2
H. Qian et al. / Tetrahedron xxx (2017) 1e4
isomerization. This reversible fluorescence intensity regulation can
be repeated multiple times by switching the configuration of the
azo-BF2 switch 1 using visible light (Fig. 1c).
2.2. Isomerization of azo-BF2
2
We evaluated the photoswitching efficiency of 2 in CD2Cl2 using
1H NMR spectroscopy. The equilibrated mixture of 2 (under dark)
consists of >99% of the E isomer (Fig. S12a). Upon 650 nm light
irradiation, the E-dominant sample reaches a photostationary state
Scheme 1. Chemical structures of azo-BF2 switches 1 and 2.
(PSS650) consisting of 88% Z isomer, with a quantum yield (F) of
52 4% (Fig. S12b). The reverse photoisomerization process (Z/E)
using 480 nm light irradiation yields a mixture consisting of 63% E
form.13 When excited at lex ¼ 508 nm,15 1-E exhibits an intense
emission band at lem ¼ 636 nm (Fig. 1b).16
isomer with FZ/E of 41
3% (Fig. S13). Previously, we demon-
We speculate that the emission is stemming from the phenan-
thridinyl BF2 core in 1 as the parent quinolinyl-based systems re-
ported so far are not emissive. Upon irradiation at 600 nm, 1-E
isomerizes to give 1-Z, accompanied with a decrease in the in-
tensity of the red emission band.17 The emission band reverts back
to its original intensity when the solution is left to equilibrate under
dark, or upon 430 nm light irradiation, which results in back
strated how aggregation influences the thermal relaxation kinetics
of 1.13 To examine whether 2 behaves similarly, concentration-
dependent kinetic studies were performed. We first used dy-
namic light scattering (DLS) to study the aggregation behavior of 2
in solution. No DLS signals were observed at a concentration of
9.0 ꢁ 10ꢀ6 M. However, when the concentration was increased to
9.0 ꢁ 10ꢀ3 M, particles with hydrodynamic radii of 17
1 nm
(Fig. S14a) were detected. Irradiation of the sample with 650 nm
light yields a Z-dominant solution, however, the particle size
(18 3 nm) is unaffected (Fig. S14b). The DLS data suggest that 2
might also exhibit different isomerization kinetics because of ag-
gregation. To our surprise, the half-life does not change when going
from high (1.9
0.3 h), to low concentrations (1.6
0.1 h)
(Fig. S10eS11). An explanation to the difference in behavior be-
tween 1 and 2 might be the prevalence of inversion, rather than
rotation mechanism in the para substituted azo-BF2 switch, 2. This
explanation is in line with recent DFT calculations, which showed
that the isomerization mechanism in the azo-BF2 switches changes
from rotation to inversion upon para-substitution.18,19
X-ray crystallographic analysis of single-crystals of 2 provided
further insight into its aggregation behavior (Fig. 2). The switch
adopts the E configuration in the solid. Each asymmetric unit
contains two crystallographically independent single-crystal
structures, which are distinguishable by the orientation of the p-
OMe group (Fig. S15). More importantly, these two structures stack
together in
(3.7658(1) Å) (Fig. 2). In addition to a different packing mode
(herringbone vs. head-to-head), the interaction in 2 is weaker
than in 1, which is illustrated by the longer interaction dis-
tance (3.7658(1) vs. 3.4956(0) and 3.4658(0) Å) and one less
a herringbone form through a p-p interaction
p p
ꢀ
pꢀp
p
interaction. This property might explain why much larger aggre-
gates are observed in 1 than in 2 (for example 92 vs. 17 nm at
~9 ꢁ 10ꢀ3 M). The less compact and smaller aggregates in addition
to the inversion mechanism, can together explain why the thermal
isomerization rate of 2 is not affected by aggregation.
Fig. 1. (a) Visible light induced E/Z isomerization of azo-BF2 1; (b) Fluorescence
(
lex ¼ 508 nm) spectra of 1-E before and after irradiation with 600 nm light; (c)
Isomerization cycles of 1 (9.0 ꢁ 10ꢀ6 M) in CH2Cl2 upon alternating irradiation using
600 and 430 nm light sources. The change in the emission intensity at lmax ¼ 636 nm
was monitored.
Fig. 2. Crystal packing through a
have been omitted for clarity.
p-p interaction (black dashes); the hydrogen atoms
Please cite this article in press as: Qian H, et al., Visible-light fluorescence photomodulation in azo-BF2 switches, Tetrahedron (2017), http://
dx.doi.org/10.1016/j.tet.2017.05.012