Topics in Current Chemistry (2018) 376:35
1 3
around 1600 cm
−1
as in trans stilbene (see above), its S 1 Raman spectrum shows
a dominating peak at 1500 cm
−1
, i.e., downshifted by 70 cm
−1
as compared to the
1570 cm
−1
mode of the t* state of stilbene. For the fluorene-based rotary machine
investigated by the Meech’s group, the 0.1-ps, viscosity-independent formation of a
transient excited state is also observed, which is characterized as a dark state, and
proposed to result from the ultrafast relaxation along a volume-conserving coordinate, such as pyramidalization of one of the carbon atoms of the isomerizing bond
[54, 162]. For both stiff-stilbene and the fluorene-based machine, the subsequent
decay to the ground state is instead viscosity-dependent like it is for trans-stilbene.
This is expected for the torsion around the central C=C double bond in these compounds, since it is a large amplitude motion displacing a significant volume of solvent. Recently, Meech’s group applied multi-VCS to follow the structural relaxation
of the transient dark state until the formation of the ground state isomer on the 1.6ps time scale [119]. FSRS employing a Raman pump resonant with the dark state
absorption around 550 nm revealed the Raman spectrum of the dark state dominated by 1345 and 1430 cm
−1
Raman peaks, strongly downshifted with respect to
the ground state Raman double peak at 1560 and 1585 cm
−1
(see Fig. 16). These
excited-state Raman signatures are discussed as being related to the isomerizing
C=C double bond by analogy to the case of stiff-stilbene. They decay on the same
time scale as the dark state population, to give rise to the Raman signature of the
ground state photoproduct (so-called “unstable rotor”, see Fig. 16) dominated by the
C=C stretch peak pair at 1510 and 1550 cm
−1
. The large frequency downshift of the
C=C Raman signature of the dark state is tentatively attributed to the elongation of
the central isomerizing bond in the excited state, in line with computational predictions [163].
Finally, another common feature of substituted trans-stilbene derivatives is the
existence of ground state, sub-populations of rotamers resulting from the thermally
activated phenyl ring rotations [164]. While the various rotamers may not easily
be distinguished by their steady-state UV–Vis spectroscopic signatures, they may
feature distinct photoreaction kinetics and vibrational signatures. Multi-VCS was
recently applied to the investigation of a family of di-fluorinated stilbene compounds
[165]. This work demonstrates the efficiency of multi-VCS at discriminating the
vibrational signatures and photoreaction kinetics of distinct subpopulations. Chemical and/or structural heterogeneity is a common feature of complex molecular systems in condensed phase. One present challenge of time-resolved non-linear spectroscopy and of physical chemistry is to resolve such heterogeneity, and multi-VCS
is a promising spectroscopic tool for that.
6 Conclusions
The detection of Raman spectra as a function of photoreaction time is one of the
most natural ways to investigate structure changes and interactions at the molecular level. Multi-VCS has achieved this goal by detecting stimulated Raman scattering (SRS) after the interaction with an actinic pulse triggering the photoreaction
of interest. SRS is a third–order non-linear spectroscopy technique which exploits
234
Reprinted from the journal
1 3
around 1600 cm
−1
as in trans stilbene (see above), its S 1 Raman spectrum shows
a dominating peak at 1500 cm
−1
, i.e., downshifted by 70 cm
−1
as compared to the
1570 cm
−1
mode of the t* state of stilbene. For the fluorene-based rotary machine
investigated by the Meech’s group, the 0.1-ps, viscosity-independent formation of a
transient excited state is also observed, which is characterized as a dark state, and
proposed to result from the ultrafast relaxation along a volume-conserving coordinate, such as pyramidalization of one of the carbon atoms of the isomerizing bond
[54, 162]. For both stiff-stilbene and the fluorene-based machine, the subsequent
decay to the ground state is instead viscosity-dependent like it is for trans-stilbene.
This is expected for the torsion around the central C=C double bond in these compounds, since it is a large amplitude motion displacing a significant volume of solvent. Recently, Meech’s group applied multi-VCS to follow the structural relaxation
of the transient dark state until the formation of the ground state isomer on the 1.6ps time scale [119]. FSRS employing a Raman pump resonant with the dark state
absorption around 550 nm revealed the Raman spectrum of the dark state dominated by 1345 and 1430 cm
−1
Raman peaks, strongly downshifted with respect to
the ground state Raman double peak at 1560 and 1585 cm
−1
(see Fig. 16). These
excited-state Raman signatures are discussed as being related to the isomerizing
C=C double bond by analogy to the case of stiff-stilbene. They decay on the same
time scale as the dark state population, to give rise to the Raman signature of the
ground state photoproduct (so-called “unstable rotor”, see Fig. 16) dominated by the
C=C stretch peak pair at 1510 and 1550 cm
−1
. The large frequency downshift of the
C=C Raman signature of the dark state is tentatively attributed to the elongation of
the central isomerizing bond in the excited state, in line with computational predictions [163].
Finally, another common feature of substituted trans-stilbene derivatives is the
existence of ground state, sub-populations of rotamers resulting from the thermally
activated phenyl ring rotations [164]. While the various rotamers may not easily
be distinguished by their steady-state UV–Vis spectroscopic signatures, they may
feature distinct photoreaction kinetics and vibrational signatures. Multi-VCS was
recently applied to the investigation of a family of di-fluorinated stilbene compounds
[165]. This work demonstrates the efficiency of multi-VCS at discriminating the
vibrational signatures and photoreaction kinetics of distinct subpopulations. Chemical and/or structural heterogeneity is a common feature of complex molecular systems in condensed phase. One present challenge of time-resolved non-linear spectroscopy and of physical chemistry is to resolve such heterogeneity, and multi-VCS
is a promising spectroscopic tool for that.
6 Conclusions
The detection of Raman spectra as a function of photoreaction time is one of the
most natural ways to investigate structure changes and interactions at the molecular level. Multi-VCS has achieved this goal by detecting stimulated Raman scattering (SRS) after the interaction with an actinic pulse triggering the photoreaction
of interest. SRS is a third–order non-linear spectroscopy technique which exploits
234
Reprinted from the journal