original state (bleaching) predominantly through a thermal
process with a reasonable kinetics (relatively fast, 10-1 s-1).
Furthermore, they should be resistant, allowing the occur-
rence of many coloring-decoloring cycles. The evaluation
of the photochromic behavior of molecules involves the
knowledge of some relevant parameters, related to the kinetic
and spectral properties, through a kinetic analysis performed
under flash photolysis or continuous irradiation. The infor-
mation obtained by the two methods can be very distinct
since the time scales of observation are completely different.
tions,9 with a Michael adduct bearing an aromatic amino
function,10 to give the thieno-2H-chromene amino acid
derivative 2 in good yield (Scheme 2).
Scheme 2
The photochromic behavior of 2H-1-benzopyrans (2H-
chromenes) is based on a photoinduced reversible opening
of the pyran ring that converts the colorless form (closed
form) into a set of isomers with the pyran ring opened (open
forms), leading to distinct absorption spectra. Much research
has been devoted to this class of compounds, and the
heteroannellation represents an interesting approach in
promoting improvements in their photochromic properties.6
Herein we describe the synthesis of a new photochromic
R-amino acid based on a 2H-chromene annellated with a
dimethylthiophene ring on the f face, using a C-N palla-
dium-catalyzed cross-coupling and the study of its photo-
chromic behavior under flash photolysis and continuous
irradiation.
(i) Pd(OAc)2 (10 mol %), BINAP (15 mol %), Cs2CO3 (1.4
equiv), dry toluene, 100 °C, 3 h.
We began with the synthesis of the precursor bromothieno-
2H-chromene 1 in good yield, by chromenization reaction
of the, also prepared, 6-bromo-5-hydroxy-2,3-dimethylbenzo-
[b]thiophene with 1,1-bis(4-methoxyphenyl)prop-2-yn-1-ol,7
using acidic Al2O3 as catalyst and drying agent (Scheme 1).
The photochromic properties of compounds 1 and 2 were
evaluated by flash photolysis, and due to the good results of
compound 2, it was also studied under continuous irradiation
and compared with a reference 2H-chromene benzoannellated
on the f face and derivative of the same propargylic
alcohol11,12 (Table 1).
The activation wavelengths of the closed form of com-
pound 2 in toluene [λmax (ꢀ) 280 (35490), 286 (26700); 309
(41300), 327sh (30500), 339 (18500), 375 (10100) nm] cor-
respond to strong bands in the near UV, which is very im-
portant, as the activation with less energetic radiation usually
improves the sensitiveness to sunlight and also fatigue resis-
tance. The photochromic behavior of compounds 1, 2, and
reference, under flash photolysis in toluene at room tempera-
ture (Table 1), was quantified by three spectrokinetic para-
meters: absorption maxima of the colored forms (λmax); ther-
mal bleaching (ring closure) rate (k∆); and “colorability” (A0).
The latter parameter, which has been defined for photochro-
mic compounds,13 is directly connected to the molar absorp-
tivity of colored species and to the quantum yield of color-
ation, and was evaluated by monitoring the absorbance (A0)
at λmax immediately after the flashgun was fired. A0 is the
experimental value corresponding to the following equation:
Scheme 1
A0 ) ꢀMC × Φcol × k × CCF (for low concentration)
These conditions were already used by us in the synthesis
of methyl-induced linear and angular thieno-2H-chromenes.8
Compound 1 was then coupled under C-N coupling condi-
ꢀMC, molar absorptivity of colored forms
Φcol, quantum yield for photocoloration
k, constant including photolysis conditions
CCF, initial concentration of the colorless form
(6) Van Gemert, B. In Organic Photochromic and Thermochromic
Compounds; Crano, J. C., Guglielmetti, R., Eds.; Plenum Press: New York,
1999; Vol. 1, Chapter 3, pp 111-140 and references therein.
(7) Gabbutt, C. D.; Heron, B. M.; Instone, A. C.; Thomas, D. A.;
Partington, S. M.; Hursthouse, M. B.; Gelbrich, T. Eur. J. Org. Chem. 2003,
1220-1230.
(8) Queiroz, M.-J. R. P.; Plasencia, P. M. S.; Dubest, R.; Aubard, J.;
Guglielmetti, R. Tetrahedron 2003, 59, 2567-2573.
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Org. Lett., Vol. 7, No. 22, 2005