  We investigate photoisomerization (PI), the shape change of a molecule upon
photoabsorption, in a dissipative environment using a simple spin-boson model.
We identify two classes of environment depending on whether it "entangles" with
the molecule. In the absence of entanglement, the environment merely causes a
blue shift of the required photon frequency and reduces the quantum efficiency
of PI. With entanglement the molecule can undergo a quantum phase transition
between a state that photoisomerizes to a state that does not.
