Prakash Chandra Jha

Spin Multiplicity Dependence of Nonlinear Optical Properties

Open-shell spin-restricted time-dependent density functional theory is applied to explore the spin multiplicity dependence of linear and nonlinear optical properties. An open-shell neutral conjugated system, the C(4)H(4)N radical in the doublet X(2)A(2), quartet X(4)A(2), and sextet X(6)A(1) states, is chosen as a model system to illustrate various aspects of the theory. It is found that irrespective of the exchange-correlation functional employed, the components of the polarizability alpha(-omega,omega) and first hyperpolarizability beta(-2 omega,omega,omega) show very different dependency with respect to the multiplicity, with an increasing trend for higher spin states. This is rationalized by the decrease in conjugation and stability of the system with increasing multiplicity, and by the way the interaction between unpaired electrons and the external field is shielded by remaining electrons of the molecule. The study suggests the applicability of open-shell systems for frequency-dependent nonlinear optical properties and for the possibility of spin control for such properties.

Searching of potential energy curves for the benzene dimer using dispersion-corrected density functional theory.

The present work aims to establish the utility of dispersion-corrected density functional theory for potential energy curves of the benzene dimer, a problem that has received significant attention for a long time. The interaction energies of parallel-stacked, T-shaped and parallel-displaced benzene dimer configurations have been evaluated using both dispersion- and normal gradient-corrected Perdew-Burke-Ernzerhof functionals along with Dunnings augmented correlation-consistent polarized valence triple-zeta (aug-cc-pVTZ) basis functions and compared with explicit correlation methods. The potential energy curves for the parallel-stacked and parallel-displaced benzene dimers are in excellent agreement with highly accurate coupled cluster (CCSD(T)) results, while for the T-shaped benzene dimer the dispersion-corrected results show a distinct deviation, being closer in that case to the MP2 level of results. The overestimation of interaction energy in the T-shaped dimer may be attributed to the presence of a permanent dipole moment in this configuration and indicates a structural dependence of the dispersion-corrected density functional method.

One- and two-photon Absorptions in asymmetrically substituted free-base porphyrins: A density functional theory study

Electronic spectra and structures of a new family of free-base porphyrin (H(2)P) derivatives with 4-(diphenylamino)stilbene (DPAS) or 4,4-bis-(diphenylamino)stilbene (BDPAS) asymmetric substituents, recently synthesized and studied by Drobizhev et al. [J. Phys. Chem. B 110, 9802 (2006)] are investigated by density functional theory (DFT) using modern density functionals and the 6-31G* basis set. The time-dependent DFT technique is applied for calculations of one- and two-photon absorption spectra, electric and magnetic dipole moments, and for prediction of electronic circular dichroism for these chiral molecules. The four-band absorption spectrum of the H(2)P molecule (Q(x), Q(y), 0-0 and 1-0 bands) is enhanced in single-bond-linked DPAS. This enhancement is explained by hyperconjugation of the almost orthogonal pi systems and by small charge-transfer admixtures. The effect is much stronger for the double-bond- and triple-bond-linked DPAS and BDPAS substituents where absorption in the Q region transforms into a two-band spectrum. These molecules with ethenyl and ethynyl bonding of the porphyrin and donor substituent show very strong two-photon absorption in the near-infrared region. DFT calculations explain this by more efficient conjugation between the H(2)P and DPAS (BDPAS) chromophores, since they are almost coplanar: Gerade states of the H(2)P molecule occur in the Soret region and transform into charge-transfer states with nonzero transition moments. They are responsible for the strong two-photon absorption effects. Mixing of excitations in both chromophores explains the broadening of the Soret band. Though the calculated two-photon absorption cross sections are overestimated, the qualitative trends are reproduced and help understanding the whole genesis of spectra of these asymmetrically substituted H(2)P derivatives.

Time-dependent density functional theory for nonlinear properties of open-shell systems

This paper presents response theory based on a spin-restricted Kohn-Sham formalism for computation of time-dependent and time-independent nonlinear properties of molecules with a high spin ground state. The developed approach is capable to handle arbitrary perturbations and constitutes an efficient procedure for evaluation of electric, magnetic, and mixed properties. Apart from presenting the derivation of the proposed approach, we show results from illustrating calculations of static and dynamic hyperpolarizabilities of small Si(3n+1)H(6n+3) (n=0,1,2) clusters which mimic Si(111) surfaces with dangling bond defects. The results indicate that the first hyperpolarizability tensor components of Si(3n+1)H(6n+3) have an ordering compatible with the measurements of second harmonic generation in SiO2/Si(111) interfaces and, therefore, support the hypothesis that silicon surface defects with dangling bonds are responsible for this phenomenon. The results exhibit a strong dependence on the quality of basis set and exchange-correlation functional, showing that an appropriate set of diffuse functions is required for reliable predictions of the first hyperpolarizability of open-shell compounds.

Modeling two photon absorption cross sections of open-shell systems.

We demonstrate the prospects of computing two photon absorption cross sections of open-shell systems by applying recently developed spin-restricted time-dependent density functional response theory using the pyrrole radical as an example. The spin multiplicity effects on two photon absorption cross sections of this species are investigated for the doublet, quartet, and sextet states. It is found that irrespective of the exchange-correlation functional employed, the two photon cross sections increase with the increase in spin multiplicity. This result indicates that two photon cross sections of paramagnetic compounds can be controlled by manipulating their spin states and this opens new possibilities for design of hybrid magneto-optical materials.

Pressure dependence of crystal structure and molecular packing in anthracene

Anthracene molecular crystal has been investigated up to a pressure of 10.5 GPa at room temperature using variable shape variable size Monte Carlo simulations in an isothermal–isobaric ensemble. We have reported various structural quantities, such as cell parameters and unit cell volume, as a function of pressure and compared them with the experimental results [J. Chem. Phys. 119, 1078 (2003)]. The pressure dependence of angles θ, δ and χ which describe the relative packing of molecules in the crystal has been calculated. We report that anthracene molecular crystal does not exhibit any first order phase transition up to a pressure of 10.5 GPa which is consistent with the experimental observations by Oehzelt et al. [Phys. Rev. B 66, 174104 (2002)]. The calculated equation of state (EOS) has been fitted to a Murnaghan-type EOS with good agreement. The calculated bulk modulus and the pressure derivative of bulk modulus are 8.2 GPa and 8.9 respectively which are in agreement with the experimentally calculated values.Anthracene molecular crystal has been investigated up to a pressure of 10.5 GPa at room temperature using variable shape variable size Monte Carlo simulations in an isothermal–isobaric ensemble. We have reported various structural quantities, such as cell parameters and unit cell volume, as a function of pressure and compared them with the experimental results [J. Chem. Phys. 119, 1078 (2003)]. The pressure dependence of angles θ, δ and χ which describe the relative packing of molecules in the crystal has been calculated. We report that anthracene molecular crystal does not exhibit any first order phase transition up to a pressure of 10.5 GPa which is consistent with the experimental observations by Oehzelt et al. [Phys. Rev. B 66, 174104 (2002)]. The calculated equation of state (EOS) has been fitted to a Murnaghan-type EOS with good agreement. The calculated bulk modulus and the pressure derivative of bulk modulus are 8.2 GPa and 8.9 respectively which are in agreement with the experimentally calculated...

Two- and three-photon absorption of organic ionic pyrylium based materials

Pyrylium dyes having the same basic chemical structure, differing only to a specific substituent, have been used as novel materials for multiphoton three-dimensional data storage. Electronic absorption spectra, two-photon and three-photon absorption properties of this class of pyrylium dyes, have been studied theoretically and compared to experimental results. The effects of the counteranion, the surrounding solvent, and electron releasing and electron withdrawing groups in specific positions of the basic structure have been explored in detail. It is argued that on grounds of the quality of experimental spectroscopic agreement, the computed two- and three-photon data may be used in pulse propagation simulations of three-dimensional recording in optical memories.

Solvation shell structure of cyclooctylpyranone in water solvent and its comparative structure, dynamics and dipole moment in HIV protease

We have investigated the solvation structure for cyclooctylpyranone (COP) in water solvent using force-field molecular dynamics (MD) and Car-Parrinello mixed quantum mechanics-molecular mechanics (CPMD) calculations. The MD calculations show that in water solvent COP can exist in two conformational states which differ with respect to the relative orientations of the three rings, namely phenyl, pyranone and cyclooctane. We report the existence of strong orientational preference for the water molecule in the first solvation shell and the orientational preference disappears for solvent molecules beyond the first solvation shell. In order to investigate the confinement effect on the structure, dynamics, charge distribution and dipole moment of COP, we have carried out MD and CPMD calculations for COP within HIV type-1 protease (PR). Interestingly, we do not see any conformational transitions for COP within the protein cavity and it remains as a single conformer. We do see a remarkable effect of confinement on few other torsional degrees of freedom such as gg to tg conformational shift for the propyl group of COP. However, the methyl group rotational dynamics remains similar in the water solvent and in the protein environment. Also, within the protein cavity, the COP molecule is more polarized when compared to water solvent. Static ab initio electronic structure calculations were performed on the COP molecule with varying torsional angle in order to investigate the angle dependence of the molecular volume and energy.

A critical examination of two-photon absorption cross-sections of some reference dyes

The electronic structure and one- and two-photon absorption spectra of four most commonly used Flurophores, namely Bis-MSB, Coumarin 307, Fluorescein and Rhodamine B has been theoretically calculated and compared with experimentally reported spectra. The possible reasons for discrepancies in two photon absorption cross-sections reported in the literature till date is discussed. Since, our calculations are independent of laser source and different calibration methods used by different experimental groups, it should provide, at least in principle, a good picture about the electronic structure. The effect of solvent on the electronic one photon as well as two photon absorption spectra is also obtained. Finally, we also discuss about the necessary precautions that one needs to take when they compare their results with the one already reported in the literature.