Noah Manz

Noah Manz

Oct 15, 2018

Group 6 Copy 311
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Lab Note 2; 10-15-18

Thank you all so much for your support! It is so exciting to feel like I'm really moving forward with, what was before, just an idea. Now, I'm seeing not only the experiment, but hopefully, my career in materials science come together! Fun stuff!

There’s a Sherlock Holmes quote which roughly conveys, “you shouldn’t postulate before you’ve observed”. In my case, I’ve adopted this to mean, “do your research before buying things”. To this end, I’ve backed up my ≈4 years of Graphene study in the last couple of months by taking the time to delve into some new topics- specifically, symmetry in molecules. I promise, it’s as dry as it sounds…

As a recap, there’s this idea that any quantized particle may be described sometimes in terms of a wave, and sometimes in terms of a particle (i.e duality). An equation called the wave-function then describes the probability of an electron being found at certain spacial coordinates at a specific time. When combined with the Schrodinger equation, what you obtain are discrete “solutions” corresponding to the available energy states of the electron which imply geometric orbital confines- that is, the energy of the electron describes the where it will be physically found.

The important take-away here is that electron energy and orbital geometry are very closely related- so much in fact, they imply each other. When electronic excited states of a molecule are formed, electron density at specific spacial coordinates are probable to change- the results, are electron densities (orbitals) which can be defined in terms of Cartesian symmetry. That is, I can define a certain axis and certain symmetry operations, and with these, I can give you an idea about what different orbitals looks like.

When we talk about a molecule (being defined as a “group” of atoms), the individual orbitals of the atoms interact to form large, delocalized regions of high electron density around the molecule. These domains are referred to as molecular orbitals (as opposed to atomic orbitals). Keeping in mind that geometries of atomic (and indeed molecular) orbitals are sensitive to electron energy, I will further point out that physical geometries (as opposed to electronic geometries) are sensitive to the shapes of the orbitals. Therefore, it can be extrapolated that the physical geometry of a molecule is sensitive to the energy of an electron. Absorption of a photon during photolysis stimulates changes in electron energy, and therefore, stimulate changes in the physical shape of a molecule. If you look at the attached photo, you’ll see that as the homolysis “reaction” progresses, the associated electronic geometries change too (the red and blue pictorials).

One important thing to realize in the case of aromatics is that the photochemistry of these compounds (at least in UV) predicates upon excitation of π-molecular orbitals- of which there are 6. Transitions between these orbitals are evident from analysis of UV-VIS spectra, and yet, excitation of aromatics at similar wavelengths probes cleavage of C-X bonds (X=H,Cl,Br,F,I). Being that the ⍺-C bonds from a Benzene ring are σ-bonds (that is, NOT part of the π system), one may ask how excitation of π electrons can stimulate homolysis of said C-X bonds. So did I.

While there is still room for interpretation here, my [rough] thoughts are as follows. Upon electronic geometry change of an aromatic from one-photon absorption, the first triplet excited state 3B1u is formed. Vibronic-electronic exchange known as intersystem crossing allows for the otherwise spin forbidden ground—>first-singlet state to form. Second photon absorption excites the now spin paired electron to a disassociate state (most likely, 1E1u) which implies a geometry such that spacial overlap occurs between the 1E1u state (comprised of 1a2u, 1e1g and 1e2u symmetries) and C-X σ-bonds- keeping in mind still that this σ-bonds nomenclature is really only for geometric purposes. These bonds exist as delocalized molecular orbitals like π electrons, only at a significantly higher energy. Anyways, the overlap that now exists between π excited states and σ-bond domain leads to something called “energy level splitting” due to spacial interaction between two orbitals. I will admit to more conceptual understanding needed here- will get back to you on this. This splitting is such that, in the absence of compensating vibrionic-electronic exchange, the energy associated with σ-bond electrons is insufficient to satisfy the decrease in stability induced by orbital overlap (overlap occurring from excitation of π electrons). Thus, homolysis occurs.

A couple quick notes with this; if the above is the mechanism for C-X bond cleavage, then a couple statements must hold true. First, spacial overlap between excited π orbitals and σ-bonds in single aromatic monomers must also occur with the π systems of graphitic carbon. That is, geometry of the molecular orbitals must be such that upon two photon excitation, overlap of σ-bond and excited π states is indiscriminate (or at least insignificant enough) between a single monomer, and a large graphitic system. Second, the energy of electrons comprising the single bonds in aromatic rings must be of sufficient energy that energy level splitting which occurs from π excitation does not cleave in-plane C-C bonds.

Anyways, I’ll leave it there for the moment- that’s a lot to digest. I am headed to Farmington for the weekend to pick up some supplies and run a few experiments- will let you all know how it turns out!


Best,


Noah


3 comments

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  • Jennifer Thompson
    Jennifer ThompsonBacker
    Great job! Thanks for the wonderful update!
    Oct 15, 2018
  • Jean Manz
    Jean ManzBacker
    Good to see how your thinking is coming along-- had a dream last night that you were holding a 5 pound hunk of graphene, seriously did! Let's take that as a great premonition :).
    Oct 15, 2018
  • George Sharpe
    George SharpeBacker
    Noah, That is an electronic mouthful. Glad smart guys like you are working on the micro crap of the future! Saw your folks this weekend. Pretty cool couple.
    Oct 15, 2018

About This Project

Langmuir Blodgett monolayers of polyhalogenated benzene will be polymerized in-situ by photon mediated sigma—>sigma* transitions. The resultants are large conjugated systems which exhibit Graphene characteristics as the molar ratio of Carbon to Hydrogen approaches infinity. I hypothesize that photolysis and polymerization of these monolayers could provide a means to synthesizing large surface area, homogenous Graphene sheets.

Blast off!

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