Optoelectronic Properties

The thin-film UV-vis absorption spectra of BTTT-1 and BTTT-2 have nearly the same shape as the dodecyl analogues previously reported in the literature.^63,79 Considering that thin-film microstructure has a considerable impact on the UV-vis spectra of π-conjugated materials, this tends to suggest that although extending the alkyl chain possibly leads to different polymorphs in the bulk crystals (as suggested by DSC), morphologies of spin-cast thin-films of the tetradecyl and dodecyl analogues are likely to be similar within the corresponding oligomer. The crystal packing observed in thin-films of dodecyl-substituted BTTT monomer and dimer are different to each other, but similar to the corresponding bulk crystal.⁶³ However, in both cases the thin-film unit cells show roughly half the d-spacing in the lamellar direction when compared to the bulk crystal, suggesting a more symmetric crystal packing.⁶³ It is likely that the fast drying imposed by spin-casting limits the influence of the sidechains on the crystal packing. Indeed, the greater degrees of freedom of alkyl sidechains compared to conjugated backbones may mean they cannot crystallise as rapidly, and this may explain the similar UV-vis features for BTTT-1 and BTTT-2 with their dodecyl analogues.

As expected, increasing the conjugation increasing the number of repeat units leads to a red-shift in the absorption due to increase conjugation. Fluorination of the oligomers has a smaller impact on the absorption of BTTT-2 than BTTT-1. While FBTTT-1 has a broader absorption than BTTT-1, the onset of absorption is slightly higher in energy. FBTTT-2 also exhibits a small blue-shift in absorption onset compared to BTTT-2, and the conclusion from

Figure 5-17 – Thin-film UV-vis spectra (left) of the monomers and dimers of BTTT and FBTTT. Schematic representing energy levels with IP measured by ultraviolet photoemission spectroscopy (UPS), and EA estimated from the optical bandgap and IP.

conjugation length, it is therefore not surprising that the corresponding polymers exhibit the same optical bandgap (see Section 5.4.2). The ionisation potential, as measured by UPS, again shows the biggest impact of the electron-withdrawing fluorine on the monomers. The IP is increased by 0.51 eV from BTTT-1 to FBTTT-1, while in the case of BTTT-2 and FBTTT-2, the difference is only 0.07 eV. Once again, the fact that the polymers exhibit a very similar IP is therefore unsurprising, and it worth noting that the IP of BTTT-2 and FBTTT-2 are within experimental error of the polymer IPs.

5.6.4 Solid-state Behaviour

In order to study the influence of fluorination on the solid-state packing, single crystal growth was attempted for each of the oligomers. BTTT-1 easily formed large yellow plate-like crystals with dimensions of ca. 1 mm after slow evaporation from hexane solution, while BTTT-2 crystallised as thin red needles of lengths on the order of 1 mm from slow cooled saturated solution in acetone/hexane/chloroform mixture. In contrast to the relatively easy crystal formation of the non-fluorinated oligomers, the FBTTTs appear to be much less prone to growing the large crystals necessary for single crystal x-ray diffraction analysis.

Approaches including slow evaporation at room temperature or 5 ºC (hexane, toluene, o-xylene, chloroform, chlorobenzene, acetone, ethanol, isopropanol, MEK, 2-methyl-2-butanol), slow cooling of saturated solution (hexane, 1,2-dichloroethane, ethanol), and a variety of solvent/anti-solvent combinations were attempted, with no success. The best attempt was obtained from slow evaporation of an o-xylene solution of FBTTT-1 which yielded thin colourless needles with a maximum width of ca. 10 µm (See Figure 5-18).

Figure 5-18 – Optical microscopy image of crystals of FBTTT-1 formed from slow evaporation of o-xylene showing the two types of crystallisation, needle and clusters of fibres.

Interestingly, a second type of crystal is produced from this method, with vastly different dimensions. The much thinner, fibre-like crystals appear as highly entangled and interconnected clusters. The difficulties of forming larger crystals in either FBTTT-1 or FBTTT-2 may be an indication that the rate of crystallisation is too slow compared to the rate of nucleation, and the high aspect-ratio suggests only one-dimensional crystal growth is favourable. Although the specific effects of fluorination on the crystal packing were not elucidated, from the differences in crystallisation behaviour of the BTTT oligomers and their fluorinated counterparts it is clear that the impact of the fluorine atoms is considerable, and merits further investigation.

Despite the unsuccessful efforts in crystallising the FBTTT molecules, a single crystal of 3,3'-difluoro-4,4'-dihexadecyl-2,2'-bithiophene was gratifyingly obtained from slow evaporation of chloroform (see Figure 5-19). The compound packs in a triclinic unit cell, with the planar bithiophene unit arranged in a 1D slipped π-stack. A center of inversion is present along the bithiophene moiety and thus the two thiophene rings are perfectly co-planar.

This contrasts with the reported crystal structures of non-fluorinated 4,4’-dialkyl-2,2’-bithiophenes, where both the methyl and nonyl-substituted bithiophenes exhibit a herringbone packing arrangement as is common in many oligothiophenes, despite also having co-planar thiophene rings.^75,83 On the other hand, the compound has a very similar molecular arrangement to that observed for 4,4’-dinonyl-2,2’-bithiazole.⁷⁵ Furthermore, whereas the

sidechains in the fluorinated bithiophene do not. In all cases, fluorinated or not, the bithiophene moiety is planar suggesting that the crystal packing forces are large enough to overcome the steric repulsion. This is particularly interesting in the case of the fluorinated bithiophene compound since the substitution of the 3-position is expected to lead to a twisting of the thiophene-thiophene link due to steric hindrance. Furthermore, the inter-ring distance between these two atoms is significantly shorter than the sum of their respective Van der Waals radii (S-F distance is 2.94 Å, sum of Van der Waals radii is 3.27 Å)⁸⁴ and thus supports the idea of a non-covalent S-F interaction postulated in the case of PFBTTT and F-P3ATs.

Figure 5-19 – ORTEP crystal structure of 3,3'-difluoro-4,4'-dihexadecyl-2,2'-bithiophene (bottom), and its crystal packing from key angles showing the planarity of the bithiophene unit, and the slipped π-stacking motif. Note that alkyl chains are cropped in two crystal packing images, and hydrogen atoms are omitted for simplification. Grey atoms represent carbon atoms, green represent fluorine, and yellow represent sulfur.

5.7 CONCLUSIONS

Building upon the structural and morphological teachings of backbone fluorination in the P3AT system, namely increased backbone planarity and rigidity, a fluorinated derivative of the high-mobility polymer PBTTT was designed and synthesised. Due to the fluorine atom on the alkylthiophene building block hindering the dimerisation via the well-established Cu(II)-mediated oxidative coupling, a different synthetic pathway was developed. After the

probe manifestations of backbone planarisation. For additional insights into the influence of fluorination on the properties of the polymer, the analogous monomer and dimers were also synthesised, via slight modifications of established routes to oligo-BTTT. Despite the degree of fluorination being roughly half that of F-P3ATs, the impact on the properties of PFBTTT are still evident. The reduced solubility and substantially increased melting point compared to the reference PBTTT polymer suggests that the fluorinated polymer has a much greater propensity to aggregate and has greater intermolecular forces. Similar behaviour in the oligomeric analogues further supports these conclusions. The UV-vis spectra of the polymers also indicate an increased vibronic structure, qualitatively indicating that the PFBTTT polymer is more aggregated and less disordered than PBTTT.

Different to the case backbone fluorination of the P3AT polymers, the measured IP of PFBTTT does not suggest any significant shift of the HOMO upon fluorination in the polymers. There are however some indirect indications from the OFET results that point towards some influence of the fluorine atom. The manifestation of contact resistance in PFBTTT implies that charge injection is hampered, as one might expect if the IP of the semiconductor is increased. In addition, PFBTTT shows greater ambient stability, and is more resistant to the formation of traps due to oxidation. While fluorination does significantly stabilise the frontier molecular orbitals of the oligomers, the effect decreases with increasing conjugation length, corroborating the behaviour observed in the polymer system.

The four-fold increase in hole-carrier mobility when fluorinating the BT unit is consistent with the conclusions drawn from the F-P3AT polymer system, namely that the fluorine atom increases backbone planarity and rigidity, thereby reducing the disorder in the energetic landscape and facilitating charge transport. This conclusion was also supported by DFT calculations on the PBTTT system, which demonstrated the fluorinated BT unit adopts a narrow distribution of conformations about the trans coplanar conformer. Single crystal X-ray diffraction of the isolated fluorinated BT unit supports this computational data, and an S-F short-contact even suggests a possible non-covalent intramolecular interaction. In addition to the minimium energy conformation of the BT unit being fully coplanar, the tapered-nature of the potential energy suggests that the BT unit is significantly rigidified upon fluorination.

This effect manifests itself in the Raman stretches of the C=C and C-C bonds of the polymers, particularly in temperature dependent measurements, which suggest that the conjugation is

In addition to increased order on the molecular scale, the morphology is also affected by fluorination. The high degree of orientational order with respect to the substrate, as observed by GIWAXS, along with the spontaneous formation of nanofibres upon spin-coating indicates a high propensity for self-assembly and good registry between polymer chains.

While it is difficult to infer any clear conclusions from the solid-state and crystallisation behaviour of the oligomers, fluorination clearly has an influence. Similar to the polymer system, the fluorine atom increases the inter and/or intramolecular interactions, though this does not necessarily manifest itself in increased crystallinity.

Overall, these results serve to further support the structure-property relationship suggested from the fluorination of P3ATs. Backbone planarisation upon the introduction of fluorine atoms along the π-system is confirmed to occur in another polythiophene derivative, and indirect evidence of an S-F intramolecular interaction is also presented. It is worth noting that in both cases, the fluorine atom points towards a sulfur atom, across a thiophene-thiophene link, therefore this relationship cannot at this point be applied to other aromatic systems or geometries.