Paul A. Fleitz

Effect of platinum on the photophysical properties of a series of phenyl-ethynyl oligomers

In this work we detail the photophysical properties of a series of butadiynes having the formula H-(C6H4-C[triple bond]C)n-(C[triple bond]C-C6H4)n-H, n=1-3 and ligands H-(C6H4-C[triple bond]C)n-H, n=1-3 and compare these to previous work done on a complimentary series of platinum-containing complexes having the formula trans-Pt[(PC4H9)3]2[(C[triple bond]-C6H4)n-H]2, n=1-3. We are interested in understanding the role of the platinum in the photophysical properties. We found that there is conjugation through the platinum in the singlet states, but the triplet states show more complex behavior. The T1 exciton, having metal-to-ligand charge-transfer character, is most likely confined to one ligand but the Tn exciton appears to have ligand-to-metal charge-transfer character. The platinum effect was largest when n=1. When n=2-3, the S0-S1,S1-S0,T1-S0, and T1-Tn spectral properties of the platinum complex are less influenced by the metal, becoming equivalent to those of the corresponding butadiynes. When n=1, platinum decreases the triplet state lifetime, but its effect diminishes as n increases to 2.

Nonlinear measurements on AF-50

The nonlinear optical properties of N,N-diphenyl-7-[2-(4- pyridinyl) ethenyl]-9,9-di-n-decylfluoren-2-amine [AF- 50] have been investigated. The nonlinear absorption of a saturated solution of this material in acetone was investigated with 430 femtosecond pulses at 790 nm. From these results, the two-photon absorption cross-section was determined to be 25 X 10-50 cm4sec/photon molecule. This number is in agreement (within a factor of 2) with theoretical calculations. Nonlinear absorption and optical limiting measurements were also made using a Nd:YAG pumped dye laser with 4.3 ns pulses at 694 nm. These results suggest inherent differences in the performance of two-photon absorbing materials in these two different geometries.

Two‐photon absorption and second hyperpolarizability measurements in diphenylbutadiene by degenerate four‐wave mixing

We have investigated two‐photon absorption (TPA) in diphenylbutadiene at 532 nm using degenerate four‐wave mixing (DFWM) of nanosecond laser pulses. We present a theory which describes the development of double‐peaked phase conjugate pulses produced in DFWM due to TPA‐induced gratings superimposed on the usual electronic third‐order processes. Our analysis suggests a novel technique for measuring both the real and imaginary parts of the third‐order susceptibility of a molecular solution, hence leading to a determination of the TPA cross section σ2 and second hyperpolarizability γ of the solute molecules. Applying this experimental technique to diphenylbutadiene in chloroform, we obtain σ2=(40±8)×10−50 cm−4 s/photon–molecule and ‖γ‖=(420±80)×10−36 esu for this diphenyl polyene. We show that these values are in agreement with related measurements in diphenylbutadiene, and that they are consistent with a two‐photon resonance near 39 500 cm−1. The DFWM measurement technique we describe is very sensitive and s...

Z-Scan Studies of Dispersion of the Complex Third-Order Nonlinearity of Nonlinear Absorbing Chromophores

We have studied dispersion of the cubic nonlinear optical properties of organic and organometallic nonlinear absorbers. The results indicate that a multitude of excited states may be contributing to the resonances seen in the real and imaginary part of the complex hyperpolarizability. Attempts to understand the relation between the real and imaginary part of the hyperpolarizability in terms of a Kramers-Kronig transform are undertaken.

TWO-PHOTON ABSORPTION AND FLUORESCENCE OF NEW DERIVATIVES OF CYCLOHEXANONE AND PIPERIDONE

We report on two-photon absorption and two-photon excited fluorescence of a selected group of new derivatives of cyclohexanone and piperidone with regard to their molecular structure. The molecular cross-section of the two-photon absorption of these compounds, as high as 3000 × 10-50 cm4 s/photon at nanosecond pumping, makes then promising nonlinear materials for optical limiting in the near-infrared region. They also exhibit an intense two-photon exciting fluorescence. Similar compounds have previously demonstrated strong anticancer effect. Both features combined in a single compound thus open the way for more efficient treatment of cancer, when biochemical destruction of malignant cells is accompanied by their two-photon fluorescent imaging.

Lithographic Microfabrication by Using Two-Photon Absorbing Phenylenevinylene Derivative

The phenylenevinylene-based two-photon absorbing chromophore(EA4BPA-VB)with a triphenyl amine as an electron donor was synthesized. The two-photon absorption(TPA)cross-section values of EA4BPA-VB measured by two-photon-induced fluorescence method with 80 fs-pulse laser were found to beσ2 = 4.7 × 10−48 cm4s/photon. By adding this chromophore into photo-reactive SCR resin, we have successfully fabricated 3-D micro-structure(a larva)with high spatial resolution at low laser power with less than 5 mW. To improve the resolution of 3-D patternings, we also investigated a voxel size and shape depending on laser exposure time and concentration of TPA chromophore in SCR resin at near-threshold exposure condition.

Investigating the nonlinear optical properties of molten organic materials

The nonlinear absorption of two substituted thiophene compounds containing benzithiazole units was investigated using the Z-scan technique. The experiments were performed using a Nd:YAG laser frequency doubled to 532nm with 24ps laser pulses. The samples were prepared as solutions in THF and as melts by heating above their melting points into the isotropic liquid phase. Both compounds show strong nonlinear absorption under all conditions. Solutions of these materials show little or no linear absorption at the laser wavelength. However, the melts do show significant linear absorption. The nonlinear absorption observed has been analyzed using methods for two-photon absorption and two-photon assisted excited state absorption and two-photon absorption cross-sections are reported.

Dispersion of the complex cubic nonlinearity in two-photon absorbing organic and organometallic chromophores

There is much interest in nonlinear absorbing chromophores for applications in photonics, nanophotonics and biophotonics. We have performed studies of dispersion of the nonlinear absorption cross sections and the refractive nonlinearities of organic and organometallic nonlinear chromophores using the technique of Z-scan, with a tunable amplified femtosecond laser system. Z-scan is less sensitive than the popular technique of two-photon induced fluorescence but has advantages of being suitable for non-fluorescent substances and providing information on both absorptive and refractive nonlinearities. We have analysed the experimental results in terms of simple models and using the Kramers-Kronig transformation as shown in this paper for Coumarine 307 and an organometallic dendrimer. The dispersion curves are often dominated by two-photon resonances but inclusion of other nonlinear mechanisms seems to be necessary for better understanding of their features.

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.