Structure and energetics of small iron clusters

Computational method on highly efficient D-π-A-π-D-based different molecular acceptors for organic solar cells applications and non-linear optical behaviour

Eight molecular structures (BT-A1 to BT-A8) with high-performance non-fullerene acceptor (NFA) were selected for organic solar cells (OSCs) and non-linear optical (NLO) applications. Their electronic, photovoltaic (PV) and optoelectronic properties were tuned by adding powerful electron-withdrawing groups to the acceptor (A) of the D-π-A-π-D structure. Using time-dependent density functional theory (TD-DFT) techniques, based on the laws of quantum chemical calculations, the absorption spectra, stability of the highest and lowest-energy molecular orbitals (HOMO/LUMOs), electron density, intramolecular charge transfer (ICT), transition density matrix (TDM), were examined. The binding energy (Eb) and density of states (DOS) were probed to realize the optoelectronic analysis of the structures BT-A1 to BT-A8. Noncovalent interactions (NCIs) based on a reduced density gradient (RDG) were used to describe the nature and strength of D-A interactions in the molecules BT-A1 to BT-A8. The new refined molecules BT-A1 to BT-A8 exhibited strong absorbance bands between 408-721 nm and high electron transfer contribution (ETC) ranges between 87-96 %, along with the smallest excitation energies (Ex) between 1.71-3.55 eV in the solvent dichloromethane. Dipolar moment strengths ranging from 0.38 to 4.72 Debye in both the excited and ground states have determined with good solubility properties of BT-A1 to BT-A8 in polar solvent. Highly effective charge mobilities and prevention of charge recombination have been demonstrated by the electron (0.18-0.41 eV) and hole RE values (0.13-0.89 eV) for the new compounds. Power conversion efficiencies (PCE) of BT-A1 to BT-A8 were nearly the same because of better outcomes compared to the molecules in the BT. Compared to poly[4.8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b: 4,5-b']dithiophene-2,6- diyl-alt-(4-2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th), the open circuit voltages (Voc) of compounds BT-A1 to BT-A8 were ranged from 1.52 to 2.13 eV. The polarizability (α) and hyperpolarizability (β) of the molecules BT-A1 to BT-A8 were used to determine the non-linear optical (NLO) properties. The results showed that BT-A2, BT-A6 and BT-A7 have good NLO activity. This computational analysis demonstrates the superiority of the molecules with NFA. Hence the compounds are advised for the use in production of high-performance OSCs and NLO activity.

Structural modification of A-C-A configured X-PCIC acceptor molecule for efficient photovoltaic properties with low energy loss in organic solar cells

Modification of terminal acceptors of non-fullerene organic solar cell molecule with different terminal acceptors can help in screening of molecules to develop organic photovoltaic cells with improved performance. Thus, in this work, seven new molecules with an unfused core have been designed and thoroughly investigated. DFT/TD-DFT simulations were performed on studied molecules to explore the ground and excited state characteristics. UV-Visible analysis revealed the red shift in the absorption spectrum (reaching 781 nm) owing to their smaller energy gap up to 1.94 eV. Furthermore, transition density matrix analysis demonstrated that peripheral acceptors extract the electron density from the core effectively. The effectiveness of our investigated molecules as materials for high-performing organic photovoltaic cells has been shown by an examination of their electron and hole mobilities for fast charge transfer. When combined with PTB7-Th, all molecules displayed high open circuit voltage. XP5 molecule exhibited highest open circuit voltage (1.70 eV) and lowest energy loss of 0.30 eV. All designed molecules exhibit the improved aforementioned parameters, which shows that these molecules can be used to develop competent solar devices in future.

Theoretical framework for achieving high V oc in non-fused non-fullerene terthiophene-based end-capped modified derivatives for potential applications in organic photovoltaics

Non-fused ring-based OSCs are an excellent choice, which is attributed to their low cost and flexibility in applications. However, developing efficient and stable non-fused ring-based OSCs is still a big challenge. In this work, with the intent to increase V _oc for enhanced performance, seven new molecules derived from a pre-existing A-D-A type A3T-5 molecule are proposed. Different important optical, electronic and efficiency-related attributes of molecules are studied using the DFT approach. It is discovered that newly devised molecules possess the optimum features required to construct proficient OSCs. They possess a small band gap ranging from 2.22-2.29 eV and planar geometries. Six of seven newly proposed molecules have less excitation energy, a higher absorption coefficient and higher dipole moment than A3T-5 in both gaseous and solvent phases. The A3T-7 molecule exhibited the maximum improvement in optoelectronic properties showing the highest λ _max at 697 nm and the lowest E _x of 1.77 eV. The proposed molecules have lower ionization potential values, reorganization energies of electrons and interaction coefficients than the A3T-5 molecule. The V _oc of six newly developed molecules is higher (V _oc ranging from 1.46-1.72 eV) than that of A3T-5 (V _oc = 1.55 eV). Similarly, almost all the proposed molecules except W6 exhibited improvement in fill factor compared to the A3T-5 reference. This remarkable improvement in efficiency-associated parameters (V _oc and FF) proves that these molecules can be successfully used as an advanced version of terthiophene-based OSCs in the future.

Effect of Oriented Electric Fields on Biologically Relevant Iron-Sulfur Clusters: Tuning Redox Reactivity for Catalysis

Enzyme-based iron-sulfur clusters, exemplified in families such as hydrogenases, nitrogenases, and radical S-adenosylmethionine enzymes, feature in many essential biological processes. The functionality of biological iron-sulfur clusters extends beyond simple electron transfer, relying primarily on the redox activity of the clusters, with a remarkable diversity for different enzymes. The active-site structure and the electrostatic environment in which the cluster resides direct this redox reactivity. Oriented electric fields in enzymatic active sites can be significantly strong, and understanding the extent of their effect on iron-sulfur cluster reactivity can inform first steps toward rationally engineering their reactivity. An extensive systematic density functional theory-based screening approach using OPBE/TZP has afforded a simple electric field-effect representation. The results demonstrate that the orientation of an external electric field of strength 28.8 MV cm^-1 at the center of the cluster can have a significant effect on its relative stability in the order of 35 kJ mol^-1. This shows clear implications for the reactivity of iron-sulfur clusters in enzymes. The results also demonstrate that the orientation of the electric field can alter the most stable broken-symmetry state, which further has implications on the directionality of initiated electron-transfer reactions. These insights open the path for manipulating the enzymatic redox reactivity of iron-sulfur cluster-containing enzymes by rationally engineering oriented electric fields within the enzymes.

Evolution of Ferromagnetic and Antiferromagnetic States in Iron Nitride Clusters FenN and FenN2 (n = 1-10)

First-principles density functional theory calculations on neutral and singly negatively and positively charged iron clusters Fe_n and iron nitride clusters Fe_nN and Fe_nN₂ (n = 1-10) in the range of 1 ≤ n ≤ 10 revealed that there is a strong competition between ferromagnetic and antiferromagnetic states especially in the Fe_nN₂^0,±1 cluster series. This phenomenon was related to superexchange via a bridging N atom between two iron atoms in the Fe_nN₂^0,±1 cluster series and to a double superexchange effect via a Fe atom shared by two N atoms in the Fe_nN₂^0,±1 series. A thorough examination of the structure-energy-spin state relationships in these clusters is conducted, leading to new insights and confirmation of available experimental results on structural parameters and dissociation energetics. The bond energies of both nitrogen atoms in the Fe_nN₂ series are approximately the same. They weakly depend on the charge of the host cluster and fluctuate around 5.5 eV when moving along the series. The energy of N₂ desorption is relatively small; it varies by about 1.0 eV and depends on the charge of the cluster. The experimental finding that N₂ dissociates on the Fe_n⁺ clusters beginning with n = 4 was supported by the results of our computations. Our computed values of the Fe_n⁺-N bonding energies agree with the experimental data within the experimental uncertainty bars. It was found that the attachment of one or two N atoms does not seriously affect the polarizability, electron affinity, or ionization energy of the host iron clusters independent of the charge.

Dissociation of dinitrogen on iron clusters: a detailed study of the Fe16 + N2 case

The coalescence of two Fe8N as well as the structure of the Fe16N2 cluster were studied using density functional theory with the generalized gradient approximation and a basis set of triple-zeta quality. It was found that the coalescence may proceed without an energy barrier and that the geometrical structures of the resulting clusters depend strongly on the mutual orientations of the initial moieties. The dissociation of N2 is energetically favorable on Fe16, and the nitrogen atoms share the same Fe atom in the lowest energy state of the Fe16N2 species. The attachment of two nitrogen atoms leads to a decrease in the total spin magnetic moment of the ground-state Fe16 host by 6 μB due to the peculiarities of chemical bonding in the magnetic clusters. In order to gain insight into the dependence of properties on charge and to estimate the bonding energies of both N atoms, we performed optimizations of Fe16N and the singly charged ions of both Fe16N2 and Fe16N. It was found that the electronic properties of the Fe16N2 cluster, such as electron affinity and ionization energy, do not appreciably depend on the attachment of nitrogen atoms but that the average binding energy per atom changes significantly. The lowering in total energy due to the attachment of two N atoms was found to be nearly independent of charge. The IR and Raman spectra were simulated for Fe16N2 and its ions, and it was found that the positions of the most intense peaks in the IR spectra strongly depend on charge and therefore present fingerprints of the charged states. The chemical bonding in the ground-state Fe16N20,±1 species was described in terms of the localized molecular orbitals.

Water Dissociation upon Adsorption onto Free Iron Clusters Is Size Dependent

Cationic iron clusters, produced through laser ablation and subsequently complexed with a water molecule Fen(+)-H2O (n = 6-15) are mass-selectively investigated via infrared multiple photon dissociation (IR-MPD) spectroscopy in the 300-1700 cm(-1) spectral range. The experimental data are complemented by density functional theory calculations at the OPBE/TZP level for the Fe13(+)-H2O system. The observed spectra can be explained by a mixture of clusters where for a majority water is adsorbed molecularly but for a small but significant fraction also dissociation of water molecules occurs. The bands observed at frequencies 300-700 cm(-1) exhibit regular, size-dependent frequency shifts, showing that (a) dissociation takes places on all cluster sizes and (b) the interaction of water with the cluster surface is not influenced much by the particular cluster structure. The intensity evolution of the absorption bands suggests that dissociation is increasingly probable for larger cluster sizes.

Gas-phase perspective on the thermodynamics and kinetics of heterogeneous catalysis

Gas-phase studies of small transition metal cluster cations provide thermochemistry of utility to surface science and heterogeneous catalysis.