Investigation of the guaiazulene–p-chloranil charge transfer complex in solution: a combined spectroscopic and quantum chemical approach

The spontaneous interaction between the electron-rich guaiazulene (D) and the electron-deficient p-chloranil (A) in solution leads to the formation of a stable donor–acceptor charge-transfer (CT) complex, which is characterized by a distinct near-infrared CT absorption band centered at 808 nm. The present study aims to elucidate the nature, strength, and electronic origin of this interaction through a combined experimental and theoretical approach. Absorption spectroscopy and fluorescence quenching experiments reveal significant donor–acceptor association, yielding binding constants of 20,830 dm³ mol⁻¹ and 43,560 dm³ mol⁻¹, respectively, thereby indicating strong intermolecular electronic coupling in solution. To gain deeper insight into the structural and electronic features of the complex, density functional theory (DFT) calculations were performed in both gas phase and non-polar solvent environments to obtain optimized geometries and analyze the ground- and excited-state electronic structures of the D–A system. The theoretical results suggest a favorable donor–acceptor alignment that facilitates efficient orbital overlap between the π-electron system of guaiazulene and the low-lying π* orbitals of p-chloranil. Furthermore, excited-state properties investigated using Time-Dependent Density Functional Theory (TDDFT) show excellent agreement with experimental findings, predicting a CT transition energy of 1.534 eV, which closely matches the experimentally observed value of 1.537 eV. The strong correlation between spectroscopic observations and theoretical predictions provides a comprehensive understanding of the electronic interactions governing the formation of the guaiazulene–p-chloranil charge-transfer complex and highlights the role of intermolecular donor–acceptor coupling in stabilizing the non-covalent assembly.