David M. Sammeth

Site selective photoselection study of indole in argon matrix: location of the 1La origin

Abstract Polarized fluorescence excitation and dispersed fluorescence spectra of indole in solid Ar at 20 K under site selective conditions have revealed that ν 28 is Herzberg-Teller (HT) active, both in the excitation and the fluorescence spectrum. The two lines 455 and 480 cm −1 above the 1 L b origin are a Fermi doublet involving 28 0 1 and the out-of-plane combination 39 0 1 41 0 1 and derive 1 L a optical character via the HT coupling by ν 28 . The true 1 L a origin appears to be about 1400 cm −1 above the 1 L b origin in vapor, as discerned by the differential red shift in the Ar matrix.

Aqueous Dissolved Gas Measurements Using Near-Infrared Raman Spectroscopy

Near-infrared Raman spectra of several gases dissolved in an aqueous medium have been acquired and analyzed for the first time. Spectra of dissolved and gaseous CO2, SO2, N2O, dimethyl ether, and O2 are presented and compared. Raman-forbidden bands in the spectrum of gaseous dimethyl ether are observed in the dissolved spectrum; this result can be explained by relaxed selection rules due to lowered symmetry. Raman scattering enhancements in the dissolved phase are measured and shown to compare well with a theoretical prediction accounting for internal field enhancements in water. Dissolved CO2 is measured quantitatively at concentrations from 0 to 250 Torr with the use of the method of partial least-squares, with an average prediction error of 12 Torr.

1La transitions of jet-cooled 3-methylindole

Abstract One-photon and polarized two-photon fluorescence excitation spectra of jet-cooled 3-methylindole (3MI) and 3-trideuterio-methylindole are reported at a resolution of ≈0.2 cm−1. On the basis of the two-photon intensity ratios for linear versus circular polarized excitation, strong lines with predominantly 1La character are identified in 3MI at 409, 420, 468, 609, 617, 739, 820, and 918 cm−1 above the 1Lb origin. The lowest-lying 1La line in 5-methylindole appears to be at 1424 cm−1. Evidence for an avoided crossing of the 1La and 1Lb surfaces in 3MI is presented.

One-pot polymorphism of nonlinear optical materials. First example of organic polytypes

Two nonlinear optical materials 1,1-dicyano-4-(4-dimethylaminophenyl)-1,3-butadiene (I) and 3-methoxydicyanovinylbenzene (II) have been investigated with regard to crystal growth, polymorphism, structure characterization and physical properties. It was found that both compounds form one-pot (concomitant) polymorphs, that might also be described as organic polytypes. Both polymorphs of compounds I and II form identical molecular layers with molecules located in the layer plane. In the two phases of compound I the layers superposition and their sequence differs significantly. This circumstance most probably determines the different color of the two crystal modifications. In the two crystal phases of compound II molecular layers and their superposition types are almost identical. The only difference between the phases is the sequence of the layer superposition. Single crystal and powder X-ray diffraction techniques, powder test for second harmonic generation, UV spectroscopy, and computational methods were used for characterization of these compounds.

Two-photon fluorescence excitation spectrum of a naphthalene norbornylog: Implications for electron transfer

Abstract Polarized two-photon fluorescence excitation spectra of 1,4-dimethoxy-2,3-dimethyl naphthalene and of a 1,4-dimethoxy-2,3-norbornolog naphthalene, in 0.002 M cyclohexane solution, are reported in the range 665-545 nm at 293 K. Intensity predictions for the 1 L b and 1 L a states based upon alternate hydrocarbon pseudoparity rules are confirmed: the weak 1 L b origin is seen as is a strong 1 L b vibronic origin attributed to the naphthalene 4b 2u vibration, but the 1 L a origin is not identified. However, consideration of the one-photon absorption spectrum then reveals that the 1 L a origin is higher in energy than is the 1 L b origin, confirming that the fast through-bridge photoinduced electron transfer seen in naphthalene—dicyanoethylene norbornologs is an electronically forbidden but vibronically allowed process. As far as we are aware, these are the first examples of such processes.

X-ray crystal structures, molecular mechanics and quantum chemical calculations, and calculations of the nonlinear optical polarizabilities in the series of monohalogen substituted derivatives of dicyanovinylbenzene

Abstract The molecular and crystal structure of a series of potential non-linear optical (NLO) compounds — o-, m- and p-halogen-substituted derivatives of dicyanovinylbenzene, has been studied. The molecular geometry was investigated with an X-ray diffraction analysis and discussed along with results of the molecular mechanics (MM3) and ab initio quantum calculations (MP2 and DFT). The influence of the crystal packing on the molecular planarity has been revealed. Calculations of the dipole moments and molecular hyperpolarizabilities for molecules in question were carried out in the finite field approach. In addition to a several acentric compounds of this series found before, in the present investigation one more acentric crystal of the p-I substituted dicyanovinylbenzene was found (space group Pna21, Z=4). Second harmonic generation (SHG) of the laser light for this compound was confirmed by the Kurtz powder test.

Spectroscopic, cyto-, and photo-toxicity studies of substituted piperidones: potential sensitizers for two-photon photodynamic therapy

Two-photon photodynamic therapy has the advantages of being highly localized in its effects and allows for deeper tissue penetration, when compared to one-photon photodynamic therapy. N-alkylated 3,5-bis(arylidene)-4-piperidones, with a donor-pi-acceptor-pi-donor structure, have the potential to be useful two-photon sensitizers. We have measured two-photon cross sections (using femtosecond excitation), fluorescence quantum yields, fluorescence lifetimes, and xray crystal structures for a number of these compounds. Most two-photon cross sections are comparable to or larger than that of Rhodamine B. However, the fluorescence quantum yields are low (all less than 10%) and the fluorescence lifetimes are less than 1 ns (with one exception), suggesting that there may be a significant energy transfer to the triplet state. This would encourage singlet oxygen formation and increase cellular toxicity. Results of dark cyto-toxicity studies with several human cancer cell lines are presented. White light photo-toxicity results are also presented, and suggest that increasing the number of double bonds, from one to two, in the piperidone wings increases the photo-toxicity with little corresponding change in the dark cyto-toxicity.

Vaporization of Nonvolatile and Matrix-Isolated Molecules Using a Novel Laser Vaporization Technique

A technique is described which uses a laser-generated high-velocity polymer flyer to vaporize nonvolatile molecules and low-temperature matrix-isolated species. Morphology studies, time-of-flight measurements, static collection methods, and real-time laser-induced fluorescence were used to characterize the new vaporization technique. Cr(CO)6 in a mineral oil glass at 77 K, rhodamine dye, and naphthalene were vaporized in these experiments.