3 Single-Molecule Studies of Electronic Excitation
Energy Transfer
Lately, the quest for investigating electronic coupling at the single molecule level has
led to a number of experimental and theoretical studies. Specific topics that have been
addressed include the investigation of coherent coupling and localization,
superradiant emission, singlet–singlet annihilation and the origin of collective “off”
states [117–123]. In this section, single-molecule experiments conducted with D–A
compounds described in Sect. 2.1 are presented. As already outlined in Sect. 2.1, the
D–A-model systems have been specifically designed to gain quantitative and novel
information by SMS on the rate and mechanism of electronic EET. One important
factor has been to arrange the donor(s) and acceptor(s) via fairly rigid oligo
(phenylene) bridges at fixed distances and orientations. The chromophores PMI,
PDI and TDI were chosen because they possess favorable properties for SMS,
with large absorption cross-sections, high emission quantum yields and low
photobleaching yields. In the dyads and triads, the donor and acceptor chromophores
can be selectively excited, and all the chromophores give rise to sharp zero-phonon
lines at cryogenic temperatures, which is a crucial prerequisite for extracting EET
times from line width measurements. These outstanding characteristics of the
multichromophores have fostered a number of novel single-molecule results, which
are discussed in the next section.
3.1 Flexibility of Donor–Acceptor Dyads
Following the discussion in Sect. 2.1, we will start with a presentation of the results
that have been gained for the dyads 1 and 2. An important parameter in the
description of EET is the mutual orientation of the transition dipoles, typically
expressed by the orientation factor κ. In Fo ¨rster theory [12] this orientation factor
appears squared [see Eq. (1)] and κ
2 assumes values of 4 for collinear aligned
transition dipoles and 1 for parallel transition dipoles. From early single-molecule
measurements on dyad 1 [1], it was known that the two transition dipoles are not
aligned in a truly collinear fashion. In these experiments, which used annular
illumination to extract the 3D orientation of the transition dipoles of PDI and
TDI, an average deviation from a collinear alignment by 22
was found. Later
experiments used rotation polarization of the excitation light rather than annular
illumination. In these studies, which allowed faster data acquisition, the PDI and
TDI chromophores were selectively excited in alternate fashion by two laser
sources [124]. The new results nicely confirmed the original distribution concerning
the relative orientation of transition dipoles. Moreover, it was found that the
flexibility of the oligo( p-phenylene) spacer is the origin of the deviations from
collinearity; each p-phenylene group on average contributes 12
to the deviation.
The experimental results were supported by quantum chemical calculations from
Optical Properties of Assemblies of Molecules and Nanoparticles
93
Energy Transfer
Lately, the quest for investigating electronic coupling at the single molecule level has
led to a number of experimental and theoretical studies. Specific topics that have been
addressed include the investigation of coherent coupling and localization,
superradiant emission, singlet–singlet annihilation and the origin of collective “off”
states [117–123]. In this section, single-molecule experiments conducted with D–A
compounds described in Sect. 2.1 are presented. As already outlined in Sect. 2.1, the
D–A-model systems have been specifically designed to gain quantitative and novel
information by SMS on the rate and mechanism of electronic EET. One important
factor has been to arrange the donor(s) and acceptor(s) via fairly rigid oligo
(phenylene) bridges at fixed distances and orientations. The chromophores PMI,
PDI and TDI were chosen because they possess favorable properties for SMS,
with large absorption cross-sections, high emission quantum yields and low
photobleaching yields. In the dyads and triads, the donor and acceptor chromophores
can be selectively excited, and all the chromophores give rise to sharp zero-phonon
lines at cryogenic temperatures, which is a crucial prerequisite for extracting EET
times from line width measurements. These outstanding characteristics of the
multichromophores have fostered a number of novel single-molecule results, which
are discussed in the next section.
3.1 Flexibility of Donor–Acceptor Dyads
Following the discussion in Sect. 2.1, we will start with a presentation of the results
that have been gained for the dyads 1 and 2. An important parameter in the
description of EET is the mutual orientation of the transition dipoles, typically
expressed by the orientation factor κ. In Fo ¨rster theory [12] this orientation factor
appears squared [see Eq. (1)] and κ
2 assumes values of 4 for collinear aligned
transition dipoles and 1 for parallel transition dipoles. From early single-molecule
measurements on dyad 1 [1], it was known that the two transition dipoles are not
aligned in a truly collinear fashion. In these experiments, which used annular
illumination to extract the 3D orientation of the transition dipoles of PDI and
TDI, an average deviation from a collinear alignment by 22
was found. Later
experiments used rotation polarization of the excitation light rather than annular
illumination. In these studies, which allowed faster data acquisition, the PDI and
TDI chromophores were selectively excited in alternate fashion by two laser
sources [124]. The new results nicely confirmed the original distribution concerning
the relative orientation of transition dipoles. Moreover, it was found that the
flexibility of the oligo( p-phenylene) spacer is the origin of the deviations from
collinearity; each p-phenylene group on average contributes 12
to the deviation.
The experimental results were supported by quantum chemical calculations from
Optical Properties of Assemblies of Molecules and Nanoparticles
93