The backbone conformation of conjugated polymers affects, to a large extent, their optical and electronic properties. The usually flexible substituents provide solubility and influence the packing behavior of conjugated polymers in films or in bad solvents. However, the role of the side chains in determining and potentially controlling the backbone conformation, and thus the optical and electronic properties on the single polymer level, is currently under debate. Here, we investigate directly the impact of the side chains by studying the bulky-substituted poly(3-(2,5-dioctylphenyl)thiophene) (PDOPT) and the common poly(3-hexylthiophene) (P3HT), both with a defined molecular weight and high regioregularity, using low-temperature single-chain photoluminescence (PL) spectroscopy and quantum-classical simulations. Surprisingly, the optical transition energy of PDOPT is significantly (∼2,000 cm or 0.25 eV) red-shifted relative to P3HT despite a higher static and dynamic disorder in the former. We ascribe this red shift to a side-chain induced backbone planarization in PDOPT, supported by temperature-dependent ensemble PL spectroscopy. Our atomistic simulations reveal that the bulkier 2,5-dioctylphenyl side chains of PDOPT adopt a clear secondary helical structural motif and thus protect conjugation, i.e., enforce backbone planarity, whereas, for P3HT, this is not the case. These different degrees of planarity in both thiophenes do not result in different conjugation lengths, which we found to be similar. It is rather the stronger electronic coupling between the repeating units in the more planar PDOPT which gives rise to the observed spectral red shift as well as to a reduced calculated electron-hole polarization.

The large majority of semiconducting polymers based on poly(alkylthiophene)s with either linear or branched alkyl side chains are reported to π-stack in their crystalline phases. In regioregular poly(3-(2,5-dioctylphenyl)thiophene) (PDOPT), however, π−π interactions are absent due to the presence of the bulky 2,5-dioctylphenyl side groups. In this work, high levels of crystallinity and orientation are created in thin films of PDOPT aligned on substrates of naphthalene by slow directional epitaxial crystallization of the side chains. Depending on molecular weight, both edge-on and flat-on lamellar crystals are obtained. As for poly(3-hexylthiophene) (P3HT), electron microscopy imaging reveals a transition from extended to folded chain crystallization in PDOPT for M n ≈ 12.7 kDa. The high orientation and crystallinity result in high anisotropy in UV−vis absorption and photoluminescence with well-defined vibronic structures. The single-crystal-like electron diffraction patterns are further used to refine a structural model of PDOPT (a = 29.09 Å, b = 10.45 Å, c = 7.72 Å, α = β = γ = 90°, and space group P21/c). Uniquely, crystalline PDOPT features perfectly planarized chains despite the absence of π-stacking between polythiophene backbones. The octyl side chains are interdigitated and crystallize in a dense subcell which is compared to that of other semiconducting polythiophenes, e.g. form I poly(3-hexylthiophene) and poly(2,5-bis(3-dodecyl-2-yl)thieno[3,2b]thiophene), to draw a trend between overall crystallinity and density of the side chain packing.

A highly active nickel catalyst with a hybrid P,N ligand is successfully used for the first time for the polymerization of a thiophene monomer with sterically very demanding side groups. The performance of this catalyst is by far not achievable with commercially available standard catalysts. Polythiophenes with special side chain patterns can thus be made with predetermined molecular weight, low dispersity, and high regioregularity, which enable the preparation of various large crystalline superstructures for the investigation of anisotropic optoelectronic properties in the absence of π–π interactions.

As-cast and slowly crystallized films of conjugated polymers can contain (partially) ordered and less ordered (amorphous) regions with structural defects. Crystallization allows to generate chains with highly planarized backbones, embedded in structures exhibiting long-range order. In the present study, we used spatially resolved optical spectroscopy to quantify differences in the degree of order of a bulky substituted poly(3-(2,5-dioctylphenyl)thiophene) (PDOPT). In particular, we compared absorption and photoluminescence (PL) measurements from large spherulitic crystals, and the same region rapidly recrystallized after melting, which allowed to identify characteristic features of ordered and less ordered regions. In addition, on the basis of temperature-dependent absorbance and PL measurements, we followed in situ melting and recrystallization processes, i.e., transitions between ordered and disordered phases. A multipeak analysis of absorption and PL spectra based on a modified Franck−Condon progression showed changes in for example the relative intensities of each peak, the excitonic bandwidth, and the vibronic energy as a function of temperature. Most importantly, at the phase transition temperature, a clear change in the positions of the peaks (i.e., their wavelengths, corresponding to the energy of the emitted photons) was detected. In particular, the relative absorption and PL intensities depended sensitively on the extent of order within PDOPT samples. Furthermore, on the basis of a comparison with calorimetric measurements, we have confirmed correlations between changes in the relative absorbance and PL intensities with variations in order/disorder occurring during melting and recrystallization.

The photophysical properties of a phenylsubstituted poly(thiophene), poly(3-(2,5-dioctylphenyl)-thiophene) (PDOPT), were studied as a function of polarization and degree of orientation of the crystalline structure. Under well-chosen controlled conditions, largesized spherulitic crystals of PDOPT were successfully prepared from the melt. From polarized optical microscopy and X-ray diffraction, the molecular orientation of PDOPT within the spherulite was determined, indicating that the fastest growth direction of the spherulite was the a-axis. This implied that crystallization of PDOPT was directed by the packing of the side chains rather than the backbones, which are significantly separated. As the crystalline lamellae were all radially oriented, the local absorbance strongly depended on the polarization of the incoming light. Compared to randomly oriented crystals in a quenched and thus rapidly crystallized sample, PDOPT spherulites displayed red-shifted absorption and emission spectra, combined with a reduced photoluminescence quantum yield. Even for these markedly separated polymer backbones (1.47 nm), the reduced photoluminescence suggests an enhancement of interchain interactions of highly ordered bulky substituted polythiophene induced by crystallization.T he optoelectronic properties of conjugated polymers are of importance for their application in organic electronic devices. 1 These properties are considerably determined by both the conformation of the chain-like molecules (intrachain arrangements of chromophores) and the way these molecules pack in the solid state (interchain arrangements of chromophores). In thin films of conjugated polymers prepared by standard techniques like spin coating, interchain energy transport is usually more rapid than intrachain transport. 2 Thus, the corresponding optoelectronic properties depend sensitively on the extent of interchain interactions. 3 It is widely accepted that interchain interactions in thin films are effectively influenced by chain aggregation and film morphology. Consequently, the relevant photophysical and electronic properties can be controlled by appropriate processing conditions. 4,5 Most previous studies have focused on conjugated polymers with short backbone stacking distances (<5 Å), such as in alkylsubstituted poly(thiophene)s (P3ATs) 5,6 or poly(phenylenevinylene)s (PPVs). 7,8 In these materials, interchain interactions are mainly determined by the overlap of π-orbitals, i.e., π−π interactions. To control interchain interactions, conjugated polymers with bulky side chains have been developed. 9 Bulky side chains can separate the polymer backbones from each other and thus may have a profound influence on the final properties of these materials. 9,10 In particular, the separation of main chains can strongly improve the photoluminescence (PL) quantum yield (Q y ) of conjugated systems. 11 Up to now, however, only a few reports have discussed the relationships between morphology and optoelectronic properties of conjugated systems having a large main c...