Donor-acceptor-substituted oligo(1,4-phenylene)s

Article abstract of DOI:10.1002/hlca.200800398

Oligo(para-phenylene)s (DAOPPs) 2a-2d (n = 1-4) with terminal donor-acceptor substitution (D = C₆H₁₃O, A = NO ₂) were prepared by applying Suzuki cross-couplings for chain extension and end capping. The push-pull effect in

Full text of DOI:10.1002/hlca.200800398

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Donor– Acceptor-Substituted Oligo(1,4-phenylene)s
by Soungkyoo Kim, Annette Oehlhof, Bernhard Beile, and Herbert Meier*
Institute of Organic Chemistry, University of Mainz, Duesbergweg 10 – 14, D-55099 Mainz
(phone: þ 49(0)61313922605; fax: þ 49(0)61313925396; e-mail: hmeier@mail.uni-mainz.de)
Oligo(para-phenylene)s (DAOPPs) 2a – 2d (n ¼ 1 – 4) with terminal donor– acceptor substitution
(D ¼ C6H13O, A ¼ NO2) were prepared by applying Suzuki cross-couplings for chain extension and end
capping. The push – pull effect induces short-reaching polarizations of the chain consisting of conjugated
but twisted benzene rings, which was studied by NMR measurements. Electron excitation from the
ground-state S0 to the more planar first-excited singlet state S1 is combined with a strong intramolecular
charge transfer (ICT), which is documented by the red shift of the long-wavelength absorption (charge-
transfer band) for short chains (one or two repeat units, n ¼ 1 or 2). The opposite influence of decreasing
ICT and increasing conjugation length leads to a bathochromic series (lmax(n þ 1) ꢀ lmax(n)) with a fast
saturation of lmax (n). The effective conjugation length nECL ¼ 4 corresponds to l1 349 nm. These results
are discussed in the context of other oligo(para-phenylene)s (OPPs).
Introduction. – Conjugated oligomers with terminal donor– acceptor substitution
(D – p – A) represent an interesting class of compounds since their unusual linear and
nonlinear optical and optoelectronic properties distinguish them as target compounds
in materials science [1].
The 1,4-phenylene (para-phenylene) building blocks are simple repeat units for
oligo- and poly(para-phenylene)s (OPPs and PPPs) 1 (Fig. 1), which belong to the
class of conjugated oligo- and poly(arylene)s [2 – 5]. Unsubstituted OPPs and PPPs 1
(R ¼ H) have a very low solubility and, therefore, a difficult process ability. Kern et al.
[6] and Heitz and Ullrich [7] were the first who improved the solubility by the
introduction of alkyl side chains. A drawback, caused by these side chains, is due to the
fact, that the conjugation of the main chain is impaired by such substituents. As already
realized in 1,1’-biphenyl, the OPPs and PPPs are nonplanar. The average torsion angle
q of 238 in the parent, unsubstituted chain is a compromise between the ideal
conjugation (q ¼ 08) and the lowest steric interaction of neighboring benzene rings (q ¼
908). From perturbation theory, the decrease of resonance energy is well described by
Fig. 1. Oligo- and poly(para-phenylene)s 1, the corresponding oligomers with terminal donor– acceptor
substitution 1’, and the special target systems 2 (D ¼ C6H13O, A ¼ NO2)
ꢀ 2009 Verlag Helvetica Chimica Acta AG, Zꢁrich

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cos2q for each torsion angle q [8]. An OPP chain of n benzene rings has therefore a
factor of (cos2q)nꢁ1. Thus, the average torsion angle q of 238 reduces the resonance
energy in a p-sexiphenyl segment (n ¼ 6) to 50%. Moreover, a q of 55.78 would
implicate a 90% decrease of the resonance energy already in a substituted p-terphenyl
(n ¼ 3).
Alkyl side chains can enhance the twist angle to 50 – 908, so that the conjugation
effect disappears more and more. When the substitution with alkyl chains is confined to
the terminal benzene rings [9] or to few selected benzene rings in the chain [10], the
solubilizing effect is limited and yet the conjugation effect impaired. Therefore, we
disclaimed alkyl side chains in our concept of DAOPPs (donor– acceptor OPPs) 1’ and
decided to use hexyloxy (C6H13O) as electron-donating (D) and solubilizing
substituents. Moreover, we assumed that the effective conjugation length of such
DAOPPs 2 with D ¼ C6H13O and A ¼ NO2 should be low, so that it is not necessary to
synthesize long conjugated chains with many repeat units.
Results and Discussion. – The 1-(hexyloxy)-4-nitrobenzene (2a) can be easily
prepared from 4-nitrophenol (3) and 1-bromohexane (4) [11]. The higher members
2b – 2d (n ¼ 2 – 4) of the DAOPP series were obtained by applying the Suzuki cross-
coupling method [12 – 17] (Scheme). Due to the better solubility of the donor part, we
Scheme. Preparation of the OPPs 2a – 2d
a) K2CO3, acetone or DMF. b) 1. BuLi, 2. H2O. c) K2CO3, [Pd(PPh3)4].

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performed the chain extension with OPPs 6, which contain the hexyloxy group at one
end and a bromo substituent at the other terminal position.
The starting compound 6a was obtained by alkylation of phenol 5 with 4. Lithiation
of 6 and reaction with fluorodimethoxyborane (7) [18] (see also [19]) yielded the
boronic acids 8 (48 – 75%). Regardless of their self-condensation, they were trans-
formed to the higher ꢂphenyloguesꢃ (6a ! 8a ! 6b ! 8b ! 6c ! 8c) by alternate
reactions with 1-bromo-4-iodobenzene (9) and fluorodimethoxyborane (7). Hensel
and co-workers found that the iodo side of 9 is much more reactive than the bromo side
[20]. Thus, a chemoselective coupling is possible under certain reaction conditions.
Therefore, a protection/deprotection technique could be avoided. The bromo
substituent can then be used subsequently for the transformation to the higher boronic
acid 8. The final step, the end capping with 1-bromo-4-nitrobenzene (10), furnished the
target DAOPPs 2b – 2d (n ¼ 2 – 4). The total yields amounted to 55% (5 ! 2b, three
steps), 17% (5 ! 2c, five steps), and 5% (5 ! 2d, seven steps). We stopped the chain
extension at the quaterphenyl 2d (n ¼ 4) because the effective conjugation length was
reached there, and the solubility became rather low. Further chain extensions (n ¼ 5,
6,...) are certainly feasible on this route when branched, that means better solubilizing
alkoxy groups are applied.
The purity of the intermediates 6a – 6c was checked by NMR spectroscopy. Their
1H- and 13C-NMR data are summarized in the Table. The target compounds 2a – 2d
were characterized by their UV, NMR, and MS data and by their elemental analyses
(Exper. Part).
Polarization of the conjugated chain by terminal donor– acceptor substitution is a
typical property of D – p – A systems. Such a push – pull effect is often described by
zwitterionic resonance structures – as shown in Fig. 2 for DAOPPs and the related
oligo(1,4-phenylenevinylene)s (DAOPVs). Apart from this characterization in the
valence-bond theory (VB), a quadrupolar description according to MO theory is often
more appropriate – in particular for longer chains. The donor group D enhances the
electron density at one end of the p chain, and the acceptor group A decreases the
electron density at the other end [1]. Strength of D and A and the nature of the p linker
decide how ꢂdeepꢃ the change of electron density ꢂgrabsꢃ into the chain.
Fig. 2. Zwitterionic resonance structures of donor– acceptor substituted oligo(1,4-phenylene)s (DAOPPs)
1’ and oligo(1,4-phenylenevinylene)s (DAOPVs) 11 in the VB theory and quadrupolar counterpart 12 for
both in MO theory

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Whereas OPVs are essentially planar (in a flat well of the potential energy related
to torsions), unsymmetrical OPPs 1’ with n ꢀ 2 repeat units exist in 2nꢁ1 nonplanar
conformations [21]. Torsions along the chain diminish certainly the mesomeric push –
pull effect and the extra resonance energy. Fig. 3 illustrates the 1H-NMR data of 2a – 2d
(n ¼ 1 – 4). Each benzene ring represents an AA’BB’ spin system with a certain Dd
difference. The Dd values are high for the ꢂouterꢃ benzene rings and very low for the
ꢂinnerꢃ benzene rings. On the whole, all chemical shifts d(H) increase with increasing n
values. This is, however, an anisotropy effect. The anisotropy cone of the aromatic
region increases with increasing number of rings.
Fig. 3. AA’BB’ Spin systems of the benzene rings in the 1H-NMR spectra of 2a – 2d (n ¼ 1 – 4).
Measurement in CDCl3, except 2d, which was measured in (D8)THF.
The superposition of anisotropy and electron-density effects plays a minor role in
13C-NMR spectroscopy, so that the d(C) values permit a reliable statement about
charge densities. Fig. 4 summarizes the d(C) values of 1,1’-biphenyl (13), the merely
acceptor- or donor-substituted model compounds 14 and 15, and the push – pull
compounds 2b and 2c. The d(C) values of 13 – 15 reveal that the electron-withdrawing
NO2 group and the electron releasing C6H13O group have a strong effect on their own
benzene ring, each, but only a minor effect on the next benzene ring. The polarization
of the p chain by the push – pull substitution is much higher in the DAOPV series 11
[22][23] than here in the DAOPP series 1’. The torsion of the benzene rings in 1’
reduces the charge transfer. Nevertheless, a certain push – pull effect is present in series
2. The ꢂinnerꢃ quaternary C-atoms of 2b and 2c have – compared to 13 (d(C) 141.1) –
alternately high and low d values, i.e., d(C) 147.2 and 130.7 in the case of 2b, and d(C)

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147.3, 136.8, 141.6, and 132.4 in the case of 2c. The same effect should be valid for 2d.
The solubility of 2d in CDCl3 is, however, too low to obtain reliable d values of the
quaternary C-atoms.
Fig. 4. 13C-NMR Chemical shifts d [ppm] of 1,1’-biphenyl (13), 4-nitro-1,1’-biphenyl (14), 4-(hexyloxy)-
1,1’-biphenyl (15), 4-(hexyloxy)-4’-nitro-1,1’-biphenyl (2b), and 4-(hexyloxy)-4’’-nitro-1,1’:4’,1’’-terphenyl
(2c). Measurement in CDCl3, with Me4Si as internal standard.
After the polarization of the DAOPPs in the ground-state S0, the charge
distribution in the more planar first-excited singlet state S1 shall be discussed here.
The band of the allowed long-wavelength electron excitation S0 ! S1 is called a charge-
transfer band because it is based on an intramolecular charge transfer (ICT). The shift
of electron density from the donor to the acceptor leads in the majority of D – p – A
compounds to very high dipole moments m (S1) [1]. The ICT lowers the transition
energy DE (S0 ! S1) by reducing the electron-correlation energy. However, when the
distance between D and A increases, the ICT decreases. Increasing length of the p
chain effects, on the other hand, a decreasing HOMO – LUMO gap. Consequently, the
increasing number n of repeat units has two opposite effects, and the crucial question is,
which of the two effects will prevail. Fig. 5 puts the lmax values, obtained for the series
2a – 2d (n ¼ 1 – 4) in the context of other OPPs. The parent OPP series 13a – 13f [24] as
well as series 16a – 16d [9] with tert-butyl groups at the terminal rings are ꢂnormalꢃ series
of conjugated oligomers. That means, their lmax values increase monotonously with

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increasing n values and approach to a limiting value l . The curves shown in Fig. 5
1
correspond to empirical exponential functions [26], which proved to be much better for
intra- and extrapolations of the transition energy DE than earlier used hyperbolic
functions (DE ¼ f(1/n)) [1][21][26] or trigonometric functions (DE ¼ f(cos p/n þ 1))
[27]. Recently Gierschner and co-workers [28] and Bednarz and co-workers [29]
Fig. 5. Long-wavelength absorption maxima of 2a – 2d in CHCl3 (.) in comparison to other OPP series:
13a – 13f in THF [24], 16a – 16d in THF [9], 17a – 17d in octane [20], and 18a – 18d in 1-methylpyrrolidin-
2-one [25]

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proposed semi- to nonempirical equations for the dependence of l or DE on the
number n of repeat units.
The effective conjugation length [30] of 13 and 16 amounts to nECL ¼ 9 and 11,
respectively. These numbers nECL, as well as l , characterize the saturation
1
phenomenon of conjugated oligomers. Series 17 [20] gave the remarkable result that
the lmax values do not depend on n, that means on the length of the chain. Two hexyl
groups at every second benzene ring are obviously sufficient for the total loss of
conjugation.
The push – pull series 2a – 2d is a bathochromic series: lmax 311, 344, 346, and 348 nm
(in CHCl3) for n ¼ 1, 2, 3, and 4, respectively, but the convergence limit (l ꢂ 1) nm [1]
1
is already reached at nECL ¼ 4 (Fig. 5). The S0 ! S2 transition approaches, with
increasing chain length, the S0 ! S1 transition. Both bands are still separate for n ¼ 3 but
have to be separated for n ¼ 4. A slightly unsymmetrical Gauss function proved to be
successful for this purpose [31]. The ICT effects a very strong red shift for n ¼ 1 and 2,
but then it decreases fast for higher n values. The bathochromic effect (lmax(n þ 1) ꢀ
lmax(n)) caused by the increased conjugation is almost cancelled by the decreasing
ICT effect. Series 18 [25] has – compared to 2 – an even stronger push – pull character
(Fig. 5). Increasing conjugation can no more compensate the decreasing ICT for n > 2.
Thus lmax of series 18 reaches a maximum for n ¼ 2. The extrapolation to l1 in such a
case would require some higher n values (n > 4) [1].
Conclusion and Outlook. – Oligo(para-phenylene)s 2a – 2d with terminal donor–
acceptor substitution (RO/NO2) can be easily prepared by applying the Suzuki –
Miyaura protocol. A protecting/deprotecting technique can be avoided, when the
different reactivity of 1-bromo-4-iodobenzene is used for the chain-extension steps.
Although the polarization of such D – p – A chains is rather low in the ground-state
S0 (NMR measurements), the intramolecular charge transfer (ICT) on electronic
excitation (S0 ! S1) is high, that means the long-wavelength absorption is considerably
red-shifted for small numbers of repeat units (n ¼ 1 and 2). Higher n values (n ¼ 3 and
4) lead very fast to a saturation (nECL ¼ 4) in the UV region (l 349 nm). The
1
calculations of DAOPPs 1’ on the CNDOVSB basis [32] or DFT basis [33] do not
completely satisfy, in this respect, but together with the results discussed here, one can
predict, that longer DAOPPs (n ¼ 5, 6,...) with strong push – pull effects will have high
dipole moments m and first-order hyperpolarizabilities b, but will still have a very good
transparency in the VIS region. Their preparation and process ability should be
facilitated by well solubilizing, strong donor groups such as bis(2-hexyloctyl)amino
[(C6H13)2CHCH2]2N) or (2-hexyloctyl)oxy groups ((C6H13)2CHCH2O).
We are grateful to the Deutsche Forschungsgemeinschaft, the Fonds der Chemischen Industrie, and
the Materialwissenschaftliche Forschungszentrum der Universitꢀt Mainz for financial support.
Experimental Part
1. General. The starting compounds 3 – 5, 9, and 10 and the model compounds 13 and 14 are
commercially available. M.p. (uncorrected): Bꢁchi apparatus. UV/VIS Spectra: Zeiss-MCS-320/340
diode array; CHCl3 solns.; lmax in nm. 1H- and 13C-NMR Spectra: Bruker AM 400 and AMX 400; CDCl3
solns.; d in ppm, rel. to Me4Si as internal standard, J in Hz. FD-MS: Finnigan MAT-95; in m/z (rel. %).

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2. Alkylation of 4-Nitrophenol (3) and 4-Bromophenol (5) with 1-Bromohexane (4). Preparations
according to [11][34][35].
1
1-(Hexyloxy)-4-nitrobenzene (2a): Yield 95%. M.p. 268 (EtOH) ([34]: 25.58). H-NMR (CDCl3):
0.89 (t, J ¼ 6.1, Me(6’)), 1.28 – 1.37 (m, CH2(4’), CH2(5’)); 1.40 – 1.48 (m, CH2(3’)); 1.75 – 1.83 (m,
CH2(2’)); 4.02 (t, J ¼ 6.4, CH2(1’)); 6.91, 8.16 (AA’BB’, HꢁC(2,6), HꢁC(3,5)). 13C-NMR (CDCl3): 13.9
(Me(6’)); 22.4 (CH2(5’)); 25.6 (CH2(3’)); 28.9 (CH2(2’), 31.4 (CH2(4’)); 68.9 (CH2(1’)); 114.4 (C(3));
125.8 (C(2)); 141.4 (C(1)); 164.3 (C(4)). The product corresponds to an authentic sample [34].
1-Bromo-4-(hexyloxy)benzene (6a): Colorless viscous oil. Yield 94%. The product corresponds to an
authentic sample [35].
3. Chain Extension 6a ! 8a ! 6b ! 8b ! 6c ! 8c. 3.1. Steps 6 ! 8: General Procedure. A soln. of the
corresponding bromo compound 6a – 6c (20.0 mmol) in Et2O (60 – 100 ml) was purged with a stream of
Ar, before 2.5m BuLi in hexane (8.4 ml, 21.0 mmol) was added at ꢁ 788 with a syringe. After 2 h stirring,
the soln. was slowly added at ꢁ 788 to fluorodimethoxyborane (7; 368 g, 40.0 mmol) [18][19]. The
cooling was stopped and the soln. allowed to come to r.t. After 12 – 24 h, H2O (50 ml) was added. The aq.
layer was extracted with Et2O (2 ꢃ 30 ml), and the combined org. phase was washed with H2O (20 ml),
dried (MgSO4), and purified by column filtration (SiO2, 15 ꢃ 8 cm, hexane/Et2O 100 :0 ! 0 :100).
Evaporation yielded the crude boronic acids 8a (75%), 8b (48%), or 8c (48%), which could be directly
used for the Suzuki reaction with 1-bromo-4-iodobenzene (9) or for the end capping with 1-bromo-4-
nitrobenzene (10).
3.2. Steps 8 ! 6: General Procedure. A soln. of the boronic acid 8a,b (6.0 mmol) and iodo component
9 (1.70 g, 6.0 mmol) in toluene/THF 1:1 (200 ml) was degassed and purged with a slow stream of Ar. Aq.
2m K2CO3 (100 ml) and [Pd(PPh3)4] (115 mg, 0.1 mmol) were added. The mixture was vigorously stirred
and refluxed under Ar for 2 – 3 d. The H2O phase was extracted with toluene (2 ꢃ 20 ml), and the
combined org. phase was washed with sat. NaCl soln. (20 ml), dried (MgSO4), and purified by column
filtration (SiO2, 15 ꢃ 8 cm, toluene). The crude products 6b (70%) or 6c (64%) could be directly used for
the next step. A purity control was based on the NMR data: Table.
4. End Capping 8a – 8c ! 2b – 2d: General Procedure. A soln. of crude boronic acid 8a – 8c
(1.0 mmol) and 1-bromo-4-nitrobenzene (10) in toluene/THF 1:1 (30 – 40 ml) was degassed and purged
with Ar. Aq. 2m K2CO3 (15 – 20 ml) and [Pd(PPh3)4] (20 mg, 17.4 · 10ꢁ3 mmol) were added. The mixture
was vigorously stirred and heated under reflux for 2 – 4 d. The H2O phase was extracted with toluene (2 ꢃ
20 ml), and the combined org. phase was washed with sat. NaCl soln. (20 ml), dried (MgSO4), and
evaporated. The crude DAOPPs 2a – 2d were purified by crystallization.
4-(Hexyloxy)-4’-nitro-1,1’-biphenyl (2b): Yield 78%. Colorless solid. M.p. 608 (PrOH). 1H-NMR
(CDCl3): 0.90 (t, J ¼ 6.1, Me(6’’)); 1.32 – 1.38 (m, CH2(4’’), CH2(5’’)); 1.42 – 1.48 (m, CH2(3’’)); 1.75 – 1.85
(m, CH2(2’’)); 4.00 (t, J ¼ 6.4, CH2(1’’)); 6.98, 7.55 (AA’BB’, HꢁC(2,6), HꢁC(3,5)); 7.67, 8.25 (AA’BB’,
HꢁC(2’,6’), HꢁC(3’,5’)). 13C-NMR (CDCl3): 13.8 (Me); 22.4 (CH2(5’’)); 25.5 (CH2(3’’)); 29.0
(CH2(2’’)); 31.4 (CH2(4’’)); 68.1 (CH2(1’’)); 115.1 (C(3)); 123.9 (C(3’)); 126.9 (C(2’)); 128.4 (C(2));
130.7 (C(1)); 146.5 (C(4’)); 147.2 (C(1’)); 160.0 (C(4)). FD-MS: 300 (100, [M þ H]þ). Anal. calc. for
C18H21NO3 (299.4): C 72.22, H 7.07, N 4.68; found: C 71.98, H 7.26, N 4.88.
4-(Hexyloxy)-4’’-nitro-1,1’:4’,1’’-terphenyl (2c): Yield 70%. Light yellow solid. M.p. 198 – 2008
(PrOH/1,2-dichlorobenzene). 1H-NMR (CDCl3): 0.93 (t, J ¼ 6.1, Me(6’’’)); 1.32 – 1.40 (m, CH2(4’’’),
CH2(5’’’)); 1.43 – 1.53 (m, CH2(3’’’)); 1.77 – 1.89 (m, CH2(2’’’)); 4.01 (t, J ¼ 6.5, CH2(1’’’)); 7.01, 7.58
(AA’BB’, HꢁC(2,6), HꢁC(3,5)); 7.67, 7.68 (AA’BB’, HꢁC(2’,6’), HꢁC(3’,5’)); 7.77, 8.31 (AA’BB’,
HꢁC(2’’,6’’), HꢁC(3’’,5’’)). 13C-NMR (CDCl3): 14.0 (Me(6’’’); 22.6 (CH2(5’’’)); 25.7 (CH2(3’’’)); 29.2
(CH2(2’’’)); 31.6 (CH2(4’’’)); 68.2 (CH2(1’’’)); 115.1 (C(3)); 124.2 (C(3’’)); 127.3 (C(2’’)); 127.5, 127.7
(C(2’), C(3’)); 128.1 (C(3)); 132.4 (C(1)); 136.8 (C(4’)); 141.6 (C(1’)); 147.3 (C(1’’)); 147.1 (C(4’’)); 159.3
(C(4)). FD-MS: 376 (100, [M þ H]þ). Anal. calc. for C24H25NO3 (375.5): C 76.77, H 6.71, N 3.73; found: C
76.99, H 7.01, N 3.75.
4-(Hexyloxy)-4’’’-nitro-1,1’:4’,1’’:4’’,1’’’-quaterphenyl (2d): Yield 62%. Light yellow solid. M.p.
> 2008 (dec.; PrOH/1,2-dichlorobenzene). 1H-NMR ((D8)THF): 0.94 (t, J ¼ 6.1, Me); 1.34 – 1.41 (m,
CH2(4’’’’), CH2(5’’’’)); 1.45 – 1.54 (m, CH2(3’’’’)); 1.76 – 1.85 (m, CH2(2’’’’)); 4.01 (t, J ¼ 6.4, CH2(1’’’’); 7.00,
7.61 (AA’BB’, HꢁC(2,6), HꢁC(3,5)); 7.70, 7.77 (AA’BB’, HꢁC(2’,6’), HꢁC(3’,5’)); 7.85, 7.86 (AA’BB’,
HꢁC(2’’,6’’), HꢁC(3’’,5’’)); 7.96, 8.33 (AA’BB’, HꢁC(2’’’,6’’’), HꢁC(3’’’,5’’’)). 13C-NMR: Solubility

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unsufficient for reliable data. FD-MS: 452 (100, [M þ H]þ). Anal. calc. for C30H29NO3 (451.6): C 79.80, H
6.47, N 3.10; found: C 80.08, H 6.33, N 2.90.
5. 4-(Hexyloxy)-1,1’-biphenyl (15). Preparation according to [36] and analogously to 3 þ 4 ! 2a.
Yield 93%. M.p. 688 ([36]: 63 – 63.58).
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Received November 4, 2008