Raman spectroscopic and optical imaging of high spin/low spin domains in a spin crossover complex

► The [FeII(bapbpy)(NCS)2] complex shows a two-stepped spin-state transition. ► Raman mapping reveals nucleation and growth of domains of molecules in the same spin state during the two transitions. ► Elastic stress and strain are evidenced throughout by polarized light microscopy. ► The first transition is characterized (on the observation scale) by a single nucleation site and domain growth along the c axis. ► An interplay between 'twin domains' and 'spin domains' is revealed for the second transition.