Anisotropy of Spin-Lattice Relaxation Time (T₁) for Oxo-Vanadium(IV) and Nitrido Chromium(V) Porphyrins

Designing molecular complexes as qubits requires understanding properties that contribute to long electron spin-lattice relaxation times. Spin-lattice relaxation was measured as a function of temperature and position in the spectrum for vanadyl tetraphenylporphyrin (VOTPP) in titanyl tetraphenylporphyin (TiOTPP) and zinc tetraphenylporphyrin (ZnTPP) hosts and for nitrido chromium(V) TPP in TiOTPP. T₁ also was measured for the vanadyl complexes of octaethylporphyrin (OEP) and tetratolylporphyrin (TTP) in ZnOEP or ZnTTP, respectively. T₁ anisotropy is defined as the ratio of T₁ when the magnetic field is along the VO or CrN bond (the z axis) to T₁ when the magnetic field is in the porphyrin plane. For these vanadyl and Cr(V) porphyrins, T₁ anisotropy increases rapidly between about 60 and 140 K and is strongly dependent on the host lattice, ranging from the unusually large maxima of 36 for VOTPP in TiOTPP and 28 for CrNTPP in TiOTPP to 3.8 for VOTPP in ZnTPP and 3.3 for VOOEP in ZnOEP. Empirical modeling of the temperature dependence of T₁ showed that phonons with energies of 170 to 225 cm^-1 made substantially larger contributions to relaxation in the perpendicular plane than along the z axis and that the magnitudes of these contributions were strongly dependent on the host lattice. These results demonstrate that optimizing molecules as qubits requires consideration of interaction with the host lattice in addition to molecular properties.