Well-defined photogenerated molecular spin systems have potential utility in spintronics and quantum information science because they can be prepared at modest temperatures. Little is known regarding the excited-state dynamics of conjugated assemblies that feature both expansive electronic delocalization and unpaired electron spin. Here meso-to-meso ethyne-bridged (porphinato)metal arrays that incorporate a Cu(II) radical center were synthesized and investigated via ultrafast transient absorption spectroscopic methods. Upon photoexcitation, a singdoublet (²S₁) state is formed, followed by internal conversion (IC) to a tripdoublet (²T₁) state, and intersystem crossing (ISC) to a tripquartet (⁴T₁) state, before relaxation to ground-state (²S₀). This work highlights the first direct observation of copper porphyrin ²T₁ → ⁴T₁ ISC from a biexponetial decay of the near infrared (NIR) ^2,4T₁ → ^2,4T_n excited-state absorption (ESA) manifold, which is spectrally well-resolved from transitions associated with the ²S₁ state. Theoretical studies using complete active space self-consistent field (CASSCF) combined with multireference n-electron valence perturbation theory (NEVPT2) elucidate expansive delocalization of the highest occupied x-polarized porphyrin π orbital, and spatial separation of the localized electron density in the singly occupied y-polarized orbital of the ⁴T₁ state and that of the Cu free spin center. These electronic structural characteristics reduce the spin-orbit coupling between the ⁴T₁ and ²S₀ states, and thus extend the quartet state lifetime in these conjugated structures. This work demonstrates photogeneration of a long-lived high multiplicity state realized through incorporating a stable radical center into a highly conjugated framework, and highlights multiple strategies for exploiting conjugated molecular wires for tuning electronically excited multiplet state lifetimes.

Chih-Hung Ko

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