DFT study of the spectroscopic behaviour of different iron(II)-terpyridine derivatives with application in DSSCs

Through a comprehensive computational analysis utilizing Density Functional Theory (DFT), we clarify the electronic structure and spectroscopic properties of modified iron(II)-terpyridine derivatives, with the aim of enhancing the efficiency of Dye-Sensitized Solar Cells (DSSCs). We optimized a series of nineteen iron(II)-terpyridine derivatives and related compounds in acetonitrile (MeCN) as the solvent using TDDFT, evaluating their potential as dyes for DSSCs. From the conducted computations on the optimized geometries of the nineteen [Fe(Lⁿ)₂]²⁺ complexes, containing substituted terpyridine and related ligands L¹-L¹⁹, we determined the wavelengths (λ in nm), transition energy (E in eV), oscillator strength (f), type of transitions, excited state lifetime (τ), light harvesting efficiency (LHE), frontier orbital character and their energies (E_LUMO/E_HOMO), natural transition orbitals (NTOs), injection driving force of a dye (ΔG_inject), and regeneration driving force of a dye (ΔG_regenerate). Results show that the theoretically calculated values for assessing dye efficiency in a DSSC correlate with available experimental values. The UV–visible spectra of [Fe(Lⁿ)₂]²⁺ exhibited a peak above 500 nm (λ_max) in the visible region, attributed to the ligand-to-metal charge transfer band (LMCT) in literature, and a significant absorbance peak at approximately 300 nm (λ_A,max) in the UV region. The M06-D3/CEP-121G method replicated all reported λ_max and λ_A,max values with a mean absolute deviation (MAD) of 21 and 18 nm, respectively. Our findings underscore the connections between electronic modifications and absorption spectra, emphasizing their impact on the light-harvesting capabilities and overall performance of DSSCs. This research contributes to the advancement of fundamental principles governing the design and optimization of novel photovoltaic materials, facilitating the development of more efficient and sustainable solar energy technologies.