A new photochromic 8π-system based on an azaheptatriene-tetrahydroazepinoisoquinoline electrocyclization

Article abstract of DOI:10.1021/jo000807+

A new photochromic family of tetrahydroazepinoisoquinolines (THAI) 4a-i has been prepared. It undergoes light-induced ring opening from spiro compounds 4 to betaines 3 that decolorize in a very fast cyclizing reaction. Depending on substitution of the precursors 1 and 6, the photochromic styryltetrahydroindolizines (THI) 5k-q are formed in a periselective way. The conformation and configuration of the new photochromic THAI 4 and THI 5 were investigated by NMR and the structure of both compounds was proven by X-ray analysis. The photochromic properties were studied by laser flash photolysis, which afforded the lifetime of the colored form 3 and 3′ in the micro- or nanosecond range.

Full text of DOI:10.1021/jo000807+

1130
J . Org. Chem. 2001, 66, 1130-1137
A New P h otoch r om ic 8π-System Ba sed on a n
Aza h ep ta tr ien e-Tetr a h yd r oa zep in oisoqu in olin e Electr ocycliza tion
Yongsheng Tan,† Thomas Hartmann,† Volker Huch,† Heinz Du¨rr,*,† Pierre Valat,‡
Veronique Wintgens,‡ and J ean Kossanyi‡
FB 11.2 Organische Chemie, Universita¨t des Saarlandes, D-66041 Saarbru¨cken, Germany, and
Laboratoire des mate´riaux mole´culaires, CNRS, 2-8 Rue Henri Dunant, 94320 Thiais, France
ch12hd@rz.uni-sb.de
Received J uly 28, 2000
A new photochromic family of tetrahydroazepinoisoquinolines (THAI) 4a -i has been prepared. It
undergoes light-induced ring opening from spiro compounds 4 to betaines 3 that decolorize in a
very fast cyclizing reaction. Depending on substitution of the precursors 1 and 6, the photochromic
styryltetrahydroindolizines (THI) 5k -q are formed in a periselective way. The conformation and
configuration of the new photochromic THAI 4 and THI 5 were investigated by NMR and the
structure of both compounds was proven by X-ray analysis. The photochromic properties were
studied by laser flash photolysis, which afforded the lifetime of the colored form 3 and 3′ in the
micro- or nanosecond range.
In tr od u ction
dolizines1,4,11,12 have been shown to be very efficient and
extensively adjustable photochromes, which cover a vast
domain of half-lives of the colored form, spanning from
10-8 s to infinite. The latter also show extreme flexibility
with regard to many photochromic properties.
However, reports on 8π-seven-atom photochromic sys-
tems (4n) are rare.1,4 Only very few molecules have been
reported, and the reversibility of the processes involved
has not always been established.13
Extending our work on dihydroindolizines, we discov-
ered recently a new photochromic system, the aza-
heptatrienes THAI 4 which constitute a new class of
molecules with interesting properties.
In this paper, we report on the synthesis of a novel
reversible photochromic system the aza-heptatriene 3/tet-
rahydroazepinoisoquinoline (THAI) 4, as well as on the
aza-heptatriene 3/tetrahydroisoquinoline (THI) 5, both
4 and 5 formed in a pericyclic process. We further present
the structure elucidation of 4 and 5 by X-ray analysis
and the photophysical properties of systems 3 S 4 and 3
S 5, respectively.
In the recent past photochromic materials steadily
gained increasing interest since their commercial ap-
plication in ophthalmological lenses.1-3 These 6π-systems
based on an electrocyclic ring-opening/ring-closure pro-
cess are the most powerful and widespread basis for
photochromic molecules.1-5 Photochromes can be used
also in many other domains such as actinometry1-3 and
information recording and storage,1-4 in light switches,2-4
in holography,6,7 or in light switchable enzymes8 to
mention just a few applications. For ophthalmic lenses
the spiroxazines and chromenes have become the most
attractive candidates.9,10 The colorization/decolorization
cycles in these molecules are based on a 6π-six-atom
electrocyclic ring-closure/ring-opening process.1,9,10
The related 4π-three-atom systems as the aziridines1
are interesting photochromics but with limited applica-
tions, while the 6π-five-atom systems, the dihydroin-
* To whom correspondence should be addressed.
† Universita¨t des Saarlandes.
‡ CNRS.
Resu lts
(1) Du¨rr, H. In Photochromism - Molecules and Systems; Du¨rr, H.,
Bouas-Laurent, H., Eds.; Elsevier: Amsterdam, 1990; p 210 ff. and p
510 ff.
(2) Guglielmetti, R. In Photochromism - Molecules and Systems;
Du¨rr, H., Bouas-Laurent, H., Eds.; Elsevier: Amsterdam, 1990; p 314
ff.
(3) McArdle, C. B. Applied photochromic polymer systems; Chapman
and Hall, New York, 1991.
(4) Du¨rr, H. In Organic photochromic and thermochromic com-
pounds Vol.1; Crano, J . C., Guglielmetti, R. J ., Eds.; Plenum Press:
New York, 1998; 223 ff.
Syn th esis. The synthesis of tetrahydroazepinoiso-
quinoline (THAI 4a -i) was envisaged from spirocyclo-
propene 1 and 1-(4-substituted-styryl)-3,4-dihydroiso-
quinolines 2 as precursors. By adding a styryl link to the
1-position of the quinoline base, an extended conjugated
betaine 3 for the known THI1,4,14 was expected to be
formed.
(5) Du¨rr, H. In Organic Photochemistry and Photobiology; Horspoole,
W. M., Song, P.-S., Eds.; CRC Press: 1995; pp 1121-1140.
(6) Zelichanok, A.; Buchholz, F.; Fischer, E.; Katur, E.; Krongausz,
V.; Ameser, H.; Bra¨uchle,C. J . Photochem. Photobiol. 1993, 76, 135.
(7) Dromikov, A. S.; Rentzepis, P. M. Mol. Cryst. Liq. Crist. 1994,
246, 379.
(8) Willner, I.; Rubin, S. Angew. Chem. 1996, 108, 419-439.
(9) Bertelson, R. C. In Organic photochromic and thermochromic
compounds Vol.1; Crano, J . C., Guglielmetti, R. J ., Eds.; Plenum
Press: New York, 1998; pp 11-83.
(10) Maeda, S. In Organic photochromic and thermochromic com-
pounds Vol.1; Crano, J . C., Guglielmetti, R. J ., Eds.; Plenum Press:
New York, 1998; pp 85-107.
The synthesis of precursor 2 was carried out according
to Bischler-Napieralski15 procedure by cyclization of
(11) Weber, C.; Rustemeyer, F.; Du¨rr, H. Adv. Mater. 1998, 10(16),
1348-1351.
(12) Andreis, C.; Du¨rr, H.; Wintgens, V.; Valat, P.; Kossanyi, J .
Chem. Eur. J . 1997, 3(4), 509.
(13) Du¨rr, H. Angew. Chem. 1989, 101, 427-445; Angew Chem., Int.
Ed. 1989, 28, 413-438.
(14) Du¨rr, H.; Gross, H. Angew. Chem. 1982, 94, 204.
(15) Bischler, A.; Napiralski, B. Ber. 1893, 26, 1903. Tietze, L. F.;
Eicher, T. Reaktionen und Synthesen; Thieme: Stuttart, 1981; p 319.
10.1021/jo000807+ CCC: $20.00 © 2001 American Chemical Society
Published on Web 02/01/2001

New Photochromic 8π-System
J . Org. Chem., Vol. 66, No. 4, 2001 1131
Ta ble 1. Su bstitu en t P a tter n of THAI 4a -i a n d THI
5k -q, Meltin g P oin ts, a n d Rea ction Yield s
Sch em e 1. P r ep a r a tion of Dih yd r oisoqu in olin e
P r ecu r sor s 2
mp
(°C)
yield
(%)
compd R1
R2
R3
R4
E
4a
4b
4c
4d
4e
4f
4g
4h
4i
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Cl
Cl
CO2CH3
CO2C2H5
CO2CH3
CO2C2H5
148-149 65
167-169 71
138-140 50
168-170 60
152-154 71
150-152 60
128-130 70
140-142 57
129-131 53
H
H
H
H
H
H
H
H
NO2 CO2CH3
NO2 CO2C2H5
CH3 CO2CH3
CO2CH3 Cl
H
CO2CH3
CO2C7H11
(norbornyl)
H
5k
5l
5m Br
5n
5o
5p
5q
Cl
Cl
Cl
Cl
Br
Br
Br
H
H
Cl
H
H
H
H
H
H
H
H
Cl
Cl
H
CO2Et
CO2Me
CO2Me
CO2Me
CO2Et
CO2Me
CO2Me
196-198 29
211-213 36
213-215 66
231-232 75
204-206 46
149-150 54
219-220 64
Br
Br
NO2
NO2 NO2
H
should be shifted to 6.0 and 6.5 ppm with coupling
constants of about 16 Hz, whereas the signals found for
THAI 4 appear around 4.3 ppm (5′-H) and 6.2 ppm (6′-
H) and both show coupling constants of about 7.0 Hz.
They result from the azepine allylic proton 5′ and the
olefinic 6′ proton.
Thus, a five-membered ring THI structure can be
excluded and the seven-membered ring structure of 4 is
supported.
N-phenethylacetamide16 that resulted in 1-methyl-6,7-
dimethoxy-3,4-dihydroisoquinoline, which gave the cor-
responding acetophenone derivative 2′′after hydrolysis in
diluted hydrochloric acid and addition of potassium
carbonate.
For the norbonylester THAI derivative 4i a chiral
center exists in the CB position of the norbonyl group (see
the Experimental Section); hence, two diastereomers are
possible in the R and S configurations. The presence of
two isomers for the norbonyl derivative is deduced from
the 1H NMR spectra, for example from the 7′ and 10′
protons, which appear as doublets (usually singlets)
around 7.1 and 6.6 ppm, respectively.
The two configurations can also be verified from the
13C NMR spectrum. The signal of the spiro carbon around
63.5 ppm is doubled just as the carbonyl carbon atoms
that arise from the 2′ and 3′ positions. The two signals
of the carbonyl methyl and ethylester derivatives at 167.5
and 167.9 ppm, respectively, split in two signals between
166.8 and 167.1 ppm.
All structures were characterized by mass spectros-
copy, elemental analysis, and IR spectroscopy. The
postulated structures 4a -i are additionally based on
homonuclear 1H,1H-COSY and 13C,1H-COSY experi-
ments.
A table of the characteristic NMR data can be found
in the Supporting Information for compounds 4a -i.
Diastereoisomers of THAI 4h and THI 5l,p ,i with R
and S configurations are due to the unsymmetrically
substituted fluorene rings. An isolation of these diaster-
eomers was found impossible due to their light sensitiv-
ity. As the azepine ring and the pyrrole ring open to a
transoid betaine form 3 the thermal back reaction to the
closed form 4 yields always a mixture of diastereomers.
The stereochemistry given for the phenyl group in
position 5′ (Scheme 2) is clearly deduced from the X-ray
structure of 4z.
Subsequent aldol condensation with benzaldehyde
derivatives gave the R,â-unsaturated ketones 2′ (see
Scheme 1), which cyclized in boiling diluted hydrochloric
acid solution into 1-(4′-styryl)-3,4-dihydroisoquinoline17
2. New p-nitro and p-methyl derivatives could be syn-
thesized according to this method.
All 1-(4′-substituted-styryl)-3,4-dihydroisoquinolines 2
were quite unstable except the p-nitrostyryl-substituted
compounds 2 (R ) NO2). Therefore, the former have been
stabilized as HCl salts and were liberated from their
hydrochlorides in situ with triethylamine.
Syn th esis of Tetr a h yd r oa zep in oisoqu in olin e Es-
ter s (THAI). Reacting spirocyclopropene 1 with 1-styryl-
phenyldihydroisoquinoline 2 at ambient temperature (dry
diethyl ether) leads to the THAI 4 via the betaine
intermediate 3′. Compounds 4a -i could be isolated in
moderate to good yields (see Scheme 1, Table 1).
In this reaction the nitrogen of the N-heterocycle adds
nucleophilically to the cyclopropene ring of 1, which opens
via a cyclopropyl-allyl conversion to intermediate A and
leads in an electrocyclic reaction to the betaine form 3
and finally to the product 4.
The NMR experiments carried out for the 7 ring THAIs
4 show a characteristic signal pattern. In all compounds
four complex multiplets can be assigned to the methylene
protons 11′, 12′ of the dihydroisoquinoline ring. They
appear in the range from 2.6 to 4.5 ppm. Since these two
methylene groups appear as two distinct triplets in the
starting material 2, it indicates that, in the THAI
molecules 4, they are no longer magnetically equivalent.
This can be attributed to a distortion of the ring to a half-
boat form (see also the crystal structure).
The diastereomeric ratio is about 1:1 in the mixtures
for 4h and THI 5l,p ,i as concluded from the NMR data.
This is in contrast to previous findings, where the five-
membered ring formation is normally diastereoselec-
tive.1,4 The diastereoisomeric ratio of the THAI 4h /THI
5l,p ,i are listed in the Supporting Information.
For styryl-substituted THI 5-rings two doublets are
observed for each proton of the trans styryl group. They
(16) Orito, K.; Matsuzaki, T.; Suginome, H. Heterocycles 1988,
27(10), 2403.
(17) Brossi, A.; Dolan, L. A.; Teitel, S. Org. Synth. 1977, 56, 3.

1132 J . Org. Chem., Vol. 66, No. 4, 2001
Sch em e 2. Syn th esis a n d Rea ction P a th w a y of Ester THAI 4 a n d THI 5
Tan et al.
Syn th esis of Tetr ah ydr oisoqu in olin e Ester s (THI).
The synthesis of the ester tetrahydroindolizines (THI, five
rings) was carried out as described for the synthesis of
the THAI esters 4 (seven rings). Only substitution in the
2,7-position of the fluorene ring afforded the formation
of the styryl-substituted five-ring THI 5 periselectively
(see Scheme 2, Table 2).
The structure of these new 1-styryl-tetrahydroindoliz-
ines (THI) was established by NMR, mass spectra, and
IR.
The NMR analysis of the five-ring THI gave a quite
different signal pattern in comparison to the seven-ring
THAI. All H NMR spectra of product THI 4r -z show
also four complex multiplets between 2.60 and 3.80 ppm
for the methylene groups in positions 5′ and 6′, but they
are shifted about 0.2-0.7 ppm to lower field. The signals
in the olefinic part of the spectrum can be assigned to
the 10′ und 7′-H of the isoquinoline phenyl ring.
The most characteristic signals are the two doublets
in the THIs of the styryl double bond at 6.10 (5l)-6.30
1

New Photochromic 8π-System
J . Org. Chem., Vol. 66, No. 4, 2001 1133
The bond angles of the pyrrole ester moiety in the THI
system show only slight distortion and exhibit, with
minor deviations of about 3°-4° for C(10)-C(11)-C(12),
C(1)-C(10)-C(11), and C(12)-C(1)-N(1), the same bond
angles as a dihydropyrrole ring that was calculated by
molecular mechanics. All further characteristics and
important structural data are listed in the Supporting
Information.
Syn th esis of Dicya n otetr a h yd r oa zep in oisoqu in o-
lin es (THAI). Cyano-substitued spiro-cyclopropenes,
which could serve as precursors, are not stable5 at room
temperature; therefore, spiropyrazoles 6 are photolyzed
in the presence of the 1-styryldihydroisoquinolines 2.13,14,18
The photolyses were carried out at room temperature
with λ > 290 nm (125 W Philips HPK, Hg-lamp, Pyrex
filter) in absolute diethyl ether. After some time, a
turquoise blue color was obtained, which disappeared
again when the reaction was finished.
F igu r e 1. X-ray structure of 5n .
Ta ble 2. Su bstitu en t P a tter n , Meltin g P oin ts a n d
Removal of the solvent and subsequent column chro-
matography of the reaction mixture on silica gel (in the
dark) affords the pure dicyanotetrahydroazepinoiso-
quinolines 4r -z in moderate yield (see Scheme 3).
Irradiation of the spiro dicyanopyrazole 6 generated a
vinylcarbene. The nitrogen of the isoquinoline attacks it
as a nucleophile to form a ring open colored betaine 3,
that is able to undergo 1,7-electrocyclization to the
desired products 4. The structures of these new tetrahy-
droazepinoisoquinolines 4r -z (THAI) have been estab-
lished by NMR, mass spectra, elemental analysis, and
Rea ction Yield s for THAI 4r -z
mp
(°C)
yield
(%)
compd
R1
R2
R3
R4
4r
4s
4t
4u
4v
4w
4x
4y
4z
H
H
H
Cl
Cl
Cl
Br
Br
Br
H
H
H
Cl
Cl
Cl
Br
Br
Br
H
H
H
H
H
H
H
H
H
H
Cl
CH3
H
Cl
CH3
H
Cl
180-182
174-176
176-178
186-188
171-173
177-179
203-205
173-175
180-182
25
38
32
28
22
33
40
33
26
CH3
1
IR. The H NMR and 13C NMR spectra of the dicyano-
substituted THAI show the same characteristic pattern
as their ester derivatives.
For 4z, an X-ray analysis was carried out that led to
the structure shown (see Figure 2).
ppm (5p ) and 6.44-6.49 ppm, respectively. They show
coupling constants of 3J ) 15.5-16.0 Hz, which correlate
very well with the literature.17 This is a major indication
for the existence of the styryl-five ring (see Table 8 for
THIs 5k -q in the Supporting Information). In contrast
to these findings, the THAI 4 show signals due to the
olefinic 5’-H and 6’-H azepine bonds in the region 4.32
(4w )-4.41 ppm (4r ) and 6.09-6.26 ppm (see Table 10
for THAIs 4r -z, Supporting Information). Compounds
4h and 5l,p ,i can exist as diastereomers (Tables 8 and
9, Supporting Information).
The final configuration and structure were additionally
proven by homonuclear 1H,1H-COSY and 13C,1H-COSY
experiments. The final structure proof of the 10′b-styryl-
THI 5 was also obtained by an X-ray analysis, which has
been performed on crystals of the 2,7 dibromofluorene
methyl ester 5n (Figure 1). The crystal structure corre-
sponds very well with the conformation and configuration
derived from NMR.
The X-ray analysis of the 2,7-dibromofluorene THAI
4z permits a definite and clear structural assignment.
As given in Figure 2, the THAI molecule 4z can be
divided into three parts A, B, and C consisting of fluorene,
dihydroazepine, and isoquinoline parts. In part B, the
existence of the seven-membered ring is clearly evident.
Bond distances, especially in the azepine ring, are of
interest. All carbon atoms C(2)-C(5) in the 7 ring are
incorporated as single bonds and have bond distances of
1.487-1.513 Å. The C(6)-N(1) shows a single bond,
which is slightly shorter however with only 1.418 Å.
The C(1)-C(2)-bond and C(5)-C(6) bond with 1.316
and 1.295 Å respectively have double bond character in
agreement with the NMR data. The short N(1)-C(1) bond
with 1.365 Å results probably from a partially delocalized
imine/ enamine structure.
All three regions A, B, and C1,13,18 of the THI can be
clearly assigned. The five-ring ester groups are perpen-
dicular to the fluorene embedded in the dihydroisoquino-
line part. The bond lengths of the pyrrolidine ring show
single bonds for C(11)-C(12), C(12)-C(1) with bond
distances of 1.520 and 1.626 Å respectively, and 1.476 Å
for C(1)-N(1).
The C(12)-C(1) bond is unusually long, having 1.63 Å
for a single bond, and hence is relatively weak; therefore,
it is easily cleaved by UV light, leading to ring opening
and to the formation of betaine 3. The 1.360 Å for the
C(10)-C(11) dihydropyrrole double bond in 5d is also
consistent with the NMR data.
From the bond angles it follows, that the dihy-
droazepine ring exists in a slightly distorted half tub
conformation. While these bond angles of 122.5° C(4)-
C(5)-C(6) and C(5)-C(6)-N(1) exhibit standard values,
the angles of N(1)-C(1)-C(2) and C(1)-C(2)-C(3) with
130° are distorted by about 10°. Both the spiro-bonded
carbon C(3) as well as carbon C(4) exhibit an sp3
configuration with bond angles of 109.8° for C(2)-C(3)-
C(4) and 111.1° for C(3)-C(4)-C(5).
The bond angle of C(6)-N(1)-C(1) matches with 119.7°
for a classical sp2 carbon and, hence, indicates a partially
delocalized polymethine system the C(2) cyano-group of
the THI system. The cyano bond C(2)-C(18) is shortened
by about 0.06 Å as compared to the C(1)-C(17) bond, and
the C(18)-N(3) bond is 0.05 Å longer than the 1.12 Å
C(17)-N(2) bond. Thus, the bond lengths indicate double
(18) Du¨rr, H.; Kranz, C.; Schulz, C.; Kilburg, H.; J o¨nsson, H. P. Proc.
Indian Acad. Sci. 1995, 107, 645-658.

1134 J . Org. Chem., Vol. 66, No. 4, 2001
Sch em e 3. Syn th esis a n d Rea ction P a th w a y of Dicya n o THAI 4
Tan et al.
shorter than the longest bond (C(2)-C(3)) of the azepine
system and even 0.12A shorter in comparison with the
corresponding C(12)-C(1) bond, which is opening in five-
membered ring THI 5n .
From the differences in bond length of the THAI 4z
(C(3)-C(4)) and THI 5n (C(12)-C(1)) systems, conclu-
sions concerning the efficiency and the facility of the ring
opening reaction may be drawn which correspond with
quantum yield determinations. Further characteristic
and important data of 4z are listed in the Supporting
Information.
Discu ssion
The nucleophilic addition of styryldihydroisoquinoline
2 to the spirocyclopropene 1, via the betaine 3, forms a
conjugated azapentadienyl system. The ring-opened be-
taine 3 has basically two possibilities to close, either via
the well-known 1,5-electrocyclization (4n + 2π ) 6
electrons) to form tetrahydroindolizines (THI) 5k -q or
by including the conjugated styryl group to undergo a
F igu r e 2. X-ray structure of 4z.
bond tendencies for C(2)-C(18) and C(18)-N(3), which
supports also the polymethine character.
The C(3)-C(4) bond with 1.504 Å is breaking in the
photochromic process of the THAI 4z. It is about 0.01 Å

New Photochromic 8π-System
J . Org. Chem., Vol. 66, No. 4, 2001 1135
Ta ble 3. Sp ectr oscop ic Da ta of THAI 4 a n d THI 5 in
CH2Cl2 (c ) 3 × 10-4 m ol/L) a t Room Tem p er a tu r e
UV/vis
fluorescence
transient lifetime
λmax
λ(exc) λ(em) φFl
λmax
(nm)
τa
(µs)
compd (nm) log ꢀ (nm) (nm) (10-4
)
4a
4b
4c
4d
4e
4f
4g
4h
4i
311
311
310
311
310
310
311
311
314
327
327
328
328
329
333
337
310
314
317
310
314
315
311
314
315
4.10
3.95
4.06
4.01
4.04
4.08
3.99
3.93
3.98
4.10
3.98
3.98
4.11
4.09
4.07
4.04
4.03
3.95
3.88
4.10
3.96
3.97
4.11
3.89
4.13
339
340
341
340
338
338
337
341
336
400
338
335
415
333
415
415
338
336
336
334
336
336
343
338
336
431
430
445
445
431
430
425
439
422
456
487
491
520
495
510
495
410
405
403
389
406
402
403
420
405
35.4
15.4
10.5
11.4
6.5
-/-
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
0.37
0.28
0.21
0.28
0.22
0.12
0.66
105
95
150
260
215
230
235
260
250
-/-
-/-
-/-
-/-
7.5
-/-
49.1
59.3
15.7
19.6
6.1
9.1
11.6
7.5
6.9
7.8
17.3
4.4
-/-
2.5
6.5
5.3
5.9
1.9
2.3
-/-
-/-
-/-
5k
5l
490/720
5m
5n
5o
5p
5q
4r
490/720
F igu r e 3. Transient absorption spectra of compound 4r in
dichloromethane solution excited at 347.5 nm with a ruby laser
and analyzed at different times.
460/700
480/740
500/680
As the 3 f 4 cyclization is very fast, the UV/vis
maxima of 3 could only be established clearly by laser
flash photolysis measurements. The photophysical data
obtained by time-resolved transient absorption spectros-
copy are summarized in Table 3. Two maxima could be
detected (measured by ns excitation Nd:YAG-laser flash
photolysis20) in the transient absorption spectra of com-
pounds 3r -z (CN-substitution), which range between 500
and 680 nm. The maxima of these transients were about
40 nm bathochromically shifted for the ester THI’s 5k -q
relative to the spectra obtained in the case of 4.
A typical example is compound 4r . The transient
spectrum obtained in dichloromethane solution by excita-
tion of compound 4r at 347 nm with a ruby laser is
depicted in Figure 3. As seen in Figure 3, the spectrum
evolves with time. The change in the transient absorption
is better visualized by examining the spectral change at
different wavelengths. The shorter lifetime from the
biexponential decay at 660 nm, that is 14 µs, can be found
again in the growing-in of the transients at 410 and 780
nm (Figure 4). As this change occurs on the two sides of
the transient absorption spectrum it is indicative of an
overlap of two different species absorbing in the same
region. These two species should correspond to the Z and
E forms of the betaine, the latter (3r ) being more
stable.12,21,22
4s
4t
4u
4v
4w
4x
4y
4z
500/680
a
For comparison τ1/2 ) ln 2 × τ (see ref 1).
1,7-electrocyclization to THAI 4 (4n ) 8 electron process).
For the THAI 4a -i and 4r -z exclusively a 1,7-electro-
cyclization was observed.
This corresponds very well with the theory for electro-
cyclic reactions where it is always the longest conjugated
system which reacts, which leads here to the seven-
membered ring (see in detail in ref 19). The THI 5 are
formed in 1,5-electrocyclization, where only part of the
conjugated system reacts. The formation of THAI 4 is
therefore a periselective reaction.19
The 1,7-electrocyclization is, however, disfavored when
acceptor groups are present such as both 2,7-halogen-
fluorenes and dicarboxylic ester as in the case of com-
pounds 3. Due to some steric hindrance but more likely
to electronic effects the esters 3k -q cyclize periselectively
to five rings and thus form the THI 5.
The comparison to the dicyano-substituted betaines
3r -z shows that the 2,7-fluorene substitution does not
lead to 4r -z by 1,5 cyclization. The strong electron
withdrawing cyano groups seem to stabilize the positive
charge on the styryl group providing in all cases 4 by a
periselective 1,7-electrocyclization reaction. Here a reso-
nance formula 3′ (positively charged in position 7) favors
the 1,7-electrocyclization.
P h otop h ysica l P r op er ties. Both the new THAI 4
and the THI 5 were investigated further with regard to
photochromic properties. Irradiation of 4a -i, 5k -q, and
4r -z in CH2Cl2 with a Xe lamp (Pyrex filter λ > 290 nm)
at room temperature showed no color change. However,
cooling the solutions to 77 K with subsequent irradiation
gave in the cases of 4r -z (CN) a turquoise blue color,
which faded on warming to room temperature. This color
is due to the colored betaine 3.
All lifetimes τ of the colored forms were established
from decay curves of the transient absorption spectra.12,21
The lifetimes τ of these betaines 3a -i range between 100
and 260 µs for the CN-substituted compounds.22 By
comparison, the lifetime of the THI 5k -q (five-membered
rings), in the ns range, are shorter by about 3 orders of
magnitude. The reversibility of the new THAI systems
3r -z f 4r -z as well as that of THI 3k -q f 5k -q has
been clearly established by performing several coloration/
decoloration cycles.
(20) Valat, P.; Tripathi, S.; Wintgens, V.; Kossanyi, J . New J . Chem.
1990, 14, 825.
(21) Bleisinger, H.; Scheidhauer, P.; Du¨rr, H.; Wintgens, V.; Valat,
P.; Kossanyi, J . J . Org. Chem. 1998, 63(4), 990-1000.
(22) Detailed transient data will be discussed in a separate paper.
(19) Fleming, I. Frontier Orbitals and Organic Chemical Reactions;
Wiley-Interscience: New York, 1990.

1136 J . Org. Chem., Vol. 66, No. 4, 2001
Tan et al.
THAI’s 4a -i could not be measured due to apparatus
limitations,20 since the kinetics of the photochromic
processes appears to be faster than 1-10 ns.
All compounds 4 and 5 showed little fluorescence
emission with fluorescence quantum yields φF ranging
from 2‚10-4 for 4z to 6‚10-3 for 4h (Table 3). Typical
fluorescence maxima of the 7-ring THAI lie between 400
and 445 nm. Going from diester (4a -i) to dicyano (4r -
z) substitution caused hypsochromic shifts of the fluo-
rescence spectra by about 30 nm. The THI 5k -q emitted
at higher wavelengths and showed maximum emission
around 500 nm (Table 3).
Con clu sion s
The azaheptatriene-dihydroazepines (THAI) 4 consti-
tute a new class of very fast cyclizing photochromic
molecules. The photochromism is based on an electrocy-
clic ring opening/ ring closure reaction of a seven-
membered ring (8π). The conformation and configuration
of 4 and 5 were determined by NMR. Definite structures
of 4z and 5n were elucidated by X-ray analysis to show
a half-boat form for the 7 ring of 4z and an almost planar
geometry for the five-membered ring of 5n . The process
of the ring closure 3 f 4 and its reverse is an 8π-seven-
atom electrocyclization. In the presence of electron-
attracting substituents as in 5k -q (ester) only five-
membered ring THIs are obtained in a 6π-five-atom
process. The transformation of betaines 3 to the THAI 4
is thus a periselective 4n process. This is one of the very
few 8π-systems for which a reversible light-induced
process is clearly established.
Exp er im en ta l Section
All NMR spectra were collected on a 500 MHz spectrometer
in CDCl3 using TMS as internal standard. Chemical shifts are
reported in ppm (δ). Column chromatography was performed
using silica gel (50-200 µm /70-270 mesh). Solvents used
were water free and dried according to standard procedures.
IR spectra were measured as powder in KBr. Melting points
are uncorrected. Flash photolysis experiments were carried out
with a frequency-double-pulsed ruby laser (347.5 nm, 20 ns
fwmh). The usual crossed beam system was used, the analyz-
ing being a conventional pulsed Xenon lamp. A detailed
description of the experimental setup has been given already.20
Gen er a l P r oced u r e for th e P r ep a r a tion of Diester
1-Aza -2,6-cycloh exa d ien o[1,7]isoqu in olin es (4a-i, 5k -q).
To a suspension of 2-styryldihydroisoquinoline HCl salt 2 (1.0
mmol) in absolute ether was added an equimolar solution of
triethylamine/ether (1.0 mmol). After 10 min of stirring at
room temperature, an equimolar ether solution of spirocyclo-
propene 1 (0.306 g, 1.0 mmol) was given to the mixture. After
further stirring for 24 h (TLC control) in the dark at room
temperature, the solvent was removed under reduced pressure.
The residue was taken up in CH2Cl2 and separated by column
chromatography (on SiO2) with eluent mixtures of [CH2Cl2/
EtOAc, 9:2] to afford compound 4. The product was recrystal-
lized from diethyl ether to yield white, colorless crystals.
11′,12′-H -2′,3′-Dim et h oxyca r b on yl-8′,9′-d im et h oxy-5′-
p h en ylsp ir o[flu or en e-9,4′-(1-a za -2,6-cycloh ep t a d ien o)-
[1,7-a ]isoqu in olin e] (4a ). Sta n d a r d P r oced u r e. Prepara-
tion: 0.306 mg (1.0 mmol) of methylester-spirocyclopropene
1, 0.33 g (1.0 mmol) of 1-styryl dihydroisoquinoline 2, 101 mg
(1.0 mmol) of triethylamine, 50 mL of diethyl ether. Reaction
time: 72 h. Yield: 389 mg (65%) colorless crystals. IR ν (cm-1):
3062-3003 (C-H, arom.), 2952, 2835 (C-H, aliph.), 1733 (Cd
O), 1714 (CdO), 1607 (CdC), 1577, 1511, 1452, 1261, 1233,
F igu r e 4. Oscilloscope trace of the growing-in and the decay
of the transient absorption of compound 4r in dichloromethane
solution analyzed (a) at 410 nm, (b) at 660 nm, and (c) at 780
nm.
For compounds 4a -i not showing photochromism with
a laser flash, we assume that the electrocyclization is too
fast owing to the fact, that the ring open betaine form of
the ester THAI’s 4a -i can be blocked in a light induced
azidochromic reaction. The decay τ of the excited esters
1
2
1207, 1046, 858, 755. H NMR δ: 2.69 (dt, 1H, J ) 15.9 Hz,
3J ) 3.1 Hz), 2.80 (s, 3H), 3.43 (m, 1H), 3.74 (m, 1H), 3.88 (s,
3
3H), 3.89 (m, 1H), 3.90 (s, 3H), 3.96 (s, 3H), 4.40 (d, 1H, J )

New Photochromic 8π-System
J . Org. Chem., Vol. 66, No. 4, 2001 1137
3
3
4
7.5 Hz), 6.20 (d, 2H, J ) 7.5 Hz), 6.23 (d, 1H, J ) 7.52 H),
Hz), 7.30-7.43 (m; 5H), 7.50-7.55 (m; 2H), 7.73 (d; 1H, J )
3
3
6.62 (s, 1H), 6.74 (t, J ) 7.52 Hz, 2H), 6.85 (t, 1H, J ) 7.5
1.8 Hz. 13C NMR δ: 29.71, 40.51, 50.59; 53.25, 55.33, 55.71,
68.62, 74.98, 101.21(q), 108.25, 110.97, 120.21, 120.62, 121.16,
125.38, 126.91, 127.29, 128.01, 128.59, 128.85, 129.66, 131.16
(q), 131.25 (q), 135.90 (q), 137.66 (q), 140.03 (q), 147.08 (q),
147.22 (q), 147.83 (q), 149.30 (q), 152.54 (q), 163.08, 163.57
(CdO). MS (C38H31NO6Br2): 758.5 (56.8), [M+ + 1], calcd
(757.48 g/mol).
Gen er a l P r oced u r e for th e P r ep a r a tion of Dicya n o-
1-a za -2,6-cycloh exa d ien o[1,7]isoqu in olin es (4r -z). To a
suspension of 2-styryldihydroisoquinoline (HCl salt) 2 (0.33
g, 1.0 mmol) in absolute ether was added an equimolar solution
of triethylamine/ether (0.101 g 1.0 mmol). After 10 min of
stirring at room temperature, an equimolar solution of dicy-
cano spiro-pyrrazole 5 (0.268 g 1.0 mmol) in ether (500 mL)
was added. After the reaction mixture was flushed for 15 min
with dry nitrogen, the solution was photolyzed (Pyrex filter,
125 W HPK mercury high-pressure lamp). After 2 h nitrogen
development ceased and the reaction was complete (TLC
control). The ether was removed and the residue purified by
column chromatography (SiO2) with eluent mixtures of [CH2-
Cl2/ EtOAc, 9:2] to afford compound 4. The product was
recrystallized from ether to yield colorless to slightly yellow
crystals 4j-r .
3
3
Hz), 7.02 (t, 1H, J ) 7.5 Hz), 7.10 (s, 1H), 7.17 (t, 1H, J )
7.5 Hz), 7.22 (d, 1H, 3J ) 7.5 Hz), 7.30 (t, 1H, 3J ) 7.5 Hz),
7.35 (t, 1H, 3J ) 7.5 Hz), 7.42 (d, 1H, 3J ) 7.5 Hz), 7.44 (d,
1H, 3J ) 7.1 Hz), 7.60 (d, 1H, 3J ) 7.1 Hz). 13C NMR δ: 29.34,
48.11, 50.41, 51.28, 52.67, 55.81, 56.27, 63.17, 106.38, 106.87
(q), 111.11, 116.68, 118.76, 122.51, 123.24, 124.77, 125.78,
126.07, 126.24, 126.50, 126.58, 126.76, 126.90, 128.46, 138.56
(q), 139.14 (q), 139.28 (q), 142.60 (q), 147.86 (q), 149.38 (q),
149.47 (q), 150.98 (q), 151.87 (q), 167.54, 167.86. MS (for
C38H33NO6): 600.1 (13.2) (M+ + 1), (calcd 599.69 g/mol).
11′,12′-H-2′,3′-Din or bor n ylester -8′,9′-d im eth oxy-5′-p h e-
n ylsp ir o[flu or en e-9,4′-(1-a za -2,6-cycloh ep ta d ien o)[1,7-a ]-
isoqu in olin e] (4i). Sta n d a r d P r oced u r e. Preparation: 0.33
g (1.0 mmol) of 1-styryl-6,7-dimethoxy-3,4-dihydroisoquinoline
(HCl salt) 2, 0.466 g (1.0 mmol) of norbornylester-spiro-
cyclopropene 1, 101 mg (1.0 mmol) of triethylamine. Reaction
time: 72 h. Yield: 0.402 g (53%) colorless crystals. IR ν (cm-1):
3060-3000 (C-H, arom.), 2955, 2871 (C-H, aliph), 1726 (Cd
O), 1686 (CdO), 1609 (CdC), 1550, 1512, 1451, 1420, 1360,
1255, 1141, 1022, 850, 747. 1H NMR δ: 0.43-0.91 (m, 4H),
1.07-1.28 (m, 7H), 1.48-1.80 (m, 7H), 2.34 (s, 1H), 2.55-2.69
(m, 2H), 3.48 (m, 1H), 3.68 (m, 1H), 3.88 (s, 3H), 3.89 (s, 3H),
3
3.92 (d, 1H), 3.99 (m, 1H), 4.38 (d, 1H. J ) 4.9 Hz), 4.91 (d,
11′,12′-H-2′,3′-Dicya n o-8′,9′-d im eth oxy-5′-(4R-p h en yl)-
sp ir o[flu or en e-9,4′-(1-a za -2,6-cycloh ep ta d ien o)[1,7-a ]iso-
qu in olin e] (4r ) Sta n d a r d P r oced u r e. Preparation: 0.33 g
(1.0 mmol) of HCl salt of 1-styryl-3,4-dihydroisoquinoline 3a ,
0.268 g (1.0 mmol) of 4′,5′-dicyanespiro[9H-fluoren-9,3′-[3H]-
pyrazole], 0.101 g (1.0 mmol) of triethylamine. Reaction time:
1 h. Yield: 0.133 g (25%) colorless crystals. IR ν (cm-1): 3065,
3045, (C-H, arom.), 2939, 2837 (C-H, aliph), 2219 (C≡N),
2199 (C≡N), 1670, 1620 (CdC), 1558, 1515, 1450, 1275, 1252,
1210, 1024, 856, 743 cm-1. 1H NMR δ: 2.84 (dt, 1H, 2J ) 15.4
Hz, 3J ) 3.6 Hz), 3.39-3.47 (m, 1H), 3.84-3.88 (m and s, 4H),
3
3
3
1H, J ) 4.4 Hz), 6.17 (d; 2H, J ) 7.5 Hz), 6.22 (d; 1H, J )
3
3
7.1 Hz), 6.62 (d; 1H), 6.72 (t; 2H, J ) 7.5 Hz), 6.84 (t; 1H, J
) 7.5 Hz), 6.98-7.02 (m; 1H), 7.09 (d; 1H), 7.13-7.18 (m; 2H),
7.27-7.33 (m; 2H), 7.37-7.44 (m; 2H), 7.59-7.62 (m; 1H). 13
C
NMR δ: 24.13, 24.19, 24.36, 27.89, 28.29, 28.35, 29.65, 34.42,
34.47, 34.79, 35.46, 35.54,38.00, 38.38, 38.43, 38.52, 38.75,
40.34, 40.54, 40.97, 41.09, 41.32 (21 aliph-C in norbornyl group
of isomeric A and B), 47.76, 51.66, 55.94, 56.37, 63.51, 63.62,
76.79, 79.42, 106.03 and 106.12, 106.41 (q) 111.23, 116.63,
116.72, 118.85, 119.03, 122.84, 122.90,123.42, 124.95, 125.01,
125.82, 126.13, 126.34, 126.54, 126.71, 126.86, 127.09, 128.73,
138.70 (q), 139.45 (q), 139.59 (q), 139.71 (q) 142.83 (q), 147.91
(q), 149.44 (q), 150.37 (q), 150.43 (q), 151.70 (q), 152.45 (q),
166.83, 166.92, 167.07. MS (for C50H49NO6): 760 (24.2) [M+
+1], (calcd 759.95 g/mol).
3
2
3.93 (s; 3H), 4.41 (d; 1H, J ) 7.1 Hz), 4.47 (dt; 1H, J ) 12.8
3
3
3
Hz, J ) 4.0 Hz), 6.26 (d; 1H, J ) 7.1 Hz), 6.36 (d; 2H, J )
3
3
7.1 Hz), 6.70 (s; 1H), 6.80 (t; 2H, J ) 7.1 Hz), 6.93 (t; 1H, J
3
) 7.1 Hz), 7.07 (s; 1H), 7.19 (t; 1H, J ) 7.5 Hz), 7.28 (t; 1H,
3J ) 7.5 Hz), 7.35 (d; 1H, J ) 7.5 Hz), 7.36 (t; 1H, J ) 7.5
3
3
3
3
1′,5′,6′-H -8′,9′-Dim et h oxy-10b ′-st yr yl-2′,3′-d im et h oxy-
ca r b on ylsp ir o[2,7]d ib r om flu or en e-9,1′-p yr r olo[2,1-a ]-
isoqu in olin e (5m ). Stan dar d P r ocedu r e. Preparation: 0.464
g (1.0 mmol) of 2,7-dibromospirocylopropene, 0.330 g (1.0
mmol) of HCl salt of 1-styryl-6,7-dimethoxy-3,4-dihydroiso-
quinoline, 0.110 g (1.0 mmol) of triethylamine. Reaction time:
72 h. Yield:210 mg (55.5%).IR ν (cm-1): 3075-3000 (C-H,
arom.), 2990, 2840 (C-H, aliph.), 1747 (CdO), 1686 (CdO),
Hz), 7.49 (d; J ) 7.5 Hz, 1H), 7.53 (d; 1H, J ) 7.5 Hz), 7.54
(d; 1H, J ) 7.5 Hz). 13C NMR δ: 28.73, 50.22, 50.42, 56.05,
3
56.52, 62.73, 104.41 (q), 106.81, 110.59, 111.49, 113.71, 115.76,
117.32, 119.98, 122.20, 124.46, 126.10, 126.74, 127.17, 127.32,
127.64, 128.18, 128.62, 128.82, 130.84, 138.01 (q), 139.65 (q),
140.14 (q), 142.89 (q), 146.79 (q), 147.74 (q), 148.58 (q), 150.31
(q). Anal. Calcd for C36H27N3O2 (533.64): C, 81.03; H, 5.10; N,
7.87. Found: C, 80.78; H, 4.87; N, 7.72.
1580, 1476, 1256, 1061, 967, 862, 770 cm-1. H NMR δ: 2.72
1
2
(dd; 1H, J ) 15.9 Hz, J ) 2.7 Hz), 3.11 (m; 1H), 3.30 (s; 3H),
Ack n ow led gm en t. Financial help from the “DFG”
and the “Fonds der chemischen Industrie” is acknowl-
edged.
3.35 (s; 3H), 3.47 (m; 1H), 3.71 (m; 1H), 3.75 (s; 3H), 4.08 (s;
3
3H), 5.56 (s; 1H), 6.15 (d; 1H, J ) 15.9 Hz), 6.46 (s; 1H), 6.47
3
4
(d; 1H, J ) 15.9 Hz), 7.10 (d; 1H, J ) 1.8 Hz, 7.21 (dd; 1H,
3J ) 8.0 Hz, J ) 1.8 Hz), 7.27 (dd; 1H, J ) 8.0 Hz, J ) 1.8
J O000807+
4
3
4