Mario Pagliaro
Istituto per lo Studio dei Materiali Nanostrutturati, CNR, Palermo, Italy

“The main barrier to using photochemistry is the fear of photochemistry itself”, (1) commented Booker-Milburn five years after introducing the FEP photoreactor that opened the route to the practically feasible photocatalytic synthesis of fine chemicals (2).

Very few industrial synthetic processes are based on visible light photochemical processes including the low-cost synthesis of rose oxide, (3) and the production of the anthelmintic drug ascaridole via solar irradiation of alpha-terpinene carried out since 1943 (4). Even more, the high cost of light due to the poor efficiency of most electric light sources (until the advent of LEDs in the early 2000s) discouraged  chemical companies from introducing light photons as chemical reagents, no matter how clean they could be (5).

The fundamental limitation to synthetic organic photochemistry, indeed, resides in the logarithmic law governing light penetration in solution, thanks to which very little light is available beyond a very short path length off the lamp irradiating the solution. For example, in a 0.05 M solution of a compound with modest extinction coefficient, 90% of the incident light i ...