Substituent Effect Induces Emission Modulation of Stilbene Photoswitches by Spatial Tuning of the N/B Electronic Constraints

Article abstract of DOI:10.1021/acs.orglett.0c01777

We present a family of stilbene derivatives (1-4) ortho-functionalized with an electron donor and acceptor. Their emission was tuned by donor substitutions, dictating charge transfer dependent on the N···B spatial separation. The steric reduction of subst

Full text of DOI:10.1021/acs.orglett.0c01777

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Substituent Effect Induces Emission Modulation of Stilbene
Photoswitches by Spatial Tuning of the N/B Electronic Constraints
Guangqian Ji, Nan Wang, Xiaodong Yin, and Pangkuan Chen*
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ABSTRACT: We present a family of stilbene derivatives (1−4) ortho-
functionalized with an electron donor and acceptor. Their emission was
tuned by donor substitutions, dictating charge transfer dependent on the
N···B spatial separation. The steric reduction of substituents on N leads
to stronger emission responses upon trans-to-cis photoisomerization.
Upon further approach, a bistable isomer was photochemically achieved
(4: R1 = Me, R2 = H) with quenched emission, which is ascribed to the
pronounced B···N constraint via B ← NH coordination in cis-4.
ignificant advances in diverse smart materials have recently
been stimulated by the marked evolution of intelligent
Scheme 1. Proposed New Photoswitchable Stilbenes with
Spatial Effect on the N/B Constraints
S
technology.1 Among numerous stimuli-responsive materials,
well-defined photoactive switches have particularly invigorated
the design and synthesis of light-driven structurally reversible
systems such as diarylethene, dimethylcethrene, spiropyran,
azobenzene, and diazocine derivatives.2−7 This could be
ascribed to the prominent prospect of easily accessible,
reliable, and macroscopic color-switching properties with a
high spatiotemporal resolution under specific light irradiation.
In contrast, the other analogues of photosensitive stilbenes can
also behave as molecular switches but with different photo-
chemistry.8
Strongly luminescent materials have attracted intense
research interests for applications in many areas such as
(opto)electronics, sensors, and devices.9 The triarylborane
(Ar3B) moiety represents an important electron-accepting
unit,10,11 which affords fascinating interactions with electron
donors either through π-conjugation or by Lewis-pair affinity
(X → B, X = N, O, P).12 We have recently reported a synthetic
approach to a para-functionalization on a half moiety of
stilbenes by Ar3B and Ar3N groups,13 leading to the isolation of
stable intermediates of 4a,4b-dihydrophenanthrenes (4a,4b-
DHPs) that drove the trans-to-cis isomerization and electro-
cyclic ring closure reactions. (See Scheme 1.)
issue as well.14 We herein propose a series of molecular
switching systems (1−4) that allow the modulation of the
spatial separation between the B/N Lewis pairs upon
photoinduced isomerization (Scheme 1). These structural
characteristics can also enable the B/N electronic interactions
to be further enhanced to circumvent the free rotation of
stilbenes due to the increment of intramolecular constraints in
the order of 1cis < 2cis < 3cis < 4cis.
Tuning the dynamic electronic interactions of Lewis pairs
(B/N) imparted by olefin chromophores (CC) is of
fundamental importance, albeit it is still challenging, likely
due to the fact that stilbenes undergo trans-to-cis isomerization
at low efficiency with thermodynamically metastable Z-
isomeric products. The uncontrollable rotations (e.g.,
rotamers) of phenyl rings could result in such an experimental
Received: May 26, 2020
https://dx.doi.org/10.1021/acs.orglett.0c01777
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

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The details of the synthesis are described in the Supporting
Information (SI). Single crystals of 1−4 were grown by slow
evaporation from solutions in CH2Cl2/methanol, and their
structures were characterized by X-ray crystallographic analysis.
As shown in Figure 1, all B centers are in sp2 trigonal geometry.
Figure 2. (a) UV−vis and emission spectra recorded in THF for
trans-1−4. (b) Solvatochromic emissions for trans-1 and trans-4.
Figure 1. Crystal structures of cis-1, trans-2, trans-3, and trans-4. The
structure of trans-3 was acquired from ref 15. The hydrogen atoms on
aromatic rings and methyl groups are omitted for clarity (50% thermal
ellipsoids).
optical gaps (Egap(opt)) determined by the experimental
absorption onsets (Table 1 and Table S5).
To show the contribution of differential substitution to the
spatial tuning of B/N electronic interactions within the
stilbene-based photoswitching systems 1−4, their electronic
structures upon trans-to-cis isomerization were monitored
using absorption and emission spectroscopy. As shown in
Figures S25 and S26, no significant change occurred in the UV
absorption spectrum of 1 and 2, suggesting a low photo-
switching efficiency, in reasonable agreement with Φconv = 40%
Configurations of trans stilbenes for crystals 2, 3, and 4 are
consistent with those in solutions with distinct coupling
3
constants of J(1HCCH) = 16, 12, and 16 Hz, respectively,
determined by H NMR data (2: 7.54/7.50 and 7.23/7.19
ppm; 3: 6.83/6.80 and 6.57/6.54 ppm; 4: 7.40/7.36 and 6.97/
6.93 ppm). These structures show an increased twisting angle
between the two phenyl rings of stilbenes and a reduced B···N
distance with the steric decrease in donor substituents on N
(inset, Figure 1), indicating the enhancement of the
1
for 1 and 15% for 2 determined by H NMR integration
3
analysis with a decrease in J(HCCH) of 4 Hz. (See the SI.)
thermodynamic stability in the order of 2trans < 3trans < 4trans
.
This low photoconversion rate may be a result of π
conjugation stabilization by Ph substituents on N that leads
to only a little change in their emission spectra after UV
irradiation. The existence of rotamers in cis-1 and cis-2 may
also contribute to their emission due to a weak B···N
interaction that is again induced by large sterics on N moieties.
In stark contrast, the absorption spectra of 3 showed a
clearly visible increase at ∼380 nm (Figure 3a), strongly
indicating that the ICT is more favorable in cis-3 after
isomerization than in the original trans-3. As a remarkable
consequence, the emission of 3 is enhanced immediately after
the UV-triggered photoswitching with an increased fluores-
cence quantum efficiency, Φfl (Figure 3b). In comparison to 1
and 2, an apparent increment in the emission of 3 was
supposed to be ascribed to the reduced sterics on the N donor
(R1 = R2 = Me) that, in general, benefits the spatial separation-
dependent CT state emission in the cis form. Interestingly,
when the steric effect of donor substitution even further
decreases (R1 = Me, R2 = H), the optical responses are
completely reversed in 4. As shown in Figure 3c, the
characteristic CT band at 388 nm in the absorption undergoes
an evident disappearance upon UV light stimulation,
concomitant with a gradually attenuated emission and a loss
of Φfl = 60 to 13% in CHCl3 (Figure 3d). Similar experiences
were also found for the emission of 4 in other organic solvents
(Figure S27).
This trend can also be reproduced by density functional theory
(DFT) computational studies (Figure S38). However, the
observation of a cis-1 isomer in crystals is distinct from that of
the trans-1 determined by 1H NMR (3J(HCCH)= 12 Hz, 6.44/
6.41 and 6.63/6.60 ppm) in solution, which is likely ascribed
to adaptive molecular packing in the solid structure of 1.
trans-1−4 display a strong π−π* absorption at 295−325 nm,
and a remarkable shoulder was observed at 388 nm for 4,
which is ascribed to intramolecular N/B charge transfer (ICT).
These molecules are strongly luminescent (Φfl > 20%, in THF)
and show huge Stokes shifts up to 13950 cm−1 with
bathochromically shifted emissions as the steric reduction of
donor substituents (λem = 498, 508, 533, and 540 nm for trans-
1, -2, -3, and -4, respectively). Solvatochromic emission was
demonstrated as a function of solvent polarity (see the SI), as
evidenced by the blue emission in hexane shifting to yellow for
1 and to red for 4 in DMSO (Figure 2b). Electronic structure
calculations via DFT (B3LYP/CAM-B3LYP, 6-31G*) and
time-dependent (TD)-DFT (B3LYP, 6-311G**) were carried
out. (See the SI.) The frontier molecular orbitals calculated for
1−4 show similar LUMOs on B-centered triaryl acceptors and
slightly different HOMOs featuring an increasing π-conjuga-
tion over the stilbene skeleton, presumably due to a smaller
repulsion between the aryl groups in the trans configurations
upon the N donor substituents being sterically decreased. The
five lowest vertical excitations exhibit integrated electronic
transitions of π−π* (HOMO − 1, HOMO − 2, and HOMO −
3 → LUMO) and ICT character (HOMO → LUMO and
LUMO + 1). Each of these excitations is close in energy to the
To gain better insights into the photochemical behaviors of
4 in comparison with 1−3, valuable structural information was
acquired from the 1H and 11B NMR spectroscopy upon
photoswitchable isomerization (Figure 4 and the SI). At room
B
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Table 1. Photophysical and Computational Data
a
a
b
c
d
d
e
f
g
λabs (nm)
λem (nm)
τav (ns)
Φcony (%)
EHOMO (eV)
ELUMO (eV)
Egap(DFT) (eV)
ETDDFT (eV)
Egap(opt) (eV)
1
2
3
4
295
300
323
498
508
533
540
4.2
7.8
7.3
7.3
b
40
15
60
70
−4.98
−1.64
3.34
3.35
3.52
3.40
2.87
2.89
2.98
2.88
2.95
2.97
3.01
−5.06
−5.16
−5.02
−1.71
−1.64
−1.62
308, 390
2.76
a
c
d
e
Recorded in THF (c = 1.0 × 10 −5 M). Emission lifetime. trans-to-cis conversion efficiency. DFT calculations (B3LYP, 6-31G*). Egap = ELUMO
− EHOMO. Vertical excitation of S0 → S1 via TD-DFT. Egap(opt): optical energy gap.
f
g
from those remaining unchanged in 1−3 (Figures S30, S32,
and S34), and it likely identified the formation of BSp3 moieties
by coordination with Lewis-basic NH in cis-4. This is
consistent with the B ← NH coordination recently observed
in a B/N ortho-functionalized phenyl system.12i Despite the
fact that we attempted to grow crystals of cis-4, this proved to
be a considerable challenge due to the coexistence of
electronically stabilized cis-4 and unconverted trans-4 in
stock solutions, as indicated by high-performance liquid
chromatography (HPLC) traces with small differences in
their retention times (Figure S28). In comparison, thermody-
namically metastable cis-1−3 completely relaxed back to trans
isomers in solutions, leading to an easy crystallization identical
to those before UV irradiation.
The redox properties of 4 before and after treatment with
UV irradiation were recorded to corroborate the impact of
steric substitution on the B···N electronic interactions in trans-
to-cis photoisomerization. A comparison of the electro-
chemical data acquired for trans- and cis-4 using cyclic and
differential pulse voltammetry (CV and DPV) revealed a
Figure 3. Absorption and emission changes with continuous UV
irradiation of trans-3 (a, b) and trans-4 (c,d) in CHCl3.
2
second oxidation (Eox = 0.87 V, vs Fc+/Fc, in CH2Cl2) and
2
reduction band (Ere = −2.93 V, in THF) in addition to the
1
1
original bands at Eox = 0.36 V and Ere = −2.54 V that are
assigned to the one-electron oxidation on N and reduction on
B, respectively (Figure 4c and the Figure S43). The occurrence
of even more positive oxidation and negative reduction bands
further reflects the formation of new species with stronger
Lewis acid−base interactions, likely resulting from the
coordination of B ← NH in the cis-4 isomer. We also
performed the fluorescence experiments as a function of T, and
its temperature dependence exhibits a great enhancement in
the broadened emission spectra of cis-4 as T increases from
298 to 368 K. Reversely, cis-3 showed a hypsochromic, slightly
reduced emission that was essentially induced by a thermal
quenching mechanism (Figure 5a,b). Again, these observations
strongly suggest that the B···NH interactions via coordination
in cis-4 are gradually cleaved upon T stimulus, leading to the
recovery of efficient ICT and strong emission. Thus all of the
11B NMR, CV and DPV, and VT emission data support the
modulation of B/N constraints imposed by properly tuned
sterics on the N donor during the photoisomerization of B,N-
functionalized stilbenes.
The backward isomerization of cis-1−4 can be thermally
induced at high T, as demonstrated by the full recovery of the
1H NMR spectra of trans isomers. (See the SI.) Under ambient
conditions, however, the cis-4 was found to be very stable
without spectroscopic changes observed in solution over 5 days
of storage (Figure S37). In comparison, cis-3 undergoes slow
inversion under similar conditions (Figure S36). The unique
stability of cis-4 was attributed to the pronounced B···N
constraints that greatly circumvent free rotations of the two
phenyl rings against the CC bond. Controlled dynamic
switching of the emission was also enabled by its remarkable
1
Figure 4. Photoisomerization of 4 monitored by (a) H NMR and
(b) 11B NMR in C6D6. (c) Cyclic voltammetry (vs Fc+/Fc) of 4
before and after UV irradiation, ν = 100 mV/s.
temperature, the 1H NMR signal corresponding to cis-4
examined by 3J(HCCH) = 8 Hz was increasing with continuous
UV light irradiation, together with a decrease in the original
11B signal at δ 76 and the development of a new band at δ = 45
(Figure 4b). This highfield-shifted 11B signal in 4 is distinctive
C
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obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
Pangkuan Chen − Beijing Key Laboratory of Photoelectronic/
Electrophotonic Conversion Materials, School of Chemistry and
Chemical Engineering, Beijing Institute of Technology of China,
Beijing 102488, People’s Republic of China; College of
Materials and Chemical Engineering, Key Laboratory of
Inorganic Nonmetallic Crystalline and Energy Conversion
Materials, China Three Gorges University, Yichang 443002, P.
R. China; orcid.org/0000-0002-8940-7418;
Email: pangkuan@bit.edu.cn
Authors
Guangqian Ji − Beijing Key Laboratory of Photoelectronic/
Electrophotonic Conversion Materials, School of Chemistry and
Chemical Engineering, Beijing Institute of Technology of China,
Beijing 102488, People’s Republic of China
Nan Wang − Beijing Key Laboratory of Photoelectronic/
Electrophotonic Conversion Materials, School of Chemistry and
Chemical Engineering, Beijing Institute of Technology of China,
Beijing 102488, People’s Republic of China; orcid.org/
0000-0001-5973-4496
Xiaodong Yin − Beijing Key Laboratory of Photoelectronic/
Electrophotonic Conversion Materials, School of Chemistry and
Chemical Engineering, Beijing Institute of Technology of China,
Beijing 102488, People’s Republic of China
Figure 5. (a,b) Variable-temperature (VT)-dependent emission
spectra of cis-3 and cis-4 isomers recorded in toluene. (c) Fatigue
resistance of emission at 502 nm of 4 with alternating UV irradiation
and heating to 120 °C.
photostability. Upon alternating the forward photoisomeriza-
tion and backward thermoisomerization, the emission intensity
was reversibly switched between two different states of trans-
and cis-4 isomers. No significant degradation of the emission
was observed over many cycles of treatment via UV irradiation
and heating in the high-boiling-point solvent toluene (Figure
5c), indicating a robust fatigue resistance of molecular switch
4. This type of bistable photoswitching system with long
lifetimes will be crucial to sustainable applications in future
smart devices.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c01777
Notes
In summary, we have described a new design and facile
access to photoswitchable luminescent stilbenes 1−4. We have
corroborated the key role of substitution sterics in controlling
the photoisomerization and B/N electronic interactions. As the
steric effect of substituents was systematically tuned, the highly
enhanced ICT and emission properties were demonstrated for
3 upon its exposure to UV light. Remarkably, the further
reduction of sterics resulted in a pronounced B ← NH
coordination in the ring strain system of cis-4. Such an
electronically stabilized B···N constraint circumvents the free
rotation of stilbene aromatic rings, enabling a long-lived
bistable system with unique emission responses. We envision
that this work will offer new opportunities for the design of
advanced luminescent materials and smart devices using
controllable photoswitching scaffolds.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the National Natural
Science Foundation of China (NSFC) (no. 21772012). We are
greatly thankful for Prof. Suning Wang at Queen’s University
for helpful discussion in the past few years. We thank the
Analysis & Testing Centre at Beijing Institute of Technology
for advanced facilities.
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