PHOTOCHEMISTRY. What for?
Photochemistry is a powerful tool in the field of organic synthesis: this approach allows to perform chemical processes usually forbidden under thermal conditions by changing the properties of the molecules upon light absorption.
Nowadays, photochemistry is gaining more and more importance in the field of “Green Chemistry”: actually, light can be considered a clean reagent since it activates the substrates, leaving no residues in the reaction mixture. This allows for a simple workup and purification of the products. Thus, photochemistry appears as a valid alternative in the frame of sustainable development.
The research carried out in our laboratories focuses on two different targets. First, we try to devise innovative and new photochemical reactions and set up the best overall conditions to make them interesting for synthetic applications. Second, these syntheses are supported by mechanistic studies, where a deeper investigation of the reaction mechanism is conducted. This last target is fulfilled by means of both steady state spectroscopy (Absorption, Fluorescence and Phosphorescence), as well as by time resolved spectroscopy (Laser Flash Photolysis, Single Photon Counter), and by computational tools (softwares: Gaussian, Gamess, ADF and Molcas).
Photochemistry in our lab
Photochemistry is often considered a different discipline with respect to traditional (thermal) chemistry. Actually, photochemical and thermal reactions employ two different approaches towards the same goal. This sharp distinction is usually seen as a barrier and makes photochemistry a rarely investigated subject (both in the academic field and in the industry). Despite its high synthetic potential (demonstrated by several examples present in the literature, e.g., A. Albini, M. Fagnoni Eds., “Handbook of Synthetic Photochemistry”, 2009, Wiley-VCH, Weinheim), organic photochemistry is far from being fully exploited, except by a small number of research groups.
Kessil lamp and set up
This short section aims to describe a photochemical laboratory from the practical point of view, showing the main instruments that are usually employed, and attempting to clarify what “making” photochemistry means and to bring this discipline closer to the generality of chemists.
Obviously, the main characteristic of a photochemical reaction is the use of light. In most cases, we use LED-based light sources, emitting in specific regions of the spectrum, ranging from the near UV to the visible. Our laboratory uses high-power Kessil LED lamps for most of our synthetic applications, as appreciated from the pictures.
In the past, we were used to adopting light sources emitting in the UV region, and quartz tubes were thus required.
Old lamps still available in our lab
From top to bottom, a low-pressure mercury lamp (emission at 254 nm), two “phosphor-coated” lamps (emission at 310 and 360 nm, respectively), a medium-pressure mercury lamp (used for immersion reactors), and a high-pressure mercury lamp.
The different kinds of light sources can be composed according to the required experimental conditions, having recourse to:
Multi-lamp apparatus
Immersion reactor
Vessels on the window led
Sometimes, there is no need to use the UV radiation, and the reactions can be conveniently carried out by using sunlight (which emits mainly in the visible part of the spectrum), and the glass reaction vessels can be exposed on the window ledge.
SolarBox in our lab
The solar radiation, however, is intermittent and varies according to the geographical position and the weather conditions. To overcome this drawback, we can use a solar simulator, which reproduces the solar emission employing a Xenon lamp.
A thesis in our lab is like a typical thesis on organic chemistry. It is possible, however, to focus on the synthetic, mechanistic, or applicative aspects of the processes.