Discrete, soluble covalent organic boronate ester rectangles

Article abstract of DOI:10.1039/c3cc42777c

The facile self-assembly of nanoscale boronate ester rectangles from linear bis-catechols and 1,4-benzene diboronic acid is described. Spectroscopic and computational analyses reveal the influence of extended π-conjugation on the rectangles' absorption an

Full text of DOI:10.1039/c3cc42777c

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Discrete, soluble covalent organic boronate ester
rectangles†
Cite this: Chem. Commun., 2013,
49, 6167
Merry K. Smith, Natalia E. Powers-Riggs and Brian H. Northrop*
Received 15th April 2013,
Accepted 25th May 2013
DOI: 10.1039/c3cc42777c
www.rsc.org/chemcomm
The facile self-assembly of nanoscale boronate ester rectangles 1,4-Benzene diboronic acid (BDBA), however, is insoluble in CDCl3
from linear bis-catechols and 1,4-benzene diboronic acid is described. where it self-condenses into boroxine anhydride species, thus pre-
Spectroscopic and computational analyses reveal the influence of venting assembly with either 1 or 2 in CDCl3. We found that the
extended p-conjugation on the rectangles’ absorption and fluorescence addition of CD3OD catalyses the dissociation of boroxine anhydride
properties. The rectangles represent a new class of discrete, organic species and, likewise, the dynamic self-assembly of BDBA with 1
soluble covalent organic polygons.
and 2. Indeed, within 10 min of mixing 1 or 2 with BDBA (10 : 1
CDCl3–CD3OD, 50 1C) the H NMR spectrum of both assemblies
1
The thermodynamically driven, dynamic covalent1 condensation of
boronic acids2 with catechols to give boronate esters has proven to be
a highly efficient means of preparing complex molecular and
macromolecular structures. Boronate ester-based covalent organic
frameworks3,4 (COFs) in particular have attracted significant interest
on account of their well-defined porosity, high thermal stabilities, low
densities, and applications in gas storage,4a,b optoelectronics,4c–f,h
and heterogeneous catalysis.4g While multiple examples of infinitely
periodic 2- and 3-dimensional boronate ester COFs have been
reported since their discovery5 in 2005 there have been few examples6
of their discrete analogues: i.e. porous, highly unsaturated covalent
organic polygons or polyhedra (COPs) built from boronic acids and
oligo-catechols.7 Herein we report the synthesis and dynamic self-
assembly of two shape-persistent,1c alkyl-functionalized covalent
organic boronate ester rectangles. Spectroscopic investigations reveal
the dynamic nature of their self-assembly as well as their structure-
dependent absorption and fluorescence properties.
become substantially disordered as dynamic covalent exchange
between BDBA and 1 or 2 initially results in the formation of various
kinetic intermediate assemblies. Within 3 h these dynamic libraries
of oligomers have largely collapsed to their most thermodynamically
favoured structures, as indicated by considerable sharpening of their
1H NMR spectra. Full conversion to rectangles 3 and 4, however, is
not complete until H2O and CD3OD are removed by stirring over 4 Å
molecular sieves,10 providing rectangles 3 and 4 in near quantitative
yield (Scheme 1). In addition to increasing the rectangles’ solubility,
the alkyl chains of 3 and 4 likely aid in rendering both assemblies
highly resistant to hydrolysis11 as hydrolytic disassembly could only
be achieved by refluxing in a 1 : 1 mixture of D2O–CD3OD.
Linear bis-catechols 1 and 2 containing at their core 2,3-bis-
(hexyloxy)phenylene or diethynyl-2,3-bis(hexyloxy)phenylene, respec-
tively, were designed to function as 01 secondary building units in
accordance with the directional-bonding8 approach to molecular
self-assembly. Hexyloxy groups were found to be essential for the
current study as neither unsubstituted nor methoxy substituted
linear bis-catechols were sufficiently soluble to allow adequate
solution phase assembly and characterization. There is literature
precedent6b,d,9 for the dynamic assembly of discrete and oligomeric
boronate esters in hydrophobic solvents such as CHCl3 and CH2Cl2.
Wesleyan University, Department of Chemistry, 52 Lawn Ave., Middletown, CT 06459,
USA. E-mail: bnorthrop@wesleyan.edu; Fax: +1 860 685-2211; Tel: +1 860 685-3987
† Electronic supplementary information (ESI) available: Full synthetic details,
characterization, and spectra as well as theoretical modeling procedures and full
results. See DOI: 10.1039/c3cc42777c
Scheme 1 Solution-phase synthesis of boronate ester rectangles 3 and 4 from
the dynamic self-assembly of linear bis-catechols 1 and 2 with BDBA.
c
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Fig. 1 Partial 1H NMR spectra (CDCl3, 300 MHz, 298 K) of bis-catechol 1 (a) and
boronate ester rectangle 3 (b) indicating diagnostic shifts of proton signals Ha–Hd upon
self-assembly.
Fig. 2 Absorption (solid lines) and normalized fluorescence (dashed lines)
spectra of boronate ester rectangles 3 and 4. All spectra were recorded in CHCl3
(1.0 Â 10À5 M). Excitation wavelengths: lex = 270 nm (3), lex = 320 nm (4).
The formation of rectangles 3 and 4 is supported by key shifts of
The most prominent structural difference between linear bis-
diagnostic signals in their 1H NMR spectra. Catechol Ar–OH signals catechols 1 and 2, and likewise rectangles 3 and 4, is the presence
at 7.61 and 6.01 ppm for tetraol 1, for example, are no longer present of ethynyl moieties in 2 and 4. These ethynyl groups have two
1
in the H NMR spectrum of rectangle 3 (Fig. 1). Catechol protons noteworthy effects: (i) catechol moieties of 2 are able to adopt syn-
Ha–Hc shift collectively downfield by 0.3 ppm while the proton signal periplanar and anti-periplanar conformations while those of 1 cannot
Hd of the central aromatic unit of 1 shifts from 7.33 ppm in the free due to steric interactions at its biaryl C–C bonds, and (ii) p-conjugation
tetraol to 7.41 ppm in assembly 3. A new singlet corresponding is increased in 2 relative to 1. UV/Vis and fluorescence spectroscopy
to the aromatic protons of the central boronate unit is observed at were used to investigate the effects of increased conjugation on the
1
8.25 ppm. Analogous changes occur in the H NMR spectrum of absorption and emission properties of rectangles 3 and 4. Plotted
rectangle 4 (ESI,† Fig. S1). The sharp signals observed in the CDCl3 in Fig. 2 are the UV/Vis and fluorescence spectra of rectangles 3 and 4
spectra of 3 and 4 suggest assemblies of high symmetry rather than (1.0 Â 10À5 M, CHCl3).14 The absorption of rectangle 3 (labs = 264 nm,
collections of oligomers.
e=3.0Â 104 MÀ1 cmÀ1) is almost identical to that of the boronate ester
Further characterization of covalent organic rectangles 3 and 4 is obtained by condensation of BDBA with catechol (labs = 282 nm,
provided by infrared (IR) spectroscopy. The IR spectra of linear e = 2.5 Â 104 MÀ1 cmÀ1),15 underscoring the fact that conjugation is
catechols 1 and 2 (ESI,† Fig. S2 and S3) reveal broad catechol CO–H broken at the twisted biaryl bonds of 3. Absorption of rectangle 4, by
stretching bands at 3272 and 3311 cmÀ1, respectively, while the IR contrast, is red-shifted by 93 nm relative to 3 (labs = 357 nm, e = 2.5 Â
spectrum of BDBA shows a BO–H band centred at 3286 cmÀ1. IR 104 MÀ1 cmÀ1) reflecting the increased conjugation provided by the
spectra of rectangles 3 and 4, however, contain no stretching bands ethynyl spacers. The effect of extended p-conjugation is again
above 3000 cmÀ1, indicating full dehydration of each species. New observed, though to a lesser extent, in the fluorescence spectra of 3
B–O stretching bands at and 1326 cmÀ1 (rectangle 3) and 1333 cmÀ1 (lem = 383 nm) and 4 (lem = 400 nm). Stokes shifts of 119 nm and
(rectangle 4) support the formation of boronate esters, but cannot rule 43 nm are observed for rectangles 3 and 4, respectively, indicating the
out the formation of anhydrides. Boronate anhydrides display a more conformationally flexible rectangle 3 undergoes greater relaxa-
characteristic12 absorption band between 570–580 cmÀ1, however tion upon excitation than the planar, more conjugated rectangle 4.
neither the IR spectra of 3 nor 4 contains a peak in this region Both boronate ester rectangles show significant emission in the blue
providing more definitive evidence that boronate esters rather than region of the spectrum, making them possible candidates for soluble,
anhydrides are formed. Furthermore, rectangles 3and 4display bands stable blue emitters for organic light emitting diode (OLED)
at 660 and 656 cmÀ1, respectively, corresponding to boronate ester out applications.16
of plane bending modes.13
Additional insight into increased p-conjugation in rectangle 4
NMR and IR spectroscopic analyses, while supportive of relative to 3 was obtained through density functional theoretical
the formation of boronate ester assemblies, cannot rule out the calculations. Models of both rectangles with hexyloxy functionalities
possibility that alternative, highly symmetric boronate ester oligo- replaced with methoxy groups were constructed and fully conforma-
mers have formed or are present. Accurate mass MALDI mass tional searched using molecular mechanics. The lowest energy struc-
spectrometry was therefore used to investigate the molecularity of ture of each rectangle was then optimized to full convergence at the
assemblies 3 and 4. The MALDI mass spectrum of rectangle 3 reveals B3LYP/6-31+g(d,p) level17 in a PCM18 solvent model for CHCl3 using
a peak of m/z = 1199.565 [M + Na]+, which is in agreement with the the program Gaussian 09.19 As can be seen in Fig. 3, the lowest energy
calculated value of 1199.560. The MALDI mass spectrum of rectangle conformation of 3is non-planar, with dihedral angles of 1291between
4 shows a peak of m/z = 1272.572 [M]+ compared to a calculated its conjugated boronate ester aromatic units and its central alkoxy-
value of 1272.571. MALDI mass spectrometry results quantitatively substituted aromatic rings. Rectangle 4, however, does adopt a planar
establish the molecularity of covalent organic rectangles 3 and 4 as conformation. NBO analysis reveals that the HOMO orbitals of
being discrete species rather than oligomers.
rectangles 3 and 4 are quite similar: both predominantly localized
c
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Notes and references
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¨
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´
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Soluble, shape-persistent covalent organic polygons bearing alkyl
functionalities are likely to allow for the design and fabrication of
new boronate ester materials – e.g. porous boronate ester mesogens,
the organized assembly of covalent organic polygons into self-
assembled monolayers, and the use of soluble covalent organic
polygons as sensors for halide anions. The violet-blue emission of
rectangles 3 and 4 opens possibilities for these and related assem-
blies to be used as soluble, thermally robust20 components of
OLEDs. Furthermore, we anticipate soluble COPs will play an
important role in understanding and optimizing structure–function
relationships of related COFs by providing valuable solution phase
analysis that cannot otherwise be easily obtained given the insoluble
nature of rigid, infinitely periodic frameworks. We are actively
investigating the solution phase assembly and sensing applications
of rectangles 3 and 4 as well as the synthesis and self-assembly of
additional soluble, discrete covalent organic polygons.
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sion (by 1H NMR) in the presence of residual H2O or methanol.
11 Alkylation of the pores of COFs has been shown to increase their
hydrolytic stability, see L. M. Lanni, R. W. Tilford, M. Bharathy and
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13 Assignments of vibrational modes were supported by frequency
analysis obtained from DFT calculations, see ESI,† Fig. S2, S3 and S7.
14 For UV/Vis and fluorescence data for 1, 2, and BDBA see the ESI†.
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Acknowledgement is made to the donors of The American
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19 M. J. Frisch, et al., Gaussian 09, Revision A.1, (see ESI† for the full reference).
20 Melting points of boronate rectangles 3 and 4 are both >200 1C.
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 6167--6169 6169