  We propose that a type of molecular rulers, which contains two electron spins
with precisely adjustable distance by varying the length of the
shape-persistent polymer chains, can be utilized to constrain the axial-vector
mediated interaction between electron spins at the nanometer scale. With
measurements of the coupling strengths between two electron spins within
different molecular rulers, an improved laboratory bound of exotic
dipole-dipole interaction between electrons is established within the force
range from 3 to 220 nm. The upper bound of the coupling $g_A^e g_A^e/4\pi\hbar
c $ at 200 nm is $|g_A^e g_A^e/4\pi\hbar c| \leq 4.9\times10^{-13}$, which is
about 20 times more stringent than previous limits.
