  We present a combined study of thermal expansion and resistance fluctuation
spectroscopy measurements exploring the static and dynamic aspects of the
charge-glass formation in the quasi-two-dimensional organic conductors
$\theta$-(BEDT-TTF)$_2$$MM^\prime$(SCN)$_4$ with $M$ = Cs and $M^\prime$ =
Co,Zn. In these materials, the emergence of a novel charge-glass state so far
has been interpreted in purely electronic terms by considering the strong
frustration of the Coulomb interactions on a triangular lattice. Contrary to
this view, we provide comprehensive evidence for the involvement of a
\textit{structural} glass-like transition at $T_{\text{g}} \sim 90-100\,$K.
This glassy transition can be assigned to the freezing of structural
conformations of the ethylene endgroups in the donor molecule with an
activation energy of $E_{\rm{a}}\approx 0.32\,$eV, and the concomitant slowing
down of the charge carrier dynamics is well described by a model of
non-exponential kinetics. These findings discolse an important aspect of the
phase diagram and renders the current understanding of the charge-glass state
in the whole family of $\theta$-(BEDT-TTF)$_2MM^\prime$(SCN)$_4$ incomplete.
Our results suggest that the entanglement of slow structural and charge-cluster
dynamics due to the intimate coupling of lattice and electronic degrees of
freedom determine the charge-glass formation under geometric frustration.

Strongly Correlated Electrons