1 3
Topics in Current Chemistry (2018) 376:35
evolution to assign Raman spectra to specific electronic states or assist the assignment of electronic states by using known Raman spectra, as will be further illustrated below. Finally, the ability of multi-VCS to reveal detailed structural information on excited electronic states makes it a very powerful experimental approach to
assess the accuracy of state-of-the-art computational developments targeting quantum chemical modeling of electronic structures and of molecular photoreactivity in
general (Table 1).
A seminal example in multi-VCS, which depicts many of the points mentioned
above, is the pioneering FSRS investigation of the photoinduced, sub-ps structural
evolution of the protonated Schiff base of retinal (PSBR) in the rhodopsin protein,
the visual sensor [120]. Upon light excitation to S 1 , the C11=C12 bond of PSBR
extends and acquires a single-bond character. This enables a very fast torsional
motion to drive the system within ~ 100 fs to a CInt [121], where it decays to S 0 , with
a predicted C11=C12 bond twist of ~ 90°. By implementing FSRS, the vibrational
frequencies associated to the hydrogen out-of-plane motions (in the 800–900 cm
−1
range) of both hydrogen atoms linked to the H–C11=C12–H isomerizing bond could
be observed as a function of time, after decay to the S 0 . This experiment revealed a
rapid (300-fs time scale) blue shift of the H wagging frequencies by ~ 100 cm
−1
,
which is the signature of a large structural reorganization of PSBR occurring on the
ground state PES S 0 . More specifically, the blue shift is shown to reveal the stiffening of these vibrations resulting from the planarization of the PSBR backbone, i.e.,
completion of the C11=C12 isomerization (torsional) motion towards the first vibrationally relaxed photoproduct intermediate, called bathorhodopsin.
5.2 Ultrafast Vibrational Dynamics in the Excited States of Carotenoids
Carotenoids have been an important class of molecules investigated by multi-VCS
due to their central role in several biological functions [122]. As chromophores in
light-harvesting complexes (LHC), for example, carotenoids are involved in the
initial absorption of light and energy transfer to other chromophores (e.g., bacteriochlorophylls). The very strong absorption from the ground state S 0 is due to the
π–π* transition to the second electronic state labeled S 2 . The transition from the S 0
to the first electronic S 1 state is not one-photon allowed, making the S 1 state a “dark”
electronic state. Upon light absorption to S 2 , the electronic relaxation to S 1 is very
fast (within 100–200 fs for all-trans-β-carotene and lycopene). Further electronic
relaxation from S 1 to S 0 takes place in the picosecond time scale, varying with the
number N of effective conjugated C=C double bonds. For example, the S 1 state in
all-trans-β-carotene (N ~ 10.5) decays with about 9 ps, while in lycopene (N = 11)
decays faster with about 4.1 ps [123]. The S 2 to S 1 relaxation mechanism has been
intensely debated in the last decades (see for example Ref. [124–128]). One source
of discussion has been, for example, the nearly identical lifetime of the S 2 state for
several open-chain carotenoids with different numbers N of effective conjugated
C=C bonds, which has been interpreted as the result of additional electronic dark
states between the S 2 and S 1 states. The experimental identification of these dark
states via transient absorption, nevertheless, has been extremely challenging due
227
Reprinted from the journal
Topics in Current Chemistry (2018) 376:35
evolution to assign Raman spectra to specific electronic states or assist the assignment of electronic states by using known Raman spectra, as will be further illustrated below. Finally, the ability of multi-VCS to reveal detailed structural information on excited electronic states makes it a very powerful experimental approach to
assess the accuracy of state-of-the-art computational developments targeting quantum chemical modeling of electronic structures and of molecular photoreactivity in
general (Table 1).
A seminal example in multi-VCS, which depicts many of the points mentioned
above, is the pioneering FSRS investigation of the photoinduced, sub-ps structural
evolution of the protonated Schiff base of retinal (PSBR) in the rhodopsin protein,
the visual sensor [120]. Upon light excitation to S 1 , the C11=C12 bond of PSBR
extends and acquires a single-bond character. This enables a very fast torsional
motion to drive the system within ~ 100 fs to a CInt [121], where it decays to S 0 , with
a predicted C11=C12 bond twist of ~ 90°. By implementing FSRS, the vibrational
frequencies associated to the hydrogen out-of-plane motions (in the 800–900 cm
−1
range) of both hydrogen atoms linked to the H–C11=C12–H isomerizing bond could
be observed as a function of time, after decay to the S 0 . This experiment revealed a
rapid (300-fs time scale) blue shift of the H wagging frequencies by ~ 100 cm
−1
,
which is the signature of a large structural reorganization of PSBR occurring on the
ground state PES S 0 . More specifically, the blue shift is shown to reveal the stiffening of these vibrations resulting from the planarization of the PSBR backbone, i.e.,
completion of the C11=C12 isomerization (torsional) motion towards the first vibrationally relaxed photoproduct intermediate, called bathorhodopsin.
5.2 Ultrafast Vibrational Dynamics in the Excited States of Carotenoids
Carotenoids have been an important class of molecules investigated by multi-VCS
due to their central role in several biological functions [122]. As chromophores in
light-harvesting complexes (LHC), for example, carotenoids are involved in the
initial absorption of light and energy transfer to other chromophores (e.g., bacteriochlorophylls). The very strong absorption from the ground state S 0 is due to the
π–π* transition to the second electronic state labeled S 2 . The transition from the S 0
to the first electronic S 1 state is not one-photon allowed, making the S 1 state a “dark”
electronic state. Upon light absorption to S 2 , the electronic relaxation to S 1 is very
fast (within 100–200 fs for all-trans-β-carotene and lycopene). Further electronic
relaxation from S 1 to S 0 takes place in the picosecond time scale, varying with the
number N of effective conjugated C=C double bonds. For example, the S 1 state in
all-trans-β-carotene (N ~ 10.5) decays with about 9 ps, while in lycopene (N = 11)
decays faster with about 4.1 ps [123]. The S 2 to S 1 relaxation mechanism has been
intensely debated in the last decades (see for example Ref. [124–128]). One source
of discussion has been, for example, the nearly identical lifetime of the S 2 state for
several open-chain carotenoids with different numbers N of effective conjugated
C=C bonds, which has been interpreted as the result of additional electronic dark
states between the S 2 and S 1 states. The experimental identification of these dark
states via transient absorption, nevertheless, has been extremely challenging due
227
Reprinted from the journal