Helicity as a steric force: Stabilization and helicity-dependent reversion of colored o-quinonoid intermediates of helical chromenes

Article abstract of DOI:10.1021/ja312027c

Photolysis of regioisomeric helical chromenes 1 and 2 leads to colored reactive intermediates. While the latter generally decay quite rapidly, they are found to be longer lived in 1 and highly persistent in 2. The remarkable stability of the otherwise fleeting transient in 2 allowed isolation and structural characterization by X-ray crystallography. The structural analyses revealed that steric force inherent to the helical scaffold is the origin of stability as well as differentiation in the persistence of the intermediates of 1 and 2 (1Q and 2Q). The structure further shows that diphenylvinyl moiety in the TT isomer of 2Q gets splayed over the helical scaffold such that it is fraught with a huge steric strain to undergo required bond rotations to regenerate the precursor chromene. Otherwise, reversion of 2Q was found to occur at higher temperatures. Aazahelical chromenes 3 and 4 with varying magnitudes of helicity were designed in pursuit of o-quinonoid intermediates with graded activation barriers. Their photogenerated intermediates 3Q and 4Q were also isolated and structurally characterized. The activation barriers for thermal reversion of 2Q-4Q, as determined from Arrhenius and Eyring plots, are found to correlate nicely with the helical turn, which decisively determines the steric force. The exploitation of helicity is thus demonstrated to develop a novel set of photoresponsive helicenes 2-4 that lead to colored intermediates exhibiting graded stability. It is further shown that the photochromism of 2-4 in conjunction with response of 2Q-4Q to external stimuli (acid, heat, and visible radiation) permits development of molecular logic gates with INHIBIT function.

Full text of DOI:10.1021/ja312027c

Article
pubs.acs.org/JACS
Helicity as a Steric Force: Stabilization and Helicity-Dependent
Reversion of Colored o‑Quinonoid Intermediates of Helical
Chromenes
Jarugu Narasimha Moorthy,* Susovan Mandal, Arindam Mukhopadhyay, and Subhas Samanta
Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India
S
* Supporting Information
ABSTRACT: Photolysis of regioisomeric helical chromenes 1
and 2 leads to colored reactive intermediates. While the latter
generally decay quite rapidly, they are found to be longer lived in
1 and highly persistent in 2. The remarkable stability of the
otherwise fleeting transient in 2 allowed isolation and structural
characterization by X-ray crystallography. The structural analyses
revealed that steric force inherent to the helical scaffold is the
origin of stability as well as differentiation in the persistence of
the intermediates of 1 and 2 (1Q and 2Q). The structure further
shows that diphenylvinyl moiety in the TT isomer of 2Q gets
splayed over the helical scaffold such that it is fraught with a
huge steric strain to undergo required bond rotations to
regenerate the precursor chromene. Otherwise, reversion of
2Q was found to occur at higher temperatures. Aazahelical
chromenes 3 and 4 with varying magnitudes of helicity were designed in pursuit of o-quinonoid intermediates with graded
activation barriers. Their photogenerated intermediates 3Q and 4Q were also isolated and structurally characterized. The
activation barriers for thermal reversion of 2Q−4Q, as determined from Arrhenius and Eyring plots, are found to correlate nicely
with the helical turn, which decisively determines the steric force. The exploitation of helicity is thus demonstrated to develop a
novel set of photoresponsive helicenes 2−4 that lead to colored intermediates exhibiting graded stability. It is further shown that
the photochromism of 2−4 in conjunction with response of 2Q−4Q to external stimuli (acid, heat, and visible radiation) permits
development of molecular logic gates with INHIBIT function.
materials.23,24 Insofar as molecular recognition is concerned,
there has been remarkable progress in harvesting exceptional
enantiodifferentiating potential of helicenes for creation of
organocatalysts; diverse catalysts based on helicenes with
INTRODUCTION
■
Helicity is ubiquitous in all forms that besiege our day-to-day
life.1,2 Aside from the fact that it constitutes the structure of a
basic unit of life, namely DNA, and gigantic galaxies that are
ingenious designs continue to be explored in pursuit of high
inherent in the composition of the universe, helicity appears to
enantioselectivity in asymmetric synthesis.25,26 Helicenes or
be indispensable in the nature’s wiliest designs and creations
be it in a plant/animal kingdom or in a materialistic world.3−5
For chemists, helical structures have traditionally represented
aesthetic marvels. The initial and pioneering work of Newman
on helical organic structures spawned a great deal of interest in
the properties of helicenes.6,7 Over the past decade or so, there
has been a swarming interest in exploring the utility of helicity
in a variety of applications. Today, the helical scaffold
essentially forms a design element in the development of
recognition-based sensors,8,9 molecular switches,10 springs,11
motors,12 tweezers,13 solenoids,14 organic electronics,15 IR-
sensing16 and display devices,17 NLO materials and thin films,18
liquid crystalline materials,19 polymers,20 etc. As applied to
biology, helicenes, by virtue of their inherent chirality, are
obvious choices for efficient chiral selection in DNA binding
and intercalation, telomerase inhibition, etc.21,22 The remark-
ably higher optical rotations confer helicenes with unique
opportunities for exploration in the development of chiroptical
helicenoids27,28 elicit tremendous excitement in the context of
photoresponsive materials.
2,2-Diarylbenzopyrans, termed generally chromenes, are a
unique class of photochromic compounds that are industrially
useful in ophthalmic lenses.29−32 Irradiation of chromenes with
UV radiation brings about C−O bond cleavage leading to the
so-called o-quinonoid intermediates that are colored; the latter
revert thermally as well as photochemically with visible light.
While the photochromic phenomenon of chromenes, spiropyr-
ans, is widely explored in ophthalmic lenses, their application as
molecular switches in optical data storage devices is limited by
the rapid thermal reversion of the photogenerated colored
forms.33 Thus, modulation of spectrokinetic properties of the
photogenerated o-quinonoid intermediates and hence the
Received: December 10, 2012
© XXXX American Chemical Society
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Chart 1
a
Scheme 1. Synthesis of Helical Chromenes 1−4
a
(a) (7-Coumaryl)methyltriphenylphosphonium bromide, 50% NaOH, DCM, 0 °C to rt, 3 h; (b) hν, I2, O2, toluene, 24 h; (c)
benzyltriphenylphosphonium bromide, 50% NaOH, DCM, 0 °C to rt, 3 h; (d) (i) PhMgBr/THF, (ii) H+, rt; (e) (6-coumaryl)-
methyltriphenylphosphonium bromide, 50% NaOH, DCM, 0 °C to rt, 3 h; (f) (4-methoxybenzyl)triphenylphosphonium bromide, 50% NaOH,
DCM, 0 °C to rt, 3 h; (g) BBr3, dry DCM, 0 °C to rt, 6 h; (h) 1,1-diphenylpropargyl alcohol, PPTS, dry DCE, reflux, 24 h; (i) (Boc)2O, TEA,
DMAP, THF, 0 °C to rt, 12 h; (j) 30% piperidine in DCM, rt, 12 h; (k) H2O2, Conc. H2SO4, DCM−MeOH, rt, 48 h.
photochromic phenomenon of chromenes has been a subject of
extensive investigations.32,34 Our own interest centers in the
phenomenon of photochromism, and we have been interested
in the modulation of photochromic property exhibited by
chromenes.35 Buoyed by our preliminary investigations on
pentahelical chromenes, which offered a glimpse into the effect
of helical scaffold as a steric force on the behavior of colored
intermediates,35a we designed regioisomeric hexahelical chro-
menes 1 and 2 (Chart 1) to unambiguously exemplify the role
of helicity in modulating the photochromic behavior. The
analogous azahelical chromenes 3 and 4 were subsequently
designed and synthesized, and their photochromic behavior
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Figure 1. Perspective drawings of the X-ray determined molecular structures of regioisomeric helical chromenes 1 and 2 (a and b), and heterohelical
chromenes 3Me (c) and 4Me (d).
investigated. Herein, we show that helical scaffold in molecular
systems, such as 2−4 (Chart 1), can be a unique steric force to
stabilize the otherwise fleeting photogenerated colored o-
quinonoid intermediates to the extent that they can be isolated
as solid materials and their structures determined by X-ray
crystallography. Reversion of these intermediates to the
colorless forms occurs upon heating. We show that the
reversion kinetics can be modulated by manipulating the
magnitude of steric force, which is determined by the curvature
of the helical arc. Subtle variations in the helicities of
chromenes 2−4 (Chart 1) are shown to manifest differently
in the stabilities of their respective o-quinonoid intermediates to
permit development of thermoresponsive materials that revert
at graded temperatures. Indeed, the photochromic phenomena
of 2−4 in conjunction with the response of their stable colored
intermediates to external stimuli, such as acid, heat, and visible
radiation, permit development of molecular logic gates with
INHIBIT function for application in optical data storage
devices.
prepared by a sequence of reactions involving Wittig
olefination, oxidative cyclization, and Dakin reaction, cf.
Scheme 1. Similarly, chromenes 4Me and 4Boc were prepared
starting from 3,6-diformylcarbazole by an analogous sequence
of reactions mentioned above. The phenols were then
transformed to the chromenes by reacting with 1,1-
diphenylpropargyl alcohol using PPTS as a catalyst. All the
helical chromenes were comprehensively characterized by IR,
NMR, and mass spectral data.
X-ray Crystal Structures of Chromenes 1−4. We were
successful in obtaining the crystals of 1, 2, 3Me, and 4Me
suitable for X-ray analysis, while persistent efforts with
crystallization of 3Boc and 4Boc were in vain. X-ray diffraction
intensity data collection for the crystals of 1, 2, 3Me, and 4Me
was carried out at 100 K on a Smart Apex CCD diffractometer.
Subsequently, the structure determinations and refinements
were performed using SHELX suit of programs. In Figure 1 are
shown the perspective drawings of the molecular structures of
these helical chromenes. In all of these structures, one observes
considerable deformation of aromatic rings from planarity
effected evidently by the helicity. In order to gauge the relative
helicities of chromenes 2−4, one may consider a number of
parameters, i.e., θ, d, ψ1, ψ2, and φtotal. The parameter ‘d’ is the
distance between the center of the benzene ring to which the
pyran ring is annulated and the center of the terminal benzene
ring. Similarly, the parameter ‘θ’ describes the angle between
the mean planes of the two aromatic rings for which the
parameter ‘d’ is calculated. The twist angles ψ1 and ψ2 refer to
the torsion angles for the atoms a−b−f−g and b−c−e−f, as
shown in Figure 1. The sum of torsion angles for the atoms of
inner helicene, i.e., a−b−c−d, b−c−d−e, c−d−e−f, and d−e−
f−g, is denoted by φtotal. The values of d (Å), θ (°), ψ1 (°), ψ2
RESULTS AND DISCUSSION
■
Synthesis of Helical Chromenes. The regioisomeric
carbohelical chromenes 1 and 2 were synthesized from
precursor helical coumarins via Grignard reaction followed by
dehydration. The helical coumarins were in turn prepared by
oxidative photocyclization of the diarylolefins accessed by
Wittig olefination, cf. Scheme 1. The azahelical chromenes 3Me
and 3Boc were prepared by subjecting the suitably function-
alized phenols to cyclocondensation with 1,1-diphenylpropargyl
alcohol in the presence of pyridinium-p-toluenesulfonate
(PPTS) as a catalyst in dry 1,2-dichloroethane at reflux. The
required hydroxy-substituted helical carbazole derivatives were
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(°), and φtotal (°) calculated for all chromenes 2−4 are collected
in Table 1 along with those for the regiohelical chromene 1 for
value for d and a larger value for θ should reflect more helicity.
It thus emerges that the helicity for 2−4 follows the trend: 2 >
3 > 4. This trend is completely corroborated by twist angles ψ1
and ψ2 as well as the sum of torsion angles φtotal
.
Table 1. Geometrical Parameters of Chromenes 1, 2, 3Me,
and 4Me
Photochemistry of Regioisomeric Helical Chromenes
1 and 2 and Heterohelical Chromenes 3 and 4. Exposure
of the solutions of chromenes 1 and 2 in mesitylene/1,2-
dichlorobenzene (5 × 10−3 M) to UV-radiation (λex ≃ 350 nm)
under N2 atmosphere led to instant blood-red coloration. While
the color in 1 was found to bleach on standing in the dark
within a few minutes, that in the case of 2 was found to be
remarkably persistent. In Figure 2 are shown the absorption
spectra before and after irradiation, which reveal that the
absorptions for 1 and 2 terminate at ∼435 nm, while strong
absorptions in the visible region extend down to 650 nm for the
colored intermediates generated subsequent to photolysis. It is
noteworthy that some fine structure is apparent for the
absorption spectra of chromenes 1 and 2 in contrast to a broad
feature that is observed for the photogenerated colored
parameters
1
2
3Me
4Me
θ (°)
d (Å)
63.99
5.20
46.84
5.05
45.76
5.49
38.06
5.63
a
ψ1 (°)
ψ2 (°)
φtotal (°)
110.15
52.59
68.00
112.05
44.48
77.22
91.28
39.62
68.75
74.73
26.99
61.09
b
c
a
b
c
The twist angle a−b−f−g. The twist angle b−c−e−f. Sum of
torsion angles: a−b−c−d, b−c−d−e, c−d−e−f, and d−e−f−g.
comparison. The expectation here is that with increasing helical
turn, the terminal benzene ring should become closer to the
diphenylpyran moiety such that steric repulsions may cause the
terminal benzene ring to twist considerably. Thus, a smaller
Figure 2. Absorption spectra of chromenes 1 (a), 2 (b), 3Me (c), 3Boc (d), 4Me (e), and 4Boc (f) before (black) and after irradiation (red). The
thermal reversion of the colored species is also shown when monitored at 298 and 373 K for 1 and 2−4, respectively.
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Table 2. Activation Energy (Ea), Pre-Exponential Factor (A), ΔH‡ ΔS‡, Rate Constant for Thermal Bleaching at 353 K, and
Half-Life (t1/2) at 298 K for Each of the o-Quinonoid Intermediates 2Q−4Q
entry
compound
2Q
3MeQ
3BocQ
4MeQ
4BocQ
1
2
3
4
5
6
Ea (kcal mol−1
)
30.52
13.20
28.48
12.64
28.83
12.83
26.80
12.22
24.36
10.86
log A
ΔH‡ (kcal mol−1
ΔS‡ (cal K−1 mol−1
k (s−1
t1/2 at 298 K
)
29.70
27.86
28.21
26.11
23.73
)
−1.01
−2.84
−1.97
−4.95
−10.98
6.23 × 10−5
94 d
)
0.20 × 10−5
46 yrs
1.20 × 10−5
4.6 yrs
0.99 × 10−5
5.3 yrs
4.17 × 10−5
253 d
Figure 3. 1H NMR spectra (500 MHz, C6D6) of helical chromene 2 and its photolysates with increasing duration of irradiation at 350 nm. Notice
the appearance of new signals that is attributable to the formation of a single product, namely, o-quinonoid intermediate 2Q.
Figure 4. X-ray determined molecular structures of the o-quinonoid intermediates derived from helical chromenes 2, 3Boc, and 4Me.
intermediates in the visible region. All of the heterohelical
chromenes 3 and 4 were found to exhibit photochemical
behavior akin to that of chromene 2. Photolysis of chromenes
3Me and 3Boc yielded colored intermediates that are highly
persistent. In comparison, the intermediates of 4Me and 4Boc
were found to be comparatively less stable; the bleaching was
found to occur slowly at room temperature over a few days in
the case of 4Boc.
Decay Kinetics of the Colored Intermediates. Given
that the photogenerated colored intermediates of chromenes
generally revert quite rapidly as observed with helical chromene
1 in the present investigation, remarkable persistence observed
for the intermediates of chromenes 2−4, i.e., 2Q−4Q, is
intriguing. Bleaching of the color was found to occur upon
heating the solutions as monitored by UV−vis absorption
spectroscopy, cf. Figure 2. Thus, temperature-dependent decay
kinetics of the colored intermediates were carried out over a
range of temperatures by following the disappearance of
absorption at 524 nm in each case, cf. Supporting Information.
From the rate constants thus extracted, Arrhenius free energy of
activation (Ea) and the pre-exponential factor (A) were
determined for reversion of each of the intermediates, i.e.,
2Q−4Q, Table 2. Similarly, the enthalpy and entropy of
activation were also determined from Eyring plots, cf. Table 2
and Supporting Information. Accordingly the barriers for
reversion of the colored intermediates of 2−4 follow the
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order: 2Q > 3MeQ ≈ 3BocQ > 4MeQ > 4BocQ. The half-
lives (t1/2) as determined from Arrhenius equation also follow
the same trend, see entry 6.
i.e., A → B, was determined based on the rate equation
developed for the Scheme 2, cf. Supporting Information.
The integrated rate law for the Scheme 2 is given by:
X-ray Structural Characterization of the Colored
Intermediates. H NMR monitoring of the photolysates of
[A]t
[A]0
1
2
1
2
1
2
=
[e−k t + e−(2k +k )t
]
2−4 revealed the formation of only one colored isomer. In
Figure 3 are shown the 1H NMR spectra of the photolysates of
chromene 2 with increasing duration of irradiation at 350 nm in
a photoreactor. As can be seen, the signals corresponding to the
colored intermediate appear distinctly, and their formation
saturates with time pointing to attainment of photostationary
state (pss) at which the relative concentration of the colored
intermediate is 28%. A similar behavior is observed for all other
chromenes 3−4, cf. Supporting Information.
The exceptional stabilities of the colored intermediates 2Q-
4Q spurred us to isolate them and establish their structures by
X-ray crystallography. From a comprehensive analytical data,
the products of all chromenes 2−4 were identified as o-
quinonoid intermediates. Crystallization of the colored photo-
products by slow evaporation led to the crystals of 2Q, 3BocQ,
and 4MeQ suitable for X-ray studies. The perspective drawings
of the molecular structures are shown in Figure 4. Clearly, they
correspond to the o-quinonoid intermediates formed sub-
sequent to C−O bond cleavage. What is noteworthy is that the
geometry of the o-quinonoid intermediate in each case
corresponds to TT configuration, vide infra.
Based on the above expression, the rates k1 and k2 were
determined from a biexponential fit for the disappearance of A
with time. Thus, the rate constants determined at different
temperatures were fitted to Arrhenius and Eyring equations to
extract the activation parameters, namely, log A, Ea, ΔH‡, and
ΔS‡, cf. Table 3, for both racemization as well as thermal
Table 3. Activation Energy (Ea), Pre-Exponential Factor (A),
ΔH‡ and ΔS‡, and Rate Constant for Racemization and
Reversion/Ring Closure at 353 K as Determined by HPLC
Analyses for a Representative o-Quinonoid Intermediate,
i.e., 3BocQ
entry
parameters
Ea (kcal mol−1
inversion/racemization thermal reversion
1
2
3
4
5
)
31.07
13.88
28.06
12.3
logA
ΔH‡ (kcal mol−1
ΔS‡ (cal K−1 mol−1
k (s−1
)
30.31
27.25
)
2.5
−4.65
)
0.54 × 10−5
0.85 × 10−5
Activation Barriers for Racemization of o-Quinonoid
Intermediates. To address the question as to whether thermal
decomposition of the colored intermediates is competed by
racemization, the enantiomers of a representative quinonoid
intermediate, namely 3BocQ, were resolved with an AD-H
chiral column using HPLC. Thus, thermal decomposition of
one of the optically pure enantiomer (let us say ‘A’, Scheme 2)
reversion/ring closure. Indeed, the Arrhenius activation barriers
for reversion of 3BocQ thus extracted from the integrated
equation above agree closely with those determined by time-
dependent thermal reversion as followed by UV−vis absorption
spectroscopy (Table 2). Further, the Arrhenius inversion/
racemization barrier determined likewise is found to compare
very well with those determined experimentally for analogous
pentahelical systems that contain a methyl group at the C1
position.36
Scheme 2. Reversion and Concomitant Inversion/
Racemization of a Representative o-Quinonoid Intermediate
3BocQ with Heating
Helicity-Dependent Stabilization and Reversion of
the Colored o-Quinonoid Intermediates at Graded
Intervals of Temperature. The mechanism of photo-
chromism of chromenes has been a subject of several
investigations.37−40 The current understanding is that the
photoexcitation of chromenes results in C−O bond cleavage
leading to the colored species, which are ascribed to the so-
called o-quinonoid intermediates, cf. Scheme 3. The instanta-
neously formed CC isomer may undergo either rapid bond
rotation to the TC isomer or collapse back to the precursor
chromene. A two-photon absorption by the TC isomer has
been shown to lead to the TT isomer, which may undergo C−
C bond rotation to the CT isomer.37−40 In general, the CC and
CT isomers are discounted as being unimportant in the
observed photochromism of chromenes at room temperature
based on the consideration that the CC isomer must be very
short-lived and that the population of CT isomers should be
negligible due to steric strain in its geometry. Accordingly, the
TC isomer is predominantly observed in laser-flash photolysis,
while both TC and TT are observed under steady-state
irradiation conditions; of course, the TC isomer corresponds
generally to the predominant species.41 In the backdrop of this
knowledge, we attribute the species responsible for color
observed upon photolysis of helical chromenes 1−4 to o-
quinonoid intermediates. Indeed, the structure determinations
by X-ray crystallography (Figure 4) unequivocally establish the
colored species to be o-quinonoid intermediates.
was followed at different intervals of time using HPLC. As the
chiral isomer ‘A’ depleted with time on heating, the amounts of
formation of its enantiomer ‘B’ and the reversion product, i.e.,
chromene, ‘C’, were determined by analyzing the reaction
mixture with a chiral HPLC. Based on the areas of the starting
chiral 3BocQ (A), its enantiomer (B), and thermally
regenerated chromene (C), the rate of inversion/racemization,
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Scheme 3. Mechanistic Details of Photochromism of Chromene (left) and the Behavior Observed for Helical Chromenes 1−4
(right)
Figure 5. Necessary bond rotations that must occur for collapse of the TT isomers of the o-quinonoid intermediates to the corresponding colorless
chromenes as shown for 2Q as a representative case.
In line with the mechanistic picture described above, the
photogenerated o-quinonoid intermediates of helical chromene
1 with its pyran oxygen located on the periphery of the helical
scaffold exhibited decay kinetics that could be fitted to a
biexponential function. The fast-decaying predominant compo-
nent is attributed to the TC isomer, while the slow-decaying
minor fraction is assigned to the TT isomer. The brick-red
color was bleached within 2−3 min after exposure of the
solution of 1 to UV radiation. In contrast, the color in the case
of its regioisomeric chromene 2 was found to be highly
persistent to permit isolation and characterization of the o-
quinonoid intermediate (2Q), cf. Figure 4 and Scheme 3. The
reversion behavior of the colored species of 1 is something that
is typically observed for chromenes for which the o-quinonoid
intermediates are stabilized by electronic or hydrogen-bonding
interactions.34,35,42,43 The question that springs up is: What is it
that causes the stability of 2Q to be so drastically different from
that of 1Q? The origin of observed differentiation in the
persistence of the regioisomeric colored intermediates should
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be traceable to the structural factors, which seemingly operate
differently.
Replacement of methyl by Boc group diverts participation of
the lone pair in the resonance structures of the photogenerated
quinonoid intermediates so as to influence the stabilities of the
latter. This electronic effect is glaring for the intermediates of
4MeQ and 4BocQ in which the nitrogen is proximate to the o-
quinonoid moiety such that its contribution to resonance
stabilization is significant in 4MeQ and virtually none in
4BocQ. As can be seen from Table 2, the activation barrier for
4BocQ is significantly lesser than that for 4MeQ. The lack of
such resonance stabilization in the former is reflected in its
rapid decay (entry 5, Table 2). It is noteworthy that the steric
hindrance is relatively lower in 4MeQ and 4BocQ so that their
thermal reversions to the colorless forms are faster than those
of 2Q and 3Q. In contrast, the influence of similar electronic
factors is not apparent for 3MeQ and 3BocQ; the activation
barriers for reversion in these cases are comparable and so are
the rates of their decay. In these cases, the nitrogen is evidently
too far away from the o-quinonoid moiety for the electronic
factors to become important.
The X-ray crystal structures of 2Q−4Q establish the
geometry to be TT. We believe that the steric and electronic
factors may influence mesomeric structures such that thermal
equilibrium is possible via C−C bond rotations between the
TC and TT isomers.44 To reconcile the regio-differentiating
thermal bleaching behavior of 1Q and 2Q, let us consider their
structures in Scheme 3. In the TT and TC isomers of 1Q, the
diphenylvinyl moiety is disposed outward of the helical scaffold,
while it is splayed over the internal curvature of the helical
scaffold in 2Q. For reversion of the TC and TT o-quinonoid
intermediates to the colorless closed form, the diphenylvinyl
moiety must undergo bond rotations. As one may readily
decipher, the reversion to the colorless form should be easy in
1Q due to lack of steric barrier for rotation of the diphenylvinyl
moiety. In contrast, the TT isomer in the case of 2Q should be
fraught with a huge barrier enforced by the helical scaffold for
its reversion.
As shown in Figure 5, the TT isomer must undergo bond
rotation first to the TC isomer and close up to the chromene in
the subsequent steps. In fact, collapse of the TC isomer to the
chromene via CC isomer may be sterically accelerated. Thus,
bond rotation in the TT isomer that leads to the TC isomer
should determine the barrier for collapse of the o-quinonoid
intermediate to the chromene. As can be seen from Figure 5,
the diphenylvinyl moiety cannot easily undergo the required
bond rotations to get ultimately to the CC isomer to revert to
the colorless precursor. The TT isomer is evidently the most
stable isomer in which the diphenyvinyl moiety is castled nicely
within the helical arc. The recent theoretical calculations44
show that the TT isomer is the most stable of all others even in
systems that are devoid of steric factors. Thus, it is the
operation of steric forceexerted by the helical scaffold in the
o-quinonoid moietyin 2Q and lack of it in 1Q that is the
origin of extreme stability or rapid reversion, respectively.
This contrasting behavior exhibited by the regioisomeric
intermediates of 1 and 2 in terms of persistence was indeed the
basis for the design of heterohelical analogs 3 and 4. The
objective, as mentioned earlier, was to modulate the helicity of
helical chromenes and examine the extent to which the
persistence of the photogenerated colored intermediates is
influenced. In 3 and 4, location of the five-membered
heterocyclic ring was envisaged to modify the helical arc and
hence the steric force. Differences in the magnitudes of steric
force extant in helical chromenes 2−4 can be assessed from d,
θ, ψ1, ψ2, and φtotal described earlier. Accordingly smaller values
of d and larger values of θ should imply severe steric crowding.
As shown in Figure 1, the distance ‘d’ is lowest for 2 followed
by 3Me and 4Me. The angle ‘θ’ is highest for 2 followed by
3Me and 4Me. In a similar manner, the other parameters, i.e.,
ψ1, ψ2, and φtotal, are highest for 2 and lowest for 4. Thus, the
photogenerated TT isomer should be expected to be more
stable in 2 followed by that in 3Me and 4Me such that the
thermal reversion is slowest for 2Q followed by 3MeQ and
4MeQ. In complete agreement with this reasoning, the
experimentally observed order of the activation barriers for
reversion is: 2Q > 3MeQ > 4MeQ. This is clearly borne out
from the decay rates of 2Q, 3MeQ, and 4MeQ at 80 °C, cf.
entry 5, Table 2; of the three intermediates, 2Q decays the
slowest and 4MeQ the fastest.
Helicenes are inherently characterized by chirality and may
undergo racemization.45 The barriers for the latter are
determined by the number of aromatic rings that the helicene
is made up of46 and the presence of substituents on the atoms
of inner helicene.36 The helical inversion may in principle
compete with thermal reversion of the colored intermediates,
i.e., 2Q−4Q. Such a process occurring simultaneously might
influence the bond rotations and favored configurations of the
o-quinonoid intermediates on heating. The barriers for
inversion/racemization as well as reversion to chromene
determined for the resolved enantiomer of 3BocQ described
earlier show that the reversion occurs much faster than
inversion/racemization. This should indeed be the case for
2Q and 3MeQ for which the activation barriers for thermal
reversion are comparable. For these cases, the slow
racemization should be expected to have no bearing on the
conformational changes explored by the o-quinonoid inter-
mediates during their reversion to chromenes. In contrast, for
intermediates with low activation barriers for thermal
reversions, i.e., 4MeQ and 4BocQ, the racemization is likely
to compete efficiently with reversion.
Molecular Logic Gate Based on Photochromic Proper-
ties of the Helical Chromenes and Stimuli-Responsive
Reversion of their Stable o-Quinonoid Intermediates.
Construction of molecular-level electronic devices47 is one of
the chief goals in the contemporary chemistry of functional
materials.48−50 Molecular switches,10,51 the chemical systems
that undergo reversible changes under the influence of
appropriate input, are exploited in the fabrication of
miniaturized systems. The ultimate objective is to obviate the
use of macroscopic semiconductors and develop molecular
switches that mimic Boolean logic52 conventions to permit
electronic computing at the molecular level through encoding
and decoding of binary digits, i.e., 0 and 1, in the form of input
and output.
The photochromic phenomenon observed with helical
chromenes in conjunction with thermal reversion of the
colored intermediates allows, in principle, adaptation of 2−4
to logic operations. We found that the colored o-quinonoid
intermediates can be readily reverted with visible radiation as
well as H+. Thus, one has four inputs, namely UV radiation (I1),
heat (I2), H+ (I3), and visible radiation (I4), and one output
(O) described by the absorption at a chosen wavelength in the
visible range. By defining an absorbance of <0.1 and >0.1 at 524
The chromenes 3Boc and 4Boc represent systems in which
the electronic factors are superposed over steric factors.
H
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Figure 6. Representation of a two-state molecular switch based on INH logic function.
nm as corresponding to the output being 0 or 1, respectively,
one deduces that the logic gate is ‘on’, i.e., O = 1, only when UV
light is present (I1 = 1) and all other inputs are absent (I2 = 0, I3
= 0, I4 = 0). Further, when the UV radiation is absent (I1 = 0)
or when any of the inputs I2−I4 is present simultaneously with
I1, the absorbance at 524 nm would be <0.1 such that O = 0.
Thus, the photochromic helical chromenes 2−4 represent two-
state molecular switches with an overall INHIBIT (INH) logic
gate function. The functioning of the molecular switch with UV
and heat as the input is shown in Figure 6 for the helical
chromene 2 along with the truth table for the INH logic
operation. Thus, the photo- and thermoresponsive helical
chromenes 2−4 should constitute novel additions to the gamut
of molecular logic gates. A variety of logic operations53 have
been fabricated at the molecular level. It turns out that the logic
operation INHIBIT54 has been explored in a range of
applications.55,56 A new and exciting dimension is the
application of INHIBIT in photonic materials with light as an
input.
quinonoid intermediates of helical chromenes 2−4 engineered
via sterics built into the helical scaffold via rational design is
remarkable. For example, the activation parameters for
reversion of the colored form of 2Q point to a half-life ≃ 46
years at 298 K, which renders them as being equivalent to well-
acknowledged photochromic materials, namely, diarylethy-
lenes59 (lifetime ≃ 45 years at 298 K). The longest lifetimes
known heretofore for some of the best examples of spiropyrans,
spirooxazines, and fulgides are ≃7 d, 1.4 h, and ≃3 d,
respectively, at 298 K.59 Thus, the stability of the photo-
generated colored intermediates observed herein for the helical
chromenes 2−4 should confer the latter with unparalleled
utility in light-written data storage devices, more so in view of
the fact that the optical information can be read out easily with
other stimuli, such as H+ and visible radiation, cf. Figure 7.
Photochromism of Helical Chromenes and Implica-
tions for Application as Molecular Switches in Optical
Data Storage. Photochromic diarylpyrans (chromenes),
spiropyrans, and spirooxazines32,57 have gained lots of
importance in terms of their applications in ophthalmic
lenses31,32 and liquid crystals.57 The advantages with these
photochromes are the ease of synthetic accessibility, structural
variability, high quantum yields of photocoloration, high fatigue
resistance, and very high two-photon cross section. Insofar as
their application in optical data storage devices57,58 is
concerned, the major disadvantage is that the energy barrier
for thermal reversion of the photogenerated colored forms is
very low.33,59 This serious drawback continues to curtail their
application to long-term storage of light-written information in
particular. Indeed, bistability of photochromic compounds is a
fundamental requirement in ‘photochemical erasable memo-
ry’.60 In spite of a large number of modifications toward
improving the kinetic parameters for thermal reversion of the
photogenerated colored merocyanine forms, chromenes,
spiropyrans, and spirooxazines appear to fail in rivaling the
bistability observed with diarylethylenes58 and fulgides.61 Only
few instances of stabilization43,62,63 of colored merocyanines by
electronic delocalization, dipole interactions, hydrogen bond-
ing, etc. are known in contrast to a number of reports that
exemplify the isolation and characterization of photogenerated
colored intermediates of diarylethylenes64 and fulgides.65
In light of the aforementioned notorious limitation with
thermal instability of the colored forms of chromenes/
spiropyrans/spirooxzines, the exceptional stability of the o-
Figure 7. Read-out of light-written information as applied to optical
data storage.
CONCLUSIONS
■
Photolysis of rationally designed regioisomeric helical chro-
menes 1 and 2 is shown to lead to colored reactive
intermediates in both cases. While the photogenerated colored
species decay quite rapidly in general, they are found to be
longer lived in the case of 1 and highly persistent in the case of
2; the calculated half-life is 46 years. The remarkable stability of
the intermediate 2Q permitted characterization by X-ray
crystallography. Structural analyses reveal that the steric force
inherent to the helical scaffold is the origin of stability as well as
differentiation in the persistence of 1Q and 2Q. In the latter,
the diphenylvinyl moiety gets splayed over the helical scaffold,
whereby it is fraught with a huge steric strain to undergo
required bond rotations to regenerate the precursor chromene.
Otherwise, reversion is found to occur at high temperatures.
Based on the helicity-dependent steric barrier for thermal
reversion, azahelical chromenes 3 and 4 that are analogs of 2
and differ in terms of the magnitude of helicity were designed
I
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(d, J = 8.6 Hz, 1H), 7.69−7.73 (m, 3H), 7.79 (d, J = 8.6 Hz, 1H), 7.85
(d, J = 8.0 Hz, 1H), 7.93 (d, J = 8.3 Hz, 1H), 8.31 (d, J = 8.3 Hz, 1H);
13C NMR (CDCl3, 125 MHz) δ 82.9, 119.8, 122.7, 123.2, 124.3, 124.5,
124.9, 125.4, 125.6, 125.7, 126.0 (×2), 126.4 (×2), 126.7 (×2), 126.8,
126.9, 127.16 (×2), 127.24, 127.5 (×2), 127.9, 129.6, 130.0, 130.5,
131.4, 132.5, 133.3, 133.9, 143.6, 144.3, 149.7; ESI-MS+ m/z calcd for
C37H25O, 485.1905 [M + H]; found, 485.1902.
Carbohelical Chromene 2. Pale-yellow crystalline solid, yield
34%; mp 151−153 °C; IR (film) cm−1 3056, 2924, 2853, 1490, 1446;
1H NMR (CDCl3, 500 MHz) δ 5.41 (d, J = 10.3 Hz, 1H), 5.86 (d, J =
in pursuit of o-quinonoid intermediates with graded activation
barriers. The helical chromenes 3 and 4 were also found to
exhibit photochromism akin to 2 such that their respective
photogenerated intermediates (3Q and 4Q) are also isolated
and structurally characterized. The thermal reversion barriers of
2Q−4Q are shown to correlate uniquely with the helical turn,
which decisively determines the steric force. The exploitation of
helicity is thus demonstrated to develop a novel set of
photoresponsive molecular systems 2−4 that lead to colored o-
quinonoid intermediates of varying stabilities and revert
thermally at graded intervals of temperature. It is shown that
the photochromic phenomena of 2−4 in conjunction with the
response of their intermediates 2Q−4Q to external stimuli,
such as acid, heat, and visible radiation allow development of
molecular logic gates with INH function. To the best of our
knowledge, exploitation of the helicity in a rational manner to
stabilize reactive intermediates and thus modulate functional
behavior is unprecedented.
10.3 Hz, 1H), 6.10 (t, J = 8.0 Hz, 1H), 7.10−7.22 (m, 6H), 7.31 (d, J =
6.9 Hz, 2H), 7.35−7.48 (m, 4H), 7.68 (d, J = 8.6 Hz, 1H), 7.72 (d, J =
8.0 Hz, 1H), 7.80 (d, J = 8.0 Hz, 1H), 7.83 (d, J = 8.6 Hz, 1H), 7.84−
7.94 (m, 5H); 13C NMR (CDCl3, 125 MHz) δ 81.5, 116.1, 117.7,
122.6, 123.4, 123.7, 124.0, 125.5, 125.7, 126.0, 126.6, 127.0 (×2),
127.1 (×2), 127.3, 127.39, 127.45, 128.0 (×2), 128.1 (×2), 128.4,
128.6, 129.0, 129.7, 130.5, 130.6, 131.3, 133.6, 144.4, 146.1, 151.1;
ESI-MS+ m/z calcd for C37H25O, 485.1905 [M + H]; found, 485.1908.
Representative Procedure for Cyclocondensation of Phe-
nols with 1,1-Diphenylpropargyl Alcohol: Synthesis of
Azahelical Chromene 3Me. An oven-dried round-bottom flask
was charged with 2-hydroxy-9-methyl-9H-naphtho[2,1-c]carbazole
(0.90 g, 3.03 mmol), 1,1-diphenylprop-2-yn-1-ol (1.26 g, 6.06
mmol), and a catalytic amount of PPTS (0.04 g, 5 mol %) under a
nitrogen gas atmosphere, and dry 1,2-dichloroethane (40 mL) was
introduced. The contents were heated at reflux for 18−24 h. After this
period, the reaction mixture was cooled and washed with saturated
Na2CO3 solution, and the organic matter was extracted with
chloroform (30 mL ×3). The combined extract was dried over
anhyd. Na2SO4, filtered, and evaporated to yield the crude compound.
Further purification by silica-gel column chromatography using 2.5%
ethyl acetate in pet ether afforded the azahelical chromene 3Me as a
colorless crystalline material in 76% yield; mp 210−212 °C; IR (KBr)
EXPERIMENTAL SECTION
■
1
General Aspects. H and 13C NMR spectra were recorded on
JEOL (400 and 500 MHz) spectrometers in CDCl3/C6D6/CD3CN as
a solvent. The ESI mass spectra were recorded on Waters QTOF
machine. UV−vis absorption spectra were recorded using a Shimadzu
UV-1800 spectrophotometer. Dry tetrahydrofuran (THF) and toluene
were freshly distilled over sodium prior to use. All the reactions were
monitored by analytical thin layer chromatography (TLC) using
commercial aluminum sheets precoated with silica gel. Chromatog-
raphy was conducted on silica gel (Acme, Mumbai, 60−120 mesh). All
the commercial chemicals were used as received.
Synthesis of Helical Chromenes. The carbohelical chromenes 1
and 2 were synthesized from precursor helical coumarins via Grignard
reaction followed by dehydration. The helical coumarins were in turn
prepared by oxidative photocyclization of the diarylolefins accessed by
Wittig olefination, cf. Supporting Information. The azahelical
chromenes 3Me and 3Boc were prepared by subjecting the suitably
functionalized phenols to cyclocondensation with 1,1-diphenylpro-
pargyl alcohol in the presence of PPTS as a catalyst in dry 1,2-
dichloroethane at reflux. The required hydroxy-substituted helical
carbazole derivatives were prepared by a sequence of reactions
involving Wittig olefination, oxidative cyclization, and Dakin reaction,
cf. Supporting Information. Similarly, chromenes 4Me and 4Boc were
prepared starting from 3,6-diformylcarbazole by an analogous
sequence of reactions mentioned above. The phenols were then
transformed to the chromenes by reacting with 1,1-diphenylpropargyl
alcohol using PPTS as a catalyst. All the helical chromenes were
comprehensively characterized by IR, NMR, and mass analyses.
Representative Procedure for Grignard Reaction Followed
by Cyclization: Synthesis of Carbohelical Chromenes 1 and 2.
To a stirred solution of excess of phenylmagnesium bromide (0.33 g,
1.8 mmol) in dry THF at 0 °C was added a solution of helical
coumarin, i.e., phenanthro[9,10-k]-3H-naphthopyran-3-one (0.10 g,
0.3 mmol), in THF (3 mL) under nitrogen gas atmosphere. After
stirring for 1 h at room temperature, the reaction mixture was cooled,
quenched with dil H2SO4, and stirred at room temperature for 1 h.
The resultant reaction mixture was extracted with ethyl acetate. The
combined organic extract was washed with brine solution, dried over
anhyd sodium sulfate, filtered, and evaporated. The crude product
obtained was purified by silica gel column chromatography to afford
chromene 2 as a pale-yellow crystalline solid (0.05 g, 34% yield). A
similar protocol was adopted to synthesize 1 (15% yield) from
phenanthro[9,10-k]-2H-naphthopyran-2-one.
1
cm−1 3401, 2975, 1702, 1615, 1371; H NMR (CDCl3, 500 MHz) δ
3.97 (s, 3H), 5.77 (d, J = 10.1 Hz, 1H), 6.39 (t, J = 7.9 Hz, 1H), 6.79
(d, J = 10.1 Hz, 1H), 7.22−7.33 (m, 6H), 7.39−7.47 (m, 3H), 7.51−
7.57 (m, 4H), 7.66 (d, J = 8.3 Hz, 1H), 7.69 (d, J = 7.7 Hz, 2H), 7.72
(d, J = 8.6 Hz, 1H), 7.86 (d, J = 8.6 Hz, 1H), 7.90 (d, J = 8.6 Hz, 1H);
13C NMR (CDCl3, 125 MHz) δ 29.4, 81.9, 107.8, 109.9, 117.1, 117.2,
118.2, 119.0, 122.3, 123.8, 124.0, 124.5, 124.6, 125.0 (×2), 126.4,
126.9 (×2), 127.3 (×2), 127.4, 127.5, 127.8, 128.1 (×2), 128.2 (×2),
128.3, 128.4, 129.4, 129.5, 139.9, 140.0, 140.5, 144.4, 151.1; ESI-MS+
m/z calcd for C36H26NO, 488.2014 [M + H]; found, 488.2019.
Azahelical Chromene 3Boc. Colorless solid, yield 79%; mp 178−
1
180 °C; IR (KBr) cm−1 3057, 2924, 1724, 1457, 1313; H NMR
(CDCl3, 500 MHz) δ 1.82 (s, 9H), 5.78 (d, J = 10.3 Hz, 1H), 6.31 (t, J
= 8.0 Hz, 1H), 6.70 (d, J = 10.1 Hz, 1H), 7.24−7.29 (m, 7H), 7.44−
7.48 (m, 3H), 7.54 (t, J = 7.2 Hz, 2H), 7.62−7.64 (m, 3H), 7.87 (d, J =
2.3 Hz, 1H), 7.89 (d, J = 2.6 Hz, 1H), 8.29 (d, J = 8.3 Hz, 1H), 8.66
(d, J = 8.6 Hz, 1H); 13C NMR (CDCl3, 125 MHz) δ 28.4 (×3), 82.1,
84.2, 115.2, 116.5, 117.2, 117.4, 122.1, 122.6, 122.8, 123.3, 123.4,
124.6, 124.8, 125.6, 125.9, 126.8 (×2), 127.1, 127.3 (×2), 127.4,
127.61, 127.65, 128.1 (×2), 128.4 (×2), 129.3, 129.7, 130.8, 137.4,
137.7, 144.4, 146.0, 151.1, 151.3; ESI-MS+ m/z calcd for C40H32NO3,
574.2382 [M + H]; found, 574.2381.
Azahelical Chromene 4Me. Colorless solid, yield 78%; mp 104−
106 °C; IR (KBr) cm−1 3054, 2920, 2850, 1729, 1589, 1435, 1263; 1H
NMR (CDCl3, 500 MHz) δ 3.93 (s, 3H), 5.84 (d, J = 9.5 Hz, 1H),
6.65 (d, J = 9.8 Hz, 1H), 7.28−7.32 (m, 4H), 7.34−7.41 (m, 5H),
7.49−7.52 (m, 2H), 7.56 (t, J = 7.1 Hz, 1H), 7.60 (d, J = 8.6 Hz, 1H),
7.72 (d, J = 8.6 Hz, 1H), 7.82−7.85 (m, 4H), 7.91 (d, J = 7.7 Hz, 1H),
8.57 (d, J = 8.3 Hz, 1H); 13C NMR (CDCl3, 125 MHz) δ 29.7, 81.3,
109.4, 109.8, 115.0, 115.1, 116.1, 121.2, 123.8, 123.9, 124.3, 126.2,
126.3, 126.8 (×2), 126.9, 127.1, 127.14, 127.17, 127.19, 127.5, 127.6
(×2), 127.9, 128.1 (×2), 128.7, 129.8, 132.7, 136.4, 141.5, 144.4,
146.0, 146.8; ESI-MS+ m/z calcd for C36H26NO, 488.2014 [M + H];
found, 488.2011.
Carbohelical Chromene 1. Off-white solid, yield 15%; mp 174−
1
177 °C; IR (film) cm−1 3052, 2925, 2835, 1447; H NMR (CDCl3,
500 MHz) δ 6.06 (d, J = 9.5 Hz, 1H), 6.18 (d, J = 7.4 Hz, 2H), 6.43
(d, J = 7.5 Hz, 2H), 6.68−6.71 (m, 2H), 6.74 (d, J = 9.4 Hz, 1H), 6.81
(t, J = 7.4 Hz, 1H), 6.96−7.00 (m, 2H), 7.08 (t, J = 7.4 Hz, 1H), 7.21−
7.30 (m, 2H), 7.43 (t, J = 7.4 Hz, 1H), 7.49 (d, J = 8.0 Hz, 1H), 7.67
Azahelical Chromene 4Boc. Pale yellow, yield 69%; mp 88−90
1
°C; IR (KBr) cm−1 3053, 2969, 2925, 1724, 1664, 1447, 1315; H
NMR (CDCl3, 500 MHz) δ 1.78 (m, 9H), 5.79 (d, J = 9.8 Hz, 1H),
J
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6.40 (d, J = 9.8 Hz, 1H), 7.13−7.16 (m, 1H), 7.22−7.30 (m, 4H),
7.34−7.39 (m, 3H), 7.49−7.54 (m, 3H), 7.71 (d, J = 7.7 Hz, 2H), 7.75
(d, J = 8.6 Hz, 1H), 7.79 (d, J = 8.6 Hz, 1H), 7.85 (d, J = 8.6 Hz, 1H),
7.89 (d, J = 7.7 Hz, 1H), 8.31 (d, J = 2.2 Hz, 1H), 8.32 (d, J = 3.4 Hz,
1H), 8.56 (d, J = 8.6 Hz, 1H); 13C NMR (CDCl3, 125 MHz) δ 28.4
(×3), 81.4, 84.3, 115.4, 115.5, 115.8, 117.2, 119.2, 124.3, 124.5, 124.9,
125.0, 125.7, 126.3, 126.4, 126.5, 126.8 (×2), 127.1, 127.3 (×2), 127.4
(×2), 127.6, 127.8, 127.9, 128.1 (×3), 129.3, 130.0, 132.4, 133.6,
139.7, 144.1, 145.7, 148.9, 150.7; ESI-MS+ m/z calcd for C40H32NO3,
574.2382 [M + H]; found, 574.2381.
o-Quinonoid Intermediate 4BocQ. Dark-red crystalline solid. IR
1
(neat) cm−1 3053, 2923, 2853, 1646, 1536; H NMR (CDCl3, 500
MHz) δ 1.78 (s, 9H), 5.78 (d, J = 9.8 Hz, 1H), 6.40 (d, J = 9.8 Hz,
1H), 7.15 (t, J = 7.0 Hz, 1H), 7.21−7.31 (m, 5H), 7.33−7.39 (m, 3H),
7.46−7.58 (m, 3H), 7.71 (d, J = 7.4 Hz, 2H), 7.74 (d, J = 8.6 Hz, 1H),
7.78 (d, J = 8.6 Hz, 1H), 7.84 (d, J = 8.9 Hz, 1H), 7.88 (d, J = 7.9 Hz,
1H), 8.31 (d, J = 9.2 Hz, 1H), 8.56 (d, J = 8.9 Hz, 1H); 13C NMR
(CDCl3, 125 MHz) δ 28.4, 84.3, 115.4, 115.5, 115.8, 117.2, 119.2,
121.5, 124.3, 124.5, 124.9, 125.0, 125.7, 126.3, 126.4, 126.5, 126.8,
127.4, 127.6, 127.8, 127.9, 128.0, 128.4, 129.2, 130.0, 130.2, 132.4,
133.6, 139.7, 144.1, 145.6, 148.9, 150.7, 185.8; ESI-MS+ m/z calcd for
C40H32NO3, 574.2382 [M + H]; found, 574.2387.
X-ray Structure Determinations. The X-ray diffraction intensity
data collection for the crystals of 1, 2, 3Me, and 4Me and the o-
quinonoid intermediates, i.e., 2Q, 3BocQ, and 4MeQ, was carried out
at 100 K on a Bruker Nonius SMART APEX CCD detector system
with Mo-sealed Siemens ceramic diffraction tube (λ = 0.7107 Å) and a
highly oriented graphite monochromator operating at 50 kV and 30
mA. Subsequently, the data were collected in a hemisphere mode and
processed with Bruker SAINTPLUS. Empirical absorption correction
was made using Bruker SADABS. The structure was solved in each
case by direct methods using SHELXL package and refined by full
matrix least-squares method based on F2 using SHELX-97 program.
The hydrogens were largely located from difference Fourier maps, and
those that could not be identified were fixed geometrically. Hydrogens
were treated as riding on their nonhydrogens and refined isotropically,
while all nonhydrogens were subjected to anisotropic refinement. The
X-ray crystallographic coordinates for all the structures have been
deposited at the Cambridge Crystallographic Data Centre (CCDC).
The deposition numbers are CCDC 901784 (for 1), CCDC 901785
(for 2), CCDC 901786 (for 2Q), CCDC 901787 for (3BocQ),
CCDC 901788 for (3Me), CCDC 901789 for (4Me), and CCDC
901790 for (4MeQ). These data can be obtained free of charge
(http://www.ccdc.cam.ac.uk/data_request/cif). Details of crystal data
and refinement are given in the Supporting Information.
Representative Procedure for Preparative Scale Photolysis
of Chromenes: Isolation of o-Quinonoid Intermediates. A
solution of carbohelical chromene 2, a representative case, in freshly
distilled DCM (∼10−4−10−5 M) was purged with nitrogen gas for 20
min. The resulting solution was irradiated in a Luzchem photoreactor
fitted with 350 nm lamps for ∼30 min. The progress of the reaction
was monitored by TLC analysis, which was characterized by the
appearance of dark-red color. The photolysate was stripped off the
solvent under reduced pressure at room temperature, and the resulting
solid was subjected to silica-gel (neutral) column chromatography to
afford pure 2Q as a dark-red solid. IR (KBr) cm−1 3053, 2923, 2853,
1
1646, 1536; H NMR (CD3CN, 500 MHz) δ 5.19 (d, J = 11.7 Hz,
1H), 6.00 (d, J = 7.7 Hz, 2H), 6.21 (d, J = 11.7 Hz, 1H), 6.39 (d, J =
9.7 Hz, 1H), 6.51 (d, J = 7.7 Hz, 2H), 7.10−7.18 (m, 4H), 7.23 (t, J =
7.4 Hz, 1H), 7.27−7.30 (m, 1H), 7.37 (d, J = 8.5 Hz, 1H), 7.41−7.45
(m, 1H), 7.60−7.64 (m, 2H), 7.69 (d, J = 8.3 Hz, 1H), 7.73 (d, J = 7.7
Hz, 1H), 7.79 (d, J = 9.4 Hz, 1H),7.87−7.89 (m, 2H), 7.99 (d, J = 7.7
Hz, 1H), 8.14 (d, J = 8.3 Hz, 1H); 13C NMR (C6D6, 125 MHz) δ
124.0, 125.7, 125.8, 126.7, 127.0, 127.1, 127.2, 128.2, 128.4, 128.7,
129.2, 130.0, 131.0, 131.9, 132.7, 133.8, 134.1, 137.6, 138.0, 138.1,
141.6, 143.0. 152.0, 188.6; ESI-MS+ m/z calcd for C37H25O,
485.1905[M + H]; found, 485.1903.
o-Quinonoid Intermediate 3MeQ. Dark-red crystalline solid; IR
(film) cm−1 2920, 2850, 1729, 1460, 1370; 1H NMR (C6D6, 500
MHz) δ 2.97 (s, 3H), 6.07 (d, J = 12.2 Hz, 1H), 6.37 (dd, J1 = 8.4 Hz,
J2 = 1.2 Hz, 2H), 6.45−6.46 (m, 2H), 6.47 (d, J = 9.4 Hz, 1H), 6.70 (t,
J = 7.9 Hz, 2H), 6.78−6.81 (m, 1H), 6.95 (dd, J1 = 7.0 Hz, J2 = 2.7 Hz,
1H), 7.00−7.12 (m, 6H), 7.27−7.32 (m, 2H), 7.49 (d, J = 12.2 Hz,
1H), 7.63 (d, J = 8.9 Hz, 1H), 7.70 (d, J = 7.9 Hz, 1H), 8.53 (dd, J1 =
6.1 Hz, J2 = 1.9 Hz, 1H); 13C NMR (CDCl3, 125 MHz) δ 29.7, 106.2,
108.9, 109.1, 111.8, 117.6, 118.1, 118.5, 119.3, 123.9, 124.1, 124.3,
124.7, 126.8, 126.9, 127.2 (×2), 127.5, 127.6 (×2), 127.8 (×2), 127.9,
128.1 (×2), 128.3, 128.9, 130.1 (×2), 130.5, 131.4, 138.2, 139.6, 144.1,
152.5, 190.9; ESI-MS+ m/z calcd for C36H26NO, 488.2014 [M + H];
found, 488.2018.
ASSOCIATED CONTENT
■
S
* Supporting Information
1
Synthesis of starting materials and their spectral data, H and
13C NMR spectra of all the compounds, details of the
determination of kinetic and thermodynamic parameters,
absorption spectra, logic gate truth table, and tables of crystal
data. This material is available free of charge via Internet at
http://pubs.acs.org.
o-Quinonoid Intermediate 3BocQ. Dark-red solid; IR (KBr)
1
cm−1 2917, 2849, 1728, 1460, 1310; H NMR (CDCl3, 500 MHz) δ
AUTHOR INFORMATION
Corresponding Author
moorthy@iitk.ac.in
■
1.70 (s, 9H), 5.88 (d, J = 12.3 Hz, 1H), 6.35 (d, J = 7.2 Hz, 1H), 6.49−
6.51 (m, 3H), 6.98 (d, J = 12.3 Hz, 1H), 7.02−7.05 (m, 2H), 7.12 (t, J
= 7.2 Hz, 1H), 7.19−7.29 (m, 4H), 7.36 (t, J = 7.5 Hz, 1H), 7.44 (t, J
= 7.2 Hz, 1H), 7.53 (d, J = 8.1 Hz, 1H), 7.64 (d, J = 9.7 Hz, 1H). 7.88
(d, J = 9.2 Hz, 1H), 7.92 (d, J = 8.1 Hz, 1H), 7.96 (d, J = 8.0 Hz, 1H),
8.26 (d, J = 8.3 Hz, 1H), 8.59 (d, J = 8.9 Hz, 1H); 13C NMR (CDCl3,
125 MHz) δ 29.7 (×3), 84.4, 115.8, 117.6, 120.3, 122.2, 123.7, 124.4,
125.1, 125.5, 125.8, 125.9, 127.2 (×2), 127.7, 127.9 ( × 3), 128.0,
128.1, 128.4, 128.6, 129.8, 130.0 (×2), 131.1, 131.7, 137.5, 137.72,
137.77, 138.6, 140.8, 140.9, 143.7, 150.6, 153.5, 190.3; ESI-MS+ m/z
calcd for C40H32NO3, 574.2382 [M + H]; found, 574.2386.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
J.N.M. is thankful to Council of Scientific and Industrial
Research (CSIR), India for funding. S.M. and A.M. are grateful
to CSIR for senior and junior research fellowships, respectively.
o-Quinonoid Intermediate 4MeQ. Dark-red crystalline solid; IR
1
(neat) cm−1 3053, 2923, 2853, 1646, 1536; H NMR (CDCl3, 500
REFERENCES
MHz) δ 4.02 (s, 3H), 5.97 (d, J = 12.6 Hz, 1H), 6.01 (d, J = 7.7 Hz,
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1H), 7.29−7.35 (m, 4H), 7.44−7.47 (m, 1H), 7.49−7.52 (m, 1H),
7.59 (d, J = 10.0 Hz, 1H), 7.63−7.67 (m, 3H), 7.85 (d, J = 8.6 Hz,
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140.2, 150.7, 189.9; ESI-MS+ m/z calcd for C36H26NO 488.2014 [M +
H]; found, 488.2018.
■
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