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fluorescence experiments further indicate that the different
emission properties of the systems under different conditions
(solution, solid state, gel) indeed result from different excited
states (see the Supporting Information).
Powder X-ray diffraction experiments (PXRD, see the
Supporting Information) were carried out with the dried gels
and revealed one strong peak in the low-angle region (1a: d =
28.3 ꢁ, 1b: d = 25.6 ꢁ, and 2b: d = 25.9 ꢁ). However, the
relatively weak and broad peaks of the dried gels cannot be
used to make unambiguous assignments. The emission spectra
of the dried gels of 1a,b and 2b are also different from those
of the corresponding hexane gels (see Figure 3 and the
Supporting Information). Therefore, the X-ray diffraction
patterns could not be utilized to study the molecular
organization of the gels. Based on the current studies,
however, instead of p–p interactions, ionic interactions are
believed to be the major driving force for the formation of the
highly fluorescent organogels, which is supported by the
variable-concentration NMR studies.[8a] A plausible molec-
ular organization for the organogels is proposed in Figure 4.
In hydrocarbon solvents, the solvophobic effect allows the
Figure 5. The solid-state emission spectra of 1a in the absence (solid
line) and presence of Rhodamine B (3:1) (dotted line) before mechan-
ical grinding (BM) and after mechanical grinding (AM) (3:1) (dashed
line). Inset: Corresponding emission decay (excitation at 466 nm and
monitoring at 533 nm) in the solid state. Solid line: 1a, dotted line:
1a and Rhodamine B (3:1) BM, crosses: 1a and Rhodamine B (3:1)
AM, IR: instrument response decay.
inset). Subsequent thermal annealing (5 min at 908C) resulted
in the recovery of the original emission spectrum. A film
containing 1b and Rhodamine (3:1) also exhibits similar
mechanically responsive energy-transfer features indicating
that the process is independent of the counteranion (see the
Supporting Information).
Encouraged by this mechanically responsive FRET, we
envisioned that the mechanically responsive emission of the
phosphole-lipid system could be further amplified through
the energy transfer from the donor to a similar acceptor
species. We thus chose 4b as the donor (blue emission) and 2b
as the acceptor (orange emission) in order to amplify the
stimulus-responsive signal. The efficient spectral overlap
between the emission of 4b and the absorption of 2b
promised for efficient FRET in the solid state. The drop-
cast film containing 4b and 2b (100:1) exhibited a typical blue
emission of 4b (l = 460 nm) with a broader red-shifted tail
(Figure 6). However, upon mechanical grinding, the fluores-
cence of the doped film shifted to orange. Figure 5a shows
that the donor emission of the ground film is considerably
quenched; the emission of the acceptor 2b (l = 567 nm) is the
main component after grinding. Importantly, the blue emis-
sion can be fully recovered by thermal annealing of the
ground film (5 min at 908C), and this process can remarkably
be repeated several times.
It is well known that FRET can occur over distances of up
to 100 ꢁ between donor and acceptor.[10] But more impor-
tantly, the FRETefficiency is highly dependent on the donor–
acceptor distance, which makes such intermolecular process
very useful for the sensing of external stimuli, particularly in
the application toward mechanical responses.[11] Based on our
studies, this mechanically responsive FRET is believed to be
mainly due to changes in the distance between the donor and
the acceptor. In the drop-cast film, the electrostatic repulsion
prevents strong intermolecular interactions between the
bulky phospholium donor (4b) and the acceptor (2b) species,
Figure 4. Plausible molecular organization of the 1D fibers of 1a,b and
2b.
phosphole-lipids to form inverse rodlike structures in which
the ionic conjugated heads aggregate through ionic interac-
tions inside the fibers and the hydrophobic tails point out into
the solvent (Figure 4, top view); this is complementary to the
aggregation of amphiphilic molecules in aqueous solution.[9]
Besides, the intermolecular interactions of the two peripheral
aromatic (phenyl or thienyl) groups of the dithieno core could
further stabilize these 1D fiberlike structures (Figure 4, side
view).[8b]
In order to investigate the light-harvesting capabilities of
the new systems, the potential energy transfer between
phosphole-lipids 1a,b as the donors and Rhodamine B as an
acceptor was studied in CHCl3 (see the Supporting Informa-
tion). Both 1a and 1b show fluorescence resonance energy
transfer (FRET) with moderate efficiencies of 43% and 46%,
respectively, at a donor/Rhodamine B ratio of 1:1 in CHCl3.
Because of the low solubility of Rhodamine B in the hydro-
carbon solvents used for this study, the energy-transfer
process was studied in the solid state instead of the gel
state. Figure 5 shows that the donor emission of 1a is partially
quenched when the drop-cast film was doped with 25% mol
of Rhodamine B. Interestingly, upon mechanical grinding of
the film, the donor emission of 1a was further quenched
significantly. Concurrently, the emission decay of donor 1a in
the mechanically ground film was shortened, indicating the
presence of a nonradiative energy-transfer process (Figure 5,
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 3964 –3968