have strong absorption in the visible light spectra, they have long excited state
lifetimes, and, importantly, they are stable under photolytic conditions. Photosensitizers based on Ru(II) and Ir(III) complexes with polypyridyl ligands have been
among the most extensively used in light-driven reactions [7, 8]. Photosensitizers
based on other transition metals such as Cu [9], Au [10], or Fe, [11] have also been
developed. Different organic chromophores have been also found to absorb light and
photocatalyze target reactions [12, 13].
The first reports on the use of a Ru-based complex as a photocatalyst appear in
1978 [14]. However, the renaissance of the photoredox catalysis applied on synthetic
organic chemistry starts in 2008 with two simultaneous publications. Both of these
works took advantage of the ability of [Ru(bpy) 3 ]Cl 2 (bpy ¼ 2,2’bipyridine) complex to absorb visible light photons. MacMillan’s group reported the combination of
a photocatalyst with an organocatalyst for the asymmetric alkylation of aldehydes
triggered by sunlight [15]. Yoon and coworkers reported a [2+2] enone cycloaddition activated by visible light [16].
It is noteworthy that despite the similarities between these two studies, an
important conceptual difference exists between them. The system reported by
Yoon needs a sacrificial reagent to complete the photocatalytic cycle; in the system
reported by MacMillan, an intermediate of the organocatalytic cycle regenerates the
ground state of the photocatalyst. Dual systems like that reported by MacMillan have
attracted huge interest in recent years [17]. Their defining characteristic is that a
photocatalyst is combined with another system which is not able to absorb light by
itself. The two systems react in a symbiotic manner merging the features of the two
individual processes, which results in a novel reactivity that cannot be accomplished
by either of the two systems acting independently [18].
Modern photochemistry usually relies on the ability of the photosensitizers to
react to carry out single electron transfer (SET) steps upon irradiation to the triplet
excited state. Single electron transfer from or to a substrate opens the door to novel
reactivity which is complementary to the more classical two-electron chemistry
[19, 20]. This approach involving photoinduced electron transfer (PET) is usually
called photoredox catalysis in the chemical literature. The excited state of a chromophore organic molecule or metal-based complex is both a stronger oxidant and
reductant species. Thus, upon irradiation, two photocatalytic cycles can exist, as
outlined in Fig. 1. If the initial electron transfer reduces the substrate, this pathway is
termed oxidative quenching because the photocatalyst evolves to an oxidized intermediate. Alternatively, if the first electron transfer is from the substrate to the excited
form of the photosensitizer, the pathway is called reductive quenching. In reductive
quenching, the substrate is oxidized while the photoredox catalyst is reduced. In both
cases, the photocatalyst is ultimately regenerated to its initial form by accepting or
donating an electron [21].
An alternative to photoinduced electron transfer is direct energy transfer. The
most common mechanism for this type of processes is the Dexter energy transfer
[22]. In Dexter energy transfer, the photosensitizer does not transfer any electron but
the excitation, as shown in Fig. 2. The key in this event is a nonradiative relaxation of
the photocatalyst synchronized with the generation of the excited state of the
Computational Modeling of Selected Photoactivated Processes
133
lifetimes, and, importantly, they are stable under photolytic conditions. Photosensitizers based on Ru(II) and Ir(III) complexes with polypyridyl ligands have been
among the most extensively used in light-driven reactions [7, 8]. Photosensitizers
based on other transition metals such as Cu [9], Au [10], or Fe, [11] have also been
developed. Different organic chromophores have been also found to absorb light and
photocatalyze target reactions [12, 13].
The first reports on the use of a Ru-based complex as a photocatalyst appear in
1978 [14]. However, the renaissance of the photoredox catalysis applied on synthetic
organic chemistry starts in 2008 with two simultaneous publications. Both of these
works took advantage of the ability of [Ru(bpy) 3 ]Cl 2 (bpy ¼ 2,2’bipyridine) complex to absorb visible light photons. MacMillan’s group reported the combination of
a photocatalyst with an organocatalyst for the asymmetric alkylation of aldehydes
triggered by sunlight [15]. Yoon and coworkers reported a [2+2] enone cycloaddition activated by visible light [16].
It is noteworthy that despite the similarities between these two studies, an
important conceptual difference exists between them. The system reported by
Yoon needs a sacrificial reagent to complete the photocatalytic cycle; in the system
reported by MacMillan, an intermediate of the organocatalytic cycle regenerates the
ground state of the photocatalyst. Dual systems like that reported by MacMillan have
attracted huge interest in recent years [17]. Their defining characteristic is that a
photocatalyst is combined with another system which is not able to absorb light by
itself. The two systems react in a symbiotic manner merging the features of the two
individual processes, which results in a novel reactivity that cannot be accomplished
by either of the two systems acting independently [18].
Modern photochemistry usually relies on the ability of the photosensitizers to
react to carry out single electron transfer (SET) steps upon irradiation to the triplet
excited state. Single electron transfer from or to a substrate opens the door to novel
reactivity which is complementary to the more classical two-electron chemistry
[19, 20]. This approach involving photoinduced electron transfer (PET) is usually
called photoredox catalysis in the chemical literature. The excited state of a chromophore organic molecule or metal-based complex is both a stronger oxidant and
reductant species. Thus, upon irradiation, two photocatalytic cycles can exist, as
outlined in Fig. 1. If the initial electron transfer reduces the substrate, this pathway is
termed oxidative quenching because the photocatalyst evolves to an oxidized intermediate. Alternatively, if the first electron transfer is from the substrate to the excited
form of the photosensitizer, the pathway is called reductive quenching. In reductive
quenching, the substrate is oxidized while the photoredox catalyst is reduced. In both
cases, the photocatalyst is ultimately regenerated to its initial form by accepting or
donating an electron [21].
An alternative to photoinduced electron transfer is direct energy transfer. The
most common mechanism for this type of processes is the Dexter energy transfer
[22]. In Dexter energy transfer, the photosensitizer does not transfer any electron but
the excitation, as shown in Fig. 2. The key in this event is a nonradiative relaxation of
the photocatalyst synchronized with the generation of the excited state of the
Computational Modeling of Selected Photoactivated Processes
133