Synthesis and photochromic behaviour under flash photolysis and continuous irradiation of novel 2H-chromenes derived from hydroxydibenzothiophenes

Article abstract of DOI:10.1016/S0040-4020(02)00023-6

The synthesis and photochromic properties, under flash photolysis and continuous irradiation, of new 2,2-diphenyl-2H-1-benzopyrans including a dibenzothiophene nucleus are described. Under flash photolysis, all compounds exhibit photochromic behaviour in solution at room temperature, but under continuous irradiation the same was not perceived. Compared to reference compounds, a general bathochromic shift and the existence of two absorption bands in the Vis spectra of the open forms, leading to a boardening in the absorption range, is observed. The heteroannellation effects on the spectrokinetic parameters are variable and depend on the position and geometry of the fused benzothiophene moiety. The effect of electron-withdrawing substituents in C(6) of the 2H-1-benzopyran skeleton, with the benzothiophene nucleus fused at the 7,8 positions, are analyzed.

Full text of DOI:10.1016/S0040-4020(02)00023-6

TETRAHEDRON
Pergamon
Tetrahedron 58 32002) 1709±1718
Synthesis and photochromic behaviour under ¯ash photolysis
and continuous irradiation of novel 2H-chromenes
derived from hydroxydibenzothiophenes
a,p
M. Manuel Oliveira, Corinne Moustrou, Luis M. Carvalho, Jose A. C. Silva, Andre Samat,
b
a
a
b
Â
Â
Robert Guglielmetti,b Roger Dubest,c Jean Aubardc and Ana M. F. Oliveira-Camposd
a
 Â
Dept. de Quõmica, Universidade de Tras-os-Montes e Alto Douro, 5000 Vila Real, Portugal
  Â
LCMOM, ESA 6114 CNRS, Universite de la Mediterranee, 13288 Marseille cedex 9, France
b
c
Â
ITODYS, Universite Denis Diderot Paris 7, 75005 Paris, France
Â
Centro de Quõmica, IBQF, Universidade do Minho, 4710 Braga, Portugal
d
Received 1 August 2001; revised 17 October 2001; accepted 3 January 2002
AbstractÐThe synthesis and photochromic properties, under ¯ash photolysis and continuous irradiation, of new 2,2-diphenyl-2H-1-
benzopyrans including a dibenzothiophene nucleus are described. Under ¯ash photolysis, all compounds exhibit photochromic behaviour
in solution at room temperature, but under continuous irradiation the same was not perceived. Compared to reference compounds, a general
bathochromic shift and the existence of two absorption bands in the Vis spectra of the open forms, leading to a boardening in the absorption
range, is observed. The heteroannellation effects on the spectrokinetic parameters are variable and depend on the position and geometry of
the fused benzothiophene moiety. The effect of electron-withdrawing substituents in C36) of the 2H-1-benzopyran skeleton, with the
benzothiophene nucleus fused at the 7,8 positions, are analyzed. q 2002 Published by Elsevier Science Ltd.
1. Introduction
chromic plastic ophthalmic lenses6±8 became the major
commercial success of these unique organic chemicals.
Important families, such as spiropyrans and spirooxazines
have been extensively developed and studied. More recently
2H-1-benzopyrans 3ˆ2H-chromenes) have been the subject
of intense research efforts in order to obtain a greater basic
understanding of the photochromic behaviour and to
achieve an improvement of the photochromic properties.
In the last decades photochromic molecules became an
important ®eld of research in organic chemistry due to the
large number of their practical and potential applications.
All these applications, based on the reversibility, con®gure
the fascinating ability of modulating given physical proper-
ties through an external stimulation, namely upon light
irradiation. Variable transmission materials, optical
switches and memories1±5 are among the most interesting.
The photochromic behaviour of 2H-1-benzopyrans is
based on a reversible pyran ring opening, induced by light,
that converts a colourless form 3usually named the `closed
form') in a set of photoisomers where the pyran ring is
opened 3the `open form') 3Scheme 1).9,10 Due to the quasi
planarity of the open forms structures, an extension of the
p-electrons conjugation is achieved, leading to distinct
absorption, usually in the visible range, after irradiation.
These chemical species undergo a reversible structural
photo-transformation, leading to states with different
absorption spectra and different physical and chemical
properties, as a result of the structural change involved.
Generally, the back reaction occurs predominantly through
a thermal pathway, which can be accompanied by a photo-
chemical process, although often not important. In some
systems, however, the bleaching time can be signi®cantly
accelerated by irradiation with visible light and this can
become the main process to return to the original state.
The photochromic properties looked for are mainly depen-
dent on the application. In the ®eld of photochromic lenses
the ideal targets are molecules that can be photo stimulated
by sunlight 3heliochromism), developing rapidly an intense
neutral colouration 3absorption covering as much as possi-
ble the visible spectrum) and returning to the original state
3bleaching) predominantly through a thermal process with a
reasonable kinetic 3not too fast, nor too slow). Furthermore
they should be fatigue resistant, allowing the occurrence of
many colouring±decolouring cycles.
Since the early 1990s, the applications in the ®eld of photo-
Keywords: photochromism; 2H-chromenes; ¯ash photolysis; continuous
irradiation; dibenzothiophene.
Corresponding author. Tel.: 1351-259-350259; fax: 1351-259-350480;
e-mail: moliveir@utad.pt
p
0040±4020/02/$ -see front matter q 2002 Published by Elsevier Science Ltd.
PII: S0040-4020302)00023-6

1710
M. M. Oliveira et al. / Tetrahedron 58 :2002) 1709±1718
Scheme 1. Photochromic equilibrium for 2H-chromenes.
The evaluation of the photochromic behaviour of molecules
involves the knowledge of some relevant parameters,
related to the kinetic and spectral properties, through a
kinetic analysis performed by photolysis under ¯ash or
continuous irradiation. The information obtained by the
two methods can be very distinct since the time scales of
observation are completely different.
phenyl or spiro¯uorene allowing extension of the
conjugation11,12
The presence of a thiophene moiety in 2H-chromene
molecules seems to give rise to interesting photochromic
properties.13±16 Molecules of 2H-chromene including
dibenzothiophene nuclei are not described in the literature
and so we decided to investigate the photochromic
behaviour of these novel systems. In this paper, we report
the synthesis and the spectrokinetic properties in solution,
under ¯ash and continuous irradiation, of a variety of 2,2-
diphenyl-2H-chromenes in which the position and geometry
of a fused benzothiophene group was varied.
The main strategies to improve the photochromic properties
of these molecules involve the modi®cation of the 2H-1-
benzopyrans structures through fusion of ®ve or six-
membered heteroaromatic rings and the appropriate
substitution on the molecule, namely the introduction of
substituents, to the sp3 carbon of the pyran ring, such as
Scheme 2. General synthesis of compounds 1a and 3a.

M. M. Oliveira et al. / Tetrahedron 58 :2002) 1709±1718
1711
Scheme 3. General synthesis of compounds 2a, 4a and 5a.
2. Results and discussion
2.1. Synthesis
melting points and spectral characteristics in accordance
with literature values.
Among several methods available to obtain hetero-
annellated 2,2-diaryl-2H-chromenes24 two synthetic
approaches were considered, both involving the transforma-
tion of the corresponding heterocyclic phenols through a
`one-pot reaction'.
Compounds 1a and 3a were prepared by known methods
from coupling of 3-methoxybenzenethiol with 2-chloro-
cyclohexanone in presence of NaOH 30% followed by
cyclisation with PPA at 1508C to yield a mixture of both
tetrahydrodibenzothiophenes 31-OMe 22% and 3-OMe
53%).17,18 After separation by column chromatography
they were aromatized with DDQ in benzene19 and demethyl-
ated with BBr3 in CH2Cl2 at 2808C20 with an overall yield
of 56% 33-OMe) and 74% 31-OMe) 3Scheme 2).
Method A is based on the organotitanium-mediated conden-
sation of a,b-unsaturated aldehydes with phenols. It
involves the titanium3IV) salt of the phenol, obtained with
Ti3OEt)4 on azeotropic distillation of formed EtOH and
leads to the C-alkylation in a ortho-position, that through
a subsequent electrocyclization yields the chromene
moiety.25,26 Method B is based on the thermal condensation
of a suitable alkynol and the phenol in an apolar solvent
Compounds 2a, 4a and 5a were obtained from dibenzo-
thiophene21±23 accordingly to Scheme 3 in 69, 59 and 72%
yields, respectively. These known compounds showed
Scheme 4. 2H-Chromenes derived from dibenzothiophene nuclei.

1712
M. M. Oliveira et al. / Tetrahedron 58 :2002) 1709±1718
at 169 ppm in the 13C NMR spectrum due to the carboxylic
acid group.
All the synthesised chromenes were fully characterised by
spectroscopic methods and elemental analysis.
2.2. Photochromic properties
All the closed forms of the 2H-1-benzopyrans synthesised
including a dibenzothiophene nucleus 3Scheme 4) show
absorption bands in the near-UV 3see Section 4 and, for
example, Fig. 1) and can, in principle, be activated by
sunlight 3heliochromism).
Spectrokinetic studies under ¯ash photolysisÐUnder ¯ash
photolysis the compounds described exhibit photochromic
behaviour, at room temperature in toluene solutions. The
usual comparative spectrokinetic parameters 3maxima
wavelengths of the coloured form, colourability and rate
constant of thermal bleaching) are summarized in Table 1
in which we included two reference naphthopyrans 3Ref 1
and Ref 2) for comparison.31,32
Figure 1. UV/Vis absorption spectra of uncoloured 3CF) 3´´´) and coloured
species 3Ð) of compound 7 in toluene.
under acid catalysis 3p-toluenesulphonic acid or pyridinium
p-toluenesulfonate). The reaction proceeds via a Claisen-
like [3,3]-sigmatropic rearrangement of the resulting
propargyl aryl ether and a subsequent [1,5]-sigmatropic
shift and electrocyclisation leading to the chromene.26±28
The effects of the heteroannellation and the introduction of
some groups in the molecule on the photoactivity of these
compounds can be evaluated and discussed through the
comparison to values obtained for reference compounds
and similar compounds with a thiophene moiety.15,32,33
Although we often obtain better yields with method B, for
compounds 1b and 3b, very complex mixtures that are very
dif®cult to purify were obtained, thus we decided to apply
method A that affords good results 28 and 32%,
respectively. To observe the effect of electron-withdrawing
groups on the photochromic behaviour, derivatives 5b, 6±8
were also synthesised 3Scheme 4).
2.2.1. Annellation effects. Comparing to the reference
naphthopyrans, the introduction of a fused benzothiophene
group led to a global bathochromic shift in the spectra of the
open forms. Moreover, an interesting broadening in the
visible absorption spectra of all the open forms can be
observed, due to the presence of two absorption bands,
located at 430±460 and 510±580 nm 3see, for example,
Fig. 1). Two bands in the visible absorption spectrum of
the open forms are also observed with the reference 7,8-
annellated naphthopyran 3Ref 2), although they are located
at shorter wavelengths. The reference naphthopyran with a
5,6-annellation 3Ref 1) is distinctly different displaying only
one single absorption band culminating at 432 nm. Another
general characteristic in chromenes with a fused thiophene
Chromene 6 was obtained in 30% yield by treatment of 5b
with Zn3CN)2 in DMF with dppf and Pd3dba)3,29 and the IR
band at 2213 cm21 was conclusive. The formyl derivative 7
was prepared in 40% yield by lithiation 3n-BuLi) followed
by treatment with DMF.23 Its identi®cation was ®rst
con®rmed by the IR band due to the carbonyl group at
1670 cm21 and the aldehyde proton that appears at
1
10.51 ppm in its H NMR spectrum. The carboxylation of
5b to obtain 8 was also effected by lithiation and dry ice
quenching30 in 48% yield, which was con®rmed by IR bands
at 3058 cm21 3±OH) and 1677 cm21 3CvO) and by a signal
Table 1. Maxima wavelengths of the coloured forms 3l1OF, l2OF), colourability 3A01, A02 and Aeq), fading rate 3kDf and kDc) of the described 2H-chromenes
and two reference compounds in toluene solutions under ¯ash photolysis 32.5£1025 M at 258C) and continuous irradiation 31£1024 M at 208C). S position is
relative to the 2H-1-benzopyran moiety. Ref 1ˆ3,3-diphenyl-3H-naphtho[2,1-b]pyran; Ref 2ˆ2,2-diphenyl-2H-naphtho[1,2-b]pyran 3from Ref. 32)
Compound
Annellation
Flash photolysis
Continuous irradiation
l1OF
A01
l2OF
A02
kDf 3amplitude %)
Aeq
kDc 3amplitude %)
3b
2b
1b
4b
4c
5b
6
7
5,6 S±3C5)
5,6 S±3C6)
7,8 S±3C7)
7,8 S±3C8)
7,8 S±3C8)
7,8 S±3C8)
7,8 S±3C8)
7,8 S±3C8)
7,8 S±3C8)
5,6
459
449
436
440
457
454
454
453
446
432
403
0.64
0.88
1.2
2.3
1.3
1.3
1.7
3.9
2.8
557
513
513
544
551
558
578
577
570
±
0.28
0.59
0.3
0.51
0.75
0.57
0.62
0.84
0.69
±
0.68 389), 0.16 311)
2.23 397), 0.05 33)
0.04 367), 0.01 333)
0.06 394), 0.01 36)
0.31 396), 0.007 34)
0.16 387), 0.02 313)
1.34 396), 0.02 34)
0.82 368), 0.01 332)
0.59 395), ,0.01 35)
0.09
Not heliochromic
Not heliochromic
0.6
0.02 382), ,0.01 318)
0.45
0.13
0.35
0.07
0.05 365), 0.02 335)
0.23 378), ,0.01 322)
0.04 396), ,0.01 34)
0.11 363), 0.01 337)
Bistable system
8
0.24
0.21
±
0.05a
0.07 380), 0.003 320)
Ref 1
Ref 2
0.84
1.08
7,8
481
1.62
,0.01
±
a
30 min after the end of the irradiation the solution remained with a residual yellowish colour corresponding to about 0.1 absorbance units, at the wavelength
of the measurement.

M. M. Oliveira et al. / Tetrahedron 58 :2002) 1709±1718
1713
or benzothiophene group, is observed: the existence of two
phases, of variable amplitudes, in the fading kinetics,
whereas for the reference compounds only one is
observed.15 The slow fading rate phase is usually attributed
to the most stable s-trans-isomer.
114 nm in the absorption band at lower wavelengths, a
larger bathochromic shift 3about 130 nm) of the absorption
band at higher wavelengths and a decrease in the stability of
the open forms, leading to an acceleration of the fading
processes. This indicates that the presence of electron-
withdrawing groups in that part of the molecule promotes
the unstability of the open forms, as it was already observed
with photochromic naphthopyrans annellated in the 5,6-
positions. It is noteworthy that the interesting kinetic
phase amplitudes of 6 3with the cyano group), which is not
observed in 7 3with the formyl group), that exhibits a slower
kinetic phase too long. Compound 8 exhibits an interesting
amplitude of the ®rst kinetic phase, but the fading process
becomes so slow that it is dif®cult to calculate its value. The
effects of these groups in the colourability are modest indi-
cating that this kind of substitution may not contribute to an
enhancement of the quantum yield for photocolouration.
-5,6-Annellation: taking into account the 2H-1-benzopyran
moiety, two structural types may be considered: one where
the sulphur atom is bound to C35) 33b) and the other one
with the sulphur atom bound to C36) 32b). Table 1 shows
that the position of the sulphur atom has a marked in¯uence
in the photochromic behaviour of these compounds.
Compound 2b 3S±C36)) exhibits a less pronounced batho-
chromic shift than 3b 3S±3C5)) but displays an enhanced
photocolouration ef®ciency 3colourability) and a faster
kinetics of ring closure 3higher thermal instability). Quali-
tatively the same conclusions can be drawn from the
literature data for 2H-chromenes with a fused dimethyl-
thiophene nucleus in the same positions, although poorer
colourabilities and slower fading rates are apparent.15
The induced modi®cations achieved with the bromo group,
although in the same direction, are not as pronounced, what
is in agreement with its weaker electron withdrawing
characteristics through inductive effect. The greater batho-
chromic shifts observed for 6±8 can be, in part, explained
by the greater extension conjugation of the additional
p-electrons of the substituents.
-7,8-Annellation: again, considering the 2H-1-benzopyran
moiety, two structural types may be considered, one
where the sulphur atom is bound to C37) 31b) and others
with the sulphur atom bound to C38) 34b and the substituted
derivatives 4c, 5b, 6±8). As it is usually observed the
compounds without substituents and with a 7,8-annellation,
exhibit a very slow thermal bleaching rate which can be
explained by the less important nonbonding interactions in
the open forms.26 It can be observed here that there are
differences in the spectrokinetic parameters when the
geometry of the fused benzothiophene moiety is reversed.
The sulphur atom in C38) led to a more pronounced batho-
chromic shift and to a signi®cant colourability exaltation.
The ring closure kinetics is not much affected but the ampli-
tude of the second phase is quite different in both
compounds. In compound 1b 3S±3C7)) the second kinetic
phase is about one third of the total bleaching process,
meaning that this compound persists with a residual colour
for a much longer time than 4b after the ¯ash irradiation.
In compound 4c one methoxy group was introduced, in the
para position, in each phenyl group attached to the sp3
carbon of the pyran ring. Relatively to 4b the effects
observed due to the presence of this electron-donor group,
were a global bathochromic shift with relevance to the
absorption band located at lower wavelengths 3117 nm)
and a faster fading process, with a magnitude that can be
interesting for applications in the ®eld of variable optical
transmission materials. The observed effects on the coloura-
tion ef®ciency are not relevant.
Spectrokinetic studies under continuous irradiationÐ
methods employing continuous irradiation operate in
experimental conditions quite comparable to those that are
found in the normal uses of photochromic lenses. A solution
containing the photochromic molecule is exposed to an
adequate light until a photochemical and thermal equi-
librium 3photostationary state) is reached. The kinetics of
decolouration can then be studied in the absence of light
3thermal process) or, if wanted, under irradiation with a
different light 3photochemical process).
2.2.2. Substituent effects. Due to the interesting photo-
chromic properties of 4b we tried to improve the slow
kinetics of ring closure, maintaining a good colourability
and an extended absorption range in the visible spectra.
The introduction of some moderately to weak electron-with-
drawing substituents in the C37) position is known to
produce this kind of modi®cations in 2,2-diphenylnaphtho-
pyrans annellated in the 5,6-positions.30 So, we decided to
introduce in C36) the bromo, formyl, cyano and carboxylic
groups 3compounds 5b, 7, 6 and 8, respectively) to evaluate
their in¯uence on the photochromic behaviour of this novel
structure. This approach is also interesting as it opens the
possibility of further modi®cations in the molecule.
In comparison with the ¯ash photolysis method a longer
irradiation time and a lower intensity light ¯ux is used.
During irradiation, the open forms are formed, but only
the photoisomers with the longest lifetimes will accumulate
and will be observed. The spectrokinetic analysis is based
on considering the photostationary state or on the reaction
progress. The method is particularly well suited for the
study of slow photochromic systems such as those used in
ophthalmic lenses that are coloured in the sun and colourless
in the dark.
The values show that this kind of substitution led to a global
acceleration of the bleaching process and a bathochromic
shift as observed before with equally substituted naphtho-
pyrans.30
The spectrokinetic parameters 3Aeq and the rate constants of
thermal bleaching, kDc) obtained under continuous irradia-
tion are summarized in Table 1 in which we included a
corresponding naphthopyran 3Ref 1) as a reference
Comparatively to the molecule without substituents 34b) the
effects of the formyl, cyano and carboxylic groups 37, 6 and
8, respectively) are similar, a bathochromic shift of about

1714
M. M. Oliveira et al. / Tetrahedron 58 :2002) 1709±1718
signi®cative difference between the stable photoisomers
produced in both methods. Compound 8 seems to have a
different fading process with only one kinetic phase. This
may not be correct because a relevant residual yellowish
colour at the end of the measurement is observed that may
be attributed to a very stable open form 3second kinetic
constant very low) or to degradation products that masked
the second kinetic phase, avoiding its evaluation.
Some correlation can be observed in the colourabilities
obtained through both methods 3A0 and Aeq), although the
observed colourabilities under continuous irradiation are
always much lower. These differences in the colourability
observed in both methods can be explained: A0 is dependent
on the quantum yield of the photochemical colouration and
on the absorptivity of the open forms; Aeq is dependent on
the absorptivity of the coloured forms, the rate coef®cient of
the photochemical colouring reaction and the rate coef®-
cient of thermal bleaching reaction. A fast thermal
decolouration means a lower Aeq and this may be the case
of all the compounds described.
Figure 2. Photoinduced absorbance change under continuous irradiation of
compound 4b showing the kinetics of the biexponential thermal relaxation
at 440 nm.
compound for comparison. Absorbance measurements were
made at the maximum wavelength of the ®rst absorption
band of the open forms 3see, for example, Fig. 2).
Compounds 3b and 2b may constitute fast photochromic
systems that cannot be studied under continuous irradiation.
The lifetime of the coloured forms must be more than
about 0.1 s for the photoisomer to accumulate under the
continuous irradiation conditions.
An analysis of these data allows some conclusions about the
results of both methods: in some cases the fading kinetics
observed under ¯ash irradiation 3kDf) can be correlated with
the one 3kDc) observed under continuous irradiation; the
absorbance attained under continuous irradiation is always
lower than the colourability under ¯ash irradiation;
compounds 2b and 3b, annellated in the 5,6-positions
could not be activated under the conditions used for
continuous irradiation 3see, for example, Fig. 3).
Under continuous irradiation the open form of compound 7
has a high thermal stability and could only be photo-
chemically bleached, thus constituting a bistable system
3Fig. 4).
3. Conclusion
In both methods the bleaching kinetics is biexponential
3except for 8) and it seems that there is some correlation
between the values of the calculated constants by the two
methods for 1b, 4b and 4c, indicating a similar nature of the
stable photoisomers in the mixture obtained after irradiation
in both methods. However, for compounds 5b and 7 there is
no correlation in the calculated constants, even the slower
ones, what may signify that for those compounds there is a
The synthesis of benzothiophene-fused 2H-1-benzopyrans
was achieved by the usual methods. All compounds
exhibited photochromic activity, in solution at room
temperature, under ¯ash photolysis. Studies made under
continuous irradiation were not in total agreement particu-
larly with compounds with the benzothiophene moiety
fused at the 5,6 position. The observed spectrokinetic para-
meters are dependent on the relative position and geometry
of the annellation. From a general point of view the hetero-
annellation led to a global bathochromic shift and a
broadening of the visible absorption range. The presence
of electron-withdrawing substituents in C36) of the 2H-1-
benzopyran skeleton, with the benzothiophene nucleus
fused at the 7,8 positions, led to a global acceleration of
the thermal bleaching process. The presence of the formyl
group led to a bistable system under continuous irradiation,
with interesting colouration ef®ciency and visible spectrum
coverage, that can be important to applications where the
transformations can be photoinduced, in both directions,
using light of different wavelengths.
4. Experimental
4.1. Materials and methods
Figure 3. UV/Vis absorption spectra of a solution of compound 2b in
toluene.
È
Solvents 3Riedel±Haen and Merck) were used without

M. M. Oliveira et al. / Tetrahedron 58 :2002) 1709±1718
1715
Figure 4. Kinetics of thermal relaxation at 453 nm of compound 7 after irradiation.
further puri®cation other than drying over sodium 3Et2O)
and over anhydrous calcium chloride 3CH2Cl2). Column
chromatography was performed on silica gel Merck 60
For measurements, thermostated 3208C) 10 mm cells were
used. The irradiation was carried out in the spectro-
photometer holder at the right angle to the monitoring
beam, using a ®ber-optic system.
1
370±230 mesh). H and 13C NMR spectra were recorded
on a Varian Unity Plus 3300 and 75.4 MHz, respectively)
and on a Bruker BM 250 3250 and 62.9 MHz, respectively)
using tetramethylsilane as internal standard. Chemical shifts
are given in ppm and coupling constants in Hz. IR spectra
were recorded on a Perkin±Elmer-FTIR-1600 spectro-
photometer using KBr disks and wavenumbers are given
in cm21. UV/Vis spectra were recorded on a CARY 50
Varian spectrophotometer using 1£1025 M toluene
solutions. Maxima wavelengths 3lmax) are given in nm
and molar absorption coef®cients of closed forms 31) are
given in l mol21 cm21. Elemental analysis was performed
on a LECO-932-CNS analyser and mass spectra on a
AutoSpecE spectrometer. Melting points 38C), measured
in capillary tubes on a BuÈchi 535 apparatus, are uncorrected.
The relaxation kinetics for both methods were calculated
using Origin 5.0 Professional program.
4.3. General method for the synthesis of pyrano-
dibenzothiophenes 1b and 3b
A solution of the hydroxydibenzothiophene 310 mmol) in
dry toluene 350 ml), under Ar, was stirred until all the
dibenzothiophene was dissolved. A solution of titanium3IV)
ethoxide 310 mmol) in dry toluene 340 ml) was added over a
period of 10 min. The mixture was re¯uxed for 30 min, and
the ethanol formed was slowly distilled 3up to 1/3 of the
initial volume). The mixture was cooled at rt and a solution
of b-phenylcinnamaldehyde 310 mmol in 40 ml of dry
toluene) was added dropwise. The mixture was re¯uxed
for a period of 2±6 h, cooled to rt, quenched with 40 ml
of 2 M aq. NaOH, and extracted with CH2Cl2 33£40 ml).
The combined org. extracts were dried 3MgSO4), evaporated
to dryness and the residue was puri®ed by CC on silica gel.
4.2. Spectrokinetic measurements
For the determination of l1 and l2, A01 and A02, and kDf,
5£1025 mol dm23 toluene solutions were used. The ¯ash
photolysis apparatus was monitored by a Warner and
Swasey rapid spectrometer, allowing to record visible
absorption spectra of coloured forms in the 400±700 nm
range 3acquisition time 1 ms, repetitivity 1.25 ms).31,34
Flashes 3duration 50 ms) were generated by two xenon
tubes with a quartz envelope. The energy of the ¯ashes
was 60 J for the whole polychromatic emission spectrum.
For measurements, thermostated 3258C) 100 mm cells were
used. The light from the analysis lamp 350 W, quartz-
iodide) was ®ltered using a Schott GC400 high-pass ®lter.
4.3.1. 2,2-Diphenyl-[2H]-pyran[6,5-a]dibenzothiophene
21b). 3 h re¯ux, 28% yield, puri®cation by column chro-
mathography, eluent CH2Cl2/pentane 31:5) gave a white
solid, mp 117±119. IR: 3057, 3023, 1631, 1594, 1486,
1444, 1419, 1305, 1226, 1105, 1062, 1051, 989, 950, 808,
755, 746, 730, 696. UV/Vis 3closed form): 316 31 6230),
336 31 6310), 352 31 8730). 1H NMR 3CDCl3): 6.28 31H, d,
Jˆ9.8 Hz, H-3); 6.88 31H, d, Jˆ9.8 Hz, H-4); 7.24 31H, d,
Jˆ8 Hz, H-6); 7.38±7.49 37H, m, H-5, 30, 40, 50, 300, 400 and
500); 7.53±7.57 32H, m, H-9 and 10); 7.65±7.69 34H, m,
H-20, 60, 200 and 600); 7.91±7.95 31H, m, H-8); 8.89±8.92
31H, m, H-11). 13C NMR 3CDCl3): 83.9 3s); 115.0 3d);
116.7 3s); 122.2 3d); 123.5 3d); 123.9 3s); 124.6 3d); 125.2
3d); 125.9 3d); 126.9 3d); 127.0 34C, C-20, 60, 200 and 600);
127.6 32C, C-40 and 400); 128.2 34C, C-30, 50, 300 and 500);
134.8 3s); 139.0 3s); 141.0 3s); 144.9 32C, C-10 and 100);
149.7 3s). MS: 390 3100, M1z), 313 372), 284 315), 191
For the determination of absorption spectra of closed form
and open form 1£1025 mol dm23 toluene solutions were
used. Determination of lmax were made with a light ¯ux
of 400 W m22. For measurements of Aeq, and kDc under
continuous irradiation, 1£1024 mol dm23 toluene solutions
were used. The apparatus was monitored by a CARY 50
Varian spectrometer 32.0 nm spectral band pass) with a
Xenon lamp 3Ushio type UXL-150SO/Oriel instruments).

1716
M. M. Oliveira et al. / Tetrahedron 58 :2002) 1709±1718
319), 165 39). Anal. calcd for C27H18OS: C 83.05, H 4.65, S
8.21; found C 83.16, H 4.90, S 8.04.
m, H-9); 7.41±7.44 31H, m, H-8); 7.51±7.54 34H, m, H-20,
60, 200 and 600); 7.66 31H, d, Jˆ7.9, H-6); 7.83±7.86 31H, m,
H-10); 8.04±8.08 31H, m, H-7). 13C NMR 3CDCl3): 83.4 3s);
114.3 3d); 118.3 3s); 121.6 3d); 123.0 3d); 123.3 3d); 123.4
3d); 124.3 3d); 126,6 3d); 126.7 34C, C-20, 60, 200 and 600);
126.8 3s); 127.5 32C, C-40 and 400); 128.1 34C, C-30, 50, 300
and 500); 128.5 3d); 135.7 3s); 137,5 3s); 140.1 3s); 144.6 32C,
C-10 and 100); 147.2 3s). MS: 390 3100, M1z); 371 38); 313
387); 284 320); 191 321); 178 38); 165 311); 77 38). Anal.
calcd for C27H18OS: C 83.05; H 4.65; S 8.21%; found C
83.25, H 4.82, S 8.02.
4.3.2. 2,2-Diphenyl-[2H]-pyran[6,5-c]dibenzothiophene
23b). 4 h 30 min re¯ux, 32% yield, puri®cation by column
chromatography, eluent CH2Cl2/pentane 31:4), gave a white
solid, mp 191±194. IR: 3058, 3024, 1633, 1577, 1490, 1444,
1251, 1220, 1170, 1049, 1000, 943, 914, 817, 752, 696.
UV/Vis 3closed form): 310 31 6360), 324 31 4500), 350
1
31 1650). H NMR 3CDCl3): 6.25 31H, d, Jˆ9.8 Hz, H-3);
6.78 31H, d, Jˆ9.8 Hz, H-4); 7.02 31H, d, Jˆ8.5 Hz, H-11);
7.22±7.31 38H, m, H-7, 8, 30, 40, 50, 300, 400 and 500); 7.39±
7.42 34H, m, H-20, 60, 200 and 600); 7.71±7.75 31H, m, H-6);
7.82 31H, d, Jˆ8.5 Hz, H-10); 7.90±7.93 31H, m, H-9). 13C
NMR 3CDCl3): 80.1 3s); 111.6 3d); 112.2 3s); 117.7 3d);
117.8 3d); 119.0 3d); 119.7 3d); 121.5 3d); 122.6 3d); 124.0
34C, C-20, 60, 200 and 600); 124.6 3s); 124.7 32C, C-40 and 400);
125.2 34C, C-30, 50, 300 and 500); 126.2 3d); 126.9 3s); 132.8
3s); 135.2 3s); 141.5 32C, C-10 and 100); 148.4 3s). MS: 390
3100, M1z), 313 373), 284 320), 191 326), 178 313), 165 310).
Anal. calcd for C27H18SO: C 83.05, H 4.65, S 8.21; found C
82.96, H 4.69, S 8.24.
4.4.3. 6-Bromo-2,2-diphenyl-[2H]-pyran[6,5-a]dibenzo-
thiophene 25b). 3 h 30 min re¯ux, 86% yield, puri®cation
by column chromatography, eluent CH2Cl2/pentane 33:7),
gave an off-white solid, mp 217±219. IR: 3058, 3023,
1637, 1594, 1486, 1469, 1438, 1392, 1346, 1251, 1214,
1170, 1062, 997, 948, 904, 769, 728, 698. UV/Vis 3closed
form): 304 31 20,290), 318 31 27,310), 346 31 5500), 362 31
5700). 1H NMR 3CDCl3): 6.33 31H, d, Jˆ10 Hz, H-3); 6.73
31H, d, Jˆ10 Hz, H-4); 7.25±7.30 32H, m, H-40 and 400);
7.32 31H, s, H-5); 7.33±7.37 34H, m, H-30, 50, 300 and 500);
7.48±7.53 36H, m, H-8, 9, 20, 60, 200 and 600); 7.85±7.89 31H,
m, H-10); 9.10±9.13 31H, m, H-7). 13C NMR 3CDCl3): 83.8
3s); 108.8 3s); 118.7 3s); 122.1 3d); 122.8 3d); 124.0 3d);
125.1 3d); 126.7 34C, C-20, 60, 200 and 600); 127.0 3d);
127.8 32C, C-40 and 400); 127.9 3d); 128.2 34C, C-30, 50, 300
and 500); 129.3 3s); 130.0 3d); 133.9 3s); 135.5 3s); 140.5 3s);
144.2 32C, C-10 and 100); 146.3 3s). MS: 470 3M1 81Br, 100);
468 3M1 79Br, 100); 393 337); 371 310); 311 316); 284 312);
239 35); 191 312); 178 314); 165 311); 105 35); 7736). Anal.
calcd for C27H17BrOS: C 69.09; H 3.65; S 6.83; found C
69.02, H 3.72, S 7.06.
4.4. General method for the synthesis of pyrano-
dibenzothiophenes 2b, 4b and 5b
To a stirred mixture of the hydroxydibenzothiophene
310 mmol) and 1,1-diphenyl-2-propyn-1-ol 311 mmol) in
dry CH2Cl2 310±25 ml), under Ar, a catalytic amount of
p-toluenesulfonic acid 3APTS) was added. The mixture
was kept at rt or heated to re¯ux, according to TLC analysis.
4.4.1. 2,2-Diphenyl-[2H]-pyran[5,6-a]dibenzothiophene
22b). 70 min rt, 54% yield, puri®cation by column chroma-
tography, eluent CH2Cl2/pentane 315:85), gave a yellow
solid, mp 138±140. IR: 3058, 1631, 1571, 1444, 1411,
1255, 1205, 1110, 989, 944, 759, 725, 698. UV/Vis 3closed
form): 306 31 12,700), 316 31 14,850), 364 31 4590), 378 31
4500). 1H NMR 3CDCl3): 6.39 31H, d, Jˆ9.9 Hz, H-3); 7.16
31H, d, Jˆ8.5 Hz, H-11); 7.22±7.28 32H, m, H-40 and 400);
7.30±7.35 34H, m, H-30, 50, 300 and 500); 7.40±7.42 31H, m,
H-7); 7.42±7.44 31H, m, H-6); 7.49±7,53 34H, m, H-20, 60,
200 and 600); 7.59 31H, d, Jˆ8.5 Hz, H-10); 7.67 31H, d,
Jˆ9.9 Hz, H-4); 7.80±7.83 31H, m, H-8); 8.31±8.35 31H,
m, H-5). 13C NMR 3CDCl3): 81.5 3s); 116.9 3d); 118.0 3s);
121.2 3d); 123.0 3d); 123.3 3d); 124.1 3d); 124.6 3d); 129.0
3d); 127.1 34C, C-20, 60, 200 and 600); 127.6 32C, C-40 and 400);
128.1 34C, C-30, 50, 300 and 500); 129.9 3d); 131.0 3s); 132.5
3s); 135.7 3s); 140.9 3s); 144.4 32C, C-10 and 100); 150.6 3s).
MS 390 3100, M1z); 313 396); 284 39); 191 39); 165 311).
Anal. calcd for C27H18OS: C 83,05; H 4,65; S 8,21%; found
C 82.95, H 4.60, S 8.10.
4.5. General method for the synthesis of pyrano-
dibenzothiophene 4c
To a stirred mixture of the hydroxydibenzothiophene 4a
310 mmol) and 1,1-bis34-methoxyphenyl)-2-propyn-1-ol
311 mmol) in dry CH2Cl2 310±25 ml), under Ar, a catalytic
amount of of pyridinium p-toluenesulfonate 3PPTS) was
added. The mixture was kept at rt or heated to re¯ux,
according to TLC analysis.
4.5.1. 2,2-Bis24-methoxyphenyl)-[2H]-pyran[5,6-c]dibenzo-
thiophene 24c). 2 h 15 min re¯ux, 56% yield, puri®cation
by column chromatography, eluent CH2Cl2/pentane
32±1:3±1), gave an off-white solid, mp 184±185. IR:
3048, 2954, 2904, 2834, 1606, 1511, 1442, 1401, 1371,
1297, 1247, 1174, 1031, 829, 752, 705, 626. UV/Vis 3closed
form): 304 31 19,150), 318 31 26,130), 356 31 5400), 372 31
5570). 1H NMR 3CDCl3): 3.78 36H, s, 2£OCH3); 6.21 31H,
d, Jˆ9.9 Hz, H-3); 6.74 31H, d, Jˆ9.9 Hz, H-4); 6.83±6.88
34H, m, H-30, 50, 300 and 500); 7.14 31H, d, Jˆ8 Hz, H-5);
7.41±7.46 36H, m, H-8, 9, 20, 60, 200 and 600); 7.66 31H, d,
Jˆ8 Hz, H-6); 7.84±7.87 31H, m, H-10); 8.06±8,08 31H, m,
H-7). 13C NMR 3CDCl3): 55.2 32C, q); 83.2 3s); 113.4 34C,
C-30, 50, 300 and 500); 114.3 3d); 118.4 3s); 121.7 3d); 122.9
3d); 123.0 3d); 0123.4 3d); 124.3 3d); 126.6 3d); 126.9 3s);
128.1 34C, C-2 , 60, 200 and 600); 128.9 3d); 135.8 3s); 137.1
32C, C-10 and 100); 137.5 3s); 140.2 3s); 147.4 3s); 158.9 32C,
C-40 and 400). MS: 450 3100, M1z); 343 346); 300 36); 271
4.4.2. 2,2-Diphenyl-[2H]-pyran[6,5-a]dibenzothiophene
24b). 1 h 45 min re¯ux, 43% yield, puri®cation by column
chromatography, eluent CH2Cl2/pentane 31:4), gave a
yellow solid, mp 134±135. IR: 3056, 3023, 2921, 1602,
1479, 1444, 1405, 1214, 1056, 1000, 960, 902, 813, 767,
742, 698. UV/Vis 3closed form): 304 31 17,190), 318 31
1
23,020), 347 31 4370), 362 31 4430). H NMR 3CDCl3):
6.27 31H, d, Jˆ9.8 Hz, H-3); 6.77 31H, d, Jˆ9.8 Hz, H-4);
7.13 31H, d, Jˆ7.9 Hz, H-5); 7.26±7.79 32H, m, H-40 and
400); 7.30±7.35 34H, m, H-30, 50, 300 and 500); 7.41±7.43 31H,

M. M. Oliveira et al. / Tetrahedron 58 :2002) 1709±1718
1717
35); 251 35); 225 37). Anal. calcd for C29H22O3S: C 77.33; H
4.89; S 7.11; found C 77.17, H 4.90, S 7.11.
191 19); 165 37). Anal. calcd for C28H18O2S: C 80.36; H
4.33; S 7.66; found C 80.01, H 4.41, S 7.41.
4.6. Synthesis of pyranodibenzothiophenes 624, 723 and 88
4.6.3. 6-Carboxy-2,2-diphenyl-[2H]-pyran[5,6-c]dibenzo-
thiophene 28). To a solution of 6 30.400 g, 0.85 mmol) in
dry ether 320 ml), n-BuLi 31.6 M in hexanes, 0.85 mmol)
was added dropwise with stirring and under Ar. The mixture
was left at rt for 1 h and an excess of dry ice was added and
left stirring for 30 min. The mixture was hydrolysed with
cold water 340 ml), washed with NaOH 5% 33£50 ml) and
acidi®ed with HCl 2N. The solid was ®ltered and recrystal-
lised from pentane to give a white solid 30.160 g, 48%). Mp
241±242. IR: 3058 3OH), 2998, 2881, 2672, 2561, 1677
3CvO), 1536, 1444, 1419, 1284, 1220, 1174, 1068, 1002,
732, 698. UV/Vis 3closed form): 325 31 10,160), 352 31
4.6.1. 6-Cyano-2,2-diphenyl-[2H]-pyran[5,6-c]dibenzo-
thiophene 26). To a solution of 5a 30.567 g; 1.20 mmol)
in DMF 39 ml), 1,10-bis3diphenylphosphin) ferrocene
3dppf) 30.133 g, 0.240 mmol) and Pd23dba)3 30.055 g,
0.060 mmol) were added. When the reaction mixture
reached 908C, Zn3CN)2 30.169 g, 1.44 mmol) was added
over a 2 h period and heating continued for 3 h. The mixture
was cooled to rt, ®ltered through Celite 545, washed with
CH2Cl2 and the solvent was evaporated. The residue was
poured into water and extracted with a saturated solution of
NaCl. The org. layer was dried 3MgSO4) and the solvent was
removed under reduced pressure. The residue was puri®ed
by column chromatography 3CH2Cl2/pentane 1:4) yielding
the product as a white solid 30.130 g, 30%). Mp 212±214.
IR: 3064, 3031, 2213 3CN), 1643, 1598, 1544, 1473, 1444,
1261, 1153, 1002, 944, 908, 736, 700. UV/Vis 3closed
form): 318 31 8966), 331 31 11,300), 351 31 4626), 367
1
4752), 369 31 3869). H NMR 3acetone): 6.33 31H, d,
Jˆ9.6 Hz, H-3); 6.79 31H, d, Jˆ9.6 Hz, H-4); 7.28±7.38
37H, m, H-9, 30, 40, 50, 300, 400 and 500); 7.45 31H, t,
J1ˆJ2ˆ7.8 Hz, H-8); 7.83 31H, s, H-5); 7.87 31H, ld,
Jˆ7.8 Hz, H-10); 8.83 31H, ld, Jˆ7.8 Hz, H-7), the
COOH proton was not observed. 13C NMR 3Acetone):
85.1 3s); 118.0 3s); 123.1 3d); 123.3 3s); 123.8 3d); 125.3
3d); 127.3 34C, C-20, 60, 200 and 600); 127.4 3d); 127.5 3d);
128.3 3d); 128.7 32C, C-40 and 400); 129.0 3s); 129.2 34C,
C-30, 50, 300 and 500); 130.7 3d); 135.5 3s); 136.0 3s); 141.2 3s);
145.3 32C, C-10 and 100); 150.1 3s); 169.1 3s). MS: 420 3100,
M1z); 390 312); 371 35); 357 353); 313 39); 284 36); 191 311);
165 37). Anal. calcd for C28H18O3S: C 77.40; H 4.18; S 7.38;
found C 72.62, H 4.02, S 7.25.
1
31 4415). H NMR 3CDCl3): 6.36 31H, d, Jˆ10 Hz, H-3);
6.77 31H, d, Jˆ10 Hz, H-4); 7.27±7.33 32H, m, H-40 and
400); 7.34±7.40 34H, m, H-30, 50, 300 and 500); 7.47±7.52 35H,
m, H-9, 20, 60, 200 and 600); 7.53 31H, s, H-5); 7.54±7.57 31H,
m, H-8); 7.89±7.92 31H, m, H-10); 8.86±8.89 31H, m, H-7).
13C NMR 3CDCl3): 84.8 3s); 98.3 3s); 117.5 3s); 118.7 3s);
121.6 3d); 123.0 3d); 123.8 3d); 125.1 3d); 126,7 34C, C-20,
60, 200 and 600); 128.0 32C, C-40 and 400); 128.1 3d); 128,4 34C,
C-30, 50, 300 and 500); 129.6 3d); 129.8 3d); 133.8 3s); 137.0
3s); 140.8 3s); 143.8 32C, C-10 and 100); 150.8 3s), the C-6b
signal was not detected. MS: 415 3100, M1z); 338 353); 309
39); 208 36); 191 314); 165 38); 77 35). Anal. calcd for
C28H17SNO: C 80.94; H 4.12; N 3.37, S 7.72; found C
80.63, H 4.20, N 3.13, S 7.46.
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377 31 5692). H NMR 3Acetone): 6.36 31H, d, Jˆ10 Hz,
H-3); 6.85 31H, d, Jˆ10 Hz, H-4); 7.29±7.39 37H, m, H-9,
30, 40, 50, 300, 400 and 500); 7.48±7.54 35H, m, H-8, 20, 60, 200
and 600); 7.75 31H, s, H-5); 7.89±7.92 31H, m, H-10);
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126.7 34C, C-20, 60, 200 and 600); 127.5 3d); 127.6 3d); 127.6
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129.0 3s); 129.3 3d); 130.9 3d); 135.2 3s); 137.1 3s); 141.0 3s);
144.0 32C, C-10 and 100); 151.7 3s); 190.1 3s). MS: 420 3100,
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