Cross-conjugation vs. linear conjugation in donor-bridge-acceptor nitrophenol chromophores

Article abstract of DOI:10.1002/ejoc.201301642

The influence of cross-conjugation vs. linear conjugation on the electronic communication between donor and acceptor groups in phenol(ate)-bridge- nitrobenzene chromophores was investigated by solution and gas-phase absorption spectroscopy, fluorescence s

Full text of DOI:10.1002/ejoc.201301642

FULL PAPER
DOI: 10.1002/ejoc.201301642
Cross-Conjugation vs. Linear Conjugation in Donor–Bridge–Acceptor
Nitrophenol Chromophores
Mikkel Andreas Christensen,[a] Eduardo Antonio Della Pia,[a] Jørgen Houmøller,[b]
Sean Thomsen,[b] Marius Wanko,[c] Andrew D. Bond,[d] Angel Rubio,[c]
Steen Brøndsted Nielsen,[b] and Mogens Brøndsted Nielsen*[a]
Keywords: Chromophores / Charge transfer / Conjugation / Donor–acceptor systems
The influence of cross-conjugation vs. linear conjugation on
the electronic communication between donor and acceptor
groups in phenol(ate)–bridge–nitrobenzene chromophores
was investigated by solution and gas-phase absorption spec-
troscopy, fluorescence spectroscopy, and quantum chemical
calculations. The compounds studied include, among others,
geminally and trans-substituted diethynylethenes prepared
by stepwise Sonogashira cross-coupling reactions, and the
cross-conjugated analogue of stilbene. A butadiyne-bridged
donor–acceptor chromophore was prepared by an unsym-
metrical Pd-catalysed coupling between a chloroalkyne and
a terminal alkyne. While the linearly conjugated chromo-
phores showed a strong and redshifted charge-transfer (CT)
absorption maximum upon deprotonation in solution, a new
redshifted absorption was either absent or present as a weak
shoulder for the cross-conjugated derivatives. Calculations
on the nonplanar 1,1-diaryl-substituted ethylene derivative
revealed that the S0–S1 absorption has a very low oscillator
strength due to insignificant coupling between the donor and
acceptor ends. The energy of this CT absorption for a nonsol-
vated molecule is smaller than that of the linearly conjugated
stilbene analogue by 0.4 eV. This result cannot directly be
inferred from the solution studies. A gas-phase absorption
spectrum of this chromophore was measured by action spec-
troscopy, which only revealed a higher-energy absorption
band. Based on calculations, this band was assigned to a
higher-lying π–π* transition. For the planar diethynylethene
chromophores, cross-conjugation leads to a decrease of
0.3 eV in the excitation energy, according to the calculations.
Finally, in contrast to the linearly conjugated chromophores,
the cross-conjugated ones did not show any fluorescence,
which may be due to deexcitation via the low-lying CT
“dark” state.
units. We have recently[2] studied a series of phenolate–
bridge–nitrobenzene chromophores by both solution and
gas-phase absorption spectroscopy, and the results revealed
a clear trend: When the donor and acceptor units are
weakly coupled via the connecting bridge, the CT absorp-
tion occurs at a lower energy. This result is also consistent
with several studies on other donor–acceptor chromo-
phores conducted, for example, by Diederich and co-
workers.[3] In other words, inefficient conjugation between
the two units leads to redshifted absorption maxima. This
may at first seem counter to the usual perception of the
influence of conjugation, and the fact that a longer linearly
conjugated oligoene/oligoenyne absorbs at lower energy
than a shorter one.[4] Indeed, extended conjugation at both
ends of donor–acceptor chromophores plays a similar role
by raising the HOMO energy of the donor and lowering
the LUMO energy of the acceptor, thereby inducing a red-
shift, but an additional mixing of these orbitals conveyed
by the bridge should lead to the opposite effect. This is, for
example, borne out by the fact that the absorption maxi-
mum of the weakly coupled m-nitrophenolate is redshifted
relative to those of the o- and p-nitrophenolates, as shown
by both solution and gas-phase studies.[2c,2d] In some cases,
Introduction
Organic molecules composed of donor and acceptor enti-
ties separated by a bridge usually show intramolecular
charge-transfer (CT) transitions, and they are of interest as
building blocks for advanced optical materials and photo-
voltaic devices, as well as in molecular electronics.[1] It is
desirable to be able to fine-tune the position of the charge-
transfer absorption for a specific donor–acceptor system by
varying the electronic nature of the bridge between the two
[a] Department of Chemistry & Center for Exploitation of Solar
Energy, University of Copenhagen,
Universitetsparken 5, 2100 Copenhagen Ø, Denmark
E-mail: mbn@kiku.dk
http://www.ki.ku.dk/Forskning/cese/
[b] Department of Physics and Astronomy, Aarhus University,
Ny Munkegade, 8000, Aarhus C, Denmark
[c] NanoBio Spectroscopy Group and ETSF Centre for Scientific
Development, Centro Mixto CSIC-UPV/EHU “Fisica de
Materiales”, University of the Basque Country UPV/EHU
Centro Joxe Mari Korta,
Avenida de Tolosa, 72, 20018 Donostia-San Sebastian, Spain
[d] Department of Pharmacy, University of Copenhagen,
Universitetsparken 2, 2100 Copenhagen Ø, Denmark
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201301642.
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Donor–Bridge–Acceptor Nitrophenol Chromophores
however, solution studies can be misleading in terms of re- known aryl iodide 10,[11] which, gratifyingly, furnished com-
vealing the coupling between donor and acceptor units, due pound 11. Finally, the ester group was cleaved to give target
to solvation effects. Thus, from gas-phase absorption spec- molecule 1.
troscopy of O–/NO2 functionalized tolane and stilbene mo-
lecules, we have found that an alkyne bridge provides a less
efficient coupling (redshifted absorption) than an alkene
bridge, whereas the opposite and wrong conclusion would
have been drawn from solution studies.[2b] Solution studies
on tetrathiafulvalene–bridge–dicyanoethylene chromo-
phores on the other hand support the less efficient coupling
conveyed by an alkyne bridge.[5]
To shed further light on the tuning of the absorption of
nitrophenol(ate)s, we decided to investigate the influence of
cross-conjugation vs. linear conjugation. Several experimen-
tal and theoretical studies have revealed that cross-conjuga-
tion, as found for example in dendralenes,[6] results in less
efficient π-electron delocalization than linear conjuga-
tion.[4,7] A too weak coupling between donor and acceptor
units may lead to the disappearance of the CT transition in
the electronic absorption spectrum or to a very weak band,
which can complicate the interpretation of spectroscopic
data. Indeed, the CT absorption of m-nitrophenolate is
much weaker than those of o- and p-nitrophenolates.[2c]
Figure 1 shows the compounds of interest for this study.
The cross-conjugated and linearly conjugated geminal- and
trans-diethynylethenes 1 and E-2 have C2 units between the
central ethylene and aryl groups, which should allow the
molecules to be planar. The properties of these compounds
were compared to those of the linearly conjugated buta-
diyne 3, the cross-conjugated and nonplanar alkene 4, and
the linearly conjugated fluorene 5, in which the two aryl
groups are forced into planarity. Protocols for the synthesis
of 1–4 are presented, as well as a modified procedure for
known[8] compound 5.
Scheme 1. Synthesis of cross-conjugated compound 1.
The syntheses of E-2 and 3 are shown in Scheme 2.
Known terminal alkyne 12[12] (readily prepared from 10, see
Supporting Information) was treated with vinyl bromide E-
13[13] under Sonogashira conditions to give diethynylethene
E-14. Cleavage of the ester group then gave compound E-2.
We have recently shown that chloroalkynes undergo cross-
coupling reactions with terminal alkynes under Sonoga-
Figure 1. Phenol–bridge–nitrobenzene compounds with cross-con-
shira conditions as readily as aryl iodides.[14] This knowl-
jugated or linearly conjugated bridges.
edge was conveniently used in the first step towards 3. Thus,
12 was treated with chloroalkyne 15[15] under Sonogashira
conditions to give butadiyne 16. Finally, the ester group was
cleaved to give phenol derivative 3.
Results and Discussion
Acetylation of ketone 17[16] gave compound 18
The synthesis of compound 1 is shown in Scheme 1. (Scheme 3), which was then subjected to a Wittig reaction
First, known vinyl triflate 6[9] was treated with p-nitrophen- followed by hydrolysis of the ester group during aqueous
ylacetylene (7) in a Sonogashira coupling[10] to give gemi- work-up of the reaction mixture to give 1,1-diarylethylene
nally substituted diethynylethene 8. Desilylation of this 4. For unknown reasons, the related benzoate of 18 (with an
compound provided terminal alkyne 9. A direct Sonoga- OBz instead of the OAc functionality) showed no reactivity
shira coupling between this compound and 4-iodophenol towards the phosphorus ylide, and only starting material
was not successful in our hands. Instead, we treated 9 with was recovered in that case. An alternative strategy for the
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Figure 2. Molecular structure of 4 (with displacement ellipsoids of
50% probability for non-hydrogen atoms; CCDC-967150).
Conversion of aniline derivative 19 into fluorene deriva-
tive 5 has been described previously.[8] However, we used
similar conditions[19] to those used to obtain the related
OH/NO2 substituted terphenyl compound (Scheme 4).
Scheme 2. Synthesis of linearly conjugated compounds E-2 and 3.
Scheme 4. Synthesis of fluorene derivative 5.
preparation of 4 by demethylation of the corresponding
methoxy derivative[17] (using either BBr3, HBr, or Me3SiI)
proved unsuccessful (other unidentified products were ob-
tained). Single crystals of 4 were grown from diethyl ether/
heptane and subjected to X-ray crystallographic analysis.
The structure revealed that the two aryl rings are rotated
out of coplanarity (Figure 2). Thus, the angle between the
ethylene plane and aryl plane C3–C8 is 32.6°, and the angle
between the ethylene plane and aryl plane C9–C14 is 48.7°.
It is well known that 1,1-diaryl ketones and ethylenes adopt
a propeller conformation to avoid the ortho-H,H clash.[18]
Solution-Phase UV/Vis Spectroscopy
The UV/Vis absorption spectra in MeOH of neutral
chromophores 1–5 are shown in Figure 3 top, while those of
the corresponding sodium phenolates are shown in Figure 3
bottom. The absorption maxima are collected in Table 1.
Dieethynylethene E-2 was observed to undergo photoisom-
erization to its Z isomer, whose longest-wavelength CT ab-
sorption has a lower intensity, as deduced from the absorp-
tion spectrum of the resulting mixture of the two isomers in
photostationary equilibrium (see Supporting Information).
The longest-wavelength absorptions of linearly conjugated
chromophores E-2, 3, and 5 become significantly redshifted
upon generation of the corresponding phenolates, consis-
tent with the assumed CT character of these absorptions.
Deprotonation of cross-conjugated compound 1 results in-
stead in a blueshift of the longest-wavelength absorption.
This observation seems to indicate that the actual CT ab-
sorption corresponding to a HOMO–LUMO transition
(S0–S1) is in fact absent from the spectrum. Generation of
the phenolate of cross-conjugated compound 4 does result
in a new redshifted absorption, which appears as a weak
shoulder/tail around 375 nm. We note that in previous stud-
ies on 1-(4-cyanophenyl)-1-[4-(dimethylamino)phenyl]ethyl-
ene and 1-(4-cyanophenyl)-1-(4-methoxyphenyl)ethylene a
band, assigned to a CT absorption, was also observed as a
Scheme 3. Synthesis of cross-conjugated compound 4.
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Donor–Bridge–Acceptor Nitrophenol Chromophores
shoulder absorption at the red edge of the strong absorp- Briefly, the number of photoinduced neutral fragment prod-
tion band in the spectrum.[20] To shed more light on the ucts was measured as a function of the excitation wave-
absorptions of the phenolate of 4, and on the character of length, and this number was used as a measure of the ab-
these absorptions, we decided to turn to gas-phase experi- sorption cross-section (more details in Experimental Sec-
ments and calculations (vide infra), thereby avoiding having tion). Figure 4 (top) shows the yield of neutral products
to take into account complicated solvation and counterion formed in the first 15 μs after photoexcitation as a function
effects for this negatively charged species.
of excitation wavelength. This signal is labeled Prompt SED
in the Figure. The signal is strongest at the lowest wave-
length of 420 nm, and it decreases almost linearly to about
480 nm. There is a long tail at higher wavelengths that ex-
tends up to about 650 nm (this tail is more clearly revealed
when corrections for laser power were not done, see Sup-
porting Information). As the vertical electron-detachment
energy is 2.86 eV (434 nm) (vide infra), both direct electron
detachment and dissociation can contribute to the signal at
the lowest wavelengths. After half a revolution and later
times, the delayed formation of neutral products was sam-
pled (Figure 4, bottom). This signal was recorded both with
the SED and the MCP detectors, and the signal from 10
revolutions was summed. The signal here is most probably
due to dissociation and not electron emission, although it is
hard to exclude thermionic emission. However, the electron
emission clearly does not occur directly from the excited
state, as we do not sample neutral products until 30 μs after
Figure 3. UV/Vis absorption spectra of chromophores in MeOH
(top, neutral chromophores) and in MeOH + 1% NaOMe (bottom,
phenolates).
Table 1. Absorption maxima (λmax) and molar absorptivities (ε) of
compounds in MeOH (sh = shoulder).
Neutral
Phenolate[a]
λmax [nm] (ε [m–1 cm–1])
λmax [nm] (ε [m–1 cm–1])
1
262 (45300), 293 (63700),
308 (58300), 346 (27300)
320 (30300), 368 (44370)
241 (15000), 273 (13900),
287 (16400), 303 (16400),
361 (15800)
272 (60800), 312 (60500),
336 (50800, sh)
315 (32800), 416 (34900)
252 (20700), 310 (23200),
418 (10900)
E-2
3
4
5
229 (37100), 269 (48000),
327 (16000, sh)
244 (5100), 364 (8000)
293 (64600),
375 (4700, sh)
268 (5400), 430 (8000)
[a] Corresponding to sodium phenolate (obtained by 1% NaOMe).
Figure 4. Gas-phase absorption spectra of the phenolate of 4. Top:
signal from neutral compounds formed within about 15 μs after
photoexcitation vs. the wavelength of the light. This signal was
measured with the secondary emission detector (SED), and is de-
noted the prompt signal. Bottom: yield of neutral compounds
formed after at least 30 μs as a function of wavelength. This signal
was measured with both the SED and microchannel plate (MCP)
Gas-Phase Action Spectroscopy
The intrinsic optical properties of the phenolate of 4 were
studied in the gas phase by action spectroscopy using the
ELectrostatic Ion Storage ring in Aarhus (ELISA).[21] detectors, and is denoted the delayed signal.
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M. Brøndsted Nielsen et al.
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photoexcitation. The two spectra reveal a band with a maxi-
mum between 430 nm and 450 nm that we ascribe to a
higher-lying π–π* state based on calculations (vide infra).
As the prompt signal was highest at 420 nm, this shows that
direct electron detachment does indeed occur to produce
neutral products. The probability of detachment is in-
creased as the population of the excited state, which acts as
a doorway state to the continuum, increases.
Excitation Energy Calculations
To shed light on the lowest-energy transitions of the phe-
nolate of 4, we subjected it to a computational study. The
geometry was optimized with MP2/TZVPP, and electronic
excitation energies were calculated using the CC2[22] and
SORCI[23] methods with the aug-cc-pVDZ basis set. The
coupled-cluster CC2 method was used in earlier studies on
nitrophenolates,[2] where it predicted the lowest π–π* transi-
tion to be 0.02–0.22 eV above the experimental absorption
maximum. SORCI is a multi-reference method, similar to
the CASPT2 method. It is more suitable for the selective
description of resonant π–π* excitations, which CC2 tends
to mix with artificial Rydberg excitations that appear in the
same energy region. Both methods agree that the S0–S1
transition is a HOMO–LUMO (π–π*) transition with
strong CT character and with an energy of 0.99 eV
(1252 nm; SORCI) or 1.63 eV (761 nm; CC2), and therefore
outside the energy window of the gas-phase measurements.
The calculated oscillator strength for this transition is very
small (0.01 in both methods), consistent with frontier or-
bital plots showing that the HOMO and LUMO orbitals
overlap only weakly (Figure 5). When comparing the CC2
CT absorption of the phenolate of 4 (760 nm) to that pre-
viously calculated for the linearly conjugated stilbene deriv-
ative (608 nm, 2.04 eV)[2a] (see Figure 5, top, and Figure 6),
it transpires that cross-conjugation does indeed cause a sig-
nificant redshift in the transition of the phenolate of 4. The
predicted S0–S2 transition of the phenolate of 4 is described
as an n–π* transition at 2.52 eV (492 nm) by CC2. The
SORCI spectrum of π–π* transitions further reveals that
the lowest transitions with significant oscillator strengths
are higher-lying π–π* transitions, which SORCI predicts at
energies of 2.70 and 2.86 eV (459 and 434 nm, respectively),
Figure 5. Frontier natural orbitals of the linearly and cross-conju-
gated ethylene- and diethynylethene-bridged phenolate chromo-
phores.
Figure 6. CT absorption energies calculated by the coupled-cluster
CC2 method for linearly conjugated and cross-conjugated deriva-
tives.
close to the peak seen in the experimental gas-phase spec- by dihedral angles of 43° (nitrophenyl) and 26° (phenolate)
trum. Similar transitions are found in the CC2 spectrum relative to the ethylene bridge.
when diffuse functions are excluded from the basis set. Un-
For the phenolate of the cross-conjugated and planar di-
der these conditions, Rydberg-like transitions are shifted to ethynylethene compound 1, the CC2 method provides ab-
higher energies, but also valence excitation energies are sorptions at 886 nm (1.40 eV; oscillator strength 0.06) and
overestimated. We calculated a vertical detachment energy 499 nm (2.49 eV; oscillator strength 0.35). The decoupling
(VDE) of 2.86 eV (434 nm) with the CEPA/1 method.[23] of the donor and acceptor ends by the cross-conjugated
Hence, the higher-lying π–π* transitions may be excitations spacer is reflected in the small oscillator strength of the CT
into resonant states.
transition at 886 nm, and is consistent with the HOMO and
It is noteworthy that the optimized structure of the LUMO presented in Figure 5 (bottom). For the phenolate
phenolate of 4 does not adopt a fully coplanar conforma- of linearly conjugated diethynylethene E-2, absorption max-
tion, as this indicates that electronic communication in the ima of 736 nm (1.69 eV; oscillator strength 1.90) and
cross-conjugated system is not strong enough to overcome 420 nm (2.95 eV; oscillator strength 0.38) are obtained.
modest steric strain. Thus, the two aryl groups are rotated Thus, again we see a significantly lower CT excitation en-
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Donor–Bridge–Acceptor Nitrophenol Chromophores
ergy for the cross-conjugated species, although the differ- by either linearly or cross-conjugated bridges. Due to the
ence (0.3 eV) is smaller than that obtained between the eth- weak coupling provided by cross-conjugated bridges, the
ylene-bridged chromophores (0.4 eV). The results are sum- HOMO–LUMO CT absorption (S0–S1) was either absent
marized in Figure 6. By comparing the two linearly conju- or very weak in the solution-phase absorption spectra of
gated phenolates, we see that the longer diethynylethene these compounds. This interpretation was supported by
bridge causes a lower CT excitation energy. This depen- quantum chemical calculations on the phenolates of 1 and
dence on the distance between donor and acceptor ends is 4, which have cross-conjugated ethylene and diethynyl-
consistent with studies on push–pull-conjugated oligomers ethene bridges between the phenolate and nitrobenzene
(HOMO–LUMO gap decreasing with donor–acceptor sep- units. The validity of the theoretical predictions of 4 was
aration).[24] Likewise, the excitation energy of the cross-con- ascertained by comparing the calculated higher-energy
jugated phenolate of E-2 is lower than that of 4.
transition to that observed experimentally in the gas phase.
In addition, the calculations show that the electronic de-
coupling between the donor and the acceptor ends does in-
deed result in lower-energy HOMO–LUMO gaps for the
phenolates of 1 and 4 relative to those of linearly conju-
gated analogues, consistent with our previous results on
cross-conjugated m-nitrophenolate vs. linearly conjugated
o/p-nitrophenolates. The most significant difference is seen
between nonplanar and planar 1,1- and 1,2-diarylethylenes.
Finally, while the linearly conjugated chromophores were
weakly fluorescent, the cross-conjugated ones were found
to be nonfluorescent, which indicates an ultrafast internal
conversion to a low-lying “dark” state (CT state).
Solution-Phase Fluorescence Spectroscopy
The fluorescent properties of the chromophores were
also investigated. Notably, cross-conjugated chromophores
1 and 4 did not show any significant emission in the range
350–850 nm, and 2 and 3 showed a weak emission. Neutral
fluorene derivative 5 showed a stronger emission at 588 nm,
which was redshifted to 740 nm for the phenolate (Fig-
ure 7). The fluorescence characteristics of the compounds
are summarized in Table 2. The absence of fluorescence for
the cross-conjugated compounds indicates ultrafast internal
conversion (IC) to a lower-lying “dark” state, consistent
with the existence of a CT state.
Experimental Section
General Methods: Chemicals were used as received from suppliers.
All metal-catalysed reactions were performed under an atmosphere
of argon in argon-flushed solvents. Dry THF was distilled from
sodium, and dry CH2Cl2 was obtained from a solvent purification
system from Innovative Technology, Inc. Flash column and dry
column vacuum chromatography were performed on silica gel
(ROCC 40–62 μm and 15–40 μm, respectively). Thin-layer
chromatography (TLC) was carried out using aluminum sheets pre-
coated with silica gel 60F (Merck 5554). NMR spectra were re-
corded with a 500 MHz Bruker instrument with a cryoprobe. The
residual solvent signal was used for calibration (CDCl3: δ = 7.26
and 77.16 ppm for 1H and 13C, respectively). Electrospray (ESI)
mass spectra were recorded with a Micromass Q-TOF mass spec-
trometer (for HRMS) with polyethylene glycol (PEG) as internal
standard, or with a Bruker Micro-TOF Q II mass spectrometer.
Gas chromatography (GC) mass spectra were recorded with an Ag-
ilent 6890A (G1530A) gas chromatograph coupled with an Agilent
mass-selective detector 5973 Network. Melting points were deter-
mined with a Stuart SMP 40 automatic melting point apparatus.
IR spectra were obtained with a Bruker ALPHA-P spectrometer
using the “ATR platinum Diamond 1 Refl” accessory. Peaks are
listed with relative intensities: vw (very weak), w (weak), m (me-
dium), s (strong). Elemental analyses were obtained at the micro-
analysis laboratory at the Department of Chemistry, University of
Copenhagen.
Figure 7. Excitation (dashed lines) and emission (bold lines) spectra
of 5 in MeOH (black lines, neutral chromophore) and in MeOH +
1% NaOMe (gray lines, phenolate).
Table 2. Fluorescence characteristics of compounds in MeOH;
emission maxima (λemis) and quantum yields (φfl).
Neutral Phenolate[a]
Compound λexc [nm] λemis [nm] λemis [nm]
Neutral
φfl
E-2
3
5
368
366
372
566
516
588
n.a.
n.a.
740
8ϫ10–4
6ϫ10–4
4ϫ10–3
UV/Vis absorption spectra were measured with a Cary 50 UV/Vis
spectrophotometer at a concentration of 10–5 m in methanol in a
quartz cuvette with a path length of 1 cm. Fluorescence measure-
ments were performed on a Fluorolog-3 (Jobin Yvon Horiba) spec-
trophotometer in methanol at room temperature. The fluorescence
quantum yields were determined considering 9,10-diphenyl-
anthracene as a standard (φfl = 0.95).
[a] Corresponding to sodium phenolate (obtained by 1% NaOMe);
n.a.: not applicable.
Conclusions
In conclusion, we have developed protocols for the syn-
Gas-phase spectroscopy was carried out using the ELectrostatic
thesis of a variety of nitrophenolate chromophores bridged Ion Storage ring in Aarhus (ELISA), as described in detail pre-
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M. Brøndsted Nielsen et al.
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viously.[21] The sample was dissolved in methanol and ionised using
the electrospray method. Ions were accumulated in a 22-pole ion
trap filled with helium buffer gas (room temperature). The ion trap
was emptied ten times per second to produce ion bunches. After
acceleration to 22 keV energies, ions of interest were selected using
an electromagnet and injected into the ring, where they were stored.
The revolution time in the ring was 60 μs. After 24 ms, the ions
were irradiated with light from an EKSPLA laser, which is an
Nd:YAG laser operated at 10 Hz. Its 1064 nm output was fre-
quency tripled to 355 nm, and this light was sent through an optical
parametric oscillator (OPO) to produce visible and infrared light.
The yield of neutral compounds produced immediately after photo-
excitation was measured by a secondary emission detector (SED)
and after successive revolutions in the ring. Another detector
[microchannel plate detector (MCP)] was used to measure the yield
of neutral compounds formed half a revolution later Ϫ this detec-
tor only measures delayed dissociation. Wavelengths from 420 nm
to 700 nm were scanned. The photoinduced yield of neutral com-
pounds was corrected for ion-beam fluctuations and divided by the
number of photons at each wavelength to obtain action spectra.
The repetition rate of the experiment was 10 Hz.
CDCl3): δ = 8.24–8.16 (m, 4 H), 7.70–7.59 (m, 3 H), 7.59–7.49 (m,
4 H), 7.21 (d, J = 8.7 Hz, 2 H), 2.74–2.57 (m, 4 H), 1.77–1.59 (m, 6
H) ppm. 13C NMR (126 MHz, CDCl3): δ = 164.89, 163.98, 150.81,
146.84, 133.77, 132.72, 132.13, 130.47, 130.21, 129.30, 128.64,
123.58, 121.86, 120.91, 97.54, 91.60, 90.83, 89.39, 85.43, 33.26,
33.20, 27.86, 27.77, 26.09 ppm. MS (FAB+): m/z = 461.62
[M + H]+. C30H23NO4 (461.51): calcd. C 78.07, H 5.02, N 3.03;
found C 77.74, H 4.90, N 3.04.
4-[3-Cyclohexylidene-5-(4-nitrophenyl)penta-1,4-diyn-1-yl]phenol
(1): Ester 11 (88 mg, 0.19 mmol) was dissolved in CH2Cl2 (10 mL)
and methanol (20 mL). K2CO3 (50 mg, 0.36 mmol) was added, and
the mixture was stirred at room temp. for 0.5 h. The reaction mix-
ture was then concentrated in vacuo and the residue was parti-
tioned between water and CH2Cl2. The combined organic extracts
were dried with Na2SO4, and the solvent was removed under re-
duced pressure. Flash column chromatography (SiO2, CH2Cl2) gave
1 (38 mg, 56%) as a yellow solid, m.p. 176–177 °C. IR (ART): ν =
˜
3397 (m), 2925 (w), 2850 (w), 2195 (w), 1592 (m), 1505 (s), 1339
(s), 1265 (m), 1165 (m 1098 m), 849 (s), 830 (s), 747 (m), 668 (m),
529 (m) cm–1. 1H NMR (500 MHz, CDCl3): δ = 8.19 (d, J = 9.0 Hz,
2 H), 7.61 (d, J = 9.0 Hz, 2 H), 7.39 (d, J = 8.8 Hz, 2 H), 6.80 (d,
J = 8.8 Hz, 2 H), 4.84 (s, 1 H), 2.68–2.59 (m, 4 H), 1.74–1.60 (m, 6
H) ppm. 13C NMR (126 MHz, CDCl3): δ = 163.23, 155.78, 146.93,
3-Cyclohexylidene-1-(4-nitrophenyl)-5-trimethylsilyl-1,4-pentadiyne
(8): Triflate 6 (100 mg, 0.294 mmol) and p-nitrophenylacetylene 7
(43 mg, 0.294 mmol) were dissolved in THF (15 mL). PdCl2- 133.33, 132.24, 130.70, 123.71, 115.76, 115.60, 97.84, 92.04, 91.52,
(PPh3)2 (10 mg, 0.014 mmol), CuI (2.8 mg, 0.015 mmol), and
iPr2NH (2 mL) were added, and the mixture was stirred under ar-
gon for 15 h. Et2O (100 mL) was added, and the mixture was
washed with saturated aqueous NH4Cl (50 mL) and brine (50 mL).
The solution was dried with Na2SO4 and concentrated in vacuo.
Flash column chromatography (SiO2, heptane/EtOAc, 9:1) gave 8
(40 mg, 40%) as a yellow oil that solidified upon standing, m.p.
90.0–91.7 °C. 1H NMR (500 MHz, CDCl3): δ = 8.18 (d, J = 8.9 Hz,
2 H), 7.59 (d, J = 8.9 Hz, 2 H), 2.59–2.55 (m, 4 H), 1.73–1.57 (m,
6 H), 0.23 (s, 9 H) ppm. 13C NMR (126 MHz, CDCl3): δ = 165.09,
146.95, 132.24, 130.66, 123.68, 100.55, 97.92, 96.98, 91.69, 89.60,
33.29, 33.19, 27.89, 27.74, 26.19, 0.14 ppm. MS (FAB+): m/z =
338.2 [M + H]+. C20H23NO2Si (337.49): calcd. C 71.18, H 6.87, N
4.15; found C 71.18, H 6.99, N 4.12.
89.35, 84.08, 33.34, 33.25, 27.99, 27.89, 26.26 ppm. HRMS (ESI–):
calcd. for C23H18NO3 [M – H]– 356.1292; found 356.1304.
–
(E)-4-[6-(4-Nitrophenyl)hexa-3-en-1,5-diyn-1-yl]phenyl Benzoate (E-
14): A mixture of vinyl bromide E-13 (173 mg, 0.686 mmol), alkyne
12 (196 mg, 0.882 mmol), PdCl2(PPh3)2 (24 mg, 0.034 mmol), and
CuI (7 mg, 0.04 mmol) was degassed, and a degassed mixture of
toluene (10 mL) and iPr2NH (3.5 mL) was added. The mixture was
stirred for 3.5 h at room temp., then saturated aqueous ammonium
chloride (30 mL) was added, and the mixture was then extracted
with CH2Cl2 (40 mL). The organic phase was washed with brine
(20 mL), dried with Na2SO4, and concentrated in vacuo. Flash col-
umn chromatography (SiO2, heptane/CH2Cl2, 1:2) gave E-14
(235 mg, 87%) as a bright yellow solid. Trace amounts of the Z-
isomer could be removed by recrystallization from MeCN, m.p.
170–173 °C (decomp.). 1H NMR (500 MHz, CDCl3): δ = 8.25–8.17
(m, 4 H), 7.68–7.63 (m, 1 H), 7.63–7.58 (m, 2 H), 7.57–7.47 (m, 4
H), 7.23 (d, J = 8.7 Hz, 2 H), 6.39 (d, J = 16.0 Hz, 1 H), 6.30 (d,
J = 16.0 Hz, 1 H) ppm. 13C NMR (126 MHz, CDCl3): δ = 164.82,
151.31, 147.15, 133.84, 132.99, 132.30, 130.23, 129.74, 129.20,
128.66, 123.68, 122.86, 122.04, 120.25, 119.66, 95.61, 93.04, 92.65,
3-Cyclohexylidene-1-(4-nitrophenyl)-1,4-pentadiyne (9): Compound
8 (350 mg, 1.04 mmol) was dissolved in THF (15 mL) and MeOH
(5 mL). K2CO3 (52 mg, 0.38 mmol) was added, and the mixture
was stirred for 7 h. The solvent was evaporated in vacuo, and the
residue was subjected to flash column chromatography (SiO2, hept-
ane/CH2Cl2, 3:2) to give 9 (267 mg, 86%) as a yellow solid, m.p.
+
126.9–128.0 °C. IR (ART): ν = 3290 (m), 2933 (m), 2856 (m), 2203
87.85 ppm. HRMS (ESI+): calcd. for C25H16NO4 [M – H]+
˜
(w), 1595 (m), 1517 (s), 1342 (s), 854 (m) cm–1
.
1H NMR 394.1074; found 394.1078.
(500 MHz, CDCl3): δ = 8.18 (d, J = 8.9 Hz, 2 H), 7.59 (d, J =
8.9 Hz, 2 H), 3.13 (s, 1 H), 2.66–2.50 (m, 4 H), 1.72–1.57 (m, 6 H)
ppm. 13C NMR (126 MHz, CDCl3): δ = 165.61, 147.02, 132.22,
130.43, 123.71, 96.72, 91.40, 89.69, 79.76, 79.66, 33.30, 33.21,
27.90, 27.82, 26.14 ppm. MS (GC-MS): m/z = 265.1 [M]+.
C17H15NO2 (265.31): calcd. C 76.96, H 5.70, N 5.28; found C
76.70, H 5.62, N 5.19.
(E)-4-[6-(4-Nitrophenyl)hexa-3-en-1,5-diyn-1-yl]phenol (E-2): Ester
E-14 (100 mg, 0.254 mmol) was dissolved in CH2Cl2 (20 mL) and
MeOH (20 mL). K2CO3 (50 mg, 0.36 mmol) was added, and the
mixture was stirred for 5 h. Saturated aqueous NH4Cl (40 mL) was
added, the phases were separated, and the aqueous phase was ex-
tracted with CH2Cl2. The combined organic extracts were washed
with brine, dried with Na2SO4, and concentrated in vacuo. Flash
column chromatography (SiO2, CH2Cl2/MeOH, 99:1) gave E-2
(43 mg, 59%) as an orange solid, m.p. 191–195 °C (decomp.). IR
4-[3-Cyclohexylidene-5-(4-nitrophenyl)penta-1,4-diyn-1-yl]phenyl
Benzoate (11): A mixture of alkyne 9 (227 mg, 0.856 mmol), iodide
10 (231 mg, 0.713 mmol), PdCl2(PPh3)2 (25 mg, 0.036 mmol), and
CuI (7 mg, 0.04 mmol) was degassed, and degassed THF (20 mL)
and iPr2NH (2.5 mL) were added. The mixture was stirred for 4 h,
then the solvent was removed in vacuo, Et2O was added, and, after
filtration, the mixture was concentrated. Flash column chromatog-
raphy (SiO2, heptane/CH2Cl2, 1:1) gave 11 (303 mg, 92%) as a yel-
low solid, m.p. 49.5–50.2 °C (decomp.). 1H NMR (500 MHz,
(ART): ν = 3414 (vw), 3106 (vw), 2850 (w), 2166 (w), 1604 (w),
˜
1569 (s), 1338 (m), 1271 (m), 1159 (m 1099 m), 925 (s), 852 (s), 830
(m), 748 (m), 687 (m), 533 (m) cm–1. 1H NMR (500 MHz, CDCl3):
δ = 8.20 (d, J = 9.0 Hz, 2 H), 7.59 (d, J = 9.0 Hz, 2 H), 7.37 (d, J
= 8.8 Hz, 2 H), 6.81 (d, J = 8.8 Hz, 2 H), 6.37 (d, J = 15.9 Hz, 1
H), 6.23 (d, J = 15.9 Hz, 1 H), 4.93 (s, 1 H) ppm. 13C NMR
(126 MHz, CDCl3): δ = 156.37, 147.21, 133.67, 132.37, 130.02,
2050
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Donor–Bridge–Acceptor Nitrophenol Chromophores
123.81, 123.42, 118.75, 115.80, 115.09, 96.70, 93.48, 92.42,
brine (25 mL), and dried with Na2SO4. The crude product was
86.89 ppm. HRMS (ESI–): calcd. for C18H10NO3 [M – H]– purified by dry column vacuum chromatography (SiO2, CH2Cl2) to
–
288.0666; found 288.0672. C18H11NO3 (289.28): calcd. C 74.73, H give 4 (250 mg, 52%) as a yellow solid, m.p. 125–128 °C. IR (ART):
3.83, N 4.84; found C 74.40, H 3.55, N 4.56.
ν = 3434 (m), 3049 (w), 2927 (w), 1590 (s), 1512 (s), 1496 (s), 1338
˜
(s), 1316 (s), 1272 (s), 1205 (s), 1175 (s), 1107 (m), 937 (s), 908 (s),
4-[(4-Nitrophenyl)buta-1,3-diyn-1-yl]phenyl Benzoate (16): To a de-
gassed mixture of toluene (5 mL) and iPr2NH (2 mL) was added a
mixture of alkyne 12 (149 mg, 0.67 mmol), chloroalkyne 15
(102 mg, 0.56 mmol), PdCl2(PPh3)2 (20 mg, 0.028 mmol), and CuI
(5 mg, 0.026 mmol). The mixture was stirred at room temp. for 3 h.
Saturated aqueous NH4Cl (40 mL) was added, and the mixture was
extracted with CH2Cl2 (40 mL). The organic phase was washed
with brine (30 mL), dried with Na2SO4, and concentrated in vacuo.
Flash column chromatography (SiO2, heptane/CH2Cl2, 1:1) gave 16
(84 mg, 41%) as a yellow solid, m.p. 200–204 °C (decomp.). 1H
NMR (300 MHz, CD2Cl2): δ = 8.29–8.11 (m, 4 H), 7.78–7.60 (m,
5 H), 7.54 (t, J = 7.6 Hz, 2 H), 7.27 (d, J = 8.8 Hz, 2 H) ppm.
13C NMR (126 MHz, CD2Cl2): δ = 165.20, 152.70, 148.18, 134.53,
134.45, 133.88, 130.65, 129.71, 129.25, 129.09, 124.21, 122.87,
119.17, 83.89, 79.81, 78.90, 73.71 ppm. MS (FAB+): m/z = 368.46
[M + H]+. C23H13NO4 (367.35): calcd. C 75.20, H 3.57, N 3.81;
found C 75.05, H 3.43, N 3.61.
1
860 (m), 833 (s), 801 (m), 768 (m), 710 (s), 506 (s) cm–1. H NMR
(500 MHz, CDCl3): δ = 8.19 (d, J = 8.9 Hz, 2 H), 7.49 (d, J =
8.9 Hz, 2 H), 7.18 (d, J = 8.7 Hz, 2 H), 6.82 (d, J = 8.7 Hz, 2 H),
5.55 (d, J = 0.7 Hz, 1 H), 5.48 (d, J = 0.7 Hz, 1 H), 4.79 (s, 1 H)
ppm. 13C NMR (126 MHz, CDCl3): δ = 155.87, 148.55, 147.95,
147.46, 133.04, 129.72, 129.18, 123.68, 116.07, 115.48 ppm. MS
(ESI–): m/z = 240.0 [M – H]+. C14H11NO3 (241.24): calcd. C 69.70,
H 4.60, N 5.81; found C 69.48, H 4.38, N 5.56.
7-Nitro-9H-fluoren-2-ol (5): Amine 19 (340 mg, 1.51 mmol) was
heated at 100 °C for 15 min with H2SO4 (2.5 m; 20 mL). Then acetic
acid (25 mL) was added, and the mixture was boiled further until
the yellow ammonium sulfate had dissolved. The ammonium sulf-
ate was next obtained in a finely divided form by rapidly cooling
the solution to 0 °C while stirring. NaNO2 (440 mg, 6.38 mmol) in
water (5.5 mL) was added to the stirred suspension at 0 °C, and
the temperature was raised to 80 °C and kept there for 2 h. Then,
sulfamic acid was added to the bright yellow suspension until no
gas was formed upon addition. The mixture was cooled to 0 °C,
and H2SO4 (40% aq. w/w; 66 mL) was then gradually added while
stirring. The mixture was heated at 80 °C for 2.5 h. The yellow solid
was isolated from the cooled mixture and washed with aqueous
NaHCO3. Recrystallization from AcOH gave 5 (115 mg, 34%) as
an orange solid, m.p. 225–228 °C (decomp.); ref.[8a] 250 °C; ref.[8b]
4-[(4-Nitrophenyl)buta-1,3-diyn-1-yl]phenol (3): Ester 16 (77 mg,
0.21 mmol) was dissolved in CH2Cl2 (15 mL) and MeOH (15 mL).
K2CO3 (50 mg) was added, and the mixture was stirred for 3 h.
Saturated aqueous NH4Cl (40 mL) was added, and the mixture was
extracted with CH2Cl2 (40 mL), washed with brine (25 mL), dried
with Na2SO4, and concentrated in vacuo. Flash column
chromatography (SiO2, MeOH/CH2Cl2, 1:99) gave 3 (33 mg, 60%)
241–251 °C. IR (ART): ν = 3407 (w), 1608 (m), 1583 (s), 1508 (s),
˜
as a yellow solid, m.p. 220–222 °C (decomp.). IR (ART): ν = 3401
˜
1454 (s), 1393 (m), 1309 (s), 1288 (s), 1102 (m 1072 s), 914 (m), 852
(m), 2206 (m), 1558 (m), 1503 (s), 1332 (s), 1271 (m), 1210 (m),
1170 (m), 1105 (m 833 s), 745 (m), 534 (m), 512 (m) cm–1. 1H NMR
(500 MHz, CDCl3): δ = 8.21 (d, J = 9.0 Hz, 2 H), 7.65 (d, J =
9.0 Hz, 2 H), 7.46 (d, J = 8.8 Hz, 2 H), 6.82 (d, J = 8.8 Hz, 2 H),
5.00 (s, 1 H) ppm. 13C NMR (126 MHz, CDCl3): δ = 157.07,
147.50, 134.75, 133.25, 129.20, 123.83, 115.95, 113.49, 84.75, 79.48,
(m), 815 (s), 758 (s), 731 (s) cm–1 1H NMR (500 MHz, [D6]-
.
DMSO): δ = 8.34 (d, J = 2.0 Hz, 1 H), 8.23 (dd, J = 8.4, 2.0 Hz,
1 H), 7.92 (d, J = 8.4 Hz, 1 H), 7.85 (d, J = 8.3 Hz, 1 H), 7.05 (d,
J = 2.2 Hz, 1 H), 6.87 (dd, J = 8.3, 2.2 Hz, 1 H), 3.97 (s, 2 H) ppm.
13C NMR (126 MHz, [D6]DMSO): δ = 159.58, 148.56, 147.54,
144.75, 143.29, 129.98, 123.09, 122.91, 120.08, 118.81, 115.19,
112.24, 36.42 ppm. HRMS (ESI+): calcd. for C13H9NO3Na+ [M +
Na]+ 250.0475; found 250.0463.
78.85, 72.33 ppm. HRMS (ESI–): calcd. for C16H8NO3 [M – H]–
–
262.0510; found 262.0513. C16H9NO3 (263.25): calcd. C 73.00, H
3.45, N 5.32; found C 72.58, H 3.21, N 5.07.
Supporting Information (see footnote on the first page of this arti-
cle): Experimental details for the synthesis of 12, absorption spectra
for the E-2/Z-2 mixture, gas-phase absorption spectrum of the phe-
4-Acetoxy-4Ј-nitrobenzophenone (18): Phenol 17 (576 mg,
2.37 mmol) was dissolved in pyridine (10 mL), AcCl (0.20 mL,
2.8 mmol) was added, and the mixture was stirred for 5 h. The
reaction mixture was poured into ice-water (100 mL), and the mix-
ture was extracted with EtOAc (3ϫ 75 mL). The combined organic
extracts were washed with saturated aqueous NaHCO3 (50 mL)
and brine (50 mL), and dried with Na2SO4. The solvent was re-
moved under reduced pressure. Purification by dry column vacuum
chromatography (SiO2, CH2Cl2) gave 18 (565 mg, 84%) as a white
1
nolate of 4 without corrections for laser power, copies of H and
13C NMR spectra of all compounds.
Acknowledgments
The Danish Council for Independent Research, Natural Sciences
(#10-082088), the Lundbeck Foundation, and the University of
Copenhagen are gratefully acknowledged for financial support.
M. W. and A. R. acknowledge financial support from the European
Research Council Advanced Grant DYNamo (ERC-2010-AdG –
267374) Spanish Grants (FIS2011-65702-C02-01 and PIB2010US-
00652), Grupo Consolidado UPV/EHU del Gobierno Vasco
(IT578-13), Ikerbasque and European Commission project
CRONOS (280879-2).
1
solid, m.p. 134.9–135.8 °C. H NMR (500 MHz, CDCl3): δ = 8.35
(d, J = 8.9 Hz, 2 H), 7.93 (d, J = 8.9 Hz, 2 H), 7.86 (d, J = 8.8 Hz,
2 H), 7.27 (d, J = 8.8 Hz, 2 H), 2.36 (s, 3 H) ppm. 13C NMR
(126 MHz, CDCl3): δ = 193.71, 168.91, 154.80, 150.00, 142.91,
133.87, 131.89, 130.73, 123.75, 122.14, 21.30 ppm. MS (GC-MS):
m/z = 285.1 [M]+. C15H11NO5 (285.25): calcd. C 63.16, H 3.89, N
4.91; found C 62.91, H 3.64, N 4.79.
4-[1-(4-Nitrophenyl)vinyl]phenol (4): nBuLi (1.39 m in hexanes:
2.85 mL, 3.95 mmol) was added to a suspension of methyltri-
phenylphosphonium bromide (1.43 g, 4.00 mmol) in THF (30 mL)
at 0 °C. After 45 min, ketone 18 (0.565 g, 1.98 mmol) was added in
one portion. After 2.5 h, the cooling bath was removed, and the
mixture was stirred at room temp. for 24 h. Saturated aqueous
NH4Cl (25 mL) was added, and the mixture was extracted with
Et2O (3ϫ 20 mL). The combined ether layers were washed with
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2051

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Received: November 1, 2013
Published Online: January 28, 2014
2052
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2014, 2044–2052