The fine structure of the neutral nitrogen-vacancy center in diamond

The nitrogen-vacancy (NV) center in diamond is a widely utilized system due to its useful quantum properties. Almost all research focuses on the negative charge state (NV−) and comparatively little is understood about the neutral charge state (NV0). This is surprising as the charge state often fluctuates between NV0 and NV− during measurements. There are potentially underutilized technical applications that could take advantage of NV0, either by improving the performance of NV0 or utilizing NV− directly. However, the fine structure of NV0 has not been observed. Here, we rectify this lack of knowledge by performing magnetic circular dichroism measurements that quantitatively determine the fine structure of NV0. The observed behavior is accurately described by spin-Hamiltonians in the ground and excited states with the ground state yielding a spin-orbit coupling of λ = 2.24 +- 0.05 GHz and a orbital g-factor of 0.0186 +- 0.0005. The reasons why this fine structure has not been previously measured are discussed and strainbroadening is concluded to be the likely reason.We would like to acknowledge various funding sources for support during this work. In particular, NM would like to acknowledge the Australian Research Council through grants DP170103098. MD would like to acknowledge the Australian Research Council through grants DP170103098 and DE170100169. EK would like to acknowledge the Australian Research Council through grants DP110104565 and DP150103137

The highly resolved electronic spectrum of the square planar CuCl₄²⁻ ion

The low temperature magnetic circular dichroism(MCD) and electron paramagnetic resonance(EPR)spectra of Cu(II) dopedCs₂ZrCl₆ are reported. The Cu(II) ion is incorporated as the square planar copper tetrachloride ion, CuCl₄²⁻, which substitutes at the Zr(IV) site in the Cs₂ZrCl₆ lattice, with a complete absence of axial coordination. Both the EPR and MCD show highly resolved spectra from which it is possible to determine the superhyperfine coupling constants and excited state geometries respectively. The Franck–Condon intensity patterns suggest that there is a substantial relaxation of the host lattice about the impurity ion. For the lowest energy ²B1g(x²-y²)→²B2g(xy) transition, both the magnetic dipole allowed electronic origin as well as vibronic false origins are observed. The high resolution of the spectra allowed the accurate determination of the odd parity vibrations that are active in the spectra. The opposite sign of the MCD of the two components of the ²Eg(xz,yz)excited state allows this splitting to be determined for the first time. Accurate and unambiguous spectral parameters for the CuCl₄²⁻ ion are important as it has become a benchmark transition metal complex for theoretical electronic structure calculations

Application of Magnetic Circular Dichroism spectroscopy to the study of the OEC in Photosystem II from cyanobacteria and higher plants.

Absorption from the OEC was expected to be too weak to be measured through basic optical spectroscopy, with a molar extinction coefficient of <100, compared to 106 for chlorophyll. We therefore used circular dichroism (CD) and magnetic circular dichroism (MCD) to search for this absorption

Assignment of the Q-Bands of the Chlorophylls: Coherence Loss via Q x - Q y Mixing

We provide a new and definitive spectral assignment for the absorption, emission, high-resolution fluorescence excitation, linear dichroism, and/or magnetic circular dichroism spectra of 32 chlorophyllides in various environments. This encompases all dat

Identification of the Chromophores in Prussian blue

Prussian blue was the world's first synthetic dye. Its structural, optical and magnetic properties have led to many applications in technology and medicine, and provide paradigms for understanding coordination polymers, framework materials and mixed-valence compounds. The intense red absorption of Prussian blue that characterises chemical and physical properties critical to many of these applications is now shown to arise from localised intervalence charge transfer transitions within two chromophoric variants (ligand isomers) of an idealised "dimer" fragment {(NC)5FeII}(mu-CN){FeIII(NC)3(H2O)2}. This fragment is only available in modern interpretations of the material's crystal structure, with the traditional motif {(NC)5FeII}(mu-CN){FeIII(NC)5} shown not to facilitate visible absorption. Essential to the analysis is the demonstration, obtained independently using absorption and magnetic circular dichroism spectroscopies, that spectra of Prussian blues are strongly influenced by particle size and (subsequent) light scattering. These interpretations are guided and supported by density functional theory calculations (CAM-B3LYP), supplemented by coupled cluster and Bethe-Salpeter spectral simulations, as well as electron paramagnetic resonance spectroscopy of Prussian blue and a model molecular dimeric ion [Fe2(CN)11]6-

Structured near-infrared Magnetic Circular Dichroism spectra of the Mn₄CaO₅ cluster of PSII in T. vulcanus are dominated by Mn(IV) d-d 'spin-flip' transitions

Photosystem II passes through four metastable S-states in catalysing light-driven water oxidation. Variable temperature variable field (VTVH) Magnetic Circular Dichroism (MCD) spectra in PSII of Thermosynochococcus (T.) vulcanus for each S-state are reported. These spectra, along with assignments, provide a new window into the electronic and magnetic structure of Mn₄CaO₅. VTVH MCD spectra taken in the S₂state provide a clear g=2, S=1/2 paramagnetic characteristic, which is entirely consistent with that known by EPR. The three features, seen as positive (+) at 749nm, negative (-) at 773nm and (+) at 808nm are assigned as ⁴A→²E spin-flips within the d³ configuration of the Mn(IV) centres present. This assignment is supported by comparison(s) to spin-flips seen in a range of Mn(IV) materials. S₃ exhibits a more intense (-) MCD peak at 764nm and has a stronger MCD saturation characteristic. This S₃ MCD saturation behaviour can be accurately modelled using parameters taken directly from analyses of EPR spectra. We see no evidence for Mn(III) d-d absorption in the near-IR of any S-state. We suggest that Mn(IV)-based absorption may be responsible for the well-known near-IR induced changes induced in S₂ EPR spectra of T. vulcanus and not Mn(III)-based, as has been commonly assumed. Through an analysis of the nephelauxetic effect, the excitation energy of S-state dependent spin-flips seen may help identify coordination characteristics and changes at each Mn(IV). A prospectus as to what more detailed S-state dependent MCD studies promise to achieve is outlined.We recognise the support of the Australian Research Council through grants DP110104565 and DP150103137 (E.K.), FT140100834 (N.C) and MEXT/JSPS of Japan through a Grant-in-Aid for Specially Promoted Research No. 24000018 (J.R.S.)

The deep red state of photosystem II in Cyanidioschyzon merolae

We identified and characterised the deep red state (DRS), an optically-absorbing charge transfer state of PSII, which lies at lower energy than P680, in the red algae Cyanidioschyzon merolae by means of low temperature absorption and magnetic circular dichroism spectroscopies. The photoactive DRS has been previously studied in PSII of the higher plant Spinacia oleracea, and in the cyanobacterium Thermosynechococcus vulcanus. We found the DRS in PSII of C. merolae has similar spectral properties. Treatment of PSII with dithionite leads to reduction of cytochrome (cyt) b559 and the PsbV-based cyt c550 as well as the disassembly of the oxygen-evolving complex. Whereas the overall visible absorption spectrum of PSII was little affected, the DRS absorption in the reduced sample was no longer seen. This bleaching of the DRS is discussed in terms of a corresponding lack of a DRS feature in D1D2/cyt b559 reaction centre preparations of PSII.Australian Research Council through grants DP110104565 and DP 15010313

The primary donor of far-red photosystem II: ChlD1 or PD2?

Far-red light (FRL) Photosystem II (PSII) isolated from Chroococcidiopsis thermalis is studied using parallel analyses of low-temperature absorption, circular dichroism (CD) and magnetic circular dichroism (MCD) spectroscopies in conjunction with fluorescence measurements. This extends earlier studies (Nurnberg et al 2018 Science 360 (2018) 1210-1213). We confirm that the chlorophyll absorbing at 726 nm is the primary electron donor. At 1.8 K efficient photochemistry occurs when exciting at 726 nm and shorter wavelengths; but not at wavelengths longer than 726 nm. The 726 nm absorption peak exhibits a 21 ± 4 cm-1 electrochromic shift due to formation of the semiquinone anion, QA-. Modelling indicates that no other FRL pigment is located among the 6 central reaction center chlorins: PD1, PD2 ChlD1, ChlD2, PheoD1 and PheoD2. Two of these chlorins, ChlD1 and PD2, are located at a distance and orientation relative to QA- so as to account for the observed electrochromic shift. Previously, ChlD1 was taken as the most likely candidate for the primary donor based on spectroscopy, sequence analysis and mechanistic arguments. Here, a more detailed comparison of the spectroscopic data with exciton modelling of the electrochromic pattern indicates that PD2 is at least as likely as ChlD1 to be responsible for the 726 nm absorption. The correspondence in sign and magnitude of the CD observed at 726 nm with that predicted from modelling favors PD2 as the primary donor. The pros and cons of PD2 vs ChlD1 as the location of the FRL-primary donor are discussed.We recognize the support of the Australian Research Councilthrough grants DP110104565 and DP150103137 (EK), FT140100834(NC). This work was supported by BBSRC grants BB/L011506/1 andBB/R001383/1 (AWR, AF and DN

Protonation state of F420H2 in the prodrug-activating deazaflavin dependent nitroreductase (Ddn) from Mycobacterium tuberculosis

The protonation state of the deazaflavin dependent nitroreductase (Ddn) enzyme bound cofactor F420 was investigated using UV-visible spectroscopy and computational simulations. The reduced cofactor F420H2 was determined to be present in its deprotonated state in the holoenzyme form. The mechanistic implications of these findings are discussed.MLC and CJJ gratefully acknowledge funding from the Australian Research Council in the form of Discovery Project funding (DP130102144) and ARC Future Fellowships. MLC also acknowledges generous allocations of supercomputing time on the National Facility of the Australian National Computational Infrastructure

Selective and differential optical spectroscopies in photosynthesis

Photosynthetic pigments are inherently intense optical absorbers and have strong polarisation characteristics. They can also luminesce strongly. These properties have led optical spectroscopies to be, quite naturally, key techniques in photosynthesis. Ho