Thermally reversible novel photochromic dihydroindoles

Article abstract of DOI:10.1002/poc.1265

Thermally reversible novel photochromic 4,5- and 6,7-dihydroindoles possessing a hexafluoropropano-bridge, a spiro-adamantane moiety and a phenyl group were synthesized and their photochemical forward as well as thermal back reactions were investigated. W

Full text of DOI:10.1002/poc.1265

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2007; 20: 851–856
Published online 17 September 2007 in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/poc.1265
Thermally reversible novel photochromic
dihydroindoles
*
Nobutaka Eguchi, Takashi Ubukata and Yasushi Yokoyama
Department of Advanced Materials Chemistry, Graduate School of Engineering, Yokohama National University, Japan
Received 10 January 2007; revised 25 June 2007; accepted 7 August 2007
ABSTRACT: Thermally reversible novel photochromic 4,5- and 6,7-dihydroindoles possessing a hexafluoropropano-
bridge, a spiro-adamantane moiety and a phenyl group were synthesized and their photochemical forward as well as
thermal back reactions were investigated. While the open coloured form of 4,5-dihydroindole had an absorption
maximum at 417 nm in toluene, that of 6,7-dihydroindole at 553 nm. Although the thermal back reaction of the former
took about 2 h at r.t., that of the latter completed within a few minutes. Copyright # 2007 John Wiley & Sons, Ltd.
KEYWORDS: photochromism; thermally reversible; dihydroindole; perfluorocyclopentene; adamantane
INTRODUCTION
isomers 7 was strong and fast and the decolouration was
reasonably quick.
Photochromic compounds1 are divided into two
categories. One is thermally irreversible, the other
thermally reversible. While the former includes only
limited families such as fulgides2 and diarylethenes,3 the
latter includes a number of classes such as spiropyrans4
and spirooxazines,4 naphthopyrans (chromens),5 azoben-
zenes,6 hexaarylbisimidazoles,7 stilbenes8 and so on.
Although some of the latter – spirooxazines and naph-
thopyrans – have already been used for the auto
light-regulating ophthalmic lenses,5 the number of
photochromic families used for practical applications is
so small.
In the previous papers, we introduced new therm-
ally reversible photochromic dihydrobenzothiophenes
(DHBTs)9,10 (1 and 2) and dihydronaphthalenes
(DHNPs)10 (3–6) with a hexafluoropropano-bridge, a
spiro-adamantane group and a phenyl group (Scheme 1).
They showed thermally reversible photochromism based
on 6p-electrocyclization (Scheme 2). However, the
decolouration rate was so slow for 1 and 2, and so fast
for 3–6 that the way of application of them would be
restricted. In this paper, the synthesis and photochromic
reactions of dihydroindoles (DHINs; 7 and 8) are
described. Both of them showed thermally reversible
photochromic reactions, and the colouration of one of the
RESULTS AND DISCUSSION
Molecular design
In the previous papers, we described the synthesis of
dihydroaromatic molecules which show photochromic
reactions. They can be produced as the open form
(O-form) possessing a 2,2,3,3,4,4-hexafluorocyclopen-
tene with an aromatic ring on its C-1 and the 1-
adamantylidene-1-phenylmethyl group on C-2. Upon
UV-light irradiation, it cyclizes to form the closed form
(C-form) by the 6p-electrocyclization, which, in order
to restore the aromaticity, changes to the rearranged
form (R-form) by the thermal 1,5-sigmatropic hydrogen
rearrangement. The thermally reversible photochromism
occurs between the R-form and the coloured quinodi-
methane form (Q-form). Irradiation of UV light to the
colourless R-form generates the coloured Q-form. The
Q-form goes back to the R-form thermally as well as by
visible-light irradiation. When the quantum yield of the
colouring reaction is large and back reaction is small, and
the thermal back reaction is quick, the compound would
show a strong colour under the sunlight which contains
both UV and visible lights, but it becomes colourless
quickly when the light is shaded.
*Correspondence to: Y. Yokoyama, Yokohama National University,
Department of Advanced Materials Chemistry, Graduate School of
Engineering, Yokohama, Japan.
We have already reported the synthesis of DHBTs 1
and 2 and DHNPs 3–6. The approximate time required for
E-mail: yyokoyam@ynu.ac.jp
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 851–856

852
N. EGUCHI, T. UBUKATA AND Y. YOKOYAMA
Table 1. Decolouration time (Q-form to R-form) at r.t.
1a
Decolouration time 2.5 h >6 h <1 s <1 s <1 s <1 s
2a
3b
4b
5b
6b
a In toluene solution.
b In amorphous polyolefin film.
Katritzky et al.11 reported the calculation-based ASE
of heteroaromatic compounds. Some of their data are
shown in Table 2.
There is a strong relationship that when the ASE is
smaller (thiophene: DHBT), the thermal back reaction
proceeds at a slow rate, and the ASE is larger (benzene:
DHNP), the thermal back reaction occurs instantaneously.
In order to obtain a thermally reversible photochromic
compound whose colour disappears within several
minutes or so while it exhibits certain coloration, we
have to look for an aromatic ring whose ASE is in
between them. We found that the ASE of pyrrole is
106.6 kJ molꢀ1 which is ideal for our purpose. Thus, we
decided to synthesize DHINs 7 and 8.
Scheme 1. Photochromism of dihydroaromatic compounds
Synthesis of 7 and 8
the Q-forms to change to the R-forms is listed in Table 1.
DHBTs take hours, while DHNPs take less than a second
at room temperature.
Synthetic pathways of 7 and 8 are shown in Scheme 3.
Regioselective bromination reactions of 1-methylpyr-
role (9) to form 3-bromo-1-methylpyrrole (10) and 2-
bromo-1-methylpyrrole (11) were carried out according
to the reported procedures by NBS in tetrahydrofuran
(THF) and by NBS with a catalytic amount of PBr3 in
THF, respectively.12 Lithiation of bromopyrroles 10 and
11 with butyllithium followed by the reaction with 1-
(1-adamantyliene-1-phenylmethyl)-2,3,3,4,4,5,5-heptafl-
uorocyclopentene9 afforded 7O and 8O in 12% and 11%
yield, respectively. Irradiation of 7O with 313-nm light in
cyclohexane or toluene gave, after 6p-electrocycliza-
tion followed by hydrogen rearrangement, 7R in 11%.
Similarly, irradiation of 8O with 313-nm light in toluene
gave 8R in 57% yield.
When the R-forms of DHBTs and DHNPs become their
Q-forms, the thiophene or benzene rings open to generate
quinodimethane-type structures. In this occasion, they
loose the aromaticity. It is easily assumed that when the
aromatic stabilization energy (ASE) of the R-form is
larger, the corresponding Q-form is more unstable. It
would make the activation energy of the thermal back
reaction smaller, and the decolouration takes place faster.
Photocolouration and thermal decolouration
of 7R and 8R
For the compounds 1R–6R, the colourless R-form
generates the coloured Q-form by UV irradiation, which
goes back to the R-form thermally. We envisioned the
similar reactions. Upon irradiation of 366-nm light at
24 8C, the toluene solution of 7R changed from colourless
to purple. The absorption maximum was 553 nm.
Apparently the coloured 7Q was formed. When the
coloured solution was left in the dark, the purple colour
disappeared completely within 5 min. The spectral
changes during UV irradiation and thermal decolouration
of 7 are shown in Figs 1 and 2, respectively.
Scheme 2. Photochromism of dihydrobenzothiophene
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 851–856
DOI: 10.1002/poc

NOVEL PHOTOCHROMIC DIHYDROINDOLES
Table 2. Aromatic stabilization energy11
853
Thiophene
93.6
Benzene
111.6
Pyrrole
106.6
Aromatic stabilization energy/kJ molꢀ1
Scheme 3. Synthesis of dihydroindoles
Similarly, the toluene solution of 8R was irradiated
with 313-nm light at 26.2 8C. The solution turned from
almost colourless to yellow, and the absorption maximum
of the new band ascribable to the formation of 8Q was
417 nm. Different from 7Q, the decolouration of 8Q took
much longer time. The change in absorption spectra of
coloration and decolouration of 8 are shown in Figs 3
and 4, respectively.
Although the absorption spectrum of 7 (7R) before UV
irradiation in Fig. 1 was identical with that after the
complete thermal back reaction from 7Q in Fig. 2 that of 8
after the thermal back reaction in Fig. 4 was not identical
with the spectrum before UV irradiation (Fig. 3). One of
the reasons should be the difference of the wavelength of
the irradiation energy. While the irradiation on 7R was
carried out with 366-nm light of mercury lamp, 313-nm
Figure 1. Change in absorption spectra of 7R in toluene at
24 8C (2.02 ꢁ 10ꢀ4 mol dmꢀ3). Irradiation time/ꢀs;2 0, 10, 20,
40, 60, 100, 200. Light intensity; 0.94 mW cm (366 nm).
Cell length; 1 cm
Figure 2. Change in absorption spectra of 7Q in toluene at
24 8C (2.02 ꢁ 10ꢀ4 mol dmꢀ3). Elapsed time/s; 0, 10, 20, 40,
60, 120, 300. Cell length; 1 cm
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 851–856
DOI: 10.1002/poc

854
N. EGUCHI, T. UBUKATA AND Y. YOKOYAMA
The structures of the Q-forms generated from 7R and
8R, which exhibit different decolouration rates, may be
the E/Z isomers of the benzylidene moiety attached to the
hexafluorocyclopentane ring of the Q-form. During the
photochemical ring opening of the R-form, the cyclohex-
adiene ring opens by way of the conrotatory mode, which
can give the geometrical isomers of the double bond.
However, it is not certain which isomer corresponds to the
faster-reacting one and the slower-reacting one.
The absorption maxima of the Q-forms of 4,5-dihydro
derivatives are shorter than that of 6,7-dihydro deriva-
tives. It is attributed to the longer conjugation of
6,7-dihydro derivatives as shown in Scheme 4. It is
reasonable that the absorption maximum of 7Q is longer
than that of 1Q because of the stronger electron donating
ability of the nitrogen atom than the sulphur atom.
However, it is interesting that the absorption maximum of
8Q is shorter than 2Q. It may be attributed to the steric
congestion caused by the methyl group on nitrogen atom
and the perfluorocyclopentene ring which would have
generated the torsion, that is, deviation from the planarity,
of the conjugation system.
In conclusion, we have synthesized new thermally
reversible photochromic 6,7- and 4,5-dihydroindoles (7R
and 8R, respectively) possessing a hexafluoropropano-
bridge, a spiro-adamantane moiety and a phenyl group.
While the coloured Q-form of 8R had an absorption
maximum at 417 nm in toluene, that of 7R at 553 nm. The
difference in the absorption maxima was explained by the
difference in the length of conjugation and the steric
congestion around the conjugation system. Although the
thermal back reaction of 8Q took more than 1 h at r.t. that
of 7Q completed within a few minutes.
Figure 3. Change in absorption spectrum of 8R in toluene
at 26 8C (1.00 ꢁ 10ꢀ4 mol dmꢀ3). Irradiation time/s; 0, 20,
60, 120, 2ꢀ420, 300, 420, 600, 960. Light intensity;
0.68 mW cm (313 nm). Cell length; 1 cm
light was used for 8R because 8R does not have a large
absorption at 366 nm. The use of 313-nm light might have
caused degradation of 8 to some extent to generate some
slightly coloured species.
In the case of DHBTs 1 and 2, the decolouration from
2Q to 2R (4,5-dihydrobenzothiophene)10 was slower than
that from 1Q to 1R (6,7-dihydrobenzothiophene).9 The
similar phenomenon occurred for 7 and 8. Namely,
8Q–8R (4,5-dihydroindole) was slower than 7Q–7R
(6,7-dihydroindole).
We then obtained the reaction rate constants of thermal
reactions of 7Q and 8Q at different temperatures. The
reaction kinetics was expressed by the sum of two
first-order reactions; a fast but less dominant (28–6%) one
and a slow and dominant (94–72%) one. The Arrhenius
activation energies (Ea) and frequency factors (A) of the
slower thermal decolouration reactions were then
calculated. The results are listed in Table 3, together
with the data of 1Q and 2Q. The data show that the
largeness of reaction rate constants of 7Q is basically
attributed to the small Ea. However, although Ea of 8Q is
smaller than that of 7Q, it is compensated by its much
smaller frequency factor.
EXPERIMENTAL
General
1H NMR spectra were recorded with JEOL JNM-EX-270
(270 MHz) spectrometer and JNM-AL-400 (400 MHz)
spectrometer in CDCl3. The signals are expressed as parts
per million down field from tetramethylsilane, used as an
internal standard (d value). Splitting patterns are indicated
as s, singlet; d, doublet; t, triplet; m, multiplet. IR spectra
were measured using a HORIBA FT-730 spectrometer.
Low- and high-resolution mass spectra were taken with a
JEOL JMS-AX-600 mass spectrometer. UV–Vis spectra
were recorded on a JASCO V-550 UV–Vis spectrometer
or a Shimadzu Multispec-1500 UV–Vis spectrometer. The
emission line of 313 nm of a 500 W high-pressure
mercury lamp (Ushio electric) was separated by filters
(5 cm water filter, a UV-D35 glass filter, 5 cm aqueous
NiSO4ꢂ6H2O solution, 1 cm aqueous K2CrO4–NaOH
solution and 1 cm aqueous potassium hydrogenphthalate
solution). The emission line of 366 nm of a 500 W
high-pressure mercury lamp (Ushio electric) was sepa-
Figure 4. Change in absorption spectra of 8Q in toluene at
26 8C (1.00 ꢁ 10ꢀ4 mol dmꢀ3). Elapsed time/s; 0, 60, 180,
300, 480, 600, 1200, 2400, 4800. Cell length; 1 cm
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 851–856
DOI: 10.1002/poc

NOVEL PHOTOCHROMIC DIHYDROINDOLES
855
Table 3. Absorption maximum and kinetic data of decolouration of Q-forms in toluene
lmax (nm)
500
T (K)
k (sꢀ1
)
Ea (kJ molꢀ1
)
A (sꢀ1
)
ꢀ4
ꢀ4
ꢀ4
ꢀ5
ꢀ4
ꢀ4
ꢀ2
ꢀ1
ꢀ1
ꢀ4
ꢀ3
ꢀ3
290
299
310
302
306
315
289
298
308
289
294
298
308
309
1.33 ꢁ 10
4.73 ꢁ 10
1.23 ꢁ 10
9.68 ꢁ 10
1.85 ꢁ 10
6.05 ꢁ 10
2.61 ꢁ 10
1.07 ꢁ 10
6.51 ꢁ 10
7.49 ꢁ 10
1.39 ꢁ 10
3.23 ꢁ 10
1Q
2Q
7Q
84.0
1.96 ꢁ 1011
2.50 ꢁ 1014
9.93 ꢁ 1011 b
468
106.3
ꢀ3
ꢀ2
ꢀ2
ꢀ5
ꢀ4
ꢀ4
ꢀ4
ꢀ4
4.18 ꢁ 10
1.17 ꢁ 10
3.38 ꢁ 10
9.86 ꢁ 10
1.66 ꢁ 10
3.18 ꢁ 10
5.72 ꢁ 10
7.91 ꢁ 10
537
79.5b
8Q
417
73.1b
2.40 ꢁ 109 b
a
—
9.27 ꢁ 10ꢀ3
a Not observed.
b Values for the slower reactions.
rated by filters (5 cm water filter, 5 cm aqueous CuSO4
solution, a UV-35 glass filter and a UV-D35 glass filter).
The silica-gel column chromatographic separation was
carried out with a Merck Kieselgel 60 (230–400 mesh)
with ethyl acetate and hexane as an eluant. The alumina
column chromatographic separation was carried out with
a Merck Aluminium oxide 90 active basic (70–230 mesh)
with ethyl acetate and hexane as an eluant. Analytical
thin-layer chromatography was performed on Merck
pre-coated silica gel 60 F-254, 0.25 mm thick TLC plates.
All of the synthetic reactions were carried out under a dry
nitrogen atmosphere. THF was freshly distilled from
benzophenone ketyl.
with saturated brine, and dried with anhydrous sodium
sulphate. After the drying agent was filtered off, the
solvent was removed in vacuo. The residue was purified
by silica-gel column chromatography to give 7O
(78.9 mg, 12%). Mp 145–147 8C. IR (KBr) n cmꢀ1
:
3133, 3081, 2948, 2929, 2914, 2853, 1635, 1610, 1450,
1427, 1366, 1337, 1303, 1277. 1H NMR (270 MHz,
CDCl3) 1.5–2.2 (12H, m), 2.59 (1H, s), 3.09 (1H, s), 3.69
(3H, s), 6.62 (1H, dd, J/Hz ¼ 2.6, 2.0), 6.81 (1H, dd, J/
Hz ¼ 2.6, 2.0), 7.2–7.3 (6H, m). MS (EI, 70 eV) m/z 477
(Mþ, 100), 356 (11). Found: m/z 477.18735. Calcd
for C27H25F6N: M, 477.18910
Synthesis of 1-(adamantylidene-
Synthesis of
1-phenylmehyl)-3,3,4,4,5,5,-hexafluoro-2-
(1-methyl-3-pyrryl)cyclopentene (7O)
1-(adamantylidene-1-phenylmehyl)-
3,3,4,4,5,5,-hexafluoro-
2-(1-mehyl-2-pyrryl)cyclopentene (8O)
To a solution of 3-bromo-1-methylpyrrole (10,12 213 mg,
1.33 mmol, 5.3 eq.) in 10 ml THF at ꢀ60 8C was added
TMEDA (0.25 ml, 1.68 mmol, 6.7 eq.) and a hexane
To a solution of 2-bromo-1-methylpyrrole (11,12 696 mg,
4.35 mmol, 4.6 eq.) in 50 ml THF at ꢀ73 8C was added a
hexane solution of butyllithium (1.60 mol dmꢀ3, 3.3 ml,
5.3 mmol, 5.6 eq.), and the mixture was stirred for 60 min
at that temperature. This mixture was added dropwise to a
THF (6.4 ml) solution of 12 (395 mg, 0.949 mmol, 1 eq.)
at ꢀ73 8C, and the resulting mixture was kept stirring over
night. After the reaction was quenched with water, the
mixture was extracted with ethyl acetate three times. The
organic layer was washed with saturated brine, and dried
with anhydrous sodium sulphate. After the drying agent
was filtered off, the solvent was removed in vacuo. The
residue was purified by silica-gel column chromatog-
raphy to give 8O (234 mg, 11%). Mp 107–110 8C. IR
(KBr) n cmꢀ1: 3084, 3024, 2938, 2883, 2854, 1612, 1599,
1374, 1281. 1H NMR (400 MHz, CDCl3) d 1.7–2.0 (12H,
m), 2.63 (2H, br s), 3.25 (3H, s), 6.14 (1H, dd, J/Hz ¼ 3.3,
2.6), 6.35 (1H, dd, J/Hz ¼ 3.3, 2.0), 6.67 (1H, dd, J/
Hz ¼ 2.6, 2.0), 6.96 (2H, m), 7.2 (3H, m). MS (EI, 70 eV)
solution of butyllithium (1.52 mol dmꢀ3
,
1.1 ml,
1.67 mmol, 6.7 eq.), and the mixture was stirred for
90 min at that temperature. This mixture was added
dropwise to a THF (4.0 ml) solution of 12 (104 mg,
0.250 mmol, 1 eq.) at ꢀ60 8C, and the resulting mixture
was kept stirring over night. After the reaction was
quenched with water, the mixture was extracted with
ethyl acetate three times. The organic layer was washed
Scheme 4. Conjugation in Q-forms.
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 851–856
DOI: 10.1002/poc

856
N. EGUCHI, T. UBUKATA AND Y. YOKOYAMA
m/z 477 (Mþ, 100), 223 (54). Found: m/z 477.18777.
Calcd for C27H25F6N: M, 477.18910.
coupling)), 7.11 (2H, t, J/Hz ¼ 7.3), 7.20 (1H, t, J/
Hz ¼ 7.3). MS (EI, 70 eV) m/z 477 (Mþ, 6), 476 (30), 475
(100), 352 (37). Found: m/z 477.19080. Calcd
for C27H25F6N: M, 477.18910.
Synthesis of 4(,5(-Hexafluoropropano-
1-methyl-6(-phenylspiro[adamantine-
2,7((6(H)-indole] (7R)
Acknowledgements
A solution of 7O (30.7 mg, 0.064 mmol) in cyclohexane
(ca 80 ml) was irradiated with 313-nm light for 21 h.
Similarly, a solution of 7O (51.2 mg, 0.107 mmol) in
toluene (ca 80 ml) was irradiated with 313-nm light for
25 h. Each solution was kept in the dark for 1 day, and the
solvent was removed in vacuo. The residue was combined
and purified by aluminium oxide column chromatog-
raphy, to give 7R (8.74 mg, 11%). Mp 190–194 8C. IR
(KBr) n cmꢀ1: 3001, 2946, 2926, 2912, 2856, 1452, 1394,
1358, 1316, 1249. 1H NMR (270 MHz, CDCl3) d 1.3–2.5
(13H, m), 2.54 (1H, d, J/Hz ¼ 13.2), 3.45 (3H, s), 4.44
(1H, d, J/Hz ¼ 4 .0 (H–F coupling)), 6.40 (1H, dd, J/
Hz ¼ 3.3, 1.3 (H—F coupling)), 6.57 (1H, d, J/Hz ¼ 3.3),
6.64 (2H, d, J/Hz ¼ 7.3), 7.12 (2H, t, J/Hz ¼ 7.3), 7.19
(1H, t, J/Hz ¼ 7.3). MS (EI, 70 eV) m/z 477 (Mþ, 100),
386 (66), 330 (22). Found: m/z 477.19061. Calcd
for C27H25F6N: M, 477.18910.
This work was supported by a Grant-in-Aid for Scientific
Research on Priority Areas (417) from the Ministry of
Education, Culture, Sports, Science and Technology
(MEXT) of the Japanese Government. The authors thank
Zeon Corp. for the generous gift of octafluorocyclopen-
tene. The Instrumental Analysis Center, Yokohama
National University, is also acknowledged for the use
of NMR and mass spectrometers.
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Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 851–856
DOI: 10.1002/poc