Enhancing one-dimensional charge transport through intermolecular pi-electron delocalization: conductivity improvement for organic nanobelts.

Electrical conductivity of organic semiconductor materials represents one of the critical parameters that control the performance efficiency of organic-based electronic and optoelectronic devices. For materials assembled from planar aromatic molecules, evidence suggests that charge carrier mobility is usually maximized along the direction of cofacial π-π stacking of the molecules.1-3 While such conductivity optimization can be realized in the single crystalline phase, it is usually challenging to prepare a single crystal at large scale, and moreover it is difficult to unveil the orientation of molecular stacking within a bulk crystal when aligning it in a device, where directional control of charge transport is often demanded. To this end, fabrication of one-dimensional nanomaterials (e.g., nanowires) with cofacial molecular stacking along the long-axis of nanowire becomes essential in both fundamental investigation and practical application development. Such nanowires enable investigations that may lead to direct correlation between the maximized conductivity and the optimized molecular stacking.2 Recently, great progress has been made in fabricating nanowires or nanobelts from large, rigid, conjugate molecules, for which the molecular π-π stacking is mostly along the long-axis of nanowire (or nanobelt).3,4 While theoretical modeling and calculation suggest efficient intermolecular π-electron delocalization (and thus enhanced conductivity) along the molecular stacking,5,6 there are few studies reported on the experimental characterization and evaluation of such π-electron delocalization and the effect on electrical conductivity.7,8 In this Communication, we report a direct probing of the delocalized charge within a PTCDI nanobelt (a special nanowire with beltlike morphology) using ESR spectrometry methods. The efficient π-electron delocalization is consistent with the high conductivity measured with the same nanobelt. The PTCDI molecule employed (shown in Scheme 1) has proven effective for cofacial π-π stacking owing to the minimal sidechain steric hindrance.9,10 Highly uniform nanobelts can be fabricated from this molecule using phase-transfer self-assembly that was previously developed in our lab.11 Compared to common nanowires with round cross section, a nanobelt will provide large area interface when deposited on electrodes, thus facilitating the fabrication of devices with improved electrical contact. When reacting with a strong reducing reagent (e.g., hydrazine, E°ox ) +0.43 V, vs SCE), PTCDI (E°red ) -0.59 V, vs SCE12) was reduced to an anionic radical (Figure 1), which is stable in an oxygen free environment. The spectral structure of the radical obtained matches the standard spectra of PTCDI radicals that were obtained by reacting with metal sodium.12,13 Under argon protection, the radical sustained for extended time (inset of Figure 1). More importantly, the transformation between the neutral and radical state of PTCDI is reversible, that is, the neutral PTCDI can be recovered from the radical simply by removing the hydrazine via solvent extraction. The high stability observed for the PTCDI radical is likely due to the efficient in-plane delocalization of the charge over the whole PTCDI skeleton. Such in-plane electron delocalization is consistent with the typical n-type character of PTCDI materials as evidenced in real devices.14 The stable charged state makes PTCDI an ideal candidate for investigating the cofacial π-electron delocalization, mimicking the vertical (interplanar) charge migration within graphite. The stable anionic radical was also measured by electron spin resonance (ESR) spectrometry, with which a hyperfine spectrum of the radical was observed (Figure S2), consistent with previous observation by others.13 When cofacially stacked into a columnar phase within a nanobelt, the PTCDI anionic radical loses the hyperfine structure in ESR spectrum. Furthermore, in contrast to the symmetric spectrum (with g-tensor of 2.0033) observed for the free radical (Figure 2), the ESR spectrum of the anionic-doped nanobelt loses the reflection symmetry about the line center, showing an anisotropic g-tensor, with g (2.0038) > g (2.0026). Similar asymmetric ESR spectrum was obtained for PTCDI stacking in films.13 The anisotropic g-tensor is consistent with the uniaxial property of the nanobelt, which is dominated by the cofacial stacking of PTCDI.9,10 This observation, along with the lack of hyperfine ESR spectrum, implies strong intermolecular π-electron delocalization along the long axis of nanobelt. † Southern Illinois University. ‡ Chinese Academy of Sciences. Figure 1. UV-vis absorption spectra showing the conversion of PTCDI molecule (1.0 μM in DMF, red line) into anionic radical (green) in the presence of hydrazine (0.1 M). The inset shows the dramatic color change indicative of radical formation in DMF.