Generation of Low-Dimensional Architectures through the Self-Assembly of Pyromellitic Diimide Derivatives

Small π-conjugated molecules can be designed and synthesized to undergo controlled self-assembly forming low-dimensional architectures, with programmed order at the supramolecular level. Such order is of paramount importance because it defines the property of the obtained material. Here, we have focused our attention to four pyromellitic diimide derivatives exposing different types of side chains. The joint effect of different noncovalent interactions including π–π stacking, H-bonding, and van der Waals forces on the four derivatives yielded different self-assembled architectures. Atomic force microscopy studies, corroborated with infrared and nuclear magnetic resonance spectroscopic measurements, provided complementary multiscale insight into these assemblies.


Synthesis and analysis
Figure S1 Synthesis of compounds 1-4.

IR spectroscopy
Infrared absorption spectra were studied within the mid-IR range (400 -4000 cm -1 ) by using Bruker Equinox 55 FT-IR spectrometer equipped with a KBr beam splitter. For temperature measurements the samples were placed on a KBr plate in THMS 600 Linkam heating stage equipped with a temperature controller. At the beginning the samples in the form of single crystal or powder were measured at room temperature. Subsequently, they were heated until melting to obtain thin films which were cooled down and heated again. Each temperature cycle was done between room temperature and temperature above melting point and the IR spectra were continuously collected. Before each measurement the sample temperature was stabilized for several minutes.

UV-Vis spectra
Absorption spectra of compounds 2-4 were recorded with a Shimadzu UV-2401 PC spectrophotometer at concentrations 1×10 -5 M in dichloromethane at the room temperature. The absorption bands and molar absorption coefficients are listed in Table S1. The electronic absorption spectra of compounds 2, 3 and 4 in the UV region showed typical bands for π-π* stacking ligand-based transitions at 230-350nm.

Density Functional Theory (DFT)
All calculations were carried out using the Gaussian 09 application which is available through the PL-Grid portal [1]. Calculations were performed, within Density Functional Theory (DFT) [2] framework at M06/6-311G* level [3,4]. The M06 method was selected based on the results from extensive comparative studies, it is also recommended for calculations for organometallic and inorganometallic compounds and for non-covalent interactions [3]. Finally, to take into account the effect of solvent we used the Polarizable Continuum Model (PCM) [5] with dichloromethane and DMSO chosen as solvents as in the experimental studies. All figures depicting orbitals were prepared with use of Avogadro computer program [6].

Charge transport properties in field-effect transistors devices
In order to unravel the charge transport ability of all the above-mentioned materials, their electrical characterization was performed by integrating them in field-effect transistor (FET) devices whose active layer was composed of different self-assembled architectures of arylene di-imide derivative (1-4). Transistors in bottom-contact bottom-gate configuration were fabricated on n ++ -Si substrates with 230 nm of thermally grown SiO 2 as the gate dielectric and pre-patterned pairs of (bottom) gold electrodes with interdigitated geometry as the source and drain. The semiconductor layer was deposited by drop-casting from a solution in DMSO for compounds 3 and 4 or a chloroform/DMSO mixture (vol:vol ratio 9:1) for compounds 1 and 2. After drop-casting, films were dried at room temperature overnight. All samples were prepared and measured under N 2 atmosphere to avoid oxidative doping of the materials and ensure reproducibility of the experiments. Further, as these molecules are mainly an electron transporter as previously shown [7], the SiO 2 substrate was chemically functionalized with hexametheyldisilazane (HMDS) previously deposited by spin-coating and followed by annealing at 100° C for 1h. The HMDS treatment of the surface hinders trapping coming from hydroxyl groups, which are present on the SiO 2 surface in the form of silanols. [8] Previous fluorinated arylene di-imide derivative showed mobilities in the order of 10 -2 cm 2 V -1 s -1 which are several order of magnitude higher than what measured in compound 1, which feature mobilities in the 10 -8 -10 -9 cm 2 V -1 s -1 range. In addition, the injection barrier from the Au electrodes into the LUMO of all the compounds can also be associated with the low device mobility recorded. Conversely, the threshold voltage (V TH ) was found to be positive and close to +20 V. The sign of Vth is consistent with the behaviour of a n-type field-effect transistor working in accumulation. Its absolute value could be, in first approximation, associated with a low number of structural/electronic defects i.e. traps. The poor solubility and the consequent necessity of employing DMSO as a solvent could additionally account for these reduced performances, as solvent traces could be still present in S21 the film even after a temperature annealing. It is believed that high boiling point and dielectric constant solvents do not allow charge carriers to easily overcome the molecule-to-molecule energetic barrier represented by the solvation sphere. By and large, we believe that the use of DMSO as well as the difficult charge injection are the major responsible for the nonfunctioning devices when compounds 2, 3 and 4 are used as the active layer.
FET Transistors Bottom-contact-bottom-gate transistors were fabricated starting from SiO 2 /Si-n ++ substrates. The thermally grown SiO 2 layer (230 nm) has a capacitance per unit area (C i ) of 15 nFcm −2 . Pre-patterned 40-nm-thick gold source and drain electrodes did not undergo any chemical functionalization. The FET substrates were cleaned with acetone (95% GC) and isopropyl alcohol (99.7% GC) in an ultrasonic bath (20 min/solvent) followed by a gentle drying under nitrogen gas. After that, the silicon oxide substrate was functionalized with hexamethyl disilazane (HMDS) by spin-coating 100 µL onto the substrate surface for 60 s at 1500 rpm, followed by thermal annealing at 100 °C for 1 h.