BioPHYSICAL chemistry

excitation, and the use of successive delays provides the time-evolution
profile of the couplings.
The ability of this technique to provide a direct mapping of the couplings
as the energy-transfer process occurs within the complex is illustrated by
the experimental 2D traces for the FMO complex. Before excitation, the
positions of the three main diagonal peaks of the 2D spectrum match the
positions of the peaks observed in the low-temperature absorption spectrum
(Figure 14.18). In addition to the diagonal peaks, several off-diagonal
features, both positive and negative, are evident in the 2D spectrum. These
off-diagonal peaks arise from transitions involving excited states. The peak
labeled A indicates that the bacteriochlorophylls identified with peaks 1
and 5 of the absorption spectrum are coupled. The presence of the off-
diagonal peak labeled B reveals a coupling for the pigment’s contribution
to peaks 2 and 5. After light excitation the energy is transferred and the
2D spectrum is seen to evolve. The main diagonal peak shifts to lower
energies, and the changes in the off-diagonal peaks indicate a change in
the couplings among the bacteriochlorophylls. The results can be inter-
preted in terms of specific pathways of energy transferfor the different
bacteriochlorophylls, rather than being simple stepwise energy transfer from
the highest energy bacteriochlorophyll to the lowest.In the two possible
pathways, certain states that are spatially close are skipped when they
are energetically distinct.
These results for the FMO complex demonstrate that 2D time-resolved
spectroscopy can overcome the problem of overlapping absorption bands
in conventional spectra just as 2D NMR is needed to interpret spectra with
many overlapping contributions. Scientists have performed 2D experiments
with NMR for many years as the required manipulations of the radio-
frequency waves have proved to be quite tractable. In contrast, such experi-
mental control of light in the optical spectrum has proved to be more
difficult until recent times. Recent efforts, including this work by Brixner

CHAPTER 14 OPTICAL SPECTROSCOPY 309

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2345 67

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Figure 14.17Experimental spectra of the FMO complex. (A) The linear absorption spectrum
of the complex, showing the presence of several overlapping contributions at 77 K. The other
three panels show the initial 2D spectrum and the spectra after 200 and 1000 fs. From Brixner
et al. (2005).