Synthesis and characterisation of BODIPY radical anions

Article abstract of DOI:10.1039/c2cc16047a

The redox processes associated with BODIPY analogues are studied by electrochemical and spectroscopic methods revealing a characteristic profile for the persistent BODIPY radical and quenching of fluorescence upon reduction.

Full text of DOI:10.1039/c2cc16047a

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COMMUNICATION
Synthesis and characterisation of BODIPY radical anionsw
Victoria J. Richards, Alexandra L. Gower, Jasper E. H. B. Smith, E. Stephen Davies,
´
Dorothee Lahaye, Anna G. Slater (nee Phillips), William Lewis, Alexander J. Blake,
Neil R. Champness* and Deborah L. Kays*
´
Received 28th September 2011, Accepted 1st December 2011
DOI: 10.1039/c2cc16047a
The redox processes associated with BODIPY analogues are
studied by electrochemical and spectroscopic methods revealing
a characteristic profile for the persistent BODIPY radical and
quenching of fluorescence upon reduction.
of spectroelectrochemistry and EPR we are able to identify key
signatures for the mono-reduced species and, in the case of 1,
we were able to monitor quenching of fluorescence associated
with BODIPY upon reduction to the radical anion 1ꢀꢁ. Our
results not only lead to a greater understanding of these
important molecules but also indicate that fluorescence
quenching may be significant in highly reducing conditions.
Compound 1 was prepared by literature methods via synthesis
of 5-phenyldipyrromethane, oxidation using 2,3-dicyano-5,6-
dichlorobenzoquinone (DDQ) and subsequent reaction with
BF3ꢂOEt2. Compound 2 was prepared by two methods, first
following literature methodology11 through reaction of 1 with
catechol in the presence of a Lewis acid, AlCl3, resulting in an
overall 27% yield based on dipyrromethane. A new, alternative,
method was also attempted via the reaction of B-bromocatechol-
borane with 5-phenyldipyrrin, generated in situ. The second
Fluorescent dyes remain the focus of an ever growing field of
research due to their application and potential relevance in a
vast array of technological areas. Amongst the most studied of
fluorescent dye molecules is the family of compounds generically
known as BODIPY.1,2 The interest in this particular moiety
arises from its strong fluorescence and robust nature that makes
it an ideal species for labelling studies. As such, BODIPY
analogues have been developed for use in biological labelling3
resulting in a large number of patents and the commercialisation
of BODIPY-based products.1,4
The use of BODIPY relies upon a thorough understanding of
its electronic properties. Extensive studies have been performed
on BODIPY and its analogues to evaluate and understand the
optical and electronic properties of this important species and its
derivatives.1,2,5,6 It is perhaps surprising, therefore, that the
reduced form of BODIPY species has not been extensively
characterised. This is particularly pertinent when considering
the use of BODIPY moieties in strongly reducing conditions,
including biological environments, and current extensive interest
in radicals containing main group elements.7–9
In this study we investigate the reduction of N,N0-difluoroboryl-
5-(phenyl)dipyrrin 1 and N,N0-(1,2-benzodioxaborol-2-yl)-5-
(phenyl)dipyrrin 2 (Scheme 1), representative examples of the
wider family of BODIPY species. The latter example, 2, was
chosen to represent a class of BODIPY analogues where the
BF2 bonds are replaced by B(OR)2 moieties, an increasingly
studied family of BODIPY analogues.6,10,11 Through the use
School of Chemistry, University of Nottingham,
University Park, Nottingham, NG7 2RD, UK.
E-mail: Deborah.Kays@nottingham.ac.uk,
Neil.Champness@nottingham.ac.uk; Fax: +44 115 9513563;
Tel: +44 115 9513491
w Electronic supplementary information (ESI) available: Further details
on the crystal structure determination of 2; synthetic and experimental
details; comparison of bond lengths from crystal structures and DFT
calculations and additional figures. CCDC 836231. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/
c2cc16047a
Scheme 1 Synthesis of BODIPY derivatives 1 and 2, including a view
of the single crystal structure of 2 showing the distorted tetrahedral
environment at the boron atom. Displacement ellipsoids are drawn at
the 50% probability level.
c
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Chem. Commun., 2012, 48, 1751–1753 1751

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strategy afforded the product, 2, in 65% yield and therefore
represents a more effective synthetic approach to this particular
compound.
Single crystals of 2 were grown by diffusion of iso-propanol
vapour into a solution of the compound in dichloromethane
and studied by single crystal X-ray diffraction methods:z
2 crystallises in the tetragonal space group I41/a with two
molecules in the asymmetric unit. The boron atoms adopt a
four-coordinate distorted tetrahedral environment with B–N
bond lengths elongated with respect to those in the previously
reported structure of 15 (see Scheme 1). The N–B–N angles are
similar in 15 and 2 and the O–B–O angle is similar to related
systems11 and notably narrower than the corresponding
F–B–F angle in 1.5
In order to establish the redox properties of 1 and 2, and in
particular the nature of the mono-reduced state of both species,
we studied the behaviour of the two compounds using cyclic
voltammetry (CV) and coupled electrolysis/spectroscopic
(UV-vis/EPR/fluorescence) methods. To further understand the
nature of the reduced and oxidised states, DFT calculations12
were performed for 1 and 2. The calculations are in good
agreement in terms of bond lengths and angles with the crystal
structures of 1 and 2 (see SI) and provide a means of assessing the
molecular orbitals for the two compounds.
The LUMO of both 1 and 2 is located almost exclusively on
the dipyrrin moiety with little electron density associated with
the B or F atoms, or in the case of 2 the catecholate group,
indicating that the reduction processes should be similar in
nature for the two compounds. The CV experiments confirm
that the two compounds exhibit similar behaviour upon
reduction, with two reduction processes in the range ꢁ1.15
to ꢁ2.17 V (Fig. 1a). The first reduction process has properties
characteristic of diffusion control; the second reduction does
not have an associated return wave and was not studied
further. The first and second reductions for 2 are 0.04 V less
negative than for 1, indicating that replacing the two fluoro
substituents in 1 with the catecholate group in 2 has an overall
greater electron withdrawing effect on the Ph-dipyrrin moiety.
Whilst there is a relatively small difference in reduction
potentials for 1 and 2, oxidation offers better discrimination
between the two compounds. Thus, CV experiments show
single oxidation processes in the range +0.80 to +1.19 V for
both compounds (Fig. 1a). Although the oxidation processes
do not have an associated return wave it is notable that 2 is
considerably easier to oxidise, by 0.39 V, than 1. DFT
calculations give insight into this difference; for 1 the HOMO
is located on the dipyrrin unit, but in contrast for 2 the HOMO
is located primarily on the catecholate group, accounting for
the significant shift in potential for this oxidation.
Fig. 1 (a) Cyclic voltammograms recorded for 1 and 2 in CH2Cl2
containing [nBu4N][BF4] (0.4 M) at 0.1 Vsꢁ1. E for first reduction =
1
2
ꢁ1.19 (1); ꢁ1.15 (2) V (vs. Fc+/Fc) (see SI for further detailsw).
(b) UV-visible spectra for 1 (arrows indicate the inter-conversion of
1 to 1ꢁ). Spectra recorded in CH2Cl2 containing [nBu4N][BF4] (0.4 M)
using spectroelectrochemical methods.
coefficients. In the range 250–350 nm, reduction leads to the
development of new bands at 291 and ca. 330 nm. Applying a
potential sufficient to reverse the reduction process regenerated
the spectral profile of the neutral species exactly for 1 and with a
small (o5%) loss of intensity for 2, indicating that for each the
reduction process is essentially chemically reversible under these
conditions and confirming the formation of a persistent BODIPY-
based radical.
ꢁ
ꢁ
The nature of the radicals 1ꢀ and 2ꢀ was probed further by
EPR spectroscopy. For both 1 and 2 coulometry measurements
confirmed that the first reduction corresponds to a oꢁne-electroꢁn
process and generated solutions of the radicals 1ꢀ and 2ꢀ
suitable for study by X-band EPR spectroscopy as fluid solutions.
Each of these spectra are of similar spectral width (ca. 30 G) and
position (giso 2.0029 and 2.0027 for 1 and 2, respectively) and
demonstrate extensive and similar hyperfine coupling, although
The UV/vis spectra of 1 and 2 are very similar and characteristic
of the B-dipyrrin-Ph framework, and this similarity is extended to
the spectra of the electrochemically generated anions (Fig. 1b).
The in situ one-electron reduction of each was followed by UV/vis
spectroelectrochemistry at an optically transparent electrode.
Generation of the one-electron reduced radical anion shows
depletion of the major bands (p -p* transition with vibrational
shoulder at higher energy) for the neutral species, at ca. 500 nm,
with the corresponding development of a series of bands in the
range 450–570 nm, all with significantly smaller absorption
ꢁ
this is better resolved in the case of 2ꢀ (Fig. 2). In both cases if
one assumes a symmetric dipyrrin environment, with no coupling
to the boron or fluorine atoms or to the cathecolate moiety, then
a total of 2430 lines will theoretically be observed. If coupling to
the spin-active B, F or catecholate atoms are assumed then a
significantly larger number of lines are expected. Unfortunately,
attempts to model the EPR spectra using theoretical values
c
1752 Chem. Commun., 2012, 48, 1751–1753
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Notes and references
z Crystal data for 2: C21H15BN2O2. Tetragonal, space group I41/a
(No. 88), a = b = 25.1377(7), c = 27.1775(8) A, V = 17173.5(8) A3,
T = 90(2)K, Z = 36, Dc = 1.177 g cmꢁ3, m = 0.606 mmꢁ1, F(000) =
6336. The crystal lattice contains a partially-occupied molecule of the
same composition as the fully-occupied molecules. This could not be
modelled in terms of discrete atoms sites, so PLATON SQUEEZE16 was
used to generate a set of diffraction intensities with the contribution from
this region removed. Of the 19 436 reflections collected, 8505 were unique,
with Rint = 0.072. Final R1 (wR2) = 0.0707 (0.186) with GOF = 0.95.
y Direct correlation of reduction potentials is difficult in part due to
different experimental conditions. Simple calculations indicate that the
reduction potential of 1, measured vs. Fc+/Fc, correlates to a potential
of ca. ꢁ0.43 V vs. NHE. Taking into account the use of non-aqueous
solvents in our experiments direct comparison is not possible, how-
ever, it should also be noted that the reductive ability of enzymes are
also difficult to correlate.17
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ꢁ
Fig. 2 X-band EPR spectra of electrochemically generated (a) 1ꢀ
ꢁ
and (b) 2ꢀ in CH2Cl2 containing [nBu4N][BF4] (0.4 M) at 291 K.
obtained from DFT calculations were unsuccessful despite using a
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4 Over 720 patents related to BODIPY have been applied for as of
May 2011, (Source: CAS SciFinder).
ꢁ
1ꢀ and 2ꢀ would indicate a common contribution to the
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It is particularly notable that upon reduction of 1 the
characteristic BODIPY fluorescence associated with excitation
at 503 nm is quenched (see SI). The loss of fluorescence upon
reduction of 1 is significant and has a bearing on the use of
BODIPY dyes in highly reducing biological environments. For
example, some iron-sulfur clusters in ferrodoxins,13 such as
spinach ferrodoxin,14 exhibit reduction potentials of a similar
magnitude to 1,y potentially leading to BODIPY reduction and
fluorescence quenching. Indeed, addition of [NEt4]2[Fe4S4(SPh)4]15
to 1 in CH2Cl2 solution does result in quenching of fluores-
cence. Thus, the use of BODIPY dyes as fluorescent imaging
agents in such environments may be unreliable.
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ꢁ
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indicate that the molecular orbitals of these reduced forms of
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of the molecules, a result consistent with the similarity of redox
potentials for reduction despite quite different functionalisation of
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ꢁ
ꢁ
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are highly complex, providing a signature for this reduced
BODIPY species and confirming the formation of persistent
boron-containing radicals based on this important class of
molecule. Quenching of fluorescence upon reduction indicates
that BODIPY dyes may be inappropriate for use in biological
labelling of highly reducing systems.
We thank the Engineering and Physical Sciences Research
Council for support; NRC gratefully acknowledges receipt of
a Royal Society Leverhulme Trust Senior Research Fellowship
and a Royal Society Wolfson Merit Award.
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c
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Chem. Commun., 2012, 48, 1751–1753 1753