Primary Steps in Photo–Biological Reactions 551
Isomerization coordinate
200 fs
460 540500
Wavelength (nm)
33 fs
400 fs
6 ps
3
2
1
0
1
2
c
580
a
200 fs
h$
h$
Energy (kcal/mole)
b
S
0
S
1
S
n
100
50
0
11-cis rhodopsin
CH
3
CH
3
CH
3
CH
3
11
11
12
12
NH
N
H
All-trans photoproduct
Figure 11.12 Schematic representation of the photo-isomerization reaction of rhodopsin.
(a) Ground and excited state potential energy curves as a function of the torsional coordinate. The
spectrum is red-shifted after absorption of a photon at 500 nm. The classical sketch of the cis-trans
transformation is shown in (b). (c) Difference spectra measurements of 11-cis-rhodopsin at various
delays following a 35-fs pump pulse at 500 nm (≈10 fs probe) (from Schoenlein et al. [44]).
The quantum efficiency of this reaction is exceptionally high (0.67). The radia-
tion lifetime of the excited state of rhodopsin is 10
−8
s. The extinction coefficient
is 6. 4 10
4
M
−1
cm
−1
, a typical value for a strongly absorbing dye.
The reaction of photo-isomerization was studied through transient transmis-
sion spectroscopy through a jet of rhodopsin [45]. Adequate spectral selectivity
was achieved with a pump pulse of 35-fs at 500 nm. A 10-fs probe pulse with
a spectrum in the range of 450 nm to 570 nm was used. The differential trans-
mission spectrum versus delay shown in Fig. 11.12 indicate disappearance of the
500 nm peak, and increased absorption at 530 nm, in the first 150 fs following
excitation. The speed of that isomerization calls for a better classical representa-
tion of the cis- versus trans-configuration than Fig. 11.12(b). It is doubtful that
the large motion of nuclei implied by the sketch could take place in a time as
short as 100 fs.
11.5.2. Photosynthesis
Photosynthesis is the process by which plants convert solar energy into chem-
ical energy. Its importance is obvious, because it is at the origin of life on our
552 Examples of Ultrafast Processes in Matter
planet. This topic is too vast to be adequately covered in a section of this book.
A general overview of the topic can be found in a review article by Fleming and
van Grondelle [46], and in topical books [1, 43].
There are pigment–protein complexes called reaction centers, where a direc-
tional electron transfer takes place across a biological membrane. Light harvest-
ing molecules (“antenna” chlorophylls) transfer electronic excitation energy to a
special pair (P in the sketch of Figure 11.13) of chlorophyll molecules, which
acts as the primary electron donor.
The latter transfers an electron to a pheophytin (H
A
) within 3 ps, and from it
to a quinone (Q
A
) in 200 ps, then to the other quinone Q
B
, hence establishing
a potential difference across a biological membrane. Biochemical reactions that
store the energy subsequently occur with these separated charges.
The energy dissipation in the first processes should be small (about 0.25 eV) as
compared to the excitation energy (1.38 eV), to minimize the waste of excitation
energy. The electron transfer should be fast to compete with fluorescence and
radiationless decay.
The complexity of the problem can be appreciated by looking at the represen-
tation of the molecular structure of a bacterium’s photosynthetic reaction center,
which was determined to atomic resolution by Deisenhofer and Michel [46,48].
A block diagram of the electron-carrying pigments in the reaction center is shown
in Fig. 11.13.
Recent transient absorption experiments have concentrated on the fast ini-
tial electron transfers [49]. In a model proposed by Zinth et al. [47, 49], the
bacteriochlorophyll anion B
−
A
is created in the first 3-ps reaction. The subsequent
B
B
B
A
H
B
H
A
Q
B
Q
A
P
Figure 11.13 Sketch of the molecular arrangement of the four bacteriochlorophylls (P, B
A
,B
B
),
the two bacteriopheophytins (H
A
,H
B
), and the two quinones (Q
A
,Q
B
) in reaction centers (from
Zinth et al. [47]).
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