  A family of molecular Mott insulators on triangular lattice provided a few
S=1/2 triangular quantum spin liquid candidates, with
$\kappa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ being the most studied material of this
group. The large number experimental works present a conflicting set of
evidence, with some suggesting spin liquid behavior, while others point towards
a valence bond solid with orphan spins. In this work we use Raman scattering
spectroscopy to probe both local charge on molecular sites and lattice phonons
as a function of temperature down to 6~K. Based on the analysis of the line
shape of the BEDT-TTF charge sensitive vibration $\nu_2$ on cooling below 40 K,
we suggest a development of disordered fluctuating charge disproportionation on
(BEDT-TTF)$_2$ dimers of amplitude as small as 0.06$e$. The lattice phonons
show strong anomalous broadening on cooling only in the (c,c) scattering
channel, associated with the developing charge disproportionation. We suggest
an interpretation, where the coupling of disordered charge dipoles on dimers to
the lattice results in anisotropic modulation of charge transfer integrals
between dimer lattice sites. Such fluctuations would result in modulation of
magnetic coupling between spins which can produce fluctuating charge ordered
spin-singlet pairs.
