Synthesis and photochromism of novel phenylene-linked photochromic bispyrans

Article abstract of DOI:10.1021/ol052587y

(Chemical Equation Presented) Phenylene-linked bisnaphthopyrans were synthesized in good yields via the one-pot reaction of bis-propargyl alcohols with naphthols. Temperature-dependent photochromism in 1,4-phenylene-linked bispyrans leads to up to 60 nm bathochromic shift between the colored species formed at room temperature and at -20°C. Better fatigue resistance and higher colorability was observed in 1,4-phenylene-linked bis-[2H]-naphtho[1,2-b] pyrans by comparison to the 1,3-phenylene linked bis-[2H]-naphtho[1,2-b]pyrans.

Full text of DOI:10.1021/ol052587y

ORGANIC
LETTERS
2006
Vol. 8, No. 1
99-102
Synthesis and Photochromism of Novel
Phenylene-Linked Photochromic
Bispyrans
Weili Zhao and Erick M. Carreira*
Laboratorium fu¨r Organische Chemie, ETH-Ho¨nggerberg HCI H335,
CH-8093 Zu¨rich, Switzerland
carreira@org.chem.ethz.ch
Received October 25, 2005
ABSTRACT
Phenylene-linked bisnaphthopyrans were synthesized in good yields via the one-pot reaction of bis-propargyl alcohols with naphthols.
Temperature-dependent photochromism in 1,4-phenylene-linked bispyrans leads to up to 60 nm bathochromic shift between the colored species
formed at room temperature and at
−20 °C. Better fatigue resistance and higher colorability was observed in 1,4-phenylene-linked bis-[2H]-
naphtho[1,2-b]pyrans by comparison to the 1,3-phenylene linked bis-[2H]-naphtho[1,2-b]pyrans.
The successful commercialization of ophthalmic lenses and
glasses using organic photochromic dyes have attracted great
interest in the design and synthesis of photochromic com-
pounds with improved properties.1,2 There has been recent
particular interest in the design, synthesis, and study of novel
bisphotochromophores.3-7 Recently, we reported oligoth-
iophene-linked bispyrans exhibiting unprecedented sequential
or temperature-dependent photochromism.6 Terthiophene-
and quaterthiophene-linked bispyrans have also been reported
by Guglielmetti et al.7 However, fundamental questions
persist regarding the role of oligothiophenes as indispensable
subunits for the observation of sequential or temperature-
dependent photochromic behavior of such bispyrans; more-
over, there is relatively little data that permits evaluation of
linker effects on the photochromic properties of these
systems. Additionally, truly general strategies for the produc-
tion of novel bispyrans are lacking. The reported synthesis
of bispyrans generally adopt either Pd-catalyzed Stille
coupling reactions of distannyl bithiophene with bromothie-
nyl pyrans or Cu-promoted homocoupling reaction of
thiophene-substituted pyrans.6,7 The use of organotin com-
pounds in the former method detract from their utility, and
in the latter case, the prescribed use of strong base limits
the extent to which the method is tolerant of various
functionalities. Herein, we report efficient synthesis and
photochromic properties of novel phenylene-linked bisnaph-
thopyrans utilizing a new and general synthetic pathway,
(1) Crano, J. C.; Guglielmetti, R. Organic Photochromic and Thermo-
chromic Compounds; Plenum Press: New York, 1999.
(2) (a) Willner, I. Acc. Chem. Res. 1997, 30, 347-356. (b) Feringa B.
L. Molecular Switches; Wiley-VCH: Weinheim, 2001. (c) Du¨rr, H.; Bouas-
laurent, H. Photochromism: Molecules and Systems; Elsevier: Amsterdam,
2003.
(3) For diarylethene, see: (a) Higashiguchi, K.; Matsuda, K.; Irie, M.
Angew. Chem., Int. Ed. 2003, 42, 3537-3540. (b) Kobatake, S.; Irie, M.
Tetrahedron 2003, 59, 8359-8364. (c) Matsuda, K.; Irie, M. J. Am. Chem.
Soc. 2001, 123, 9896-9897. (d) Chen, B.; Wang, M.; Luo, Q.; Tian, H.
Synth. Met. 2003, 137, 985-987. (e) Luo, Q.; Chen, B.; Wang, M.; Tian,
H. AdV. Funct. Mater. 2003, 13, 233-239. (f) Higashiguchi, K.; Matsuda,
K.; Tanifuji, N.; Irie, M. J. Am. Chem. Soc. 2005, 127, 8922-8923.
(4) For benzodihydropyrene, see: (a) Mitchell, R. H.; Bandyopadhyay,
S. Org. Lett. 2004, 6, 1729-1732. (b) Mitchell, R. H.; Ward, T. R.; Chen,
Y.; Wang, Y.; Weerawarna, S. A.; Dibble, P. W.; Marsellam M. J.; Almutair,
A.; Wang, Z. J. Am. Chem. Soc. 2003, 125, 2974-2988. (c) Mitchell. R.
H.; Ward, T. R.; Wang. Y.; Dibble, P. W. J. Am. Chem. Soc. 1999, 121,
2601-2602.
(5) For dihydroindolizines, see: Bleisinger, H.; Scheidhauer, P.; Du¨rr,
H.; Wintgens, V.; Valat, P.; Kossanyi, J. J. Org. Chem. 1998, 63, 990-
1000.
(6) (a) Zhao, W.; Carreira, E. M. J. Am. Chem. Soc. 2002, 124, 1582-
1583. (b) Zhao, W.; Carreira, E. M. J. Am. Chem. Soc. Submitted for
publication.
(7) (a) Yassar, A.; Galy, N. R.; Frigoli, M.; Moustrou, C.; Samat, A.;
Guglielmetti, R.; Jaafari, A. Synth. Met. 2001, 124, 23-27. (b) Yassar, A.;
Jaafari, H.; Galy, N. R.; Frigoli, M.; Moustrou, C.; Samat, A.; Guglielmetti,
R. Eur. Phys. J. Appl. Phys. 2002, 18, 3-8.
10.1021/ol052587y CCC: $33.50
© 2006 American Chemical Society
Published on Web 12/03/2005

which permits the preparation of both phenylene-linked bis-
[3H]-naphtho[2,1-b]pyrans and bis-[2H]-naphtho[1,2-b]-
pyrans.
The starting point of our synthesis was the commercially
available terphthaloyl or isophthaloyl chlorides. Friedel-
Crafts acylation afforded the required bisketones (4a-c and
4′a/4′b) in good to excellent yields (Scheme 1).8 The next
of modified conditions, wherein sodium acetylide with excess
acetylene gas in DMSO was used, the amount of such side
products could be reduced significantly, allowing for the
isolation of bispropargyl alcohols (5a-c and 5′a/5′b) in
preparatively useful yields. Following this step, we prepared
the bispyrans under acid-catalyzed conditions, which we have
previously documented.10,11 Good to excellent yields of
phenylene-linked bispyrans (1a-c/2a-c/3a-c, 1′a/1′b/2′a/
2′b/3′a/3′b) were obtained (Scheme 1). The synthesis strategy
delineated herein represents its first general application to
the preparation of bispyrans linked through sp3-hybridized
centers and complements strategies involving Pd- and Cu-
mediated couplings.6,7
Scheme 1. Synthesis of the Phenylene-Linked Compounds
With the targeted compounds in hand, we focused our
efforts on the study of the photophysical properties of the
bis-[3H]-naphtho[2,1-b]pyrans 1a-c and 1′a/1′b. The spec-
trum of 1a has four absorption bands in the region of 300-
400 nm at 304, 317, 347, and 361 nm and the same
absorption maxima as 3,3-diphenyl-[3H]-naphtho[2,1-b]-
pyran (6),12 which was chosen as a standard for comparison,
but with higher extinction coefficient (e.g., 9000 versus 4900
dm3‚mol-1‚cm-1 at 361 nm). This indicates that the two
photochromic units in 1a are well insulated and the absence
of any interaction between the two pyran units in its closed
forms.
Compound 1a exhibits temperature-dependent photo-
chromism, whereas 3,3-diphenyl-[3H]-naphtho[2,1-b]pyran
6 lacks this feature. The absorption spectra of compound 1a
prior to and after UV irradiation at room temperature and 5
and -20 °C are shown in Figure 1. At room temperature,
the colorless solution of 1a in toluene displays rapid yellow
coloration (λmax ) 434 nm) under UV irradiation, similar to
3,3-diphenyl-[3H]-naphtho[2,1-b]pyran 6 (λmax ) 432 nm).
In contrast, at -20 °C under UV irradiation, 1a displays red
coloration (λmax ) 494 nm) while 3,3-diphenyl-[3H]-naphtho-
[2,1-b]pyran 6 remains yellow. The latter (6) displayed
increased intensity at -20 °C compared to that observed at
room temperature. This likely results from the fact that the
second photochromic unit in the bispyrans undergoes opening
when the thermal fading of the first opened pyran unit
becomes slow at low temperature. It is surprising that the
simple phenylene linker could afford as much as 60 nm
bathochromic shift between the colored species formed at
(9) Coelho, P. J.; Salvador, M. A.; Oliveira, M. M.; Carvalho, L. M.
Tetrahedron 2004, 60, 2593-2599.
(10) Zhao, W.; Carreira, E. M. Org. Lett. 2003, 5, 4153-4154.
(11) The method for pyran preparation from a propargyl alcohol and a
naphthol in the presence of an acid catalyst is known to generate
naphthopyran in moderate yields in most cases [e.g., 55-63% for compound
6. See: (a) Tanaka, K.; Aoki, H.; Hosomi, H.; Ohba, S. Org. Lett. 2000, 2,
2133. (b) Heller, H. G.; Levell, J. R. U.S. Patent 5,955,520, 1999. (c) Harie,
G.; Samat, A.; Guglielmetti, R. Tetrahedron Lett. 1997, 38, 3075. However,
for the generation of bispyrans wherein each pyran is fused onto a common
naphthalene moiety only low yields have been reported (12-17%). See:
(d) Tanaka, K.; Aoki, H.; Hosomi, H.; Ohba, S. Org. Lett. 2000, 2, 2133.
(e) Knowles, D. B.; Von Gemert, B. U.S. Patent 5,464,567, 1995].
(12) The structures of the reference compounds (6 and 7) used in this
study are:
step in the sequence requires the generation of bispropargyl
alcohols.
A general problem that we encountered in the preparation
of the requisite propargyl alcohols is that significant amounts
of side products (2-butyne-1,4-diol derivatives) result from
double addition of acetylene to the ketone.9 Through the use
(8) Gilb, W.; Gro¨tsch, G.; Schubert, H. U.S. Patent 5,300,693, 1994.
100
Org. Lett., Vol. 8, No. 1, 2006

1b with extinction coefficients of 21300 dm3‚mol-1‚cm-1
for closed-TC and 18300 dm3‚mol-1‚cm-1 for closed-TT.
The photocoloration quantum yield for the generation of
closed-TC in dichloromethane was then measured to be 0.72
by the standard procedure using potassium ferrioxalate as
actinometer (see Supporting Information for details).13
The 1,3-linked bispyran 1′b has identical quantum yield
as 1,4-linked bispyran 1b. However, the colorability14 (A0,
the absorbance at photostationary state under white light
irradiation), thermal fading rate (k-1, the bleaching rate of
the colored form in the dark), and fatigue resistance (T1/2,
time required to decrease to the half of the colorability) of
1′a/1′b are quite different from those of 1a/1b (see Table
1).
Figure 1. UV-vis spectra of compound 1a (2.5 × 10-5 mol/L in
toluene) prior to and after UV (366 nm) irradiation for 4 min at
various temperature. (a) prior to UV irradiation; (b) UV (366 nm)
irradiation for 4 min at ambient temperature; (c) UV irradiation
for 4 min at 5 °C; (d) UV irradiation for 4 min at -20 °C.
Table 1. Colorability (A0), Absorption Maxima, Thermal
Bleaching Rate (k-1), and Fatigue Resistance (T1/2) of Bispyran
at Room Temperature in Toluene
entry
compd
A0/λmax (nm)
k-1 (s-1
)
T1/2 (min)
1
2
3
4
5
6
7
8
1a
1b
1c
1′a
1′b
2a
2b
2c
0.411/434
0.353/433
0.310/465
0.377/432
0.355/430
0.232/454
0.252/452
0.221/477
0.277/450
0.232/446
0.294/432
0.087
0.135
0.199
0.103
0.158
0.187
0.288
0.422
0.197
0.275
0.101
90
93
room temperature and at -20 °C. Temperature-dependent
photochromism was also observed for the other dimers (1-3
and 1′-3′); however, such effect is less pronounced for those
bispyrans with fast fading rates. The photochromic trans-
formations of 1 are shown in Scheme 2. The original state
266b
16
20
76
108
396b
40
9
10
11
2′a
2′b
6a
53
117
Scheme 2. Photochromic Reaction of Compounds 1a-c
a 3,3-Diphenyl-[3H]-naphtho[2,1-b]pyran. b Estimated from 1 h result.
As shown in Table 1 the colorability of 1a is much higher
than that of reference compound 3,3-diphenly-[3H]-naphtho-
[2,1-b]pyran 6 (compare entry 1 to entry 11); however, the
thermal fading rate and fatigue resistance of 1a are slightly
lower than that of the reference. para-Fluoro substitution on
the 3-phenyl group (1b) has little effect on the absorption
maxima of the colored form (closed-open); however, it
induces increased thermal fading rate (compare entry 2 to
entry 1). para-Methoxy substitution results in much faster
thermal fading rate and much improved fatigue resistance,
albeit lower colorability (compare entry 3 to entry 1). Bromo
substitution on the naphthalene subunit inevitably leads to
much faster thermal fading rate and improved lifetime in
most cases (compare entry 6 to entry 1, entry 7 to entry 2,
and entry 8 to entry 3). Increased thermal fading could also
of 1 is the closed-closed form (closed-closed); upon UV
irradiation, one pyran unit opens to generate closed-open
forms (closed-open) which constitute the fast fading closed-
trans-cis isomer (closed-TC) and closed-trans-trans isomer
(closed-TT), which display yellow color. Under UV irradia-
tion at low temperature, the second pyran units undergo
opening to generate open-open forms (open-open, only one
isomer is shown), which display red color.
(13) Murov, S. L.; Carmichael, L.; Hug, G. L. Handbook of Photochem-
istry; Marcel Dekker: New York, 1993; pp 298-305.
(14) Colorability (A0) is defined as the absorbance at the photostationary
state under conditions of the experiment. The photochromic behavior of
compounds was studied in 1 cm light path quartz cells as 2.5 × 10-5 mol/L
toluene solutions (2.5 mL) at 25 °C under continuous irradiation with white
light (320-700 nm) from an 150 W ozone-free Xe lamp (the light was
passed through a water circulated heat trap and a C3C26 optical filter to
eliminate heat, infrared light, and UV light less than 320 nm. The output
of the white light was measured to be 1940 W/m-2 with an EPM-1000
power meter).
Using low-temperature chromatography, we could obtain
the pure monocolored closed-TC and closed-TT forms in
Org. Lett., Vol. 8, No. 1, 2006
101

be realized by changing linking sites from 1,4 to 1,3 (1 versus
1′ and 2 versus 2′); however, at the unexpected expense of
fatigue resistance. The improved photostability observed for
methoxy-substituted bispyrans is attributed to the low equi-
librium intensity of the colored forms. meta-Phenylene-linked
bispyrans displayed increased fading rate and thus a lower
colorability presumably due to the steric congestion in the
open forms. The unexpected observation of lower fatigue
resistance in this case is presumably attributed to the
unwanted interaction of the radical products generated in the
degradation process on the pyran fragment within the same
structure.15
Table 2. Colorability (A0), Absorption Maxima, Thermal
Bleaching Rate (k-1), and Fatigue Resistance (T1/2) of Bispyrans
3a-c and 3′a/3′b at Room Temperature in Toluene
entry
compd
A0/λmax (nm)
k-1 (s-1
)
T1/2 (min)
1
2
3
4
5
6
3a
3b
3c
3′a
3′b
7a
0.808/487
0.759/487
0.710/499
0.768/480
0.656/479
0.516/476
0.0062
0.0103
0.0260
0.0152
0.0190
0.0106
379b
486b
860b
215b
192
332b
a 2,2-Diphenyl-5-ethoxycarbonyl-6-phenyl-[2H]-naphtho [1,2-b]pyran.
b Estimated from 1 h measurement.
We then turned our attention to the identification of a
bispyran with higher colorability and improved fatigue
resistance. This could be accomplished by changing the pyran
portion from [3H]-naphtho-[2,1-b]pyran (as in 1a-c, 2a-
c, 1′a-b, and 2′a-b) to [2H]-naphtho-[1,2-b]pyran (3a-c
and 3′a-b) as shown in Scheme 1.16 For 3a-c and 3′a-b
the photochromic properties are collected in Table 2, and
2,2-diphenyl-5-ethoxycarbonyl-6-phenyl-[2H]-naphtho [1,2-
b]pyran (7) was used as the reference for comparison.12
improved fatigue resistance over bispyrans 1a-c and 2a-c
(Table 1, entries 1-3 and entries 6-8).
From Table 2, it could also be concluded that para-fluoro
substitutions on the 3-phenyl subunit induce increased
thermal fading rate (compare entry 2 to entry 1, and entry 5
to entry 4). para-Methoxy-substituted bispyran 3c displays
much faster thermal fading rate and much improved fatigue
resistance over 3a lacking substitution (compare entry 3 to
entry 1). These observations are similar to the cases for
bispyrans 1a-c and 2a-c in Table 1.
In conclusion, we have synthesized novel phenylene-linked
photochromic bispyrans displaying temperature-dependent
photochromism. Up to 60 nm bathochromic shift between
the colored species formed at room temperature and was
observed at low temperature of -20 °C. The high colorability
and excellent fatigue resistance of bispyrans 3a-c provide
new avenues for the design of the next generation of longer-
lasting, fatigue-resistant photochromic compounds.
From Table 2, it could be concluded that the colorabilty
of the bispyrans is higher than reference 7. The most notable
features for bispyrans 3a-c are that they display higher
colorability and better fatigue resistance than the reference
7 (compare entries 1-3 to entry 6), and 1,4-linked bispyran
3a/3b are more fatigue-resistant than 1,3-linked species 3′a/
3′b (compare entries 1 and 2 to entries 4 and 5). Bispyrans
3a-c (Table 2, entries 1-3) also display significantly
(15) (a) Strokach, Y.; Alfimov, M.; Barachevsky, V.; Arsenov, V.;
Gorelik, A. Mol. Cryst. Liq. Cryst. 1997, 298, 97-103. (b) Malatesta, V.
Photodegradation of Organic Photochromes. In Organic Photochromic and
Thermochromic Compounds; Crano, J. C., Guglielmetti, R., Eds.; Plenum
Press: New York, 1999; Vol. 2.
Acknowledgment. We thank the ETH-Z for support.
(16) (a) The fatigue resistance of photochromic compounds can be
dramatically affected by substituents: Coelho, P. J.; Carvalho, L. M.;
Abrantes, S.; Oliveira, M. M.; Oliveira-Campos, A. M. F.; Samat, A.;
Guglielmetti, R. Tetrahedron 2002, 58, 9505-9511. (b) It is known that
unsubstituted [2H]-naphtho-[1,2-b]pyrans have much lower fatigue resistance
than the [3H]-naphtho-[2,1-b]pyran due to the high abundance of the colored
forms for the former under irradiation. However, an alkoxycarbonyl
substituent at the 5-position of [2H]-naphtho-[1,2-b]pyrans is known to result
in much improved fatigue resistance while still providing sufficient
colorability. Such alkoxycarbonyl-substituted pyrans are commercially used
in plastic ophthalmic lenses: (c) Lin, J. WO 01/51483, 2001. (d) Van
Gemert, B.; Kish, D. G. U.S. Patent 5,753,146, 1998.
Supporting Information Available: Detailed synthesis
and characterization data for bispyrans and their precusors.
Isolation of the colored forms of the photochromic bispyrans,
kinetics of photocoloration and thermal fading, and measure-
ments for the fatigue resistance of photochromic compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OL052587Y
102
Org. Lett., Vol. 8, No. 1, 2006