A non-smooth continuous unitary representation of a Banach-Lie group

In this note we show that the representation of the additive group of the Hilbert space $L^2([0,1],\mathbb{R})$ on $L^2([0,1],\text{C})$ given by the multiplication operators $\pi (f):=e^{if}$ is continuous but its space of smooth vectors is trivial. This example shows that a continuous unitary representation of an infinite dimensional Lie group need not be smooth.