NANO LETTERS
Oriented Nanostructures from Single Molecules of a Semiconducting Polymer: Polarization Evidence for Highly Aligned Intramolecular Geometries
2003 Vol. 3, No. 5 603-607
A. Mehta,† P. Kumar,‡ M. D. Dadmun,‡ J. Zheng,§ R. M. Dickson,§ T. Thundat,† B. G. Sumpter,| and M. D. Barnes*,⊥ Life Sciences, Computer Sciences and Mathematics, and Chemical Sciences DiVisions, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6142, Department of Chemistry, UniVersity of Tennessee, KnoxVille, Tennessee 37996-1600, and School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400 Received February 6, 2003; Revised Manuscript Received March 24, 2003
ABSTRACT We report the observation of uniformly oriented transition moments perpendicular to the support substrate in single molecules of a conjugated polymer (MEH-PPV) isolated by ink-jet printing techniques. Fluorescence imaging combined with atomic force microscopy and polarization modulation studies, supported by molecular mechanics simulation, provides compelling evidence of polymer nanoparticle (single-molecule) structures with an extraordinary degree of intramolecular order. This is a general technique for preparing oriented nanostructures from structurally similar polymers.
Techniques for achieving a uniform transition-moment orientation of luminescent nanoscale species are of significant importance in the emerging new field of nanophotonics and molecular-scale optoelectronics.1,2 As demonstrated by recent work by Sandoghdar and co-workers,3 an exploitation of coherent interactions between adjacent molecules with the same dipole orientation can be used to tailor the entanglement between excited states and, on particle/molecule separation scales comparable to the transition wavelength, could ultimately be used to prepare active photonic structures (i.e., gates or switches). In a photonics context, where the manipulation of photons in the xy plane is desirable, the ability to orient molecular species with dipole transition moments perpendicular to a supporting substrate is of special importance because the emission is naturally suppressed into (unwanted) k states perpendicular to the substrate. However, given the level of experimental difficulty in the gas-phase orientation of simple molecules such as symmetric tops,4 it * Corresponding author. E-mail:
[email protected]. † Life Sciences Division, Oak Ridge National Laboratory. ‡ University of Tennessee. § Georgia Institute of Technology. | Computer Sciences and Mathematics Division, Oak Ridge National Laboratory. ⊥ Chemical Sciences Division, Oak Ridge National Laboratory. 10.1021/nl0340733 CCC: $25.00 Published on Web 04/23/2003
© 2003 American Chemical Society
might seem impossible to orient structurally complex species such as conjugated polymers.5 The first problem is that, because the optical transition moment is nearly collinear with a localized conjugated radiative recombination site within the polymer chain, the macromolecule needs to possess a high degree of internal structural order. Second, assuming that it is possible to form single molecules of conjugated polymers with such a high degree of internal order, there is the question of how to orient such nanostructures in the laboratory reference frame and to maintain that orientation in a stable sample format. Indeed, in conventional thin-film processing formats, the transition dipoles of these species are well known to be randomly oriented in the xy plane, with a fairly broad distribution of intramolecular morphologies that are reflected in modest polarization contrast.6 In this letter, we describe a straightforward method for isolating single (charged) semiconducting polymer chains with uniform orientation perpendicular to the substrate plane on the basis of ink-jet printing techniques. The uniform z orientation of the transition moments are clearly manifested in distinct spatial fluorescence intensity patterns, and the luminescence dynamics show all of the hallmarks of singlemolecule fluorescence (on-off blinking and discrete photo-
bleaching) as well as significantly enhanced photostability and narrow spectral signatures.7 Here, the z orientation is derived largely from an electrostatic interaction of the charged cylindrical nanoparticle with the coverglass surface. Polarization spectroscopy of these species, combined with molecular mechanics simulation, points to single macromolecular structures with an extraordinary degree of internal order. We have observed similar results for different conjugated polymers, indicating a generality in the method for structurally similar polymers. These results demonstrate a completely “tabletop” method of preparing nanoscale polymeric or composite species with distinctly different properties than their thin-film counterparts. Polyphenylene vinylene (PPV) and similar conjugated polymers have received a great deal of attention in polymerbased optoelectronic device contexts8,9 because of their efficient luminescence and charge-transport properties as well as the convenience in solution-phase processing afforded by side-chain derivatization.5 One of the most common of these is (poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene]) or MEH-PPV, which is composed of conjugated stiff-chain segments (6-12 monomer units long) connected by structural defects (saturated vinylene linkages) that allows for a variety of morphologies depending on the molecular weight, solvent, and thin-film host material.10 An important and well recognized issue in polymer-based optoelectronic device performance is the chain organization and alignment of these species within the film,11 and strategies to control these parameters have stimulated intense research effort by a number of groups.8,9,12 Recently, single-molecule spectroscopic and polarization measurements have been used to probe photophysical properties and intramolecular chain alignment and have revealed fascinating behavior such as on-off blinking, multiple luminescence intensity levels, and interesting time-dependent spectral characteristics.12-15 Although luminescence dynamics carry important information on the nature of the emissive site within the molecule, polarization-modulated single-molecule fluorescence excitation measurements have provided new insight into the intramolecular structure of conjuagted polymers.6 The connection between excitation polarization anisotropy and intramolecular structure in these systems is based on the idea that excitonic excitation can occur in any one of many local conjugated segments within the molecule, followed by rapid intramolecular energy transfer to lower-lying radiative trap states. That is, the excitation may occur in many different possible sites within the molecule, where the degree of intramolecular order is manifested in the contrast in luminescence efficiency between orthogonal polarizations. In the case of single-molecule measurements in dilute thin films,6 the broad distribution of polarization contrast parameters suggests a wide variation in polymer chain conformations with varying degrees of intramolecular order. The question then arises as to whether these measurements suggest a fundamental entropic limit associated with the intramolecular organization of the polymer or whether the degree of collapse is affected by external parameters such as the substrate and host-polymer interactions. 604
Our experiments were designed to elucidate an answer to this question by isolating single chains of MEH-PPV16 in microdroplets of ultradilute polymer solution using piezoelectric droplet generation17 where the polymer chains dry in the absence of substrate or host polymer interactions. A stream of highly monodisperse 5-µm droplets of 10-11-10-13 M MEH-PPV solution in doubly distilled tetrahydrofuran (THF) was confined in a vented drying tube (20 cm long) to ensure that all of the solvent evaporated before the particles encountered the substrate. The dry nanoparticles were collected on clean glass coverslips and probed with a combination of high-spatial-resolution fluorescence imaging and atomic force microscopy to measure dipole orientation and nanoparticle structural characteristics. In our experiments, we used an intraobjective total internal reflection (TIR) geometry where the excitation beam is aligned off-axis so that the expanded beam encounters the air-coverglass interface at the critical angle and is retroreflected back through the 1.4-NA, 100× oil objective.18 This excitation geometry allows for the simultaneous excitation of all possible transition dipole orientations in 3-D space and combined with a small (∼200 nm) defocusing of the objective provides straightforward imaging of the dipole emission pattern from which the precise dipole orientation (φ, θ) can be obtained.18,19 Figure 1A shows an example of high-magnification (200×) fluorescence images of MEH-PPV nanoparticles prepared from microdroplets of 10-12 M THF solution. The uniform “donut”-like spatial fluorescence intensity patterns are characteristic of single dipoles aligned perpendicular to the glass substrate and are seen both in focus as well as in slight defocusing.18 Unlike a multichromophoric fluorescent source (such as a dye-doped nanoparticle) that emits isotropically in 4π steradians, fluorescence from single dipoles are emitted with a sine-squared angular distribution with respect to the dipole axis. The toroidal spatial intensity patterns we observe are derived from the fact that emission is forbidden at angles along the optic axis for the z orientation of the transition dipole, resulting in the central intensity node for a given nanoparticle fluorescence image. This result is the exact opposite of the dipole orientation of these species in spun-cast thin films that lie randomly oriented (in φ) nearly parallel to the substrate. We observe surface coverage that scales with polymer concentration as dilute as 10 fM (corresponding to
