New benzopyranocarbazoles: Synthesis and photochromic behaviour

Article abstract of DOI:10.1016/j.tet.2004.12.055

The synthesis of three new benzopyranocarbazoles (=[indole]naphthopyrans) from hydroxybenzo[a]carbazoles is described. The photochromic properties of these novel compounds were investigated under flash photolysis and continuous irradiation. Compared to known [indole]benzopyrans these new compounds showed a significant bathochromic shift in the spectra of the open forms, an increase in colourabilities and slower ring closure kinetics. The photochromic behaviour of compound 4 has been further investigated. Continuous near-UV irradiation led to the formation of one photoisomer (TC) that was subsequently partially converted, to the other (TT). Thermal reversion of the preirradiated system to the original form was only partial and followed a monoexponential decay involving the back-conversion of the TC-isomer to the uncoloured closed form (CF). The thermally stable TT-isomer could only be photobleached with visible light. This process was shown to proceed through a fast photoconversion TT→TC followed by the thermal path TC→CF. Thermal relaxation of the activated system was also studied at various temperatures.

Full text of DOI:10.1016/j.tet.2004.12.055

Tetrahedron 61 (2005) 1681–1691
New benzopyranocarbazoles:
synthesis and photochromic behaviour
*
´
M. Manuel Oliveira, Maria A. Salvador, Paulo J. Coelho and Luıs M. Carvalho
´
´
Centro de Quımica-Vila Real, Universidade de Tras-os-Montes e Alto Douro, 5001-911 Vila Real, Portugal
Received 28 September 2004; revised 7 December 2004; accepted 23 December 2004
Available online 15 January 2005
Abstract—The synthesis of three new benzopyranocarbazoles (Z[indole]naphthopyrans) from hydroxybenzo[a]carbazoles is described.
The photochromic properties of these novel compounds were investigated under flash photolysis and continuous irradiation. Compared to
known [indole]benzopyrans these new compounds showed a significant bathochromic shift in the spectra of the open forms, an increase in
colourabilities and slower ring closure kinetics. The photochromic behaviour of compound 4 has been further investigated. Continuous near-
UV irradiation led to the formation of one photoisomer (TC) that was subsequently partially converted, to the other (TT). Thermal reversion
of the preirradiated system to the original form was only partial and followed a monoexponential decay involving the back-conversion of the
TC-isomer to the uncoloured closed form (CF). The thermally stable TT-isomer could only be photobleached with visible light. This process
was shown to proceed through a fast photoconversion TT/TC followed by the thermal path TC/CF. Thermal relaxation of the activated
system was also studied at various temperatures.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
phenomenon and has resulted in the determination of
intrinsic properties of the process which is responsible for
the reversible change.7–9 Normally, however, the UV
spectroscopy only gives global information about the
photochromic system and does not give individual infor-
mation about each of the various forms obtained under
irradiation.
Benzo- and naphthopyrans derivatives are one of the most
studied and important class of photochromic molecules.1,2
They were originally studied for their potential in offering
complementary colours (yellow to orange) to the well
known (blue) indolinospironaphthoxazines.3 Nowadays, it
is possible to obtain any colour using only naphthopyrans.4
Owing to their good photochromic properties, associated
with high fatigue resistance, this family of compounds has
been used in recent years in the manufacture of photo-
chromic lenses that darken in sunlight.5
Recently, the photochromic process has been deeply studied
by 1H, 19F and 13C NMR spectroscopy. This technique
allowed the elucidation of important mechanistic and
structural features along with the calculation of kinetic
and thermodynamic parameters. From these studies, it is
apparent that benzo- and naphthopyrans, under continuous
UV irradiation, usually generate two major photoproducts,
namely the trans–cis (TC) and the trans–trans (TT) forms.
Both are responsible for the colour obtained after
irradiation; however, usually, the latter isomer was found
to be the most thermally stable.10–12
Under UV irradiation these molecules, in solution or
incorporated in polymer matrices, undergo a pyran-ring
opening due to the breaking of the C(sp3)–O bond leading to
an equilibrium between the uncoloured closed form (CF)
and a set of coloured stereoisomers of the open form (OF),
having different stabilities (Scheme 1).6 Usually the
phenomenon is thermally reversible, although it is well
known that in some cases it can be also photoinduced with
visible light.
Benzo- and naphthopyrans containing a fused carbazolo or
indolo group exhibit interesting photochromic properties
and have been patented.13–16 In previous works, we
described the photochromic properties of indolo-fused
benzopyrans.17,18 These compounds showed some interest-
ing properties such as a high photocolouration efficiency in
the near-UV and two absorption bands in the visible range.
Since naphthopyrans are more photochromic and less
fatigue prone than benzopyrans (because they are activated
Typically, UV–Vis spectroscopy has been used to study this
Keywords: Photochromism; Benzopyranocarbazoles; Naphthopyrans;
Heterocycles; Spectrokinetic; Hydroxybenzo[a]carbazoles.
* Corresponding author. Tel.: C351 259 350284; fax: C351 259 350480;
e-mail: lcarv@utad.pt
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.12.055

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M. M. Oliveira et al. / Tetrahedron 61 (2005) 1681–1691
Scheme 1.
with less energetic light) and the photochromic properties
can be dramatically changed by structural features1,4,19 we
decided to synthesise and study new indolo-fused
naphthopyrans.
(p-chloroanil) in xylene21,22 affording methoxybenzo[a]-
carbazoles 2a–c in very good yields (80–96%). This two
step procedure represents an easier route to prepare
methoxybenzo[a]carbazoles over the Fischer–Borsche16
method because it avoids bubbling the hydrazone inter-
mediate with gaseous HCl. Demethylation of methoxyl
groups using pyridine hydrochloride21–23 afforded
the hydroxybenzo[a]carbazoles 3a–c in good yields
(63–85%).
During the evaluation of spectrokinetic parameters, we have
found that one of the compounds was particularly suitable to
make further studies that allowed to elucidate some
interesting features about the photochromic mechanism in
the naphthopyrans family.
Naphthopyrans are usually prepared through reaction of
naphthols with 1,1-diarylprop-2-yn-1-ol under acid
catalysis. For basic naphthols, such as hydroxybenzo[a]-
carbazoles 3a–c, an alternative method involving the
organotitanium mediated condensation with an a,b-unsatu-
rated aldehyde is more adequate. The reaction of a-phenyl-
cinnamaldehyde with a TiIV ‘phenolate’, obtained by adding
Ti(OEt)4 to the hydroxybenzo[a]carbazoles 3a–c and
separating the ethanol formed by azeotropic distillation,
leads to C-alkylation in ortho-position that through a
subsequent electrocyclisation yielded the naphthopyrans 4
(63%), 5 (24%) and 6 (77%) (Scheme 3).24
2. Results and discussion
2.1. Synthesis
The new benzopyranocarbazoles 4–6 were prepared in four
steps from methoxytetralones 1a–c as outlined in Schemes 2
and 3. The 5-, 6- and 7-methoxytetralones 1a–c were
refluxed 1 h with phenylhydrazinium chloride in ethanol, in
the presence of a few drops of acetic acid20 and were then
dehydrogenated with tetrachloro-1,4-benzoquinone
Scheme 2.

M. M. Oliveira et al. / Tetrahedron 61 (2005) 1681–1691
1683
Scheme 3.
2.2. Photochromic properties
to the apparition, in the closed forms, of strong UV
absorption bands shifted further towards the visible (see
Figure 1 for comparison of Ref1 and compound 4). This
feature is important as the activation with less energetic
radiation improves both the sensitiveness to solar light and
also the fatigue resistance.2
Although benzopyranocarbazoles with an indole moiety
fused to the f-face (5,6-positions) of a 2,2-diphenyl-2H-
naphtho[1,2-b]pyran nucleus were already described in
prior art,16 the compounds 4–6, 2,2-diphenyl-2H-
naphtho[1,2-b]pyran fused to the i-face and 3,3-diphenyl-
3H-naphtho[2,1-b]pyran fused to the i- and k- faces are now
described for the first time. The new naphthopyrans exhibit
photochromic behaviour at room temperature in toluene
solutions. The relevant spectrokinetic parameters (acti-
vation wavelengths of closed forms, maxima wavelengths
of the coloured forms, colourability and thermal bleaching
rates) were evaluated under flash photolysis and continuous
near-UV irradiation. Both methods are extensively used to
quantify spectrokinetic parameters of organic photo-
chromes, but the information obtained by the two methods
can be very distinct because time scales of observation and
light flux intensities applied are completely different.
Methods employing continuous irradiation use a longer
irradiation time and a lower intensity light flux and are
usually well suited to the study of slow systems. Moreover
they operate in experimental conditions quite comparable to
those that are found in applications where sunlight
activation is intended. The data obtained are summarized
in Table 1 together with data obtained with the correspond-
ing pyranocarbazoles (Ref1, Ref3 and Ref4)18 and the
reference naphthopyrans (Ref2 and Ref5)25,26 for compara-
tive purposes.
2.2.2. Maxima wavelengths of coloured forms. Compared
to the corresponding 2H- and 3H-naphthopyrans, the fusion
of an indole ring to naphthopyrans led to a global
bathochromic shift in the spectra of the open forms. This
effect was already observed for 2H-1-benzopyrans fused to
an indole moiety.17,18 From a general point of view
comparing the two sets of compounds with a fused indole
moiety, both under flash photolysis and continuous
irradiation, the introduction of an additional benzene ring
led to the loss of the two band profile in the visible
absorption spectra of the coloured forms. One exception was
observed, under continuous irradiation, for the open form of
compound 4 (2,2-diphenyl-2H-naphtho[1,2-b]pyran with
the indole fused to 7,8 positions) that displays two
significant absorption bands in the visible range (Fig. 1).
The same was observed for 2,2-diphenyl-2H-naphtho[1,2-
b]pyran with the indole group fused to the 5,6 positions.24
No clear electronic effect could be readily observed due to
the presence of the indole nitrogen atom. Electron-donating
substituents at the 7-position of diphenylnaphthopyrans
have little effect on the maxima absorption wavelength of
the open form and at the 10-position have not been
reported.2,4
2.2.1. Activation wavelengths of uncoloured forms.
Compared to the reference pyranocarbazoles, the introduc-
tion of an additional benzene ring, in the f- and h-faces (5,6-
and 7,8-positions) of the 2H-1-benzopyran ring system, led
2.2.3. Colourability and thermal bleaching rate. All the
new described compounds exhibit better colourabilities than
the corresponding 2H-1-benzopyrans fused to an indole

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M. M. Oliveira et al. / Tetrahedron 61 (2005) 1681–1691
Table 1. Maxima wavelengths of the coloured forms (lmax, nm), colourability (A0 and Aeq), fading rate (kD,sK1) of compounds 4–6 and five reference
compounds in toluene solutions under flash photolysis (2.5!10K5 M at 25 8C) and continuous irradiation (1!10K4 M at 20 8C)
Compound
Flash photolysis open form
Continuous irradiation open form
lmax
A0
kD (%)
lmax
Aeq
kD (%)
415
542
1.8
0.26
1!10K2 (38)
4!10K3 (62)
4!10K3 (71)
2!10K3 (29)
Ref1
415
2.22
414
517
1.34
1.38
1!10K3 (100)
(414 and 517 nm)
4
531
1.1
1!10K3 (100)
403
481
1.08
1.62
6!10K4 (98)
3!10K4 (2)
Ref2
!0.01
469
0.72
460
549
0.68
0.61
Ref3
4!10K2 (100)
460
0.28
0.04 (100)
5
490
3.2
3!10K2 (100)
485
0.37
0.13 (100)
443
590
1.1
0.29
0.17 (94)
2!10K2 (6)
0.10 (80)
6!10K3 (20)
Ref4
443
0.15
4!10K2 (84)
9!10K3 (16)
6
474
1.9
467
0.61
3!10K2 (100)

M. M. Oliveira et al. / Tetrahedron 61 (2005) 1681–1691
1685
Table 1 (continued)
Compound
Flash photolysis open form
Continuous irradiation open form
lmax
A0
kD (%)
lmax
Aeq
kD (%)
7!10K2 (80)
3!10K3 (20)
Ref5
432
0.84
0.09
432
0.21
Ref1Z2,2-diphenyl-2H-pyran[3,2-c]carbazole,18 Ref2Z2,2-diphenyl-2H-naphtho[1,2-b]pyran,25 Ref3Z2,2-diphenyl-2H-pyran[6,5-c]carbazole,18 Ref4Z
2,2-diphenyl-2H-pyran[5,6-a]carbazole18 and Ref5Z3,3-diphenyl-3H-naphtho[2,1-b]pyran.26
moiety, but slower thermal bleaching kinetics. This points to
an increase in the thermal stability of the open forms, due to
the extension of p-conjugation, and results in higher
concentrations of coloured forms in the mixture obtained
upon irradiation. The effect can be explained by the
decrease of nonbonding interactions in the open forms,
promoted by the spacing of the indole moiety imposed by
the additional benzene ring.
photoproducts that possess very similar absorption spectra,
but one is thermally unstable and the other has a high
thermal stability.8 According to recent studies these
photoproducts can be identified, as the trans–cis (TC) and
trans–trans (TT) isomers (Scheme 1) and the fast fading
phase, of higher amplitude, can usually be attributed to the
TC-isomer.11
2.3. Mechanistic studies
As generally observed for photochromic naphthopyrans,
compounds 5 and 6 (5,6 annellation) are less thermally
stable than compound 4 (7,8 annellation). Compared to
compounds 4 and 5, under flash photolysis compound 6
exhibits a different bi-exponential bleaching kinetic.
Observing the structures of the TT and TC open forms for
compound 6 it is apparent, particularly for the TT-isomer,
the possible existence of steric interactions, between the NH
function and one ethylenic H-atom, that can affect the pyran
ring opening/closing process, leading to two thermally
unstable isomers with two observable kinetic constants.
2.3.1. The effect of different times of irradiation. The
most interesting observed feature is that compounds 4 and 5,
under flash photolysis or continuous irradiation, follow a
bleaching monoexponential model. Compound 6 exhibits a
biexponential bleaching kinetic under flash photolysis, but
follows a monoexponential model under continuous
irradiation. This is indicative that, for this compound,
there is a significant difference in the relative amounts of
photoisomers in the mixture obtained after irradiation in
both methods.
After continuous irradiation, all the new compounds
exhibited thermal bleaching kinetics that follow the
monoexponential law AZA0eKktCR (R is the residual
absorption of the solution) from which the kinetic constants
were determined. Consequently, after the UV irradiation has
ceased all the solutions were only partially bleached and, in
the dark, a significant residual colour remained for a long
time. This behaviour indicates the formation of two
For compound 4 it was observed a noteworthy agreement
between the bleaching kinetic obtained in both under flash
photolysis and continuous irradiation. This may indicate a
close similarity between the photoproducts formed in the
two methods. A series of experiments was performed with
compound 4, in the dark at 293 K, evaluating the bleaching
kinetics after different times of continuous irradiation
Figure 1. Absorbance spectra of compound 4 and Ref1 before and after UV–Vis irradiation.

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M. M. Oliveira et al. / Tetrahedron 61 (2005) 1681–1691
Figure 2. Absorbance decrease at 517 nm after different times of irradiation (toluene solution 1!10K4 M of compound 4 at 293 K). Residual absorbances
were measured after the system has reached an apparent constant value.
during the colouration process. The results are depicted in
Figure 2.
investigated by spectrophotometric methods8 and 19F
NMR spectroscopy.10
In all experiments the system exhibited the same kinetic
behaviour. However, as the time of irradiation became
longer, the residual colour of the solution increased. The
residual colour can be attributed to the presence of slow
decaying coloured species and/or to the formation of
coloured photodegradation products. The last assumption
was not considered because degradation was estimated to be
less than 3%, based on the colourabilities obtained in
successive colouration/decolouration cycles. The
residual coloured system, on irradiating with visible light
(O420 nm), returned to the uncoloured state and recovered
almost the same photochromic behaviour as before. From
these observations, it is apparent that the residual colour
should be attributed to the presence of a thermally stable
isomer.
2.3.2. Visible irradiation and temperature effects. The
bleaching was complete only by irradiating with visible
light (O420 nm). A series of experiments was performed in
order to study the effect of visible light irradiation on the
kinetic behaviour of the system at different temperatures.
The results are summarized in Table 2.
A monoexponential model could be considered for all the
bleaching processes, and examination of the results in
Table 2 shows that all the bleaching processes were
significantly accelerated by temperature increase (this
acceleration was accompanied by an expected decrease in
the colourability) (Fig. 3).
A remarkable feature in the results is that, for the same
temperature, the observed kinetic constants remained at the
same order of magnitude whether or not visible light was on.
The visible light, although promoting the complete bleach-
ing of solutions, has a minor effect on the values of kinetic
constants whatever the path considered: photostationary
state/residual colour, photostationary state/uncoloured
state or residual colour/uncoloured state. These results
suggest that, for this particular system, the return to the
closed uncoloured form (CF) is essentially thermal and
corresponds predominantly with a TC/CF pathway. The
TT-isomer does not proceed directly to CF but undergoes a
These results suggest that, in this system, the faster decaying
isomer (TC) is first produced and, subsequently, partially
converted, through light absorption, to the slower decaying
isomer (TT). This is in accordance with the expected
different energies required to produce each isomer. After the
C–O cleavage, promoted by UV irradiation, a one-bond
rotation is required to produce the TC-isomer, whereas
the TT-isomer requires a subsequent E–Z isomerisation of
a double bond also induced by light. The same was
already observed for photochromic 2H-chromenes
Table 2. Bleaching rate constants for compound 4 at different temperatures considering various paths
Path
Temperature (K)
Visible light (O420 nm)
k (sK1
)
Photostationary state/residual colour
Residual colour/uncoloured state
Photostationary state/uncoloured state
Photostationary state/residual colour
Residual colour/uncoloured state
Photostationary state/uncoloured state
Photostationary state/residual colour
Photostationary state/residual colour
283
283
283
293
293
293
313
323
Off
On
On
Off
On
On
Off
Off
2.6!10K4
2.5!10K4
2.6!10K4
1.0!10K3
8.1!10K4
7.9!10K4
8.6!10K3
2.3!10K2

M. M. Oliveira et al. / Tetrahedron 61 (2005) 1681–1691
1687
Figure 3. Kinetic behaviour of compound 4 at two different temperatures.
fast photoisomerization TT/TC, induced by visible
irradiation. Complete bleaching is then achieved through
the slow rate determining thermal process TC/CF. This
suggestion is further confirmed with additional experiments
irradiating the system with visible light for short time
periods, after the first thermal fading. It was observed that it
was unnecessary to irradiate with visible light during the
whole decolouration process to decolourize the solution
completely. With our irradiating device, 180 s of visible
irradiation was enough for the system to proceed to the
complete bleaching in the dark (Fig. 4).
process, using a Eyring plot (Fig. 6): DH‡Z82.2G
3.4 kJ molK1 and DS‡ZK22.3G5.6 J molK1 KK1. The
enthalpy of activation value agrees with those referred for
photochromic compounds (40–120 kJ molK1).11 The nega-
tive value obtained for DS‡ points to an expected loss of
freedom in the recyclization into the original closed form
but is difficult to interpret as it includes an unknown
contribution from the rearrangement of the solvent
(toluene).
3. Conclusion
Based on these informations, the major processes occurring
during a photocolouration/decolouration cycle are depicted
in Figure 5.
The inclusion of an additional benzene ring in pyrano-
carbazoles yielded three new photochromic benzopyrano-
carbazoles. All the compounds described exhibited strong
absorption bands in the range 370–390 nm which made
them readily activated with solar light and potentially less
prone to photodegradation. The fusion of an indole moiety
2.3.3. Activation energies. The experiments carried out at
different temperatures allowed us to evaluate the standard
entropy and enthalpy of activation, for the TC/CF
Figure 4. Thermal and photochemical bleaching for compound 4 at 293 K (toluene solution 1!10K4 M; visible light O420 nm).

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M. M. Oliveira et al. / Tetrahedron 61 (2005) 1681–1691
Figure 5. Isomerization processes occurring during a photocolouration/decolouration cycle for compound 4.
Figure 6. Eyring plot for the thermal relaxation process TC/CF.
at the i-face of 2,2-diphenyl-2H-naphtho[1,2-b]pyran or i-
and k- faces of 3,3-diphenyl-3H-naphtho[2,1-b]pyran
induced some thermal instability to the open forms.
However, it is apparent that electronic effects are modest
and no relevant additional steric strain is promoted in open
forms. Compared to pyranocarbazoles, no relevant enhance-
ment in the colourabilities was observed, and the effects
seem to be related to the increase in the thermal stabilities of
the open forms due to the extension of conjugation. In the
dark, all the UV-irradiated solutions exhibited a partial
monoexponential thermal bleaching and a persistent
relevant residual colour, which could be removed only
through the irradiation with visible light. This points to a
three-component system with an original uncoloured
compound (CF) giving, upon UV-irradiation, two coloured
photoisomers (TC and TT) with very different thermal
stabilities.
The photochromic mechanism was investigated for com-
pound 4 and, based on the results, the following plausible
reaction mechanism could be proposed (Scheme 4).
In solution, UV irradiation induces the opening of the pyran
ring through the C–O bond breakage followed by a one-
bond rotation leading to the TC-isomer. Subsequently,
this isomer is partially converted to the TT-isomer through a
E–Z isomerisation of a double bond, also induced by UV
light. At the photostationary state, the coloured mixture
includes CF, TC and TT and, when the light was turned off,
Scheme 4.

M. M. Oliveira et al. / Tetrahedron 61 (2005) 1681–1691
1689
the thermal bleaching observed is mainly due to the process
TC/CF, and a thermal process TT/CF can be excluded.
Irradiation with visible light promotes the disappearance of
the persistent residual colour through the fast back-
conversion TT/TC followed by the thermal conversion
TC/CF.
thermostated (10, 20, 40 and 50 8C) 10 mm quartz cell,
containing the sample solution (3.5 mL), equipped with
magnetic stirring was used. In a preliminary experiment, the
visible absorption spectrum of the closed form and the lmax
of the open form were determined. In a second experiment
the absorbance at photostationary equilibrium, Aeq, was
measured at lmax and then the decrease in the absorbance
with the time was monitored. The rate constants were
calculated using mono and multiexponential models.
4. Experimental
4.1. General remarks
4.3. General method for the synthesis of compounds 2a–c
¨
Solvents (Riedel–Haen and Merck) were used without
A mixture of the appropriate methoxytetralone 1a–c (1.8 g,
10 mmol), phenylhydrazinium chloride (1.1 g, 7.6 mmol)
and a few drops of acetic acid was refluxed in ethanol
(15 mL) for 1 h. After cooling, the solid formed was filtered,
washed with water (2!20 mL) and dissolved with dichloro-
methane (10 mL). The combined organic extracts were
dried (Na2SO4) and evaporated to dryness. To the crude
(dihydrobenzocarbazoles), chloroanil (tetrachloro-1,4-
benzoquinone) (1.4 equiv) and dry xylene (50 mL) were
added and the mixture was refluxed for 2 h, under Ar. After
cooling the tetrachlorohydroquinone was filtered off by
suction and washed with Et2O. The filtrate was washed with
NaOH (10%), water and dried (Na2SO4). After solvent
evaporation Et2O was added and a precipitate was formed
that corresponded to 5-methoxy-, 6-methoxy- and
7-methoxybenzo[a]carbazole in each case.
further purification other than drying over sodium (Et2O) or
anhydrous calcium chloride (CH2Cl2). Column chromato-
graphy (CC) was performed on silica gel Merck 60 (70–230
mesh). 1H and 13C NMR spectra were recorded on a Bruker
ARX (400 and 100.5 MHz, respectively) using tetramethyl-
silane as internal standard in acetone-d6 or DMSO-d6,
respectively. Chemical shifts are given in ppm and coupling
constants in Hz. IR spectra were recorded on a Perkin–
Elmer-FTIR-1600 spectrophotometer using KBr disks and
wavenumbers are given in cmK1. UV–Vis spectra were
recorded on a CARY 50 Varian spectrophotometer using
1!10K4 M toluene solutions. Maxima wavelengths (lmax
)
are given in nm and molar absorption coefficients of closed
forms (3) in L molK1 cmK1. Mass spectra were obtained
under electronic impact (EIZ70 eV) on a AutoSpecE
spectrometer. Melting points (8C), measured in capillary
tubes on a Bu¨chi 535 apparatus, are uncorrected. All new
compounds were determined to be O95% pure by 1H NMR
spectroscopy. taZapparent triplet; slZlarge singlet; dlZ
large duplet.
4.3.1. 4-Methoxybenzo[a]carbazole 2a. Light brown
powder. Yield 90%. Mp 166–168 (lit20 168). 1H NMR
(acetone-d6): 4.11 (3H, s, –OCH3), 7.08 (1H, dl, JZ8.4 Hz,
H-3), 7.29 (1H, dt, JZ7.6, 1.2 Hz, H-8), 7.44 (1H, dt, JZ
7.6, 1.2 Hz, H-9), 7.58 (1H, t, JZ8.4 Hz, H-2), 7.66 (1H, dd,
JZ7.6, 1.2 Hz, H-10), 8.06–8.09 (m, 2H), 8.22–8.25 (m,
2H), 10.25 (1H, sl, –NH).
4.2. Spectrokinetic measurements
4.2.1. Spectrokinetic studies under flash photolysis. For
the determination of lmax, A0, and kD, 5!10K5 mol dmK3
toluene solutions were used. The flash photolysis experi-
ments were monitored by a Warner and Swasey rapid
spectrometer, allowing to record visible absorption spectra
of coloured forms in the 400–700 nm range (acquisition
time 1 ms, repetitivity 1.25 ms).27,28 Flashes (duration
50 ms) were generated by two xenon tubes with a quartz
envelope. The energy of the flashes was 60 J for the whole
polychromatic emission spectrum. For measurements,
thermostated (25 8C) 100 mm cells were used. The light
from the analysis lamp (50 W, quartz–iodide) was filtered
using a Schott GC 400 high-pass filter.
4.3.2. 3-Methoxybenzo[a]carbazole 2b. Light brown
powder. Yield 80%. Mp 243–244 (lit23 245).1H NMR
(acetone-d6): 4.00 (3H, s, –OCH3), 7.24–7.31 (2H, m, H-2
and H-8), 7.40 (1H, dt, JZ8.0, 1.0 Hz, H-9), 7.52 (1H, d,
JZ2.0 Hz, H-4), 7.62–7.64 (2H, m, H-5 and H-10), 8.17
(1H, dd, JZ8.0 Hz, 1.0, H-7), 8.20 (1H, d, JZ8.7 Hz, H-6),
8.42 (1H, d, JZ9.0 Hz, H-2), 11.20 (1H, sl, –NH).
4.3.3. 2-Methoxybenzo[a]carbazole 2c. Brown powder.
Yield 96%. Mp 186–187 (lit23 190).1H NMR (acetone-d6):
4.03 (3H, s, –OCH3), 7.22 (1H, dd, JZ9.0, 2.4 Hz, H-3),
7.27 (1H, dt, JZ7.5, 1.0 Hz, H-8), 7.43 (1H, dt, JZ7.5,
1.0 Hz, H-9), 7.63–7.67 (2H, m, H-5 and H-10), 7.95 (1H, d,
JZ2.4 Hz, H-1), 7.99 (1H, d, JZ9.0 Hz, H-4), 8.10 (1H, d,
JZ8.4 Hz, H-6), 8.20 (1H, dd, JZ7.5 Hz, 1.0, H-7), 11.25
(1H, sl, –NH).
4.2.2. Spectrokinetic studies under continuous
irradiation. For measurements of lmax, Aeq and kD under
continuous irradiation, 1!10K4 M toluene solutions were
used. Irradiation experiments were made using a CARY 50
Varian spectrometer coupled to a 150 W ozone free xenon
lamp (6255 Oriel Instruments). The light from the UV lamp
was filtered using a water filter (61945 Oriel Instruments)
and then carried to the spectrophotometer holder at the right
angle to the monitoring beam using a fiber-optic system
4.4. General method for the synthesis of compounds 3a–c
A mixture of methoxybenzo[a]carbazole 2a–c (1.24 g,
5 mmol) and pyridine hydrochloride (3.47 g, 30 mmol)
was gently boiled for 30–40 min. After cooling, water
(120 mL) was added, and the precipitate thus obtained was
filtered off. The solid was redissolved in acetone and
evaporated to dryness affording the hydroxybenzo-
[a]carbazoles.
(77654 Oriel Instruments). A light flux of 40 W mK2
,
measured with a Goldilux Photometer with a UV-A probe
was used. Visible irradiation experiments were performed
using a long-pass filter, Schott GG 420 (Oriel 59480). A

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M. M. Oliveira et al. / Tetrahedron 61 (2005) 1681–1691
4.4.1. 4-Hydroxybenzo[a]carbazole 3a. Light brown
powder. Yield 94%. Mp O250 (lit20 266). 1H NMR
(acetone-d6): 6.92 (1H, dd, JZ7.5, 2.3 Hz), 7.13–7.17
(1H, m), 7.28–7.35 (2H, m), 7.52 (1H, d, JZ7.8 Hz, 7.86
(1H, d, JZ8.2 Hz), 7.96 (1H, d, JZ9.0 Hz), 8.06–8.09 (2H,
m), 9.00 (1H, s, –OH), 11.00 (1H, sl, –NH).
368.0 (6270), 386.0 (7090). 1H NMR: 6.53 (1H, d JZ
10.0 Hz, H-3), 7.07 (1H, t, JZ7,5 Hz), 7.11–7.14 (m, 2H),
7.20–7.26 0(0m,05H), 7.30 (1H, d, JZ8.9 Hz), 7.39–7.41 (4H,
m, H-20, 2 , 6 and 600), 7.45 (1H, d JZ10.0 Hz, H-4), 7.46
(1H, d, JZ8.0 Hz), 7.69 (1H, d, JZ8.9 Hz), 8.00 (1H, d,
JZ7.8 Hz), 8.07 (1H, d, JZ8.8 Hz), 8.26 (1H, d, JZ
8.9 Hz), 11.2 (1H, sl, NH). 13C NMR: 82.3 (C-4), 111.7 (d),
113.4 (d), 115.6 (s), 116.8 (s), 117.2 (d), 117.4 (s), 119.6 (d),
119.9 (d), 120.4 (d), 120.9 (d), 123.9 (s), 124.2 (d), 124.7
(d), 126.9 (4C, C-20, 2000, 60 and 600), 127.8 (2C, C-40 and 400),
128.6 (4C, C-30, 300, 5 and 5000), 128.8 (s), 128.9 (d), 136.7
(s), 139.5 (s), 145.4 (2C, C-1 and 100), 150.4 (s). MS: m/z
(%): 423 (100), 346 (81), 315 (5), 257 (6), 211 (7), 173 (15),
165 (9). Exact mass for C31H21NO: 423.1623. Found
423.1639.
4.4.2. 3-Hydroxybenzo[a]carbazole 3b. Light brown
powder. Yield 94%. Mp O250 (lit23 265).1H NMR
(acetone-d6): 7.22–7.29 (2H, m, H-2 and H-8), 7.38 (1H,
dt, JZ7.5, 1.2 Hz, H-9), 7.42 (1H, d, JZ2.4 Hz, H-4), 7.51
(1H, d, JZ8.7 Hz, H-5), 7.62 (1H, dl, JZ8.1 Hz, H-10),
8.13–8.16 (2H, m, H-6 and H-7), 8.38 (1H, d, JZ9.0 Hz,
H-1), 8.67 (1H, s, –OH).
4.4.3. 2-Hydroxybenzo[a]carbazole 3c. Light brown
powder. Yield 65%. Mp 237–239 (lit23 246). 1H NMR
(acetone-d6): 7.10–7.17 (2H, m), 7.30 (1H, ta, JZ7.7 Hz,
H-9), 7.49–7.53 (2H, m), 7.71 (1H, sl, H-1), 7.84 (1H, dd,
JZ7.7 Hz, 1.1), 7.92 (1H, dd, JZ8.5 Hz, 1.3), 8.06 (1H, d,
JZ7.9 Hz), 8.68 (1H, s, –OH).
4.5.3. 3,13-Dihydro-3,3-diphenyl-1-benzopyran[5,6-
a]carbazole 6. White solid. Yield 77%. Mp 217.3–218.4.
IR: 3425 (NH), 3056, 1450, 1390, 1240, 809, 746, 699. UV–
Vis (closed form): 335.0 (7540), 351.1 (6950), 371.0 (5750),
390.1 (6350). 1H NMR: 6.51 (1H, d, JZ9.8 Hz, H-2), 7.23–
7.30 (3H, m, H-10 or H-11, H-40 and 400), 7.32–7.36 (5H, m,
H-30, 300, 50, 500 and H-7), 7.43 (1H, dt, JZ8.0, 1.2 Hz, H-11
or H-10), 7.59 (1H, d, JZ8.5 Hz, H-5), 7.61–7.64 (4H, m,
H-20, 200, 60, 600), 7.71 (1H, d, JZ8.0 Hz, H-12), 7.91 (1H, d,
JZ8.8 Hz, H-8), 7.99 (1H, d, JZ9.8 Hz, H-1), 8.09 (1H, d,
JZ8.5 Hz, H-6), 8.18 (1H d, JZ8.0 Hz, H-9), 10.82 (1H, sl,
N–H). 13C NMR: 82.2 (C-3), 112.6 (d), 115.3 (s), 117.6 (d),
118.0 (d), 119.8 (s), 120.4 (d), 120.5 (d), 121.5 (s), 121.7
(d), 1020 3.7 (d), 124.1 (s0), 125.7 (d), 127.7 (4C, 0C-20, 200, 0060
and 6 ), 128.3 (2C, C-4 and 400), 128.9 (4C, C-3 , 300, 50, 5 ),
129.0 (d), 129.9 (s), 132.1 (d), 135.1 (d), 140.8 (d), 145.7
(2C, C-10 and 100), 151.9 (s). MS: m/z (%): 423 (100), 346
(40), 315 (5), 257 (9), 228 (6), 212 (7), 191 (6), 173 (8), 165
(11), 77 (5). Exact mass for C31H21NO: 423.1623. Found
423.1610.
4.5. General method for the synthesis of compounds 4–6
A suspension of hydroxybenzo[a]carbazole 3a–c, (2.33 g,
10 mmol) in dry toluene (50 ml), under Ar, was stirred until
all the hydroxybenzocarbazole was dissolved. A solution of
titanium (IV) ethoxide (2.28 g, 10 mmol) in dry toluene
(40 ml) was added over a period of 10 min. The mixture was
refluxed for 30 min, and the ethanol formed was slowly
distilled (up to 1/3 of the initial volume). The mixture was
cooled to r.t. and a solution of b-phenylcinnamaldehyde
(2.08 g, 10 mmol) in 40 ml of dry toluene was added
dropwise. The mixture was refluxed for a period of 2–6 h,
cooled to r.t., quenched with NaOH (2M aq, 40 mL), and
extracted with CH2Cl2 (3!40 ml). The combined organic
extracts were dried (MgSO4), evaporated to dryness and the
residue was purified by CC on silica gel.
Acknowledgements
4.5.1. 5,13-Dihydro-5,5-diphenyl-1-benzopyran[7,8-
a]carbazole 4. Light brown solid. Yield 63%. Mp 230.5–
231.5. IR: 3424 (NH), 3056, 1517, 1490,1457, 1398, 1232,
821, 746, 700. UV–Vis (closed form): 314 (32210), 332
(14020), 347 (12040), 366 (10810), 385 (13860). 1H NMR:
6.46 (1H, d, JZ9.7 Hz,0H-4), 6.91 (1H, d, JZ9.7 Hz, H-3),
7.22–7.270(3H, m, H-4 , 400 and H-12), 7.32–7.42 (6H, m,
H-30, 30,050 , 500, H-10 and H-11), 7.61–7.63 (4H, m, H-20, 200,
60 and 6 ), 7.66 (1H, d, JZ8.3 Hz, H-1), 8.03 (1H, d, JZ
8.3 Hz, H-2), 8.18 (1H, d, JZ7.8 Hz, H-9), 8.22 (1H, d, JZ
8.8 Hz, H-7) and 8.27 (1H, d, JZ8.8 Hz, H-8) AB system,
11.30 (1H, sl, N–H). 13C NMR: 83.8 (C-5), 112.2 (d), 113.9
(d), 115.4 (d), 116.9 (d), 119.4 (s), 120.0(d), 120.3 (d), 120.5
(d), 123.5 (s0)0, 123.8 (s), 124.6 (s), 124.9 (d), 125.6 (d), 127.4
(4C, C-20, 2 , 600 and 6000), 128.3 (2C, C-40 and 400), 128.5 (d),
129.4 (4C, C-3 , 300, 5 and 500), 136.4 (s), 140.3 (s), 146.3
(2C, C-10 and 100), 149.4 (s). MS: m/z (%): 423 (100), 346
(43), 317 (6), 212 (7), 191 (7), 165 (6). Exact mass for
C31H21NO: 423.1623. Found 423.1635.
We are grateful to FCT and FEDER for financial support
through Project POCTI/Qui/38771/2001 and to R. Dubest
and J. Aubard (ESA CNRS 7086, University of Paris VII)
for the evaluation of spectrokinetic parameters by flash
photolysis.
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