Impact of Axial and Peripheral Perfluoro-Substitution on the Photophysical Properties of Titanium Phthalocyanines

While peripheral fluorination and axial substitution are established strategies in phthalocyanine chemistry, their combined effect on titanium oxide phthalocyanine (TiOPc) derivatives remains largely unexplored. To address this, we designed and synthesized two novel TiOPc derivatives: CYLY6–0, featuring peripheral perfluoro-substituents, and CYLY6–1, which further incorporates an axial 3,4-dihydroxybenzaldehyde ligand coordinated to the central titanium atom. Most significantly, the nonlinear optical (NLO) results revealed that the two-photon absorption (TPA) cross-section of CYLY6–0 is 2.8 times larger than that of the parent TiOPc. Contrary to the commonly held view that axial substitution enhances NLO properties, the TPA cross-section of CYLY6–1 (with both substitutions) was only 2 times larger than that of TiOPc, indicating that the axial moiety suppresses the enhancement of NLO properties, by peripheral fluorination. This unanticipated antagonistic effect provides a critical revision to the structure–property relationship in TiOPc derivatives and offers a clear, refined design strategy for advanced organic NLO materials: peripheral fluorination is highly beneficial, while further axial coordination can be counterproductive. These insights provide valuable references for the molecular design of high-performance TiOPc-based materials.