then passed onto a detector where the amount of fluorescence is measured.
In the case of fluorescence spectrophotometers, one typically scans the
monochromator associated with the detector. Alternatively, the excitation
monochromator can be scanned if one wants to measure the spectrum
of the material absorbing the light that results in fluorescence.
Phosphorescence
As discussed above, there are certain types of electronic transition that
are not allowed. One such case is a transition that involves both a
change in electronic orbital and a flip of electron spin. When the spins
of the two electrons in the highest orbitals are antiparallel, we refer to
them as being in a singlet state (Chapter 12). If their spins are aligned
(and in different orbitals, since you cannot have two electrons with the
same spin in the same orbital), we refer to them as being in a triplet state.
Sometimes, the spin of an electron in the excited state will flip, usually
due to interactions with the magnetic moments of the surrounding nuclei.
The electrons are then in a triplet state and a direct transition to the ground
state, which is usually a singlet state, is not allowed, resulting in the
triplet state having a long lifetime as short as microseconds but as long
as seconds. When the transition does occur, light is given off and this is
called phosphorescence. Some molecules readily undergo phosphorescence
and are used in glow-in-the-dark toys.
RESEARCH DIRECTION: PROBING ENERGY TRANSFER USING
TWO-DIMENSIONAL OPTICAL SPECTROSCOPY
In photosynthetic bacteria, light is captured by the light-harvesting com-
plexes and funneled to the reaction center, which performs the primary
photochemistry of converting the light energy into chemical energy (Chap-
ter 20). Green sulfur bacteria have a large peripheral antenna complex,
known as a chlorosome, which collects the energy of the light, using up to
10,000 bacteriochlorophylls. After light excitation, the light energy is effici-
ently transferred to the reaction center through a water-soluble pigment–
protein complex that is termed the FMO protein after Richard Fenna, Brian
Matthews, and John Olson, who led the early efforts to characterize the
structure and function of this complex (Matthews & Fenna 1980; Olson
1998). The structure of the complex has been determined from two organ-
isms using X-ray crystallography (Li et al. 1997). The protein consists of
three identical subunits that form a trimer with a central symmetry axis
(Figure 14.16). Each subunit has the distinctive feature of a large βsheet
that is folded into a “taco shell” around seven bacteriochlorophylls. The
relative distances between pairs of the bacteriochlorophylls range from 4