Article

Photoinduced random molecular reorientation by nonradiative energy relaxation: An experimental test

INFM-Coherentia and Università Federico II, Dipartimento di Scienze Fisiche, Complesso di Monte S. Angelo, via Cintia, 80126 Naples, Italy.
Physical Review E (Impact Factor: 2.33). 12/2004; 70(5 Pt 1):051702. DOI: 10.1103/PhysRevE.70.051702
Source: PubMed

ABSTRACT By measuring the time-resolved fluorescence depolarization as a function of light excitation wavelength we address the question of a possible photoinduced orientational randomization of amino-anthraquinone dyes in liquid solutions. We find no significant dependence within the experimental uncertainties of both the initial molecule anisotropy and of the subsequent rotational diffusion dynamics on the photon energy. This indicates that this effect, if present, must be very small. A simple model of photoinduced local heating and corresponding enhanced rotational diffusion is in accordance with this result. This null result rules out some recent proposals that photoinduced local heating may contribute significantly to molecular reorientation effects in different materials. A small but statistically significant effect of photon energy is instead found in the excited-state lifetime of the dye.

0 Followers
 · 
63 Views
  • [Show abstract] [Hide abstract]
    ABSTRACT: In a photoresponsive Langmuir monolayer comprised of smectic-C-like domains of mesogenic trans-azobenzene derivative embedded within an isotropic matrix of its cis isomer counterpart, several structurally differing circular droplets were irradiated with linearly polarized light. This report describes the structural rearrangements that occurred in these droplets upon illumination followed by Brewster angle microscopy analysis. Starting from initial well-characterized and symmetric states, final photoaligned situations were reached in which the azimuth angles of the rod-shaped elongated molecules were found to be perpendicular to the electric component of the excitation light. The dynamical aspects of the photoalignments, including their transient patterns, are captured by a theoretical model that couples a relaxational principle incorporating long-range elastic forces with a kinetic formalism presenting an anisotropic rate law.
    The Journal of Chemical Physics 07/2005; 122(24):244722. DOI:10.1063/1.1927518 · 3.12 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We report on the large wavelength range and light-initiated thermal tuning of the reflection of a liquid crystal (LC) formulation (S811/ZLI-2806) near a smectic (SmA) to cholesteric (CLC) phase transition enabled by the use of a high order parameter heat transfer dye (anthraquinone, AQ). Upon irradiation with a 647 nm Krypton ion (Kr+) laser line, absorption by AQ generates heat that is transferred to the surrounding LC host. In the S811/ZLI-2806 formulation examined here, the optically generated increase in temperature serves to transition the phase from SmA to CLC. As has been documented, the SmA→CLC transition is typified by a pitch contraction that blueshifts the position of the CLC reflection, in this case a shift from 2500 to 700 nm that can occur in less than 100 s. The tuning range and speed are dependent on the laser power and the amount of dye in the cell.
    Applied Physics Letters 07/2010; 97(1):011107-011107-3. DOI:10.1063/1.3459957 · 3.52 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Light-stimulated transformation of a molecule in a liquid crystal (LC) greatly affects the ordering of its nearest neighbors. At early stage of excitation this local order parameter modification is characterized with extremely short times, while a minor concentration of photo-modified molecules could contribute a lot into modulation of refractive indices of LC. Dispersion of optical activity at the edge of the selective reflection band of a chiral LC enhances significantly the nonlinear optical response. Combination of these effects has a strong potential in different aspects of optical signal processing, all-optical switching and photonics.
    Molecular Crystals and Liquid Crystals 06/2011; 543(1):143/[909]-150/[916]. DOI:10.1080/15421406.2011.569453 · 0.49 Impact Factor

Preview

Download
2 Downloads
Available from