Electronic transitions are the backbone of photochemistry, determining how molecules interact with light. These transitions follow specific rules that dictate their likelihood and intensity, shaping the behavior of excited molecules.

Understanding electronic transitions is crucial for predicting and explaining photochemical reactions. From singlet-singlet to charge transfer transitions, each type has unique characteristics that influence absorption spectra and excited state properties.

Electronic Transitions in Photochemistry

Selection rules for electronic transitions

• Selection rules govern allowed and forbidden electronic transitions
  • Spin selection rule determines transitions based on spin multiplicity
    • Allowed transitions occur between states with same spin multiplicity
    • Forbidden transitions occur between states with different spin multiplicity
  • Laporte selection rule applies to centrosymmetric molecules (octahedral complexes)
    • Allowed transitions occur between orbitals of different parity
    • Forbidden transitions occur between orbitals of same parity
• Allowed transitions exhibit higher probability and stronger absorption intensity (π-π*, n-π* in non-centrosymmetric molecules)
• Forbidden transitions show lower probability and weaker absorption intensity (n-π in centrosymmetric molecules, singlet-triplet transitions)

Singlet-singlet vs singlet-triplet transitions

• Singlet-singlet transitions occur between states with same spin multiplicity
  • Allowed by spin selection rule, fast process (picosecond timescale)
  • Higher absorption intensity (S0 → S1 transition)
• Singlet-triplet transitions occur between states with different spin multiplicity
  • Forbidden by spin selection rule, slow process (microsecond to millisecond timescale)
  • Lower absorption intensity (S0 → T1 transition)
• Intersystem crossing facilitates singlet-triplet transitions
  • Enhanced by spin-orbit coupling, more prominent in molecules with heavy atoms (iodine, bromine)

Characteristics of n-π and π-π transitions

• n-π transitions involve promotion of non-bonding electron to antibonding π orbital
  • Occur in molecules with lone pairs (carbonyls, amines)
  • Lower energy transition, weak absorption intensity
  • Appear as shoulder peaks in absorption spectra
  • Sensitive to solvent polarity (blue shift in polar solvents)
• π-π transitions involve promotion of π-bonding electron to antibonding π orbital
  • Occur in molecules with conjugated systems (benzene, polyenes)
  • Higher energy transition, strong absorption intensity
  • Appear as well-defined peaks in absorption spectra
  • Less sensitive to solvent polarity than n-π
• Comparison of n-π* and π-π* transitions
  • Relative energies: $E(π-π*) > E(n-π*)$
  • Absorption intensities: $ε(π-π*) > ε(n-π*)$
  • Solvatochromic effects: n-π* more pronounced than π-π*

Electronic transitions in inorganic complexes

• d-d transitions occur between d orbitals in transition metal complexes
  • Laporte-forbidden in centrosymmetric complexes, weak absorption intensity
  • Responsible for colors of many transition metal complexes (copper sulfate)
• Charge transfer transitions involve electron movement between metal and ligand
  • Metal-to-ligand charge transfer (MLCT): electron moves from metal d orbital to ligand π orbital (ruthenium bipyridine complexes)
  • Ligand-to-metal charge transfer (LMCT): electron moves from ligand to metal d orbital (permanganate ion)
• Intraligand transitions occur within the ligands of the complex
  • Similar to organic molecule transitions (π-π*, n-π*)
• f-f transitions occur in lanthanide and actinide complexes
  • Involve f orbitals, very weak absorption due to Laporte and spin forbidden nature
• Vibronic coupling relaxes selection rules
  • Allows some forbidden transitions to gain intensity through molecular vibrations