Increase in Spin-Phonon Coupling in Molecular Qubit CuPc from Substrate Effects*

Molecular qubits are a promising platform for quantum information systems (QIS). It is understood that to function in devices, regular arrays of qubits supported by a substrate are needed. The substrate imposes mechanical and electronic boundary conditions on the molecule, however the impact of these effects on spin lattice relaxation times (T₁) is not well understood. We perform electronic structure calculations to assess the impact of a graphene substrate (C_gr) on the molecular qubit copper phthalocyanine (CuPc). We separate the impacts arising from the mechanical boundary and electronic boundary conditions of the substrate on the spin-phonon coupling (SPC) of CuPc. Then we use a simple thermal model to predict the impact of the changes in SPC from 0-300K. Our analysis of the vibrational modes with and without C_gr shows that the character and amplitude of the modes that impact T₁ changes, leading to an overall increase in SPC with the surface. We explain these changes by examining how the presence of the substrate alters the symmetry of CuPc. Our work shows a surface can have a large impact on SPC and that ways to reduce this coupling need to be found to fully utilize arrays of molecular qubits.