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the vast majority of ruthenium(II) chromophores bearing a
terpy ligand, even though the mesoionic carbene ligand
imposes a more strained ligand environment around the
metal center (∠N−Ru−N = 154.3°) than terpy (∠N−Ru−N =
158.4°).25 Moreover, the lifetime of 4 exceeds the longest
3MLCT lifetimes achieved so far by any mononuclear
ruthenium(II) complex.8−10,17,28 Interestingly, the data for 1−
4 does not strictly conform to the energy gap law: The shortest
τ value is observed in the case with the largest energy gap (i.e.,
1) while the longest τ was measured for 4, which is
characterized by the smallest E0−0. We therefore surmise that
the longer τ values of 2−4 relative to 1 are governed primarily
by the increased separation between the emitting 3MLCT state
(10) Breivogel, A.; Forster, C.; Heinze, K. Inorg. Chem. 2010, 49,
̈
7052.
(11) Hammarstrom, L.; Johansson, O. Coord. Chem. Rev. 2010, 254,
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2546.
3
and the deactivating MC state (Figure 2). Lending support to
(12) Abrahamsson, M.; Jager, M.; Osterman, T.; Eriksson, L.;
̈
3
a thermally inaccessible MC state is the lifetimes at 77 K for
Persson, P.; Becker, H.-C.; Johansson, O.; Hammarstrom, L. J. Am.
̈
complexes 2−4 (13−18 μs; Table S1, SI) being similar to that
exhibited by [Ru(terpy)2](PF6)2 (11 μs) at 77 K. The
conjugated substituents of the terpy ligand may also have an
auxiliary role on the τ values (0.39, 1.72, and 6.98 μs where R1
= −H, −CO2Me, and −2-furyl, respectively), but the increase
in the τ value with the terminal bromides present in 4 is
indicative of inductive effects on the carbene ligand playing a
significant role in the photophysics.
Chem. Soc. 2006, 128, 12616.
̈
(13) Osterman, T.; Abrahamsson, M.; Becker, H.-C.; Hammarstrom,
L.; Persson, P. J. Phys. Chem. A 2012, 116, 1041.
̈
(14) Abrahamsson, M.; Jager, M.; Kumar Rohan, J.; Osterman, T.;
̈
Persson, P.; Becker, H.-C.; Johansson, O.; Hammarstrom, L. J. Am.
̈
Chem. Soc. 2008, 130, 15533.
(15) Robson, K. C. D.; Koivisto, B. D.; Yella, A.; Sporinova, B.;
Nazeeruddin, M. K.; Baumgartner, T.; Gratzel, M.; Berlinguette, C. P.
̈
Inorg. Chem. 2011, 50, 5494.
To the best of our knowledge, the lifetimes of 3 and 4 are
among the highest for any monometallic ruthenium(II)
(16) Koivisto, B. D.; Robson, K. C. D.; Berlinguette, C. P. Inorg.
Chem. 2009, 48, 9644.
3
complexes where emission originates from a MLCT state.
(17) Schulze, B.; Escudero, D.; Friebe, C.; Siebert, R.; Gorls, H.;
̈
These long-lived excited states are manifest in the strongly σ-
donating and π-accepting ability of the carbene ligand resolving
́
Sinn, S.; Thomas, M.; Mai, S.; Popp, J.; Dietzek, B.; Gonzalez, L.;
Schubert, U. S. Chem.Eur. J. 2012, 18, 3785.
3
3
the MLCT and MC states, while the presence of EWGs or
EDGs about the ligands helps to further separate these states.
These collective results demonstrate the enormous potential of
using mesoionic carbene complexes to modify the photo-
physical properties of ruthenium(II) complexes. Investigations
are underway to gain a better understanding of the photo-
physical properties of these complexes and to examine their
potential in light harvesting applications.
(18) Medlycott, E. A.; Hanan, G. S. Chem. Soc. Rev. 2005, 34, 133.
(19) Collin, J. P.; Beley, M.; Sauvage, J. P.; Barigelletti, F. Inorg. Chim.
Acta 1991, 186, 91.
(20) Djukic, J. P.; Sortais, J. B.; Barloy, L.; Pfeffer, M. Eur. J. Inorg.
Chem. 2009, 817.
(21) Chi, Y.; Chou, P.-T. Chem. Soc. Rev. 2007, 36, 1421.
(22) Constable, E. C.; Thompson, A.; Tocher, D. A.; Daniels, M. A.
M. New J. Chem. 1992, 16, 855.
(23) Collin, J. P.; Kayhanian, R.; Sauvage, J. P.; Calogero, G.;
Barigelletti, F.; DeCian, A.; Fischer, J. Chem. Commun. 1997, 775.
(24) Son, S. U.; Park, K. H.; Lee, Y.-S.; Kim, B. Y.; Choi, C. H.; Lah,
M. S.; Jang, Y. H.; Jang, D.-J.; Chung, Y. K. Inorg. Chem. 2004, 43,
6896.
ASSOCIATED CONTENT
* Supporting Information
■
S
Full experimental details and TD-DFT data for 2−4. This
material is available free of charge via the Internet at http://
(25) Schulze, B.; Escudero, D.; Friebe, C.; Siebert, R.; Gorls, H.;
̈
Kohn, U.; Altuntas, E.; Baumgaertel, A.; Hager, M. D.; Winter, A.;
̈
Dietzek, B.; Popp, J.; Gonzalez, L.; Schubert, U. S. Chem.Eur. J.
́
2011, 17, 5494.
AUTHOR INFORMATION
Corresponding Author
■
(26) Ung, G.; Bertrand, G. Chem.Eur. J. 2011, 17, 8269.
(27) For the T1-optimized geometry of 1, the excited electron is
located on the carbene (see ref 25); similar studies are underway for
2−4.
(28) There are examples of mononuclear ruthenium complexes with
slightly longer lifetimes where emission is primarily ligand-based, e.g.:
Wang, X. Y.; Del Guerzo, A.; Baitalik, S.; Simon, G.; Shaw, G. B.;
Chen, L. X.; Schmehl, R. Photosynth. Res. 2006, 87, 83.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
C.P.B. is grateful for support from the Canadian Natural
Science and Engineering Research Council, Canada Research
Chairs, Canadian Foundation for Innovation, and Canada
School of Energy and Environment. B.S. is grateful to the
Fonds der Chemischen Industrie for a Ph.D. scholarship.
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