M. M. Szostak

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.

1H-NMR, dielectric and calorimetric studies of molecular motions in m-nitroaniline crystal

Abstract Spin-lattice relaxation time, T 1 , spin-lattice relaxation time in the rotating frame, T 1 ρ , and the second moment of the resonance line measurements at 80 MHz and over the 190–380 K temperature range are reported for protons in the optically nonlinear material m -nitroaniline ( m -NA). T 1 has also been measured for samples irradiated by low energy and low intensity radiation. The real and imaginary parts of electric permittivity as well as tan  δ have been recorded in the 80–380 K temperature range at frequencies ranging from 100 Hz to 1.0 MHz. DSC measurements have been performed in the 110–387 K temperature range. Two phase transitions have been found: A glassy to rotative transition at 160 K and a plastic to plastic transformation at 365 K. The main feature of the m -NA crystal is that its plasticity continues to grow as the temperature increases. The reorientations of phenyl rings, the –NH 2 group proton 180° jumps, the lattice distortions caused by anisotropic thermal expansion and the cooperative reorientations of big molecular aggregates are thought to be the reasons for phase transitions and for the subsequent intermolecular charge transfer. The results are discussed with respect to optical second harmonic generation and near-IR photochemical reaction found in the m -NA crystal.

Vibronic couplings in vibrational spectra and nonlinear electrooptic properties of meta-nitroaniline crystal

Abstract It is concluded from the linkage of the theory of the nonlinear electrooptic properties (NEP) with the vibronic theories of IR and Raman intensities that a relation between the IR spectra and the NEP of aromatic crystals must exist. Reanalysis of metanitroaniline( m -NA) spec detection of vibronic couplings in vibrational spectra has been proposed. It seems that the stronger couplings in non-centrosymmetriccrystals correspon

Unpaired Spin Generation Under Near IR Radiation in Electrooptic m-Nitroaniline Crystal. NIR Absorption, Dilatometric and EPR Studies

Abstract Near IR spectra in the 5700–7000 cm−1 region and thermal expansion coefficients of the meta-nitroaniline (m-NA) crystals in the (010) cleavage plane have been measured within the 290–380 K temperature range. From the temperature dependence of the NIR absorption results that at 320 K the conformations of the -NH2 group and phenyl ring start to change. The change of slope on the thermal expansion curves indicates 330 K as possible phase transition temperature. The irradiation of the sample in the NIR spectrophotometer at 6800 cm−1 by 72 houres causes the weak EPR signal which gains strongly the intensity after heating at 323 K. The results are discussed in terms of an infrared photochemical reaction and intra- and inter-molecular interactions and motions.

Helical superstructure and charged polarons contributions to optical nonlinearity of 2-methyl-4-nitroaniline crystals studied by resonance Raman, electron paramagnetic resonance, circular dichroism spectroscopies, and quantum chemical calculations.

The Raman excitation profiles of solid 2-methyl-4-nitroaniline (MNA) reveal several band enhancements by intermolecular and intramolecular charge transfer states. Calculated excited- and ground-state molecular geometries and excited state distortions qualitatively determined from Raman spectra indicate multiple vibrations leading to MNA dissociation. Also, overtones and combination tones can generate charged polarons, as detected by electron paramagnetic resonance after the exposure to 980 and 1550 nm laser diodes. The MNA space group Ia (C(s)(4)) is nonchiral; however, the electronic circular dichroism (CD) spectra of solution, KBr pellet, and single crystal were recorded. The crystal chirality is elucidated by room-temperature dynamic disorder, possible helical superstructure along the [102] polar axis, and charged polarons presence. The CD spectra ab initio calculations for MNA neutral and negatively charged monomers, dimers, and trimers, lying along the helix, confirmed the chirality. The role of these findings toward efficient optical nonlinearity and electric conductivity failure is discussed.

Dynamical disorder in 2-methyl-4-nitroaniline and its deuterated analogue crystals studied by Fourier transform infrared and nuclear magnetic resonance.

The Fourier transform infrared spectra of the thin layers of 2-methyl-4-nitroaniline (MNA) and its deuterated analog were recorded in the 500-4000 cm(-1) region in the 10-300 K temperature range. Activation energies of the -CH(3), -NH(2), and -NO(2) groups reorientations were estimated. The (1)H-NMR spin-lattice relaxation time, T(1), and the second moment of (1)H-NMR resonance line, M(2), measured in the 80-298 K temperature range, were used to determine the parameters of the -CH(3) group motion. The experimental potential barriers for the amine, nitro, and methyl group reorientations are considered in the context of strengths of the N-H([ellipsis (horizontal)])O, C-H([ellipsis (horizontal)])O intermolecular hydrogen bonds, and other short contacts, recognized recently [U. Okwieka et al., J. Raman Spectrosc. 39, 849 (2008)], and they agree with the barriers calculated by quantum chemical methods. The dynamical disorder found in the MNA crystal in the large temperature range seems to be important from the point of view of its nonlinear optical and other properties.

LO-TO splittings, effective charges and interactions in electro-optic meta-nitroaniline crystal as studied by polarized IR reflection and transmission spectra

Abstract The polarized IR reflection spectra of the meta-nitroaniline ( m -NA) single crystal along the a, b and c crystallographic axes as well as the b and c polarized transmission spectra have been measured in the 100–400 cm −1 region. The LO-TO splitting values have been calculated from the reflection spectra by fitting them with the four parameter dielectric function. The dipole moment derivatives, relevant to dynamic effective charges, of the vibrations have also been calculated and used to check the applicability of the oriented gas model (OGM) to reflection spectra. The discrepancies from the OGM have been discussed in terms of vibronic couplings, weak hydrogen bondings (HB) and intramolecular charge transfer.

Spectroscopic evidence of hydrogen bonding and intramolecular charge transfer cooperation in polar m-aminophenol crystal

Abstract The polarized infrared (IR) and Raman spectra of the m-aminophenol (m-AP) single crystal and the vibrational spectra of its solutions and powders in the 200–4000 cm−1 and 5000–7000 cm−1 regions as well as the electronic spectra were measured. on the basis of previous data and the oriented gas-model calculations, assignments of the internal vibrations are proposed. Discrepancies from the model relative intensities are explained with the help of quantum chemical INDO calculations by vibronic couplings and by the delocalization of charge through the chain of molecules connected by intermolecular OH⋯N bonding. The possibility of proton transfer in the excited state was excluded. The findings were considered in view of the non-linear electrooptic properties of the m-AP crystal.

ESR studies of near infrared induced unpaired spins in two polymorphs of meta-nitrophenol

Abstract The ESR spectra in the X-band region of two polymorphs of m-nitrophenol: the noncentrosymmetrical and optically nonlinear orthorhombic form and the monoclinic one were measured after the previous irradiation with the near IR beams corresponding to the overtones of the OH and CH stretching vibrations and the following heating to about 340K. The evolution of the strong signals as a function of time and temperature suggests that photochemical reactions occur in the studied crystals. The possible interpretation of the ESR signals is presented.

Thermally induced rearrangement of hydrogen-bonded helices in solid 4-isopropylphenol as studied by calorimetric, proton NMR, dielectric and near IR spectroscopic methods

Abstract Calorimetric, dielectric and Fourier transform near infrared (IR) spectroscopic methods were used to study molecular dynamics and structural transition in solid 4-isopropylphenol (4IP) above room temperature. Pulse proton nuclear magnetic resonance (NMR) measurements were performed in the 100–340 K temperature range. A phase transition was found at 331.5 K, 1.5 K below the melting point. Energetically inequivalent methyl groups reorientations were observed in differently prepared samples and this suggested that a high-temperature polymorph occurs below the transition point as a metastable phase. Dielectric relaxation measurements showed an electric conductivity similar in value to that in water. This was detected as a pronounced contribution to the imaginary part of dielectric permittivity at temperatures higher than 310 K. Near IR spectra revealed that hydrogen bondings are stronger in the high-temperature phase than in the room-temperature-stable one. We propose that thermally induced molecular rearrangements enable proton transfer in hydrogen bonds (HBs) and this stimulates protonic conduction.