Molecular Photochemistry Quiz

Module Structure

• Fundamentals of light
• Propagation of light in waveguides
• Light interaction with matter
• Lasers
• Photobiology basics
• Biophotonics applications
  • Bioimaging
  • Tissue engineering

Topics to be Covered

• Atoms and Molecules
• Molecular level interactions
  • Absorption, spontaneous emission, stimulated emission
  • Fate of excited molecules
  • Fluorescence
  • Light scattering
    • Rayleigh, Mie, Raman, spectroscopy

Introduction

• Light interaction with bulk matter includes reflection, refraction, and diffraction
• At a molecular level, time-varying electric fields of light interact with matter's electric charges and dipoles
• Forces cause charges and dipoles to accelerate
  • Rotational - microwaves or far infrared light
  • Vibrational - infrared light
  • Electronic - visible and ultraviolet light
• As energy increases (from microwaves to UV), more movements become possible, making spectra more complex
• Absorption and emission involve quantized electronic and vibrational states of molecules

Absorption

• Absorption is a transition from a lower energy state to a higher one, powered by a photon
• The energy difference between the states (En-Em) must equal the photon energy (hf)
• The photon is annihilated in the process
• If the photon energy doesn't match the energy difference, the photon might be re-radiated (e.g., in 10 femtoseconds) causing phenomena like Rayleigh scattering

Spontaneous Emission

• Spontaneous emission is a random process
• An excited state returns to a stable lower energy state by emitting a photon
• The phase and direction of the emitted photon are random and independent of other photons

Stimulated Emission

• A photon with energy equal to the energy gap triggers an excited state to return to a lower energy state, emitting a second photon
• The second photon has identical frequency, phase, direction, and polarization to the original photon
• This is how lasers work, with the original photon being retained

Potential Energy Curves

• Diagrams show ground and excited electronic energy states of a molecule
• Both have vibrational states (labeled v')
• Photon absorption generally starts from v"=0 ground state
• Photon emission generally starts from an excited electronic state with v'=0

Excited States of Molecules

• Molecules (or aggregates) enter excited states when absorbing photons
• Excited states are unstable and return to ground states as quickly as possible
• Energy release can be radiative or non-radiative
• Several processes compete for deactivation
• Excited states are denoted by A* and ground state by A

Excited States of Molecules - 2

• Photoinduced electron transfer
  • Photoionization (A* → A+ + e-)
  • Electron transfer (D* + A → D+ + A)
• Energy transfer (A* + B → A + B*)
• Excited state complexes
  • Excimer (A* + A → (A-A)*)
  • Exciplex (A* + B → (A-B)*)
• Various processes like state-to-state crossing (internal conversion), vibrational relaxation.

Photochemistry

• Photochemistry describes the absorption of light by molecules and resulting chemical reactions leading to stable compounds
• Excited molecules can release energy through photoassociation (A* + B → A-B) or photodecomposition (A* → B + C)
• These processes play roles in biology (e.g., photosynthesis, vision)

Photophysical Process

• Jablonski diagrams show energy transitions
• Singlet states (e.g., S0, S1, S2) with increasing state number, minimum energy increases
• So represents the ground state
• Short lines represent quantized vibrational states
• Straight lines represent transitions connected to photon absorption or emission
• Vertical 'squiggly' lines on Jablonski diagrams indicate vibrational relaxation

Vibrational Relaxation

• Energy loss through vibrational relaxation occurs on vertical 'squiggly' lines
• Unless between zero-point vibrational states, excess vibrational energy exists and is converted into translational energy through collisions (heating)
• Energy can also be lost through emission in the infrared range when collisions are infrequent

State-to-State Crossing

• Horizontal lines represent changes in quantum states without energy changes
• A non-radiative process is internal conversion (IC)
• Internal conversion is rapid and involves vibrational relaxation to the lowest vibrational level of the excited state (S1)

Tonic Water

• Quinine absorbs light at 250nm and 350nm
• Quinine emits light at 450nm

Fluorescence

• Fluorescence is the process where an excited singlet state (S1) returns to the singlet ground state (S0) by emitting a photon
• The absorption spectrum has a higher wavelength than fluorescence spectra
• Fluorescence is rapid (nanoseconds) and used in numerous applications, such as environmental monitoring, clinical chemistry, and DNA sequencing

Fluorescence - 2

• Initial photon absorption takes the molecule from the zero vibrational state of the ground state to an excited electronic state with vibrational energy
• Energy loss through collisions leads to vibrational relaxation
• The excited molecule returns to the ground state by emitting a photon, resulting in a fluorescence spectrum
• Vibrational states play significant roles in the absorption and emission spectra

Fluorescence - 3

• 0-0 absorption and fluorescence transitions can occur simultaneously (same wavelength)
• Emitted photon has lower energy than the absorbed photon, leading to a higher wavelength in the fluorescence spectrum
• Fluorescence is a rapid process, occurring within nanoseconds