Satadal Paul

Interplay among Aromaticity, Magnetism, and Nonlinear Optical Response in All-Metal Aromatic Systems

All-metal aromatic molecules are the latest inclusion in the family of aromatic systems. Two different classes of all-metal aromatic clusters are primarily identified: one is aromatic only in the low spin state, and the other shows aromaticity even in high-spin situations. This observation prompts us to investigate the effect of spin multiplicity on aromaticity, taking Al(4)(2-), Te(2)As(2)(2-), and their copper complexes as reference systems. Among these clusters, it has been found that the molecules that are aromatic only in their singlet state manifest antiaromaticity in their triplet state. The aromaticity in the singlet state is characterized by the diatropic ring current circulated through the bonds, which are cleaved to generate excess spin density on the atoms in the antiaromatic triplet state. Hence, in such systems, an antagonistic relationship between aromaticity and high-spin situations emerges. On the other hand, in the case of triplet aromatic molecules, the magnetic orbitals and the orbitals maintaining aromaticity are different; hence, aromaticity is not depleted in the high-spin state. The nonlinear optical (NLO) behavior of the same set of clusters in different spin states has also been addressed. We correlate the second hyperpolarizability and spin density in order to judge the effect of spin multiplicity on third-order NLO response. This correlation reveals a high degree of NLO behavior in systems with excess spin density. The variance of aromaticity and NLO response with spin multiplicity is found to stem from a single aspect, the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), and eventually the interplay among aromaticity, magnetism, and NLO response in such materials is established. Hence, the HOMO-LUMO energy gap becomes the cornerstone for tuning the interplay. This correlation among the said properties is not system-specific and thus can be envisaged even beyond the periphery of all-metal aromatic clusters. Such interplay is of crucial importance in tailoring novel paradigm of multifunctional materials.

Interpretation and Quantification of Magnetic Interaction through Spin Topology.

This work develops a formalism to quantify the interaction among unpaired spins from the ground state spin topology. Magnetic systems where the spins are coupled through direct exchange and superexchange are chosen as references. Starting from a general Hamiltonian, an effective Hamiltonian is obtained in terms of spin density which is utilized to compute exchange coupling constants in magnetic systems executing direct exchange. The high-spin-low-spin energy gap, required to extract the coupling constant, is obtained through the broken symmetry approach within the framework of density functional theory. On the other hand, a perturbative approach is adopted to address the superexchange process. Spin transfer in between the sites in the exchange pathway is found to govern the magnetic nature of a molecule executing superexchange. The metal-ligand magnetic interaction is estimated using the second order perturbation energy for ligand to metal charge transfer and spin densities on the concerned sites. Using the present formalism, the total coupling constant in a superexchange process is also partitioned into the contributions from metal-ligand and metal-metal interactions. Sign and magnitude of the exchange coupling constants, derived through the present formalism, are found to be in parity with those obtained using the well-known spin projection technique. Moreover, in all of the cases, the ground state spin topology is found to complement the sign of coupling constants. Thus, the spin topology turns into a simple and logical means to interpret the nature of exchange interaction. The spin density representation in the present case resembles McConnells spin density Hamiltonian and in turn validates it.

The impact of surface structure and band gap on the optoelectronic properties of Cu2O nanoclusters of varying size and symmetry

A systematic characterization of Cu2O nanoclusters using classical electrodynamics and time-dependent density functional theory (TDDFT) is performed to investigate their response to light with the alteration of size and symmetry. Absorption and scattering play a crucial role in tuning the surface plasmon resonance (SPR), which is the focal feature of optoelectronic properties. In larger dimensions the SPR is found to be strongly influenced by scattering and in smaller NPs it is dominated by absorption. A blue shift of the SPR peak is observed with decreasing cluster size. The optical properties of Cu2O nanoclusters are also affected by the symmetry aspect. With the variation of size and symmetry the associated surface structure and band gap are also varied. The TDDFT calculation is performed to explore the impact of these two fundamental factors on the optoelectronic nature of (Cu2O)n clusters. The TDDFT study on Cu2O nanoclusters reveals the nature of electronic excitations in photoirradiated (Cu2O)n clusters for n = 1, 2, and 3. The transitions involved in (Cu2O)n are basically categorized as ligand to metal charge transfer (LMCT) and metal to metal charge transfer (MMCT) processes. The change in absorption with varying cluster dimension and symmetry is found to be critically controlled by the relative probabilities of LMCT and MMCT processes. A competing surface reconstruction effect and occupied–virtual energy gap are also found to govern the SPR pattern of the Cu2O nanoclusters. All of these observations provide an appropriate guideline to tune SPR of Cu2O NPs for specific applications.

Exchange interactions in systems with multiple magnetic sites.

Nonequivalent magnetic interactions in systems with multiple magnetic centers can be explored through a proper description of exchange coupling. The magnetic exchange coupling constant (J) in systems with two magnetic sites is reliably estimated using Heisenberg-Dirac-van Vleck (HDVV) model through broken symmetry approach (BS) within a density functional theory (DFT) framework. However, in case of systems with multiple magnetic centers, exchange coupling constants, evaluated through state-of-the-art techniques, are often found to be inadequate to produce a correct fingerprint of the nature of magnetic interactions therein. This work suggests a new scheme to estimate exchange coupling constants in such systems. In this strategy, distribution of spins on magnetic sites in the ground state of systems with multiple magnetic centers is computed. On the basis of this spin mapping, exchange coupling constants between specific pairs are estimated through BS-DFT approach while keeping all other paramagnetic atoms magnetically inactive. Nonetheless, the effect of magnetically inert paramagnetic sites is already taken into account by the process of spin mapping, which is further justified through expressing the HDVV Hamiltonian in terms of spin density operators. We employ this technique to hypothetical benchmark systems, H(3)He(3) and H(4)He(4) followed by real molecules, cationic manganese trimer, 1,3,5-benzenetriyltris (N-tert-butyl nitroxide), and a pentanuclear manganese complex. Results are found to be concordant with the already established nature of magnetic interaction in these systems. This strategy is different from the most popular scheme to compute J in systems with multiple magnetic centers in the sense that it avoids the formation of a large matrix out of different spin configurations and thus provides a reliable and computationally economic way to address the magnetic interactions in non isotropic systems with multiple magnetic sites.

Concurrent loss of aromaticity and onset of superexchange in Mg3Na2 with an increasing Na–Mg3 distance

Gradual migration of Na+ from Mg32− brings about fascinating change in aromatic and magnetic behavior of inorganic Mg3Na2 cluster, which is addressed at the B3LYP and QCISD levels. During this process, Na+ takes away the electron density from Mg32− causing a net decrease in aromaticity. A tug-of-war between the Pauli repulsion and the aromaticity is shown to be responsible for the observed stability and aromaticity trends in singlet and triplet states. Implications of a spin crossover vis-à-vis a possible superexchange are also explored.

Noncomparative scaling of aromaticity through electron itinerancy

Aromaticity is a multidimensional concept and not a directly observable. These facts have always stood in the way of developing an appropriate theoretical framework for scaling of aromaticity. In the present work, a quantitative account of aromaticity is developed on the basis of cyclic delocalization of π-electrons, which is the phenomenon leading to unique features of aromatic molecules. The stabilization in molecular energy, caused by delocalization of π-electrons is obtained as a second order perturbation energy for archetypal aromatic systems. The final expression parameterizes the aromatic stabilization energy in terms of atom to atom charge transfer integral, onsite repulsion energy and the population of spin orbitals at each site in the delocalized π-electrons. An appropriate computational platform is framed to compute each and individual parameter in the derived equation. The numerical values of aromatic stabilization energies obtained for various aromatic molecules are found to be in close agreement with available theoretical and experimental reports. Thus the reliable estimate of aromaticity through the proposed formalism renders it as a useful tool for the direct assessment of aromaticity, which has been a long standing problem in chemistry.

Ligand-induced symmetry breaking and concomitant blueshift in the emission wavelength of an octahedral chromium complex

AbstractThe resulting distortion of the octahedral symmetry of the complex [CrIII(NH3)6]3+ upon replacing the axial ligands with halides (i.e., weaker ligands) affects the stability of the doublet state with respect to that of the quartet ground state. This substitution affects the doublet-to-quartet transition responsible for phosphorescence. The position of the halide with respect to ammonia in the spectrochemical series is a major influence on the emission wavelength of the complex. The close proximity of fluorine and ammonia in the spectrochemical series leads to a blueshift in the emission wavelength when fluoride ions are introduced into the complex, thus providing a rational approach to the design of blue-phosphorescent materials, which are desirable for OLEDs used in full-color displays. Graphical abstractShifts in the phosphorescence emission wavelength of an octahedral Cr(III) complex caused by axial ligand substitution. Replacing the axial ligands leads to a change in the relative positions of the axial and equatorial ligands in the spectrochemical series, which in turn induces a redshift or a blueshift in the emission wavelength

Effect of charge transfer and periodicity on the magnetism of [Cr(Cp*)2][ETCE]

Magnetism in metallocene based donor–acceptor complexes stems from the donor to acceptor charge transfer. Thus, to correlate the exchange coupling constant J and the charge transfer integral, a formalism is developed which enables one to obtain the coupling constant from the value of the charge transfer integral and the spin topology of the system. The variance in the magnetic interaction between donor and acceptor is also investigated along two perpendicular directions in the three dimensional crystal structure of the reference system, decamethylchromocenium ethyl tricyanoethylenecarboxylate [Cr(Cp*)2][ETCE]. These donor–acceptor pairs (V-pair and H-pair), oriented along vertical and horizontal directions respectively, are found to have different extents of J, which is attributed to the difference in exchange coupling mechanisms, viz., direct exchange and superexchange. Next, V-pair and H-pair are taken together to treat both the intrachain and interchain magnetic interactions, since this competition is necessary to decipher the overall magnetic ordering in the bulk phase. In fact, this truncated model produces a small positive value of J supporting the weak ferromagnetic nature of the complex. Lastly, a periodic condition is imposed on the system to comprehend the nature of magnetism in the extended system. Interestingly, the ferromagnetism, prevailing in the aperiodic system, turns into weak antiferromagnetism in the periodic environment. This is explained through the comparison of density of states (DOS) plots in aperiodic and periodic systems. This DOS analysis reveals proximity of the donor and acceptor orbitals, facilitating their mixing in periodic conditions. This mixing causes the antiferromagnetic interaction to prevail over the ferromagnetic one, and imparts an overall antiferromagnetic nature in periodic conditions. This change over in magnetic nature with the imposition of periodicity may be useful to understand the dependence of magnetic behavior with dimensionality in extended systems.