Adsorption and Assembly of Photoelectronic TiOPc Molecules on

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Adsorption and Assembly of Photoelectronic TiOPc Molecules on Coinage Metal Surfaces Wenhui Zhao, Hao Zhu, Huanjun Song, Jing Liu, Qiwei Chen, Yuan Wang, and Kai Wu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12673 • Publication Date (Web): 26 Mar 2018 Downloaded from http://pubs.acs.org on March 26, 2018

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The Journal of Physical Chemistry

Adsorption and Assembly of Photoelectronic TiOPc Molecules on Coinage Metal Surfaces Wenhui Zhao, Hao Zhu, Huanjun Song, Jing Liu, Qiwei Chen, Yuan Wang*, Kai Wu* BNLMS, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China Abstract The adsorption and assembly of individual and sub-monolayered TiOPc on Ag(111), Cu(111) and Au(111) have been investigated by scanning tunneling microscopy (STM) and spectroscopy. High resolution STM imaging as well as dI/dV and I-z measurements reveal that TiOPc adsorbed on Ag(111) adopts either O-up or O-down configuration. An intermolecular dipole-dipole interaction leads to that neighboring TiOPc molecules in alternating O-up and O-down configurations form a highly ordered checkerboard assembly structure on Ag(111). However, no large size TiOPc assemblies are observed on Cu(111) and Au(111) due to low surface mobility and diffusivity caused by strong TiOPc-Cu(111) interaction and the templating effect by the reconstructed herringbone structure of Au(111), respectively. Instead, molecular dimers on both Au(111) and Cu(111) as well as molecular aggregates on Au(111) are routinely observed in experiments, indicating that the intermolecular dipole-dipole attraction and weak hydrogen bonding co-play an important role in steering the adsorption and assembly of the TiOPc molecules on the coinage metal surfaces.

KEYWORDS TiOPc, Molecular Assembly, Dipole-dipole Attraction, MoleculeSubstrate Interaction, Coinage Metal Surfaces

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1. Introduction Due to its high photoelectronic activity in IR region,1 titanyl phthalocyanine (TiOPc) is an excellent photocarrier generation material.2 It’s been widely explored because of its potential applications in organic photovoltaic solar cells,3-5 organic light emitting diodes6, 7 and field effect transistors.8 Bulk or thin film TiOPc materials exhibit four polymorphs: monoclinic phases I, C and Y as well as triclinic phase II.9-12 The carrier generation efficiency in different TiOPc crystal form is remarkably different because the intermolecular interaction between the TiOPc molecules has a profound impact on its photo response.13, 14 In order to obtain a uniform crystal with a high efficiency, it’s vital to understand the growth mechanism and structure of the TiOPc molecular films. In earlier reports, the adsorption and thin film growth of TiOPc on various surfaces have been investigated by density functional theory (DFT) calculations, scanning tunneling microscopy (STM), low energy electron diffraction (LEED), infrared absorption spectroscopy (IRAS) and other analytical techniques.15-23 According to these studies, the non-planar TiOPc molecule in pyramidal structure adsorbs on metal surfaces with its Pc plane parallel to substrate surface and its ending O atom pointing toward either vacuum (O-up configuration) or substrate (O-down configuration).15, 21 In specific, TiOPc adsorbs into three different phases at different coverages on Ag(111) held at high temperatures.18 LEED, IRAS and STM measurements indicated a two-dimensional (2D) gas-like phase at a coverage below 0.6 monolayer (ML), a highly ordered commensurate phase (c-phase) above 0.6 ML and a point-on-line (POL) phase at 1.0 ML. All TiOPc molecules on Ag(111) adopted the O-up configuration according to IRAS measurements.18 In this study, detailed STM and scanning tunneling spectroscopy (STS) measurements were performed to explore the adsorption and assembly of the TiOPc molecules on Ag(111) held at room temperature, leading to a new adsorption model where the TiOPc molecules adsorb on Ag(111) in two different configurations, i.e., O-up and O-down ones. At low coverage, a disordered 2D-gas phase appeared. At a high coverage, orderly arranged O-up and O-down molecules assembled into a checkerboard-like structure, similar to that formed by Sn-up and Sn-down SnPc molecules on Ag(111).24-25 To explore the intermolecular interaction and substrate effect, Au(111) and Cu(111) were also employed for the adsorption and assembly of 2 ACS Paragon Plus Environment

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the TiOPc molecules. Our experimental results indicated that the intermolecular dipole-dipole interaction turned out to be the main driving force for the formation of the checkerboard assembly pattern on Ag(111).

2. Experimental Methods All measurements were performed on a commercial low temperature scanning tunneling microscope (Unisoku, USM1200S) with a base pressure below 10-10 torr. The Ag(111), Au(111) and Cu(111) substrates were cleaned by cycles of 1.5 keV Ar+ ion bombardment and subsequent annealing at ~760 K , ~750 K and ~730 K, respectively. The TiOPc molecule was synthesized according to the method reported previously.26 The purity was characterized by elemental analysis: calculated for C32H16N8OTi, C 66.68, H 2.80, N 19.44; found, C 66.31, H 2.95, N 19.06%.26 The molecule was evaporated at ~573K before degassed at ~563K for more than 4 hours. The evaporation rate was around 0.6 ML/min, as monitored by a quartz crystal microbalance (Inficon, SQM-160) and calibrated by STM imaging. 1.0 ML was defined as the substrate surface was saturatedly covered by a layer of the TiOPc molecules. The substrates were held at room temperature during the evaporation of the TiOPc molecule. An electrochemically etched tungsten tip was used as the STM tip that underwent thermal cleaning by an e-beam heater prior to use. All STM images (except Figure 4b which was acquired at ~78K) and STS measurements were achieved at ~4.6 K and afterwards processed with WSxM27 and Origin 8.1 softwares, respectively.

3. Results and Discussion 3.1 Molecular adsorption on Ag(111). The TiOPc molecules were thermally deposited on Ag(111) held at room temperature. As shown by the inset in Figure 1a, two inequivalent molecular species appear in the STM image. High resolution STM image shows an obvious difference in these two molecules. The one marked by the yellow arrow possesses a four-fold symmetry with a bright protrusion at its center, while the other marked by the green arrow shows a four-fold symmetry without the 3 ACS Paragon Plus Environment


central protrusion. These observations indicate that the molecules adsorbed on the Ag(111) substrate in two different configurations, namely, O-up (marked by the yellow arrow in Figure 1a) in which the Pc plane lies parallel to the surface and the ending O atom points to the vacuum, and O-down (marked by the green arrow in Figure 1a) in which the ending O atom points towards the substrate, according to previous reports of the same molecule on other substrates.15, 21, 23

Figure 1. STS measurements to individual TiOPc molecules on Ag(111). (a) dI/dV spectra measured at the centers of O-up and O-down molecules and bare Ag(111) as well. Inset: high resolution STM image of the O-up and O-down molecules marked by the yellow and green arrows, respectively. Imaging size: 2.3 nm × 2.9 nm, Vbias = 80 mV, I = 690 pA. (b) Molecular structure of TiOPc (left) and its adsorption models for the O-up and O-down TiOPc configurations (right). (c) I-z spectra acquired above the centers of the O-up (the blue curve) and O-down (the red curve) molecules and on bare Ag(111) (the black curve). (d) lnI-z curves. To disclose the adsorption scenario, STS measurements were carried out on the identified molecules in Figure 1a. The dI/dV spectra were collected at the molecule center. The STS spectrum measured on the O-up molecule shows a distinct broad 4 ACS Paragon Plus Environment

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peak around 1.0 V (as shown by the blue curve in Figure 1a) while that measured on the O-down molecule depicts no obvious features over the bias voltage range from 2.0 V to +2.0 V (as shown by the red curve in Figure 1a), illustrating that the electronic structures of these two molecules are different from each other. Direct evidence for the co-existence of two molecular configurations comes from the local work function (LWF) measurements over the concerned molecules. As shown in Figure 1b, the TiOPc molecule exhibits a large vertical dipole moment of 3.7 Debye orienting from Ti to O (marked by the yellow arrow in Figure 1b).22 Since the Ag(111) substrate possesses a permanent dipole moment orienting from bulk to vacuum (marked by the red arrow in Figure 1b), the adsorbed O-up and O-down molecules would cause different LWF changes acquired above the center of each molecule.28, 29 An efficient method to determine the LWF with STM is to measure the local barrier height (LBH) which is defined as

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(

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݈݊‫)ܫ‬ଶ ≅ 0.952(

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where I, z, me, h, and Φ are correspondingly tunneling current, tip-sample separation, electron mass, Plank constant and the measured LBH. The measured LBH is the average of the work functions for the tip Φt and the sample Φs, i.e., Φ = (Φt + Φs)/2. According to the given equation, the LBH can be calculated from the measured I-z curve.30 Figure 1c shows the I-z curves measured at the centers of the O-up and Odown molecules with respect to that measured on the bare Ag(111) substrate. Figure 1d shows the linear lnI-z curves whose slopes yield the LWFs. The experimental results pointed out that the LWF on the O-up molecule increased by 0.24 ± 0.03 eV with respect to that on the bare Ag(111) substrate while on the O-down molecule it decreased by 0.58 ± 0.03 eV. Both results confirmed the coexistence of the O-up and O-down TiOPc molecules on Ag(111).

3.2 Checkerboard assembly structure on Ag(111). As mentioned above, the TiOPc molecules assembled into different phases upon adsorption on Ag(111) at different coverages. In Figure 2a, the disordered 2D-gas phase appears at low coverage (< 0.1



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ML). The ordered checkerboard-like assembly phase develops at higher coverage (> 0.2 ML, Figure 2b) and coexists with the 2D-gas phase until 1.0 ML.

Figure 2. STM images of two phases formed by TiOPc on Ag(111). (a) Disordered 2D-gas phase at 0.1 ML. Vbias = -0.5 V, I = -50 pA. (b) Coexisted 2D-gas and assembly phases at 0.2 ML. Vbias = -1.0 V, I = -40 pA. (c) Close-up of the checkerboard structure. The yellow square denotes the square unit cell with a = b = 2.11 nm. The adjacent O-up and O-down molecule rotate against each other by an angle α = (16 ± 1)o. Vbias = -1.0 V, I = -40 pA. The high resolution STM image in Figure 2c indicates that the assembly phase is an ordered checkerboard assembly structure24 with a square unit cell where a = b = 2.11 ± 0.01 nm, as highlighted by the yellow square. Since there are two possible adsorption configurations for adsorbed TiOPc molecules, the checkerboard structure is actually formed by alternating O-up and O-down molecular rows. The angle α between the equivalent axes of symmetry along the molecular lobes of two adjacent O-up and O-down molecules is (16 ± 1)o, as shown in Figure 2c. The highly ordered alternating packing of the O-up and O-down molecules implies the existence of a specific interaction between these two kinds of molecules. The aggregates in the 2Dgas phase indicate the trend that the O-up and O-down molecules adsorb in neighborhood. As described earlier, the TiOPc molecule possesses a large vertical dipole moment pointing from Ti to O,22 the checkerboard assembly of the TiOPc molecules on Ag(111) is therefore ascribed to the dipole-dipole attraction between neighboring O-up and O-down molecules.


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3.3 Thermal treatment effect on the adsorption and assembly of the TiOPc molecules on Ag(111)

Figure 3. STM images of the 2D-gas phase before (a) and after (b) thermal annealing at 473 K for 30 min, and the statistic histogram (c) of the O-up (red) and O-down (grey) molecules in (a) and (b). The ratio of the O-up to O-down molecules changes from 1: 1 to 7: 1 as the coverage decreases from 0.1 ML to 0.04 ML. STM images of the assembly phase before (d) and after (e) flash annealing of the sample up to 553 K for 1 minute, and the statistic histogram (f) of the O-up (red) and O-down (grey) molecules in (d) and (e). The new annealing phase (e) possesses a rectangle unit cell with a’ = 1.41 ± 0.01 nm and b’ = 1.49 ± 0.01 nm, as shown by the yellow rectangle in (e). The angle between one of the diagonals of the molecules and a’ is ϕ = (30 ± 1) o

. Imaging conditions: (a) Vbias = -0.5 V, I = -52 pA; (b) Vbias = -0.2 V, I = -70 pA; (d)

Vbias = -1 V, I = -40 pA; (e) Vbias = -1 V, I = -40 pA. The statistical data were acquired by counts of at least 800 molecules. Thermal treatment can change the adsorption geometry and assembly pattern of the TiOPc molecules on Ag(111). At low coverage (i.e., 0.1 ML), the numeric ratio of the O-up to O-down molecules in the 2D-gas phase changed from nearly 1:1 to 7:1 as more O-down molecules (which interact with the substrate less strongly than the O-up ones) desorbed upon thermal annealing of the sample at 473 K for 30 minutes, as shown in Figures 3a, 3b and 3c. At high coverage (i.e., 0.98 ML), the occupancy of 7 ACS Paragon Plus Environment


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the O-down molecules in the assembly phase decreased from 46.8% to 1.3% upon flash annealing of the sample up to 553 K for 1 minute, as shown in Figures 3d, 3e and 3f. Simultaneously, a new assembly structure named the annealing phase appears, as shown in Figure 3e. This structure possesses a rectangular unit cell with a’ = 1.41 ± 0.01 nm and b’ = 1.49 ± 0.01 nm, as highlighted by the yellow rectangle in Figure 3e. All O-up molecules in the new phase adopt the same orientation with one of their diagonals deviated from a’ by ϕ = (30 ± 1) o, as shown in Figure 3e. Such a flash annealing did not obviously change the total coverage, which evidences that the Odown molecules flip upon thermal treatment. Therefore, the effect of tentative thermal treatment is two-fold: relatively preferential desorption and flip of the adsorbed Odown molecules on Ag(111). Both lead to that the ratio of the O-up to O-down molecules changes upon thermal treatment on this surface.

3.4 Adsorption and assembly on Cu(111) and Au(111). In order to verify the dipole-dipole attraction between neighboring O-up and O-down molecules, adsorption and assembly of the TiOPc molecules on Cu(111) and Au(111) were also scrutinized for comparison. As shown in Figure 4a, the TiOPc molecules adsorbed on Cu(111) adopt both O-up and O-down configurations. The O-up molecule shows a bright central protrusion surrounded by four dim lobes and maintains the four-fold symmetry. Meanwhile the O-down molecule possesses a two-fold symmetry with no central protrusion and its two diagonal lobes are brighter and wider than the other two ones. The symmetry reduction of MPcs (where M stands for hydrogen or metal) has been observed for CoPc31 and FePc32 on Cu(111), CuPc on Ag(100)33 and H2Pc on Ag(111).34 The incommensurability of the four-fold symmetric Pc and six-fold symmetric substrate lattice Cu(111)31, 32 should be responsible for the symmetry reduction on Cu(111). In the present study, on the one hand, the planar Pc of the TiOPc molecule in O-down configuration is lifted up by the O atom and hence invokes a weak interaction with the substrate. On the other hand, the O-up configuration with the Pc plane in direct contact with the substrate has largely reserved the four-fold symmetry. Therefore, the symmetry reduction observed in the present study does not mainly originate from the interaction between the Pc plane and the substrate, but is rather similar to that for the 8 ACS Paragon Plus Environment

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Cl-down ClAlPc molecule35 and O-down VOPc36 molecule on Cu(111). The symmetry reduction of the O-down configuration is accordingly attributed to the strong interaction between the O atom and the Cu(111) substrate, leading to charge redistribution within the Pc which is related to Jahn-Teller distortion.35

Figure 4. STM images of the TiOPc molecules on Cu(111) acquired at liquid helium temperature (~4.6 K) (a) and liquid nitrogen temperature (~78 K) (b). (a) Individual O-down molecule with a two-fold symmetry (left) and O-up molecule (right) with a four-fold symmetry. Vbias = 0.5 V, I = 10 pA. (b) The TiOPc molecules on Cu(111) at 0.3 ML. The O-up molecules are stable and clear (as marked by the yellow circle) under the perturbance of the STM tip while the O-down molecules are mobile and blurry (as marked by the red circle). Vbias = -1 V, I = -40 pA. The interaction between the O-up molecules and the Cu(111) substrate becomes even stronger, which is judged by the facts that the molecules are immobile during the STM imaging at liquid nitrogen temperature (as marked by the yellow circle in Figure 4b) while the isolated O-down TiOPc molecules appear blurry and diffuse under the same imaging conditions (as marked by the red circle in Figure 4b). On Ag(111), the diffusivity of the O-up or O-down molecules in the 2D-gas phase was so large that they became quite mobile and no clear STM image of an individual TiOPc molecule could be acquired at 78 K. Such a strong interaction between the molecules and the Cu(111) substrate result in a different molecular morphology for the O-down molecule and a lower surface diffusivity of the O-up molecule, compared with their counterparts on Ag(111), so that the TiOPc molecules sparsely reside on the surface 9 ACS Paragon Plus Environment


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without formation of any assembly structures up to 0.5 ML, as shown in Figure 5a. Above 0.7 ML, the O-up molecules remain dispersed while some O-down molecules tilt to manifest bright protrusions, as depicted in Figure 5b. High resolution STM imaging of the area marked by the white frame in Figure 5b identifies that dimers are formed by paired O-up and tilted O-down molecules, as shown in the upper panel of Figure 5c. In each dimer, the adsorbed O-down molecule overlaps one of its smaller lobes above the O-up one. Based on the high resolution STM images, a proposed geometric model for the dimer is given in the lower panel of Figure 5c. These two molecules in each dimer rotate against each other by an angle of 41o (as marked by the yellow dashed lines in Figure 5c), which brings closer the distance between one of the H atoms in the lifted peripheral indole ring of the O-down molecule and the O atom of the O-up molecule. Hence, a possible hydrogen bond between the mentioned H and O atoms37 is established as marked by the green dashed lines in Figure 5c. Additionally, dipole-dipole attraction between the O-up and O-down molecules may further stabilize the dimer. No dimers between two adjacent O-up or O-down molecules were experimentally observed, presumably due to repulsive dipole interactions. This further confirms that the dipole-dipole interaction between the TiOPc molecules plays an important role in mediating their adsorption behavior at surfaces.

Figure 5. STM images of the TiOPc molecules on Cu(111) at different coverages. (a) 0.1 ML. Vbias = 0.5 V, I = 30 pA. (b) 0.7 ML. The white square denotes the imaging area in (c). Vbias = 0.5 V, I = 40 pA. (c) Upper panel: Enlarged STM image showing the dimers formed by paired O-up and one O-down molecules in (b). Vbias = 0.5 V, I = 40 pA. Lower panel: Schematic geometric models for the dimer in top (left) and side (right) view. The rotational angle β between two neighboring molecules against each 10 ACS Paragon Plus Environment

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other is 41o, as marked by the yellow dashed lines. The green dashed lines in green circles highlight the weak hydrogen bond formed between the O atom in the O-up molecule and a nearest H atom in the peripheral indole ring of the O-down molecule. The herringbone structure and elbows of the Au(111) substrate also have a significant influence on the molecular adsorption and assembly.38, 39 At 0.08 ML, the O-down TiOPc molecules predominantly reside at the elbow sites, as pointed out by the yellow arrows in Figure 6a, while the O-up ones sparsely disperse in the fcc regions (the broader region divided by the dashed lines), as shown in Figure 6b. Such an adsorption behavior should be dictated by two interactions: the strong one between the O atom in an O-down molecule and the low-coordinated Au atom at the elbow site, and the other between the molecular π-orbitals of the O-up molecules and d-electrons of Au.40 Dimers similar to that observed on Cu(111) are labeled by the green circles in Figures 6a and 6c. The TiOPc molecules in the dimer could be dragged apart via the STM tip manipulation (as depicted by the red arrow in Figure 6c) while the single molecules and other dimers nearby (as labeled by the numbers in Figures 6c and 6d) remain unchanged or slightly change their positions, which verifies that the dimer does consist of a pair of O-up and O-down molecules, as denoted by the green arrows in Figure 6d. Again, these two molecules in the dimer rotate against each other by 41o, which is the same as the situation for the dimer on Cu(111). In our experiments, dimers rather than individual O-down molecules could be identified in the fcc and hcp regions at low coverage on Au(111), implying that the dipole-dipole attraction and weak hydrogen bonds37 between the opposite TiOPc molecules in a dimer be responsible for the stabilization of the O-down molecules.



Figure 6. STM images of the TiOPc molecules adsorbed on Au(111) at various coverages. (a) 0.08 ML. The yellow arrows mark the O-down molecules positioned at the elbow sites. The green circle highlights the dimer formed by paired O-down and O-up molecules. Vbias = 0.1 V, I = 45 pA. (b) Zoom-in STM image in (a). The orange dashed lines mark the herringbones of Au(111). The broader region divided by the dashed lines is the fcc region and the narrow one, the hcp region.41 Vbias = 0.1 V, I = 45 pA. (c) STM image showing the dimer to be manipulated (as highlighted by the green circle) and the single molecules labeled with numbers around the dimer. The red arrow indicates the dragging direction of the dimer by the tip. Another dimer is labeled with d1. (d) STM image of the manipulated dimer which breaks into O-up and O-down molecules (as marked by the green arrows). The results were reproducible for more than 10 dimers manipulated. The single molecules labeled as 1, 2, 3, 6, 7 remained unchanged, while the single molecules named 4, 5, 8 and the dimer d1 changed their position or rotated under the disturbing of the STM tip during the manipulation. Imaging conditions: Vbias = 0.5 V, I = 40 pA. (e) 0.7 ML. Vbias = 0.5 V, I = 25 pA. The white square marks the imaging area in (f). (f) Zoom-in of the STM image in (e). Vbias = 0.5 V, I = 15 pA. Once their coverage reaches 0.7 ML (Figure 6e), the TiOPc molecules densely packed in the fcc and hcp regions, leaving the herringbones clearly exposed. A close12 ACS Paragon Plus Environment

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up of the STM image (Figure 6f) discloses an obvious tendency that the O-up and Odown TiOPc molecules inch in towards each other, resembling to their packing mode in the assembly phase on Ag(111). Thus, the dipole-dipole attraction between adjacent O-up and O-down molecules remarkably stabilizes the O-down molecules in the fcc and hcp regions and results in the checkerboard fragments. The varying TiOPc molecular adsorption and assembly behaviors in sub-monolayer regime on different coinage metal substrates can be rationalized by different molecule-substrate interaction strengths. The interaction between the TiOPc molecule and Cu(111) is the strongest, leading to the striking distortion of the O-down molecule. Such a stronger molecule-substrate interaction on Cu(111) lowers the molecular mobility and diffusivity of the O-up molecule, leading to no molecular assembly. The molecule-substrate interaction on Ag(111) is, however, relatively weaker so that the TiOPc molecules self-assemble into a highly ordered checkerboard structure via the dipole-dipole attraction between adjacent O-up and O-down molecules. The molecule-substrate interaction on Au(111) heavily depends on the molecular adsorption regions in the herringbone structure. Such an asymmetric interaction results in no large size assembly but merely some fragments of the checkerboard pattern on Au(111). In addition, the inter-molecular interactions such as dipole-dipole attraction and hydrogen bonding also co-play an important role in steering the molecular adsorption and assembly behaviors, i.e., formation of the dimers by paired O-up and O-down molecules on Au(111) and Cu(111), aggregates consisting of alternating O-up and O-down molecules in fcc region on Au(111) and the checkerboard assembly structure on Ag(111). 4. Conclusions In summary, the adsorption and assembly of the TiOPc molecules in sub-monolayer regime on Ag(111), Cu(111) and Au(111) were explored by LT-STM and STS. High resolution STM imaging as well as dI/dV and I-z spectral measurements revealed that the TiOPc molecule adsorbed on Ag(111) adopted two configurations, i.e., the O-up and the O-down ones. The TiOPc molecules formed into disordered 2D-gas phase at low coverage and ordered assembly phase at high coverage upon adsorption on Ag(111). The assembly phase was experimentally confirmed to be a highly ordered checkerboard assembly structure that was formed by alternating O-up and O-down 13 ACS Paragon Plus Environment


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molecular rows. The checkerboard assembly was mainly formed via the dipole-dipole attraction between adjacent O-up and O-down molecules. The lowered molecular mobility and diffusivity of the O-up TiOPc due to the strong molecule-substrate interaction on Cu(111) and the templating effect by the herringbone structure on Au(111) prevented the TiOPc molecules from being assembled into large size structure at these surfaces. However, the experimentally observed dimers formed by paired O-up and O-down molecules on both Au(111) and Cu(111) and fragments of the checkerboard pattern on Au(111) indicated that the dipole-dipole interaction and weak hydrogen bonding worked together to mediate the molecular adsorption on Cu(111) and Au(111). Our experimental results clearly demonstrated that TiOPc adsorption, aggregation and assembly were dictated and steered by the moleculesubstrate interaction strength and inter-molecular interactions such as dipole-dipole attraction and weak hydrogen bonding on various substrates. Unravelling the molecular packing patterns of functional organic molecules may advance our understanding of the unique properties and extraordinary performances of the photoelectronic devices42, 43 based on the photoelectronic molecules like TiOPc. Corresponding Author *E-mail: [email protected] (KW), [email protected] (YW) ACKNOWLEDGMENT This work is jointly supported by MOST (2017YFA0204702) and NSFC (91527303, 21333001), China. REFERENCES 1. Zhang, X.; Wang, Y.; Niu, L. Titanyl Phthalocyanine and Its Soluble Derivatives: Highly Efficient Photosensitizers for Singlet Oxygen Production. J. Photochem. Photobiol., A 2010, 209, 232-237. 2. Saito, T.; Sisk, W.; Kobayashi, T.; Suzuki, S.; Iwayanagi, T. Photocarrier Generation Processes of Phthalocyanines Studied by Photocurrent and Electroabsorption Measurements. J. Phys. Chem. 1993, 97, 8026-8031. 3. Brumbach, M.; Placencia, D.; Armstrong, N. R. Titanyl Phthalocyanine/C60 Heterojunctions: Band-Edge Offsets and Photovoltaic Device Performance. J. Phys. Chem. C 2008, 112, 3142-3151. 14 ACS Paragon Plus Environment

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Table of Contents

Room temperature adsorption and assembly of optoelectronic TiOPc molecules on Cu(111), Ag(111) and Au(111) at nearly full coverage.


Adsorption and Assembly of Photoelectronic TiOPc Molecules on

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