  This paper is devoted to logarithmic Hardy-Littlewood-Sobolev inequalities in
the two-dimensional Euclidean space, in presence of an external potential with
logarithmic growth. The coupling with the potential introduces a new parameter,
with two regimes. The attractive regime reflects the standard logarithmic
Hardy-Littlewood-Sobolev inequality. The second regime corresponds to a reverse
inequality, with the opposite sign in the convolution term, that allows us to
bound the free energy of a drift-diffusion-Poisson system from below. Our
method is based on an extension of an entropy method proposed by E. Carlen, J.
Carrillo and M. Loss, and on a nonlinear diffusion equation.
