Fluorescence and Phosphorescence

The Energy Well

Going down to ground state means going down in NRG and is more stable. PE increases from the electrostatic force

Quantized

Certain discrete magnitudes (light is quantized as packets of energy)

Pauli Exclusion Principle

2 e- cannot have the same four quantum numbers

• orbital contains n,l,m

• e- spin: ms = ½ (up) and -½ (down)

• most of the time it’s focused on how the spin is the same and that is forbidden

Absorption

UV or Vis strikes a molecule and just the right amount of NRG will move it up to an excited state (from either ground → excited or excited → excited

Vibrational Structure

Each (different orbital) vibrational state has a ground excited state and multiple excited states and many rotational states. This causes a broad range where photons can be absorbed and e- can sit.

Fluorescence

Photon must be absorbed and a photon must be emitted. Fast process

Stockpiles

These happen in ground vibrational or ground excited states where most of the e- stay and require some type of NRG to move up or down. Massive _____ in the ground state.

Internal Conversion

Uses heat or some other type of NRG to move an e-. Predominant way of moving between states

Excited State Lifetimes vs. Ground Vibrational Lifetimes

Happens in the pico realm (10^-12) vs. happening in the nano realm (10^-9)

Vibrational Relaxation

Happens due to the excited state lifetime not lasting long, and the e- travels down from each excited state until it hits the ground excited state

Stokes Shift

The difference in the wavelength at which a molecule emits light is relative to the wavelength at which the molecule was excited

Phosphorescence

Photon must be absorbed and a photon must be emitted. Slow process that happens due to the forbiddenness of phosphorescence

Intersystem Crossing

e- undergoes vibrational relaxation and then is converted to an excited triplet state. This happens by inverting the spin of the excited state e-. This means the e- has the same spin and it’s counterpart, but in different orbitals, so the Pauli Exclusion Principle is not violated.

Forbidden

Can happen at a very low probability event and is a very slow process

Excited Singlet vs Excited Triplet

Excited Triplet state is a lower NRG state than the excited singlet state

Chromophores

Very common functional groups that absorb light, absorb UV and Vis

Fluorophores

An uncommon functional group that fluoresces

Relating Quantum Yield to Stockpile Competition

Slower rates of internal conversion (knr) mean that the rate of fluor. may be able to win the kinetic battle and emit fluorescence

Structure vs. knr

rigid planar structures have slow internal conversions

Phosp. quantum yield

kp is really small, so the knr needs to be extremely small. longer lifetime due to the slow kinetics. Happens a lot with solids in cryogenic states—cold decreases knr

Excited state lifetime

Time between ex. and em.

Fluor→ most time spent in stockpile state, em. of photon main player

Phos→time spent is some other more significant steps, forbidden states