Single-Molecule Microscopy Studies of Electric-Field Poling in Chromophore−Polymer Composite Materials

Abstract

One strategy for increasing the efficiency of organic electrooptic devices based on chromophore-polymer composite materials is to improve chromophore ordering. In these materials, ordering is induced through the interaction of the chromophore dipole moment with an external electric field, applied at temperatures near the Tg of the polymer host, a process referred to as "poling". To provide insight into the molecular details of the poling process under conditions representative of device construction, the rotational dynamics of single 4-dicyano-methylene-2-methyl-6-(p-(dimethylamino)styryl)-4H-pyran (DCM) molecules in poly(methyl acrylate) at T = Tg + 11 degrees C in the presence and absence of an electric field are investigated using single-molecule confocal fluorescence microscopy. Single-molecule rotational dynamics are monitored through the time evolution of the fluorescence anisotropy. The anisotropy correlation function demonstrates nonexponential decay, with beta values derived from fits using the Kohlrausch-Williams-Watts law ranging from 0.7 to 1 with beta(KWW) = 0.83. This observation is consistent with previous studies of molecular rotation dynamics in polymer melts and reflects the dynamical heterogeneity provided by the polymer host. The rotational dynamics of DCM are weakly perturbed in the presence of a 50 V/microm electric field, typical of the field strength employed in device construction. The expected perturbation of the rotational dynamics is determined and found to be consistent with the alignment potential created by the electric field relative to the amount of thermal energy available. The relevance of these findings with respect to current models of the poling process is discussed. This work demonstrates the utility of polarization-sensitive single-molecule microscopy in elucidating the details of molecular reorientation during poling.

Spatial and temporal heterogeneity in supercooled glycerol: Evidence from wide field single molecule imaging

“…LD is the SM analog of polarization anisotropy as measured in ensemble measurements and has therefore been widely used to probe SM orientational dynamics in supercooled liquids. 20,21,[23][24][25]35,36 In polarization anisotropy measurements with spherical probes, nonexponentiality in the decay of the ACF is interpreted as arising from spatial and/or temporal heterogeneity. 37,38 For SM LD ACFs, such nonexponentiality cannot arise from averaging over many molecules and may thus be assumed to arise from temporal heterogeneity.…”

Massive Faculty Donations and Institutional Conflicts of Interest

“…In 2006, chemistry professor Phil Reid had confirmed in sensitive experiments on single molecules what we had seen by looking; dye molecules were poorly ordered and subject to degradation. 102 Later, he wrote to BK, Robinson, Hopkins, and others, "[S]ingle-molecule work from my group demonstrated that the field-induced perturbation on the chromophore [dye] rotational dynamics were very modest…this paper blew a relatively large hole [enlarged an existing hole] in the belief that the poling field provided a serious alignment potential." He continued, "I would argue the most important scientific contributions in this field have come from workers performing fundamental research.…”

Analysis of orientational dynamics of single fluorophore trajectories from three-angle polarization experiments

“…The techniques developed for detecting molecular orientations have been applied in many materials and biological systems. [1][2][3][4][5][6][7][8][9] An easy method to acquire orientation information with polarization spectroscopy of single fluorescent molecules, which relates the intensity profile and the polarization distribution of the collected fluorescence radiation to the emitting source orientation. 10,11 In practice, many detection schemes have been proposed, and each has its own design for measuring the polarization with the collected fluorescence.…”