  CMB photons redshift and blueshift as they move through gravitational
potentials $\Phi$ while propagating across the Universe. If the potential is
not constant in time, the photons will pick up a net redshift or blueshift,
known as the Integrated Sachs-Wolfe (ISW) effect. In the $z \ll 1000$ universe,
$\dot{\Phi}$ is nonzero on large scales when the Universe transitions from
matter to dark energy domination. This effect is only detectable in
cross-correlation with large-scale structure at $z \sim 1$. In this paper we
present a 3.2$\sigma$ detection of the ISW effect using cross-correlations
between unWISE infrared galaxies and Planck CMB temperature maps. We use 3
tomographic galaxy samples spanning $0 < z < 2$, allowing us to fully probe the
dark energy domination era and the transition into matter domination. This
measurement is consistent with $\Lambda$CDM ($A_{\rm ISW} = 0.96 \pm 0.30$). We
study constraints on a particular class of dynamical dark energy models (where
the dark energy equation of state is different in matter and dark energy
domination), finding that unWISE-ISW improves constraints from type Ia
supernovae due to improved constraints on the time evolution of dark energy.
When combining with BAO measurements, we obtain the tightest constraints on
specific dynamical dark energy models. In the context of a phenomenological
model for freezing quintessence, the Mocker model, we constrain the dark energy
density within 10% at $z < 2$ using ISW, BAO and supernovae. Moreover, the ISW
measurement itself provides an important independent check when relaxing
assumptions about the theory of gravity, as it is sensitive to the
gravitational potential rather than the expansion history.
