Donor-acceptor 1,4-fluorenylene chromophores: Photophysics, electrochemistry, and synthesis through a route for asymmetric chromophore preparation

Article abstract of DOI:10.1002/ejoc.201402181

Fourteen chromophores of the form 1-(4-X-styryl)-4-(4-nitrostyryl)benzene or 1-(4-X-styryl)-4-(4-nitrostyryl)fluorene have been synthesized with X = CF₃, Cl, I, H, CH₃, OCH₃, or OCH(CH₃)₃. An innovative synthetic route was developed in the course of this work wherein phosphonium-phosphonate ester bifunctional precursors were employed. By exploiting steric considerations and the difference in acidity of protons in the position alpha to the phosphonium and phosphonate ester, target asymmetric chromophores were assembled in a straightforward fashion with high selectivity. The photophysical characteristics of the chromophores are described, including notable solvatochromism revealed by measuring photophysical properties in acetonitrile, dichloromethane, tetrahydrofuran, and toluene. Electrochemical analysis by cyclic voltammetry, along with DFT calculations (B3LYP-6-31G) were employed to reveal the HOMO and LUMO distributions and energies, thus providing further understanding of the properties of these materials and the similarities/differences between phenylene-centered and fluorenylene-centered chromophores.

Full text of DOI:10.1002/ejoc.201402181

FULL PAPER
DOI: 10.1002/ejoc.201402181
Donor–Acceptor 1,4-Fluorenylene Chromophores: Photophysics,
Electrochemistry, and Synthesis through a Route for Asymmetric Chromophore
Preparation
[
a]
[b]
[b]
[b]
Brynna J. Laughlin, Mary K. Burdette, Chadwick R. Powell, Benjamin E. Levy,
Andrew G. Tennyson,*[
a,b,c,d]
and Rhett C. Smith*
[a,b,c]
Keywords: Chromophores / Electrochemistry / Donor–acceptor systems / Fluorene / Photophysics
Fourteen chromophores of the form 1-(4-X-styryl)-4-(4-
nitrostyryl)benzene or 1-(4-X-styryl)-4-(4-nitrostyryl)fluorene
The photophysical characteristics of the chromophores are
described, including notable solvatochromism revealed by
measuring photophysical properties in acetonitrile, dichloro-
methane, tetrahydrofuran, and toluene. Electrochemical
analysis by cyclic voltammetry, along with DFT calculations
(B3LYP-6-31G*) were employed to reveal the HOMO and
LUMO distributions and energies, thus providing further un-
derstanding of the properties of these materials and the simi-
larities/differences between phenylene-centered and fluor-
enylene-centered chromophores.
have been synthesized with X = CF
3 3 3
, Cl, I, H, CH , OCH ,
or OCH(CH . An innovative synthetic route was developed
3 3
)
in the course of this work wherein phosphonium-phos-
phonate ester bifunctional precursors were employed. By ex-
ploiting steric considerations and the difference in acidity of
protons in the position alpha to the phosphonium and phos-
phonate ester, target asymmetric chromophores were as-
sembled in a straightforward fashion with high selectivity.
Introduction
logical sensing. Polymeric systems such as PPV can, how-
ever, be difficult to characterize because of high molecular
There has been a recent surge in interest in π-conjugated weights, poor solubility, polydispersity, and structural de-
molecules and polymers that feature both electron-releasing fects, among other problems. To develop a deeper under-
and electron-withdrawing subunits: so-called donor–ac- standing of how properties change as the effective conjuga-
ceptor (D–A) chromophores. This trend stems, in part, tion length of a π-system increases, many insightful studies
from the advantages that such materials hold for certain have focused on analogous oligomers that can be fully char-
[
13]
organic photonic and electronic applications such as non- acterized.
The simplest examples of small segments of
One D–A OPVs include asymmetrically substituted stilbene and
type of conjugated backbone system that is of interest in 1,4-distyrylbenzene (DSB) derivatives, which have been ex-
linear optics (NLO)[
2–8]
and organic photovoltaics.
[9–12]
[
17]
this field of research are poly(p-phenylenevinylene) and oli- tensively studied for several decades. In the case of NLO
go(p-phenylenevinylene) (OPV) derivatives.[ These mate- applications, it is ultimately the manner in which dipoles
rials are also useful in applications including laser dyes, derived from asymmetric substitution of the chromophore
13]
[14,15]
field-effect transistors (FETs),
memory storage devices are aligned that eventually dictates the utility of the mate-
and electrochemical supercapacitors,[ and as dyes for bio- rial. In photovoltaics, the asymmetric substitution of the
16]
[18]
chromophore not only provides a means for tuning the ab-
sorption wavelength, but can also influence the electron af-
finity and ionization potential of the material, which are
key considerations for leveraging charge transport through
the active layer of organic bulk heterojunction solar cells.
Both NLO and photovoltaic applications also rely on the
molecular-level orientation and film morphology for proper
alignment of materials.
[
[
a] Department of Chemistry, Clemson University,
Clemson, SC 29634, USA
b] Laboratory for Creative Inquiry in Chemistry, Clemson
University,
Clemson, SC 29634, USA
E-mail: rhett@clemson.edu
http://www.clemson.edu/chemistry/people/Tennyson.html
http://www.clemson.edu/chemistry/people/rsmith.html
c] Center for Optical Materials Science and Engineering
Technology, Clemson University,
[
[
As part of an effort to improve structure/morphology
Anderson, SC 29634, USA
d] Department of Materials Science and Engineering, Clemson control in PPV and OPV systems, we previously reported
University,
Clemson, SC 29634, USA
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201402181.
[19,20]
[21]
several polymers
and small-molecule models
wherein the commonly employed 1,4-phenylene π-systems
[
22–26]
are replaced by 1,4-fluorenylene
moieties (Figure 1).
5998
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2014, 5998–6009

Donor-Acceptor 1,4-Fluorenylene Chromophores
This substitution leads to greatly improved solubility of the electrochemical properties of their composite chromo-
chromophores in many cases, without significantly altering phores. We were therefore interested in preparing asymmet-
the absorption, photoluminescence, solvatochromic, or rically substituted chromophores, including D–A systems,
incorporating the 1,4-fluorenylene unit for comparison to
known 1,4-phenylene-containing analogues.
One challenge facing researchers interested in exploring
asymmetrically substituted chromophores is that the se-
quential placement of two or more differently substituted
chromophore subunits on a central core is often required
for their synthesis. Taking DSBs as an example, this prob-
lem has led to the development of several routes to achieve
the asymmetric substitution. It is possible to substitute a
styryl units for one of two substituents of a 1,4-X -benzene
2
precursor, but this approach generally requires tedious sep-
aration of starting materials, monosubstituted and disubsti-
tuted materials and consequently produces poor overall
[
27]
yields.
synthesize asymmetrically substituted DSB derivatives
Scheme 1, A) involves synthesis of a phosphonium salt,
One of the common site-selective procedures to
(
Wittig reaction of the salt with an aldehyde, radical bromin-
ation of a benzylic methyl para to the substituted position,
a second substitution of a phosphonium group on the benz-
ylic bromomethyl, followed by a second Wittig reaction. Al-
ternatively, a Wittig reaction can be carried out to yield a
stilbene moiety having a halogen substituent that can be
Figure 1. Selected polymers (left) and small-molecule models
right) containing the 1,4-fluorenylene unit (R = n-hexyl, EtHx =
-ethylhexyl).
(
2
Scheme 1. Representative generalized routes to asymmetrically substituted 1,4-distyrylbenzene chromophores based on current literature
precedent.
Eur. J. Org. Chem. 2014, 5998–6009
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5999

A. G. Tennyson, R. C. Smith et al.
FULL PAPER
subsequently converted into either an aldehyde for the sec- Results and Discussion
ond Wittig-type coupling or used for Heck coupling to yield
the second olefin linkage (Scheme 1, B).[ Other multistep Design Rationale and Synthesis of Asymmetric Molecules
28]
and consequently low-yielding syntheses have been devizsed
The target molecules for this study were chosen to fea-
involving iterative Pd-catalyzed C–C bond formation/Wit-
ture a strongly electron-withdrawing nitro substituent at
tig-type coupling, sometimes requiring additional protec-
one end of the chromophore and a substituent X at the
tion/deprotection sequences (Scheme 1, C)[ or tethering
other end of the chromophore. Substituents X were chosen
29]
[
30]
precursors to a support such as a perfluorous alkyl unit
to sample a range of electron-donating or -releasing abili-
ties, as quantified by their Hammett substituent constants
or silica-immobilized linker[ to allow for separation of re-
action mixtures followed by removal of the molecule from
the support. These routes are often low-yielding after so
many steps and are sometimes not viable due to the sensi-
tivity of alkene units to halogenation under the radical bro-
mination conditions.
31]
[
1]
(
Table 1). To efficiently place the two differently substi-
tuted styryl moieties onto the 1,4-phenylene or 1,4-fluoren-
ylene core, a difunctional core structure from which all tar-
gets could be easily derived was desired to avoid an un-
necessarily long, low-yielding route. The synthetic route
shown in Scheme 3 was thus devised as a potentially conve-
nient and general route to asymmetric chromophores bear-
ing phenylenevinylene subunits.
One interesting report[ describing the preparation of
asymmetrically derivatized 2,2Ј-bipyridyl chromophores
made use of the low solubility of phosphonium salt Bipy-P
32]
(Scheme 2) to isolate the monophosphonium/monobromo-
methyl bipyridyl derivative, which was then employed in a Table 1. Hammett substituent constants for relevant functional
Wittig reaction to append the first stilbenyl unit. This al- groups.[
1]
lowed the remaining benzylic bromide position to be modi-
fied with different chromophoric subunits (Scheme 2). Al-
though this strategy worked well on the bipyridine deriva-
Substituent
σ
para
NO
CF
2
0.78
0.54
0.23
0.18
3
tive, it is not directly as useful for preparing 1,4-distyryl- Cl
benzene derivatives because the electronic communication
between the two benzylic sites leads to side reactions such
as polymerization through Gilch-like pathways[ upon ex-
posure to the base required for the Wittig reaction stage.
I
H
0.00
CH
3
–0.17
–0.27
–0.45
–0.83
33]
OCH
3 2
OCH(CH )
3
The overall strategy of exploiting the ready isolation of a N(CH
monophosphonium salt from a bis(benzylic bromide), how-
ever, could be an excellent step to improving asymmetric
substitution. Despite the attractiveness of this approach,
however, there do not appear to be additional applications
of it to the synthesis of other asymmetrically substituted
chromophores. In the current work, we report a general
strategy for the preparation of asymmetrically substituted
3 2
)
1,4-phenylene and 1,4-fluorenylene chromophores wherein
we exploit differences in solubility, pK values of deproton-
a
ation sites, and reaction rates, to attain high selectivity. By
using these routes, fourteen chromophores were prepared
and then studied by photophysical and electrochemical
methods as well as by DFT calculations.
Scheme 3. Route to asymmetrically substituted distyrylbenzene de-
rivatives.
To our surprise, key intermediate α-phosphonium-αЈ-
(phosphonate ester)-p-xylene M3 does not appear to have
been reported previously, even though its immediate precur-
sor, M2, is commercially available. For our study, M2 was
synthesized by reaction of triphenylphosphine with M1 in
Scheme 2. Synthetic route to asymmetrically substituted 2,2Ј-bipyr-
idyl chromophores.
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Donor-Acceptor 1,4-Fluorenylene Chromophores
toluene. Isolation of M2 was straightforward because the ery case, 4 precipitated out of solution after quenching the
target product precipitates upon monosubstitution. Com- base with water, allowing facile collection by filtration. For
1
pound M2 was thus collected by simple filtration in Ͼ 95% each molecule, the olefin units were observed by H NMR
purity and was used without further purification.
spectroscopic analysis to exist in both E and Z isomeric
The next step was a Michaelis–Arbuzov reaction between forms. The E,E isomers were readily obtained by isomeriza-
M2 and triethylphosphite, which afforded M3 in good yield tion mediated by heating the crude solids to reflux in tolu-
(Ͼ 75%). Precursor M3 provided the difunctional scaffold ene with a catalytic amount of iodine. The compounds were
necessary for sequential placement of the two different styr- then either collected by filtration or were extracted from the
yl groups desired to access the asymmetric chromophore. cooled reaction mixture. Compounds 4 were obtained in
On the basis of the difference in acidity of benzylic protons modest to good yields, ranging from 39–82%, and included
adjacent to a phosphonium (pK ca. 17.6) vs. those in a previously unreported DSB derivatives 4a–c and 4g. It is
a
phosphonate ester α-position (pK ca. 27.6, Figure 2), we noteworthy that the route outlined in Scheme 3 allows easy
a
hypothesized that the phosphonium site could be selectively access to a wide array of asymmetrically substituted DSBs
deprotonated to afford the phosphonium ylide under Wittig in only three-steps from commercially available M2 and any
conditions in tetrahydrofuran (THF) en route to the requi- commercial aldehydes that are stable to Wittig condensa-
site olefin, leaving the phosphonate ester in place for subse- tion. Preparation of symmetrically substituted 1,4-di(4-
quent reaction. Indeed, when M3 and 4-nitrobenzaldehyde nitrostyryl)benzene (5b; Figure 3) did not require use of the
react in the presence of KOtBu, the major product was tar- new synthetic protocol, but it was prepared as previously
[
21]
get 3. Solubility is also an important factor in this step, reported
for comparison of its properties to asymmetri-
with improved yields obtained when ethanol was employed cally substituted molecules 4 (see below).
as solvent in place of THF, owing to the better solubility
of M3 in ethanol.
Figure 2. The pK
a
values[20] for a benzyl phosphonate ester (left)
and benzyl phosphonium salt that model the possible potential de-
protonation sites of M3.
Figure 3. Symmetrically substituted fluorenylene and phenylene
compounds with nitro groups.
To assess the selectivity for reaction at the phosphonium
site over the phosphonate ester site, the crude reaction mix-
ture of 3 was analyzed by H NMR spectroscopy. The H rivatized 1,4-fluorenylene chromophores (Scheme 4) is more
NMR spectrum of product 3 showed that the integration complex than the route shown in Scheme 3 because, unlike
The synthetic route required to access asymmetrically de-
1
1
ratio of the proton signals corresponding to –CH P(O)- the phenylene core, the 1- and 4-positions of the fluorenyl-
2
+
(
OEt) and –CH P Ph was 13:1, corresponding to 93% of ene core are inequivalent. Scheme 4 thus illustrates the syn-
2
2
3
the desired product 3 and only 7% of the product resulting thetic route devised to access the target asymmetrically sub-
from olefination at the phosphonate ester site.
stituted fluorene derivatives 2.
The purification of 3 requires removal of the side product
The starting material M4 was first prepared as previously
[
35]
triphenylphosphine oxide, which proved to require a reported. After heating M4 to reflux in toluene with tri-
multistep process. By using column chromatography with phenylphosphine for 2 h, the reaction mixture was cooled
an eluent of CH Cl with 2% methanol, a majority of prod- and product M5 precipitated out of solution as a white
2
2
uct was isolated. Unfortunately, 3 and triphenylphosphine powder, which was collected by filtration. As with the 1,4-
oxide have similar retention factors (R ), which lead to co- phenylene system, this route exploits the selective precipi-
f
elution of the two compounds in several fractions. These tation of the mono-phosphonium salt as a convenient isola-
fractions were thus collected and repurified by preparative tion of target M5 without any of the bis(phosphonium) ob-
TLC (CH Cl with 2% methanol) to collect the rest of the servable by NMR spectroscopic analysis. Unlike the 1,4-
2
2
desired product 3. It was observed that 3 was isolable as a phenylene system, the two benzylic sites in M4 are inequiva-
mixture of E and Z isomers, which rapidly interconvert on lent. The inequivalency of the two benzylic sites means that
exposure to light; this is a common issue for these types of two products are possible, but, on the basis of steric consid-
stilbene derivatives.[
34]
erations, chemical intuition would suggest that placing a
The target reference molecules 4a–h were then conve- bulky triphenylphosphine moiety at the 1-position of the
niently synthesized by reaction of common precursor 3 with fluorene ring, adjacent to the quaternary 9-position carbon,
the requisite para-substituted benzaldehydes. In almost ev- would face a significant kinetic barrier compared with sub-
Eur. J. Org. Chem. 2014, 5998–6009
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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A. G. Tennyson, R. C. Smith et al.
FULL PAPER
by using KOC(CH ) as base. Under these conditions, how-
3
3
ever, most of the starting materials remained unreacted
even after heating the mixture for over 12 h. When n-butyl-
lithium was used as base in place of KOC(CH ) , M7 was
3
3
obtained in 36% yield.
It is well-known that metal ions can influence the E/Z
isomer ratio and product yield in the Horner–Wadsworth–
[
21,22]
+
Emmons variation of the Wittig reaction,
and that Li
in particular has a significant beneficial influence on yield.
This aspect of the reaction was therefore explored in more
detail. In one experiment, 5 mol equiv. of LiCl per mol of
KOC(CH ) was added to the reaction mixture, keeping all
3
3
other conditions the same as the first failed attempt that
had employed KOC(CH ) as base. In the second experi-
3
3
ment, 5 mol equiv. of LiCl was used as an additive and a
significantly weaker base, 1 equiv. of N(CH ) , was em-
3
3
ployed. In both cases where LiCl was present as an additive,
the reaction went to at least 50% completion, similar to the
reaction with n-butyllithium, after the same period of time.
This data suggests that this particular Horner–Wadsworth–
Emmons variation of the Wittig reaction is significantly de-
pendent on cation effects to drive the reaction to comple-
tion, more so than the base strength or reaction tempera-
ture.
As observed in the preparation of 3 (Scheme 3), it was
found that the deprotonation is selective for the phosphon-
ium α-carbon over the phosphonate α-carbon, with no
1
other products observable in a H NMR spectrum of the
crude reaction mixture. The pure product was isolated by
column chromatography. Even after purification, however,
the proton NMR spectrum showed evidence for the pres-
ence of both E and Z isomers, which interconvert rapidly
upon exposure to light or thermally upon standing in the
dark at room temperature. Once purified by column
Scheme 4. Route to asymmetrically substituted fluoreneylene deriv-
atives 2.
stitution at the benzylic site at the 4-position of the fluorene chromatography, M7 was used, with the unavoidable con-
ring. Indeed, when M4 was treated with triphenylphosphine tamination of some Z isomer, for synthesis of the target
1
in a 1:0.9 molar equivalence ratio, the H NMR spectrum asymmetrically substituted fluorenylene molecules 2a–g.
of the crude reaction mixture showed only one phosphon-
Compounds 2a–g were successfully synthesized by com-
ium product, indicating Ն95% selectivity for one monosub- bining M7 with the requisite para-substituted benzaldehyde
stituted product over the other. To confirm the anticipated in anhydrous THF. The desired Horner–Wittig reaction was
regiochemistry of the product, NMR experiments using the carried out by addition of n-butyllithium to the reaction
Nuclear Overhauser Effect (NOE) were employed. These mixture, just as with the phenylene derivatives 4. Once
experiments confirmed the hypothesis that the C-4 benzylic again, when the reaction was attempted with potassium
site was the site that underwent selective substitution to tert-butoxide as the base, most of the starting material was
form the phosphonium salt, giving the regioisomer of M5 recovered unreacted. Thus, nBuLi was employed and the
depicted in Scheme 4.
reactions proceeded well upon stirring at room temperature
The Michaelis–Arbuzov reaction of M5 with excess tri- for 18 h or more. Workup involved quenching the excess
ethylphosphite was a facile reaction requiring heating for base with water, followed by extraction with CH Cl . Purifi-
2
2
3
h, followed by vacuum distillation to remove remaining cation involved either column chromatography on silica or
excess triethylphosphite. The reaction of M5 with triethyl- preparative thin-layer chromatography on silica, giving
[
34]
phosphite afforded M6, a crystalline solid, in high yield yields of 22–36%. As previously reported, compounds 2
ca. 94%). isomerize readily upon exposure to light as well as ther-
The synthesis of M7 was carried out by reaction of M6 mally at room temperature, so although the molecules were
(
with p-nitrobenzaldehyde in THF. Phosphonium-phos- isolated predominantly as all-E isomers, it was not possible
phonate ester M6, unlike phenylene analogue M3, was solu- to isolate them without small amounts of Z isomer at room
ble in THF, presumably due to the presence of two hexyl temperature.
groups on M6. The synthesis of M7 was thus first at-
Symmetrically substituted 9,9-dihexyl-1,4-bis(4-nitro-
tempted by reacting M6 and p-nitrobenzaldehyde in THF styryl)fluorene (5a; Figure 3) was also prepared as pre-
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Eur. J. Org. Chem. 2014, 5998–6009

Donor-Acceptor 1,4-Fluorenylene Chromophores
viously reported for comparison to asymmetrically substi- Table 3. Photophysical characterization of asymmetrically substi-
[
21]
tuted distyrylbenzene derivatives 4a–h and 5b.
tuted molecules 2.
Solvent
λ
max
λ
em
Φ (Ϯ0.02) Stokes’ shift
[
nm]
[nm]
[nm]
Photophysical Properties
5
b
CH
CH
THF
3
CN
Cl
397
402
400
397
577
540
488
478
0.32
0.43
0.13
0.03
180
138
88
2
2
Table 2 and Table 3 provide data from the photophysical
characterization of series 2 and 4. For comparison, the pho-
tophysical properties of 5a and 5b[ (Table 3) are included
in the tables. One of the main purposes of evaluating the
photophysical properties of these molecules was to evaluate
the effect not only of the additional steric encumbrance re-
sulting from fluorenylene attachment but also the effect of
varying the donor or acceptor strength of the group at the X = Cl
opposite end of the chromophore from the nitro group.
There was also interest in evaluating any solvatochromic
CH
3
Ph
CN
Cl
81
23]
4
a
CH
CH
THF
3
2
380
386
383
385
584
559
508
460
0.08
0.32
0.08
0.01
204
173
125
75
X = CF
3
2
CH
3
Ph
CN
Cl
4
b
CH
CH
THF
3
2
383
388
387
392
620
587
520
483
0.02
0.04
0.29
0.02
237
199
133
91
2
CH
3
Ph
CN
Cl
4
c
CH
CH
THF
3
2
386
392
390
393
620
583
524
480
0.02
0.17
0.23
0.03
234
191
134
87
effects, so the photophysical characteristics of each mole-
cule were evaluated in four solvents sampling differing po-
larities (CH CN, CH Cl , THF, and toluene).
X = I
2
3
2
2
CH
3
Ph
CN
Cl
Table 2. Selected photophysical properties of asymmetrically sub-
stituted fluorenylene-bearing molecules 2a–g and 5a.
4d
X = H
CH
CH
THF
3
2
382
390
388
390
622
588
634
479
0.01
0.14
0.29
0.04
240
198
246
89
2
Solvent
λ
max
λ
[nm]
em
Φ (Ϯ0.02) Stokes’ shift
CH
3
Ph
CN
Cl
[
nm]
[nm]
4
e
CH
CH
THF
3
2
388
394
392
396
638
611
541
491
0.01
0.05
0.34
0.35
250
217
149
95
5
a
CH
CH
THF
3
2
CN
Cl
391
393
391
380
591
564
500
465
0.06[a]
0.24
200
171
109
85
X = CH
3
2
2
0.22
CH
3
Ph
CN
Cl
0.06[
a]
CH
3
Ph
CN
Cl
4
f
CH
CH
THF
3
2
388
394
392
388
467
454
467/564
500
0.03
0.03
0.25
0.18
79
60
2
a
CH
CH
THF
3
370
376
374
388
607
577
515
487
0.11
0.27
0.35
0.11
237
201
141
99
X = OCH
3
2
X = CF
3
2
2
Ph[a]
CH
3
112
CH
3
Ph
CN
Cl
4
g
CH
CH
THF
3
CN
396
402
400
404
450
604
582
515
0.00
0.00
0.12
0.53
54
2
b
CH
CH
THF
3
2
369
373
377
388
610
590
532
491
0.04
0.20
0.38
0.16
241
217
155
103
X = OiPr
2
Cl
2
202
182
111
X = Cl
2
CH
3
Ph
CN
Cl
CH
3
Ph
CN
Cl
4
X =
N(CH )
3 2
h
CH
CH
3
2
392
460
438
588
0.03
0.01
46
128
2
c
CH
CH
THF
3
381
381
379
376
615
598
533
489
0.03
0.21
0.25
0.12
234
217
154
113
2
X = I
2
2
THF
CH Ph
426
430
541
594
0.02
0.20
115
164
CH
3
Ph
CN
Cl
3
2
d
CH
CH
THF
3
376
385
376
373
606
594
526
486
0.02
0.16
0.21
0.12
230
209
145
113
[a] Poor solubility of the molecule in solvent.
X = H
2
2
CH
3
Ph
CN
Cl
by only about 5–12 nm regardless of substituent X. The
most strongly electron-donating group [X = OCH(CH ) ]
caused the largest shift in λmax [e.g., in THF, λmax undergoes
a progressive shift from 376 to 388 nm when X changes
2
e
CH
CH
THF
3
2
378
385
382
390
470
469
543
499
0.05
0.15
0.43
0.54
92
84
161
109
3
2
X = CH
3
2
CH
3
Ph
CN
Cl
from H to OCH(CH ) ]. In contrast, for 4a–h, a stronger
2
X =
f
CH
CH
3
2
373
381
459
466
0.04
0.05
86
85
3 2
trend emerged in the λmax values. Regardless of solvent,
when electron-donor strength increased, the λmax values
were redshifted compared with 4d, in which X = H. Specifi-
cally, in CH Cl , λ = 394, 394, 402, and 460 nm for X =
CH , OCH , OCH(CH ) , and N(CH ) , respectively. This
2
OCH
3
THF
379
394
568
530
0.18
0.47
189
136
CH
3
Ph[a]
2
2
max
2
g
CH
3
2
CN
388
392
388
391
474
586
574
517
0.01
0.01
0.21
0.41
86
3
3
3 2
3 2
X = OiPr CH
Cl
2
194
186
126
would be the expected trend based on the contributions of
the electron-rich donor groups.
THF
CH Ph
3
When comparing chromophores in series 2 to the analo-
gously substituted members of series 4, it was found that
[a] Poor solubility of the molecule in solvent.
For 2a–g, regardless of solvent, no observable trend was the λmax for molecules 2a–g were blueshifted compared with
seen in λmax when the X group was varied between donor those of 4a–g, which is attributable to twisting of adjacent
and acceptors; the change in λmax from 2d (X = H), varied π-conjugated segments of the chromophore in the fluoren-
Eur. J. Org. Chem. 2014, 5998–6009
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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6003

A. G. Tennyson, R. C. Smith et al.
FULL PAPER
ylene derivatives out of coplanarity due to the steric encum- DFT Calculations and Electrochemistry
brance of dihexyl substitution at C-9.
The ground state HOMO and LUMO distributions are
Based on previous work[ in which it was observed that
the photoluminescence maxima (λem) of symmetrically sub-
stituted fluorenylene and phenylene chromophores varied
in response to changes in solvent polarity, it was of interest
to examine whether similar solvatochromic effects were ex-
hibited by series 2 and 4. Indeed, a significant solvatoch-
romic effect is evident visually when cuvettes of the com-
pounds in different solvents are observed under illumina-
tion from a handheld UV light (λexcit = 365 nm; Figure 4).
23]
shown in Figure 5 and Figure 6, respectively, for 2/4a–g cal-
culated by density functional theory at the B3LYP-6-31G*
level. It can be seen that, generally, the localization for both
series is on the DSB unit, although for series 2, the HOMO
is spread out onto the fluorenylene unit. As the X group
increases in donor strength, the localization of the HOMO
shifts more to the side of the DSB backbone to which the
X group is attached and away from the second aryl ring of
the fluorenylene unit. On the other hand, the LUMO is
localized on the side of the chromophore with the nitro
Figure 4. A significant solvatochromic effect in the photolumines-
cence of 4b in CH
3 2 2 3
CN, CH Cl , THF, and CH Ph (left to right)
under a handheld UV lamp (λexcit = 365 nm).
For molecules 2a–d, the consistent trend was that as the
solvent polarity increased, the λem became progressively
more redshifted (e.g., for 2a, λem = 487 nm in toluene and
6
07 nm in CH CN). This is due to the fact that in the ex-
3
cited state the dipole moment of the molecule increases,
therefore a solvent with a larger dipole better stabilizes the
excited state, leading to a lower energy emission (red-
[
36,37]
shift).
Interestingly, with the molecules substituted with donor
groups (i.e., 2e–g), the λem blueshifted as the solvent po-
larity increased from toluene to acetonitrile (e.g., for 2f, λem
=
530 and 474 nm in toluene and CH CN, respectively).
3
This is a phenomenon that is commonly termed “negative
[
38]
solvatochromism”.
Negative solvatochromism occurs
when charge is transferred back to the donor on the mole-
cule in the excited state, diminishing the dipole moment, in
[4,38]
contrast to the more typical case described above.
Molecules 4a–h exhibit similar behavior in λem to the
trend observed in series 2. For 4a–e, the molecules with X
=
acceptor groups, or inductive donating groups, the λem
increases as the solvent polarity increases; this again can be
explained by stabilization of the increased dipole moment
of the molecule in the excited state by the more polar sol-
vents. As with 2g and 2f [X = OCH , OCH(CH ) ], 4g–h
3
3 2
[
for 4h, X = N(CH ) ], exhibit behavior this is opposite to
3 2
the trend observed when X = acceptor groups. As solvent
polarity increases, the λem for these molecules shows a hyp-
sochromic shift (e.g., for 4f, λem = 500 and 467 nm for tolu-
ene and acetonitrile, respectively), which is again attribut-
able to charge in the molecule being transferred back to the
donor in the excited state.
Figure 5. HOMO plots obtained by DFT calculations (B3LYP-6-
3
1G*) for compounds 2a–g (left) and 4a–g (right). Hexyl groups
for series 2 were truncated to methyl groups to simplify calcula-
tions.
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2014, 5998–6009

Donor-Acceptor 1,4-Fluorenylene Chromophores
group, which is explained by the fact that the areas of occu- to –1.61 V) and the second 80 mV (–1.98 to –2.16 V). The
pation by the LUMO should have a greater ability to gain similar reduction potentials for both reduction processes
electrons. As the nitro group is the most electron-with- are consistent with a LUMO that is essentially equivalent
drawing unit, these calculations are consistent.
to a 4-nitrostyrylbenzene unit for all members of a series.
Table 4. Electrochemical properties of 2, 4, and 5.
E1/2 [V]
Eonset [V]
E1/2 [V]
Eonset [V]
2
a
–1.65,
irrev.
–1.58,
–2.11
4a
4b
4c
4d
4e
4f
–1.61,
–2.10
–1.54,
–1.98
X = CF
3
2b
–1.66,
–2.15
–1.57,
–2.10
–1.61,
–2.17
–1.54,
–2.09
X = Cl
2c
–1.65,
irrev.
–1.58,
–2.10
–1.67,
irrev.
–1.60,
–2.16
X = I
2d
–1.68,
–2.20
–1.61,
–2.03
–1.64,
–2.23
–1.57,
–2.07
X = H
2e
–1.63,
–2.20
–1.59,
–2.15
–1.63,
–2.18
–1.54,
–2.08
X = Me
2f
–1.63,
–2.14
–1.54,
–2.03
–
–
X = OMe
2
g
–1.66,
–2.21
–1.58,
–2.09
4g
–
–
X = OiPr
Conclusions
We have synthesized a new series of molecules incorpo-
rating a 1,4-fluorenylene scaffold with varying D–A charac-
ter. Additionally, new DSBs were synthesized that were
analogous to the fluorenylene molecules to study the effect
on synthesis and properties resulting from the larger π-con-
jugated system and asymmetry of the 9,9-dihexyl-1,4-fluor-
enylene unit in series 2 compared with the 1,4-phenylene
unit in 4. It was found that both the DSB derivatives and
analogous fluorenylene molecules exhibit solvatochromic
behavior in the excited state. Further investigation of these
molecules including electrochemical characterization and
experiments to examine NLO behavior could show other
interesting features of the 1,4-fluorenelyne scaffold mol-
ecules, making them useful for further applications.
Experimental Section
Reagents and General Methods: Reagents were obtained from
Aldrich Chemical Co., TCI America, Acros or Alfa Aesar and used
without further purification. Air-sensitive reactions were carried
out in solvents purified by passage through alumina columns under
Figure 6. LUMO plots obtained by DFT calculations (B3LYP-6-
3
1G*) for compounds 2a–g (left) and 4a–g (right). Hexyl groups for
a dry N
system. Air-sensitive operations were done in an MBraun dry box
or by using standard Schlenk line techniques under N . NMR spec-
tra were obtained with a Jeol 300 spectrometer operating at
00 MHz for protons, 75 MHz for carbon, and 121.47 MHz for
phosphorus. All spectra were collected at 25 °C and referenced to
2
atmosphere employing an MBraun solvent purification
series 2 were truncated to methyl groups to simplify calculations.
2
The calculations are supported by electrochemical analy-
ses (Table 4). Cyclic voltammetry of 2 and 4 revealed highly
conserved redox processes among all functional groups for
3
both scaffolds. All variants of 2 and 4 exhibited quasirevers- residual solvent signals. Coupling constants are reported in Hz.
ible first reduction peaks that fell within a narrow range
Electrochemistry: All electrochemical experiments were performed
with a CH Instruments Electrochemical Workstation 660D using
an airtight three-electrode cell under a nitrogen atmosphere. A gold
(–1.63 to –1.68 V), and all but 2a, 2c and 4c had quasi-
reversible second reduction peaks ranging from –2.10 to
–2.23 V. The reduction onset potentials for 2 and 4 show working electrode and tungsten wire counter electrode were used,
similar behavior, where the first onset spans 70 mV (–1.54 along with a silver wire quasi-reference electrode. Measurements
Eur. J. Org. Chem. 2014, 5998–6009
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
6005

A. G. Tennyson, R. C. Smith et al.
FULL PAPER
were made using anhydrous CH
2
Cl
2
with 0.1 m [nBu
4
N][PF
6
] as
(d, J = 14.4 Hz, 2 H), 6.90 (t, J = 6.5 Hz, 1 H), 7.06–7.34 (m, signal
supporting electrolyte. Ferrocene (Fc) was chosen as the internal
overlaps solvent signal; spectrum indicates 5 H), 7.53–7.86 (m, 15
–
1
31
standard. All reported potentials were collected with a 100 mVs
H) ppm. P NMR (CDCl
3
, 121.5 MHz): δ = 20.97 ppm. HRMS:
+
/0
51BrP [M]+ 701.2912; found 701.2912.
scan rate and were referenced to Fc by shifting [Fc] to 0 V.
m/z calcd. for C45H
General Spectroscopic Methods: Photoluminescence (PL) spectra
were acquired with a Varian Cary Eclipse fluorescence spectropho-
tometer. Absorption spectra were recorded with a Varian Cary 50
Bio absorption spectrophotometer. Samples for all absorbance and
PL spectra were prepared in Spectrosil quartz cuvettes having a
path length of 1 cm. The solvents for all optical measurements were
purified and made anhydrous/anaerobic by passage through alu-
Synthesis of M6: To M5 (0.300 g, 0.383 mmol), triethylphosphite
(0.284 g, 1.71 mmol) was added. The reaction was heated to reflux
under nitrogen for 3 h at 110 °C, then the excess triethylphosphite
was removed by vacuum distillation to give an off-white crystalline
1
solid (0.304 g, 94.4%); m.p. 58–69 °C.
H NMR (CDCl3,
300 MHz): δ = 0.19–0.51 (m, 3 H), 0.71–0.80 (m, 6 H), 1.05 (br. s,
14 H), 1.24–1.39 (m, 5 H), 1.87–2.20 (m, 4 H), 3.34 (d, J = 22 Hz,
2 H), 4.02–4.13 (m, 4 H), 5.92 (d, J = 15 Hz, 2 H), 6.86–6.91 (m,
1 H), 7.08–7.20 (m, 2 H), 7.28–7.34 (m, partially overlaps solvent
signal; spectrum indicates 3 H), 7.52–7.71 (m, 9 H), 7.77–7.84 (m,
2
mina columns under a N atmosphere employing an MBraun sol-
vent purification system. Photoluminescence quantum yields were
measured relative to quinine bisulfate (Φ = 0.564) in 1 n aqueous
[
39]
31
sulfuric acid.
6 H) ppm. P NMR (CDCl , 121.5 MHz): δ = 20.82, 27.18 ppm.
3
+
HRMS: m/z calcd. for C49
H
61
O
3
P
2
[M] 759.4096; found 759.4084.
Synthesis of M2: To a stirring solution of M1 (10.0 g, 37.9 mmol)
in toluene (100 mL), triphenylphosphine (8.95 g, 34.1 mmol) was
added. The reaction mixture was heated to reflux under nitrogen
for 1 h and then cooled to room temperature. A precipitate formed
during the reaction, which was collected by vacuum filtration. The
solid was then washed with toluene (50 mL), followed by diethyl
ether (100 mL). The resulting white solid was used without any
further purification. Spectroscopic properties match those pre-
viously reported.
Synthesis of M7: In a dry box, M6 (0.810, 0.964 mmol) and 4-
nitrobenzaldehyde (0.0541 g, 0.964 mmol) were dissolved in THF
(
(
125 mL) in a pressure flask. A solution of potassium tert-butoxide
0.180 g, 0.964 mmol) in THF (20 mL) was added dropwise to the
reaction mixture. The reaction mixture turned a turbid gold-green
upon complete addition of base. The reaction mixture was sealed
with a Teflon® screw cap and removed from the dry box. The solu-
tion was stirred at room temperature for 2 h, then heated in an oil
bath at 65 °C for 17 h. The reaction mixture was then cooled to
room temperature and methanol (50 mL) was added to the mixture.
The solvents were removed under reduced pressure, yielding a yel-
low oil. The crude product was further purified by column
Synthesis of M3: To triphenyl(4-bromomethylbenzyl)phosphonium
bromide (M2; 1.00 g, 2.24 mmol), triethylphosphite (1.86 g,
1
1.2 mmol) was added. The reaction mixture was heated to reflux
at 110 °C under N for 3 h. The excess triethylphosphite was then
removed by vacuum distillation at 115 °C for 5 h. A white crystal-
2
2 2
chromatography on silica, eluting with CH Cl with 1% methanol
1
(R = 0.46). It was found that to get full separation from other
line powder (1.03 g, 78.6%) was collected; m.p. 201–204 °C.
NMR (CDCl , 300 MHz): δ = 1.21 (t, J = 7.2 Hz, 6 H), 3.05 (d, J
20.2 Hz, 2 H), 3.93–4.03 (m, 4 H), 7.06 (s, 4 H), 7.58–7.78 (m,
H
f
impurities, a column with a diameter of 2 inches and 8 inches of
silica were needed. Other columns tried did not provide sufficient
separation of impurities from target compound. After column
3
=
1
C
6
5 H) ppm. 3 P NMR (CDCl
33BrO (583.44): calcd. C 61.76, H 5.70, N 0.00; found C
1.68, H 5.87, N 0.00.
1
3
, 121.5 MHz): δ = 23.61, 26.36 ppm.
chromatography, the compound was collected as a bright-yellow oil
30
H
3 2
P
1
(
0.412 g, 67.7%). H NMR (CDCl
3
, 300 MHz): δ = 0.37–0.58 (m,
4
1
H), 0.71–0.79 (m, 6 H), 1.01–1.14 (m, 12 H), 1.25–1.32 (m, 6 H),
.99–2.28 (m, 4 H), 3.39–3.48 (m, 2 H), 4.00–4.19 (m, 4 H), 6.84
Synthesis of M4: To a solution of M3 (0.100 g, 0.171 mmol) in
ethanol (15 mL), a solution of 4-nitrobenzaldehyde (0.0300 g,
(
d, J = 12 Hz, 1 H), 6.94 (d, J = 7.9 Hz, 1 H), 7.11 (d, J = 16 Hz,
0.198 mmol) in THF (2 mL) was added. The reaction mixture was
1
2
8
H), 7.18 (d, J = 8.9 Hz, 2 H), 7.29–7.36 (m, 5 H), 7.43–7.48 (m,
placed under N and stirred for 10 min. To this solution, potassium
2
H), 7.60–7.64 (m, 1 H),7.70–7.77 (m, 2 H), 7.87–7.92 (m, 2 H),
tert-butoxide (0.0192 g, 0.171 mmol) in ethanol (2 mL) was added.
The colorless solution turned bright-yellow once the base was
3
1
.04 (d, J = 18 Hz, 1 H), 8.28 (d, J = 8.9 Hz, 2 H) ppm. P NMR
, 121.5 MHz): δ = 27.72 ppm. HRMS: m/z calcd. for
P [M]+ 631.3426; found 631.3421.
(CDCl
3
added. The reaction was stirred under N
.5 h, then H O (5 mL) was added to quench the base. The solvents
were then removed by rotary evaporation. The crude product was
further purified by preparative TLC using CH Cl with 2% meth-
anol as the eluent (R = 0.46). This afforded a yellow oil (0.0525 g,
1.6%) that was isolable as a mixture of E and Z isomers, which
2
at room temperature for
C
38 5
H50NO
2
2
General Method for Synthesis of Asymmetric Fluorenylene Mol-
ecules: For each molecule, 1 and the corresponding aldehyde were
combined in anhydrous THF in a dry box. To this reaction mixture,
n-butyllithium was added and the reaction was stirred under nitro-
gen for several hours. Purification techniques involved preparative
TLC or passing through silica. Due to the light-sensitivity of the
series 2 molecules that leads to rapid interconversion between E
2
2
f
8
was observed to rapidly interconvert on exposure to light or heat.
Due to the low stability of the product to isomerization, it was
employed as a mixture for the preparation of compounds 4 without
additional purification; m.p. 99–111 °C. HRMS: m/z calcd. for
1
3
1
and Z isomers, C NMR spectroscopic data is not reported; for H
NMR analysis, integrations in the aromatic region are not always
reported as whole integers, because it was difficult to differentiate
peaks resulting from either E or Z isomers.
19 5
C H22NO
P [M]+ 375.1236; found 375.1232.
Synthesis of M5: Triphenylphosphine (0.475 g, 1.81 mmol) was
added to M4 (1.05 g, 2.01 mmol) in toluene (11 mL). The reaction
mixture was stirred under nitrogen for 1 h and then heated to reflux
for 1 h. After allowing the reaction mixture to cool, the solution
was filtered through a fritted funnel. The collected solid was
3
Synthesis of 2a (X = CF ): In a dry box, to a solution of 1 (0.100 g,
0.158 mmol) in THF (10 mL), 4-(trifluoromethyl)benzaldehyde
(0.0300 g, 0.174 mmol) was added in a pressure tube. The reaction
mixture was stirred for 5 min, then n-butyllithium (2.5 m in hex-
anes, 0.070 mL, 0.174 mmol) was added. The solution became
dark-red when the n-butyllithium was added dropwise. The reac-
2
washed with toluene (30 mL), followed by Et O (75 mL). The solid
was collected and dried, yielding a white powder (1.15 g, 73.0%);
1
3
m.p. 202.8–205.8 °C. H NMR (CDCl , 300 MHz): δ = 0.18–0.47
(
m, 4 H), 0.77 (t, J = 7 Hz, 6 H), 1.05–1.15 (m, 12 H), 1.93 (td, J tion mixture was capped with a Teflon® screw cap and stirred in
=
4, 13 Hz, 2 H), 2.19 (td, J = 4, 13 Hz, 2 H), 4.66 (s, 2 H), 6.02
the dry box for 23 h. H
2
O (15 mL) was added, the compound was
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Donor-Acceptor 1,4-Fluorenylene Chromophores
extracted with CH
with H O (3ϫ 50 mL). The solvents were removed by rotary evapo-
ration and the compound was further purified by passage through
a silica plug (hexanes/CH Cl , 1:1). This afforded 2a (0.023 g, 22%)
as a yellow oil. H NMR (CDCl , 300 MHz): δ = 0.41–0.76 (m, 9
H), 0.89–1.06 (m, 13 H), 2.08 (td, J = 4.4, 12 Hz, 2 H), 2.32 (td, J
4.5, 12 Hz, 2 H), 7.06–7.19 (m, 1.75 H), 7.33–7.48 (m, 4.5 H),
2
Cl
2
(30 mL), and the organic layer was washed
resulting oil was reddish-brown. The compound was further puri-
fied by preparative TLC, using CH Cl as eluent (R = 0.92), af-
fording 2d (0.033 g, 35%) as a reddish-brown oil. H NMR
(CDCl , 300 MHz): δ = 0.39–0.74 (m, 9 H), 0.80–1.08 (m, 13 H),
2.07 (td, J = 4.4, 12 Hz, 2 H), 2.28–2.41 (m, 2 H), 6.54 (d, J =
12 Hz, 0.3 H), 6.62 (d, J = 12 Hz, 0.3 H), 6.78–6.82 (m, 0.2 H),
6.86 (d, J = 12 Hz, 0.1 H), 6.95–7.21 (m, 1.9 H), 7.24–7.96 (m, 13.7
2
2
2
f
1
2
2
3
1
3
=
7
1
.54–7.75 (m, 10.75 H), 7.80–7.83 (m, 1.25 H), 7.94 (d, J = 8.6 Hz, H), 8.00–8.03 (m, 0.2 H), 8.11 (d, J = 16.1 Hz, 0.3 H), 8.22 (d, J =
.38 H), 8.10 (d, J = 16.1 Hz, 0.8 H), 8.30 (d, J = 8.6 Hz, 1.75 8.9 Hz, 0.1 H), 8.29 (d, J = 8.9 Hz, 0.8 H), 8.34–8.43 (m, 0.2) ppm.
+
+
H) ppm. HRMS: m/z calcd. for C42
H44NO
2
F
3
[M] 651.3324;
HRMS: m/z calcd. for C41
2
H45NO [M] 583.3450; found 583.3443.
found 651.3316.
Synthesis of 2e (X = CH ): In a dry box, 1 (0.100 g, 0.158 mmol)
3
Synthesis of 2b (X = Cl): In a pressure tube in a dry box, 1 (0.100 g,
.158 mmol) and 4-chlorobenzaldehyde (0.0240 g, 0.174 mmol)
and p-tolualdehyde (0.0210 g, 0.174 mmol) were combined in THF
(20 mL) in a pressure tube, and n-butyllithium (2.5 m in hexanes,
0
were combined in THF (20 mL), and n-butyllithium (2.5 m in hex- 0.070 mL, 0.174 mmol) was added (the solution became black).
anes, 0.070 mL, 0.174 mmol) was added. The solution turned from
yellow to dark-brown/black when the base was added. The reaction
mixture was sealed with a Teflon® screw cap and stirred under
The reaction mixture was sealed with a Teflon® screw cap and
stirred under N for 71 h. H O (10 mL) was added to the solution
and the compound was extracted with CH Cl (2ϫ 50 mL). The
organic layer was washed with H O (2ϫ 100 mL), the organic layer
was collected, and all solvents were removed by rotary evaporation.
The compound was further purified by dissolving in a solution of
2
2
2
2
nitrogen for 72 h. H
which appeared to turn cloudy, but no precipitate formed, so the
compound was extracted with CH Cl (50 mL) and washed with
O (3ϫ 50 mL). The organic layer was collected and all solvents
were removed by rotary evaporation. The compound was further
purified by dissolving in a solution of CH Cl /hexanes (1:1) and
2
O (15 mL) was then added to the solution,
2
2
2
H
2
CH
2 2
Cl /hexanes (1:1) and passing through silica. The solution was
collected and solvents were removed by rotary evaporation to yield
1
2
2
3
2e (0.034 g, 36%). H NMR (CDCl , 300 MHz): δ = 0.40–0.75 (m,
passing through silica. The silica was washed several times with
this eluent, which was then collected. After removing the solvents
by rotary evaporation, 2b (0.021 g, 22%) was obtained. H NMR
10 H), 0.89–1.09 (m, 12 H), 2.07 (td, J = 3.8, 12 Hz, 2 H), 2.28–
2.40 (m, 5 H), 6.85 (d, J = 12 Hz, 0.15 H), 6.98–7.10 (m, 0.6 H),
7.15 (d, J = 16.1 Hz, 0.6 H), 7.22–7.68 (m, 9.8 H), 7.73 (d, J =
1
(
2
6
CDCl
3
, 300 MHz): δ = 0.40–0.76 (m, 9 H), 0.98–1.08 (m, 13 H), 8.9 Hz, 1.2 H), 7.78–7.82 (m, 0.6 H), 7.91–7.95 (m, 0.5 H), 8.11 (d,
.07 (td, J = 4.4, 12 Hz, 2 H), 2.32 (td, J = 4.4, 12 Hz, 2 H), 6.84– J = 15.8 Hz, 0.6 H), 8.29 (d, J = 8.6 Hz, 1.4 H) ppm. HRMS: m/z
.93 (m, 0.15 H), 6.98–7.03 (m, 0.90 H), 7.15 (d, J = 16.1 Hz, 0.60 calcd. for C42
+
H47NO
2
[M] 597.3607; found 597.3600.
H), 7.23–7.49 (m, 11.8 H), 7.54–7.66 (m, 1.9 H), 7.73 (d, J = 8.6 Hz,
Synthesis of 2f (X = OCH ): In a dry box, 1 (0.132 g, 0.210 mmol)
3
1.1 H), 7.79–7.82 (m, 0.54 H), 7.91–7.95 (m, 0.41 H), 8.09 (d, J =
was dissolved in anhydrous THF (25 mL) and 4-methoxybenzalde-
hyde (0.0314 g, 0.231 mmol) was added. The reaction mixture was
stirred for several minutes, then n-butyllithium (2.5 m in hexanes,
0.092 mL) was slowly added dropwise. The golden-yellow solution
slowly became dark-brown/black as the base was added. The reac-
tion mixture was stirred at room temperature for 5 days, then re-
1
6.1 Hz, 0.63 H), 8.29 (d, J = 8.6 Hz, 1.3 H) ppm. HRMS: m/z
+
calcd. for C41
H44NO
2
Cl [M] 617.3061; found 617.3057.
Synthesis of 2c (X = I): In a dry box, 1 (0.100 g, 0.158 mmol) and
-iodobenzaldehyde (0.0400 g, 0.174 mmol) were combined in THF
20 mL) in a pressure tube, and n-butyllithium (2.5 m in hexanes,
.070 mL, 0.174 mmol) was added. The solution turned from yel-
4
(
0
moved from the dry box. H
became orange and cloudy. The organic products were extracted
with CH Cl (75 mL) and the organic layer was washed with H
2
O (30 mL) was added and the mixture
low to golden-brown when the base was added. The reaction mix-
ture was sealed with a Teflon® screw cap and stirred under nitrogen
2
2
2
O
for 72 h. H
2
O (15 mL) was added and the compound was extracted
(60 mL) and washed with H O (3ϫ 50 mL). The or-
(3ϫ 100 mL). The organic layers were collected and solvent was
removed by rotary evaporation. The resulting reddish-brown oil
was washed with methanol (ca. 5 mL), which helped to remove
some of the excess aldehyde. The oil was then dissolved in minimal
with CH Cl
2
2
2
ganic layer was collected and all solvents were removed by rotary
evaporation. The compound was further purified by dissolving in
a solution of CH
silica was washed several times with this solution mixture. After
removing the solvents by rotary evaporation, 2c (0.027 g, 24%) was
obtained as a viscous oil. H NMR (CDCl
2
Cl
2
/hexanes (1:1) and passing through silica. The
CH
the sides of the glass of an Erlenmeyer flask, so the methanol was
decanted away. The oil was dissolved in CH Cl and recollected.
The methanol wash was also collected and the solvent was removed
by rotary evaporation. The resulting oil was then washed again
2 2
Cl and added dropwise to methanol. The product stuck to
2
2
1
3
, 300 MHz): δ = 0.39–
0.59 (m, 3 H), 0.67–0.74 (m, 6 H), 0.89–1.08 (m, 13 H), 2.06 (td, J
=
4.4, 12 Hz, 2 H), 2.31 (td, J = 4.4, 12 Hz, 2 H), 6.60 (d, J = with methanol (3ϫ 10 mL) and was then recollected. It was deter-
1
(
1.6 Hz, 0.06 H), 6.84–6.91 (m, 0.15 H), 6.95–7.00 (m, 0.1 H), 7.15 mined by NMR spectroscopy that both collected oils were the de-
d, J = 16.1 Hz, 0.63 H), 7.23–7.42 (m, 9.6 H), 7.48–7.62 (m, 1.5 sired product. All product was collected to yield 2f (0.0548 g,
H), 7.66 (d, J = 16.1 Hz, 0.75 H), 7.72–7.81 (m, 3.8 H), 7.91–7.95 42.6%) as a reddish brown sticky oil; the compound was isolated
m, 0.25 H), 8.09 (d, J = 16.1 Hz, 0.75 H), 8.29 (d, J = 8.9 Hz, 1.5 as a mixture of E and Z isomers that interconvert rapidly upon
(
+
1
H) ppm. HRMS: m/z calcd. for C41
09.2409.
H44NO
2
I [M] 709.2417; found
3
exposure to light. H NMR (CDCl , 300 MHz): δ = 6.95–7.00 (m,
7
4 H), 7.09 (d, J = 18.8 Hz, 1 H), 7.29–7.58 (m, 13.25 H), 7.72 (d,
J = 8.9 Hz, 2.50 H), 7.80–7.83 (m, 0.5 H), 8.11 (d, J = 16.1 Hz,
Synthesis of 2d (X = H): In a pressure tube, 1 (0.100 g, 0.158 mmol)
and benzaldehyde (0.0185 g, 0.174 mmol) were combined in THF
0.5 H), 8.29 (d, J = 8.6 Hz, 2.75 H) ppm. Due to the presence of
both E and Z isomers, NMR integrations in the aromatic region
(15 mL) in a dry box, and n-butyllithium (2.5 m in hexanes,
are not reported as whole integers relative to the alkyl-region peaks.
0.070 mL, 0.174 mmol) was added (the reaction mixture turned
+
HRMS: m/z calcd. for C42
H47NO
3
[M] 613.3556; found 613.3563.
®
dark-brown). The pressure tube was sealed with a Teflon screw
cap and stirred for 24 h. H O (5 mL) was added to the reaction
mixture and all solvents were removed by rotary evaporation. The
2
Synthesis of 2g [Y = OCH(CH
3
)
2
]: In a pressure tube, 1 (0.100 g,
0.158 mmol) and 4-isopropoxybenzaldehyde (0.0286 g, 0.174 mmol)
Eur. J. Org. Chem. 2014, 5998–6009
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
6007

A. G. Tennyson, R. C. Smith et al.
FULL PAPER
were combined in THF (15 mL) in a dry box. To this solution, n-
butyllithium (2.5 m in hexanes, 0.070 mL, 0.174 mmol) was added
and the reaction mixture turned turbid green-brown. The pressure
mixture. After shaking vigorously, the red-brown solution turned
pale-yellow. The compound was extracted with CHCl
with H O (3ϫ 50 mL). The organic layers were collected and the
solvent was removed under reduced pressure to yield 4b (0.066 g,
3
and washed
2
tube was sealed with a Teflon® screw cap and stirred for 24 h. H
O
2
1
(
15 mL) was then added to the reaction mixture and all solvents
were removed by rotary evaporation. The compound was further
purified by preparative TLC, using CH Cl as the eluent (R
.92), affording 2g (0.035 g, 35%) as a yellow-brown oil. H NMR
CDCl
3
70%) as a yellow solid; m.p. 241–244 °C. H NMR (CDCl ,
300 MHz): δ = 7.09–7.26 (m, includes overlap from solvent signal,
2
2
f
=
structure implies 4 H), 7.30–7.47 (m, 4 H), 7.54 (s, 4 H), 7.64 (d, J
1
13
0
(
1
3
= 8.9 Hz, 2 H), 8.23 (d, J = 8.6 Hz, 2 H) ppm. C NMR (CDCl ,
3
, 300 MHz): δ = 0.39–0.77 (m, 9 H), 0.85–1.08 (m, 13 H), 75 MHz): δ = 124.30, 126.30, 126.94, 127.13, 127.55, 127.83,
.37 (d, J = 5.9 Hz, 6 H), 2.06 (td, J = 4.4, 12 Hz, 2 H), 2.37 (td, 128.14, 128.62, 129.03, 132.86, 133.55, 135.70, 135.77, 137.66,
143.91, 146.83 ppm. HRMS: m/z calcd. for C22 Cl [M +
J = 4.4, 12 Hz, 2 H), 4.57–4.65 (m. 1 H), 6.92–7.04 (m, 3.5 H), 7.14
d, J = 15.8 Hz, 1 H), 7.35–7.42 (m, 3.5 H), 7.47–7.58 (m, 6 H), 1] 362.0948; found 362.0952.
H17NO
2
+
(
7
2
6
.79–7.82 (m, 1 H), 8.11 (d, J = 16.1 Hz, 1 H), 8.29 (d, J = 8.9 Hz,
H) ppm. HRMS: m/z calcd. for C44H51NO [M] 641.3869; found
3
41.3860.
Synthesis of 4c (Y = I): To a solution of 3 (0.100 g, 0.172 mmol)
in ethanol (15 mL), 4-iodobenzaldehyde (0.0440 g, 0.190 mmol) in
THF (3 mL) was added. The mixture was stirred for 5 min under
+
N
2
and then KOtBu (0.039 g, 0.345 mmol) was added. After adding
the base, the reaction mixture turned a murky brown. This reaction
mixture was then stirred under N for 21.5 h, then H O (20 mL)
was added and a precipitate formed. The precipitate was collected
by vacuum filtration and washed with additional H O (20 mL).
General Method for Synthesis of Asymmetric Phenylene Molecules:
For each molecule, 3 was dissolved in ethanol and stirred under
nitrogen. To this solution, the corresponding benzaldehyde was
added, then KOtBu was added and the reaction mixture was stirred
over a period of several hours. The reaction was quenched with
2
2
2
The crude material was dissolved in toluene (20 mL) and a catalytic
amount of iodine was added. The reaction was heated to reflux
under N for 4 h. After cooling to room temperature, a precipitate
2
H
2
O. Workup involved collection by filtration or extraction with
CHCl . The E,E-isomers of the molecules were obtained by heating
to reflux in toluene with a catalytic amount of iodine, then collect-
3
formed that was collected by vacuum filtration. This afforded the
[35]
[40]
[41]
ing by filtration or extraction. Compounds 4d, 4e, 4f, and
pure compound (0.051 g, 65%) as a yellow solid; m.p. 314–317 °C.
[
41]
4h
have been reported previously and their preparative pro-
1
8
H NMR ([D ]THF, 300 MHz): δ = 7.13–7.21 (m, 2 H), 7.26–7.34
cedures are not repeated here.
(
m, 3 H), 7.42 (d, J = 16.5 Hz, 1 H), 7.55–7.62 (m, 4 H), 7.68 (d,
3
Synthesis of 4a (X = CF ): To a solution of 3 (0.100 g, 0.172 mmol)
J = 8.6 Hz, 2 H), 7.76 (d, J = 8.9 Hz, 2 H), 8.20 (d, J = 8.6 Hz, 2
1
3
in ethanol (15 mL), 4-(trifluoromethyl)benzaldehyde (0.0330 g,
.190 mmol) in THF (3 mL) was added. The mixture was stirred be obtained. HRMS: m/z calcd. for C22
for 5 min under N , then KOtBu (0.039 g, 0.345 mmol) was added.
found 453.0220.
When the base was added, the bright-yellow solution became am-
Synthesis of 4g [Y = OCH(CH ) ]: To a solution of 3 (0.100 g,
ber-brown. The reaction mixture was then stirred under N for
(2ϫ
0 mL) followed by vacuum evaporation of solvents afforded the
H) ppm. Due to poor solubility, reasonable C NMR could not
H16NO
2
I [M]+ 453.0226;
0
2
3
2
2
0.172 mmol) in ethanol (15 mL), 4-isopropoxybenzaldehyde
(0.0312 g, 0.190 mmol) in THF (2 mL) was added. This reaction
mixture was stirred and then KOtBu (0.0390 g, 0.345 mmol) was
added. When the base was added, the solution turned a cloudy
brown-green. The reaction was stirred under N for 6 h, then H O
2
5
2 2 2
1.5 h, then H O (20 mL) was added. Extraction with CH Cl
crude product, which was a mixture of E and Z isomers, as indi-
cated by NMR spectroscopy. The crude material was dissolved in
toluene (20 mL) and a catalytic amount of iodine was added. The
2
2
(15 mL) was added and a yellow precipitate formed in the flask.
The solid was collected by vacuum filtration. To obtain all E iso-
mer, the compound was then added to toluene with a catalytic
amount of iodine. This reaction mixture was heated to reflux under
reaction was heated to reflux under N
room temperature, aq. Na SO (10 mL) was added to reduce the
iodine. The product was extracted with CHCl and the organic lay-
ers were washed with H O (3ϫ 50 mL). The organic layer was col-
lected and solvents were removed by rotary evaporation, yielding
a (0.056 g, 82%) as a yellow solid; m.p. 196–199 °C. 1H NMR
CDCl , 300 MHz): δ = 7.15–7.20 (m, 2 H), 7.23–7.31 (m, includes
overlap from solvent signal, structure implies 3 H), 7.43–7.49 (m,
2
for 4 h. After cooling to
2
3
3
2
N for 15.5 h, and then it was cooled to room temperature. Once
2
cool, the compound precipitated out of the toluene. The solid was
4
again collected by vacuum filtration, yielding 4g (0.026 g, 39%) as
1
(
3
a yellow solid; m.p. 263–265 °C. H NMR (CDCl , 300 MHz): δ =
3
1.35 (d, J = 5.8 Hz, 6 H), 4.56–4.63 (m, 1 H), 6.89 (d, J = 8.6 Hz,
2
H), 7.52–7.57 (m, 3 H), 7.62–7.70 (m, 5 H), 8.23 (d, J = 8.6 Hz, 1 2 H), 6.97 (d, J = 16.1 Hz, 1 H), 7.09–7.17 (m, 2 H), 7.21–7.30 (m,
13
H) ppm. C NMR (CD
2
Cl
26.52, 126.75, 126.98, 127.31, 127.52, 128.50, 128.66, 130.51,
31.94, 132.07, 132.60, 136.29, 137.19, 143.86 ppm. HRMS: m/z
2
, 75 MHz): δ = 124.13, 125.65, 125.78, includes overlap from solvent signal, structure implies 2 H), 7.45
1
1
(d, J = 8.6 Hz, 2 H), 7.49–7.56 (m, 3 H), 7.64 (d, J = 8.9 Hz, 2 H),
1
3
8.22 (d, J = 8.6 Hz, 2 H) ppm. C NMR (CDCl , 75 MHz): δ =
3
+
calcd. for C23
H16NO
2
F
3
[M] 395.1133; found 395.1126.
22.15, 70.03, 116.10, 124.29, 125.76, 125.82, 126.78, 126.88, 127.51,
1
1
27.97, 129.16, 129.71, 133.07, 135.04, 138.49, 144.06, 146.72,
57.96 ppm. HRMS: m/z calcd. for C25H23NO [M] 385.1678;
3
Synthesis of 4b (Y = Cl): To a solution of 3 (0.150 g, 0.258 mmol)
in ethanol (20 mL), 4-chlorobenzaldehyde (0.0406 g, 0.289 mmol)
in THF (4 mL) was added, followed by potassium tert-butoxide
+
found 385.162.
Supporting Information (see footnote on the first page of this arti-
(
0.0580 g, 0.517 mmol). The solution turned green-brown when ad-
dition of the base was complete. The reaction mixture was stirred
under N for 17.5 h, then the solution was removed from the dry
box and H O (20 mL) was added. The mixture appeared to be a
1
31
13
cle): Copies of H, P, and C NMR spectra, NOE experiments,
UV/Vis absorption spectra and photoluminescence spectra in all
four solvents..
2
2
fine suspension, so it was centrifuged, then the solvents were de-
canted away from the fine solid. The solid was then dissolved in
toluene (10 mL) and a catalytic amount of iodine was added. The
reaction was heated to reflux under nitrogen for 16.5 h. After cool-
Acknowledgments
The authors would like to thank the US National Science Founda-
tion (NSF) (CAREER Award CHE-0847132 to R. C. S., and a
2 3
ing to room temperature, aq. Na SO (50 mL) was added to the
6008
www.eurjoc.org
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2014, 5998–6009

Donor-Acceptor 1,4-Fluorenylene Chromophores
Graduate Research Fellowship to B. J. L.), the Clemson University
Creative Inquiry Program, and the University Research Grants
Committee and Calhoun Honors College for their generous sup-
port. Samantha J. El Homsi is thanked for laboratory support.
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Received: March 3, 2014
Published Online: August 5, 2014
842.
Eur. J. Org. Chem. 2014, 5998–6009
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
6009