Direct observation of backbone planarization via side-chain alignment in single bulky-substituted polythiophenes

Abstract

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.

Structural and Photophysical Templating of Conjugated Polyelectrolytes with Single-Stranded DNA

“… 1 − 5 In particular, the strong correlation of the photophysical properties of conjugated polythiophenes to their backbone conformation has been under intensive investigation. 6 − 10 Achieving a highly ordered and extended backbone conformation is of considerable relevance, since intrachain coupling is in this case dominant, and important delocalization as well as coherent long-range migration of excitons and charges can be realized. 11 − 15 Such directional energy and charge transport along the chain can be exploited in electrical components, e.g., nanowires for molecular devices, 16 − 18 cathodes in next generation batteries, 19 capacitors, 20 , 21 nanosensors, 22 nanophotonics, quantum information technologies, or artificial light-harvesting systems.…”

Managing Local Order in Conjugated Polymer Blends via Polarity Contrast

“…[1][2][3][4] This can affectoften stronglycontingent optoelectronic properties, such as charge-carrier transport [5][6][7] or absorption/emission features. [8][9][10] Originally, alkyl side chains were introduced to aid solubility and, generally, processability; 11 while more recently polar side chains, such as those based on oligo(ethylene oxide) groups, have attracted interest as they render the base material more compatible with socalled 'green' solvents. 12,13 Moreover, polymer semiconductors, substituted with polar side chains, are interesting for bioelectronic applications, for instance, where mixed electron/ion conduction is required (e.g., in electrochemical transistors used for biomimetic signal transduction, ion pumps, bioactive sensing elements), or where high biocompatibility is needed, including biointegrated electronics and wearable devices.…”

Carborane‐Induced Excimer Emission of Severely Twisted Bis‐o‐Carboranyl Chrysene

“…The calculated emission wavelengths of these all span the range of 296–500 nm, in good agreement with observed LE PL emission. It appears that, depending on the relative dihedral geometries of the carboranes, chrysene is twisted to varying degrees, leading to significant differences in absorption and emission energies, that is, higher degrees of twisting leads to shorter absorption and emissive wavelengths and vice versa 50. As such, the complexity of the vibronic region of 2 is explained by the flexing of the chrysene core as the carboranes rotate around ϕ .…”