This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 10301–10303 10301

Cite this: Chem. Commun., 2012, 48, 10301–10303

Benzobisoxazole fluorophore vicariously senses amines, ureas, anionsw

Jaebum Lim and Ognjen S. Miljanic*

Received 2nd August 2012, Accepted 31st August 2012

DOI: 10.1039/c2cc35626k

A benzobisoxazole-based cruciform fluorophore forms fluores-

cent complexes with simple boronic acids. Through the changes

of their fluorescence emission colours, these complexes can sense

and qualitatively distinguish among structurally similar organic

nitrogen compounds (amines and ureas) and small organic and

inorganic anions. Preliminary results suggest that the intensity

of this hybrid sensor’s fluorescent response to chloride anions can

be quantitatively correlated to chloride concentration.

Owing to the electron-deficient character of boron and its ability

to reversibly bind electron-rich species, organoboronic acids

(R–B(OH)2) have been prominently used as fluorescent sensors

for saccharides,1 anions2 and other nucleophilic analytes.3 To

permit a modulation of fluorescent response upon analyte binding

to boron, a fluorophore must be incorporated into the R group of

the boronic acid, typically requiring a non-trivial synthesis.

Alternatively, non-fluorescent boronic acids can be used as

sensors through the indicator displacement strategy,2c,3a,4

wherein the boronic acid transiently binds—and thus quenches—a

fluorescent indicator. Upon its displacement by a better-binding

analyte, the indicator recovers its fluorescence, which can be

measured and correlated with the analyte concentration. This

methodology often allows simplifications in the synthesis of the

sensor, but can be qualitatively limited to the emission responses of

the indicator. In this communication, we report a self-assembled

fluorescent sensing system comprised of a benzobisoxazole

fluorophore and commercially available boronic acids. In its

response to organic nitrogen compounds and small inorganic

and organic anions, this hybrid sensor offers a range of

qualitatively diverse emission responses, consistent with both

indicator displacement and fluorescence modulation pathways.

We have previously shown that the fluorescence emission colours

of benzobisoxazole cruciform5 1 (Fig. 1) dramatically change upon

addition of simple boronic acids such as B1–B7.6 These changes are

presumably caused by the formation of complexes between the

pyridine nitrogens in 1 and boron atoms in B1–B7; this

complexation stabilizes the LUMO of 1, usually leading to

red shifts in fluorescence emission.7 We speculated that these

transient complexes could sense other nucleophilic analytes

that could interact with electron-deficient boron nuclei of

coordinated boronic acids. This hypothesis was tested on two

analyte classes: (1) nitrogen-containing organic compounds,

including amines and ureas (Fig. 2), and (2) small organic

and inorganic anions of varying basicity (Fig. 3).

Evaluation of analyte binding was performed using simple

photography of emission colours, pioneered by Bunz et al.8

A 10�6 M solution of cruciform 1 in 1,2,4-trichlorobenzene

(TCB) was first exposed to a B20 000-fold excess of boronic

acids B1–B7. Emission colours of the complexes developed

instantaneously and were photographed in a darkroom setup

(see ESIw for details). All but two boronic acids (B4 and B6)

quenched the fluorescence of 1 (Fig. 2A, row #1). Subsequently,

2 mL aliquots of these seven stock solutions were transferred to

cuvettes and exposed to 40 mg of analytes shown in Fig. 2A.

Photographs of the developed emission colours are shown in

Fig. 2A, rows #2–13. Several trends are apparent. Analytes more

basic than pyridine-based cruciform 1 (rows #2–4)9 appear to

simply regenerate the fluorescence of pure 1, manifesting indicator

displacement behaviour. On the other hand, less basic analytes

appear to modulate the strength of the complex between 1 and

boronic acids,10 resulting in a change in fluorescence colour.

Finally, boronic acid B4 appears to be self-complexed: it does

not significantly affect the fluorescence of 1, regardless of

whether an analyte is present. Overall, qualitative distinction

among analytes in rows #2–13 could be achieved in some, but

not all cases.

Next, we turned our attention to the variation of the solvent

in which emission colours were observed; cruciform 1 and its

Fig. 1 Structure of cruciform fluorophore 1 and its emission colours

in 1,2,4-trichlorobenzene (TCB), cyclohexane (CH), dichloromethane

(DCM), chloroform (CF) and acetonitrile (AN). In the box, structures

of boronic acids B1–B7 used as additives in this study.

University of Houston, Department of Chemistry, 136 FlemingBuilding, Houston, TX 77204-5003, USA. E-mail: miljanic@uh.edu;Fax: +1 (713) 743-2709; Tel: +1 (832) 842-8827w Electronic supplementary information (ESI) available: Descriptionsof procedures for photographing emission colours, numerical analysisof R/G/B values for those colours, control experiments. See DOI:10.1039/c2cc35626k

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10302 Chem. Commun., 2012, 48, 10301–10303 This journal is c The Royal Society of Chemistry 2012

complexes with boronic acids show highly solvatochromic beha-

viour. Here, only two boronic acids—B1 and B2—were studied as

additives to 1, and the fluorescence response was examined in five

solvents: TCB, cyclohexane (CH), dichloromethane (DCM),

chloroform (CF) and acetonitrile (AN). The results are summarized

in the two panels shown in Fig. 2B. Similarly to the solvent

variation experiments, amines more basic than the pyridine-based

cruciform 1 show emission colours that suggest at least partial

expulsion of 1 from its boronic acid complexes (Fig. 2A, rows

#2–4). On the other hand, less basic analytes show a diverse range

of emission colours. This diversity was sufficient to permit qualita-

tive discrimination between these species, based on the fact that no

two of them will have all ten identical emission colours. This notion

was confirmed by the statistical analysis of the numeric Red/Green/

Blue values of emission colours, which is detailed in the ESI.w

Following the successful demonstration that organic nitrogen

compounds can be qualitatively identified using cruciform 1 in

tandem with boronic acid additives, small organic and inorganic

anions were examined next. These analytes are of broad signifi-

cance in environmental, physiological and industrial applications.11

The methodology used was analogous to the one described for the

analysis of organic amines and ureas. The results of the anion

analysis are shown in Fig. 3. Again, basic anions such as

fluoride or benzoate lead to the apparent decomplexation of 1

from the boronic acid (Fig. 3A, rows #5 and #13). The

qualitative discrimination between the different anions is

possible and statistically significant, as detailed in the ESI.wThe use of this hybrid sensing system, coupled with the

photographic identification of emission responses, allows rapid

Fig. 2 (A) Row #1 shows emission colours of complexes of cruciform 1

and boronic acids B1–B7 in TCB as the solvent. Rows #2–13 show the

emission colours resulting from the exposure of these complexes to the

specified organic nitrogen compounds. (B) Row #1 shows emission colours

of complexes of cruciform 1 and boronic acids B1 (left) and B2 (right) in

five different solvents. Rows #2–13 show the emission colours resulting

from the exposure of these complexes to the specified amine analytes. In all

experiments, the molar ratio of 1 vs. B1–B7 was B1 : 20000. For all

photographs, lexcitation was 365 nm and shutter speed was 0.5 s.

Fig. 3 (A) Row #1 shows emission colours of complexes of cruciform

1 and boronic acids B1–B7 in TCB as the solvent. Rows #2–13 show

the emission colours resulting from the exposure of these complexes to

the specified anions, added as their tetrabutylammonium (TBA) salts.

(B) Row #1 shows emission colours of complexes of cruciform 1 and

boronic acids B1 (left) and B2 (right) in five different solvents. Rows

#2–13 show the emission colours resulting from the exposure of these

complexes to the specified anions, added as their TBA salts. In all

experiments, the molar ratio of 1 vs. B1–B7 was B1 : 20 000. For all

photographs, lexcitation was 365 nm and shutter speed was 0.5 s.

This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 10301–10303 10303

screening of potential sensors for a given analyte. In this

context, the behaviour of complexes of 1 with B1–B7 in TCB

was particularly intriguing: initially quenched fluorescence of

these complexes was restored upon exposure to analytes. Such

OFF - ON behaviour12 is desirable in sensing applications

because of the enhanced contrast. We thus investigated whether

the 1�B1 complex could be used as a quantitative sensor for

chloride anions. The results of the titration of the 1 : 20 000

mixture of 1 and B1 in TCB with concentrated solutions of

tetrabutylammonium chloride (TBACl) are shown in Fig. 4.

A calibration curve (Fig. 4B) suggests an exponential corre-

lation (R2 = 0.911) between the fluorescence enhancement and

the amount of added TBACl. We are currently investigating

whether quantification using the 1�B1 sensing system is possible

for other analytes as well.

In conclusion, we have developed a self-assembled two-

component fluorescent sensing system which can respond to

diverse analytes through either fluorescence modulation or

indicator displacement pathways. This versatility is achieved

by chemically decoupling the boronic acid—which engages into

the chemical reaction, from the cruciform fluorophore—which

reports on this change. The two come together only in a

complex; thus, the synthesis of the sensing system is simplified,

and its generality is increased, as selectivity and optical response

can be independently engineered. Given the available diversity

of cruciform structures and potential additives, this ‘‘vicarious

sensing’’ strategy could allow rapid screening and optimization

of new sensors for many chemically diverse species.

This research was generously supported by the National

Science Foundation CAREER program (CHE-1151292), the

Welch Foundation (grant no. E-1768), the University of Houston

(UH) and the Texas Center for Superconductivity at UH.

Notes and references

1 For reviews, see: (a) T. D. James, Top. Curr. Chem., 2007, 277,107–152; (b) T. D. James, M. D. Phillips and S. Shinkai, BoronicAcids in Saccharide Recognition, RSC Publishing, Cambridge,2006; (c) T. D. James, in Boronic Acids: Preparation, Applicationsin Organic Synthesis and Medicine, ed. D. G. Hall, Wiley-VCH,Weinheim, 2005, pp. 441–480; (d) H. Fang, G. Kaur and B. Wang,J. Fluoresc., 2004, 14, 481–489; (e) S. Striegler, Curr. Org. Chem.,

2003, 7, 81–102; (f) T. D. James and S. Shinkai, Top. Curr. Chem.,2002, 218, 159–200.

2 For examples and reviews, see: (a) R. Nishiyabu, Y. Kubo,T. D. James and J. S. Fossey, Chem. Commun., 2011, 47,1106–1123; (b) C. R. Wade, A. E. J. Broomsgrove, S. Aldridgeand F. P. Gabbaı, Chem. Rev., 2010, 110, 3958–3984; (c) Y. Kubo,A. Kobayashi, T. Ishida, Y. Misawa and T. D. James, Chem.Commun., 2005, 2846–2848; (d) R. Badugu, J. R. Lakowicz andC. D. Geddes, Curr. Anal. Chem., 2005, 1, 157–170;(e) N. DiCesare and J. R. Lakowicz, Anal. Biochem., 2002, 301,111–116.

3 For examples, see: (a) Z. Guo, I. Shin and J. Yoon, Chem.Commun., 2012, 48, 5956–5967, and the references therein;(b) L. T. Gallagher, J. S. Heo, M. A. Lopez, B. M. Ray, J. Xiao,A. P. Umali, A. Zhang, S. Dharmarajan, H. Heymann andE. V. Anslyn, Supramol. Chem., 2012, 24, 143–148;(c) A. E. Hargrove, R. N. Reyes, I. Riddington, E. V. Anslynand J. L. Sessler, Org. Lett., 2010, 12, 4804–4807.

4 (a) L. Zhu, Z. L. Zhong and E. V. Anslyn, J. Am. Chem. Soc.,2005, 127, 4260–4269; (b) S. L. Wiskur, H. Ait-Haddou,J. J. Lavigne and E. V. Anslyn, Acc. Chem. Res., 2001, 34,963–972; (c) L. A. Cabell, M.-K. Monahan and E. V. Anslyn,Tetrahedron Lett., 1999, 40, 7753–7756; (d) S. Arimori,H. Murakami, M. Takeuchi and S. Shinkai, J. Chem. Soc., Chem.Commun., 1995, 961–962.

5 For other classes of cruciform fluorophores, see, inter alia:(a) A. J. Zucchero, P. L. McGrier and U. H. F. Bunz, Acc. Chem.Res., 2010, 43, 397–408; (b) A. J. Zucchero, J. N. Wilson andU. H. F. Bunz, J. Am. Chem. Soc., 2006, 128, 11872–11881;(c) J. A. Marsden, J. J. Miller, L. D. Shirtcliff and M. M. Haley,J. Am. Chem. Soc., 2005, 127, 2464–2476.

6 J. Lim, D. Nam and O. S. Miljanic, Chem. Sci., 2012, 3, 559–563.7 (a) J. Lim, T. A. Albright, B. R. Martin and O. S. Miljanic, J. Org.Chem., 2011, 76, 10207–10209. See also: (b) J. Lim, K. Osowska,J. A. Armitage, B. R. Martin and O. S. Miljanic, CrystEngComm,2012, 14, 6152–6162; (c) K. Osowska and O. S. Miljanic, Chem.Commun., 2010, 46, 4276–4278; (d) A. K. Feldman,M. L. Steigerwald, X. Guo and C. Nuckolls, Acc. Chem. Res.,2008, 41, 1731–1741; (e) J. E. Klare, G. S. Tulevski andC. Nuckolls, Langmuir, 2004, 20, 10068–10072; (f) J. E. Klare,G. S. Tulevski, K. Sugo, A. de Picciotto, A. K. White andC. Nuckolls, J. Am. Chem. Soc., 2003, 125, 6030–6031;(g) B. C. Tlach, A. L. Tomlinson, A. Bhuwalka and M. Jeffries-EL,J. Org. Chem., 2011, 76, 8670–8681.

8 (a) E. A. Davey, A. J. Zucchero, O. Trapp and U. H. F. Bunz,J. Am. Chem. Soc., 2011, 133, 7716–7718. For other examples ofamine sensing using cruciforms, see: (b) J. Kumpf and U. H. F.Bunz, Chem.–Eur. J., 2012, 18, 8921–8924; (c) C. Patze, K. Broedner,F. Rominger, O. Trapp and U. H. F. Bunz, Chem.–Eur. J., 2011, 17,13720–13725; (d) P. L. McGrier, K. M. Solntsev, S. Miao,L. M. Tolbert, O. R. Miranda, V. M. Rotello and U. H. F. Bunz,Chem.–Eur. J., 2008, 14, 4503–4510.

9 Pyridine is a weaker base (pKa = 5.25 for the conjugated acid)than triethylamine, piperidine, and piperazine, whose conjugatedacids have pKa values of 10.75, 11.12 and 9.83, successively. See:http://www.zirchrom.com/organic.htm.

10 K. Koumoto, M. Takeuchi and S. Shinkai, Supramol. Chem., 1998,9, 203–210.

11 (a) J. L. Sessler, P. Gale and W.-S. Cho, Anion Receptor Chemistry,RSC Publishing, Cambridge, 2006; (b) P. A. Gale, Chem. Commun.,2011, 47, 82–86; (c) S. R. Bayly and P. D. Beer, Top. Curr. Chem.,2008, 129, 45–94; (d) R. Martinez-Manez and F. Sancenon, Chem.Rev., 2003, 103, 4419–4476. For a previous example of anion-responsive cruciform fluorophores, see: (e) S. M. Brombosz,A. J. Zucchero, R. L. Phillips, D. Vazquez, A. Wilson and U. H. F.Bunz, Org. Lett., 2007, 9, 4519–4522.

12 (a) Y. Abrahan, H. Salman, K. Suwinska and Y. Eichen, Chem.Commun., 2011, 47, 6087–6089; (b) C. N. Carroll, B. A. Coombs,S. P. McClintock, C. A. Johnson II, O. B. Berryman,D. W. Johnson and M. M. Haley, Chem. Commun., 2011, 47,5539–5541; (c) D.-S. Kim and K. H. Ahn, J. Org. Chem., 2008, 73,6831–6834; (d) A. P. de Silva, H. Q. N. Gunaratne andC. P. McCoy, Chem. Commun., 1996, 2399–2400.

Fig. 4 (A) Stacked fluorescence emission spectra for the titration of a

mixture of 1 and B1 (1 : 20 000 molar ratio) with TBACl. (B) Enhance-

ment in the emission intensity of a 1�B1 solution (at 501 nm) is plotted

as a function of added TBACl. Equivalents of TBACl are given relative

to the amount of cruciform 1, and lexcitation was 365 nm.