the bis-chromenes 3 and 4 undergo mono-heterolysis leading to
o-quinonoid intermediates, whose absorption properties parallel
those of the model aryl analogues 6 and 7, respectively. Whereas
the intermediate derived from 3 was found to exhibit an ab-
sorption maximum at 550 nm (purple), the absorption of the o-
quinonoid intermediate derived from 4 was found be highly blue-
shifted with kmax ca. 430 nm (orange-red). The cross-linked and
substitutionally-biased bis-chromene 5, which may be viewed as
a 6-(7-chromenyl)chromene as well as 7-(6-chromenyl)chromene,
lends itself to novel photobehaviour in that the irradiation leads
to differing proportions (depending on the temperature) of the
intermediates of 6 C–O and 7 C–O heterolysis. The unique
absorption spectral distribution arising from the formation of
two o-quinonoid intermediates derived from 6 C–O and 7 C–O
heterolyses gives a dark brown colour to the solutions of irradiated
Laurent, Elsevier, Amsterdam, 1990, ch. 26; (c) R. Guglielmetti, in
Photochromism: Molecules and Systems, ed. H. Durr and H. Bouas-
Laurent, Elsevier, Amsterdam, 1990, ch. 8 and 23, and references cited
therein; (d) B. V. Gemert, in Organic Photochromic and Thermochromic
Compounds, ed. J. C. Crano and R. Guglielmetti, Plenum Press, New
York, 1999, vol. 1, p. 111.
J. C. Crano, T. Flood, D. Knowles, A. Kumar and B. V. Gemert, Pure
Appl. Chem., 1996, 68, 1395.
(a) R. S. Becker and J. Michl, J. Am. Chem. Soc., 1966, 88, 5931;
(b) Y. Kodama, T. Nakabayashi, K. Segawa, E. Hattori, M. Sakuragi,
N. Nishi and H. Sakuragi, J. Phys. Chem. A, 2000, 104, 11478, and
references cited therein.
J. N. Moorthy, P. Venkatakrishnan, S. Samanta and D. Krishnakumar,
Org. Lett., 2007, 9, 919.
5 Molecular Switches, ed. B. L. Feringa, Wiley, Weinheim, 2001.
For examples, see: (a) L. Gobbi, P. Seiler and F. Diedrich, Angew.
Chem., Int. Ed., 1999, 38, 674; (b) R. H. Mitchell, T. R. Ward, Y. Wang
and P. W. Dibble, J. Am. Chem. Soc., 1999, 121, 2601; (c) T. Mrozek,
H. Gorner and J. Daub, Chem.–Eur. J., 2001, 7, 1028.
2
3
4
6
5. In other words, the photobehaviour of 5 is equivalent to
7 A. Samat, V. Lokshin, K. Chamontin, D. Levi, G. Pepe and R.
Guglielmetti, Tetrahedron, 2001, 57, 7349.
a composite of that observed for 3 as well as 4. In view of
the fact that naphthopyrans with a neutral grey colour are
important for applications in conjunction with spiro-oxazines
for the development of ophthalmic plastic lenses and that there
exists tremendous interest currently in the development of multi-
addressable photoswitchable systems, the bis-chromenes with
novel photochromism represent a new class of molecular systems.
We are presently exploring the behaviour of analogous bis-
naphthopyrans toward bis-photochromic materials.
8
(a) S. Delbaere, B. L.-Houze, C. Bochu, Y. Teral, M. Campredon and G.
Vermeersch, J. Chem. Soc., Perkin Trans. 2, 1998, 1153; (b) H. Gorner
and A. K. Chibisov, J. Photochem. Photobiol., A, 2002, 149, 83; (c) G.
Ottavi, G. Favaro and V. Malatesta, J. Photochem. Photobiol., A, 1998,
1
15, 123; (d) J. Hobley, V. Malatesta, K. Hatanaka, S. Kajimoto, S. L.
Williams and H. Fukumura, Phys. Chem. Chem. Phys., 2002, 4, 180;
(e) M. Frigoli, V. Pimienta, C. Moustrou, A. Samat, R. Guglielmetti,
J. Aubard, F. Maurel and J.-C. Micheau, Photochem. Photobiol. Sci.,
2
003, 2, 888.
9
(a) The C–O bond heterolysis of both pyran rings synergistically to
yield highly conjugated bis-opened forms does not appear to occur
in 3–5 as reflected from the absorption spectra of the intermediates,
which closely resemble those of the intermediates of the model analogs
Acknowledgements
6
and 7. In general, considerable shifts in the absorption maxima have
been observed for bis-opened forms relative to those of the mono-
opened forms, see: W. Zhao and E. M. Carreira, J. Am. Chem. Soc.,
2002, 124, 1582; (b) W. Zhao and E. M. Carreira, Org. Lett., 2006, 8,
We are thankful to DST, India for finanical support. PV and SS
are grateful to CSIR for their research fellowships.
9
9.
1
0 It is intriguing that the decay of the o-quinonoid intermediate of
3 follows an almost monoexponential function as compared to the
behaviour observed for those of 4 and 5. We believe that the electronic
factors intrinsic to the aryl rings are transmitted differently in 6- and
7-arylchromenes.
Notes and references
1
(a) R. C. Bertelson, in Photochromism, ed. G. H. Brown, Wiley, New
York, 1971, ch. 10, and references cited therein; (b) K. Ichimura, in
Photochromism: Molecules and Systems, ed. H. Durr and H. Bouas-
11 C. Lenoble and R. S. Becker, J. Photochem., 1986, 33, 187.
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