Through-space control of the persistence of photogenerated o-quinonoid intermediates in naphthalenes containing cofacially oriented chromenes and arenes

Article abstract of DOI:10.1021/ol300449p

Remarkable modulation of the persistence of the photogenerated colored o-quinonoid intermediates via a through-space interaction has been demonstrated in chromenes 1-4 based on 1,8-diarylnaphthalenes. Polar/π interaction is shown to stabilize the closed form of 4 to such an extent that photoinduced coloration is virtually invisible, while the same stabilization in the opened form of 2 permits ready coloration with a long-lived o-quinonoid intermediate.

Full text of DOI:10.1021/ol300449p

ORGANIC
LETTERS
2012
Vol. 14, No. 10
2438–2441
Through-Space Control of the Persistence
of Photogenerated o-Quinonoid
Intermediates in Naphthalenes Containing
Cofacially Oriented Chromenes and Arenes‡
Jarugu Narasimha Moorthy,* Susovan Mandal, and Keshaba Nanda Parida
Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India
moorthy@iitk.ac.in
Received February 23, 2012
ABSTRACT
Remarkable modulation of the persistence of the photogenerated colored o-quinonoid intermediates via a through-space interaction has been demonstrated
in chromenes 1À4 based on 1,8-diarylnaphthalenes. Polar/π interaction is shown to stabilize the closed form of 4 to such an extent that photoinduced
coloration is virtually invisible, while the same stabilization in the opened form of 2 permits ready coloration with a long-lived o-quinonoid intermediate.
Molecule-based electronic materials are relevant in min-
iaturized hybrid devices that can perform magnification,
rectification, and switching operations similar to those
exhibited by microscopic silicon-based analogs. In parti-
cular, photoresponsive materials are of significant impor-
tance from the point of view of their applications in vari-
able transmission glasses, nanoscale sensors, high density
optical data storage, molecular machines, etc.1À3 Among
diverse classes of photochromic compounds, arylchro-
menes (2H-benzopyrans and 2H-naphthopyrans) have
been extensively investigated in view of their industrial
application in ophthalmic lenses.1c 2,2-Diarylchromenes
readily undergo photoinduced CÀO bond heterolysis to
ring-opened colored o-quinonoid intermediates, which
revert back to the closed form (colorless) either by expo-
sing to visible light or by keeping in the dark.4,5 We have
been interested in the modulation of spectrokinetic proper-
ties of the colored o-quinonoid intermediates;6 we have
shown that simple arylation at 6- and 7-positions of the
chromene nucleus can manifest, via mesomeric effects in
dramatically distinct absorptions of the intermediates and
associated decay kinetics.6b,d We have also shown that the
helical environment around the chromene moiety also
modifies the spectrokinetic properties of the photogenerated
‡ Dedicated to Prof. Gautam R. Desiraju on the occasion of his 60th
birthday.
(1) (a) Photochromism: Molecules and Systems; Durr, H., Bouas-Laurent,
H., Eds.; Elsevier: Amsterdam, 1990. (b) Organic Photochromic and
Thermochromic Compounds; Crano, J. C., Guglielmetti, R. J., Eds.; Plenum
Press: New York, 1999; Vols. 1 and 2. (c) Crano, J. C.; Flood, T.; Knowles,
D.; Kumar, A.; Gemert, B. V. Pure Appl. Chem. 1996, 68, 1395. (d) Corns,
S. N.; Partington, S. M.; Towns, A. D. Color. Technol. 2009, 125, 249.
(2) (a) Willner, I. Acc. Chem. Res. 1997, 30, 347. (b) Irie, M. Chem.
Rev. 2000, 100, 1683. (c) Feringa, B. L. Acc. Chem. Res. 2001, 34, 504. (d)
Lemieux, V.; Branda, N. R. Org. Lett. 2005, 7, 2969. (e) Raymo, F. M.;
Tomasulo, M. Chem. Soc. Rev. 2005, 34, 327. (f) Paramonov, S. V.;
Lokshinb, V.; Fedorova, O. A. J. Photochem. Photobiol., C 2011,
12, 209.
(4) Lenoble, C.; Becker, R. S. J. Photochem. 1986, 33, 187.
(5) (a) Kodama, Y.; Nakabayashi, T.; Segawa, K.; Hattori, E.;
Sakuragi, M.; Nishi, N.; Sakuragi, H. J. Phys. Chem. A 2000, 104,
11478. (b) Hobley, J.; Malatesta, V.; Hatanaka, K.; Kajimoto, S.;
Williams, S. L.; Fukumura, H. Phys. Chem. Chem. Phys. 2002, 4, 180.
(c) Gentili, P. L.; Danilov, E.; Ortica, F.; Rodgers, M. A. J.; Favaro, G.
Photochem. Photobiol. Sci. 2004, 3, 886.
(6) (a) Moorthy, J. N.; Venkatakrishnan, P.; Sengupta, S.; Baidya,
M. Org. Lett. 2006, 8, 4891. (b) Moorthy, J. N.; Venkatakrishnan, P.;
Samanta, S.; Kumar, D. K. Org. Lett. 2007, 9, 919. (c) Moorthy, J. N.;
Venkatakrishnan, P.; Samanta, S. Org. Biomol. Chem. 2007, 5, 1354. (d)
Moorthy, J. N.; Koner, A. L.; Samanta, S.; Roy, A.; Nau, W. M.
Chem.;Eur. J. 2009, 15, 4289. (e)Mandal, S.;Parida, K. N.; Samanta, S.;
Moorthy, J. N. J. Org. Chem. 2011, 76, 7406.
(3) (a) Kawata, S.; Kawata, Y. Chem. Rev. 2000, 100, 1777. (b)
Bertarelli, C.; Biancoc, A.; Castagnaa, R.; Pariania, G. J. Photochem.
Photobiol., C 2011, 12, 106.
r
10.1021/ol300449p
Published on Web 04/27/2012
2012 American Chemical Society

o-quinonoid intermediates.6a In our recent investigations,
we delineated the effect of electronic coupling via toroidal
delocalization on the photochromism of chromenes that are
constrained to be part of the hexaphenylbenzene scaffold.6e
During these investigations, our attention was drawn to the
voluminous investigations by Siegel et al.7 on establishing
the nature of interaction between cofacially oriented aryl
rings substituted at the 1,8-positions of the naphthalene
core. While scant evidence has been offered in favor of
the charge-transfer interaction between the aryl rings
experimentally,8 polar/π effect (a through-space Coulombic
interaction) has been shown to be responsible for the
behavior and stability of 1,8-diarylnaphthalene derivatives.
We wondered as to how the nature of interaction, i.e.,
charge-transfer or polar/π effect, may manifest in the
photochemistry of naphthalenes substituted at 1,8-positions
with chromene and an aryl ring that contains different
substituents. As the chromenes and their photogenerated
o-quinonoid intermediates are complementary to each other
in terms of electronic properties (the former being electron-
rich and the latter being electron-poor), the spectrokinetic
properties of the intermediates were a priori expected to
relay information on the true nature of the interaction.
Herein, we report the photochemistry of chromenes based
on naphthalenes 1À4 that contain cofacially oriented aryl
rings and remarkable through-space control of the lifetimes
of quinonoid intermediates (Figure 1).
Scheme 1
Figure 2. The absorption spectra of chromenes (a) 1, (b) 2, (c) 3,
and (d) 4 before (black) and after irradiation (red) in toluene
with λ = 350 nm at 298 K (1 and 2) and at 278 K (3 and 4). The
concentrations of the chromenes in toluene were typically 4À5 Â
10À3 M. Also shown in the insets are decay profiles of the
intermediates of 1 and 2.
Figure 1. Structures of the chromenes based on naphthalene
core.
Steady-state irradiation of the solutions of chromenes 1
and 2 in toluene (4À5 Â 10À3 M) contained in 3-cm quartz
cuvettes at λ = 350 nm in a photoreactor for a few minutes
led to purple-red coloration at 298 K. However, similar
color changes were not observed for the solutions of
chromenes 3 and 4 unless the irradiation was carried out
at 278 K. In Figure 2 are shown the absorption spectra of
the solutions before and after irradiation; the spectra for 1
and 2 were recorded at rt, while for 3 and 4, the spectral
acquisition was done at 278 K. Clearly, all the chromenes
1À4 lead to photogenerated intermediates for which the
differences in absorptions are unexceptional.
The naphthalene-based chromenes 1À4 were readily
synthesized from their precursor 1-(4-hydroxyphenyl)-8-
arylnaphthalenes by PPTS (pyridinium-p-toluenesulfo-
nate)-mediated cyclocondensation reaction, Scheme 1.
The latter were accessed by Pd(0)-catalyzed Suzuki cou-
pling reactions; see the Supporting Information. All chro-
menes 1À4 were thoroughly characterized by IR, 1H
NMR, 13C NMR, and ESI-MS spectroscopic analyses.
One observes that the photogenerated intermediates
exhibit uniformly a 2-band pattern between 400 and 440 nm
followed by a broad absorption band that spans from
450 to 700 nm (Table 1). While the purple-red color in 1
and 2 was found to disappear in 2À3 min at 298 K, that in
the case of 3 and 4 was found to revert to the colorless form
too rapidly to preclude spectral monitoring of their decays
(7) (a) Cozzi, F.; Cinquini, M.; Annunziata, R.; Dwyer, T.; Siegel,
J. S. J. Am. Chem. Soc. 1992, 114, 5729. (b) Cozzi, F.; Cinquini, M.;
Annunziata, R.; Siegel, J. S. J. Am. Chem. Soc. 1993, 115, 5330. (c) Cozzi,
F.; Siegel, J. S. Pure Appl. Chem. 1995, 67, 683. (d) Cozzi, F.; Ponzini, F.;
Annunziata, R.; Cinquini, M.; Siegel, J. S. Angew Chem., Int. Ed. 1995,
34, 1019.
(8) (a) Beaumont, T. G.; Davis, K. M. C. J. Chem. Soc. B 1967, 1131.
(b) Kaneta, N.; Mitamura, F.; Uemura, M.; Murata, Y.; Komatsu, K.
Tetrahedron Lett. 1996, 37, 5385.
Org. Lett., Vol. 14, No. 10, 2012
2439

state irradiation conditions, second photon absorption
may populate CT and TT isomers (Scheme 2).9À12 In
Table 1. Absorption Properties of Arylchromenes 1À4 and
Kinetic Data of Their Photogenerated o-Quinonoid
Intermediates in Toluene
Scheme 2
absorption properties
before hν
(nm) (ε)
after hν
max (nm)a
rate k1
and k2 (sÀ1
)
compound
λ
1
2
3
4
287 (14370)
303 (8150)
288 (7490)
286 (6945)
420, 524
420, 514
420, 529
417, 537
0.021b
0.010
c
c
a The long wavelength absorption maximum corresponds to the
broad band. b The decay curve was fitted to biexponential function;
the rate constant refers to the major component. c The very short lifetime
of the transient o-quinonoidintermediate disallowed kinetic monitoring;
see text.
at room temperature. In Figure 2 are also shown (inset) the
decay kinetics for the disappearance of the colored species
derived from chromenes 1 and 2 (Table 1). Of the two
colored intermediates of chromenes 3 and 4, that of the
latter was found to decay too rapidly. Thus, the lifetimes of
transient intermediates derived from 1À4 were found to
follow the order 2 > 1 > 3 > 4. The data in Table 1 show
that the intermediate of 1 decays at least two times faster
than that of 2, while that of 3 is much faster and 4 the
fastest.
To gain insights concerning relative dispositions of the
aryl rings that are cofacial in each of the chromenes 1À4,
we sought to establish their X-ray crystal structures. While
the crystals suitable for X-ray investigations were obtained
for 1À3, our persistent efforts togrow the crystals of 4 were
in vain. In Figure 3 are shown the X-ray determined
general, the CC isomer is known to decay quite rapidly
within a few picoseconds.9À12 It is the TC and TT isomers
that are generally observed over a few milliseconds to
minutes time scale; the formation of CT is generally ruled
out on the basis of steric considerations. Thus, the color
that is observed upon photolysis of chromenes is generally
attributed to TC and TT isomers; formation of the latter is
important only under prolonged durations of irradiation.
Otherwise, the species that decays predominantly when
chromens are subjected to irradiation for brief periods of
irradiation is the TC isomer. The longer-lived TT isomer,
formed in small amounts, may interfere with decay
kinetics.6d In the backdrop of this mechanistic exemplar,
let us now consider the spectrokinetic behavior of the
photogenerated intermediates of 1À4. As mentioned ear-
lier, the spectral features for all of the intermediates of 1À4
are unexceptional. While the colored species attributed to
o-quinonoid intermediates of 1 and 2 are readily observa-
ble at room temperature, those for 3 are barely observable
and virtually not for 4. Furthermore, the fact that the
chromene 1 leads to colored intermediates that persist for a
few minutes at room temperature is remarkable in light of
the fact that the parent chromene, i.e., 2,2-diphenylbenzo-
pyran, does not lend itself to readily observable colored
intermediates.4 It is thus compellingly evident that the
substituent(s) in the aryl ring that is cofacially oriented
on the naphthalene scaffold with respect to the chromene
moiety exert(s) remarkable influence on the behavior of
Figure 3. The X-ray determined molecular structures of chro-
menes (a) 1, (b) 2, and (c) 3.
molecular structures of 1À3. The distance between the
˚
two aryl moieties ranges from 3.4 to 3.6 A. It is also
(10) (a) Moine, B.; Buntinx, G.; Poizat, O.; Rehault, J.; Moustrou, C.;
Samat, A. J. Phys. Org. Chem. 2007, 20, 936. (b) Coelho, P. J.; Carvalho,
L. M.; Abrantes, S.; Oliveira, M. M.; Oliveira-Campos, A. M. F.; Samat,
A.; Guglielmetti, R. Tetrahedron 2002, 58, 9505.
(11) (a) Day, P. N.; Wang, Z.; Pachter, R. J. Phys. Chem. 1995, 99,
9730. (b) Jacquemin, D.; Perpete, E. A.; Maurel, F.; Perrier, A. Phys.
Chem. Chem. Phys. 2010, 12, 13144.
(12) (a) Delbaere, S.; Houze, B. L.; Bochu, C.; Teral, Y.; Campredon,
M.; Vermeersch, G. J. Chem. Soc., Perkin Trans. 2 1998, 1153. (b)
Delbaere, S.; Micheau, J. C.; Vermeersch, G. J. Org. Chem. 2003, 68,
8968. (c) Delbaere, S.; Micheau, J. C.; Frigoli, M.; Mehl, G. H.;
Vermeersch, G. J. Phys. Org. Chem. 2007, 20, 929.
noteworthy that the angles between the planes of the aryl
rings and the naphthalene core range between 48 and 66°.
The photochemistry of chromenes is well established.
Accordingly, CÀO bond heterolysis in the singlet excited
state leads instantaneously to o-quinonoid intermediate
CC, which may isomerize to TC isomer. Under steady-
(9) Sousa, C. M.; Pina, J.; de Melo, J. S.; Berthet, J.; Delbaere, S.;
Coelho, P. J. Org. Lett. 2011, 13, 4040.
2440
Org. Lett., Vol. 14, No. 10, 2012

photogenerated o-quinonoid intermediates. That the inter-
action between the aryl ring and the closed chromene/
ring-opened o-quinonoid moiety can indeed be possible via
through-space is evident from the X-ray determined mo-
lecular structures of chromenes 1À3. The distance between
the centers of the cofacially oriented aryl ring and the
To understand the kinetic behavior of o-quinonoid
intermediates of chromenes 1À4, let us consider 2 and 4;
the former contains an electron-rich arene ring, while the
latter an electron-poor ring (Scheme 3). In closed and open
forms, the chromene and its photogenerated o-quinonoid
intermediate should constitute electron-rich and electron-
poor rings, respectively. Thus, the polar interaction be-
tweenthecofaciallyoriented ringsshouldbestrongerinthe
open form than in the closed form in 2, while the opposite
˚
benzene ring of the chromene varies between 3.4 and 3.6 A.
This is the distance at which the geometry between two
aromatic rings can be effectively stabilized or destabilized
or may even lead to reactions, e.g., photodimerizations in
the solid state.13 Although the aryl rings are somewhat
tilted with respect tothe naphthalene core, their geometries
should inprinciple allow considerable face-to-faceoverlap.
How does the through-space interaction bring about a
dramatic influence on the decay/persistence of the photo-
generated o-quinonoid intermediates of chromenes based
on 1,8-napthalene? A lot of investigation has been
carried out to gain insights into the nature of interaction
between cofacially oriented arenes. In particular, the em-
phasis has centered on resolving the dichotomy between
electrostatic nature and charge-transfer.7 It is well-known
that the preferred geometry for benzene dimer is slipped/
offset parallel geometry in both solution as well as solid
state.14 The interaction between benzenes substituted with
electron-donating and electron-withdrawing groups can
be understood on the basis of electrostatic force between
the quadrupole moments. Indeed, it has been established
that the quadrupoles of benzene and hexafluorobenzene
are almost equal in magnitude but opposite in sign,15
so that parallel-stacked geometry is favored for the
benzeneÀhexafluorobenzene dimer. In the case of 1,8-diaryl-
naphthalenes in which the two aryl rings that are con-
strained to be cofacial contain electron-donating and
electron-withdrawing groups, the interactions can be of
two types, i.e., (i) polar (electrostatic and induction) and
(ii) van der Waals (dispersion).7c As the contact area is
small in the case of 1,8-diarylnaphthalenes, the polar term
is expected to dominate. The experimental evidence7 as
well as the theoretical calculations14,15 show that the
interaction between the cofacially oriented aryl moieties
is polar/π in nature. Further, no strong absorption above
400 nm in the UVÀvis absorption spectra has been ob-
served in 1,8-diarylnaphthalenes containing aryl rings that
are electronically complementary, which rules out the
possibility of charge-transfer.7 In the present investigation,
we did not observe any absorptions that may be attributed
to charge-transfer bands in all of the chromenes and their
photogenerated intermediates, cf. Figure 2.
Scheme 3
should hold true in the case of 4. In other words, the polar/
π interaction should stabilize the open form in 2 and
destabilize the same in 4. The behavior of parent chromene
1 and the cyano-substituted derivative 3 should be ex-
pected to be within the extremes represented by 2 and 4. In
complete accordance with this reasoning, one observes the
persistence of color due to the photogenerated reactive
intermediates in the order 2 > 1 > 3 > 4.
In summary, remarkable modulation of the persistence
of o-quinonoid intermediates by through-space interac-
tions has been demonstrated in a variety of chromenes
based on 1,8-diarylnaphthalenes. In general, the modula-
tion of kinetic property has been possible via annulation or
electronic substituent/mesomeric effects. Through-space
control of the kinetic decay of o-quinonoid intermediates
is heretofore unprecedented, and the results described
herein illustrate the finesse with which the behavior of
reactive intermediates can be controlled.
Acknowledgment. J.N.M. acknowledges the financial
support from CSIR, India. S.M. and K.N.P. are grateful
to CSIR for their research fellowships. We thank Prof.
Venugopalan for help with the crystal structures.
Supporting Information Available. Experimental pro-
cedures, spectral data (UVÀvis and NMR), spectral
reproductions, and crystal structure refinement details.
This material is available free of charge via the Internet
at http://pubs.acs.org.
(13) Ramamurthy, V.; Venkatesan, K. Chem. Rev. 1987, 87, 433.
(14) (a) Jennings, W. B.; Farrell, B. M.; Malone, J. F. Acc. Chem. Res.
2001, 34, 885. (b) Tsuzuki, S.; Honda, K.; Uchimaru, T.; Mikami, M.;
Tanabe, K. J. Am. Chem. Soc. 2002, 124, 104.
(15) Tsuzuki, S.; Uchimaru, T.; Mikami, M. J. Phys. Chem. A 2006,
110, 2027 and references therein.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 10, 2012
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