Methoxyl-induced fluorescence quenching in N,N′-bridged 9H-dipyrrinones and an X-ray crystal structure

Article abstract of DOI:10.1007/s00706-008-0924-2

Strongly fluorescent dipyrrinones can be prepared by bridging the pyrrole and lactam nitrogens with a carbonyl group, from reaction with N,N′-carbonyldiimidazole in the presence of a strong, non-nucleophilic base. The yellow, N,N′-carbonyl-bridged dipyrrinones typically have fluorescent quantum yields (φ_F) approaching 1.0. Thus, in chloroform, N,N′-bridged 9H-dipyrrinones with β-alkyl substituents: 2,3-diethyl-7,8-dimethyl has φ_F = 0.90 (λ_em = 465 nm) and 2,3-dimethyl-7,8-dimethoxy has φ_F = 0.84 (λ_em = 482 nm). In contrast, 2,3-dimethoxy-7,8-dimethyl and 2,3,7,8-tetramethoxy show red-shifted λ_em but with strongly reduced φ_F: φ_F = 0.10 (λ_em = 511 nm) and 0.08 (λ_em = 511 nm), respectively. Methoxy substituents on the lactam, but not the pyrrole ring act to quench the fluorescence and shift the emission and excitation wavelengths bathochromically. The first X-ray crystal structure of an N,N′-carbonyl-bridged dipyrrinone was obtained from 7,8-dimethoxy-2,3-dimethyl-10H-dipyrrin-1-one.

Full text of DOI:10.1007/s00706-008-0924-2

Monatsh Chem 139, 1377–1385 (2008)
DOI 10.1007/s00706-008-0924-2
Printed in The Netherlands
Methoxyl-induced fluorescence quenching in N,N9-bridged 9H-
dipyrrinones and an X-ray crystal structure
Sanjeev K. Dey, David A. Lightner
Department of Chemistry, University of Nevada, Reno, Nevada, USA
Received 26 February 2008; Accepted 6 March 2008; Published online 2 June 2008
# Springer-Verlag 2008
Abstract Strongly fluorescent dipyrrinones can be
prepared by bridging the pyrrole and lactam nitrogens
with a carbonyl group, from reaction with N,N0-car-
bonyldiimidazole in the presence of a strong, non-
nucleophilic base. The yellow, N,N0-carbonyl-bridged
dipyrrinones typically have fluorescent quantum
yields (ꢀF) approaching 1.0. Thus, in chloroform,
N,N0-bridged 9H-dipyrrinones with ꢁ-alkyl substitu-
phore has been used to probe chirality in circular
dichroism (CD) spectroscopy [2a, 2b] and in fluores-
cence-detected CD [2c]. Recently analogs for asses-
sing Ca2þ concentrations [3], for incorporation into
biomolecules [4], and with potential medical applica-
tions in detecting cholestasis [5] and use in fluores-
cence imaging were prepared [5]. In only a few
instances we have found markedly reduced fluores-
cence in this chromophore: analogs with –CH¼
C(CN)CO2CH2CH3 and –CH¼C(CO2CH2CH3)2
substituents at the pyrrole ꢁ-position, where ꢀF drop-
ped to ꢀ0.1 in cyclohexane and ꢀ10ꢁ3 in DMSO
[1d], and those with substituents at the pyrrole ꢂ-po-
sition, –CH¼CHCO2CH3, C(¼O)CH3, and CHO, al-
so with large decreases in ꢀF [1g].
In order to explore the influence of heteroatoms on
the fluorescence of dipyrrinones, we prepared four
9H-dipyrrinones with a combination of ꢁ-methoxyls
and alkyls (5–80) and converted them to the carbonyl-
bridged analogs 1–40 (Fig. 1). In the following, we
describe their syntheses and characterization, includ-
ing the X-ray structure determination, first and we
report on the surprising fluorescence quenching effect
of the ꢁ-methoxyls located on the lactam ring.
ents: 2,3-diethyl-7,8-dimethyl has ꢀF ¼ 0.90 (lem
¼
465 nm) and 2,3-dimethyl-7,8-dimethoxy has ꢀF ¼
0.84 (lem ¼ 482 nm). In contrast, 2,3-dimethoxy-7,8-
dimethyl and 2,3,7,8-tetramethoxy show red-shifted
lem but with strongly reduced ꢀF: ꢀF ¼ 0.10 (lem
¼
511 nm) and 0.08 (lem ¼ 511 nm), respectively. Me-
thoxy substituents on the lactam, but not the pyrrole
ring act to quench the fluorescence and shift the emis-
sion and excitation wavelengths bathochromically.
The first X-ray crystal structure of an N,N0-carbonyl-
bridged dipyrrinone was obtained from 7,8-dime-
thoxy-2,3-dimethyl-10H-dipyrrin-1-one.
Keywords Pyrrole; Synthesis; Fluorescence; X-Ray.
Introduction
In contrast to the parent dipyrrinones, N,N0-carbonyl-
bridged dipyrrinones are typically intensely fluores-
cent [1, 2], with fluorescence quantum yields (ꢀF)
approaching unity in organic solvents. The chromo-
Results and discussion
Synthesis aspects
Correspondence: David A. Lightner, Department of Chemis-
try, University of Nevada, Reno, Nevada 89557-0020, USA.
E-mail: lightner@scs.unr.edu
For the preparation of 5–7 (Scheme 1) and 1–3
(Fig. 1), 3,4-dimethoxy-1H-pyrrole was the required

1378
S. K. Dey, D. A. Lightner
were converted in excellent yields to their N,N0-car-
bonyl-bridged analogs using CDI and DBU to give 4
from 8 in 92% yield, and 40 from 80 in 80% yield.
Structural aspects
The dipyrrinone structures 5–80 and their deriva-
tives 1–40 follow from the method of synthesis and
the known structures of their monopyrrole precur-
sors 9–13. The structures were confirmed by their
13C NMR spectra (Table 1). Characteristic of N,N0-
carbonyl-bridged dipyrrinones, the new C¼O group
appears near 140 ppm and the lactam carbonyl is
more shielded (by ꢀ5–10 ppm) than that in the par-
ent dipyrrinone. Other carbon signals remain close to
those observed for the parent dipyrrinones. Clearly, the
location of the methoxy groups plays a role in shifting
the carbon resonances. Thus, when on the pyrrole ꢁ-
positions, C(7) and C(8) are deshielded by ꢀ20 ppm,
and C(6) and C(9) are shielded by ꢀ15ppm. Sub-
stituting the lactam ꢁ-positions with methoxyls im-
parts no similar strong shifts to the lactam ring
carbons: C(1) becomes more shielded by ꢀ5 ppm.
Fig. 1 The target N,N0-carbonyl-bridged 9H-dipyrrinones of
this work (1–40) and their dipyrrinone precursors (5–80)
synthetic relay compound. It was known from pub-
lished work [6], and previously we had shown how
to convert it to the corresponding pyrrolin-2-one (9)
[7], which serves as the precursor to 5 and 6, and 1
and 2. Conversion of 9 to dipyrrinones 5 and 6 with
methoxy groups on the lactam ring required either
3,4-dimethoxy-2-formyl-1H-pyrrole (12) or 3,4-di-
methyl-2-formyl-1H-pyrrole (13). The former was
prepared in 52% yield by a Vilsmeier reaction [8]
on 9; the latter was prepared by treatment of the
known ethyl 3,4-dimethyl-1H-pyrrole-2-carboxylate
first with KOH to saponify, then with trifluoro-
acetic acid and ethyl orthoformate. Base-catalyzed
condensation of 9 and 12 yielded the tetramethoxy-
dipyrrinone 5 in 40% yield, from which the carbon-
yl-bridged analog 1 was prepared in 91% yield by
reaction of 1,10-carbonyldiimidazole (CDI) and 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU). Similarly, 6
was prepared from 9 and 13 in 55% yield and con-
verted to 2 in 92% yield, and 7 was prepared from 10
and 12 in 87% yield and converted to 3 in 91% yield.
Dipyrrinone 8 was prepared in 60% yield from 3,4-
diethyl-3-pyrrolin-2-one (11) and 3,4-dimethyl-2-for-
myl-1H-pyrrole (13), and 80 [9] was prepared from 10
and 13 in 52% yield. As above, these dipyrrinones
Molecularity in solution from vapor pressure
osmometry
As determined in chloroform solution, and consistent
with the behavior of other dipyrrinones [10, 11], the
parent 9H-dipyrrinones 5–80 are dimeric in solution
(Table 2). The dimers are held together by intermo-
lecular hydrogen bonds [10, 11]. However, when the
dipyrrinone nitrogens are connected via a carbonyl
bridge (1–40), intermolecular hydrogen bonding is
no longer possible, and compounds 1–40 all are
monomers over the concentration range studied. This
is understandable in terms of 5–80 being intermolec-
ularly hydrogen-bonded dimers involving the NHs
Scheme 1

Fluorescence quenching in N,N0-bridged 9H-dipyrrinones
1379
13
Table 1 Assignments and comparison of the C NMR chemical shifts of 1–80 in (CD3)2SO at 23ꢂC
R1
R2
1
2
3
4
40
OCH3
OCH3
CH3
CH2CH3
CH3
OCH3
CH3
OCH3
CH3
CH3
5
6
7
8
80
Carbon
Chemical Shifts (ꢃ=ppm) for
1
2
3
4
40
5
6
7
8
80
1
2
3
4
5
6
7
8
C¼O
¼C–
¼C–
¼C–
¼C–
¼C–
¼C–
¼C–
¼CH
C¼O
CH2=CH3
CH3
CH2=CH3
CH3
CH3
CH3
CH3
161.4
126.8
147.0
123.4
95.9
114.7
135.8
144.7
99.5
139.8
–
60.6
–
59.6
–
60.9
–
58.1
161.6
126.8
147.3
126.1
97.4
123.7
121.9
125.8
116.9
140.0
–
60.9
–
59.6
8.6
–
9.9
–
166.8
125.9
142.5
131.6
96.6
115.7
135.8
145.0
99.2
140.6
8.2
–
9.6
–
–
60.7
–
166.6
130.8
147.8
130.6
98.3
126.9
122.0
126.0
116.8
140.9
16.3
13.3
17.1
14.5
8.7
166.8
125.9
142.5
131.9
97.9
126.8
121.8
125.8
116.6
140.7
8.2
–
9.6
–
8.6
–
10.0
–
165.9
127.0
146.6
122.5
96.3
114.6
135.2
140.6
104.3
–
–
61.0
–
59.7
–
61.8
–
166.0
126.1
146.4
121.6
96.7
123.2
118.2
121.0
120.1
–
–
60.3
–
59.1
9.1
–
171.6
125.7
141.5
131.5
97.5
115.1
134.9
140.7
104.1
–
9.1
–
10.2
–
–
61.9
–
171.6
129.1
146.9
128.4
97.9
123.7
118.2
121.8
120.0
–
16.3
13.8
17.0
15.7
9.1
171.8
124.1
141.5
129.7
97.8
123.6
118.1
121.7
119.8
–
8.3
–
9.5
–
9.1
–
9
10
21
22
31
32
71
72
81
82
–
10.0
–
–
10.0
–
10.0
–
10.0
–
CH3
58.1
58.6
58.6
and lactam carbonyl [10, 11]. When the NHs are no
longer available, as in 1–40, hydrogen bonding is no
longer possible, and monomers prevail.
less: ꢀ5 nm. Locating methoxy groups on the lactam
ring apparently has a much greater influence on
the UV-visible lmax than locating them on the pyr-
role ring. The trends seen for the parent dipyrrinones
generally carry over to the bridged analogs 1–40. In
addition, the long wavelength UV-visible absorbance
coefficients of 1–40 are greatly reduced compared
with the parent dipyrrinones.
UV-visible spectra
Consistent with measurements from other dipyrrin-
ones, the UV-visible spectra (Table 3) of 9H-di-
pyrrinones 5–80 show a long wavelength maximum
absorption of 30,000–40,000 near 400 nm. Absent
an alkyl substituent at C(9), the 9H-dipyrrinones 8
and 80 exhibit a small (ꢀ10 nm) hypsochromic shift.
Methoxyl substituents on the lactam ring of 5 and 6
cause a 10–15 nm hypsochromic shift in lmax rela-
tive to 8 and 80; when the methoxyl substituents are
located at the pyrrole ꢁ-positions, the shift is much
Fluorescence
We noticed immediately that to the eye bridged di-
pyrrinone 1 seemed to be less fluorescent than is typ-
ical of other bridged dipyrrinones. And, in fact, when
the fluorescence quantum yield (ꢀF) was determined
(Table 4), it became clear that severe fluorescence

1380
S. K. Dey, D. A. Lightner
Table 2 Molecular weights (MWs) of carbonyl-bridged 9H-dipyrrinones 1–40 and parent 9H-dipyrrinones 5–80 determined by
vapor pressure osmometrya at 45ꢂC in CHCl3 solutionb
R1
R2
R3
R4
Formula weight
(FW=g molꢁ1
Measured
MW=g molꢁ1
)
1
2
3
4
40
5
6
7
8
80
OCH3
OCH3
CH3
CH2CH3
CH3
OCH3
OCH3
CH3
CH2CH3
CH3
OCH3
OCH3
CH3
CH2CH3
CH3
OCH3
OCH3
CH3
CH2CH3
CH3
OCH3
CH3
OCH3
CH3
CH3
OCH3
CH3
OCH3
CH3
CH3
OCH3
CH3
OCH3
CH3
CH3
OCH3
CH3
OCH3
CH3
CH3
306
274
274
270
242
280
248
248
244
216
305 ꢃ 3
277 ꢃ 6
281 ꢃ 11
275 ꢃ 7
235 ꢃ 13
570 ꢃ 15
490 ꢃ 12
496 ꢃ 8
485 ꢃ 4
430 ꢃ 7
a
b
Calibrated with benzil (FW ¼ 210 g molꢁ1, MW ¼ 220 ꢃ 15 g molꢁ1); Conc. range 0.5–2.0ꢄ10ꢁ3 mol kgꢁ1
Table 3 Solvent dependence of the UV-visible spectra of 1–80
Compound
l
max=nm ("=dm3 molꢁ1 cmꢁ1
)
Cyclohexane
C6H6
CHCl3
CH3OH
(CH3)2SO
1
2
3
398 (12700)
393 (12700)
419 (7400)sh
396 (8200)
421 (15700)sh
398 (15900)
419 (17500)sh
397 (18000)
380 (37200)
398 (14900)
398 (14900)
417 (12100)sh
398 (12900)
419 (16000)sh
404 (16500)
416 (16500)sh
402 (16800)
382 (22800)
367 (22500)sh
383 (32000)
392 (27000)
377 (26000)sh
395 (33900)
402 (15300)
402 (14700)
401 (11800)
398 (14500)
398 (14100)
406 (12500)
400 (14900)
399 (15600)
394 (11800)
4
412 (14800)
411 (15500)
379 (23300)
412 (14800)
410 (16000)
382 (26000)
412 (12700)
411 (17000)
382 (24500)
40
5
6
7
381 (37000)
412 (21400)sh
390 (35800)
415 (24700)sh
394 (42500)
395 (41000)
381 (29400)
391 (28700)sh
377 (30100)
392 (29300)
386 (34200)
395 (32900)
385 (32700)
396 (31900)
384 (31000)sh
400 (32400)
8
400 (33900)
400 (32600)
80
394 (33600)
393 (30000)
400 (32000)
Table 4 Solvent dependence of the fluorescence excitation (lex=nm) and emission (lem=nm) wavelengths and quantum yields
(ꢀF) of 1–40
Pigment
Cyclohexane
C6H6
CHCl3
CH3OH
ꢀF
(CH3)2SO
lem
ꢀF
lem
ꢀF
lem
ꢀF
lem
lem
ꢀF
1a
2a
3a
4a
479
480
470
454
456
0.27
0.37
1.0
1.0
1.0
491
493
468
482
468
0.30
0.42
0.92
1.0
511
511
482
465
482
0.08
0.10
0.84
0.90
0.99
545
540
512
514
519
0.03
0.02
0.39
0.44
0.52
516
511
503
494
494
0.13
0.11
0.90
1.0
a
40
0.95
0.94
a
lexc ¼ 398 nm; reference standard: 9,10-diphenylanthracene

Fluorescence quenching in N,N0-bridged 9H-dipyrrinones
1381
quenching had occurred. The behavior may be com-
pared with carbonyl-bridged all-alkyl-substituted 9H-
dipyrrinones 4 and 40 where ꢀF approaches unity over
the range of solvents studied. In order to probe the
cause of the fluorescence quenching found in 1, we
synthesized and measured ꢀF values for analogs 2 and
3 each with two methoxy groups and two methyl
groups. In 2 the methoxy groups are located on the
lactam ring; in 3 they are located on the pyrrole ring.
And the fluorescence data clearly show (Table 4) that
ꢁ-methoxy groups on the pyrrole ring have only a
minor effect in quenching fluorescence, while meth-
oxy groups on the lactam ring exert the major effect.
The reason for the strong fluorescence quenching is
unclear. Methoxy groups are not known to quench
fluorescence in fluorogenic substances, such as in
stilbenes [12a], coumarins [12b, 12c], quinolinones
[12c], and fluoranthenes [12d]. Dipole–dipole effects
between the methoxy groups and the lactam chromo-
phore may play a role.
of methoxy groups on the lactam ring (as in 1 and 2)
causes lem to shift toward the red by ꢀ25 nm relative
to 4 and 40, but with methoxy groups only on the
pyrrole ring (3), the shift is less pronounced (ꢀ10–
15 nm relative to 4 and 40).
X-Ray crystal structure of 3
As might be anticipated from its drawn structure
(Fig. 1), 3 (and we assume other N,N-carbonyl-
bridged dipyrrinones) has a planar polycyclic skele-
ton. Molecules of 3 in the crystal stack in pairs
(Fig. 2A) that lie in layers of parallel sheets some
˚
3.5 A apart (Fig. 2B, C). The molecules stack one
above each other such that the relative orientation
of molecules in each stack is orientated by 180ꢂ in
˚
plane. The new C¼O group is slightly longer (1.214 A)
˚
than the lactam C¼O (1.212 A), and the associated
˚
N–C¼O single bonds of the former (1.380 A) are
˚
much shorter than that in the lactam (1.436A).
All bridged dipyrrinones show a clear solvatochro-
mic effect in their fluorescence spectra, with lem
shifting strongly toward the red with increasing sol-
vent dielectric. For 1 and 2, the shift is accompanied
by a strongly reduced ꢀF. For 3 and 4, ꢀF remains
near unity, except in CH3OH solvent, where contact
between solvent and pigment probably involves hy-
drogen bonding to the pigment carbonyl group(s). In
the most polar solvent ((CH3)2SO), ꢀF remains high
for 3, 4, and 40 but subdued for 1 and 2. The presence
Carbon–carbon double bond alternation C(2)¼C(3):
˚
˚
˚
1.349 A; C(3)–C(4): 1.452 A; C(4)¼C(5): 1.345 A;
˚
˚
C(5)–C(6): 1.422 A; C(6)¼C(7): 1.379 A; C(7)–
˚
˚
C(8): 1.427 A; C(8)¼C(9): 1.356A is well estab-
lished. Curiously, only in the pyrrole ring do the
double bonds differ much in length, with the long
C(6)¼C(7) bond. The other C¼C double bonds are
remarkably similar in length. And in this sense they
differ from those found in dipyrrinones, e.g., 2,3,8-
tetraethyl-7,9-dimethyl-10H-dipyrrin-2-one, C(2)¼
Fig. 2 Crystal structure drawings of 3 showing (A) the numbering system used and orientation of molecules lying in a common
plane and the stacking pattern in face (B) and edge (C) views. The three molecules of (A), as viewed from top to bottom, may
be located in the left column of structures in (B) as the topside structures in the upper and middle pairs and the structure
sandwiched in the middle of the bottom set of three

1382
S. K. Dey, D. A. Lightner
13C NMR spectra. Vapor-pressure osmometry (VPO) experi-
ments were run on an OSMOMAT 070 instrument (Gonotek,
Berlin). Radial chromatography was carried out on Merck
silica gel PF254 with CaSO4 binder preparative layer grade,
using a Chromatotron (Harrison Research, Inc, Palo Alto, CA)
with 1, 2, or 4 mm thick rotors and analytical thin-layer chro-
matography was carried out on J.T. Baker silica gel IB-F
plates (125ꢄm layer). Melting points were determined on a
Mel-Temp capillary apparatus and are corrected. Combustion
analyses were carried out by Desert Analytics, Tucson, AZ,
and found to be within ꢃ0.3% of theoretical values.
˚
˚
C(3): 1.303 A; C(3)–C(4): 1.443 A; C(4)¼C(5):
˚
˚
˚
1.347 A; C(5)–C(6): 1.405 A; C(6)¼C(7): 1.399 A;
˚
˚
C(7)–C(8): 1.376 A, and C(8)¼C(9): 1.367 A, where
bond alternation was previously noted [13], and where
it can be seen that the C(2)¼C(3), C(5)–C(6), and
C(7)–C(8) are shorter than in 3, whereas C(6)¼C(7)
is longer. These differences are presumably due to the
ꢁ-methoxy substituents on the pyrrole ring of 3.
The internal bond angle at C(5) in 3 (119ꢂ) is much
smaller than that in the dipyrrinone (133ꢂ) [12], indi-
cating that strain is imposed here in forming the third
ring of 3. The adjacent internal bond angles, at C(6)
and C(4), 117 and 121ꢂ, respectively, of 3 are also
more compressed than those of the dipyrrinone, 126
and 127ꢂ, respectively [13], all apparently due to steric
compression in forming the N,N0-carbonyl bridge.
The spectral data were obtained in spectral grade solvents
(Aldrich or Fischer) which were distilled under Ar stream just
prior to use. Before the distillation CHCl3 was passed through
a basic alumina column. Distillation of (CH3)2SO solvent was
carried out at 0.5mmHg vacuum collecting the solvent at
0ꢂC and thawing it under Ar. The starting compounds 3,4-
dimethoxy-3-pyrrolin-2-one (9) [7], 3,4-dimethyl-3-pyrrolin-
2-one (10) [9], 3,4-diethyl-3-pyrrolin-2-one (11) [7], and
3,4-dimethyl-2-formyl-1H-pyrrole (13) [9] were prepared as
described previously.
Concluding comments
1,2,8,9-Tetramethoxy-3H,5H-dipyrrolo[1,2-c:20,10-f]-
pyrimidine-3,5-dione (1, C14H14N2O6)
In our attempts to prepare strongly-fluorescent N,N0-
carbonyl-bridged dipyrrinones with lem>500 nm,
from methoxy-substituted dipyrrinones 5–7 we
prepared analogs (1–3) whose UV-visible long wave-
length absorption was shifted ꢀ10–15 nm hypsochro-
mically relative to the all-alkyl substituted analogs (4,
40). Yet the fluorescent emission wavelength (lem) of
1 and 2 is bathochromically shifted relative to 4 and 40
by ꢀ25nm. Unexpectedly, the derivatives with meth-
oxy groups on the lactam ring (1 and 2) were only
poorly-fluorescent, whereas, with methoxy groups on
the pyrrole ring no such quenching is observed. It is
unclear why 1 and 2 are such weak fluorophores, while
3 remains strongly fluorescent. An X-ray crystal struc-
ture of 3 indicates a planar polycyclic framework with
the molecules paired up and lying in parallel sheets.
To a solution of 280 g (1.00 mmol) dipyrrinone 5 was added
0.81 (5.0mmol) CDI, 80 cm3 anhydrous CH2Cl2 and 0.75 cm3
(0.20 mmol) DBU, and the mixture was heated at reflux under
nitrogen for 16h. After cooling, the mixture was washed with
100 cm3 of 1% aqueous HCl, then with H2O (1ꢄ100 cm3),
and dried over anhydrous Na2SO4. After filtration and removal
of the solvent under vacuum, the residue was purified by radial
chromatography (eluent: 2% CH3OH in CH2Cl2, vol=vol) and
recrystallized from n-hexane-ethyl acetate to give pure 1.
Yield: 270 mg (90%); mp 188–190ꢂC; 1H NMR (CDCl3):
ꢃ ¼ 3.82 (3H, s), 3.96 (3H, s), 3.98 (3H, s), 4.18 (3H, s), 6.62
(1H, s), 7.12 (1H, s) ppm; 13C NMR (CDCl3): ꢃ ¼ 58.4, 59.8,
61.7, 61.18, 96.2, 99.5, 115.7, 123.9, 127.4, 136.5, 145.6,
147.1, 162.3ppm.
1,2-Dimethoxy-8,9-dimethyl-3H,5H-dipyrrolo[1,2-c:20,10-f]-
pyrimidine-3,5-dione (2, C14H14N2O4)
Following the procedure for 1, 248 mg (1.00 mmol) dipyrri-
none 6, (0.81 g, 5.0mmol) CDI, and 0.75cm3 (0.20 mmol)
DBU gave pure 2 after crystallization from ethyl acetate-n-
hexane. Yield: 255 mg (93%); mp 159–160ꢂC; 1H NMR
(CDCl3): ꢃ ¼ 2.05 (3H, s), 2.08 (3H, s), 3.96 (3H, 2), 4.19
(3H, s), 6.52 (1H, s), 7.43 (1H, s) ppm; 13C NMR (CDCl3):
ꢃ ¼ 9.2, 10.5, 59.8, 61.2, 97.2, 117.8, 122.3, 126.3, 126.5,
127.4, 141.0, 147.3, 162.4ppm.
Experimental
All fluorescence spectra were measured as reported previously
[14] using 9,10-diphenylanthracene as reference standard on a
Jobin Yvon Fluorolog 3 model FL 3–22 instrument by using
constant spectral parameters: step resolution (increment) of
1 nm, both excitation and emission slits of 2 nm, and integra-
tion time of 0.5 sec and were uncorrected. The UV-visible
spectra were recorded on a Perkin-Elmer Lambda 12 spectro-
photometer. NMR spectra were acquired on a Varian Unity
Plus spectrometer at 11.75 T magnetic field strength operating
1,2-Dimethyl-8,9-dimethoxy-3H,5H-dipyrrolo[1,2-c:20,10-
f[pyrimidine-3,5-dione (3, C14H14N2O4)
As in the procedure for 1, a mixture of 248 mg (1.00 mmol)
dipyrrinone 7, 0.81 g (5.0mmol) of CDI, and 0.75 cm3
(2.0mmol) DBU in 80 cm3 anhydrous CH2Cl2 was heated at
reflux under nitrogen for 16h. Work-up afforded pure 3, after
purification by radial chromatography (eluent: 2% CH3OH in
CH2Cl2, vol=vol) and recrystallization from n-hexane-ethyl
1
at H frequency of 500 MHz and 13C frequency of 125 MHz
in solutions of CDCl3 (referenced at 7.26 ppm for 1H and
77.00 ppm for 13C or (CD3)2SO (referenced at 2.49ppm for
1H and 39.50 ppm for 13C). J-modulated spin-echo (Attached
Proton Test) and gHMBC experiments were used to assign the

Fluorescence quenching in N,N0-bridged 9H-dipyrrinones
1383
acetate. Yield: 240 mg (91%); mp 245–247ꢂC; 1H NMR
(CDCl3): ꢃ ¼ 1.93 (3H, s), 2.11 (3H, s), 3.82 (3H, s), 3.97
(3H, s), 6.49 (1H, s), 7.11 (1H, s) ppm; 13C NMR (CDCl3):
ꢃ ¼ 8.8, 10.0, 58.4, 61.2, 95.8, 99.3, 116.5, 127.5, 132.3,
136.4, 141.5, 145.8, 167.6 ppm.
gen for 12h. Work-up as before gave pure 6. Yield: 0.11 g
1
(55%); mp 240–242ꢂC; H NMR (CDCl3): ꢃ ¼ 2.03 (3H, s),
2.12 (3H, s), 3.93 (3H, s), 4.17 (3H, s), 6.30 (1H, s), 6.77 (1H,
d, J ¼ 2.5 Hz), 9.98 (1H, br.s), 18.43 (1H, br.s) ppm; 13C NMR
(CDCl3): ꢃ ¼ 9.6, 10.3, 59.4, 60.9, 101.1, 119.6, 120.6, 121.3,
124.3, 124.6, 126.3, 148.2, 168.7 ppm.
1,2-Diethyl-8,9-dimethyl-3H,5H-dipyrrolo[1,2-c:20,10]-
pyrimidine-3,5-dione (4, C16H18N2O2)
7,8-Dimethoxy-2,3-dimethyl-10H-dipyrrin-1-one
Following the synthesis procedure for 1, to a solution of
260 mg (1.00 mmol) dipyrrinone 8 in 80 cm3 anhydrous
CH2Cl2 was added 0.81 mg (5.0mmol) CDI and 0.75cm3
(0.50 mmol) DBU, and the mixture was heated at reflux under
nitrogen for 16 h. Work-up and purification as before gave
pure 4. Yield 240 mg (92%); mp 198–199ꢂC; 1H NMR
(CDCl3): ꢃ ¼ 1.13 (3H, t, J ¼ 7.5 Hz), 1.24 (3H, t, J ¼
7.5 Hz), 2.05 (3H, s), 2.11 (3H, s), 2.40 (2H, q, J ¼ 7.5 Hz),
2.55 (2H, q, J ¼ 7.5 Hz), 6.42 (1H, s), 7.44 (1H, s) ppm; 13C
NMR (CDCl3): ꢃ ¼ 9.3, 10.5, 13.7, 14.8, 17.1, 18.1, 97.2,
117.6, 122.2, 126.5, 126.9, 131.4, 132.3, 141.9, 147.0,
167.5ppm.
(7, C13H16N2O3)
Following the same procedure for synthesis of 5, 500 mg
(3.23 mmol) of pyrrole aldehyde 12 and 0.716g (6.45 mmol)
pyrrolinone 10 were dissolved in 60 cm3 methanol to which
ꢀ2.8 g solid KOH were added. Reflux for 30h under nitrogen
was followed by cooling and quenching by the addition of
50cm3 water. After extraction with 600 cm3 CH2Cl2 (6ꢄ
100 cm3), the organic phases were combined, dried over an-
hydrous Na2SO4, and evaporated. The residue was dissolved
in a little ethyl acetate, and after addition of n-hexane, pure
product 7 precipitated. Yield: 700 mg (87%); mp 220–222ꢂC;
1H NMR (CDCl3): ꢃ ¼ 1.90 (3H, s), 2.10 (3H, s), 3.89 (3H, s),
4.02 (3H, s), 6.50 (1H, s), 6.9 (1H, s), 9.48 (1H, br.s) ppm; 13C
NMR (CDCl3): ꢃ ¼ 8.6, 10.1, 58.7, 61.8, 99.21, 104.3, 115.6,
125.3, 131.3, 137.0, 140.9, 142.1, 173.6 ppm.
1,2,8,9-Tetramethyl-3H,5H-dipyrrolo[1,2-c:20,10-f]-
pyrimidine-3,5-dione (40, C14H14N2O2)
Following the procedure outlined for 1, to a solution of
216 mg (1.00 mmol) dipyrrinone 80 in 80 cm3 anhydrous
CH2Cl2 was added 81 mg (5.0mmol) CDI and 0.75cm3
(0.50 mmol) DBU and the mixture was heated at reflux under
nitrogen for 16 h. Work-up and purification as before gave
pure 40. Yield: 193 mg (80%); mp 240–242ꢂC; 1H NMR
(CDCl3): ꢃ ¼ 1.93 (3H, s), 2.05 (3H, s), 2.10 (3H, s), 2.1
(3H, s), 6.93 (1H, s), 7.42 (1H, s) ppm; 13C NMR (CDCl3):
ꢃ ¼ 8.8, 9.3, 10.0, 10.6, 97.0, 117.5, 126.5, 126.9, 127.4,
132.4, 141.7, 141.7, 167.7 ppm.
2,3-Diethyl-7,8-dimethyl-10H-dipyrrin-1-one
(8, C15H22N2O)
Following the procedure for the synthesis of 5, the residue was
purified by radial chromatography (eluent: 1–3% CH3OH in
CH2Cl2, vol=vol) and then recrystallized from CH3OH in
CH2Cl2 to give pure dipyrrinone 8. Yield: 1.1 gm (60%); mp
1
118–120ꢂC; H NMR (CDCl3): ꢃ ¼ 1.13 (3H, t, J ¼ 7.6 Hz),
1.20 (3H, t, J ¼ 7.6 Hz), 2.00 (3H, s), 2.34 (3H, s), 2.55 (2H, q,
J ¼ 7.6 Hz), 2.56 (2H, q, J ¼ 7.6 Hz), 6.16 (1H, s), 6.80 (1H,
d, J ¼ 2.5 Hz), 10.49 (1H, br.s), 11.17 (1H, br.s) ppm; 13C
NMR (CDCl3): ꢃ ¼ 9.7, 10.4, 14.1, 16.1, 17.2, 18.0, 101.7,
119.6, 121.5, 124.5, 124.6, 128.4, 129.2, 148.2, 174.3 ppm.
2,3,7,8-Tetramethoxy-10H-dipyrrin-1-one (5, C13H16N2O5)
To a mixture of 0.50 g (3.2mmol) 3,4-dimethoxy-2-formyl-
1H-pyrrole (12), 1.0 mg (7.0mmol) 3,4-dimethoxy-1,5-dihy-
dro-2H-pyrrol-2-one (9) in 80 cm3 of methanol was added a
solution of 5.6g (0.10 mmol) KOH, and the mixture was heat-
ed at vigorous reflux for 48 h. After 24 h cooling, an additional
equivalent of 9 was added. After cooling, the solvent was
evaporated under vacuum, the residue was diluted with
10cm3 H2O, cooled in an ice bath, and extracted with
CH2Cl2 (5ꢄ100 cm3). The organic layer was evaporated
(rotovap) and purified by flash chromatography to give dipyr-
rinone 5, which was pure enough for use in the next step.
Yield: 0.36g (40%); mp 150–152ꢂC; 1H NMR (CDCl3):
ꢃ ¼ 3.79 (3H, s), 3.89 (3H, s), 3.92 (3H, s), 4.1 (3H, s) 5.85
(1H, s), 6.35 (1H, dd, J ¼ 2.0 Hz), 8.10 (1H, br.s), 9.30 (1H,
br.s) ppm; 13C NMR (CDCl3): ꢃ ¼ 58.4, 59.2, 60.8, 61.7, 97.8,
102.8, 115.2, 123.6, 127.1, 135.0, 141.1, 146.4, 166.6ppm.
3,4-Dimethoxy-2-formyl-1H-pyrrole (12, C7H9NO3)
3,4-Dimethoxy-1H-pyrrole (500 mg, 4.00mmol) was dis-
solved in 50cm3 anhydrous ether in a 250 cm3 r.b. flask. To
it was added 0.36cm3 dry DMF and 0.43 cm3 POCl3. Ayellow
solid precipitated, and the reaction was allowed to stir at 0ꢂC
for 6 h, then kept at 5ꢂC overnight. The organic layer was
evaporated (rotovap) and to the solid residue was added
ꢀ2 g KOH in minimal amount of water while cooling in an
ice bath. The resulting solution was allowed to stir for 30min,
then it was extracted with 6ꢄ100 cm3 CH2Cl2. The combined
organic layers were dried over anhydrous Na2SO4, filtered,
and evaporated (rotovap) to obtain crude pyrrole aldehyde 9,
which was purified using column chromatography. Yield:
320 mg (52%); mp 47–48ꢂC; 1H NMR (CDCl3): ꢃ ¼ 3.68
(3H, s), 3.98 (3H, s), 6.58 (1H, dd, J ¼ 2.0 Hz), 8.4 (1H,
br.s), 9.52 (1H, s) ppm; 13C NMR (CDCl3): ꢃ ¼ 59.3, 61.5,
111.7, 119.6, 139.1, 144.4, 176.9 ppm.
2,3-Dimethoxy-2,8-dimethyl-10H-dipyrrin-1-one
(6, C13H16N2O4)
Following the preparation of 5, to a solution of 100 mg
(0.80 mmol) pyrrole aldehyde 13 and pyrrolinone 9 (120mg,
0.80mmol) in 30 cm3 methanol was added 2.5 cm3 5 N metha-
nolic KOH, and the mixture was heated at reflux under nitro-
X-Ray structure and solution
Crystals of 3 were grown by slow diffusion of n-hexane into a
solution of CH2Cl2. A crystal was placed into the tip of a

1384
S. K. Dey, D. A. Lightner
0.1 mm diameter glass capillary and mounted on a Bruker
SMART Apex system for data collection at 100(2)K. A pre-
liminary set of cell constants was calculated from reflections
harvested from 3 sets of 20 frames. These initial sets of frames
were oriented such that orthogonal wedges of reciprocal space
were surveyed (final orientation matrices determined from
global least-squares refinement of 4984 reflections). The data
reflections from the actual data collection after integration
(SAINT 6.45, 2003) [16]. Crystal data and refinement infor-
mation given in Table 5.
The structure was solved and refined using SHELXL-L
[17]. The monoclinic space group P2(1)=c was determined
based on systematic absences and intensity statistics. A di-
rect-methods solution was calculated which provided most
non-hydrogen atoms from the E-map. Full-matrix least
squares=difference Fourier cycles were performed for struc-
ture refinement. All non-hydrogen atoms were refined with
anisotropic displacement parameters unless stated other-
wise. Hydrogen atom positions were placed in ideal posi-
tions and refined as riding atoms with relative isotropic
˚
collection was carried out using MoKꢂ radiation (0.71073 A
graphite monochromator) with a frame time of 20s and a
detector distance of 4.94cm. A randomly oriented region of
reciprocal space was surveyed to the extent of 2 hemispheres
˚
and to a resolution of 0.66A. Four major sections of frames
were collected with 0.5ꢂ steps in ! at 600 different ꢀ settings
and a detector position of 27ꢂ in 2ꢅ. The intensity data were
corrected for absorption and decay (SADABS) [15]. Final cell
constants were calculated from the xyz centroids of strong
˚
displacement parameters (a C–H distance fixed at 0.96 A
and a thermal parameter 1.2 times the host carbon atom).
Tables of atomic coordinates, bond lengths and angles, ani-
sotropic displacement parameters, hydrogen coordinates,
and isotropic displacement parameters have been deposited
at the Cambridge Crystallographic Data Centre, CCDC No.
677268 (Table 5).
Table 5 Crystal data and structure refinement for 3
Compound
3
Empirical formula
Formula weight
Temperature
C14H14N2O4
274.28
100(2) K
Acknowledgement
We thank the U.S. National Institutes of Health (HD 17779)
for generous support of this research. We also thank the
National Science Foundation (CHE-0226402 and CHE-
0521191) for providing funding to purchase the X-ray diffrac-
tometer used in this work and acquiring a 400 MHz NMR
spectrometer and upgrading an existing unit. We thank Prof.
T.W. Bell for use of the vapor pressure osmometer.
˚
Wavelength
0.71073 A
Monoclinic
P2(1)=c
Crystal system
Space group
Unit cell dimensions
˚
a ¼ 6.9495(6) A
ꢂ ¼ 90ꢂ
˚
b ¼ 21.2529(19) A
ꢁ ¼ 106.366(5)ꢂ
˚
c ¼ 8.5671(6) A
ꢆ ¼ 90ꢂ
3
˚
1214.07(17) A
Volume
Z
Density (calculated)
Absorption coefficient
F(000)
Crystal size
Theta range for data
collection
References
4
1.358 Mg=m3
0.098 mmꢁ1
528
1. a) Brower JO, Lightner DA (2002) J Org Chem 67:2713;
b) Pavlopoulos TG, Lightner DA, Brower JO (2003)
Applied Optics – LP 42:3555; c) Boiadjiev SE, Lightner
DA (2004) J Phys Org Chem 17:675; d) Boiadjiev SE,
Lightner DA (2004) J Heterocyclic Chem 41:1033; e)
Boiadjiev SE, Lightner DA (2005) J Org Chem 70:688; f)
Boiadjiev SE, Lightner DA (2005) Monatsh Chem
136:553; g) Boiadjiev SE, Lightner DA (2008) Monatsh
Chem 139:50
2. a) Boiadjiev SE, Lightner DA (2005) Monatsh Chem
136:489; b) Boiadjiev SE, Lightner DA (2005) Chirality
17:316; c) Nehira T, Boiadjiev SE, Lightner DA (2008)
Monatsh Chem 139:591
0.24ꢄ0.10ꢄ0.05 mm3
1.92 to 22.50ꢂ
Index ranges
ꢁ8 ꢅ h ꢅ 5, ꢁ17 ꢅ k ꢅ 24,
ꢁ10 ꢅ l ꢅ 10
Reflections collected
Independent reflections
Completeness to
Theta ¼ 25.00ꢂ
6259
2104 [R(int) ¼ 0.0406]
98.2%
Absorption correction
Max. and min.
Transmission
None
0.9951 and 0.9770
3. Coskun A, Deniz E, Akkaya EU (1005) J Mat Chem
15:2908
4. a) Berry & Associates Inc., 2434 Bishop Circle, Dexter,
MI 48130 USA; b) Chemical Abstracts registry numbers
850220-49-4, 850220-26-7, and 850220-22-3
5. a) Woydziak ZR, Boiadjiev SE, Norona WS, McDonagh
AF, Lightner DA (2005) J Org Chem 70:8417; b)
Boiadjiev SE, Woydziak ZA, McDonagh AF, Lightner
DA (2006) Tetrahedron 62:7043
Refinement method
Full-matrix least-
squares on F2
2104=0=186
Data=restraints=parameters
Goodness-of-fit on F2
Final R indices [I>2ꢇ(I)]
R indices (all data)
Extinction coefficient
Largest diff. peak and hole
1.048
R1 ¼ 0.0393, wR2 ¼ 0.0915
R1 ¼ 0.0591, wR2 ¼ 0.0981
0.0109(18)
0.236 and ꢁ0.223 eA
6. Merz A, Meyer T (1999) Synthesis:94
7. Dey SK, Lightner DA (2007) Monatsh Chem 138:687
ꢁ3
˚

Fluorescence quenching in N,N0-bridged 9H-dipyrrinones
1385
8. a) Xie M, Lightner DA (1993) Tetrahedron 49:2185; b)
Lightner DA, Low LK (1975) J Heterocyclic Chem
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9. a) Montforts F-P, Schwartz UM (1985) Liebig’s Ann
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14. BoiadjievSE, LightnerDA(2004) JPhys OrgChem17:675
15. Sheldrick GM (2003) SADABS, V6.14, Bruker Analyti-
cal X-ray Systems, Madison, WI, USA
10. a) For leading references, see Falk H (1989) The Chem-
istry of Linear Oligopyrroles and Bile Pigments. Springer,
¨
Wien; b) Falk H, Grubmayr K, Hollbacher G, Hofer O,
Leodolter A, Neufinger F, Ribo JM (1977) Monatsh
´
Chem 108:1113; c) Falk H, Schlederer T, Wolschann P
(1981) Monatsh Chem 112:199
11. a) Trull FR, Ma JS, Landen GL, Lightner DA (1983)
Israel J Chem (Symposium-in-Print on Chemistry and
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MT, Lightner DA (2001) Monatsh Chem 132:203
16. SAINT V6.45, Bruker Analytical X-ray Systems,
Madison, WI, USA
17. Sheldrick GM (2003) SHELXL-L V6.14, Bruker Analyt-
ical X-ray Systems, Madison, WI, USA