Józef Lipiński

Theoretical model for the double fluorescence of p-cyano-N,N-dimethylaniline in polar solvents

Abstract The energies of the low lying electronic states of p -cyano-N,N-dimethylaniline (CDMA) in its planar and in 90° twisted conformation have been calculated using the modified INDO method. Solvent effects have been taken into account. The results agree reasonably well with the experimental data and support the hypothesis of the dual fluorescence of CDMA as being related to the intramolecular rotation of the N(CH 3 ) 2 group, with a decisive role of the solvent polarity.

Conformation and solvent dependence of the first and second molecular hyperpolarizabilities of charge-transfer chromophores. Quantum-chemical calculations

Abstract The results of semiempirical quantum-chemical calculations of the first- and second-order hyperpolarizabilities (β and γ, respectively) for molecules in the gas phase and in chloroform and aqueous solvents are presented. The finite-field formalism and the sum-over-states method was used to calculate individual components of the β and γ tensors. The solvent effect was included via the discrete quantum-mechanical Langevin dipoles/Monte Carlo approach and the continuum virtual charge method. The calculations demonstrate the existence of large geometry/conformation and solvent effects on the hyperpolarizabilities of the charge-transfer type chromophores. The calculated β and γ values including the solvent effect are compared to respective experimental data determined in solution phase.

Derivation of theoretical formulas for resonance integrals from Heisenberg equation of motion

On the basis of the Heisenberg equation of motion and Linderberg-Seamans approximations useful formulas for β-resonance integrals in the NDO-like semiempirical methods have been derived. The case ofs,p-electron elements has been investigated. The possibility of inclusion of Rydberg and/or inner orbitals in the basis set of the valence orbitals has been taken into consideration. The results of the test INDOL/R method calculations of vertical transition energies for H2 and H2O molecules has been presented.

Quantumchemical electronegativity and hardness indices for bonded atoms

Abstract An analysis of the electronegativity (χ A ) and hardness (η A ) of a bonded atom (A) is given on the ground of the Hartree-Fock Hamiltonian and based on Mulliken population analysis. Practical expressions for χ A and η A are developed within the Koopmans approximation. Electronegativity equalization (EE) has not been pursued, partial EE has been obtained, though. The resulting indices χ A and η A describe correctly atoms in simple molecules. The directional effect of the substituent in the benzene ring is properly rationalized and enhanced by means of the electronegativity and hardness of the ring carbons.

Single- and Two-Photon Excited Fluorescence in Organic Nonlinear Optical Single Crystal 3-(1,1-Dicyanoethenyl)-1-phenyl-4, 5-dihydro-1H-pyrazole

Fluorescence of nonlinear optical organic single crystal of 3-(1,1-dicyanoethenyl)-1-phenyl-4,5-dihydro-1H-pyrazole (DCNP) excited by a nonabsorbed light pulses from Q-switched Nd:YAG laser λ = 1064 nm as well as absorbed λ = 532 nm light is reported. Two mechanisms of two-photon excited fluorescence are considered: (i) direct two-photon excited fluorescence and (ii) single-photon excitation due to reabsorption of light generated in process of second harmonic generation (SHG) by the crystal due to its nonlinear optical properties. Strong anisotropy of fluorescence that has been observed is linked with uniaxial molecular alignment. Fluorescence decay profile shows two- exponential decay with lifetimes of emitting species of 3.7 and 5.6 ns at 293 K. The excitation and fluorescence spectra of the DCNP single crystal have been measured at 294 K and in function of temperature down to 77.4 K. The strong bathochromic shift of fluorescence spectrum in crystal with respect to fluorescence of DCNP molecule in solution is observed and interpreted with possible formation of molecular aggregates.

Photoluminescence and quantum chemical studies of electronic and optical properties of m-nitroaniline and m-nitrophenol crystals

Fluorescence, phosphorescence and their excitation spectra of single crystals of m-nitroaniline (m-NA) and of m-nitrophenol (m-NPh) were measured at 5 K. m-NA and m-NPh were also studied in an n-hexane Shpolskii matrix. Phosphorescence and phosphorescence decays were also collected at higher temperatures. On the basis of band position analysis and quantum chemical calculations the phosphorescence is assigned to the molecular emission whereas the fluorescence is supposed to originate from crystal defects—radical ions. Radical ions are photogenerated in both materials and in m-NA by crystal freezing below the glassy phase transition. Computed values of the first order hyperpolarizability, βvec, are larger for radical ions than for neutral molecules which suggests that radical ions are intermediates in the molecular mechanism of optical nonlinearity generation.

Polarization justified Fukui functions

New Fukui functions have been derived within the conceptual density functional theory by the analysis of the polarization effect of a system in static electric field. Resulting Fukui functions accurately reproduce the global softness and electronic dipolar polarizability; they meet the condition integral[f(r)/r]dr = -(partial differential mu/partial differential Z)(N) and lead to very reasonable values of the global hardness for atoms for the group of 29 main group elements. Computational clarity makes the new Fukui functions a promising tool in studies of molecular reactivity.

Solvent effect on the nonlinear optical properties of para-nitroaniline studied by Langevin dipoles–Monte Carlo (LD/MC) approach

Abstract Polarizabilities and hyperpolarizabilities and their frequency dispersions are calculated for para-nitroaniline in the water solvent using Langevin dipoles–Monte Carlo (LD/MC) approach. The sum-over-states formalism is used to calculate individual components of linear and nonlinear polarizability tensors for para-nitroaniline. The calculated values of hyperpolarizability including the solvent effect are compared with experimental results from solution phase EFISH measurements.

Charge density flow as a driving force of distortion in excited protonated azaaromatics

Abstract The INDO method was used to calculate electronic charge densities in the ground and lowest excited singlet and triplet states of neutral and protonated 2,3- and 1,4-diaza derivatives of naphthalene and phenanthrene, and benzo (a) phenazine. Experiniental changes of pKa upon excitation can be correlated with the values of electron density flow into the heterocy- clic ring. Electron-density increase turns out to be a major factor which causes distortion in some excited protonated species.

Finite-field calculations of the second-order hyperpolarizabilities γ of molecules in solutions

Abstract The results of the semi-empirical computation of the second-order hyperpolarizabilities γ for molecules in the gas phase and in solutions are presented. The finite-field formalism was used to calculate individual components of γ tensors. The solvent effect was included via the quantum-mechanical Langevin dipoles/Monte Carlo approach. The calculations demonstrate the existence of the large solvent effect on the second-order hyperpolarizabilities of the charge-transfer type of non-linear optical chromophores. The calculated γ values, including the solvent effect, are compared with experimental data determined in solution phase THG measurements.