BOIMPYs: Rapid Access to a Family of Red-Emissive Fluorophores and NIR Dyes

Article abstract of DOI:10.1002/anie.201606883

A fundamental, highly fluorescent, and easily accessible scaffold derived from the BODIPY core is reported. The use of benzimidazole as a bridging ligand at the meso position enables the binding of two BF₂units to provide sufficient rigidity and enhanced electron-withdrawing strength. Absorption and emission events thus take place in the red (λ≈600 nm); the fluorescence quantum yields can reach unity (0.96) and show little dependence on solvent polarity. The synthetic route was shortened to two steps starting from commercially available precursors while the preparation is modular and tolerates various pyrrole and benzimidazole moieties. Fluoride replacement by propynyl groups, various halogenations, as well as Knoevenagel-type condensations were applied to extend the versatility of these new photostable fluorophores, which we termed BOIMPYs.

Full text of DOI:10.1002/anie.201606883

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Communications
Chemie
International Edition: DOI: 10.1002/anie.201606883
German Edition: DOI: 10.1002/ange.201606883
Fluorophores
BOIMPYs: Rapid Access to a Family of Red-Emissive Fluorophores
and NIR Dyes
Lukas J. Patalag, Peter G. Jones, and Daniel B. Werz*
Dedicated to Professor Rolf Gleiter on the occasion of his 80th birthday
Abstract: A fundamental, highly fluorescent, and easily
accessible scaffold derived from the BODIPY core is reported.
The use of benzimidazole as a bridging ligand at the meso
position enables the binding of two BF2 units to provide
sufficient rigidity and enhanced electron-withdrawing strength.
Absorption and emission events thus take place in the red
(l ꢀ 600 nm); the fluorescence quantum yields can reach unity
(0.96) and show little dependence on solvent polarity. The
synthetic route was shortened to two steps starting from
commercially available precursors while the preparation is
modular and tolerates various pyrrole and benzimidazole
moieties. Fluoride replacement by propynyl groups, various
halogenations, as well as Knoevenagel-type condensations
were applied to extend the versatility of these new photostable
fluorophores, which we termed BOIMPYs.
Our initial considerations about novel fluorogenic motifs
arose from the idea of efficiently exploiting the crucial meso
position of the parent BODIPY core. Its high LUMO
coefficient can be addressed by derivatization with electron-
withdrawing substituents to narrow the HOMO–LUMO gap
and to achieve a red shift in absorption and emission. Even
though this approach has already been applied with striking
impact (aza-BODIPYs, cyano substitution),[13] no aromatic
residues fully conjugated and in plane with the core structure
have yet been installed at the meso position because of steric
À
shielding by the adjacent C H bonds at positions 1 and 7
(Figure 1).[14]
S
ince their first preparation in 1968,[1] BODIPY fluoro-
phores[2] have contributed heavily to numerous research
fields, such as the development of biological labels,[3] tunable
laser dyes,[4] solid-state solar concentrators,[5] electrolumines-
cence devices,[6] photodynamic therapy,[7] and diverse indica-
tors[8] usually based on fluorescence modulation mechanisms.
At the same time, various analogues have come into focus[9] as
the red edge of the visible spectrum has become gradually
more appealing. Aza-BODIPYs[10] filled this gap elegantly
despite their restriction to redundant substitution patterns.
Similarly, a range of aromatic-ring-fused and dimeric frame-
works or combinations thereof were introduced[11] although
synthetic efforts, also for precursor formation, often increased
disproportionately with sophisticated structural complex-
ity.[12] The ability to shift the beneficial photophysics of the
BODIPY core (such as high fluorescence quantum yields,
good photostability, and brightness) to longer wavelengths
while still benefitting from the synthetic repertoire available
for its functionalization would greatly strengthen this vibrant
field of research.
Figure 1. Geometrical derivation of the BOIMPY motif and its modes
of functionalization.
Our intention was to circumvent this repulsion by trans-
forming it into a coordination pattern, with the assistance of
two BF2 units and one imidazole auxochrome as a bridging
ligand. As the imidazole moiety is now contributing signifi-
cantly to the framework geometry, we consider the term
BOIMPY (bis(borondifluoride)-8-imidazodipyrromethene)
to be suitable for such motifs. Benzannulation was chosen
to extend the newly accessible p system further and to
enhance stability by steric encasement of the contiguous
binding pockets. A mirror plane, as depicted in Figure 1,
provides an alternative visualization of the structural design.
The 2-hydroxymethylbenzimidazoles 1a–i were smoothly
converted into the respective carbaldehydes with ortho-
iodoxybenzoic acid (IBX) and condensed in situ with ethyl-
pyrrole to furnish dipyrromethanes 2a–i in yields of up to
59% (Scheme 1). The evolving basicity of products 2a–i,
additionally hampered by the very poor solubility of the
carbaldehyde intermediates, was overcome with a buffering
DMSO/NMP solvent system (NMP = N-methyl-2-pyrrolidi-
none) and an excess of trifluoroacetic acid (TFA). Extensive
studies of the final step into fluorophores 3a–i revealed low
[*] Dipl.-Chem. L. J. Patalag, Prof. Dr. D. B. Werz
Technische Universitꢀt Braunschweig
Institute of Organic Chemistry
Hagenring 30, 38106 Braunschweig (Germany)
E-mail: d.werz@tu-braunschweig.de
Prof. Dr. P. G. Jones
Technische Universitꢀt Braunschweig
Institute of Inorganic and Analytical Chemistry
Hagenring 30, 38106 Braunschweig (Germany)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201606883.
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Figure 2. BOIMPYs with various (halogenated) pyrrole moieties.
[a] Yields of isolated products after IBX oxidation and condensation
with benzimidazole 1a. [b] Yields of isolated products for the fluoro-
phore syntheses. [c] Different procedure. [d] A non-fluorescent ana-
logue with one BF2 unit was first obtained in 53% yield; the second
BF2 unit was inserted in an additional step (44% overall yield).
Scheme 1. Two-step synthesis of BOIMPYs. [a] First value: yield of the
one-pot oxidation/condensation sequence; second value in red: yield
of the fluorophore synthesis.
temperatures (À788C) and strong, hindered bases to be
prerequisites for efficient oxidation and twofold BF2 coordi-
nation. Whereas 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)
raised the yields to a level of about 20%, Schwesingerꢀs non-
cationic phosphazene base (BEMP) established reproducible
yields of up to 45% for this two-step one-pot process. The
dipyrrins formed after 2,3-dichloro-5,6-dicyano-1,4-benzoqui-
none (DDQ) oxidation suffer from significant instability and
have not yet been isolated. Surprisingly, only traces of the
BODIPY congeners with a non-participating benzimidazole
auxochrome were formed, which underlines its crucial role
during the initial stage of the coordination process under the
strongly basic reaction conditions.
Owing to its enhanced electron deficiency, the BOIMPY
core can be halogenated in a controlled fashion to furnish
substrates for cross-coupling reactions in high yields
(Figure 2). The b-positions of the ethyl groups are strongly
favored for substitution. An excess of N-bromosuccinimide
(NBS) led to a triply brominated product (3n) with an
increased torsion angle of 35.78 (Figure 4) whereas
N-iodosuccinimide (NIS) iodination occurred far more selec-
tively, making singly and doubly iodinated species (3o, 3p)
accessible upon varying the temperature.
Scheme 2. Fluoride substitution to furnish the corresponding E-BOIM-
PYs. [a] Aside from 3a, 3 f was also subjected to the reaction (see the
Supporting Information). [b] Grignard reagent (12 equiv). [c] Grignard
reagent (5 equiv). [d] The diol was employed as the corresponding
acetonide, and deprotection was achieved in the same process; the
overall yield is given.
boron centers in a well-defined synthetic process. When
acetal-protected 3-butyne-1,2-diol was employed as the
Grignard reagent, the fluoride replacement can be completed
with an acidic deprotection step to furnish the water-soluble
variant 7 in a one-pot process. This approach constitutes
a general method of converting variously derivatized fluo-
rophores into hydrophilic congeners. As an extension to the
broad portfolio of introduced water-soluble (aza-)BODIPYs
(e.g., ammonium salts,[16] sulfonated species),[17,10e] fluoro-
phore 7 is uncharged and offers eight hydroxy groups for
further derivatizations or specific bioconjugation (e.g., glyco-
sylation).
To demonstrate the analogy to BODIPY chemistry
further, we studied the replacement of the fluorides with
carbon nucleophiles to obtain so-called E-BOIMPYs
(Scheme 2). Depending on the excess of 1-propynylmagne-
sium bromide, tetrasubstituted derivatives (5a, 5 f) and their
unsymmetrically disubstituted counterparts (6a, 6 f) were
obtained in excellent overall yields. A cooperative effect
seems to promote the complete conversion of one BF2 group
before the second one is attacked. The option of addressing
the boron centers sequentially sets the stage for preparing
complex cascade-type dyads[15] or triads of suitable fluoro-
phores, such as pyrenes or perylenes, attached at neighboring
Styryl-substituted BODIPYs are known to show large
bathochromic shifts in absorption and emission. When
involving bulky, hydrophobic aromatic aldehydes, BOIMPY
3j proved to be ideally suited for Knoevenagel-type con-
densations with yields of up to 90% and very short reaction
times, a feature that might be attributed to the amplified
acidity at the a-methyl groups of precursor 3j (Figure 3).
However, this reactivity can be tamed by replacing commonly
used acetic acid with TFA to favor singly condensed NIR
fluorophores, such as the fluorescent fatty acid analogue 13 or
the azido-equipped congener 12 as a labeling agent for click
chemistry (labs ꢀ 700 nm). Whereas these singly styryl-deriv-
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Figure 4. X-ray crystal structure[18] of BOIMPY 3a and calculated
interplanar angles (DFT). [a] First value: between the pyrrole moieties;
second value: between the pyrrole and the benzimidazole moiety.
[b] Determined from the crystal structure.
Figure 3. Styryl-containing NIR-BOIMPYs by Knoevenagel condensa-
tion.
atized species outpace corresponding BODIPY variants by
a bathochromic shift of Dl = 120 nm in absorption, a twofold
condensation with dialkoxy-substituted aromatic aldehydes
leads to wavelengths of 780 nm (10, 11) and even 951 nm (9)
with 2,2’-bisthiophene moieties. Compared with similar
BODIPY variants, the pronounced shift of electron density
to the core-centered LUMO upon excitation and the con-
comitantly polarized styryl double bond might be reasons for
the disappointing decrease in fluorescence quantum yields for
all styryl-containing NIR BOIMPYs. No significant fluores-
cence could be detected for NIR dye 9 (Table 1).
The exact geometry of the fluorophore was determined by
single-crystal X-ray diffraction of the parent BOIMPY 3a
(Figure 4). Repulsion between H2 and H6 results in an
interplanar angle of 9.48 (DFT: 19.28) between the pyrrole
moieties. Thus the symmetry of the minimum-energy struc-
ture is C2, which was supported by DFT calculations (B3LYP/
6–311G(d,p)), even though the energy difference to a C2v
the LUMO, which is partially delocalized outside the pyrrole
units, the distribution of electron density in the HOMO
corresponds to the pattern calculated for a typical BODIPY
core (Figure 5). More remarkable, however, is the energy
difference between the frontier orbitals in comparison to
those of
a
BODIPY prototype (DELUMOs = À0.76 eV,
DEHOMOs = À0.35 eV) or an aza-BODIPY congener
(DELUMOs = À0.23 eV, DEHOMOs = À0.17 eV), rendering the
chromophore highly electron-deficient. Despite its conjuga-
tion-based connectivity, the additional meso p system is
therefore not responsible for the bathochromic shift of
100 nm with respect to a comparable BODIPY variant.
Instead, the second BF2 unit with its strongly electron-
withdrawing nature narrows the HOMO–LUMO gap down
to 2.70 eV, which is somewhat smaller than that calculated for
an aza-BODIPY (2.76 eV) and, to the best of our knowledge,
the smallest value hitherto reported for a dipyrromethene
core. Electronic saturation of the pyrroles (b-alkyl groups, 3l)
is consequently effective and triggers a shift of about 20 nm in
absorption and a sharp increase in the extinction coefficient
(Figure 6).
geometry as a vibrational transition state is quite small (ca.
À1
À
1 kcalmol ). Standard N B bond lengths (ca. 1.55 ꢁ)
indicate a tight, symmetric mode of chelation. Unlike for
The Stokes shifts were small
(ca. 400 cmÀ1) as expected, reflect-
Table 1: A selection of prepared compounds and their spectroscopic data at room temperature.[a]
ing a singlet emission event and the
rigidity enforced by the two boron
centers, preventing non-radiative
channeling of excitation energy.
This results in excellent quantum
yields of up to 0.96 (3a–h, 3j–l)
with slightly lower values (5–10%)
in MeCN than in dichloromethane
(DCM).
Distal substituents at the benz-
imidazole moiety (3b–h) had only
a negligible effect on the maximum
absorption wavelength (Table 1),
which illustrates the silent role of
the auxochrome with respect to the
absorption and emission energies.
However, a crucial influence on the
emission quality was observed
when the interplay of the frontier
orbital energies is appropriately
tuned with substituents to match
À1
lA [nm]
lFmax [nm]
e00 [104 mÀ1 cmÀ1
]
]
fF(RT)
~
Dn [cm
max
3a
3c
3d
3i
598 (592) 602[c] 612 (608) 618[c] 81.4 (67.9)[b] 75.0[c] 383 (445) 430[c] 0.96 (0.83) 0.94[c]
599 (593)
600 (593)
615 (608)
622 (615)
611 (606)
615 (607)
n.d.[f]
75.7 (68.1)[b]
75.0 (67.7)[b]
62.5[a] (49.8)[b]
111 (82.4)[b]
117
79.2
72.8
92.7
59.2 (58.0)
51.9/40.2[d]
190 (241)[c]
210 (231)[c]
149 (155)[c]
174 (180)[c]
84.8 (83.9)
91.3[e]
328 (362)
407 (389)
–
0.79 (0.74)
0.80 (0.70)
n.d.[f]
3l
633 (627)
632 (627)
630 (625)
628 (622)
649 (642)
607 (604)
602/604[d]
838 (828)[c]
n.d.[f]
279 (311)
280 (391)
387 (447)
337 (397)
365 (398)
361 (393)
281/393[d]
520 (376)[c]
–
0.78 (0.72)
0.39 (0.36)
0.11 (0.11)
0.10 (0.09)
0.04 (0.03)
0.83 (0.77)
0.65/0.58[d]
0.05 (0.06)[c]
n.d.[f]
3m 621 (612)
3n
3o
3p
5a
7
8
9
10
11
12
13
615 (608)
615 (607)
634 (626)
594 (590)
592/590[d]
803 (803)[c]
951 (935)[c]
784 (788)[c]
783 (790)[c]
694 (683)
695[e]
824 (816)[c]
810 (813)[c]
722 (715)
724[e]
619 (435)[c]
426 (358)[c]
559 (655)
576[e]
0.04 (0.05)[c]
0.05 (0.07)[c]
0.10 (0.08)
0.10[e]
[a] First value in DCM, second value in MeCN (brackets), unless otherwise stated. [b] Inaccurate
attenuation coefficient because of poor solubility. [c] Value in toluene. [d] First value in MeOH, second
value in PBS buffer (pH 7.4). [e] Value in THF. [f] n.d.=not detected, lA =absorption wavelength,
lF =fluorescence wavelength.
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fluorescence modulation. Cyclic voltammetry experiments
gave two quasi-reversible reduction potentials (vs. Fc+/Fc) at
À390 mVand À1100 mV, respectively, without any indication
of a spectral change or decomposition at slow scan rates
(Figure 7). The absence of corresponding oxidation steps is in
accordance with the low-energy HOMO discussed above.
Figure 5. Frontier orbital energies (DFT) and their pictorial representa-
tions for an unsubstituted prototype of 3a.
Figure 7. Cyclic voltammogram of 3a in DCM at room temperature vs.
Fc+/Fc.
To estimate the relative photostability of the BOIMPYs,
a BODIPY fluorophore was used as a reference. Except for
3k, the selected BOIMPYs exhibited enhanced stabilities
while the propynyl-substituted variant 5a was strikingly stable
(Scheme 2); 91% had remained intact after irradiation for
90 min with
a high-intensity mercury-vapor lamp (ca.
1 WcmÀ2 at the probe) in MeCN.
In conclusion, we have conducted an extensive study of
a novel, red-emissive, and highly fluorescent scaffold that
might be regarded as a promising structural extension to the
well-established BODIPY set of fluorophores. A fully con-
jugated meso arene substituent bridged by two adjacent BF2
units is an unprecedented feature of the new motif, which
offers a relative bathochromic shift of 100 nm, exceptionally
high quantum yields, improved photostability, a fluorescence
modulation mechanism, and, furthermore, rapid synthetic
access. Modes of post-functionalization known for BODIPYs
are widely applicable to the new motif, as the tunable
halogenations, the selective fluoride substitutions, and the
variable single or twofold Knoevenagel-type condensations
suggest. The latter are able to shift the absorption up to l =
951 nm, providing easily accessible NIR dyes and fluoro-
phores in only three linear synthetic steps.
Figure 6. Absorption and emission spectra of 3a, 3l, 10, and 12 and
the absorption spectrum of 9 in DCM at room temperature. The
extinctions of 9, 10, and 12 were normalized.
energy transfer processes. Despite its conjugation with the
chromophore p system, the auxochromic moiety might still be
sufficiently decoupled to allow for photoinduced electron
transfer (PET). It remains controversial whether such reduc-
tive PET-like processes account for the slightly diminished
fluorescence efficiencies of the electron-rich methoxy-sub-
stituted species 3c (0.79) and 3g (0.77). The total depletion of
emission for 3i (meso-naphthimidazole), however, seems to
be governed by the crucial interaction of the energetically
raised, reshaped HOMO (À6.07 eV), now located at the
naphthimidazole moiety, with the excited chromophoric
dipyrromethene
system,
namely
the
half-vacant
Acknowledgements
HOMO-1 (À6.63 eV), from which the main transition still
occurs (see the Supporting Information). Considering the
altered relative orbital energies, a reductive PET-like process
is thus conceivable although the meso conjugation may
somehow contradict the original definition of this process.
In contrast to BODIPYs with distorted meso arene units, this
mechanism is realized with exact coplanarity, which should
turn the meso auxochrome into an equally efficient site for
L.J.P. thanks Martin Brçring, Kai Brandhorst, Sebastian
Hçfler, Miguel Silva Valverde, Emma Bales, Eliza Tarco-
veanu, and Katharina Bolsewig for kind support and the DFG
(SFB 803, A05) for funding.
Keywords: BODIPY · dyes · fluorophores · luminescence ·
near-infrared
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[18] CCDC 1438952 contains the supplementary crystallographic
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Received: July 15, 2016
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Fluorophores
L. J. Patalag, P. G. Jones,
D. B. Werz*
&&&& — &&&&
BOIMPYs: Rapid Access to a Family of
Red-Emissive Fluorophores and NIR
Dyes
A convenient leap into the red: Two
synthetic steps provide access to a new
family of highly fluorescent fluorophores
termed BOIMPYs. Two BF2 units enable
the efficient exploitation of the meso
position and lead to absorption at l
ꢀ600 nm. Owing to the structural sim-
ilarity to well-studied BODIPY dyes,
common modes of post-functionalization
can be easily transferred to the new motif.
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!