Thomas L. Penner

Absorption and second-harmonic generation of monomer and aggregate hemicyanine dye in Langmuir–Blodgett films

Absorption spectra and second-harmonic generation of Langmuir–Blodgett monolayers containing a hemicyanine dye with a large second-order hyperpolarizability are reported. The dye is found to be in the form of H aggregates in a pure-dye film and mostly monomeric in a 1:4 dye–arachidic acid film. This observation explains a report in the literature that mixed dye–arachidic acid films display greater second-harmonic generation efficiency than a pure-dye film. It may also be a factor in the reported failure of second-harmonic generation to increase quadratically with the number of layers in noncentrosymmetric multilayer films.

Silica nanoparticles encapsulating near-infrared emissive cyanine dyes

We show that efficient near-infrared (NIR) cyanine fluorophores (1 and 2) can be encapsulated into silica nanoparticles providing a highly versatile and unique platform for in vivo diagnostics. Utilizing this platform, multiple fluorophores can be loaded within a single particle allowing the light absorption and emission properties of the nanoparticle to be controlled independent of particle size. Furthermore, such dyed nanoparticles may have extinction coefficients as high as about 100 x 10(6) Lmol(-1)cm(-1) in the NIR (on a per mole of particles basis), with quantum yields from about 8-10%. A simple synthetic method for varying particle size and dye-loading level is presented, and a modified Stober synthesis reduces deleterious exposure of the dye to the highly alkaline conditions used. The cyanine dyes are encapsulated in silica in a non-aggregated state and the fluorescence brightness is largely maintained to nominal dye concentrations approaching 50 muM. The ability to control light absorption and emission properties independent of particle size, and convenient access to particle sizes in the range of 20-100 nm (a size regime difficult to access with other nanoparticle approaches such as quantum dots), are important features for anatomical targeting in in vivo diagnostics and targeted therapeutic applications.

Energy transfer between J-aggregated dye monolayers

Abstract Energy transfer between two Langmuir-Blodgett films has been measured for the first time under conditions where both layers of dye are highly concentrated and J aggregated. The distance dependence of energy transfer was examined by the technique of interposing of a varying number of fatty acid layers between the dye layers. The efficiency of energy transfer was found to decrease with the fourth power of the distance. This is the behavior expected when the donor excitation is localized. For extensively delocalized excitation a quadratic dependence has been predicted. The strong resonance coupling between donor and acceptor in the present case results in efficient energy transfer over distances that are comparable with the expected delocalization of excitation in the donor aggregate so that the localized excitation model is appropriate. A three-dye system of J-aggregated dyes is also described. The efficiency of energy transfer from initial donor to ultimate acceptor is on the order of 90% over about 100 A.

The formation of mixed J aggregates of cyanine dyes in Langmuir-Blodgett monolayers.

Certain combinations of N-octadecyl-substituted cyanine dyes will form mixed J aggregates when spread on a Langmuir trough from solution mixtures and transferred to glass support. These mixed aggregates show single absorption and fluorescence bands intermediate between those of the J aggregates of the individual component dyes. Quite specific compatibility appears to be necessary to form such mixed aggregates, since relatively small structural changes destroy the effect. These results confirm Scheibes observations on such aggregates formed in solution which were reported nearly 50 years ago.

Blue and green Čerenkov-type second-harmonic generation in a polymeric Langmuir–Blodgett waveguide

Blue (430-nm) and green (532-nm) wavelengths of Cerenkov radiation were generated in an all-polymeric Langmuir–Blodgett (LB) waveguide. A nonzero second-order nonlinear-optical (NLO) susceptibility was obtained by alternate deposition of an NLO active and an NLO passive polymer. The Cerenkov condition was fulfilled through control of the thickness of the LB film and the modal dispersion relations for waveguides. The second-harmonic light, generated in the waveguide, leaks into the substrate at the Cerenkov angle. The Cerenkov-type guided-mode-to-radiation-mode phase-matching scheme reduces the waveguide attenuation for the second harmonic and provides a convenient way of separating the second harmonic from the fundamental.

Ultrafast optical modulation of quadratic non-linearity from an Ru(II)-bipyridine complex in Langmuir-Blodgett assemblies

Abstract Alternate Langmuir-Blodgett (LB) films consisting of a ruthenium(II)-bipyridine complex and ammonium amphiphiles exhibit a strong second-harmonic generation (SHG) on irradiation with nanosecond neodymium-doped yttrium aluminium garnet 1064 nm laser pulses. We successfully demonstrated that the SHG intensity from the LB films was reversibly modulated when the films were exposed to nanosecond 355 nm laser pulses. The switching time for the optical modulation of SHG was determined to be less than 2 ps from a subpicosecond dye laser pump-probe SHG experiment. The mechanism for the optical modulation of SHG was ascribed to a change in the molecular hyperpolarizability of the ruthenium complex on going from the ground state to the excited state.

Langmuir-Blodgett films for second-order nonlinear optics

Langmuir-Blodgett films have been fabricated of a prepolymerized amphiphile containing a chromophore of high second-order nonlinear optical activity. Thick films, containing as many as 262 layers of noncentrosymmetric structure have been prepared by alternate deposition of the dye polymer and several different materials. In contrast to previous literature reports on Langmuir-Blodgett films of other polymeric dyes, these films show the theoretically expected quadratic dependence of second harmonic generation with film thickness. Polarization studies of the second harmonic generation demonstrate that the chromophores are highly ordered in these films. In addition, by using all-polymeric materials it has been possible to fabricate a waveguide of these films with optical attenuation of about 1 dB cm-1, substantially lower than has been previously reported for Langmuir-Blodgett films even without a noncentrosymmetric structure.

Phase-matched second-harmonic generation in waveguides of polymeric Langmuir-Blodgett films

Langmuir-Blodgett (LB) films have been fabricated from a preformed acrylate amphiphilic copolymer with nonlinear optical (NLO) active chromophores covalently attached to the polymer backbone. By alternate deposition of this NLO active polymer with an NLO passive amphiphilic polymer, high quality multilayer films have been obtained with sufficient thickness to allow waveguiding in the LB films. The observed optical attenuation is between 1 and 3 dB cm-1. The linear and nonlinear optical characteristics of these films have been determined, both in transmission (or reflection) and in waveguide format. The high degree of order in LB films makes this type of supramolecular structure attractive for NLO applications. Based on the observed parameters (dispersion of the refractive indices, birefringence, tilt angle of the chromophore, hyperpolarizability), different phase-matching schemes can result in efficient second harmonic generation.

Linear and nonlinear optical properties of polymeric Langmuir-Blodgett films

We have prepared Langmuir-Blodgett monolayer and multilayer films of a polymeric amphiphile with a side chain containing a chromophore of high molecular hyperpolarizability for second order nonlinear optics. Using optical techniques including polarized transmission and attenuated total reflectance spectroscopies, second harmonic generation, and ellipsometry, we have characterized the orientational order of the chromophore and other polymer groups in the films. The high degree of chromophoric order seen in these films indicates that such polymers are good candidates as materials to fabricate Langmuir-Blodgett films for nonlinear optical applications.

EXCITED STATE FORMATION IN THE IRRADIATION OF 1,3-CYCLOHEXADIENE

Abstract Irradiation of 1,3-cyclohexadiene with high energy radiation leads to dimerization. Relative amounts of these dimers vary widely with reaction conditions but the composition of the mixtures can be expressed in terms of variable amounts of two groups each of which contains the dimers in a fixed proportion. The evidence indicates that one of these groups arises from dimerization of the cyclohexadiene cation and that the other originates from diene triplets. The latter in turn appear to be produced by neutralization of ions. Singlet product, 1, 3, 5-hexatriene, is also formed but is not ionic in origin. The effect of a change in linear energy transfer on the radiation processes is also investigated.