arrangement, while irradiation induces conversion to O-bound η
1 -OSO MS1 or sidebound η
2 -(O,SO) MS2 isomers similar to the nitrosyl linkage isomer species. The
first evidence for sulphur dioxide linkage isomerism was obtained in 1979 from
solution spectroscopy and proposed two distinct metastable isomers [37]. The
side-bound isomer was confirmed in 2002 from steady-state photocrystallographic
measurements on trans-[Ru(NH 3 ) 4 Cl(SO 2 )]Cl with 300–500 nm light at temperatures below 150 K [38, 39]. The MS1 isomer was confirmed by Bowes et al. in
2006 from photocrystallographic measurements on [Ru(NH 3 ) 4 (H 2 O)(SO 2 )]
[MeC 6 H 4 SO 3 ] 2 , which shows 36% conversion to the η
1 -OSO form on irradiation
at 13 K [40].
From these early studies, several other ruthenium sulphur dioxide compounds
have been designed and studied using photocrystallography to establish the effects
of the crystal environment on the isomerisation process. In particular, systematic
studies on [Ru(SO 2 )(NH 3 ) 4 X]Y, varying both the ligand X and the counter-anion Y,
have shown that the excited-state geometry is strongly influenced by the crystal
environment and the nature of the trans ligand [41–43]. More recent studies have
also demonstrated photoisomerisation in osmium SO 2 complexes [44].
2.4 Dinitrogen (N 2 ) Systems
Though comparatively less studied, photoinduced linkage isomerism can also occur
with dinitrogen (N 2 ) ligands. N 2 binds to transition metal centres in both mono- and
bidentate coordination geometries (c.f. Fig. 3), and the earliest reports of dinitrogen
coordination complexes date back to the 1960s [45]. A variety of complexes with
terminal and bridging N 2 ligands have since been investigated, with recent interest in
their use for catalytic nitrogen fixation [46–48].
Single-crystal dinitrogen linkage isomerism was first demonstrated in the osmium
complex [Os(NH 3 ) 5 (N 2 )](PF 6 ) 2 ] [49], for which photocrystallographic measurements showed that a maximum of 17.4% of the molecules in the crystal could be
converted from a ground-state “end-on” η
1 -N 2 arrangement to a side-bound η
2 -N,N
isomer by irradiation with 325 nm laser light at 100 K. The side-bound isomer was
confirmed to be metastable and found to persist for at least a day at this temperature.
2.5 Linkage Isomer Device Development
The design of linkage isomers with high conversion has been motivated by the
possibility of using them in molecular devices. For example, one could consider a
molecular data storage application similar to an optical disc where the state of a
binary digit (bit) is encoded by the majority isomer (ground state or excited state;
GS/ES) in small spots on a thin film. Bits are set by photoactivating a spot with a
laser, reset by localised heating and read with a suitable probe. To be able to write
208
L. E. Hatcher et al.
1 -OSO MS1 or sidebound η
2 -(O,SO) MS2 isomers similar to the nitrosyl linkage isomer species. The
first evidence for sulphur dioxide linkage isomerism was obtained in 1979 from
solution spectroscopy and proposed two distinct metastable isomers [37]. The
side-bound isomer was confirmed in 2002 from steady-state photocrystallographic
measurements on trans-[Ru(NH 3 ) 4 Cl(SO 2 )]Cl with 300–500 nm light at temperatures below 150 K [38, 39]. The MS1 isomer was confirmed by Bowes et al. in
2006 from photocrystallographic measurements on [Ru(NH 3 ) 4 (H 2 O)(SO 2 )]
[MeC 6 H 4 SO 3 ] 2 , which shows 36% conversion to the η
1 -OSO form on irradiation
at 13 K [40].
From these early studies, several other ruthenium sulphur dioxide compounds
have been designed and studied using photocrystallography to establish the effects
of the crystal environment on the isomerisation process. In particular, systematic
studies on [Ru(SO 2 )(NH 3 ) 4 X]Y, varying both the ligand X and the counter-anion Y,
have shown that the excited-state geometry is strongly influenced by the crystal
environment and the nature of the trans ligand [41–43]. More recent studies have
also demonstrated photoisomerisation in osmium SO 2 complexes [44].
2.4 Dinitrogen (N 2 ) Systems
Though comparatively less studied, photoinduced linkage isomerism can also occur
with dinitrogen (N 2 ) ligands. N 2 binds to transition metal centres in both mono- and
bidentate coordination geometries (c.f. Fig. 3), and the earliest reports of dinitrogen
coordination complexes date back to the 1960s [45]. A variety of complexes with
terminal and bridging N 2 ligands have since been investigated, with recent interest in
their use for catalytic nitrogen fixation [46–48].
Single-crystal dinitrogen linkage isomerism was first demonstrated in the osmium
complex [Os(NH 3 ) 5 (N 2 )](PF 6 ) 2 ] [49], for which photocrystallographic measurements showed that a maximum of 17.4% of the molecules in the crystal could be
converted from a ground-state “end-on” η
1 -N 2 arrangement to a side-bound η
2 -N,N
isomer by irradiation with 325 nm laser light at 100 K. The side-bound isomer was
confirmed to be metastable and found to persist for at least a day at this temperature.
2.5 Linkage Isomer Device Development
The design of linkage isomers with high conversion has been motivated by the
possibility of using them in molecular devices. For example, one could consider a
molecular data storage application similar to an optical disc where the state of a
binary digit (bit) is encoded by the majority isomer (ground state or excited state;
GS/ES) in small spots on a thin film. Bits are set by photoactivating a spot with a
laser, reset by localised heating and read with a suitable probe. To be able to write
208
L. E. Hatcher et al.