🌟 What Is Fluorescence?

Fluorescence is a type of luminescence, meaning light emission that does not come from heat. Instead, it comes from how molecules interact with light at the electronic level.

Historically:

• In 1845, Frederick William Herschel observed a blue glow from quinine when illuminated with light.
• Later, Georg Gabriel Stokes discovered that once light had passed through one quinine solution, it could no longer make another glow.
• This led to the realization that light carries discrete packets of energy that can be “used up” — a key idea later explained by quantum mechanics.

⚛️ Molecular Orbitals Refresher (Organic Chemistry Core)

Molecules contain electrons in molecular orbitals, arranged by energy:

• Low-energy orbitals: σ (sigma)
• Higher-energy orbitals: π (pi)
• Antibonding orbitals: σ*, π*
• Non-bonding orbitals: sometimes in between

Key terms:

• HOMO → Highest Occupied Molecular Orbital
• LUMO → Lowest Unoccupied Molecular Orbital

Electrons always fill the lowest energy orbitals first.

🌈 How Light Interacts with Molecules

Light is electromagnetic radiation, often written as:

E = h u

Where:

• E = energy
• h = Planck’s constant
• ν = frequency

If light has exactly the right energy, it can:

• Be absorbed
• Promote an electron from HOMO → LUMO

This creates an excited electronic state.

⏱️ Excited States and Energy Release

An excited molecule is unstable and must relax back to equilibrium.

Two possible outcomes:

1. Emission of light → fluorescence ✨
2. Release of heat → no light (non-radiative decay)

Only molecules where light emission competes successfully with heat loss are fluorescent molecules.

🧪 Example: Quinine (Tonic Water Glow)

Observations:

• Absorbs UV light (~300–350 nm)
• Emits blue light (~440–490 nm)
• Quinine solutions appear colorless in visible light

⚠️ This raises a puzzle:

If absorbed and emitted light involve the same electron transition, why are the wavelengths different?

🔄 The Stokes Shift

The difference between:

• Absorption wavelength
• Emission wavelength

is called the Stokes shift.

Since energy must be conserved, this means some energy is lost before emission — but where?

📊 Absorption vs Emission Spectra (Perylene Example)

Key features:

• Multiple absorption peaks
• Multiple emission peaks

This cannot be explained by electronic states alone.

📐 Jablonski Diagram: The Full Picture

A Jablonski diagram (named after a Polish physicist) shows:

• Electronic states:
  • Ground state (S₀)
  • First excited state (S₁)
• Vibrational sub-levels within each electronic state

⚠️ Energy increases upwards, even if the axis is not drawn.

🔧 Vibrational States: The Missing Piece

Important insight:

• Electronic excitation happens in femtoseconds
• Atoms cannot move that fast

So:

• An electron is promoted
• Bond strength changes
• But bond length cannot adjust instantly

This creates vibrational excitation.

Key consequences:

• Excitation goes to excited electronic + excited vibrational states
• Vibrational energy spacings are much smaller than electronic ones

⚡ Relaxation Pathways (Rates Matter!)

Relaxation happens in this order:

1. Vibrational relaxation (very fast)
   • Energy lost as heat to the surroundings
2. Electronic relaxation
   • Can emit light (fluorescence)

➡️ Result:

• Emission always comes from the lowest vibrational level of the excited state
• Emitted light has lower energy (longer wavelength) than absorbed light

✔️ This explains the Stokes shift

🎶 Why Spectra Have Multiple Peaks

• Absorption excites different vibrational levels
• Emission can end in different vibrational levels of the ground state

This creates:

• Multiple absorption peaks
• Multiple emission peaks

🚫 Why Some Molecules Do NOT Fluoresce

Some molecules lose energy by:

• Radiationless transitions (heat only)

Shown in diagrams as wavy arrows.

Fluorescence depends on competition between rates:

• Radiative (light-emitting) vs non-radiative (heat)

Rules of thumb:

• Similar rates → weak fluorescence
• Faster radiative rate → high fluorescence yield

🧠 Big Picture Summary

✔ Fluorescence is quantum-mechanical ✔ Light excites electrons (HOMO → LUMO) ✔ Vibrational relaxation causes energy loss ✔ Emitted light has lower energy (Stokes shift) ✔ Fluorescence depends on relative rate constants