ground-state NO isomer, an excited-state ON isomer and a second side-bound
excited-state isomer (Fig. 2), with the different metastable isomers accessed by
varying the irradiation wavelength. The combination of SCXRD and in situ photoexcitation was coined “photocrystallography” and is now commonly used to
describe this research area.
In this chapter, we will review the historical development and current state of the
art in photocrystallography in the context of linkage isomers. We will start from
early photocrystallographic experiments to identify metastable species and proceed
to discuss the development of low-temperature dynamic SCXRD experiments to
probe the isomerisation kinetics. We will then finish with a discussion of how
modern synchrotron facilities and photon-counting detectors allow the structures
of species with shorter lifetimes to be determined.
This chapter is organised as follows. In Sect. 2 we will give a brief overview of
the major classes of linkage isomer systems and some of the key research studies in
each area, and in Sect. 3, we will introduce the photocrystallography experiments
used to study them. Finally, in Sect. 4 we will discuss the main considerations for
designing time-resolved SCXRD experiments to follow the isomerisation processes
in real time, leading to recent developments towards sub-second SCXRD experiments on linkage isomers.
2 Linkage Isomer Systems
Several families of linkage isomer systems have been identified with ligands including NO 2
À (nitrite), NO (nitrosyl), sulphoxide (SO 2 ) and (di-)nitrogen (N 2 ). Some of
the known binding configurations of these ligands in transition metal complexes are
summarised in Fig. 3.
In most of these systems, the complexes crystallise with the ligand in its energetic
ground-state geometry, and can be excited into one or more metastable configurations by photoactivation. Metastable isomers typically have long lifetimes at cryogenic temperatures and are thus kinetically trapped, but decay rapidly back to the
ground state on warming to room temperature.
2.1 Nitrite (NO 2
2 ) Systems
Following the discovery of [Co(NH 3 ) 5 (NO 2 )]
2+ in the 1850s [5], nitrites remain one
of the best studied classes of linkage isomer materials to date. Though a number of
coordination geometries are theoretically possible for the nitrite ligand [6], in
practice the monodentate nitro (η
1 -NO 2 ), endo-nitrito (η
1 -ONO) and exo-nitrito
(η
1 -ONO) arrangements are the most commonly observed in molecular crystals.
Photochemical conversion between the nitro and nitrito isomers of [Co
(NH 3 ) 5 (NO 2 )]
2+ in solution was first reported in the 1940s [7], while solid-state
measurements were first performed on microcrystalline powders in the 1970s. As
Watching Photochemistry Happen: Recent Developments in Dynamic Single-Crystal. . .
203
excited-state isomer (Fig. 2), with the different metastable isomers accessed by
varying the irradiation wavelength. The combination of SCXRD and in situ photoexcitation was coined “photocrystallography” and is now commonly used to
describe this research area.
In this chapter, we will review the historical development and current state of the
art in photocrystallography in the context of linkage isomers. We will start from
early photocrystallographic experiments to identify metastable species and proceed
to discuss the development of low-temperature dynamic SCXRD experiments to
probe the isomerisation kinetics. We will then finish with a discussion of how
modern synchrotron facilities and photon-counting detectors allow the structures
of species with shorter lifetimes to be determined.
This chapter is organised as follows. In Sect. 2 we will give a brief overview of
the major classes of linkage isomer systems and some of the key research studies in
each area, and in Sect. 3, we will introduce the photocrystallography experiments
used to study them. Finally, in Sect. 4 we will discuss the main considerations for
designing time-resolved SCXRD experiments to follow the isomerisation processes
in real time, leading to recent developments towards sub-second SCXRD experiments on linkage isomers.
2 Linkage Isomer Systems
Several families of linkage isomer systems have been identified with ligands including NO 2
À (nitrite), NO (nitrosyl), sulphoxide (SO 2 ) and (di-)nitrogen (N 2 ). Some of
the known binding configurations of these ligands in transition metal complexes are
summarised in Fig. 3.
In most of these systems, the complexes crystallise with the ligand in its energetic
ground-state geometry, and can be excited into one or more metastable configurations by photoactivation. Metastable isomers typically have long lifetimes at cryogenic temperatures and are thus kinetically trapped, but decay rapidly back to the
ground state on warming to room temperature.
2.1 Nitrite (NO 2
2 ) Systems
Following the discovery of [Co(NH 3 ) 5 (NO 2 )]
2+ in the 1850s [5], nitrites remain one
of the best studied classes of linkage isomer materials to date. Though a number of
coordination geometries are theoretically possible for the nitrite ligand [6], in
practice the monodentate nitro (η
1 -NO 2 ), endo-nitrito (η
1 -ONO) and exo-nitrito
(η
1 -ONO) arrangements are the most commonly observed in molecular crystals.
Photochemical conversion between the nitro and nitrito isomers of [Co
(NH 3 ) 5 (NO 2 )]
2+ in solution was first reported in the 1940s [7], while solid-state
measurements were first performed on microcrystalline powders in the 1970s. As
Watching Photochemistry Happen: Recent Developments in Dynamic Single-Crystal. . .
203