Synthesis and Ultrafast Excited-State Dynamics of Zinc and Palladium Triply Fused Diporphyrins

We report the synthesis and ultrafast excited-state dynamics of two new meso-meso, beta-beta, beta-beta) triply fused diporphyrins, Zn-3DP and Pd-3DP. Both compounds were found to have short excited-state lifetimes: Zn-3DP possessed an average SI lifetime of 14 ps before nonradiative deactivation to the ground state, whereas Pd-3DP displayed a longer average SI lifetime of 18 ps before crossing to the T-1 state, which itself possessed a very short triplet lifetime of 1.7 ns. The excited-state dynamics of Zn-3DP, compared to similar zinc(II) diporphyrins reported in the literature, suggests that a conical intersection of the SI and So potential energy surfaces plays a major role as a deactivation pathway of these molecules. Furthermore, the short triplet lifetime of Pd-3DP, compared to other diporphyrins that also exploit the intramolecular heavy atom effect, reveals that the position of the heavy atom within the diporphyrin framework influences the strength of spin-orbit coupling. The implications for employing triply fused diporphyrins as NIR-absorbing triplet sensitizers are discussed.