Exciton chirality. (A) origins of and (B) applications from strongly fluorescent dipyrrinone chromophores

Article abstract of DOI:10.1007/s00706-004-0278-3

(A) The origin of exciton interaction and examples of its application to organic stereo-chemistry are presented. (B) N,N′-Carbonyl-bridged dipyrrinones constitute a new class of highly fluorescent chromophores suitable for investigations of stereochemistr

Full text of DOI:10.1007/s00706-004-0278-3

Monatshefte f€ur Chemie 136, 489–508 (2005)
DOI 10.1007/s00706-004-0278-3
Exciton Chirality. (A) Origins of and (B)
Applications from Strongly Fluorescent
Dipyrrinone Chromophores
ꢀ
Stefan E. Boiadjiev and David A. Lightner
Department of Chemistry, University of Nevada, Reno, Nevada 89557, USA
Received October 5, 2004; accepted (revised) November 10, 2004
Published online March 8, 2005 # Springer-Verlag 2005
Summary. (A) The origin of exciton interaction and examples of its application to organic stereo-
chemistry are presented. (B) N,N0-Carbonyl-bridged dipyrrinones constitute a new class of highly
fluorescent chromophores suitable for investigations of stereochemistry and absolute configuration.
N,N0-Carbonylxanthobilirubic acid esters are strongly fluorescent, with a fluorescence quantum yield
(ꢀF) ꢁ0.8, but produce only weak exciton CD from the trans-1,2-cyclohexanediol template. The ester
of an analog with benzoic acid replacing propionic, N,N0-carbonyl-8-(4-carboxyphenyl)-3-ethyl-2,7,9-
trimethyl-(10H)-dipyrrin-1-one, exhibits strong fluorescence (ꢀF ¼ 0.68, ꢁem ¼ 493 nm, ꢁex ¼ 422 nm
in CHCl3) and UV-Vis absorption (" ꢁ 21000 at 424 nm) in organic solvents. Its diester with (1S,2S)-
cyclohexanediol is fluorescent and exhibits exciton circular dichroism (D" ¼ þ15dm3 ꢂ molꢃ1 ꢂ cmꢃ1
,
ꢁ ¼ 432 nm; D" ¼ ꢃ4 dm3 ꢂ molꢃ1 cmꢃ1, ꢁ ¼ 380nm) that correlates with the Exciton Chirality Rule.
Keywords. Circular Dichroism; Fluorescence; Exciton; Pyrroles.
Part A. Fundamentals of Exciton Chirality
Exciton Coupling
Exciton Spectra. Excitation of a molecule from its electronic ground state to an
electronically excited state – promoted usually by the absorption of ultraviolet or
visible light – involves movement of an electron, a process called an electronic
transition. In near ultraviolet-visible light absorption, the electron typically moves
from a high-energy occupied molecular orbital to a low-energy unoccupied mole-
ꢀ
cular orbital, such as in ꢂ ! ꢂ excitations. This movement of an electron creates
an instantaneous dipole or polarization of charge (called an electric transition
moment or electric transition dipole), a vector quantity with both a direction
(orientation) and a magnitude (intensity) that vary according to the nature of
the particular electronic transition and the chromophore involved. Two or more
ꢀ
Corresponding author. E-mail: lightner@scs.unr.edu

490
S. E. Boiadjiev and D. A. Lightner
Fig. 1. Diagrammatic representation of exciton coupling between two chromophores (A and B)
fastened together by covalent bonding or intermolecular forces; local excitations are shown (left and
ꢀ
ꢀ
right) for the chromophores in their locally excited monomer states (A or B ); when the two
chromophores lie sufficiently close to one another, or the local excitations are sufficiently intense,
excitation is delocalized between the two chromophores and the excited state (exciton) is split by
resonance interaction of the local excitations in the composite system or molecule (center); exciton
coupling may take place between identical chromophores (A ¼ B) or non-identical chromophores
(A ¼ B) and is less effective when the excitation energies are very different, i.e., when the relevant
UV-Vis absorption bands do not overlap
chromophores, brought into proximity to one another, may interact with each other
(even when orbital overlap and electron exchange are negligible) when one
chromophore is excited electronically. That is, when the electronic transitions
associated with two chromophores have similar energies (e.g., ꢁ) and intensities
(e.g., "), the electric transition dipole of one chromophore can interact with that of
the other chromophore, and the excitation energy becomes delocalized over the
two chromophores. This dipole–dipole coupling of locally excited states produces
a delocalized excitation (called an exciton) and results in a splitting (called exciton
splitting) of the locally excited states (Fig. 1). Such interaction is called exciton
coupling [1] from which stereochemical information may be extracted [1–5], as
will be shown in the following.
Exciton Spectra
Exciton coupling is manifested in two distinct bands in the UV-Vis spectrum, one red-
shifted from the center of the local excitation, the other blue-shifted. However, unless
the exciton splitting is large, a single broad band is more typically seen if both exciton
transitions are allowed (Fig. 2). When only one exciton transition is allowed, the
absorption spectrum may appear as either a red-shifted or blue-shifted single band.
When the interacting chromophores are oriented to form a chiral array, the exciton
transitions may be detected by circular dichroism (CD) spectroscopy, with spectra
typically taking on a characteristic bisignate form. Each of the UV-Vis exciton bands
has a corresponding CD band when the two chromophores are arranged in a chiral
orientation. Since the CD transitions from an exciton couplet are always oppositely
signed, exciton interaction is generally detected much more easily by CD spec-
troscopy than by UV-Vis spectroscopy. Important for analyzing stereochemistry,
the signed order of the bisignate CD spectrum can be correlated with the absolute

Strongly Fluorescent Dipyrrinone Chromophores
491
Fig. 2. Exciton coupling detected by simulated UV (lower) and CD (upper) spectroscopy; the observed
UV curve comes from summing the UV curves for the two exciton transitions; the observed bisignate
CD curve arises from summing the two oppositely-signed CD curves from the two exciton transitions
orientation of the interacting chromophores, according to the Exciton Chirality Rule
of Harada at Tohoku University and Nakanishi at Columbia University [2, 3].
Both UV and CD exciton spectra arise from the electronic spectral proper-
ties of the component chromophores, the distance between the chromophores
and their relative orientation [2, 3, 6]. Because excitons originate in electric
transition dipole–dipole coupling, strongly-allowed transitions with large transi-
tion dipoles are intrinsically more effective than weakly-allowed transitions,
given that dipole–dipole interaction falls off as the inverse cube of the separa-
tion distance. Electronic transition intensity and interchromophoric distance
are important factors in exciton coupling, as is the relative orientation of the
chromophores (specifically, the relative orientation of the relevant transition
dipoles). The orientation of two transition dipoles (and chromophores) is impor-
tant because it determines the allowedness of the two exciton transitions,
because it represents stereochemistry and relates absolute stereochemistry to the
signed order of the exciton CD couplet. The relative orientation of two dipoles
may be broken down into three limiting cases (Fig. 3) that correlate orientation
with predicted UV-Vis and CD spectra. Chromophores stacked with transition
moments parallel are predicted to exhibit a blue-shifted UV-Vis band because
only the higher energy, shorter wavelength exciton transition is allowed [1]. In
contrast, chromophores with transition moments in an in-line orientation exhibit
a red-shifted UV band because only the lower-energy (longer wavelength) exci-
tation is allowed. In these two limiting cases, the transition dipoles do not form
a chiral array; thus no CD results. In the more general orientation (oblique)
both exciton transitions are allowed, and the composite UV-Vis band is split or
unsplit, depending on whether the exciton splitting energy (V) is large or small.
With a chiral oblique orientation one sees a bisignate CD curve, a composite
from overlapping positive and negative Cotton effects.

492
S. E. Boiadjiev and D. A. Lightner
Fig. 3. Orientation dependence in exciton coupling between two chromophores (ellipsoids) and their
long-axis transition dipoles (represented by double-headed arrows); the solid single-headed arrows
connecting ground (G) and excited (X) states represent allowed transitions; dashed arrows represent
forbidden transitions; the consequences of the orientation may be found in wavelength-shifted UV-
Vis spectra; two limiting orientations lead to red-shifted (in-line) and blue-shifted (parallel) UV-Vis
bands; in the oblique orientation, both transitions are allowed and lead to broadened or split UV-Vis
spectra; allowed CD transitions are found only when the dipoles have a chiral, oblique orientation or
form a chiral array; V is the exciton splitting
Orientation Dependence of Chromophores in Exciton Coupling UV and CD
A useful example of how orientation dependence affects coupled spectra may be
seen clearly in the UV-Vis spectra of bis-porphyrins [7] (Fig. 4). Mono-porphyrin
spectra (Fig. 4A) show a typical sharp and narrow, very intense Soret band near
400 nm and weak long wavelength bands near 500 nm. When two porphyrin chro-
mophores are present, however (Figs. 4B–4D), the Soret band is usually split and
shifted, which may be taken as evidence for interchromophoric interaction (exciton

Strongly Fluorescent Dipyrrinone Chromophores
493
Fig. 4. UV-Vis absorption spectra of zinc etioporphyrins attached covalently to a naphthalene; (A) is
the reference spectrum for monomeric 1-naphthyl and 2-naphthyl porphyrins: 1-NP and 2-NP; (B)–
(D) compare the spectra (dashed line) of the monomer porphyrins of (A) to those of variously
oriented porphyrin dimers (redrawn from Ref. [7])
coupling). Absent such interaction, the spectra of Figs. 4B–4D would look exactly
like that of Fig. 4A, at twice the intensity.
Figures 4B–4D illustrate the dependence of the UV-Vis spectra on the orienta-
tion of the two chromophores. When the two porphyrin chromophores lie in a
stacked or parallel arrangement (Fig. 4B), a blue-shifted single band is observed,
exactly as predicted (Fig. 3, left). The Soret band is blue-shifted and broadened
relative to the monochromophore. At the other extreme, when the porphyrin chro-
mophores are arranged approximately in-line (Fig. 4C), exciton coupling theory
predicts a red-shifted band (Fig. 3, right). In this example, one sees (Fig. 4C) two
bands, with the more intense band being red-shifted. The presence of the weak,
shorter wavelength band indicates that the transition dipoles are not arranged per-
fectly in-line. In the oblique orientation of Fig. 4D, two bands are observed, as
predicted (Fig. 3, middle). Other examples of exciton-coupling in UV-Vis spectro-
scopy may be found [1–5], often with splittings not as large as those seen in

494
S. E. Boiadjiev and D. A. Lightner
Figs. 4B–4D. In such cases, when the chromophores constitute a chiral array, CD
can be an excellent tool for detecting and studying exciton coupling [2–5]. Por-
phyrins are becoming a potent chromophore for adjunct exciton chirality CD and
the determination of absolute configuration [3, 4, 8].
Anthracene (A) is also a particularly good chromophore for detecting exciton
coupling and examining orientation dependence. It has intense UV transitions,
hence large electric transition dipole moments: one oriented along its long axis
1
1
ð BbÞ, another oriented along the short axis ð LaÞ, as shown in Fig. 5. The more
intense (1Bb, " ꢁ 200000 dm3 ꢂ molꢃ1 ꢂ cmꢃ1) is polarized along the long axis of
the chromophore; the less intense (1La " ꢁ 7500 dm3 ꢂ molꢃ1 ꢂ cmꢃ1) is oriented
along the short axis, with dipole strengths of 88ꢄ1036 and 2.7ꢄ1036 cgs [2]. In the
Fig. 5. (A) UV spectra of anthracene (A) and the di-anthracene (DA) fused to bicyclo[2.2.2]octane;
the double-headed arrows ( !) shown on the structure of A are the in-plane orientations of the
electric dipole transition moment vectors associated with an intense short wavelength UV absorption
(1Bb) near 250 nm and a weak long wavelength absorption (1La) near 360 nm; ꢁmax for the exciton
bands of bis-anthracene is red shifted; the absorption ꢁmax of DA is red-shifted relative to that of A
because the former derives from an exciton interaction where the transition dipoles are oriented
approximately in-line (see Fig. 3); (B) CD spectrum of the chiral bis-anthracene (DA); the in-plane
long-axis oriented electric transition dipoles ( !) correspond to the ꢁ250 nm UV transition of each
A chromophore and in DA intersect at a positive torsion angle þ ꢁ151ꢅ; the signed order of the CD
Cotton effects of the ꢁ250 nm exciton couplet indicates a (þ)-helical orientation of the two electric
transition dipoles, confirming the absolute configuration shown (redrawn from Ref. [2])

Strongly Fluorescent Dipyrrinone Chromophores
495
chiral example (DA) of Fig. 5, with anthracenes fused to the bicyclo[2.2.2]octane
framework [2], the strong long axis-polarized transition dipoles of the chromo-
phores are not oriented parallel. They lie one in each of two planes that intersect
at a dihedral angle of ꢁ120ꢅ and intersect nearly in-line at an obtuse angle of
ꢁ150ꢅ. The UV spectrum of DA clearly resembles that of its individual component
chromophores but differs significantly; 1) It is not simply the sum of two indepen-
dent anthracene transitions, which one would obtain if the two A chromophores did
not interact. 2) Instead, characteristic of two overlapping exciton transitions, the
1
DA spectrum shows a broadened and intensified Bb band that is red-shifted from
252 to 267 nm. Broadening is due to unresolved exciton splitting, and the bath-
1
ochromic shift is due to an alignment of the Bb transition dipoles approaching
in-line: at an ꢁ150ꢅ intersection angle.
More noticeable in CD spectroscopy (Fig. 5B), exciton splitting in the DA seen
as a single band in the UV spectrum (Fig. 5A) is seen as oppositely-signed
members of a characteristic exciton couplet. The intensity (D") of the exciton
CD transitions is very large compared to ordinary CD spectra (jD"j ꢁ 1 to
10 dm3 ꢂ molꢃ1 ꢂ cmꢃ1 [5]) from non-interacting chromophores, and the signed
order of the exciton couplet CD Cotton effects may be correlated with the absolute
stereochemistry of the molecule, specifically, with the helicity of the relevant
electric dipole transition moments, according to the Exciton Chirality Rule
[2–4]. The long-axis transition dipoles make a positive dihedral or torsion angle
in DA (Fig. 5B), and a positive torsion angle should correlate with a long-wave-
length positive Cotton effect of the exciton CD couplet.
The Exciton Chirality Rule and Absolute Configuration
According to the Harada and Nakanishi’s Exciton Chirality Rule [2–3], when the
relevant transition moments are oriented to form a positive ðþÞ torsion angle
(positive chirality), the long wavelength component of the associated exciton cou-
plet can be expected to exhibit a positive Cotton effect (Fig. 6), as found in DA of
Fig. 5B. When they are oriented to form a negative ðꢃÞ torsion angle (negative
chirality), the long wavelength Cotton effect is negative. The CD spectrum of DA
(Fig. 5B) clearly shows an exciton splitting as two oppositely-signed, very intense
Cotton effects near 267 nm. The intensity (D") surpasses that from dissymmetric
vicinal action, indicating that exciton systems might be recognized by very intense
bisignate CD Cotton effects. The signed order of the Cotton effects can be corre-
1
lated with the relative orientation of the Bb transition moments from the two
anthracene chromophores. The extraordinarily intense exciton couplet of DA has a
ðþÞ component at 268 nm and ðꢃÞ component at 250 nm, thus positive ðþÞ exciton
chirality, which predicts a ðþÞ helical orientation of the transition dipoles – as is
characteristic of the absolute configuration shown. In general, from analyses of CD
(or ORD) spectral data, one can determine the absolute configuration when the
conformation is known, and one can obtain information on the conformation when
the absolute configuration is known. Exciton CD spectroscopy is typically used to
determine absolute configuration. Thus, from an exciton CD spectrum, one can
deduce the helical orientation of the transition moments, and with knowledge of the
electronic structure and orientation of the relevant electric transition dipoles, one

496
S. E. Boiadjiev and D. A. Lightner
Fig. 6. Graphical illustration of the Exciton Chirality Rule that relates the torsion angle or helicity of
two interacting electric dipole transition moments ( !) to the signed order of the CD Cotton effects
can elicit the absolute configuration of the bichromophore system or molecule
when the constitutional structure is known.
One may find many examples in which Exciton Chirality Rule is used to
determine absolute configuration [2–5]. In most cases, a chiral compound is deri-
vatized to introduce one or more suitable chromophores. Hydroxyl groups are
commonly derivatized as esters with acids having appropriate, usually aromatic
chromophores. The ideal chromophore would have a very intense UV-Vis electro-
nic transition located in a convenient spectral window, with the orientation of its
electric transition moment being well-defined relative to the ester R–O bond. 4-
Dimethylaminobenzoate, with an intense (" ꢁ 30000 dm3 ꢂ molꢃ1 ꢂ cmꢃ1) transition
(dipole oriented along the long axis of the molecule from nitrogen to carboxyl), is
an easily accessible chromophore, absorbing in a generally non-interfering UV
region (near 310 nm). Although the ester chromophore can adopt a large number
of different conformations, the s-cis predominates; thus, the relevant transition
dipole is typically aligned parallel to the ester O–R bond. Consequently, for a
1,2-diol diester the relative helicity (þ or ꢃ) of the transition dipoles can be
determined by inspection, and one can make an assignment of absolute configura-
tion from the CD spectrum. A good derivatizing agent is thus a symmetric acid in
which the alignment of strongly allowed electric dipole transition moment is
known with a high degree of certainty [5].
Application of the Exciton Chirality Rule to determine the absolute configura-
tion of trans-1,2-cyclohexanediol is straightforward. Assume a chair conformation,
which means both OH groups are equatorial. Looking down the C–C bond con-
necting the carbons bearing the OH groups (Fig. 7), one sees that the O–C–C–O
torsion angles have opposite helicities: ðþÞ for the (1S,2S) enantiomer, ðꢃÞ for the
(1R,2R). In the bis-4-dimethylaminobenzoate derivative, the relevant electric

Strongly Fluorescent Dipyrrinone Chromophores
497
Fig. 7. Conformational structure and Newman projection diagrams of (1S,2S)- and (1R,2R)-
cyclohexanediol and their bis-4-dimethylaminobenzoate derivatives
dipole transition moments align parallel to the C–O bonds of the diol and thus the
torsion angle and helicity made by the electric transition dipoles is the same as that
of the O–C–C–O torsion angle, with ðþÞ and ðꢃÞ exciton chirality as above. That
is, according to the Exciton Chirality Rule, the (1S,2S) diester will exhibit a posi-
tive exciton chirality, with long wavelength positive and short wavelength negative
Cotton effects for the ꢁ310 nm electronic transition(s), and the (1R,2R) diester will
exhibit a negative exciton chirality with a long wavelength negative and short
wavelength positive Cotton effects (Fig. 7).
As may be seen in Fig. 8, the bis-4-dimethylaminobenzoate ester of (1S,2S) and
(1R,2R)-cyclohexanediol exhibit a broadened UV spectrum, with a nearly split
long wavelength absorption near 310 nm, lying between the components of the
Fig. 8. CD (————) and UV (————) spectra of (1R,2R)- and (1S,2S)-cyclohexanediol bis-4-
dimethylaminobenzoate; CD (- - - - - - - -) and UV (- - - - - - - -) spectra of mono-4-dimethylamino-
benzoate in cyclohexane solvent at 22ꢅC

498
S. E. Boiadjiev and D. A. Lightner
CD couplets, as predicted from Figs. 2 and 3. Mirror image bisignate exciton CD
curves are seen, and the signed order of each couplet correlates with the helicity of
the diol O–C–C–O torsion angle and, more exactly, with the transition moments of
the diester, consistent with the predictions of the Exciton Chirality Rule. The CD
spectrum of the mono ester (mono, Fig. 8) is very different and shows an extremely
weak monosignate curve. Clearly, the presence of two 4-dimethylaminobenzoate
chromophores makes an enormous difference: the diester spectra are seen to be
neither the sum of two identical mono-ester UV curves, which would be coincident
and not broadened or shifted (Fig. 3), or two identical CD curves, which would be
net mono-signate. A striking contrast that implies a different origin for the diester
transitions – one diagnostic for and consistent with exciton coupling.
Part B. Exciton CD from Fluorescent Dipyrrinones
Introduction
A few years ago, we showed that the yellow dipyrrinone chromophore (Fig. 9), with
3
its intense long wavelength ꢂ–ꢂ absorption ("max ꢁ 30000 dm ꢂ molꢃ1 ꢂ cmꢃ1,
ꢀ
ꢁmax ꢁ 410 nm) electric transition dipole moment oriented along the long axis of
the molecule [9, 10] can be an effective chromophore for exciton chirality CD
studies in the visible region [11]. Positions C(2) and C(8) lie approximately along
the long axis of the molecule and thus constitute appropriate sites for attachment to
chiral molecules such as 1,2-cyclohexanediol. In the earlier studies we compared
exciton CD spectra of 1,2-cyclohexanediol and 1,2-diaminocyclohexane deriva-
tives with dipyrrinones with acetic acid and propionic acid belts attached to
C(8). The shorter belt, with fewer degrees of motional freedom, gave the more
intense CD, and both acetic and propionic derivatives correlated well with the
Exciton Chirality Rule [11]. However, we could not prepare a derivative with
the carboxylic acid group attached directly to C(8) and thereby reduce the con-
formational mobility.
More recently we showed that dipyrrinones with a carbonyl bridge between the
pyrrole and lactam are highly fluorescent, with fluorescence quantum yields
ꢀF>0.5 [12]. Strong fluorescence arises by preventing the energy-wasting
4Z ! 4E dipyrrinone carbon–carbon double bond isomerization [13]. For those
Fig. 9. (Left) The 10H-dipyrrin-1-one chromophore in its syn conformation with the approximate
orientation of the long wavelength (~410 nm) electric transition dipole moment ( !); (Right) the
highly fluorescent N,N0-carbonyl-bridged dipyrrinone chromophore (dipyrrolo[1,2-c:20,10-f]pyrimi-
dine-3,5-dione), with dipyrrinone numbering systems

Strongly Fluorescent Dipyrrinone Chromophores
499
Fig. 10. (A) The chromophores of this work: 4-dimethylaminobenzoic acid (DMAB), xantho-
bilirubic acid (XBR), fluorescent xanthoglow (XG), and its benzoic acid analog 1; (B) (1R,2R)-
cyclohexanediol (R1 ¼ R2 ¼ H) and its mono- and di-esters (ME and DE) with the chromophoric
carboxylic acids of (A); (C) Newman drawing looking down the 1,2 carbon-carbon bond of the diol
derivatized as a diester (DE) with the chromophores of (A)
studies [12] we prepared a derivative of xanthobilirubic acid (XBR, Fig. 10A),
which we call xanthoglow (XG, Fig. 10A). To extend our dipyrrinone chromophore
stereochemical investigations into exciton CD of fluorescent chromophores, we
prepared a new fluorescent dipyrrinone derivative (1), with fewer orientational
degrees of freedom than XG, a benzoic acid replacing propionic at C(8). With
the carboxyl group attached to a rigid benzene ring spacer at C(8), the dipyrrinone
long wavelength transition moment is expected to lie approximately parallel to the
long axis of the benzoic acid. In the current study, we prepared cyclohexanediol
esters with XG and 1 and compared the CD of mono- and di-esters (ME and DE) of
XBR, XG, and 1 with optically active 1,2-cyclohexanediol (Fig. 10B). Although a
fluorescent chromophore is not required for exciton chirality absorption CD stu-
dies, a strongly fluorescent chromophore might open up the possibility for com-
plementary CD studies in the emission mode, where increased sensitivity (from
very low sample concentrations) could be an advantage.
Results and Discussion
Synthesis Aspects
Xanthoglow (XG, Fig. 10A) was known from earlier studies; its benzoic acid
analog 1 was prepared as outlined in the Scheme. The known dipyrrinone 2

500
S. E. Boiadjiev and D. A. Lightner
Scheme
[14], which is rather insoluble in most organic solvents served as the starting
material, and procedures for converting it to 1 were investigated. Direct reaction
with 1,10-carbonyldiimidazole (CDI) works best with esters, although conditions
were recently developed to convert acid 2 into 1 in moderate yield after mild
hydrolysis of the intermediate acyl imidazole. To react an ester with CDI, an appro-
priate ester had to be chosen so as to facilitate conversion of the N,N0-carbonyl-
bridged dipyrrinone ester to its acid 1 without loss of the carbonyl bridge.
Saponification of the methyl or ethyl ester of 1 proved unsatisfactory; so, more
easily removed acid protecting groups were investigated: tert-butyl and benzyl.
Conversion of 2 to its tert-butyl ester 3a by activation of the acid with CDI then
treatment with tert-butyl alcohol gave only a 27% yield of 3a, but (on a small
scale) the yield could be improved to 47% by esterifying in the presence of sulfuric
acid adsorbed onto anhydrous magnesium sulfate [15]. Attempts to scale up the
reaction led to lower yields. Thus, although the tert-butyl ester 3a could be
converted to the N,N0-carbonyl-bridged dipyrrinone 4a, we abandoned using the
acid-sensitive tert-butyl protecting group in favor of the hydrogenolyzable benzyl

Strongly Fluorescent Dipyrrinone Chromophores
501
protecting group. Thus, reaction of 2 with cesium carbonate followed by benzyl
bromide afforded 3b in 64% yield, and 3b was smoothly converted to 4b in 78%
yield. However, deprotection of 4b led to both 1 as well as its 2,3-dihydro analog,
and this procedure, too, was abandoned in favor of a more successful route using
the allyl protecting group. Thus, the cesium carboxylate of 2 was reacted with allyl
bromide to give 3c in 83% yield, and 3c was converted to 4c in 96% yield.
Deprotection of 4c using tetrakis(triphenylphosphine)palladium(0) and pyrrolidine
[16] afforded 1 in 96% yield. Conversion of 1 to the di- and mono-esters of (1S,2S)
and (1R,2R)-cyclohexanediol was accomplished using procedures developed ear-
lier [11] and modified for the current work.
Circular Dichroism
Consistent with an earlier report [11b], the circular dichroism (CD) spectrum of
XBR-DE (Fig. 11) in CH2Cl2 clearly shows a negative chirality (D" ¼ ꢃ16.0 dm3 ꢂ
molꢃ1 ꢂ cmꢃ1, ꢁ ¼ 439 nm; D" ¼ þ22.3 dm3 ꢂ molꢃ1 ꢂ cmꢃ1, ꢁ ¼ 382 nm) exciton
transition – as predicted from the Exciton Chirality Rule [2, 3] and simple molec-
ular models. In the same solvents, the N,N0-carbonyl-bridged dipyrrinone analog
(XG-DE) also shows a negative chirality exciton CD spectrum, but of a clearly
weaker magnitude: D" ¼ ꢃ5.2 dm3 ꢂ molꢃ1 ꢂ cmꢃ1, ꢁ ¼ 433 nm; D" ¼ þ2.4 dm3 ꢂ
molꢃ1 ꢂ cmꢃ1, ꢁ ¼ 394 nm (Fig. 11). Thus, XG would appear to be a poor substitute
for XBR as an exciton coupling chromophore. More optimistically for circularly
polarized luminescence (CPL) or fluorescence detected CD (FDCD), ꢀF of XG-DE
remained high: ꢀF ¼ 0.37 (CHCl3).
Suspecting that by stiffening the linker between the chromophore enhanced CD
intensities would be found, we used 1 to prepare and examine 1-DE. The reasoning
Fig. 11. CD spectra in CH2Cl2 of XBR-DE ( ꢂ ꢂ ꢂ ꢂ ), XG-DE (———), 1-DE ( - - - - ), and 1-ME
(– ꢂ – ꢂ ) at room temperature

502
S. E. Boiadjiev and D. A. Lightner
Table 1. Comparison of the circular dichroism and UV-Vis spectral dataa from the esters of (1R,2R)-
cyclohexanediol with 1, XG, and XBR
Diester of
Solvent
Circular dichroism
D"1 max (ꢁ1)
ꢁ at D" ¼ 0
D"2 max (ꢁ2)
1
cyclohexane
benzene
ꢃ19.0 (429)
ꢃ5.1 (443)
ꢃ17.4 (431)
ꢃ3.8 (442)
ꢃ13.6 (445)
ꢃ15.2 (432)
ꢃ5.2 (438)
ꢃ15.3 (431)
ꢃ4.6 (437)
ꢃ16.0 (439)
ꢃ17.2 (426)
ꢃ3.1 (436)
ꢃ16.7 (429)
ꢃ2.4 (451)
ꢃ15.1 (432)
ꢃ3.0 (438)
ꢃ12.6 (419)
400
411
401
416
409
403
411
400
410
404
394
411
402
425
400
410
397
þ8.8 (380)
þ5.1 (394)
þ3.9 (381)
þ3.4 (388)
þ18.9 (386)
þ3.9 (380)
þ2.6 (393)
þ3.6 (380)
þ2.2 (396)
þ22.3 (382)
þ4.0 (388)
þ2.0 (398)
þ6.3 (381)
þ3.0 (401)
þ3.4 (380)
þ2.4 (397)
þ6.7 (382)
XG
1
XG
XBRc
1
CHCl3
XG
1
CH2Cl2
XG
XBRc
1
CH3CN
XG
1
CH3OH
(CH3)2SO
XG
1
XG
XBRc
Diester of
Solvent
UV-Vis
(ꢁ)
"
"
max
(ꢁ)
"
max
(ꢁ)b
"
max
(ꢁ)b
max
1
cyclohexane
benzene
38200 (429)
25800 (434)sh
40600 (428)sh
29100 (434)sh
41000 (407)
34100 (409)
41100 (416)
33900 (413)
61400 (393)
42200 (421)
34300 (421)
42600 (421)
36000 (421)
61400 (389)
43000 (415)
34600 (414)
40600 (420)
32600 (417)
41000 (421)
34000 (419)
51900 (408)
22500 (430)
19200 (432)
20800 (425)
18400 (428)
22700 (409)
19200 (412)
20900 (417)
18200 (417)sh
XG
1
XG
XBRc
1
CHCl3
21000 (424)
18800 (427)
21500 (421)
19200 (426)
XG
1
32300 (435)sh
34100 (434)sh
CH2Cl2
XG
XBRc
1
26700 (400)
CH3CN
21600 (417)
19100 (421)
21000 (421)
18300 (426)
20700 (421)
18900 (427)
XG
1
31500 (430)sh
CH3OH
(CH3)2SO
XG
1
XG
XBRc
a
b
c
" and D" in dm3 ꢂ molꢃ1 ꢂ cmꢃ1, ꢁ in nm; data for the mono-ester; data from Ref. [11b]
was at least qualitatively correct, as 1-DE gave an exciton CD spectrum (Fig. 11)
with D" ¼ ꢃ15.6 dm3 ꢂ molꢃ1 ꢂ cmꢃ1 (ꢁ ¼ 431 nm); D" ¼ þ3.7 dm3 ꢂ molꢃ1 ꢂ cmꢃ1
(ꢁ ¼ 383 nm), with the CD intensities being closer to those observed for XBR-DE.

Strongly Fluorescent Dipyrrinone Chromophores
503
Table 2. Fluorescence quantum yields (ꢀF) of monoesters (ME) and Diesters (DE) of 1 and XG with
trans-1,2-cyclohexanediol
c
Solventb
1-ME (ꢁem
)
1-DE (ꢁem
)
XG-ME (ꢁem
)
XG-DE (ꢁem)
Cyclohexane
Benzene
0.77 (458)
0.76 (483)
0.65 (492)
0.40 (526)
0.66 (504)
0.47 (473)
0.62 (481)
0.59 (493)
0.17 (524)
0.43 (503)
0.64 (474)
0.74 (487)
0.68 (497)
0.34 (530)
0.65 (506)
0.15 (501)
0.18 (501)
0.37 (501)
0.15 (509)
0.13 (507)
Chloroform
Methanol
Dimethyl sulfoxide
a
Relative quantum yield determined (Ref. [17]) vs. 9,10-diphenylanthracene standard: ꢀF ¼ 0.90 in
b
c
cyclohexane; concentration 6.4ꢄ10ꢃ7 to 1.2ꢄ10ꢃ6 mol ꢂ dmꢃ3
;
emission wavelength at maxi-
mum intensity
Although the CD intensities of 1-DE were disappointingly low, the data for 1-DE
and XG-DE suggest an attenuation due to large distances between the chro-
mophores and=or multiple conformations. Consistent with fluorescence measure-
ments reported for the methyl ester of XG (ꢀF ¼ 0.67, ꢁem ¼ 497 nm, ꢁex ¼ 419 nm,
CHCl3) [17] and the isobutyl ester of 1 (ꢀF ¼ 0.66, ꢁem ¼ 493 nm, ꢁex ¼ 420 nm,
CHCl3) [18], the fluorescence quantum yields of the mono esters of 1 and XG with
1,2-cyclohexanediol are also comparably large (Table 2). In the diesters, as might
be expected, the ꢀF values drop, for XG-DE down to about 25–50% of the ꢀF values
from XG-ME. Unexpectedly but gratifyingly, the drop is far smaller with the more
rigid diester, 1-DE. A comparison of the solvent dependence of ꢀF from 1-DE and
XG-DE (Table 2) indicates that the ꢀF values of the former remain much higher than
those of the latter, often comparable to those of 1-ME. For example, ꢀF ¼ 0.59
(CHCl3) for 1-DE, or some 50% higher than ꢀF (0.37) from XG-DE, thus indicating
that 1-DE might be a worthy candidate for investigations of CPL or FDCD.
Experimental
Nuclear magnetic resonance spectra were measured on a Varian Unity Plus spectrometer at 11.75 T
magnetic field strength operating at 1H frequency of 500 MHz and 13C frequency of 125 MHz. Unless
otherwise noted, CDCl3 solvent was used throughout, and chemical shifts are reported in ꢃ ppm
referenced to residual CHCl3 1H signal at 7.26ppm and CDCl3 13C signal at 77.00 ppm. J-modulated
spin-echo (Attached Proton Test) and gHMBC experiments were used to assign the 13C NMR signals.
All UV-Vis spectra were recorded on a Perkin Elmer Lambda 12 spectrophotometer and the circular
dichroism spectra were recorded on a JASCO J-600 dichrograph. The fluorescence spectra were
measured on a Jobin Yvon FluoroMax 3 instrument, and quantum yields were determined according
to literature procedures [17]. All optical spectral measurements were carried out using 1 cm quartz

504
S. E. Boiadjiev and D. A. Lightner
cells. 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 chromatography was carried out on J. T. Baker silica gel IB-F plates
(125 ꢄm layers). Melting points were determined on a Mel-Temp capillary apparatus and are uncor-
rected. The combustion analyses for carbon, hydrogen, and nitrogen were carried out by Desert
Analytics, Tucson, AZ, and gave results within ꢆ0.3% of the theoretical values. Commercial reagents
were used as received from Acros or Aldrich. The solvents for optical spectroscopy were HPLC grade
and were distilled under a stream of argon just prior to use. Before the distillation, CHCl3 was passed
through a basic alumina column. Dimethyl sulfoxide was distilled at 0.5 mm Hg vacuum collecting the
solvent at 0ꢅC and thawing it under Ar. Dipyrrinone 2 was available from previous work [14].
8-[4-(t-Butyloxycarbonyl)phenyl]-3-ethyl-2,7,9-trimethyl-1,10-dihydro-11H-dipyrrin-1-one
(3a, C25H30N2O3)
To a vigorously stirred suspension of 4.8 g anhydrous MgSO4 (40 mmol) in 40cm3 anhydrous CH2Cl2
was slowly added conc H2SO4 (0.55 cm3, 10mmol) and the mixture was stirred for 20min [15]. Then
175 mg acid 2 (0.5mmol) were added followed by 4.8 cm3 t-butanol (50 mmol) and the mixture was
stirred in a tightly closed flask for 19 h. Saturated aqueous NaHCO3 solution was added slowly until
effervescence ceased and the mixture was partitioned between 200 cm3 H2O and 150 cm3 CHCl3. The
organic layer was washed with H2O (2ꢄ100 cm3), then dried (Na2SO4), filtered, and the solvent was
evaporated under vacuum. The residue was purified by radial chromatography followed by recrystal-
lization from CH2Cl2–CH3OH. Yield 95 mg (47%); mp 275–279ꢅC (dec); 1H NMR: ꢃ ¼ 1.20
(t, J ¼ 7.6 Hz, 3H), 1.62 (s, 9H), 1.96 (s, 3H), 2.19 (s, 3H), 2.49 (s, 3H), 2.57 (q, J ¼ 7.6 Hz, 2H),
6.21 (s, 1H), 7.34 (m, J ¼ 8.1 Hz, 2H), 8.04 (m, J ¼ 8.1Hz, 2H), 10.66 (br.s, 1H), 11.40 (br.s, 1H) ppm;
13C NMR: ꢃ ¼ 8.6, 10.4, 12.4, 15.0, 18.0, 28.2, 80.8, 100.9, 122.9, 123.0, 123.2, 123.7, 127.9, 129.29,
129.32, 129.5, 132.0, 140.4, 148.5, 165.9, 174.3ppm.
8-[4-(Benzyloxycarbonyl)phenyl]-3-ethyl-2,7,9-trimethyl-1,10-dihydro-11H-dipyrrin-1-one
(3b, C28H28N2O3)
A mixture of 0.70 g dipyrrinone 2 (2.0mmol), 0.57 g Cs2CO3 (1.7mmol), 15cm3 anhydrous dimethyl-
formamide, and 0.7 cm3 benzyl bromide (6.0mmol) was stirred under Ar at 50ꢅC for 4 h. After
cooling, the mixture was partitioned between 150 cm3 CHCl3 and 100 cm3 1% aq HCl. The organic
layer was washed with 4ꢄ100 cm3 H2O, then dried (Na2SO4), filtered, and the solvent was evaporated
under vacuum. The residue was purified by radial chromatography followed by recrystallization from
1
CH2Cl2–CH3OH. Yield 0.56g (64%); mp 273–275ꢅC (dec); H NMR: ꢃ ¼ 1.20 (t, J ¼ 7.7 Hz, 3H),
1.96 (s, 3H), 2.19 (s, 3H), 2.49 (s, 3H), 2.58 (q, J ¼ 7.7 Hz, 2H), 5.40 (s, 2H), 6.21 (s, 1H), 7.36 (m,
3H), 7.40 (m, 2H), 7.47 (m, J ¼ 8.3 Hz, 2H), 8.12 (m, J ¼ 8.3 Hz, 2H), 10.64 (br.s, 1H), 11.36 (br.s, 1H)
ppm; 13C NMR: ꢃ ¼ 8.6, 10.4, 12.5, 15.0, 18.0, 66.6, 100.9, 122.7, 123.1, 123.2, 123.6, 127.3, 128.0,
128.1, 128.2, 128.6, 129.6, 129.7, 132.1, 136.2, 141.1, 148.6, 166.5, 174.3ppm.
8-[4-(Allyloxycarbonyl)phenyl]-3-ethyl-2,7,9-trimethyl-1,10-dihydro-11H-dipyrrin-1-one
(3c, C24H26N2O3)
A mixture of 1.75 g dipyrrinone 2 (5.0mmol), 1.63 g Cs2CO3 (5.0mmol), 30cm3 anhydrous dimethyl-
formamide, and 1.3cm3 allyl bromide (15.0mmol) was stirred under Ar in a tightly closed flask at 80–
85ꢅC for 16 h. After cooling, the mixture was partitioned between 300 cm3 CHCl3 and 100 cm3 1%
aq HCl. The organic layer was washed with 4ꢄ100 cm3 H2O, then dried (Na2SO4), filtered, and the
solvent was evaporated under vacuum. The residue was purified by radial chromatography followed by
1
recrystallization from CH2Cl2–CH3OH. Yield 1.63 g (83%); mp 280–282ꢅ (dec); H NMR: ꢃ ¼ 1.20

Strongly Fluorescent Dipyrrinone Chromophores
505
(t, J ¼ 7.7 Hz, 3H), 1.96 (s, 3H), 2.20 (s, 3H), 2.50 (s, 3H), 2.57 (q, J ¼ 7.7 Hz, 2H), 4.85
3
4
3
4
2
(ddd, J ¼ 5.7 Hz, J ¼ 1.5, 1.4 Hz, 2H), 5.31 (ddt, J ¼ 10.4Hz, J ¼ 1.4 Hz, J ¼ 1.2 Hz, 1H), 5.44
3
4
2
(ddt, J ¼ 17.2 Hz, J ¼ 1.5 Hz, J ¼ 1.2Hz, 1H), 6.07 (ddt, J ¼ 17.2, 10.4, 5.7 Hz, 1H), 6.21 (s, 1H),
7.36 (m, J ¼ 8.3 Hz, 2H), 8.11 (m, J ¼ 8.3Hz, 2H), 10.66 (br.s, 1H), 11.39 (br.s, 1H) ppm; 13C NMR:
ꢃ ¼ 8.5, 10.4, 12.4, 15.0, 17.9, 65.4, 100.8, 118.1, 122.6, 123.0, 123.2, 123.6, 127.3, 127.9, 129.53,
129.54, 132.1, 132.4, 141.0, 148.5, 166.3, 174.2ppm.
General Procedure for N,N-Cyclization Leading to 4a–4c
A mixture of 1 mmol of dipyrrinone 3a–3c, 0.81g 1,10-carbonyldiimidazole (5mmol), 0.75cm3 1,8-
diazabicyclo[5.4.0]undec-7-ene (5mmol), and 80cm3 anhydrous CH2Cl2 was heated under N2 at reflux
for 16 h. After cooling, the mixture was washed with 1% aq HCl (2ꢄ70cm3), then with H2O
(3ꢄ70 cm3), and the solution was dried (MgSO4). After filtration and evaporation of the solvent under
vacuum, the residue was purified by radial chromatography followed by recrystallization from ethyl
acetate-hexane to afford bright yellow tricyclic compounds 4a–4c.
8-[4-(t-Butyloxycarbonyl)phenyl]-1-ethyl-2,7,9-trimethyl-3H,5H-
dipyrrolo[1,2-c:20,10-f]pyrimidine-3,5-dione (4a, C26H28N2O4)
Yield 0.41g (94%); mp 201–202ꢅC; 1H NMR: ꢃ ¼ 1.23 (t, J ¼ 7.7 Hz, 3H), 1.61 (s, 9H), 1.98 (s, 3H),
2.09 (s, 3H), 2.56 (q, J ¼ 7.7Hz, 2H), 2.65 (s, 3H), 6.46 (s, 1H), 7.29 (m, J ¼ 8.4 Hz, 2H), 8.05
(m, J ¼ 8.4 Hz, 2H) ppm; 13C NMR: ꢃ ¼ 8.5, 9.7, 13.76, 13.82, 18.0, 28.2, 81.1, 97.0, 119.7, 126.6,
126.7, 129.3, 129.5, 129.9, 130.8, 130.9, 131.9, 138.0, 143.4, 146.6, 165.6, 167.7 ppm.
8-[4-(Benzyloxycarbonyl)phenyl]-1-ethyl-2,7,9-trimethyl-3H,5H-
dipyrrolo[1,2-c:20,10-f]pyrimidine-3,5-dione (4b, C29H26N2O4)
Yield 0.36g (78%); mp 177–178ꢅC; 1H NMR: ꢃ ¼ 1.23 (t, J ¼ 7.7 Hz, 3H), 1.97 (s, 3H), 2.09 (s, 3H),
2.56 (q, J ¼ 7.7 Hz, 2H), 2.65 (s, 3H), 5.39 (s, 2H), 6.46 (s, 1H), 7.32 (m, J ¼ 8.4 Hz, 2H), 7.35 (m, 1H),
7.40 (m, 2H), 7.46 (m, 2H), 8.14 (m, J ¼ 8.4 Hz, 2H) ppm; 13C NMR: ꢃ ¼ 8.5, 9.7, 13.76, 13.80, 18.0,
66.7, 96.9, 119.6, 126.66, 126.68, 128.1, 128.2, 128.6, 128.8, 129.1, 129.8, 130.0, 130.9, 131.9, 136.0,
138.7, 143.4, 146.6, 166.2, 167.6 ppm.
8-[4-(Allyloxycarbonyl)phenyl]-1-ethyl-2,7,9-trimethyl-3H,5H-
dipyrrolo[1,2-c:20,10-f]pyrimidine-3,5-dione (4c, C25H24N2O4)
Yield 0.40g (96%); mp 195–196ꢅC; 1H NMR: ꢃ ¼ 1.24 (t, J ¼ 7.7 Hz, 3H), 1.98 (s, 3H), 2.10 (s, 3H),
2.57 (q, J ¼ 7.7 Hz, 2H), 2.66 (s, 3H), 4.85 (ddd, 3J ¼ 5.7 Hz, 4J ¼ 1.5, 1.4 Hz, 2H), 5.30 (ddt,
3J ¼ 10.4Hz, 4J ¼ 1.4 Hz, 2J ¼ 1.2 Hz, 1H), 5.43 (ddt, 3J ¼ 17.3Hz, 4J ¼ 1.5 Hz, 2J ¼ 1.2 Hz, 1H),
6.05 (ddt, J ¼ 17.3, 10.4, 5.7 Hz, 1H), 6.46 (s, 1H), 7.32 (m, J ¼ 8.5 Hz, 2H), 8.13 (m, J ¼ 8.5 Hz,
2H) ppm; 13C NMR: ꢃ ¼ 8.4, 9.6, 13.70, 13.73, 17.9, 65.5, 96.9, 118.2, 119.6, 126.5, 126.6, 128.8,
129.0, 129.6, 129.9, 130.8, 131.8, 132.1, 138.5, 143.3, 146.5, 165.9, 167.5ppm.
8-(4-Carboxyphenyl)-1-ethyl-2,7,9-trimethyl-3H,5H-
dipyrrolo[1,2-c:20,10-f]pyrimidine-3,5-dione (1, C22H20N2O4)
To an Ar-protected suspension of 1.67 g allyl ester 4c (4.0mmol) in 25cm3 anhydrous acetonitrile was
added 0.23g tetrakis(triphenylphosphine)palladium(0) (0.2mmol) and 0.10 g triphenylphosphine
(0.4mmol), followed by a solution of 0.37 cm3 pyrrolidine (4.4 mmol) in 3 cm3 acetonitrile, and the
mixture was stirred for 6 h [16]. Then it was diluted with 400 cm3 CH2CH2:CHCl3 ꢁ 1:1 (by volume)
and washed with 1% aq HCl (2ꢄ50 cm3) and H2O (2ꢄ100 cm3). After drying (Na2SO4), filtration, and

506
S. E. Boiadjiev and D. A. Lightner
evaporation of the solvents under vacuum, the residue was triturated with benzene (30 cm3) and the
product was separated by filtration. Washing with 2ꢄ5 cm3 benzene afforded acid 1 with sufficient
purity for use in the next steps. Small portion of the crude product was purified by radial chromato-
graphy followed by recrystallization from ethyl acetate-hexane to give analytically pure acid 1. Yield
1
1.44 g (96%); mp 319–321ꢅC; H NMR (DMSO-d6): ꢃ ¼ 1.13 (t, J ¼ 7.6Hz, 3H), 1.86 (s, 3H), 2.09
(s, 3H), 2.54 (s, 3H), 2.59 (q, J ¼ 7.6 Hz, 2H), 7.11 (s, 1H), 7.40 (m, J ¼ 8.3 Hz, 2H), 8.00 (m, J ¼ 8.3 Hz,
2H), 12.96 (br.s, 1H) ppm; 13C NMR (DMSO-d6): ꢃ ¼ 8.1, 9.4, 13.6, 13.7, 17.2, 98.1, 119.3, 125.4,
126.9, 128.3, 129.3, 129.4, 129.9, 130.5, 130.6, 137.8, 142.5, 147.3, 166.9, 167.1 ppm.
General Procedure for Syntheses of Diesters
To an Ar-protected solution of 0.75 mmol of acid 1 or XG [12] in 3 cm3 anhydrous DMSO was added
130 mg CDI (0.80 mmol), and the mixture was stirred at 45ꢅC for 1.5 h. Then (1S,2S)- or (1R,2R)-
trans-cyclohexanediol or diaminocyclohexane (0.25mmol) followed by 19mm3 DBU (0.125 mmol)
were added, and the mixture was stirred at 85–90ꢅC for 16h. After cooling, it was diluted with 150cm3
CHCl3 and washed with 1% aq HCl (50 cm3) and H2O (4ꢄ100 cm3). The solution was dried (Na2SO4),
filtered, and after evaporation of the solvent, the residue was purified by radial chromatography and
recrystallization from CH2Cl2–CH3OH or ethyl acetate-hexane.
General Procedure for Syntheses of Monoesters
The syntheses of monoesters followed the same procedure as for diesters above except for the ratio
acid 1 or XG:cyclohexanediol was 0.50mmol:1.00 mmol.
(þ)-(1S,2S)-Cyclohexylidene bis[1-ethyl-2,7,9-trimethyl-3H,5H-
dipyrrolo[1,2-c:20,10-f]pyrimidine-3,5-dione-8-(1,4-phenylenecarboxylate)]
(ent-1-DE, C50H48N4O8)
20
Yield 0.19 g (90%); mp 306–309ꢅC (dec); ½ꢅꢇD ¼ þ460.2 10ꢃ1 ꢂ deg ꢂ cm2 ꢂ gꢃ1 (c ¼ 0.15, CHCl3); 1H-
NMR: ꢃ ¼ 1.23 (t, J ¼ 7.7 Hz, 6H), 1.52 (m, 2H), 1.65 (m, 2H), 1.86 (m, 2H), 1.97 (s, 6H), 2.05 (s, 6H),
2.28 (m, 2H), 2.56 (q, J ¼ 7.7 Hz, 4H), 2.61 (s, 6H), 5.28 (m, 2H), 6.44 (s, 2H), 7.26 (m, J ¼ 8.4 Hz,
4H), 8.05 (m, J ¼ 8.4 Hz, 4H) ppm; 13C NMR: ꢃ ¼ 8.4, 9.7, 13.7, 13.8, 17.9, 23.5, 30.2, 74.4, 96.9,
119.6, 126.6, 126.7, 128.9, 129.0, 129.7, 129.9, 130.9, 131.8, 138.5, 143.4, 146.6, 165.8, 167.6 ppm.
(ꢃ)-(1R,2R)-Cyclohexylidene bis[1-ethyl-2,7,9-trimethyl-3H,5H-
dipyrrolo[1,2-c:20,10-f]pyrimidine-3,5-dione-8-(1,4-phenylenecarboxylate)]
(1-DE, C50H48N4O8)
20
Yield 0.17 g (81%); mp 301–308ꢅC (dec); ½ꢅꢇD ¼ ꢃ470.0 10ꢃ1 ꢂ deg ꢂ cm2 ꢂ gꢃ1 (c ¼ 0.23, CHCl3).
(þ)-(1S,2S)-Cyclohexylidene 1-[1-ethyl-2,7,9-trimethyl-3H,5H-
dipyrrolo[1,2-c:20,10-f]pyrimidine-3,5-dione-8-(1,4-phenylenecarboxylate)]-2-ol
(ent-1-ME, C28H30N2O5)
20
Yield 0.18 g (77%); mp 167–169ꢅC; ½ꢅꢇD ¼ þ18.9 10ꢃ1 ꢂ deg ꢂ cm2 ꢂ gꢃ1 (c ¼ 0.42, CHCl3); 1H NMR:
ꢃ ¼ 1.24 (t, J ¼ 7.7Hz, 3H), 1.42 (m, 4H), 1.77 (m, 2H), 1.98 (s, 3H), 2.10 (s, 3H), 2.17 (m, 2H), 2.57
(q, J ¼ 7.7 Hz, 2H), 2.66 (s, 3H), 3.76 (m, 1H), 4.88 (m, 1H), 6.46 (s, 1H), 7.33 (m, J ¼ 8.4 Hz, 2H),
8.12, (m, J ¼ 8.4Hz, 2H) ppm; 13C NMR: ꢃ ¼ 8.5, 9.7, 13.76, 13.81, 18.0, 23.7, 23.9, 30.0, 33.0, 72.8,
78.7, 96.9, 119.6, 126.68, 126.69, 129.0, 129.1, 129.7, 130.0, 130.9, 131.9, 138.7, 143.4, 146.6, 166.5,
167.7ppm.

Strongly Fluorescent Dipyrrinone Chromophores
507
(ꢃ)-(1R,2R)-Cyclohexylidene 1-[1-ethyl-2,7,9-trimethyl-3H,5H-
dipyrrolo[1,2-c:20,10-f]pyrimidine-3,5-dione-8-(1,4-phenylenecarboxylate)]-2-ol
(1-ME, C28H30N2O5)
20
Yield 0.16 g (69%); mp 168–170ꢅC; ½ꢅꢇD ¼ ꢃ20.4 10ꢃ1 ꢂ deg ꢂ cm2 ꢂ gꢃ1 (c ¼ 0.27, CHCl3).
(þ)-(1S,2S)-Cyclohexylidene bis[1-ethyl-2,7,9-trimethyl-3H,5H-
dipyrrolo[1,2-c:20,10-f]pyrimidine-3,5-dione-8-(1,2-ethylenecarboxylate)]
(ent-XG-DE, C42H48N4O8)
20
Yield 0.13 g (71%); mp 165–167ꢅC; ½ꢅꢇD ¼ þ129.7 10ꢃ1 ꢂ deg ꢂ cm2 ꢂ gꢃ1 (c ¼ 0.17, CHCl3); 1H
NMR: ꢃ ¼ 1.19 (t, J ¼ 7.7 Hz, 6H), 1.35 (m, 2H), 1.38 (m, 2H), 1.72 (m, 2H), 1.88 (s, 6H), 1.99
(m, 2H), 2.08 (s, 6H), 2.34 (m, 4H), 2.51 (q, J ¼ 7.7 Hz, 4H), 2.54 (m, 4H), 2.60 (m, 6H), 4.84 (m, 2H),
6.32 (s, 2H) ppm; 13C NMR: ꢃ ¼ 8.3, 9.0, 13.0, 13.7, 17.9, 19.2, 23.4, 30.2, 34.7, 73.8, 96.9, 120.4,
125.9, 126.0, 126.3, 130.2, 131.3, 143.1, 146.6, 167.6, 171.9 ppm.
(ꢃ)-(1R,2R)-Cyclohexylidene bis[1-ethyl-2,7,9-trimethyl-3H,5H-
dipyrrolo[1,2-c:20,10-f]pyrimidine-3,5-dione-8-(1,2-ethylenecarboxylate)]
(XG-DE, C42H48N4O8)
20
Yield 0.13 g (73%); mp 164–166ꢅC; ½ꢅꢇD ¼ ꢃ133.2 10ꢃ1 ꢂ deg ꢂ cm2 ꢂ gꢃ1 (c ¼ 0.25, CHCl3).
(þ)-(1S,2S)-Cyclohexylidene 1-[1-ethyl-2,7,9-trimethyl-3H,5H-
dipyrrolo[1,2-c:20,10-f]pyrimidine-3,5-dione-8-(1,2-ethylenecarboxylate)]-2-ol
(ent-XG-ME, C24H30N2O5)
20
Yield 0.11 g (53%); mp 159–160ꢅC; ½ꢅꢇD ¼ þ24.6 10ꢃ1 ꢂ deg ꢂ cm2 ꢂ gꢃ1 (c ¼ 0.25, CHCl3); 1H NMR:
ꢃ ¼ 1.21 (t, J ¼ 7.7 Hz, 3H), 1.29 (m, 4H), 1.70 (m, 2H), 1.95 (s, 3H), 2.01 (m, 2H), 2.07 (br.s, 1H), 2.13
(s, 3H), 2.47 (t, J ¼ 7.7 Hz, 2H), 2.53 (q, J ¼ 7.7Hz, 2H), 2.66 (s, 3H), 2.75 (t, J ¼ 7.7 Hz, 2H), 3.53 (m,
1H), 4.58 (m, 1H), 6.38 (s, 1H) ppm; 13C NMR: ꢃ ¼ 8.4, 9.2, 13.1, 13.8, 17.9, 19.4, 23.7, 23.9, 30.0, 33.1,
34.7, 72.8, 78.5, 96.9, 120.4, 125.8, 126.27, 126.30, 130.4, 131.6, 143.4, 146.5, 167.8, 172.9ppm.
(ꢃ)-(1R,2R)-Cyclohexylidene 1-[1-ethyl-2,7,9-trimethyl-3H,5H-
dipyrrolo[1,2-c:20,10-f]pyrimidine-3,5-dione-8-(1,2-ethylenecarboxylate)]-2-ol
(XG-ME, C24H30N2O5)
20
Yield 0.10 g (47%); mp 159–160ꢅC; ½ꢅꢇD ¼ ꢃ26.9 10ꢃ1 ꢂ deg ꢂ cm2 ꢂ gꢃ1 (c ¼ 0.30, CHCl3).
Acknowledgments
We thank the U.S. National Institutes of Health (HD-17779) for generous support of this work. Dr.
S. E. Boiadjiev is on leave from the Institute of Organic Chemistry, Bulgarian Academy of Sciences.
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