Highly Conjugated Porphyrin Arrays Enable Optical Resolution of Ferromagnetic and Antiferromagnetic Aligned States of the Triplet Exciton and an Incorporated Stable Radical

Well-defined photogenerated molecular spin systems have potential utility in spintronics and quantum information science (QIS). Because molecular magnetic, optical, and electronic properties can be controlled by design, diverse spin systems can be prepared at modest temperatures. Photogenerated molecular spin systems often involve states prepared from the interaction of excitons and charges. Resolving the nature of electron spin alignment in photogenerated spin states described by the coupling of a triplet exciton and a stable radical commonly relies on EPR spectroscopy. We describe ethyne-bridged (porphinato)metal (PMn) oligomers that incorporate a macrocycle-bound Cu(II) radical center. Upon photoexcitation of such PMn arrays, a singdoublet (2S1) state is formed; ultrafast internal conversion (IC) then produces a tripdoublet (2T1) state, which undergoes intersystem crossing (ISC) to produce a tripquartet (4T1) state, before relaxation to the ground state (2S0). These highly conjugated Cu(II) radical-containing PMn arrays enable direct observation of copper porphyrin 2T1 → 4T1 ISC dynamics from the biexponential decay of the near-infrared (NIR) 2,4T1 → 2,4Tn transient absorption manifold. Multireference n-electron valence perturbation theory (NEVPT2) computations illuminate how PMn electronic structure controls the relaxation dynamics of these long-lived (>10 ns) electronically excited multiplet states. These studies show that highly conjugated and polarizable porphyrin arrays incorporating stable spin centers provide rare π-delocalized systems where the ferromagnetic and antiferromagnetic alignment between a triplet exciton and a stable radical are both spectrally resolved and addressable using transient optical spectroscopy at wavelengths exceeding 1 μm, providing new opportunities to QIS.