3.3.1 Studies with Monochromatic X-Ray Radiation
Pump-probe X-ray experiments on molecular species, using monochromatic radiation rather than Laue techniques, were first described in 2001. Techert et al. reported
a time-resolved study of transient changes in N,N-dimethylaminonitrile (DMABN)
using stroboscopic X-ray powder diffraction (XRPD) methods [108]. Photoactivation
of DMABN at λ ¼ 267 nm resulted in an ultra-fast structural change as a result of the
electronic excitation, and relaxation back to the ground state occurs on the picosecond
timescale. The relaxation process was structurally analysed using synchrotron radiation with the beam being chopped mechanically and synchronised with light pulses
from a Ti:sapphire laser, with pump-probe delay times ranging from À240 to 1,500 ps,
allowing for several data points to be collected. The reflections that showed the
greatest intensity changes during the structural rearrangement were identified, and
the authors followed these intensity changes with increasing delay times. The data
showed a conversion level to the excited state of between 28 and 32% at short time
delays, which is consistent with spectroscopic data. Refinement of the powder data
using the Rietveld method showed that the relaxation occurred via a change in the
inversion angle of the molecule and a rotation of the methyl group (as shown in Fig. 6),
with a total structural relaxation time of 520 ps.
Coppens was among the first to take advantage of the higher intensity of
synchrotron X-ray radiation and in 2002 reported the first single-crystal X-ray
diffraction study of a species with a microsecond lifetime. The results of pumpprobe experiments on salts of the [Pt 2 (pop) 4 ]
4À anion (pop ¼ [H 2 P 2 O 5 ]
2À ) showed
that structural distortions in the anion are induced by photoactivation with 355 nm
light, producing a triplet excited state with a microsecond lifetime [83]. The key
change in the structure upon excitation was the shortening of the Pt–Pt bond which
has previously been proposed from electronic [109] and Raman spectroscopy [110]
and XAFS [111] studies, the latter suggesting a reduction in Pt–Pt bond length of
0.52 Å. The pump-probe time-resolved single-crystal X-ray study was carried out on
the tetra-tetraethylammonium salt of [Pt 2 (pop) 4 ]
4À at a temperature of 17 K and
33 μs wide light pulses from a Nd/YAG laser, at a repetition rate of 5,100 Hz, and
using the “light-on/light-off” data collection strategy. Analysis of the excited state
data showed a 2% level of excitation under the experimental conditions. Upon
refinement of the Pt positions, the rest of the structure being treated as a rigid
group, the Pt–Pt distance was found to shorten by 0.28(9) Å with a concomitant
rotation of 3
about the Pt–Pt vector. The Pt–Pt distance has subsequently been
confirmed by a second diffraction experiment [112] on the related salt
(n-Bu 4 N) 2 H 2 [Pt 2 (pop) 4 ] where the Pt–Pt reduction in length was recorded at
0.23 Å. More recently, further confirmation has come from scattering measurements
of an aqueous solution [113] and a time-resolved EXAFS study [114] that showed
the reduction in Pt–Pt distance from that found in the ground state structure were
0.24 Å and 0.31 Å, respectively.
Another substantial reduction in metal-metal bond length was reported for the Rh–
Rh bond length in [Rh 2 (dimen) 4 ][PF 6 ] 2 .MeCN (dimen ¼ 1,8-diisocyanomethane)
Time-Resolved Single-Crystal X-Ray Crystallography
255
Pump-probe X-ray experiments on molecular species, using monochromatic radiation rather than Laue techniques, were first described in 2001. Techert et al. reported
a time-resolved study of transient changes in N,N-dimethylaminonitrile (DMABN)
using stroboscopic X-ray powder diffraction (XRPD) methods [108]. Photoactivation
of DMABN at λ ¼ 267 nm resulted in an ultra-fast structural change as a result of the
electronic excitation, and relaxation back to the ground state occurs on the picosecond
timescale. The relaxation process was structurally analysed using synchrotron radiation with the beam being chopped mechanically and synchronised with light pulses
from a Ti:sapphire laser, with pump-probe delay times ranging from À240 to 1,500 ps,
allowing for several data points to be collected. The reflections that showed the
greatest intensity changes during the structural rearrangement were identified, and
the authors followed these intensity changes with increasing delay times. The data
showed a conversion level to the excited state of between 28 and 32% at short time
delays, which is consistent with spectroscopic data. Refinement of the powder data
using the Rietveld method showed that the relaxation occurred via a change in the
inversion angle of the molecule and a rotation of the methyl group (as shown in Fig. 6),
with a total structural relaxation time of 520 ps.
Coppens was among the first to take advantage of the higher intensity of
synchrotron X-ray radiation and in 2002 reported the first single-crystal X-ray
diffraction study of a species with a microsecond lifetime. The results of pumpprobe experiments on salts of the [Pt 2 (pop) 4 ]
4À anion (pop ¼ [H 2 P 2 O 5 ]
2À ) showed
that structural distortions in the anion are induced by photoactivation with 355 nm
light, producing a triplet excited state with a microsecond lifetime [83]. The key
change in the structure upon excitation was the shortening of the Pt–Pt bond which
has previously been proposed from electronic [109] and Raman spectroscopy [110]
and XAFS [111] studies, the latter suggesting a reduction in Pt–Pt bond length of
0.52 Å. The pump-probe time-resolved single-crystal X-ray study was carried out on
the tetra-tetraethylammonium salt of [Pt 2 (pop) 4 ]
4À at a temperature of 17 K and
33 μs wide light pulses from a Nd/YAG laser, at a repetition rate of 5,100 Hz, and
using the “light-on/light-off” data collection strategy. Analysis of the excited state
data showed a 2% level of excitation under the experimental conditions. Upon
refinement of the Pt positions, the rest of the structure being treated as a rigid
group, the Pt–Pt distance was found to shorten by 0.28(9) Å with a concomitant
rotation of 3
about the Pt–Pt vector. The Pt–Pt distance has subsequently been
confirmed by a second diffraction experiment [112] on the related salt
(n-Bu 4 N) 2 H 2 [Pt 2 (pop) 4 ] where the Pt–Pt reduction in length was recorded at
0.23 Å. More recently, further confirmation has come from scattering measurements
of an aqueous solution [113] and a time-resolved EXAFS study [114] that showed
the reduction in Pt–Pt distance from that found in the ground state structure were
0.24 Å and 0.31 Å, respectively.
Another substantial reduction in metal-metal bond length was reported for the Rh–
Rh bond length in [Rh 2 (dimen) 4 ][PF 6 ] 2 .MeCN (dimen ¼ 1,8-diisocyanomethane)
Time-Resolved Single-Crystal X-Ray Crystallography
255