Article pubs.acs.org/JPCC
Tailoring the Reaction Path in the On-Surface Chemistry of Thienoacenes Laurentiu E. Dinca,† Jennifer M. MacLeod,*,†,‡ Josh Lipton-Duffin,†,‡,∥ Chaoying Fu,§ Dongling Ma,† Dmitrii F. Perepichka,§,⊥ and Federico Rosei*,†,⊥,# Centre Énergie, Matériaux et Télécommunications, Institut National de la Recherche Scientifique, Université du Québec, 1650 boulevard Lionel-Boulet, Varennes, Québec J3X 1S2, Canada ‡ School of Chemistry, Physics, and Mechanical Engineering and ∥Central Analytical Research Facility (CARF), Institute for Future Environments, Queensland University of Technology (QUT), 2 George Street, Brisbane, 4001 QLD, Australia § Department of Chemistry and ⊥Center for Self-Assembled Chemical Structures, McGill University, 801 Sherbrooke Street West, Montreal, Quebec H3A 0B8, Canada # Institute for Fundamental and Frontier Science, University of Electronic Science and Technology of China, Chengdu 610054, PR China †
S Supporting Information *
ABSTRACT: Oligothiophenes provide rich opportunities for surface confined reactions that can lead to two-dimensional materials. We have performed systematic studies of tetrathieno-anthracene (TTA) based molecules on different transition metal surfaces to reveal the details of their on-surface chemistry. On the (111) surfaces of Ni, Pd, and Cu, we observe the sulfur abstraction from the monomer following thermal activation, whose yield varies with the type of metal surface. On Ni(111) and Pd(111) the internal design of the 2TTA isomer promotes intramolecular rebonding to produce pentacene, whereas geometrical constraints prevent the 3TTA isomer from intramolecular rebonding, promoting oligomerization. On Cu(111), desulfurization is preceded by dehydrogenation, which introduces metal-mediated intermolecular coupling in 2TTA. This organometallic phase is stable up to 200 °C. On all surfaces, the desulfurization and dehydrogenation of the molecules are important reaction pathways which define the bonding geometries of the products.
1. INTRODUCTION
containing molecules react on a range of catalytic metal surfaces. We recently reported a new reaction involving the catalytic abstraction of sulfur from trithieno[2′,3′:5,6:3′,2′:3,4:3′,2′:7,8]anthra[1,2-b]thiophene (2TTA) (I in Scheme 1a), with subsequent intramolecular cyclization on Ni(111).13 By depositing the same molecule on Cu(111) we were able to initiate dehydrogenation using the highly biased tip of a scanning tunneling microscope (STM). Under this regime, the 2TTA molecules were reductively dehydrogenated while leaving the aromatic core intact.14 Following dehydrogenation, the activated 2TTA molecules formed intermolecular organometallic assemblies mediated by Cu adatoms (II, Scheme 1a). Here, we describe a systematic investigation of 2TTA and its isomer 3TTA (trithieno-[2′,3′:3,4:2′,3′:7,8:3′,2′:5,6]anthra[2,1-b]thiophene (Scheme 1b) molecule on a series of transition metal surfaces, where they may provide a useful model system for studying the effect of the substrate on interand intramolecular reactions. This type of study can provide new insights into the mechanisms underlying the on-surface
A deeper understanding of surface-confined reactions of molecules with multiple reactive sites is a key factor in tailoring the synthesis of sophisticated molecular architectures in two dimensions.1−4 Identifying the processes that occur at the submolecular level (e.g., bond breaking and covalent coupling) can be quite subtle, yet is a critical step toward predicting and controlling on-surface chemistry, and has been successfully achieved using the tools available within the realm of surface science.5−9 Thiophene moieties are often incorporated in the design of molecules for applications in organic electronics, because of their favorable electronic properties.10,11 However, upon adsorption on transition metal surfaces, the sulfur can be abstracted,12 which leads to opening of the thiophene ring and degradation or transformation of the employed building blocks. Such reactions are detrimental for the synthesis of thiophenebased polymers on surface templates and is a potential concern for bulk polymerization reactions, most of which are catalyzed by transition metals (as soluble organometallic complexes or nanoparticles). On the other hand, they offer a mechanism for creating new structural moieties from thiophene precursors. Therefore, it is important to understand how thiophene© 2015 American Chemical Society
Received: June 7, 2015 Revised: August 24, 2015 Published: August 25, 2015 22432
DOI: 10.1021/acs.jpcc.5b05418 J. Phys. Chem. C 2015, 119, 22432−22438
Article
The Journal of Physical Chemistry C
2. EXPERIMENTAL SECTION STM experiments were carried out in an ultrahigh vacuum (UHV) chamber with a base pressure of 10−10 mbar. The Ni, both single crystal and foil (99% purity), and the Pd substrates were cleaned by repeated cycles of sputtering with 1 keV Ar+ for 10 min followed by annealing at 850 °C for 20 min. The Cu substrate was cleaned in similar conditions, except that annealing was performed at 500 °C. The 2TTA and 3TTA molecules were synthesized as described elsewhere,26 and purified by vacuum sublimation (10−1 mbar, 300 °C). In the present experiments, they were sublimated from a Knudsen-type effusion cell at a temperature of approximately 180 °C using pyrolytic boron nitride (PBN) and Al2O3 crucibles. During deposition, the substrates were held at room temperature (RT). After deposition and characterization of the molecular layer, the samples were annealed at specific temperatures for 15 min. STM characterization was performed using a commercial variable-temperature instrument (Aarhus 150, SPECS GmbH) equipped with cut Pt/Ir tips. Unless otherwise noted, measurements were performed at RT. All STM data were collected in constant-current mode. Bias voltages are reported with respect to the STM tip. To compensate for instrumental drift and creep, the STM images were corrected to reflect lattice parameters of known structures wherever possible using WSxM software.27 Time-of-flight secondary ion mass spectrometry (TOFSIMS) was carried out using an ION-TOF SIMS IV with a base pressure of 10−10 mbar. A 15 keV Bi+ beam was used to sample an area of approximately 50 × 50 μm2. Both positive and negative SIMS were performed at three different locations on the surface. The optimized shapes of precursor molecular structures result from the gas phase calculations using Gaussian 09,28 at the B3LYP level of theory by using 6-31G(d) basis set (for C, H, S), complemented where needed with an effective core potential LANL2DZ (for Cu and Pd). Avogadro,29 and Discovery Studio Visualization,30 open-source molecular builders and visualization tools, were used to produce basic molecular models.
Scheme 1. Reaction of 2TTA (a) and 3TTA (b) on Transition Metal Surfaces as Observed by STMa
a
Roman numbers, from I to V, label representative intermediate products or structures.
reactions of these molecules, leading to an improved understanding of how to tailor molecule/substrate pairs to produce desired products.15−18 More precisely, investigating relationships between the structure of the molecule and the resulting products could lead to predictive control over these surfaceconfined reactions. Of particular interest are the prediction of selectivity for intra- or intermolecular reactions, and the facilitation of strategies to identify and design precursors capable of region-specific reactions. We examined three metals, Ni, Pd, and Cu, which were chosen to represent a range of reactivity and are the most commonly used catalysts in C−C coupling reactions, particularly in the synthesis of conjugated polymers.19−21 The reactivity of a given metal is correlated to its d-shell occupancy or the d-band center of mass (εd) with respect to the position of the Fermi level (EF).22,23 Nickel has the closest εd to its EF,24 and is particularly interesting since it is located critically in the periodic table with respect to the other two metals: Pd (4d series) and Cu (3d 4s series, with the 3d filled and 4s unfilled).25 This array of substrates allowed us to explore the effect of surfaces on the resulting products. We selected the (111) facet because it has the lowest surface energy for a monatomic fcc crystal, and because of its relative isotropy compared to the other low-index facets.
3. RESULTS AND DISCUSSION 3.1. 2TTA. 3.1.1. Intramolecular Cyclization to Pentacene. In our previous experiments, the surface of Ni(111) was observed to desulfurize 2TTA, leaving the anthracene core intact (structure I in Scheme 1a).13 The process started at temperatures as low as RT, which was sufficient to transform the 2TTA into a mixed organic phase comprising partially desulfurized intermediates and the newly formed pentacene (Pc) product (Scheme 1a).13 The cyclization process is driven to completion below 100 °C. In the present experiments, we investigated whether this novel cyclization reaction could also take place on a polycrystalline substrate. The TOF-SIMS spectrum of a sample deposited on a clean Ni foil and annealed at 50 °C matches our previous observations on Ni(111),13 showing a strong peak with m/z = 278 Da corresponding to the pentacene molecular ion (C22H14)+ (Figure 1).31 On Cu(111) and Pd(111), higher temperatures are necessary to carry out the cyclization reaction, indicating a higher activation energy. When annealed to temperatures below 100 °C for 15 min on Pd(111), the 2TTA molecules appear intact. 22433
DOI: 10.1021/acs.jpcc.5b05418 J. Phys. Chem. C 2015, 119, 22432−22438
Article
The Journal of Physical Chemistry C
Table 1. Summary of the Most Common Reaction Products of 2TTA on the (111) Facet of Ni, Pd, and Cu, at Various Annealing Temperaturesa T(°C)
Ni(111)
Ni(foil)
Pd(111)
Cu(111)
×
TTA
diffusing TTA
100 150 200
1/2(TTA), Pc, 1/2 Pc Pc Pc, Pc dimer Pc multimer
Pc × ×
250
×
×
TTA Pc, Pc+(S/Pd) Pc, Pc+(S/Pd), Pc dimer ×
× TTA, TTA+Cu TTA+Cu, Pc, Pc +Cu Pc, Pc multimer
RT
The phases marked with “ × ” were not investigated. The “1/2” stands for an incomplete reaction: desulfurization (of 2TTA) or cyclization (to Pc). a
Figure 1. TOF-SIMS showing the positive ion spectrum of the 2TTA/ Ni-foil sample. The peaks corresponding to nickel and pentacene (C22H14)+ ions have been labeled, as well as the peaks of Al+ and Pb+ impurities.
STM images reveal molecular features with a shape similar to that calculated for 2TTA in the gas phase, where the four thiophene rings of 2TTA emerge as characteristic protrusions (Figure 2a).
Figure 3. Room temperature STM images of 2TTA on Pd(111). 10 × 10 nm2 in (a) (pentacene), and 7 × 7 nm2 in (b) (pentacene and pentacene dimers, with associated molecular models presented in Figure S2, SI) after annealing at 150 °C for 15 min (3.5 × 2.5 nm2 for the inset image in (b) which shows one Pc and one Pc trimer). Scanning parameters: It = 0.49 nA, Ub = 19 mV for (a); It = 0.48 nA, Ub = 362 mV for (b) (It = −0.66 nA, Ub = −21 mV for the inset in (b)). Figure 2. STM images showing whole 2TTA: on Pd(111) in (a) 4.3 × 5.7 nm2 annealed at 100 °C for 15 min and imaged at RT; and on Cu(111) in (b) 5 × 5 nm2 by deposition at −160 °C, annealing at RT, and imaging at −160 °C. Scanning parameters: It = 0.47 nA, Ub = 524 mV for (a); and It = −0.52 nA, Ub = −883 mV for (b).
On Cu(111) the 2TTA is highly diffusive at RT, as noted in our previous reports.14 Following RT deposition, only streaky and indistinct features could be imaged by STM.32 After cooling to −160 °C, the STM images reveal intact 2TTA molecules assembled into small ordered domains (Figure 2b).33 A systematic study of annealing temperatures revealed that the temperature required to produce pentacene from 2TTA increases in the range Ni(111), Pd(111), and Cu(111), as shown in Table 1. On Ni(111) the conversion is complete below 100 °C, whereas Pd(111) and Cu(111) require annealing up to 150 °C (Figure 3) and 250 °C, respectively (Figure 4). In the latter case, the reaction appears to start only above 200 °C (Figure 4a−c). The sulfur atoms extracted from 2TTA reconstruct in triangular domains or decorate the steps of the Cu surface (Figure S1, SI).34 The sulfur atoms aligned along ⟨11̅0⟩ are spaced by 0.50 ± 0.05 nm, which is identical, within experimental error, to the (2 × 2) reconstruction reported by Wahlström et al.35 At high molecular coverage, the individual pentacene moieties form two types of coassemblies with this 2 × 2 sulfur reconstruction (Figure 4a). The molecules arrange with their long axes along ⟨11̅0⟩, forming concentric triangular structures. The end-to-side arrangement
Figure 4. STM images of 2TTA on Cu(111), after annealing for 15 min at 200 °C in (a−c) and for 20 min at 250 °C in (d). In the 18 × 18 nm2 image (a) recorded at RT, the green (solid) and orange (dashed) circles surround two enantiomer phases in which pentacene makes assemblies (∼9.7 nm2) directed by the 2 × 2 sulfur reconstruction (2TTA coverage close to 1 ML). The two 3.9 × 2.5 nm2 STM images (b) (Figure S4b, SI) and (c) (Figure S4d, SI) show at −160 °C common assemblies formed by three pentacene-like products (starting coverage of 2TTA < 0.25 ML). The image (d) of 5 × 7 nm2 comprising pentacene coassemblies with sulfur in domains of ∼12.6 nm2 was recorded at RT. Scanning parameters: It = 0.16 nA, Ub = 341 mV for (a), It = −0.89 nA, Ub = −968 mV for (b,c), and It = −0.81 nA, Ub = −202 mV for (d).
of the three pentacene moieties at the center of these structures (Figure 4a) is chiral; both enantiomers were found in roughly equal abundances on the surface.36 The structures remain 22434
DOI: 10.1021/acs.jpcc.5b05418 J. Phys. Chem. C 2015, 119, 22432−22438
Article
The Journal of Physical Chemistry C qualitatively similar after annealing to 250 °C, although the higher temperature annealing can lead to an increase in their abundance and expansion of their internal ordering to up to three concentric triangles (from 9.7 to ∼12.6 nm2), as shown in Figure 4d. 3.1.2. Cu-Mediated Intermolecular Phase. While on Ni and Pd the transformation of 2TTA to pentacene is direct, on Cu(111) it is preceded at 150 °C by the formation of an intermediate structure. Although the structure shown in Figure 5 is incomplete, it clearly implies a kagome-like lattice, which
desulfurization. Our present experiments further elucidate the structure of this desulfurized organometallic phase. In addition to the chiral concentric triangular structures, two additional types of triangular motifs were identified on Cu(111), covered with
