Akamptisomerism as a Switching Element: Substituent Effects on Bond Angle Reflection and Photophysical Properties of B-O-B Porphyrins.

Developing novel molecular switches requires structural modifications that yield isomers with distinct photophysical properties. Akamptisomerism ─ a bond angle reflection (BAR) process occurring in low-symmetry porphyrins bridged by (F)B-O-B(F) units ─ induces distortions from the porphyrin pseudoplane, offering a promising strategy for optoelectronic switching. Herein, we report the first quantum chemical investigation of BAR in β- and -substituted porphyrinoid systems as a potential switching mechanism. Density functional theory (DFT) calculations (B3LYP-D3/def2-QZVP//B3LYP-D3/6-31+G**) confirm the occurrence of ()-akamptisomerism in all evaluated compounds, featuring a thermal interconversion barrier ( → ) of 26.6 ± 2.1 kcal mol that is largely unaffected by porphyrin substitution across 28 compounds. Simulated UV-vis spectra (TD-CAM-B3LYP/6-31+G**) reveal that β,β-push-pull systems bearing -NMe and -NO groups on opposite pseudoplanes, combined with -Bu substitution, exhibit significant spectral differentiation between akamptisomers (Δ / = 320.6 meV, at the S state). This shift arises from intramolecular charge transfer between pseudoplanes and steric distortion induced by the -Bu group. These findings establish akamptisomerism as a viable platform for constructing molecular building blocks with distinct optical signatures. Moreover, they underscore how substitution patterns can be exploited to tune the photophysical properties of BAR systems, providing valuable insights for the rational design of next-generation switchable materials.