  We study how CMB bispectrum, produced by weak gravitational lensing and
structure formation, can constrain the redshift evolution of the dark energy
equation of state independently on its present value. Analyzing the line of
sight contribution to the angular CMB bispectrum, we find that the relevant
redshift at which the structure formation contributes to the signal is $0.1
\lsim z \lsim 2$ for multipoles $1000 \gsim l \gsim 100$: just the epoch when
the dark energy starts to dominate the cosmological expansion rate. For
scenarios having the same equation of state at the present, this turns out to
be a new observable capable to discriminate between models of dark energy with
different time evolution of equation of state. We assess the strength of this
effetc within the framework of tracking Quintessence trajectories obeying SUGRA
and Ratra-Peebles potentials.
