.
Angewandte
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distance between them was measured to be approximately
.65 ꢀ, which corresponds to the formation of weak p–p
approximately 125 nm from 528 nm at 0 GPa to 652 nm at
8 GPa. The range of this shift totally covered the correspond-
ing spectra of the three crystals C1 (527 nm), C2 (579 nm),
and C3 (618 nm). The density of the three crystal polymorphs
increased from C1 to C3, and increasing pressure would be
expected to shift the equilibrium to the densest form. Thus,
the application of a higher pressure may lead to a material
with a much greater density than that of the C3 polymorph.
The piezochromic luminescence observed indicated that
when pressure was applied, BP2VA powder underwent
a transformation between the molecular aggregation states
of the three crystals (Figure 3). In this process, external
3
interaction between the anthracene planes (Figure 2b). Thus,
C2 was an orange-emissive crystal (lmax = 579 nm). In the case
of C3, molecules were bound together by a strong p–p
interaction to form pairs in which the adjacent anthracene
planes overlapped almost in a face-to-face stack with
a distance between them of approximately 3.52 ꢀ (Figure 2c).
This strong p–p interaction induced the red emission of C3
with lmax = 618 nm. The red shift of the emission relative to
those of C1 and C2 can be ascribed to the smaller band gap of
the BP2VA molecule in C3 owing to the increased band
widths of both the HOMO- and the LUMO-derived band as
a result of the increased p–p interaction, according to a tight-
[
9]
binding model. Nonetheless, another important factor for
the red-shifted fluorescence should be taken into consider-
ation, that is, the increase in exciton coupling and orbital
overlap between neighboring molecules from C1 to C2 to C3.
Increased exciton coupling and orbital overlap could lead to
a strong red shift of the emission of the lowest state of the
[3c]
coupled chromophores.
This enhanced intermolecular
coupling is consistent with the increased density of the three
crystal polymorphs (see Table S1 in the Supporting Informa-
tion). Therefore, the PL emission of BP2VA in the aggregate/
solid state could be changed by altering its molecular stacking
Figure 3. Stacking modes and corresponding emission colors for the
various molecular aggregation states in BP2VA powder.
[
10]
mode.
To gain more insight into the origin of the piezochromic
properties of BP2VA powder, we studied the phase character-
istics of the BP2VA powder by powder X-ray diffraction
pressure impelled the molecular aggregation state of BP2VA
powder to transform from J-type aggregation, such as that of
C1, to H-type aggregation, as found for C2, and further to
aggregated dimers stacked in a more tightly bound face-to-
face arrangement, as in the case of C3. Meanwhile, the
intermolecular p–p interaction strengthened gradually and
thus induced the PL spectrum of the powder to change from
a green emission (no p–p interaction) to an orange emission
(weak p–p interaction) and then to a red emission (strong p–p
interaction). Therefore, the ability of the molecular aggrega-
tion state to change upon grinding or under pressure leads to
the changeable fluorescence color and the piezochromic
effect in BP2VA powder.
In conclusion, we have discovered a novel example of
piezochromic luminescence in the molecule BP2VA, which
exhibits distinctly different fluorescence emission in its three
crystals. On the basis of single-crystal structural, photophys-
ical, and computational studies, we found that enhanced p–p
interaction between adjacent anthracene planes in the crystal
and increased exciton coupling and orbital overlap between
neighboring molecules induced the shift in fluorescence
emission from green to red. The PL spectra showed that
changes in the molecular aggregation state in BP2VA powder
upon grinding or under external pressure led to an enhanced
intermolecular p–p interaction and thus induced the piezo-
chromic luminescence. This color-switchable feature of
BP2VA may have potential for application in optical-record-
ing and temperature- or pressure-sensing materials.
(
PXRD) analysis (see Figure S5), with simulated patterns
from the single-crystal data as a reference. The PXRD pattern
of unground BP2VA powder agreed well with the simulated
XRD pattern obtained from the crystal data of C1. It
suggested that the initial samples adopt the same molecular
arrangement as that of the C1 polymorph, with J-type
aggregation along the molecular long axis and no effective
p–p interaction between the central anthracene planes.
Although some resolvable peaks of the ground sample were
consistent with those of the unground sample, the intensity
was weaker, which indicates that the initial aggregation state
was changed by grinding. The orange emission of ground
powder with lmax = 561 nm was similar to the emission of C2
(lmax = 579 nm), which showed red-shifted fluorescence and
a blue-shifted absorption relative to the fluorescence and
absorption of the unground powder (see Figure S6). These
results suggested that H-type aggregation similar to the
molecular packing of C2 possibly occurred during the grind-
[
11]
ing process, although some of the initial aggregation state
remained after grinding. The red shift observed upon grinding
of the powder possibly originated from enhanced exciton
coupling between the pairs of neighboring chromophores;
exciton coupling already occurred over a long distance in the
unground powder, despite the very small orbital overlap. This
explanation is supported by the blue shift observed from the
emission of the unground powder to that of BP2VA in
solution at 77 K, under which conditions the molecule is
[
12]
completely in a single-molecular state.
As mentioned above, the PL spectrum of BP2VA powder
under external pressure underwent a larger red shift of
1
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 10782 –10785