Kartenstapel

- Excited singlet state may reach lower energy triplet state through the influence of spin-orbital coupling.

- Direct transmission into triplet state is "forbidden" (Wigner's rule).

- Triplet state chemistry may differ from singlet state chemistry!

Typical Sensitizers: Ketones with ET (S) > ET (R)

Benzophenone

Acetone

Requirement: ET (R) > ET (Q) (Q should have low triplet energy)

Application: ensure that photoreaction proceeds via singlet state

O2

LP: 254 nm

MP: 254, 313, 366, 405, 436, 546 nm

HP: continuous >280 nm

Reaction rate [mol h-1] = Einstein h-1 absorbed × F

• Absorption of light by photoactive compound, based on its extinction coefficient ε, at a given wavelengths l, e.g. an emission line of the chosen light source, as a function of the path length and concentration.

High dilution necessary for strongly absorbing compounds (high e).

Problem: chlorination gives di-, tri- and tetrachlorinated compounds

Relative reactivity: CH3 (1) < CH2 (3.9) < CH (5.2)

Photochlorination best if all Hydrogens in alkene are equivalent