We address the role of excitonic coupling on the nature of photoexcitations in the conjugated polymer regioregular poly(3-hexylthiophene). By means of temperature-dependent absorption and photoluminescence spectroscopy, we show that optical emission is overwhelmingly dominated by weakly coupled H aggregates. The relative absorbance of the 0-0 and 0-1 vibronic peaks provides a powerfully simple means to extract the magnitude of the intermolecular coupling energy, of approximately 5 and 30 meV for films spun from isodurene and chloroform solutions, respectively.

https://arxiv.org/pdf/cond-mat/0702663.pdf

Role of Intermolecular Coupling in the Photophysics of Disordered Organic Semiconductors: Aggregate Emission in Regioregular Polythiophene

Stacking of poly(3-alkylthiophene)s P3RThs and poly(4-alkylthiazole)s P4RTzs has been studied. Light scattering analysis indicates that head-to-tail (HT) type HT-P3HexTh (R = n-C6H13) gives a degree of depolarization (ρv) of 0.26 in CHCl3, which reveals that HT-P3HexTh takes a stiff structure even in the good solvent. Addition of CH3OH to CHCl3 solutions of HT-P3HexTh and head-to-head (HH) type HH-P4HepTz (R = n-C7H15) leads to π-stacking of the polymer molecules to form stable colloidal particles. The light scattering analysis of the colloidal solution of HT-P3HexTh in a 2:1 solution of CHCl3 and CH3OH reveals that HT-P3HexTh is aggregated in a parallel style. Results of filtration experiments using membranes with 0.20 and 0.02 μm pores agree with the degree of the aggregation. P3HexThs with irregular structures (P3HexTh (Fe) and P3HexTh (Ni) with HT/HH ratios of about 7/3 and 1/2, respectively) show a weaker trend to aggregate; however, P3HexTh (Fe) is considered to stack in a surface region of a stretc...

Extensive studies on pi-stacking of poly(3-alkylthiophene-2,5-diyl)s and poly(4-alkylthiazole-2,5-diyl)s by optical spectroscopy, NMR analysis, light scattering analysis, and X-ray crystallography

Poly(3-alkylthiophenes) are conducting polymers that have good solubility, environmental stability, and processability.1 The synthesis of regioregular polythiophenes has produced defect-free, structurally homogeneous, head-to-tail coupled poly(alkylthiophenes) (HT-PATs) that have greatly improved electronic and photonic properties over regiorandom analogues.2,3 Regioregular polythiophenes have led to a multitude of important and novel nanoand microscale electronic materials and devices.4-7 Very recently, we have developed an endgroup functionalization methodology of HT-PATs that allows for the synthesis of a plethora of well-defined block copolymers that form nanowires with high electrical conductivity.7 Here, we have discovered that the nickel-initiated regioregular polymerization of alkylthiophenes proceeds by a chain growth mechanism and does not occur by the accepted step growth mechanism.8 We also observed that the degree of polymerization of the synthesized poly(alkylthiophenes) increases with the conversion and can be predicted by the molar ratio of monomer to nickel initiator. On the basis of our experimental results, we predict that nickel-initiated crosscoupling polymerization is essentially a living system, giving PATs with low polydispersities (PDIs). In a cross-coupling step polymerization catalyzed by Ni(dppp)Cl2, one would expect a fast disappearance of the monomer and increase of the polymer molecular weight toward the end of the polymerization.8-10 On the basis of the experimental results, we observed that relatively high molecular weight polymer forms almost immediately. As a model reaction, we have also found that 2 equiv of a variety of aryl dibromides and 1 equiv of an aryl organometallic (either magnesium or zinc) always gives a near quantitative yield of the trimeric aryl and minor amounts ( 98% H-T coupling).12 The mechanism of the cross-coupling chaingrowth polymerization is outlined in Scheme 1. The first step in the mechanism, where the 2-bromo-5-chlorozinc3-hexylthiophene monomer (1) generated in situ from 2-bromo-3-hexylthiophene reacts with Ni(dppp)Cl2, yielding the organonickel compound (2), is as it has been described by others.9,13 We differ in our mechanism in that reductive elimination of 2 immediately forms an associated pair [3‚4] of the tail-to-tail aryl halide dimer (4) and nickel (0) (3). The dimer 4 undergoes fast oxidative addition to the nickel center generating 5, in view of the fact that the formation of the complex 3‚4 eliminates potential separation of 4 from 3. Subsequently, growth of the polymer chain occurs by insertion of one monomer at a time as shown in the reaction cycle (5 f 6 f [3‚7] f 5), where the Ni(dppp) moiety is always incorporated into polymer chain as an end group. Addition of various Grignard reagents (R′MgX) at the end of polymerization results in end-capping of HT* Corresponding author: e-mail rm5g@andrew.cmu.edu. Figure 1. Conversion (filled symbols) and logarithm of monomer concentration (open symbols) vs time plots for 2-bromo-3-hexylthiophene polymerization at different concentration of Ni(dppp)Cl2 initiator (23-25 °C); [M]0 ) 0.075 mol/ L: (9, 0) [M]0:[Ni(dppp)Cl2] ) 136:1; (b, O) [M]0:[Ni(dppp)Cl2] ) 57:1; (2, 4) [M]0:[Ni(dppp)Cl2] ) 49:1.

/pdf/chain-growth-mechanism-for-regioregular-nickel-initiated-32zax9qdsl.pdf

Chain Growth Mechanism for Regioregular Nickel-Initiated Cross-Coupling Polymerizations

The development of polythiophene (PT) derivatives with essentially 100% head-to-tail (HT) couplings1,2 has led to the discovery of materials that self-assemble both in solution and in the solid state. Light-scattering studies on regioregular, HTpoly(3-dodecylthiophene) (HT-PDDT) show that a supramolecular organization occurs via intermolecular aggregation.3 This solution aggregation is precursive to a microcrystalline, self-assembled structure which has been characterized by X-ray studies.1,2,4 Polythiophenes possessing this metastable organization evidence high electrical conductivities1,2,5-8 and very small bandgaps. Previous work in our lab has shown that both conformational order and solid state organization in regioregular PT derivatives are remarkably sensitive to the placement and nature of the substituent chains.1,6,7,9 This sensitivity provides the opportunity to design new conducting polymers in which self-assembly is controlled by environmental factors. One remarkable example is HT-2,5-poly(thiophene-3-propionic acid) (3, PTPA). This polymer was designed such that upon deprotonation, water soluble polythiophene salts are formed.10 Both 3 and its partially deprotonated derivative 5 undergo protein-like hydrophobic assembly, with subsequent H-bond stabilization, to generate a self-assembled conducting polymer aggregate. Varying the size of the counterion in the carboxylate polymer changes the effective steric bulk of the substituent. Small cations favor a self-assembled (purple) state and large cations can completely disrupt the aggregated phase. In fact, a remarkable chemoselective counterion size-dependent chromaticity (Figure 1) is observed for polymer 5. The UV-vis λmax can be varied over a 130 nm range (from purple to yellow) simply by changing the countercation. Initially, we prepared the carboxylate precursor polymer 2 using the Ni(dppp)Cl2 cross-coupling method for the synthesis of HT polythiophenes.5,6,9 This method leads to low molecular weights (Mn ) 1.5-3.0 K), low yields (<5%), and polymers that show variable ionic chromaticity. However, the regioregular oxazoline polythiophene (2) (Mn ) 8 K, PDI ) 1.2) is prepared (Scheme 1) from monomer11 1 in high yields (84%)

/pdf/self-assembly-and-disassembly-of-regioregular-water-soluble-233iphkdhu.pdf

Self-Assembly and Disassembly of Regioregular, Water Soluble Polythiophenes: Chemoselective Ionchromatic Sensing in Water

The molecular orientation of regioregular poly(3-hexylthiophene) varies from edge-on to plane-on depending on the film drying rate controlled by spin-coating speed. The field effect hole mobility v...

/pdf/variations-in-semiconducting-polymer-microstructure-and-hole-2crvt497tb.pdf

Variations in semiconducting polymer microstructure and hole mobility with spin-coating speed

Static and dynamic polarized and depolarized light scattering characterization of dilute solutions of regioregular poly(3-dodecylthiophene) is described to study intermolecular association as a function of the thermal history of the solutions. It is shown that metastable aggregation obtains under all of the conditions studied, including temperatures as high as 65 °C. Under some conditions, the aggregated moiety appears to have a disklike shape at room temperature, with appreciable depolarized scattering, attributed to an extended chain structure, consistent with the formation of the lamellar suprastructure characteristic of the bulk, with the polythiophene chains adopting an extended conforma- tion and forming a nematic phase in a polythiophene lamella faced by lamellae rich in alkyl chains. Under other conditions, especially at low temperature, elongated supramolecular structures are formed, with the polythiophene chains in an extended conformation. The reversible thermochromic effect is associated with enhanced order of the alkyl side chains with decreasing temperature, facilitating coplanar conformers in the polythiophene backbones, with the attendant enhancement in the ﷿-﷿* transitions of the thiophene ring electronic absorption spectra. This behavior is analogous to the events in the thermochromic event in the bulk. Such supramolecular structures could intervene in normal film casting solution processing, with effects on the electronic or optical properties of the cast film. This interpretation of the light scattering data suggests strategies to enhance or suppress lamella formation in solvent-cast films.

/pdf/intermolecular-association-and-supramolecular-organization-2jtzmvf7yt.pdf

S. Yue

Papers

Intermolecular Association and Supramolecular Organization in Dilute Solution. 1. Regioregular Poly(3-dodecylthiophene)