Metal coordinated porphyrin systems are one of the most fundamental and common units consisting of the system of nature and human body as the representative forms of enzyme and catalyst. Their distinct metabolic and molecular properties are developed by metal-ligand interaction. The metal-ligand interaction is developed by the electronic states of metal and ligand, molecular geometry, and corresponding orbital interactions. Therefore, elucidation of the role of metalation effect and the orbital interaction between metal and ligand depending on the molecular conformation is crucial to an in-depth understanding of their functional molecular properties and precise control for promising applications.
In this study, we have focused on two different aromatic mono-Pd(II) hexaphyrins, PdH and PdM, which exhibit Hückel and Möbius aromaticity, respectively. These aromatic mono-Pd(II) hexaphyrins have the same palladium metal and aromatic hexaphyrin skeleton, but their conformational geometries are different. In the ground state, the FT-IR spectra of two Pd(II) hexaphyrins in the region of C=C stretching modes showed intense IR bands at 1500 cm-1 similar to those of meso-C6F5 hexaphyrins.  However, their TRIR spectral evolutions upon photoexcitation were in a sharp contrast with each other. The combination of GSB and ESA for C6F5 group IR bands appear in the range of 1490 ~ 1530 cm-1 at the initial spectrum. The TRIR spectra of PdH (Fig. 2e) exhibited a dynamic shift of ESA within 100 ps and relaxation to the ground state which is corresponding to the lifetime of 500 ps. However, those of PdH (Fig. 2f) exhibited only residual signals for the triplet state after the intersystem crossing dynamics without any spectral shift of ESA band at 1510 cm-1.