Seth R. Marder

Three-dimensional microfabrication using two-photon polymerization

Photopolymerization initiated by the simultaneous absorption of two photons is unique in its ability to produce complex three-dimensional (3D) structures from a single, thick photopolymer film. Strong 3D confinement of the polymerization process is not possible in other polymer microfabrication techniques such as LIGA, rapid prototyping, and conventional photoresist technology. Two-photon polymerization also permits the fabrication of 3D structures and the definition of lithographic features on non-planar surfaces. We have developed a wide array of chromophores which hold great promise for 3D microfabrication, as well as other applications, such as two-photon fluorescence imaging and 3D optical data storage. These materials are based on a donor- (pi) -donor, donor-acceptor-donor, or acceptor-donor-acceptor structural motif. The magnitude of the two-photon absorption cross-section, (delta) , and the position of the two-photon absorption maximum, (lambda) (2)max, can be controlled by varying the length of the conjugated bridge and by varying the strength of the donor/acceptor groups. In this way, chromophores have been developed which exhibit strong two- photon absorption in the range of 500 - 975 nm, in some cases as high as 4400 X 10-50 cm4 s/photon-molecule. In the case of donor-(pi) -donor structures, quantum-chemical calculations show that the large absorption cross-sections arise from the symmetric re-distribution of charge from the donor end-groups to the conjugated bridge, resulting in an electronic excited-state which is more delocalized than the ground state. For many of these molecules, two-photon excitation populates a state which is sufficiently reducing that a charge transfer reaction can occur with acrylate monomers. The efficiency of these processes can be described using Marcus theory. Under suitable conditions, such reactions can induce radical polymerization of acrylate resins. Polymerization rates have been measured, and we show that these two-photon chromophores display increased sensitivity and recording speed over conventional photoinitiators. Complex 3D structures can be fabricated in acrylate films doped with these chromophores using tightly focused near-infrared femtosecond laser pulses. A 3D periodic array of polymeric columns has been produced for use in photonic bandgap applications. Tapered waveguide structures for interconnecting disparate-sized optical components have been constructed. More traditional MEMS structures, such as cantilevers, have also been produced. Such structures may be useful for organic vapor sensors. The two-photon photopolymerization process can be extended to other material systems, such as metallic, ceramic, and composite materials, by templating the photopolymer structures.

Comparison of the Optical and Electrochemical Properties of Bi(perylene diimide)s Linked through Ortho and Bay Positions

The Ullmann homocoupling of 2-bromo-perylene diimides (PDIs) gave [2,2′-biperylene]-3,4:9,10:3′,4′:9′,10′-tetrakis(dicarboximide)s, 2,2′-bi(PDI)s, and the Suzuki coupling of a PDI-2-boronic ester and a 1-bromo-PDI gave a [1,2′-biperylene]-3,4:9,10:3′,4′:9′,10′-tetrakis(dicarboximide), 1,2′-bi(PDI). These were compared with [1,1′-biperylene]-3,4:9,10:3′,4′:9′,10′-tetrakis(dicarboximide)s, 1,1′-bi(PDI)s. Solution absorption spectra suggest that the PDIs in 2,2′-bi(PDI)s are more planar and less strongly coupled than those in 1,1′-bi(PDI)s, which is consistent with density functional theory calculations. 2,2′-Bi(PDI)s are less easily reduced than 1,1′- and 1,2′-bi(PDI)s by ca. 70–90 mV. Bulk heterojunction organic solar cells incorporating a 2,2′-bi(PDI) acceptor behaved similarly to those employing its 1,1′-bi(PDI) analogue.

Synthesis and structural characterization of multifunctional polysiloxanes for photorefractive effect

We have synthesized a novel kind of multifunctional polysiloxanes containing charge-transporting agent and electro-optical chromophobe as side chains for photorefractive application. The structural characterization of this kind of polymer is presented by 1H-NMR, IR spectra and elemental analysis.

Redox-active molecules as electrical dopants for OLED transport materials (Conference Presentation)

Electrical doping of organic semiconductors increases conductivity and reduces injection barriers from electrode materials, both of which effects can improve the performance of organic light-emitting diodes (OLEDs). However, the low electron affinities of typical OLED electron-transport materials make the identification of suitable n-dopants particularly challenging; electropositive metals such as the alkali metals are not easily handled and form monoatomic ions that are rather mobile in host materials, whereas molecular dopants that operate as simple one-electron reductants must have low ionization energies, which leads to severe air sensitivity. This presentation will discuss approaches to circumventing this issue by coupling electron transfer to other chemical reactivity. In particular, dimers formed by certain highly reducing organometallic sandwich compounds and organic radicals can be handled in air, yet have effective reducing potentials, corresponding to formation of the corresponding monomeric cations and contribution of two electrons to the semiconductor, of ca. –2.0 V vs. ferrocene. These values fall a little short of what is required for typical OLED materials; approaches to further extending the doping reach of these dimers will be described. One such approach involving photoirradiation of a dimer:semiconductor blend leads to metastable doping of a material with a redox potential of –2.24 V, which allows the fabrication of efficient OLEDs in which even high-workfunction electrodes, such as indium tin oxide, can be used as electron-injection contacts.

Organic Materials for All-Optical Signal Processing and Optical Limiting

Third-order nonlinearities and optical switching and limiting figures of merit are reported for several conjugated organic materials. Polymethines with large real to imaginary hyperpolarizability ratios and conjugated polymers with strong nonlinear absorption will be discussed.

Nonlinear Absorption Spectroscopy of a Bis(Porphyrin)-Substituted Squaraine

The nonlinear absorption mechanisms of a bis(porphyrin)-substituted squaraine have been studied with femtosecond, picosecond, and nanosecond pulsewidths. The two-photon absorption is ~10× larger than those of the constituents and is explained by intra-molecular charge transfer.

Temporal and Spectral Nonlinear Absorption Characterization of a Hybrid Porphyrin-Squaraine-Porphyrin Macromolecule

The nonlinear absorption mechanisms of a porphyrin-squaraine-porphyrin macromolecule have been studied with femto/pico/nanosecond pulsewidths. Two-photon absorption of the macromolecule is ~10× larger than the constituents and is explained by intra-molecular charge transfer.