[Pd(Bu 4 dien)(NO 2 )]BPh 4 system from the fit in Fig. 9b [59]. Rather strikingly, the
excited-state lifetime varies by nine orders of magnitude, from ~10
8 s at 175 K to
100 ms at 325 K. This range of timescales may be compared to the time required to
collect a full X-ray dataset. For a well-diffracting crystal, a typical laboratory X-ray
system with a microfocus or rotating anode source and a CCD detector might take in
the region of 1–2 h to collect a full dataset and could therefore reasonably access
decay measurements up to ~230 K as in Fig. 10. A high-flux synchrotron source, on
the other hand, may take only a few minutes to collect a dataset, extending the range
of temperatures that can be measured up to around 250 K. Towards room temperature, the lifetime is on the order of seconds, which would require pump-probe
measurements to access as we discuss in the next section.
3.5 Pseudo-Steady-State Measurements
To perform the excitation and decay measurements outlined in the previous subsections, the experimental conditions (i.e. temperature and illumination) are adjusted
so as to study the excitation and decay processes independently. If the crystal is
illuminated continuously at a temperature where the two processes are competitive, a
steady-state equilibrium will be reached. This experiment is illustrated schematically
in Fig. 11.
Slightly different behaviour is obtained depending on whether a continuous or
pulsed excitation source is used. In the experiment depicted in Fig. 11a, the sample is
illuminated continuously, and the excited-state population builds smoothly to a
steady-state value determined by the balance of the excitation and decay rates at
the measurement temperature. In Fig. 11b, on the other hand, a pulsed illumination
source is used, and the excited-state population builds during the light pulses and
decays between them, oscillating about an equilibrium value. Provided the repetition
rate of the excitation pulses is much shorter than the timescale of the structural
measurements, the refined structure will show the averaged value. In the
photocrystallographic literature, the term “pseudo-steady-state” is usually used to
describe both of these scenarios, although when discussing the kinetics, it is useful to
distinguish the behaviour obtained with pulsed and continuous excitation sources as,
e.g. “pseudo-steady-state” and “steady-state”, respectively.
In most systems, the decay rate increases with temperature, while the excitation
rate remains approximately constant, with the result that the steady-state excitedstate population falls from a maximum to zero over a range of temperatures.
Figure 12 shows a set of pseudo-steady-state measurements on the [Pd(Bu 4 dien)
(NO 2 )]BPh 4 system. In this experiment, the decay becomes competitive with the
excitation at around 230 K. Between 230 and 290 K, the increasing decay rate causes
the steady-state excited-state population to drop from 100% to zero, and above
290 K the excited-state population is no longer measurable.
This behaviour can be understood intuitively as follows. For a given excitation
wavelength and power, the excitation rate is approximately constant, or at best is
Watching Photochemistry Happen: Recent Developments in Dynamic Single-Crystal. . .
219
excited-state lifetime varies by nine orders of magnitude, from ~10
8 s at 175 K to
100 ms at 325 K. This range of timescales may be compared to the time required to
collect a full X-ray dataset. For a well-diffracting crystal, a typical laboratory X-ray
system with a microfocus or rotating anode source and a CCD detector might take in
the region of 1–2 h to collect a full dataset and could therefore reasonably access
decay measurements up to ~230 K as in Fig. 10. A high-flux synchrotron source, on
the other hand, may take only a few minutes to collect a dataset, extending the range
of temperatures that can be measured up to around 250 K. Towards room temperature, the lifetime is on the order of seconds, which would require pump-probe
measurements to access as we discuss in the next section.
3.5 Pseudo-Steady-State Measurements
To perform the excitation and decay measurements outlined in the previous subsections, the experimental conditions (i.e. temperature and illumination) are adjusted
so as to study the excitation and decay processes independently. If the crystal is
illuminated continuously at a temperature where the two processes are competitive, a
steady-state equilibrium will be reached. This experiment is illustrated schematically
in Fig. 11.
Slightly different behaviour is obtained depending on whether a continuous or
pulsed excitation source is used. In the experiment depicted in Fig. 11a, the sample is
illuminated continuously, and the excited-state population builds smoothly to a
steady-state value determined by the balance of the excitation and decay rates at
the measurement temperature. In Fig. 11b, on the other hand, a pulsed illumination
source is used, and the excited-state population builds during the light pulses and
decays between them, oscillating about an equilibrium value. Provided the repetition
rate of the excitation pulses is much shorter than the timescale of the structural
measurements, the refined structure will show the averaged value. In the
photocrystallographic literature, the term “pseudo-steady-state” is usually used to
describe both of these scenarios, although when discussing the kinetics, it is useful to
distinguish the behaviour obtained with pulsed and continuous excitation sources as,
e.g. “pseudo-steady-state” and “steady-state”, respectively.
In most systems, the decay rate increases with temperature, while the excitation
rate remains approximately constant, with the result that the steady-state excitedstate population falls from a maximum to zero over a range of temperatures.
Figure 12 shows a set of pseudo-steady-state measurements on the [Pd(Bu 4 dien)
(NO 2 )]BPh 4 system. In this experiment, the decay becomes competitive with the
excitation at around 230 K. Between 230 and 290 K, the increasing decay rate causes
the steady-state excited-state population to drop from 100% to zero, and above
290 K the excited-state population is no longer measurable.
This behaviour can be understood intuitively as follows. For a given excitation
wavelength and power, the excitation rate is approximately constant, or at best is
Watching Photochemistry Happen: Recent Developments in Dynamic Single-Crystal. . .
219