Highly luminescent, electron-deficient bora-cyclophanes

Article abstract of DOI:10.1021/ja209602z

A highly luminescent conjugated organoboron macrocycle containing six Lewis acidic boron centers was synthesized. Comparison of the optical and electronic properties with those of a hexameric linear oligomer revealed important differences due to delocaliz

Full text of DOI:10.1021/ja209602z

Communication
pubs.acs.org/JACS
Highly Luminescent, Electron-Deficient Bora-cyclophanes
Pangkuan Chen and Frieder Jakle*
̈
Department of Chemistry, Rutgers UniversityNewark, 73 Warren Street, Newark, New Jersey 07102, United States
S
* Supporting Information
duced triarylamine-based aza-cyclophanes.2,8 We also discuss
ABSTRACT: A highly luminescent conjugated organo-
boron macrocycle containing six Lewis acidic boron
centers was synthesized. Comparison of the optical and
electronic properties with those of a hexameric linear
oligomer revealed important differences due to delocaliza-
tion within the highly symmetric cyclic conjugated
structure. Exposure of this unique electron-deficient
bora-cyclophane to fluoride or cyanide results in amplified
fluorescence quenching and can be exploited to switch
between an electron-deficient macrocycle and a highly
charged, electron-rich borate cycle.
the effect of cyclization on optical and electronic characteristics
and present preliminary results on the complexation of anions,
which can act as a stimulus to turn the electron-deficient
macrocycle into an electron-rich one.
Treatment of Br2B-Fl-BBr2 (Fl = 9,9-dihexyl-2,7-fluorene-
diyl) with Me3Sn-Fl-SnMe3 is known to lead to polycondensa-
tion, and the resulting polymer [Fl‑B(Br)]n can be converted
with TipCu (Tip = 2,4,6-triisopropylphenyl) to a Lewis acidic
organoboron polymer [Fl‑B(Tip)]n that is highly luminescent
and exhibits good stability in air.9 Very recently we have
succeeded in the preparation of a series of well-defined
conjugated fluoreneborane oligomers Me3Si-[Fl‑B(Tip)]n-Fl-
SiMe3 (LFn, n = 1−6; Fl = 9,9-dimethylfluorene-2,7-diyl) using
a novel iterative procedure.10 We reasoned that some of the
longer monodisperse oligomers should be promising as
precursors for macrocycles because the formation of so-called
“overshooting oligomers”11 that lead to larger linear structures
rather than cyclics can be prevented. Moreover, the number of
possible cyclic products is limited since ring closure of, for
example, a tetraboryl precursor with a fluorene linker can only
lead to a tetramer, octamer, dodecamer, etc., while a hexaboryl
species would result in a hexamer, dodecamer, etc., without any
of the intermediate ring sizes. This should greatly facilitate
isolation of a specific desired cyclic product.
We first converted the oligo(fluoreneborane)s LFn (n =
2,4)10 into species Br2B-[Fl‑B(Tip)]n-Fl-BBr2 (n = 2, 4) by
reaction with 2 equiv of BBr3 in CH2Cl2 at RT (Scheme 1).
The resulting BBr2-terminated species were then treated in situ
with 2,7-distannyl-9,9-dimethylfluorene under dilute condi-
tions, followed by reaction with TipCu, which serves to replace
the reactive Br substituents with bulky groups that sterically
stabilize the borane centers. The gel permeation chromatog-
raphy (GPC) trace of the product obtained from the precursor
LF2 revealed a polymodal profile with distinct peaks at 1300,
2000, 3800, 6500 Da, and MALDI-TOF MS data suggested the
presence of multiple linear and cyclic species, including the
tetrameric and octameric macrocycles (MC4, MC8).12 In
contrast, a similar reaction sequence starting from the longer
oligomer LF4 led to highly efficient cyclization with formation
of MC6 as the major product in >80% yield based on NMR
and GPC analysis of the crude mixture.13 Column chromatog-
raphy on silica gel followed by crystallization from a 10/1
mixture of hexanes/toluene at −35 °C gave monodisperse
MC6 as a white microcrystalline solid.
acrocycles continue to attract tremendous interest, in
M
part because of their ability to act as hosts for guest
molecules and their pronounced tendency to form well-defined
porous supramolecular structures in the solid state.1 The
channels generated during the assembly process can, for
example, be used for selective ion transport, as catalytically
active sites or for photochemical reactions in a confined
environment, leading to unexpected selectivities.2 Conjugated
macrocycles in particular have received much attention in
recent years as they also offer desirable optical, electronic, or
sensory properties; in addition, they are important from a
fundamental standpoint, because they represent polymer chains
without the presence of end groups, which tend to influence
the photophysical and electronic characteristics of the linear
counterparts.3
Conjugated organoborane macrocycles would be especially
interesting in that they feature an “anti-crown”-like structure, in
which more commonly encountered donor atoms (e.g., O, S,
N, P) are replaced with Lewis acid sites.4 Such an arrangement
could be beneficial for the selective detection of anions and
other electron-rich substrates.5 In addition, the unusual
electronic properties of conjugated organoborane oligomers
and polymers that result from p-π overlap of the empty p-
orbital on boron with π-conjugated organic groups continue to
fascinate researchers as they suggest potential applications in
optoelectronic devices.6 However, although several reports on
boron-containing macrocycles have appeared in the literature,
in most cases the Lewis acidity of boron is low due to π-overlap
with amino or alkoxy substituents or the borons are
tetracoordinate, and thus no significant electron delocalization
is present.7 Herein we describe a rational approach to hitherto
unprecedented bora-cyclophanes that feature multiple highly
electron-deficient organoborane moieties as an integral part of
the ring system and thus constitute the first “charge-reverse”
analogues of electron-donating macrocycles such as porphyrins,
phthalocyanines, calixarenes, oligopyrrols, and recently intro-
The identity of macrocycle MC6 was unequivocally
confirmed by GPC analysis, high-resolution mass spectrometry,
Received: October 12, 2011
Published: November 21, 2011
© 2011 American Chemical Society
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Communication
2437.667 (calcd for [M−H]− 2437.701) and an isotope pattern
that suggests an overlap of [M]− and [M−H]− ions (Figure
1b). A smaller peak to the left corresponds to fragment ions of
[M−CH3]− overlapping with [M−CH3−H]−. No other signals
were observed.
Scheme 1. Synthesis of the Bora-cyclophane MC6
Analysis of the electronic structure of the macrocycle
revealed characteristic differences in comparison to the
respective linear oligomers. Both cyclic13 and square wave
voltammograms show six distinct peaks, where the first two
processes to give [MC6]2− occur almost simultaneously,
whereas reduction of the following four boron centers occurs
at increasingly cathodic potentials (Figure 2). In comparison to
Figure 2. Comparison of square wave voltamograms of MC6 and the
linear hexamer LF6 (THF, 0.1 M Bu4NPF6, 100 mV/s).
the linear analogue LF6, all reductions except for the first one
occur at more negative potentials, which is attributed to larger
Coulombic repulsion in the cyclic framework (in the fully
reduced cyclic species every boron center experiences the effect
of two neighboring boron radical anions, while in the linear
species the terminal borons only have one neighboring boron
radical anion).
and multinuclear NMR spectroscopy; moreover, the cyclic
arrangement of six Lewis acidic boron centers in MC6 is
reflected in the electrochemical and anion-binding behavior.14
Based on GPC against PS standards (Figure 1a), MC6 has a
The macrocycle MC6 absorbs at a slightly shorter wave-
length than linear LF6 (Figure 3). This effect could be due to a
Figure 3. Comparison of the UV−vis and fluorescence spectra of MC6
with those of linear LF6 in CH2Cl2 (excited at λmax). Inset:
Photograph of MC6 in CH2Cl2 exposed to a UV lamp (365 nm).
Figure 1. (a) GPC trace of MC6 in THF. (b) MALDI-TOF MS data
of MC6. Insets: Comparison of experimental (top) and calculated
(bottom: [M−H]−) peak patterns.
more restricted conformation, which may not allow for optimal
overlap of the empty p orbitals on B with the adjacent fluorene
π-systems. However, this is unlikely as the calculated structure
(DFT, B3LYP 6-31G(d); alkyl groups were omitted)15 shows
little strain with endocyclic B−C bond lengths of 1.568 Å and
angles about B that range from 120.3° to 120.4°.12,16 Moreover,
the HOMO for MC6-calc is localized on the six fluorene
moieties, while the LUMO shows delocalization of all six empty
p orbitals on B with the organic π-systems (Figure 4). This
stands in contrast to the observations for LF6-calc,10 for which
the LUMO is mostly concentrated on just four of the six
available boron centers. The difference is attributed to the
absence of fluorene end groups and the resulting higher
symmetry in the cyclic system.
molecular weight of Mn = 2470 Da (Mw/Mn = 1.01), which is
close to the expected value of 2437 Da but significantly lower
than that of the respective linear hexamer LF6 (Mn = 3360 Da).
This observation is consistent with a more compact cyclic
structure that gives rise to a relatively smaller hydrodynamic
volume. The structure of MC6 was further confirmed by
1
multinuclear NMR. Most strikingly, the H and 13C NMR
spectra show only one distinct set of signals for the fluorene
and Tip moieties, which is in stark contrast to the linear
precursor, for which complex patterns arise from multiple
nonequivalent repeating units.13 In the 11B NMR a single broad
resonance is observed at 70 ppm, which is in the expected
chemical shift range and consistent with the presence of only
one type of triarylborane moieties. Finally, the negative ion
mode MALDI-TOF MS shows a major peak with a mass of
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Communication
Figure 5. Emission spectra (λex = 366 nm) for the titration of MC6
(4.474 × 10−6 M) with F− in THF at RT and illustration of the
charged macrocycle [MC6•X6]6− (X = F− or CN−) obtained in the
presence of an excess of the anion X−. The amounts of F− added per
cycle (from top to bottom) are as follows: 0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2,
1.6, 2.0, 4.0, 6.0 equiv.
Figure 4. HOMO and LUMO orbital representations for MC6-calc.
TD-DFT calculations13 were also performed, and they
suggest another reason for the differences in the absorption
profiles, which is that, in contrast to LF6-calc, the lowest
energy absorption for MC6-calc is symmetry-forbidden
(HOMO→LUMO, 415 nm, f = 0.000), making higher energy
transitions that involve contributions primarily from the
HOMO−1 and HOMO−2 to the LUMO orbital (389 nm, f
= 2.439; 388 nm, f = 2.434) and the 0−1 vibronic transition
dominant.17−19 Indeed, the experimental absorption onsets
(LF6, 402 nm vs MC6, 404 nm) and the calculated HOMO−
LUMO gaps follow the expected trend with a decrease from
linear LF6-calc (3.51 eV) to the “infinite chain” of MC6-calc
(3.49 eV). Also consistent is that similar blue emissions are
observed in both cases and the maxima are even slightly red-
shifted for the cyclic species. A remarkable quantum yield of
0.98 was measured for MC6. A relatively modest solvatochro-
mic effect is consistent with some degree of polarization of the
excited state, and the emission profiles proved to be slightly
concentration-dependent, suggesting possibly the formation of
aggregates at high concentrations.13
The presence of electron-deficient boron centers allows for
binding of electron-rich substrates. For instance, borate cycles
are generated in the presence of F− or CN−.5 This process is
very effective with large stepwise binding constants that are in
the range typical of highly electron-deficient triarylboranes
(∼105−108 M−1) and decrease slightly as the cycle becomes
more highly charged.20 Another interesting aspect is that the
presence of only 1 equiv of the anion for each hexameric
macrocycle results in ca. 75% quenching of the luminescence,
although the absorbance decreases by less than 40% (Figure 5).
Based on the fitting of the absorption data (Figure S11,
Supporting Information), at a 1:6 ratio of F/B, the relative
abundance of species [MC6], [MC6]F, [MC6]F2, [MC6]F3,
[MC6]F4, [MC6]F5, [MC6]F6 is 29:49:20:2:0:0:0, while the
amount of free [MC6] decreases to 4% after addition of 2 equiv
of F−. This suggests that the emission of all the cycles that are
complexed with even just one anion is effectively quenched,
consistent with an amplified quenching mechanism.21 Addition
of an excess of [Bu4N]F or [Bu4N]CN leads to highly charged
species {[(MC6)X6](Bu4N)n}(6−n)− (X = F, CN), which were
also identified by high resolution ESI-MS analysis (Figure S10,
Supporting Information).13
is the presence of multiple electron-deficient organoborane
moieties, which leads to high affinity for electron-rich
substrates. There has been immense recent interest in the
binding of anions to organoborane Lewis acid receptors, and
MC6 binds strongly to F− and CN− as evidenced by UV−vis
titration and ESI FT-MS, thus suggesting potential use in anion
recognition and as stimuli-responsive optoelectronic materials.
Anion binding to MC6 results in amplified fluorescence
quenching with formation of a highly charged hexaborate
species. This offers a facile means to convert an electron-
deficient macrocycle into an electron-rich one using anion
addition as an external stimulus.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures and data; complete ref 15. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
fjaekle@rutgers.edu
ACKNOWLEDGMENTS
■
Financial support by the National Science Foundation (NSF
CHE-080942, CHE-1112195) is gratefully acknowledged. F.J.
thanks the Alfred P. Sloan Foundation for a research fellowship
and the Alexander-von-Humboldt Foundation for a Friedrich
Wilhelm Bessel Research Award.
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(12) In comparison to MC6, the formation of MC4 is considerably
less favorable. This is attributed to significant ring strain in MC4 as
confirmed by DFT calculations on MC4-calc. The calculated
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(see the Supporting Information).
(13) See the Supporting Information for further details.
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