In the high temperature regime, spin-lattice relaxation (T1) often represents the dominant contributor to decoherence in molecular electron spin quantum bits (qubits). Prior research has pointed to the coupling of the electron wavefunction to phonon modes (spin-phonon coupling) as a driver of T1. Through investigations of the effects of systematic and synthetically controllable geometric distortions on anisotropic T1, we demonstrate direct control over the specific vibrational modes that govern T1 in the two-phonon Raman regime. This has been achieved through the ability to systematically control the porphyrin ring symmetry in copper porphyrins. In a non-ruffled, planar porphyrin, the symmetry of the ruffling vibrational mode transforms as B1u. However, upon ruffling, the mode changes symmetry and transforms as A1. Due to spin-phonon coupling selection rules, this allows the ruffling vibrational mode to undergo spin-phonon coupling only in the ruffled structure. Interestingly, this vibrational control comes without significant changes to the first coordination sphere of the metal. While this mode activation accelerates relaxation, it demonstrates that longer range perturbations to molecular symmetry can control which modes are allowed to undergo spin-phonon coupling at the metal center, further guiding synthetic design approaches seeking to incorporate molecular qubits into extended two- or three-dimensional arrays. Finally, we discuss further rationalization of the experimental results through correlations to a newly developed theoretical approach utilizing the electron spin-orbit coupled wavefunctions.

Spin-phonon control over electron spin relaxation via vibrational symmetry perturbations in ruffled copper porphyrins

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