Stimuli-responsive cyclopenta[ef]heptalenes: Synthesis and optical properties

Article abstract of DOI:10.1002/ejoc.201500059

We report the one-pot synthesis, 1D and 2D NMR characterization, and UV/Vis study of a series of cyclopenta[ef]heptalenes 4a-c that exhibit strong stimuli-responsive behavior, with a tunable energy gap as a result of perturbation of HOMO, LUMO, and LUMO+1 energies upon doping/dedoping with TFA/Et₃N. The approach employed allows for the extension of conjugation at C-4 of the cyclopenta[ef]heptalene skeleton from X = H (4a) to X = CN (4b) and X = 2-thiophenyl (4c), resulting in longer absorption maxima and smaller optical energy gaps of the cyclopenta[ef]heptalenium cations 4a-c⁺. Additionally, in the presence of a UV-activated (< 300 nm) photoacid generator (PAG), protonation of 4c can be indirectly achieved by intermolecular photoinduced electron transfer (PeT) from the excited state of 4c to the PAG, upon which the latter undergoes photodecomposition resulting in the generation of acid. This phenomenon facilitates the use of cyclopenta[ef]heptalenes 4a-c as visible sensitizers of commercial PAGs.

Full text of DOI:10.1002/ejoc.201500059

Job/Unit: O50059
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Date: 17-02-15 18:03:00
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FULL PAPER
DOI: 10.1002/ejoc.201500059
Stimuli-Responsive Cyclopenta[ef]heptalenes: Synthesis and Optical Properties
[
a]
[a]
[a]
Ebrahim H. Ghazvini Zadeh, Adam W. Woodward, David Richardson,
[b]
[c,d]
Mykhailo. V. Bondar, and Kevin D. Belfield*
Keywords: Cyclopenta[ef]heptalene / Stimuli response / Azulene / Electron transfer / Photoacid generator / Sensitizers /
Photochromism / Natural products / Dyes/pigments
We report the one-pot synthesis, 1D and 2D NMR characteri- smaller optical energy gaps of the cyclopenta[ef]heptalenium
+
zation, and UV/Vis study of a series of cyclopenta[ef]heptal-
enes 4a–c that exhibit strong stimuli-responsive behavior,
with a tunable energy gap as a result of perturbation of
cations 4a–c . Additionally, in the presence of a UV-activated
(Ͻ 300 nm) photoacid generator (PAG), protonation of 4c can
be indirectly achieved by intermolecular photoinduced elec-
HOMO, LUMO, and LUMO+1 energies upon doping/dedop- tron transfer (PeT) from the excited state of 4c to the PAG,
ing with TFA/Et
3
N. The approach employed allows for the
upon which the latter undergoes photodecomposition re-
sulting in the generation of acid. This phenomenon facilitates
the use of cyclopenta[ef]heptalenes 4a–c as visible sensitizers
of commercial PAGs.
extension of conjugation at C-4 of the cyclopenta[ef]hepta-
lene skeleton from X = H (4a) to X = CN (4b) and X = 2-
thiophenyl (4c), resulting in longer absorption maxima and
Introduction
owing to its fused electron-rich cyclopentadiene and elec-
[
9,10]
tron-poor heptatriene skeleton (Figure 1).
Electronic
There is extensive growth in the research and develop-
ment of organic materials whose optical, electronic, and
conductive properties can be modulated upon application
of external stimuli. The design of such chromophores has
been based on various π-conjugated building blocks that
are functionalized with electron-donor or -acceptor moie-
properties of the azulene core result in the domination of
fluorescence from the S excited state to the S state, in
2
[4]
0
violation of Kasha’s rule, and very low emission intensity
from the S to S state.
1
0
[
1]
ties, leading to extended push-pull chromophores. Ex-
panded π-conjugated organic materials are often known for
poor physical and chemical stability, limiting their scope of
[
1,2]
application.
To address these drawbacks, efforts have
been reported to exploit the azulene framework as a modu-
lar building block for the synthesis of chromophores having
stimuli-responsive behavior.[
Figure 1. Resonance structures of guaiazulene with numbering
scheme and permanent dipole moment.
3]
Similar to the various azure-blue derivatives of the bicy-
clic sesquiterpene azulene, guaiazulene exhibits unique elec-
tronic and optical properties that allow for its use in charge-
Treatment of guaiazulene with strong acids leads to the
protonation of the cyclopentadiene ring at the unsubsti-
tuted 3-position and yields the tropylium cation, which is
accompanied by a change in the radiative decay pathway
from the S ǞS to the S ǞS transition in the protonated
[
4,5]
[6,7]
transport,
nonlinear-optics,
and sensor applica-
[
8]
tions. Unlike other small aromatic hydrocarbons, guaia-
zulene exhibits a relatively large permanent dipole moment,
2
0
1
0
[
11]
species.
Various substitution patterns on the azulene core have
been tailored in order to alter the azulene structure and
[
[
[
a] Department of Chemistry, University of Central Florida,
Orlando, FL 32816, USA
b] Institute of Physics NASU,
understand the effect of acid doping on the optical and elec-
Prospect Nauki 46, Kiev 28, 03028, Ukraine
c] College of Science and Liberal Arts, New Jersey Institute of
Technology, University Heights,
Newark, NJ 07102, USA
E-mail: belfield@njit.edu
http://csla.njit.edu/people/belfield.php
d] School of Chemistry and Chemical Engineering, Shaanxi
Normal University,
[12–15]
tronic properties of azulene-containing systems.
The
selective synthesis of azulene derivatives having a single iso-
meric arrangement of functional groups at the 4- and 7-
[12]
positions of the seven-membered ring was reported. Such
[
a connectivity pattern allowed for the incorporation of two
thiophenyl groups at these positions by Stille coupling. The
corresponding azulenium cation showed a significantly
smaller energy gap with a concomitant redshift of the ab-
Xi’an 710062, PRC
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201500059.
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K. D. Belfield et al.
FULL PAPER
sorption maximum and a relatively strong S ǞS fluores- route was reexamined in order to afford 4a–c in a one-pot
1
0
cence. Recently, a similar reversible color change and a synthesis. Accordingly, condensation and ring formation
strong redshift of the absorption maximum along with a were achieved when excess acetonitriles 2a–c and tBuOK
smaller optical band gap was reported that can be tuned by were used, leading to yields higher than those obtained by
the nature of the donor connected to the seven-membered the step-wise approach (see Experimental Section).
ring (C4–C7 substitution).[13–15]
There have been limited reports on the extension of the
azulene framework by annulation. As early as 1959, Hafner
et al. used the immonium salts of 4,6,8-trimethylazulene-1-
formaldehyde and 4,6,8-trimethylazulene-1-propenal to
form by intramolecular cyclization the corresponding cy-
clopenta[cd]azulene and cyclopenta[ef]heptalene, respec-
[
16]
tively. Recently, Wu and co-workers demonstrated a rela-
tively lengthy method for the synthesis of two function-
alized cyclopenta[ef]heptalenes,[17] and none of the prepared
derivatives were fully conjugated throughout the skeleton.
In addition, the doping of these tricyclic systems with acids
has not yet been reported. We envisioned that a stimuli-
responsive cyclopenta[ef]heptalene framework can be
achieved by adopting a synthetic approach similar to the
one reported by Wu.[ As depicted in Scheme 1, the inter-
mediate aldehyde is attacked by the enolate, leading to 7,8-
dihydrocyclopenta[ef]heptalene. In this work, we employed
the acidic protons of the Me group at C-4 of prepared guai-
azulene-based acrylonitriles for the annulation of the fused
16]
Scheme 2. Syntheses of 4a–c.
We predicted that protonation with trifluoroacetic acid
seven-membered ring. This concomitantly introduces a ba- (TFA) would occur on the amine, as observed with various
sic amine on C-5 that is thought to be feasibly protonated other aromatic compounds having exocyclic amines. How-
1
13
by acids to generate a halochromic system. We concluded ever, a thorough NMR study, including COSY, H- C
1
15
that, although protonation takes place at C-6 rather than HSQC, H- N HSQC, HMBC, and INADEQUATE ex-
at the amine, the system exhibits strong reversible stimuli- periments, shows that protonation occurs at C-6, as evi-
responsive behavior. Furthermore, when compared to pre- denced by the presence of a CH signal in the HSQC spec-
2
vious literature procedures, the approach employed in this tra of the protonated forms (Figure 2, inset). Further analy-
work is rather versatile in that it allows for various alkyl or sis of spectra corresponding to the neutral and protonated
1
(
hetero)aryl groups to be installed on the C-4 position of forms of 4 shows that all aromatic H resonances appear at
the cyclopenta[ef]heptalene skeleton, hence yielding an ar- lower frequency than expected for aromatic systems, but
ray of cyclopenta[ef]heptalenes that may enable the study shift significantly to higher frequency when protonated
of photophysical structure-property trends.
(Figure 2). This suggests that the NH group does not be-
2
have as an aromatic amine, but rather as an enamine. This
phenomenon can be interpreted by comparing the relative
hardness of cyclopenta[ef]heptalene (0.285), azulene
(0.439), and benzene (1.000). Coined by Parr and Zhou,
relative hardness is an index used to identify aromatic, non-
aromatic, and antiaromatic character of cyclic conjugated
molecules, and incorporates high stability, low reactivity,
and sustained induced ring current as defining measures of
[
18]
Scheme 1. Ring formation through enol attack on the aldehyde aromaticity.
Accordingly, the values of relative hardness
group vs. methylene anion attack on the cyano group.
suggest that 4 exhibits low aromatic character, which is con-
sistent with the unusual low frequencies of aromatic H res-
onances. Additionally, the higher frequencies of the H res-
onances of 4a–c are comparable to those seen in azulene
1
1
+
Results and Discussion
derivatives, suggesting that a larger sustained induced ring
The syntheses of cyclopenta[ef]heptalenes 4a–c are current exists, and hence a larger aromatic character for the
shown in Scheme 2. Initially, guaiazulene-3-carblaldehyde protonated species. The resonances of cyclopenta[ef]heptal-
(1) was condensed with acetonitriles 2a–c in the presence of enes are assigned by analyzing the 1D and 2D NMR spec-
morpholine as a base to afford the corresponding guai- tra, which became important to resolve any ambiguities, es-
1
azulene-based acrylonitriles 3a–c. Treatment of 3a–c with pecially at C-2a, C-2a , and C-10a in the cyclopenta[ef]hep-
2
equiv. of tBuOK led to the formation of the fused seven- talene skeleton (see Scheme 2 for numbering). The assign-
membered ring compounds 4a–c. The two-step synthetic ment was validated by directly observing the 13C- C con-
13
2
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Stimuli-Responsive Cyclopenta[ef]heptalenes
1
) and 4b+ (top, CDCl
1
13
+
Figure 2. H NMR spectra for 4b (bottom, CDCl
3
3
+ 1% TFA) with aliphatic H- C HSQC spectrum for 4b
(inset).
nectivity with an INADEQUATE experiment on 4b (see
Supporting Information). In the case of 4a, protonation
could potentially occur at either C-4 or C-6, which are at
the β-position of the amino group. However, HMBC corre-
lation between the C resonance of C-7 and the CH2 1
1
3
H
resonance indicate that protonation with TFA occurs at C-
6.
Examination of the UV/Vis spectra of 4a–c (Figure 3)
in their neutral states (DCM + 1% Et N) shows that all
3
derivatives exhibit an absorption maximum at ca. 400 nm
that corresponds to the S –S transition, consistent with
0
2
other reported azulenes (Figure 3a, Table 1).[ It is no-
table, however, that there is little to no indication of the
long-wavelength absorption that corresponds to the S ǞS
18]
0
1
transition seen in azulene derivatives.[ Upon protonation
of 4a–c, there is a distinct bathochromic shift relative to
the original π–π* transitions (Figure 3b). The absorption
maxima of the new broad peaks are correlated with the ex-
13]
+
tent of conjugation of the protonated heptalenes 4a–c ,
where λmax = 574, 587, and 609 nm for 4a–c , respectively.
+
Table 1. Spectral data for compounds 4a–c in DCM and DCM/
TFA (9:1, v/v).
4
Neutral
λ
Protonated
λ
abs
max
abs
onset
[a]
abs
max
abs
onset
λ
E
g
λ
E
g
[nm]
[nm]
[eV]
[nm]
[nm]
[eV]
a
b
c
390
406
400
498
510
521
2.49
2.43
2.38
574
586
609
647
692
752
1.92
1.79
1.65
Figure 3. (a) Absorption spectra of 4a–c in DCM and 1% Et
(b) Absorption spectra of 4a–c+ in DCM/TFA (9:1, v/v).
3
N.
[
g
a] E = optical energy gap.
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K. D. Belfield et al.
FULL PAPER
By extrapolating the onset wavelength of the absorption quantum chemical calculations, and similar to other azu-
[
12]
spectra of cyclopenta[ef]heptalenes in their neutral 4a–c and lene derivatives previously reported.
+
protonated forms 4a–c , it is possible to estimate their cor-
Inspired by reports that discuss the intramolecular pho-
It can be toinduced electron transfer (PeT) from the S state of azu-
[19]
responding optical energy gap, E (Table 1).
g
2
seen that the energy gap decreases through the series of de- lene to the excited state of various electron acceptors,[ the
rivatives. These values are consistent with the difference be- possible intermolecular PeT of 4 to the excited state of a
tween the HOMO and LUMO energies determined through commercial PAG, diphenyliodonium hexafluorophosphate
20]
(DPIHFP), was investigated. Such a PAG is photoactivated
at wavelengths ranging from vacuum ultraviolet (VUV,
[
22]
Ͻ200 nm) to UV.
This presents a serious drawback, es-
pecially in certain applications as it limits their use to a
spectral region that may overlap with functional groups that
undergo, e.g., free radical photoinitiated polymerization.[
As a proof of concept, sensitization of DPIHFP with 4 was
investigated when 4c was initially excited in the range of
the S –S transition (365–410 nm). Photoexcitation of 4c/
21]
0
2
DPIHFP was recorded at 10 s intervals (Figure 4a). Acid
generation was evident as the band at 580 nm, commonly
+
observed in the UV/Vis profile of 4c , gradually emerges
with a concomitant decrease in intensity at 400 nm. An
isosbestic point observed at 465 nm is indicative of the pres-
ence of two inter-converting optically different phases in the
solution.
Interestingly, excitation of a 4c/DPIHEP solution at
532 nm is similar in manner to that observed in Figure 4a,
yet provides better controlled sensitization of DPIHFP as a
result of lower absorptivity at longer wavelength (Fig-
ure 4b). On the other hand, the use of a 254 nm excitation
source allows for a direct excitation of DPIHFP. This was
+
observed when 4c was obtained by phototitration of an
equimolar mixture of 4c/DPIHFP in DCM by using a
2
54 nm hand-held UV lamp (average irradiance ca.
–2
12.5 mWcm ) at 20 s intervals. Complete protonation was
achieved in 80 s by using direct excitation at 254 nm, while
requiring ca. 3 min when excited at 405 nm, and ca. 8 min
for 532 nm excitation. This suggests that 4a–c can be used
as sensitizers for commercial UV-activated PAGs.
Conclusions
We report a new approach towards the efficient synthesis
of stimuli-responsive cyclopenta[ef]heptalenes 4 from natu-
rally occurring guaiazulene. Upon protonation with TFA,
perturbation of the electronic states resulted in a switch
from the dominant S ǞS transition in the neutral state
0
2
to the S ǞS transition as seen in other reported azulene
0
1
+
derivatives, where E of 4a–c is finely tunable by substitu-
g
ents at C-4. In addition, UV/Vis spectroscopy demonstrated
that 4 can act as a sensitizer of DPIHFP. This suggests that
such cyclopenta[ef]heptalenes may be employed as visible
sensitizers in a variety of applications of PAGs.
Figure 4. Absorption spectra recorded at (a) 10 s intervals of
–2
4
c/DPIHFP (3.7ϫ10–4 m, 12.5 mWcm , 405 nm), (b) at 40 s inter- Experimental Section
–3
–2
vals of 4c/DPIHFP (1.25ϫ10 m, 12.5 mWcm , 532 nm) and
c) at 20 s intervals of 4c/DPIHFP (1.1ϫ10 m, 12.5 mWcm , 3-Formylguaiazulene (1): Phosphorus oxychloride (5.00 mL,
–3
–2
(
2
54 nm).
53.0 mmol) was added to a solution of guaiazulene (5.00 g,
4
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Stimuli-Responsive Cyclopenta[ef]heptalenes
5.0 mmol) in DMF at 0 °C under an inert gas. After stirring in an
ice bath for 1.5 h, the solution was carefully neutralized with concd.
aqueous KOH solution. The purple solution was extracted with
2
of 1 (1.00 g, 4.4 mmol), 2a (1.5 mL, 8.9 mmol) and morpholine
(0.78 mL, 8.9 mmol) in dry THF (25 mL) and the mixture heated
to reflux for 3 h. The formation of 4a was monitored by TLC;
interestingly, exposure of the developed TLC to TFA vapor resulted
in a rapid change in the color of the spot corresponding to 4a
from pale yellow to purple-blue. The solution was cooled to room
temperature and then diluted with DCM (50 mL) and washed with
CH
Na
2
Cl
SO
2
(3ϫ 50 mL), and the organic fractions were dried with
and concentrated under reduced pressure. The resulting
2
4
crude oil was purified by column chromatography (hexanes/EtOAc,
3
7
=
(
(
:1) to afford the title product 2 as purple solid (4.55 g, 20.1 mmol,
1
9%). H NMR (CDCl
2.1 Hz, 1 H), 8.23 (s, 1 H), 7.59 (dd, J = 10.8, 2.1 Hz, 1 H), 7.43
d, J = 10.8 Hz, 1 H), 3.16 (p, J = 7.0 Hz, 1 H), 3.15 (s, 1 H), 2.59
s, 1 H), 1.39 (d, J = 7.0 Hz, 6 H) ppm. 13C NMR (CDCl
3
, 400 MHz): δ = 10.64 (s, 1 H), 8.29 (d, J dilute HCl (2 n, 3 ϫ 50 mL). The organic layer was dried with
Na SO and then concentrated under reduced pressure. The crude
2
4
product was purified by silica column chromatography using hex-
3
,
anes/EtOAc (5:1, v/v) to afford the title product as a dark yellow
1
1
1
00 MHz): δ = 186.9, 147.5, 147.1, 144.1, 140.0, 139.7, 136.6, 136.0,
33.0, 128.0, 127.7, 38.7, 30.7, 25.1, 13.6 ppm.
oil (15 % for step-wise; 59 % for one-pot). H NMR (CDCl
3
,
400 MHz): δ = 6.74 (d, J = 10.8 Hz, 1 H), 6.64 (s, 1 H), 6.39 (d, J
2.0 Hz, 1 H), 5.70 (dd, J = 12.4, 2.0 Hz, 1 H), 5.43 (d, J =
=
2
-[(5-Isopropyl-3,8-dimethylazulen-1-yl)methylene]malononitrile
1
1
2.4 Hz, 1 H), 4.94 (dd, J = 10.8, 2.4 Hz, 1 H), 4.84 (d, J = 2.4 Hz,
H), 3.72 (br. s, 2 H), 2.24–2.17 (m, 4 H), 1.04 (d, J = 6.8 Hz, 6
(3a): tBuOK (0.56 g, 5.0 mmol, 1.25 equiv.) was added in small por-
tions to a solution of 3-formylguaiazulene (1) (0.90 g, 4.0 mmol)
and diethyl (cyanomethyl)phosphonate (0.80 mL, 5.0 mmol) in an-
hydrous THF at 0 °C. The reaction mixture was warmed to room
temperature and then heated at 50 °C for 1 h. The resulting green
solution was dried under reduced pressure and subsequently puri-
fied by column chromatography (hexanes/EtOAc, 4:1) to afford the
1
3
H) ppm. C NMR (CDCl
3
, 100 MHz): δ = 155.8, 144.2, 141.3,
1
2
2
38.7, 133.7, 133.1, 129.9, 129.3, 129.2, 123.8, 111.4, 109.9, 36.5,
+
2.8, 13.8 ppm. MS: calcd. for C18H19N [M + H] 250.1591; found
50.1579.
Synthesis of 8-Amino-4-isopropyl-2-methylcyclopenta[ef]heptalene-9-
carbonitrile (4b) from 3b: tBuOK (0.22 g, 2.0 mmol) was added to
a solution of 3b (0.55 g, 2.0 mmol) in anhydrous THF (10 mL).
The solution was heated to reflux for 12 h, and then cooled to
room temperature. The solvent was evaporated, and the resulting
crude product was dissolved with DCM (20 mL) and washed with
dilute HCl (2 n, 3 ϫ 50 mL). The organic layer was dried with
1
title product 3a as a green-blue solid (1.00 g, 3.6 mmol, 92%). H
NMR (CDCl
3
, 400 MHz, trans/cis = 2:1): δ = 8.46 (s, 0.34 H), 8.22
(d, J = 16.0 Hz, 0.63 H), 8.12 (dd, J = 14.4 Hz, 1 H), 7.94 (d, J =
1
7
1.6 Hz, 0.38 H), 7.72 (s, 0.62 H), 7.39 (dd, J = 10.8, 2.0 Hz, 1 H),
.07 (d, J = 10.8 Hz, 1 H), 5.53 (d, J = 15.6 Hz, 0.62 H), 5.12 (d,
J = 11.6 Hz, 0.36 H), 3.09–3.02 (m, 1 H), 2.93 (s, 3 H), 2.59 (s, 1
H), 2.56 (s, 2 H), 1.34 (d, J = 6.8 Hz, 6 H) ppm. 13C NMR (CDCl
,
3
Na
2 4
SO and concentrated to dryness. Column chromatography
100 MHz): δ = 151, 146.7, 146.6, 146.4, 145.0, 145.0, 144.6, 143.1,
(
hexanes/DCM, 3:1) was used to isolate the title product as a dark
141.5, 137.8, 137.0, 136.5, 136.4, 135.9, 135.3 135.0, 131.3, 131.9,
1
yellow solid (23% for stepwise vs. 71 % for one-pot). H NMR
CDCl , 400 MHz): δ = 6.74 (s, 1 H), 6.44 (d, J = 2.0 Hz, 1 H),
.88 (dd, J = 12.4, 2.0 Hz, 1 H), 5.62 (d, J = 12.4 Hz, 1 H), 4.90
s, 1 H), 4.45 (br. s, 2 H), 2.29 (quint, J = 6.8 Hz, 1 H), 2.20 (s, 3
1
28.1, 127.4, 122.6, 122.3, 121.3, 120.5, 89.9, 88.4, 38.4, 29.2, 25.0,
(
5
(
3
+
24.9, 13.5, 13.5 ppm. MS: calcd. for C18
H
19N [M + H] 250.1591;
[
23]
found 250.1564.
1
3
2
3
-[(5-Isopropyl-3,8-dimethylazulen-1-yl)methylene]malononitrile H), 1.07 (d, J = 6.8 Hz, 6 H) ppm. C NMR (CDCl , 100 MHz):
(3b): Compound 1 (0.90 g, 4.0 mmol) was added to a solution of
δ = 154.7, 154.4, 144.2, 142.6, 142.5, 138.2, 137.8, 134.2, 132.3,
malononitrile 2b (0.33 g, 5.0 mmol, 1.2 equiv.) and N-methyl-
morpholine (1 mL, 2.5 equiv.) in methanol. After heating the re-
sulting solution at 50 °C for 1 h, the volatile materials were evapo-
rated under reduced pressure. The crude solid was purified by col-
umn chromatography (hexanes/EtOAc, 4:1) to afford the title prod-
130.1, 129.9, 128.2, 127.2, 127.7, 125.1, 124.3, 114.0, 109.5, 36.2,
+
22.5, 13.5 ppm. MS: calcd. for C19
H
18
N
2
[M + H] 275.1543; found
275.1503.
Isopropyl-2-methyl-9-(thiophen-2-yl)cyclopenta[ef]heptalen-8-amine
1
(4c): Golden yellow solid (21% for stepwise vs. 92% for one-pot).
uct 3b as dark yellow solid (1.00 g, 3.6 mmol, 90 %). H NMR
1
3
H NMR (CDCl , 400 MHz): δ = 7.25 (dd, J = 5.2, 1.2 Hz, 1 H),
(
CDCl
3
, 400 MHz): δ = 8.63 (s, 1 H), 8.46 (s, 1 H), 8.28 (d, J =
.2 Hz, 1 H), 7.66 (dd, J = 10.8; 2.2 Hz, 1 H), 7.48 (d, J = 10.8 Hz,
H), 3.18 (sept, J = 6.9 Hz, 1 H), 3.10 (s, 3 H), 2.59 (s, 3 H), 1.41
, 100 MHz): δ = 151.2,
7
1
.00 (dd, J = 5.2, 3.6 Hz, 1 H), 6.95 (s, 1 H), 6.93 (dd, J = 3.6,
.2 Hz, 1 H), 6.62 (s, 1 H), 6.36 (d, J = 2.0 Hz, 1 H), 5.71 (dd, J =
2
1
d, J = 6.9 Hz, 6 H) ppm. 13C NMR (CDCl
12.4 Hz, 1 H), 5.46 (d, J = 12.4 Hz, 1 H), 4.92 (s, 1 H), 4.14 (br. s,
(
3
1
3
2
H), 2.26–2.17 (m, 4 H), 1.05 (d, J = 6.8 Hz, 6 H) ppm. C NMR
1
1
50.4, 147.9, 145.7, 141.7, 138.0, 137.6, 136.0, 135.8, 130.8, 121.3,
17.8, 116.8, 38.5, 29.6, 24.7 ppm.[
24]
(CDCl , 100 MHz): δ = 154.8, 154.2, 144.3, 142.8, 142.6, 138.4,
3
1
1
37.9, 133.9, 132.5, 130.1, 130.1, 128.3, 127.1, 126.7, 125.1, 124.3,
14.0, 109.3, 36.2, 22.4, 13.5 ppm. MS: calcd. for C22 21NS [M +
3
-(5-Isopropyl-3,8-dimethylazulen-1-yl)-2-(thiophen-2-yl)acrylo-
H
nitrile (3c): The synthetic procedure for 3c is similar to that de-
scribed for 3a. Purification of the crude reaction mixture by column
+
H] 332.1468; found 332.1440.
chromatography (hexanes/EtOAc, 5:1) afforded the title product as
Spectroscopic Measurements: Absorption spectra were recorded in
spectral-grade solvents and 10 mm quartz cuvettes with an Agilent
8453 spectrophotometer. Phototitrations of 4c were performed in
the presence of DPIHFP, and excited by using a LOCTITE 97034
1
dark yellow solid (1.12 g, 3.4 mmol, 85 %). H NMR (CDCl
3
,
400 MHz): δ = 10.42 (s, 1 H), 8.22 (s, 1 H), 8.13 (s, 1 H), 7.75 (dd,
J = 16.0, 2.0 Hz, 1 H), 7.61–7.60 (m, 1 H), 7.21 (dd, J = 16.0,
2
3
.0 Hz, 1 H), 6.98–6.96 (m, 1 H), 6.90 (dd, J = 4.4, 1.2 Hz, 1 H), UV lamp, a 532 nm diode laser, and a 254 nm hand-held UV lamp.
.14 (sept, J = 7.2 Hz, 1 H), 2.56 (s, 3 H), 1.38 (d, J = 7.2 Hz, 6
13
H) ppm. C NMR (CDCl
3
, 100 MHz): δ = 185.5, 147.4, 144.1,
44.0, 143.5, 140.3, 137.6, 136.0, 135.3, 132.6, 130.8, 128.0, 127.9,
27.2, 126.7, 126.5, 113.5, 38.3, 24.4, 12.9 ppm. MS: calcd. for
21NS [M + H]+ 332.1468; found 332.1440.
1
1
Acknowledgments
22
C H
We wish to acknowledge support from the National Science Foun-
dation (CBET-1517273 and CHE-0832622) and the U.S. National
Academy of Sciences (PGA-P210877).
One-Pot Synthesis of 9-Isopropyl-1-methylcyclopenta[ef]heptalen-5-
amine (4a): tBuOK (2.24 g, 20.0 mmol) was added to a hot solution
Eur. J. Org. Chem. 0000, 0–0
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/KAP1
Date: 17-02-15 18:03:00
Pages: 7
K. D. Belfield et al.
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6
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Eur. J. Org. Chem. 0000, 0–0

Job/Unit: O50059
/KAP1
Date: 17-02-15 18:03:00
Pages: 7
Stimuli-Responsive Cyclopenta[ef]heptalenes
Chromophores
Cyclopenta[ef]heptalenes 4a–c, prepared by
annulation of 3-formylguaiazulene (1), re-
vealed a stimuli-responsive behavior upon
treatment with strong acids. 1D and 2D
NMR spectroscopy indicated that pro-
tonation occurred on C-6, while photoacid
titrations demonstrated that the reported
tricyclic systems can serve as efficient vis-
ible sensitizers of UV-activated photoacid
generators (PAGs).
E. H. Ghazvini Zadeh, A. W. Woodward,
D. Richardson, M. V. Bondar,
K. D. Belfield* .................................. 1–7
Stimuli-Responsive Cyclopenta[ef]heptal-
enes: Synthesis and Optical Properties
Keywords: Cyclopenta[ef]heptalene / Stim-
uli response / Azulene / Electron transfer /
Photoacid generator / Sensitizers / Pho-
tochromism / Natural products / Dyes/pig-
ments
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7