3.4 Rates and Mechanism of Energy Transfer
At cryogenic temperatures, the combination of confocal microscopy and frequencyselective SMS is a powerful tool for the investigation of multichromophoric
aggregates because it allows for individually addressing molecules whose spatial
distance is much smaller than the optical resolution of the imaging system
[130, 131]. Such experiments strongly rely on the occurrence of sharp zero-phonon
transitions and an inhomogeneous broadening much larger than the homogeneous
width of the former. These features open up a number of interesting perspectives for
the study of proximate and interacting molecules.
To image individual multichromophoric assemblies, a home-built lowtemperature laser scanning confocal microscope was used, which is described in
detail elsewhere [28]. Excitation spectra of single molecules within an individual
multichromophore are measured by using a ring-dye laser pumped by an argon ion
laser as tunable light source. Excitation spectra are obtained by scanning the laser
across the molecular absorption lines while detecting the red-shifted emission from
either the same chromophore or from chromophores to which the excitation energy
has been transferred. Typically, excitation spectra have been recorded at a temperature of 1.4 K. In principal, any temperature between 1.4 K and room temperature is
accessible with the cryostate in which the sample and part of the microscope are
inserted.
As has been shown recently, for systems exhibiting weak electron–phonon
coupling, the rate constant of EET can be extracted from the widths of the sharp
purely electronic zero-phonon lines (ZPLs) of donor chromophores observed at low
temperatures [2, 9, 26]. Under these conditions the homogeneous line width of a
ZPL is composed of the following contributions:
Δν hom ¼
1
2π T 1
þ
1
π T
∗
2
þ
1
2π τ ET
(3)
The first two terms are the contributions from the lifetime of the excited state T 1
(population decay) and pure dephasing processes T 2
* ; the third term results from the
EET, which is treated as an T 1 -type energy relaxation process that causes line
broadening of the donor excitation spectrum. In case the third contribution
dominates the line width, EET time constants are easily accessible from the simple
relation Δν ¼ (2πτ EET )
À1 . Actually, it has to be checked for every particular case
that the pure dephasing and lifetime contribution can be neglected compared to the
EET contribution. In addition, it has to be assured that the excitation lines are not
power broadened and that the band width of the dye laser is substantially smaller
than the width of the ZPL. With a laser band width of 2 GHz, which corresponds to
the broad band mode of a ring dye laser, line widths are accessible in the range from
5 to roughly 500 GHz, corresponding to EET rates between 3 Â 10
10 and
3 Â 10
12 s
À1 .
98
T. Basche ´ et al.
At cryogenic temperatures, the combination of confocal microscopy and frequencyselective SMS is a powerful tool for the investigation of multichromophoric
aggregates because it allows for individually addressing molecules whose spatial
distance is much smaller than the optical resolution of the imaging system
[130, 131]. Such experiments strongly rely on the occurrence of sharp zero-phonon
transitions and an inhomogeneous broadening much larger than the homogeneous
width of the former. These features open up a number of interesting perspectives for
the study of proximate and interacting molecules.
To image individual multichromophoric assemblies, a home-built lowtemperature laser scanning confocal microscope was used, which is described in
detail elsewhere [28]. Excitation spectra of single molecules within an individual
multichromophore are measured by using a ring-dye laser pumped by an argon ion
laser as tunable light source. Excitation spectra are obtained by scanning the laser
across the molecular absorption lines while detecting the red-shifted emission from
either the same chromophore or from chromophores to which the excitation energy
has been transferred. Typically, excitation spectra have been recorded at a temperature of 1.4 K. In principal, any temperature between 1.4 K and room temperature is
accessible with the cryostate in which the sample and part of the microscope are
inserted.
As has been shown recently, for systems exhibiting weak electron–phonon
coupling, the rate constant of EET can be extracted from the widths of the sharp
purely electronic zero-phonon lines (ZPLs) of donor chromophores observed at low
temperatures [2, 9, 26]. Under these conditions the homogeneous line width of a
ZPL is composed of the following contributions:
Δν hom ¼
1
2π T 1
þ
1
π T
∗
2
þ
1
2π τ ET
(3)
The first two terms are the contributions from the lifetime of the excited state T 1
(population decay) and pure dephasing processes T 2
* ; the third term results from the
EET, which is treated as an T 1 -type energy relaxation process that causes line
broadening of the donor excitation spectrum. In case the third contribution
dominates the line width, EET time constants are easily accessible from the simple
relation Δν ¼ (2πτ EET )
À1 . Actually, it has to be checked for every particular case
that the pure dephasing and lifetime contribution can be neglected compared to the
EET contribution. In addition, it has to be assured that the excitation lines are not
power broadened and that the band width of the dye laser is substantially smaller
than the width of the ZPL. With a laser band width of 2 GHz, which corresponds to
the broad band mode of a ring dye laser, line widths are accessible in the range from
5 to roughly 500 GHz, corresponding to EET rates between 3 Â 10
10 and
3 Â 10
12 s
À1 .
98
T. Basche ´ et al.