  We consider certain families of Hecke characters $\phi$ over a quadratic
imaginary field $F$. The order of vanishing of the $L$-function $L(\phi,s)$ at
the central point $s=-1$, according to the Beilinson conjectures, should be
equal to the dimension of the space of extensions of the Tate motive
$\mathbb{Q}(1)$ by the motive associated with $\phi$. In this article,
candidates for the corresponding extensions of Hodge structure are constructed,
under the assumptions that the sign of the functional equation of $L(\phi,s)$
is $-1$ and that $L^\prime(\phi,-1) \neq 0$. This is achieved by means of the
cohomology of variations of Hodge structures over Picard modular surfaces
attached to $F$ and Harder's theory of Eisenstein cohomology. Moreover, we
provide a criterion for the non-triviality of these extensions, based on the
non-vanishing of a height pairing that we define and study.
