Ultrafast intersystem crossing and spin dynamics of zinc meso-tetraphenylporphyrin covalently bound to stable radicals

Tert-Butylphenylnitroxide (BPNO^•) and α,γ- bisdiphenylene-β-phenylallyl (BDPA^•) stable radicals are each attached to zinc meso-tetraphenylporphyrin (ZnTPP) at a fixed distance using one of the ZnTPP phenyl groups. BPNO^• and BDPA ^• are oriented para (1 and 3, respectively) or meta (2 and 4, respectively) relative to the porphyrin macrocycle. Following photoexcitation of 1-4, transient optical absorption spectroscopy is used to observe excited state quenching of ¹ZnTPP by the radicals and time-resolved electron paramagnetic resonance (TREPR) spectroscopy is used to monitor the spin dynamics of the paramagnetic product states. The presence of BPNO^• or BDPA^• accelerates the intersystem crossing rate of ¹ZnTPP about 10- to 500-fold in 1-4 depending on the structure compared to that of ¹ZnTPP itself. In addition, the lifetime of ³ZnTPP in 1 is shorter than that of ³ZnTPP itself as a result of enhanced intersystem crossing (EISC) from ³ZnTPP to the ground state. The TREPR spectra of the three unpaired spins produced within 1 and 2 show spin-polarized excited doublet (D₁) and quartet (Q) states and subsequent formation of a spin-polarized ground state radical (D ₀). All three signals are absorptive for 1 and emissive for 2. Polarization inversion of the Q state is observed on a tens of nanoseconds time scale in 2, while no polarization inversion is observed for 1. The lack of polarization inversion in 1 is attributed to the short lifetime of the doublet-quartet manifold as a result of the very large exchange interaction. The TREPR spectra of 3 and 4 show ground state radical polarization at X-band (9.5 GHz) at room temperature, but not at 85 K, and similarly no polarization is observed at W-band (94 GHz). No evidence of excited doublet or quartet states is observed, indicating that the exchange interaction is both weak and temperature dependent. These results show that although ultrafast EISC produces ³ZnTPP within 1-4, the magnitude of the exchange interactions between the three relevant spins in the resulting ³ZnTPP-BPNO ^• and ³ZnTPP-BDPA^• systems dramatically alters their spin dynamics.