Going beyond red with a Tri- and tetracoordinate boron conjugate: Intriguing near-IR optical properties and applications in anion sensing

Article abstract of DOI:10.1021/ic402441w

The design and synthesis of a new tri- and tetracoordinate boron conjugate is reported. The conjugate shows broad near-IR emission (～625-850 nm) and is found to be a selective colorimetric and ratiometric sensor for fluoride ions.

Full text of DOI:10.1021/ic402441w

Communication
pubs.acs.org/IC
Going beyond Red with a Tri- and Tetracoordinate Boron Conjugate:
Intriguing Near-IR Optical Properties and Applications in Anion
Sensing
Samir Kumar Sarkar, Sanjoy Mukherjee, and Pakkirisamy Thilagar*
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India
S
* Supporting Information
project is to develop TAB-based NIR materials and utilization of
their properties for potential applications.
ABSTRACT: The design and synthesis of a new tri- and
tetracoordinate boron conjugate is reported. The con-
jugate shows broad near-IR emission (∼625−850 nm) and
is found to be a selective colorimetric and ratiometric
sensor for fluoride ions.
In the present study, we considered decoration of a styryl−
BODIPY moiety with TAB units to be a rational approach for
achieving NIR-emissive TABs. Also, we reasoned that the
incorporation of an iodothiophene moiety at the meso position
of the BODIPY unit may lengthen effective conjugation and
provide sites for further functionalization. Accordingly, we
designed and synthesized a (TAB)2−styryl−BODIPY conjugate
(1) (Scheme 1; see the Supporting Information, SI). To
n recent years, luminescent organic dyes have emerged as
1
Iindispensable components of modern materials chemistry. In
particular, near-infrared (NIR) luminescent dyes have attracted a
lot of attention compared to other classes of luminogens (e.g.,
blue/green emissive dyes) in view of their biological, photo-
voltaic, and optoelectronic applications.2 NIR monitoring of any
target analyte is a challenging task. In the past decade,
tetracoordinate boron-containing styryl−BODIPY dyes, primar-
ily developed by Daub et al.3a and Akkaya et al.,3b,c have emerged
as a versatile class of NIR luminogens, which find applications in
bioimaging, photodynamic therapy, molecular recognition,
artificial light-harvesting systems, and organic optoelectronic
devices.3,4 BODIPYs have been the subject of extensive studies in
the past few years owing to their inherent sharp absorption and
emission bands, high quantum yields, high photostability, and
ease of functionalization.4,5
a
Scheme 1. Synthesis of 1
a
(a) 2-Methylpyrrole/DCM, DDQ, Et3N, and BF3·Et2O. (b) 4-
(Dimesitylboryl)benzaldehyde (1) or benzaldehyde (2), piperidine,
and PTSA/toluene.
understand the effects of TAB entities on the optical properties
of 1, model system 2 (Scheme 1) was also synthesized and
studied (see the SI). Compounds 1 and 2 were synthesized using
the procedure shown in Scheme 1. Condensation of 5-
iodothiophene-2-carbaldehyde (1a) with 2-methylpyrrole, fol-
lowed by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoqui-
none (DDQ) and the subsequent addition of Et3N and BF3·Et2O
in CH2Cl2, gave 1b. Further, condensation of 1b with 4-
(dimesitylboryl)benzaldehyde (or benzaldehyde for 2) in the
presence of p-toluenesulfonamide (PTSA) and piperidine
resulted in compound 1. Compound 1 was characterized by
NMR (1H, 13C, and 19F) and MALDI-TOF techniques (see the
SI). The molecular structure of 1 was confirmed by single-crystal
X-ray diffraction (Figure 1) studies.
The molecular structure of 1 is shown in Figure 1. The
molecule adopts a “Y” shape with spatially separated TAB units
[the B2(TAB)−B3(TAB) distance is 12.69 Å]. The boradiazain-
dacene unit forms a near-planar arrangement with the two Ar3B
planes (dihedral angles of 13.0 and 20.2°). Density functional
theory (DFT; B3LYP)10 calculations almost reproduce the
structure determined experimentally (see the SI).
Tricoordinate boron-containing triarylboranes (TABs) have
also attracted significant attention in past few years mainly due to
their potential applications in organic light-emitting diodes,6,7a,b
anion sensing,7c−f and organic thin-film transistors,7g etc. In the
absence of suitable donor units, TABs commonly show
photoluminescence at the blue end of the visible spectrum.6,7
However, the incorporation of TAB units in rationally designed
molecular architectures can be employed to achieve functional
luminescent materials with longer-wavelength emissions. In this
regard, TAB-based donor−acceptors dyad/triads and polymeric
systems have been studied.6−8 In the present decade, the
combination of tri- and tetracoordinate boron conjugates (TAB−
BODIPY dyad/triad) has received considerable attention.9 In
2008, Lee et al. reported the first borane−BODIPY conjugate,
which shows borane-to-BODIPY energy transfer (ET), resulting
in single-channel emission from the BODIPY moiety, and the ET
process was also utilized for selective recognition of cyanide
ions.9a Very recently in 2013, we demonstrated unique di- and
tricolor emissive properties of borane−BODIPY conjugates and
their applications as fluoride-selective fluorescent sensors.9f,g In
spite of these efforts, TAB-based NIR luminogens are not well
documented in the literature. An important objective of this
Received: September 26, 2013
Published: February 13, 2014
© 2014 American Chemical Society
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Inorganic Chemistry
Communication
nm) duplicates its absorption spectrum, which strongly supports
our previous argument.
Consecutively, we became interested in exploring its potential
as an anion sensor. Compound 1 was found to be selectively
responsive toward fluoride ions. Fluoride plays both an
advantageous (e.g., dental care, anesthetics, osteoporosis, and
psychiatric drugs) and injurious (e.g., water contamination, etc.)
role in health care and environment.11a In this context,
colorimetric and NIR recognition of fluoride has become an
important issue to address.11 To evaluate the fluoride binding
ability of 1, it was titrated against tetra-n-butylammonium
fluoride [TBAF; in a 10 μM dichloromethane (DCM) solution].
Conjugate 1 shows colorimetric as well as ratiometric fluorescent
response in the presence of fluoride (Figure 2).
Figure 1. Thermal ellipsoid probability plot of 1 (left, thermal ellipsoids
are at a 30% probability level) and FMOs of 1 (isovalue = 0.02). Color
code: C, black; B, magenta; N, blue; F, green; S, yellow; I, purple.
The absorption spectrum of 1 (SI, Figure S24) shows bands at
∼400 (ε = 5.15 × 104), ∼480 (ε = 1.56 × 104), ∼615 (ε = 4.5 ×
104), and ∼675 nm (ε = 1.06 × 105). TAB units in conjugate 1
cause a red shift of ∼15−25 nm with respect to 2. This red shift
possibly results from the lower HOMO−LUMO gap induced by
the TAB (2.06 eV) in 1 compared to that in 2 (2.12 eV), as shown
by DFT calculations (Figure 1 and the SI). The nonlocalized
distribution of the frontier molecular orbitals (FMOs; Figure 1
and the SI) over the molecular framework is indicative of effective
conjugation in 1. Compound 1 shows stronger absorption
characteristics in the region ∼350−450 nm (ε = 5.15 × 104 at λabs
= 400 nm) compared to 2 (ε = 2.75 × 104 at λabs = 400 nm). The
difference in the absorption characteristics can be attributed to
the presence of a TAB unit in 1. Time-dependent DFT (TD-
DFT) calculations suggest that the ∼400 nm band in 1 is
dominated by the transition from the styryl−BODIPY-centered
HOMO to the boryl-centered LUMO+1 (f = 1.1010) level (see
the SI).
Figure 2. Absorption and emission (10 μM DCM solution, λex = 350
nm) spectral changes of 1 upon addition of TBAF. The residual signal
700 nm is the overtone of excitation (350 nm).
The addition of fluoride to compound 1 causes a considerable
red shift in its absorption spectrum. The characteristic styryl−
BODIPY band is shifted from ∼675 to ∼700 nm. The absorption
bands at ∼325 and ∼500 nm gradually gain in intensity. Further,
the characteristic boryl absorption bandat ∼400nm showspartial
quenching. Saturation of these optical responses occurs after the
addition of ∼2.0 equiv of fluoride, indicating the formation of a
1:2 complex. The overall fluoride association constant (K2) for
compound 1 was found to be 8.87 × 109. The changes in the
absorption spectrum result in a visually observable color change
from dark green to light greenish-yellow in the solution state.
However, the appearance of any characteristic ICT band was not
observed. TD-DFT studies (see the SI) show that the ∼500 nm
band arises from the fluoride-bound borane-centered HOMO−5
to styryl-centered LUMO (568.32 nm; f = 0.3689). DFT studies
(see the SI) also suggest that the addition of fluoride to the TAB
entity (forming 1·2F−) results in a significant lowering of the
HOMO−LUMO gap, although the FMOs are delocalized over
the entire molecular backbone, indicating the absence of any
possible ICT processes in 1·2F−.
When conjugate 2 is excited at 400 nm, the emission spectrum
shows a sharp band at ∼675 nm with a shoulder at ∼720 nm
(Figure S24). Upon excitation at 400 nm, 1 shows a sharp
emission peak at ∼700 nm with two shoulders at ∼625 and ∼755
nm. The emission bands of 1 are ∼25 nm red-shifted compared to
2 (λem = 675 nm). The bathochromic shift is due to the
involvement of low-energy FMOs in 1 compared to 2. This
inference is supported by computational studies. Time-resolved
emission decay profiles (λem = 700 nm) show that 1 has an
excited-state fluorescence lifetime of 3.2 ns, evidently suggesting
the nature of this luminescence as fluorescence (see the SI). The
emission wavelengths (λem) for compounds 1 and 2 are
unaffected in a wide range of concentrations, signifying the
absence of any intermolecular interactions (see the SI). The
absorption and emission features of 1 (and 2) are also insensitive
to the solvent polarity (see the SI), indicating the absence of any
intramolecular charge-transfer (ICT) processes. Although
excitations at different spectral regions (λex) alter the emission
intensities of 1, they do not affect λem (see the SI). Interestingly,
upon excitation (λex) at 375−400 nm, compound 1 shows the
highest intense emission bands (see the SI). As stated vide supra,
TD-DFT studies and a comparison with 2 indicate that the
∼375−400 nm absorption bands in 1 are dominated by the Ar3B-
centered transitions (Figure S24 and the SI). The results show
that a highly efficient electronic energy-transfer (EET) process
(from TAB to BODIPY) is present in 1, which may be facilitated
by the near-coplanar orientations of the styryl−BODIPY core
and pendant Ar3B units. This borane-to-BODIPY ET, which can
be considered to involve a Foster resonance energy-transfer
(FRET) process, is probably responsible for the absence of any
higher-energy Ar3B-centered emission, which is in contrast to the
previously reported meso-substituted dual-emissive borane−
BODIPY conjugates.9f The excitation spectrum of 1 (λem = 700
Conjugate 1 (in a 10 μM DCM solution) shows a ratiometric
fluorescent response upon the addition of fluoride (Figure 3).
Figure 3. Ratiometric fluorescence response (I440/I740 in a 10 μM DCM
solution; λex = 350 nm) of 1 upon the addition of TBAF in a 10 μM DCM
solution (left). Photographs of 1 under ambient light (right top) and UV
light (right bottom) in the presence of different species.
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Communication
With the gradual addition of TBAF, the broad emission bands at
∼625−800 nm display significant quenching (Stern−Volmer
quenching constant Ksv = 3.7 × 107). The quenching of this
emission band may possibly occur from the stopping of the EET
process from the TAB moiety to BODIPY fluorophore.
Interestingly, a distinctly new broad emission band appears at
∼400 nm and gradually increases upon the addition of fluoride.
Dual-emissive behavior is prominent in the fluoride-bound form
1·2F− rather than in the free form (1). This dual emission
possibly results from the diminished EET processes (from the
fluoride-bound TAB units to the BODIPY core) in 1·2F−,
allowing emission from higher-energy states. The saturation of
emission occurs upon the addition of 2.0 equiv of fluoride. Such a
high sensitivity is uncommon in other TAB-based fluoride
receptors. During fluoride addition, monitoring the I440/I740 ratio
(Figure 3) clearly demonstrates the ratiometric-response
behavior of 1. The I440/I740 intensity ratio is initially 0.0, which
gradually increase to ∼1.5 and reaches saturation after the
addition of only 2.0 equiv of fluoride. This type of ratiometric
response provides the opportunity in molecular sensors to
intrinsically rectify measurement errors. The plot also suggests
the absence of any significant negative cooperative bonding effect
after the addition of 1.0 equiv of fluoride. Such behavior may be
rationalized considering the spatial separation of the two TAB
units and the absence of any strong donor−acceptor ICT process,
which affect the binding of the second fluoride anion.
Competitive binding studies (Figure 3 and the SI) demonstrate
that the photophysical response of conjugate 1 is highly selective
toward fluoride. Interestingly, 1 is insensitive even toward the
presence of large quantities of CN− (>10.0 equiv), which is a
general interfering species in TAB-based anion receptors. The 1H
NMR titrations reveal that while compound 1 binds with fluoride
anions, it remains practically inert toward the presence of even
large amounts of cyanide anions (see the SI).
REFERENCES
■
(1) (a) Binnemans, K. Chem. Rev. 2009, 109, 4283. (b) Freeman, R.;
Willner, B.; Willner, I. J. Phys. Chem. Lett. 2011, 2, 2667. (c) Sasabe, H.;
Kido, J. Chem. Mater. 2011, 23, 621. (d) Li, D.; Zhang, H.; Wang, Y.
Chem. Soc. Rev. 2013, 42, 8416. (e) Wang, X.; Wolfbeis, O. S.; Meier, R. J.
Chem. Soc. Rev. 2013, 42, 7834. (f) Wu, P.; Yan, X.-P. Chem. Soc. Rev.
2013, 42, 5489. (g) Bruel, A. M.; Hager, M. D.; Schubert, U. S. Chem. Soc.
Rev. 2013, 42, 5366. (h) Ciardelli, F.; Ruggeri, G.; Pucci, A. Chem. Soc.
Rev. 2013, 42, 857. (i) Huang, X.; Han, S.; Huang, W.; Li, X. Chem. Soc.
Rev. 2013, 42, 173.
(2) (a) Luo, S.; Zhang, E.; Su, Y.; Cheng, T.; Shi, C. Biomaterials 2011,
32, 7127. (b) Pansare, V. J.; Hejazi, S.; Faenza, W. J.; Prud’homme, R. K.
Chem. Mater. 2012, 24, 812. (c) Xiang, H.; Cheng, J.; Ma, X.; Zhou, X.;
Chruma, J. J. Chem. Soc. Rev. 2013, 42, 6128.
(3) (a) Rurack, K.; Kollmannsberger, M.; Daub, J. New J. Chem. 2001,
25, 289. (b) Atilgan, S.; Ekmekci, Z.; Dogan, A. L.; Guc, D.; Akkaya, E. U.
Chem. Commun. 2006, 4398. (c) Buyukcakir, O.; Bozdemir, O. A.;
Kolemen, S.; Erbas, S.; Akkaya, E. U. Org. Lett. 2009, 11, 4644.
(4) (a) Ozlem, S.; Akkaya, E. U. J. Am. Chem. Soc. 2009, 131, 48.
(b) Atilgan, S.; Ozdemir, T.; Akkaya, E. U. Org. Lett. 2008, 10, 4065.
(c) Ela, S. E.; Yilmaz, M. D.; Icli, B.; Dede, Y.; Icli, S.; Akkaya, E. U. Org.
Lett. 2008, 10, 3299. (d) Bura, T.; Leclerc, N.; Fall, S.; Leveque, P.;
Heiser, T.; Ziessel, R. Org. Lett. 2011, 13, 6030. (e) Bura, T.; Leclerc, N.;
́
Fall, S.; Leveque, P.; Heiser, T.; Retailleau, P.; Rihn, S.; Mirloup, A.;
Ziessel, R. J. Am. Chem. Soc. 2012, 134, 17404.
(5) (a) Loudet, A.; Burgess, K. Chem. Rev. 2007, 107, 4891. (b) Ulrich,
G.; Harriman, A.; Ziessel, R. Angew. Chem., Int. Ed. 2008, 47, 1202.
(6) (a) Yamaguchi, S.; Akiyama, S.; Tamao, K. J. Am. Chem. Soc. 2001,
123, 11372. (b) Yamaguchi, S.; Shirasaka, T.; Akiyama, S.; Tamao, K. J.
Am. Chem. Soc. 2002, 124, 8816. (c) Sundararaman, A.; Victor, M.;
Varughese, R.; Jakle, F. J. Am. Chem. Soc. 2005, 127, 13748. (d) Chiu, C.-
̈
W.; Gabbaï, F. P. J. Am. Chem. Soc. 2006, 128, 14248. (e) Liu, X.-Y.; Bai,
D.-R.; Wang, S. Angew. Chem., Int. Ed. 2006, 45, 5475. (f) Parab, K.;
Venkatasubbaiah, K.; Jakle, F. J. Am. Chem. Soc. 2006, 128, 12879.
̈
(g) Sakuda, E.; Funahashi, A.; Kitamura, N. Inorg. Chem. 2006, 45,
10670. (h) Yuan, M.-S.; Liu, Z.-Q.; Fang, Q. J. Org. Chem. 2007, 72, 7915.
(i) Bai, D.-R.; Liu, X.-Y.; Wang, S. Chem.Eur. J. 2007, 13, 5713.
(j) Zhao, S.-B.; McCormick, T.; Wang, S. Inorg. Chem. 2007, 46, 10965.
(k) Kim, Y.; Gabbaï, F. P. J. Am. Chem. Soc. 2009, 131, 3363.
(7) (a) Entwistle, C. D.; Marder, T. B. Chem. Mater. 2004, 16, 4574.
(b) Yamaguchi, S.; Wakamiya, A. Pure Appl. Chem. 2006, 78, 1413.
(c) Wade, C. R.; Gabbai, F. P. Dalton Trans. 2009, 9169. (d) Hudnall, T.
W.; Chiu, C.-W.; Gabbai, F. P. Acc. Chem. Res. 2009, 42, 388. (e) Wade,
C. R.; Broomsgrove, A. E. J.; Aldridge, S.; Gabbai, F. P. Chem. Rev. 2010,
110, 3958. (f) Kim, Y.; Huh, H.-S.; Lee, M. H.; Lenov, I. L.; Zhao, H.;
Gabbai, F. P. Chem.Eur. J. 2011, 17, 2057. (g) Huang, W.; Besar, K.;
LeCover, R.; Rule, A. M.; Breysse, P. N.; Katz, H. E. J. Am. Chem. Soc.
2012, 134, 14650.
In conclusion, we have developed a NIR-emitting TAB-
decorated styryl−BODIPY (1) via a facile synthetic route. The
incorporation of TAB entities results in a significantly red-shifted
broad emission in 1 (compared to 2). The near-coplanar
orientation of Ar3B planes the BODIPY core results in a highly
efficient (TAB-to-BODIPY) EET process in 1. Conjugate 1 acts
as a highly selective and sensitive fluoride sensor with a naked eye
visual response as well as ratiometric fluorescent response. The
dual emission in fluoride-bound 1 possibly results from the
restricted partial TAB-to-BODIPY ET. Further investigations are
underway.
(8) (a) Hudson, Z. M.; Wang, S. Acc. Chem. Res. 2009, 42, 1584.
(b) Jakle, F. Chem. Rev. 2010, 110, 3985. (c) Tanaka, K.; Chujo, Y.
̈
Macromol. Rapid Commun. 2012, 33, 1235.
ASSOCIATED CONTENT
* Supporting Information
■
(9) (a) Huh, J. O.; Do, Y.; Lee, M. H. Organometallics 2008, 27, 1022.
(b) Fu, G.-L.; Pan, H.; Zhao, Y.-H.; Zhao, C.-H. Org. Biomol. Chem.
2011, 9, 8141. (c) Sun, H.; Dong, X.; Liu, S.; Zhao, Q.; Mou, X.; Yang, H.
Y.; Huang, W. J. Phys. Chem. C 2011, 115, 19947. (d) Yin, Z.; Tam, A. Y.-
Y.; Wong, K. M.-C.; Tao, C.-H.; Li, B.; Poon, C.-T.; Wu, L.; Yam, V. W.-
W. Dalton Trans. 2012, 41, 11340. (e) Lu, J.; Ko, S.-B.; Walters, N. R.;
Wang, S. Org. Lett. 2012, 14, 5660. (f) Swamy, P. C. A.; Mukherjee, S.;
Thilagar, P. Chem. Commun. 2013, 49, 993. (g) Sarkar, S. K.; Thilagar, P.
Chem. Commun. 2013, 49, 8558.
S
Synthesis, characterizations, and computational data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: thilagar@ipc.iisc.ernet.in. Tel: 0091-80-22933353. Fax:
0091-80-23601552.
(10) Frisch, M. J.; et al. Gaussian 09; Gaussian Inc.: Wallingford, CT,
2009 ( see the SI for complete citation).
(11) (a) Jagtap, S.; Yenkie, M. K.; Labhsetwar, N.; Rayalu, S. Chem. Rev.
2012, 112, 2454. (b) Cametti, M.; Rissanen, K. Chem. Soc. Rev. 2013, 42,
2016.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
P.T. thanks the DST and CSIR, New Delhi, for financial support.
S.K.S. thanks IISc, and S.M. thanks the CSIR for SPMF.
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