Q1(o). Photosensitised reactions

Answer in brief

Reactions in which the light is absorbed by another species, the photosensitiser, which passes the energy to the reactant and returns unchanged. Examples: Hg-sensitised dissociation of H₂; photosynthesis (chlorophyll).

Key points

• The reactant A does not absorb the light; the photosensitiser S does: S + hν → S*, S* + A → S + A*, A* → products.
• Mercury: Hg (6¹S₀) + hν (253.7 nm) → Hg* (6³P₁); Hg* + H₂ → Hg + 2H; 471.5 kJ mol⁻¹ > D(H–H) = 436 kJ mol⁻¹.
• Photosynthesis: chlorophyll absorbs red (≈ 680 nm) and blue light; CO₂ and H₂O do not absorb visible light.
• Singlet oxygen: rose bengal or methylene blue (triplet) + ³O₂ → dye + ¹O₂; dyes also sensitise AgBr in photography.