Highly selective phosphorescent chemosensor for fluoride based on an iridium(III) complex containing arylborane units

Article abstract of DOI:10.1021/ic800500c

A new phosphorescent iridium(III) complex [Ir(Bpq)₂(bpy)] ⁺PF₆^- based on cyclometalated ligands (Bpq) containing a dimesitylboryl group was synthesized and characterized by photophysical and electrochemical studies. The excited-state properties of Bpq and [Ir(Bpq)₂(bpy)]⁺PF₆^_ were investigated using molecular orbital calculations. Importantly, both Bpq and [Ir(Bpq)₂(bpy)]⁺PF₆^- could be used as highly selective chemosensors for a fluoride anion (F^-) detected by the naked eye, owing to the interaction of the dimesitylboryl group (BMes₂) with F^-. For the Bpq ligand, a red shift of the emission spectrum was observed upon the addition of F^-, which could be attributed to an excited-state switch from a π-π* transition to a charge-transfer transition upon complexation with F^-. The addition of F_- to a solution of [Ir(Bpq)₂(bpy)]⁺PF ₆^- induced a change in the solution color from yellow to orange-red and phosphorescent quenching, indicating that [Ir(Bpq) ₂(bpy)]⁺PF₆^- could act as an excellent ON-OFF-type phosphorescent chemosensor for F^-.

Full text of DOI:10.1021/ic800500c

Inorg. Chem. 2008, 47, 9256-9264
Highly Selective Phosphorescent Chemosensor for Fluoride Based on
an Iridium(III) Complex Containing Arylborane Units
Qiang Zhao,† Fuyou Li,*,† Shujuan Liu,‡ Mengxiao Yu,† Zhiqiang Liu,†,§ Tao Yi,† and Chunhui Huang†
Department of Chemistry & Laboratory of AdVanced Materials, Fudan UniVersity, Shanghai
200433, People’s Republic of China, and Institute of AdVanced Materials (IAM), Nanjing
UniVersity of Posts & Telecommunications, Nanjing 210003, People’s Republic of China
Received March 19, 2008
A new phosphorescent iridium(III) complex [Ir(Bpq)2(bpy)]+PF6- based on cyclometalated ligands (Bpq) containing
a dimesitylboryl group was synthesized and characterized by photophysical and electrochemical studies. The excited-
state properties of Bpq and [Ir(Bpq)2(bpy)]+PF6- were investigated using molecular orbital calculations. Importantly,
both Bpq and [Ir(Bpq)2(bpy)]+PF6- could be used as highly selective chemosensors for a fluoride anion (F-) detected
by the naked eye, owing to the interaction of the dimesitylboryl group (BMes2) with F-. For the Bpq ligand, a red
shift of the emission spectrum was observed upon the addition of F-, which could be attributed to an excited-state
switch from a π-π* transition to a charge-transfer transition upon complexation with F-. The addition of F- to a
solution of [Ir(Bpq)2(bpy)]+PF6- induced a change in the solution color from yellow to orange-red and phosphorescent
quenching, indicating that [Ir(Bpq)2(bpy)]+PF6- could act as an excellent ON-OFF-type phosphorescent chemosensor
for F-.
Introduction
recognition and detection are of growing interest, and there
is a need to develop new selective and sensitive methods
for fluoride detection in various environments. Up to now,
a large number of synthetic chromophores have been
designed as chemosensors for this important analyte.2 It is
well-known that (thio)ureas,2 amides,3 pyrrole,4 and imida-
zolium5 are particularly effective in reacting with F- through
the formation of a hydrogen bond between the active N-H
group and F-. Recently, specific Lewis acid-base interac-
tions, such as the strong affinity of a boron atom toward F-,
Fluoride anion (F-), the smallest anion, plays an essential
role in a broad range of biological, medical, and chemical
processes and in applications such as dental care, treatment
of osteoporosis, fluorination of water supplies, and even
chemical and nuclear warfare agents.1 As a result, its
*Towhomcorrespondenceshouldbeaddressed.E-mail:fyli@fudan.edu.cn.
Tel: 86-21-55664185. Fax: 86-21-55664621.
†
Fudan University.
Nanjing University of Posts and Telecommunications.
Permanent address: Shandong University, Jinan 250100, People’s
‡
§
Republic of China.
(3) For example, see: (a) Gale, P. A. Acc. Chem. Res. 2006, 39, 465–
475. (b) Bondy, C. R.; Loeb, S. J. Coord. Chem. ReV. 2003, 240,
77–99.
(4) (a) Sessler, J. L.; Camiolo, S.; Gale, P. A. Coord. Chem. ReV. 2003,
240, 17–55. (b) Plitt, P.; Gross, D. E.; Lynch, V. M.; Sessler, J. L.
Chem.sEur. J. 2007, 13, 1374–1381. (c) Lin, C. I.; Selvi, S.; Fang,
J. M.; Chou, P. T.; Lai, C. H.; Cheng, Y. M. J. Org. Chem. 2007, 72,
3537–3542. (d) de Namor, A. F. D.; Shehab, M.; Abbas, I.; Withams,
M. V.; Zvietcovich-Guerra, J. J. Phys. Chem. B 2006, 110, 12653–
12659.
(1) (a) Kirk, K. L. Biochemistry of the Halogens and Inorganic Halides;
Plenum Press: New York, 1991; p 58. (b) Riggs, B. L. Bone and
Mineral Research; Annual 2; Elsevier: Amsterdam, The Netherlands,
1984; p 366. (c) Wiseman, A. Handbook of Experimental Pharmacol-
ogy XX/2; Springer-Verlag: Berlin, 1970; Part 2, p 48.
(2) For recent reviews on anion receptors, see: (a) Coord. Chem. ReV.
2003, 240,issues 1 and 2 (Special Issues on anion receptors). (b)
Mart´ınez-Ma´n˜ez, R.; Sanceno´n, F. Chem. ReV. 2003, 103, 4419–4476.
(c) Suksai, C.; Tuntulani, T. Chem. Soc. ReV. 2003, 32, 192–202. (d)
Best, M. D.; Tobey, S. L.; Anslyn, E. V. Coord. Chem. ReV. 2003,
240, 3–15. (e) Beer, P. D.; Gale, P. A. Angew. Chem., Int. Ed. 2001,
40, 486–516. (f) Snowden, T. S.; Anslyn, E. V. Curr. Opin. Chem.
Biol. 1999, 740–746. (g) Arnendola, V.; Bonizzoni, M.; Esteban-
Gomez, D.; Fabbrizzi, L.; Licchelli, M.; Sancenon, F.; Taglietti, A.
Coord. Chem. ReV. 2006, 250, 1451–1470. (h) Gunnlaugsson, T.;
Glynn, M.; Tocci, G. M.; Kruger, P. E.; Pfeffer, F. M. Coord. Chem.
ReV. 2006, 250, 3094–3117. (i) Amendola, V.; Esteban-Gomez, D.;
Fabbrizzi, L.; Licchelli, M. Acc. Chem. Res. 2006, 39, 343–353.
(5) Yoon, J. Y.; Kim, S. K.; Singh, N. J.; Kim, K. S. Chem. Soc. ReV.
2006, 35, 355–360.
(6) (a) Yamaguchi, S.; Akiyama, S.; Tamao, K. J. Am. Chem. Soc. 2001,
123, 11372–11375. (b) Yamaguchi, S.; Akiyama, S.; Tamao, K. J. Am.
Chem. Soc. 2000, 122, 6335–6336. (c) Yamaguchi, S.; Shirasaka, T.;
Akiyama, S.; Tamao, K. J. Am. Chem. Soc. 2002, 124, 8816–8817.
(d) Kubo, Y.; Yamamoto, M.; Ikeda, M.; Takeuchi, M.; Shinkai, S.;
Yamaguchi, S.; Tamao, K. Angew. Chem., Int. Ed. 2003, 42, 2036–
2040.
9256 Inorganic Chemistry, Vol. 47, No. 20, 2008
10.1021/ic800500c CCC: $40.75  2008 American Chemical Society
Published on Web 09/24/2008

Highly SelectiWe Phosphorescent Chemosensor for Fluoride
have been adopted as an efficient approach for fluoride
detection.6-11 Some three-coordinated boron compounds
with a donor-π conjugation-acceptor (D-π-A) chemical
structure,12 in which the dimesitylboryl (BMes2) group was
adopted as the electron acceptor, were reported to be highly
selective colorimetric chemosensors. In the presence of F-,
strong B-F interaction can interrupt the extended π conjuga-
tion of these organoboron derivatives, thereby causing a
dramatic change in the photophysical properties. To the best
of our knowledge, however, the output signals of most
fluoride chemosensors are limited to absorption and fluo-
rescence signals. Recently, Gabbai et al. reported the first
example of a phosphorescent anion sensor based on a
heteronuclear B/Hg bidentate Lewis acid.7a
in the local environment, significant Stokes shifts for easy
separation of excitation and emission, and relatively long
lifetimes compared to purely organic luminophores. Espe-
cially, the long lifetime of phosphorescence can eliminate
the interference of the short-lifetime background fluorescence
and scattering light by using a time-resolved luminescence
technology. The selectivity is enhanced because it is a less
usual phenomenon than that of fluorescence. Importantly,
the long lifetime also allows the excited state ample time to
sample its environment, and it is this interaction that makes
these materials sensitive reporters. Some complexes, such
as platinum(II), rhenium(I), and ruthenium(II) complexes,
have been successfully utilized as phosphorescent chemosen-
sors for anions,13 oxygen concentration,14 and metal ions.15
As one of the best classes of phosphorescent dyes,16-25
iridium(III) complexes exhibited tunable luminescent color
(from blue to red), high luminescent quantum yield (∼0.7
in organic solvent), and remarkable structure-function
relationships and have been used as highly efficient elec-
troluminescent emitters in organic light-emitting diodes.
Recently, the photophysical properties of iridium(III) com-
plexes were reported to be affected by external stimuli such
Recently, the use of phosphorescent heavy-metal com-
plexes as chemosensors has attracted considerable interest
because of advantageous photophysical properties of heavy-
metal complexes, such as emission colors shifts with changes
(7) (a) Melaimi, M.; Gabbai, F. P. J. Am. Chem. Soc. 2005, 127, 9680–
9681. (b) Sole, S.; Gabbai, F. P. Chem. Commun. 2004, 1284–1285.
(c) Lee, M. H.; Agou, T.; Kobayashi, J.; Kawashima, T.; Gabbai, F. P.
Chem. Commun. 2007, 1133–1135. (d) Chiu, C. W.; Gabbai, F. P.
J. Am. Chem. Soc. 2006, 128, 14248–14249. (e) Hudnall, T. W.;
Melaimi, M.; Gabbai, F. P. Org. Lett. 2006, 8, 2747–2749. (f) Hudnall,
T. W.; Gabbaı, F. P. J. Am. Chem. Soc. 2007, 129, 11978–11986.
(8) (a) Liu, X. Y.; Bai, D. R.; Wang, S. Angew. Chem., Int. Ed. 2006, 45,
5475–5478. (b) Bai, D. R.; Liu, X. Y.; Wang, S. Chem.sEur. J. 2007,
13, 5713–5723. (c) Sun, Y.; Ross, N.; Zhao, S. B.; Huszarik, K.; Jia,
W. L.; Wang, R. Y.; Macartney, D.; Wang, S. J. Am. Chem. Soc.
2007, 129, 7510–7511. (d) Zhao, S. B.; McCormick, T.; Wang, S.
Inorg. Chem. 2007, 46, 10965–10967.
(9) (a) Liu, Z. Q.; Shi, M.; Li, F. Y.; Fang, Q.; Chen, Z. H.; Yi, T.; Huang,
C. H. Org. Lett. 2005, 7, 5481–5484. (b) Zhou, Z. G.; Xiao, S. Z.;
Xu, J.; Liu, Z. Q.; Shi, M.; Li, F. Y.; Yi, T.; Huang, C. H. Org. Lett.
2006, 8, 3911–3914. (c) Zhou, Z. G.; Yang, H.; Shi, M.; Xiao, S. Z.;
Li, F. Y.; Yi, T.; Huang, C. H. ChemPhysChem 2007, 8, 1289–1292.
(d) Zhou, Z. G.; Li, F. Y.; Yi, T.; Huang, C. H. Tetrahedron Lett.
2007, 48, 6633–6636.
(13) (a) Anzenbacher, P., Jr.; Tyson, D. S.; Jursıkova, K.; Castellano, F. N.
J. Am. Chem. Soc. 2002, 124, 6232–6233. (b) Fillaut, J.-L.; Andries,
J.; Perruchon, J.; Desvergne, J.-P.; Toupet, L.; Fadel, L.; Zouchoune,
B.; Saillard, J.-Y. Inorg. Chem. 2007, 46, 5922–5932. (c) Jose, D. A.;
Kar, P.; Koley, D.; Ganguly, B.; Thiel, W.; Ghosh, H. N.; Das, A.
Inorg. Chem. 2007, 46, 5576–5584.
(14) (a) Huynh, L.; Wang, Z.; Yang, J.; Stoeva, V.; Lough, A.; Manners,
I.; Winnik, M. A. Chem. Mater. 2005, 17, 4765–4773. (b) Brin˜as,
R. P.; Troxler, T.; Hochstrasser, R. M.; Vinogradov, S. A. J. Am. Chem.
Soc. 2005, 127, 11851–11862. (c) Han, B.-H.; Winnik, M. A. Chem.
Mater. 2005, 17, 4001–4009. (d) McGee, K. A.; Veltkamp, D. J.;
Marquardt, B. J.; Mann, K. R. J. Am. Chem. Soc. 2007, 129, 15092–
15093. (e) Lei, B.; Li, B.; Zhang, H.; Zhang, L.; Li, W. J. Phys. Chem.
C 2007, 111, 11291–11301.
(10) (a) Dusemund, C.; Sandanayake, K. R. A. S.; Shinkai, S. J. Chem.
Soc., Chem. Commun. 1995, 333–334. (b) Yamamoto, H.; Ori, A.;
Ueda, K.; Dusemund, C.; Shinkai, S. Chem. Commun. 1996, 407–
408.
(15) Tang, W. S.; Lu, X. X.; Wong, K. M. C.; Yam, V. W. W. J. Mater.
Chem. 2005, 15, 2714–2720.
(16) For recent review, see: (a) Chou, P. T.; Chi, Y. Chem.sEur. J. 2007,
13, 380–395. (b) Ma, B.; Djurovich, P. I.; Thompson, M. E. Coord.
Chem. ReV. 2005, 249, 1501–1510. (c) Forrest, S. R.; Thompson, M. E.
Chem. ReV. 2007, 107, 923–925. (d) Huang, C. H.; Li, F. Y.; Huang,
W. Introduction to Organic Light-Emitting Materials and DeVices;
Press of Fudan University: Shanghai, China, 2005. (e) Grushin, V. V.
Chem, ReV. 2004, 104, 1629–1662.
(11) (a) Shiratori, H.; Ohno, T.; Nozaki, K.; Osuka, A. Chem. Commun.
1999, 2181–2182. (b) Sundararaman, A.; Victor, M.; Varughese, R.;
Ja¨kle, F. J. Am. Chem. Soc. 2005, 127, 13748–13749. (c) Aldridge,
S.; Bresner, C.; Fallis, I. A.; Coles, S. J.; Hursthouse, M. B. Chem.
Commun. 2002, 740–741. (d) Badugu, R.; Lakowicz, J. R.; Geddes,
C. D. Curr. Anal. Chem. 2005, 1, 157. (e) Bresner, C.; Day, J. K.;
Coombs, N. D.; Fallis, I. A.; Aldridge, S.; Coles, S. J.; Hursthouse,
M. B. Dalton Trans. 2006, 3660–3667. (f) Bresner, C.; Aldridge, S.;
Fallis, I. A.; Jones, C.; Ooi, L.-L. Angew. Chem., Int. Ed. 2005, 44,
3606–3609. (g) Cooper, C. R.; Spencer, N.; James, T. D. Chem.
Commun. 1998, 1365–1366. (h) Arimori, S.; Davidson, M. G.; Fyles,
T. M.; Hibbert, T. G.; James, T. D.; Kociok-Koehn, G. I. Chem.
Commun. 2004, 1640–1641. (i) DiCesare, N.; Lakowicz, J. R. Anal.
Biochem. 2002, 301, 111–116. (j) Parab, K.; Venkatasubbaiah, K.;
Ja¨kle, F. J. Am. Chem. Soc. 2006, 128, 12879–12885. (k) Williams,
V. C.; Piers, W. E.; Clegg, W.; Elsegood, M. R. J.; Collins, S.; Marder,
T. B. J. Am. Chem. Soc. 1999, 121, 3244–3245. (l) Agou, T.;
Kobayashi, J.; Kawashima, T. Org. Lett. 2005, 7, 4373–4376. (m)
Sakuda, E.; Funahashi, A.; Kitamura, N. Inorg. Chem. 2006, 45,
10670–10677. (n) Oehlke, A.; Auer, A. A.; Jahre, I.; Walfort, B.;
Ru¨ffer, T.; Zoufala´, P.; Lang, H.; Spange, S. J. Org. Chem. 2007, 72,
4328–4339.
(17) Sprouse, S.; King, K. A.; Spellane, P. J.; Watts, R. J. J. Am. Chem.
Soc. 1984, 106, 6647–6653.
(18) (a) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.;
Kwong, R.; Tsyba, I.; Bortz, M.; Mui, B.; Bau, R.; Thompson, M. E.
Inorg. Chem. 2001, 40, 1704–1711. (b) Tamayo, A. B.; Garon, S.;
Sajoto, T.; Djurovich, P. I.; Tsyba, I. M.; Bau, R.; Thompson, M. E.
Inorg. Chem. 2005, 44, 8723–8732. (c) Lo, S.-C.; Anthopoulos, T. D.;
Namdas, E. B.; Burn, P. L.; Samuel, I. D. W. AdV, Mater. 2005, 17,
1945–1948. (d) Lo, S.-C.; Richards, G. J.; Markham, P. J.; Namdas,
E. B.; Sharma, S.; Burn, P. L.; Samuel, I. D. W. AdV. Funct. Mater.
2005, 15, 1451–1458. (e) Bettington, S.; Tavasli, M.; Bryce, M. R.;
Beeby, A.; Al-Attar, H. A.; Monkman, A. P. Chem.sEur. J. 2007,
13, 1423–1431. (f) King, S. M.; Al-Attar, H. A.; Evans, R. J.;
Congreve, A.; Beeby, A.; Monkman, A. P. AdV. Funct. Mater. 2006,
16, 1043–1050. (g) Zhou, G.; Ho, C. L.; Wong, W. Y.; Wang, Q.;
Ma, D.; Wang, L.; Lin, Z.; Marder, T. B.; Beeby, A. AdV. Funct.
Mater. 2008, 18, 499–511.
(12) (a) Entwistle, C. D.; Marder, T. B. Chem. Mater. 2004, 16, 4574–
4585. (b) Entwistle, C. D.; Marder, T. B. Angew. Chem., Int. Ed. 2002,
41, 2927–2931. Angew. Chem. 2002, 114, 3051-3056. (c) Yuan, Z.;
Entwistle, C. D.; Collings, J. C.; Albesa-Jove´, D.; Batsanov, A. S.;
Howard, J. A. K.; Taylor, N. J.; Kaiser, H. M.; Kaufmann, D. E.;
Poon, S.-Y.; Wong, W.-Y.; Jardin, C.; Fathallah, S.; Boucekkine, A.;
Halet, J.-F.; Marder, T. B. Chem.sEur. J. 2006, 12, 2758–2771. (d)
Huh, J. O.; Do, Y.; Lee, M. H. Organometallics 2008, 27, 1022–
1025.
(19) (a) Slinker, J. D.; Gorodetsky, A. A.; Lowry, M. S.; Wang, J.; Parker,
S.; Rohl, R.; Bernhard, S.; Malliaras, G. G. J. Am. Chem. Soc. 2004,
126, 2763–2767. (b) Lowry, M. S.; Bernhard, S. Chem.sEur. J. 2006,
12, 7970–7977.
(20) (a) Yeh, S.-J.; Wu, M.-F.; Chen, C.-T.; Song, Y.-H.; Chi, Y.; Ho,
M.-H.; Hsu, S.-F.; Chen, C. H. AdV. Mater. 2005, 17, 285–289. (b)
Yang, C.-H.; Li, S.-W.; Chi, Y.; Cheng, Y.-M.; Yeh, Y.-S.; Chou,
P.-T.; Lee, G.-H.; Wang, C.-H.; Shu, C.-F. Inorg. Chem. 2005, 44,
7770–7780.
Inorganic Chemistry, Vol. 47, No. 20, 2008 9257

Zhao et al.
as biomolecules (such as estrogen receptor R)24 and chemical
compounds (such as anions, alkali cations, and amino acid).26
Up to now, however, only a few examples have been reported
of iridium(III) complexes used as chemosensors to selectively
sense heavy-metal ions such as Hg2+ or Pb2+ over other metal
cations with high sensitivity.27
Scheme 1. Chemical Structures of Ligand and Iridium(III) Complexes
It is well-known that the photophysical and electrochemi-
cal properties of iridium(III) complexes are dependent on
the chemical structures of cyclometalated ligands (C∧N
ligands). When a cyclometalated ligand of an iridium(III)
complex contains a specific coordinating element for an
analyte, the presence of this analyte can lead to dramatic
changes in the photophysical and electrochemical properties
of the iridium(III) complex. In the present study, on the basis
of the strong interaction between the three-coordination boron
atom and F-, a BMes2 group was introduced into a
cyclometalated ligand (Bpq), and a novel iridium(III) com-
plex salt [Ir(Bpq)2(bpy)]+PF6 based on Bpq (see Scheme
-
1) was synthesized. The photophysical and elelctrochemical
properties of Bpq and [Ir(Bpq)2(bpy)]+PF6 have been
-
investigated and compared with those of a model complex
(21) (a) Zhao, Q.; Jiang, C.-Y.; Shi, M.; Li, F.-Y.; Yi, T.; Cao, Y.; Huang,
C.-H. Organometallics 2006, 25, 3631–3638. (b) Zhao, Q.; Liu, S.;
Shi, M.; Wang, C.; Yu, M.; Li, L.; Li, F.; Yi, T.; Huang, C. Inorg.
Chem. 2006, 45, 6152–6160. (c) Liu, S. J.; Zhao, Q.; Chen, R. F.;
Deng, Y.; Fan, Q. L.; Li, F. Y.; Wang, L. H.; Huang, C. H.; Huang,
W. Chem.sEur. J. 2006, 12, 4351–4361. (d) Liu, Z. W.; Guan, M.;
Bian, Z. Q.; Nie, D. B.; Gong, Z. L.; Li, Z. B.; Huang, C. H. AdV.
Funct. Mater. 2006, 16, 1441–1448. (e) Li, X. H.; Chen, Z.; Shen,
L.; Li, F. Y.; Zhao, Q.; Yi, T.; Cao, Y.; Huang, C. H. Inorg. Chem.
2007, 46, 5518–5527. (f) Liu, S. J.; Zhao, Q.; Deng, Y.; Xia, Y. J.;
Lin, J.; Fan, Q. L.; Wang, L. H.; Huang, W. J. Phys. Chem. C 2007,
111, 1166–1175. (g) Zhao, Q.; Li, L.; Li, F. Y.; Yu, M. X.; Liu, Z. P.;
Yi, T.; Huang, C. H. Chem. Commun. 2008, 685–687. (h) Yu, M. X.;
Zhao, Q.; Shi, L. X.; Li, F. Y.; Zhou, Z. G.; Yang, H.; Yi, T.; Huang,
C. H. Chem. Commun. 2008, 2115–2117.
(22) (a) Jiang, J. X.; Xu, Y. H.; Yang, W.; Guan, R.; Liu, Z. Q.; Zhen,
H. Y.; Cao, Y. AdV. Mater. 2006, 18, 1769–1773. (c) Chen, L. Q.;
You, H.; Yang, C. L.; Ma, D. G.; Qin, J. Q. Chem. Commun. 2007,
1352–1354. (c) Rehmann, N.; Ulbricht, C.; Ko¨hnen, A.; Zacharias,
P.; Gather, M. C.; Hertel, D.; Holder, E.; Meerholz, K.; Schubert,
U. S. AdV. Mater. 2008, 20, 129–133. (d) Liu, S. J.; Zhao, Q.; Fan,
Q. L.; Huang, W. Eur. J. Inorg. Chem. 2008, 2177–2185. (e) You,
Y.; Seo, J.; Kim, S. H.; Kim, K. S.; Ahn, T. K.; Kim, D.; Park, S. Y.
Inorg. Chem. 2008, 47, 1476–1487.
salt [Ir(pq)2(bpy)]+PF6 without the BMes2 group. Impor-
-
tantly, further investigation shows that [Ir(Bpq)2(bpy)]+PF6
-
can act as a highly selective phosphorescent chemosensor
for F-.
Experimental Section
Materials. Acetylacetone, dimesitylboron fluoride, and 2-ethoxy-
ethanol were obtained from Acros. 2-Aminobenzaldehyde, ac-
etophenone, 2,2-bipyridine, 4-bromoacetophenone, IrCl3 ·3H2O, and
Pd(PPh3)4 were industrial products and were used without further
purification.
1
General Experiments. H NMR spectra were recorded with a
Varian spectrometer at 400 MHz. Mass spectra were obtained on
a Shimadzu matrix-assisted laser desorption/ionization time-of-flight
mass spectrometer (MALDI-TOF-MS). The UV-visible spectra
were recorded on a Shimadzu UV-2550 spectrometer. Steady-state
emission experiments at room temperature were measured on an
Edinburgh FLS 920 spectrometer. Lifetime studies were performed
with an Edinburgh FLS 920 photocounting system with a hydrogen-
filled pulse lamp as the excitation source. The data were analyzed
by iterative convolution of the luminescence decay profile with the
instrument response function using the software package provided
by Edinburgh Instruments. Quantum yields were determined with
a Hamamatsu C9920-01 calibrated integrating sphere system. The
solution was degassed by three freeze-pump-thaw cycles.
Calculation of Binding Constants. The binding constant (K)
of Bpq with F- was calculated from the flowing equation A0/(A -
A0) ) [a/(b - a)][(l/K[F-]) + 1],28a where A0 and A are the
absorbances at 333 nm in the absence and presence of F-,
(23) Coppo, P.; Duati, M.; Kozhevnikov, V. N.; Hofstraat, J. W.; De Cola,
L. Angew. Chem. 2005, 117, 1840–1844. Angew. Chem., Int. Ed. 2005,
44, 1806-1810.
(24) (a) Lo, K. K.-W.; Chung, C.-K.; Zhu, N. Chem.sEur. J. 2003, 9,
475–483. (b) Lo, K. K.-W.; Chan, J. S.-W.; Lui, L.-H.; Chung, C.-K.
Organometallics 2004, 23, 3108–3116. (c) Lo, K. K.-W.; Li, C.-K.;
Lau, J. S.-Y. Organometallics 2005, 24, 4594–4601. (d) Lo, K. K.-
W.; Chung, C.-K.; Zhu, N. Chem.sEur. J. 2006, 12, 1500–1512. (e)
Lo, K. K. W.; Lau, J. S. Y. Inorg. Chem. 2007, 46, 700–709. (f) Lo,
K. K.-W.; Zhang, K. Y.; Chung, C.-K.; Kwok, K. Y. Chem.sEur. J.
2007, 13, 7110–7120. (g) Lo, K. K. W; Hui, W. K.; Chung, C. K.;
Tsang, K. H. K.; Lee, T. K. M.; Li, C. K.; Lau, J. S. Y.; Ng, D. C. M.
Coord. Chem. ReV. 2006, 250, 1724–1736.
(25) (a) Yersin, H. Top. Curr. Chem. 2004, 241, 1–6. (b) You, Y. M.;
Park, S. Y. J. Am. Chem. Soc. 2005, 127, 12438–12439. (c)
Tsuboyama, A.; Iwawaki, H.; Furugori, M.; Mukaide, T.; Kamatani,
J.; Igawa, S.; Moriyama, T.; Miura, S.; Takiguchi, T.; Okada, S.;
Hoshino, M.; Ueno, K. J. Am. Chem. Soc. 2003, 125, 12971–12979.
(d) Polson, M.; Ravaglia, M.; Fracasso, S.; Garavelli, M.; Scandola,
F. Inorg. Chem. 2005, 44, 1282–1289. (e) Obara, S.; Itabashi, M.;
Okuda, F.; Tamaki, S.; Tanabe, Y.; Ishii, Y.; Nozaki, K.; Haga, M.
Inorg. Chem. 2006, 45, 8907–8921. (f) Wilkinson, A. J.; Puschmann,
H.; Howard, J. A. K.; Foster, C. E.; Williams, J. A. G. Inorg. Chem.
2006, 45, 8685–8699.
(26) (a) Ho, M. L.; Hwang, F. M.; Chen, P. N.; Hu, Y. H.; Cheng, Y. M.;
Chen, K. S.; Lee, G. H.; Chi, Y.; Chou, P. T. Org. Biomol. Chem.
2006, 4, 98–103. (b) Goodall, W.; Williams, J. A. G. J. Chem. Soc.,
Dalton Trans. 2000, 2893–2895. (c) Zhao, Q.; Liu, S. J.; Shi, M.; Li,
F. Y.; Jing, H.; Yi, T.; Huang, C. H. Organometallics 2007, 26, 5922–
5930. (d) Chen, H. L.; Zhao, Q.; Li, F. Y.; Wu, Y. B.; Yang, H.; Yi,
T.; Huang, C. H. Inorg. Chem. 2007, 46, 11075–11081.
respectively. In addition, a ) ꢀLλexc(dΦL/dλ)λ ∆λ and b )
em
ꢀMLλexc(dΦML/dλ)λ ∆λ, where ∆λ is the band pass of the emission
em
monochromator, L and ML refer to Bpq before and after binding
with F-, respectively, ΦL and ΦML are the respective quantum yields
of these two species, and ꢀL and ꢀML are the corresponding molar
absorption
coefficients.
The
binding
constant
of
-
[Ir(Bpq)2(bpy)]+PF6 with F- was calculated from the absorption
titration data by using the Hyperquad2006 program.28b
Electrochemical Measurements. Electrochemical measurements
were performed with an Eco Chemie’s Autolab. All measurements
were carried out in a one-compartment cell under nitrogen gas,
equipped with a glassy-carbon working electrode, a platinum wire
counter electrode, and an Ag/Ag+ reference electrode. The sup-
ported electrolyte was a 0.10 mol L-1 dichloromethane solution of
tetrabutylammonium hexafluorophosphate (Bu4NPF6). The scan rate
(27) (a) Zhao, Q.; Cao, T. Y.; Li, F. Y.; Li, X. H.; Jing, H.; Yi, T.; Huang,
C. H. Organometallics 2007, 26, 2077–2081. (b) Ho, M. L.; Cheng,
Y. M.; Wu, L. C.; Chou, P. T.; Lee, G. H.; Hsu, F. C.; Chi, Y.
Polyhedron 2007, 26, 4886–4892. (c) Zhao, Q.; Liu, S. J.; Li, F. Y.;
Yi, T.; Huang, C. H. Dalton Trans. 2008, 3836–3840.
was 50 mV s-1
.
9258 Inorganic Chemistry, Vol. 47, No. 20, 2008

Highly SelectiWe Phosphorescent Chemosensor for Fluoride
b
Scheme 2. Synthesis of Ligand Bpqa and of Complex [Ir(Bpq)2(bpy)]+PF6
-
a (i) NaOH, ethanol, reflux; (ii) nBuLi, THF, -78 °C; (iii) Mes2BF, THF, -78 °C. b (iv) 2-Ethoxyethanol:H2O ) 3:1, 110 °C, N2; (v) bipyridine, CH2Cl2,
reflux; (vi) KPF6, RT.
Theoretical Calculations. The calculation was performed using
the Gaussian 03 suite of programs.29 The optimizations of the ligand
and complex structures were performed using B3LYP density
functional theory (DFT). The LANL2DZ basis set was used to treat
the iridium atom, whereas the 6-31G* basis set was used to treat
all other atoms. The contours of the highest occupied molecular
and lowest unoccupied molecular orbitals (HOMOs and LUMOs)
were plotted.
Synthesis of pqBr. To a mixture of 4-bromoacetophenone (4.0
mmol) and 2-aminobenzadehyde (4 mmol) in ethanol was added
saturated ethanolic NaOH, and the mixture was refluxed overnight
(see Scheme S1 in the Supporting Information). After cooling, the
precipitate was collected by filtration and recrystallized from a
CH2Cl2 solution to obtain white crystals (2.8 mmol) in 70% yield.
1H NMR (400 MHz, CDCl3, 298 K): δ 8.23 (d, 1H), 9.13 (d, 1H),
8.04 (d, 2H), 7.82-7.85 (m, 2H), 7.73 (t, 1H), 7.64 (d, 2H), 7.54
(t, 1H). Anal. Calcd for C15H10NBr: C, 63.40; H, 3.55; N, 4.93.
Found: C, 63.72; H, 3.34; N, 4.55.
filtered to give crude cyclometalated iridium(III) chloro-bridged
dimer. The solution of cyclometalated iridium(III) chloro-bridged
dimer (0.079 mmol) and bpy (0.158 mmol) in CH2Cl2/MeOH [30
mL, 2:1 (v/v)] was heated to reflux. After 4 h, the red solution was
cooled to room temperature and then a 10-fold excess of potassium
hexafluorophosphate was added. The suspension was stirred for 2 h
and then was filtered to remove insoluble inorganic salts. The
solution was evaporated to dryness under reduced pressure. It was
chromatographed by using CH2Cl2/acetone (15:1) to afford an
1
orange-yellow solid in 70% yield. H NMR (400 MHz, CDCl3,
298 K): δ 8.36 (d, 2H), 8.11 (d, 2H), 8.00 (t, 2H), 7.90-7.96 (m,
4H), 7.82 (d, 2H), 7.58 (d, 2H), 7.32-7.39 (m, 4H), 7.24 (d, 2H),
7.10 (d, 2H), 6.95 (t, 2H), 6.48 (s, 8H), 6.24 (s, 2H), 2.24 (s, 12H),
1.62 (s, 24H). Anal. Calcd for IrC76H70B2F6N4P: C, 65.28; H, 5.05;
N, 4.01. Found: C, 65.68; H, 5.46; N, 4.45. MS (MALDI-TOF):
m/e 1254.0 (M - PF6).
Results and Discussion
Synthesis of Bpq. Under a nitrogen atmosphere, n-butyllithium
(1.32 mL, 2.11 mmol, 1.6 M in n-hexane) was added dropwise to
a stirred solution of Brpq (0.500 g, 1.76 mmol) in absolute
tetrahydrofuran (THF; 18 mL) at -78 °C. The reaction mixture
was stirred for 1 h at -78 °C, and then a solution of dimesitylboron
fluoride (0.566 g, 2.11 mmol) in THF was added (see Scheme 2).
After stirring for 18 h at room temperature, the reaction mixture
was quenched with H2O (50 mL) and extracted three times with
CH2Cl2 (50 mL). The combined organic extracts were dried over
Na2SO4, and the solvent was removed in vacuo. The crude product
was purified by chromatography with CH2Cl2/petroleum ether (1:
4) to obtain white needles in 60% yield. 1H NMR (400 MHz,
CDCl3, 298 K): δ 8.13-8.25 (m, 4H), 7.54-7.93 (m, 6H), 6.85 (s,
4H), 2.33 (s, 6H), 2.05 (s, 12H). Anal. Calcd for C33H32NB: C,
87.41; H, 7.11; N, 3.09. Found: C, 87.79; H, 7.55; N, 3.52.
Synthesis of [Ir(Bpq)2(bpy)]+PF6-. A mixture of 2-ethoxy-
ethanol and water (3:1, v/v) was added to a flask containing
IrCl3 ·3H2O (1 mmol) and Bpq (2.5 mmol). The mixture was
refluxed for 24 h. After cooling, the orange solid precipitate was
Synthesis. The synthesis procedure for Bpq and
[Ir(Bpq)2(bpy)]+PF6- is outlined in Scheme 2. The precursor
pqBr (Scheme 2) was easily obtained in good yield by a
Friedla¨nder condensation reaction of 2-aminobenzadehyde
with 4-bromoacetophenone according to a previously pub-
lished procedure.30 Ligand Bpq was synthesized in 60% yield
by lithiating pqBr, followed by the addition of Mes2BF at
-78 °C.31 The bis-cyclometalated iridium(III) complex salt
[Ir(Bpq)2(bpy)]+PF6 containing bipyridine (bpy) and the
-
Bpq ligand with the BMes2 group were prepared by a two-
step reaction via iridium(III) µ-chloro-bridged dimer com-
plexes according to a conventional procedure.32 The dinu-
clear cyclometalated iridium(III) chloro-bridged precursor
[Ir(Bpq)2Cl]2 was synthesized using the same methods as
those reported by Nonoyama.33 By bridge splitting reactions
of [Ir(Bpq)2Cl]2 and subsequent complexation with 2,2-
bipyridine, [Ir(Bpq)2(bpy)]+PF6- was synthesized in a yield
1
of 70% and characterized by elemental analysis, H NMR
(28) (a) Fery-Forgues, S.; Le Bris, M.; Guette´, J.; Valeur, B. J. Phys. Chem.
1988, 92, 6233–6237. (b) Gans, P.; Sabatini, A.; Vacca, A. Talanta
1996, 43, 1739–1753.
spectroscopy, and MALDI-TOF-MS spectroscopy.
Photophysical and Electrochemical Properties of
Bpq. Data on the UV-vis absorption and photoluminescent
(PL) properties of the ancillary ligand Bpq are shown in
Table 1. Ligand Bpq shows two absorption bands centered
at 277 and 333 nm with molar extinction coefficients (ꢀ) of
∼104 (Table 1). In addition, Bpq displays an intense blue
fluorescent emission, with a maximal emission wavelength
(29) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene,
M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev,
O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala,
P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;
Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas,
O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;
Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision
C.02; Gaussian, Inc.: Wallingford, CT, 2004.
(30) Hu, Y. Z.; Zhang, G.; Thummel, R. P. Org. Lett. 2003, 5, 2251–2253.
(31) Stahl, R.; Lambert, C.; Kaiser, C.; Wortmann, R.; Jakober, R.
Chem.sEur. J. 2006, 12, 2358–2370.
(32) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Lee,
H. E.; Adachi, C.; Burrows, P. E.; Forrest, S. R.; Thompson, M. E.
J. Am. Chem. Soc. 2001, 123, 4304–4312.
(33) Nonoyama, K. Bull. Chem. Soc. Jpn. 1974, 47, 467.
Inorganic Chemistry, Vol. 47, No. 20, 2008 9259

Zhao et al.
Table 1. Photophysical Properties of Bpq and [Ir(Bpq)2(bpy)]+PF6 Measured in a CH3CN Solutiona
-
compound
λabs (log ꢀ) (nm)
λem (nm)
Φem
τ (µs)
Bpq
[Ir(Bpq)2(bpy)]+PF6
277 (4.46), 333 (4.56)
289 (4.84), 351 (4.73), 368 (4.65), 465 (3.77)
439
592, 637 (sh)
0.51
0.49
-
2.67
a The solution was degassed by three freeze-pump-thaw cycles.
Figure 1. Cyclic voltammograms of Bpq and [Ir(Bpq)2(bpy)]+PF6- before
and after the addition of F-.
-
Figure 2. Absorption and PL spectra of [Ir(Bpq)2(bpy)]+PF6 (20 µM)
and [Ir(pq)2(bpy)]+PF6- in different conditions. The solution was degassed
by three freeze-pump-thaw cycles.
at 439 nm. As shown in Figure 1, Bpq shows two reduction
waves at -2.01 and -2.50 V.
Electrochemical Properties of [Ir(Bpq)2(bpy)]+PF6 .
-
The electrochemical properties of [Ir(Bpq)2(bpy)]+PF6- were
studied by cyclic voltammetry. As shown in Figure 1,
[Ir(Bpq)2(bpy)]+PF6- exhibits an irreversible oxidation wave.
be assigned to a mixture of ligand-to-ligand charge transfer
(LLCT), metal-to-ligand charge transfer (MLCT), and spin-
forbiden ligand-centered (3LC) transitions. The BMes2 groups
playasignificantroleintheexcitedstateof[Ir(Bpq)2(bpy)]+PF6-.
In addition, [Ir(Bpq)2(bpy)]+PF6 exhibits three reduction
-
waves at -1.90, -2.46, and -2.72 V. According to a
previous report,34 the waves at -2.46 and -2.72 V are
assigned to the reduction of pq fragments. Also, according
to previous reports on cationic iridium(III) complexes
containing bpy,35 we tentatively assign the wave at -1.90
V to reduction of the bpy ligand.
[Ir(Bpq)2(bpy)]+PF6 shows an intense emission band at
-
592 nm with a shoulder at 637 nm, and the PL color is
orange-red at room temperature in a CH3CN solution. The
phosphorescence quantum yield in degassed solution is 0.49
and that in an aerated solution is 0.17 measured by a
calibrated integrating sphere system, indicating that the
emission is sensitive to oxygen. The moderately high
phosphorescence quantum yield (0.17) in an aerated solution
ensures that this complex can function as a phosphorescence
fluoride ion sensor. The emission lifetimes and excitation
spectra monitored at 592 and 637 nm are almost the same
(see Table 1), indicating that the two bands relate to the same
Photophysical Properties of [Ir(Bpq)2(bpy)]+PF6 . To
-
better understand the photophysical properties of
[Ir(Bpq)2(bpy)]+PF6 , DFT calculations for this complex
-
were performed and molecular orbitals were studied (see
Figure 3). The HOMO distribution primarily resides on the
BMes2 fragments of ligand Bpq. Also, the HOMO-2
distribution resides on the iridium center and phenyl groups
of ligand Bpq. HOMO-6 and HOMO-3 distributions are
located at the iridium center and BMes2 fragments of Bpq.
In addition, the LUMO distribution is dominated by the
ligand bpy, and LUMO+2 distribution is located at the Bpq
ligand.
excited state. The PL spectrum of [Ir(Bpq)2(bpy)]+PF6 is
-
broad and featureless, and thus it can be concluded that the
emission of [Ir(Bpq)2(bpy)]+PF6- primarily originates from
CT states. Upon comparison with the model complex
[Ir(pq)2(bpy)]+PF6 , it is seen that the emission wavelength
-
of [Ir(Bpq)2(bpy)]+PF6- is significantly red-shifted, suggest-
ing that the BMes2 group makes a significant contribution
Time-dependent DFT (TDDFT) calculations were then
performed to estimate the corresponding transition energy
-
to the excited state of [Ir(Bpq)2(bpy)]+PF6 .
of complex [Ir(Bpq)2(bpy)]+PF6 . The calculated parameters
-
PL spectra of [Ir(Bpq)2(bpy)]+PF6 in different solvents
-
for the lowest-lying transitions are listed in Table 2. The
lowest triplet state originates from HOMO-6 f LUMO,
HOMO-3 f LUMO, HOMO-2 f LUMO, and HOMO-2
f LUMO+2, which apparently possess a mixture of [πC∧N
f π*N∧N] 3LLCT and [dπ(Ir) f π*N∧N] 3MLCT transitions
and at different temperatures were also measured (Figure
2). A blue shift of the emission maximum was observed from
a fluid solution at room temperature to a rigid matrix at 77
K, which is often observed for cyclometalated iridium(III)
diimine MLCT and LLCT emitters.36 The blue shift is caused
by fast solvent reorganization in a fluid solution at room
temperature, which can stabilize the CT states before
emission takes place. At 77 K, however, this stabilization
3
with some contribution from LC (πC∧N f π*C∧N). Hence,
the weak absorption bands in the range of 400-600 nm can
(34) Wu, F. I.; Su, H. J.; Shu, C. F.; Luo, L.; Diau, W. G.; Cheng, C. H.;
Duan, J. P.; Lee, G. H. J. Mater. Chem. 2005, 15, 1035–1042.
(35) Neve, F.; La Deda, M.; Crispini, A.; Bellusci, A.; Puntoriero, F.;
Campagna, S. Organometallics 2004, 23, 5856–5863.
(36) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; von
Zelewsky, A. Coord. Chem. ReV. 1988, 84, 85–277.
9260 Inorganic Chemistry, Vol. 47, No. 20, 2008

Highly SelectiWe Phosphorescent Chemosensor for Fluoride
Figure 4. Change in the UV-vis absorption spectra of Bpq (20 µM) in a
CH3CN solution with various amounts of F-. Inset: absorption titration
profile at 333 nm versus 1 equiv of F- in solution for Bpq.
Figure 5. Change in the emission spectra of Bpq (20 µM) in a CH3CN
solution with various amounts of F- (λex ) 358 nm). Inset: emission color
observed in a CH3CN solution of Bpq (500 µM) in the absence (left) and
presence (after) of 1 equiv of F-.
Figure 3. HOMO-6, HOMO-3, HOMO-2, HOMO, LUMO, and
-
LUMO+2 distributions of [Ir(Bpq)2(bpy)]+PF6
.
with a B(Mes)2 group in the ligand. Both complexes have
longer emission lifetimes and lower quantum yields in
Table 2. Calculated Energy Levels of the Lowest Singlet (S1) and
8d
-
Triplet States (T1) for [Ir(Bpq)2(bpy)]+PF6
solution compared with [Ir(Bpq)2(bpy)]+PF6 .
-
a
state
S5
excitation
Ecal (eV) λcal (nm)
fb
Optical and Electrochemical Responses of Bpq to
F-. It has been demonstrated that the vacant p orbital on the
boron atom in the triarylborane derivative coordinates
selectively with fluoride anions, giving rise to changes in
both the absorption and emission spectra of the triarylborane
derivative. In the present study, the ability of Bpq to complex
with F- was investigated using UV-vis absorption and
emission titration experiments. After the addition of F-, the
absorption band at 333 nm was red shifted (see Figure 4).
As shown by absorption and emission titration curves, Bpq
needs approximately 1 equiv of F- to reach saturation. The
binding constant (K) was calculated to be 4.70 × 104 M-1
from the absorption titration curve. Furthermore, the addition
of F- induces an obvious variation in the electrochemical
properties of Bpq (Figure 1). Taken together, these experi-
mental results indicate an intense interaction between the
boron center and the fluoride anion.
HOMO-6 f LUMO+2 (11%)
HOMO-2 f LUMO+2 (67%)
HOMO-6 f LUMO (11%)
HOMO-3 f LUMO (10%)
HOMO-2 f LUMO (67%)
HOMO-2 f LUMO+2 (10%)
2.53
490
0.0456
T1
2.08
596
0
a Ecal: calculated energy. b f: calculated oscillator strength.
of CT states is hampered. PL measurement in different
solvents revealed that the emission maximum of
[Ir(Bpq)2(bpy)]+PF6- occurs at higher energy in the less polar
CH2Cl2 than in the more polar CH3CN (Figure 2), which is
also very common for cyclometalated iridium(III) diimine
MLCT and LLCT emitters. Hence, the excited state of
[Ir(Bpq)2(bpy)]+PF6 can be assigned to a CT state.
-
Recently, Marder et al. have reported a neutral iridium(III)
complex Ir-B with a B(Mes)2 group on the C∧N ligand
(2-phenylpyridine).18g Compared with the complex without
a B(Mes)2 group, an evident red shift in the emission
wavelength can be observed, which is similar to our complex
Importantly, the addition of F- caused a decrease in the
emission intensity and a red shift of the emission band from
439 to 485 nm (Figure 5), corresponding to a visual change
in the emission color from bright blue to weak blue-green
(Figure 5, inset). In many previous reports,6-11 coordination
of fluoride to the empty p orbital of the boron atom would
[Ir(Bpq)2(bpy)]+PF6 . In addition, both Ir-B and
-
[Ir(Bpq)2(bpy)]+PF6- exhibit red emission and the emission
originates from the CT state. In addition, Wang et al. also
reported a copper(I) complex and a platinum(II) complex
Inorganic Chemistry, Vol. 47, No. 20, 2008 9261

Zhao et al.
Table 3. Calculated Energy Levels of the Lowest Singlet (S1) for Bpq
and Bpq-F-
a
state
Bpq
excitation
Ecal (eV) λcal (nm)
fb
HOMO-1 f LUMO (42%)
HOMO f LUMO (53%)
HOMO f LUMO+1 (11%)
3.39
366
0.0627
Bpq-F- HOMO f LUMO (70%)
2.81
441
0.0419
a Ecal: calculated energy. b f: calculated oscillator strength.
center and the pq unit, so the first reduction wave of Bpq is
tentatively assigned to the reduction of the boron center and
the pq unit. The LUMO+1 orbital is mainly localized on
the pq fragment with some distribution on the boron center.
So, the second reduction wave of Bpq was tentatively
assigned to the reduction of the pq fragment with some
contribution from the boron center. The lowest singlet state
originates from HOMO-1 f LUMO, HOMO f LUMO,
and HOMO f LUMO+1 (see Table 3). So, the lowest
electronic transition in Bpq can mainly be attributed to the
π-π* transition of Bpq with CT from the mesityl group to
the pq unit. Compared to Bpq, evident variation in the
molecular orbital distributions is observed for the adduct
Bpq-F-. The HOMO distribution is located on the BMes2
unit, and the LUMO distribution resides on the pq unit. The
lowest electronic transition originates from HOMO f
LUMO. These facts indicate that the lowest electronic
transition of Bpq-F- can be attributed to CT from BMes2 to
the pq unit. Hence, after complexation with F-, the excited
state is switched from the π-π* transition of Bpq to the
CT transition, resulting in variation in the photophysical and
electrochemical properties of Bpq. In addition, surface charge
analysis based on DFT indicates that the charge on the boron
atom after binding of F- increased significantly compared
with that before binding of F- (Supporting Information),
which also shows a reversal of the roles of the acceptor and
donor for the boron atom upon F- binding.
Figure 6. Orbital distributions of Bpq and Bpq-F-.
block intramolecular CT or interrupt the π conjugation
extended through the boron atom, which led to fluorescence
quenching or a dramatic blue shift of the emission bands. In
this case, however, we observed a red shift of emission
spectra induced by complexation with F-. According to the
calculation results (see below), the complexation with F-
switches the excited state from the π-π* transition of Bpq
to the CT transition of the adduct Bpq-F-, which is different
from many other reports. Also, it is this switch of the excited
state that leads to the red shift of the emission spectra. A
similar switch of the excited state has also been reported by
DiCesare and Lakowicz11i and Spange et al.11n In these
systems, boronic acid (or boranate ester) groups, which act
as electron-withdrawing groups, were used as binding sites
for F-. Upon complexation with F-, they can be changed
into electron-donating groups. When these groups and other
electron-withdrawing groups were introduced into the same
π-electron system, the complexation with F- could induce
CT interaction between the anionic boron acid (or boronate)
ester and the electron-withdrawing groups.
Optical Responses of [Ir(Bpq)2(bpy)]+PF6- to F-. Con-
sidering the significant response of ligand Bpq to F-, it is
reasoned that the photophysical and electrochemical proper-
ties of [Ir(Bpq)2(bpy)]+PF6 could be affected upon the
-
addition of F-. Figure 7 shows the variation in the absorption
-
spectra of [Ir(Bpq)2(bpy)]+PF6 upon the addition of F-.
When F- was added to a solution of [Ir(Bpq)2(bpy)]+PF6 ,
-
the absorbance at 290 and 350 nm decreased gradually
whereas the absorbance at 400 nm increased, corresponding
to an isobestic point at 379 nm. Importantly, the absorption
band in the range of 420-600 nm was red-shifted with an
increase in the absorbance, corresponding to a change in the
solution color from yellow to orange-red (Figure 7, inset),
indicating that [Ir(Bpq)2(bpy)]+PF6- can serve as a “naked-
eye” indicator of F-.
Moreover, a very weak variation in the absorption and
emission spectra of Bpq was observed upon the addition of
an excess of other anions, such as Br-, Cl-, I-, ClO4 , NO3 ,
-
-
CH3COO-, and H2PO4 (see the Supporting Information),
-
indicating that Bpq displays high selectivity in sensing F-.
Mechanism of the Bpq Response to F-. To understand
the mechanism by which Bpq responds to F-, molecular
orbital calculations for Bpq and the adduct Bpq-F- were
performed using TDDFT calculations (see Figure 6 and Table
3). For Bpq, the BMes2 group and the quinoline (pq) segment
have a planar arrangement. The HOMO-1 and HOMO
distributions of Bpq are predominantly located on the mesityl
group and the pq unit. The LUMO is localized on the boron
As shown in Figure 8, emission spectra of [Ir(Bpq)2-
(bpy)]+PF6 also displayed obvious changes when F- was
-
added. A decrease in the emission intensity of [Ir(Bpq)2-
(bpy)]+PF6 was observed upon the addition of F-. After
-
the addition of approximately 2 equiv of F-, the emission
of [Ir(Bpq)2(bpy)]+PF6 was almost completely quenched,
-
which could be observed by the naked eye (Figure 9). Hence,
9262 Inorganic Chemistry, Vol. 47, No. 20, 2008

Highly SelectiWe Phosphorescent Chemosensor for Fluoride
Figure 10. HOMO and LUMO distributions of [Ir(Bpq)2(bpy)]+PF6--2F-.
Table 4. Calculated Energy Levels of the Lowest Singlet (S1) and
Triplet States (T1) for [Ir(Bpq)2(bpy)]+PF6--2F-
state
excitation
fa
S1
T1
HOMO f LUMO
HOMO f LUMO
0.0045
0
Figure 7. Change in the UV-vis absorption spectra of [Ir(Bpq)2-
-
(bpy)]+PF6 (20 µM) in a CH3CN solution with various amounts of F-.
-
Inset: solution color observed in a CH3CN solution of [Ir(Bpq)2(bpy)]+PF6
a f: calculated oscillator strength.
(500 µM) in the absence (left) and presence (after) of 2 equiv of F-.
Molecularorbitalcalculationsfortheadduct[Ir(Bpq)2(bpy)]+-
PF6 -2F- were performed using TDDFT calculations (see
-
Figure 10 and Table 4). HOMO distribution resides on the
BMes2-F- fragments. Also, LUMO distribution resides on
the bpy fragment. The lowest triplet state originates from
HOMO f LUMO, which shows that the complexation of
BMes2 with F- changes the exicited-state property of the
complex.
The influence of F- on the electrochemical properties of
[Ir(Bpq)2(bpy)]+PF6- was also investigated. Upon the addi-
tion of F-, the oxidation wave disappeared. The reason for
the disappearance of the oxidation wave is not very clear.
For [Ir(Bpq)2(bpy)]+PF6 , the HOMO distribution resides on
-
the BMes2 fragment, so the irreversible oxidation process
of [Ir(Bpq)2(bpy)]+PF6 can be tentatively assigned to the
-
oxidation of BMes2 groups. After complexation with F-, the
HOMO distribution resides on the BMes2-F- fragment. So,
the disappearance of the oxidation wave of BMes2 groups
can be tentatively attributed to the formation of a BMes2-F
adduct, which prevents the oxidation of BMes2 groups.
High selectivity is necessary for an excellent chemosensor.
Herein, the selective coordination studies of [Ir(Bpq)2(bpy)]+-
PF6- by absorption and emission spectra were then extended
to other anions in a CH3CN solution. As shown in Figure
11, only the addition of F- results in prominent changes in
the ratio of absorbance at 350 and 400 nm, whereas the
addition of a large excess of other anions (such as Cl-, Br-,
Figure 8. Change in the emission spectra of [Ir(Bpq)2(bpy)]+PF6- (20 µM)
in an air-equilibrated CH3CN solution with various amounts of F- (λex
)
379 nm). Inset: fluorescent titration profile at 592 nm versus 1 equiv of F-
-
in solution for [Ir(Bpq)2(bpy)]+PF6
.
Figure 9. Emission color observed in a CH3CN solution of [Ir(Bpq)2-
(bpy)]+PF6- (500 µM) in the absence (left) and presence (right) of 2 equiv
of F-.
[Ir(Bpq)2(bpy)]+PF6 could be used as an ON-OFF-type
I-, ClO4 , NO3 , H2PO4 , and CH3COO-) causes slight
-
-
-
-
phosphorescent probe for F-.
changes. Therefore, [Ir(Bpq)2(bpy)]+PF6 displayed a high
-
Considering that there are two boron centers in one
complex molecule, it is possible for [Ir(Bpq)2(bpy)]+PF6 to
form a 1:2 complex with 2 equiv of F-. This has been
verified by variation in the 11B NMR spectra of
[Ir(Bpq)2(bpy)]+PF6 before and after the addition of F- (see
Figure S7 in the Supporting Information). After complexation
with F-, the signal was changed completely, suggesting that
both of the two boron centers in the complex bind with F-.
Using the UV-vis titration data, the binding constants K1
and K2 of [Ir(Bpq)2(bpy)]+PF6 with F- were determined to
be 1.29 × 106 and 4.27 × 105 M-1, respectively.
selectivity in sensing F-. Achieving high selectivity for the
analyte of interest over a complex background of potentially
competing species is a challenge in sensor development.
Thus, the competition experiment was also carried out by
the addition of F- to solutions of [Ir(Bpq)2(bpy)]+PF6 in
-
the presence of other anions. As shown in Figure 11, whether
in the absence or presence of other anions, obvious spectral
changes were observed for [Ir(Bpq)2(bpy)]+PF6 upon the
-
addition of F-. The results indicate that the sensing of F-
by [Ir(Bpq)2(bpy)]+PF6 is hardly affected by these com-
-
monly coexistent ions. Therefore, [Ir(Bpq)2(bpy)]+PF6- can
Inorganic Chemistry, Vol. 47, No. 20, 2008 9263

Zhao et al.
chemical properties of ligand Bpq and complex [Ir(Bpq)2-
(bpy)]+PF6- shows that they can be used as highly selective
chemosensors for F- detectable by the naked eye. The
dimesitylboryl group is believed to play very important roles
in these unique responses. For the cyclometalated ligand Bpq,
an evident red shift of the emission spectra was observed
after the addition of F-. This variation can be explained by
the excited-state switch from π-π* to CT transition. For
[Ir(Bpq)2(bpy)]+PF6 , the addition of F- induces an evident
-
change in the solution color from yellow to orange-red and
a pronounced ON-OFF-type phosphorescent signaling be-
havior. Hence, [Ir(Bpq)2(bpy)]+PF6- can act as an excellent
phosphorescent chemosensor for F- with naked-eye detec-
tion. This work provides a useful design strategy for the
synthesis of new phosphorescent chemosensors by simple
modification of the chemical structure of ligands to contain
a specific recognition site.
Figure 11. Absorption (a) and fluorimetric (b) responses of
[Ir(Bpq)2(bpy)]+PF6- (20 µM) in the presence of 2 equiv of F- or 5 equiv
of other anions in a MeCN solution. Bars represent the ratio A350 nm/A400 nm
of the absorbance intensity at 350 and 400 nm (a) and the emission intensity
(I592 nm) at 592 nm (b). Gray and white represent A350 nm/A400 nm and I592 nm
before and after the addition of anions, respectively. Bars: 1, F-; 2, Cl-; 3,
Br-; 4, I-; 5, ClO4-; 6, NO3-; 7, H2PO4-; 8, CH3COO-.
Acknowledgment. The authors are thankful for financial
support from National Natural Science Foundation of China
(Grants 20490210, 20501006, and 20775017), NHTPC
(Grant 2006AA03Z318), NCET-06-0353 and Shanghai Sci-
ence and Technology Community (Grant 06QH14002), Huo
Yingdong Education Foundation (Grant 104012), and Shang-
hai Leading Academic Discipline Project (Project B108).
act as a highly selective phosphorescent chemosensor for
F-. It should be pointed out that all systems containing
BMes2 groups that have been examined as fluoride ion
sensors are ion specific and that some workers have also
commented on their CN- binding affinities.12d
Supporting Information Available: Excitation spectra of
[Ir(Bpq)2(bpy)]+PF6-, UV-vis and fluorescence spectra of Bpq and
-
[Ir(Bpq)2(bpy)]+PF6 upon titration with other anions, selective
Conclusion
absorption and fluorimetric response of Bpq to various anions, and
surface charge distributions of Bpq and Bpq-F-. This material is
available free of charge via the Internet at http://pubs.acs.org.
In summary, we have synthesized a new iridium(III)
complex [Ir(Bpq)2(bpy)]+PF6 containing bismesitylboryl
-
groups. The investigation of the photophysical and electro-
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9264 Inorganic Chemistry, Vol. 47, No. 20, 2008