Switching of a single boryl center in π-conjugated photochromic polyboryl compounds and its impact on fluorescence quenching

Article abstract of DOI:10.1002/anie.201003144

All for one and one for all: Molecules with multiple conjugated photochromic boryl units undergo photoisomerization on a single boryl unit only. This single boryl switching causes fluorescence quenching of the entire molecule, which is greatly amplified w

Full text of DOI:10.1002/anie.201003144

Communications
DOI: 10.1002/anie.201003144
Photoisomerization
Switching of a Single Boryl Center in p-Conjugated Photochromic
Polyboryl Compounds and Its Impact on Fluorescence Quenching**
Chul Baik, Stephen K. Murphy, and Suning Wang*
Many materials consisting of a p-conjugated organic back-
bone attached to either a tetrahedral or trigonal-planar boron
center display enhanced fluorescence and charge-transport
properties. This phenomenon can be attributed to either
chelation-enhanced p-conjugation of the organic backbone
by a tetrahedral boron center[1–2] or to the strong electron-
accepting ability of a trigonal-planar boron center.[3] Hence,
boron-containing molecules have found applications in
OLEDs, organic transistors, and nonlinear optical materi-
als.[1–3] We recently reported an unusual photochromic switch
involving tetrahedral N,C-chelating boron chromophores
such as B(ppy)Mes2 (ppy = 2-phenylpyridine, Mes = mesi-
tyl).[4] These compounds undergo a thermally reversible
photoisomerization from a colorless or light colored state to
a dark colored state upon irradiation with 350–450 nm light
(Scheme 1). These compounds are highly fluorescent in the
light-colored state with tunable emission colors, whereas the
dark-colored isomers are nonemissive.
cules as shown in Scheme 2. Our study has shown that
isomerization of one chromophore prevents isomerization of
the others, leading to amplified and reversible fluorescence
quenching of the entire molecule. The details of our study are
presented herein.
Scheme 2. Syntheses of B2, B2’, B3, and B6. General conditions:
10 mol% CuI, 5 mol% [Pd(PPh3)4] (except for (a)), 30 equiv NEt3, THF
with a) O2 bubbling, b) 1,4-diiodobenzene, c) 1,3,5-tribromobenzene,
and d) hexakis(4-iodophenyl)benzene. B1 is shown for comparison.
The synthesis of the monoboryl B1 was reported pre-
viously.[4b] Diboryl B2 and B2’, triboryl B3, and hexaboryl B6
were synthesized from a common intermediate 1 (see the
Supporting Information). B2’ was prepared by Hay cou-
pling,[5] and B2, B3, and B6 were prepared by Sonogashira
coupling[6] with the appropriate aryl halides.
Scheme 1. Photoisomerization of B(ppy)Mes2 derivatives.
We have recently shown that this photoisomerization can
be suppressed when the system is conjugated to an olefinic
bond, which can dissipate the excitation energy through
trans–cis isomerization.[4b] While this conjugation has the
benefit of stabilizing the system towards photodegradation
and preserving its luminescence properties, it also renders the
system photochromically inert. To elucidate the impact of
incorporating multiple photochromic boron centers into these
materials, we prepared a series of new p-conjugated mole-
The crystal structures of B2, B2’, and B6 (Figure 1)[7]
reveal several important features of these materials. B2’ and
B2 have good coplanarity between the two boron chromo-
phores and dihedral angles of 08 between the two ppy rings for
B2’ and 23.98 between the ppy and the central phenyl ring for
B2, which is consistent with strong p conjugation and
electronic communication between the boron centers in
these molecules. In contrast, the electronic communication
between the boron centers in B6 appears to be much weaker
as a result of the high degree of steric congestion, as
evidenced by the large dihedral angles of 22.38, 59.58, and
82.88 between the ppy chelates and adjacent phenyl rings.
These trends in electronic communication are manifested
in the reduction potentials and absorption spectra of these
materials. The polyboryl compounds except B3 all show two
well-resolved reduction peaks, and the first reduction poten-
tials are more positive than that of B1. All compounds display
a low-energy shoulder band in their absorption spectra,
attributable to charge transfer from the mesityl groups to the
conjugated backbone based on DFT calculations. The energy
[*] Dr. C. Baik, S. K. Murphy, Prof. Dr. S. Wang
Department of Chemistry, Queen’s University
Kingston, Ontario, K7L 3N6 (Canada)
Fax: (+1)613-533-6669
E-mail: wangs@chem.queensu.ca
[**] This work was supported financially by the Natural Sciences and
Engineering Research Council of Canada. We thank Zachary M.
Hudson for help in the preparation of this manuscript.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201003144.
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Figure 1. Crystal structures of B2’ (top left), B2 (top right), and B6
(bottom). B: red. N: blue. Solvent molecules omitted.
of this transition decreases in the order B1 > B3 > B6 > B2 >
B2’, which is consistent with 1) the strong conjugation of the
boron centers in B2’ owing to the coplanarity of its two ppy
chelates, 2) improved conjugation of para-substituted B2
relative to meta-substituted B3, and 3) weak conjugation in
B6 caused by steric congestion. All compounds display bright
fluorescence in the 490–510 nm range with B2’ having the
lowest emission energy.
Figure 2. Changes in the absorption (left) and emission (right) spectra
Upon irradiation with UV light (365 nm), all polyboryl
compounds undergo structural and color changes to isomers
B2b, B2’b, B3b, and B6b, respectively. This photoisomeriza-
tion process was monitored by 1H NMR, UV/Vis, and
fluorescence spectroscopy. In the UV/Vis spectra, a broad
band appears at around 650 nm and grows with irradiation
time for all compounds in the same manner as monoboryl B1.
However, as shown in Figure 2, the ratio of the intensity of
this broad band to the strongest absorption band at the
saturation point (reached within minutes at ca. 10ꢀ5 m)
decreases with the number of boryl chromophores (20% for
B1b, ca. 9% for B2b, ca. 6% for B3b, and ca. 3% for B6b).
Since the 20% ratio represents 100% conversion of mono-
boryl B1 to B1b,[4b] the 9%, 6%, and 3% ratios for B2, B3,
and B6 indicates that only one boryl unit in these molecules
undergoes photoisomerization.
of B1, B2, B3, and B6 in toluene upon irradiation at 365 nm. Inset:
Photographs showing the color of the compounds before and after
irradiation.
This result was confirmed by 1H NMR studies, which show
characteristic peaks for the isomerized and nonisomerized
1
Figure 3. 1H NMR spectral change of B2 upon switching to B2b by
irradiation at 365 nm and B2b thermal reversal back to B2 by heating
at 358C in C6D6.
chromophores. Considering B2 as an example, its H NMR
spectrum in C6D6 displays a new set of well-resolved peaks
originating from B2b, shown in Figure 3. The characteristic
ꢀ
peaks Ha and Hb are consistent with the formation of the C
C-coupled structure B2b shown in Figure 3.[4a] Additionally,
peaks H1’ and H1’’ that maintain a 1:1 ratio with extended
irradiation, which may be assigned to a nonisomerized and
the singlet proton H1 disappears and is replaced with two new
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Communications
isomerized boron chromophore, respectively (see the Sup-
porting Information). Furthermore, 1H NMR photolysis
experiments confirmed that B2’, B3, and B6 also undergo
single boryl switching upon irradiation and reach 100%
conversion, given sufficient irradiation time.
NMR competition experiments revealed that the poly-
boryl compounds isomerize much faster than monoboryl B1
at a given concentration. For example, using B(ppy)Mes2 as
the internal standard, B2 isomerizes approximately 1.7 times
more rapidly than B1 (see the Supporting Information). This
result may be attributed to the “antenna effect”,[8] whereby
the multiple boron chromophores harvest more photons at a
given concentration leading to more rapid isomerization.
This single boryl switching phenomenon displayed by the
conjugated polyboryl compounds can be explained as follows
using B2 as an example. Upon excitation, there are two
possible isomerization pathways: k1 to form the monoiso-
merized product and k2 to form the diisomerized product.
Because of the substantial structural rearrangement that
occurs around the boron center upon isomerization, simulta-
neous switching of both boron centers in the same molecule
must be much slower than a single boryl switching, thus
making the single boryl switching kinetically favored. Once
the monoisomerized product is formed, the energy of its
excited state can be dissipated by fast intramolecular energy
transfer to the low-energy absorption band of the dark species
according to the Kashaꢀs rule, effectively quenching any
further isomerization.[9] The single boryl switching behavior
of the polyboryl compounds is in sharp contrast to the well
known diarylethene (DTE) systems, which usually show
simultaneous multichromophore switching.[10] The lack of a
photostationary state and the relatively slow isomerization
rate of the boryl system may be responsible for the observed
single boryl transformation. This effect can be considered
beneficial if the integrity of the conjugated system is
important for applications such as organic charge-transport
materials or transistors, and of course undesired if the
material is intended to achieve maximum photochromic
effects.
Figure 4. The plot of the ratio of total emission intensity change jDIj
versus the total absorption changes jDAj for the monoboryl and
polyboryl compounds.
As observed previously for related monoboryl com-
pounds, the isomerization process is fully thermally reversible
for all polyboryl compounds in this series. The fluorescence
1
and H NMR spectra of all compounds can be fully restored
by heating the solution of the dark species at 358, or by
keeping the solution at room temperature for an extended
period of time. Kinetic data obtained from 1H NMR studies of
the thermal reversal are shown in Figure 5 and Table 1, and
are consistent with a first-order process. All dark isomers of
Figure 5. Kinetic plot of the natural logarithm of the ratio of photo-
isomerized product to the initial amount of photoisomerized product
versus time during heating at 358C for 1 mg/0.75 mL degassed
solution in C6D6.
Since the isomerized chromophore is nonemissive, this
process effectively quenches fluorescence, as shown in
Figure 2. Interestingly, as the
number of boron chromophores is
Table 1: Photophysical and electrochemical properties.
increased, maximum fluorescence
quenching is achieved with progres-
sively lower absorption spectral
changes. This amplification effect
on fluorescence quenching is illus-
trated by the plot of fluorescence
intensity change (j DI j ) of the light-
colored species over the absorbance
change of the low energy band of
the dark species (j DA j ; Figure 4).
Electronic communication between
the boron chromophores in the
polyboryl systems is clearly respon-
sible for the amplified fluorescence
quenching.
Cmpd
lmax [nm]
Optical
energy
gap [eV]
lem
[nm][a]
Ff[b]
E1/2red [V][c]
Thermal
reversal
kthermal ꢀ106
[sꢀ1 [d]
(e) [mꢀ1 cmꢀ1 [a]
]
[%]
]
t
1/2 [min][d]
B1
B2
B2’
B3
B6
313 (24900)
377 (11700)
366 (55800)
393 (42400)
372 (43000)
408 (31800)
319 (61000)
385 (23000)
326 (151100)
385 (58500)
2.88
2.72
2.68
2.70
2.73
490
495
508
505
494
37
39
27
20
34
ꢀ2.03
360
240
95
30.9
44.2
ꢀ1.93, ꢀ2.09
ꢀ1.73, ꢀ2.09
ꢀ1.96[e]
115
155
310
78.4
37.3
ꢀ2.01, ꢀ2.23
[a] At 10ꢀ5 m in toluene. [b] 9,10-Diphenylanthracene (F=90%) used as the standard in toluene.
[c] Relative to [FeCp2]0/+ (Cp=cyclopentadienyl) in DMF. [d] At 358C in C6D6. [e] Not resolved because of
very poor solubility.
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M. Esteghamatian, N. X. Hu, Z. Popovic, G. Enright, S. Breeze,
the polyboryl compounds undergo thermal reversal faster
than the monoboryl B1b, with rate constants increasing in the
order B1b < B6b < B2b < B3b < B2’b, suggesting that p con-
jugation and steric congestion together determine the rate of
the reverse process.
In summary, we have established that conjugated systems
with multiple switchable boryl chromophores only undergo
photoisomerization at a single boryl center, while the
remaining chromophores accelerate this process by intra-
molecular energy transfer. Furthermore, the isomerization of
one chromophore prevents isomerization of the others,
leaving the remainder of the p system intact even upon
prolonged irradiation. Using this approach, the preparation of
photochromic materials with tunable intensity and fluores-
cence quenching properties is possible.
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Experimental Section
General procedure: All experiments were performed under a nitro-
gen atmosphere. 2-Bromo-5-(triisopropylsilylethynyl)pyridine was
prepared by a literature procedure.[11] The synthetic details for all
polyboryl compounds can be found in the Supporting Information.
Quantum yields of all compounds were measured in toluene using
9,10-diphenylanthracene as the standard at 298 K (Fr = 0.90) and
calculated using previously known procedures.[12] Molecular orbital
and molecular geometry calculations were performed using the
Gaussian03 program suite,[13] with crystal structures used as the
starting points for geometry optimizations where possible. Calcula-
tions were performed at the B3LYP level of theory using 6-31G* as
the basis set for B2 and B2’ and STO-3G for B3 and B6.
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Received: May 24, 2010
Published online: September 20, 2010
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[7] CCDC 777277 (B2), 777278 (B2’), and 777279 (B6) contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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Keywords: boron · conjugation · fluorescence · isomerization ·
.
photochromism
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