An Electric Field Stimulated Spin Crossover Transition in a Molecular Adsorbate

We have investigated the occupied and unoccupied electronic structure of ultra thin films of the spin crossover [Fe(H$_2$B(pz)$_2)_2$(bipy)] complex (with H$_2$B(pz)$_2 =$ bis(hydrido)bis(1H-pyrazol-1-yl)borate and bipy $=$ 2,2'-bipyridine) by ultraviolet photoelectron spectroscopy (UPS), inverse photoemission (IPES) and X-ray absorption spectroscopy (XAS). A bandgap of 2-3 eV is deduced from combined UPS and IPES measurements of the films on Au substrates. The shift of the unoccupied density of states seen in IPES is consistent with the thermally induced spin crossover transition for molecules deposited on the organic ferroelectric copolymer polyvinylidene fluoride with trifluoroethylene (PVDF-TrFE). Perhaps more significant is the fact that the spin crossover transition, and certainly the unoccupied electronic structure, is influenced by the ferroelectric polarization direction of PVDF-TrFE substrates at temperatures in the vicinity of the thermally driven spin crossover transition.