Characterization of 2,7,12,17-tetra-tert-butyl-porphycene and its Pd2+ complex: from synthesis to the photoexcited triplet state.

Porphycenes as constitutional isomers of porphyrins require more demanding synthesis but exhibit fascinating and useful properties. In free-base porphycenes, tautomerism in the ground state and strong fluorescence from the photoexcited singlet state are established. Production of reactive oxygen species (ROS) via the photoexcited triplet state is observed for both free-base and metalloporphycenes. These aspects have been extensively studied, whereas the properties of the photoexcited triplet states remain underexplored. Here, we present an improved synthetic protocol and characterization of 2,7,12,17-tetra-tert-butyl porphycene and its Pd2+ complex. We characterize the ground states of both compounds with cyclic voltammetry and UV/VIS spectroscopy, and use transient EPR (trEPR) spectroscopy to investigate their photoexcited triplet states. For free-base porphycenes, tautomerization effects are observed via temperature-dependent trEPR measurements, whereas these are absent in the Pd2+ complex. The zero-field splitting (ZFS) parameters demonstrate that the Pd2+ complex retained its π-π* character. In contrast, a reorientation of spin polarization from in-plane in the free-base compound to out-of-plane in the Pd2+ complex is observed. The experimental data are complemented by DFT and CASSCF calculations. These results provide a basis for selective modification of the porphycene macrocycle to control its triplet properties. These findings are relevant to applications such as photodynamic therapy (PDT) and spin polarization transfer via the radical triplet pair mechanism (RTPM).