Pyrene-Azobenzene Dyads and Their Photochemistry

Article abstract of DOI:10.1002/ejoc.201301301

The facile synthesis of three new folding azobenzene-pyrene systems 1-3, connected together by a serendipitously obtained and unpredicted ester linkage, is reported. Additional characterization of the photochemistry of these systems revealed variations in azobenzene photoisomerization (trans-cis and cis-trans) and quenching of pyrene fluorescence, as a result of intra-excitation energy transfer from the pyrene chromophore to an azobenzene. Through the use of aryl substituent electronic effects to tune the absorption properties of the azobenzene relative to the pyrene, we show that efficient photo-switching can be achieved when the trans-azobenzene absorbance band is well separated from that of the pyrene (compound 1), whereas overlap of the corresponding absorbance bands in the cases of 2 and 3 significantly compromises trans-cis isomerization by enhancing cis-trans interconversion. The facile synthesis and preliminary characterization of three pyrenebutyric acid azobenzene esters (compounds 1-3) revealed the capacity of the systems to fold, resulting in the modulation of azobenzene photoisomerization through excitation energy transfer from the pyrene chromophore to azobenzene.

Full text of DOI:10.1002/ejoc.201301301

FULL PAPER
DOI: 10.1002/ejoc.201301301
Pyrene–Azobenzene Dyads and Their Photochemistry
Olaf Nachtigall,[a][‡] Reiner Lomoth,[b] Christian Dahlstrand,[a] Anna Lundstedt,[a]
Adolf Gogoll,[a] Matthew J. Webb,*[a] and Helena Grennberg[a]
Keywords: Azo compounds / Pyrene / Synthesis / Photochemistry / Isomerization / Sensitization
The facile synthesis of three new folding azobenzene-pyrene
systems 1–3, connected together by a serendipitously ob-
tained and unpredicted ester linkage, is reported. Additional
characterization of the photochemistry of these systems re-
vealed variations in azobenzene photoisomerization (trans-
cis and cis-trans) and quenching of pyrene fluorescence, as
a result of intra-excitation energy transfer from the pyrene
chromophore to an azobenzene. Through the use of aryl sub-
stituent electronic effects to tune the absorption properties of
the azobenzene relative to the pyrene, we show that efficient
photo-switching can be achieved when the trans-azo-
benzene absorbance band is well separated from that of the
pyrene (compound 1), whereas overlap of the corresponding
absorbance bands in the cases of 2 and 3 significantly com-
promises trans-cis isomerization by enhancing cis-trans in-
terconversion.
functionalization of graphene[13] we chose this approach
and designed a series of pyrene-appended azobenzenes.
However, in our preliminary studies, without any involve-
ment of graphene, it became obvious to us that the photo-
isomerization of azobenzene systems can be affected by an
intramolecular interference effect from the appended pyrene
unit. Here we describe the synthesis and characterization of
three compounds with a generic structure consisting of a
pyrenebutyric acid moiety covalently bonded to a para-sub-
stituted azobenzene unit (–NH2, –OH and –H) through an
ester linkage (Figure 1) and use the results of our analyses
to help explain the variations in photoisomerization and
suggest how to avoid, as well as benefit from, the pyrene
excitation energy transfer.
Introduction
The photochromism of azobenzene units,[1] together with
their remarkable photostability, offers a chemically non-in-
vasive route to significant conformational and electronic
changes in molecular systems through the simple absorp-
tion of a photon. As a result, azobenzenes have lent them-
selves to use in photochromic surfaces,[2] polymers,[3] den-
drimers,[4] liquid crystals,[5] biomolecules,[6] supramolecular
systems[6d,7] and molecular machines,[8] as well as in indus-
trial dyes and non-linear optical devices.[9] In order to
achieve many of these applications successfully, the photo-
switching capacities of the molecules have been manipu-
lated by the choice of electron-donating/-withdrawing aryl
substituents. Three categories of azobenzene have been rec-
ognized as a result of their distinguishing spectral and
photo-physical characteristics (pseudo-stilbenes, amino-
azobenzenes and azobenzenes).[1c] More recently, new sys-
tems have made photoisomerization possible without the
need for UV light,[10] and bridged structures have been
shown to enhance switching further.[11] Often, for systems
to benefit from these properties of azobenzenes, the mo-
lecules have been covalently coupled to large aromatic
groups, such as pyrene systems, to allow anchoring of the
molecular systems to surfaces through electrostatic interac-
tion.[12] In our studies directed towards the noncovalent
Figure 1. The pyrenebutyric acid azobenzene esters synthesized.
[a] Department of Chemistry – BMC, Uppsala University,
Husargatan 3, Box 576, 75123 Uppsala, Sweden
E-mail: matthew.j.webb@kemi.uu.se
Results and Discussion
http://www.kemi.uu.se/
[b] Department of Chemistry – Ångström Laboratories, Uppsala
University,
Syntheses, Characterization and Conformation
Regementsvägen 1, 752 37 Uppsala, Sweden
[‡] Visiting Erasmus Student from Freie Universität Berlin
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201301301.
Compound 1 was prepared in three steps in 44% overall
yield. In the first steps the 4-[(4-aminophenyl)diazenyl]-
phenol was prepared by a literature procedure.[14] The origi-
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Pyrene–Azobenzene Dyads and Their Photochemistry
nal synthetic target was the amide-coupled pyrenebutyric
In order to gain further insight into the possible confor-
acid azobenzene,[12d] yet after trying DCC/HOBt (N,NЈ-di- mations favoured by these systems we performed a confor-
cyclohexylcarbodiimide/1-hydroxybenzotriazole hydrate), mational search on compound 1, at the semi-empirical
as well as basic conditions (triethylamine) with the pyrene- AM1 level. The ten conformers of lowest energy were then
butyryl chloride, no significant coupling was achieved. geometry-optimized at the M062X/6-31G(d) level of
Finally, when HBTU/DIPEA (O-benzotriazol-1-yl- theory[16] and compared with the folded conformers opti-
N,N,NЈ,NЈ-tetramethyluronium hexafluorophosphate/N,N- mized at the same level (Figure S12 in the Supporting Infor-
diisopropylethylamine) conditions were employed and the mation). We found that the energy difference between the
reaction mixture was left for three days, a single major folded and unfolded conformations of 1 was 6.5 kcalmol–1
product was identified and isolated (Figure 1). The compar- in favour of the folded system (Figure 3). We also found
able nucleophilicities of the free amine and the phenolate two trans isomers of the folded system, determined by
anion (under the basic experimental conditions) meant that whether the N=N bond was turned into or away from the
a mixture of substituted azobenzenes was expected; how- pyrene unit (Figure S13 in the Supporting Information);
ever, instead of the anticipated amide-linked product (which these differed in energy by 0.27 kcalmol–1. With regard to
we did not recover) the ester compound 1 was obtained. the NMR ROESY experiments, the calculated distances be-
The lack of amide formation was attributed to a stabilized tween protons (a) and (h) and (e) and (h) were 3.72 Å and
tautomer involving the free amine and the azo component 4.02 Å, respectively, in the folded conformation, and there-
that reduced the amine’s nucleophilicity relative to that of fore at the limit of detection. According to the gas-phase
the phenolate anion species under our conditions.
geometries these were not the closest intramolecular dis-
Compound 2 was prepared with 4,4Ј-(diazene-1,2-diyl)- tances but those that were closest to the ester “hinge” con-
diphenol,[15] under the same coupling conditions as for 1, necting the two π-systems. The closest gas-phase distance
to generate the originally intended phenolic substituted was 2.95 Å, between protons (g) and (b*). No nOe was ob-
product. Despite the potential to generate the bis-pyrenebu- served, however, probably due to a high degree of molecular
tyric acid azobenzene diester, 2 was recovered in a respect- motion.
able overall yield (60%).
The preparation of compound 3 (79%) confirmed the
applicability of this synthetic protocol to the generation of
this family of molecules whilst also providing a control sys-
tem for 1 and 2. The compounds were readily purified by
column chromatography on silica and characterized by
NMR, UV/Vis, fluorescence and IR spectroscopy, together
with mass spectrometry.
In addition to the standard NMR experiments, two-di-
mensional ROESY experiments were carried out in an at-
tempt to identify any through-space cross-peaks between
protons on the different azobenzene systems and the pyrene
unit that might indicate the solution-state conformations of
these molecules. Compounds 1 and 2 were prepared in [D6]-
DMSO as solvent whereas 3 was prepared in CDCl3. In the
end, the through-space proton correlation experiments were
able to identify partial cross-peaks between protons (a) and
(h) as well as (e) and (h) in all three compounds (Figure 2
and Figures S3, S6 and S9 in the Supporting Information).
Although the observed signals were very weak they indi-
cated a folded arrangement around the ester linker in solu-
tion.
Figure 3. The most stable isomers of compound 1 in its unfolded
(open) and folded conformations. The folded 1 is also shown from
the side for clarity. C (grey), H (white), O (red), N (blue). Geometry
optimized at the M062X/6–31G(d) level.
Absorption Spectra
The UV/Vis absorption spectra of compounds 1, 2 and
3 in methanol in their initial trans-azobenzene isomeric
forms are shown in Figure 4 (a–c) together with the spectra
of their starting materials (i.e., 1-pyrenebutyric acid and the
appropriate para-substituted azobenzene). The pyrene chro-
mophore is characterized by two absorption bands peaking
at 274 and 340 nm that feature the typical vibronic structure
Figure 2. Structure of compound 1, showing the proton labelling
scheme.
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and is unaffected by the coupled azobenzenes. The azoben- bands due to the allowed πǞπ* excitation (highlighted in
zene chromophores give rise to weaker absorption bands in Figure 4, a). The πǞπ* excitation absorbance band dis-
the visible region (ca. 450 nm), arising from the symmetry- plays a high degree of dependence on the electronic charac-
forbidden nǞπ* transition, and more intense absorption ter of the azobenzene substituents: the electron-donating
amino group of compound 1, for example, facilitates a sig-
nificant red shift of the πǞπ* absorbance band (388 nm)
relative to the less electron-donating phenolic substituent of
2 (355 nm, Figure 4, b), which is itself red-shifted relative
to the unsubstituted aryl group of 3 (Figure 4, c). The shifts
can readily be seen in the three azobenzene building blocks,
as well as in the linked compounds 1 and 2. The obscured
azobenzene πǞπ* absorption of compound 3 effectively
corresponds to the spectrum of regular azobenzene with the
transition at ca. 320 nm. As a consequence of the variation
in azobenzene absorption characteristics, the πǞπ* band
of the azobenzene moiety is only well separated from the
pyrene absorption bands in the case of compound 1,
whereas severe or complete overlap of these bands is ob-
served in the cases of 2 and 3, respectively.
Photoisomerization
Photoexcitation of azobenzenes to either excited state, S1
(πǞπ*) or S2 (nǞπ*), is known to result in both transǞcis
and cisǞtrans isomerization. The cis/trans ratio in the
photostationary state is determined by the relative quantum
yields (φ) of the two processes and the relative molar ab-
sorptivities (ε) of the two isomers at the excitation wave-
length.
(cis/trans)pss = φtransǞcis εtrans/φcisǞtrans εcis
Irradiation of the πǞπ* absorption band of the trans
isomer of unsubstituted azobenzene, for example, results in
a photostationary state consisting predominantly of the cis
isomer (ca. 80%) as identified by a significant blue shift of
the πǞπ* absorption band and a decrease in intensity rela-
tive to the trans isomer.[9] The nǞπ* absorption bands of
the two isomers largely overlap, but the cis form has a
higher molar absorptivity. Consequently, illumination at a
wavelength corresponding to the nǞπ* absorption band
yields a photostationary state dominated by the trans iso-
mer (ca. 90% for unsubstituted azobenzene).[9] Therefore,
starting from the stable trans isomer of an azobenzene and
irradiating the corresponding πǞπ* absorption band can
be expected to transform most of the system into the cis
isomer, whereas subsequent irradiation of the correspond-
ing nǞπ* absorption band should recover at least part of
the trans form.
A streamlined account of illumination experiments car-
ried out with compounds 1, 2 and 3 is given below. Table 1
lists the irradiation wavelengths, photon flux and solvents
used to probe the basic photochemistry of these systems.
Quantum yield measurements based on our UV/Vis absorp-
tion data required the determination of the molar absorp-
tivities of the corresponding isomeric forms, but our efforts
to isolate the trans and cis species by HPLC were unsuccess-
ful (Figure S18 in the Supporting Information). Attempts
Figure 4. UV/Vis spectra of (a) 1 (20.7 μm), (b) 2 (20.6 μm), and
(c) 3 (21.3 μm) and their corresponding building blocks in methanol
at 298 K. Note: the πǞπ* and the nǞπ* absorption bands of the
azobenzene component of 1 were observed at 388 and 445 nm
respectively. The absorbance bands of the corresponding pyrene
and azobenzene components were well separated, allowing the indi-
vidual contributions to be clearly observed.
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Pyrene–Azobenzene Dyads and Their Photochemistry
to verify isomeric compositions in more concentrated solu-
tions by NMR spectroscopy were also unsuccessful. In
place of quantum yields we detail the experimental condi-
tions used to produce the recorded photostationary states
for each system (Table 1 and Figure 5). Although this
analysis does not take account of the photon absorption at
each illumination wavelength and variations in molar ab-
sorptivity between the isomers, with all else being equal
(sample volume, concentration and path length), the behav-
iour observed in each case is representative of the relevant
system, under the stated conditions, and therefore offers a
relative insight into the photochemistry of the three dif-
ferent dyads. Initially, all the samples were prepared in
methanol, but no switching at all was observed in this sol-
vent. Compounds 1 and 2 were then prepared in aceto-
nitrile and 3 in chloroform (stabilized with 0.2% ethanol)
for solubility, which allowed the following observations to
be made.
Table 1. The wavelengths of light and corresponding photon flux
per unit area that allowed photoisomerization (Z to E and E to Z)
of compounds 1, 2 and 3 in various solvents at 298 K.
Solvent
Wavelength (nm)[b]
[photon flux (1020) /s–1 m–2]
trans to cis
cis to trans
1
2
3
CH3CN
CH3CN
CHCl3 0.2% EtOH 320 (4.3)
385 (8.4)
380 (8.3)
445 (5.7) and 326 (4.8)[a]
440 (5.6)
440 (5.6)
[a] Secondary irradiation wavelength. [b] 14 nm bandwidth.
Illumination of compound 1 at 385 nm (πǞπ*) (Fig-
ure 5, a) rapidly established a photostationary state domi-
nated by the cis isomer. Subsequent illumination at 445 nm
recovered approximately 40% of the trans form, relative to
the trans-cis conversion, due to the less favourable ratio of
molar absorptivities at this wavelength relative to unsubsti-
tuted azobenzene. Further recovery of the trans isomer was
observed upon illumination at 326 nm. This additional re-
covery might be explained either conventionally, in terms
of the relatively stronger absorption of the cis isomer and
the more favourable relative quantum yield in the S2 state
that is known for unsubstituted azobenzene,[9] or less con-
ventionally, in terms of an intricate pyrene sensitization ef-
fect that enhances the cis-trans process (vide infra). Irre-
spective of this, the overlaid spectra identify a clear isosbes-
tic point indicative of the presence of only two isomeric
species in solution despite the alternative irradiation proto-
col.
Figure 5. UV/Vis spectra of (a) 1 (20.7 μm) and (b) 2 (20.6 μm) in
acetonitrile and of (c) 3 (21.3 μm) in chloroform (0.2% ethanol)
after irradiation at various wavelengths to achieve trans-cis and
then cis-trans isomerization. Note: some overlapping spectra and
labelled isosbestic points (circles).
The overlapping absorbance bands of compound 2
meant that to irradiate the azobenzene component was also
to irradiate the pyrene unit. Consequently, the direct πǞπ*
excitation at 360 nm did not result in any significant forma-
tion of cis product (not shown). In order to achieve any
photoisomerization, from the trans to the cis form, it was clear isosbestic point can be seen in the series of spectra. In
necessary to excite the sample at the red edge of the πǞπ* further comparison with compound 1 it can be observed
absorption (380 nm, Figure 5, b). Substantial recovery of that, despite irradiation at similar photon flux for the trans
the trans isomer was then brought about in the usual way to cis isomerization process (Table 1), the photostationary
by illumination at 440 nm (nǞπ*). As with compound 1 a state was achieved only after a threefold increase in the irra-
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diation time. One explanation for the failure to accumulate a result of excitation energy transfer from the pyrene chro-
significant quantities of the cis isomer of compound 2 mophore to the azobenzene. The amino-substituted com-
would be interference resulting from the concomitant exci- pound 1 was able to achieve near quantitative trans-cis
tation of the pyrene chromophore.
isomerization as a result of well-separated pyrene and azo-
To offer further insight into this pyrene-interference ar- benzene absorbance bands, but the overlapping absorbance
gument, compound 3 was illuminated. In this system the bands of 2 and 3 meant that the trans-cis isomerization in
total overlap of pyrene and azobenzene absorbance bands these cases was significantly compromised by pyrene sensiti-
meant that the πǞπ* transition of the azobenzene could zation of the corresponding azobenzene components, which
not be selectively excited. As a consequence, illumination enhanced the reverse cis-trans process.
at 320 nm (πǞπ*) generated a photostationary state with
The intriguing photophysical properties of these systems
apparently only small quantities of the cis isomer (Figure 5, should be of interest to those working with azobenzene sys-
c) affirming switching inhibition in overlapping systems. cis tems, particularly when they are linked to large aromatic
to trans conversion was possible through irradiation at groups. Furthermore, the structural characteristics of these
440 nm (nǞπ*) but, unlike in the cases of compounds 1 systems might also allow further chiral resolution of the
and 2, no isosbestic point was observed, indicating the for- two azobenzene cis isomers,[21] insight into the switching
mation of side products in solution.
mechanism,[22] and potentially a new approach to preparing
In our view, the trans-isomer-biased photostationary push-pull azobenzene systems.
states of compounds 2 and 3 can be explained by excitation
energy transfer from the pyrene chromophore to the azo-
benzene system.[17] This pyrene sensitization, which might
also be responsible for the cis-trans enhancement of com-
pound 1, is corroborated by the strongly quenched pyrene
fluorescence of 1 and 2 relative to the 1-pyrenebutyric acid
starting material observed in our fluorescence spectroscopy
measurements (Figures S10 and S11 in the Supporting In-
formation).
Experimental Section
General Methods: All reagents from commercial sources were used
1
without further purification. Solution-state H NMR spectra were
recorded with Varian Unity 400 (399.98 MHz for 1H) or Varian
Unity Inova 500 (499.94 MHz for 1H) spectrometers. All 13C NMR
spectra were recorded with
a Varian Unity 400 instrument
(100.58 MHz for 13C). Unless otherwise stated, NMR spectra were
recorded in deuterated dimethyl sulfoxide ([D6]DMSO) at 298 K.
All 1H NMR assignments were derived from COSY,[23] P. E . -
COSY,[24] gHSQC,[25] gHMBC[26] and ROESY[27] spectra and were
labelled according to the systems shown. Chemical shifts (δ) were
referenced to the solvent resonances (1H at 2.50 and 13C at δ =
39 ppm). Coupling constants (J) are given in Hz. For convenience,
the following abbreviations are used: s singlet, d doublet, t triplet,
dd doublet of doublets, dt doublet of triplets, m multiplet, br.
broad, Cq quaternary carbon. High-resolution mass spectrometry
data were acquired with an Agilent Technologies LC/MSD TOF
mass spectrometer (Agilent Technologies, Santa Clara, CA, USA).
Samples were directly infused into the ESI ion source with the aid
of a syringe pump (Harvard Apparatus, Holliston, MA, USA) at
a flow rate of 10 μLmin–1. MS parameters were set to: gas tempera-
ture 300 °C, drying gas (N2) 5 Lmin–1, nebulizer gas (N2, Neb)
15 psig, capillary potential (Cap) 4000, fragmenting potential
(Frag) 215 V, skimmer potential 60 V, OCT RFV 250 V. UV/Vis
spectra were recorded with Varian CARY 3 or 5000 UV/Vis diode
array spectrophotometers with use of a 1 cm path-length quartz
cell versus a pure solvent reference. The data were reported in units
of nm for the wavelength λ and m–1 cm–1 for the molar absorptivity
ε. Room temperature fluorescence emission spectra were obtained
with a Horiba/Jobin Yvon Fluorolog fluorescence spectrometer.
The excitation wavelength was 310 nm and emission spectra were
recorded from 320 to 600 nm with 1 nm steps. For the photoiso-
merisation experiments approx. 20 μm solutions (2 mL) were stirred
and irradiated with specific wavelengths of light with a Horiba/
Jobin Yvon Fluorolog fluorescence spectrometer and a slit width
corresponding to a spectral bandwidth of 14 nm. IR spectra were
recorded with a Perkin–Elmer FT-IR Universal TR sampling Ac-
cessory. HPLC-grade solvents were used as received for all UV/Vis
and fluorescence spectroscopy experiments. Deuterated solvents of
at least 99.9% purity were used for NMR experiments.
The well-known preference of the triplet state of azo-
benzene for cis-trans isomerization (φcisǞtrans/φtransǞcis = 65)
allows triplet energy donors with ET Ͼ 45 kcalmol–1, such
as pyrene (ET = 48 kcalmol–1),[18] to be able to sensitize the
transformation with unity quantum yield.[19] However, the
triplet quantum yield of ca. 0.35 for pyrene is drastically
lowered (20ϫ) in the dyads by the efficient quenching of
the excited singlet state, so triplet sensitization might not be
wholly sufficient to explain the dominance of the cis-trans
process upon pyrene excitation. Analogous effects on the
photoisomerization reported for azobenzene-dansyl dyads
were interpreted in terms of differences in excited state ge-
ometry between direct and sensitized excitation that in the
latter case can only deactivate to the trans isomer.[20]
The inability to achieve a cis-dominated photostationary
state effectively either with compound 2 or with compound
3 suggests that when pyrene and azobenzene πǞπ* ab-
sorbance bands overlap (2 and 3) trans-cis isomerization is
outcompeted by the more effective pyrene-sensitized cis-
trans process. In order to achieve significant net trans-cis
transformation, as in 1, the corresponding absorbance
bands need to be well separated. In contrast, if the intention
is to modulate the cis photostationary state, then overlap-
ping pyrene–azobenzene absorbance bands would be desir-
able.
Conclusions
In conclusion, the pyrene moiety, originally under con-
sideration only as an anchor group, appears to play a sig-
General Experimental Procedure: The appropriate azobenzene
nificant role in effecting azobenzene photoisomerization as (1.0 mmol), 1-pyrenebutyric acid (346 mg, 1.2 mmol) and HBTU
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Pyrene–Azobenzene Dyads and Their Photochemistry
(417 mg, 1.1 mmol) were placed in a two-necked round-bottomed
flask (50 mL). The flask was sealed and subjected to three vacuum/
argon cycles. Anhydrous DMF (12 mL) and DIPEA (226 μL,
1.3 mmol) were then added with maintenance of the argon atmo-
sphere. Once addition was complete the reaction mixture was
stirred under argon at room temperature for 72 h. The progress of
the reaction was followed by TLC (dichloromethane/cyclohexane/
methanol 70:30:1). Once the reaction was complete the solvent was
removed in vacuo, after which the crude mixture was dissolved in
dichloromethane, washed with HCl (0.1 m, 2ϫ 100 mL) and water
(3ϫ 100 mL) and dried with magnesium sulfate. The solvent was
removed in vacuo.
122.3, 34.0, 32.8, 26.8 ppm. IR: ν = 3039 (w), (CHarom), 2910 (w)
˜
(C–Halkyl), 1758 (s) (C=O) cm–1. UV/Vis (MeOH): λmax (ε,
m
–1 cm–1) = 341 (58000), 324 (48000), 274 (52000), 263 (29500) nm.
HRMS (ESI, iPrOH): calcd. 491.1735 [M + Na]+; found 491.1740.
Computational Methods: The conformational search was performed
at the AM1 level of theory by use of the Spartan program package.
The resulting geometries were then geometry optimized with the
aid of density functional theory (DFT) and the M062X level with
use of the 6–31G(d) Pople-type basis set.[16] The electronic transi-
tions were calculated with TD-M062X and the 6–311+G(2d, p)
basis set because this basis set has been shown to yield results
equivalent to those obtained with the much larger 6-311++G(3df,
3pd) basis set.[29] All DFT calculations were performed with
Gaussian 09.[30]
4-[(4-Aminophenyl)diazenyl]phenyl 4-(Pyren-1-yl)butanoate (1): This
compound was prepared from 4-[(4-aminophenyl)diazenyl]-
phenol,[14] purified by column chromatography over silica (DCM/
pentane/MeOH, 70:30:1) and recovered as a yellow solid (44%),
Supporting Information (see footnote on the first page of this arti-
cle): Spectroscopy data for the systems are provided together with
further results from the computational analysis and HPLC study.
1
m.p. 134–138 °C. H NMR ([D6]DMSO, 400 MHz): δ = 8.48 (d, J
= 12 Hz, 2 H), 8.30 (m, 4 H), 8.19 (m, 2 H), 8.10 (dd, J = 7.6 Hz,
1 H), 8.05 (d, J = 7.6 Hz, 1 H), 7.80 (d, J = 8.4 Hz, 2 H), 7.70 (d,
J = 8.4 Hz, 2 H), 7.29 (d, J = 8.4 Hz, 2 H), 6.71 (d, J = 8.4 Hz, 2
H), 6.15 (s, 2 H), 3.50 (m, 2 H), 2.83 (dd, J = 7.5 Hz, 2 H), 2.22
(m, 2 H) ppm. 13C NMR ([D6]DMSO, 100 MHz): δ = 172.5, 153.8,
151.9, 150.0, 143.6, 136.9, 131.8, 131.3, 130.3, 129.1, 128.5, 128.3,
128.2, 127.5, 127.0, 126.1, 125.9, 125.7, 125.1, 125.0, 124.3, 123.5,
Acknowledgments
Giovanny Parada for HPLC support. Financial support from the
Swedish Research Council (Vetenskapsrådet), Uppsala University
KoF07 Graphene and KoF07 Molecular Electronics and, for O. N.
studentships, the European Commission and the German Fried-
rich-Ebert-Stiftung (FES).
123.4, 114.2, 34.1, 32.7, 27.4 ppm. IR: ν = 3402 (w), 3305 (w),
˜
3205 (w), 3044 (w) (N–H, C–Harom.), 1730 (s) (C=O) cm–1. UV/Vis
(MeOH): λmax (ε, m–1 cm–1) = 388 (25891), 341 (45033), 325 (29908),
275 (45242), 264 nm (28988). HRMS (ESI, MeOH): calcd.
484.2025 [M + H]+; found 484.2019.
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4-[(4-Hydroxyphenyl)diazenyl]phenyl 4-(Pyren-1-yl)butanoate (2):
This compound was prepared from 4,4Ј-(diazene-1,2-diyl)di-
phenol[15] Purification was performed by column chromatography
on silica (DCM/pentane/MeOH 70:30:1), after which the product
was recrystallized from ethanol layered with heptane. The recrys-
tallized product was recovered as bright orange crystals (60%),
1
m.p. 193–197 °C. H NMR ([D6]DMSO, 500 MHz): δ = 10.35 (s,
1 H), 8.46 (d, J = 9.0 Hz, 1 H), 8.29 (m, 4 H), 8.17 (m, 2 H), 8.08
(dd, J = 7.5 Hz, 1 H), 8.03 (d, J = 7.5 Hz, 1 H), 7.85 (d, J = 8.5 Hz,
2 H), 7.81 (d, J = 8.5 Hz, 2 H), 7.32 (d, J = 8.5 Hz, 2 H), 6.96 (d,
J = 8.5 Hz, 2 H), 3.48 (m, 2 H), 2.82 (dd, J = 7.5 Hz, 2 H), 2.20
(m, 2 H) ppm. 13C NMR ([D6]DMSO, 100 MHz): δ = 172.4, 160.7,
152.8, 150.6, 146.0, 136.9, 131.7, 131.3, 130.3, 129.1, 128.5, 128.3,
128.2, 127.5, 127.0, 125.9, 125.7, 125.1, 125.0, 124.3, 124.0, 123.6,
123.4, 116.8, 34.1, 32.7, 27.4 ppm. IR: ν = 3412 (m) (O–H), 3045
˜
(w) (C–Harom.), 1724 (s) (C=O) cm–1. UV/Vis (MeOH): λmax (ε,
m
–1 cm–1) = 341 (51900), 325 (35439), 275 (38720), 264 nm (21803).
HRMS (ESI, MeOH): calcd. 485.1865 [M + H]+; found 485.1874.
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use of dichloromethane to elute the product before recrystallization
from the minimum amount of dichloromethane layered with hept-
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m.p. 131–133 °C. 1H NMR (CDCl3, 500 MHz): δ = 8.35 (d, J =
9.3 Hz, 1 H), 8.18 (dd, J = 7.6 Hz, 2 H), 8.15 (d, J = 7.8 Hz, 1 H),
8.14 (d, J = 9.3 Hz, 1 H), 8.05 (d, J = 1.2 Hz, 2 H), 8.00 (t, J =
7.6 Hz, 1 H), 7.95 (d, J = 8.9 Hz, 2 H), 7.92 (d, J = 7.8 Hz, 1 H),
7.91 (d, J = 7.4 Hz, 2 H), 7.51 (m, 3 H), 7.24 (d, J = 8.9 Hz, 2 H),
3.52 (t, J = 7.8 Hz, 2 H), 2.75 (t, J = 6.2 Hz, 2 H), 2.35 (m, J =
7.8 Hz, 2 H) ppm. 13C NMR (CDCl3, 125 MHz): δ = 171.8, 152.8,
152.7, 150.4, 135.5, 131.6, 131.2, 131.0, 130.2, 129.2, 128.9, 127.7,
127.6, 127.6, 127.0, 126.0, 125.3, 125.1, 125.0, 124.2, 123.4, 123.0,
Eur. J. Org. Chem. 2014, 966–972
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
971

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Received: August 28, 2013
Published Online: December 5, 2013
972
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Eur. J. Org. Chem. 2014, 966–972