Diazocine Derivatives: A Family of Azobenzenes for Photochromism with Highly Enhanced Turn-On Fluorescence

Article abstract of DOI:10.1021/acs.orglett.9b01215

π-Conjugated azobenzene derivatives were reported to couple azo moieties with aryl fluorophore. Using the photochemistry of the C₂-bridged azobenzene, we have accomplished two diazocine-based photoswitching molecules (cAz1 and cAz2). In additio

Full text of DOI:10.1021/acs.orglett.9b01215

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Diazocine Derivatives: A Family of Azobenzenes for Photochromism
with Highly Enhanced Turn-On Fluorescence
†
†,‡
,†
Qian Zhu, Suning Wang,
and Pangkuan Chen*
†Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, Key Laboratory of Cluster Science of the
Ministry of Education, School of Chemistry and Chemical Engineering, Beijing Institute of Technology of China, Beijing 102488,
People’s Republic of China
‡
Department of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada
*
S Supporting Information
ABSTRACT: π-Conjugated azobenzene derivatives were
reported to couple azo moieties with aryl fluorophore.
Using the photochemistry of the C -bridged azobenzene, we
2
have accomplished two diazocine-based photoswitching
molecules (cAz1 and cAz2). In addition to remarkable
photochromic properties, they show a turn-on fluorescence,
and the emission was highly enhanced with quantum
efficiency increasing up to 0.22 for cAz1. The emission
remains persistent and can only be switched off via thermal deactivation. They display greater optical responses, compared with
the linear analogues (Az1−Az4). This work may provide a new scaffold toward advanced light-emitting materials using
photoswitchable systems.
hotochromic molecular systems represent an important
class of stimuli-responsive materials and have gained
of the NN bond on a picosecond time scale can deactivate
1
5
P
the excited fluorophore. Consequently, the universal design
principle applied to azo-based luminescent materials remains
considerably elusive. To date, several limited pathways have
been proposed for azobenzene luminescence, such as the
formation of X ← NN coordination bonds (X = B, Si, M;
tremendous attention for broad applications in optical sensors,
1
−3
memories, and photoswitching devices.
These molecules
reversibly switch configurations between their isomers, leading
to distinct color change concomitant with photoisomerization
processes modulated by the key photoswitchable functionality.
Among components of this type, the dyes of azobenzene and
derivatives have been well-established as being the most
frequently utilized photochromic compounds, because of easier
access and highly controllable photochemistry, compared to
16−18
see Scheme 1),
self-assembled chromophore aggrega-
Scheme 1. Selected Systems of Strong Fluorescent
Azobenzene Derivatives
4
−11
some other counterparts.
They usually undergo a very
efficient and reversible cis−trans photoconversion upon
exposure to UV-vis light irradiation, leading to optical
characteristics in the photophysical absorption.
Diazocines, which are new derivatives of azobenzene bridged
by ethylene in the ortho positions, were developed by
Siewertsen et al. for further improved photoswitching
1
2
behavior. As unconventional cyclic azobenzene motifs,
diazocines have been convinced that the large separation
(
∼85 nm) of n−π* transitions in the absorption bands for the
cis and trans isomers is intrinsically ascribed to the geometrical
ring constraints.
6
,13
19
20
The unique ring structure of diazocines
tion, and decoupling fluorescence mechanism. However,
azobenzene-based systems that emit intense luminescence
without shutdown of the photoswitching properties are
fundamentally important for new light-emitting materials.
Herein, we first describe the synthetic strategy of diazocine-
based conjugated luminescent materials using the ring-
plays a key role in pursuing excellent photochemical efficiency,
and quantitative photoconversions were achieved between the
two configurations. Very recently, heterodiazocines (e.g., O-
and S-diazocines) were also reported to unveil new photo-
14
chemistry in the literature.
However, the photochromic switching of azobenzene
generally proceeds at the expense of photoluminescence
capability, since the ultrafast nonradiative photoisomerization
Received: April 6, 2019
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01215
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
constrained molecular systems. We expect these compounds
might be considered as advanced photoswitchable emitters,
because their luminescence behavior could be switched at no
expense of the photochromic properties upon cis−trans
isomerizations.
shown in Figure 2a, cAz1 displays a bow-shaped ribbon
structure in good agreement with density functional theory
The synthetic procedures for cAz1, cAz2, and Az1−Az4 are
shown in Scheme 2 and Supporting Information (SI),
a
Scheme 2. Synthetic Route of cAz1 and cAz2
a
Reagents and conditions: (i) t-BuOK, Br (1.3 equiv), THF, 0 °C, 12
2
h; (ii) zinc powder, Ba(OH) (4.0 equiv), EtOH/H O, reflux, 200
2
2
Figure 2. (a) Single-crystal X-ray structures of cAz1 and Az1. H
atoms and solvent molecules are omitted. (b) Vertical excitation
energy (eV), oscillator strength (f), and key orbitals involved in
transitions to S and S states (isovalue = 0.02, TD-DFT, B3LYP/6-
min; (iii) 4-Ph NC H SnMe (2.0 equiv), Pd(PPh ) , toluene, reflux,
2
6
4
3
3 4
1
2 h, under N ; and (iv) diphenylamine (2.5 equiv), Pd(OAc) (5%
mol), Xantphos (5% mol), t-BuONa, toluene, reflux, 12 h, under N2.
2
2
4
5
3
11G**) for cAz1-Z.
respectively. We started the synthesis of diazocines by the
homocoupling reaction of 4-bromo-1-methyl-2-nitrobenzene
(
DFT) computations for the Z isomer that is thermodynami-
to give compound 1 in the presence of Br under basic
2
cally more stable than the E isomer, because of the ring
constraint of ethylene bridge.
unit leads to a bending angle (∼73.7°) between the two
moieties determined by the Ph rings of diazocines (see the SI).
The CNNC moiety is almost coplanar with a slight twist of
conditions. Subsequently, the core building block 2 was
obtained via intramolecular reductive cyclization of the
11−13
The highly curved cyclic
resulting C -bridged intermediate in dilute basic solutions of
2
EtOH/H O. Next, the Pd-catalyzed Stille and Buchwald
2
coupling reactions of 2 and aryl amines under standard
conditions led to products of cAz1 and cAz2, respectively. As
detailed in the SI, the Suzuki coupling of (E)-1,2-bis(3-
iodophenyl)diazene with Ar N- or Ar B-substituted borane
∼
7.4° and NNC angles of ∼120°. While the CCCC moiety
deviates from planarity with a pronounced torsion angle of
36.5°, leading to some configurational flexibility required for
∼
3
3
further photoisomerization. The NN bond length of
moieties resulted in the linear azobenzene derivatives Az1 and
Az3 (see structures in Figure 1). Similarly, two other analogues
1
1
.265(6) Å was found to be much shorter than that of
.531(8) Å for C−C single bond of the C bridge (Figure S28
2
in the SI). In contrast, the linear analogues of Az1 and Az2
show E isomeric crystal structures (Figure 2a (right) and the
SI), consistent with those commonly observed in parent
azobenzenes.
DFT and time-dependent density functional theory (TD-
DFT) calculations (B3LYP, 6-31G*, and 6-311G**) were
performed, and the results are summarized in the SI. Once aryl
substituents are coupled with azobenzene moieties, the
HOMO energy levels are strongly increased from −6.11 eV
for the model to approximately −4.95 eV for cAz1, cAz2, and
Az1−Az4, and the HOMO−LUMO energy gaps are hence
squeezed (Table S5 in the SI). For cAz1 and cAz2, the lowest
unoccupied molecular orbital (LUMO) levels are reduced by
Figure 1. Structures of N- and B-functionalized π-conjugated azo
compounds.
∼
0.2 eV as the configurations are switched from the Z-isomer
to the E-isomer, leading to a stronger electron-accepting
diazocine functionality, together with a red-shifted optical
excitation by 50 nm. cAz1-Z shows the highest occupied
Az2 and Az4 were prepared through Buchwald C−N coupling
reactions. These molecules were fully characterized by nuclear
1
13
11
magnetic resonance ( H, C, and B NMR) and high-
resolution mass spectroscopy (HRMS) spectroscopic analyses.
The solid-state structures of cAz1, Az1, and Az2 were
examined by single-crystal X-ray diffraction (XRD) analysis.
The crystals of cAz1 were obtained by slow evaporation from
the solution of dichloroethane/methanol (DCE/MeOH). As
molecular orbital (HOMO) mainly localized on terminal Ar N
3
moieties, while the LUMO is fully delocalized over the π-
conjugated system involving diazocines and the N moieties
(Figure 2b). However, the stretched cAz1-E isomer has a less
efficient π-conjugation of the HOMO and LUMO (Figure S43
in the SI). Vertical excitations to the S , S , and S states have
1
2
3
B
DOI: 10.1021/acs.orglett.9b01215
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
similar energies and weak oscillator strengths, and other
transitions are likely much more effective (Table S7 in the SI).
As a representative example, cAz1 shows the n−π* transition
nm in the E configuration (Figure 3a). Similar spectral changes
were observed for cAz2 but with an even faster isomerization,
within 50 s, in comparison to cAz1 and Az1−Az4 (see Figure
3b, as well as Figure S29 in the SI). Surprisingly, under the
visible light irradiation at 410 nm, the cAz1-E isomer was
found to be highly photochemically stable without significant
to S (HOMO → LUMO+1, 356 nm, 3.49 eV, f = 0.5555) and
4
π−π* transition to S state (HOMO−1 → LUMO+1, 346 nm,
5
3
.59 eV, f = 0.6920) (Figure 2b; also see Table S6 in the SI).
To explore the switchable photochromic properties, the
1
changes in absorption and H NMR signals. Alternatively, the
photophysical behaviors of these compounds were investigated
back conversion to the Z isomers can be accomplished for
cAz1 and cAz2 upon heating to ∼70 °C in 10 min. The full
1
by UV-vis absorption and H NMR spectroscopy (see Figures
1
3
and 4, as well as Figures S29−S35 in the SI). Solutions of
recovery of Z configurations was confirmed by H NMR
studies (see Figure 4 and the SI). They behave distinctively
from the photoreversible parent diazocines and heterodiazo-
12,14
cines reported previously,
which may be attributed to the
electronic stabilization via π-conjugation. The photoisomeriza-
tion efficiencies (Φconv) were acquired from the UV-vis
absorbance determined at the photostationary state of THF
solutions and are summarized in Table 1. Compared with
parent molecules, the relatively small photoconversion from
14% to 46% could also be induced by π-extension of the
conjugated skeleton.
Table 1. Summary of the Absorption, Photoluminescence
(
PL), and Fluorescence Quantum Efficiency (Φ)
a
b
b
a
b
b
c
λabs
λabs
λPL
(nm)
390
ΦPL
ΦPL
τav
Φconv
(
nm)
(nm)
(%)
(%) (ns)
(%)
cAz1
cAz2
Az1
Az2
Az3
Az4
336
300
328
302
331
311
335, 386
300, 380
327, 388
302, 368
331, 393
310, 375
<1
<1
<1
1.0
<1
1.0
22
6
14
9
8
16
1.9
2.5
1.3
5.5
3.7
7.4
14
19
14
39
18
46
366, 461
394
Figure 3. Absorption and photoluminescence (PL) spectral change of
a,b) cAz1 and (c,d) cAz2 upon irradiation under UV light (λ = 300
nm) in THF (c = 1.0 × 10 M, λex = λabs(max)).
(
362
−
5
390, 488
363, 472
a
−5
Recorded in THF (c = 1.0 × 10 M) before irradiation under UV
b
light (λ = 300 nm). Recorded after UV irradiation. λ and λPL are
abs
c
shown with maximum values. Photoisomerization efficiency
determined by UV-vis absorption data.
Interestingly, these molecules show luminescence after
photoisomerization in contrast to common azobenzene
derivatives (Figure 5). Upon UV irradiation at λ = 300 nm,
Figure 5. (a) Emission spectra of cAz1, cAz2, and Az1−Az4 recorded
1
Figure 4. Comparison of aromatic H NMR signals of cAz1 between
−5
in tetrahydrofuran (THF) (c = 1.0 × 10 M, λ = λ
) after the
ex
abs(max)
the cis and trans isomers on exposure to the UV light (λ = 300 nm)
and thermal recovery in 10 min. Inset photographs show photo-
chromic visualization of solutions.
UV irradiation (λ = 300 nm). (b) Photographs of the emissive
solutions in THF under UV lamp (λex = 365 nm).
for example, a highly enhanced emission band quickly develops
at 390 nm, with a remarkable increase in the fluorescence
quantum efficiency from Φ < 1% for cAz1-Z to 22% for cAz1-
cAz1 and cAz2 are light yellow under ambient conditions.
Upon UV irradiation at 300 nm, the colors turn dark brown,
because of the forward photoisomerization. The THF solution
of cAz1 shows a decrease in the strong n−π* transition at λ =
E (Table 1). The average fluorescence lifetimes (τ ) were
av
measured on the nanosecond scale after photoisomerization.
To explore the fluorescence mechanism, we additionally
investigated the photophysical properties of nonsubstituted
diazocine and acyclic azobenzene for comparison with
corresponding further functionalized derivatives. Although
3
36 nm, together with an increase in the bathochromically
shifted absorption at λ = 386 nm. This features the typical
12
switching for the Z → E isomerization of diazocines. In
addition, the π−π* excitation is further enhanced around 310
C
DOI: 10.1021/acs.orglett.9b01215
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
the two parent systems show pronounced photochromism in
their absorption spectra with characteristic optical responses
(
Figure S30 in the SI), almost no individual emission could be
identified upon photoisomerization. Therefore, we conclude
that the functionalization of π-extension with aryl fluorophore
strongly contributes to the turn-on fluorescence, including
triphenylamino and triarylborane moieties (Az1−Az4) and
other groups.
To examine the responsive validity of diazocine-based
materials, we compared the cis−trans isomerization rate and
photochromic sensitivity. Previous studies on the cis−trans
photoisomerization of azobenzenes have shown that the
photoconversion follows the first-order kinetics and the rate
constant k can be acquired from the equation
Figure 7. (a) Photoluminescence durability based on emission
measurements for cAz1 in THF. Inset shows the expansion of the
emission spectra before UV irradiation and after heating. (b)
Photographs of the solution cAz1 in THF: before UV irradiation
under ambient light (left, Z), after UV irradiation under ambient light
(middle, E), and with irradiation using a UV hand lamp (right, E),
respectively.
Ä
É
Å
Ñ
Å
Ñ
Å (A − A ) Ñ
Å
0
∞ Ñ
lnÅ
Ñ = kt
Å
Ñ
Å
Ñ
Å (A − A ) Ñ
Å
Ç
t
∞ Ñ
Ö
1
was observed in H NMR over several cycles. A similar
where A , A , and A are the absorbances before UV
0
t
∞
property was also deduced for cAz2 (see the SI). Such a
phenomenon is in contrast to those of UV-vis photoswitching
systems, which is of considerable advantage for sustainable
light-emitting applications. The effective extension of π-
conjugation may have a great impact on this.
irradiation, at time t, and after irradiation for prolonged time,
8
respectively. The forward isomerization absorption data were
fitted into the equation for cAz2, Az2, and Az4 with similar π-
conjugation length. As shown in Figure 6a, the derived
In conclusion, we have successfully synthesized the first
examples of conjugated photochromic materials based on the
unique ring-constrained photoswitchable diazocine moieties.
Spectroscopic studies elucidated that cAz1 and cAz2 undergo
a faster photoswitching isomerization, in comparison to the
parent azobenzene analogues, indicating that the diazocine-
based molecular systems would ideally serve as highly efficient
photochromic switches. Meanwhile, these molecules can
persistently remain in the fluorescence “on” state rather than
energy-transfer-induced luminescence quenching upon photo-
isomerization. Instead, the stable fluorescence can only be
switched off via thermal quenching due to the back
isomerization. We envision that the key findings on
uncommon diazocines provide a proof of concept for the
design of new light-emitting materials, based on multifunc-
tional conjugated azobenzene derivatives.
Figure 6. (a). Data fitting for comparison of the cis−trans
isomerization rate based on the absorbances at λ
of the initial
abs(max)
bands for cAz2, Az2, and Az4 with similar π-conjugation length. (b)
Comparison of the photochromic sensitivity based on the absorbance
changes at λabs(max) of the new bands (c = 1.0 × 10− M, THF).
5
conversion rate constants (k) were found to be 2.4 × 10 s−1
−
1
for cAz2, 6.6 × 10−3
−1
−3 −1
ASSOCIATED CONTENT
Supporting Information
s
for Az2, and 3.7 × 10
s
for Az4,
■
*
S
indicative of a highly efficient photoswitching of cAz2. Data
fitting of other molecules is shown in Figure S40 in the SI, and
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b01215.
−
2
−2
we found their k values to be 1.1 × 10 , 5.2 × 10 , and 1.1 ×
−
2
−1
1
0
s
for cAz1, Az1, and Az3, respectively. We also
inspected the absorbance difference upon the completion of
photoisomerization (Figure 6b). A greater color change and
more sensitive response were achieved for cAz1 and cAz2, with
respect to other linear analogues. Hence, the diazocine-based
molecular systems are superior in photochromic switches.
To evaluate the photostability of the fluorescence properties
of these molecules, we chose cAz1 and cAz2 as representative
examples and monitored the photoluminescence spectra (see
Figure 7, as well as Figure S41 in the SI). Similar to the trend
Experimental procedures, analytic data (1H, 13C, 11B,
and MS) for all products and intermediates (PDF)
Accession Codes
CCDC 1902679, 1902683, and 1906983 contain the
supplementary crystallographic data for this paper. These
data can be 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.
1
observed in H NMR for cAz1, the highly enhanced emission
after UV irradiation remains unchanged on the visible-light
irradiation (e.g., at λ = 410 nm), and no fluorescence decay
occurred over 10 days of exposure under ambient conditions.
However, the new emission band at 390 nm was fully
quenched after heating at 70 °C. These observations strongly
suggest that the fluorescence “on” state of cAz1-E is light-
tolerant and can only be switched back to the “dark” state of Z
isomer in response to temperature T. No fatigue/degradation
AUTHOR INFORMATION
Corresponding Author
■
*
E-mail: pangkuan@bit.edu.cn.
ORCID
Suning Wang: 0000-0003-0889-251X
D
DOI: 10.1021/acs.orglett.9b01215
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(
17) (a) Yoshino, J.; Kano, N.; Kawashima, T. Chem. Commun.
Pangkuan Chen: 0000-0002-8940-7418
Notes
2
007, 559. (b) Yoshino, J.; Kano, N.; Kawashima, T. Chem. Lett.
008, 37, 960. (c) Yoshino, J.; Kano, N.; Kawashima, T. Dalton Trans
2
The authors declare no competing financial interest.
2013, 42, 15826. (d) Yoshino, J.; Furuta, A.; Kambe, T.; Itoi, H.;
Kano, N.; Kawashima, T.; Ito, Y.; Asashima, M. Chem. - Eur. J. 2010,
1
6, 5026. (e) Kano, N.; Yamamura, M.; Kawashima, T. Dalton Trans
015, 44, 16256.
ACKNOWLEDGMENTS
Q.Z., S.W., and P.C. thank the National Natural Science
Foundation of China for financial support (Grant Nos.
1571017 and 21772012). We thank Prof. Nan Wang and
■
2
(
6
(
18) Gon, M.; Tanaka, K.; Chujo, Y. Angew. Chem., Int. Ed. 2018, 57,
546.
19) (a) Tsuda, K.; Dol, G. C.; Gensch, T.; Hofkens, J.; Latterini, L.;
2
Yun Li (Beijing Institute of Technology) for their help with
XRD analysis. We thank the Analysis & Testing Centre at the
Beijing Institute of Technology for advanced facilities.
Weener, J. W.; Meijer, E. W.; De Schryver, F. C. J. Am. Chem. Soc.
2000, 122, 3445. (b) Han, M.; Hara, M. J. Am. Chem. Soc. 2005, 127,
10951.
(20) Jacquart, A.; Williams, R. M.; Brouwer, A. M.; Ishow, E. Chem. -
Eur. J. 2012, 18, 3706.
REFERENCES
■
(
1) (a) Bertleson, R. C.; Maeda, S.; Van Gemert, B. In Organic
Photochromic and Thermochromic Compounds, Vol. 1; Crano, J. C.,
Guglielmetti, R. J., Eds.; Plenum Press: New York, 1999. (b) Nigel
Corns, S.; Partington, S. M.; Towns, A. D. Color. Technol. 2009, 125,
2
49. (c) Feringa, B. L.; Brown, W. R. Molecular Switches, 2nd Edition;
Wiley−VCH: Weinheim, Germany, 2011.
2) Intelligent Stimuli-Responsive Materials; Li, Q.; Ed.; John Wiley &
Sons: Hoboken, NJ, 2013.
3) (a) Russew, M. M.; Hecht, S. Adv. Mater. 2010, 22, 3348.
b) Saha, S.; Stoddart, J. F. Chem. Soc. Rev. 2007, 36, 77. (c) Fihey, A.;
Perrier, A.; Browne, W. R.; Jacquemin, D. Chem. Soc. Rev. 2015, 44,
719. (d) Szymanski, W.; Beierle, J. M.; Kistemaker, H. A. V.; Velema,
W. A.; Feringa, B. L. Chem. Rev. 2013, 113, 6114.
4) Galanti, A.; Diez-Cabanes, V.; Santoro, J.; Valas
A.; Mayor, M.; Cornil, J.; Samorì, P. J. Am. Chem. Soc. 2018, 140,
6062.
5) (a) Moreno, J.; Gerecke, M.; Grubert, L.; Kovalenko, S. A.;
Hecht, S. Angew. Chem., Int. Ed. 2016, 55, 1544. (b) Bleger, D.;
Schwarz, J.; Brouwer, A. M.; Hecht, S. J. Am. Chem. Soc. 2012, 134,
0597.
6) Hecht, S.; Bleg
7) Zacharias, P. S.; Ameerunisha, S.; Korupoju, S. R. J. Chem. Soc.,
Perkin Trans. 2 1998, 2, 2055.
8) Surampudi, S. K.; Patel, H. R.; Nagarjuna, G.; Venkataraman, D.
Chem. Commun. 2013, 49, 7519.
9) (a) Beharry, A. A.; Sadovski, O.; Woolley, G. A. J. Am. Chem. Soc.
011, 133, 19684. (b) Samanta, S.; Beharry, A. A.; Sadovski, O.;
(
(
(
3
(
ek, M.; Minoia,
́ ̌
1
(
́
2
(
(
́
er, D. Angew. Chem., Int. Ed. 2015, 54, 11338.
(
(
2
McCormick, T. M.; Babalhavaeji, A.; Tropepe, V.; Woolley, G. A. J.
Am. Chem. Soc. 2013, 135, 9777. (c) Dong, M.; Babalhavaeji, A.;
Samanta, S.; Beharry, A. A.; Woolley, G. A. Acc. Chem. Res. 2015, 48,
2
662.
10) (a) Yang, Y.; Hughes, R. P.; Aprahamian, I. J. Am. Chem. Soc.
012, 134, 15221. (b) Yang, Y.; Hughes, R. P.; Aprahamian, I. J. Am.
Chem. Soc. 2014, 136, 13190.
11) (a) Julia-Lopez, A.; Hernando, J.; Ruiz-Molina, D.; Gonzal
Monje, P.; Sedo, J.; Roscini, C. Angew. Chem., Int. Ed. 2016, 55,
(
2
(
̀
́
́
ez-
́
1
2
5044. (b) Perrier, A.; Maurel, F.; Jacquemin, D. Acc. Chem. Res.
012, 45, 1173.
(
12) Siewertsen, R.; Neumann, H.; Buchheim-Stehn, B.; Herges, R.;
Nather, C.; Renth, F.; Temps, F. J. Am. Chem. Soc. 2009, 131, 15594.
13) Samanta, S.; Qin, C.; Lough, A. J.; Woolley, G. A. Angew.
Chem., Int. Ed. 2012, 51, 6452.
14) Hammerich, M.; Schutt, C.; Stah
F.; Hoppner, R.; Herges, R. J. Am. Chem. Soc. 2016, 138, 13111.
15) (a) Cembran, A.; Bernardi, F.; Garavelli, M.; Gagliardi, L.;
̈
(
(
̈
ler, C.; Lentes, P.; Rohricht,
̈ ̈
̈
(
Orlandi, G. J. Am. Chem. Soc. 2004, 126, 3234. (b) Bandara, H. M. D.;
Burdette, S. C. Chem. Soc. Rev. 2012, 41, 1809.
(
16) (a) Wakatsuki, Y.; Yamazaki, H.; Grutsch, P. A.; Santhanam,
M.; Kutal, C. J. Am. Chem. Soc. 1985, 107, 8153. (b) Ghedini, M.;
Pucci, D.; Calogero, G.; Barigelletti, F. Chem. Phys. Lett. 1997, 267,
́
41. (c) Babic, D.; Curic, M.; Molcanov, K.; Ilc, G.; Plavec, J. Inorg.
́ ́ ̌
3
Chem. 2008, 47, 10446. (d) Srivastava, K.; Chakraborty, T.; Singh, H.
B.; Butcher, R. J. Dalton Trans 2011, 40, 4489. (e) Bartwal, G.;
Aggarwal, K.; Khurana, J. M. New J. Chem. 2018, 42, 2224.
E
DOI: 10.1021/acs.orglett.9b01215
Org. Lett. XXXX, XXX, XXX−XXX