Matthew Farrow

University of Colorado at Boulder, Boulder, Colorado, United States

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Publications (12)29.7 Total impact

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    ABSTRACT: Azobenzene and its derivatives are among the most important organic photonic materials, with their photo-induced trans-cis isomerization leading to applications ranging from holographic data storage and photoalignment to photoactuation and nanorobotics. A key element and enduring mystery in the photophysics of azobenzenes, central to all such applications, is athermal photofluidization: illumination that produces only a sub-Kelvin increase in average temperature can reduce, by many orders of magnitude, the viscosity of an organic glassy host at temperatures more than 100 K below its thermal glass transition. Here we analyse the relaxation dynamics of a dense monolayer glass of azobenzene-based molecules to obtain a measurement of the transient local effective temperature at which a photo-isomerizing molecule attacks its orientationally confining barriers. This high temperature (T(loc)~800 K) leads directly to photofluidization, as each absorbed photon generates an event in which a local glass transition temperature is exceeded, enabling collective confining barriers to be attacked with near 100% quantum efficiency.
    Nature Communications 02/2013; 4:1521. DOI:10.1038/ncomms2483 · 10.74 Impact Factor
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    ABSTRACT: The initial development of photoinduced anisotropy in highly photosensitive monolayers of an aminoazobenzene molecule (dMR, a derivative of o-methyl red) that are initially randomized using circularly polarized light is found to be significantly slower than in monolayers randomized by thermal relaxation. We propose that this is a direct consequence of the slow thermal relaxation of isomers from the cis to the trans state and suggest that such considerations are important in designing even more sensitive photoactive monolayers and in understanding their photodynamics.
    Journal of Applied Physics 05/2011; 109(10):103521-103521-5. DOI:10.1063/1.3587572 · 2.19 Impact Factor
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    ABSTRACT: Photosensitive surfaces treated to have in-plane structural anisotropy by illumination with polarized light can be used to orient liquid crystals (LCs). Here we report a detailed study of the dynamic behavior of this process at both short and long times, comparing the ordering induced in the bare active surface with that of the LC in contact with the surface using a high-sensitivity polarimeter that enables detailed characterization of the anisotropy of the active surface. The experiments were carried out using self-assembled monolayers (SAMs) made from dimethylaminoazobenzene covalently bonded to a glass surface through a triethoxysilane terminus. This surface gives planar alignment of the liquid crystal director with an azimuthal orientation that can be controlled by the polarization of actinic light. We find a remarkable long-term collective interaction between the orientationally ordered SAM and the director field of the LC: while an azobenzene based SAM in contact with an isotropic gas or liquid relaxes to an azimuthally isotropic state in the absence of light due to thermal fluctuations, an orientationally written SAM in contact with LC in the absence of light can maintain the LC director twist permanently, that is, the SAM is capable of providing azimuthal anchoring to the LC even in the presence of a torque about the surface normal. We find that the short-time, transient LC reorientation is limited by the weak azimuthal anchoring strength of the SAM and by the LC viscosity.
    Langmuir 10/2010; 26(22):17482-8. DOI:10.1021/la102788j · 4.38 Impact Factor
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    ABSTRACT: We describe a new type of optically controlled liquid crystal alignment layer that demonstrates unprecedented performance. It consists of an aminoazobenzene-type material with a very simple molecular structure, which is derived from methyl red by a one-step synthesis. We have devised a method of forming covalently attached monolayers of this material on glass by an amine-assisted condensation reaction involving the triethoxysilane end of the molecule. A nematic liquid crystal (LC) cell made with the monolayer and a rubbed polymer layer was switched from a uniform state to a twisted state with a polarized 450 nm control beam having a dose of 5.5 mJ/cm(2). This is equivalent to an average of only one absorbed photon per azobenzene group. Through atomic force microscopy, absorption spectroscopy, spectroscopic ellipsometry, and second harmonic generation experiments, we have confirmed that layers of this type are smooth and uniform with a surface coverage consistent with a monolayer and that the azobenzene groups are tilted, on average, 55 degrees with respect to the surface normal. These characteristics lead to a large interaction energy density between the layer and LC. The monolayer's rapid response in developing anisotropy in this property can be attributed to a large absorption cross section, as well as the favorable tilt angle, which allows for sufficient photoisomerization free volume in a dense layer.
    Langmuir 01/2009; 25(2):997-1003. DOI:10.1021/la803491g · 4.38 Impact Factor
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    ABSTRACT: Self-Assembled Monolayers (SAMs) synthesized on glass and incorporating azobenzene are illuminated with green light at normal incidence to study the effect of photo-isomerization on in-plane molecular orientation. Measurements of the monolayer birefringence at the glass-gas interface show that the SAM orientation and order parameter dynamics are subdiffusive, characterized by a stretched exponential relaxation with a distribution of relaxation times. Order decays with power law relaxation and exponents that decrease with increased initial writing intensity and/or duration, indicative of orientational trapping wells with a distribution of depths. Results on dynamics of reorientation at the glass-solvent interface will also be presented. This work was supported by NSF Grant CHE-0079122 and NSF MRSEC Grant DMR-0213918.
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    ABSTRACT: A simple procedure for the preparation of octadecylsiloxane self-assembled monolayers (SAMs) on float glass substrates is described. The method utilizes commercial octadecyltriethoxysilane, OTE: n-C18H37Si(OCH2CH3)3, as the SAM precursor, with deposition accomplished in toluene solution using n-butylamine as catalyst. This synthetic approach obviates the use of the problematic trichlorosilanes typically required for the preparation of high quality SAMs, and is characterized by a wide ‘process window,’ utilizing off-the-shelf reagents without special handling.
    Liquid Crystals 04/2004; 31(4-4):481-489. DOI:10.1080/02678290410001666075 · 2.35 Impact Factor
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    ABSTRACT: We present dynamic measurements of the in-plane optic axis orientation birefringence photo-induced in an azo-benzene-based self-assembled monolayer (azo-SAM) to be used for liquid crystal (LC) alignment. Our experiment avoids the complications of traditional measurements where the SAM orientation is inferred from the optical response of a nematic LC in a cell with one SAM surface. Times for photo-induced optical changes are found to vary inversely with the incident intensity, down to 100 mus time scales. The measurements indicate that both the orientational order parameter and the optic axis orientation change during photo-buffing. This behavior is confirmed by computer simulations solving a non-linear diffusion equation with photo-excitation terms. We also find that heating the SAM randomizes the alignment. This work was supported by NSF Grant CHE-0079122 and NSF MRSEC Grant DMR-0213918.
  • YW Yi, T. E. Furtak, M. J. Farrow, D. M. Walba
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    ABSTRACT: Noncontact alignment of liquid crystal displays offers the advantage of reduced contamination and minimal surface charging. This approach also provides a means of reversible alignment after a device has been assembled. With this objective we have synthesized self-assembled monolayers based on dimethylaminoazobenzene units covalently attached to a glass surface by means of a short alkylsiloxane anchor, a derivatized version of methyl red (d-MR). The resulting architecture favors an orientation in which the axis of the azobenzene group should be nearly parallel to the surface with an isotropic azimuthal distribution. Under illumination with polarized light the trans-cis isomerization and subsequent relaxation serves to wiggle the molecule into an orientation perpendicular to the treatment polarization. We have tested this scenario using optical second harmonic generation and supporting optical techniques. We are able to identify a surface order parameter that characterizes the photoalignment of the azobenzene group, and have shown that the treatment illumination promotes the formation of H aggregates of the azobenzene species. We have also demonstrated that the treated d-MR layer successfully aligns a nematic liquid crystal.
    Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 09/2003; 21:1770-1775. DOI:10.1116/1.1603282 · 2.14 Impact Factor
  • Talanta 07/2003; · 3.51 Impact Factor
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    ABSTRACT: Aminoazobenzene (aAbz) molecules undergo rapid photoisomerization and relaxation that is strongly dependent on the direction of the polarization of the incident light. This promotes rotation of the axis of aAbz units by a Brownian ratchet mechanism. The result is light-induced structural anisotropy in the ensemble. We have synthesized aAbz-containing self-assembled monolayers on glass using a straightforward preparation starting with methyl red. We have studied these layers using optical second harmonic generation and high-resolution absorption dichroism spectroscopy. The results show that the aAbz axes are inclined by approximately 15 degrees with respect to the surface plane, and that polarized treatment illumination induces a surface in-plane order parameter up to 0.10. We also have demonstrated that this layer creates anisotropic anchoring of a nematic liquid crystal (LC), and that this anchoring can be remotely and rapidly switched with very low intensity treatment illumination. This is sufficient to induce and remove a 90-degree twist in a LC cell (with its two interior windows coated by the aAbz layer and rubbed nylon, respectively) depending on the polarization of the viewing light.
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    ABSTRACT: We have prepared Self-Assembled Monolayers (SAMs) incorporating azobenzenes to study the effect of photo-isomerization on liquid crystal (LC) alignment. To study the effects of irradiating the SAMs with polarized UV light, we have carried out in situ tests of transmission of nematic cells which consist of one fresh azo-SAM substrate and one rubbed nylon substrate. Cells are initially uniform, with the director aligning parallel to the nylon rubbing direction. When illuminated with UV light polarized parallel to the rubbing direction, the LC orientation at the SAM surface rotates gradually through 90^circ and the cell becomes twisted. The reorientation occurs more quickly when the UV intensity is increased. The long axes of the LC molecules at the SAM boundary thus align perpendicular to the polarization direction of the UV light. The alignment effect disappears gradually when the sample is heated to approximately 120^circC. This work is supported by NSF grant CHE-0079122 and NSF MRSEC grant DMR-9809555.
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    ABSTRACT: Chemisorbed monolayers containing chromophores have been used in the past for non-contact switching of liquid crystal devices. However, most of these systems have required large optical doses to develop the necessary anisotropic molecular orientation distribution. We used an aminoazobenzene (aAzb) material that is derived from methyl red to make self-assembled monolayers that become anisotropic under very small illumination levels of polarized blue light. A nematic liquid crystal (5CB) cell, with this monolayer on one window, requires only 5.5 mJ/cm2, or 1.8 absorbed photons per molecule, to reach 90 20-micron thickness of the device. This unprecedented sensitivity is due to rapid relaxation of the aAzb photoisomerization (enabling an effective Brownian ratchet process), a favorable in-plane orientation and non-aggregated nature of the aAzb groups, and an optimal surface density with uniform coverage of the monolayer.