which a comparable flexibility was predicted (see Sect. 4.2). These results are also
consistent with experimental data obtained for dyad 2 and with earlier studies of the
persistence length of poly( p-phenylenes) [125–127].
3.2 Control of the Energy Transfer Pathway by Dual Pulse
Excitation
The reversible switching of the transition frequency [128] or the turning on and off
of EET [129] by light irradiation are prominent examples for the controlled
modification of the optical response of single molecules. In this section, experiments will be described that allowed control of the EET process in single molecules
of dyad 1. In particular, the direction of EET could be partially reverted in the sense
that the former donor became the acceptor and vice versa [4]. The experiments were
based on selective excitation of both PDI and TDI in 1 by short light pulses of
different color and with variable time delay. Different excitation and energy
transfer scenarios were created and monitored by the corresponding fluorescence
signals.
In Fig. 18a, the relevant photophysical processes that may occur in 1 are
depicted. Exciting TDI to its S 1 state leads to TDI fluorescence. On the other
hand, photons absorbed by PDI are rapidly transferred to TDI, also resulting in
TDI fluorescence. The latter energy transfer pathway will be denoted as EET.
Obviously, due to energetic reasons, it is not possible to transfer excitation energy
from TDI to PDI after exciting TDI to its S 1 state. The situation is different if two
excitations are present at the same time. Then, the EET pathway is not available
anymore because TDI is already in the excited state. Under these conditions,
singlet–singlet annihilation (SSA) as well as donor fluorescence may arise. In
SSA, energy is transferred from one electronically excited state to the other,
resulting in a higher excited state and a ground state (see Fig. 18a). The higher
excited singlet state (S n ) typically quickly relaxes to the first excited singlet state
(S 1 ), effectively quenching one photon. Please note that EET as well as SSA are
both manifestations of resonant electronic EET.
In the experiments, red (635 nm) and green (523 nm) pulsed laser sources were
utilized to selectively excite TDI and PDI, respectively. Whereas the red laser was
set to maximum power to saturate the S 0 ! S 1 transition of TDI, the green
excitation intensity had to be set considerably lower to prevent fast photobleaching
of PDI. Three different pulse sequences were implemented (A, B, C) as depicted in
Fig. 18b. In the course of period A, only green pulses were used for excitation. In
the second sequence (B), the green and red lasers were fired and both chromophores
were excited successively with a time delay of 12.5 ns (3–4 times longer than the
fluorescence decay times of the chromophores). The third sequence (C) was
optimized to prepare both chromophores in the excited state at the same time.
Applying the excitation cycle to single TDI molecules yielded fluorescence time
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T. Basche ´ et al.
consistent with experimental data obtained for dyad 2 and with earlier studies of the
persistence length of poly( p-phenylenes) [125–127].
3.2 Control of the Energy Transfer Pathway by Dual Pulse
Excitation
The reversible switching of the transition frequency [128] or the turning on and off
of EET [129] by light irradiation are prominent examples for the controlled
modification of the optical response of single molecules. In this section, experiments will be described that allowed control of the EET process in single molecules
of dyad 1. In particular, the direction of EET could be partially reverted in the sense
that the former donor became the acceptor and vice versa [4]. The experiments were
based on selective excitation of both PDI and TDI in 1 by short light pulses of
different color and with variable time delay. Different excitation and energy
transfer scenarios were created and monitored by the corresponding fluorescence
signals.
In Fig. 18a, the relevant photophysical processes that may occur in 1 are
depicted. Exciting TDI to its S 1 state leads to TDI fluorescence. On the other
hand, photons absorbed by PDI are rapidly transferred to TDI, also resulting in
TDI fluorescence. The latter energy transfer pathway will be denoted as EET.
Obviously, due to energetic reasons, it is not possible to transfer excitation energy
from TDI to PDI after exciting TDI to its S 1 state. The situation is different if two
excitations are present at the same time. Then, the EET pathway is not available
anymore because TDI is already in the excited state. Under these conditions,
singlet–singlet annihilation (SSA) as well as donor fluorescence may arise. In
SSA, energy is transferred from one electronically excited state to the other,
resulting in a higher excited state and a ground state (see Fig. 18a). The higher
excited singlet state (S n ) typically quickly relaxes to the first excited singlet state
(S 1 ), effectively quenching one photon. Please note that EET as well as SSA are
both manifestations of resonant electronic EET.
In the experiments, red (635 nm) and green (523 nm) pulsed laser sources were
utilized to selectively excite TDI and PDI, respectively. Whereas the red laser was
set to maximum power to saturate the S 0 ! S 1 transition of TDI, the green
excitation intensity had to be set considerably lower to prevent fast photobleaching
of PDI. Three different pulse sequences were implemented (A, B, C) as depicted in
Fig. 18b. In the course of period A, only green pulses were used for excitation. In
the second sequence (B), the green and red lasers were fired and both chromophores
were excited successively with a time delay of 12.5 ns (3–4 times longer than the
fluorescence decay times of the chromophores). The third sequence (C) was
optimized to prepare both chromophores in the excited state at the same time.
Applying the excitation cycle to single TDI molecules yielded fluorescence time
94
T. Basche ´ et al.