Covalent Functionalization by Cycloaddition Reactions of Pristine Defect-Free Graphene.

Based on a low-temperature scanning tunneling microscopy study, we present a direct visualization of a cycloaddition reaction performed for some specific fluorinated maleimide molecules deposited on graphene. Up to now, it was widely admitted that such a cycloaddition reaction can not happen without pre-existing defects. However, our study shows that the cycloaddition reaction can be carried out on a defect-free basal graphene plane at room temperature. In the course of covalently grafting the molecules to graphene, the sp2 conjugation of carbon atoms was broken, and local sp3 bonds were created. The grafted molecules perturbed the graphene lattice, generating a standing-wave pattern with an anisotropy which was attributed to a (1,2) cycloaddition, as revealed by T-matrix approximation calculations. DFT calculations showed that while both (1,4) and (1,2) cycloadditions were possible on free-standing graphene, only the (1,2) cycloaddition could be obtained for graphene on SiC(0001). Globally averaging spectroscopic techniques, XPS and ARPES, were used to determine the modification in the elemental composition of the samples induced by the reaction, indicating an opening of an electronic gap in graphene.

Tunneling spectroscopy measurements on hydrogen-bonded supramolecular polymers.

We studied the formation of hydrogen-bonded supramolecular polymers of Ethyl Hexyl Urea Toluene (EHUT) on a gold (111) surface by low temperature scanning tunneling microscopy. Tunneling spectroscopy performed along an individual molecule embedded in a self-assembled layer revealed strong changes in the value of the HOMO-LUMO gap. A variation of the LUMO state is attributed to the effect of space charge accumulation resulting from anisotropic adhesion of the molecule. In addition, for specific tunneling conditions, changes induced through the formation of hydrogen bonds became visible in the differential conductance (dI/dV) maps; isolated molecules, hydrogen bonded dimers and supramolecular polymers of EHUT were distinguishable through their electronic properties.

One- and two-photon absorption and emission properties of an oligo(phenylenethienylene)s series.

The photophysical and nonlinear absorption properties of an oligo(phenylenethienylene)s series (nTBT) are investigated in this article. The length of the chromophore is gradually increased from one to four phenylenethienylene repeating units in order to evaluate the effects of the electronic delocalization on the two-photon absorption cross sections (δ). According to the excitation anisotropy measurements and quantum chemical calculations, two electronic transitions with distinctive symmetries, 1Ag → 1Bu and 1Ag → 2Ag, are present in the low energy region of the linear absorption spectrum. The lowest-energy transition 1Ag → 1Bu is one-photon allowed but two-photon forbidden and implies an electronic charge delocalization all along the oligomer segment whereas the weakly-allowed 1Ag → 2Ag transition exhibits a transition moment perpendicular to the average plane of the chromophore. The latter transition mainly contributes to the two-photon absorption ability of the oligomers. All derivatives are poorly solvatochromic and the breakdown of the mirror symmetry rule observed between absorption and fluorescence spectra at room temperature has been attributed to a photoinduced geometrical relaxation leading to a very efficient planarization process of the oligomer irrespective of its size. Increasing the oligomer length results in a slight shift of the two-photon absorption band (∼1300 cm(-1)) and in a drastic increase of δ from 2 ± 1 GM up to 802 ± 160 GM for 1TBT and 4TBT respectively. Based on a three-level model, it was found that main contributions to the strong increase of δ stem from the transition moments Mge and Mee' which are multiplied by a factor of 2.8 and 5 when going from 1TBT to 4TBT.

Generating long supramolecular pathways with a continuous density of states by physically linking conjugated molecules via their end groups.

Self-assembly of conjugated 2,5-dialkoxy-phenylene-thienylene-based oligomers on epitaxial monolayer graphene was studied in ultrahigh vacuum by low-temperature scanning tunneling microscopy (STM). The formation of long one-dimensional (1D) supramolecular chain-like structures has been observed, associated to a physical linking of their ends which involved the rotation of the end thiophene rings in order to allow π-π stacking of these end-groups. dI/dV maps taken at an energy corresponding to the excited states showed a continuous electronic density of states, which tentatively suggests that within such molecular chains conjugation of electrons is preserved even across physically linked molecules. Thus, in a self-organization process conjugation may be extended by appropriately adapting conformations of neighboring molecules. Our STM results on such self-organized end-linked molecules potentially represent a direct visualization of J-aggregates.