Raman Phonon Spectra of Isotopic Mixed Naphthalene Crystals: Librational Exciton Model and the Amalgamation Limit

We present an experimental Raman study of lattice modes in neat and mixed (10%–90%) crystals of naphthalene‐h8 and naphthalene‐d8 at 100°K with 1 cm−1 resolution. The spectral features of the neat crystals are preserved in the heavily doped mixed crystals, with small shifts and broadenings characteristic of an amalgamation limit that assumes weakly coupled excitation bands in the restricted Frenkel‐Davydov limit. Rotation‐translation interaction does not affect the mixed crystal spectra, thus making the Raman technique uniquely suited for the investigation of the librational (rotational) phonon band structure. The evidence is against localized or pseudolocalized phonons in these isotopic mixed crystals.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70356/2/JCPSA6-57-2-863-1.pd

Temperature dependence of a vibrational exciton: Some methyl motions of durene

An intramolecular vibrational frequency in a molecular crystal has been found to show a significant temperature dependence. The low lying intramolecular mode at 272 cm−1 (room temperature value) of durene‐h14 (and the corresponding durene‐d14 frequency at 242 cm−1) shows an increase of [invertedlazys]9 cm−1[invertedlazys]9cm−1 in frequency in going from room temperature to 100°K. A theoretical discussion is presented on the role of various interactions that may give rise to the temperature dependence of the intramolecular mode. The theoretical considerations suggest that, although the thermal expansion of the lattice may be contributing to this temperature dependence, the dominant contribution is due to the cubic anharmonic interactions involving methyl torsions. It is suggested that in complex molecular crystals, where there are some very low lying intramolecular modes, the distinction between lattice modes and intramolecular modes based on the criterion that only lattice modes show frequency shift with temperature may be misleading. In such cases our mixed crystal criterion that the phonons obey the amalgamation limit in isotopic (H�D)(H�D) mixed crystals, whereas intramolecular vibrations are in the separated band limit, should be more reliable. This is clearly demonstrated in the case of durene.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70780/2/JCPSA6-58-11-5031-1.pd

Ab initio studies of two-photon absorption of some stilbenoid chromophores

Two-photon absorption of a series of donor-acceptor trans-stilbene derivatives is studied by means of density functional theory applied to second-order response function. Several important issues in modeling are highlighted which must be addressed for a reliable reproduction of the experimental results. It is evident that the correct order of magnitude of calculated two-photon absorption cross sections can only be obtained if proper account is taken of vibrational broadening of the absorption profiles. A comparison of the theoretical results with the experimental ones indicates that the computed two-photon absorption cross sections are in rough agreement with our previous report, although the observed systematic increase of the cross sections with the electron acceptor strength is not well reproduced. It is suggested that this disagreement may be due not only to the deficiencies of the computations but also to a variety of factors contributing to the experimental value of the effective two-photon absorption cross section, which are not taken into account in the ab initio calculations

Phonon sidebands of localized excitons in molecular crystals with methyl torsions: Hexamethylbenzene

Fluorescence and phosphorescence phonon sidebands of isotopic mixed hexamethylbenzene crystals at 2°K are presented. The external phonons can be observed separately from the semi‐internal (methyl torsion) ones. The nature of the electronic or vibronic state has observable but not drastic effects on the exciton‐phonon coupling function. Likewise, the exciton delocalization is of minor importance to the exciton‐phonon function in hexamethylbenzene. The coupling between external and internal vibrations is also weak in this system. The exciton‐phonon coupling appears comparable for the optical and acoustic phonons in hexamethylbenzene. The phonon sidebands give some of the phonon singularities of the low‐temperature crystal.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70173/2/JCPSA6-60-6-2365-1.pd

Phonon Raman spectra, molecular motions, and phase transitions of dimethylacetylene crystal

The phonon Raman spectra of crystals of dimethylacetylene, perdeutero-dimethylacetylene and their mixtures have been investigated down to 20[deg]K. The high temperature phase shows only one line and the low temperature phase only two. Neither of these is related to the Rz (methyl) rotation, which seems to remain practically free. However, the latter motion is found to be restricted in a newly discovered metastable phase, which is reported here first, displaying 5-6 phonon bands. The well-known [lambda]-point transition appears little related to methyl rotations. The internal rotation of dimethylacetylene seems to stay practically free down to 20[deg]K in the stable phases.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/33846/1/0000104.pd

Negative refractivity assisted optical power limiting

We propose an approach for achieving optical power limiting (OPL) using planar multilayered media. The approach involves the use of multiple bilayer structures that consist of a negative index (NIM) layer followed by a nonlinear two-photon absorbing (TPA) layer. The NIM layers refocus light in the TPA layers to increase intensity and nonlinear absorption. We perform parametric two-dimensional finite difference time domain simulations and compute the power transmittance of the media as a function of the incident light intensity, the number of layers, the layer thickness, and the material properties. Our analysis demonstrates proof-of-concept and indicates that the TPA-based OPL can be enhanced and optimized using NIM focusing.This work was in part supported by a grant from the office of Vice-President for Research at the University at Buffalo and in part by the Chemistry and Life Sciences Directorate of the Air Force Office of Scientific Research

Charge carrier mobility in an organic-inorganic hybrid nanocomposite

Organic-inorganic hybrid materials are media for electronic and optoelectronic applications. We present a study of the electronic transport in such a modelnanoparticle-sensitizedhybridorganic-inorganic photorefractive host system, consisting of poly(N-vinylcarbazole) doped with quantum dots of cadmium sulfide, using standard time-of-flight techniques. The photocurrent transients exhibit features typical of dispersive transport in an amorphous semiconductor. The hole mobility depends strongly on the electric field and temperature indicating Poole–Frenkel-like activated hopping transport; a thickness dependence of the mobility is observed. The presence of nanoparticles does not lead to increased trapping of holes. Conversely, a surprising result is observed: the mobility actually increases with the increase of nanoparticle concentration even though it is well below the percolation limit.This study was supported by a NSF, DMR Solid State and Polymer Chemistry Grant No. DMR0075867. Partial support by a Defense Research Initiative on Nanotechnology (DURINT), Contract No. F496200110358, through the Directorate of Chemistry and Life Sciences of the Air Force Office of Scientific Research is also acknowledged

Triplet exciton percolation and superexchange: Naphthalene C10H8–C10D8

The phosphorescence of betamethylnaphthalene doped into a naphthalene−h8/naphthalene−d8 mixed crystal has been measured. The results demonstrate that (1) dynamical exciton percolation does occur (i.e., a transition from an exciton insulator to an exciton conductor), that (2) it is very useful for the investigation of energy transfer in molecular aggregates, and that (3) it is a critical test of our current knowledge of exciton exchange and superexchange. (AIP)Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70872/2/JCPSA6-62-1-292-1.pd

Spectroscopic evidence for a continuous change in molecular and crystal structure: deformation of biphenyl in the low temperature solid

A Raman temperature study of biphenyl-h10 and biphenyl-d10 crystals reveals an unusual spectral change over a wide temperature region. Many internal and external infrared-active modes appear gradually as the temperature is lowered from about 75 to 15[deg]K. We interpret this as a mild and gradual change in both crystal and molecular structure: a loss of center of inversion, probably involving the partial return of the molecule to its non-planar shape (D2) in the vapor phase.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/22435/1/0000886.pd

Second-Harmonic and Sum-Frequency Imaging of Organic Nanocrystals with Photon Scanning Tunneling Microscope

Second-harmonic generation and sum-frequency generation with photon scanning tunneling microscopy and shear-force detection are used to map the nonlinear optical response and the surface topograph of N-(4-nitrophenyl)-(L)-prolinol crystals with a subdiffraction-limited resolution. The domain-size dependence of the spatial feature is obtained, which shows the local orientational distribution of the optical near field radiated by nonlinear nanocrystals and reveals the difference between nanoscopic and macroscopic second-order optical nonlinearities of molecular crystals