BF2-chelate complexes of 6-(4-iodophenyl)-2,3,4,8,9,10- hexamethyldipyrrin and 2-(4-iodobenzoyl)-3,4,5-trimethylpyrrole: Fluorescent dyes with a chemical anchor group

Article abstract of DOI:10.1002/zaac.200800112

The one-pot condensation/coordination reaction of 4-iodobenzoylchloride, 2,3,4-trimethylpyrrole and BF₃ × Et₂O yields the BF₂ chelate complexes of the 1:1 condensation product 2-(4-iodobenzoyl)-3,4,5-trimethylpyrrole and of the 1:2 product 6-(4-iodophenyl)-2,3,4,8,9,10-hexamethyldipyrrin, as separable compounds in 6 and 38 % yield, respectively. Both new boron derivatives are fluorescent already upon exitation with ambient light. While the fluorescence quantum yield of the benzoyl derivative is very low, this value is significantly higher for the related boron dipyrrin (BODIPY) derivative. Single crystal X-ray diffraction studies of both compounds reveal that the reason for these deviating physical properties are structural in nature. For the BODIPY an essentially flat structure of the fluorophor has been established, in addition to restricted rotation of the 4-iodophenyl substituent, so that no conformational dynamic facilitates radiationless deactivations. The 1:1 condensation product on the other hand allows a fast equilibration of the photophysical exitation by dynamic processes and therefore exhibits a low fluorescence quantum yield. Both luminophores contain an iodoaryl moiety with potential uses for further functionalization and bioconjugation.

Full text of DOI:10.1002/zaac.200800112

DOI: 10.1002/zaac.200800112
BF2-Chelate Complexes of 6-(4-Iodophenyl)-2,3,4,8,9,10-hexamethyldipyrrin and
2-(4-Iodobenzoyl)-3,4,5-trimethylpyrrole: Fluorescent Dyes with a Chemical
Anchor Group
Martin Bröring*, Robin Krüger, and Christian Kleeberg
Marburg, Philipps-Universität, Fachbereich Chemie
Received March 12th, 2008.
Abstract. The one-pot condensation/coordination reaction of
4-iodobenzoylchloride, 2,3,4-trimethylpyrrole and BF3 ϫ Et2O
yields the BF2 chelate complexes of the 1:1 condensation product
2-(4-iodobenzoyl)-3,4,5-trimethylpyrrole and of the 1:2 product
6-(4-iodophenyl)-2,3,4,8,9,10-hexamethyldipyrrin, as separable
compounds in 6 and 38 % yield, respectively. Both new boron de-
rivatives are fluorescent already upon exitation with ambient light.
While the fluorescence quantum yield of the benzoyl derivative is
very low, this value is significantly higher for the related boron
dipyrrin (BODIPY) derivative. Single crystal X-ray diffraction
studies of both compounds reveal that the reason for these deviat-
ing physical properties are structural in nature. For the BODIPY
an essentially flat structure of the fluorophor has been established,
in addition to restricted rotation of the 4-iodophenyl substituent,
so that no conformational dynamic facilitates radiationless deacti-
vations. The 1:1 condensation product on the other hand allows a
fast equilibration of the photophysical exitation by dynamic pro-
cesses and therefore exhibits a low fluorescence quantum yield.
Both luminophores contain an iodoaryl moiety with potential uses
for further functionalization and bioconjugation.
Keywords: Fluorescent dyes; BODIPY®s; Oligopyrroles; Boron;
Structure-Function correlation
Introduction
2,2Ј-bidipyrrins [30, 31] seem particularly appealing in this
context due to their straight-forward synthesis, high sta-
bility, and known ability to form mono- and dinuclear tran-
sition metal chelates of different structure [32Ϫ39]. In the
course of our latest investigations in such boron chelate
species of open-chain oligopyrrolic ligands [40] it became
apparent that the spectroscopy as well as the chemistry of
the covalent dimers deviates significantly from what is
known for BODIPYs. We therefore became interested in
BODIPYs as spectroscopic reference and reaction testing
systems. In this contribution we report the preparation as
well as the structural and general photophysical investi-
gation of such a compound carrying a 4-iodophenyl anchor
group for further functionalization [41].
Due to their outstanding photophysical properties and light
stability the class of BODIPY fluorophors (Fig. 1) currently
attract multifacetted interest in many research areas.
BODIPYs constitute a class of boron chelates with a dipyr-
rin ligand [1] (BOron DIPYrrin, also called boron dipyrro-
methene or boraindacene). This particular class of boron
chelate species is very versatile and has found application
as functional dyes in laser dyes [2, 3], light harvesters [4Ϫ6],
fluorescent switches [7], biomolecular labels [8], cation sen-
sors [9Ϫ17], and other fields [18Ϫ24]. Very recently, two
comprehensive reviews of the field have appeared in the lit-
erature [25, 26].
Results and Discussion
The one-pot synthesis of meso-aryl substituted BODIPYs
is straightforward and follows a well established procedure
as depicted in Scheme 1. If 2,3,4-trimethylpyrrole (1) and
4-iodobenzoylchloride (2) are used as starting materials the
desired BODIPY 3 can be isolated in 38 % yield. As de-
scribed before for a related system [42], a minor fluorescent
product, the ketopyrrole BF2 chelate 4, forms as a by-prod-
uct under these conditions and can be isolated after chrom-
atographic purification in 6 % yield. The elemental compo-
sition of the products was established by high-resolution
ESIϩ mass spectrometry which gave evidence for [MϩNa]ϩ
ions in both cases.
Fig. 1 Basic structure of the BODIPY chromophor.
Oligomeric dipyrrins are of interest as ligands for BF2
groups because of the potential formation of covalent
BODIPY aggregates [27Ϫ29]. Formal dipyrrin dimers like
* Prof. Dr. M. Bröring
Fachbereich Chemie d. Universität
D-35032 Marburg
E-Mail: Martin.broering@chemie.uni-marburg.de
Z. Anorg. Allg. Chem. 2008, 634, 1555Ϫ1559
© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1555

M. Bröring, R. Krüger, C. Kleeberg
F···H, F···C or F···F contacts and has been analyzed in sev-
eral instances in the past [43Ϫ46]. The respective F···H
coupling with the protons of the terminal methyl groups
can not be observed, presumably due to the fast rotation of
these substituents. The same holds true for F···C and F···H
couplings in 4 which may be undetectable due to the larger
distance between these groups and/or a reduced molecular
rigidity.
The most prominent feature of the new compounds, how-
ever, is their intense color and the fluorescence of particu-
larly the BODIPY (3). Both, absorption and emission spec-
tra, have been recorded in ethanol for both species and are
shown in Figure 3. The absorption spectrum of 3 is charac-
terized by two major bands at 332 and 525 nm which are
typical for all BODIPY dyes. The smaller chromophor of 4
absorbs at higher energy and shows broad bands at 299 and
387 nm. Emission spectra were recorded in ethanol after
irradiation into the major absorbtion bands at 525 and
387 nm, respectively, and have been corrected for solvent
emission. A single fluorescence band has been found for 3
and 4 at 542 and 522 nm, which is intense and narrow for
3, and weak and broad for 4.
Scheme 1 Preparation of 4-iodophenyl substituted fluorophors 3
and 4.
The presence of four easily detectable nuclei 1H, 13C, 19F
and 11B in 3 and 4 allows for detailed NMR investigations
1
on the new compounds. The H NMR spectra bear no sur-
prises and can simply be understood as composed of inde-
pendent signal groups for the alkylpyrrole and the 4-iodo-
phenyl subunits. The same applies for the 19F and 11B NMR
spectra. Due to the effective symmetry of both molecules
the fluorine atoms in the BF2 groups of both compounds
are magnetically equivalent. In the 19F NMR spectra of 3
and 4 broadened four line signals appear for these nuclei at
1
Ϫ146.5 and Ϫ159.0 ppm, respectively, due to JB,F coup-
lings with the I ϭ 3/2 nucleus 11B. The 11B NMR spectra on
the other hand present the anticipated broadened triplets at
0.41 and 2.92 ppm for the BODIPY (3) and the ketopyrrole
chelate (4), respectively.
Fig. 2 Details of the 13C NMR spectra of 3 (left) and 4 (right)
showing a “through space” C···F coupling for the BODIPY
(75 MHz, CD2Cl2).
An unexpected feature was found in the proton decou-
pled 13C NMR spectrum of BODIPY (3). The signal associ-
ated with the terminal methyl carbon nuclei at 12.5 ppm is
split to a triplet with a small coupling constant of J ϳ 2 Hz,
while the equivalent signal in the 13C NMR spectrum of 4
shows no such splitting (Fig. 2). This splitting can be under-
stood on the basis of a “through space” magnetic interac-
tion between the fluorine nucleus and the methyl group in
3. “Through space” coupling (as opposed to “through bond”
coupling) is known for compounds with close and rigid
Fig. 3 Electronic absorption (top) and emission spectra (bottom)
of 3 and 4 in ethanol. Note the different scales for extinction and
intensity for both dyes.
While naked eye can clearly distinguish the strong fluor-
escence of 3 from the weak one of 4 the determination of
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© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2008, 1555Ϫ1559

BF2-Chelate Complexes
fluorescence quantum yields has been found quite difficult
in this case. The extremely weak fluorescence emission of 4
can hardly be distinguished from instrumental noise and is
visible only for relatively high dye concentrations. Aggre-
gation phenomena can therefore not be excluded in this
case [47]. A quantum yield analysis performed on the ob-
tained data [48] results in relative fluorescence intensities of
about 120:1 for 3 and 4.
3 and 4 display advantageous crystallization properties
and could be investigated in the solid state by single crystal
X-ray diffraction. 3 forms red crystals in the monoclinic
system, space group P21/c, with Z ϭ 4. The yellow 4 also
crystallizes readily from pentane layered dichloromethane
solutions in the monoclinic system, space group P21/n, with
Z ϭ 4. Illustrations of the molecular structures and selected
metrical data are given in Figures 4 and 5.
ates from the ideal tetrahedral angle and therefore leads to
a slightly unsymmetric tetrahedral coordination of boron.
The 4-iodophenyl and the BF2 groups are situated in an
almost orthogonal arrangement with dihedral angles of
77.25° and 91.69°, respectively, with respect to the BODIPY
main plane. A rigid molecule with a single minimum con-
formation can be assumed from the small anisotropy of the
ellipsoids even at a probability level of 90 % (Fig. 4). This
latter observation is usually a precondition for strong fluor-
escence as observed for 3 and other BODIPY dyes. The
distances between the F atoms and the carbon atoms of the
˚
methyl termini are found between 3.045 and 3.251 A which
is clearly below the van der Waals limit [49]. In combination
with the rigid molecular structure this small distance allows
magnetic interactions between the 19F and 13C nuclei and
therefore leads to the observed coupling in the 13C NMR
spectrum of 3.
The C5BNO core unit of the ketopyrrole chelate 4 is also
essentially planar with a maximum displacement of an
˚
atom from the mean squares plane of only 0.026 A, and
presents the BF2 subunit plane in almost orthogonal ar-
rangement (89.75°). Other than in 3, however, the 4-iodo-
phenyl substituent of 4 is found in conjugation with the
C5BNO fluorophor and deviates by only 2.38° from the re-
spective mean squares plane. The coordination polyhedron
of the boron atom is more distorted from a tetrahedron
than in 3 due to the bonding within a five-membered ring
chelate with a N-B-O angle of only 98.5°. Several ellipsoids
show strongly anisotropic displacements (Figure 5). The
sites of these ellipsoids are found within the 4-iodophenyl
substituent and within the C2BNO chelate and indicate ro-
tational and conformational freedom at these positions,
respectively. It is very likely that the low fluorescence inten-
sity of 4 is caused by an intramolecular quenching of the
S1 exited state through these movements, and that in par-
ticular the dynamics within the C2BNO chelate ring lead to
a significant weakening of magnetic coupling between the
fluorine atoms and the terminal methyl group. In combi-
nation with the observed elongation of the F···Cterm dis-
Fig. 4 Molecular structure of BODIPY (3) (ellipsoids set at 90 %
probability, hydrogen atoms removed for clarity).
˚
Selected bond lengths /A and angles /°: B-F1 1.382(4), B-F2 1.401(4), B-
N1 1.539(4), B-N2 1.547(4), F1-B-F2 109.3(2), F1-B-N1 110.9(3), F1-B-N2
110.5(2), F2-B-N1 109.8(2), F2-B-N2 109.1(3), N1-B-N2 107.1(2).
˚
tances to 3.587 and 3.612 A the structural features of 4 thus
provide a satisfactory explanation for the lack of a “through
space” coupling in this case.
Fig. 5 Molecular structure of ketopyrrole-BF2 chelate 4 (ellip-
soids set at 90 % probability, hydrogen atoms removed for clarity).
Conclusion
˚
Selected bond lengths /A and angles /°: B-F1 1.379(5), B-F2 1.373(5), B-N
1.547(4), B-O 1.507(4), F1-B-F2 110.4(3), F1-B-N 113.4(3), F1-B-O 110.7(3),
F2-B-N 112.8(3), F2-B-O 110.4(3), N-B-O 98.5(2).
In summary we have described new examples for a general
preparative entry to 4-iodophenyl functionalized or-
ganoboron fluorophors, characterized the new compounds
by spectroscopic and X-ray crystallographic means and an-
alyzed deviating spectroscopic and photophysical behaviour
of two fluorophors with respect to their molecular struc-
tures. Future work will attempt metal catalyzed homo- and
cross coupling reactions as well as halide-metal exchange
studies on these functional dyes in order to produce novel
building blocks for biohybrid syntheses.
The differences between the fluorophors 3 and 4 found
in the C···F through space-coupling and in fluorescence
intensity can be understood from the results of the crystal-
lographic analyses. The BODIPY framework of 3 is essen-
tially flat with a maximum displacement of a single atom
from the mean squares C9N2B plane of only 0.065 A. Since
the boron atom is bound by two nitrogen atoms in a six-
membered ring chelate the angle N1-B-N2 of 107.1° devi-
˚
Z. Anorg. Allg. Chem. 2008, 1555Ϫ1559
© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
1557

M. Bröring, R. Krüger, C. Kleeberg
Crystal data for 3: C21H22BF2IN2, 478.12, monoclinic, space group
P21/c, a ϭ 8.4442(8), b ϭ 7.5710(9), c ϭ 31.096(4) A, β ϭ
Experimental Section
˚
Materials and Methods: 2,3,4-Trimethylpyrrole (1) [50] was pre-
pared according to a published procedure. All other chemicals were
purchased from Aldrich and used without further purification. Sol-
vents were dried by standard procedures and distilled from appro-
priate drying agents. NMR spectra were obtained with a Bruker
ARX-300 or a Bruker DRX 400 spectrometer. Chemical shifts (δ)
are given in ppm relative to residual protio solvent resonances (1H,
13C NMR spectra) or to external standards (BF3 ϫ Et2O for 11B
and CFCl3 for 19F NMR spectra). High resolution ESI mass spec-
tra were recorded with a QStarPulsar i. UV/Vis spectra were meas-
ured on a Shimadzu UV-1601 PC in concentrations of about 10Ϫ5
mol LϪ1. Emission spectra were obtained on a Varian Cary
Eclipse spectrophotometer.
3
94.674(12)°, V ϭ 1981.4(4) A , Z ϭ 4, ρcalc ϭ 1.603 g cmϪ3
,
˚
μ(Mo-Kα) ϭ 1.642 cmϪ1, θmin ϭ 2.42°, θmax ϭ 25.85°, 13138 reflec-
tions measured, 3804 independent, 2712 observed with I > 2σ(I),
250 parameters, 0 restraints, R1 (I > 2σ(I)) ϭ 0.0260, wR2 (all
Ϫ3
˚
data) ϭ 0.0568, max./min. peak ϭ 0.488/Ϫ0.402 eA
.
Crystal data for 4: C14H13BF2INO, 386.96, monoclinic, space
˚
group P21/n, a ϭ 7.7594(7), b ϭ 15.760(2), c ϭ 11.6942(10) A, β ϭ
3
101.529(10)°, V ϭ 1401.2(3) A , Z ϭ 4, ρcalc ϭ 1.834 g cmϪ3
,
˚
μ(Mo-Kα) ϭ 2.301 cmϪ1, θmin ϭ 2.20°, θmax ϭ 26.01°, 10700 reflec-
tions measured, 2565 independent, 2075 observed with I > 2σ(I),
184 parameters, 0 restraints, R1 (I > 2σ(I)) ϭ 0.0287, wR2 (all
Ϫ3
˚
data) ϭ 0.0708, max./min. peak ϭ 1.010/Ϫ0.575 eA
.
Preparation of the fluorophors 3 and 4: 2,3,4-Trimethylpyrrole (1)
(436 mg , 4 mmol) is dissolved in dichloromethane (80 mL) and
treated with 4-iodobenzoylchloride (534 mg , 2 mmol). The mixture
is stirred for 4 days at ambient temperature, and the brown solution
Acknowledgements. We gratefully acknowledge financial support
for this work by the Volkswagen-Foundation.
gradually turns red. Triethylamine (606 mg
, 6 mmol) and
References
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λ /nm (εrel) ϭ 332 (0.10), 525 (1). Ϫ Emission (EtOH): λ /nm ϭ 542. Ϫ
HRMS (ESIϩ): m/z 501.0789; calcd. for [C21H22BF2IN2Na]ϩ: 501.0792.
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