The control of photochromism of [3H]-naphthopyran derivatives with intramolecular CH-π bonds

Article abstract of DOI:10.1021/ol301812e

The photochromism of [3H]-naphthopyran derivatives can be switched from T-type to inverse- or P-type through the manipulation of relative thermodynamic stabilities of open isomers with intramolecular CH-π bonds.

Full text of DOI:10.1021/ol301812e

ORGANIC
LETTERS
2012
Vol. 14, No. 16
4150–4153
The Control of Photochromism of
[3H]‑Naphthopyran Derivatives with
Intramolecular CHꢀπ Bonds
,†
‡
§
§
ꢀ ^
ꢀ
Michel Frigoli,* Francois Maurel, Jerome Berthet, Stephanie Delbaere,
†
ꢀ ^
Jerome Marrot, and Maria. M. Oliveira
ꢀ
ILV, CNRS UMR 8180, Universite de Versailles, Versailles, France, ITODYS, CNRS
UMR 7086, Universite Paris, Paris, France, UDSL, CNRS UMR 8516, University Lille
ꢀ
ꢀ
Nord de France, Lille, France, and CQ-VR, Universidade de Tras-os-Montes e Alto
Douro, Vila Real, Portugal
michel.frigoli@chimie.uvsq.fr
Received July 2, 2012
ABSTRACT
The photochromism of [3H]-naphthopyran derivatives can be switched from T-type to inverse- or P-type through the manipulation of relative
thermodynamic stabilities of open isomers with intramolecular CHꢀπ bonds.
Photoswitchable molecules such as azo compounds and
dithienylethenes have been extensively investigated in re-
cent years, due to their potential in technological advanced
applications ranging from molecular electronics to bio-
imaging.1 These applications rely on the capacity of the
photochromic compounds to switch reversibly between
two states thermally (meta)stable having different proper-
ties by light (P-type photochromism).2 In contrast, photo-
chromic organic naphthopyrans have attracted much
attention due to their industrial application in ophthalmic
technology.3 The reversible change of color under solar
exposition that ceases after the removal of the illumination
constitutes the first required property for the use of
naphthopyrans in plastic photochromic lenses (T-type
photochromism). Changing the photochromism of naph-
thopyrans from T- to P-type is very challenging and would
offer a newfamilyofphotochromes that couldbeof critical
use in advanced applications and constitute an interesting
alternative to azocompounds and dithienylethenes. The
photochromism of [3H]-naphthopyrans is believed to start
with CꢀO bond cleavage of the closed pyran ring (CF)
induced by UV absorption that leads after some internal
rearrangement of double bond to an open colored isomer
transoid-cis (TC) which returns thermally to the initial
state (Scheme 1).4 Under continuous irradiation, TC can
eventually produce a transoid-trans open isomer (TT)
following isomerization of one double bond.4 The re-
versible change of color comes from the fact that the TC
isomer isthe majorproduct undersolarorUVirradiation.5
In fact, TC and TT isomers differ from their thermal and
thermodynamic stabilities. In the case of the reference
compound namely the 3,3-diphenyl-[3H]-naphtho[2,1-b]-
pyran at rt in toluene solution, TC is less thermally stable
by ∼3ꢀ4 orders of magnitude compared to TT while its ther-
modynamic stability is ∼8.9 kJ higher than that of TT.6
†
ꢀ
Universite de Versailles.
‡
ꢀ
Universite Paris.
§ University Lille Nord de France.
ꢀ
Universidade de Tras-os-Montes e Alto Douro.
(1) Selected reviews and books on photochromism and applications:
(a) Irie, M. Chem. Rev. 2000, 100, 1685. (b) Molecular Switches; Feringa,
B. L., Ed.; Wiley-VCH: Weinheim, 2001. (c) Tian, H.; Feng, Y. J. Mater.
Chem. 2008, 18, 1617. (f) Tian, Z.; Wu, W. A.; Li, D. Q. ChemPhysChem
2009, 10, 2577. (d) Russew, M.-M.; Hecht, S. Adv. Mater. 2010, 22, 3348.
(e) Beharry, A. A.; Woolley, G. A. Chem. Soc. Rev. 2011, 40, 4422.
€
(2) Bouas-Laurent, H.; Durr, H. Pure Appl. Chem. 2001, 73, 639.
(3) Reviews on photochromic naphthopyrans: (a) Van Gemert, B. In
Organic Photochromic and Thermochromic Compounds; Crano, J. C.,
Guglielmetti, R. J., Eds.; Kluwer Academic/Plenum Publishers: New York,
1999; Vol. 1, Chapter 3. (b) Hepworth, J. D. Heron, B. M. Fuctional
Dyes; Sung-Hoon, K., Ed.; Elsevier: Amsterdam, 2006. (c) Corns, N. S.;
Partington, S. M.; Towns, A. D. Color. Technol. 2009, 125, 249.
(4) (a) Delbaere, S.; Luccioni-Houze, B.; Bochu, C.; Teral, Y.;
Campredon, M.; Vermeersch, G. J. Chem. Soc., Perkin Trans. 2 1998,
1153. (b) Ottavi, G.; Favaro, G.; Malatesta, C. J. Photochem. Photobiol.,
A 1998, 115, 123.
r
10.1021/ol301812e
Published on Web 07/30/2012
2012 American Chemical Society

Scheme 1. Photochromic Interconversion in
[3H]-Naphthopyran
Scheme 2. Synthesis of Photochromic Compounds 1 and 2
Indeed, a H-bond is established between proton H2 and
the oxygen of the ketone group stabilizing TC while the
same proton in TT is in a repulsive interaction with the
naphthalene ring, consequently destabilizing TT. The iso-
merization of the double bond involved in the formation of
TT from TC is rendered more difficult due to the presence
of this H-bond that should be broken in this transforma-
tion. Thus, the photochromism of naphthopyran is con-
trolled by a H-bond allowing the accumulation of the most
thermodynamic stable open isomer TC. According to this
argument, the manipulation of the relative thermodynamic
stabilities of open isomers compared to the closed form
would in principle modify the thermo-/photoequilibrium
between all species involved in the photochromic processes.
Herein, we report the synthesis and the properties of two
naphthopyran derivatives 1 and 2 displaying unusual and
intriguing inverse- and P-type photochromism respectively,
through the adjustment of relative thermodynamic stabi-
lities of open isomers with CHꢀπ bonds. To the best of our
knowledge, the control of the photochromism of naphtho-
pyrans with CHꢀπ bonds is the first time demonstrated and
sheds further insight into the molecular design of naphtho-
pyrans with specific and innovative properties. Compounds 1
and 2 namely belong to benzopyrano[6,5-c]carbazole deriv-
atives which differ from one substituent located at the
3-position. 1 has an ethynylphenyl group while this group
is replaced by a phenyl substituent in 2. This little structural
change has a tremendous effect on the thermo-/photoequili-
brium between the closed and open isomers.
Figure 1. X-ray molecular structure of CTC isomer.
obtained by a “one-pot reaction” with 7-phenyl-7H-
benzo[c]carbazol-2-ol and the corresponding prop-2-yn-1-
ol derivatives in the presence of a catalytic amount of
PPTS.7 For 2, the reaction proceeded smoothly and the
closed form CF2 was obtained as a pure product in 75%
yield. In the case of 1, the reaction mixture became redder,
and after 24 h of stirring, analysis with a TLC plate
revealed a dark red spot as the main product. This was
very surprising and indicative that colored open isomers
were stable in the reaction mixture. 1 could be purified by
column chromatography with a mixture of petroleum
ether and ethylacetate (90/10) as eluent and obtained as a
dark red solid in 54% yield. 1H NMR spectrum at 20 °C in
toluene solution amazingly revealed the presence of only
two isomers (80/20 ratio). Four open isomers could be
expected for naphthopyrans having a chiral center at the
3-position. The minor isomer was assigned to CF1 due to the
doublet signal at 6.00 ppm (J = 9.1 Hz), characteristic of
proton H2 of the pyran ring (Figure 2S and Table 1S,
Supporting Information (SI)). For the major isomer, two
pseudodoublets at 9.26 and 9.39 ppm with a coupling
constant of 12.2 Hz in the AB system were observed. The
measured values of the coupling constant and downfield
chemical shifts make it possible to attribute these signals
to protons H2 (deshielded by CdO) and H1 (deshielded
by the concomitant effect of the ethynyl and indolino group)
in TC-type compound isomers.4a,5cꢀ5f The major isomer was
found to be a CTC isomer, and this assignment was based
on a 2D-NOESY experiment (Figure 6S, SI).
The preparation of 1 and 2 is outlined in Scheme 2. The
building of the naphthopyran scaffold in 1 and 2 was
(5) Selected references: (a) Guo, K. P.; Chen, Y. J. Mater. Chem.
2010, 20, 4193. (b) Frigoli, M.; Pimienta, V.; Moustrou, C.; Samat, A.;
Guglielmetti, R.; Aubard, J.; Maurel, F.; Micheau, J. C. Photochem.
Photobiol. Sci. 2003, 8, 888. (c) Venec, D.; Delbaere, S.; Micheau, J. C.;
Frigoli, M.; Moustrou, C.; Samat, A.; Vermeersch, G. Photochem.
Photobiol., A 2006, 181, 174. (d) Tulyakova, E. V.; Fedorova, O. A.;
Micheau, J. C.; Paramonov, V.; Lokshin, V.; Vermeersch, G.; Delbaere,
S. ChemPhysChem 2011, 12, 1294. (e) Gabbutt, C. D.; Heron, B. M.;
Instone, A. C. Tetrahedron 2006, 62, 737. (f) Ortica, F.; Smimmo, P.;
Favaro, G.; Mazzucato, U.; Delbaere, S.; Venec, D.; Vermeersch, G.;
Frigoli, M.; Moustrou, C.; Samat, A. Photochem. Photobiol. Sci. 2004,
3, 878. (g) Frigoli, M.; Mehl, G. H. Chem. Commun. 2004, 2040.
(h) Oliveira, M. M.; Salvador, M. A.; Delbaere, S.; Berthet, J.; Vermeersch,
G.;Micheau, J. C.; Coelho, P. J.;Carvalho, L. M. J. Photochem. Photobiol.,
A 2008, 198, 242. (i) Coelho, P. J.; Salvador, M. A.; Oliveira, M. M.;
Carvalho, L. M. J. Photochem. Photobiol., A 2005, 172, 300. (j) Sallenave,
X.; Delbaere, S.; Vermeersch, G.; Saleh, A.; Pozzo, J.-L. Tetrahedron Lett.
2005, 46, 3257.
The structure of the CTC isomer was nicely confirmed
by X-ray crystallography (Figure 1).8 Wewere verypleased
(6) (a) Demadrille, R.; Rabourdin, A.; Campredon, M.; Giusti., G. J.
Photochem. Photobiol., A 2006, 168, 143. (b) Gabbutt, C. D.; Heron, M.;
Instone, A. C.; Kolla, S. B.; Mahajan, K.; Coelho, P. J.; Carvalho, L. M.
Dyes Pigm. 2008, 76, 24. Relative energies of all isomers for the reference
was calculated by DFT calculation (SI).
(7) Frigoli, M.; Moustrou, C.; Samat, A.; Guglielmetti, R. Eur. J.
Org. Chem. 2003, 2799.
(8) X-ray data file is available in Cambridge database under deposi-
tion number CCDC 885269.
Org. Lett., Vol. 14, No. 16, 2012
4151

to get the first X-ray molecular structure of one open isomer
in the naphthopyran series. It is worth noting that the
˚
distance between H2 and the ketone group is ∼2.31 A, the
angle C2ꢀH2ꢀO is 122.25°, and the dihedral angle between
C4aꢀOꢀH2ꢀC2 is 21.37°, confirming a strong H-bond is
taking place in the TC isomer.9 The alternate bond lengths
˚
of the diene unit between the double (C3ꢀC2 = 1.36 A),
˚
pseudosingle (C2ꢀC1 = 1.43 A), and double bond
˚
(C1ꢀC13d = 1.36 A) are in agreement with a conjugated
bridge and a quinoidal structure of the open form.8 This is
also consistent with the bond length of the conjugated ketone
9
group (1.22 A). Due to the repulsive interaction between H1
˚
and the indoline group, a dihedral angle (C13bꢀC13cꢀ
C13dꢀC1) of 32° between the diene and the pseudonaphtha-
lene units is observed. Thanks to the ethynyl group, the
anisole is almost perfectly planar with the conjugated bridge
(C2ꢀC3ꢀC10ꢀC20 =4.65°) and the phenyl group is slightly
more twisted from planarity (C2ꢀC3ꢀC100ꢀC200 = 7.53°).
Indeed, the twisting of the phenyl group comes from the
dedication of this group to be in a strong edge-to-
face interaction with the phenyl of the carbazole unit. The
geometry of the edge-to-face interaction is as follows: the
Figure 2. (a) Thermo- and photoequilibrium for 1. (b) Absorp-
tion spectra before and after visible irradiation. (c) Absorption
spectra of CTC and CTT in the visible range.
strong upfield effect is the NMR signature of a strong
CHꢀπ bond between H200 and the phenyl of the carbazole
in CTC.10 A similar upfield effect was observed between
the chemical shifts of proton H20 (7.11 ppm) in CTT and
H20 (8.01 ppm) in CTC. This demonstrates that a strong
edge to face interaction between the anisole and the phenyl
of the carbazole is taking place in the CTT isomer as well.
In our experimental conditions, at PSS state, the photo-
equilibrium was comprised of CF1 (10%), CTC (37%),
and CTT (53%). Knowing the percentage of all isomers
before and after irradiation, it was possible to calculate
the absorption property of CTT in the visible region. The
absorption maximum of CTT is 453 nm with an ɛ value of
13 400 molꢀ1 L cmꢀ1. Compared to CTC, the absorption
00
˚
proton H2 in interaction to ring centroid distance is 2.70 A,
and the dihedral angle between the interacting ring planes is
51.77°. These bond lengths and angles are typical for strong
edge-to-face interactions with face-tilted-T geometry.10
According to the X-ray structure, the introduction of the
ethynyl group at the 3-position leads to a very good
conjugation throughout the entire molecule by limiting
the steric hindrance at the strict minimum. The concomi-
tant effect of a planar structure highly conjugated and the
withdrawing character of the ethynyl group11 with the
presence of a strong CHꢀπ bond led to the first thermo-
equilibrium between CF1 and CTC wherein the CTC is the
most abundant isomer (Figure 2a).
3
3
maximum is blue-shifted with a lower ɛ value. This is
indicative of a lower conjugation which might result from
the fact that the steric repulsion between H-2 and the
carbazole is more important in CTT than that between H-1
and the carbazole in CTC. Optimized geometries at the
CAM-B3LYP/6-311G(d,p) level of CTT and CTC indicate
that the dihedral angle between the diene unit and the
pseudonaphthalene core (C13b-C13c-C13d-C1) for CTT is
40.8° and 34.4° for CTC, respectively (Table 2S, SI). There-
fore, the overlapping of the π-electrons should be less efficient
in TT than in TC isomers and might be responsible for the
absorption difference observed between CTC and CTT.
Under UV irradiation, CTT could be converted back to
CTC. After 20 min of irradiation, the photoequilibrium
was comprised of CF1 (10%), CTC (75%), and CTT
(15%) (Figure 8S, SI). Moreover, the thermal stability of
CTT is amazingly high as, after 1 month at rt, only 24% of
CTT disappeared to give CTC (Figure 9S, SI). In order to
gain insight into the effect of the ethynyl group and CHꢀπ
bonds, we carried out DFT calculations to determine the
relative energy between all isomers (Figure 2). The order of
The photochemical interconversion of 1 was carried out
in toluene solution at 20 °C and followed by UV/vis and
NMR spectroscopies. Before irradiation, the system 1
displays a strong red color centered at 482 nm, ascertained
to the absorption behavior of CTC. Knowing the concen-
tration of CTC, an ɛ value of 17800 molꢀ1 L cmꢀ1 at
3
3
482 nm was deduced. Upon irradiation with UV light, the
absorption did not change at all. In contrast, upon visible
irradiation, a blue shift of the maximum band was ob-
served (Figure 2b). The change of absorbance was indica-
tive that CTC photoisomerized into a less conjugated open
isomer. NMR spectrum at the PSS state obtained after
5 min of irradiation, indicates the exclusive formation of
CTT isomer (Figure 2, Figure 5S, SI). The difference in the
NMR spectra of CTC and CTT confirmed the presence of
the CHꢀπ bond between proton H200 and the phenyl of the
carbazole unit in CTC as seen in the X-ray molecular
structure. The chemical shift of proton H200 (6.78 ppm) in
CTC is upfield compared to H200 in CTT (7.70 ppm). This
stability given in kJ molꢀ1 was as follows: CF1 > CTC
3
(ΔH = 2.1) > CTT (ΔH = 5.7) > TTC (ΔH = 18.9) >
TTT (ΔH = 20.3). The order of stability is correlated in
some way to the efficiency of the conjugation throughout
the whole molecule which is well represented by the
(9) Desiraju, G. Chem. Res. 1991, 24, 290.
(10) Jennings, W. B.; Farrell, B. M.; Malone, J. F. Acc. Chem. Res.
2001, 34, 885.
~
ꢁ
(11) Lucas, X.; Quinonero, D.; Frontera, A.; Deya, P. M. J. Phys.
Chem. A 2009, 113, 10367.
4152
Org. Lett., Vol. 14, No. 16, 2012

canbeobtainedphotochemically. EventhoughTTisomers
revert back to the closed form thermally, this thermal
process is rather slow with the half-life comparable to
those of azocompounds.1f To confirm the photoproducts
produced under UV irradiation, NMR spectra before and
after irradiation at rt were recorded. After UV irradiation,
two open isomers were observed and were attributed to
TTT and CTT isomers (Figures 10Sꢀ11S, SI). At this
temperature, short-lived TC species cannot be followed in
the NMR time scale. Yet, they were detected at 228 K
(Figure 13S, SI). At PSS, the conversion ratio from the
closed form to the open form was 40%, comprising 26% of
TTT and 14% of CTT. The conversion ratio is rather low.
The chemical shifts of protons in TTT and CTT are very
similar, except those belonging to the anisole and phenyl
group at the 3-position and in particular H20 and H200
protons of interest(Table 1S, Figure 13S, SI). For instance,
H20 (6.57 ppm) in TTT is upfield compared to H20 (7.24
ppm) in CTT. For H200, a reverse scenario is observed. H200
(6.69 ppm) in CTT is upfield compared to H200 (7.32 ppm)
in TTT. As in CTT for 1, H20 and H200 in TTT and CTT
respectively interact in edge-to-face fashion with the phe-
nyl group of the carbazole unit. To gain more information
about the effect of these CHꢀπ bonds in the energy
difference between all isomers, we carried out DFT calcu-
lations. CF2 is the most thermodynamically stable isomer
(Figure 3). More importantly, due to the CHꢀπ bond,
TT isomers becomes more thermodynamically stable
than their TC counterparts. In the reference compound
displaying T-type photochromism, a reverse scenario is
observed in which TC is the most stable open isomer. This
finding indicates that P-type photochromic naphthopyr-
ans are obtained for compounds in which TT isomers are
the most stable open isomers. In 2, this has been possible
due to the stabilizing energy through intramolecular
CHꢀπ bonds in TT isomers that counterbalances in
some way the stabilizing energy of the CHꢀO bond in
TC isomers.
Figure 3. (a) Photochemical interconversion for 2. (b) Absorp-
tion spectra before and after UV irradiation. (c) Absorption
changes at 419 nm upon UV then upon visible light.
dihedral angle between the anisole and the diene unit
(C2ꢀC3ꢀC10ꢀC20) (Table 3S, SI). Also, CTC and CTT
are stabilized by a CHꢀπ bond. This explains why CF1
and CTC are in thermal equilibrium. The small difference
in energy between these two forms indicates a barrierless
thermoisomerization between CF1 and CTC.12 System 1 is
the first example of inverse-type photochromism in the
naphthopyran series in which visible irradiation induces
isomerization from CTC to CTT that leads to a blue shift
of the absorption and the original absorbance is retrieved
by UV irradiation. This original behavior has been possi-
ble due to the electronic and steric effect of the ethynyl
group at the 3-position and the important presence of the
CHꢀπ bond occurring in open isomers.
For 2, before irradiation, the closed form absorbs
exclusively in the UV region and the solution is colorless.
Upon continuous UV irradiation, the solution turns from
colorless to yellow. The maximum of the visible band was
419 nm (Figure 3b). After reaching the PSS state, the
irradiation was removed and the change of absorbance
was monitored at 419 nm at rt in the dark (Figure 3c). A
fast bleaching rate constant (k = 0.21 sꢀ1) was first ob-
served, accounting for 7% of total absorbance, attributed
to TC isomers. 93% of the residual absorbance was found to
In conclusion, we showed for the first time that [3H]-
naphthopyrans can display unusual inverse- or P-type
photochromism through the manipulation of relative
thermodynamic stabilities of open isomers with CHꢀπ
bonds. The relative energy of all isomers is an important
parameter that should be taken into account in the design
of naphthopyrans with specific properties.13 P-type photo-
chromic naphthopyrans rely on the introduction of fused
rings in the k-face. Such photoswitchable molecules with
improved properties could be an interesting alternative
to azomolecules in applications requiring among others
a large geometry change upon photoisomerization. The
modulation of fused (hetero) rings in the k-face of [3H]-
naphthopyrans is underway in our laboratory.
very stable for a long period of time (k = 1.01 ꢁ 10ꢀ6 sꢀ1
,
1/2 = 380 h). The stable coloration was attributed to the
t
absorption of TT isomers. The reaction back from TT to
CF2 can be induced photochemically with visible light.
This photochemical process involves two consecutive
transformations, TTfTCfCF, the first one being the
rate limiting step.5h,i 2 can be considered as a P-type
photochromic system in which coloration and bleaching
Supporting Information Available. Experimental pro-
cedures, spectroscopic data, and DFT calculations. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(12) Rosaria, M.; Nunzio, di; Gentili, P. L.; Romani, A.; Favaro, G.
J. Phys. Chem. C 2010, 114, 6123.
(13) Song, L.; Yang, Y.; Zhang, Q.; Tian, H.; Zhu, W. J. Phys. Chem.
B 2011, 115, 14648.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 16, 2012
4153