Synthesis of homoleptic ruthenium "star" complexes by click reaction for TiO2 sensitization

Article abstract of DOI:10.1002/ejic.201200896

Homoleptic Ru^II complexes carrying rigid linkers for attachment to semiconductor surfaces were synthesized by using the click reaction. Star-shaped complexes KStar-TzC-pBn (1a) and KStar-TzC-Ipa (1b), composed of 2,2'-bipyridine ligands 4,4'-di

Full text of DOI:10.1002/ejic.201200896

SHORT COMMUNICATION
DOI: 10.1002/ejic.201200896
Synthesis of Homoleptic Ruthenium “Star” Complexes by Click Reaction for
TiO2 Sensitization
Keyur P. Chitre,[a] Elena Guillén,[b] Audri Soojin Yoon,[a] and Elena Galoppini*[a]
Keywords: Click chemistry / Ruthenium / Sensitizers / TiO2 nanoparticles / N ligands
Homoleptic RuII complexes carrying rigid linkers for attach-
ment to semiconductor surfaces were synthesized by using
the click reaction. Star-shaped complexes KStar-TzC-pBn
(1a) and KStar-TzC-Ipa (1b), composed of 2,2Ј-bipyridine li-
gands 4,4Ј-disubstituted with 1,2,3-triazole rings connected
to the Ru complex through the C4 of the 1,2,3-triazole (Tz)
ring and capped with p-benzoic (pBn) and isophthalic (Ipa)
methyl esters, respectively, as well as KStar-TzN-pBn (1c),
connected to the Ru complex through the N1 of the Tz ring,
were studied in solution and bound to semiconducting TiO2
and insulating ZrO2 nanoparticle films.
alkyne cycloaddition to form a 1,2,3-triazole ring.[11] The
click approach, which is only very recently being applied to
the synthesis of inorganic complexes,[12] has two advan-
tages. First, the linker can be “clicked” to different anchor
units, and, second, it will be possible to probe the electron-
mediating function of triazole rings, changes in the dipole
moment, and the structural conformation effects of the
linker in relation to the ring connectivity with the Ru com-
plex moiety.[13] Here we describe the synthesis of com-
pounds 1a, 1b, and 1c shown in Figure 1. Compounds 1a
and 1b have C-connectivity to the Ru(bpy)3 core but differ
in the anchor group (R = pBn and Ipa, respectively),
whereas 1c is attached through N-connectivity to the
Ru(bpy)3 core and can be compared with 1a, which has the
same pBn anchor group.
Introduction
Chromophores attached through rigid linker-anchor
units to nanostructured TiO2 or other semiconductor sur-
faces are of interest for the development of dye-sensitized
solar cells,[1] photocatalysts,[2] and other applications. Syn-
thetic modifications of the linkers can be used to tune the
properties of the chromophore unit,[3] to modify surface
binding, and to control charge-transfer processes.[4] Over
the years, the linker strategy has led to a better understand-
ing of interfacial charge transfer[5] and to improved de-
vices.[6] However, the surface heterogeneity of nanostruc-
tured semiconductors can prevail over molecular design,
and preventing binding disorder remains a significant chal-
lenge.[7]
Recently, we reported that “shielding” strategies for the
adsorption of photo- or redox-active molecules on semi-
conductors result in improved binding control.[8] One ap-
proach involved the development of homoleptic RuII com-
plexes (Star complexes) with octahedral symmetry,[9] in
which the chromophore unit is shielded from direct con-
tacts with the surface through the symmetrical arrangement
of oligophenylenethynylene linkers. The initial Star com-
plexes exhibited very low injection yields;[9b] however, recent
computational studies suggest that judicious variations of
the linker and anchor could improve the electronic coupling
with the surface.[10]
In this paper we report the first Star complexes prepared
by using the click reaction, that is, Cu-catalyzed azide–
Figure 1. Molecular structure of 1a, 1b, and 1c.
[a] Department of Chemistry, Rutgers University
73 Warren Street, Newark, NJ 07032, USA
E-mail: galoppin@rutgers.edu
Homepage: http://galoppinigroup.com
[b] Department of Organic Chemistry
Results and Discussion
University of Málaga,
Campus de Teatinos, 29071 Málaga, Spain
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201200896.
The synthesis of KStar-TzC-pBn (1a) and KStar-TzC-
Ipa (1b) was carried out as shown in Scheme 1. Azides 3a
Eur. J. Inorg. Chem. 2012, 5461–5464
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5461

K. P. Chitre, E. Guillén, A. S. Yoon, E. Galoppini
SHORT COMMUNICATION
and 3b were prepared in about 80% yield by conversion of
the corresponding amines into diazonium salts and reaction
with sodium azide.[14] Ligands 4a and 4b were synthesized
by the click reaction[12a,15] between azides 3a or 3b and 4,4Ј-
diethynyl-2,2Ј-bipyridine (2) in 43 and 39% yields, respec-
tively, with use of catalytic CuII in the presence of sodium
ascorbate to generate CuI in situ. The modest yields may
be due to complexation of CuI with bipyridine. The reaction
was carried out in the presence of small amounts of
(iPr)2EtN to promote the formation of the copper acetylide
intermediate.[16] In the absence of base, the reaction yielded
mainly the monosubstituted product. A more attractive ap-
proach, involving the click reaction of the hexaethynyl-sub-
stituted Ru complex 5a with the azide-substituted anchors
(Scheme 1), was unsuccessful.[17] Possible explanations in-
clude the occurrence of alkyne homocoupling in the pres-
ence of copper salts or the formation of copper acetylide
complexes on one or more of the alkynes (CuI satura-
Scheme 2. Click synthesis of N-connectivity Star complexes. Rea-
gents and conditions: (i) CuSO4·5H2O, sodium ascorbate,
(iPr)2EtN, H2O, tBuOH, MeOH, room temp., 18 h; (ii) PCl3, aceto-
tion),[18] a process that would prevent cyclization with the nitrile, reflux, 4 h; (iii) Ru(DMSO)4Cl2, ethylene glycol, MeOH,
H2O, 120 °C, 3 d.
azide.
and 1c show the νas(C=O) stretch characteristic of the ester
group at 1728, 1733, and 1710 cm–1, respectively. The spec-
tra of 1a and 1b bound to TiO2 films (Figures 2 and S9)
show broad bands characteristic of the carboxylate group
in the 1500–1650 cm–1 region. The observed spectral
changes are consistent with chelating and/or bidentate
binding modes of the carboxyl group to TiO2, presumably
as a result of surface hydrolysis or other interactions be-
tween the ester groups and the TiO2 surface.[9] The presence
of the carbonyl band assigned to free COOMe indicates
that, as expected, not all the anchor groups bind to TiO2.
Scheme 1. Click synthesis of C-connectivity Star complexes. Rea-
gents and conditions: (i) CuSO4·5H2O, sodium ascorbate,
(iPr)2EtN, H2O, tBuOH, MeOH, room temp., 18 h; (ii) Ru-
(DMSO)4Cl2, ethylene glycol, MeOH, H2O, 120 °C, 3 d.
Similarly, the synthesis of KStar-TzN-pBn (1c)[19] was
carried out as shown in Scheme 2. 4,4Ј-Diazido-2,2Ј-bipyr-
idine 1,1Ј-dioxide (6) was prepared from the nitro precursor
in 97% yield and was then reacted with the corresponding
alkyne 8 for the click reaction to obtain the N,NЈ-dioxide
form of ligand 9, which was reduced with phosphorus tri-
chloride to obtain ligand 9 in 65% overall yield. However,
Figure 2. FTIR-ATR spectra of KStar-TzC-pBn (1a) neat (bottom)
the approach involving click reaction on diazidobipyridyl-
and bound to nanoparticle TiO2 films (top). The broad band above
substituted Ru complex 11 was unsuccessful because of the
instability of 11 in a variety of reaction conditions.[17]
FTIR-ATR spectra of azides 3a, 3b, and 6 showed the
3100 cm–1 is assigned to water adsorbed on the film.
The solution (THF, 0.9 mm) UV/Vis absorption and
characteristic N3 stretching bands at about 2100 (νas) and steady-state photoluminescence (PL) spectra of 1a, 1b, and
1250 (νsym) cm–1 (Figures S2, S3, and S6). Bipyridine 2 and 1c are shown in Figures S24 and S26, and selected photo-
alkyne 7 showed a strong νas(ϵC–H) band at 3100 cm–1 and physical properties are listed in Table 1. The UV/Vis ab-
a weak νsym(CϵC) band at about 2100 cm–1 (see Support- sorption spectra for all complexes exhibit bands in the 280–
ing Information). The FTIR-ATR spectra of ligands 4a, 4b, 310 nm region assigned to the πǞπ* and nǞπ* transitions
and 8, as well as the corresponding Ru complexes 1a, 1b, of the bpy and triazole moieties. The broad, structureless
5462
www.eurjic.org
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2012, 5461–5464

Homoleptic Ruthenium “Star” Complexes by Click Reaction
MLCT band typical of Ru(bpy)32+ complexes is centered at
These interesting observations suggest that the photolu-
about 450 nm. The values of the molar extinction coeffi- minescence properties could be the result of multiple factors
cients (ε1a = 2.3ϫ104 m–1 cm–1, ε1b = 2.1ϫ104 m–1 cm–1, and (different linker conformations or electronic delocalization
ε1c = 0.2ϫ104 m–1 cm–1) are close to those observed for typ- influencing the excited state electronic structure, for in-
ical bpy complexes of RuII.[9] The steady state PL spectra stance). While this is beyond the scope of this communica-
for 1a and 1b show a broad emission centered at 637 nm, tion, we will probe the origin of this behavior in our future
but the emission quantum yields (Φ1a = 0.7% and Φ1b
=
studies.
Star complexes 1a and 1b were bound onto TiO2 and
Compound 1c is not photolu- ZrO2 nanoparticle mesoporous films. The 1a/TiO2 and 1b/
0.4%) are about one order of magnitude lower than that
2+ [20]
observed for Ru(bpy)3
.
minescent. To probe the possibility that electron donation TiO2 films showed the characteristic MLCT band in the
from the triazole ring may quench the emission, particularly UV/Vis spectra (Figure S25) and full quenching of the PL
in the N-connectivity Star compound 1c, acid was added to emission (Figure 4). Since ZrO2 has a wider band gap (Ebg
the solution. Protonation of the triazole ring resulted in a ≈ 5 eV) compared to TiO2 (Ebg ≈ 3 eV), it behaves as an
twofold increase in PL, and titration of the acidic solution insulator, and the PL emission was observed on films pre-
with base led to reversible behavior, with an approximately pared from this material (Figure 4).
twofold decrease in PL for complexes 1a and 1b,[21] as
shown in Figures 3 and S23. Surprisingly, no photolumines-
cence was observed for 1c under acidic conditions, although
the UV/Vis spectra of acidic solutions of 1c showed in-
creased intensity (Figure S26). Additionally, titration of
Ru(bpy)32+ 2Cl– with 4-phenyl-1H-1,2,3-triazole did not re-
sult in intermolecular photoluminescence quenching, sug-
gesting that the role of the triazole ring on the emissive
properties of 1a–c is complex and needs to be further inves-
tigated.
Table 1. Selected photophysical properties in THF at room temp.
[a]
[a]
Complex
λabs (ε [m–1 cm–1]) λPL
Φem (ϫ10–2)
KStar-TzC-pBn (1a) 485 (2.3ϫ104)
637
638
–
0.7
0.4
–
KStar-TzC-Ipa (1b) 485 (2.1ϫ104)
KStar-TzN-pBn (1c) ca. 500 (0.2ϫ104)
Figure 4. PL spectra of 1a and 1b in solution and bound on TiO2
and ZrO2 nanoparticle thin films.
2+[b]
Ru(bpy)3
450 (1.2ϫ104)
610
6.2
[a] Wavelengths in nm. [b] Data from ref.[9]
Conclusions
Three star-shaped ruthenium complexes, KStar-TzC-pBn
(1a), KStar-TzC-Ipa (1b), and KStar-TzN-pBn (1c), con-
taining the triazole unit were synthesized by the click reac-
tion and characterized. Titration with acid and base showed
reversible behavior with twofold increase in the PL emission
and vice-versa for C-connectivity complexes 1a and 1b.
FTIR, UV/Vis, and photoluminescence studies of the com-
pounds in solution and bound on metal oxide semiconduc-
tor nanoparticle thin films (TiO2 and ZrO2) showed that
the complexes bind to the semiconductors. On TiO2 the
photoluminescence was completely quenched, whereas the
functionalized ZrO2 films showed a broad emission band.
Ongoing studies include studying interfacial charge transfer
and theoretical calculations for 1a–c to assess the structural
and electron-mediating function of 1,2,3-triazole rings and
the role of the anchor group.
Supporting Information (see footnote on the first page of this arti-
Figure 3. Photoluminescence spectrum of a 9ϫ10–4 m THF solu-
tion of 1a upon titration with aliquots of 6 n HCl. Inset: PL spec-
trum of a 9ϫ10–4 m THF solution of 1b upon titration with ali-
quots of 6 n NaOH. λexc = 485 nm.
1
cle): Synthesis and characterization of 1a, 1b, and 1c. H and 13C-
NMR spectra for 4a, 4b, 9, 1a, 1b, and 1c. FTIR-ATR spectra for
2, 6, 3a, 3b, 8, 4a, 4b, 9, 1b neat and adsorbed on TiO2, PL spectra
Eur. J. Inorg. Chem. 2012, 5461–5464
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5463

K. P. Chitre, E. Guillén, A. S. Yoon, E. Galoppini
SHORT COMMUNICATION
[8] a) M. Porel, A. Klimczak, M. Freitag, E. Galoppini, V. Rama-
murthy, Langmuir 2012, 28, 3355–3359; b) M. Freitag, E. Ga-
loppini, Energy Environ. Sci. 2011, 4, 2482–2494.
of 1b upon titration with acid, UV/Vis and PL spectra of 1a and
1b, UV/Vis spectrum of 1c in solution and upon titration with acid.
[9] a) Y. Zhang, E. Galoppini, P. G. Johansson, G. J. Meyer, Pure
Appl. Chem. 2011, 83, 861–868; b) P. G. Johansson, Y. Zhang,
M. Abrahamsson, G. J. Meyer, E. Galoppini, Chem. Commun.
2011, 47, 6410–6412.
Acknowledgments
[10] P. Persson, in private communication (publication in prepara-
tion).
E. G. thanks Prof. Randolph Thummel for suggesting the develop-
ment of homoleptic complexes. This work was supported through
a grant from the Office of Basic Energy Sciences of the U.S. De-
partment of Energy (DE-FG02-01ER15256). E. Guillén thanks the
Spanish Ministry of Science and Innovation for a fellowship (BES-
2008-004786).
[11] M. Meldal, C. W. Tornoe, Chem. Rev. 2008, 108, 2952–3015.
[12] a) S. Cicchi, P. Fabbrizzi, B. Cecconi, Synlett 2011, 223–226;
b) A. Baron, C. Herrero, A. Quaranta, M. F. Charlot, W. Leibl,
B. Vauzeilles, A. Aukauloo, Inorg. Chem. 2012, 51, 5985–5987.
[13] a) G. de Miguel, M. Wielopolski, D. I. Schuster, M. A. Fazio,
O. P. Lee, C. K. Haley, A. L. Ortiz, L. Echegoyen, T. Clark,
D. M. Guldi, J. Am. Chem. Soc. 2011, 133, 13036–13054; b) C.
Katan, M. Blanchard-Desce, S. Tretiak, J. Chem. Theory Com-
put. 2010, 6, 3410–3426.
[14] S. W. Kwok, J. R. Fotsing, R. J. Fraser, V. O. Rodionov, V. V.
Fokin, Org. Lett. 2010, 12, 4217–4219.
[15] S. Paek, C. Baik, M. S. Kang, H. Kang, J. Ko, J. Organomet.
Chem. 2010, 695, 821–826.
[16] a) P. Mobian, J. P. Collin, J. P. Sauvage, Tetrahedron Lett. 2006,
47, 4907–4909; b) G. Zhang, L. Fang, L. Zhu, D. Sun, P. G.
Wang, Bioorg. Med. Chem. 2006, 14, 426–434.
[17] a) C. D. Hein, X. M. Liu, D. Wang, Pharm. Res. 2008, 25,
2216–2230; b) V. D. Bock, H. Hiemstra, J. H. van Maarseveen,
Eur. J. Org. Chem. 2006, 51–68.
[18] E. H. Ryu, Y. Zhao, Org. Lett. 2005, 7, 1035–1037.
[19] A. Baron, C. Herrero, A. Quaranta, M. F. Charlot, W. Leibl,
B. Vauzeilles, A. Aukauloo, Chem. Commun. 2011, 47, 11011–
11013.
[20] D. Wang, R. Mendelsohn, E. Galoppini, P. G. Hoertz, R. A.
Carlisle, G. J. Meyer, J. Phys. Chem. B 2004, 108, 16642–16653.
[21] a) M. Atmeh, N. R. Russell, R. J. Forster, T. E. Keyes, J. In-
clusion Phenom. Macrocyclic Chem. 2007, 57, 607–612; b) H. A.
Nieuwenhuis, J. G. Haasnoot, R. Hage, J. Reedijk, T. L.
Snoeck, D. J. Stufkens, J. G. Vos, Inorg. Chem. 1991, 30, 48–54;
c) S. Fanni, T. E. Keyes, C. M. O’Connor, H. Hughes, R. Y.
Wang, J. G. Vos, Coord. Chem. Rev. 2000, 208, 77–86.
Received: August 3, 2012
[1]
[2]
A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson,
Chem. Rev. 2010, 110, 6595–6663.
a) A. Kubacka, M. Fernandez-Garcia, G. Colon, Chem. Rev.
2012, 112, 1555–1614; b) W. J. Youngblood, S. H. Lee, K.
Maeda, T. E. Mallouk, Acc. Chem. Res. 2009, 42, 1966–1973;
c) D. Gust, T. A. Moore, A. L. Moore, Acc. Chem. Res. 2009,
42, 1890–1898.
[3]
A. Mishra, M. K. Fischer, P. Bäuerle, Angew. Chem. 2009, 121,
2510; Angew. Chem. Int. Ed. 2009, 48, 2474–2499.
[4] a) A. Kopecky, E. Galoppini in Specialist Periodical Report on
Photochemistry, Vol. 37 (Ed.: A. Albini), The Royal Society
of Chemistry, Cambridge, 2009, pp. 362–392; b) E. Galoppini,
Coord. Chem. Rev. 2004, 248, 1283–1297.
[5] a) R. Ernstorfer, L. Gundlach, S. Felber, W. Storck, R. Eich-
berger, F. Willig, J. Phys. Chem. B 2006, 110, 25383–25391; b)
N. A. Anderson, X. Ai, D. T. Chen, D. L. Mohler, T. Q. Lian,
J. Phys. Chem. B 2003, 107, 14231–14239.
[6] D. P. Hagberg, J. H. Yum, H. Lee, F. De Angelis, T. Marinado,
K. M. Karlsson, R. Humphry-Baker, L. Sun, A. Hagfeldt, M.
Grätzel, M. K. Nazeeruddin, J. Am. Chem. Soc. 2008, 130,
6259–6266.
[7] a) S. Thyagarajan, E. Galoppini, P. Persson, J. M. Giaimuccio,
G. J. Meyer, Langmuir 2009, 25, 9219–9226; b) C.-W. Chang,
L. Luo, C.-K. Chou, C.-F. Lo, C.-Y. Lin, C.-S. Hung, Y.-P. Lee,
E. W.-G. Diau, J. Phys. Chem. C 2009, 113, 11524–11531.
Published Online: October 4, 2012
5464
www.eurjic.org
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2012, 5461–5464