C O M M U N I C A T I O N S
(Figure S12). This suggests that the yellow emission after smearing
is due to an excited-state effect rather than ground-state association.
In summary, we have reported morphology-dependent fluores-
cence and unusual reversible mechanochromic luminescence for
solid-state BF2AVB. Unlike previously reported BF2dbm(s) deriva-
tives that typically exhibit strongly red-shifted and significantly
broadened fluorescence spectra, BF2AVB(s) shows unexpectedly
sharp emission spectra that can be tuned via the solid form, such
as single crystals, dendritic solid, or spin-cast film. Single-crystal
XRD revealed that BF2AVB molecules can form multiple emissive
aggregation states with different intermolecular interactions. The
fluorescence color is dramatically altered after crushing or physically
smearing BF2AVB crystals or upon scratching or rubbing annealed
film samples. The mechanochromic fluorescence recovers repro-
ducibly. From a material design standpoint, the fact that the
scratched or rubbed region emission fades over time and merges
with the background color makes BF2AVB a self-healing optical
material. The relationship between dye structure, solid-state emis-
sion color, and recovery time is under exploration.
Figure 6. Photos (A inset) showing the BF2AVB spin-cast films under
UV excitation and their corresponding (A) emission spectra and (B) XRD
patterns for unannealed (UA) and thermally annealed (TA) samples.
Figure 7. AFM images of BF2AVB spin-cast films on glass substrates
(A) before and (B) after thermal annealing.
Acknowledgment. We thank the National Science Foundation
atoms,20,21 and other interactions could be involved. After green
BF2AVB crystals were thoroughly ground, there were no major
differences in the XRD patterns; only very minor peak shifts (<0.5°)
and relative intensity changes were observed (Figure S10).
We also attempted to create a more stable amorphous BF2AVB
film on a glass substrate by spin-coating from CH2Cl2 solution.
The emission spectrum of the resulting film resembles that of the
smeared BF2AVB solid. It is significantly red-shifted and broadened
(λF ) 530 nm, τ ) 24.7 ns) (Figure 6A). Thermal annealing at
110 °C for 5 min, however, turned the film emission blue (λF )
459 nm, τ ) 3.87 ns). BF2AVB film microstructures before and
after thermal treatment were examined with XRD and atomic force
microscopy (AFM). Figure 6B shows a sharp peak at 2θ ) 5.3°
only for the heat-treated film, suggestive of a crystalline transforma-
tion during the annealing process. AFM images reveal well-defined
rectangular sheetlike aggregates after thermal treatment (Figure 7).
Some regions of the amorphous film appeared crystalline even
before heating, suggesting a strong tendency for ordering.
Data suggest that the yellow emission may arise from the
amorphous state of BF2AVB, whereas excimers may form in
mechanically perturbed regions as a result of greater rotational
freedom.6,22,23 Furthermore, excimers may serve as low-energy
traps for exciton transfer in the crystals, which may help to explain
the observed emission color changes and relative retention of XRD
features. This hypothesis is also supported by fluorescence spec-
troscopy at 77 K, where the freshly smeared regions (which are
yellow at room temperature) become green and the spectra match
the background fluorescence of the unscratched annealed film. The
yellow emission is restored in the scratched regions as the sample
is warmed back to room temperature (Figure S11). If BF2AVB
excimer formation requires a conformational change, rigidifying
the solid may suppress this process and efficiently reduce the yellow
emission from the smeared regions. Finally, although the emission
of the smeared region is red-shifted, its excitation spectrum is
actually blue-shifted relative to that of the blue dendritic solid
(CHE 0718879) for support of this work.
Supporting Information Available: Experimental methods, sup-
porting figures, and crystallographic data (CIF). This material is
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