Thomas M. Cooper

Investigation of Transition Metal-Xanthate Complexes for Nonlinear Optical Applications

We have synthesized several metal xanthate complexes from metal salts and xanthate potassium salts. Powders of several of the materials were sorted and tested for their nonlinear optical properties. Cadmium xanthate demonstrated phase matchability for frequency doubling of 1.06 micron light and generated a very strong second harmonic signal.

Structure-optical property relationships of porphyrins

Porphyrins are attractive compounds for optical applications. We have been investigating the relationship between molecular structure and optical properties of a number of porphyrin compounds. Structural variations explored include insertion of metal ions, extension of conjugation, halogenation and alkylation either at the pyrrole position or the meso-aryl groups. The characterization of these chromophores includes measurement of UV/Vis, fluorescence and fluorescence lifetimes. Furthermore, we have investigated their nonlinear absorption, excitation dynamics. The significant factors influencing limiting behavior appear to be the heavy atom effect, electron donating and withdrawing substituents conformation distortion and changes in conjugation. Detailed understanding will be gained from measurements of photophysical parameters underlying limiting behavior.

Degenerate Frequency Two-Beam Coupling in Organic Media via Phase Modulation with Nanosecond Pulses

This work presents a theoretical treatment using population redistribution and the thermo-optic effect to mediate degenerate frequency two-beam coupling (TBC) in the nanosecond regime in third-order nonlinear organic solutions. We show experimentally that the energy transfer is indeed a result of TBC and can be modeled using self- and cross-phase modulation to produce the required frequency shift. As a result of the relatively long lifetimes and large phase shifts induced by population redistribution and thermo-optic effects, the coupling efficiency can be significant. For the special case when a single input beam is aligned to overlap with the Fresnel reflection of the sample/air interface, coupling efficiencies can easily exceed 50% of the incident pump energy, which can account for a severe deleterious effect in nonlinear transmission experimentation.

Development of novel two-photon absorbing chromophores

There has been much interest in the development of two-photon absorbing materials and many efforts to understand the nonlinear absorption properties of these dyes but this area is still not well understood. A computational model has been developed in our lab to understand the nanosecond nonlinear absorption properties that incorporate all of the measured one-photon photophysical parameters of a class of materials called AFX. We have investigated the nonlinear and photophysical properties of the AFX chromophores including the two-photon absorption cross-section, the excited state cross-section, the intersystem crossing quantum yield, and the singlet and triplet excited state lifetimes using a variety of experimental techniques that include UV-visible, fluorescence and phosphorescence spectroscopy, time correlated single photon counting, ultrafast transient absorption, and nanosecond laser flash photolysis. The model accurately predicts the nanosecond nonlinear transmittance data using experimentally measured parameters. Much of the strong nonlinear absorption has been shown to be due to excited state absorption from both the singlet and triplet excited states. Based on this understanding of the nonlinear absorption and the importance of singlet and triplet excited states we have begun to develop new two-photon absorbing molecules within the AFX class as well as linked to other classes of nonlinear absorbing molecules. This opens up the possibilities of new materials with unique and interesting properties. Specifically we have been working on a new class of two-photon absorbing molecules linked to platinum poly-ynes. In the platinum poly-yne chromophores the photophysics are more complicated and we have just started to understand what drives both the linear and non-linear photophysical properties.

Spectroscopy and nonlinear optical absorption of bis(diphenylamino) diphenyl polyenes

The spectroscopy and nonlinear absorption of bis(diphenylamino) diphenyl polyenes have been studied in octane and dichloromethane solvents. The amines exhibit high fluorescence quantum yield and two photon excited emission. Two photon absorption cross section, (sigma) 2, was measured by Z-scan experiments. Strong two photon absorption is indicated by high values for (sigma) 2. Solvent has strong influence in the measurement of (sigma) 2 values.

Spectroscopic investigations of dithienyl polyenes

To understand the photophysics of nonlinear absorbers, we have investigated the photophysics of a series of di(2- thienyl-3,3,4,4-butyl)polyenes. Spectroscopic measurements, including UV/Vis, fluorescence, fluorescence lifetimes, fluorescence quantum yields, triplet state lifetime, solvent effects and two-photon absorption coefficient were obtained as a function of the number of double bonds (n equals 1 - 5). Trends in the data reflected the ordering, energy gap between and mixing of 1Bu* and 1Ag* excited state configurations. We investigated the solvatochromism of a series of (alpha) ,(omega) -di(2- dithienyl 3,3,4,4-butyl) polyenes. Absorption (n equals 1 - 5 double bonds) were collected in a series of aprotic solvents. The absorption energy dispersion effect sensitivity increased smoothly with n, reaching asymptotic behavior as n approached 5. The emission energy had less solvent sensitivity, giving evidence for a polar 1Bu* absorbing state and a nonpolar 1Ag* emitting state. We observed sensitivity of the absorbing and emitting states to solute-solvent dipole-dipole interactions, suggesting the dithienyl polyenes had a polar syn ground state conformation.

Change of electric dipole moment in charge transfer transitions of ferrocene oligomers studied by ultrafast two-photon absorption

Change of permanent electric dipole moment in the lower-energy charge transfer transitions for a series of symmetrical and non-symmetrical ferrocene-phenyleneethynylene oligomers were studied by measuring the corresponding femtosecond two-photon absorption cross section spectra, and were determined to be in the range Δμ = 3 – 10 D. Quantum-chemical calculations of Δμ for the non-symmetrical oligomers show good quantitative agreement with the experimental results, thus validating two-photon absorption spectroscopy as a viable experimental approach to study electrostatic properties of organometallics and other charge transfer systems.

Structure-spectroscopic property relationships in a series of platinum acetylides

In order to understand electronic and conformational effects on structure-spectroscopic property relationships in platinum acetylides, we synthesized a model series of chromophores trans-Pt(PBu3)2(Cuf0baCPhenyl-X)2, where X = NH2, OCH3, diphenylamino, t-Bu, methyl, H, F, benzothiazole, trifluoromethyl, CN and nitro. We collected linear spectra, including ground state absorption, phosphorescence and phosphorescence excitation spectra. We also performed DFT and TDDFT calculations on the ground and excited state properties of these compounds. The calculations and experimental data show the excited state properties are a function of the electronic properties of the substituents and the molecular conformation.

Synthesis of dual NIR two-photon absorptive [60]fullerenyl multiadducts for nonlinear light-transmittance reduction application

Synthesis of several C60-(antenna)x conjugates was performed to demonstrate high flexibility in the design of organic nonlinear optical (NLO) nanostructures showing broadband characteristics with capability to absorb light over a wide range of wavelengths. It was achieved by covalent attachment of a hybrid combination of two types of light-harvesting fluorescent antenna chromophores on a C60 cage. Ultrafast photoresponsive intramolecular Föster resonance energy-transfer among antenna units and shared excited energy-accepting C60 cage is proposed as a plausible mechanism to enhance the broadband NLO ability. Characterization of the branched triad C60(>DPAF-C18)(>CPAF-C2M) and the tetrad C60(>DPAF-C18)(>CPAF-C2M)2 was carried out by various spectroscopic techniques. These compounds showed approximately equal extinction coefficients of optical absorption over 400‒550 nm that corresponds to near-IR two-photon based excitation wavelengths at 780‒1100 nm. These nanomaterials may be utilized in NLO coatings for achieving efficient light-transmittance reduction at the same NIR wavelengths.

Exciplex formation in solid state blends of charge-transfer type AFX dyes and bisimide compounds

We have been studying exciplex formation in nonlinear optical materials containing a high concentration of 2PA chromophores (AFX dyes) as a means to enhance the nonlinear optical properties. A number of dipolar AFX dyes having various electron-accepting moieties (π-excess and π-deficient examples) and three bisimide compounds having substituents with varying electron-withdrawing power were synthesized to study exciplex formation in their solid state blends. In substrate supported thin films of various equimolar blends containing an AFX dye and bisimide, the dye monomer emission was severely quenched, and a new emission, red-shifted by up to 110 nm, appeared. The emission energies were consistent with the charge recombination energy calculated from the energy levels of the donor and acceptor present in the blend, which confirmed the emission was from an exciplex. Time resolved emission measurements also indicated the presence of much longer lived transients in the blends, consistent with exciplex formation. Spectroelectrochemistry confirmed that the radical cations of these dyes had strong absorption in the NIR region, so exciplex formation is a means to enhance nonlinear optical absorption of the dyes in this spectral region.