Photochromism of acetyl-cyclophanochromene: Intriguing stabilization of photogenerated colored o-quinonoid intermediates

Article abstract of DOI:10.1002/ejoc.201403403

The photochromism of a chromene annulated on [2.2]paracyclophane, Ac-CPC, was investigated. Whereas the parent chromene 2,2-diphenylbenzopyran shows photoinduced coloration only at very low temperatures, acetyl-cyclophanochromene Ac-CPC was found to exhib

Full text of DOI:10.1002/ejoc.201403403

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201403403
Photochromism of Acetyl-Cyclophanochromene: Intriguing Stabilization of
Photogenerated Colored o-Quinonoid Intermediates
Susovan Mandal,*[a] Arindam Mukhopadhyay,[b] and Jarugu Narasimha Moorthy[b]
Keywords: Photochromism / Cyclophanes / Chromenes / Phane effect / Conjugation
The photochromism of a chromene annulated on [2.2]paracy-
clophane, Ac-CPC, was investigated. Whereas the parent
chromene 2,2-diphenylbenzopyran shows photoinduced col-
oration only at very low temperatures, acetyl-cyclophanoch-
romene Ac-CPC was found to exhibit room-temperature pho-
tochromism. With no extended conjugation due to absence
of any substituents in two cofacially-oriented aryl rings, the
through-space delocalization of π-electrons, i.e., the phane
effect, manifests nicely leading to the stabilization of the
photogenerated colored o-quinonoid intermediates. The
interaction between cofacial aryl rings is supported by the
distance between them, as revealed by X-ray crystal struc-
ture analysis. Additionally, electrophile···nucleophile interac-
tions are surmised to be important for intriguingly longer
persistence of the photogenerated intermediates of Ac-CPC
relative to the lifetime of the photogenerated intermediate
generated from the parent cyclophanochromene, which is
devoid of an electron-withdrawing acetyl group.
pyrans.[6] Aromatic annulation, electronic effects trans-
mitted by arylation, and toroidal conjugation extant to
hexaphenylbenzenes have been shown to influence the
photochromic phenomena.[7,8] In a novel series of helical
chromenes, helicity – as a steric force – has been shown to
guide the persistence of the photogenerated colored inter-
mediates.[9] The influence of through-space electronic ef-
fects has also been demonstrated in chromenes that contain
cofacially oriented aryl rings based on 1,8-diarylnaphthal-
ene[10] and in paracyclophanes grafted with chromenes (i.e.,
cyclophanochromenes, CPCs).[11] In continuation of ongo-
ing investigations on CPCs, we examined the intriguing
photochromic behavior observed for a novel chromene, that
is, acetyl-cyclophanochromene (Ac-CPC, Figure 1).
Introduction
The demand for functional materials is on the rise.[1]
A
profound obsession for functional materials is reflected
from explosive research on solar cells, optoelectronic mate-
rials, sensors, biomaterials, data storage devices, and so
on.[2] Organic photochromic molecules are a class of func-
tional materials that have been extensively used in a variety
of applications, for instance, in UV-protective laser goggles,
ophthalmic lenses, display systems, variable transmission
glasses, information recording and storage devices, optical
switches, and nonlinear device components.[2e,2f,3,4] The
phenomenon of reversible interconversion of a substance
between two physically and chemically distinct states by the
influence of light and heat is known as photochromism.[3]
2,2-Diarylbenzopyrans – popularly known as chromenes –
are important members of the family of organic photochro-
mic molecules.[3] Chromenes undergo C–O bond heterolysis
in the presence of UV light to give rise to colored reactive
open forms, termed o-quinonoid intermediates. The latter
can be reverted back to their original colorless closed forms
by the influence of visible light or by action of heat.[5] Mod-
ulation of the spectrokinetic properties of the colored inter-
mediates derived from chromenes has been thoroughly in-
vestigated by one of us.[6–11] We explored the influence of
electronic effects in a series of substituted 2,2-diarylbenzo-
Figure 1. Molecular structures of CPC, Ac-CPC, and MeO-CPC.
[2.2]Paracyclophane is a widely used scaffold that has
been exploited in the development of sensors,[12] asymmet-
ric catalysts,[13] organic light-emitting diodes,[14] and solar
cells.[14] For the first time, we exploited this novel scaffold
to demonstrate the phane effect on the photochromic phe-
nomenon.[11] During these investigations, we found that the
new acetylated derivative, Ac-CPC, also exhibits photo-
chromism at ambient temperatures, which is not observed
[a] Department of Chemistry, Rajiv Gandhi University of
Knowledge and Technology-IIIT-Nuzvid,
Nuzvid 521201, India
E-mail: susovanm6@gmail.com
http://www.rgukt.in/profile_SusovanMandal.html
[b] Department of Chemistry, Indian Institute of Technology,
Kanpur 208016, India
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201403403.
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S. Mandal, A. Mukhopadhyay, J. N. Moorthy
SHORT COMMUNICATION
for the parent chromene, that is, 2,2-diphenylbenzopyran; corresponding phenol. This phenol was annulated with 1,1-
the latter shows photocoloration only at very low tempera- diphenylpropargyl alcohol by using PPTS (pyridinium p-
tures (173–263 K).[15] We herein report that photoexcitation toluenesulfonate) as a catalyst to afford Ac-CPC, which was
1
of Ac-CPC leads to colored o-quinonoid intermediates that thoroughly characterized by IR, H NMR, and 13C NMR
are intriguingly more stable than those derived from the spectroscopic data and ESI mass spectrometry. The struc-
parent CPC; indeed, persistence of the colored intermedi- ture of Ac-CPC was unequivocally established by single-
ates is comparable to that of the methoxy-substituted crystal X-ray structure determination (Figure 2).
cyclophanochromene (MeO-CPC, Figure 1). Besides the
X-ray Single-Crystal Structure Determination of Ac-CPC
and Structural Analysis
phane effect, electrophile···nucleophile interactions are
additionally surmised to be responsible for the increased
persistence of the colored o-quinonoid intermediate(s) of
Ac-CPC.
To examine the relative orientations of the two aryl rings,
we undertook crystal structure determination of cyclo-
phanochromene Ac-CPC. Crystals of this compound were
readily obtained by slow evaporation of its solution in
CHCl3/CH2Cl2 at room temperature. X-ray diffraction data
collection and subsequent structure determination revealed
that the crystals belong to the monoclinic crystal system
with the P21/n (No. 11) space group. In Figure 2 is shown
a perspective drawing of the molecular structure of Ac-
CPC. The crystal packing analyses show that the structure
is stabilized by weak C–H···O hydrogen bonds.
A closer inspection of the molecular structure in Figure 2
shows that the two aryl rings of the cyclophane are cofacial
with a center-to-center distance of 2.97 Å. This distance is
lower than the sum of the van der Waals radii of two
carbon atoms, which attests to closer interaction between
the two cofacially oriented aryl rings. The X-ray-deter-
mined structure reveals further that the acetyl functionality
and the pyran moiety lie on the same side of the cyclo-
phane. One observes that the pyran oxygen atom ap-
proaches the carbonyl carbon atom almost orthogonally,
and indeed lies at a distance of 3.23 Å. The angle of ap-
proach of the pyran oxygen atom to the carbonyl group
turns out to be 102.3° (Figure 2).
Results and Discussion
Synthesis of Cyclophanochromene Ac-CPC
The synthetic methodology employed for the preparation
of Ac-CPC is depicted in Scheme 1. Commercially available
[2.2]paracyclophane was acetylated with acetyl chloride/
AlCl3 in dry CH2Cl2. Formylation of the product with
CHCl2OCH3/TiCl4 followed by Dakin oxidation led to the
Figure 2. (a) Perspective drawing of the X-ray-determined molec-
ular structure of Ac-CPC; hydrogen atoms are omitted for clarity.
Notice the cofacial orientation of the two aryl rings and the prox- Spectrokinetic Behavior of the Photogenerated o-Quinonoid
imity of the pyran oxygen atom to the carbonyl carbon atom.
(b) Geometrical parameters for the approach of the pyran oxygen
atom to the acetyl carbonyl carbon atom, as reported by Dunitz
and coworkers (see below).
Intermediate of Chromene Ac-CPC
The photobehavior of Ac-CPC was examined in toluene
(5ϫ10–3 m) at 298 K. Upon exposure to UV irradiation (λex
Scheme 1. Synthesis of Ac-CPC (DCE = 1,2-dichloroethane).
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Photochromism of Acetyl-Cyclophanochromene
≈ 350 nm), the colorless toluene solution was found to turn for example, as applied to CPCs in Scheme 2. The colored
orange-red. The absorption spectra of the solution before o-quinonoid reactive intermediates, owing to their instabil-
and after UV irradiation are shown in Figure 3 along with ity, revert to initial colorless closed forms either by heating
the observed color changes. Whereas the absorption of Ac- or by irradiation with visible light. The parent chromene,
CPC is characterized by a strong shoulder at 336 nm, which that is, 2,2-diphenylbenzopyran, is known to exhibit photo-
tapers off at about 390 nm, that of the photogenerated in- chromism only at low temperatures (173–263 K).[15] Rever-
termediate is found to exhibit a two-band feature; one ob- sion of the open forms to the closed form is so fast in this
serves a relatively sharp band toward the shorter wave- case that it is practically impossible to detect the formation
length region with a maximum at 416 nm and another in of o-quinonoid intermediates at room temperature.[15] In
the longer wavelength region spanning the entire region principle, four isomeric o-quinonoid intermediates may re-
from 450 to 700 nm (Figure 3). The orange-red color was sult upon photolysis of chromenes, as shown in Scheme 2.
found to bleach within 2–3 min upon standing in the dark It has been established from laser flash photolysis investi-
(Figure 3). The decoloration of the colored o-quinonoid in- gations that the CC isomer reverts rapidly on the nano-
termediates was monitored spectrophotometrically at second to millisecond timescale through thermal 6π con-
416 nm. Decay of the colored intermediate, as monitored rotatory ring closure.[5] Before its decay to the closed form,
by a change in the absorption with time, is also shown in the CC isomer may produce the TC isomer by C–C bond
Figure 3. The kinetic decay trace was best fitted to a mono- rotation. The TT isomer can be generated by a two-photon
exponential function (Figure 3), which permitted determi- absorption process from chromene passing through the TC
nation of the rate constant for reversion of the colored form isomer. It has been firmly established that only the TC and
to the colorless form. The rate constant thus extracted was TT isomers are the ones that are mainly responsible for the
0.011 s–1 at 298 K.
observed coloration upon UV irradiation of chromenes.
Population of the CT isomers is believed to be abysmally
small because of severe steric crowding, which renders it the
least thermodynamically stable. Because of the two-photon
requirement, the TT isomer is likely to be formed only mini-
mally upon exposure of the chromene to UV irradiation for
short periods of time. At shorter durations of irradiation,
it is the TC isomer that is predominantly formed, and the
colored species is generally attributed to this isomer. We
consider that the TC isomer is responsible for the observed
color as a result of photoirradiation of Ac-CPC.
Figure 3. (a) Absorption spectra of Ac-CPC in toluene (5ϫ10–3 m)
before (black) and after (red) photoirradiation. (b) Decay profile
for thermal bleaching of the colored form of Ac-CPC at 298 K in
toluene; kinetics were followed by monitoring the change in the
absorbance with time at 416 nm at 298 K subsequent to photoexcit-
ation of the colorless solution of Ac-CPC at 350 nm for a brief
period.
Scheme 2. Photochromism of cyclophanochromene Ac-CPC.
Stabilization of o-Quinonoid Intermediates by a
Combination of the Phane Effect and
Electrophile···Nucleophile Interactions
We have shown in our previous investigation that CPC
and MeO-CPC also exhibit photochromism, and this was
Mechanistic investigation of the photochromism of 2,2- attributed to the phane effect;[11] notably, coloration due to
diarylbenzopyrans is well documented.[5] Photoirradiation o-quinonoid intermediates of the simple 2,2-diphenylbenzo-
of colorless pyrans (closed form) leads to coloration, and pyran was simply not observable at room temperature but
the color is attributed to the formation of open forms, only at low temperatures (173–263 K). The photogenerated
namely, o-quinonoid intermediates, derived through colored o-quinonoid intermediates of CPC and MeO-CPC
heterolysis of the C(sp3)–O bond in the singlet-excited state; were found to decay at 298 K with rate constants of about
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S. Mandal, A. Mukhopadhyay, J. N. Moorthy
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0.023 and 0.012 s–1, respectively.[11] In stark contrast, the photogenerated o-quinonoid intermediates of Ac-CPC as
colored intermediate in Ac-CPC was found to revert more well. As shown in Figure 4, we envision that the carbonyl
slowly (k = 0.011 s–1) than that of CPC and with a rate group of the o-quinonoid intermediate may function as a
almost comparable to that of MeO-CPC. This is intriguing nucleophile toward the acetyl carbonyl carbon atom. Thus,
in view of the fact that in cofacially oriented systems, one electrophile···nucleophile interactions and the phane effect
would expect the electron-withdrawing groups to destabilize presumably operate in concert to render the o-quinonoid
the electron-deficient o-quinonoid intermediates.[10] What is intermediates of Ac-CPC more stable than those derived
it that stabilizes the intermediate of Ac-CPC in a manner from CPC and comparable to those derived from MeO-
that is counterintuitive?
CPC. Lest, the persistence of the o-quinonoid intermediate
It is evident that the chromene is electron rich in nature, in contrary to the expectation cannot be explained. Al-
whereas the open form, that is, the o-quinonoid intermedi- though limited to a single substrate, the observed photo-
ate, is electron deficient. Hence, electron-releasing groups chromic phenomenon brings out how subtle interactions
attached to the cofacially oriented arenes can stabilize the superposed on the phane effect may lead to counterintuitive
o-quinonoid forms to the extent that the color is observable manifestations.
at room temperature.[10] Several studies have revealed that
there are two distinct types of delocalization of charge den-
sity in [2.2]paracyclophanes, namely, chromophore state
and phane state.[14,16] The first category relates to delocal-
ization through ethylene C–C bonds, and it is mainly ob-
served in paracyclophanes containing long conjugated sub-
stituents. The phane state is attributed to through-space
delocalization of electronic charge density between the co-
facial aryl rings, and it comes into picture if the rings are
properly substituted with simple functional groups.[14,16]
The cyclophanochromene Ac-CPC described herein con-
tains simple acetyl group, and the interactions in it can be
better described by the phane state. Through-space interac-
tions are indeed supported from the X-ray-determined crys-
tal structure of Ac-CPC. The center-to-center distance be-
Figure 4. Presumed electrophile···nucleophile interactions in the
closed and open forms of Ac-CPC.
Conclusions
tween the two aryl rings is as short as 2.97 Å, which is a lot
lesser than the sum of the van der Waals’ radii of two
carbon atoms.
The photochromism of [2.2]paracyclophane annulated
with 2,2-diphenylpyran, namely, Ac-CPC, was investigated.
The photogenerated colored form was found to afford re-
markable spectrokinetic properties at room temperature. In
contrast to the behavior of the photogenerated intermedi-
ates of the parent cyclophane annulated with 2,2-di-
phenylpyran, that is., CPC, Ac-CPC exhibited coloration at
ambient temperatures upon UV irradiation with consider-
able persistence. The X-ray-determined crystal structure
clearly reveals the cofacial arrangement of the two aryl moi-
eties such that the phane effect is amply operative. Ad-
ditionally, electrophile···nucleophile interactions are sur-
mised to be important for the persistence of the photo-
generated o-quinonoid intermediates of Ac-CPC in com-
parison to that of the o-quinonoid intermediates generated
from the parent cyclophanochromene, that is, CPC, which
is devoid of an electron-withdrawing acetyl group.
The phane effect stabilizes the o-quinonoid intermediates
of Ac-CPC to the extent that they are accessible and visible
at ambient temperature. As depicted in Figure 3, the col-
ored intermediate of Ac-CPC exhibits absorption in the vis-
ible region (400–650 nm) with a decay rate constant of
0.011 s–1 at 298 K. However, what is intriguing, as men-
tioned earlier, is that the persistence of the o-quinonoid in-
termediates of Ac-CPC is longer than that of the intermedi-
ates of CPC. As shown in Figure 2, the acetyl group lies in
close proximity of the pyran oxygen atom. Indeed, one can
readily recognize an electrophile···nucleophile interaction
between the pyran oxygen atom and the acetyl carbonyl
carbon atom in a fashion akin to that of the approach of a
nucleophile toward an electrophilic carbonyl group. Pion-
eering studies by Bürgi and Dunitz have shown that the
approach of a nucleophile toward the electrophilic carbonyl
group is directional and occurs along what is termed the
“Bürgi–Dunitz trajectory”.[17] The geometrical parameters
(developed by Bürgi and Dunitz) for the nucleophilic ap-
proach of the pyran oxygen atom toward the electrophilic
acetyl carbonyl group were calculated from the X-ray-deter-
mined molecular structure of Ac-CPC and are shown in
Figure 2. Clearly, an additional interaction in the form of
an electrophile···nucleophile interaction is operative in the
closed form of Ac-CPC besides the cyclophane effect. We
Experimental Section
General Aspects: 1,2-Dichloroethane was distilled from calcium
hydride under an atmosphere of nitrogen prior to use. Anhydrous
THF and toluene were freshly distilled from sodium under a nitro-
gen gas atmosphere. All other solvents were distilled before use.
Column chromatography was performed with silica gel of 60–120
μ mesh.
Synthesis of Acetyl-cyclophanochromene (Ac-CPC): 13-Acetyl-4-
believe that a similar scenario will likely operate for the hydroxy[2.2]paracyclophane (0.20 g, 0.75 mmol), 1,1-diphenylprop-
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Photochromism of Acetyl-Cyclophanochromene
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2-yn-1-ol (0.32 g, 1.50 mmol), and a catalytic amount of PPTS
(5 mol-%) were added to dry DCE (10 mL). The mixture was
heated at reflux under a nitrogen gas atmosphere for 24 h. After
this period, the contents were cooled and washed, and the organic
matter was extracted with chloroform (3ϫ 20 mL). The combined
organic extract was dried with anhydrous Na2SO4 and concen-
trated under reduced pressure. After removal of the solvent, the
crude mixture was subjected to silica gel column chromatography
(4% ethyl acetate in petroleum ether) to obtain pure Ac-CPC, yield
60%. Colorless solid, m.p. 115–117 °C. 1H NMR (500 MHz,
CDCl3): δ = 7.54 (d, J = 7.2 Hz, 2 H), 7.40 (t, J = 7.5 Hz, 2 H),
7.21–7.26 (m, 3 H), 6.96–7.09 (m, 4 H), 6.72 (d, J = 7.8 Hz, 1 H),
6.67 (d, J = 9.5 Hz, 1 H), 6.57 (d, J = 7.5 Hz, 1 H), 6.42 (d, J =
9.5 Hz, 1 H), 6.38 (d, J = 7.7 Hz, 1 H), 6.11 (d, J = 7.7 Hz, 1 H),
4.46–4.50 (m, 1 H), 3.63–3.69 (m, 1 H), 3.36–3.40 (m, 1 H), 3.20
(t, J = 12.3 Hz, 1 H), 2.85–2.92 (m, 2 H), 2.69–2.77 (m, 2 H),
2.10 ppm (s, 3 H). 13C NMR (125 MHz, CDCl3): δ = 27.1, 28.1,
31.7, 33.8, 34.8, 80.4, 122.8, 122.9, 124.9, 125.7, 126.1, 126.5, 126.8,
127.4, 127.5, 127.9, 128.1, 128.8, 132.8, 134.0, 135.5, 135.9, 136.0,
[2]
[3]
[4]
137.5, 142.6, 144.3, 144.5, 151.2, 198.6 ppm. IR (KBr): ν = 3027,
˜
2856, 1662, 1580, 1550, 1491, 1453 cm–1. MS (ESI+): calcd. for
C33H28O2 [M + H] 457.2168; found 457.2162.
X-ray Single-Crystal Structure Determination of Ac-CPC: The X-
ray diffraction intensity data for crystals of Ac-CPC were collected
at 100 K with a Bruker SMART APEX CCD detector system hav-
ing a Mo-sealed Siemens ceramic diffraction tube (λ = 0.7107 Å)
and a highly oriented graphite monochromator operating at 50 kV
and 30 mA. The data collection was done in a hemisphere mode
and was analyzed with Bruker’s SAINTPLUS. The structure was
determined by direct methods by using the SHELXL package, and
refinement was done by full-matrix least-squares method based on
F2 by using the SHELX-97 program. The hydrogen atoms were
largely located from difference Fourier map, and those that could
not be identified were fixed geometrically. Hydrogen atoms were
refined isotropically and were treated as riding on their non-
hydrogen atoms, whereas all non-hydrogen atoms were subjected to
anisotropic refinement. Refinement details and other data are given
in the Supporting Information.
[5]
CCDC-1040065 (for Ac-CPC) contains the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Supporting Information (see footnote on the first page of this arti-
cle): Crystal data and copies of the 1H NMR and 13C NMR spectra
of all the compounds.
Acknowledgments
S. M. and J. N. M. thankfully acknowledge financial support from
the Rajiv Gandhi University of Knowledge and Technology-IIIT-
Nuzvid and the Council of Scientific and Industrial Research
(CSIR), New Delhi, respectively. A. M. is grateful to CSIR for a
research fellowship. The authors thank Mr. Amrit Kumar for his
help during preliminary investigations.
[6]
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Received: November 21, 2014
Published Online: January 27, 2015
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