Multi-Orbital Charge Transfer into Nonplanar Cycloarenes Revealed with CO-Functionalized Tips

This study combines CO-functionalized tip STM simulations with orbital tomography to reveal multi-orbital charge transfer from the Cu(110) surface into nonplanar kekulene and isokekulene molecules, validating a robust method for characterizing complex adsorbed systems with low yields.

The Gist

The Big Picture: Building Molecular Lego on a Trampoline

Imagine you are trying to build a very specific, intricate shape out of Lego bricks. In the world of chemistry, scientists often build these shapes (molecules) directly on a metal surface, like a trampoline. Sometimes, the trampoline changes the shape of the Lego structure, or the structure changes the trampoline.

In this study, scientists were building two very similar, ring-shaped molecules: Kekulene (which is flat, like a pancake) and Isokekulene (which is wobbly and non-flat, like a crumpled piece of paper). They built these on two different types of "trampolines" (copper surfaces): a smooth one called Cu(111) and a slightly rougher one called Cu(110).

The Mystery: Why Do the Pictures Look Weird?

The scientists used a super-powerful microscope called a Scanning Tunneling Microscope (STM). To get a really sharp picture, they put a tiny carbon monoxide (CO) molecule on the tip of their microscope, like putting a fine brush on a paintbrush.

When they looked at the molecules on the rougher copper surface (Cu(110)), they saw something strange. The images didn't just show the shape of the molecule; they showed extra "glow" or complex patterns.

• The Analogy: Imagine taking a photo of a car at night. You expect to see the shape of the car. But instead, you see the car's shape plus a weird, glowing aura around it. The scientists knew this "aura" wasn't just the shape; it was caused by electricity (electrons) moving between the copper trampoline and the molecule. But they didn't know exactly how much electricity was moving or where it was going.

The Investigation: Two Different Detective Tools

To solve the mystery of this "glow," the team used two different detective tools:

1. The "Crowd Photo" (POT/ARPES)
First, they used a technique called Photoemission Orbital Tomography (POT).

• The Analogy: Imagine trying to figure out what a crowd of people is wearing by taking a single, wide-angle photo of the whole stadium. You can see the general colors and patterns of the whole group, but you can't see individual faces.
• What it told them: This method confirmed that the molecules were indeed absorbing extra electrons from the copper surface. It also confirmed that on the rougher copper, the scientists had successfully built almost entirely the "wobbly" Isokekulene molecules, not the flat Kekulene ones.

2. The "Flashlight" (STM with CO tips)
Next, they went back to their high-powered microscope to look at single molecules one by one.

• The Analogy: This is like walking up to a single person in that crowd and shining a flashlight on them to see exactly what they are wearing.
• The Problem: The "glow" (the extra electrons) was so strong and mixed up that it was hard to tell which specific part of the molecule was holding the extra electricity. It was like trying to hear a single instrument in a loud orchestra.

The Solution: The "Digital Recipe"

Since the microscope pictures were a mix of many different things, the scientists created a digital recipe to decode them.

1. The Ingredients: They used computer simulations (DFT) to calculate what the "empty" parts of the molecule's energy levels looked like.
2. The Mixing: They realized the "glow" wasn't just one thing. It was a mixture of several different energy levels (orbitals) that had become partially filled with electrons from the copper.
3. The Simulation: They built a computer model that mixed these different energy levels together, weighting them based on how much electron density the copper was actually giving them.

The Result:
When they compared their "mixed recipe" simulation to the actual microscope photos, it was a perfect match!

• The Discovery: They proved that the copper surface was dumping a significant amount of extra electrons into the molecules. It wasn't just filling one bucket; it was filling up several different "buckets" (orbitals) at the same time.

The Twist: One Molecule Was Tricky

While the method worked perfectly for the flat Kekulene and the "upside-down" wobbly Isokekulene, it struggled with the "right-side-up" wobbly Isokekulene.

• The Analogy: Imagine you have a recipe for a cake that tastes perfect every time, except for one specific version where the cake keeps collapsing in the middle. You know the ingredients are right, but the shape of the pan (the geometry) must be slightly wrong in your recipe.
• What it means: The computer simulation predicted the molecule should sit in a certain spot on the copper, but the actual microscope photo showed it sitting slightly differently. The "recipe" (the simulation) needed a tweak to match reality. This told the scientists that their computer models need to be more precise about exactly how these wobbly molecules sit on the metal.

Summary

• What they did: They studied how electrons move between a copper surface and special ring-shaped molecules.
• How they did it: They combined a super-microscope (that sees single molecules) with a "crowd photo" technique and advanced computer simulations.
• What they found: The copper surface gives extra electrons to these molecules, filling up multiple empty spots at once.
• Why it matters: They created a new way to "decode" these complex microscope images. This method works even when the molecules are wobbly, stick tightly to the surface, or are very hard to make in large quantities. It helps scientists understand exactly how these tiny structures behave when they touch metal.

Technical Summary

Technical Summary: Multi-Orbital Charge Transfer into Nonplanar Cycloarenes Revealed with CO-Functionalized Tips

Problem Statement
On-surface synthesis has enabled the creation of diverse molecular systems, including the highly selective synthesis of kekulene and its nonplanar isomer, isokekulene, on copper surfaces. While scanning tunneling microscopy (STM) with CO-functionalized tips can resolve the geometric structure of these molecules at low coverage, images of isokekulene and kekulene on Cu(110) exhibit complex features near the Fermi energy. These features suggest strong molecule-surface interactions and potential charge transfer, but their specific electronic origin remains unclear. Furthermore, standard area-integrating techniques like photoemission orbital tomography (POT) are limited to ordered monolayers and cannot easily resolve single-molecule properties or distinguish between different nonplanar adsorption configurations (e.g., "up" vs. "down" isokekulene) when they coexist.

Methodology
The authors employed a combined experimental and theoretical approach to investigate the electronic structure of kekulene and isokekulene on Cu(110):

1. Experimental Imaging:

   • STM: Constant-height STM images were recorded using CO-functionalized tips at low bias voltages (near the Fermi energy) for single molecules at low coverage and for full monolayers.
   • ARPES/POT: Angle-resolved photoemission spectroscopy (ARPES) and photoemission orbital tomography (POT) were performed on full monolayers to measure momentum maps ($k$-maps) and confirm charge transfer and adsorption geometry at the macroscopic scale.

2. Theoretical Modeling:

   • DFT Calculations: Density Functional Theory (DFT) was used to calculate relaxed adsorption geometries and the Molecular Orbital Projected Density of States (MOPDOS) for kekulene and isokekulene in various configurations (up/down) and adsorption sites (long bridge, hollow) on Cu(110).
   • STM Simulation: Simulated STM images were constructed by calculating the modulus squared of the lateral gradient of the wave function (simulating a $p$-wave CO tip). Crucially, these simulations were not based on single orbitals but on a weighted sum of multiple molecular orbitals (LUMO through LUMO+3). The weights were derived directly from the calculated MOPDOS to reflect the occupancy of formerly unoccupied orbitals due to charge transfer.
   • Iterative Refinement: To test the accuracy of the theoretical predictions, specific orbitals were manually removed from the weighted sum in the simulations to see if the agreement with experimental images improved.

Key Results

1. Charge Transfer Confirmation: Both experimental STM and POT data, supported by DFT, confirm a significant charge transfer from the Cu(110) surface into multiple formerly unoccupied molecular orbitals (LUMO to LUMO+3) of kekulene and isokekulene. This results in a strong hybridization between molecular and substrate states.
2. Orbital Deconvolution via Weighted Sum: The study demonstrates that single constant-height STM images of these strongly interacting systems cannot be explained by a single molecular orbital. Instead, the complex contrast patterns are reproduced only by simulating a weighted sum of multiple orbitals, where the weights correspond to the calculated MOPDOS.
3. Validation and Discrepancy:
   • For kekulene and isokekulene (up configuration), the weighted sum simulation (using MOPDOS-derived weights) qualitatively matches the experimental STM images.
   • For isokekulene (down configuration), the initial simulation showed discrepancies (specifically, excessive contrast in the central pore). Removing the LUMO+2 orbital from the weighted sum improved the agreement, suggesting that the theoretical MOPDOS may overestimate the occupation or hybridization of this specific orbital.
4. Adsorption Geometry Insights:
   • POT analysis of the full monolayer confirmed that the film consists mainly of isokekulene occupying the long bridge site, consistent with single-molecule STM findings.
   • However, a persistent mismatch between the simulated and experimental STM images for the isokekulene (down) configuration suggests that the DFT-calculated adsorption geometry (specifically the degree of bending or the exact adsorption site) may not be fully accurate, as the simulated images remained more bent than the experimental reality.
5. Differentiation of Isomers: The area-integrating POT technique successfully distinguished between planar kekulene and nonplanar isokekulene in the monolayer, confirming the high selectivity of the synthesis.

Significance and Claims
The paper claims to provide an experimental confirmation of multi-orbital charge transfer into nonplanar cycloarenes on strongly interacting surfaces. The primary contribution is the development of an STM-based approach that utilizes weighted sums of theoretical orbitals (based on MOPDOS) to interpret complex electronic features in single-molecule images.

The authors state that this method is applicable to a wide range of adsorbed molecular systems, particularly those that are:

• Strongly interacting with the substrate.
• Nonplanar.
• Prepared at extremely low yields (where single-molecule analysis is necessary, and area-integrating methods like ARPES are insufficient).

The study highlights that while POT is powerful for monolayers, the proposed STM simulation approach offers a necessary tool for resolving the electronic structure of individual molecules and identifying discrepancies between theoretical predictions (geometry and orbital occupation) and experimental reality.