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Topics in Current Chemistry (2018) 376:35
The vibrational relaxation during the internal conversion between the S 2 and S 1
states for all-trans-β-carotene has been also explained by pure vibrational cooling,
challenging the presence of any additional electronic dark state assisting this relaxation. The first FSRS measurements of all-trans-β-carotene showed that the S 1 C=C
stretching mode at 1798 cm
−1
relaxes via a two-step process identified by two distinct frequency up-shift kinetics [133]. A fast time constant (200 fs) was related to a
strong coupling followed by a slower equilibration process (450 fs) to the complete
set of vibrational normal modes. Although these first FSRS measurements were not
able to resolve the initial 250 fs of the dynamics, more recent comprehensive studies
using pump-DFWM and pump-IVS with improved temporal resolution corroborated
the ultrafast initial step of the vibrational cooling model of the S 1 C=C. A similar
ultrafast vibrational cooling step was also observed for the C=C 1525 cm
−1
mode
for all-trans-β-carotene [101] and in other carotenoids [58]. Cooling of the S 1 C=C
mode was also observed for carotenoids in a light-harvesting complex by applying
FSRS to spirilloxanthin in the native LH1 of Rhodospirillum rubrum [134]. After
the actinic excitation, the frequency at 1740 cm
−1
evolved to 1767 cm
−1
with a time
constant of 300 fs.
The vibrational relaxation after deactivation of the S 2 state has been recently further addressed for a series of open-chain carotenoids (like lycopene) with increasing conjugated length N by using pump-DFWM [104]. The simple picture of vibrational cooling, accounted for by a bi-exponential frequency up-shift as discussed
in the previous paragraph for all-trans-β-carotene, is actually only observed for
longer open-chain carotenoids (N = 11 and 13). Short open-chain carotenoids (N = 9
and 10) show a down-shift of the C=C stretching mode from about 1580 to about
1510 cm
−1
, which has been explained as a further indication of coupling between
the S 2 and an additional electronic dark state (between the S 1 and S 2 states).
A very intriguing observation in the vibrational dynamics of carotenoids is the
additional observation of two vibrational bands in the 1800 cm
−1
spectral region in
several experiments. These bands have been observed with e.g., FSRS at 1770 and
1800 cm
−1
as well as at 1770 and 1790 cm
−1
[132, 135], and at 1740 and 1785 cm
−1
with pump-IVS and pump-DFWM [58] for all-trans-β-carotene. In the latter case,
they have been explained as the result of vibrationally hot C=C levels of the typical
S 1 C=C mode contributing to the signal (agreeing with the above picture). More
recently, FSRS has been applied to spirilloxanthin (N = 13) and two bands at 1743
and 1771 cm
−1
have been also observed [56]. However, the much longer life time
(3 ps) detected for 1771 cm
−1
in comparison to the S 1 lifetime (1.5 ps) has been
interpreted as the signature of an electronic state of another carotenoid conformer
present already in the ground state and also excited by the actinic pulse. Whether
these two bands are due to inhomogeneous S 0 conformational distributions or to
vibrationally hot bands in the S 1 manifold is still unclear.
The first attempt to follow the evolution of the high-frequency Raman spectrum of the S 2 state directly was done by FSRS using a Raman pump and probe
spectrally resonant with the S 2 ESA of all-trans-β-carotene [136]. A very broad
spectrally unresolved Raman band was disentangled with two bands at 1654 and
1739 cm
−1
. While the 1739 cm
−1
band was assigned to the C=C mode in S 1 , the
mode at 1654 cm
−1
was due to the C=C in the S 2 state. A similar frequency for the
229
Reprinted from the journal
Topics in Current Chemistry (2018) 376:35
The vibrational relaxation during the internal conversion between the S 2 and S 1
states for all-trans-β-carotene has been also explained by pure vibrational cooling,
challenging the presence of any additional electronic dark state assisting this relaxation. The first FSRS measurements of all-trans-β-carotene showed that the S 1 C=C
stretching mode at 1798 cm
−1
relaxes via a two-step process identified by two distinct frequency up-shift kinetics [133]. A fast time constant (200 fs) was related to a
strong coupling followed by a slower equilibration process (450 fs) to the complete
set of vibrational normal modes. Although these first FSRS measurements were not
able to resolve the initial 250 fs of the dynamics, more recent comprehensive studies
using pump-DFWM and pump-IVS with improved temporal resolution corroborated
the ultrafast initial step of the vibrational cooling model of the S 1 C=C. A similar
ultrafast vibrational cooling step was also observed for the C=C 1525 cm
−1
mode
for all-trans-β-carotene [101] and in other carotenoids [58]. Cooling of the S 1 C=C
mode was also observed for carotenoids in a light-harvesting complex by applying
FSRS to spirilloxanthin in the native LH1 of Rhodospirillum rubrum [134]. After
the actinic excitation, the frequency at 1740 cm
−1
evolved to 1767 cm
−1
with a time
constant of 300 fs.
The vibrational relaxation after deactivation of the S 2 state has been recently further addressed for a series of open-chain carotenoids (like lycopene) with increasing conjugated length N by using pump-DFWM [104]. The simple picture of vibrational cooling, accounted for by a bi-exponential frequency up-shift as discussed
in the previous paragraph for all-trans-β-carotene, is actually only observed for
longer open-chain carotenoids (N = 11 and 13). Short open-chain carotenoids (N = 9
and 10) show a down-shift of the C=C stretching mode from about 1580 to about
1510 cm
−1
, which has been explained as a further indication of coupling between
the S 2 and an additional electronic dark state (between the S 1 and S 2 states).
A very intriguing observation in the vibrational dynamics of carotenoids is the
additional observation of two vibrational bands in the 1800 cm
−1
spectral region in
several experiments. These bands have been observed with e.g., FSRS at 1770 and
1800 cm
−1
as well as at 1770 and 1790 cm
−1
[132, 135], and at 1740 and 1785 cm
−1
with pump-IVS and pump-DFWM [58] for all-trans-β-carotene. In the latter case,
they have been explained as the result of vibrationally hot C=C levels of the typical
S 1 C=C mode contributing to the signal (agreeing with the above picture). More
recently, FSRS has been applied to spirilloxanthin (N = 13) and two bands at 1743
and 1771 cm
−1
have been also observed [56]. However, the much longer life time
(3 ps) detected for 1771 cm
−1
in comparison to the S 1 lifetime (1.5 ps) has been
interpreted as the signature of an electronic state of another carotenoid conformer
present already in the ground state and also excited by the actinic pulse. Whether
these two bands are due to inhomogeneous S 0 conformational distributions or to
vibrationally hot bands in the S 1 manifold is still unclear.
The first attempt to follow the evolution of the high-frequency Raman spectrum of the S 2 state directly was done by FSRS using a Raman pump and probe
spectrally resonant with the S 2 ESA of all-trans-β-carotene [136]. A very broad
spectrally unresolved Raman band was disentangled with two bands at 1654 and
1739 cm
−1
. While the 1739 cm
−1
band was assigned to the C=C mode in S 1 , the
mode at 1654 cm
−1
was due to the C=C in the S 2 state. A similar frequency for the
229
Reprinted from the journal