Electronic interactions in tertiary oligophenylureas

Article abstract of DOI:10.1021/jp0448090

The syntheses, structures, and spectroscopy of a series of oligomeric tertiary oligophenylureas possessing one to five phenyl rings are reported. A convergent synthetic method employing tertiary monoamine and diamine building blocks is employed. NMR and molecular modeling are indicative of folded structures for all of the oligophenylureas in which adjacent phenyl rings have a splayed face-to-face geometry. NMR chemical shifts, absorption and emission maxima, and electrochemical oxidation potentials are all dependent upon the number of phenyl rings. The addition of a first inner phenyl has a pronounced effect on the chemical shifts, while a second and third inner phenyl have diminished effects. The oxidation potentials of the oligophenylureas display an abrupt decrease upon the addition of the second inner phenyl. The absorption and emission spectra are relatively insensitive to the addition of one to three inner phenyl rings. The electronic structures of the oligophenylureas possessing one to eight rings have been analyzed using ZINDO calculations. The frontier orbitals of the ureas with one to three phenyl rings are localized on a single phenyl ring (the inner ring for the three-ring urea), whereas the frontier orbitals of the higher oligomers are delocalized over two phenyl rings. In all cases, urea-localized n,π* transitions are lower in energy than the phenyl-localized n,π* transitions. The changes in properties with added phenyl rings parallel those previously observed for multilayered cyclophanes; however, they are less pronounced because of weaker coupling between the phenyl rings of the oligophenylureas.

Full text of DOI:10.1021/jp0448090

J. Phys. Chem. B 2005, 109, 4893-4899
4893
Electronic Interactions in Tertiary Oligophenylureas
Frederick D. Lewis,* Todd L. Kurth, and Grace B. Delos Santos
Department of Chemistry, Northwestern UniVersity, EVanston, Illinois 60208-3113
ReceiVed: NoVember 12, 2004; In Final Form: January 17, 2005
The syntheses, structures, and spectroscopy of a series of oligomeric tertiary oligophenylureas possessing
one to five phenyl rings are reported. A convergent synthetic method employing tertiary monoamine and
diamine building blocks is employed. NMR and molecular modeling are indicative of folded structures for
all of the oligophenylureas in which adjacent phenyl rings have a splayed face-to-face geometry. NMR chemical
shifts, absorption and emission maxima, and electrochemical oxidation potentials are all dependent upon the
number of phenyl rings. The addition of a first inner phenyl has a pronounced effect on the chemical shifts,
while a second and third inner phenyl have diminished effects. The oxidation potentials of the oligophenylureas
display an abrupt decrease upon the addition of the second inner phenyl. The absorption and emission spectra
are relatively insensitive to the addition of one to three inner phenyl rings. The electronic structures of the
oligophenylureas possessing one to eight rings have been analyzed using ZINDO calculations. The frontier
orbitals of the ureas with one to three phenyl rings are localized on a single phenyl ring (the inner ring for
the three-ring urea), whereas the frontier orbitals of the higher oligomers are delocalized over two phenyl
rings. In all cases, urea-localized n,π* transitions are lower in energy than the phenyl-localized π,π* transitions.
The changes in properties with added phenyl rings parallel those previously observed for multilayered
cyclophanes; however, they are less pronounced because of weaker coupling between the phenyl rings of the
oligophenylureas.
Introduction
CHART 1
Arrays of aromatic chromophores are commanding increasing
attention as potential components in molecular electronic
devices.1 These arrays can be divided into two major structural
types: linear arrays and π-stacked arrays. The linear p-
oligophenylenes have attracted the most attention as a conse-
quence of the desirable electronic properties of both the neutrals
and their radical ions and the relative ease of their synthetic
modification with electron-donor or -acceptor substituents.2,3 The
multilayered cyclophanes (Chart 1) provide the only example
of rigorously π-stacked multiple aromatic rings.4 Modifications
of the multilayered cyclophanes with electron-donating and
-withdrawing substituents on the outside rings have been
reported, but require lengthy synthetic procedures.
thalenes have aryl groups which are face-to-face, but have
splayed geometries with the shortest distance between rings at
the point of attachment to naphthalene.9 Therien et al. have
reported the synthesis of π-stacked porphyrin-bridge-quinone
systems with one or two phenyl bridging units.10 Recently,
Nakano11,12 and Rathore13 have independently reported the
synthesis of oligomeric oligofluorenes, in which the fluorene
rings are connected by a methylene linker and self-organize into
folded arrays with splayed fluorene rings (Chart 1). Oligomers
with one to four fluorene chromophores display continuous red-
shifted absorption and decreased oxidation potentials. The
absorption maxima approach a constant value with five or six
fluorenes.
Tertiary oligomeric diarylureas (Chart 2) also adopt folded
structures with face-to-face splayed benzene rings. Yamaguchi
et al.14 reported the crystal structure of a folded phenyl urea
oligomer possessing five phenyl rings, and Krebs and Jensen15
recently reported the synthesis of an oligomeric urea possessing
nine naphthalene rings. However, the nature of the electronic
interactions in these systems has not been systematically
investigated.
The electronic structures of both the oligophenylenes and
layered cyclophanes are strongly dependent upon the number
of phenyl rings.2,5-7 Both systems display a large red-shift in
the absorption and fluorescence maxima upon addition of the
third phenyl and progressively smaller red-shifts with additional
phenyls. In the case of the oligophenylenes, the large shift for
terphenyl is accompanied by a change in the character of the
lowest singlet state from 1Lb to 1La, which results in a
pronounced increase in the fluorescence rate constant and
quantum yield.5 Semiempirical MO calculations describe the
lowest singlet states of both the oligophenylenes and the three-
and four-layered cyclophanes as resulting from a HOMO-
LUMO transition in which both orbitals are largely localized
on the internal rings.2,6-8
A degree of π-stacking can also be achieved for molecules
which are less rigid than the cyclophanes. The 1,8-diarylnaph-
* Corresponding author. E-mail: lewis@chem.northwestern.edu.
10.1021/jp0448090 CCC: $30.25 © 2005 American Chemical Society
Published on Web 02/23/2005

4894 J. Phys. Chem. B, Vol. 109, No. 11, 2005
Lewis et al.
CHART 2
N,N′-Dimethyl-N,N′-p-phenylenedicarbamoyl Chloride. In
a well-ventilated hood, N,N′-dimethylphenylenediamine was
placed in a round-bottom flask with 5% excess phosgene in
toluene (20% in toluene, Fluka). Excess triethylamine was then
added, and the solution was refluxed for 1 h. The solution was
purged with N2 for one-half hour to displace residual phosgene.
The solvent was removed under vacuum yielding a yellow solid.
80% typical yield. NMR shows pure dicarbamoyl chloride
product: 1H NMR (CDCl3, 400 MHz): δ 7.33 (4H, s), 36.0
(6H, s). MS m/z 260 (M+).
N,N′′-1,4-Phenylenebis[N,N′-dimethyl-N′-phenyl]urea (3).
The triphenylurea was synthesized by adding two equivalents
of N-methylaniline to a toluene solution of the p-phenylenedi-
carbamoyl chloride with excess triethylamine and refluxing for
3 h. The solution was rotovapped to dryness and then dissolved
in methylene chloride and washed with water. An alumina
column (hexane/acetone) was run, resulting in several pure
fractions of product. 80% yield. 3: mp 197-199 °C. 1H NMR
(CDCl3, 500 MHz): δ 7.01 (4H, t, 8 Hz), 6.90 (2H, t, 8 Hz),
6.74 (4H, s), 6.49 (4H, s), 3.11 (6H, s), 3.06 (6H, s). MS calcd
for C24H26N4O2 m/e 402.6; found, 403.1.
Our interest in the electronic interactions between the
π-stacked nucleobases in the well-defined base-pair domains
of synthetic DNA hairpins led us to seek synthetically accessible
organic systems possessing π-stacked aromatic rings.16 Oligoaryl
ureas were chosen for these studies on the basis of the ease of
synthesis of dimers and oligomers possessing either a single
type of arene or mixtures of arenes, including donor-bridge-
acceptor systems. We have previously reported the results of
our studies of the electronic interactions between aryl groups
in tertiary diarylureas17-19 and of the reductive elimination of
diarylureas and several oligomeric arylureas.20,21 We report here
the results of an investigation of the electronic interactions in a
family of tertiary oligophenylureas 1-5 possessing one to five
phenyl rings (Chart 1). Weak coupling between the π-stacked
phenyl groups is responsible for the unique properties of these
molecules.
N,N′-Dimethyl-N,N′-bis[4-[methyl[(methylphenylamino)-
carbonyl]amino]-phenyl]urea (4). The tetraphenylurea was
synthesized via the self-coupling of the product of reaction, as
described for 2, between the N-methyl-phenylcarbamoyl chloride
and N,N′-dimethylphenylenediamine yielding N,N′-dimethyl-N-
(4-methylaminophenyl)-N′-phenylurea (D), 70% typical yield:
1H NMR (CDCl3, 500 MHz): δ 7.07 (3H, t, 8 Hz), 6.94 (1H,
t, 8 Hz), 6.80 (2H, d, 8 Hz), 6.57 (2H, d, 8 Hz), 6.27 (2H, d, 8
Hz), 3.60 (1H, s), 3.14 (3H, s), 3.12 (3H, s), 2.74 (3H, s).
Addition of this compound to a half equivalent solution of
phosgene yields the tetraphenylurea in 50% yield. 4: mp 194-
1
196 °C. H NMR (CDCl3, 500 MHz): δ 7.01 (4H, t, 8 Hz),
6.91 (2H, t, 8 Hz), 6.73 (4H, d, 8 Hz), 6.47 (8H, s), 3.13 (6H,
s), 3.04 (6H, s), 3.00 (3H, s). MS calcd for C33H36N6O3 m/e
564.7; found, 565.1.
Experimental Section
1
General. H NMR spectra were measured on an Inova 500
N,N′′-1,4-Phenylenebis[N,N′-dimethyl-N′-[4-[methyl[(meth-
ylphenylamino)-carbonyl]amino]phenyl]-urea (5). The pen-
taphenylurea was synthesized via the reaction, as described for
2, of N,N′-dimethyl-N-(4-methylaminophenyl)-N′-phenylurea
(D) with the N,N′-p-phenylenedicarbamoyl chloride (B). 50%
spectrometer. UV-vis spectra were measured on a Hewlett-
Packard 8452A diode array spectrometer using a 1-cm path
length quartz cell. Total emission spectra were measured on a
SPEX Fluoromax spectrometer. Low-temperature spectra were
measured in a Suprasil quartz “EPR” tube (i.d. ) 3.3 mm) using
a quartz liquid nitrogen coldfinger dewar at 77 K. Oxidation
potentials were estimated from the irreversible oxidation waves
of cyclic voltametric measurements performed in CH2Cl2
containing 5 × 10-4 M TMAPF6 with a scanning rate of 0.1
V/s using a Pt working electrode. Reduction waves were not
observed within the operating range of the solvent and elec-
trolyte.
1
yield. 5: mp 227-229 °C. H NMR (CDCl3, 500 MHz): δ
6.99 (4H, t, 8 Hz), 6.94 (2H, t, 8 Hz), 6.72 (4H, d, 8 Hz), 6.57
(4H, d, 8 Hz), 6.27 (4H, d, 8 Hz), 3.60 (3H, s), 3.14 (3H, s),
3.12 (3H, s), 2.74 (3H, s). MS calcd for C42H46N8O4 m/e 726.8;
found, 727.2.
Results and Discussion
INDO/S-CIS-SCF (ZINDO) calculations (26 occupied and
26 unoccupied frontier orbitals) were performed on a PC with
the ZINDO Hamiltonian as implemented by CAChe Release
6.0.22 All molecular models used in the semiempirical calcula-
tions were based on AM1 optimized ground-state geometries
by using the MOPAC suite of programs as implemented under
CAChe Release 6.0. All data-fitting procedures were carried
out by using Origin (version 6.1).23
Materials. Aniline, N-methylaniline, p-phenylenediamine,
and N,N′-dimethyl-N,N′-diphenylurea (2) were commercial
materials. N,N,N′-trimethyl-N′-phenylurea (1) was prepared by
the method of Lapore.24 All solvents were spectrograde.
N,N′-Dimethyl-p-phenylenediamine was prepared in ap-
proximately 60% yield from p-phenylenediamine, by the method
of Krishnamurthy.25 1H NMR (CDCl3, 500 MHz): δ 6.56 (4H,
s), 3.20 (2H, s), 2.79 (6H, s). MS m/e 136 (M+).
Synthesis and Structure. The oligomeric oligophenylureas
3-5 were prepared using the building block approach outlined
in Scheme 1 (see Experimental Section). The use of N-
methylated building blocks provides higher yields of the
oligomers than those obtained by exhaustive methylation of the
corresponding secondary ureas.17 The building block approach
can also be readily adapted to the synthesis of structurally
diverse oligoarylureas which incorporate different aryl units.21
Yamaguchi and co-workers have reported the crystal struc-
tures of 2 and 5, both of which adopt folded, face-to-face
geometries in the solid state.14,26 The solution NMR and UV
spectra of 2 and other tertiary diarylureas have been analyzed
by Lepore et al. using NMR and UV spectra.27 They conclude
that 2 adopts a folded structure in solution, as well as in the
crystal. The preference for folded versus extended structures is
steric in origin, the nonbonded repulsion of two methyls in the

Electronic Interactions in Tertiary Oligophenylureas
J. Phys. Chem. B, Vol. 109, No. 11, 2005 4895
Figure 1. Mimimized folded structure for the pentaphenylurea 5.
SCHEME 1: Convergent Synthetic Route to the
Oligophenylureas
extended conformation being larger than that between two
phenyls in the folded conformation.
The gas-phase geometries of the oligophenylureas were
optimized using the Hartree-Fock semiempirical Austin Model
1 (AM1) method, which provides reliable estimated geometries
for organic molecules.28 Local minima with folded structures
such as that shown for 5 in Figure 1 are typically 2-3 kcal/
mol more stable than extended (unfolded) structures, as previ-
ously observed for several tertiary diarylureas.17,19 Multiple local
minima with different dihedral angles between the long axes
of adjacent phenyl rings were encountered for the folded
structures, indicative of shallow ground-state potential energy
surfaces. The perfectly aligned structure of Figure 1 should be
entropically disfavored compared to less symmetric folded
structures in solution.
The 1H NMR spectra of 2-5 in CDCl3 are shown in Figure
2. In the case of 2, the aromatic protons are shifted upfield when
compared to that of the secondary diphenylurea or the tertiary
monophenylurea 1 (Chart 1). For example, the ortho protons
of 2 and the secondary diphenylurea appear as doublets at 6.78
δ and 7.42 δ, respectively. A further upfield shift, accompanied
by increased line-broadening, is observed for the “outer” phenyl
protons of 3-5. The “inner” phenyl protons of 3 appear as a
singlet, consistent with either a single symmetric conformation
or rapidly interconverting conformations. The inside protons of
4 display some splitting, plausibly reflecting a difference in
chemical shift for the internal protons which are ortho with
respect to the central versus outside urea linkages. Further
splitting is observed for the inner phenyl protons of 5, in accord
with the nonequivalence of the central phenyl and flanking
internal phenyls.
Figure 2. Aromatic region of the 1H NMR spectra of oligophenylureas
2-5 in CDCl3 solution, downfield signals from outer rings and upfield
signals from inner rings (low field singlet from solvent).
TABLE 1: Chemical Shift Data for Oligophenylureas,
Layered Cyclophanes, and Oligofluorenesa
layers
2
3
4
5
urea
outer
inner
6.81
6.74
6.49
6.73
6.47
6.72
6.57, 6.27b
cyclophane
outer
inner
oligofluorene
outer
inner
6.47
6.77
6.19
5.40
6.09
5.15
6.07
5.07, 4.92b
6.58
6.25
6.50
6.10
6.42
6.00, 5.90b
a Chemical shifts in CD2Cl2 for ortho protons of oligophenylureas,
and in CDCl3 for the aromatic protons of layered cyclophanes (data
from ref 29), and H1 protons of oligofluorenes (data from ref 12).
b Chemical shift for central inner ring.
the third ring and more gradual changes for addition of a second
and third inner ring. The inner rings display modest upfield shifts
upon incremental addition of inner rings, the largest shifts being
observed for the inner rings of 5-layered systems. The upfield
shifts are attributed to shielding by the π-electrons of the
adjacent aromatic rings. The enforced coplanar geometry of the
layered cyclophanes results in larger upfield shifts than does
the splayed geometry of the phenylureas or fluorenes. The larger
shifts for the fluorenes versus phenylureas plausibly are a
consequence of the larger aromatic rings or less perfect stacking,
either of which would provide greater shielding. The inner
protons of 3-layered arylurea with naphthyl outer rings have a
chemical shift of 5.81 δ, substantially upfield from the value
The chemical shifts of the ortho protons of 2-5 are reported
in Table 1 along with those for the corresponding multilayer
cyclophanes29 and oligofluorenes.12 The outer-ring protons for
all three systems display a large upfield shift upon addition of

4896 J. Phys. Chem. B, Vol. 109, No. 11, 2005
Lewis et al.
Figure 3. Normalized ultraviolet absorption spectra of the oligophen-
ylureas 1-5 in methylcyclohexane solution.
TABLE 2: Absorption and Emission Spectral Data for the
Oligophenylureasa
λ
abs, nm Absorbance
(MC)
λ
abs, nm
λfl
λphos
urea
×10-3
(MTHF) (MTHF) (MTHF)
1
2
3
4
5
253
249
258
260
260
8.4
9.4
3.7
2.8
b
339
341
349
351
354
407
420
428
429
430
246
263
270
275
a Absorption maxima in methylcyclohexane (MC) or methyltetrahy-
drofuran (MTHF) solution at room temperature and emission maxima
in a MTHF glass at 77 K. b Limited solubility.
Figure 4. Selected frontier molecular orbitals for the oligophenylureas
1 (a) and 2 (b).
for 3.21 This analysis is consistent with the preference of 2-5
for folded structures obtained in AM1 calculations.
oligophenylureas and oligofluorenes is significantly greater than
that for the π-stacked bases of DNA.30
Electronic Absorption Spectra. The normalized absorption
spectra of 1-5 in methylcyclohexane (MC) are shown in Figure
3. Compared to the monophenylurea 1, the absorption maximum
of 2 is blue-shifted and the absorption band is broadened. The
maxima of 3-5 are red-shifted and their bands further broad-
ened. The molar absorbance for 2 is similar to that of 1, whereas
the absorbance of 2-4 decreases with each additional phenyl
ring, indicative of substantial hypochromism. The absorption
maxima in MC and methyltetrahydrofuran (MTHF) solutions
are reported in Table 2. Except in the case of 2, the maxima
are red-shifted in the more polar MTHF, suggestive of partial
charge-transfer character for the transition(s) responsible for the
absorption band. The absorption bands of 3-5 can be fit to the
sum of two Gaussians: a short-wavelength band which is
independent of the number of phenyl rings and a long-
wavelength band which is red-shifted as the number of rings
increases.
ZINDO Calculations. The electronic structures and spectra
of the oligophenylureas were explored using the semiempirical
Hartree-Fock intermediate neglect of differential overlap
(INDO) method as parametrized by Zerner and co-workers
(ZINDO)31 and implemented in CAChe 6.0.22 Selected frontier
orbitals for 1 and 2 are shown in Figure 4 , and the HOMO and
LUMO orbitals of 3-5 and the octaphenylurea are shown in
Figure 5.32 The calculated wavelength, oscillator strength, and
description of the selected transitions are reported in Table 3.
The highest-energy occupied orbitals of 1 and 2 are localized
either on the phenyl ring (aniline-like) or on the urea group
(Figure 4). The lowest-energy unoccupied orbital are either
phenyl-localized or delocalized over the phenyl and urea
carbonyl groups. The weakly allowed lowest energy transition
for both 1 and 2 has extensive configuration interaction
involving single electron transitions from several urea-localized
filled orbitals to both urea-localized and delocalized vacant
orbitals. These states are described in Table 1 as having n,π*
character. The next two transitions for 1 can be described as
phenyl-localized π,π* 1Lb and 1La transitions by analogy to the
two bands of substituted benzenes, including aniline. The former
band has a low oscillator strength and extensive configuration
interaction, whereas the latter transition is domination by the
HOMO f LUMO single electron transition and has a much
larger calculated oscillator strength than either S1 or S2. The
latter band dominates the long-wavelength absorption of 1. In
the case of 2, the presence of two phenyl rings gives rise to
two weak π,π* transitions (S2 and S3) and two allowed (S4 and
The absorption spectra of the oligophenylureas are less
structured than those of the layered cyclophanes6 or oligoflu-
orenes.12 The cyclophanes have a resolved long-wavelength
band which undergoes a large red-shift upon addition of an inner
ring and more modest red-shifts for additional inner rings. The
intensity of this band is similar for cyclophanes with 3-5 phenyl
rings. The oligofluorenes also have resolved bands: a weak
long-wavelength band which undergoes a progressive red-shift
with added fluorenes and a stronger short-wavelength band
which does not shift appreciably. The oligofluorenes display
significant hypochromism, approaching approximately 70% for
higher oligomers. The percent hypochromism for both the

Electronic Interactions in Tertiary Oligophenylureas
J. Phys. Chem. B, Vol. 109, No. 11, 2005 4897
Figure 5. (a) LUMO and (b) HOMO orbitals of the oligophenylureas possessing 3-5 and 8 phenyl rings.
TABLE 3: Calculated Properties of Selected Singlet Statesa
(Table 3) can be attributed to the loss in oscillator strength with
increasing configuration interaction.
urea
1
state
λ, nm
A (1/mol/cm)
character
The electronic spectra of the multilayered cyclophanes have
previously been investigated using semiempirical calculations.6,7
The addition of a second and third phenyl layer results in large
red-shifts of the weak long-wavelength absorption band; how-
ever, additional phenyl layers effect relatively minor changes
in the absorption spectra. The S1 transition of the 3- and
4-layered cyclophanes is assigned to configuration interaction
of transitions localized on the inner rings, as is the case of the
oligophenylureas. All of the multilayered cyclophanes have low
S1 oscillator strengths (f ∼ 0.005). The much larger shifts for
the cyclophanes vs ureas is attributed to stronger electronic
coupling in the former vs the latter. The spectra of the
multilayered cyclophanes can be modeled by stacked planar
benzene rings with π-stacking distances equal to those of the
cyclophanes but lacking the connecting methylene bridges. In
contrast, ZINDO calculations for two phenyl rings with the
splayed geometry of 2 but lacking the urea linker display
essentially no electronic interaction. Thus, the urea linkers turn
on the weak interactions between adjacent phenyl rings.
To our knowledge, the electronic spectra of the oligofluorenes
have not been analyzed. However, the appearance of their
absorption and fluorescence spectra (vide infra) suggest that
electronic interactions between adjacent fluorenes are excitonic
in nature.12
Emission Spectra. The total emission spectra of 1-5
obtained at 77 K in a MTHF glass display two bands assigned
to fluorescence and phosphorescence of the oligoarylureas
(Figure 6). The fluorescence and phosphorescence band maxima
are reported in Table 2. We have previously reported that the
quantum yields of fluorescence and phosphorescence for 2 are
low at 77 K (0.003 and 0.008, respectively).17 The relative
intensities for 3-5 are somewhat higher than those of 2;
however, quantum yields remain low. Weak room-temperature
fluorescence is observed for 5, but not for 2-4.
The fluorescence of 1-5 is assigned to their urea-localized
n,π* states. The minor red-shift observed upon addition of
phenyl rings plausibly results from the presence of additional
nearly-degenerate urea-localized states. The weakly structured
phosphorescene is attributed to a phenyl-localized π,π* triplet
state, in accord with the results of ZINDO calculations.17 The
modest red-shift with increasing number of phenyl rings parallels
the calculated energy of the lowest-energy singlet state (Table
3). A state-energy diagram for the oligophenylureas (Scheme
2) resembles that for nitrogen heterocycles.34 Rapid internal
conversion of the π,π* singlet states populated upon long-
wavelength excitation should yield the lowest urea-localized
n,π* singlet state, which can either undergo internal conversion
or intersystem crossing to the lowest-energy π,π* triplet state.
S1
S2
S3
S1
S2
S3
S4
S5
S1
S3
S5
S1
S4
S8
S1
S5
S10
317
279
251
314
281
278
265
241
314
293
271
313
293
274
313
291
272
311
1060
10900
365
620
890
14600
27300
343549
1540
22900
352
297
30430
332
n,π*
1st π,π*
HOMO-LUMO
n,π*
2
1(π,π*)
2(π,π*)
3(π,π*)
4(π,π*)
n,π*
3
4
5
1(π,π*)
3(π,π*)
n,π*
1(π,π*)
5(π,π*)
n,π*
400
41400
1(π,π*)
6(π,π*)
a Singlet state energy, molar absorbance, and description based on
ZINDO calculations. See text for description of transitions and Figure
4 for selected molecular orbitals.
S5) π,π* transitions (Table 2). This results in broadening of
the long-wavelength absorption band and a blue-shift in its
absorption maximum.
The number of low-energy transitions n,π* and π,π* for the
oligophenylureas increases with the addition of each additional
ring. The frontier orbitals of 3 (Figure 5) are localized on the
inner phenyl ring, whereas those of 4, 5, and 8 are localized on
two of the inner phenyl rings. ZINDO calculations for higher
oligomers, including the octamer (Figure 5), show no evidence
for increased delocalization of the frontier orbitals. Fully
delocalized filled and vacant orbitals and orbitals localized on
the outside phenyl rings have lower or higher energies,
respectively, compared to the frontier orbitals. Configuration
interaction becomes more extensive with each additional phenyl
ring, with no single configuration dominating the description
of any of the lowest singlet states of 3-5. Oligomers with n
phenyl rings have n - 1 nearly-degenerate urea-localized
transitions and multiple π,π* transitions.
The lowest-energy n,π*, π,π* and first-allowed π,π* transi-
tions are reported in Table 3. The calculated energies of these
transitions are relatively insensitive to the number of phenyl
1
1
rings. Inversion of the La and Lb bands observed for the
oligophenylenes33 does not occur for the oligophenylureas. The
apparent broadening of the absorption spectra at longer wave-
lengths reflects an increase in the number of partially allowed
n,π* and π,π* transitions between 250 and 300 nm as the
number of phenyl rings increases, rather than lowering of the
S1 energy. The decrease in absorbance with added phenyl rings

4898 J. Phys. Chem. B, Vol. 109, No. 11, 2005
Lewis et al.
Figure 7. Cyclic voltamograms of oligophenylureas 2-5 in CD2Cl2
solution.
Figure 6. Luminescence spectra of oligophenylureas 1-5 in a MTHF
glass at 77 K.
SCHEME 3: Reductive Elimination of the Urea Linker
SCHEME 2: State Diagram for the Oligophenylureas (ic
) internal conversion, isc ) intersystem crossing)
larger than those for 4 and 5 (0.86 ( 0.01 V). It is interesting
to note that the large change in Eox occurs upon addition of the
second inner phenyl ring, whereas the second inner phenyl ring
causes relatively minor changes in the NMR, absorption, and
emission spectra. Inspection of the HOMO orbitals of 3 and 4
(Figure 5) provides one plausible explanation for this seeming
anomaly. The HOMO of 3 is largely localized on the inner
phenyl ring, whereas the HOMO of 4 (and the higher oligomers)
is delocalized over both inner phenyl rings.
No reductive waves are observed in the cyclic voltamograms
of the oligophenylureas in CH2Cl2. We have previously reported
that reduction of the oligophenylureas with potassium metal in
HMPA solution results in reductive elimination of all of the
urea linkers (Scheme 3).21 The resulting oligophenylene anion
radicals were characterized by EPR spectroscopy. The delocal-
ized character of the low-lying vacant MO’s of the polyarylureas
2-5 (Figures 4 and 5) may facilitate C1-C1′ bonding between
adjacent arene rings.
To our knowledge, the oxidation potentials of the layered
cyclophanes have not been reported. In the case of the
oligofluorenes, the decrease in oxidation potential versus
oligomer number is approximately linear for 2-5 arenes, with
smaller decreases for longer oligomers.12,13 Plausibly, hole
delocalization is more effective in the oligofluorenes, which have
weakly interacting, but equivalent, arene units. In contrast, the
oligophenylurea HOMO is localized on the inner phenyl rings
of 4 and the higher oligomers (Figure 5).
Concluding Remarks. The nature of the electronic interac-
tions within the oligophenylureas is influenced by an unusual
folded geometry in which the phenyl rings adopt a splayed face-
to-face relationship. This geometry is responsible for the
shielding of phenyl protons by neighboring rings, which are
stronger for inner versus outer rings. This behavior parallels
that of the multilayered cyclophanes, which display stronger
shielding as a consequence of having parallel phenyl rings with
In view of the low phosphorescence quantum yields, internal
conversion appears to be the dominant decay pathway for the
n,π* singlet states of 2-5.
The luminescence properties of 2-5 are, in some respects,
similar to those of the multilayer cyclophanes possessing 2-5
phenyl rings.35 Neither the cyclophanes nor oligophenylureas
2-4 display fluorescence at room temperature. A large red-
shift in the fluorescence and phosphorescence spectra is
observed for the 3-layer vs 2-layer cyclophanes at 77 K, but
only modest changes in the fluorescence and no change in the
phosphorescence are observed for the higher oligomers. Otsubo
et al. reported a decrease in the phosphorescence lifetime from
3.5 s for paracyclophane to 0.45 ( 0.04 s for its higher
oligomers.35
The luminescence spectra of the π-stacked oligofluorenes
differ markedly from those of the oligophenylureas and multi-
layer cyclophanes.12 The oligoflourenes display strong excimer
fluorescence at room temperature in solution, independent of
the oligomer size. This behavior is characteristic of weak
coupling between fluorene chromophores, as opposed to the
highly delocalized singlet states of the multilayer cyclophanes.
The rigid splayed geometry for the oligophenylureas may
prevent formation of a sandwich geometry, necessary for the
observation of intramolecular excimer formation between phenyl
rings.36
Oxidation and Reduction. Cyclic voltamograms for 2-5
are shown in Figure 7. The half-wave oxidation potentials of 2
and 3 are similar (1.12 ( 0.01 V vs SCE) and substantially

Electronic Interactions in Tertiary Oligophenylureas
J. Phys. Chem. B, Vol. 109, No. 11, 2005 4899
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shorter average π-stacking distances. The absorption and
emission spectra of the oligophenylureas display only small
shifts upon addition of inner rings.
The lowest energy transition is urea-localized and has n,π*
character, accounting for the absence of room-temperature
emission. Phosphorescence is assigned to a phenyl-localized
π,π* state. The electronic spectral shifts are less pronounced
than those of the corresponding multilayered cyclophanes.
Convergence of the spectral properties of the phenylureas
requires only three phenyl rings, whereas convergence for the
cyclophanes requires 5 or 6 layers. Convergence of the spectral
properties of the linear polyphenylenes requires an even larger
number of phenyl rings.2
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We have previously reported that the oligophenylureas 2-5
undergo reductive elimination of their urea linkers upon reaction
of potassium in HMPA. In the absence of experimental values
for reduction potentials and reaction rates, it is not possible to
compare the reactivity of the phenyl ureas anion radicals.
The oxidation potentials of the oligophenylureas decrease
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(Figure 7). We attribute this change to charge delocalization
involving the two inner rings of 4 and the higher oligomers, on
the basis of the appearance of their HOMO orbitals (Figure 5).
In contrast, the HOMO’s of 1-3 are localized on a single phenyl
ring (Figures 4,5). Hole delocalization in 4 suggests that the
oligophenylureas may serve as bridging elements for hole
transport in molecular electronic devices. Studies of donor-
bridge-acceptor systems with oligophenylurea bridges are
currently in progress in our laboratory.
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Acknowledgment. Funding for this project was provided
by NSF grants CHE-0100596 and CHE-0400663 and by a
Undergraduate Research Grant from Northwestern University
to G.B.D.
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