While two classes of non-heme iron enzymes use ferric centers to activate singlet organic substrates for the spin forbidden reaction with 3O2, most classes use high spin ferrous sites to activate dioxygen. These FeII active sites do not exhibit intense absorption bands and have integer spin ground states thus are mostly EPR inactive. We have developed new spectroscopic methodologies that provide geometric and electronic structural insight into the ferrous centers and their interactions with cosubstrates for dioxygen activation and into the nature of the intermediates generated in these reactions. First, we present our variable-temperature variable-field magnetic circular dichroism (VTVH MCD) methodology to experimentally define the geometric and electronic structure of the high spin ferrous active site. Then, we focus on using Nuclear Resonance Vibrational Spectroscopy (NRVS, performed at SPring-8) to define geometric structure and VTVH MCD to define the electronic structure of the FeIII-OOH and FeIV=O intermediates generated in O2 activation and the spin state dependence of their frontier molecular orbitals (FMOs) in controlling reactivity. Experimentally validated reaction coordinates are derived for the anticancer drug bleomycin in its cleavage of DNA and for an α-ketoglutarate dependent dioxygenase in its selective halogenation over the thermodynamically favored hydroxylation of substrate.
Recent years have seen a large increase in the number of reported framework materials, including the nowadays-ubiquitous metal–organic frameworks (MOFs). Many of these materials show flexibility and stimuli-responsiveness, i.e. their structure can undergo changes of large amplitude in response to physical or chemical stimulation. We describe here a toolbox of theoretical approaches, developed in our group and others, to shed light into these materials’ properties. We focus on their behavior under mechanical constraints, temperature changes, adsorption of guest molecules, and exposure to light. By means of molecular simulation at varying scale, we can now probe, rationalize and predict the behavior of stimuli-responsive materials, producing a coherent description of soft porous crystals from the unit cell scale all the way to the behavior of the whole crystal. In particular, we have studied the impact of defects in soft porous crystals, and developed a methodology for the study of their disordered phases (presence of correlated disorder, MOF glasses, and liquid MOFs)
Photon upconversion (UC) is the process that converts longer wavelength (lower energy) light to shorter wavelength (higher energy) light. Among different UC mechanisms, the UC based on triplet-triplet annihilation (TTA-UC) has attracted much attention due to its advantage of high efficiency at low excitation intensity. In the typical mechanism of TTA-UC, the triplet excited state of the donor (sensitizer), generated via intersystem crossing (ISC) from its photo-excited singlet state, transfers the triplet energy to the acceptor (emitter), and the sensitized acceptor triplets collide to undergo TTA, producing the emissive acceptor excited singlet state. The efficient TTA-UC has been achieved for donor-acceptor pairs molecularly dissolved in organic solvents, however, it has several problems such as the difficulty to show efficient TTA-UC at low chromophore concentration. To solve these fundamental issues, we introduced the concept of triplet energy migration in dense chromophore assemblies and achieved highly efficient photon upconversion at low chromophore concentration by constructing coordination copolymers of donor- and acceptor-containing ligands. Another challenge of TTA-UC is to upconvert near-infrared (NIR) light to visible light. By employing Os complexes with singlet-to-triplet (S–T) direct transition and interface complexes between semiconductor nanocrystals and chromophore ligands, we have achieved an efficient NIR-to-visible TTA-UC in the solid state.