π-Extended tetrathiafulvalene BODIPY (ex-TTF-BODIPY): A redox switched on-off-on electrochromic system with two near-infrared fluorescent outputs

Article abstract of DOI:10.1039/c4cc02567a

A π-extended tetrathiafulvalene-boradiazaindacene chimera, ex-TTF-BODIPY, has been prepared. The resulting system undergoes sequential one-electron oxidations, allowing access to both the mono-oxidized radical cationic and dicationic states. Additionally, ex-TTF-BODIPY displays electrochromic and electrofluorochromic behaviour in the near-IR portion of the electromagnetic spectrum and functions as a redox switched on-off-on emissive system. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc02567a

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p-Extended tetrathiafulvalene BODIPY
(ex-TTF-BODIPY): a redox switched ‘‘on–off–on’’
electrochromic system with two near-infrared
fluorescent outputs†
Cite this: Chem. Commun., 2014,
50, 6758
Received 8th April 2014,
Accepted 7th May 2014
Nathan L. Bill,a Jong Min Lim,b Christina M. Davis,a Steffen Bahring,c
¨
DOI: 10.1039/c4cc02567a
Jan O. Jeppesen,c Dongho Kim*b and Jonathan L. Sessler*ac
www.rsc.org/chemcomm
A p-extended tetrathiafulvalene-boradiazaindacene chimera, ex-TTF-
BODIPY (e.g., 2) and its derivatives are a class of molecules that
BODIPY, has been prepared. The resulting system undergoes sequential exhibit large absorption coefficients, high fluorescence quantum yields,
one-electron oxidations, allowing access to both the mono-oxidized and excellent photostability and are thus frequently employed as dyes.5
radical cationic and dicationic states. Additionally, ex-TTF-BODIPY However, in unmodified form they absorb and emit at wavelengths
displays electrochromic and electrofluorochromic behaviour in the shorter than 600 nm, precluding their use in NIR applications. Recent
near-IR portion of the electromagnetic spectrum and functions as a efforts to reduce the optical band gaps of BODIPYs, thereby pushing
redox switched ‘‘on–off–on’’ emissive system.
the emission to the red, include: (i) replacing the meso carbon with a
nitrogen atom (forming aza-BODIPYs), (ii) extending the p-conjugation
The development of materials that absorb and/or emit light in the pathway, via substitution or annulation, (iii) substituting the central
near-infrared (NIR) portion of the electro-magnetic spectrum is of pyrrolic core with electron-donating groups, and (iv) rigidifying the
current interest on account of their applicability in materials science, BODIPY skeleton.5,6 As yet, however, no redox switchable NIR-emitting
biochemistry, and medical applications.1 Of particular utility would systems have been reported as the result of these kinds of synthetic
be systems whose emission features could be controlled through modifications.
application of an external stimulus. Such NIR active systems might
Recently, several redox active fluorophore systems based on
see application as biological sensors or as sources of light at key BODIPYs were published. These include ferrocene-,7 flavin-8 and
communication wavelengths, among others.1 One approach to TTF-appended9 electrofluorochromic sensors whose optical response
achieving control over the fluorescence emission is through the is ‘‘switched-on’’ upon oxidation of the molecules respective electron
use of redox active fluorophores.2 These systems could, in principle, donor moieties. Unfortunately, the optical window of these systems
allow for the creation of reversible luminescence switches with lies outside (below) the NIR window. To address this latter deficiency,
multiple fluorescent ‘‘on’’ states coupled with a single ‘‘off’’ state. we have elected to ‘‘insert’’ a BODIPY subunit directly ‘‘into’’ the TTF
However, to date, only a small number of three-state electrofluoro- skeleton. The resulting functionalized boradiazaindacene (ex-TTF-
chromic organic systems have been reported.3 Moreover, to the BODIPY, 1) was expected to exhibit red-shifted absorption and
best of our knowledge, three-state organic systems that emit in the emission properties, as a result of the appended electron donating
NIR4 or which display redox-switchable ‘‘on–off–on’’ (as opposed dithiolidene rings and extended conjugation pathway. Further, the
to ‘‘on–on–off’’) features have yet to be described. Here we report conjugated BODIPY core was expected to confer electronic commu-
an extended tetrathiafulvalene (TTF) boradiazaindacene (BODIPY) nication between the dithiolidene rings, similar to other p-extended
analogue, ex-TTF-BODIPY (1), that permits access to a strong ‘‘on,’’ TTFs (‘‘exTTFs’’).10 The result, we anticipated, would be a NIR-
‘‘off,’’ and a second, weaker ‘‘on’’ NIR emissive state.
emitting system whose features could be tuned via redox modulation.
As detailed below, these design expectations were met. Moreover, we
have found that the ex-TTF-BODIPY approach allows controlled
access to a set of strong ‘‘on,’’ ‘‘off,’’ and weaker ‘‘on’’ fluorescent
states through stepwise oxidation of the dithiolidene subunits.
The synthesis of 1 (Scheme 1) entails a Horner–Wadsworth–
Emmons type coupling of phosphonate ester 311 with 1,9-diformyl-5-
phenyl BODIPY 4.12 Deprotonation of 3 with n-BuLi,13 followed by
addition of 4, resulted in the formation of 1 in 85% yield. The
ultraviolet/visible/near infrared (UV/Vis/NIR) absorption spectrum
of 1 was recorded in CH2Cl2 at 298 K and is shown in Fig. 1a.
a Department of Chemistry, The University of Texas at Austin, 105 E. 24th Street-
Stop A5300, Austin, Texas 78712-1224, USA. E-mail: sessler@cm.utexas.edu
b Department of Chemistry, Yonsei University, Seoul 120-749, Korea.
E-mail: dongho@yonsei.ac.kr
c Department of Physics, Chemistry and Pharmacy, University of Southern Denmark,
5230 Odense M, Denmark
† Electronic supplementary information (ESI) available: Details of the methods,
synthesis and characterization of all compounds, as well as spectroelectrochem-
ical, and fluorescence quantum yield data. See DOI: 10.1039/c4cc02567a
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Scheme 1 Synthesis of ex-TTF-BODIPY 1.
Fig. 2 Emission spectra of 1 (red) and 12+ (violet) as recorded in CH2Cl2 at
298 K. Excitation was carried out at 442 nm.
The reduction chemistry of 1 was also investigated by cyclic
voltammetry and was found to be similar to that reported for
unsubstituted BODIPY 2.12 For both 1 and 2, the first reduction
(E1red) processes are reversible and occur at nearly identical
potentials (À1.24 and À1.25 V17 vs. Fc/Fc+ for 1 and 2, respec-
tively). Additionally, the second reduction potentials (E2red) of
both 1 and BODIPY 2 are irreversible. However, that of 2 is
shifted cathodically (À2.29 V17 vs. ca. À2.0 V).
Fig. 1 (a) UV/Vis/NIR absorption spectra of 1 recorded in CH2Cl2 at 298 K
and (b) CV of a 1 mM CH2Cl2 solution of 1 at 298 K recorded in the
presence of TBAPF6 (100 mM) as the supporting electrolyte. Glassy carbon
working electrode, Pt wire auxiliary electrode and Ag/AgCl reference
electrode. Values reported are referenced to an internal Fc/Fc+ standard.
Although the reduction chemistries of 1 and 2 are relatively
similar, the oxidation chemistries of the two compounds are
Ex-TTF-BODIPY 1 is characterized by an absorption maximum in the
NIR spectral region (lmax = 754 nm) with less intense bands being drastically different. Explicitly, the first oxidation wave for 2 appears
observed throughout the visible region. The molar absorptivity at at +1.23 V vs. Fc/Fc+, a value 1.05 V higher than that for 1. The net
754 nm is e = 96 000 MÀ1 cmÀ1, a value leading us to propose that 1 result is that 1 has a reduced band gap compared to 2.
absorbs NIR radiation efficiently. The steady-state emission
Oxidative titrations of 1 with tris(4-bromophenyl)aminium
spectrum of 1 was measured (Fig. 2) by exciting the sample at the hexachloroantimonate (‘‘magic blue’’) in CH2Cl2 at 298 K were
absorption maximum (754 nm) in CH2Cl2 at 298 K. A sharp followed (Fig. 3) by absorption spectroscopy. Upon treatment of
fluorescence emission signal was observed under these conditions
with a lmax = 803 nm and an estimated fluorescence quantum yield
1 with increasing amounts of magic blue, the intensity of the
band at lmax = 754 nm was attenuated. Concurrently, increases
of 0.43 (Table S1, ESI†). This lmax value is among the most red in the intensities of a sharp, strong band at lmax = 886 nm and a
shifted (lowest energy) reported for BODIPY derivatives.14
weaker, broad band at lmax = 1572 nm were observed.18 These
The redox chemistry of 1 was studied (Fig. 1b) by cyclic changes in optical signature continued until one molar equivalent
voltammetry (CV) in CH2Cl2 at 298 K. Two, one-electron reversible of magic blue had been added. At this point the original 754 nm
oxidation waves are observed. The first (E1ox) is centred at ca. +0.17 V band was no longer observed. Additionally, the newly formed peaks
(vs. Fc/Fc+), while the second (Eo2x) is seen at ca. +0.44 V. The reached maximal intensity. Broad absorption bands at NIR wave-
significant separation (40.25 V) between E1ox and Eo2x is taken as an
indication that the mono-oxidized radical cation state is accessible
under these experimental conditions.
lengths are diagnostic of TTF radical-cations.19 Thus, the spectra
obtained after addition of one equivalent of magic blue is assigned
+
to the one-electron oxidized species 1ꢀ .
In previously reported p-extended TTFs with conjugated bridging
The addition of further oxidant resulted in a decrease in the
moieties, the degree of Coulombic repulsion between the positive intensity of the peaks assigned to 1ꢀ+. However, the intensity of
charges localized on the oxidized dithiole rings was cited as the key a broad band with lmax = 1032 nm was seen to increase. This
factor leading to experimentally observed separation of the oxidation latter spectral feature is assigned to the two-electron oxidized
waves.10 Although a large decrease in the E2ox À Eo1x difference is seen dication 12+. After two molar equivalents of oxidant had been
for 1 (B0.27 V) as compared to pristine TTF (0.39 V),15 the separation
between the oxidation waves found in the case of 1 is larger
added, the peak attributed to 12+ reached maximal intensity.
Further addition of oxidant resulted in no additional spectral
than other p-extended TTFs separated by just a single double bond changes associated with 1 or its oxidized daughter products.
(0.16 V).15,16 On this basis, we conclude that the separation in 1 is However, the intensity of the peak with lmax = 700 nm, attributed
greater than what would be expected based on purely Columbic to unreacted magic blue, increases as further equivalents of
repulsion effects and most likely reflects the formation of a quinoidal oxidant were added (data not shown).20
structure (Scheme 2) in the case of 12+ and commensurate
destruction of the aromaticity of one of the pyrrolic rings.
To correlate the changes in the absorption spectral features
produced upon chemical oxidation with those seen under
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Scheme 2 Stepwise oxidation of ex-TTF-BODIPY 1 to 12+
.
oxidized forms of 1. However, in all cases the calculated band gaps
were substantially larger than the experimentally derived optical
and/or electrical band gaps (see Table S2, ESI†). This could reflect
the fact that the calculations do not account for solvation or other
effects associated with the actual experimental measurements.
The calculations also revealed the presence of a reversed CT
effect, involving charge transfer from the BODIPY moiety to the
dithiolidene rings. This finding may account for the energetically
stabilized dithiole rings generated upon oxidation.
The change in the fluorescence intensity of 1 was also monitored
under conditions of chemical oxidation. Addition of magic blue to a
CH2Cl2 solution of 1 at 298 K, led to the attenuation of the signal at
803 nm. At the point when one molar equivalent of oxidant had
+
been added (corresponding to the full conversion to 1ꢀ ), the
fluorescence signal at 803 nm was no longer detectable. Additionally,
+
no new fluorescence signature was observed for 1ꢀ . It is thus
considered to be a non-fluorescent ‘‘off’’ state of 1.
Upon the addition of a second molar equivalent of oxidant,
leading to the formation of 12+ as inferred from the absorbance
titration studies noted above, the appearance of a new, NIR
fluorescence signal, albeit weak, was evident (Fig. 2). This
Fig. 3 Oxidative titrations of 1 with tris(4-bromophenyl)aminium hexachloro-
antimonate (‘‘magic blue’’) in CH2Cl2 at 298 K. Of particular note are the spectra
of pristine 1 (red), that recorded after one molar equivalent of magic blue has
been added (yielding 1ꢀ+, yellow-green), and the spectrum produced when
two equivalent of magic blue have been added (giving 12+, violet).
conditions of electrochemical analysis, spectroelectrochemical emission peak extends from approximately 950 nm to 1450 nm
analyses were carried out (Fig. S1, ESI†). Good correspondence with a lmax E 1185 nm and an estimated fluorescence quantum
between the methods was observed. Specifically, spectra closely yield of 4.8 Â 10À4 (Table S1, ESI†). To the best of our knowledge,
+
resembling those assigned to 1, 1ꢀ , and 12+ from the spectral this is the longest wavelength emission for a BODIPY derivative
titration with magic blue were recorded after subjecting a sample recorded to date.14 The weak fluorescence is thought to result
of 1 to bulk-electrolysis at À0.05 V, +0.35 V, and +0.75 V, from a combination of factors, such as an increase inÀmolecular
respectively. Further increases in the voltage produced no rotation, a heavy atom effect attributed to the SbCl6 counter-
further spectral changes, as would be expected based on both anions, and the charges present on the dithiolium rings as
the CV and chemical oxidation data. When the potential was revealed by the calculated MO structures (cf. ESI†).
brought back to À0.05 V and the sample was subjected to bulk
In summary, ex-TTF-BODIPY 1 displays electrochromic and
electrolysis for several minutes, the original spectral features electrofluorochromic behavior that extends well into the NIR
were restored. We thus propose that the response is reversible. region of the electromagnetic spectrum. The neutral and dica-
In agreement with the spectral changes seen during the tionic states of 1 are fluorescent, whereas the one-electron
oxidative titration and electrochemical analyses, molecular oxidized state, 1ꢀ+, is non-fluorescent. As a result, 1 acts as a
orbital (MO) calculations revealed reduced energy gaps between redox switchable ‘‘on–off–on’’ fluorophore. The ‘‘on’’ states
the HOMO and LUMO upon oxidation (Fig. S2 and S3; Table S2, emit at dissimilar wavelengths, allowing for the states to be
ESI†). The unprecedented low-lying absorption features of ex- distinguished from one another. Optimization of the electro-
TTF-BODIPY 1 are supported by the calculated MO structures. nics and improvement of the fluorescence quantum yields
Specifically, these analyses reveal that an extension of the could allow these types of molecules to find use in a variety
conjugation pathway, along with charge transfer (CT) interac- of application areas, such as Boolean logic-gates, optical limit-
tions involving the electron donating dithiolidene rings, serve ers, smart windows, and biochemical probes.
+
to lower the optical band gap. In the case of 1ꢀ and 12+,
This work was supported by the U.S. National Science
localization (and/or delocalization) of electron density onto Foundation (CHE-1057904 to J.L.S.), the Robert A. Welch Foun-
the dithiolidene rings of the ex-TTF-BODIPY most likely results dation (F-1018 to J.L.S.), a Mid-carrier Researcher Program
in the observed variations in the absorption spectra of the two grant (2010-0029668 to DK) from the National Research
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