Chemical control of photoinduced charge-transfer direction in a tetrathiafulvalene-fused dipyrrolylquinoxaline difluoroborate dyad

Article abstract of DOI:10.1039/d0cc05736c

A new approach for a compact annulation of tetrathiafulvalene (TTF) and dipyrrolylquinoxaline difluoroborate (QB) is presented, leading to strong electronic interactions between the TTF and QB units. Regulation of distinct photoinduced charge flows within this dyad is achieved by external stimuli, which is also verified by TD-DFT calculations. This journal is

Full text of DOI:10.1039/d0cc05736c

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Chemical control of photoinduced charge-
transfer direction in a tetrathiafulvalene-fused
Cite this:DOI: 10.1039/d0cc05736c
dipyrrolylquinoxaline difluoroborate dyad†
Received 24th August 2020,
Accepted 4th October 2020
Ping Zhou,a Ulrich Aschauer, *a Silvio Decurtins,a Thomas Feurer,b
a
a
Robert Haner
and Shi-Xia Liu
*
¨
DOI: 10.1039/d0cc05736c
rsc.li/chemcomm
A new approach for a compact annulation of tetrathiafulvalene acceptor.10 In stark contrast to the previously reported
(TTF) and dipyrrolylquinoxaline difluoroborate (QB) is presented, TTF–BODIPY ensembles,11 TTF–QB absorbs strongly in the
leading to strong electronic interactions between the TTF and QB visible spectral region due to a strong effective ICT dominated
units. Regulation of distinct photoinduced charge flows within this by one-electron excitation from the HOMO localized on the TTF
dyad is achieved by external stimuli, which is also verified by moiety to the LUMO on the QB core, indicative of largely
TD-DFT calculations.
enhanced electronic communication between them. On the
other hand, TTF–QB featured with a peripheral pyrrolic NH
Inspired by the unidirectionality of electron transfer evidenced unit, is envisaged to form a hydrogen bond with a fluoride,
in photosystem II, special attention has been devoted to leading to an ICT from the HOMO mainly localized on the
efficient control of electron flow in p-conjugated ensembles pyrrolylquinoxaline–fluoride adduct to the LUMO on the pyrrolyl-
consisting of electron donor (D) and acceptor (A) subunits.1 quinoxaline difluoroborate coordination moiety. Moreover,
However, regulating the direction of intramolecular charge upon chemical oxidation of the TTF subunit to its radical
transfer (ICT) over multiple pathways present in such D–A cation TTFꢀ+, a reverse ICT from the QB to the TTF moiety
systems remains a big challenge.2 An in-depth understanding would occur. As a consequence, chemical regulation of photo-
of this key issue is a prerequisite for the successful implemen- induced charge flow within this compact multicomponent dyad
tation of organic molecules in photovoltaics, electronics and is fulfilled.
photonics.3 Their intrinsic electronic properties are governed
In the present contribution, the synthesis and electronic
by the extent of communication between D and A units which properties of TTF–QB in response to external stimuli including
can be fine-tuned by the meticulous choice and modification of H-bonded fluoride adduct and chemical oxidation are
the D–A architectures as well as external stimuli such as discussed.
coordination,4 protonation,5 redox6,7 and light.8 Very recently,
a fullerene–phenothiazine dyad was reported to show in situ of 60% via
The target dyad TTF–QB was readily prepared in a yield
reaction of TTF-fused 2,3-di(1H-2-
a
a
switchable molecular photodiode-like behaviour by regulating pyrrolyl)quinoxaline (TTF–PQ)12 with BF3OEt2 using DBU as a
the redox state of phenothiazine.7 Our strategy involves a direct base (Scheme S1, ESI†). A coplanar conformation of the
annulation of a redox-active p-electron donor TTF9 (black part,
Chart 1) and a strong chromophore dipyrrolylquinoxaline
difluoroborate (QB, blue part, Chart 1) leading to the
formation of the TTF–QB dyad. QB is analogous to BODIPY
(boron dipyrromethene) with high extinction coefficient and
fluorescence quantum yield and can act as a p-electron
a Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3,
CH-3012 Bern, Switzerland. E-mail: ulrich.aschauer@dcb.unibe.ch,
liu@dcb.unibe.ch
b Institute of Applied Physics, University of Bern, Sidlerstrasse 5, CH-3012 Bern,
Switzerland
Chart 1 Chemical structure of the target dyad TTF–QB and chemical
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
regulation of ICT direction in the presence of fluoride and oxidant NOSbF6.
d0cc05736c
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resultant pyrrolide with the quinoxaline core renders
a
than that of TTF–QB (1.69 eV). On the other hand, its absorp-
chelating ligand forming a stable five-membered ring with a tion spectrum looks much simpler due to the decreased
boron atom, which leads to extended p-conjugation in the dyad p-conjugation of the molecule induced by the non-planarity
TTF–QB. Purification by chromatographic separation on silica of two pyrrole rings with the quinoxaline core. In other words,
gel afforded the analytically pure product which was fully BF2 complexation leads to an extended p-conjugation, which
characterized. 1H NMR and mass spectrometric data match renders a series of electronic transitions observable in the
well with its predicted molecular structure.
visible spectral region. This assumption is corroborated by
The electrochemical properties of TTF–QB, the free ligand the computational results (vide infra). All these spectroscopic
TTF–PQ and the reference compounds dipyrrolylquinoxaline results are in good agreement with the aforementioned electro-
(PQ) or QB in CH2Cl2 were investigated by cyclic voltammetry chemical properties.
(Fig. S1, S2 and Table S1, ESI†). TTF–QB undergoes two
A successive addition of tetrabutylammonium fluoride tri-
reversible oxidation processes at 0.78 and 1.10 V to the TTF hydrate (TBAF) in CH2Cl2 leads to a profound change in the
radical cation and dication states and one reversible reduction absorption spectrum of TTF–QB (Fig. 2). The occurrence of
process at À0.81 V, corresponding to the reduction of the QB clearly defined isosbestic points at 345 nm, 530 nm and 655 nm
subunit. Upon complexation of the PQ unit with one equivalent indicates the conversion of TTF–QB to its corresponding
of BF2, both oxidation and reduction potentials are positively H-bonded species with FÀ. The low-energy ICT absorption band
shifted, indicative of the enhanced p-electron withdrawing decreases at the benefit of a new ICT transition at 650 nm
ability of the quinoxaline core due to B–N bond formation which is bathochromically shifted by 40 nm in good agreement
between the nitrogen lone pair and the vacant p-orbital of the with a significant colour change from blue to yellow-green. This
boron atom. It can therefore be deduced that the resultant QB behaviour is ascribed to the formation of N–HÁ Á ÁF hydrogen
subunit acts as a stronger p-electron acceptor than the quinoxa- bonds, as previously observed during the titration of the ligand
line unit itself. A separation between the onset oxidation and TTF–PQ with the fluoride anion.12 Concomitantly, the emer-
reduction potentials leads to an electrochemical HOMO–LUMO gence of a strong absorption band at 410 nm occurs at the
gap of 1.37 eV for TTF–QB, which is significantly reduced expense of the absorption band around 310 nm. Upon addition
compared to that for TTF–PQ (1.91 eV).
of 80 equiv. of TBAF, no further noticeable spectral changes are
The dark blue TTF–QB strongly absorbs over an extended observed. The final spectrum resembles that of the reference
range within the UV-Vis spectral region (Fig. 1). A strong and QB in the presence of 80 equiv. TBAF, by exhibiting three
broad absorption band (e E 2.3 Â 104 MÀ1 cmÀ1) peaked at distinct electronic transitions (Fig. S4, ESI†). As expected,
610 nm is attributed to an ICT transition from the TTF unit to all these transitions are bathochromically shifted and much
the QB fragment as confirmed theoretically (vide infra). A series stronger in TTF–QB than in QB, by virtue of the large extended
of less-intense absorption bands is observed between 550 nm p-conjugation that substantially lowers the energy level of the
and 350 nm, followed by a very intense absorption band LUMO. In the following, quantum-chemical calculations
centered at 310 nm. These spectral features are in stark contrast demonstrate that the lowest energy transition of TTF–QB, when
with those of the free ligand TTF–PQ. On the one hand, its H-bonded to fluoride, corresponds to an ICT from the
lowest energy absorption band is hypsochromically shifted by pyrroleÁ Á ÁFÀ unit to the QB moiety, suggesting that the
120 nm, as evidenced by its purple-red colour. This observation direction of the ICT in TTF–QB can be regulated by adding FÀ.
is attributed to a weaker electron-withdrawing effect of the
quinoxaline itself than the QB subunit. Consequently, the
optical HOMO–LUMO gap of TTF–PQ (2.23 eV) is much larger
Fig. 1 The UV-Vis absorption spectra of TTF–PQ and TTF–QB (1.6 Â 10À5 M)
Fig. 2 Variation of UV-Vis absorption spectra of TTF–QB (1.5 Â 10À5 M) in
in CH2Cl2 at room temperature.
CH2Cl2 upon successive addition of aliquots of TBAF at room temperature.
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Fig. 3 Variation of UV-Vis-NIR absorption spectra of TTF–QB (1.5 Â 10À5 M)
in CH2Cl2 upon successive addition of aliquots of NOSbF6 at room
temperature.
Intramolecular electronic interactions between the TTF and
QB units in the TTF–QB dyad have further been explored by
UV-Vis-NIR spectroscopy, whereby a chemical oxidation of
TTF–QB was carried out by successive addition of NOSbF6
aliquots at room temperature. As depicted in Fig. 3, a progres-
sive reduction of the absorbance of both p–p* and ICT
transitions at 310 nm and 610 nm, respectively, is accompanied
with a concomitant emergence of new absorption bands at
736 nm and around 1040 nm which reach their maximum
values upon addition of 2.5 equiv. of NOSbF6. These new
transitions are characteristic of the TTF radical cation species
Fig. 4 Molecular orbitals of TTF–QB that are involved in the ICT
transition, in the neutral state (a), in the presence of fluoride (b) and in
the oxidised state (c). The red circle in (b) points to the H-bonded fluoride.
the calculated excitations at higher energy match well with the
observed absorption spectrum. In the actual case of the fluoride
adduct, however, the fluoride binding on the acceptor side
destabilises the occupied MO localised on the QB site such that
it represents now the HOMO of the dyad (Fig. 4b). Conse-
quently, the corresponding HOMO–LUMO transition (96% of
the calculated S0 - S1 excitation) corresponds to a charge flow
predominantly in an orthogonal direction and at longer wave-
length compared to the former one, in full agreement with the
observed bathochromic shift of the ICT band by 40 nm. The
latter energy shift is well substantiated by the calculation. Also
in the blue/UV spectral region, the calculated excitations
represent closely the experimental absorption bands. Upon
oxidation to the radical cation species TTFꢀ+–QB, a broad and
clearly asymmetric absorption feature emerges in the NIR
region at wavelengths above 800 nm, which points to two new
excitations. In accordance with it, the calculation of the open-
shell molecule reveals two electronic transitions in the NIR
part, D0 - D1 (98% b-HOMO to b-LUMO) and D0 - D2 (97%
b-HOMOÀ1 to b-LUMO) at 1483 nm and 1075 nm, respectively.
Importantly, both excitations reveal that upon photoexcitation,
the charge flow is directed in the reverse direction, hence from
the QB to the TTFꢀ+ unit which is now the acceptor site (Fig. 4c,
Tables S6 and S7, ESI†). Moreover, the calculated excitations
D0 - Dn (n Z 3) cover well the experimental absorption
features at wavelengths around 800 nm and lower.
+
TTFꢀ within a D–A ensemble.13 We note that in previously
reported studies on oxidised D–A compounds,14 the lowest
energy absorption band is assigned to the dimeric radical
cation species arising from enforced intermolecular interac-
tions. In the actual multicomponent molecule, however, the
two lowest energy electronic transitions in the NIR around the
asymmetric 1040 nm absorbance show a back CT character,
which is corroborated by TD-DFT calculations in the following.
To characterise and verify the various electronic transitions,
TD-DFT calculations were accomplished using the Gaussian
1615 package at the B3LYP/6-31G(d,p) level of theory. After DFT
relaxation to within default thresholds, TD-DFT calculations of
the 40 lowest excited states were carried out and the absorption
spectra extracted with GaussSum 3.0.16 The predicted absorp-
tion spectra of TTF–QB, its fluoride adduct and its oxidised
form are in fairly good agreement with experimental results
(Tables S2–S7 and Fig. S11, ESI†). Compared to TTF–PQ,
TTF–QB reveals in particular an absorption onset at a much
larger wavelength. This lowest energy absorption band, peaked
at 610 nm, is attributed to the S0 - S1 excitation, almost
exclusively (99%) dominated by the HOMO–LUMO transition
that involves CT from the TTF to the QB moiety, as clearly
shown by the associated molecular orbitals in Fig. 4a. This
directionality of the longest wavelength ICT is a kind of
standard for fused TTF–acceptor ensembles.13 Similarly, also
In conclusion, the key finding is that, through external
stimuli like fluoride binding or oxidation reaction, the
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This work was financially supported by the Swiss NSF
through the NCCR MUST ‘‘Molecular Ultrafast Science and
Technology’’, the Swiss National Foundation (200020_
188468), and SNF professorship grants (PP00P2_157615 and
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Conflicts of interest
There are no conflicts to declare.
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