Thermally stable fatigue resistant near infrared active photochromic
compounds, exemplified by 6-amino-7-cyano-3-(dicyclopropylmethylene)-4-
(2,5-dimethyl-3-furyl)-benzofuran-2(3H)-one
Harry G. Heller,* David S. Hughes, Michael B. Hursthouse and Neil G. Rowles
Chemistry Department, Cardiff University, PO Box 912, Cardiff, UK CF1 3TP. E-mail: heller@cardiff.ac.uk
Received (in Cambridge, UK) 2nd May 2000, Accepted 15th June 2000
Published on the Web 7th July 2000
The title pale yellow photochromic compound 11b in
toluene, on irradiation at 366 nm, cyclises to the thermally
stable infrared active blue–green photochrome, 8-amino-
7-cyano-4,4-dicyclopropyl-3a,4-dihydro-6-hydroxy-2,3a-di-
methylnaphtho[2,1-b]furan-5,6-carbolactone, 12b which has
a lmax value of 720 nm for its broad long wavelength
absorption band.
ethylidene-4-isopropylidene tetrahydrofurandione 1c and (E)-
3-[2-methyl-5-phenyl-3-thienyl)ethylidene-4-(dicyclopropyl-
methylene)tetrahydrofuran-2,5-dione 1d reacted with mal-
ononitrile and diethylamine in THF to give exclusively
(E)-2-dicyanomethylene derivatives 3a–d which, in toluene,
cyclised to photochromes 4a–d on irradiation at 366 nm. The
thermally stable blue photochromes 4a–d underwent reverse
reactions on exposure to white light. Previous papers1,2 reported
erroneously that E-fulgides (e.g. 1a) gave the corresponding
(E)-5-dicyanomethylene derivatives (e.g. 8a) on reaction with
malononitrile (1 equiv.) and diethylamine (2 equiv.) in THF,
followed by cyclisation with acetyl chloride.
E-Fulgides, (E)-3-[1-(2,5-dimethyl-3-furyl)ethylidene]-4-iso-
propylidenetetrahydrofuran-2,5-dione 1a, (E)-3-[1-(2,5-dime-
thyl-3-furyl)ethylidene]-4-(cyclopropylmethylene)tetrahydro-
furan-2,5-dione
1b,
(E)-3-[2,5-dimethyl-3-thienyl)-
Z-Fulgides 6a–d gave exclusively the corresponding pale
yellow (Z)-5-dicyanomethylene derivatives 7a–d under similar
experimental conditions. The latter, in toluene, isomerised to E-
isomers 8a–d and cyclised to photochromes 9a–d on irradiation
at 366 nm. The thermally stable blue photochromes 9a–d
underwent reverse reactions to pale yellow E-isomers 8a–d on
exposure to white light.
Deprotonation of the methyl group syn to the dicyano-
methylene group in (Z)-2-dicyanomethylene derivative 3b, by
boiling with diisopropylamine in THF, gave anion 5b, which
reacted with the adjacent cyano group to form imine 10b, which
isomerised to 6-amino-7-cyano-3-(dicyclopropylmethylene)-
4-(2,5-dimethyl-3-furyl)-1-benzofuran-2(3H)-ones 11b,† ob-
tained as bright yellow crystals (from chloroform–petrol). Its
structure was confirmed by X-ray crystallographic analysis
(Fig. 1). Amines 11a–d were prepared in a similar manner.
All new compounds were fully characterised. Spectral data,
melting points and yields are given in Table 1.
On irradiation at 366 nm, amines 11a–d in toluene cyclised to
thermally stable blue-green photochromes 12a–d (Table 2)
which underwent the reverse reactions on exposure to white
light.
Photochromes 2a–d, 4a–d, and 9a–d showed bathochromic
shifts when the push-pull effect is enhanced, as reported for
infrared active dyes.3 Photochromes 12a–d have increased
intramolecular charge-transfer character, due to the tendency to
Fig. 1 The X-ray structure of the photochromic compound 11b.
DOI: 10.1039/b003496g
Chem. Commun., 2000, 1397–1398
This journal is © The Royal Society of Chemistry 2000
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