  We study the spin evolution of close-in planets in compact multi-planetary
systems. The rotation period of these planets is often assumed to be
synchronous with the orbital period due to tidal dissipation. Here we show that
planet-planet perturbations can drive the spin of these planets into
non-synchronous or even chaotic states. In particular, we show that the transit
timing variation (TTV) is a very good probe to study the spin dynamics, since
both are dominated by the perturbations of the mean longitude of the planet. We
apply our model to KOI-227b and Kepler-88b, which are both observed undergoing
strong TTVs. We also perform numerical simulations of the spin evolution of
these two planets. We show that for KOI-227b non-synchronous rotation is
possible, while for Kepler-88b the rotation can be chaotic.
