Imaging the Predicted Isomerism of Oligo(aniline)s: A Scanning Tunneling Microscopy Study

The self-assembly of two emeraldine base tetra(aniline) derivatives is investigated using scanning tunneling microscopy. A combination of the scanning tunneling microscopy data and calculations reveals the presence of predicted cis/trans isomerism in this oxidation state. This isomerism is shown to hinder self-assembly into ordered structures, and provides indications as to why the properties of these materials, and their parent polymer, polyaniline, remain unfulfilled.

emeraldine base (EB), and pernigraniline base (PB) states. PANI also possesses a conducting emeraldine salt (ES), easily accessible by acid doping (from EB) or oxidative doping (from LEB). Thin fi lms of PANI have been investigated for use in fl ash-memory [ 8 ] and sensors [ 9 ] and have found commercial application in printed circuit board manufacturing and anti-static coatings. However, conductivity in PANI has been observed at values signifi cantly below the theoretically predicted maximum value, [ 10 ] due to molecular and mesoscopic structural defects. [ 11 ] Values as high as 10 3 S cm −1 have only been recorded in exceptional cases. As model systems for PANI, tetra(aniline) (TANI) and its derivatives can be prepared as chemically pure, discrete units that are soluble in a range of organic solvents and amenable to a host of preparation methods, whilst retaining the unique redox properties of PANI. [ 12 ] Despite the short conjugated chain length, microstructures of ES TANI, prepared by acid-doping of EB structures, are able to approach the conductivities of PANI, owing to high sample crystallinity. [ 13 ] In this report, monolayer formation of EB TANIs on a Cu(110) surface is investigated at the single-molecule level using STM. MacDiarmid et al. predicted, due to the presence of the quinoid ring in the molecular backbone, a number of different positional, geometric, and conformational isomers for EB TANIs ( Figure 1 ). [ 14 ] Whereas the positional isomers have been confi rmed and observed with NMR in solution, [ 15 ] and related to reports of inconsistent conductivity in PANI, the cis / trans isomers have not been directly experimentally observed in TANIs. The cis / trans isomerization produces a much more profound 2D change in molecular shape, and is likely to signifi cantly reduce the ability of these materials to form crystalline structures. Here we present STM data showing, for the fi rst time, the presence of these cis / trans isomers in two different TANI-based materials, and relate their presence to the lack of structure formation and consequent absence of structural data in the literature.
Monolayers of phenyl capped TANI (Ph/Ph TANI) were formed by evaporation onto a clean Cu(110) surface in ultra-high vacuum (UHV) conditions (see the Supporting Information for experimental details). Diffusion of the molecules rendered imaging impossible at room temperature; cooling the surface down to 30 K using a continuous-fl ow liquid helium cryostat however ensured immobilization and enabled imaging of the Ph/Ph TANI.  Figure 2 c (isomer 3 ) also corresponds well, with a length of 2.2 nm and an angle of α STM ≈ 43° slightly reduced from that observed in the optimized structure in Figure 2 e (α DFT ≈ 48°, length DFT of 2.3 nm). [ 16 ] The structures shown in Figure 2 b,c can be assigned unambiguously as cis and trans isomers of Ph/Ph TANI, and highlights the inability of these cis / trans EB TANIs to self-assemble. It is noteworthy that isomer 4 , with a lower energy barrier and "intermediate" conformation, could not be identifi ed unambiguously within the monolayer.
From earlier studies, it was expected that the copper substrate would play a role in stabilizing the structures formed, with the electron-rich metal likely to form a strong interaction with the electron-defi cient quinoid ring. Experimental observation corroborated such behavior: whilst annealing the EB Ph/Ph TANI monolayers above 370 K in an attempt to traverse energy barriers and access a thermodynamically more stable state, desorption of the majority of the material, possibly via fragmentation, occurred, leaving isolated double-lobed structures on the surface. These double-lobed structures were observed at both 30 K ( Figure 3 a) and 300 K (Figure 3 b). The double-lobed structures at 300 K could be imaged for over 30 min without appreciable diffusion, suggesting some degree of chemisorption had taken place. It is proposed that the two lobes are the -C 6 H 4 -NH-C 6 H 5 fragments ("arms") of TANI, shown in Figure 3 d, with the quinoid ring planar to the surface (similar to behavior calculated for azobenzene molecules on copper). [ 17 ] The line profi les over the structures also support this conclusion, with the apparent size reduced from ≈2.3 to ≈2.0 nm, suggesting that the molecule is anchored and able to curl into the z -direction.  The growth of oligomeric, as well as polymeric materials, into 3D structures is strongly directed by the initial 2D selfassembly of molecules on the substrate. This initial interaction can dominate the fi nal structure, and consequently the properties, of such fi lms. [ 5a , 18 ] Confi nement of bis (2-ethylhexyl) hydrogen phosphate (BEHP)-doped ES Ph/Ph TANI to thin fi lms and analysis by GIXS has shown ES state fi lms form a lamellar structure with long range order of up to 50 molecular layers with a d-spacing of 2.15 nm; however, the analysis of EB Ph/Ph TANI thin fi lms prepared in the same manner shows minimal evidence of ordering. [ 19 ] Crystal structures of HClO 4 -doped and HBF 4 -doped ES Ph/Ph TANI, [ 20 ] as well as LEB Ph/Ph TANI, [ 21 ] have been reported; whilst ES and LEB TANI can exhibit conformational isomerism due to out-of-plane twisting of the benzenoid rings, crucially, it seems, the positional and geometric isomers are no longer present. After inspection of the STM data presented here, it is hardly surprising that no EB Ph/Ph TANI has been crystallized and that structural studies on thin fi lms show little order. We believe our experimental results provide hints toward an explanation of the unfulfi lled properties of PANI and TANI systems, as discussed by MacDiarmid. [ 14 ] Previous work has shown that the addition of an alkyl tail moiety to TANI leads to the formation of crystalline microstructures in the EB state. [ 22 ] We therefore prepared a dodecyl-functionalized TANI, Ph/C 12 TANI, which was deposited in the same fashion as the Ph/Ph TANI ( Figure 4 ). The addition of an alkyl chain, connected by an amide bond, was however not suffi cient to induce ordered self-assembled monolayers, but did provide enough of an attraction and registration with the underlying Cu(110) substrate to allow imaging at room temperature and gentle thermal annealing (up to 400 K). Again it was evident that the molecules formed a disordered layer with a range of 2D shapes. The straighter, trans molecules showed limited alignment along the step edges. Examples of structures optimized by DFT are presented in Figure 4 (see also the Supporting Information for a more comprehensive list), and presented alongside tentative matches to shapes found within the molecular monolayer. DFT-optimized structures of Ph/C 12 TANI have a molecular length of 3.4 nm, which compares well with the length measured from STM images of 3.5 nm for both isomers given in Figure 4 c, and 3.4 nm for the two molecules shown in Figure 4 d. Internal angles of these isomers compared with the calculated values show some deviation from the experimental data, and are summarized in Table 1 . This establishes  that the calculated molecular lengths and those measured using STM correspond excellently, as expected. However some angles measured are subject to signifi cant variations, likely due to interactions with coplanar molecules and the underlying substrate, as was seen for Ph/Ph TANI. Another feature of the Ph/C 12 TANI system that Figure 4 c,d displays is the additional cis / trans isomerism around the amide bond. This feature is observed despite the lower thermodynamic stability of the cis -amide in gas phase structures (typically by around ≈15 kJ mol −1 ), and adds another array of potential 2D shapes. The nonequilibrium nature of thermal evaporation as a sample preparation technique is also highlighted by these observations.
In conclusion, monolayers of EB state Ph/Ph and Ph/C 12 TANI were deposited under UHV conditions and shown to form disordered structures. The lack of order in these monolayers can be attributed to number of isomers available to this particular oxidation state of TANIs. [ 14 ] These isomers were revealed for the fi rst time by our low temperature UHV-STM investigations, and fully supported by calculations. These results highlight the importance of isomerism in the self-assembly of small molecules, and the infl uence of the presence of isomers on the formation of ordered 3D structures, i.e., aspects that should be considered during the design of small molecules for thin fi lm applications. In addition, the data support predicted reasons for the inability of PANI to achieve its theoretical conductivity and therefore fulfi l its potential, despite intensive research efforts.

Supporting Information
Supporting Information is available online from the Wiley Online Library or from the author.