Enhanced coherence by coupling spins through a delocalized π-system: Vanadyl porphyrin dimers by Pozo, I, Huang, Z, Alexandropoulos, DI, Slota, M, Tkach, I, Bennati, M, Deng, J-R, Stawski, W, Horton, PN, Myers, WK, Kong, F, Lombardi, F, Bogani, L, Anderson, HL, Coles, SJ