High level ab initio correlated (CASPT2) computations have been used to elucidate the details of the photoinduced molecular motion and decay mechanisms of a realistic phytochrome chromophore model in vacuo and to explore the reasons underneath its photophysical/photochemical properties. Competitive deactivation routes emerge that unveil the primary photochemical event and the intrinsic photoisomerization ability of this system. The emerged in vacuo based static (i.e., nondynamical) reactivity model accounts for the formation of different excited state intermediates and suggests a qualitative rationale for the short (picosecond) excited state lifetime and ultrafast decay of the emission, its small quantum yield, and the multiexponential decay observed in both solvent and phytochromes. It is thus tentatively suggested that this is a more general deactivation scheme for photoexcited phytochrome chromophores that is independent of the surrounding environment. Spectroscopic properties have also been simulated in both isolated conditions and the protein that satisfactorily match experimental data. For this purpose, preliminary hybrid QM/MM computations at the correlated (CASPT2) level have been used in the protein and are reported here for the first time.

A simple model structure of the room-temperature magnetic semiconductor V͑TCNE͒ 2 is proposed on the basis of available experimental data. The structural, electronic, and magnetic properties are investigated using hybrid-exchange density functional theory within periodic boundary conditions. A spin-polarized ferrimagnetic ground state with a total spin of 1 B per formula unit is identified. The analysis of the corresponding electronic band structure and spin distribution reveals strong interactions between the V ions and the ͓TCNE͔ ·− radicals, identified as spin carrying units. Within a simple Ising Hamiltonian, a strong antiferromagnetic coupling between the metal and its nearest-neighbor ligands is predicted which is consistent with the observed hightemperature magnetic ordering. The computed results provide useful insight into the physical origin of the exceptional magnetic behavior of V͑TCNE͒ 2 .

Hybrid density-functional calculations performed on the metal-organic compound Nb͑TCNE͒ 2 ͑TCNE= tetracyanoethylene͒ confirm it to be a ferrimagnet with a high ordering temperature for a material of its class. Most interestingly, inspection of the electronic band structure reveals that the material is a half-metal. The structure investigated is formed by layers of Nb 2+ ions and TCNE molecules, which are bridged by additional TCNE ligands. A delicate balance between strong on-site electron correlation characteristic of the electronic d states in Nb 2+ and off-site hybridization with the TCNE molecules in the layers produces a half-metallic state. These spin-polarized Nb 2+ -͓TCNE͔ ·− layers are then coupled ferrimagnetically with the TCNE ligands above and below. The coexistence of intrinsic high-temperature ferrimagnetism and halfmetallicity in a metal-organic compound is very relevant for the emerging area of half-metallic and organic spintronics. Metal-organic magnets have been researched extensively over the last few decades because they offer the prospect of realizing magnets fabricated through controlled, lowtemperature solution-based chemistry, as opposed to hightemperature metallurgical routes. In addition, in these materials magnetic properties can be tuned through synthesis. Unfortunately many of the compounds that have been synthesized and tested display magnetic order only at very low temperatures. The most prominent exception is V͑TCNE͒ 2 ͑TCNE= tetracyanoethylene͒, which to date has the highest magnetic ordering temperature recorded ͑T C Ϸ 400 K͒ for a material of this class. [1][2][3] Due to the paucity of structural data for this system, theoretical studies have played an important role in understanding the relationship between its magnetic, electronic, and structural properties. 4 Unfortunately, the extreme air sensitivity of this material is hampering its practical exploitation.Attempts to advance the state of the art in the field of metal-organic compounds have been fuelled recently by the emerging area of spintronics, where the exploitation of the electronic spin, in addition to the charge, has the potential to increase processing speeds, storage densities, and to lower power consumption compared to conventional electronics. 5The use of organic-based magnets in the area of spintronics is potentially revolutionary because organic materials can preserve the spin information over extremely long times due to their inherently weak spin-orbit coupling.6,7 Many challenges need to be addressed, however, in the emerging field of organic spintronics.8 One crucial issue is the injection of spin-polarized current and half-metallic materials are obvious candidates for achieving this. These materials support spin-polarized current only because a semiconducting or insulating gap is present for electrons with one spin orientation but is absent for those with the opposite spin orientation. Half-metallicity has been observed in inorganic materials such as manganese perovskites and Heusler alloys, 9-11 and in m...