  We consider the influence of the dark energy dynamics at the onset of cosmic
acceleration on the Cosmic Microwave Background (CMB) bispectrum, through the
weak lensing effect induced by structure formation. We study the line of sight
behavior of the contribution to the bispectrum signal at a given angular
multipole $l$: we show that it is non-zero in a narrow interval centered at a
redshift $z$ satisfying the relation $l/r(z)\simeq k_{NL}(z)$, where the
wavenumber corresponds to the scale entering the non-linear phase, and $r$ is
the cosmological comoving distance. The relevant redshift interval is in the
range $0.1\lsim z\lsim 2$ for multipoles $1000\gsim\ell\gsim 100$; the signal
amplitude, reflecting the perturbation dynamics, is a function of the
cosmological expansion rate at those epochs, probing the dark energy equation
of state redshift dependence independently on its present value. We provide a
worked example by considering tracking inverse power law and SUGRA Quintessence
scenarios, having sensibly different redshift dynamics and respecting all the
present observational constraints. For scenarios having the same present
equation of state, we find that the effect described above induces a projection
feature which makes the bispectra shifted by several tens of multipoles, about
10 times more than the corresponding effect on the ordinary CMB angular power
spectrum.
