the lattice for the ligand to switch between one form and the other, and
intermolecular interactions should either not be particularly strong or be flexible
perhaps through a change in temperature [88, 96]. The accessible volume within the
lattice and the flexibility of the lattice under experimental conditions relates back to
the concept of the reaction cavity first proposed by Cohen [72] and then exemplified
by Ohashi who showed subsequently that the reaction cavity could flex during the
course of the reaction and that changes in temperature could significantly affect the
process [97]. For example, lowering the temperature would cause lattice contraction,
reducing the cavity size, and might thus “switch off” the reaction.
Light-induced excited spin-state trapping (LIESST) studies map the low spinhigh spin interconversions in transition metal complexes with d
4 -d
7 electronic
configurations. The spin crossover phenomenon, with the associated change in
magnetic properties, is usually activated thermally with the high spin (HS) to low
spin (LS) interconversion occurring on cooling the complex to below some critical
temperature. A significant number of these complexes can then undergo a
photoactivated conversion from the LS state to a metastable HS state, via a longlived triplet excited state. The photoinduced phenomenon was first identified in a
study of [Fe(1-ptopyltetrazole) 6 ][BF 4 ] 2 using Mössbauer spectroscopy [98]. The
phenomenon is observable in both solution and in the solid state.
Single-crystal crystallographic and powder diffraction studies of the LIESST
phenomenon soon followed [99–105], taking advantage of cryoscopic advances
that allowed crystals to be relatively easily cooled below liquid nitrogen temperatures; many LIESST transitions occurring below 80 K. Since the early 2000s, the
topic has continued to develop with a range of techniques being used to analyse the
LIESST phenomenon [13, 106]. Of additional interest are LIESST complexes that
display reverse switching from their photoinduced HS state back to a low temperature LS state which is induced via both temperature changes and “reverse-LIESST”
processes, involving further irradiation of the excited LIESST state with a different
excitation wavelength [107]. This two-way switching using different wavelengths of
light has potential applications, making these metastable state species potential
candidates for photoswitchable molecular devices.
3.3 Time-Resolved Molecular Photocrystallographic Studies
For successful pump-probe molecular photocrystallographic studies, a material in
which the molecules can undergo a fast, fully reversible switching process is
required, and there should only be a small change in unit cell dimensions during
the process. The robustness of the crystal under light and X-ray radiation is of
primary importance if sufficient data is to be obtained and the excited state structure
solved to atomic resolution.
254
P. R. Raithby
intermolecular interactions should either not be particularly strong or be flexible
perhaps through a change in temperature [88, 96]. The accessible volume within the
lattice and the flexibility of the lattice under experimental conditions relates back to
the concept of the reaction cavity first proposed by Cohen [72] and then exemplified
by Ohashi who showed subsequently that the reaction cavity could flex during the
course of the reaction and that changes in temperature could significantly affect the
process [97]. For example, lowering the temperature would cause lattice contraction,
reducing the cavity size, and might thus “switch off” the reaction.
Light-induced excited spin-state trapping (LIESST) studies map the low spinhigh spin interconversions in transition metal complexes with d
4 -d
7 electronic
configurations. The spin crossover phenomenon, with the associated change in
magnetic properties, is usually activated thermally with the high spin (HS) to low
spin (LS) interconversion occurring on cooling the complex to below some critical
temperature. A significant number of these complexes can then undergo a
photoactivated conversion from the LS state to a metastable HS state, via a longlived triplet excited state. The photoinduced phenomenon was first identified in a
study of [Fe(1-ptopyltetrazole) 6 ][BF 4 ] 2 using Mössbauer spectroscopy [98]. The
phenomenon is observable in both solution and in the solid state.
Single-crystal crystallographic and powder diffraction studies of the LIESST
phenomenon soon followed [99–105], taking advantage of cryoscopic advances
that allowed crystals to be relatively easily cooled below liquid nitrogen temperatures; many LIESST transitions occurring below 80 K. Since the early 2000s, the
topic has continued to develop with a range of techniques being used to analyse the
LIESST phenomenon [13, 106]. Of additional interest are LIESST complexes that
display reverse switching from their photoinduced HS state back to a low temperature LS state which is induced via both temperature changes and “reverse-LIESST”
processes, involving further irradiation of the excited LIESST state with a different
excitation wavelength [107]. This two-way switching using different wavelengths of
light has potential applications, making these metastable state species potential
candidates for photoswitchable molecular devices.
3.3 Time-Resolved Molecular Photocrystallographic Studies
For successful pump-probe molecular photocrystallographic studies, a material in
which the molecules can undergo a fast, fully reversible switching process is
required, and there should only be a small change in unit cell dimensions during
the process. The robustness of the crystal under light and X-ray radiation is of
primary importance if sufficient data is to be obtained and the excited state structure
solved to atomic resolution.
254
P. R. Raithby