Angewandte
Chemie
Table 2: Fluorescence decay of 1a and 2a in CH3CN, CH3CN–water
mixture (1:9, v/v), and in the solid state.
band seen for both 1a and 2a is not a result of exciton
coupling between the chromophores (Figure S20).
[a]
t1 (f1)[b] [ns]
t2 (f2)[b] [ns]
tavg[c] [ns]
The details of the molecular conformation and packing
structure were obtained by single-crystal X-ray diffraction
study.[11] Compounds 1a and 2a are nearly planar in the
crystal, and arrange into offset columnar stacks of antiparallel
dimers with close intermolecular stacking distances (3.48 ꢁ
for 1a and 3.37 ꢁ for 2a; Figure 3). Given that the transition
Compd.
lem [nm]
1a
A
B
C
D
A
B
C
D
520
630
630
630
550
650
650
650
<0.1 (1)
0.92 (1)
<0.1
0.92
4.38
4.98
0.29
1.54
2.63
2.91
1.61 (0.42)
2.42 (0.28)
0.29 (1)
0.42 (0.77)
1.42 (0.59)
1.32 (0.64)
5.04 (0.58)
5.41 (0.72)
2a
2.25 (0.23)
3.36 (0.41)
3.87 (0.36)
[a] Measured in A) CH3CN solution; B) suspended aggregates in
CH3CN:water (1:9, v/v); C) semicrystalline powder; D) single crystal.
[b] Lifetime (t) and fraction (f) of shorter (1) or longer (2) lived species.
[c] Weighted mean lifetime.
emission. In this regard, these features mirror those found for
the excimer emission of pyrene in the solid state.[1] Herein for
1a and 2a, we employ Winnikꢀs terminology of a preassoci-
ated excimer, also akin to a static excimer as opposed to
a dynamic excimer as—just as for pyrene in the solid state—
the chromophores also interact in the ground state, albeit to
a limited degree. Such interactions should be absent in
a genuine dynamic excimer according to the classical
definition in which the ground state must be dissociated
instead of only dissociative;[1] this situation prevails in
concentrated solutions of these dyes in good solvents (Fig-
ure S9). Quantum chemical calculation results demonstrate
that, despite the dark nature of their S1 state, p-stacked
dimers of 1a can emit light from the S2 state, which would be
responsible for the excimer-like emission (Figures S39–S42).
Given that both of these states are lower in energy by
approximately 0.5–0.6 eV than the S1 state of the monomer,
and that the energy offset between them is as small as circa
70 meV, it is highly probable for 1a dimers to be thermally
activated to the S2 state and to fluoresce bathochromically
therefrom, which is also consistent with the rather long
lifetime of the fluorescence of the 1a crystal.
In this context, other examples relevant to our work must
be mentioned. A first example is the observation by Weder
et al.,[13] following earlier results by Hadziioannou et al.,[14] of
a strong tendency toward excimer formation in crystalline
solid cyano-substituted oligo(p-phenylene vinylene) deriva-
tives. This tendency is facilitated by pronounced p–p inter-
actions of planarized molecules stacked in a cofacial arrange-
ment. Gierschner, Bazan, and co-workers also reported that
for fluorinated distyrylbenzenes, cofacial p stacks of slightly
offset chromophores resulted in a structureless and strongly
red-shifted excimer-like emission analogous to that found for
1a and 2a.[15] Similar observations were reported by Wꢂrth-
ner et al. for H aggregates of merocyanine dyes.[16] The recent
work of Gierschner and Park on the photophysics of
cyanodistyrylbenzenes that exhibit AIEE is also illustrati-
ve.[8g] Their observations highlight the importance of the
cyanovinyl group to achieve tight packing arrangements
in which the probability of nonradiative deactivation is
significantly decreased.
Figure 3. X-ray crystal structures of 1a and 2a drawn with thermal
ellipsoids set at 50% probability and with hydrogen atoms omitted for
clarity. a, b) Top view of the two nearest neighbor molecules in the
crystal of 1a (a) and 2a (b). The molecules of 1a (c) and 2a (d) stack
in offset columns of antiparallel dimers with displacement angles
between nearest neighbors of 768 for 1a and 478 for 2a. Atom colors:
C=gray; N=blue; S=yellow.
dipole in dyes 1a and 2a rests along the long molecular axis,
the band splittings and blue or red shifts that arise in the UV/
Vis absorption spectra of 1a and 2a upon formation of
aggregates are consistent with the observed stacking
angles.[10,12] The features present in the emission spectra of
single crystals of 1a and 2a can be directly correlated with the
packing structure determined by X-ray diffraction. AIE
features are detected prominently in the single crystals, with
strongly emissive broad bands centered at l = 624 nm (1a)
and 640 nm (2a), respectively. Since the new bands appear
under either a H-type (1a) or a J-type packing (2a), these
results also rule out exciton coupling as the origin of the new
red-shifted emission band.
The excimer-like nature of the solid-state emission was
finally established from the fluorescence lifetime of dyes 1–2
in varying media (Table 2). In dilute CH3CN solution, where
emission strictly occurs from the monomeric dyes, the decay
of the excited states of 1a and 2a is single exponential with
a short lifetime (t1a < 0.1 ns, t2a = 0.29 ns). In contrast, the
excited-state decay dynamics of 1a and 2a in both the crystal
and in semicrystalline powders are longer than those of
solution (t = 1–5 ns) and are multiexponential in character.
Moreover, suspended aggregates of 1a and 2a in CH3CN–
water mixtures (1:9, v/v) also show extended lifetimes (t1a
=
0.92 ns; single exponential). The extended excited-state life-
times that increase in prominence alongside increasing
packing quality, and the offset p-stacked arrangement exhib-
ited in the crystal, are entirely consistent with preassociated
excimer emission being the origin of the new red-shifted
We propose that 1a and 2a, which are readily excited by
visible light and show excimer emission when brought in close
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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