Conformational Switching of Verdazyl Radicals on Au(111)

Zhichao Huang, Yazhong Dai, Xiaojie Wen, Dan Liu, Yuxuan Lin, Zhen Xu, Jian Pei, Kai Wu

Beijing National Laboratory for Molecular Sciences (BNLMS), College of Chemistry and Molecular Engineering, Peking University,Beijing 100871, P.R.China.

Abstract:Pure organic radical molecules on metal surfaces are of great significance in exploration of the electron spin behavior.However, only a few of them are investigated in surface studies due to their poor thermal stability.The adsorption and conformational switching of two verdazyl radical molecules, namely, 1,5-biisopropyl-3-(benzo[b]benzo[4,5]thieno[2,3-d]thiophen-2-yl)-6-oxoverdazyl (B2P)and 1,5-biisopropyl-3-(benzo[b]benzo[4,5]thieno[2,3-d]thiophen-4-yl)-6-oxoverdazyl (B4P), are studied by scanning tunneling microscopy(STM) and density functional theory (DFT).The adsorbed B2P molecules on Au(111) form dimers, trimers and tetramers without any ordered assembly structure in which two distinct appearances of B2P in STM images are observed and assigned to be its “P” and “T” conformations.The “P” conformation molecules appear in the STM image with a large elliptical protrusion and two small ones of equal size, while the “T” ones appear with a large protrusion and two small ones of different size.Likewise, the B4P molecules on Au(111) form dimers at low coverage, strip structure at medium coverage and assembled structure at high coverage which also consists of above-mentioned two conformations.Both B2P molecules and B4P molecules are held together by weak intermolecular interaction rather than chemical bond.STM tip induced conformational switching of both verdayzl radicals is observed at the bias voltage of +2.0 V.The “T” conformation of B2P can be switched to the “P” while the “P” conformation of B4P can be switched to the “T”one.For both molecules, such a conformational switching is irreversible.The DFT calculations with Perdew-Burke-Ernzerhof version exchange-correlation functional are used to optimize the model structure and simulate the STM images.STM images of several possible molecular conformations with different isopropyl orientation and different tilt angle between verdazyl radical and Au(111) surface are simulated.For conformations with different isopropyl orientation, the STM simulated images are similar, while different tilt angles of verdazyl radical lead to significantly different STM simulated images.Combined STM experiments and DFT simulations reveal that the conformational switching originates from the change of tilting angle between the verdazyl radical and Au(111) surface.The tilt angles in “P” and “T” conformations are 0° and 50°, respectively.In this study, two different adsorption conformations of verdazyl radicals on the Au(111) surface are presented and their exact adsorption structures are identified.This study provides a possible way to study the relationship between the electron spin and configuration conversion of pure organic radical molecules and a reference for designing more conformational switchable radical molecules that can be employed as interesting molecular switches.

Key Words: Verdazyl radical;Scanning tunneling microscopy;Density functional theory;Electron spin

1 Introduction

Exploration of magnetic molecules on metal surfaces is of great importance in understanding the molecular spin properties including spin interaction.Scanning tunneling microscopy(STM) is one of the most popular and powerful tools in surface spin research due to its capability of measuring the Kondo effect1and its ultrahigh spatial resolution in real-space.The magnetic molecules employed in STM measurements are abundant2,including single molecule magnets containing transition metals3,4,phthalocyanines5-8, porphyrins9, and pure organic radical molecules10.Pure organic radical molecules have been extensively studied in organic chemistry.However, owing to their poor thermal stability, only a few of them are practically applied in surface studies.The first observation of the Kondo resonance for a stable neutral pure organic radical is 1,3,5-Triphenyl-6-oxoverdazyl adsorbed on Au(111)11.Moreover,organic molecules containing the radical nitronyl-nitroxide side group also exhibit the Kondo resonance on metal surfaces10,12,13.In recent years, people have adoptedin situpulse-induced method to create free radical molecules on surface14.These created new organic radical molecules on surface well serve as the models for systematical studies to understand the spin-spin interactions.

In this study, the adsorption and conformational conversion of two 6-oxoverdazyl radical molecules were investigated by using STM and density functional theory (DFT).The molecular structures of these two molecules, namely, 1,5-biisopropyl-3-(benzo[b]benzo[4,5]thieno[2,3-d]thiophen-2-yl)-6-oxoverdazyl(B2P) and 1,5-biisopropyl-3-(benzo[b]benzo[4,5]thieno[2,3-d]thiophen-4-yl)-6-oxoverdazyl (B4P), are showed in Fig.1a, b,respectively.The thiophene protecting group (termed BTBT)and eight-electron-conjugated six-membered ring15in B2P or B4P lead to high thermal stability of the radicals which are suitable for surface study.Both B2P and B4P remained intact when evaporated on Au(111).More interestingly, they formed two similar conformations named as “P” and “T”.In the “P”conformation, the angle between the BTBT and verdazyl radical ring is near 0°.In the “T” conformation, however, the angle is about 50°.The “T” conformation of B2P could be switched to“P” by tip induction at the bias of +2.0 V, while the “P”conformation of B4P could be switched to “T” under similar condition.Conformational switching of neutral pure organic radicals has hardly been reported in surface science, which offers the opportunity to explore the switching properties of spin carriers.

2 Experimental and calculation methods

Sample preparation and data acquisition were performed on commercial STM (Unisoku USM-1300, Japan) working under ultrahigh vacuum (UHV) conditions.The Au(111) single crystal was purchased (MaTeck, 99.999% in purity, Germany), and cleaned and ordered by several cycles of Ar+bombardment (1.5 keV, 10 μA, 10 min) followed by annealing(～750 K, 15 min).The B2P and B4P molecules were synthesized in one of our laboratories (JP group), and evaporated onto the single crystal from a home-made Ta boat.The evaporation rate was monitored by an SQM-160 quartz balance.The Au (111) substrate was kept at room temperature (about 298 K) during the evaporation process.All STM images were acquired at 4.2 K with a background pressure below 2 × 10-10mbar (1 mbar =100 Pa).The tip was simply prepared by cutting of a commercial Pt-Ir wire.

Fig.1 Molecular structures of (a) B2P and (b) B4P.

Fig.2 STM topography of B2P molecules on Au(111).(a) Large scale STM image of the B2P molecules on Au(111);(b) B2P monomer with its molecular structure superimposed;(c) dimer; (d) trimer; and (e) tetramer.

The STM image simulation was performed with the CASTEP module16in the Materials Studio software.A (6 × 6) supercell with 3 layers in thickness was used to simulate the Au(111)substrate.Periodic boundary conditions were used in the calculations.The DFT with Perdew-Burke-Ernzerhof (PBE)17version of the generalized gradient approximation (GGA) for the exchange-correlation functional was employed to optimize the structure and simulate the STM images of the molecules on Au(111).

3 Results and discussion

Fig.2a shows a typical STM topographic image of the B2P molecules adsorbed on Au(111).The B2P molecules are randomly dispersed on the Au(111) surface and form monomer,dimer, trimer and tetramer.A close-up of the B2P monomer is given in Fig.2b.Based on the dimension of the B2P molecule,the elliptical large bright and two small round protrusions are attributed to be the BTBT protecting group and the verdazyl radical ring, respectively.With the assignment of the single B2P molecule in the STM image, its dimer, trimer and tetramer are correspondingly depicted in Fig.2c through 2e.

To verify the interaction in multimers, STM tip manipulation was applied to the B2P dimer.The experimental results showed that the B2P trimer could be feasibly separated into one monomer and one dimer, demonstrating that the B2P molecules are actually held together by weak intermolecular interaction rather than chemical bond.For most B2P molecules scrutinized in this work, the monomer exhibited two conformations, namely,the “T” and “P” ones.The former appeared in the STM image with a large elliptical protrusion and two small ones of different size, while the latter appeared with a large protrusion and two small ones of equal size.

Interestingly, the “T” conformation of B2P can be switched to the “P” one by the tip disturbance.As shown in Fig.3a, b, the STM images of B2P before the switching are nearly identical at the bias of +0.5 V and +1.5 V, demonstrating the same conformation at both biases.When the scanning bias changes to+2.0 V, the “T” conformation turns into the “P” one, as shown in Fig.3c.Subsequent STM images of the B2P molecule at biases of +2.0 V and +0.5 V confirm the successful conformational switching from “T” to “P”, as pictorially shown in Fig.3d and e,respectively.However, the switch from the “P” conformation cannot be switched back to the “T” one at all biases tried experimentally.

To verify the ubiquitousness of the conformational switching,adsorbed B4P molecules on Au(111) were also investigated by STM.As shown in Fig.4, B4P can form various structures at different coverages.The topographic STM images of the B4P molecules at low, medium and high coverages are correspondingly shown in Fig.4a, b and c.At low coverage,most B4Ps exist in dimers and adsorb at the elbow sites on Au(111).The high-resolution STM image and proposed model of the dimer are given in Fig.4d where the verdazyl radical in one B4P faces toward that in the other.At medium coverage, the dimers tend to assemble into a strip structure while some B4Ps form a trimer-like structure, as shown in Fig.4e.At high coverage, the B4Ps form an assembled structure.The basic unit in this structure is marked by the blue circle in Fig.4f which consists of one B4P molecule.In Fig.4f, a unit cell (a= 1.4 nm,b= 1.3 nm,θ= 121°) in blue diamond can also be identified.The STM tip manipulation can also feasibly separate the B4P dimer or other structure into monomers, showing that the B4P molecules stick to each otherviaweak intermolecular interaction rather than chemical bond.As can be seen in Fig.4e and f, the B4P absorbed on Au(111) exhibits two conformations, namely,the “T” and “P” ones, in their STM images which display two protrusions in different and similar brightness, respectively.For B4P, the “T” conformation is more stable upon tip disturbance at the bias of +2.0 V and below while the “P” conformation undergoes conformational switch under the same condition.

Fig.3 The conformational switching process of B2P molecules from “T” conformation to “P” conformation.For each part, the top section is the STM image with a regular color bar and the bottom with a more prominent color bar.(a) B2P before switching, bias: +0.5 V.(b) B2P before switching, bias: +1.5 V.(c) B2P is switched at +2.0 V.(d) B2P after switching, bias: +2.0 V.(e) B2P after switching, bias: +0.5 V.

Fig.4 STM topographic images of B4P molecules on Au(111).Image of B4P molecules on Au(111) at (a) low coverage, (b) medium coverage and (c) high coverage.High-resolution STM images of B4P molecules on Au(111) at (d) low coverage, (e) medium coverage and (f) high coverage.

Fig.5 demonstrates the conformational switch from “P” to“T”.As shown in Fig.5a and b, all B4P molecules maintain their conformation during the tip scanning over the bias range of +0.5 to +1.5 V.As the bias rises to +2.0 V, one B4P molecule in “P”conformation (marked by the dashed circle) is disturbed by the tip and transforms into the “T” conformation (Fig.5c), which is verified by the STM image after the switching (Fig.5d).It should be noted that such a conformational switch does not necessarily work for all molecules in “P” conformation.It’s experimentally shown that conformational switching of the molecules in the assembled structure turns out to be quite difficult, but rather easy for the molecules in the trimer-like assemblies.

In order to determine the “P” and “T” conformations, DFT calculations are employed to simulate the STM images of several possible structures.The used models are first optimized prior to the STM image simulations.Obviously, the main imaging difference between “P” and “T” conformations stems from the feature of the nitrogen heterocycle in two aspects: one is the rotation of the isopropyl and the other, the tilting angle of the nitrogen heterocycle against the Au(111) surface.To discern the above-mentioned two situations, DFT simulation is performed.In terms of the isopropyl rotation, B2P is used an example in the simulation, as shown in Fig.6.In Fig.6a, both the dihedral angles between two nitrogen heterocycles and the isopropyl CN-C-H are 0°.Meanwhile, the methylene group in isopropyl is nearly parallel to the substrate surface.Fig.6b and c show the modelled and simulated results of other two possible conformations.In Fig.6b, the isopropyl marked by the red dashed circle differs from the one in Fig.6a in the dihedral angle C-N-C-H which is about 180°.The green dashed circle in Fig.6c highlights the difference that the dihedral angle C-N-C-H is about 90° and the methylene group in isopropyl is perpendicular to the surface.There exists no apparent difference among these structures, which accordingly suggests that the difference between the “P” and “T” conformations is not the reason for the rotation of the isopropyl moiety.

Fig.5 The conformational switch of B4P molecules from “P” conformation to “T” conformation.For each part, the top section is the STM image with a regular color bar, the bottom with a prominent color bar.(a) B4P before switching, bias: +0.5 V.(b) B4P before switching, bias: +1.5 V.(c) B4P is switched at +2.0 V.(d) B4P after switching, bias: 0.5 V.

Fig.6 Models and STM simulation images of B2P with three isopropyl conformations.(a) Model and simulation image of first conformation in which two C-N-C-H dihedral angle are both 0°.(b) Model and simulation image of second conformation in which two C-N-C-H dihedral angle are 0° and 180°, respectively.(c) Model and simulation image of third conformation in which two C-N-C-H dihedral angle are 90° and 0°, respectively.

Therefore, the conformations are simulated as a function of the tilting angle of the nitrogen heterocycle.In all three conformations simulated in Fig.6, the structure in Fig.6a possesses the lowest energy.Hence subsequent simulation maintains restricts the isopropyl moiety in the conformation in Fig.6a.Fig.7 shows the stimulated results of different 6-oxoverdazyl ring conformations in the B4P model.The dihedral angle between BTBT and the nitrogen heterocycle is 0° (Fig.7a),+50° (Fig.7b) and -50° (Fig.7c), correspondingly.Once the nitrogen heterocycle is parallel to the Au(111) surface (Fig.7a),the brightness of each part in the radical appears similar.However, the tilted nitrogen heterocycle (Fig.7b and c) results in a large brightness difference in the STM images.The farthest part from the surface is brightest in the simulated STM image,which is in excellent agreement with the experimental STM image for the “T” conformation.Thus, the structures of the “P”and “T” conformations for B4P correspond to the modelled ones in Fig.7a and b, respectively.The DFT calculation results also show that the energy for the “T” conformation of B4P is about 0.23 eV lower than the “P” conformation, indicating that the former is more stable.The reason is likely to be that a steric hindrance exists between the nitrogen heterocycle and the BTBT in the “P” conformation.It echoes the experimental result that only the “P” conformation for B4P can be converted to the “T”one.However, the “P” conformation becomes more stable due to the absence of the steric hindrance in B2P.

4 Conclusions

The adsorption of two verdazyl radicals like B2P and B4P molecules on Au(111) are explored with STM.Experimental results show that adsorbed B2P exists in monomers, dimers,trimers and tetramers at surface.B4P forms dimers or assembled structures, depending on its coverage.Both B2P and B4P molecules adopt two conformations, namely, the “P”conformation where the nitrogen heterocycle lies parallel to the surface and the “T” one where the nitrogen heterocycle tilts against the surface.At the bias voltage of +2.0 V, the STM tip exerts a disturbance that converts the B2P molecule from “T” to“P” conformation or transforms the B4P from “P” to “T”conformation.For both molecules, such a conformational transition is irreversible.The main difference in the most stable conformation for both molecules originates from the steric hindrance of the BTBT group.The conformational change can be employed as the working principle to achieve molecular switches based on these molecules.

Fig.7 Models and STM simulation images of B4P with three 6-oxoverdazyl ring conformation.(a) Model and simulation image of first conformation in which the dihedral angle of 6-oxoverdazyl radical and BTBT is about 0°.(b) Model and simulation image of second conformation in which the dihedral angle of 6-oxoverdazyl radical and BTBT is about 50°.(c) Model and simulation image of third conformation in which the dihedral angle of 6-oxoverdazyl radical and BTBT is about -50°.