Lowest Energy Singlet States Research Articles

Planar hexacoordinate phosphorous and arsenic

Here the first planar hexacoordinate phosphorous and arsenic (phP and phAs) atoms are reported. Neutral 18 valence electron EBe3Li3H6 clusters (E = P, As) adopt planar D3h symmetry in their lowest energy singlet state. Chemical bonding reveals partial double bond character in P(As)-Be bonds and single bond character in P(As)-Li bonds. Born-Oppenheimer molecular dynamics simulation as well as their reluctance towards decomposition reveals high kinetic stability of these clusters.

Degenerate and non-degenerate two-photon absorption of coumarin dyes.

Two-photon absorption (2PA) spectroscopy is a robust bioimaging tool that depends on the determined cross-sections (σ2PA). The absorption of both photons occurs simultaneously with equivalent (degenerate) or different (non-degenerate) photon energies, D-2PA and ND-2PA, respectively. The former has been investigated experimentally and computationally for many systems, while the latter remains relatively unexplored computationally and limited experimentally. In this study, response theory using time-dependent density functional theory (TD-DFT) and the 2-state model (2SM) have been utilized to investigate σD-2PA and σND-2PA for the excitation to the lowest energy singlet state (S1) of coumarin, coumarin 6, coumarin 120, coumarin 307, and coumarin 343. Solvents involved were methanol (MeOH), chloroform (ClForm), and dimethylsulfoxide (DMSO), where the latter leads to the largest σ2PA. Values of σ2PA are largest for coumarin 6 and lowest for coumarin, which illustrates the effect of substituents. The 2SM clarifies how the largest cross-sections correspond to molecules with the largest transition dipole moments, μ01. In general, σD-2SM computations agree with σD-2PA. Moreover, σND-2SM are in qualitative agreement with σND-2PA with comparable enhancement relative to σD-2PA. Overall, σND-2PA are larger than σD-2PA where the increase is in the range of 22% to 49%, depending on the coumarin as well as the relative energies of the two photons. This work aids in future investigations into various fluorophores to understand their photophysical properties for ND-2PA.

Melatonin photoreactivity: phosphorescence formation and quenching processes

Studies of melatonin photoreactivity in water solutions: An effect of an external heavy atom I− on UV/Vis absorption, fluorescence and phosphorescence spectra is explored. The data allowed determination of relevant energetics for the system.The heavy atom effect (HAE) of I− on melatonin is clearly found to induce an intersystem crossing from the lowest energy singlet state to the lowest energy triplet state (T1) by a state mixing. Lifetime for the first excited triplet states of melatonin in association with I− and quenching rates for halomethanes (CH2X2, CHX3, CY4, X = Cl, Br, Y = Cl) are determined from Time-Correlated Single-Photon Counting decay curves for the phosphorescence. Dramatic alterations in quenching rate constants with quenchers as CH2X2 < CHX3 < CX4 and Cl < Br are attributed to energy transfer from an I−…Me*(T1) complex to low-lying electronic states of the halomethanes followed by dissociation to form R and X fragments. Relevance of the melatonin photoreactivity to photosensitizer properties in organic media is discussed.Graphical abstract

Chemistry of the spin- 12 kagome Heisenberg antiferromagnet

We believe that a necessary first step in understanding the ground state properties of the spin-${\scriptstyle\frac{1}{2}}$ kagome Heisenberg antiferromagnet is a better understanding of this model's very large number of low energy singlet states. A description of the low energy states that is both accurate and amenable for numerical work may ultimately prove to have greater value than knowing only what these properties are, in particular when these turn on the delicate balance of many small energies. We demonstrate how this program would be implemented using the basis of spin-singlet dimerized states, though other bases that have been proposed may serve the same purpose. The quality of a basis is evaluated by its participation in all the low energy singlets, not just the ground state. From an experimental perspective, and again in light of the small energy scales involved, methods that can deliver all the low energy states promise more robust predictions than methods that only refine a fraction of these states.

Impact of non-equilibrium molecular packings on singlet fission in microcrystals observed using 2D white-light microscopy.

Singlet fission, the process of splitting a singlet exciton into two triplet excitons, has been proposed as a mechanism for improving the efficiency of future photovoltaic devices. In organic semiconductors exhibiting singlet fission, the geometric relationship between molecules plays an important role by setting the intermolecular couplings that determine the system energetics. Here, we spatially image TIPS-pentacene microcrystals using ultrafast two-dimensional white-light microscopy and discover a low-energy singlet state sparsely distributed throughout the microcrystals, with higher concentrations at edges and morphological defects. The spectra of these singlet states are consistent with slip-stacked molecular geometries and increased charge-transfer couplings. The picosecond-timescale kinetics of these low-energy singlet states matches that of the correlated triplet-pair state, which we attribute to singlet/triplet-pair interconversion at these sites. Our observations support the conclusion that small populations of geometries with favourable energetics can play outsized roles in singlet fission processes.

Electron pairing in mirror modes: surpassing the quasi-linear limit

Abstract. The mirror mode evolving in collisionless magnetised high-temperature thermally anisotropic plasmas is shown to develop an interesting macro-state. Starting as a classical zero-frequency ion fluid instability it saturates quasi-linearly at very low magnetic level, while forming elongated magnetic bubbles which trap the electron component to perform an adiabatic bounce motion along the magnetic field. Further evolution of the mirror mode towards a stationary state is determined by the bouncing trapped electrons which interact with the thermal level of ion sound waves and generate attractive wake potentials which give rise to the formation of electron pairs in the lowest-energy singlet state of two combined electrons. Pairing preferentially takes place near the bounce-mirror points where the pairs become spatially locked with all their energy in the gyration. The resulting large anisotropy of pairs enters the mirror growth rate in the quasi-linearly stable mirror mode. It breaks the quasi-linear stability and causes further growth. Pressure balance is either restored by dissipation of the pairs and their anisotropy or inflow of plasma from the environment. In the first case new pairs will continuously form until equilibrium is reached. In the final state the fraction of pairs can be estimated. This process is open to experimental verification. To our knowledge it is the only process in which high-temperature plasma pairing may occur and has an important observable macroscopic effect: breaking the quasi-linear limit and, via pressure balance, generation of localised diamagnetism.

Heterogeneous CPU + GPU Algorithm for Variational Two-Electron Reduced-Density Matrix-Driven Complete Active-Space Self-Consistent Field Theory.

We present a heterogeneous central processing unit (CPU) + graphical processing unit (GPU) algorithm for the direct variational optimization of the two-electron reduced-density matrix (2RDM) under two-particle N-representability conditions. This variational 2RDM (v2RDM) approach is the driver for a polynomially scaling approximation to configuration-interaction-driven complete active-space self-consistent field (CASSCF) theory. For v2RDM-based CASSCF computations involving an active space consisting of 50 electrons in 50 orbitals, we observe a speedup of a factor of 3.7 when the code is executed on a combination of an NVIDIA TITAN V GPU and an Intel Core i7-6850k CPU, relative to the case when the code is executed on the CPU alone. We use this GPU-accelerated v2RDM-CASSCF algorithm to explore the electronic structure of the 3,k-circumacene and 3,k-periacene series (k = 2-7) and compare indicators of polyradical character in the lowest-energy singlet states to those observed for oligoacene molecules. The singlet states in larger circumacene and periacene molecules display the same polyradical characteristics observed in oligoacenes, with the onset of this behavior occurring at smallest k for periacenes, followed by the circumacenes and then the oligoacenes. However, the unpaired electron density that accumulates along the zigzag edge of the circumacenes is slightly less than that which accumulates in the oligoacenes, while periacenes clearly exhibit the greatest buildup of unpaired electron density in this region.

Dissipative cooling of spin chains by a bath of dipolar particles

We consider a spin chain of fermionic atoms in an optical lattice, interacting with each other by super-exchange interactions. We theoretically investigate the dissipative evolution of the spin chain when it is coupled by magnetic dipole–dipole interaction to a bath consisting of atoms with a strong magnetic moment. Dipolar interactions with the bath allow for a dynamical evolution of the collective spin of the spin chain. Starting from an uncorrelated thermal sample, we demonstrate that the dissipative cooling produces highly entangled low energy spin states of the chain in a timescale of a few seconds. In practice, the lowest energy singlet state driven by super-exchange interactions is efficiently produced. This dissipative approach is a promising alternative to cool spin-full atoms in spin-independent lattices. It provides direct thermalization of the spin degrees of freedom, while traditional approaches are plagued by the inherently long timescale associated to the necessary spatial redistribution of spins under the effect of super-exchange interactions.

Electronic structure and stability of the SiO2+ dications produced in tomographic atom probe experiments.

The molecular electronic states of the SiO2+ dication have been investigated in a joint theoretical and experimental analysis. The use of a tip-shaped sample for tomographic atom probe analysis offers the unique opportunity to produce and to analyze the lifetime of some excited states of this dication. The perturbation brought by the large electric field of the polarized tip along the ion trajectory is analyzed by means of molecular dynamics simulation. For the typical electric fields used in the experiment, the lowest energy triplet states spontaneously dissociate, while the lowest energy singlet states do not. We show that the emission process leads to the formation of some excited singlet state, which dissociates by means of spin-orbit coupling with lower-energy triplet states to produce specific patterns associated with Si+ + O+ and Si2+ + O dissociation channels. These patterns are recorded and observed experimentally in a correlated time-of-flight map.

H4: A model system for assessing the performance of diffusion Monte Carlo calculations using a single Slater determinant trial function.

A model H4 system is used to investigate the accuracy of diffusion Monte Carlo (DMC) calculations employing a single Slater determinant to fix the nodal surface. The lowest energy singlet state of square H4 is a diradical which is poorly described by DMC calculations using a single determinant (SD) trial function. Here we consider distortions to rectangular structures, which decrease the amount of diradical character. The falloff of the error in the SD-DMC energy with increasing separation between the two H2 molecules is found to be much more rapid for small distortions away from square than for large distortions. This behavior is shown to be correlated with the extent of mixing between the two configurations needed to properly describe the diradical character. The error in the SD-DMC energy is found to be sizeable (∼0.1 eV) even for separations at which the coefficient of the dominant configuration in a four-electron, four-orbital complete active space self-consistent-field wave function is as large as 0.9.

Oxidative cleavage of exo-alkylidene xanthenes

exo-Alkylidene xanthenes were easily excited by irradiation with white light to generate their lowest-energy triplet state via intersystem crossing from the lowest-energy singlet state. Ground-state oxygen was then transformed by the lowest-energy triplet state of the xanthene to active singlet oxygen. The singlet oxygen reacted with the exo-alkylidene xanthene to afford the corresponding xanthone. During the transformation, the exo-alkylidene xanthenes acted as an effective photosensitizer as well as a substrate of singlet oxygen.

Electronic Structure, Spin-States, and Spin-Crossover Reaction of Heme-Related Fe-Porphyrins: A Theoretical Perspective

The electronic structures, spin-states, and geometrical parameters of tetra-, penta-, and hexa-coordinated iron-porphyrins are investigated applying density functional theory (DFT) based calculations, utilizing the plane-wave pseudopotential as well as localized basis set approaches. The splitting of the spin multiplet energies are investigated applying various functionals including recently developed hybrid meta-GGA (M06 family) functionals. Almost all of the hybrid functionals accurately reproduce the experimental ground state spins of the investigated Fe-porphyrins. However, the energetic ordering of the spin-states and the energies between them are still an issue. The widely used B3LYP provides consistent results for all chosen systems. The GGA+U functionals are found to be equally competent. After assessing the performance of various functionals in spin-state calculations, the potential energy surfaces of the oxygen binding process by heme is investigated. This reveals a "double spin-crossover" feature for the lowest energy reaction path that is consistent with previous CASPT2 calculations but predicting a lowest energy singlet state. The calculations have hence captured the spin-crossover as well as spin-flip processes. These are driven by the intra-atomic orbital polarization on the central metal atom due to the atomic and orbitals rearrangements. The nature of the chemical bonding and a molecular orbital analysis are also performed for the geometrically simple but electronic structurally complicated system tetra-coordinated planar Fe porphyrin in comparison to the penta-coordinated systems. This analysis explains the observed paradoxical appearance of certain peaks in the local density of states (DOS).

Vibrational Energy Flow Controls Internal Conversion in a Transition Metal Complex

Internal conversion (IC) between excited electronic states is a fundamental photophysical process that is important for understanding protection from UV radiation, energy transfer pathways and electron injection in artificial photosynthetic systems and organic solar cells. We have studied IC between three singlet MLCT states in an iridium complex using femtosecond fluorescence spectroscopy. Very fast IC with a time constant of <20 fs is observed from the highest state and a much slower relaxation to the lowest energy singlet state on a 70 fs time scale. The abrupt slowdown of the relaxation rate occurs when there is >0.6 eV of vibrational energy stored in the complex that has to be dissipated by intramolecular vibrational redistribution before further IC to the lower energy states can occur. These results show that the ability to dissipate vibrational energy can control the relaxation process in this class of materials.

Linear and nonlinear optical properties of a series of Ni-dithiolene derivatives

Some linear and nonlinear optical (NLO) properties of Ni(SCH)(4) and several of its derivatives have been computed by employing a series of basis sets and a hierarchy of methods (e.g., HF, DFT, coupled cluster, and multiconfigurational techniques). The electronic structure of Ni(SCH)(4) has been also analyzed by using CASSCF/CASPT2, ab initio valence bond, and DFT methods. In particular we discuss how the diradicaloid character (DC) of Ni(SCH)(4) significantly affects its NLO properties. The quasidegeneracy of the two lowest-energy singlet states 1 (1)A(g) and 1 (1)B(1u), the clear DC nature of the former, and the very large number of low-lying states enhance the NLO properties values. These particular features are used to interpret the NLO properties of Ni(SCH)(4). The DC of the considered derivatives has been estimated and correlated with the NLO properties. CASVB computations have shown that the structures with Ni(II) are the dominant ones, while those with Ni(0) and Ni(IV) have negligible weight. The weights of the four diradical structures were discussed in connection with the weight of the structures, where all the electrons are paired. Comparative discussion of the properties of Ni(SCH)(4) with those of tetrathia fulvalene demonstrates the very large effect of Ni on the properties of the Ni-dithiolene derivatives. A similar remarkable effect on the NLO properties is produced by one or two methyl or C(3)S groups. The considered Ni-dithiolene derivatives have exceptionally large NLO properties. This feature in connection with their other physical properties makes them ideal candidates for photonic applications.

Threshold Photoionization and Density Functional Theory Studies of the Niobium Carbide Clusters Nb3Cn (n = 1−4) and Nb4Cn (n = 1−6)

We have used photoionization efficiency spectroscopy to determine ionization potentials (IP) of the niobium-carbide clusters, Nb3C(n) (n = 1-4) and Nb4C(n) (n = 1-6). The Nb3C2 and Nb4C4 clusters exhibit the lowest IPs for the two series, respectively. For clusters containing up to four carbon atoms, excellent agreement is found with relative IPs calculated using density functional theory. The lowest energy isomers are mostly consistent with the development of a 2 x 2 x 2 face-centered cubic structure of Nb4C4. However, for Nb3C4 a low-lying isomer containing a molecular C2 unit is assigned to the experimental IP rather than the depleted 2 x 2 x 2 nanocrystal isomer. For Nb4C5 and Nb4C6, interpretation is less straightforward, but results indicate isomers containing molecular C2 units are the lowest in energy, suggesting that carbon-carbon bonding is preferred when the number of carbon atoms exceeds the number of metal atoms. A double IP onset is observed for Nb4C3, which is attributed to ionization from the both the lowest energy singlet state and a meta-stable triplet state. This work further supports the notion that IPs can be used as a reliable validation for the geometries of metal-carbide clusters calculated by theory.

Ultrafast Study of 9-Diazofluorene: Direct Observation of the First Two Singlet States of Fluorenylidene

Ultrafast photolysis of 9-diazofluorene (DAF) produces a broadly absorbing transient within the instrument time resolution (300 fs), which is assigned to an excited state of the diazo compound. The diazo excited state fragments to form fluorenylidene (Fl) in both its lowest energy singlet state (1Fl, 405-430 nm, depending on the solvent) and a higher energy singlet state (370 nm, 1Fl*). The excited singlet carbene has a lifetime of 20.9 ps in acetonitrile and decays to the lower energy singlet state (1Fl), which relaxes to the triplet ground state (3Fl) in acetonitrile, cyclohexane, benzene, and hexafluorobenzene. The equilibrium mixture of singlet and triplet fluorenylidene reacts with these solvents. Singlet fluorenylidene reacts with methanol and cyclohexene in competition with relaxation to 3Fl. One of the reaction products in methanol is the 9-fluorenyl cation. The rate of intersystem crossing (ISC) in hexafluorobenzene and other halogenated solvents is remarkably slow given that carbene ISC rates are generally fastest in nonpolar solvents. An explanation of this effect is advanced.

Donor Stabilized Borylnitrene: A Highly Reactive BN Analogue of Vinylidene

A donor atom stabilized borylnitrene, 2-nitreno-1,3,2-benzodioxaborole 4c, is characterized by matrix isolation IR, UV, and ESR spectroscopy as well as multiconfiguration SCF and CI computations. UV irradiation (lambda = 254 nm) of the corresponding azide 6c, isolated in solid argon at 10 K, produces 4c in high yield. The oxygen donor atoms in 4c result in a triplet ground state (|D/hc| = 1.492 cm(-)(1), |E/hc| = 0.004 cm(-)(1)) for the borylnitrene. The lowest energy singlet state ((1)A(1)) is 33 kcal mol(-)(1) higher in energy and closely related to the ground state of vinylidene. Under the conditions of matrix isolation, triplet 4c is photochemically and thermally stable toward rearrangement to the corresponding cyclic iminoborane. Photochemical irradiation (lambda > 550 nm) of 4c rather causes an efficient reaction with molecular nitrogen, lying in matrix sites nearby, to give 6c. Similarly, photochemical, but not thermal, trapping of 4c with CO is possible and results in the corresponding isocycanate 9c. Thermal reaction of 4c with O(2) in doped argon matrixes at 35 K could be observed by IR spectroscopy to result in borylnitroso-O-oxide 17c as shown by (18)O(2) labeling experiments and DFT computations. The diradical 17c is very photolabile and quickly rearranges to the nitritoborane 16c upon irradiation.

Quantum Chemical Characterization of the Vertical Electron Affinities of Didehydroquinolinium and Didehydroisoquinolinium Cations

Vertical electron affinities (EA) are predicted for the lowest energy singlet states of the 21 didehydroquinolinium cation isomers and the 21 didehydroisoquinolinium cation isomers, as well as the doublet states of the seven dehydroquinolinium cation isomers, the seven dehydroisoquinolinium cation isomers, the seven N-methyldehydroquinolinium cations, and the seven N-methyldehydroisoquinolinium cations, by using density functional theory. For the monoradicals, the calculated EA of the radical site depends only on the distance from the (formally charged) nitrogen atom, and is reduced by 0.14-0.24 eV when the NH+ group is replaced with an NCH3+ group. Nearly all of the calculated EAs for the ortho biradicals are lower (by 0.04-0.72 eV) than those for either of the corresponding monoradicals. For the meta biradicals, the calculated EAs lie either between the EAs of the corresponding monoradicals or higher (by 0.07-0.58 eV), and they are extremely sensitive to the separation (distance) between the two dehydrocarbon atoms. For the biradicals that do not have either an ortho or meta relationship the calculated EAs are all higher (by 0.02-1.93 eV) than those for either of the corresponding monoradicals. The EAs are examined to gain insight into the nature of inductive/field and resonance effects that influence the electrophilicity of the radical site(s), which is a major factor controlling the reactivity of these types of (bi)radicals.

The aromatic character and resonance stabilization energies of substituted cyclopentadienyl and indenyl cations

Calculations (B3LYP/6-31G(d)) have been used to assess the aromaticity of 5-X substituted indenyl (4) and cyclopentadienyl (5) cations with X = O–, NH2, OCH3, CH3, F, H, CN, and N2+. Two criteria were used, the aromatic stabilization energy (ASE), as determined by isodesmic reactions, and bond alternation, as determined from the Julg index (A) on the basis of carbon–carbon bond lengths. Substituent effects on the singlet state of the cyclopentadienyl cations resulted in significant decreases in antiaromatic character for electron-donating groups as indicated by larger A values (A = –0.25 for X = H and +0.26 for X = NH2). These decreases paralleled increases in the C-2—C-3 bond length and good linear correlations were obtained between A vs. the C-2—C-3 bond length and A vs. the ASE. These effects were rationalized by the stabilization by the electron-donating groups of the positive charge at C-5 generated as a consequence of a Jahn–Teller distortion leading to a lowest energy singlet state with a HOMO of a2 symmetry. In contrast, the lowest energy triplet state for each of the substituted cyclopentadienyl cations has little bond alternation (A &gt; 0.9) and, by this criterion, is not significantly antiaromatic. The triplet state is more stable than the singlet state for the unsubstituted case and those with electron-withdrawing groups (ΔEST = –11.3 and –9.3 kcal/mol for X = H and CN, respectively) (1 cal = 4.184 J), but less stable for electron-donating groups (ΔEST = +15.0 kcal/mol for X = NH2). For the indenyl cations 4, the ASE values were almost independent of the substituent and the A values only decreased slightly for electron-donating groups. The A values also indicated that the indenyl cations could be divided into two moieties, an X-substituted pentadienyl cation with considerable delocalization and little bond alternation, and a 2,3-butadiene one with considerable bond alternation. This separation also placed the major portion of the positive charge on the pentadienyl part. The lack of symmetry in the substituted indenyl cations rationalizes the selective reactivity of the 5-methoxy-substituted cation at C-1. Finally, the resonance stabilization energies (RSE) of the substituted cations gave a linear correlation with the RSEs of 4-substituted benzylic cations.Key words: indenyl cations, cyclopentadienyl cations, substituent effects, stabilization energies.

Effective quantum dimer model for trimerized kagomé antiferromagnet

An effective spin-orbit Hamiltonian is derived for a spin-1/2 trimerized kagome antiferromagnet in the second-order of perturbation theory in the ratio of two coupling constants. Low-energy singlet states of the obtained model are mapped to a quantum dimer model on a triangular lattice. The quantum dimer model is dominated by dimer resonances on a few shortest loops of the triangular lattice. Characteristic energy scale for the dimer model constitutes only a small fraction of the weaker exchange coupling constant.