  We present a study on the isoscalar quadrupole strength in tin nuclei,
focusing mainly on the low-energy region. The calculations are performed using
the Skyrme type energy density functionals within the fully self-consistent
quasiparticle random phase approximation, allowing for a good description of
the experimental data for the first 2$^+$ state and the isoscalar giant
quadrupole resonance. It is found that the first $2^+$ state and the low-energy
quadrupole states between 3 and 6 MeV display an opposite behavior with
increasing neutron number. While the strength of the first $2^+$ state
decreases, some excited states start to accumulate between 3 and 6 MeV, and
increase their strength with increasing neutron number. This low-energy region
between 3 and 6 MeV is quite sensitive to the changes in the shell structure
with increasing neutron number. In particular, between $^{116}$Sn and
$^{132}$Sn, the filling of the neutron orbitals with large values of $j$, has
an important impact on the low-energy region. Our analysis shows that the
low-energy states have a non-collective character, except the first 2$^+$
state. In addition, the states in the low-energy region above 5 MeV display an
interesting pattern: with the increase of the neutron number, their strength
increases and their nature changes, namely they switch from proton excitations
to neutron-dominated one. We conclude that the low-energy quadrupole states
between 3 and 6 MeV can provide information about the shell evolution in
open-shell nuclei.
