4.4 Photochemical Reactions

Photochemical reactions in coordination compounds involve light absorption, leading to excited states and electronic transitions. These reactions can result in luminescence, non-radiative decay, or chemical changes like ligand dissociation and isomerization.

Factors like spectral overlap, solvent choice, and reaction conditions affect photochemical efficiency. Applications include synthesis of novel compounds, photodynamic therapy, photocatalysis, and development of light-emitting devices and photochromic materials.

Principles of Photochemical Reactions

Absorption of Light and Electronic Transitions

Top images from around the web for Absorption of Light and Electronic Transitions

• 

  Crystal Field Theory | Introduction to Chemistry View original

  Is this image relevant?

• 

  9: Coordination Chemistry I - Structure and Isomers - Chemistry LibreTexts View original

  Is this image relevant?

• 

  19.3 Spectroscopic and Magnetic Properties of Coordination Compounds – Chemistry View original

  Is this image relevant?

• 

  Crystal Field Theory | Introduction to Chemistry View original

  Is this image relevant?

• 

  9: Coordination Chemistry I - Structure and Isomers - Chemistry LibreTexts View original

  Is this image relevant?

1 of 3

Top images from around the web for Absorption of Light and Electronic Transitions

• 

  Crystal Field Theory | Introduction to Chemistry View original

  Is this image relevant?

• 

  9: Coordination Chemistry I - Structure and Isomers - Chemistry LibreTexts View original

  Is this image relevant?

• 

  19.3 Spectroscopic and Magnetic Properties of Coordination Compounds – Chemistry View original

  Is this image relevant?

• 

  Crystal Field Theory | Introduction to Chemistry View original

  Is this image relevant?

• 

  9: Coordination Chemistry I - Structure and Isomers - Chemistry LibreTexts View original

  Is this image relevant?

1 of 3

• Photochemical reactions in coordination compounds involve the absorption of light by the complex, leading to electronic transitions and excited states
• The absorption of light can lead to various types of transitions:
  1. Ligand field transitions (d-d transitions)
  2. Charge transfer transitions (metal-to-ligand or ligand-to-metal)
  3. Intraligand transitions (π-π* or n-π*)

Excited State Processes and Quantum Yield

• The excited state of the coordination compound can undergo various processes:
  1. Luminescence (fluorescence or phosphorescence)
  2. Non-radiative decay
  3. Photochemical reactions (ligand dissociation, isomerization, or redox reactions)
• The quantum yield of a photochemical reaction is the ratio of the number of molecules undergoing the desired reaction to the number of photons absorbed
• The mechanism of photochemical reactions can involve the formation of reactive intermediates, such as radicals or radical ions, through electron transfer or homolytic bond cleavage
• The excited state of the coordination compound can also engage in energy transfer processes, such as sensitization or quenching, with other molecules

Factors Affecting Photochemical Efficiency

Spectral Overlap and Solvent Effects

• The efficiency of photochemical reactions depends on the overlap between the absorption spectrum of the coordination compound and the emission spectrum of the light source
• The choice of solvent can affect the efficiency of photochemical reactions by influencing:
  1. The stability of the excited state
  2. The formation of reactive intermediates
  3. The rates of competing processes
• The presence of quenchers (oxygen or electron acceptors) can decrease the efficiency of photochemical reactions by deactivating the excited state or trapping reactive intermediates

Wavelength, Photosensitizers, and Reaction Conditions

• The selectivity of photochemical reactions can be controlled by the wavelength of the light source, as different electronic transitions can be selectively excited
• The use of photosensitizers or photocatalysts can enhance the efficiency and selectivity of photochemical reactions by facilitating energy or electron transfer processes
• The temperature and pressure can affect the efficiency and selectivity of photochemical reactions by influencing the rates of competing processes or the stability of reactive intermediates

Applications of Photochemical Reactions

Synthesis and Photodynamic Therapy

• Photochemical reactions are used in the synthesis of novel coordination compounds with unique properties, such as light-responsive materials or photoswitchable complexes
• Photodynamic therapy (PDT) employs photochemical reactions of coordination compounds (porphyrins or ruthenium complexes) for the selective destruction of cancer cells or pathogenic microorganisms

Photocatalysis and Light-Emitting Devices

• Photocatalysis using coordination compounds (titanium dioxide or ruthenium complexes) is applied in environmental remediation, such as water splitting, CO2 reduction, or pollutant degradation
• Photochemical reactions of coordination compounds are used in the development of light-emitting devices, such as organic light-emitting diodes (OLEDs) or light-emitting electrochemical cells (LECs)

Photochromic Materials and Biological Processes

• Photochromic materials based on coordination compounds (diarylethenes or spiropyrans) find applications in smart windows, optical data storage, or molecular switches
• Photochemical reactions of coordination compounds are employed in the study of biological processes, such as photosynthesis, vision, or DNA damage, using model systems or biomimetic approaches