Synthesis and photochromic behaviour of a series of benzopyrans bearing an: N -phenyl-carbazole moiety: Photochromism control by the steric effect

Article abstract of DOI:10.1039/d0pp00202j

Five new N-phenyl-carbazole benzopyrans bearing different substitutions on one of the phenyl rings at the sp3 carbon have been synthesized. Their molecular structures were investigated by X-ray and NMR analyses and through quantum chemical calculations. T

Full text of DOI:10.1039/d0pp00202j

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ARTICLE
Synthesis and photochromic behaviour of a series of benzopyrans
bearing an N-phenyl-carbazole moiety: photochromism control by
steric effect
Received 00th January 20xx,
Accepted 00th January 20xx
Michel Frigoli,*a Tanguy Jousselin-Oba,a Masashi Mamada,b Jérôme Marrot,a Agnese Zangarelli,c
DOI: 10.1039/x0xx00000x
Danilo Pannacci,c Chihaya Adachi,b,d and Fausto Ortica*c,e
Five new N-phenyl-carbazole benzopyrans bearing different substitution on one of the phenyl rings at the sp3 carbon have
been synthesized. Their molecular structures were investigated by X-ray and NMR analyses and through quantum
chemical calculations. The photochromic mechanism under UV irradiation in toluene, consisting in the consecutive
formation of transoid-cis and (TC) and transoid-trans (TT) isomers, was studied by UV-vis spectral and kinetic analyses.
These molecules have been specifically design to ascertain the possibility of favouring the formation of the less
thermodynamically stable TT at the photostationary state, upon exploiting steric hindrance effects on the diene part of the
molecule. The spectrokinetic study allowed most of the spectrokinetic parameters, such as molar extinction coefficients,
quantum yields of UV colouration and visible photobleaching, the rate constants of the fast and slow thermal bleaching
processes to be estimated. Peculiar effects of substituents with different donor strengths on one phenyl ring located at the
3-position were observed on the spectrokinetic properties.
.
bleaching to the starting material can only be achieved by both
thermal and photochemical pathways. The TC forms are more
thermally labile than the TT ones, whilst their thermodynamic
Introduction
Benzo- and naphthopyrans are well known classes of photochromic
compounds1,2 that, in consequence of their usual fatigue resistance,
have received great attention over the latest years in light of their
possible practical and industrial applications,3 especially in the field
of ophthalmic lens production. The mechanism of photochromic
reaction of benzo- and naphthopyrans was first investigated by
Becker and coworkers4,5 and, unlike other photochromic materials
which usually display either P-Type or T-Type behaviour,6 they
exhibit a more complicate reaction mechanism. Indeed, upon
steady UV irradiation, the phenomenology of these compounds can
be conveniently described in terms of photoproduction of transoid-
cis (TC) and transoid-trans (TT) structures7,8 and their complete
stability is generally higher than that of TT.9,10 This behaviour has
also been observed for the reference molecule having this skeleton,
3,3-diphenyl-3H-naphtho[2,1-b]pyran (herein named RN, Scheme
1), that was extensively reported in the literature, especially over
the last three decades.5,7,11-27 The transformation of a benzo- or a
naphthopyran into a photochromic system possessing only a T-Type
or a P-Type behaviour might undoubtedly be of interest from both
points of view of basic research and practical applications. This
could be achieved by choosing a suitable solvent, the temperature
range or introducing modifications of the molecular structure.
a. Université Paris-Saclay, UVSQ, UMR CNRS 8080, Institut Lavoisier de Versailles, 45
av des Etats-Unis - 78035 Versailles, France. Email: michel.frigoli@uvsq.fr
b. Center for Organic Photonics and Electronics Research (OPERA), JST ERATO
Adachi Molecular Exciton Engineering Project, and Academia-Industry Molecular
Systems for Devices Research and Education Center, Kyushu University, Kyushu
University, Nishi, Fukuoka, 819-0395 Japan.
c. Dipartimento di Chimica, Biologia e Biotecnologie, Università degli Studi di
Perugia, Via Elce di Sotto 8 - 06123 Perugia, Italy. Email: fausto.ortica@unipg.it
d. International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu
University, Nishi, Fukuoka 819-0395, Japan.
Scheme 1 Mechanism of the photochromic reaction of 3,3-
diphenyl-3H-naphtho[2,1-b]pyran.
e. Istituto Nazionale di Fisica Nucleare (INFN), Sezione di Perugia - Via Pascoli -
06123 Perugia, Italy.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
On the one hand, a fully thermoreversible photochromic
naphthopyran might be interesting in the realization of fast
responsive optical switches while, on the other hand, a completely
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photoreversible molecule could be useful to produce bistable All reactions were carried out under argon. Dichloromethane was
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DOI: 10.1039/D0PP00202J
devices and systems for optical storage. Some examples have been distilled from P2O5, THF and toluene were distilled over
recently reported in the literature, especially on the possibility to Na/benzophenone. Dichloromethane and toluene were kept over
transform naphtopyrans into thermally reversible compounds, activated 3Å molecular sieves. 1,2-Dichloroethane was dried over
either by substituting the pyran ring with a furan one28,29 or the activated 3Å molecular sieves. All commercial reagents were used
synthesis of fused-naphthopyrans30,31 or through an on-demand without further purification. 1H and 13C NMR spectra were recorded
control of the photochromic properties, achieved by increasing the at room temperature on Bruker Avance-300 MHz NMR
1
ring-size of the alkylenedioxy moiety on the naphthopyrans.32
spectrometer. H NMR spectra were recorded at 300 MHz and 13C
One alternative approach, based on the attempt to synthesize a NMR spectra were recorded at 75 MHz. Chloroform residual peak
prevalently photoreversible naphthopyran, has been reported by was taken as internal reference at 7.26 ppm for 1H NMR and 77
one of us, carried out through the control of the photochromism of ppm for 13C NMR. Acetone residual peak was taken as internal
1
naphthopyran derivatives by means of intramolecular CH-π bonds.33 reference at 2.09 ppm for H NMR and 205.87 ppm for 13C NMR.
Indeed, the photochromism of naphthopyran was shown to be DMSO residual peak was taken as internal reference at 2.54 ppm
under thermodynamic control. In the framework of this project, we for 1H NMR and 40.45 ppm for 13C NMR. High-resolution mass
have also reported afterwards some studies on helicene- spectra were obtained by using Waters Xevo Q-Tof using positive
naphtopyrans,34,35 confirming that the improvement of the mode. Infrared spectra were recorded from Nicolet 6700 FT-IR
thermodynamic stability of the TT isomers, compared to the TC spectrometer.
ones, played an important role to increase the spectrokinetic
contribution of the TT to the overall mechanism of photochromic Instruments and methods
reaction, along with a reduced importance of the TC role at the UV-vis absorption spectra were recorded on a Perkin-Elmer Lambda
photostationary state. In both cases mentioned above, even though 800 double-beam spectrophotometer. For monitoring the spectral
TT was more thermodynamically stable than TC, we could argue modifications of the molecules in real time under either UV or
that the return from TT to TC then to CF through isomerization of visible irradiation, a HP 8453 diode-array spectrophotometer was
one double bond was made more difficult due to the steric used. Photoreaction measurements were carried out in toluene
hindrance of the helical part of the photochrome on the dienic (purchased from Merck, spectrophotometric grade, used as
moiety. One question is raised: is it possible to favour the formation received) in a 1 cm cell path in the spectrophotometer holder with
of TT compared to TC at the photostationary state while TT is less the irradiating and the probe beams forming a right angle. A 125 W
thermodynamically stable than TC just by crowding the diene part xenon lamp, coupled with a Jobin-Yvon H10 UV monochromator for
of the molecule?
the wavelength choice and a fibre-optic system, was employed as
Herein, we have specifically designed a photochromic system, the irradiating source for the photoreaction studies. The quantum
namely
3,3,13-triphenyl-3,13-dihydrochromeno[5,6-a]carbazole yields of the photocolouration and photobleaching processes were
(CN-Ph-H), to respond to the above question and prepare some determined by spectrophotometry, using potassium ferrioxalate
derivatives for which one phenyl group located at the 3-position actinometry to measure the radiation intensity, typically of the
order of 510-6 moles of photons dm-3 s-1.
was systematically functionalised with different donor groups, such
as di(pyridin-2-yl)amine (CN-Ph-N(Py)2), diphenylamine (CN-Ph-
N(Ph)2), methoxy (CN-Ph-OMe) and dimethylamine (CN-Ph-N(Me)2),
as shown in Figure 1, in order to study their impacts on the
spectrokinetic properties and more specifically to see whether is
there any relationship between the rate constants of bleaching of
TC and TT.
Fluorescence measurements were recorded on
a Spex
Fluorolog-2 1680/1 instrument, equipped with a system for spectral
correction; the quantum yields were determined by using quinine
sulphate in 1N H2SO4 (quantum yield F = 0.54636) as the standard.
The corrected areas of the sample and the standard emissions were
compared by using the following equation (1), which accounts for
the differences in absorbance and refraction index of the sample
(AF, nF) and standard (ASt, nSt) solutions.
(퐴푟푒푎)퐹 퐴푆푡 푛2퐹
휙퐹 = 휙푆푡
(1)
2
(퐴푟푒푎)푆푡 퐴퐹 푛
푆푡
Sample concentrations were always adjusted to keep the
absorbance below 0.1. The accuracy in the F values is estimated to
be within 10%.
Fig. 1 Molecular structure of the N-phenyl carbazole benzopyrans
X-Ray diffraction (XRD) analysis was conducted on Single-crystal
diffractometer Bruker AXS X8 APEX I using Mo radiation.
For the HPLC analysis, either a Gemini C18, 5 μm, 4.6×150 mm
column or a Prontosil 200-3-C30, 3 μm, 4.6×150 mm column were
alternatively used on a Waters HPLC facility, equipped with a
(CN-Ph-R).
Experimental
General Methods for Synthesis
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Waters 600 controller and a Waters 996 photodiode array detector.
Pure acetonitrile or an acetonitrile/water mixture up to 95/5
volume ratio were used as the eluent.
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DOI: 10.1039/D0PP00202J
Results and discussion
Synthesis
Scheme 4 Final steps in the synthesis of the photochromic
materials.
The synthesis of the photochromic materials relies first on the
preparation of 11-phenyl-11H-benzo[a]carbazol-2-ol, which was
obtained in two steps from the known 2-methoxy-11H-
benzo[a]carbazole, as depicted in Scheme 2.37 The first step was a
N-arylation of the carbazole moiety using catalyst derived from CuI
and trans-1,2-cyclohexanediamine as proposed in 2002 by
Buchwald and co-workers. Under this condition, N-arylated
product, compound 2 was obtained in 80% yield.38 The second step
X-ray analysis
Suitable monocrystal of CN-Ph-H for X-ray analysis was obtained.
The crystals have a monoclinic symmetry with C2/c space group.
Two enantiomers are present in the crystals. They differ from the
position of the phenyl ring C compared to the chromene ring of the
molecule. In Figure 2a, the ring C is behind the chromene ring. The
molecule is stabilized with three hydrogen bonds. There is a CH-
interaction between proton 3’’ and the ring C. The geometry of the
edge-to-face interaction is as follows: the distance between the
proton and the ring centroid of the ring C is of 3.228 Å and the
dihedral angle between the interacting ring planes is of 53.44°. A
CH-O bond between proton 3’ and oxygen 4 is observed with a
distance of 2.308 Å and the angle between C-H3’ and O4 is of
102.81°. There is also a CH-N bond between proton 1 and the
nitrogen 13 with a distance of 2.583 Å and the angle between C-H1
and N14 is of 120.83°. The distance between protons 1 and 2 is of
1.318 Å, which is typical of a localized double bond. The dihedral
angle between the two plans formed by the two phenyl rings A and
B is of 77.92°. It should be noted that for the TC form, ring A is
slightly less twisted with the diene part of the molecule compared
to ring B whereas for TT, it is ring B which is less twisted. Actually,
the phenyl group facing the proton is more planar.
was
a
simple demethylation using boron tribromide as
demethylation agent and the desired product was prepared in 95 %
yield.
Scheme 2 Preparation of 11-phenyl-11H-benzo[a]carbazol-2-ol.
The introduction of 2,2′-dipyridylamine and diphenylamine in one
phenyl of the photochromic materials lead the preparation of the
ketones 4 and 5 respectively, which were obtained by using
Ullmann condensation between 4-Bromobenzophenone and the
corresponding amine (Scheme 3). The reaction worked well with
2,2′-dipyridylamine since the corresponding ketone 4 was obtained
in 80% yield. The ketone 5 functionalised with diphenylamine was
obtained in only 33% yield.39
Scheme
3
Preparation of 2,2′-dipyridylamine- and 2,2’-
diphenylamine-benzophenones.
From commercially available 4-methoxybenzophenone and 4-
dimethylaminobenzophenone and the benzophenone 4 and 5, the
corresponding propargylic alcohols were prepared by the addition
of the lithium-trimethylsilylacetylene on the benzophenone,
followed by the cleavage of the trimethylsilyl group with an excess
of potassium carbonate in MeOH / THF, as shown in Scheme 4.
Then, the condensation between compound
3 and 1,1-
diphenylprop-2-yn-1-ol commercially available or the propargylic
alcohols 6-9 provided the desired photochromic materials (Scheme
4).
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TC and TT are stabilized by at least three hydrogen bonds as for the
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DOI: 10.1039/D0PP00202J
closed form. As for RN, TT is less thermodynamically stable than TC
because the diene part of the molecule interacts less with the rest
of the molecule. Indeed, the dihedral angle between carbons 2 and
13a is of 41.62° for TT, larger than that TC for which the dihedral
angle is of 31.45°. This difference also explains why TT absorbs at
lower wavelengths than TC. Since RN is known to display T-type
photochromism in solution, would it be the same for CN-Ph-H and
the other derivatives?
Photochromic properties
The quantitative absorption spectra of the five CN-Ph benzopyrans
in toluene are shown below (Fig.3). All compounds exhibit similar
features showing a structure that is probably due to at least two
different electronic transitions. The small differences are related to
the changes in the substituent on one of the benzene rings located
in the 3-position. The absorption spectra of the photochromes
correspond nearly to the sum of the naphthopyran core and the
substituted phenyl at the 3 position.
Fig. 2 a) X-ray structure of CN-Ph-H; Optimized structures at CAM-
B3LYP of b) TC form and c) TT form.
Thermodynamic stability
So far, P-type photochromism based on [3H]-naphthopyran was
observed for systems for which the TT form was more
thermodynamically stable than TC form. Therefore, theoretical
calculations were performed at CAM-B3LYP/6-31G(d,p) and M06-
2X/6-31G(d,p) level of theory since they are optimized to reproduce
well weak interactions that are essential in the investigated
compound, and compared with the reference [3H]-naphthopyran
(RN) (see Table 1)
Fig.
3 Quantitative absorption spectra of the five N-phenyl
carbazole benzopyrans in toluene.
CN-Ph-H When a toluene solution containing one of the N-phenyl
carbazole benzopyrans is exposed to UV light, a common trend is
observed for the five molecules. Upon UV irradiation, an initial
steep rise of absorbance in the visible region is followed by a
longer-lived further increase, up to the attainment of
a
Table 1 Relative Gibbs free energies [kcal mol-1] in toluene
computed by using DFT for the different systems under
investigation. All values were calculated by using the most stable
isomer as a reference CF.
photostationary state. The kinetic behaviour of the
photocolouration curve, characterized by a clear biexponential
trend, is shown in Fig.4a for CN-Ph-H. Once the UV irradiation is
interrupted, a fast thermal decay occurs, along with a rapid
decrease of the absorbance in the visible, which stops after a few
hundreds of seconds (second part of the kinetics in Fig.4a). The
residual colouration only bleaches if visible irradiation is carried out,
up to a complete fading of the colour (third part of the kinetics in
Fig.4a). Otherwise, if the coloured chromene is kept in the dark
after the fast thermal decay following the attainment of the
photostationary state and it is not subjected to any kind of further
irradiation, it undergoes a very slow thermal decolouration, which
only becomes barely perceptible after a few months (Fig.S3).
Compound CF
TC
TT
TT − TC
Level
of theory
RN
CN-Ph-H
CAM-
B3LYP
8.39
M06-
2X
11.41
9.67
CAM-
B3LYP
9.55
M06-
2X
12.21
10.62
CAM-
B3LYP
1.16
1.55
M06-
2X
0.80
0.95
/
0
0
6.74
8.29
For RN and CN-Ph-H, the closed form is the most
thermodynamically stable, followed by the TC form, then by TT
form, whatever the level of theory. As shown in Figure 2 b) and c),
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In addition to that, when the solution containing the chromene a consecutive 2-step reaction in a transoid-cis open coloured isomer
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was only exposed to UV light for a short time, usually about a (TC) which in turn, upon prolonged irradiation, eventually evolves
hundred of seconds, and then its thermal decay was monitored into the more thermally stable transoid-trans open coloured isomer
once the irradiation had been discontinued, a complete rapid (TT). The latter is responsible for the residual colouration that
decolouration of the system was observed (Fig. 4b).
persists over months if the system is kept stored in the dark and
bleaches completely to the starting material upon visible
irradiation. This reaction mechanism is further supported by the
short irradiation experiment reported in Figure 4b, where the
thermally unstable TC open isomer, immediately formed upon UV
photocolouration, does not further evolve into the TT form once
the irradiation is interrupted, but undergoes thermal bleaching up
to a complete fading. Indeed, after a prolonged UV irradiation,
upon injection of the solution containing the photostationary state
mixture of CN-Ph-H into the HPLC column, only two peaks were
revealed, related to the residual initial chromene and the TT isomer,
respectively. Owing to its relatively fast thermal decolouration
process, the presence of the TC species could not be detected in the
chromatogram.
In the light of all these findings, HPLC separation of the
photostationary state mixture allowed the pure TT isomer to be
collected. The complete quantitative absorption spectrum of the TT
isomer could then be obtained upon selecting a wavelength where
only very slight absorbance changes under irradiation are observed
and setting the TT molar extinction coefficient equal to that of the
initial closed form at that wavelength. A more precise approach
would be that based on the use of a real isosbestic point, that
however, in the present case, owing to the mechanism of reaction
involving three different species could not be detected. The
quantitative absorption spectrum of the TT isomer of CN-Ph-H is
displayed in Fig.5.
An algorithm was then used to determine also the quantitative
absorption spectrum of the TC isomer. After UV irradiation,
followed by HPLC separation, the TT concentration could be
obtained by means of the TT, evaluated according to the
aforementioned procedure, and the absorbance value of the kinetic
profile at 500 nm, measured after the thermal decay following the
photostationary state had occurred. This would also allow the
conversion % of the starting molecule into the TT isomer to be
evaluated. Then, considering the ratio between the peak areas of
the closed form and the TT open isomer observed at the HPLC once
the TC decay is complete, at a wavelength that best approximates
an isosbestic point, the following relation holds:
Fig. 4 Kinetic profile monitored at 500 nm for CN-Ph-H obtained
upon: a) consecutive UV irradiation at  = 340 nm, thermal decay
and final visible decolouration at  = 500 nm; b) short UV irradiation
at  = 340 nm, followed by complete thermal bleaching.
The decay rate constants for both the “fast” and the “slow”
processes, obtained through a first order treatment of the kinetics,
are reported in Table 3. The foregoing results can be explained in
terms of the following reaction scheme (Scheme 5), which is based
on the aforementioned experimental data and on results previously
reported in the literature on similar naphthopyrans.33-35,40
퐴푟푒푎푇푇
[푇푇]
=
(2)
퐴푟푒푎푐표푙표푢푟푙푒푠푠
[푐표푙표푢푟푙푒푠푠′]
Therefore, the concentration of the closed form after the complete
TC thermal decay, [colourless’], could be determined. The
absorbance decrease at 500 nm on going from the photostationary
state to the complete decay of the TC species is correlated to the
corresponding increase in the colourless form concentration
originated from the TC thermal bleaching, therefore:
Scheme 5 Mechanism of the photochromic reaction, showing the
chromene (CF), and the main coloured isomers, transoid-cis (TC)
and transoid-trans (TT).
Based on this reaction mechanism, the initial chromene, in its
closed colourless form, upon UV irradiation is transformed through
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′′
퐴
― 퐴′500 푛푚
[푐표푙표푢푟푙푒푠푠′] ― [푐표푙표푢푟푙푒푠푠′′]
500 푛푚
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=
(3)
[푐표푙표푢푟푙푒푠푠′′]
′′
500 푛푚
퐴
In eq.6, the first term refers to the finite variation of absorbance
in the early time interval of the UV irradiation kinetics, therefore
corresponds to the formation of the TC coloured isomer. TC is the
molar extinction coefficient of the TC, CF→TC is the quantum yield of
TC formation, A’TC is the total absorbance at the irradiation
wavelength and I° is the intensity of the irradiation light,
determined by chemical actinometry. Once the TC at the
wavelength where the kinetic has been recorded is known, the
quantum yield value of TC formation from the initial closed form,
where the prime symbol refers to the situation after the TC thermal
decay has occurred, while the second one is related to the
photostationary state. Once the concentration of the closed form at
the photostationary state, [colourless”], has been determined from
(3), the concentration of the TC isomer can be evaluated, and the
molar extinction coefficient of the TC isomer at a particular
wavelength can be derived:

CF→TC = 0.43, could be obtained (Fig. S4).
Even though the TT isomer exhibits a notable thermal stability
[푻푪] = [풄풐풍풐풖풓풍풆풔풔′] ― [풄풐풍풐풖풓풍풆풔풔′′]
(4)
in the dark at room temperature in toluene solution, its
transformation to the CF via TC can be accelerated by means of
visible light irradiation. In our case, the CN-Ph-H was irradiated at
435 nm, and the corresponding photobleaching kinetics at 500 nm
was recorded (Fig.6).
′′
푨
― 푨′ퟓퟎퟎ 풏풎
ퟓퟎퟎ 풏풎
ퟓퟎퟎ
=
푻푪

(5)
[푻푪]
However, once the TC concentration has been obtained (eq.4), one
can subtract the contributions of the CF and the TT species from the
total absorption spectrum at the photostationary state, using their
quantitative spectra and their concentrations at the
photostationary state itself ([colourless”] and [TT]). The complete
quantitative spectra of the TC isomer can then be derived and it is
shown in Fig.5. The contribution of the TC isomer is clearly higher at
longer wavelengths, as expected from the literature.
Fig. 6 Kinetics of visible photobleaching (irr. = 435 nm) of CN-Ph-H
in toluene, monitored at 500 nm. Inset: linear interpolation of the
first points of the decay (see below).
The TT absorption signal follows a first order decay and a procedure
similar to that already employed to determine the quantum yield of
UV photocolouration can also be used for visible
photodecolouration data treatment, by taking into account the first
experimental points of the bleaching kinetics, where only the TT
isomer significantly contributes to total absorbance:
Fig. 5 Quantitative absorption spectra of the TC and TT coloured
isomers obtained upon UV irradiation of CN-Ph-H in toluene.
As clearly visible from the first part of the kinetic profile in Fig.4a,
the UV photocolouration kinetics of the CN-Ph molecules exhibit a
biexponential behaviour, as expected in such systems characterized
by a consecutive three-step reaction mechanism. A complete
quantitative treatment to provide the quantum yields of all the
reaction steps involved, by solving the kinetic equations in a
rigorous way, is practically impossible in this situation.8 However, if
one takes into account only the early instants of the UV irradiation,
when only the TC isomer is substantially formed, as also pointed out
by the short irradiation tests, the initial rate method is helpful to
obtain the quantum yield  for the conversion of CF into TC:
Δ퐴푇푇
′
푇푇
= 휀푇푇
휙
푇푇⟶퐶퐹퐼0
(
1 ― 10 ― 퐴
)
(7)
Δ푡
In eq.7, TT is the molar extinction coefficient of the TT, TT→CF is the
quantum yield of visible photobleaching, A’TT is the total
absorbance at the irradiation wavelength and I° is the intensity of
the irradiation light, determined by chemical actinometry. The
value of the quantum yield of visible irradiation, TT→CF, is reported
in Table 3. Finally, as previously mentioned, from the concentration
value of the TT isomer, determined after the complete thermal
decay of the TC following the photostationary state attainment, the
Δ퐴푇퐶
′
푇퐶
= 휀푇퐶
휙
퐶퐹⟶푇퐶퐼0
(
1 ― 10 ― 퐴
)
(6)
Δ푡
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percentage of transformation of the initial chromene into the stable
coloured form could be calculated (Table 3).
View Article Online
DOI: 10.1039/D0PP00202J
CN-Ph-R The photobehaviour of the four substituted molecules
(CN-Ph-R; with R = -OMe, -N(methyl)2, -N(phenyl)2 and –N(pyridyl)2)
is substantially similar to that of the unsubstituted compound (CN-
Ph-H). The UV photocolouration was characterized by
a
biexponential trend and, once the photostationary state had been
attained, interruption of the irradiation only caused a partial
bleaching of the system, and the residual colouration persisted
throughout several weeks. Full decolouration was only achieved
upon visible irradiation of the solution with complete restoration of
the initial chromene. However, unlike CN-Ph-H, when the solution
at the photostationary state was injected into the HPLC column,
three peaks were observed instead of two, corresponding to three
different thermally stable molecules, as shown in Fig.7 for CN-Ph-
OMe.
Scheme 6 Mechanism proposed for the photochromic reaction of
the CN-Ph-R chromenes, showing the initial closed form (CF), and
the main coloured transoid-cis (CTC and TTC) and transoid-trans
(CTT and TTT) isomers.
HPLC analysis allowed the two TT coloured isomers to be separated
and then selectively irradiated by visible light to study their
photobleaching behaviour. The photodecolouration from TT to CF
occurred through a clear pathway and the presence of an isosbestic
point allowed the quantitative spectra of the stable TT coloured
isomers to be measured, using the same procedure already
described for CN-Ph-H. The quantitative absorption spectra of the
TT isomers are displayed in Fig.8, while their absorption maxima are
collected in Table 2, along with the normalized absorption and
TCs/TTs absorption ratio at the photostationary state. The ratios
between CTT and TTT are different between OMe and the couple of
N-(Py)2 and N-(Ph)2 derivatives. This can be explained by the
contribution of the lone pair in the -conjugation of the heteroatom
in anisole and triphenylamine groups. The oxygen in anisole has a
sp3 hybridization and therefore the lone pair of oxygen makes an
angle of 120° with the phenyl, whereas, in triphenylamine, the
nitrogen is more planar, with nitrogen-torsion angles varying
between ca. 160 and 180°, indicating the lone pair of nitrogen is
more involved in the -conjugated system.41 Since ring B in TTT is
slightly less twisted than ring A (see Fig.2), the lone pair of the
groups is slightly more involved in the whole conjugated system
when the group is attached to ring B, favouring the formation of the
TTT form. The effect is more pronounced when the nitrogen group
is involved since the lone pair of the nitrogen is more implicated in
the conjugation.
Fig. 7 Chromatogram obtained at the HPLC upon injection of a
solution of CN-Ph-OCH3 in toluene, after UV irradiation until
attainment of the photostationary state.
One signal corresponds to the residual initial chromene present at
the photostationary state, while the other peaks refer to two
different thermally stable TT isomers produced upon UV irradiation,
as clearly indicated by the spectral features observed for the two
peaks at the diode array detector of the HPLC. Indeed, in the case of
the CN-Ph-R compounds, the presence of a substituent on one of
the phenyl rings on the sp3 carbon, gives rise to two different
transoid isomers upon UV photocolouration, namely the cis-
transoid-trans (CTT) and the trans-transoid-trans (TTT); the latter
formed through the corresponding transoid-cis isomers (CTC and
TTC) (Scheme 6).
Table 2 Absorption maxima (max (nm)) of the TT isomers obtained
upon UV irradiation of the CN-Ph molecules at 340 nm; absorption
at the photostationary state measured at the longest wavelength in
the visible region (AmaxPSS/A’340nm) and at 500 nm (A500PSS/A’340nm
)
normalized to the initial absorbance of the CF at the irradiation
wavelength (340 nm); TCs/TTs absorption ratio at the
photostationary state measured at the longest wavelength in the
visible region (A%maxTC/TT) and at 500 nm (A%500TC/TT).
AmaxPSS/A’340nm
A%max
A500PSS/A’340nm
A%500
TC/TT

max (nm)
TC/TT
-H
345(sh); 384;
471
1.01
15.8
0.95
15.8
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Journal Name
-N(pyridyl)2
350;
424;
498
363;
422;
490
1.02
29.5
1.02
29.5
View Article Online
DOI: 10.1039/D0PP00202J
-N(phenyl)2
-OMe
392; 443; 518
1.00
1.04
28.7
23.1
0.92
1.03
28.6
23.1
350;
410;
490
350;
410;
486
-N(methyl)2
435; 533
0.84
38.0
0.67
37.1
CN-Ph-H has an absorption maximum (max) at 471 nm which is
bathochromic shifted by 15 nm and 19 nm for CN-Ph-OMe (486 nm)
and CN-Ph-N(pyridyl)2 (490 nm), respectively. Larger bathochromic
shifts of 47 nm and 62 nm are observed for CN-Ph-N(phenyl)2 (518
nm) and CN-Ph-N(methyl)2 (533 nm) respectively. Indeed, the max
is related to the HOMO-LUMO level, which in turn is mainly due to
the oxidation potential of the substituent (HOMO) as the acceptor
part of the molecule is the same.
Fig. 8. a) Quantitative absorption spectra of the TTT (peak 1, see
text below) forms produced upon irradiation of the CN-Ph-R
molecules, along with that of the TT obtained for the unsubstituted
CN-Ph-H; the two TTT (peak 1) and CTT (peak 2, see text) coloured
isomers produced upon irradiation of CN-Ph-N(pyridyl)2 (b) and CN-
Ph-OMe (c); d) absorption spectrum of the photostationary state
mixture obtained upon irradiation of CN-Ph-N(methyl)2.
The colourability defined as the absorbance reached at PS state
is the same for all compounds (around 1) except for CN-Ph-
N(methyl)2 which is slightly less important (0.84). At PS state, the
absorption contribution of TC form (15.8%) for CN-Ph-H is less
important than that obtained for CN-Ph-OMe (23.1%). For CN-Ph-
N(pyridyl)2 and CN-Ph-N(phenyl)2, the absorption contribution of TC
form increases to around 30% and increases even further for CN-
Ph-N(methyl)2 to 38%.
As can be inferred by Fig. 8, the molar extinction coefficients of
each pair of TT isomers for a given molecule exhibit values of the
same order of magnitude, even though the amounts formed upon
irradiation, is different (see Fig.7 for of CN-Ph-OMe). These is clearly
visible for CN-Ph-N(pyridyl)2 and the aforementioned CN-Ph-OMe.
In the case of CN-Ph-N(phenyl)2 the unbalancing between the two
TT coloured forms was so large that the area of one of the two
peaks was only barely perceptible (less than 1% of the total signal of
the coloured isomers). Therefore, it was possible isolating only one
of the two TT isomers to perform its selective photobleaching
experiment and recording its quantitative absorption spectrum.
8 | J. Name., 2012, 00, 1-3
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N(methyl)2
0.068
1.4×10-4
Irradiation of CN-Ph-N(methyl)2 did not allow any of the two TT
species to be isolated at the HPLC, due to their relatively fast
thermal decay. The different behaviour of this compound originates
only from the stronger electronic effect of the substituent, since the
steric hindrance is substantially the same for all the molecules
investigated. In consequence, it was not possible to measure the
quantitative absorption spectra of the TT isomers and only a
spectrum at the photostationary state, including of course the
residual closed form and the TT coloured species, could be reported
(Fig.8 d). For all compounds, the major TT photoproduct eluates
first using silica-C18 as non-polar stationary phase, indicating that
the same photoisomer is produced in larger quantity and
corresponds to the more polar product. Theoretical calculations
indicate that TTT has a dipolar moment higher than CTT (see table
S3 in the Supporting Information). Therefore, TTT is likely to be the
most abundant species produced at PS state. According to
theoretical calculations, TTT should also have a slightly higher molar
extinction coefficient than the one of CTT, confirming that peak 1
corresponds to TTT isomer.
View Article Online
DOI: 10.1039/D0PP00202J
From the data in Table 3, it can be seen that the rate constant of
the back reaction from TC to CF (k1) is similar for CN-Ph-H and CN-
Ph-N(pyridyl)2 even though the latter absorbs at longer wavelengths
(see Fig.8 and Table 2). k1 increases twice when N-dipyridyl group
(0.018 s-1) is changed to N-diphenyl (0.035 s-1) and increases four
times when dimethylamine (0.068 s-1) is used. Clearly, increasing
the donor strength reduces the stability of the TC form. Surprisingly,
k1 of CN-Ph-OMe (0.042 s-1) is similar to the one of CN-Ph-
N(phenyl)2 (0.035 s-1) indicating that a methoxy and diphenylamine
groups have similar electronic effect. This result is very interesting
since diphenylamine group allows to shift the absorption behaviour
by 32 nm in comparison with a methoxy functionalisation, while the
rate constants are similar for both groups. The same trend is
observed for the rate constant of the back reaction from TT to CF
(k2) as that obtained for k1. It is worthwhile to notice that upon
increasing k1 by a factor 2 between CN-Ph-N(pyridyl)2 and CN-Ph-
N(phenyl)2, k2 increases by two orders of magnitude. In the similar
manner, upon increasing k1 by 4 between CN-Ph-N(pyridyl)2 and
CN-Ph-N(methyl)2, k2 increases by four orders of magnitude. These
results indicate that the electronic effect is much more pronounced
on the stability of TT form than that of TC form. The photochemical
quantum yield from TTT to CF follows the same trend than that
obtained for k1. For the percentage conversion from CF to TT
derivatives obtained at PSS state, CN-Ph-H presents the highest
conversion of 64%. Even though the stability of the photoproducts
of CN-Ph-N(pyridyl)2 is similar to CN-Ph-H, the conversion ratio is
only 44%. The reduced conversion ratio is due to the presence of TC
forms in larger quantity at PSS state (see Table 2). Decreasing the
thermal stability of the photoisomers in CN-Ph-N(phenyl)2,
decreases even further the conversion ratio to 29%. Even if the
stability of photoisomers of CN-Ph-N(phenyl)2 and CN-Ph-OMe is
similar, the latter shows a higher conversion ratio of 43%. Once
again, this result is due to the fact that the contribution of TC form
for CN-Ph-OMe is less important (see Table 2).
The visible photobleaching reaction was also performed at 435
nm for each separated TT isomer, using potassium ferrioxalate as a
chemical actinometer. The same procedure based on the initial rate
method, already employed for the TT form of CN-Ph-H (eq.7), was
then used to measure the visible photobleaching quantum yield,

TT→CF, of the various TT isomers isolated at the HPLC for the CN-Ph-
R molecules. The values are collected in Table 3.
The percentage of transformation of the initial colourless
chromene into the stable TT isomers was also measured through
the analysis of the chromatographic traces at a suitable wavelength
where the absorbance values of all the species involved (CF and the
two TTs) were easily measurable. By taking into account the molar
extinction coefficients of the CF and the TT coloured species and
the peak areas, the % values of the stable TT isomers formed could
be calculated (Table 3).
When the thermally stable TT isomers were not subjected to
visible irradiation, their residual colour faded in the dark, at room
temperature, over relatively long times. However, the rate
constants for this thermal bleaching process exhibited a certain
dependence on the kind of substituent present on one of the
phenyl rings on the sp3 carbon of the chromene structure (Table 3).
Decay rate constants ranged from the order of 10-8 s-1, as in the
case of CN-Ph-H (Fig.S5a) and CN-Ph-N(pyridyl)2 to 10-4 s-1 for CN-
Ph-N(methyl)2 (Fig.S5b).
Owing to the presence of two TC forms in the reaction
mechanism, in the cases of the CN-Ph-R substituted molecules it
was impossible to use the algorithm previously illustrated for CN-
Ph-H to obtain the molar extinction coefficients and the
quantitative absorption spectra of the TC isomers. In consequence,
quantum yields CF→TC for the conversion of CF into TC could not be
measured. Nevertheless, upon short-term UV irradiation followed
by complete thermal decay we could follow the bleaching kinetics
of the signal corresponding to the sum of the two TC isomers
converting back to the starting material. The rate constant values of
this thermal bleaching are influenced by the presence of the various
substituents on one of the phenyl rings linked to the sp3 carbon
atom (Fig.9 and Table 3) and exhibit a trend that is similar to that
observed for the thermal decay of the TT species.
Table 3 Percentage of TT formation from CF, quantum yields of TT
visible photobleaching and decay rate constants for the thermal
bleaching processes of TC (k1) and TT (k2) isomers.
%
CFTT1
%
CFTT2
TT1CF
TT2CF
k1
k2(TT1)
(s-1
k2(TT2)
(s-1
(s-1
)
)
)
H
64
38
26
31
0.010
0.012
0.019
0.029
0.022
0.018
0.035
0.042
3.9×10-8
3.0×10-8
1.0×10-6
8.4×10-7
N(pyridyl)2
N(phenyl)2
OMe
6
3
0.042
0.042
3.6×10-8
1.0×10-6
12
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algorithm was used to obtain the complete quantitative absorption
View Article Online
DOI: 10.1039/D0PP00202J
spectrum of the fast decaying TC isomer. For CN-Ph-H, the highest
percentage of conversion (64%) of the initial chromene into the
stable TT coloured isomer was measured, under our experimental
conditions.
We investigated the effect of substituents with different donor
strengths on one phenyl ring located at the 3-position on the
spectrokinetic properties. While increasing the donor strengths
shifts the absorption maximum of the photoproducts, the stability
of TC and TT forms is decreasing. Reducing the stability of TC forms
by two or four times, the stability of the corresponding TT forms is
reduced by two- or four orders of magnitude, respectively.
Interestingly, the electronic effects of N-diphenyl and methoxy
groups on the stability of photoproducts are similar, while N-
diphenyl group leads to a significant shift of the absorption
maximum. This N-diphenyl functionalisation at the 3-position can
be interesting for future design of naphthopyrans.
Fig. 9 Normalized decay kinetics of the TC isomers obtained after
short-time UV irradiation of the five CN-Ph-R molecules.
Fluorescence studies Given that the N-phenyl carbazole is a
chromophore that exhibit fluorescence emission42 and that
photochromism usually takes place from the lowest excited singlet
state, we explored the possible fluorescence emission upon UV
excitation of the five molecules investigated. This analysis was
carried out to ascertain a possible competition between these two
pathways in the deactivation of the lowest singlet excited state. The
molecules were excited at 340 nm, the same wavelength employed
for the photochromic studies, and the integrated emission spectra
were compared to that obtained with quinine sulphate, used as a
standard, under the same experimental conditions. However, for all
five molecules very weak and noisy emission spectra were
recorded, with very low fluorescence quantum yields of the order
of F  710-4. Therefore, the possible detrimental contribution of
fluorescence to the photochromic reaction can be neglected under
our experimental conditions.
If on the one hand the unsubstituted compound CN-Ph-H, and
to a lesser extent also CN-Ph-N(pyridyl)2, CN-Ph-N(phenyl)2 and CN-
Ph-OMe point towards a P-type photochromism, on the other hand
CN-Ph-N(methyl)2, functionalised with the strongest electron
donating group, rather goes in the direction of
a T-type
photochromic compound. These results indicate the possibility to
tune the type of photochromism from P-type to T-type by
functionalisation. In this perspective, the effect of the substituent is
of notable importance in providing some useful hints for the
synthesis of new benzopyrans possessing a long term stability of the
TT(s) coloured isomer(s) in the dark.
Conflicts of interest
There are no conflicts to declare.
Conclusions
Acknowledgements
In this paper, the photochromism of
a series of newly
F.O., A.Z. and D.P. gratefully acknowledge the financial support
of the Ministero per l'Università e la Ricerca Scientifica e
Tecnologica (Rome, Italy), the University of Perugia [PRIN
2010-2011, 2010FM738P], the Istituto Nazionale di Fisica
Nucleare (I.N.F.N.); M.M. and C.A. acknowledge financial
support from JST ERATO Grant Number JPMJER1305, and JSPS
KAKENHI Grant Number 19H02790; M.F. and J.M.
acknowledge financial support from the Agence Nationale de
la Recherche ANR-16-CE07-0024 (GATE). T. J-O. acknowledges
CNRS for PhD grant.
synthesized N-phenyl carbazole benzopyrans has been investigated
in toluene solution. The unsubstituted compound, CN-Ph-H, show
P-type photochromism, even though the TC form is more
thermodynamically stable than TT form, indicating that the
photochromism is not always under thermodynamic control. In the
present case, the P-type photochromism is obtained thanks to the
steric hindrance provided by the phenyl group connected to the
nitrogen on the diene part of TT forms, making the photo- and
thermal-isomerisation more difficult.
The mechanism of the photochromic reaction, consisting of two
consecutive steps where the TC(s) and the TT(s) coloured isomers
are consecutively formed upon UV irradiation, has been elucidated.
The spectrokinetic study allowed most of the molar extinction
coefficients, the quantum yields of visible photobleaching, and the
rate constants of the fast and slow thermal bleaching processes to
be estimated.
References
1
B. Van Gemert in Organic Photochromic and Thermochromic
Compounds, Vol.1 (J. C. Crano, R. J. Guglielmetti), New York,
Kluwer Academic/Plenum Publishers, 1999, pp. 111–140.
J. D. Hepworth, B. M. Heron in Functional Dyes (Ed.: S.-H.
Kim), Amsterdam, Elsevier, 2006, pp. 85–135.
2
3
In the case of the unsubstituted compound, CN-Ph-H, the
quantum yield of UV photocolouration was also determined and an
N. S. Corns, S. M. Partington and A. D. Towns, Industrial
10 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 11 of 13
Photochemical & Photobiological Sciences
Please do not adjust margins
Journal Name
ARTICLE
organic photochromic dyes, Color. Technol., 2009, 125, 249–
naphtho[2,1-b]pyran, J. Phys. Org. Chem., 20V0ie7w, A2rt0ic,le 9O3nl6in–e
261.
943.
DOI: 10.1039/D0PP00202J
4
5
J. Kolc and R. S. Becker, Proof of structure of the colored 22 B. Moine, J. Réhault, S. Aloïse, J.-C. Micheau, C. Moustrou, A.
photoproducts of chromenes and spiropyrans, J. Phys.
Chem., 1967, 71, 4045–4048.
C. Lenoble and R. S. Becker, Photophysics, photochemistry
and kinetics of photochromic 2H-pyrans and chromenes, J.
Photochem., 1986, 33, 187–197.
Samat, O. Poizat and G. Buntinx, Transient Absorption
Studies of the Photochromic Behavior of 3H-Naphtho[2,1-
b]pyrans Linked to Thiophene Oligomers via an Acetylenic
Junction, J. Phys. Chem. A, 2008, 112, 4719–4726.
23 J. Harada, K. Ueki, M. Anada, Y. Kawazoe and K. Ogawa,
Solid-state Photochromism of Chromenes: Enhanced
Photocoloration and Observation of Unstable Colored
Species at Low Temperatures, Chem. Eur. J., 2011, 17,
14111–14119.
6
7
H. Bouas-Laurent and H. Dürr, Organic photochromism, Pure
Appl. Chem., 2001, 73, 639–665.
G. Ottavi, G. Favaro and V. Malatesta, Spectrokinetic study of
2,2-diphenyl-5,6-benzo(2H)chromene:
a thermoreversible
and photoreversible photochromic system, J. Photochem. 24 T. T. Herzog, G. Ryseck, E. Ploetz and T. Cordes, The
Photobiol. A, 1998, 115,123–128.
photochemical ring opening reaction of chromene as seen by
transient absorption and fluorescence spectroscopy,
Photochem. Photobiol. Sci., 2013, 12, 1202–1209.
8
9
S. Delbaere, G. Vermeersch and J.-C. Micheau, Quantitative
analysis of the dynamic behaviour of photochromic systems,
J. Photochem. Photobiol. C, 2011, 12, 74–105.
25 Y. Inagaki, Y. Kobayashi, K. Mutoh, and J. Abe, A Simple and
Versatile Strategy for Rapid Color Fading and Intense
Coloration of Photochromic Naphthopyran Families, J. Am.
Chem. Soc., 2017, 139, 13429–13441.
R. Demadrille, A. Rabourdin, M. Campredon and G. Giusti,
Spectroscopic characterisation and photodegradation
studies
of
photochromic
spiro[fluorene-9,3′-[3′H]-
naphtho[2,1-b]pyrans], J. Photochem. Photobiol. A, 2004, 26 S. Brazevic, S. Nizinski, R. Szabla, M. F. Rode and G.
168, 143–152.
Burdzinski, Photochromic reaction in 3H-naphthopyrans
studied by vibrational spectroscopy and quantum chemical
calculations, Phys. Chem. Chem. Phys., 2019, 21, 11861–
11870.
10 C. D. Gabbutt, M. Heron, A. C. Instone, S. B. Kolla, K.
Mahajan, P. J. Coelho and L. M. Carvalho, Synthesis and
photochromic properties of symmetrical aryl ether linked bi-
and tri-naphthopyrans, Dyes Pigm., 2008, 76, 24–34.
11 R. S. Becker and J. Michl, Photochromism of Synthetic and
Naturally Occurring 2H-Chromenes and 2H-Pyrans, J. Am.
Chem. Soc., 1966, 88, 5931–5933.
27 S. Brazevic, M. Baranowski, M. Sikorski, M.Rode and G.
Burdzinski, Ultrafast dynamics of the transoid-cis isomer
formed in photochromic reaction from 3H-naphthopyran,
ChemPhysChem, 2020, 10.1002/cphc.202000294.
12 B. Luccioni-Houze, M. Campredon, R. Guglielmetti and G. 28 C. M. Sousa, J. Berthet, S. Delbaere, and P. J. Coelho,
Giusti, Kinetic Analysis of Fluoro-[2h]-Chromenes at the
Photostationary States, Mol. Cryst. Liq. Cryst., 1997, 297,
161–165.
Synthesis of 1-Vinylidene-naphthofurans: A Thermally
Reversible Photochromic System That Colors Only When
Adsorbed on Silica Gel, J. Org. Chem., 2013, 78, 6956–6961.
13 J. J. Luthem, The Effect of Substituents on the Ring Closure 29 C. Sousa, S. Saraiva, H. Macedo and P. Coelho, Grey colouring
Reaction of Photo Activated Diarylnaphthopyrans, Mol.
Cryst. Liq. Cryst., 1997, 297, 155–160.
thermally
reversible
photochromic
1-vinylidene-
naphthofurans, Dyes Pigm., 2017, 141, 269–276.
14 A. Kumar, The Relationship Between the Structure and the 30 C. M. Sousa, J. Berthet, S. Delbaere, A. Polonia, and P. J.
Absorption Spectra of Naphtho[2,1-B]pyran, Mol. Cryst. Liq.
Cryst., 1997, 297, 139–145.
Coelho, Control of the Switching Speed of Photochromic
Naphthopyrans through Restriction of Double Bond
Isomerization, J. Org. Chem., 2017, 82, 12028−12037.
15 J.Hobley, V. Malatesta, R. Millini, W. Giroldini, L. Wis, M.
Goto, M. Kishimotoa and H. Fukumura, Photochromism of 31 C. M. Sousa and P. J. Coelho, Joining High Coloration and Fast
chromene crystals; a new property of old chromenes, Chem.
Commun., 2000, 1339–1340.
Color Fading with Photochromic Fused-Naphthopyrans, Eur.
J. Org. Chem., 2020, 2020, 985–992.
16 J. Aubard, F. Maurel, G. Buntinx, O. Poizat, G. Levi, R. 32 H. Kuroiwa, Y. Inagaki, K. Mutoh, and J. Abe, On-Demand
Guglielmetti and A. Samat, Femto/Picosecond Transient
Absorption Spectroscopy of Photochromic 3,3-
Diphenylnaphtho[2,1-b]pyran, Mol. Cryst. Liq. Cryst., 2000, 33 M. Frigoli, F. Maurel, J. Berthet, S. Delbaere, J. Marrot and M.
345, 215–220.
Control of the Photochromic Properties of Naphthopyrans,
Adv. Mater., 2019, 31, 1805661_1–1805661_5.
M. Oliveira, The Control of Photochromism of
[3H]‑Naphthopyran Derivatives with Intramolecular CH-π
Bonds, Org. Lett., 2012, 14, 4150–4153.
17 S. Delbaere and G. Vermeersch, NMR characterization of
allenyl-naphthol in the photochromic process of 3,3-
diphenyl-[3H]-naphtho[2-1,b]pyran,
Photobiol., 2003, 159, 227–232.
J.
Photochem. 34 M.Frigoli, J.Marrot, P.L.Gentili, D.Jacquemin, M.Vagnini,
D.Pannacci and F.Ortica, P-type Photochromism of new
18 P. L. Gentili, E. Danilov, F. Ortica, M. A. J. Rodgers and G.
Favaro, Dynamics of the excited states of chromenes studied
by fast and ultrafast spectroscopies, Photochem. Photobiol.
Sci., 2004, 3, 886–891.
19 R.G. Brown, M. Maafl, P. Foggi and L. Bussotti, The ultrafast
dynamics of some photochromic naphthopyrans, in
Femtochemistry and Femtobiology, M.M. Martin and J. T.
Hynes (editors) (2004) 283–286.
Helical Naphthopyrans: Synthesis, and Photochemical,
Photophysical and Theoretical Study, ChemPhysChem, 2015,
16, 2447–2458.
35 F.Ianni, S.Scorzoni, P.L.Gentili, A.Di Michele, M.Frigoli,
E.Camaioni, F.Ortica and R.Sardella, Chiral separation of
helical chromenes with chloromethyl phenylcarbamate
polysaccharide-based stationary phases, J. Sep. Sci., 2018,
41, 1266–1273.
20 F. Maurel, S. Lau Troung, J. P. Bertigny, R. Dubest, G. Lévi, J. 36 S. R. Meech and D. Phillips, Photophysics of some common
Aubard, S. Delbaere and G. Vermeersch, SERS Study of 3,3- fluorescence standards, J. Photochem., 1983, 23, 193–217.
Diphenyl-naphtho[2,1-b]pyran: Another Evidence for Allenyl- 37 M.M. Oliveira, M.A. Salvador, P.J. Coelho and L.M. Carvalho,
Naphthol Involvement in the Photochromic Mechanism, Mol.
Cryst. Liq. Cryst., 2005, 430, 235–241.
New benzopyranocarbazoles: synthesis and photochromic
behavior, Tetrahedron, 2005, 61, 1681–1691.
21 B. Moine, G. Buntinx, O. Poizat, J. Rehault, C. Moustrou and 38 J. C. Antilla, A. Klapars, S. L. Buchwald, The Copper-Catalyzed
A. Samat, Transient absorption investigation of the
photophysical properties of new photochromic 3H-
N-Arylation of Indoles, J. Am. Chem. Soc., 2002, 124, 11684–
11688.
This journal is © The Royal Society of Chemistry 20xx
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Page 12 of 13
ARTICLE
Journal Name
39 J. Pang, Y. Tao, S. Freiberg, X-P. Yang, M. D’Iorioa and S.
Wang, Syntheses, structures, and electroluminescence of
new blue luminescent star-shaped compounds based on
1,3,5-triazine and 1,3,5-trisubstituted benzene, J. Mater.
Chem., 2002, 12, 206–212.
View Article Online
DOI: 10.1039/D0PP00202J
40 M. M. Oliveira, M. A. Salvador, S. Delbaere, J. Berthet, G.
Vermeersch, J.C. Micheau, P. J. Coelho and L. M. Carvalho,
Remarkable thermally stable open forms of photochromic
new N-substituted benzopyranocarbazoles, J. Photochem.
Photobiol. A., 2008, 198, 242–249.
41 P. R. Andrews, S. L. A. Munro, M. Sadek, M. G. Wong, The
hybridization state of nitrogen as a conformational variable
in biologically active molecules, J. Chem. Soc., Perkin Trans.
2, 1988, 711-718
42 I. B. Berlman, Handbook of fluorescence spectra of aromatic
molecules, Academic Press, 1971, p. 213.
12 | J. Name., 2012, 00, 1-3
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The phenyl group in blue controls the photochromism of [3H]-naphthopyran derivatives by favouring
the photoinduced formation of TT isomer over TC at the photostationary state one by steric
hindrance.