2.2 Nitrosyl (NO) Systems
The seminal study that helped establish the field of photocrystallography was the
work on the iron-nitrosyl complex sodium nitroprusside (SNP; Fig. 2) [1]. The 1997
study by Coppens et al. confirmed the presence of two unexpectedly long-lived
metastable states in SNP, previously identified by Mössbauer spectroscopy [23], and
assigned them as photogenerated linkage isomers. Irradiation of a single crystal at
50 K with 488 nm light produces a 37% conversion from the ground-state η 1 -NO
isomer to the inverted η 1 -ON isomer (MS1). Further irradiating the partially
converted crystal at 1064 nm further induces conversion to 10% of a second sidebound η 2 -NO isomer (MS2).
SNP has since been used as a model system to explore the electronic landscape of
the photochemical linkage isomerisation in the solid state, using a variety of structural, theoretical, spectroscopic and thermal analysis methods to explore the relationships between the three isomers [22, 24–26]. It is now believed that irradiation with
visible light causes initial conversion to the side-bound MS2 form, which then
converts to the O-bound MS1 isomer under prolonged exposure. The subsequent
excitation at 1,064 nm then causes depopulation of MS1, which decays via MS2.
Other reported transition metal-nitrosyl linkage isomers investigated using
photocrystallography include systems based on Rh [27], Ni [28] and Ru. Early work
by Coppens in 1996 investigated the photogenerated MS1 and MS2 isomers in [Ru
(NO 2 ) 4 (OH)(NO)] [29], which was later followed by studies on ruthenium porphyrin
nitrosyls [30]. In parallel, Schaniel and Woike demonstrated photoactivation in trans[RuCl(py) 4 (NO)][PF 6 ] 2 .0.5H 2 O using infrared (IR) spectroscopy [31], and subsequent
photocrystallographic studies confirmed a 92% conversion to MS1 at 80 K on
irradiation with 673 nm laser light, followed by 48% conversion to MS2 at 980 nm
[32]. These successes have led to a series of related complexes with different halides,
pyridine ligands and counterions, enabling the influence of the surrounding crystalline
environment on the isomerisation to be established [19].
Another interesting example is the [Ru(NO 2 )(bipy) 2 (NO)] (bipy ¼ 2,2
0 -bipyridine)
complex, for which photoactivation of single crystals facilitates a double isonitrosylnitrito linkage isomerisation involving the intramolecular transfer of an oxygen atom
between the NO 2 and NO ligands [33]. This highlights the potential for competing
photo-release reactions, although in practice these are more likely to be observed in
solution [34] and/or in studies conducted at, or close to, room temperature. NO release
has been investigated in solid-state nitrosyl materials [35, 36], but is not typically
observed in nitrite and sulphur dioxide systems.
2.3 Sulphur Dioxide (SO 2 ) Systems
Sulphur dioxide (SO 2 ) linkage isomerism has been demonstrated both in solution
and in the single crystal. The ground-state structure is typically an S-bound
Watching Photochemistry Happen: Recent Developments in Dynamic Single-Crystal. . .
207
The seminal study that helped establish the field of photocrystallography was the
work on the iron-nitrosyl complex sodium nitroprusside (SNP; Fig. 2) [1]. The 1997
study by Coppens et al. confirmed the presence of two unexpectedly long-lived
metastable states in SNP, previously identified by Mössbauer spectroscopy [23], and
assigned them as photogenerated linkage isomers. Irradiation of a single crystal at
50 K with 488 nm light produces a 37% conversion from the ground-state η 1 -NO
isomer to the inverted η 1 -ON isomer (MS1). Further irradiating the partially
converted crystal at 1064 nm further induces conversion to 10% of a second sidebound η 2 -NO isomer (MS2).
SNP has since been used as a model system to explore the electronic landscape of
the photochemical linkage isomerisation in the solid state, using a variety of structural, theoretical, spectroscopic and thermal analysis methods to explore the relationships between the three isomers [22, 24–26]. It is now believed that irradiation with
visible light causes initial conversion to the side-bound MS2 form, which then
converts to the O-bound MS1 isomer under prolonged exposure. The subsequent
excitation at 1,064 nm then causes depopulation of MS1, which decays via MS2.
Other reported transition metal-nitrosyl linkage isomers investigated using
photocrystallography include systems based on Rh [27], Ni [28] and Ru. Early work
by Coppens in 1996 investigated the photogenerated MS1 and MS2 isomers in [Ru
(NO 2 ) 4 (OH)(NO)] [29], which was later followed by studies on ruthenium porphyrin
nitrosyls [30]. In parallel, Schaniel and Woike demonstrated photoactivation in trans[RuCl(py) 4 (NO)][PF 6 ] 2 .0.5H 2 O using infrared (IR) spectroscopy [31], and subsequent
photocrystallographic studies confirmed a 92% conversion to MS1 at 80 K on
irradiation with 673 nm laser light, followed by 48% conversion to MS2 at 980 nm
[32]. These successes have led to a series of related complexes with different halides,
pyridine ligands and counterions, enabling the influence of the surrounding crystalline
environment on the isomerisation to be established [19].
Another interesting example is the [Ru(NO 2 )(bipy) 2 (NO)] (bipy ¼ 2,2
0 -bipyridine)
complex, for which photoactivation of single crystals facilitates a double isonitrosylnitrito linkage isomerisation involving the intramolecular transfer of an oxygen atom
between the NO 2 and NO ligands [33]. This highlights the potential for competing
photo-release reactions, although in practice these are more likely to be observed in
solution [34] and/or in studies conducted at, or close to, room temperature. NO release
has been investigated in solid-state nitrosyl materials [35, 36], but is not typically
observed in nitrite and sulphur dioxide systems.
2.3 Sulphur Dioxide (SO 2 ) Systems
Sulphur dioxide (SO 2 ) linkage isomerism has been demonstrated both in solution
and in the single crystal. The ground-state structure is typically an S-bound
Watching Photochemistry Happen: Recent Developments in Dynamic Single-Crystal. . .
207