Photochromism of arylchromenes: Remarkable modification of absorption properties and lifetimes of o-quinonoid intermediates

Article abstract of DOI:10.1021/ol0701621

(Chemical Equation Presented) A significant π-conjugation in 6- and 7-arylchromenes manifests dramatically in the absorption properties of their photogenerated o-quinonoid intermediates. This in conjunction with facile synthesis via Suzuki coupling may render a myriad of photochromic arylchromenes with wide-ranging spectrokinetic properties readily accessible.

Full text of DOI:10.1021/ol0701621

ORGANIC
LETTERS
2007
Vol. 9, No. 5
919-922
Photochromism of Arylchromenes:
Remarkable Modification of Absorption
Properties and Lifetimes of o-Quinonoid
Intermediates
Jarugu Narasimha Moorthy,*,† Parthasarathy Venkatakrishnan,†
Subhas Samanta,† and D. Krishna Kumar‡
Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India, and
Analytical Science Discipline, Central Salt and Marine Chemicals Research Institute,
BhaVnagar 364 002, India
moorthy@iitk.ac.in
Received January 22, 2007
ABSTRACT
A significant
π-conjugation in 6- and 7-arylchromenes manifests dramatically in the absorption properties of their photogenerated o-quinonoid
intermediates. This in conjunction with facile synthesis via Suzuki coupling may render a myriad of photochromic arylchromenes with wide-
ranging spectrokinetic properties readily accessible.
Design and synthesis of molecular systems that respond to
photons as an external stimulus constitute an important
endeavor in the development of organic photochromic
materials, which have assumed significant contemporary
interest due to their diverse applications in variable transmis-
sion glasses, high-density optical storage and switching,
biological phenomena such as photomorphogenesis, vision
process, etc., imaging devices and smart windows.1 Among
various photochromic systems known, the molecules that
contain 2H-chromene unit, e.g., naphthopyrans and spiro-
pyrans,2 have proven to be of rich industrial value in the
production of ophthalmic lenses.3
Mechanistically, photoirradiation of colorless pyrans (closed-
forms) leads to the formation of colored o-quinonoid
intermediates (open-forms) via heterolytic cleavage of the
C(sp3)-O bond in their singlet-excited states (Scheme 1).
The colored quinonoid intermediates may revert to the
Scheme 1
† Indian Institute of Technology.
‡ Central Salt and Marine Chemicals Research Institute.
(1) (a) Bertelson, R. C. In Photochromism; Brown, G. H., Ed.; Wiley:
New York, 1971; Chapter 10 and references therein. (b) Irie, M., Ed. Special
Issue: Photochromism: Memories and Switches. Chem. ReV. 2000, 100,
1683-1890. (c) Raymo, F. M.; Tomasulo, M. Chem.sEur. J. 2006, 12,
3186.
10.1021/ol0701621 CCC: $37.00
© 2007 American Chemical Society
Published on Web 02/09/2007

starting colorless closed-forms either by heating or by
irradiation with a visible light (Scheme 1).4 Unfortunately,
simple derivatives of pyran such as benzopyrans exhibit
photochromism only at low temperatures (263-173 K);5
reversion of the open forms to the closed forms is too fast
in benzopyrans to preclude detection of the quinonoid
intermediates at the room temperature. In order to modify
the kinetic behavior of the photogenerated quinonoid inter-
mediates and to develop photochromic materials with
improved properties, various design strategies have been
explored.6
dered as to how the mesomeric effects transmitted by the
aryl substituents in 5-, 6-, and 7-arylbenzopyrans would
influence the photochromic behavior. Indeed, we were
inspired to investigate such effects based on the fact that a
library of diversely substituted molecules can be readily
accessed via Pd(0)-mediated Suzuki-coupling protocol.10
Herein, we demonstrate that substitution of the parent
benzopyran nucleus with aryl rings (see 5-7, Figure 1) leads
to dramatic modification of the photochromism via extended
π-conjugation.
The arylchromenes 5-7 were conveniently synthesized
starting from arylphenols, which were, in turn, prepared in
a facile manner by Suzuki coupling, see SI. Treatment of
the arylphenols with 1,1-diphenylprop-2-yn-1-ol in the
presence of PPTS (pyridinium p-toluenesulfonate) as a
catalyst in 1,2-dichloroethane afforded the aryl-substituted
chromenes 5-7 in respectable yields.
In the course of our recent studies on helical pyrans7 that
are photochromic, our attention was drawn toward indeno-
fused benzopyrans 1-3 (Figure 1), whose photogenerated
The X-ray crystal structure analyses carried out for
5-MetNap, 6-Nap, 6-DpaPh, and 7-MetNap show that the
angle between the planes of the aryl and the chromene rings
is maximum (ca. 42°) for 5-MetNap, while it varies from 6
to 32° for 6- and 7-aryl analogues (Figure 2). The theoretical
Figure 1. Structures of literature-reported indeno-fused chromenes
and presently examined arylchromenes.
quinonoid intermediates were reported to exhibit remarkable
variations in their absorption properties due to extended
π-conjugation.8 Given that the two phenyl rings in biphenyls
and the phenyl/naphthyl rings in R- and â-phenylnaphtha-
lenes9 are not entirely orthogonal, and that there exists only
a diminished conjugation between the two rings, we won-
Figure 2. X-ray determined molecular structures of arylchromenes.
Notice that the angle (θ/deg) between the planes of the benzene
part of the chromene ring and the aryl ring is larger for 5-aryl-
chromenes. The values in parentheses refer to the angles calculated
for AM1-minimized structures.
(2) (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.
AM1-calculated angles for the same molecules are relatively
higher, cf. Figure 2. As mentioned earlier, it is known that
the two phenyl rings in biphenyl are coplanar in the solid
state, while they are neither orthogonal nor coplanar in the
solution state.11 Therefore, although the angles between the
aryl rings as determined from X-ray studies as well as AM1
calculations are not necessarily applicable in the solution
state, they are otherwise a useful guide to gauge qualitatively
the inhibition of resonance in a closely related series. Based
on the angle between the planes, the steric inhibition of
(3) Crano, J. C.; Flood, T.; Knowles, D.; Kumar, A.; Gemert, B. V. Pure
Appl. Chem. 1996, 68, 1395.
(4) Becker, R. S.; Michl, J. J. Am. Chem. Soc. 1966, 88, 5931.
(5) Lenoble, C.; Becker, R. S. J. Photochem. 1986, 33, 187.
(6) (a) Pozzo, J.-L.; Lokshin, V.; Samat, A.; Guglielmetti, R.; Dubest,
R.; Aubard, J. J. Photochem. Photobiol. A: Chem. 1998, 114, 185. (b)
Olivera, M. M.; Salvador, M. A.; Coelho, P. J.; Carvalho, L. M. Tetrahedron
2005, 61, 1681 and references therein. (c) Coelho, P. J.; Carvalho, L. M.;
Abrantes, S.; Oliveira, M. M.; Olievera-Campos, A. M. F.; Samat, A.;
Guglielmetti, R. Tetrahedron 2002, 58, 9505.
(7) Moorthy, J. N.; Venkatakrishnan, P.; Sengupta, S.; Baidya, M. Org.
Lett. 2006, 8, 4891.
(8) (a) Coelho, P. J.; Carvalho, L. M.; Rodrigues, S.; Olievera-Campos,
A. M. F.; Dubest, R.; Aubard, J.; Samat, A.; Guglielmetti, R. Tetrahedron
2002, 58, 925 and references therein. (b) Martins, C. I.; Coelho, P. J.;
Carvalho, L. M.; Oliveira-Campos, A. M. F.; Samat, A.; Guglielmetti, R.
HelV. Chim. Acta 2003, 86, 570.
(10) Miyaura, N. Top. Curr. Chem. 2002, 219, 11.
(11) Eliel, E. L.; Wilen, S. H.; Mander, L. N. Stereochemistry of Organic
Compounds; John Wiley: New York, 1994; p 1146.
(9) Huang, H.; Drueckhammer, D. G. Chem. Commun. 2006, 2995.
920
Org. Lett., Vol. 9, No. 5, 2007

resonance is expected a priori to be maximum for 5-substi-
tuted chromenes, while that in the case of 6- and 7-substituted
analogues may parallel the trend observed for biphenyl or
â-phenylnaphthalene. In line with this reasoning, the spectral
attributes are found to be entirely different for the three
differently substituted analogues, viz., 5-Nap, 6-Nap, and
7-Nap (Figure 3 and Table 1). A similar trend in the
5 × 10-4 M) exhibited readily observable color changes,
which varied from orange-yellow to blue. In Figure 3 are
shown the absorption spectra of â-naphthyl-substituted
chromenes 5-7 in toluene recorded immediately after
irradiation at 350 nm for a brief period (30 s) at 283 K;
extended duration of irradiation for ca. 6 min was found to
have no effect on spectral features. A similar trend was
observed for the photolysis of phenyl-, p-diphenylamino-
phenyl-, and 6-methoxynaphthyl-substituted chromenes 5-7;
see the Supporting Information. Interestingly, the irradiation
of 6-arylchromenes led to a purple-to-blue color (λmax ca.
520-570 nm), while that of the 5- and 7-arylchromenes led
to yellow-to-orange red color (λmax ca. 420-480 nm). The
color due to the photogenerated intermediates was found to
disappear thermally as well as photolytically with a long
wavelength radiation. It is noteworthy that the transients of
6-arylchromenes display red-shifted absorption by ca. 50-
100 nm relative to those of the transients of 5- and
7-substituted regioisomers. The substitution of a methoxy
group in the naphthyl moiety appears to result only in a
marginal red shift by 10-20 nm. The absorption properties
of all the colored intermediates of arylchromenes 5-7 are
given in Table 1.
The absorptions of the transients in the visible region are
typical of o-quinone methide intermediates, which are formed
by the heterolysis of precursor chromenes in their singlet-
excited states (Scheme 1). Based on a comparison of the
absorption spectral features to those reported in the litera-
ture,12 we attribute the absorptions of the photogenerated
transients in Figure 3 to their respective trans-cis and trans-
trans o-quinone methide intermediates.13 In general, these
two isomers are implicated as being responsible for the
observed color (Scheme 1); the cis-cis isomer is known to
decay quite readily, while the cis-trans isomer can only be
formed with difficulty.
For thermal decay kinetics of the transient o-quinonoid
intermediates, the solutions of 6- and 7-arylchromenes in
toluene were irradiated at their λmax until the photostationary
state (PSS) was reached. At this stage, the change in
absorbance at the absorption maximum with time was
monitored for all cases at 288 K; the duration of irradiation
for the attainment of PSS was determined from coloration
plots (see the Supporting Information). For all of the
5-substituted derivatives, the decays were too fast to be
followed at 288 K. The decays for the transients of 6- and
7-arylchromenes followed a biexponential pattern (Figure
4).14 The rate constants for the decay of the photogenerated
intermediates of 6- and 7-arylchromenes, extracted from
biexponential fits, are collected in Table 1. For comparison,
the rate constants reported for the transients of 1-3 are also
Figure 3. Absorption spectra of 5-, 6-, and 7-naphthylchromenes
before (left) and after irradiation (right).
absorption profiles was uniformly observed for phenyl-,
6-methoxynapththyl-, and p-diphenylaminophenyl-substituted
chromenes 6 and 7. The absorption properties of all aryl-
chromenes are recorded in Table 1. One observes ca. 10-
Table 1. Absorption Properties and Lifetime Data of
Arylchromenes 5-7 and Their Respective Quinonoid
Intermediates
absorption propertiesa,b
before hν
(nm) (ꢀ)
after hν
(nm)
decay ratesc
k1 and k2 (s-1
)
substrate
1
2
3
427, 524
0.160, 0.025d
439, 505
457, 538
416 (br)
426 (br)
0.120, 0.011d
0.091, 0.017d
5-Ph
5-Nap
317 (br, 4026)
328 (sh, 4110)
e
e
e
e
5-MetNap 301 (sh, 13524) 436 (br)
5-DpaPh
6-Ph
6-Nap
6-MetNap 302 (33466)
313 (28435)
322 (br, 2669)
304 (30681)
484
414 (br), 522 (br) 0.061, 0.0017
422 (br), 526 (br) 0.024, 0.0054
415 (br), 543 (br) 0.040, f
6-DpaPh
7-Ph
7-Nap
322 (39944)
325 (12740)
306 (16724)
330 (16784)
424, 564 (br)
422, 530 (sh)
430, 528 (sh)
0.072, 0.0108
0.140, 0.0019
0.029, 0.0069
7-MetNap 332 (28072)
7-DpaPh 355 (33237)
439, 528 (sh)
474
0.018, 0.0037
0.105, 0.0075
a Based on absorption spectra of chromenes 5-7 in dry toluene solutions
(5 × 10-5 M). b br ) broad and sh ) shoulder. c Unless otherwise
mentioned, the decays were monitored at 288 K after steady-state irradiation
for 3-5 min. d From ref 8 at 298 K. e The decay kinetics were too fast.
f The contribution of slow-decaying component was negligible.
(12) (a) Delbaere, S.; Houze, B. L-.; Bochu, C.; Teral, Y.; Campredon,
M.; Vermeersch, G. J. Chem. Soc., Perkin Trans. 2 1998, 1153. (b) Gorner,
H.; Chibisov, A. K. J. Photochem. Photobiol. A 2002, 149, 83. (c) Ottavi,
G.; Favaro, G.; Malatesta, V. J. Photochem. Photobiol. A 1998, 115, 123.
(13) The two isomers exhibit similar absorptions with very marginal
differences, see: Hobley, J.; Malatesta, V.; Hatanaka, K.; Kajimoto, S.;
Williams, S. L.; Fukumura, H. Phys. Chem. Chem. Phys. 2002, 4, 180.
(14) At a given temperature, 7-arylchromenes, upon photolysis, exhibited
highest colorability; the intermediates derived from 7-arylchromenes were
found to decay with virtually minimum absorbance as compared those of
the 5- and 6-substituted analogues.
40 nm variation in the absorption maximum, which attests
to the fact that there exist varying degrees of mesomeric
effect between aryl and chromene rings in 5-7.
Under steady-state irradiation conditions at 283 K, the
colorless solutions of all arylchromenes 5-7 in toluene (ca.
Org. Lett., Vol. 9, No. 5, 2007
921

Scheme 2
Figure 4. Comparative picture of the thermal normalized decay
profiles of the photogenerated o-quinonoid inermediates of 6- (left)
and 7-arylchromenes (right). The wavelengths at which the decays
were monitored are given in parentheses.
given in Table 1. We attribute fast- and the slow-decaying
transients to trans-cis and trans-trans isomers (cf. Scheme
1), repectively, in analogy to the previous assignments for
the transients of 1-3.8 It is known that different isomer
distributions may be produced depending on the irradiation
technique and that the relative thermal stabilities of the
isomers as well as their propensities to interconvert may
cause the biexponential character of fading more significant
or less relevant.15 Otherwise, a perusal of the data in Table
1 reveals that transients of 6- and 7-arylchromenes exhibit
spectrokinetic properties that parallel those of 1-3.
Let us now consider the absorption properties of the
transients of 6- and 7-arylchromenes in particular. It can be
readily gleaned from the data in Table 1 and from Figure 3
that the absorption maximum in the case of 6-arylchromenes
is blue-shifted as compared to that of the 7-arylchromenes,
but the former yield quinonoid intermediates with more
extended conjugation and, hence, red-shifted absorption
maximum as compared to those of the latter. These differ-
ences should be reconciled from the way the electronic
effects are transmitted from the aryl ring to the chromene
and quinonoid moiety. In Scheme 2 are shown the mesomeric
structures for DpaPh derivatives of 6- and 7-chromenes as
representative cases and also for their respective quinonoid
intermediates. It can be seen from the resonance structures
that there exists more extended mesomeric effect in the case
of o-quinonoid intermediates of 6-arylchromenes as com-
pared to those of the 7-aryl analogues.
be modulated by a variation of the substitution pattern
(position 6 or 7) as well the substituents is evident from the
results observed for 6- and 7-arylchromenes; while the
transients of 7-arylchromenes exhibit orange-red color, those
of 6-aryl analogues exhibit blue color (see Table 1 and Figure
3).
In summary, we have shown that a variety of simple
arylchromenes can be easily synthesized via a Suzuki-
coupling protocol and that they exhibit photochromism in a
manner that parallels the trends reported for more complex
indeno-fused chromenes. In accordance with X-ray crystal-
lographic analyses of 5-MetNap, 6-Nap, 6-DpaPh, and
7-MetNap, the aryl substituents at positions 5, 6, and 7 exert
varying degrees of mesomeric effects to modify the absorp-
tion properties of chromenes and their photogenerated
quinonoid intermediates. The facile synthesis via Suzuki
coupling should render a myriad of photochromic aryl-
chromenes with wide ranging spectrokinetic properties
readily accessible.
It is compelling to compare the absorptions of the
transients of fused 1-3 with those of the corresponding free
arylchromenes 6-Ph, 7-Ph, and 7-Nap, respectively. The
absorption maxima for the intermediates of the latter parallel
very closely to those reported for the transients of 1-3, cf.
Table 1, which attests to the fact that the simple aryl
analogues are as good as their synthetically tedious cyclic
counterparts. Clearly, there exists considerable π-conjugation
between the aryl ring and the chromene nucleus in 6- and
7-arylchromenes. That the absorption properties can indeed
Acknowledgment. J.N.M. is thankful to DST, India, for
funding. P.V. and S.S. are grateful to CSIR for research
fellowships. We thank Dr. P. Dastidar (CSMCRI, Bhavna-
gar), our collaborator, for allowing his student D.K.K. to
carry out the single-crystal X-ray diffraction studies. We
thank the reviewers for their invaluable suggestions.
Supporting Information Available: Experimental pro-
cedures, spectral data (UV and NMR), crystal data and
structure refinement details, and AM1 output files. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(15) Frigoli, M.; Pimienta, V.; Moustrou, C.; Samat, A.; Guglielmetti,
R.; Aubard, J.; Maurel, F.; Micheau, J.-C. Photochem. Photobiol. Sci. 2003,
2, 888.
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