Unsymmetrical squaraine dyes with extended conjugation for second-order nonlinear optics and TiO2 sensitization to far-red light: a computational quantum chemical study

Abstract

Conjugation elongation of squaraines is a potential approach to optimize their performance as nonlinear optical (NLO) chromophores and TiO₂-photosensitizers in dye-sensitized solar cells (DSCs). This study investigates the impact of integrating π-conjugated heteroaromatic spacers on the optoelectronic properties of four modeled unsymmetrical squaraine derivatives. Density functional theory (DFT) and time-dependent TD-DFT computations revealed the dual functionality of all four π-extended squaraine dyes, with the capability to sensitize TiO₂ for far-red light harvesting and amplify second-order NLO response at the molecular-level. Dye SQ-N incorporating an ethyl-dithienopyrrole π-spacer emerged as the optimal photosensitizer for TiO₂-based DSCs, exhibiting a hyperchromic S₁ transition and a light harvesting efficiency (LHE) of 98% at 697 nm (λ_max), the most thermodynamically driven electron injection (ΔG_inj), robust adsorption (E_ads) onto the TiO₂ nanocluster, enhanced orbital coupling (ΔE_oi) and hybridization between virtual molecular π* orbitals of SQ-N and 3d-orbitals of Ti atoms, in addition to superior charge transfer at the SQ-N–TiO₂ interface, under deep red-to-NIR photoexcitation. Conversely, the P-acetyl dithienophosphole oxide π-linker in SQ-P led to a sixfold enhancement in off-resonant hyperpolarizability (β₀) compared to the π-spacer-free parent dye, in addition to manifesting the maximal dynamic electro-optic Pockels (EOP) β₁₀₆₄ and β₁₄₆₀, second-harmonic generation (SHG) β₁₀₆₄ and hyper-Rayleigh scattering β₁₀₆₄. Analytical DFT-predicted SHG activity of the modeled dyes showed simultaneous potential for NIR-to-green and telecom E-band (1460 nm) to red light conversion. β scans demonstrated dual EOP and optical rectification functionality, while dyes SQ-N and SQ-Th further displayed significant sum/difference frequency generation (SFG/DFG) output at ω₁ ± ω₂. Polarization-resolved SHG analysis revealed a hybrid dipolar–octupolar NLO symmetry across all the dyes, with maximal harmonic intensity at Ψ = ±90°—a signature of synergistic dipole alignment and 3D charge delocalization.