Correlated Electronic States of a Few Polycyclic Aromatic Hydrocarbons: A Computational Study.

In recent years, polycyclic aromatic hydrocarbons (PAHs) have been studied for their electronic properties as they are viewed as nanodots of graphene. They have also been of interest as functional molecules for applications such as light-emitting diodes and solar cells. Since the last few years, varying structural and chemical properties corresponding to the size and geometry of these molecules have been studied both theoretically and experimentally. Here, we carry out a systematic study of the electronic states of several PAHs using the Pariser-Parr-Pople model, which incorporates long-range electron correlations. In all of the molecules studied by us, we find that the 2A state is below the 1B state and hence none of them will be fluorescent in the gaseous phase. The singlet-triplet gap is more than half of the singlet-singlet gap in all cases, and hence, none of these PAHs can be candidates for improved solar cell efficiencies in a singlet fission. We discuss in detail the properties of the electronic states, which include bond orders and spin densities (in triplets) of these systems.

A Model Exact Study of the Properties of Low-Lying Electronic States of Perylene and Substituted Perylenes.

There is a resurgence of interest in the electronic structure of perylene for its applications in molecular devices such as organic photovoltaics and organic light-emitting diodes. In this study, we have obtained the low-lying singlet states of perylene by exactly solving the Parisar-Parr-Pople model Hamiltonian of this system with 20 sites and 20 electrons in the VB basis where dimensionality is ∼5.92 billion. The triplet states of perylene are obtained using a DMRG scheme with symmetry adaptation. The one- and two-photon states are very close in energy ∼3.2 eV while the lowest triplet state is slightly below 1.6 eV indicating that perylene is a good candidate for singlet fission. To explore the tunability of the electronic states, we have studied donor-acceptor substituted perylenes. The two donors and two acceptors are substituted symmetrically at either the four bay sites or four peri sites. In all the bay substitution and one peri substitution at moderate D/A strength, the optical gap is lowered to about 2.8 eV. These molecules can be used as blue emitters. We have also reported bond orders in all the cases, and perylene as well as substituted perylenes can be viewed as two weakly coupled naphthalenes in the singlet states, but in triplets these bonds tend to be comparable to other bonds in strength. The charge densities in substituted perylenes are mostly localized around the substitution sites in the ground state. The positive spin densities in triplets are concentrated around the peri and bay sites with the remaining sites having small spin densities of either sign.

Exact solution of the PPP model for correlated electronic states of tetracene and substituted tetracene.

Tetracene is an important conjugated molecule for device applications. We have used the diagrammatic valence bond method to obtain the desired states, in a Hilbert space of about 450 million singlets and 902 million triplets. We have also studied the donor/acceptor (D/A)-substituted tetracenes with D and A groups placed symmetrically about the long axis of the molecule. In these cases, by exploiting a new symmetry, which is a combination of C2 symmetry and electron-hole symmetry, we are able to obtain their low-lying states. In the case of substituted tetracene, we find that optically allowed one-photon excitation gaps reduce with increasing D/A strength, while the lowest singlet-triplet gap is only weakly affected. In all the systems we have studied, the excited singlet state, S1, is at more than twice the energy of the lowest triplet state and the second triplet is very close to the S1 state. Thus, donor-acceptor-substituted tetracene could be a good candidate in photovoltaic device application as it satisfies energy criteria for singlet fission. We have also obtained the model exact second harmonic generation (SHG) coefficients using the correction vector method, and we find that the SHG responses increase with the increase in D/A strength.

Linear and nonlinear optical properties of expanded porphyrins: a DMRG study.

We study absorption spectra and two photon absorption coefficient of expanded porphyrins (EPs) by the density matrix renormalization group (DMRG) technique. We employ the Pariser-Parr-Pople (PPP) Hamiltonian which includes long-range electron-electron interactions. We find that, in the 4n+2 EPs, there are two prominent low-lying one-photon excitations, while in 4n EPs, there is only one such excitation. We also find that 4n+2 EPs have large two-photon absorption cross sections compared to 4n EPs. The charge density rearrangement in the one-photon excited state is mostly at the pyrrole nitrogen site and at the meso carbon sites. In the two-photon states, the charge density rearrangement occurs mostly at the aza-ring sites. In the one-photon state, the C-C bond length in aza rings shows a tendency to become uniform. In the two-photon state, the bond distortions are on C-N bonds of the pyrrole ring and the adjoining C-C bonds which connect the pyrrole ring to the aza or meso carbon sites.

A density matrix renormalization group method study of optical properties of porphines and metalloporphines.

The symmetrized density matrix renormalization group method is used to study linear and nonlinear optical properties of free base porphine and metalloporphine. Long-range interacting model, namely, Pariser-Parr-Pople model is employed to capture the quantum many-body effect in these systems. The nonlinear optical coefficients are computed within the correction vector method. The computed singlet and triplet low-lying excited state energies and their charge densities are in excellent agreement with experimental as well as many other theoretical results. The rearrangement of the charge density at carbon and nitrogen sites, on excitation, is discussed. From our bond order calculation, we conclude that porphine is well described by the 18-annulenic structure in the ground state and the molecule expands upon excitation. We have modeled the regular metalloporphine by taking an effective electric field due to the metal ion and computed the excitation spectrum. Metalloporphines have D(4h) symmetry and hence have more degenerate excited states. The ground state of metalloporphines shows 20-annulenic structure, as the charge on the metal ion increases. The linear polarizability seems to increase with the charge initially and then saturates. The same trend is observed in third order polarizability coefficients.

A comparative study of aromaticity in substituted tetracyclic and hexacyclic thiophenes.

We have studied the nature of aromaticity in expanded porphyrinic analogues of thiophenes formed by four and six thiophenes. Using density functional theory (DFT) we have analyzed the aromaticity of both the macrocycle and individual molecular fragments. We find paramagnetic annulenic ring currents in the case of tetracyclic molecules and diamagnetic annulenic ring currents for hexacyclic molecules. We have also studied the effect of substitution of benzene rings within the macrocycle. We find that as the number of benzene rings is increased the aromaticity increases for tetracyclic systems and decreases for hexacyclic systems. All the results have been analyzed with various microscopic parameters, including geometry, excitation gap, and NMR criteria.

Prevention and treatment of urinary tract infection with probiotics: Review and research perspective.

The spiralling costs of antibiotic therapy, the appearance of multiresistant bacteria and more importantly for patients and clinicians, unsatisfactory therapeutic options in recurrent urinary tract infection (RUTI) calls for alternative and advanced medical solutions. So far no sufficient means to successfully prevent painful and disabling RUTI has been found. Even though long-term oral antibiotic treatment has been used with some success as a therapeutic option, this is no longer secure due to the development of bacterial resistance. One promising alternative is the use of live microorganisms (probiotics) to prevent and treat recurrent complicated and uncomplicated urinary tract infection (UTI).The human normal bacterial flora is increasingly recognised as an important defence to infection. Since the advent of antibiotic treatment five decades ago, a linear relation between antibiotic use and reduction in pathogenic bacteria has become established as medical conventional wisdom. But with the use of antibiotics the beneficial bacterial flora hosted by the human body is destroyed and pathogenic bacteria are selectively enabled to overgrow internal and external surfaces. The benign bacterial flora is crucial for body function and oervgrowth with pathogenic microorganisms leads to illness. Thus the concept of supporting the human body's normal flora with live microorganisms conferring a beneficial health effect is an important medical strategy.

Depth of focus and the moment expansion.

We introduce the so-called moment expansion of a defocused image as a tool for analyzing and improving the depth of focus in optical imaging. It is shown that a number of previously noted defocus phenomena can be readily derived or explained in terms of moment expansion. Some potential applications of the moment expansion to phase-shifting mask and pupil filter design for optical lithography are also briefly noted.

Analysis and synthesis of feedforward neural networks using discrete affine wavelet transformations.

A representation of a class of feedforward neural networks in terms of discrete affine wavelet transforms is developed. It is shown that by appropriate grouping of terms, feedforward neural networks with sigmoidal activation functions can be viewed as architectures which implement affine wavelet decompositions of mappings. It is shown that the wavelet transform formalism provides a mathematical framework within which it is possible to perform both analysis and synthesis of feedforward networks. For the purpose of analysis, the wavelet formulation characterizes a class of mappings which can be implemented by feedforward networks as well as reveals an exact implementation of a given mapping in this class. Spatio-spectral localization properties of wavelets can be exploited in synthesizing a feedforward network to perform a given approximation task. Two synthesis procedures based on spatio-spectral localization that reduce the training problem to one of convex optimization are outlined.