Roberto
Costantini
ab,
Luciano
Colazzo
cd,
Laura
Batini
e,
Matus
Stredansky
ab,
Mohammed S. G.
Mohammed
cd,
Simona
Achilli
e,
Luca
Floreano
b,
Guido
Fratesi
e,
Dimas G.
de Oteyza
cdf and
Albano
Cossaro
*b
aPhysics Department of University of Trietse, via A. Valerio 2, 34127 Trieste, Italy
bCNR-IOM, Area Science Park, Strada Statale 14, km 163,5, 34149 Trieste, Italy. E-mail: cossaro@iom.cnr.it
cDonostia International Physics Center, Paseo Manuel Lardizabal 4, E-20018 Donostia-San Sebastián, Spain
dCentro de Física de Materiales (CSIC-UPV/EHU) – MPC, Paseo Manuel de Lardizabal, 5 – E-20018 Donostia-San Sebastián, Spain
eDipartimento di Fisica “Aldo Pontremoli”, Università degli Studi di Milano, Milano, Italy
fIkerbasque, Basque Foundation for Science, E-48011 Bilbao, Spain
First published on 6th February 2020
The self-assembly of leucoquinizarin molecules on Au(111) surfaces is shown to be characterized by the molecules mostly being in their keto–enolic tautomeric form, with evidence of their temporary switching to other tautomeric forms. This reveals a metastable chemistry of the assembled molecules, to be considered for their possible employment in the formation of more complex hetero-organic interfaces.
Fig. 2 (a) STM images of a LQZ submonolayer showing the presence of nanometric pores (0.05 V, 100 pA). (b and c) Tip manipulation allows the separation of a trimer from the assembly, which is identified as the building block of the architecture (0.1 V, 50 pA). (d) Bond resolution image reveals a distorted termination of the LQZ molecules, which can be related to the diketo phenol of the T1 tautomer. The superimposed overlayer model results from the DFT calculations of the assembly, obtained as detailed in the ESI.† |
A further important aspect of LQZ assemblies, observed by STM measurements, is their unstable imaging. This is shown by way of example in Fig. 3. Panel (b) displays the image of a disordered chain of molecules, measured on a low coverage LQZ layer deposited on Au(111) held at 120 K. Maintaining the probe at the point marked by the blue cross and monitoring the current under open feedback conditions provides a trace as shown in panel (a), characterized by the presence of telegraph noise that is absent on the bare surface. By comparison with similar observations in previous studies, this effect can be related to the switching of the molecule to different tautomeric forms,22–24 although switching to different adsorption configurations cannot be excluded either. The curve exhibits two switching levels beside the most stable one, as evidenced by the blue dotted lines in the graph.
However, the switching rate and the different level's relative dominance greatly vary as the tip is positioned differently (Fig. S4, ESI†). While this proves the molecule-related origin of the switching, it also reveals the influence of the location-dependent tip-molecule interactions, in line with other well-known tautomerization switches revealed in porphycenes.24 Another similarity between the two systems is the promotion of the switching events by vibrational excitations of the molecule, which becomes obvious from the notably increased instabilities at bias values above ±50 meV, and a further increase at around ±350 meV. The symmetric thresholds for both bias polarities are a fingerprint of inelastic tunneling processes that we associate, in the absence of electronic orbitals and transitions at these low energies, to molecular vibrational modes. It should be noted, however, that instabilities in the tunneling gap are observed even at 1 meV (Fig. 3a), whereby tip-induced effects driven by inelastic tunneling excitations are minimized. The DFT energy difference between T1 and the other tautomers, as calculated for a free molecule, is too large for a tautomeric switch to be at the origin of the tunneling current behavior presented in Fig. 3, which has been measured at 4.3 K. However, it has to be considered that experimentally, both the intermolecular hydrogen bonding and the interaction with the substrate can considerably lower this energy barrier. Moreover, transitions to intermediate tautomers, where only one of the enolic terminations of T1 has switched, can be considered at the origin of the telegraph noise, with consequent reduced effort in terms of energy barrier.
To further investigate the chemistry and the morphology of the system we performed X-ray spectroscopy measurements at the ALOISA beamline25 and at its ANCHOR-SUNDYN endstation26 at the Elettra Synchrotron. Fig. 4 reports the O and C K-edge NEXAFS spectra taken on an LQZ monolayer. The carbon spectra resemble the anthracene ones,27 apart from the intensity ratio between the first two resonances, at around 285 eV. Both edges present a strong dichroism, with the lower energy transitions having maximum intensity when the polarization of the electric field is perpendicular to the surface. We calculated the eigenfunctions for the ground state of the four tautomers, as well as the density of states projected onto states of π and σ symmetry. The four conformers exhibit the same symmetry of electronic states around the HOMO. In particular, the HOMO, LUMO and LUMO+1 are π states, whereas σ-symmetry can be found for the HOMO−1 (see the ESI†). The almost perfect dichroism indicates therefore that molecules are adsorbed flat on the surface and that neither the interaction with the substrate, nor the hydrogen bonding scheme the molecules are involved in, introduce relevant distortion of the electronic structure. The lower panels of Fig. 4 report the calculated spectra for the T1 tautomer, which provides a better match to the data than other configurations. Photoemission spectroscopy (XPS) also corroborates the presence of T1 species on the surface. The O1s and C1s peaks are shown in Fig. 5, as measured on an LQZ monolayer, and as calculated for the T1 molecule. No significant differences have been found in the XPS profiles taken on films with lower coverage. The O1s peak presents two broad components, of the same intensity, found at 531.3 eV and 532.7 eV, which can be assigned to carbonyl (O) and to hydroxyl (–OH) oxygen species respectively.28 The profile of C1s is more complex and can be fitted by five components. The calculated spectra for T1 nicely reproduce the experimental profiles and confirm the O1s assignment. Regarding the C1s peak, the highest energy component of C1s at ∼289 eV is not reproduced by calculations and can be assigned to energy loss due to inner excitations of the molecule. The C1s components 3 and 4 are due to the carbon atoms linked to the hydroxyl and carbonyl oxygen atoms respectively. We ascribe both the broadness of the O1s components and the minor discrepancies between experimental and calculated C1s profiles to the intermolecular bonds that are not considered by the calculation, as well as to the morphologic disorder of the assembly and to the possible presence of different tautomers on the surface. In this regard, we remark that, the X-ray spectroscopy measurements have been taken at room temperature, whereas STM has been performed at 4 K. We can speculate that these conditions promote a relevant portion of LQZ molecules in a tautomeric form different from T1.
Fig. 4 O1s and C1s XPS spectra measured on the LQZ monolayer (top panels) and simulated for the T1 tautomer (bottom). |
Fig. 5 XPS spectra measured (top) and calculated for the T1 molecule (bottom). See the ESI,† for the comparison with the calculations of the other tautomers. |
In conclusion, the assembly of LQZ molecules on Au(111) has been characterized in terms of the morphology and of the electronic properties of the supramolecular architecture. A trimer of hydrogen bonded molecules has been identified as a recurring motif of the assembly and as a building block of differently ordered superstructures. The assembly scheme that can be inferred from the STM images on the highly ordered structures is consistent with the LQZ molecules predominantly in their keto–enolic tautomeric form. The same configuration is compatible with the X-ray spectroscopy results, as simulated by DFT calculations. We ascribe a certain degree of unpredictability of the morphology, as the molecular density increases, to the tautomeric switch the molecules can experience on the surface. This is corroborated by the observation of telegraph noise in the STS measurements. As different tautomers present different chemical properties of their oxygen terminations, our findings have to be taken into account in the perspective of employing LQZ as a building block of more complex interfaces and of the possible on-surface reactions it can be involved in.
We acknowledge: the CINECA award under the ISCRA initiative (Grant No. HP10CB0ZW2), the MIUR SIR grant SUNDYN (RBSI14G7TL, CUP B82I15000910001), and funding from the European Union's Horizon 2020 programme (Grant Agreement No. 635919) and from the Spanish MINECO (Grant No. MAT2016-78293-C6).
Footnote |
† Electronic supplementary information (ESI) available: Experimental and theoretical details, NEXAFS spectra, additional STM images, and DFT calculation. See DOI: 10.1039/c9cc09915h |
This journal is © The Royal Society of Chemistry 2020 |