Synthesis and photochromic properties of a bis(diarylethene)-naphthopyran hybrid

Article abstract of DOI:10.1016/j.dyepig.2014.12.015

The synthesis and photochromic properties of a triphotochromic molecule consisting of one naphthopyran flanked by two diarylethene units investigated by UV-Visible and Nuclear Magnetic Resonance spectroscopies are reported. Six different states resulting from the open/closed naphthopyran associated with one or two open/cyclized diarylethenes have been characterized. The photochemical and thermal interconversion between the six states of the triphotochromic target was followed allowing to determine the existing pathways upon irradiation with 313 and 365 nm light excitation, and during thermal evolution. Switching of the naphthopyran group is possible, independently of the state of the diarylethene groups. However, the diarylethene groups cannot be closed if the naphthopyran group is open.

Full text of DOI:10.1016/j.dyepig.2014.12.015

Accepted Manuscript
Synthesis and Photochromic properties of a bis(Diarylethene)-Naphthopyran Hybrid
Firealem G. Erko, Liliana Cseh, Jérôme Berthet, Georg H. Mehl, Stéphanie Delbaere
PII:
S0143-7208(14)00469-0
10.1016/j.dyepig.2014.12.015
DYPI 4615
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 13 November 2014
Revised Date: 9 December 2014
Accepted Date: 16 December 2014
Please cite this article as: Erko FG, Cseh L, Berthet J, Mehl GH, Delbaere S, Synthesis and
Photochromic properties of a bis(Diarylethene)-Naphthopyran Hybrid, Dyes and Pigments (2015), doi:
10.1016/j.dyepig.2014.12.015.
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ACCEPTED MANUSCRIPT
Synthesis and Photochromic properties of a bis(Diarylethene)-Naphthopyran Hybrid
Firealem G. Erko[a], Liliana Cseh[b,c], Jérôme Berthet [a], Georg H. Mehl[b], Stéphanie Delbaere*[a]
[a]
[b]
[c]
University of Lille, UDSL, CNRS UMR 8516, BP83, Lille, France;
University of Hull, Department of Chemistry, Hull, HU6 7RX, UK;
Institute of Chemistry of Romanian Academy, Timisoara, Romania.
* Corresponding author: stephanie.delbaere@univ-lille2.fr
Abstract
The synthesis and photochromic properties of a triphotochromic molecule consisting of one
naphthopyran flanked by two diarylethene units investigated by Ultraviolet-Visible and Nuclear
Magnetic Resonance spectroscopies are reported. Six different states resulting from the
open/closed naphthopyran associated with one or two open/cyclized diarylethenes have been
characterized. The photochemical and thermal interconversion between the six states of the
triphotochromic target was followed allowing to determine the existing pathways upon
irradiation with 313 and 365 nm light excitation, and during thermal evolution. Switching of the
naphthopyran group is possible, independently of the state of the diarylethene groups.
However, the diarylethene groups cannot be closed if the naphthopyran group is open.
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1. Introduction
Photochromism is a reversible transformation of a chemical species induced in one or both
directions by absorption of electromagnetic radiation between two isomeric states, A and B,
having different absorption spectra [1]. Usually the thermodynamically stable state A is
transformed by irradiation with UV light into state B. The back reaction can occur thermally
(photochromism of type T) or photochemically with visible light (photochromism of type P). The
two isomers differ from one another not only in the absorption spectra but also in various
physical and chemical properties, such as refractive indices, dielectric constants, oxidation–
reduction potentials, geometrical structure, conductivity and dipole moment [2].
Application of photochromic compounds to devices such as optical memory media and
photoswitches requires thermal stability of the two isomers, fatigue-resistant property, high
sensitivity and a rapid response. Diarylethenes fulfill all the above requirements simultaneously
[3-4]. Naphthopyrans represent another family of photochromic compounds which exhibit
excellent photochromic properties and are successfully applied in ophthalmic glasses as T-type
compounds [5].
As the demand for optical memories with high memory density, large storage capacity and high
data process/transfer rate is increasing, multi-wavelength (or multi-frequency) optical storage
is one of the promising approaches to increase the recording density and data capacity [6]. In
two-state photochromic systems, interconversion is possible between only two states. On the
other hand, multicomponent systems can result in reversible multimode switching between the
states [7]. N-wavelength memory can increase the recording density by N times of that of
single-wavelength memory [8]. Multi-color systems can be achieved by mixing photochromic
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compounds with different colors in solution [8-9], in polymer matrices [10], in single crystals
[11] and by incorporating two or more photochromic units in one molecule. The latter is
referred as single molecule multiphotochromism [12]. Some advantages of a single-molecule
system over mixture systems are high image resolution arising from local homogeneity,
constant color balance in a large area, and long term storage stability [6, 13]. Multi-frequency
optical memories in hybrid systems consisting of photochromic units of different kinds have a
potential advantage in the ease of selecting and designing photochromic moieties with
different absorption bands [14]. In this regard, hybrid systems between dihydroazulene and
diarylethene [15], phenoxynaphthacene–diarylethene [16], spirooxazine-naphthopyran [17]
have been reported. Association of diarylethene and naphthopyran units was first introduced
by Frigoli and Mehl and then extended to two naphthopyrans connected via a diarylethene
group, leading to the formation of four and six different states respectively [18-20]. The most
extensively studied class of multiphotochromic materials consists of linked diarylethene based
systems [12]. For covalently bound electronically disconnected diarylethene systems,
sequential closing has been confirmed [21-22] as well as for supramolecular arrays [23]. For
materials where the diarylethene groups are electronically connected, the current data does
not yet allow to identify an overall trend. On the one hand, there are a number of clear and
unambiguous reports for linear molecules, showing that the closing of one diarylethene group
results in ring-opening of the adjacent system [24-25], allowing for the rational design of
multicolor diarylethene systems [13]. At the other hand, there are reports showing that in
almost similar chemical structures [26] or in stable organic radicals containing diarylethene
dimers [27], all the diarylethene groups can be closed. Results for a star shaped hexameric
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systems [28-29] suggest that a combination of electronic effects such as energy transfer
between diarylethenes or steric effects, such as ring twists determine the switching behavior in
multi-diarylethene systems.
Considering the technological and scientific importance of this class of molecules and the effect
of conjugation and molecular linearity on their properties, we decided to explore the issue for
photoreactivity in multiphotochromic systems systematically. In this contribution we report on
the investigation of a triphotochromic system (OD-CN-OD, Chart 1) where two diarylethenes
(OD as Open Diarylethene) are linked to a naphthopyran (CN as Closed Naphthopyran) group.
The linkage is selected in such a manner that in the closed naphthopyran, the two diarylethenes
are separated by an sp3 hybridized carbon. In the open naphthopyran, this carbon is sp2
hybridized and conjugated and each of the diarylethene units are linked to the naphthopyran
moiety by π-conjugation, though the linkage between the diarylethene restricts their electronic
communication to a certain degree. The aim is here to investigate and characterize the six
different states which are theoretically expected upon irradiation under suitable conditions of
these three electronically linked photochromic units. Crucially, some of the states involve two
electronically linked diarylethene groups. Using a construction where the two diarylethenes are
electronically separated, when the naphthopyran group is closed; or linked, when it is open, the
optical linking of the conjugation of multiple diarylethene groups is explored.
The photochromic behavior of the synthesized system was investigated by UV-Vis Spectroscopy
and NMR spectroscopy. As open naphthopyrans are thermally unstable at ambient
temperature while cyclized diarylethenes are thermally stable, we first studied the
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photocyclization of diarylethene units upon excitation with 313 nm irradiation at room
temperature. Then the photochemical ring opening of the naphthopyran moiety was
investigated by irradiating with 365-nm light at rt by UV-visible spectroscopy and at low
temperature by NMR spectroscopy.
<Chart1>
2. Experimental
2.1. Equipment
Unless otherwise stated, reagents were used as supplied. UV–visible spectra were recorded for
spectroscopic grade toluene solutions of the sample (10 mm pathlength quartz cuvette, ca. 5 ×
10−5 mol.l-1) using a Cary 50 Probe spectrophotometer. NMR spectra of target compound OD-
CN-OD were recorded on a Bruker Avance 500 MHz instrument (1H NMR 500 MHz, 13C NMR 125
MHz) equipped with a TXI probe for sample solutions in toluene-d8. Irradiation for UV-visible
was provided by a 200 W high pressure Hg/Xe lamp. Irradiation for NMR measurements was
carried out directly into the NMR tube in a home-built thermoregulated (295, 243, 233, 223 and
213K) apparatus with a 1000W high-pressure Hg – Xe lamp. Monochromatic UV (313 or 365nm)
or visible light (577 nm) was obtained by passing the light through a first filter to select UV or
visible band, then through an interferential one. When required, NMR experiments were
carried at various low temperatures using a N2 cooling setup connected to the 500MHz
spectrometer.
Elemental analyses of products were carried out using an EA 1108 CHNS analyzer. All reactions
were monitored by thin-layer chromatography carried out on 0.2 nm silica gel plates (60F-254).
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Column chromatography was performed on silica gel (35-70u 60A). The purity of all final
compounds were checked by GLC analysis using a 9001 capillary gas chromatograph fitted with
a fused silica column (CP-Sil5 CB 0.12 m, 10 m long, 0.25 mm internal diameter) and using
nitrogen as the carried gas and by HPLC analysis using a system fitted with a 5μm silica column
(25 cm long, 4.6 mm internal diameter) and were found to be > 99.5 %. 1H and 13C NMR spectra
of intermediate compounds in chloroform-d1 were recorded at 400 and 100 MHz, respectively,
1
for H and 13C. Mass spectrum was recorded on Autoflex Speed MALDI TOF/TOF MSMS
instrument using 3,5-dimethoxy-4-hydroxycinnamic acid as matrix.
2.2 Preparation of intermediate compounds
2.2.1 Reagents.
All reagents used were of analytical grade and were purchased from Sigma Aldrich (most of
them) and Fluorochem (octaflurocyclopentene). The THF was dried by distillation on
Na/benzophenone. The other solvents were used without further purification other than drying
over molecular sieves. The intermediate 4-bromo-5-methyl-2-thienylboronic acid (5) and 2,5-
dimethyl-3-(perfluorocyclopent-1-enyl)thiophene (7) were obtained following the procedure
described previously ((5) [30-32]; (7) [32, 33]).
2.2.2 Bis(4-bromophenyl)methanone (1).
A mixture of 4-bromobenzoyl chloride (6.58 g, 30 mmol) in bromobenzene (25 mL) was added
at a solution of AlCl3 (4 g, 30 mmol) in bromobenzene (26 mL) under nitrogen. The mixture was
stirred at 55 °C for 3 days and then, 100 g ice and 50 mL HCl conc. were added. The resulting
mixture was extracted twice with diethyl ether, washed with aqueous NaOH 5% and then with
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water, dried over magnesium sulfate, then concentrated under vacuum. The crude product
1
recrystallized from methanol gave white powder (8.45 g, 83 %). H-NMR (CDCl3, 400 MHz, δ
[ppm]): 7.6 (s. 8H); 13C-NMR (CDCl3): 194.7, 136.0, 131.9, 131.5, 128.0; Elemental analysis calcd.
(%) for C13H8Br2O: C 45.92, H 2.37; found C 45.84, H 2.26.
2.2.3 1,1-Bis(4-bromophenyl)prop-2-yn-1-ol (2).
A solution of bis(4-bromophenyl)methanone (1) (5.1 g, 15 mmol) in dry THF (100 mL) was
added to a solution of lithium acetylide-ethylenediamine (13.8 g, 150 mmol) in dry THF (60 mL)
under nitrogen at 5 °C. The mixture was stirred at room temperature for 23h and then a
solution KOH (10 g) in ethanol (125 mL) was added. The mixture was stirred at r.t. for 15 min.
The acetic acid glacial was added until pH = 7. Water (400 mL) was added and the organic
compound was extracted with diethyl ether three times and dried over MgSO4. The crude
product was obtained by removing the solvent under pressure. Column chromatography (silica,
CH2Cl2) gave colourless liquid (2.25 g, 41 %). 1H-NMR (CDCl3, 400 MHz, δ [ppm]): 7.45 (s, 8H,
HAr), 2.90 (s, 1H, C≡CH), 2.80 (s, 1H, -OH); Elemental analysis calcd. (%) for C15H10Br2O: C 49.22,
H 2.75; found C 49.34, H 2.69.
2.2.4 3,3-Bis(4-bromophenyl)-3H-benzo[f]chromene (3).
A mixture of 2-naphthol (2.0 g, 0.014 mol) and the 1,1-bis(4-bromophenyl)prop-2-yn-1-ol (2)
(5.0 g, 0.014 mol) in CH2Cl2 (30 mL) in the presence of PPTS catalytic amount,
trimethylorthoformate (20 mL) was refluxed for 24 h. The solvent was removed and the crude
1
product was purified by column chromatography (silica, CH2Cl2); yield 45 ℅ (3.1 g). H-NMR
(CDCl3, 400 MHz, δ [ppm]): 7.94 (d, 1H, J = 8.0), 7.71 (dd, 1H, J = 8.8), 7.66 (d, 1H, J = 8.8), 7.45
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(m, 1H), 7.43 (m, 4H), 7.31-7.36 (m, 6H), 7.15 (dd, 1H, J = 8.8), 6.13 (d, 1H, J = 9.9), 13C-NMR
(CDCl3, 100 MHz): 150.0, 143.3, 131.3, 130.2, 129.4, 128.7, 128.5, 126.9, 126.5, 123.9, 121.9,
121.3, 120.4, 118.1, 114.2, 82.4. Elemental analysis calcd. (%) for C25H16Br2O: C 60.01, H 3.28;
found C 59.87, H 3.19.
2.2.5 3,3-Bis(4-(4-bromo-5-methylthiophen-2-yl)phenyl)-3H-benzo[f]chromene (6).
Compound 3,3-bis(4-bromophenyl)-3H-benzo[f]chromene (3) (1.47 g, 3 mmol) was dissolved in
THF (12 mL) and after addition of Pd(PPh3)4 (0.23 g, 0.2 mmol), the solution was stirred at r. t.
for 15 min. Aqueous Na2CO3 (5.3 g in 25 mL H2O) and 6 drops of ethylene glycol were then
added, and the resulting two phase system was heated until reflux. A solution of 5-bromo-2-
methylthiophen-3-yl-3-boronic acid (5) (1.54 g, 7 mmol) in THF (15 mL) was added drop wise.
After the addition was complete, the reaction mixture was refluxed for 8 h and then allowed to
cool down at r. t. 25 mL of water were added and the crude product was extracted by diethyl
ether (3 X 25 mL) and dried over MgSO4. Evaporating the solvent, the crude product was
purified on the column (silica, hexane for 30 min. and then hexane/CH2Cl2, 9/1).
1
Recrystallization from ethyl acetate gave a white powder with yield of 30 % (0.6 g). H-NMR
(CDCl3, 400 MHz, δ [ppm]): 7.95 (d, 1H, J = 8.2), 7.71 (d, 1H, J = 8.1), 7.67 (d, 1H, J = 8.8), 7.21-
7.50 (m, 9H), 7.30-7.37 (m, 2H), 7.20 (d, 1H, J = 8.8), 7.05 (s, 2H), 6.24 (d, 1H, J = 9.9), 2.39 (s,
6H); Elemental analysis calcd. (%) for C35H24Br2OS2: C 61.41, H 3.53, S 9.37; found C 61.72, H
3.17, S 9.4
2.2.6 3,3-Bis(4-(4-(3,3,4,4,5,5,-hexafluoro-2-(2,5-dimethylthiophene-3-yl)cyclopent-1-enyl)-5-
methylthiophen-2-yl)phenyl)-3H-benzo[f]chromene (OD-CN-OD).
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To
a
solution of compound 3,3-bis(4-(4-bromo-5-methylthiophen-2-yl)phenyl)-3H-
benzo[f]chromene (6) (200 mg, 0.293 mmol) in dry THF (15 mL) was added n-BuLi (0.4 mL of 1.6
M solution in hexane, 0.586 mmol) under nitrogen at -78 °C. This solution was stirred 1 h and
then compound 2,5-dimethyl-3-(perfluorocyclopent-1-enyl)thiophene (7) (178.3 mg, 0.586
mmol) was added in one portion. The mixture was stirred at -78 C for 1 h, and then allowed to
warm-up to r. t. After stirring for additional 12h, diethyl ether (20 mL) was added, and the
mixture was washed with water. The organic layer was dried over MgSO4 and solvent
evaporated. The crude product was purified by 2 columns (silica, CH2Cl2/hexane, 7/3; silica,
CH2Cl2/hexane, 0.5/9.5) and afforded 115.5 mg (36℅) finale compound.
1H-NMR (CDCl3, 400 MHz, δ [ppm]): 7.96 (d, 1H, H10, J = 8.3), 7.72 (d, 1H, H7, J = 8.1), 7.68 (d,
1H, H6, J = 8.8), 7.46-7.54 (m, 9H, H1,2,9), 7.30-7.37 (m, 2H, H5,8), 7.21 (s, 2H, H11), 7.20 (d, 1H, H4,
J = 8.8), 6.71 (m, 2H, H13), 6.25 (d, 1H, H3, J = 9.9), 2.40 (s, 6H, H14), 1.89 (s, 6H, H12) 2.81 (s, 6H,
H15). Elemental analysis calcd. (%) for C57H38F12OS4: C 62.51, H 3.50, S 11.71; found C 62.68, H
3.46, S 11.57. 13C-NMR (CDCl3, 100 MHz): 150.5, 144.3, 141.5, 141.4, 139.9, 132.8, 130.2, 129.9,
129.5, 128.7, 127.7, 127.1, 126.9, 126.1, 125.5, 124.7, 124.6, 123.9, 122.8, 121.4, 120.4, 118.3,
116.3, 115.6, 114.2, 113.6, 11.1, 82.1, 15.2, 14.5, 14.4. Mass spectrum: m/z 1095.49.
3. Synthesis of target compound OD-CN-OD
The preparation of the triphotochromic system was carried out in nine steps. The synthetic
path to this new molecule is outlined in Scheme 1. The synthesis started with the
preparation of ketone (1) which was obtained (yield: 83 ℅) from the reaction of
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bromobenzene with 4-bromobenzoyl chloride in the presence of AlCl3. The subsequent
condensation of lithium acetylide-ethylenediamine with (1) resulted in the corresponding
propargylic alcohol (2) (41 ℅ yield). The intermediate naphthopyran (3) was obtained by a
“one-pot reaction” of (2) and 2-naphthol in the presence of a catalytic amount of
pyridinium p-toluenesulfonic acid and trimethyl ortoformate as a dehydrating agent ((3), 45
℅ yield). The intermediate 4-bromo-5-methylthiophen-2-yl-2-boronic acid (5) was obtained
in a 72 % yield from 2-methylthiophene by using bromine in acetic acid followed by the
addition of trimethoxy borate, according to the procedure described in the literature [31-
33].
<Scheme 1>
The intermediate naphthopyran (6) was obtained by a Suzuki coupling reaction between (3) and
(5) in the presence of Pd(PPh3)4 catalyst which afforded the naphthopyran (6) (30 ℅ yield). The
intermediate 2,5-dimethyl-3-(perfluorocyclopent-1-enyl)thiophene (8) was obtained in three
steps. The reaction sequence started with the bromination in the 3-position using bromine in
chloroform, resulting in 3-bromo-2,5-dimethylthiophene (7). This introduces the functionality
required for the assembly of the photochromic skeleton. The treatment of (7) in THF with n-
butyllithium at –78°C, followed by the addition of three equivalents of hexafluorocyclopentene
yields the compound (8) (53 ℅ yield). Lithiation of the intermediate naphthopyran (6) by
bromine/lithium exchange using n-BuLi in THF at -78°C, follow by the addition of the
intermediate (8) afforded the symmetrical triphotochromic system (OD-CN-OD, 36 ℅ yield).
4. Photochemical and thermal reactions
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4.1 UV-visible spectroscopy investigations
4.1.1 Irradiation at 313 nm
Upon irradiation at 313 nm, the toluene solution of OD-CN-OD (state 1) turned from colorless
to deep purple characterized by an absorption band located at λmax = 559 nm (Figure 1a) which
remained thermally stable. Complete photobleaching is achieved upon irradiation with 577 nm
light (Figure 1b). The observed maximal absorption wavelength, the thermal stability and the
photochemical bleaching with visible light are characteristic of the cyclized diarylethene.
However, as OD-CN-OD is constituted by two diarylethene units, it is not possible by UV-vis
spectroscopy to conclude if one or two underwent ring closure.
4.1.2 Irradiation with 365 nm
Upon irradiation at 365 nm at room temperature, one absorption band appears at 471 nm with
a shoulder at 580 nm (Figure 1c). The band at 471 nm thermally decreased when the light is off,
while the band at 580 nm persisted throughout thermal relaxation, although with a shift toward
559 nm, and was completely bleached with visible light (577 nm).
<Figure 1>
Thus, the appearance of an absorption band at 471 nm that relaxes thermally is an indication of
the naphthopyran opening and the thermally stable second band characterize the diarylethene
closing. The positions of the absorption bands correspond to the pathways from photoproducts
with open naphthopyran and one or two cyclized diarylethene to the corresponding
compounds with closed naphthopyran. As OD-CN-OD is constituted of three photochromic
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entities, UV-visible studies did not allow for distinguishing if one or two diarylethenes
underwent ring closure, and if the opening of naphthopyran is associated with diarylethene
being cyclized or not. Consequently, the studies were carried out by NMR spectroscopy to
determine if it is possible to reach each of the six possible states.
4.2 NMR investigations
4.2.1 Irradiation with 313 nm at room temperature
A fresh solution of OD-CN-OD (state 1) in toluene-d8 (Figure 2a) was subjected to irradiation at
1
room temperature with 313-nm light and H NMR spectra were recorded after each period of
irradiation. New signals which remain thermally stable are observed (Figure 2b). More
particularly, we observed the appearance of a ‘doublet of doublets’ like signal at 6.00 ppm and
a ‘doublet of triplet’ like signal at 5.97 ppm, all with a 3J=9.9 Hz. Such chemical shift and vicinal
coupling constant values are typical of the H-2 in closed naphthopyrans, which is expected to
be a doublet signal with a vicinal coupling constant of 9.9 Hz. The observed multiplicity at 6.00
and 5.97 ppm is explained by the formation of diastereomers, as reported for previously
investigated diarylethene-naphthopyran hybrid photochromic systems [19-20].
<Figure 2>
The sp3 carbon C-3 of the closed naphthopyran is either in the R or S configuration when one
diarylethene is closed. The closing of diarylethene is itself in RR or SS configuration. Then, this
results in two possible diastereomers (RR-R/SS-S, RR-S/SS-R). Here, they are associated with the
two doublets at 6.00ppm (state 2).
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Similarly, the cyclization of both diarylethenes leads to the possible formation of four couples of
diastereomers : RR-R-RR/SS-S-SS, SS-R-SS/RR-S-RR, SS-R-RR/RR-S-SS and RR-R-SS/SS-S-RR. The
latter two are the identical sets. Hence, it results in the three doublets observed at 5.97 ppm,
one being twice intense. Consequently, irradiation with 313-nm light induces the cyclization of
one diarylethene unit, producing the state 2 = CD-CN-OD, and the cyclization of both
diarylethene units forming the state 3 = CD-CN-CD (Scheme 2). This cyclization process was
followed by irradiating the sample by successive periods, thus allowing to obtain the time-
evolution of concentrations (Figure 3a). As the signals between 5.97, 6.00 and 6.04 ppm
represent the H-2 protons of naphthopyran part of states 3, 2 and 1, respectively, their
intensity variations were measured to characterize the time-evolution of the concentrations.
The first periods of irradiation generate state 2, and state 3 could be detected only when
irradiation is prolonged. Also, it can be observed that the concentration of state 2 reaches a
plateau, while that of state 3 is always increased. When the light source is replaced with visible
light at 577 nm, the photobleaching of the solution is observed (Figure 3b). State 3 decreased
following a mono-exponential decay, whereas state 2 is characterized by a bi-exponential
evolution. Thus, the kinetic behavior upon UV and upon visible irradiation is a clear indication
of the pathways from state 1 to state 3 by the intermediary of state 2. In other words, UV light
induced the cyclization of one diarylethene from initial OD-CN-OD (1) into CD-CN-OD (2). The
latter absorbs UV light and the cyclization of its second diarylethene unit occurs to form CD-CN-
CD (3). The reverse processes, decyclization of one diarylethene of CD-CN-CD (3) to produce
CD-CN-OD (2), followed by the ring-opening of the second diarylethene unit of CD-CN-OD
towards the initial state 1, are fully achieved with visible light irradiation. Thus, at room
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temperature, the photochromic reaction of diarylethene has been underlined, with a high
conversion of cyclization, as 90 % of states with cyclized structures, 35% of state 2 and 55% of
state 3, has been measured.
<Figure 3>
4.2.2 Irradiation with 365 nm at low temperature
Irradiation with 365-nm light was applied to a fresh solution of 1 cooled at 223 K. As displayed
in Figure 2c, new signals are detected and among them, the emergence of one doublet signal is
observed at 9.43 ppm with a coupling constant of 11.9 Hz, which is characteristic of H2 proton
in Transoid-Cis (TC) photomerocyanine [34]. The time-evolution of solution underlined its
thermal conversion into the initial state 1, with a rate constant of relaxation (223k∆ = 1.2×10-5 s-1)
similar to values generally reported for TC-isomer [35]. Consequently, the state 4 was here
proved to be produced from 365-nm light irradiation of state 1. However, it must be noted that
the conversion is low, as no more than 16% of open naphthopyran could be produced. In
3
addition, another very low-intense doublet signal (less than 3%) at 9.41 ppm with J=11.9 Hz is
also present. Its thermal evolution was also followed and the signal decreased in intensity
while that of the doublet at 5.96 ppm (state 2) increased. A relaxation rate, 223k∆ = 1.2×10-5 s-1,
equal to that measured for 4ꢀ1 was extracted. Therefore, this indicates that cyclization of one
diarylethene can be induced with 365 nm light, as it was observed with UV-visible spectroscopy,
The doublet at 9.41 ppm is therefore assigned to the state 5, corresponding to the opening of
naphthopyran from state 2 and thermally converting into state 2. Experiments were repeated
at 233 and 243K. The same phenomenon was observed, ie, photochemical formation of state 4
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from 1, and state 5 from 2, by the intermediary of 1. Thermal relaxation resulted again in equal
rate constants: 233k∆ = 6.5×10-5 s-1 and 243k∆ = 3.4×10-4 s-1 for 4ꢀ1 and 5ꢀ2. This means that
the thermal closing of photomerocyanine is not affected by the status, open or cyclized, of
diarylethene.
Irradiation with 365 nm light at 223K was carried out with a fresh sample and instead of
recording the thermal relaxation, irradiation with 313 nm light was applied to check if
conversion 4ꢀ5, ie the closing of diarylethene unit when naphthopyran is already open, is
possible. However, no change was observed, thus, there is no cyclization of diarylethene
observed when naphthopyran is in open configuration.
4.2.3 Irradiation of mixture of states 1, 2 and 3 with 365 nm at low temperature
Irradiation with 365 nm-light at low temperature was applied to a sample previously irradiated
at room temperature with 313 nm to investigate the possible opening of naphthopyrans 2ꢀ5
and 3ꢀ6. Irradiation at room temperature with 313 nm produced a mixture containing 46%
CD-CN-CD (3), 41% CD-CN-OD (2) and the remaining 13% OD-CN-OD (1). This sample was
1
cooled at 223K and submitted to irradiation with 365 nm light. In the H NMR spectrum (Fig.
2d), the total conversion is less than 20% and could not be improved even when increasing the
duration of irradiation. The concentrations of the three initial states are 8% (1); 35% (2) and
38% (3). In addition, three superimposed doublet signals around 9.4 ppm, with the
characteristic coupling constant of 11.9 Hz, are detected. They are straightforwardly attributed
to H2 proton in TC photomerocyanines. The signals at 9.43 ppm (5%) and 9.41 ppm (6%) were
previously identified as H2 in state 4 and state 5, respectively. The third doublet at 9.38 ppm
15

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(8%) is assumed to be H2 in state 6. This was fully confirmed by recording thermal evolution,
these signals decreased in intensity while the signals of H2 for the closed configuration of
naphthopyran increased. Hence it is possible to associate them in pairs, the doublets at 9.43
ppm (H-2 in state 4) and 6.03 ppm (H-2 in state 1); the doublets at 9.41 ppm (H-2 in state 5) and
5.96 ppm (H-2 in state 2); and the doublets at 9.38 ppm (H-2 in state 6) and 5.92 ppm (H-2 in
state 3). Therefore, signals at 9.41 and 9.38 ppm are characteristic of the formation, upon
irradiation, of states 5 and 6 respectively, underlining the opening of naphthopyrans upon
irradiation of states 2 and 3 respectively. Also, it should be noted that the three states with
open naphthopyrans, namely, 4, 5 and 6 evolved thermally with the same rate constant of
relaxation (223k∆ = 1.2×10-5 s-1), thus indicating again that the status of diarylethene (open or
closed) has no effect on the thermal stability of open naphthopyrans.
5.
Discussion
Combining all the experiments performed, the overall mode of action of OD-CN-OD is outlined
in Scheme 2. Six different states resulting from the open/closed naphthopyran associated with
one or two open/cyclized diarylethenes have been structurally identified by 1H NMR
spectroscopy. The cyclization of diarylethene with 313 nm light and at room temperature could
be reached with more than 90%. The irradiation time to get such conversion is high (3 hours)
but it concerns a sample with a concentration of 1 mM for NMR spectroscopy. We could
observe the formation of state 2, followed by the formation of state 3. This means that the
cyclization of each diarylethene unit can be achieved with 313 nm light. When investigation of
the same process is performed by UV-Visible spectroscopy with a sample at 5x10-5M, the
16

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photostationary state was reached in 15 minutes. Using the percent of conversion (90% of
states 2 and 3) measured in NMR spectrum, it is possible to estimate the averaged molar
extinction coefficient at λmax = 559nm of states 2 and 3 at ca 9000 l.mol-1.cm-1. Then, the
presence of naphthopyran has no effect on the photochromic response of diarylethene, as the
values are very close to that reported for 1,2-bis(2,4-dimethyl-5-phenylthiophene-3-
yl)perfluorocyclopentene in hexane [36]. The reverse ring-opening is achieved with visible light
irradiation. Irradiation with 365 nm, followed by UV-visible spectroscopy allowed to detect the
formation of photomerocyanine and cyclization of diarylethene, but switching off the
irradiation source leads to fast relaxation of the open form of naphthopyran as expected, due
to the established thermal ring closure. Repeating this experiment with NMR spectroscopy at
low temperature enabled the production and stabilization of photomerocyanines. According to
the initial composition of sample (state 1 or mixture of states 1, 2 and 3), it was possible to
observe the formation with 365nm light of the corresponding states with open naphthopyran
(States 4, 5 and 6). From a general overview, they behave similarly as they are all formed with
relatively low conversion, and they all decreased thermally with the same rate constant of
relaxation. The thermal relaxation processes were followed at 223, 233 and 243K, from which
the values of k was deduced and used to determine, using Arrhenius plot, activation energy
equal to 75 kJ.mol-1. Such results indicate that the status of the diarylethene unit (open or
cyclized) has no effect on the thermal stability of photomerocyanines, and activation energy
determined for the closing of naphthopyrans is in complete agreement with values generally
reported [34]. It should be also mentioned that the opening of naphthopyran entity is always
with the Transoid-Cis (TC) isomery, while the Transoid-Trans could also be expected. However,
17

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in our conditions, no TT-photomerocyanine was observed. This is not completely surprising as
the TT-isomer is usually, when produced, very low concentrated [19, 20, 34, 35]. Finally, no
cyclization of diarylethene unit could be reached if naphthopyran is in an open configuration, ie
4ꢀ5ꢀ6. This means that the reactivity of diarylethene is cancelled when associated with
photomerocyanine. Such an observation was also reported with previously investigated 3-(1,2-
bis-(2,5-dimethyl-3-thienyl)perfluorocyclopentene) 3-phenyl-naphthopyran [19] and 3,3-(1,2-
bis-(2,5-dimethyl-3-thienyl)perfluorocyclopentene)-naphthopyran [20] which showed the same
non-reactivity of diarylethene when associated with open naphthopyran. These two
compounds had the diarylethene directly connected to the spiro carbon of naphthopyran. Here
a phenyl ring plays the role of spacer but it doesn’t allow for activating the cyclization of
diarylethene unit in open naphthopyran.
<Scheme 2>
6.
Conclusions
The synthesis and photochromic properties of a triphotochromic molecule consisting in one
naphthopyran flanked by two diarylethene units have been described in detail. The cyclisation
of the diarylethene at 313 nm and the reverse process with visible light does only occur when
the linking naphthopyran is in the closed configuration. The naphthopyran moiety can generate
photomerocyanine species both with open or closed diarylethene groups and the states with
open naphthopyrans (4, 5 and 6) have very similar thermal stability. In other words, switching
of the naphthopyran group is possible, independently of the state of the diarylethene groups.
However, the diarylethene group cannot be closed if the naphthopyran group is open. The
18

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presence of the phenyl groups in the open naphthopyran group, forming the link to the
diarylethene group, suggests that subtle electronic effects could be responsible for the gating of
the photoswitching of diarylethene. More generally, the results open the perspective of multi-
or poly-diarylethene–naphthopyran species where other types of spacer could be envisaged to
favor the reactivity of diarylethene when associated with open naphthopyran.
Acknowledgements
L.C. acknowledges funding through the EPSRC grant EP/D058066 and the NMSF, Swansea (UK),
for mass spectrometry data.
Supplementary data
1H NMR spectra of synthesized precursor; 1H, 13C NMR and MS spectra for target compound
OD-CN-OD; 1H, 13C NMR spectra of states 2 and 3.
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List of captions
Chart1: Structure of the target compound
Scheme 1. Synthesis of the triphotochromic hybrid OD-CN-OD; Reagent and conditions: a)
AlCl3; b) lithium acetylide-ethylenediamine, THF; c) 2-naphthol, pyridinium p-toluenesulfonic
acid (PPTS), CH(OCH3)3, CH2Cl2; d) Br2, acetic acid; e) n-BuLi, THF, B(OCH3)3; f) Pd(PPh3)4,
Na2CO3, THF; g) Br2, CHCl3; h) n-BuLi, THF, 3 eq of octafluorocyclopentene; i) n-BuLi, THF.
Scheme 2: Photochemical and thermal interconversion of the six states of triphotochromic OD-
CN-OD
Figure 1: UV-Vis absorption spectra (295 K, toluene, 5 x 10-5 M) of state 1 a) upon irradiation at
313 nm, b) followed by irradiation with 577 nm; c) upon irradiation with 365 nm (black line :
before irradiation ; dashed line: light is off)
1
Figure 2: H NMR Spectra of 1 (1 mM in toluene) a) at rt, b) after irradiation with 313 nm at rt,
c) after irradiation with 365 nm at 223K, d) after irradiation with 313 nm at rt + 365 nm at 223K
1
Figure 3: Time-evolution of concentrations of the states 1, 2 and 3 followed by H NMR during
irradiation (a) with 313 nm and (b) with 577 nm light at 295 K in toluene-d8
23

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Illustrations
Graphical abstract
24

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H1
O
H2
3
F6
F6
S
S
OD-CN-OD
state 1
S
S
O
H2
S
H1
F6
S
Open naphthopyran ON
Cyclized diarylethene CD
Chart1: Structure of the target compound
25

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O
OH
ClOC
+
a)
c)
b)
O
Br
Br
Br
Br
Br
Br
2
1
Br
Br
3
Br
S
S
(HO)2B
e)
S
d)
f)
5
4
Br
Br
i)
S
O
8
F F
F F
F
F
F
F
O
F
F
F
F
F
S
S
F
F
S
F
F
F
F
Br
S
S
S
6
Br
h)
S
S
g)
7
Br
Scheme 1. Synthesis of the triphotochromic hybrid OD-CN-OD; Reagent and conditions: a)
AlCl3; b) lithium acetylide-ethylenediamine, THF; c) 2-naphthol, pyridinium p-toluenesulfonic
acid (PPTS), CH(OCH3)3, CH2Cl2; d) Br2, acetic acid; e) n-BuLi, THF, B(OCH3)3; f) Pd(PPh3)4,
Na2CO3, THF; g) Br2, CHCl3; h) n-BuLi, THF, 3 eq of octafluorocyclopentene; i) n-BuLi, THF.
26

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O
O
F F
F F
F F
F F
UV
F
F
F
F
F
F
F
F
S
S
S
S
F
F
F
F
∆
F
OD-ON-OD
F
F
F
OD-CN-OD
4
S
S
S
S
1
vis.
UV
O
O
∆
F F
F F
F
F
F
F
F
F
F
F
F
F
F
S
S
F
F
UV
S
S
F
F
F
F
CD-ON-OD
CD-CN-OD
F
F F
S
S
S
S
5
2
UV
Vis.
O
O
F
F
F
F
F
F
F
∆
F
F
F
S
S
F
F
F
F
S
S
F
F
F
F
UV
F
CD-ON-CD
F
F
S
S
CD-CN-CD
F
6
F
F
S
S
3
Scheme 2: Photochemical and thermal interconversion of the six states of triphotochromic OD-
CN-OD
27

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Figure 1: UV-Vis absorption spectra (295 K, toluene, 5 x 10-5 M) of state 1 a) upon irradiation at
313 nm, b) followed by irradiation with 577 nm; c) upon irradiation with 365 nm (black line :
before irradiation ; dashed line: light is off)
28

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1
Figure 2: H NMR Spectra of 1 (1 mM in toluene) a) at rt, b) after irradiation with 313 nm at rt,
c) after irradiation with 365 nm at 223K, d) after irradiation with 313 nm at rt + 365 nm at 223K
29