  A central goal in quantum technologies is to maximize $G$T$_{2}$, where $G$
stands for the coupling of a qubit to control and readout signals and T$_{2}$
is the qubit's coherence time. This is challenging, as increasing $G$ (e.g. by
coupling the qubit more strongly to external stimuli) often leads to
deleterious effects on T$_{2}$. Here, we study the coupling of pure and
magnetically diluted crystals of HoW$_{10}$ magnetic clusters to microwave
superconducting coplanar waveguides. Absorption lines give a broadband picture
of the magnetic energy level scheme and, in particular, confirm the existence
of level anticrossings at equidistant magnetic fields determined by the
combination of crystal field and hyperfine interactions. Such 'spin clock
transitions' are known to shield the electronic spins against magnetic field
fluctuations. The analysis of the microwave transmission shows that the
spin-photon coupling becomes also maximum at these transitions. The results
show that engineering spin-clock states of molecular systems offers a promising
strategy to combine sizeable spin-photon interactions with a sufficient
isolation from unwanted magnetic noise sources.
