Disentangling “Bright” and “Dark” Interactions in Ordered Assemblies of Organic Semiconductors

We report on spatially correlated wavelength-resolved photoluminescence and Kelvin probe force microscopy to probe ground state charge-transfer coupling and its correlation with pi-stacking order in nanoscale assemblies of a small molecule n-type organic semiconductor, tetraazaterrylene (TAT). We find a distinct upshift in surface potential contrast (SPC) corresponding to a decrease in work function in TAT in the transition from disordered spun-cast films to ordered crystalline nanowire assemblies, accompanied by a nanowire size dependence in the SPC shift suggesting that the shift depends on both ground state charge transfer interaction and a size (volume)-dependent intrinsic doping associated with the nitrogen substitutions. For the smallest nanowires studied (surface height ≈ 10–15 nm), the SPC shift with respect to disordered films is +110 meV, in close agreement with recent theoretical calculations. These results illustrate how “dark” (ground-state) interactions in organic semiconductors can be distinguished from “bright” (excited-state) exciton coupling typically assessed by spectral measurements alone

Electrostatic Force Microscopy and Spectral Studies of Electron Attachment to Single Quantum Dots on Indium Tin Oxide Substrates

We report electrostatic force microscopy (EFM) studies combined with wavelength-resolved photoluminescence imaging of electron attachment to individual CdSe/ZnS quantum dots (QDs) coupled to semiconducting tin-doped indium oxide (ITO) substrates. Quantitative EFM measurements show unambiguous signatures of 2–3 excess electrons on individual QDs on ITO, while the distribution of measured recombination energies of QDs coupled to ITO shows ≈ −35 meV red shift (compared to QDs drop-cast on clean glass), the signature of a second-order quantum-confined Stark effect resulting from multiple-electron attachment to the QDs. We also show that the extent of QD charging can be tuned by modulating the ITO bias: EFM measurements show that ≈4 electrons are added to QDs under −2 V applied ITO bias, whereas only ≈2 electrons can be removed from the QDs for +2 V applied bias arising from Fermi level mismatch of ITO with respect to the QDs. Voltage-correlated spectral measurements on ITO coupled QDs showed a spectral modulation in their peak fluorescence energies, which can be attributed to addition or removal of electrons from the QDs

Effect of Polymer Chain Folding on the Transition from H- to J‑Aggregate Behavior in P3HT Nanofibers

A combination of wavelength-, time-, and polarization-resolved photoluminescence imaging on isolated P3HT nanofibers of varying molecular weight (from 10 to 65 kDa) has revealed a transition in dominant exciton coupling from primarily interchain (H-aggregation) for low molecular weight nanofibers, to predominantly intrachain (J-aggregation) coupling for high molecular weight nanofibers. Based on nanofiber width measurement from TEM imaging, the driving force for this transition appears to be folding of individual polymer chains within the lamellae, resulting in enhanced chain planarity and reduced torsional disorder

Tuning Aggregation of Poly(3-hexylthiophene) within Nanoparticles

Nanoparticles derived from π-conjugated polymers have gained widespread attention as active layer materials in various organic electronics applications. The optoelectronic, charge transfer, and charge transport properties of π-conjugated polymers are intimately connected to the polymer aggregate structure. Herein we show that the internal aggregate structure of regioregular poly(3-hexylthiophene) (P3HT) within polymer nanoparticles can be tuned by solvent composition during nanoparticle fabrication through the miniemulsion process. Using absorption spectra and single-NP photoluminescence decay properties, we show that a solvent mixture consisting of a low boiling good solvent and a high boiling marginal solvent results in polymer aggregate structure with a higher degree of uniformity and structural order. We find that the impact of solvent on the nature of P3HT aggregation within nanoparticles is different from what has been reported in thin films

Cross-Linked Functionalized Poly(3-hexylthiophene) Nanofibers with Tunable Excitonic Coupling

We show that mechanically and chemically robust functionalized poly(3-hexylthiophene) (P3HT) nanofibers can be made <i>via</i> chemical cross-linking. Dramatically different photophysical properties are observed depending on the choice of functionalizing moiety and cross-linking strategy. Starting with two different nanofiber families formed from (a) P3HT-<i>b</i>-P3MT or (b) P3HT-<i>b</i>-P3ST diblock copolymers, cross-linking to form robust nanowire structures was readily achieved by either a third-party cross-linking agent (hexamethylene diisocyanate, HDI) which links methoxy side chains on the P3MT system, or direct disulfide cross-link for the P3ST system. Although the nanofiber families have similar gross structure (and almost identical pre-cross-linked absorption spectra), they have completely different photophysics as signaled by ensemble and single nanofiber wavelength- and time-resolved photoluminescence as well as transient absorption (visible and near-IR) probes. For the P3ST diblock nanofibers, excitonic coupling appears to be essentially unchanged before and after cross-linking. In contrast, cross-linked P3MT nanofibers show photoluminescence similar in electronic origin, vibronic structure, and lifetime to unaggregated P3HT molecules, <i>e.g.</i>, dissolved in an inert polymer matrix, suggesting almost complete extinction of excitonic coupling. We hypothesize that the different photophysical properties can be understood from structural perturbations resulting from the cross-linking: For the P3MT system, the DIC linker induces a high degree of strain on the P3HT aggregate block, thus disrupting both intra- and interchain coupling. For the P3ST system, the spatial extent of the cross-linking is approximately commensurate with the interlamellar spacing, resulting in a minimally perturbed aggregate structure

Chiroptical Dissymmetries in Fluorescence Excitation from Single Molecules of (M-2) Helicene Dimers

We report on the single-molecule chiroptical properties of “right”-handed bridged triaryl amine helicene dimers, MH2. Using an experimental setup to precisely define the circular excitation polarization at the sample plane, we investigated the circular dichroic response in luminescence from individual molecules in which induced ellipticity from microscope optics is minimized. Our results comparing circular anisotropies in fluorescence excitation from MH2 and perylene diimide (PDI), an achiral, centrosymmetric chromophore, demonstrate a significant reduction in the breadth of the distribution of circular dissymmetry parameters obtained from modulation of the circularly polarized excitation source (457 nm). For PDI, we observe a symmetric distribution of circular anisotropy parameters centered about zero, with a fwhm of 0.25. For MH2, we observe an asymmetric distribution peaked at <i>g</i> = −0.09, with a slightly larger width as the corresponding PDI distribution. These results indicate that the large dissymmetry parameters (|<i>g</i>| > 0.5) in fluorescence excitation described in our original report (Hassey, R.; et al. Chirality 2008, 20, 1039−1046 and Hassey, R.; et al. Science 2006, 314, 1437–1439) were indeed affected by (at the time, unknown) linear polarization artifacts. However, the present results on MH2 provide compelling evidence for single-molecule circular dissymmetries much larger than solution or thin-film ensemble values, defined primarily by the enhanced rotatory strength (relative to the monomer), and restricted orientation at the sample surface

Carpenter’s Rule Folding in Rigid–Flexible Block Copolymers with Conjugation-Interrupting, Flexible Tethers Between Oligophenylenevinylenes

Rigid–flexible segmented block copolymers were synthesized and characterized as 4.5-oligo­phenylene­vinylene chromophores tethered by flexible, conjugation-interrupting 1,2-ethanedioxy or 1,4-butanedioxy units. The flexible tethers allow the possibility of collapsed order chromophore assemblies within individual polymers by chain folding at specific sites much like an old fashioned, folding carpenter’s rule. Our results indicate that using a short, flexible tether in a rigid–flexible segmented copolymer can result in collapsed rodlike structures as signaled by strongly quenched photoluminescence, even after thermal annealing. Such ability to “program” folding and tertiary structure in conjugated copolymers is important for solid-state organic light emitting materials and understanding of organic chromophore self-assembly

Time- and Polarization-Resolved Photoluminescence of Individual Semicrystalline Polythiophene (P3HT) Nanoparticles

We report on a remarkable size and internal structure dependence on time- and polarization-resolved photoluminescence (PL) from individual regioregular rrP3HT (poly-3-(hexylthiophine)) nanoparticles. For the smallest particles (∼34 nm) with relatively low crystallinity (40%), the time evolution of polarization contrast is nearly stationary; for intermediate-sized particles (∼ 65 nm), depolarization occurs on a 1–2 ns time scale. The largest and most crystalline particles studied (118 nm, 70%) show a PL depolarization on a time scale of <50 ps. In every time regime, we observe P3HT nanoparticle PL dynamics that are qualitatively different from those of extended films and single-polymer chains, highlighted by intriguing differences in power law dynamics in the PL intensity at long times. This work may support the hypothesis that hierarchical assemblies of conducting polymer nanoparticles could offer a route to higher efficiency in organic photovoltaic systems

Poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2‑<i>b</i>]thiophene] Oligomer Single-Crystal Nanowires from Supercritical Solution and Their Anisotropic Exciton Dynamics

Supercritical fluids, exhibiting a combination of liquid-like solvation power and gas-like diffusivity, are a relatively unexplored medium for processing and crystallization of oligomer and polymeric semiconductors whose optoelectronic properties critically depend on the microstructure. Here we report oligomer crystallization from the polymer organic semiconductor, poly­[2,5-bis­(3-dodecylthiophen-2-yl)­thieno­[3,2-<i>b</i>]­thiophene] (PBTTT) in supercritical hexane, yielding needle-like single crystals up to several microns in length. We characterize the crystals’ photophysical properties by time- and polarization-resolved photoluminescence (TPRPL) spectroscopy. These techniques reveal two-dimensional interchromophore coupling facilitated by the high degree of π-stacking order within the crystal. Furthermore, the crystals obtained from supercritical fluid were found to be similar photophysically as the crystallites found in solution-cast thin films and distinct from solution-grown crystals that exhibited spectroscopic signatures indicative of different packing geometries