Journal of the American Chemical Society
Article
Figure 2. Microcrystal electron crystallographic analysis of BPPV2 at
0.95 Å resolution at −180 °C. (a) A fine powder of an isolated
sample. Scale bar 1 cm. (b) Low-magnification TEM image. Scale bar
1 μm. (c) Packing structure of BPPV2 in a microcrystal. (d)
Representative structural data (in Å and °; substituents are omitted)
of BPPV2 (EC). Data in brackets are calculated at the B3LYP/6-31G
(d, p) level.
analysis of a microcrystalline powder18,19 or of crystals with a
low degree of crystalline order such as organic solid
solutions.6b Microcrystals obtained by concentration of the
eluate from silica gel chromatography (Figure 2a) were placed
on an electron microscope grid coated with a 20 nm
after another (Figure 2b) and from 29 data sets acquired an
optimum three-dimensional molecular structure of BPPV2 (
see Figure S3) and the crystal packing shown in Figure 2c,d.
One unit cell contains two molecules, each of which maintains
3.39 Å π−π interaction with its neighbor on the terminal
pyridine groups (Figure 2c). The ring strain of the cyclopenta-
[c][1,2]azaborole system is highlighted by the extremely large
bond angles around the tetrasubstituted olefin (Figure 2d,
blue). In contrast to BPPV2, the molecules of BBPV1 in the
crystal are held together by CH−π interactions at a distance of
2.95 Å instead of the π−π interaction as shown by X-ray
2.3. Photophysical Properties. The photophysical
properties of B,N-p-arylenevinylenes as well as the parent
COPV compounds are summarized in Figure 3 and Table 1,
where we see significant bathochromic shifts by 42 and 83 nm
in solution for BPPV1 and BQPV1, respectively, relative to
COPV1 absorbing at 336 nm. We find a comparable shift for
BPPV2 (44 nm) and BQPV2 (100 nm) over COPV2. Thus,
the absorption and emission of BQPV2 at 520 and 540 nm,
respectively, are comparable to those of COPV6 (522 and 536
nm).4b These molecules undergo light absorption with high
molar extinction coefficients (εmax = 1.40−5.45 × 104), and
light emission with nearly quantitative FLQY (ΦF = 0.96−
1.00) and with a fluorescence lifetime reaching τ = 6.7 ns for
BPPV1 and BQPV1 (see Figure S5). The Stokes shift (SS)
values are generally small for B,N-p-arylenevinylenes. As found
for COPVs, the rigid structure enhances the rate of radiative
decay (kr) relative to that of nonradiative decay (knr), which is
negligibly small (Table 1). The SS values are very small for the
Figure 3. Photophysical properties of BPPVs and BQPVs in solution
and thin film. (a) UV−vis absorption and (b) emission in DCM. (c)
UV−vis absorption and (d) emission in film. (e) Comparison of
absorption and emission wavelengths of BPPVs and BQPVs with
COPV1−6. (f) Photographs of colors in DCM under room light. (g)
Photographs in DCM and in film under 365 nm light irradiation.
C2-symmetric BPPV2 and BQPV2 (19−20 nm), further
attesting to the rigidity of the π-system.
Notably, the FLQY values of BPPV1 and BQPV1 are nearly
quantitative in their drop-cast films (ΦF = 1.00 and 0.95 in a
film for BPPV1 and BQPV1, respectively) as they are nearly so
in solution (ΦF = 1.00 and 0.96). Other absorption/emission
profiles in the film state are summarized on the right side of
Table 1. The data in the film state compare favorably with their
solution counterparts, reflecting the very small intermolecular
interactions found for the crystal of BPPV1 (see Figure S4).
On the other hand, for the doubly cyclized congeners, the
FLQY of BPPV2 and BQPV2 decreased from ΦF = 0.96 and
0.98 in solution to 0.75 and 0.56 in film, respectively, which we
may ascribe to π−π interactions (3.39 Å, Figure 2c) of the
terminal aromatic rings as found in the BPPV2 crystal (Figure
2c). The observed bathochromic shift of BQPV1 and BQPV2
upon going from solution to film may also stem from this π−π
interaction.
The electrochemical properties were studied using cyclic
voltammetry (CV) and differential pulse voltammetry (DPV)
in dichloromethane (DCM). Unlike COPV1 (0.86 V) and
COPV2 (0.53 V), which undergo reversible oxidations,
BPPV1 and BQPV1 undergo a single irreversible oxidation
at 0.87 and 0.90 V, respectively. In contrast, the doubly B/N-
C
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX