  The neck dynamics in Fermi-energy heavy-ion collisions, to probe the nuclear
symmetry energy in the domain of sub-saturation densities, is investigated
within an isospin dependent transport model. The single and double ratios of
neutron/proton from free nucleons and light clusters (complex particles) in the
isotopic reactions are analyzed systematically. Isospin effects of particles
produced from the neck fragmentations are explored, which are constrained
within the midrapidities ($|y/y_{proj}|<$0.3) and azimuthal angles
(70$^{o}\sim$110$^{o}$, 250$^{o}\sim$290$^{o}$) in semiperipheral nuclear
collisions. It is found that the ratios of the energetic isospin particles
strongly depend on the stiffness of nuclear symmetry energy and the effects
increase with softening the symmetry energy, which would be a nice probe for
extracting the symmetry energy below the normal density in experimentally. A
flat structure appears at the tail spectra from the double ratio distributions.
The neutron to proton ratio of light intermediate mass fragments (IMFs) with
charged number Z$\leq$8 is related to the density dependence of symmetry energy
with less sensitivity in comparison to the isospin ratios of nucleons and light
particles.
