1. Photophysical Properties of Molecules

Photophysical properties describe how molecules interact with light by absorbing, emitting, and dissipating energy. These interactions are key in spectroscopy, optoelectronics, and biological imaging.

Key Processes:

• Absorption: Photon promotes electron to excited state.
• Fluorescence: Fast emission from singlet excited state (S₁ → S₀).
• Phosphorescence: Slow emission from triplet state (T₁ → S₀).
• Internal Conversion: Non-radiative transition between states of same spin.
• Intersystem Crossing: Transition between singlet and triplet states.
• Non-Radiative Decay: Energy lost as heat without light emission.

Quantitative Parameters:

• Quantum Yield (Φ): Efficiency of photon emission.
• Lifetime (τ): Duration molecule remains excited.
• Stokes Shift: Energy/wavelength difference between absorption and emission maxima.

2. Molecular Orbital (MO) Diagrams

Sample 1 (Without Quencher)

• Ground State: Electron in HOMO.
• Excitation: Electron promoted to LUMO.
• Fluorescence: Electron returns to HOMO, emitting a photon.

Result: Strong fluorescence.

Dyad (Sample 1 + Covalently Linked Quencher)

• Same ground state as Sample 1.
• After excitation, electron transfers from donor’s LUMO to acceptor’s LUMO.
• Forms charge-separated state: Donor⁺ — Acceptor⁻.
• Competes with fluorescence, reducing its intensity and lifetime.

Result: Weaker fluorescence due to charge transfer dominance.

3. Jablonski Diagrams

Sample 1

• Excitation: S₀ → S₁ upon light absorption.
• Relaxation:
  • Vibrational relaxation to S₁ minimum.
  • Fluorescence: S₁ → S₀ radiative decay.

Result: Efficient fluorescence with no competing pathways.

Dyad

• Excitation: Donor excited from S₀ to S₁.
• Competing Processes:
  • Charge Transfer (CT) from donor to acceptor suppresses fluorescence.
  • Fluorescence partially occurs but with reduced intensity and faster decay.
• Possible Transitions:
  • S₁ → CT state (non-radiative)
  • CT → S₀ (slow recombination or quenching)

Result: Partially suppressed fluorescence with faster decay.

4. Conclusion

Without a quencher, molecules show strong fluorescence due to direct radiative relaxation. In the dyad system, fluorescence is diminished because of electron transfer to the acceptor, forming a charge-separated state. These principles are critical in designing photophysical systems for technologies like sensors, photovoltaics, and imaging.