Intramolecular excimer fluorescence from folded ground state rotamers of N,N′dimethyl-N,N′-dipyrenylurea protophanes

Article abstract of DOI:10.1139/v03-059

The molecular structure, absorption, and fluorescence spectra of N,N′-dimethyl-N,N′-di-1-pyrenylurea and N,N′ -dimethyl-N,N′-di-2-pyrenylurea have been investigated and compared to the properties of N,N,N′-trimethyl-N′-pyrenylurea model compounds. Di-1-pyrenylurea exists as a mixture of folded (E,E) syn- and anti-rotamers that interconvert via flipping of one of the pyrene rings to an unfolded (E,Z) rotamer geometry. The symmetric di-2-pyrenylurea exists as a single folded (E,E) conformation which is in equilibrium with a less-stable, unfolded (E,Z) rotamer. The absorption and fluorescence spectra of the dipyrenylureas at 77 K in a rigid glass are similar to those of monopyrenylurea model compounds. However, in fluid solution, the dipyrenylureas exhibit excimer fluorescence and very weak monomer fluorescence which have identical decay times. This behavior is attributed to fast rotational equilibrium between folded rotamers which exhibit excimer fluorescence and unfolded rotamers which exhibit monomer fluorescence. The behavior of the dipyrenylureas is compared with that of other systems that form intramolecular pyrene excimers and diarylureas.

Full text of DOI:10.1139/v03-059

770
Intramole cular e xcime r fluore s ce nce from folde d
ground s tate rotame rs of N,N′dime thyl-N,N′-
dipyre nylure a protophane s
Fre de ric k D. Le wis a nd Todd L. Kurth
Abstract: The molecular structure, absorption, and fluorescence spectra of N,N′-dimethyl-N,N′-di-1-pyrenylurea and
N,N′-dimethyl-N,N′-di-2-pyrenylurea have been investigated and compared to the properties of N,N,N′-trimethyl-N′-
pyrenylurea model compounds. Di-1-pyrenylurea exists as a mixture of folded (E,E) syn- and anti-rotamers that
interconvert via flipping of one of the pyrene rings to an unfolded (E,Z) rotamer geometry. The symmetric di-2-
pyrenylurea exists as a single folded (E,E) conformation which is in equilibrium with a less-stable, unfolded (E,Z)
rotamer. The absorption and fluorescence spectra of the dipyrenylureas at 77 K in a rigid glass are similar to those of
monopyrenylurea model compounds. However, in fluid solution, the dipyrenylureas exhibit excimer fluorescence and
very weak monomer fluorescence which have identical decay times. This behavior is attributed to fast rotational equi-
librium between folded rotamers which exhibit excimer fluorescence and unfolded rotamers which exhibit monomer
fluorescence. The behavior of the dipyrenylureas is compared with that of other systems that form intramolecular
pyrene excimers and diarylureas.
Key words: excited state rotamers, dipyrenylureas, pyrene excimers, ureaphane.
Résumé : On a étudié la structure moléculaire et les spectres d’absorption et de fluorescence de la N,N′-diméthyl-
N,N′-di-1-pyrénylurée et de la N,N′-diméthyl-N,N′,-di-2-pyrénylurée et on les a comparés aux propriétés correspondan-
tes de composés modèles de la famille des N,N,N′-triméthyl-N′-pyrénylurée. La di-1-pyrénylurée existe sous la forme
de mélange de rotamères syn- et anti- repliés (E,E) qui donnent lieu à une interconversion par le biais de l’inversion
d’un des noyaux pyrène vers un rotamère de géométrie dépliée (E,Z). La di-2-pyrénylurée existe sous la forme d’une
seule conformation repliée qui est en équilibre avec un rotamère déplié (E,Z) moins stable. Les spectres d’absorption et
de fluorescence des dipyrénylurées à 77 K, dans un verre rigide, sont semblables à ceux des composés modèles de la
famille des monopyrénylurées. Toutefois, en solution fluide, les dipyrénylurées présentent une fluorescence d’excimère
et une très faible fluorescence de monomère qui ont des temps de dégénérescence identiques. On attribue ce comporte-
ment à un rapide équilibre rotationnel entre les rotamères repliés qui présentent de la fluorescence d’excimère et les ro-
tamères dépliés qui présente de la fluorescence de monomère. On compare le comportement des dipyrénylurées avec
celui des diarylurées et d’autres systèmes qui forment des excimères pyrènes intramoléculaires.
Mots clés : rotamères à l’état excité, dipyrénylurées, excimères de pyrène, uréephane.
[Traduit par la Rédaction] Lewis and Kurth 776
Introduc tion
play either monomer fluorescence or excimer fluorescence
with small monomer–excimer spectral shifts. Examples in-
clude 1,2-di(1-pyrenyl)ethane (3), which cannot adopt a par-
allel sandwich structure, and [2.2](1,3)pyrenophane (6) (8)
and [2.2](1,8)pyrenophane (7) (9), which have extended and
splayed ground state structures, respectively. A particularly
instructive example of the effect of overlap on pyrene
fluorescence is provided by the 1,8-dipyrenyl-naphthalenes
(8–10) investigated by Staab and co-workers (10). Both the
di-2-pyrenylnaphthalene (10) and the syn rotamer of the di-
1-pyrenylnaphthalene (8) have extensive overlap between
pyrenes and display excimer fluorescence, whereas the anti-
Pyrene excimer fluorescence is among the most widely
used probes of molecular conformation and structure (1).
The calculated optimum excimer geometry for two inde-
pendent pyrene molecules is a slightly slipped sandwich
structure (2). Molecules possessing two pyrene chromo-
phores that can adopt sandwich structures display large
monomer–excimer spectral shifts. Examples include the 1,3-
dipyrenylpropanes (1 and 2) (3–6) and [n.n](2,7)pyreno-
phanes (n = 2, 3, or 4) (3–5) (7). Conversely, molecules in
which only partial overlap of the two pyrenes is possible dis-
Received 6 February 2003. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 24 June 2003.
This paper is dedicated to Donald R. Arnold in recognition of his contributions to the study of radical ions in photochemistry.
F.D. Lewis1 and T.L. Kurth. Department of Chemistry, Northwestern University, Evanston, IL 60208-3113, U.S.A.
1Corresponding author (e-mail: lewis@chem.northwestern.edu).
Can. J. Chem. 81: 770–776 (2003)
doi: 10.1139/V03-059
© 2003 NRC Canada

Lewis and Kurth
771
Fig. 1. HF/6-31G** optimized rotamers (a) syn-E,E-N,N′-dimethyl-N,N′-di-1-pyrenylurea, (b) low energy anti-E,E-N,N′-dimethyl-N,N′-
di-1-pyrenylurea, and (c) high energy anti-E,E-N,N′-dimethyl-N,N′-di-1-pyrenylurea. Each structure is depicted with the urea unit in
the Y-Z plane.
Chart 1.
Fig. 2. Folded and extended conformations of N,N′-dimethyl-N,N′-
di-2-pyrenylurea (12) (a) depiction with urea in the X-Y plane,
(b) urea in the X-Z plane, (c) urea in the Y-Z plane, and (d) ex-
tended conformation with urea in the X-Y plane.
rotamer of the di-1-pyrenylnaphthalene (9) has minimal
overlap and displays monomer fluorescence. Vögtle refers to
systems in which two aryl groups are constrained to a face-
to-face geometry by a single linker as “protophanes”, to dis-
tinguish them from cyclophanes in which aryl groups are
connected by two or more linkers (11). Structures of 1–10
are shown in Chart 1.
Several N,N′-dimethyl-N,N′-diarylureas adopt folded
(E,E) structures, both in the solid state and in solution, in
which the two aryl groups are face-to-face (e.g., Fig. 1) (12).
Computational studies of the ground state conformational
energy surface of N,N′-dimethyl-N,N′-di-1-naphthylurea in-
dicate that the folded (E,E) rotamers are more stable than the
(E,Z) isomers by 2 to 3 kcal/mol.2 The tertiary diarylureas
are unique among conformationally mobile, singly linked
diaryl systems in their preferential formation of face-to-face
conformers.
(11) (Fig. 1), N,N′-dimethyl-N,N′-di-2-pyrenylurea (12)
(Fig. 2), and their monoaryl analogs N,N,N′-trimethyl-N′-1-
pyrenylurea (11m) and N,N,N′-trimethyl-N′-2-pyrenylurea
(12m). The dipyrenylureas exhibit both monomer and exci-
mer fluorescence, which are attributed to the major folded
rotamer (excimer) and a minor extended rotamer (mono-
mer), respectively, (e.g., Fig. 2d). The monomer–excimer
spectral shifts are dependent upon the strength of the intra-
molecular pyrene–pyrene interaction in the relaxed excited
state.
We have previously observed that several diarylureas dis-
play excimer fluorescence in solution and monomer fluores-
cence in rigid glasses at 77 K (13). Evidently, large
amplitude geometric relaxation of the locally excited,
Franck–Condon singlet state of the folded (E,E) rotamers is
necessary for the formation of a fluorescent excimer. Re-
ported here are the syntheses, calculated structures, and fluo-
rescence spectra of N,N′-dimethyl-N,N′-di-1-pyrenylurea
Re s ults a nd dis c us s ion
Molecular structures
HF/6-31G** geometry optimizations (14) confirm that, as
in the case of other tertiary diarylureas, conformers in which
the Py-N-C=O geometry is E are of lower energy than their
Z isomers (12, 13, 15). The E,E-di-1-pyrenylurea 11 is pre-
2 Manuscript in preparation.
© 2003 NRC Canada

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Can. J. Chem. Vol. 81, 2003
1
Fig. 3. Low-field H NMR spectra (298 K, CDCl3) of the ter-
tiary mono- (11m and 12m) and dipyrenylureas (11 and 12).
dicted to exist as a mixture of syn- and anti-rotamers, the
former being more stable by ca. 1.0 kcal/mol (see Figs. 1a
and 1b). The more symmetric E,E-di-2-pyrenylurea 12 exists
in a single conformation (Fig. 2a–2c). For 11 and 12, the
calculated energy of the E,E-rotamer is lower than that of
the E,Z by ca. 1.5–2.5 kcal/mol. The minimized structures
shown in Figs. 1 and 2 illustrate that the tertiary dipyrenyl-
ureas have large angles between the pyrene planes. While
the short axes of the pyrenyl planes are approximately paral-
lel, the rotational (θ) and splay (φ) angles are roughly 50°
and 60°, respectively, (see Figs. 2a and 2c). These geome-
tries presumably result from alleviation of the repulsive
aryl–aryl interactions, causing a slight perturbation of the
loosely constrained urea linker. A slightly higher energy
minimum (+0.72 kcal/mol), corresponding to inversion of
the nitrogen centers and having more extensive pyrene–
pyrene interaction, exists for anti-11 (Fig. 1c). The barrier to
urea nitrogen inversion is very low (16), thus, creating a
broad potential energy minimum in which urea linker strain
is balanced against the repulsive through space aryl–aryl in-
teractions. Although symmetry prevents the observation of
similar multiple conformations for syn-11 and 12, their po-
tential energy surfaces are undoubtedly similar.
Fig. 4. Absorption spectra in MTHF of the mono- (11m and 12m)
(dashed lines) and dipyrenylureas (11 and 12) (solid lines).
The ab initio optimized E,E geometries of 11 and 12 are
consistent with crystal structures of several tertiary diaryl-
ureas that adopt folded geometries (12). Further evidence for
1
protophane structures of 11 and 12 is provided by their H
NMR spectra (Fig. 3). Comparison of the spectra of the
mono- and dipyrenylureas show that the aryl protons of the
latter lie up-field of the former, as expected for folded con-
formations with face-to-face aromatic groups (17). In the
case of 11, exchange broadening is observed in the room
1
temperature H NMR spectrum, in accord with the restricted
interconversion of the syn- and anti-E,E-rotamers. Having
only a single E,E conformation, no such broadening is ob-
served in the spectrum of 12.
Absorption and fluorescence spectra
The long-wavelength portions of the absorption spectra of
the mono- and dipyrenylureas in methyltetrahydrofuran
(MTHF) are shown in Fig. 4. They all have long-wavelength
maxima near that of pyrene (see Table 1) and weaker tails at
longer wavelengths. The absorption spectra of 11 and 11m
appear to be broadened and display increased intensity at
longer wavelengths when compared to the spectra of 12 and
12m. This is as expected due to the greater electronic inter-
action of the pyrene frontier orbitals with substituents at the
1- vs. 2-position (18). ZINDO calculations (19) for the
mono-arylureas also correctly predict a small red shift for
the first maximum and a stronger long-wavelength tail in the
spectrum of 11m relative to that of 12m. Vibrational struc-
ture similar to that of pyrene is observed in the tails of the
absorption spectra of 11m and 12m. The band shifts and
broadening in the UV spectra of 11 and 12 prevent the ob-
servation of such long-wavelength structure. We previously
suggested that the broader spectrum of 11 vs. 11m might be
a consequence of weak exciton coupling between the two
pyrenes (13). This may also be the case in the spectra of 12
with respect to 12m.
shown in Fig. 5. The excitation spectra are independent of
the wavelength of emission monitored. The 298 K excitation
spectra are similar in appearance to the absorption spectra
(Fig. 4). The fluorescence maxima of the 1-pyrenylureas are
at shorter wavelengths than those of the 2-pyrenylureas. The
deceptive Stokes shifts in the spectra of 12m and 12 are a
consequence of the long-wavelength absorption bands hav-
ing very low oscillator strength. Well-resolved vibronic
structure is observed in the emission spectra of all four
pyrenylureas at 77 K. The spectra of 11 and 12 are similar to
those of their monoarylurea analogs 11m and 12m. No
phosphorescence is observed for any of the pyrenylureas.
The fluorescence maxima, quantum yields, and decay
times for pyrene and the pyrenylureas are reported in Ta-
ble 1. The 77 K fluorescence decays of all the ureas are best
fit by dual exponentials. The multiple exponential decays are
most likely a consequence of an array of closely related con-
formations that are frozen in the solvent matrix, the behavior
of which can be effectively fit with either distributed or the
The fluorescence excitation and emission spectra of the
mono- and dipyrenylureas at 77 and 298 K in MTHF are
© 2003 NRC Canada

Lewis and Kurth
Table 1. Absorption and fluorescence spectral data for pyrene and the ureaphanes.
773
T (K)
λabs (nm)
λfl (nm)
Φfl
τ1 (ns)a
τ2 (ns)a
Pyreneb
11m
298
77
298
77
298
298
77
298
77
334
385
378
390
378
400
480
395
422
0.32
0.37
0.31
0.19
0.0008
0.01
0.10
0.03
0.21
0.002
0.02
450
61 (6.9)
21
54 (3.6)
21
21
65 (4.1)
36
60 (8.4)
36
16 (7.5)
14 (8.8)
348
11
334
334
12m
12
10 (3.4)
9 (2.0)
338
395
417c
455, 460c
298
298
340
340
36
aPreexponentials in parenthesis.
bData from ref. (2).
cDeconvolution results.
Fig. 5. Excitation and emission spectra in MTHF of the mono-
(11m and 12m) and dipyrenylureas (11 and 12). Solid lines rep-
resent 77 K data. Dashed lines represent 298 K data.
Scheme 1.
that for 12. These shifts are attributed to the formation of
intramolecular pyrene excimers. The appearance of excimer
fluorescence at 298 K but not 77 K indicates that geometric
relaxation of the Franck–Condon singlet state occurs in fluid
solution, presumably resulting in contraction of both the
splay and rotational angles of the dipyrenylureas. Weak
short-wavelength shoulders are observed in the 298 K fluo-
rescence spectra of both 11 and 12. The positions of these
shoulders are similar to the maxima of the monoarylureas
11m and 12m, respectively (see Table 1). Deconvolution of
the spectrum of 12 results in slightly shifted monomer and
excimer maxima. The monomer and excimer emission have
identical excitation spectra.
The decay times for both the excimer fluorescence and the
weak monomer fluorescence from 11 and 12 at 298 K are
the same, indicative of rapid equilibrium between the spe-
cies responsible for excimer and monomer fluorescence. We
attribute these emissions to the folded (E,E) and extended
(E,Z) ground state rotamers, respectively. In the case of 11
both the syn- and anti-(E,E)-rotamers must be in equilibrium
with the E,Z rotamer (Scheme 1). A similar kinetic scheme
was proposed by Zachariasse et al. (4) to account for the ob-
servation of a single decay time for the extended and
twofolded excited state rotamers of 1 in toluene at tempera-
tures above 20°C. The very weak monomer emission ob-
served for 11 and 12 is indicative of a low population of the
extended (E,Z) rotamer in the excited state as well as in the
ground state. The decay times for 11 and 12 are identical to
those of the monopyrenylureas 11m and 12m (Table 1). This
agreement may simply be fortuitous, as the E,Z rotamers of
multiple component kinetics. In the case of 11, different de-
cay times might be expected for the syn- and anti-rotamers,3
whereas 12 is expected to exist predominantly as a single
rotamer.
The 298 K fluorescence emission spectra of 11m and 12m
are broadened and red-shifted when compared to their 77 K
spectra. These changes reflect the occurrence of geometric
relaxation prior to emission in fluid solution and may be at-
tributed to increased molecular planarity in the singlet state.
The calculated (HF/6-31G**) (14) dihedral angles between
the pyrene and urea planes are 69° and 118° in the ground
states of 11m and 12m, respectively. Upon excitation,
planarization and thus, an increase in the pyrene—N bond
order would result in more effective conjugation leading to a
decrease in the HOMO–LUMO gap and a bathochromic
shift in the fluorescence spectrum.
Large red shifts are observed for the fluorescence of 11
and 12 at 298 vs. 77 K, the red shift for 11 being larger than
3 Variable low-temperature fluorescence decay analyses led to the resolution of lifetimes corresponding to those of the syn and anti rotamers
of N,N′-dimethyl-N,N′-di-1-naphthylurea. Manuscript submitted.
© 2003 NRC Canada

774
Can. J. Chem. Vol. 81, 2003
Table 2. Comparison of dipyrenyl fluorescence data in MTHF.
11 and 12 possess two nonequivalent pyrenes, whereas 11m
and 12m possess a single pyrene.
Ground state structure
λfl (nm)a
∆ν (cm–1)b
Equilibrium between excited state rotamers requires that
the rate of equilibration be faster than the decay time of the
shortest-lived conformer (ca. 3–5 × 107 s–1 for 11 and 12).
Assuming that the conversion of E,E to E,Z conformers is
isoentropic, a barrier of 5 to 6 kcal/mol would result in a
rate constant of ~1 × 109 s–1. The line broadening observed
in the NMR spectrum of 11 is consistent with a ground state
barrier of this magnitude. Rotational barriers of ca. 10–
28 kcal/mol have been determined for protophane systems
(20). Linker geometry and repulsive polar and steric interac-
tions between the opposing aryl groups can lower this bar-
rier.
1c
2c
Extended
Extended
498
487
6500
6250
7850
6450
4450
0,f 3800
1500
5800
1000
3d
Parallel cyclophane
Parallel cyclophane
Parallel cyclophane
Extended cyclophane
Splayed cyclophane
Sandwich (extensive overlap)
Sandwich (minimal overlap)
Sandwich (extensive overlap)
Splayed (partial overlap)
Splayed (partial overlap)
555c
515c
469c
385,f 475
410
4d
5d
6e
7g
8h
9h
10h
11i
12i
4000
0,f 5620
0,f 3340
417,f 460
400,f 480
aFluorescence maxima at 298 K. Multiple entries indicate deconvoluted
Comparison with other intramolecular excimer-forming
molecules
components.
bDifference between the monomer and excimer band maxima at room
The excimer fluorescence maxima and monomer–excimer
spectral shifts for 11 and 12 and those of several other dipy-
renyl systems are summarized in Table 2. The 1,3-dipyrenyl-
propanes 1 and 2 have nonfolded structures in the ground
state, and they display monomer fluorescence at 77 K and
excimer fluorescence at 298 K. They can readily adopt un-
strained face-to-face sandwich excimer geometires and have
large values of ∆ν, which are similar for the 1,1-, 2,2-, and
1,2-pyrenyl isomers (5). The [n.n](2,7)pyrenophanes 3–5
have parallel sandwich-like structures in the ground state
and display excimer fluorescence both in a 1.3 K glass and
at room temperature (7).4 Their ∆ν values decrease as the
length of the alkyl tethers increases, presumably because of
decreased electronic interaction between the two pyrenes.
The [2.2](1,3)pyrenophane 6 has an extended ground state
structure and displays pyrene monomer fluorescence in non-
polar solvents, both at 77 and 298 K (8). In polar solvents, it
displays overlapping monomer and excimer fluorescence.
The observation of excimer fluorescence in polar solvents is
attributed to a “harpooning” mechanism in which photo-
induced charge transfer triggers folding. The value of ∆ν in
polar solvents is smaller that that for either 1 or 3, suggest-
ing that it does not adopt a fully overlapping structure. The
[2.2](1,8)pyrenophane 7 has a splayed structure and displays
a smaller value of ∆ν than those for the other cyclophanes
(9).
temperature.
cData from ref. (5).
dData from ref. (7).
eData from ref. (8).
fMonomer fluorescence.
gData from ref. (8).
hData from ref. (9).
iData from the present study.
display excimer fluorescence in a 77 K glass. The formation
of a fluorescent excimer at 298 K presumably requires a de-
crease in the average pyrene–pyrene separation, which
would result in increased strain in the urea linkage. Thus,
the difference in energy between folded E,E and extended
E,Z conformations may actually be smaller in the excited
state than in the ground state. The value of ∆ν for 11, which
exists predominantly as the syn conformer, is similar to that
for syn-1,8-di-1-pyrenylnaphthalene (8). The value of ∆ν for
12 is similar to that for 1,8-di-2-pyrenylnaphthalene (10).
Thus, we might expect that the relaxed pyrene excimer ge-
ometries of 11 and 12 are similar to those of 8 and 10, re-
spectively.
The behavior of the dipyrenylureas is qualitatively similar
to that of several other diarylureas that we have studied. Ter-
tiary di-2-naphthylurea and di-2-anthrylurea also display
monomer fluorescence at 77 K and broadened, red-shifted
emission in solution. The values of ∆ν for these tertiary
ureas are 2200 and 2900 cm–1, respectively, significantly
smaller than the values for 11 and 12. As a consequence of
these small spectral shifts, it is not possible to resolve mono-
mer and excimer fluorescence. The fluorescence decays of
the dinaphthyl- and dianthrylureas are also single exponen-
tials, indicative of either emission from a single rotamer or
rapid equilibration between rotamers.
The crystal structures of the 1,8-di-1-pyrenylnaphthalenes
display nearly parallel pyrene rings, with splay angles <3°
and rotational angles of <15° (10). The two pyrene rings of
8 and 10 have slightly slipped sandwich geometries, where 9
has only partial pyrene–pyrene overlap. The barrier for ther-
mal isomerization of 9 to 8 is 28 kcal/mol, much higher than
the value obtained for the interconversion of syn- and anti-
E,E-N,N′-dimethyl-N,N′-di-1-naphthylurea, ca. 12 kcal/mol
1
(calculated via H NMR exchange simulations).2 The values
Conc luding re m a rks
of ∆ν for 8–10 in a 1.3 K MTHF glass reflect their structural
differences. The smallest value is observed for 9, which has
the least extensive overlap in the ground state, and 8, with
maximal overlap, has the greatest.
The calculated ground state structures of 11 and 12 have
rather poor pyrene–pyrene overlap and a large splay angle,
similar to that of 7. Thus, it is not surprising that they do not
The fluorescence properties of excimer-forming dipyrenyl
systems are highly dependent upon the nature of the linker
connecting the two pyrene chromophores. Systems such as
the pyrenophanes 3–5 and the 1,8-dipyrenylnaphthalenes in
which the two pyrenes are parallel and strongly overlapping
display excimer fluorescence both in low temperature
4 F. Diederich. Personal communication, 2002.
© 2003 NRC Canada

Lewis and Kurth
775
glasses and in room temperature solutions. Systems with
poor overlap and rigid linkers such as the pyrenophanes 6
and 7 and the 1,8-dipyrenylnaphthalene 9 display monomer
fluorescence both in glasses and in solution. Systems with
highly flexible linkers such as the 1,3-dipyrenylpropanes 1
and 2 display monomer fluorescence in rigid glasses and
excimer fluorescence in solution.
The protophanes 11 and 12 possess a linker which
restricts ground state conformational mobility, resulting in
weak electronic interactions between poorly stacked pyrenes.
As a consequence, 11 and 12 display monomer fluorescence
in a rigid glass, unlike the 1,8-dipyrenylnaphthalenes 8 and
10 which display excimer emission at 1.3 K. In fluid solu-
tion the locally excited states of 11 and 12 can undergo geo-
metric relaxation to form excimers with monomer–excimer
spectral shifts similar to those of 8 and 10 (Table 2). The
unique conformational properties of the tertiary diarylureas,
as well as their ease of synthesis, make them well suited for
the study of intramolecular excimer and exciplex behavior in
“ureaphane” systems with two or more arene layers.
2-Aminopyrene
2-Aminopyrene was synthesized via the method of
Streitwieser et al. (24). This method consists of a potassium
metal reduction of liquid ammonia, forming a potassium am-
ide solution, to which 1-bromopyrene is added. The result-
ing mixture of 1- and 2-aminopyrene is purified by HCl
extraction. The isomers are then separated by crystallization
and multiple column and preparative TLC. Our yields were
1
less than those reported by Streitwieser (<10%). H NMR
(CDCl3, 500 MHz) δ: 4.13 (s, 2H), 7.46 (s, 2H), 7.86 (t, 3J =
8 Hz, 1H), 7.87 (d, J = 9 Hz, 2H), 7.98 (d, J = 9 Hz, 2H),
8.09 (d, J = 8Hz, 2H). MS m/z: 217 (M+).
N,N′-Diarylureas
To a solution of amine (10 mmol) in dichloromethane
(20 mL), was added 2.2 equiv of triethylamine, followed by
1.6 mmol of triphosgene. After the mixture had been stirred
for 10 min, the solvent was removed using a rotary evapora-
tor. The solid residue was washed with water and recry-
stallized from DMF–water (10:1). The resulting crystalline
amide was washed with water and dried under vacuum.
N,N′-Di-1-pyrenylurea was characterized and reported previ-
ously (13). Due to its insolubility, NMR data was not obtain-
able for N,N′-di-2-pyrenylurea.
Expe rim e nta l
General
1H NMR spectra were measured on an Inova 500 spec-
trometer. 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 (id =
3.3 mm) using a quartz liquid nitrogen cold finger dewar at
77 K. Total emission quantum yields were measured by
comparing the integrated area under the total emission curve
at an equal absorbance and the same excitation wavelength
as an external standard (9-methylanthracene) (Φf = 0.35 at
298 K in cyclohexane) (21). Emission spectra are uncor-
rected and the estimated error for the quantum yields is
±10%. Fluorescence decays were measured using a Photon
Technologies International (PTI) stroboscopic detection in-
strument with a hydrogen or nitrogen lamp using a scatter-
ing solution to profile the instrument response function.
Nonlinear least-squares fitting of the decay curves employed
the Levenburg–Marquardt algorithm as described by James
et al. (22) and implemented by the PTI Timemaster software
(23). Goodness-of-fit was determined by simultaneously
judging the χ2 (<1.2 in all cases), the residuals, the Durbin–
Watson parameter (>1.5 in all cases), and the Runs Test
(>–1.9 in all cases). Multiple wavelength detection and
global analyses were applied in relevant cases. All solutions
were either purged with nitrogen for 30 min or degassed un-
der vacuum (<0.1 torr (1 torr = 133.322 Pa)) through five
freeze–pump–thaw cycles.
N,N-Dimethyl-N′-2-pyrenylurea:
1
NMR (DMSO-d6, 500 MHz) δ: 3.08 (s, 6H), 8.01 (t, J =
7.5 Hz, 2H), 8.09 (d, J = 9 Hz, 2H), 8.16 (d, J = 9 Hz, 2H),
8.27 (d, J = 7 Hz, 1H), 8.45 (s, 2H), 8.90 (s, 1H).
N,N′-Dimethyl-N,N′-diarylureas
To a solution of N,N′-diarylurea (10 mmol) in 20 mL
DMF was added drop-wise 1.5 equiv of NaH in 10 mL
DMF, followed by 1.5 equiv of MeI. The mixture was stirred
at room temperature until the starting material was con-
sumed (ca. 2 to 3 h), and was then mixed with water, and ex-
tracted with CH2Cl2. The organic layer was washed with
water and dried with anhydrous potassium carbonate. After
the solvent was removed, the residue was purified by column
chromatography using mixed solvent (acetone and hexane).
The N,N′-dimethyl-N,N′-di-1-pyrenylurea (11) was charac-
terized and reported previously (13).
N,N′-Dimethyl-N,N′-di-2-pyrenylurea (12):
1H NMR (CDCl3, 500 MHz) δ: 3.46 (s, 6H), 7.26 (d, J =
9 Hz, 2H), 7.36 (s, 2H), 7.52 (d, J = 9 Hz, 2H), 7.90 (t, J =
7.7 Hz, 1H), 7.97 (d, J = 7.7 Hz, 2H).
N,N,N′-Trimethyl-N′-arylureas
To a solution of aminopyrene (0.66 g, 3 mmol) in di-
chloromethane (20 mL) was added 1.4 mL triethylamine
(10 mmol), followed by 0.3 g triphosgene (1 mmol). After
the mixture had been stirring for 30 min, a mixture of 0.6 g
dimethylamine hydrochloride (7 mmol), 1.4 mL triethyl-
amine (10 mmol), and 20 mL CH2Cl2 was added. The mix-
ture was refluxed for 10 min, the solvent was then removed
using a rotary evaporator. The solid residue was washed with
water and recrystallized from acetone–hexane (1:1) to give
the relevant N,N-dimethyl-N′-pyrenyl urea. The N,N-
dimethyl-N′-1-pyrenylurea was characterized and reported
previously (13).
Ma te ria ls
All reagents are commercially available and were used as
received. Anhydrous MTHF containing 200 ppm 2,6-di-tert-
butyl-4-methylphenol (BHT, Aldrich) was distilled from po-
tassium hydroxide under a nitrogen atmosphere immediately
prior to use.
© 2003 NRC Canada

776
Can. J. Chem. Vol. 81, 2003
Mataga, Y. Sakata, and S. Misumi. Chem. Phys. Lett. 41, 325
N,N-Dimethyl-N′-2-pyrenylurea:
1H NMR (DMSO-d6, 500 MHz) δ: 3.08 (s, 6H), 8.01 (t,
J = 7.5 Hz, 2H), 8.09 (d, J = 9 Hz, 2H), 8.16 (d, J = 9 Hz,
2H), 8.27 (d, J = 7 Hz, 1H), 8.45 (s, 2H), 8.90 (s, 1H). This
was methylated with MeI–NaH as described above. The
crude N,N,N′-trimethyl-N′-pyrenylureas were purified by
column chromatography. N,N,N′-Trimethyl-N′-1-pyrenyl-
urea (11m) was characterized and reported previously (13).
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N,N,N′-Trimethyl-N′-2-dipyrenylurea (12m):
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3
7.85 (s, 2H), 7.97 (t, J = 7.4 Hz, 1H), 7.971 (d, J = 9 Hz,
2H), 8.06 (d, J = 9 Hz, 2H), 8.16 (d, J = 9 Hz, 2H).
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Ac knowle dge m e nts
Funding for this project was provided by NSF grant CHE-
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© 2003 NRC Canada