexes

Abstract

A comparative DFT/TD-DFT study at the B3LYP-D3BJ/def2-TZVP level, employing CPCM with methanol as solvent, was conducted to elucidate the electronic structure and photophysical properties of 2-(2-furyl)-3-hydroxychromone (FHC) and its deprotonated complexes with Zn²⁺, Ca²⁺, and Cu²⁺. In the free ligand, the enol form is stabilized by an intramolecular O–H· · · O(furan) hydrogen bond. TD-DFT calculations predict a bright S1 state at 339 nm (f = 0.801). Geometry optimization of the S1 state for both enol and keto tautomers yields vertical emissions at 380.7 nm (f = 0.831) and 456.0 nm (f = 0.847), respectively, thereby supporting the ESIPT character of the free fluorophore. Bidentate coordination via the carbonyl (O21) and deprotonated hydroxyl (O22) oxygen atoms yields stable [M(FHC-H)]⁺ complexes. Natural population analysis reveals significant metal-dependent ligand-to-metal charge transfer (Cu²⁺> Zn²⁺> Ca²⁺), with notable spin delocalization in the Cu²⁺ complex (spin population ≈ 0.61 on Cu). TD-DFT predicts intense absorptions at 358 nm (Zn²⁺, f = 0.682) and 382 nm (Ca²⁺, f = 0.783), whereas the Cu²⁺ complex exhibits only very weak transitions in the near-infrared region, rendering it effectively optically silent in the UV-visible range. In all complexes, metal coordination induces deprotonation and eliminates the intramolecular hydrogen bond, thereby structurally suppressing the ESIPT pathway. These computational results suggest distinct metal-dependent photophysical responses that may be exploited for the development of ESIPT-based fluorescent probes, although experimental validation is required. 

https://doi.org/10.70974/mat10126093