Exploring Redox States, Doping and Ordering of Electroactive Star‐Shaped Oligo(aniline)s

Abstract We have prepared a simple star‐shaped oligo(aniline) (TDPB) and characterised it in detail by MALDI‐TOF MS, UV/Vis/NIR spectroscopy, time‐dependent DFT, cyclic voltammetry and EPR spectroscopy. TDPB is part of an underdeveloped class of π‐conjugated molecules with great potential for organic electronics, display and sensor applications. It is redox active and reacts with acids to form radical cations. Acid‐doped TDPB shows behaviour similar to discotic liquid crystals, with X‐ray scattering investigations revealing columnar self‐assembled arrays. The combination of unpaired electrons and supramolecular stacking suggests that star‐shaped oligo(aniline)s like TDPB have the potential to form conducting nanowires and organic magnetic materials.


Introduction
p-Conjugated materials are being used in an increasingly wide range of organic electronic devices, such as field-effect transistors, [1] light-emitting diodes, [2] solar cells [3] and gas sensors, [4] to name but afew.One of the best-known and most versatile systems is poly(aniline) (PANI), owing to its unique acid-induced conductivity,i ts wide range of oxidation states with different colours [5] and its reactive nitrogen atoms that can bind small molecules. [6] Oligo(aniline)sa re well-defined oligomers of PANI [7] that overcomes ome of the factors limiting PANI's conductivity,s uch as polydispersity [8] and microphase segrega-tion. [9] They have as trong propensity to self-assemble, which can be tuned through av ariety of routes. For example, carefully chosen acid surfactant dopants [10] were shown to improve their in-and out-of-plane order,a ttractive for thin film applications. [11] Modifying the periphery of oligo(aniline)s provides af urther route to tune self-assembly:a ttachment of alkyl tails promotes the formation of ordered conductingt hin films, [12] while the addition of as urfactant head-group to tetra(aniline) causesi tt os elf-assemble into conducting nanowires in water. [13] Optoelectronic properties can also be tuned using an oligomer approach: variation of the aromatic ring system at the centreo ft he molecule leads to systematic changes of optoelectronic properties. [14] Finally,t he presence of external templates such as graphene has been used to direct the crystallization of oligo(aniline)sv ertically,o rienting the molecules into stacksa nd increasing the conductivity by an order of magnitude. [15] This approachh as recently been extended to other pconjugated molecules to form optical microcavities for use in nanolasers. [16] One promisinga nd somewhat underdeveloped approach to tuning self-assembly and optoelectronic properties is through exploring the use of non-linear p-conjugated structures, [17] such as star-shaped architectures (i.e.,m olecules with three or more linear "arms" connected to ac entral" core"). [18] Starshaped p-conjugated oligomers have already received some attention, [19] particularly those based on thiophenes, [20] fluorenes [21] and triarylamines. [22] Compared with their linear counterparts, they show marked differences in their absorption spectra,s olubility,s elf-assembly and thermal stability.R eviews have highlighted the use of star-shaped oligomers as active layers in organict hin-film transistors [23] (OTFTs) and solutionprocessable organic photovoltaics (OPVs). [24] Star-shaped oligomers also share structuralf eaturesw ith discotic mesogenst hat can allow them to form ordered liquid-crystalline phases, [25] of great interestf or organic electronics. [26] However,t here are very few reports of star-shapedo ligo(aniline)s, [27] leaving many unanswered questions about the fundamentalb ehaviour of this class of p-conjugated molecules. In light of the potentially desirable properties and applications of star-shaped oligo(aniline)s, and the lack of attentiong iven to them in the literature, we present here the resultso fadetailed study of the redox behaviour,a cid doping, optoelectronic properties and self-assembly of as imple star-shaped oligo(aniline) derivative. From these investigations we aim to develop tools to understand, design and exploit this class of materials for future applications.
TDPB is redox active:asasolid andinsolution it is gradually oxidisedi na ir,c hanging in colourf rom pale brown to redorange. MALDI-TOF mass spectrometry revealed that am ixture of redox states is alwaysp resent. Species with zero, one, two and three arms in the oxidised quinoidf orm [28] (Scheme 2) were observed (see SupportingI nformation Figure S4-S9). The spectra indicate ac omplex mixture of ionised and protonated speciest hat arise from electron and protont ransfers between matrix and analyte molecules. See Supporting Information Ta ble S1 for ad etailed listing of these species and their masses, depending on redoxs tate. Strongo xidants such as Ag 2 Oe nsured ad istribution in favour of the more highly oxidised states, but also led to some decomposition. Oxidised samples of TDPB are easily identified by their UV/Vis/NIR spectra (Figure 1), which show ac haracteristic peak at 469 nm, and no further absorption at higherwavelengths.
In order to characterise this seeminglyr andom mixture of redox states, we turned to simulations to provide further insight. Time-dependent density functional theory (TD-DFT) simulations are very useful for identifying electronic states of oligo(aniline)s in solution that are difficult to isolate or characterise in full. [13,14] We used atwo-step process to model TDPB:geometry minimisation, followed by simulation of the UV/Vis/NIR spectrum. We based our initial structures on the isomers and conformations of other oligo(aniline)sd etermined by X-ray diffraction, [29] NMR spectroscopy, [30] and STM, [31] then minimised the geometries in Gaussian [32] with aw idely used DFT method:t he B3LYP functional, [33] the 6-31G* basis set, [34] and ap olarisable continuum model [35] (PCM) to accountf or solvation. The output of this first modelling step can be examined in av arietyo fw ays, [36] some of which are illustrated in Figure 2f or the radicalc ation of TDPB.Int he second step, we used the CAM-B3LYP functional [37] to simulate UV/Vis/NIR spectra, since it accountsf or charge-transfer excitationsa nd polarizabilityi ne xtended pconjugated systems better than B3LYP does, and therefore provides more accurate simulated spectra. [38] TD-DFT simulations yielded similar UV/Vis/NIR absorption maxima( in the range 445-450 nm) for all the possible oxidised Scheme1.Synthetic route to the star-shaped oligo(aniline) TDPB in its fully reducedstate.
Scheme2.Structuralchanges upon full oxidation of dianiline arms. states of TDPB,t hat is, with one, two or three arms oxidised. These resultsi mply that anym ixture of these states should have practically indistinguishable UV/Vis/NIR spectra. The spectral similarity of the oxidised forms of TDPB shows the arms have little electronic influence on one another,a nd act as isolated dianiline species. These observations are consistent with absorption spectra of other p-conjugated molecules with meta substitution at abenzene ring. [40] Full details of the DFT calculations are included in the Supporting Information, including comparisons with the simple dimericl inear analogue, DPPD, which shows very similar experimental ands imulated UV/Vis/ NIR spectra of the reduced ando xidiseds tates (Supporting Information Figure S39-S44 and Table S15).
Oxidised solid samples of PANI and oligo(aniline)s typically form ac onducting state (emeraldine salt or ES) upon addition of Brønsted or Lewis acids. The semiconducting to conducting transition is marked by ac olour change from blue to green and accompanied, in PANI, by an increase in DC conductivity values of approximately 10 to 13 orders of magnitude. [41] Addition of Brønsted acids to the oxidised form of TDPB in THF or ethanol solutions causedacomparable colourc hange from red to green. When as trong aqueous mineral acid such as HCl, HBr or H 2 SO 4 was used, ap recipitate formed within af ew seconds, leaving ac olourless supernatant after af ew minutes (see Supporting Information Figure S1). In contrast, protonic doping with organic Brønsted acids (Scheme 3) led to stable, clear solutionst hat did not form ap recipitate. Three protonic dopants, widely used in previous studies of PANI and oligo(aniline)s, werea lso compared here:( AE)-camphor-10-sulfonic acid (CSA), bis(2-ethylhexyl)sulfosuccinic acid (AOT)a nd bis(2-ethylhexyl) hydrogenphosphate (BEHP). All three reactedw ith TDPB in THFt of orm solutionss howing aU V/Vis/NIR absorption band with am aximum at approximately 775 nm. See Figure 1f or ar epresentative spectrumo ft he CSA-dopeds pecies (and Supporting Information, Figure S16 and S17 for AOT and BEHP data, respectively).
TD-DFTs imulations assisted in identifying the speciesr esponsiblef or the experimentally observed UV/Vis/NIR absorption (Figure 3). The radical cation of TDPB is ag ood match for the observeds pectra,w ith ac alculated UV/Vis/NIR maximum of 772 nm, compared to the experimental maximao fa pproximately 775 nm. Phenyl-capped dianiline (DPPD)f orms as imilar speciesw ith as imilara bsorption spectrumi na cidic conditions. [28] The singlet dication can be ruled out with av ery high level of confidence as the major doped state of TDPB,a si ts simulated UV/Vis/NIR maximum of 923 nm is 150 nm higher than the experimentalv alue. Other possibilities that cannotb e ruled out include at riplet dicationic diradical, and doublet and quartet tricationic triradical species [42] (calculated l max valueso f 755, 750 and 751 nm, respectively;s ee Supporting Information for furtherd etails). These results clearly show that TDPB forms speciesc ontaining unpaired electrons when oxidised samples are exposed to acids.
Electron paramagnetic resonance (EPR) spectroscopy showednor esponse for acid-free oxidised solutions, as expected for ac losed-shell, diamagnetics pecies (Figure4,s olid line). However,t he presence of radicals in acid-doped solutions of TDPB was confirmed:a fter CSA was added, as ignal appeared at 3370 G, indicating the formation of an open-shell, paramagnetic doped species (Figure 4, dotted line).
TDPB's triphenylbenzene coreg ives it the potential to act as ad iscotic mesogen. [43] Although TDPB is not able to form mesophases on its own due to the lack of soft and flexible peripheral alkyl groups, such groups can be introduced through doping. It has been shown that oligo(aniline)s dopedw ith acid surfactants can form supramolecular thermotropic liquid-crystalline phases, [10] whilst lyotropic PANI liquid crystals were obtained when doped by CSA in a m-cresols olution. [44] Following as imilars trategy,s olutions of oligomers doped with CSA or AOT were drop-cast onto hydrophobized glass slides and investigated by polarized light microscopy (PLM) to reveal any birefringent textures and thus anisotropic organization. When low-boiling solvents such as THF,e thanol and propan-1-ol were used, the solvents evaporated quickly to leave very viscous films that could not be sheared by hand. When using octan-1-ol (a higherb oiling points olventu sed in previous tetra(aniline)-based studies [45] )s low evaporation led to the formation of soft phases that exhibited strong birefringence when sheared (Figure5). The sheared films did not undergo any visible phaset ransitions upon heating until their decomposition around180 8C.
To determinet he exact nature of supramolecular ordering leadingt ot he observed birefringence, we performedt wo-di- In short, solutionso f TDPB in THF were doped with CSA or AOT and the solvent allowed to evaporate. Samples were then prepared according to ap reviously published method, [46] and the solid residue heated to 70 8C. At hin filament of the heated material was extruded (a process similar to shearing of soft films), placed in an X-ray beam, and the resulting X-ray scattering pattern recorded with a2 Darea detector (insets in Figure 6).
An umber of important features are notable in the 2D patterns:1 )there is very little orientational dependence in the scattering patterns;2 )a relatively intense amorphous halo is observedi nb oth cases, consistent with disordered acid dopantm olecules surrounding more ordered aggregates of TDPB molecules;3)two prominent but isotropic reflections are obvious in the patterns for TDPB doped with CSA and AOT. The features in the small-angle region (A) are attributedt o stacks that are formed by the TDPB molecules, while the wideangle reflection (B) is attributed to the typical intermolecular p-stacking distance of 0.36 nm. The scattering patterns are clearly influenced by the nature and stericb ulk of the dopant, as reflected by the inter-stackdistance (Table 1). The p-stacking distance is independento ft he dopant.T aken together, the Xray data suggest al oosely ordered material containing stacks of TDPB molecules (stacked with the typical p-stacking distance of 0.36 nm), separatedb yr egions of disordered acid dopants.
Further experiments with tailor-made pro-mesogenic protonating agents and varying doping ratios are envisaged to gain furtheri nsight into this new class of conducting star-shaped oligo(aniline)s and any liquid-crystalline phases formed. In ad- Figure 3. Comparison of simulated [39] (CAM-B3LYP,dashed lines)a nd experimentals pectra (THF,s olid lines) for the reduced,o xidiseda nd dopeds tates of TDPB. For simplicity,c ounterionsw ere not modelled.

Conclusion
In conclusion, the effect of chemicals tructure on spectra,s pin and supramolecular ordering of the star-shaped oligo(aniline) derivative TDPB has been determined. TD-DFT simulation of the UV/Vis/NIR spectra of all the oxidation states and doped forms of TDPB shows that computational chemistry can be used to characterise and distinguish p-conjugated materials with complex behaviour.T he length of the arms and the pconjugation pattern at the core of TDPB causesi tt of orm radical cationics peciesi na cidic conditions. The ability to create molecular systemsw ith unpaired electronsi si mportant for applications that rely on electrical conductivity or magnetism. When doped, TDPB shows at endency to self-assemble into weakly ordered supramolecular columnar aggregates. In future,t hrough furtherd esign and structural modifications of this motif (such as planarization of the core of the star-shaped oligo(aniline) or by using pro-mesogenic protonatinga gents), novel radical-containing supramolecular nanostructures can be created. Thisw ork thus provides opportunitiesf or focused future investigations aimed at further developing the full potentialofs emiconducting andconducting star-shaped p-conjugated molecules, and exploring applications, for example, in sensing and generation of reactive oxygen species.

Acknowledgements
This work wass upported by the United Kingdom Engineering and Physical SciencesR esearch Council (EPSRC) (grant EP/ K502996/1). Mass spectrometric analysis was performed on instrumentation bought through the Core Capability for Chemistry Research -S trategic Investment in Mass Spectrometry EPSRC grant (EP/K03927X/1). The facilities and staff of the Mass Spectrometry service and the NMR service at the Universityo f Bristol,a sw ell as computing resources from the Centre for Computational Chemistry,a re kindly acknowledged. In particular,w ew ould like to thank Dr Craig Butts for discussing the NMR data. We also thank the EPSRC NationalS ervice for Electron Paramagnetic Resonance Spectroscopy at the University of Manchester,f undedu nder EPSRC grant NS/A000014/1. All underlying data are provided in the Supporting Information.