Topics in Current Chemistry (2018) 376:45
1 3
1 Introduction
The sun constitutes the most sustainable light source available for photochemistry. However, the use of solar light in photochemistry comes with significant
hurdles associated with its polychromatic nature and fluctuating ground intensity.
These limitations, coupled with problematic access to natural sunlight in a laboratory setting and the safety issues connected with the use of chemicals outdoors,
constitute significant entry barriers to the chemists willing to step into the solar
photochemistry arena.
Notably, none of those restraints are of a fundamental nature, therefore, technical and technological solutions can be devised to avoid or circumvent these
issues. For example, the commercial availability of artificial lamps that accurately
mimic the characteristics of solar light (the so-called “solar simulators” originally
developed for photovoltaics testing [1]) can significantly ease lab investigations
on photochemical reactions at high photon fluxes. Similarly, to efficiently use the
solar photons and to maintain control over the reaction temperature, several solar
photoreactor designs have been developed. In this regard, flow chemistry imposes
itself as the ideal solution to efficiently deliver the solar photons to the reaction
medium. Flow setups allow for an easier integration of solar collectors, devices
often adopted to compensate for the relatively low intensity of the solar irradiance. Furthermore, the adoption of flow also comes with additional benefits in
terms of high heat transfer, thanks to the higher surface-to-volume ratio, and simple interface with analytical instruments for both reaction control and automation. It is a natural consequence that the majority of photoreactors specifically
designed for solar application are continuous-flow reactors.
In this chapter, the applications of continuous-flow chemistry to organic solar
photochemistry will be described. The first section offers a brief historical perspective and a description of the characteristic of the solar irradiance relevant to
the chemist. After that, the different solar photoreactor designs will be presented
with their characteristics and applications. Finally, given the evident sustainability premises of the field [2], an outlook on the future role of solar photochemistry in the context of a general trend towards greener chemistry solutions will be
provided.
2 Historical Perspective
At the beginning of photochemistry, the sun was the only light source available
[3–5]. Due to its abundance and ease of access, solar light endured as a prime
light source for photochemical reactions from the earlier pioneers of the nineteenth century until the beginning of the twentieth century. With the introduction
of increasingly cheaper and more powerful artificial light sources, however, the
preference of chemists rapidly changed. By 1968, the list of light sources available to photochemists included in the second edition of Schönberg’s “preparative
2
Reprinted from the journal
1 3
1 Introduction
The sun constitutes the most sustainable light source available for photochemistry. However, the use of solar light in photochemistry comes with significant
hurdles associated with its polychromatic nature and fluctuating ground intensity.
These limitations, coupled with problematic access to natural sunlight in a laboratory setting and the safety issues connected with the use of chemicals outdoors,
constitute significant entry barriers to the chemists willing to step into the solar
photochemistry arena.
Notably, none of those restraints are of a fundamental nature, therefore, technical and technological solutions can be devised to avoid or circumvent these
issues. For example, the commercial availability of artificial lamps that accurately
mimic the characteristics of solar light (the so-called “solar simulators” originally
developed for photovoltaics testing [1]) can significantly ease lab investigations
on photochemical reactions at high photon fluxes. Similarly, to efficiently use the
solar photons and to maintain control over the reaction temperature, several solar
photoreactor designs have been developed. In this regard, flow chemistry imposes
itself as the ideal solution to efficiently deliver the solar photons to the reaction
medium. Flow setups allow for an easier integration of solar collectors, devices
often adopted to compensate for the relatively low intensity of the solar irradiance. Furthermore, the adoption of flow also comes with additional benefits in
terms of high heat transfer, thanks to the higher surface-to-volume ratio, and simple interface with analytical instruments for both reaction control and automation. It is a natural consequence that the majority of photoreactors specifically
designed for solar application are continuous-flow reactors.
In this chapter, the applications of continuous-flow chemistry to organic solar
photochemistry will be described. The first section offers a brief historical perspective and a description of the characteristic of the solar irradiance relevant to
the chemist. After that, the different solar photoreactor designs will be presented
with their characteristics and applications. Finally, given the evident sustainability premises of the field [2], an outlook on the future role of solar photochemistry in the context of a general trend towards greener chemistry solutions will be
provided.
2 Historical Perspective
At the beginning of photochemistry, the sun was the only light source available
[3–5]. Due to its abundance and ease of access, solar light endured as a prime
light source for photochemical reactions from the earlier pioneers of the nineteenth century until the beginning of the twentieth century. With the introduction
of increasingly cheaper and more powerful artificial light sources, however, the
preference of chemists rapidly changed. By 1968, the list of light sources available to photochemists included in the second edition of Schönberg’s “preparative
2
Reprinted from the journal