To examine the mechanism of a reproducible on-off switching of light emission from a novel organic electroluminescent (EL) device consisting of a spin crossover complex of [Fe(dPP)(2)](BF(4))(2) (dpp=2,6-di (pyrazol-1-yl)pyridine) and chlorophyll a, we have investigated the EL properties of an ITO/chlorophyll a: [Fe(dPP)(2)](BF(4))(2)/Al device. The comparison with a reference device not containing [Fe(dpp)(2)](BF(4))(2) revealed that the introduction of [Fe(dpp)(2)](BF(4))(2) results in small threshold voltage and large external quantum efficiency. Moreover, the EL spectra in the low-temperature region, where [Fe(dpp)(2)](BF(4))(2) is in the low-spin state, showed the emission from the ITO electrode, suggesting that injected electrons pass through the active layer via [Fe(dPP)(2)](BF(4))(2). (c) 2008 Elsevier B.V. All rights reserved.