Intramolecular charge transfer in ferrocene appended Al(III), Si(IV) and Sb(V) porphyrin panchromatic dyes

A series of ferrocene-appended free-base porphyrin (TFcH) and metallo/metalloid-porphyrins incorporating aluminum(III), silicon(IV), and antimony(V) (TFcAl, TFcSi, and TFcSb) were synthesized and comprehensively characterized using spectroscopic, electrochemical, and analytical techniques. The optical properties reveal a strong dependence on the nature and oxidation state of the central metal ion, with systematic redshifts observed in the absorption spectra as the oxidation state increases. In particular, the antimony(V) derivative exhibits an intense and broad charge-transfer band extending into the near-infrared region (∼1100 nm), indicative of pronounced intramolecular charge transfer (ICT). Fluorescence studies show significant quenching across the series, with TFcAl and TFcSb being essentially non-emissive, attributed to efficient nonradiative decay pathways such as electron/charge transfer from the ferrocene units to the porphyrin core. Electrochemical investigations further support these findings, showing that redox potentials shift to more positive values as the oxidation state increases, consistent with enhanced electron-withdrawing character. Overall, the insertion of main-group ions with various oxidation states induces an asymmetric distribution of electron density within the porphyrin framework, leading to a push-pull electronic system that promotes ICT, which is maximized in the TFcSb.