4
Tetrahedron
coordination site,14,17 it would not be so favorable in the case of
12. Cui, C.; Heilmann-Brohl, J.; Perucha, A. S.; Thomson, M. D.;
Roskos, H. G.; Wagner, M.; Jäkle, F. Macromolecules 2010, 43,
5256.
the boronium complexes due to the absence of -bonds available
for rearrangement at the substituents on the boron atom and to
the presence of ESR active species not expected in the
rearrangement mechanism. According to this hypothesis, a
sufficiently low energy level of the HOMO of the cationic moiety
to oxidize the counteranion in the excited state, provided by a
boryl moiety such as BBN found in the relatively strong Lewis
acidic tricoordinate boron compounds, and an appropriate level
of the oxidation potential of the counteranion would be required
to gain photoresponsive capability in the boronium complexes.
For color-quenching, the detailed mechanism is unclear and
whether the transformation between the noncolored and colored
states is reversible or not has yet to be determined, while an
oxygen-participating irreversible reaction may mainly contribute
the quenching under air/oxygen.
13. McGovern, G. P.; Zhu, D.; Aquino, A. J. A.; Vidović, D.;
Findlater, M. Inorg. Chem. 2013, 52, 13865.
14. Rao, Y.-L.; Amarne, H.; Zhao, S.-B.; McCormick, T. M.; Martić,
S.; Sun, Y.; Wang, R.-Y.; Wang, S. J. Am. Chem. Soc. 2008, 130,
12898.
15. Ansorg, K.; Braunschweig, H.; Chiu, C.-W.; Engels, B.; Gamon,
D.; Hügel, M.; Kupfer, T.; Radacki, K. Angew. Chem. Int. Ed.
2011, 50, 2833.
16. Iida, A.; Saito, S.; Sasamori, T.; Yamaguchi, S. Angew. Chem. Int.
Ed. 2013, 52, 3760.
17. Rao, Y.-L.; Hörl, C.; Braunschweig, H., Wang, S. Angew. Chem.
Int. Ed. 2014, 53, 9086.
18. All measurements and photoirradiation were perfomed under air.
19. Crystallographic data (excluding structure factors) for the
structures in this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication nos.
CCDC 1453149 and 1453150. Copies of the data can be obtained,
free of charge, on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ UK, (fax: +44-(0)1223-336033 or e-mail:
deposit@ccdc.cam.ac.uk).
In conclusion, photoinduced solid-state coloring behavior of
bpyBBN+TfO− (1) and its analogues was revealed. Compound 1
itself, its phenanthroline-ligand analogue phenBBN+TfO− (2), and
their halide salt analogues 1′ , 1′′, and 2′ exhibited reversible
color change in the solid state upon UV irradiation, while
bpyB(catecholato)+Cl− (6′′) had no photoresponsive capability.
Modifying the position and electronic properties of substituents
on the bipyridine ligand of 1 provided wide-ranging color
variation of the photoirradiated solids. Based on the experimental
and computational results, a viologen-like photoredox process is
suggested as a possible mechanism for photoinduced coloring.
The structure–property relationships determined in this study
may provide a basis for the development of novel and easily
modifiable photoresponsive materials.
20. Compound 3d shows irregularity in this trend, probably due to the
ambivalent nature of a fluoro group in the inductive and
mesomeric effects.
21. NMR spectra of the solutions prepared from the photoirradiated
boronium complex solids showed no other peaks than those
attributed to the original complex. X-ray crystallographic analysis
of single crystals of the complexes after exposure to UV light also
gave no indication of additional chemical species. In addition, the
new absorption appearing after irradiation in the diffuse reflection
spectra was very weak in comparison with the original absorption
of the complex. These results suggest that the yields of the
photochemical products are very low. An acetone-d6 solution of 2
showed color change from colorless to red in 3 min upon
irradiation, but no notable peaks of photoproducts could be
observed by NMR. As for other conditions, solutions of 1 and 2 do
not necessarily show color change. In all cases, prolonged
irradiation in the solution state resulted in irreversible
decomposition of the complexes. In the case of 2 in acetone, there
is a possibility of the occurrence of a reaction similar to that in the
solid state, but what occurs here is still unclear.
Acknowledgments
We thank Dr. Hideki Ohtsu, University of Toyama, for his
help in measuring the ESR spectra. This research was financially
supported by JSPS KAKENHI Grant Number JP15K21012. J.Y.
thanks the Yamaguchi Educational and Scholarship Foundation
for financial support.
22. Both peaks appear in the visible region with a broad shape. The
absorption of the photoproduct in the solid state is somewhat red-
shifted from that of the radical in DMF. This shift could be
attributed to differences in the state of the radicals, in solution or
in the solid.
Supplementary data
Supplementary
data
(experimental
procedures,
characterization data, computational studies, Table S1–3, Scheme
S1, and Figures S1–17) associated with this article can be found,
in the online version.
References and notes
1.
2.
Kamogawa, H.; Sato, S. Bull. Chem. Soc. Jpn. 1991, 64, 321.
Kamogawa, H.; Nanasawa, M. Bull. Chem. Soc. Jpn. 1993, 66,
2443.
3.
4.
Kamogawa, H.; Nanasawa, M. Chem. Lett. 1988, 373.
Dorman, S. C.; O'Brien, R. A.; Lewis, A. T.; Salter, E. A.;
Wierzbicki, A.; Hixon, P. W.; Sykora, R. E.; Mirjafari, A.; Davis,
J. H., Jr. Chem. Commun. 2011, 47, 9072.
5.
6.
7.
8.
Davidson, J. M.; French, C. M. Chem. Ind. 1959, 750.
Hünig, S.; Wehner, I. Heterocycles 1989, 28, 359.
Ma, K.; Bats, J. W.; Wagner, M. Acta Crystallogr. E 2001, o846.
Thomson, M. D.; Novosel, M.; Roskos, H. G.; Muller, T.;
Scheibitz, M.; Wagner, M.; Fabrizi de Biani, F.; Zanello, P. J.
Phys. Chem. A 2004, 108, 3281.
9.
Wood, T. K.; Piers, W. E.; Keay, B. A.; Parvez, M. Chem.
Commun. 2009, 5147.
10. Mansell, S. M.; Adams, C. J.; Bramham, G.; Haddow, M. F.;
Kaim, W.; Norman, N. C.; McGrady, J. E.; Russel C. A.; Udeen,
S. J. Chem. Commun. 2010, 46, 5070.
11. Kaim, W. Chem. Ber. 1981, 114, 3789.