  Optically addressable spins are a promising platform for quantum information
science due to their combination of a long-lived qubit with a spin-optical
interface for external qubit control and read out. The ability to chemically
synthesize such systems - to generate optically addressable molecular spins -
offers a modular qubit architecture which can be transported across different
environments, and atomistically tailored for targeted applications through
bottom-up design and synthesis. Here we demonstrate how the spin coherence in
such optically addressable molecular qubits can be controlled through
engineering their host environment. By inserting chromium (IV)-based molecular
qubits into a non-isostructural host matrix, we generate noise-insensitive
clock transitions, through a transverse zero-field splitting, that are not
present when using an isostructural host. This host-matrix engineering leads to
spin-coherence times of more than 10 microseconds for optically addressable
molecular spin qubits in a nuclear and electron-spin rich environment. We model
the dependence of spin coherence on transverse zero-field splitting from first
principles and experimentally verify the theoretical predictions with four
distinct molecular systems. Finally, we explore how to further enhance
optical-spin interfaces in molecular qubits by investigating the key parameters
of optical linewidth and spin-lattice relaxation time. Our results demonstrate
the ability to test qubit structure-function relationships through a tunable
molecular platform and highlight opportunities for using molecular qubits for
nanoscale quantum sensing in noisy environments.
