1 3
Topics in Current Chemistry (2018) 376:35
in the excited state. This was exploited to investigate the mechanism of solute–solvent energy dissipation [98]. Sub-ps decay kinetics of the t* Raman peak intensities reported with FSRS were eventually identified to be controlled by experimental
conditions (Raman depletion effect) [46], and indeed not confirmed by populationcontrolled pump-IVS [77].
Multi-VCS of cis-stilbene was also carried out similarly, but its very short c*
lifetime increases the experimental challenge [37, 148, 149], as portrayed above for
the excited states of carotenoids. A seminal pump-IVS experiment demonstrated the
ability of multi-VCS to reveal quantitative structural information along the ultrafast
isomerization pathway from c* to the CInt [37]. Following actinic UV excitation,
a 620-nm, 11-fs pulse (repump) on resonance with ESA and SE of c* was used to
impulsively trigger and subsequently probe a S 1 vibrational wavepacket. A dominant
oscillation is detected with a frequency around 240 cm
−1
, which down-shifts by up
to 30 cm
−1
on the ps time scale as a function of the waiting time between actinic and
repump pulses. This mode, previously observed in the form of a Franck–Condon
activated vibrational wavepacket in a conventional pump-probe experiment [156]
and by ps-resolved Raman spectroscopy [148], is attributed to a spectator mode
whose frequency shift reveals the anharmonicity and gradual topography change of
the excited state PES as the system evolves from the FC point towards the CInt.
FSRS experiments performed with a Raman pump on resonance with the c* ESA
band [149] revealed Raman peaks broadened by the very short c* lifetime, but the
background subtraction (inherent to FSRS data, see above) is in this case particularly challenging, leading the authors to consider the data with caution.
While both c and t photoisomerizations have long been postulated to occur
via the same transient perpendicular p state [141], its experimental evidence and
spectroscopic characterization in condensed phase came four decades later [143].
Recently, further characterization of the p* state was made by investigating stilbene derivatives where the formation and decay time of the p* state vary by more
than two orders of magnitude, while its characteristic UV absorption band is invariably observed at 350 ± 20 nm depending on the substitutions [111, 158–159]. The
dependence of the p* lifetime on the solvent polarity is interpreted as the signature
of its zwitterionic nature. Multi-VCS was performed on the trans- 1,1′-dicyanostilbene, which features a nearly 30-ps-long-lived p* state in n-hexane, thus facilitating
the elucidation of some of its vibrational spectroscopic signatures thanks to a very
sensitive FSRS set-up [111]. Three Raman peaks of a few μOD signal amplitude
and characterized by a 20–30 ps decay kinetics were attributed to the transient p*
species, including a 1558 cm
−1
mode which was tentatively assigned to the phenyl
quadrant vibrations, in the absence of any computational prediction of the p* vibrational modes.
The synthetic molecular rotary machines developed by Feringa’s group are based
on so-called crowded derivatives of stilbene and stiff-stilbene [139, 160], such as
the fluorene-based compound displayed in Fig. 14. The photoisomerization of such
compounds also involves a transient excited state species. For stiff-stilbene, a subps, viscosity-independent excited state relaxation is observed but the UV absorption signature of a stilbene-like perpendicular “phantom” state is not detected [161].
While the S 0 Raman spectrum of stiff-stilbene also features the same double peak
233
Reprinted from the journal
Topics in Current Chemistry (2018) 376:35
in the excited state. This was exploited to investigate the mechanism of solute–solvent energy dissipation [98]. Sub-ps decay kinetics of the t* Raman peak intensities reported with FSRS were eventually identified to be controlled by experimental
conditions (Raman depletion effect) [46], and indeed not confirmed by populationcontrolled pump-IVS [77].
Multi-VCS of cis-stilbene was also carried out similarly, but its very short c*
lifetime increases the experimental challenge [37, 148, 149], as portrayed above for
the excited states of carotenoids. A seminal pump-IVS experiment demonstrated the
ability of multi-VCS to reveal quantitative structural information along the ultrafast
isomerization pathway from c* to the CInt [37]. Following actinic UV excitation,
a 620-nm, 11-fs pulse (repump) on resonance with ESA and SE of c* was used to
impulsively trigger and subsequently probe a S 1 vibrational wavepacket. A dominant
oscillation is detected with a frequency around 240 cm
−1
, which down-shifts by up
to 30 cm
−1
on the ps time scale as a function of the waiting time between actinic and
repump pulses. This mode, previously observed in the form of a Franck–Condon
activated vibrational wavepacket in a conventional pump-probe experiment [156]
and by ps-resolved Raman spectroscopy [148], is attributed to a spectator mode
whose frequency shift reveals the anharmonicity and gradual topography change of
the excited state PES as the system evolves from the FC point towards the CInt.
FSRS experiments performed with a Raman pump on resonance with the c* ESA
band [149] revealed Raman peaks broadened by the very short c* lifetime, but the
background subtraction (inherent to FSRS data, see above) is in this case particularly challenging, leading the authors to consider the data with caution.
While both c and t photoisomerizations have long been postulated to occur
via the same transient perpendicular p state [141], its experimental evidence and
spectroscopic characterization in condensed phase came four decades later [143].
Recently, further characterization of the p* state was made by investigating stilbene derivatives where the formation and decay time of the p* state vary by more
than two orders of magnitude, while its characteristic UV absorption band is invariably observed at 350 ± 20 nm depending on the substitutions [111, 158–159]. The
dependence of the p* lifetime on the solvent polarity is interpreted as the signature
of its zwitterionic nature. Multi-VCS was performed on the trans- 1,1′-dicyanostilbene, which features a nearly 30-ps-long-lived p* state in n-hexane, thus facilitating
the elucidation of some of its vibrational spectroscopic signatures thanks to a very
sensitive FSRS set-up [111]. Three Raman peaks of a few μOD signal amplitude
and characterized by a 20–30 ps decay kinetics were attributed to the transient p*
species, including a 1558 cm
−1
mode which was tentatively assigned to the phenyl
quadrant vibrations, in the absence of any computational prediction of the p* vibrational modes.
The synthetic molecular rotary machines developed by Feringa’s group are based
on so-called crowded derivatives of stilbene and stiff-stilbene [139, 160], such as
the fluorene-based compound displayed in Fig. 14. The photoisomerization of such
compounds also involves a transient excited state species. For stiff-stilbene, a subps, viscosity-independent excited state relaxation is observed but the UV absorption signature of a stilbene-like perpendicular “phantom” state is not detected [161].
While the S 0 Raman spectrum of stiff-stilbene also features the same double peak
233
Reprinted from the journal