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T. Fukaminato et al.
in a suspension of THF/H 2 O mixture (Fig. 21.6d) with the same molar concentration
of 1a (1.0 × 10
−5 mol/L), respectively. Fluorescence was imaged in the dark under
continuous excitation with 436 nm light. Owing to similar fluorescence quantum
yield and emission color of 1a in THF solution and in the nanoparticle state, the
initial picture of both series of cuvettes shows almost same bright orange color.
Fluorescence in THF solution was quenched gradually and reached a dim state at
PSS. In the contrary, the nanoparticles suspension rapidly switched off to a dark state
by UV light within less than 2.5 s. This observation suggests significantly different
mechanisms of energy transfer processes of dyad 1 molecules in THF solution and
in the nanoparticle state.
Such a fascinating “giant amplification effect” can be ascribed by a very efficient intermolecular FRET process within the nanoparticles. The packing of dyad 1
molecules being quite dense in the solid matrix of nanoparticles, a single 1b molecule
can actually play the role of energy acceptor for many neighboring 1a molecules
located within the Förster radius. The intermolecular distance of dyad 1 in diluted
THF solution is too large to allow energy transfer between two dyad 1 molecules. For
example, in a dyad 1 solution (10
−6 M), the average distance between two molecules
is around 100 nm, which is much larger than its R 0 (6.8 nm). However, in nanoparticles, the molecular distance of dyad 1 molecules in the same nanoparticles is short
enough to allow efficient intermolecular energy transfer.
To quantify this giant quenching effect in nanoparticles, simplified estimations have been made in two different approaches. The fluorescence intensity
versus conversion yield profile in nanoparticles was calculated using straightforward assumptions, considering spherical nanoparticles composed of punctual and
evenly space-distributed dyad 1 molecules. This simplified model assumes that a
given 1a molecule is not quenched when the neighboring dyad 1 molecules located
at a distance shorter than R 0 are all in the 1a state, and fully quenched in all other cases.
The principle of “FRET-induced giant amplification effect,” based on intermolecular
dipole-dipole interactions at long distances, is considered to be the main photophysical process for the strong nonlinear behavior of the fluorescence quenching in dyad
1 nanoparticles. The number of quenched 1a molecules per the population of 1b unit
can be deduced from the initial slope of the curve plotted in Fig. 21.6d, providing
a huge amplification factor: around 420 1a molecules are estimated experimentally
(resp. 310 from the numerical simulation) to be quenched by intermolecular energy
transfer to a single 1b acceptor molecule.
Another way to figure out this amplification effects is to express it in terms of
photons needed to quench the whole fluorescence of the nanoparticles. Based on the
previous discussion, the condition is as following: (i) the typical size of nanoparticles
is around 25 nm containing about 7000 dyad 1 molecules each; (ii) based on the
experimental result, only 1% 1b is enough to quench more than 90% of the whole
fluorescence; (iii) each photon absorbed by DAE leads to the conversion to 1b with
an intrinsic quantum yield 1a→1b = 0.46. Consequently, we can deduce that only
150 photons absorbed by the photochromic DAE species induce the quenching of
6300 fluorophores BTD. Then, we can deduce that 42 dyad 1 molecules are quenched
per absorbed photon.
T. Fukaminato et al.
in a suspension of THF/H 2 O mixture (Fig. 21.6d) with the same molar concentration
of 1a (1.0 × 10
−5 mol/L), respectively. Fluorescence was imaged in the dark under
continuous excitation with 436 nm light. Owing to similar fluorescence quantum
yield and emission color of 1a in THF solution and in the nanoparticle state, the
initial picture of both series of cuvettes shows almost same bright orange color.
Fluorescence in THF solution was quenched gradually and reached a dim state at
PSS. In the contrary, the nanoparticles suspension rapidly switched off to a dark state
by UV light within less than 2.5 s. This observation suggests significantly different
mechanisms of energy transfer processes of dyad 1 molecules in THF solution and
in the nanoparticle state.
Such a fascinating “giant amplification effect” can be ascribed by a very efficient intermolecular FRET process within the nanoparticles. The packing of dyad 1
molecules being quite dense in the solid matrix of nanoparticles, a single 1b molecule
can actually play the role of energy acceptor for many neighboring 1a molecules
located within the Förster radius. The intermolecular distance of dyad 1 in diluted
THF solution is too large to allow energy transfer between two dyad 1 molecules. For
example, in a dyad 1 solution (10
−6 M), the average distance between two molecules
is around 100 nm, which is much larger than its R 0 (6.8 nm). However, in nanoparticles, the molecular distance of dyad 1 molecules in the same nanoparticles is short
enough to allow efficient intermolecular energy transfer.
To quantify this giant quenching effect in nanoparticles, simplified estimations have been made in two different approaches. The fluorescence intensity
versus conversion yield profile in nanoparticles was calculated using straightforward assumptions, considering spherical nanoparticles composed of punctual and
evenly space-distributed dyad 1 molecules. This simplified model assumes that a
given 1a molecule is not quenched when the neighboring dyad 1 molecules located
at a distance shorter than R 0 are all in the 1a state, and fully quenched in all other cases.
The principle of “FRET-induced giant amplification effect,” based on intermolecular
dipole-dipole interactions at long distances, is considered to be the main photophysical process for the strong nonlinear behavior of the fluorescence quenching in dyad
1 nanoparticles. The number of quenched 1a molecules per the population of 1b unit
can be deduced from the initial slope of the curve plotted in Fig. 21.6d, providing
a huge amplification factor: around 420 1a molecules are estimated experimentally
(resp. 310 from the numerical simulation) to be quenched by intermolecular energy
transfer to a single 1b acceptor molecule.
Another way to figure out this amplification effects is to express it in terms of
photons needed to quench the whole fluorescence of the nanoparticles. Based on the
previous discussion, the condition is as following: (i) the typical size of nanoparticles
is around 25 nm containing about 7000 dyad 1 molecules each; (ii) based on the
experimental result, only 1% 1b is enough to quench more than 90% of the whole
fluorescence; (iii) each photon absorbed by DAE leads to the conversion to 1b with
an intrinsic quantum yield 1a→1b = 0.46. Consequently, we can deduce that only
150 photons absorbed by the photochromic DAE species induce the quenching of
6300 fluorophores BTD. Then, we can deduce that 42 dyad 1 molecules are quenched
per absorbed photon.