Orbital Diagram:Excited States - Online Tutor, Practice Problems & Exam Prep

Conjugated molecules can absorb light energy, exciting electrons to higher energy states, which alters their Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO). For example, in 1,3-butadiene, irradiation promotes an electron from the HOMO (ψ₂) to a new HOMO (ψ₃), while the LUMO shifts from ψ₃ to ψ₄. This manipulation of orbitals is crucial for understanding reactivity and the implications of light in chemical processes.

Ground vs. Excited States

Hey guys, in this video we're going to discuss a very unique phenomenon that happens when a conjugated molecule is exposed to light. Let's get started. So guys, conjugated molecules have the ability to absorb light energy and convert that light energy into mechanical energy by kicking electrons up to a higher energy state. And when this happens, that means that your HOMO and LUMO orbitals are going to change because now you have electrons resting in different orbitals than they initially were in their ground state. And this actually will have big implications on reactivity, but we're going to get to that later because right now you don't even really know how HOMO and LUMO act, but later when we talk more about how HOMO and LUMO react with each other, the fact that you can excite electrons to a new HOMO and LUMO really will be important. Okay.

So here's an example. 1,3-Butadiene is irradiated with photons, exciting an electron up to a higher energy molecular orbital. So photons are light; you're shining the light and we're exciting an electron, okay. Predict the identity of the HOMO and LUMO orbitals after irradiation. So what we would start off with in the ground state, guys, is just 4 π electrons, right, because you have 2 here, you have 2 here, 4 π electrons, and I would fill them in order of principle. 1, 2, 3, 4 and I already have the HOMO and LUMO written out for us. HOMO (highest occupied) is ψ₂ , LUMO (lowest unoccupied) is ψ₃ and that's what we're used to. This is what the molecule looks like in its ground state. But if you shine enough light on this molecule, what's going to wind up happening is that it absorbs 1 of the photons and it's going to basically convert that energy into an electron being kicked up to a higher energy state. So what it's going to look like after irradiation is this. So let me just get out of the way here. These 2 electrons stay the same, but now I only have 1 electron here and now I have 1 electron here. Isn't that interesting? Where basically what we just did is we just took one of the electrons and moved it up here. Now I know that before I had an up spin and a down spin and now I'm just making it 2 up spins, that doesn't matter. Don't worry about whether it's up spin or down spin, that you don't need to worry about that for any of predicting HOMO and LUMO. All you need to worry about is which orbital it's in. So that means that which orbital is now the HOMO? The HOMO is now ψ₃ and which orbital is now the LUMO? The LUMO is now ψ₄ . So, guys, this is very important because this means that we can use light to manipulate which orbitals act as a HOMO and which act as a LUMO. Cool? Awesome. So that's it for this video. Let's move to the next one.

4-Methylbenzylidene camphor (4-MBC) is used by the cosmetic industry for its ability to protect the skin against UV-B radiation. Circle the part of the molecule that you theorize is responsible for its effects on UV light.

4-Methylbenzolidine camphor, or also known as 4-MBC, is used by the cosmetic industry for its ability to protect the skin against UVB radiation. This is an additive that's found in some makeup and sunscreen. If you look at the back of your products, you may find 4-MBC, and this is the structure. So, circle the part of the molecule that you theorize might be responsible for its effects in protecting human skin from UV light. So guys, why do you think that the cosmetic industry might like this molecule to be used as an additive to their cosmetics? What is it about this molecule that might actually have an effect on UV radiation? And the part that I am going to go ahead and circle is the conjugated part because notice guys, we have this whole conjugated section where I have a conjugated benzene ring, conjugated with a double bond and conjugated to a carbonyl. This entire area is all conjugated, so when UV light interacts with it, it can actually absorb those photons and kick up electrons into a higher energy state and basically prevent those photons and that UV radiation from negatively impacting the skin. So, what I would do is I would just circle this part right here. This is the conjugated portion that they would strategically use to make their cosmetics a little better and a little safer for human use. Wonderful guys, so we're done with this practice problem. Let's move on.

Here’s what students ask on this topic:

What happens to the HOMO and LUMO orbitals when a conjugated molecule absorbs light?

When a conjugated molecule absorbs light, it excites electrons to higher energy states. This changes the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO). For example, in 1,3-butadiene, irradiation promotes an electron from the HOMO (ψ2) to a new HOMO (ψ3), while the LUMO shifts from ψ3 to ψ4. This manipulation of orbitals is crucial for understanding reactivity and the implications of light in chemical processes.

How does light irradiation affect the reactivity of conjugated molecules?

Light irradiation affects the reactivity of conjugated molecules by altering their HOMO and LUMO orbitals. When electrons are excited to higher energy states, the HOMO and LUMO change, which can significantly impact how these molecules react. For instance, in 1,3-butadiene, irradiation shifts the HOMO from ψ2 to ψ3 and the LUMO from ψ3 to ψ4. This change in orbital configuration can influence the molecule's reactivity in various chemical reactions.

Can you explain the concept of HOMO and LUMO in the context of excited states?

HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) are key concepts in molecular orbital theory. In the context of excited states, when a molecule absorbs light, electrons are promoted from the HOMO to higher energy orbitals, creating a new HOMO and LUMO. For example, in 1,3-butadiene, irradiation promotes an electron from ψ2 (HOMO) to ψ3, making ψ3 the new HOMO and ψ4 the new LUMO. This shift is essential for understanding the molecule's reactivity and behavior under light exposure.

What is the significance of manipulating HOMO and LUMO orbitals using light?

Manipulating HOMO and LUMO orbitals using light is significant because it allows chemists to control the reactivity and properties of molecules. By exciting electrons to higher energy states, the HOMO and LUMO can be altered, which can change how the molecule interacts in chemical reactions. For instance, in 1,3-butadiene, light irradiation shifts the HOMO from ψ2 to ψ3 and the LUMO from ψ3 to ψ4. This ability to control orbital configurations is crucial for applications in photochemistry and materials science.

How does the excitation of electrons in 1,3-butadiene illustrate the concept of excited states?

The excitation of electrons in 1,3-butadiene illustrates the concept of excited states by showing how light energy can promote electrons to higher energy orbitals. In its ground state, 1,3-butadiene has its HOMO at ψ2 and LUMO at ψ3. When irradiated with light, an electron is excited from ψ2 to ψ3, making ψ3 the new HOMO and ψ4 the new LUMO. This shift demonstrates how excited states are formed and how they can alter the molecule's electronic structure and reactivity.