Organic photochemistry

Early examples were often uncovered by the observation of precipitates or color changes from samples that were exposed to sunlights.

The first reported case was by Ciamician that sunlight converted santonin to a yellow photoproduct:[3] An early example of a precipitate was the photodimerization of anthracene, characterized by Yulii Fedorovich Fritzsche and confirmed by Elbs.

[6][7] Illustrative, these rules help rationalize the photochemically driven electrocyclic ring-closure of hexa-2,4-diene, which proceeds in a disrotatory fashion.

[8][9] Parallel to the structural studies described above, the role of spin multiplicity – singlet vs triplet – on reactivity was evaluated.

[11] In general, it is necessary to bring the reactants close to the light source in order to obtain the highest possible luminous efficacy.

For this purpose, the reaction mixture can be irradiated either directly or in a flow-through side arm of a reactor with a suitable light source.

In addition to the radiation, light sources generate plenty of heat, which in turn requires cooling energy.

[16] In case of a stirred tank reactor, the lamp (generally shaped as an elongated cylinder) is provided with a cooling jacket and placed in the reaction solution.

One objective in the design of reactors is therefore to determine the economically most favorable dimensioning with regard to an optimization of the quantum current density.

[18] Quite parallel to the santonin to lumisantonin example is the rearrangement of 4,4-diphenylcyclohexadienone[9] Here the n-pi* triplet excited state undergoes the same beta-beta bonding.

In photoredox catalysis, the photon is absorbed by a sensitizer (antenna molecule or ion) which then effects redox reactions on the organic substrate.