  Single molecule magnets distinguish themselves in the field of quantum
magnetism through the ability to combine fundamental research with promising
applications, the evolution of quantum spintronics in the last decade
exemplifying the potential held by molecular based quantum devices. Notably,
the read-out and manipulation of the embedded nuclear spin states was used in
proof of principle studies of quantum computation at the single molecule level.
In this paper we study the relaxation dynamics of the $^{159}$Tb nuclear spins
in a diluted molecular crystal by using recently acquired understanding of the
nonadiabatic dynamics of TbPc$_2$ molecules. We find that phonon modulated
hyperfine interaction opens a direct relaxation channel between the nuclear
spins and the phonon bath. We highlight the potential importance of the
discovered mechanism for the theory of spin bath and the relaxation dynamics of
the molecular spins at crossover temperatures.
