Appending Diverse π-Extended Acceptors with Tetrathiafulvalene/Dithiafulvalene Donors: Multistate Redox Properties, Radical Ion Generation, and Mid-IR-Absorbing Mixed-Valence States

Article abstract of DOI:10.1002/chem.201704604

Three classes of donor–acceptor (D–A) π-extended chromophores (1–12) were synthesized through a phosphite-mediated cross-coupling reaction, in which the anhydride- or imide-based π-As and number of tetrathiafulvalene (TTF)/dithiafulvalene (DTF) Ds were systematically changed. Large π rings, such as benzoperylene and coronene, were integrated into the TTF/DTF unit, for the first time, to overcome their high insolubility. The anhydride and imide groups in the π acceptors can significantly alter the frontier orbitals and influence the optoelectronic properties. The D moieties allow the formation of radical cations (D^.+) and the π-extended A moieties aid the formation of radical anions (A^.?) by oxidation/reduction under ambient conditions. The molecules revealed UV/Vis/near-IR absorption, fluorescence extending into the near-IR region, and amphoteric electrochemical properties. Chromophores 10 and 12 show solvatochromism in a wide range of solvents. The π-As with anhydride functionality allow easier electron uptake, relative to the imide groups, whereas the increasing number of D TTF/DTF units make them easy to oxidize. Interestingly, the trans-TTF-fused molecules (1, 6, and 11) exhibited a mixed-valence state in the mid-IR region ((Formula presented.) =5130–4000 cm^?1). Moderate electron coupling between the redox centers is inferred to the compounds being of Robin–Day class II. The multistate redox activity along with panchromism and near-/mid-IR optical absorption of these systems can be attractive towards advanced switchable materials.

Full text of DOI:10.1002/chem.201704604

A Journal of
Accepted Article
Title: Appending Diverse ꢀ-Extended Acceptors with TTF Donors:
Multistate Redox, Radical Ion Generation and Mid-IR Absorbing
Mixed-Valence States
Authors: Pritam Mukhopadhyay, Sudhir Kumar Keshri, Deepak
Asthana, Sonam Chorol, and Yogendra Kumar
This manuscript has been accepted after peer review and appears as an
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using the Digital Object Identifier (DOI) given below. The VoR will be
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to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201704604
Link to VoR: http://dx.doi.org/10.1002/chem.201704604
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Appending Diverse π-Extended Acceptors with TTF/DTF Donors:
Multistate Redox, Radical Ion Generation and Mid-IR Absorbing
Mixed-Valence States
Sudhir Kumar Keshri,[a] Deepak Asthana,[a] Sonam Chorol,[a] Yogendra Kumar,[a] and Pritam
Mukhopadhyay*[a]
Abstract: Three classes of donor-acceptor π-extended chromophores (1–12) were synthesized via phosphite-mediated cross-coupling
reaction in which the anhydride or imide-based π-acceptors (A) and the number of tetrathiafulvalene (TTF)/ dithiafulvalene (DTF) donors (D)
were systematically changed. Large π-rings like benzoperylene and coronene were integrated with the TTF/DTF unit for the first time,
overcoming their high insolubility. The anhydride and the imide groups in the π-acceptors can significantly alter the frontier orbitals and
influence the opto-electronic properties. The D moieties allow the formation of radical cations (D•+) and the π-extended A′s aid formation of
radical anions (A•−) by oxidation/reduction under ambient conditions. The molecules revealed UV-Vis-NIR absorption, fluorescence extending
to the NIR region and amphoteric electrochemical properties. The chromophores (10 and 12) show solvatochromism in a wide range of
solvents. The π-acceptors having anhydride functionality allow easier electron uptake compared to the imide groups, while the increasing
number of donor TTF/DTF units make them easily oxidizable. Interestingly, the trans-TTF-fused molecules (1, 6 and 11) exhibited mixed-
valence state in the mid-infrared (MIR) region (5130-4000 cm-1). A moderate electron coupling between the redox centers is inferred to the
Robin-Day class II. The multistate redox activity along with panchromism and NIR/MIR optical absorption of these systems can be attractive
towards advanced switchable materials.
versatile TTF-based building blocks towards self-assembled
structures, luminescent materials, single-molecule magnets,
Introduction
solar cells, semiconductors and redox-switchable systems.10-11
The synthetic protocol for furano-quinonoid extended TTFs were
explored further to integrate
naphthopyranone,11b
Multi-state organic redox active systems and, in particular,
electrochemically amphoteric molecules, for which both oxidized
and reduced states are available within an accessible potential
window, are of great interest due to their potential applications in
molecular electronics, optoelectronics, charge transfer, mixed
valence state, optical switches and so on.1 An effective
communication between the donor-acceptor (D–A) scaffolds has
to be ensured through a π-bridge. A π-bridge that can double its
role as a redox active D or A scaffold can have significant
implications in its optoelectronic properties.
Organic π-acceptors (A) such as anhydride/imide-based
scaffolds such as the naphthalenedianhydride (NDA)/
naphthalenediimide (NDI), pyromelliticdianhydride (PMDA)/
pyromelliticdiimide (PMDI) are capable of accepting electrons
from mild reducing agents. The radical anions and the dianions
formed can be applied as NIR probes, sensors, semiconductors,
etc.2-5 On the other hand, tetrathiafulvalene (TTF) is an electron
donor scaffold which forms radical cation (TTF•+) and dication
(TTF2+) in a step-wise manner to form the mixed valence
states.6-8 These radical species enabled with facile mixed
valence state generation, provides a platform to investigate
electron transfer mechanisms.9 In this context, development of
the 1,3-dithiole-2-thione-4,5-dithiolate group offers a number of
benzothiadiazole,11c dipyridophenazine,11d quinoxaline-based11e
π-acceptors. The TTF analogues with extended π-conjugated
systems have also been reviewed to evaluate the effect of
spacer length on the conduction mechanism of mixed-valence
charge-transfer compounds.12
Although diverse ranges of π-conjugated TTFs have been
synthesized, studies on furano- and pyrano-anhydride-based π-
acceptors have been explored scarcely. Further, their imide-
based congeners are yet to be investigated. We were interested
to explore the design and synthetic protocols towards pre- and
post-imidization of the TTF/DTF-integrated π-systems and also
vary the number of TTF/DTF donors. These systems are
envisaged to have subtly-tuned opto-electronic properties and
can also be promising precursors to radical anions and radical
cations. We were also interested to integrate the TTF/DTF
scaffold with very large π-extended systems, e.g.,
benzoperylene and coronene, which significantly suffers from
high insolubility in common solvents.
Electron coupling between the D-A scaffolds provides
further opportunities to modulate their mixed valence states and
have new generation of low-energy absorbers. According to the
Robin-Day formalism, the organic mixed valence compounds
are classified into three classes depending on the extent of the
electron coupling element, Hab between the two redox centers.
The mixed valence compounds in which the interaction is too
small, is referred to as class I. Class II refers to the moderate
interaction between the redox centers and the class III is
characterized by strong interaction and full delocalization of the
charge over the entire system.13 The intervalence charge-
transfer (IV-CT) band of organic π-conjugated mixed-valence
[a]
Sudhir Kumar Keshri, Dr. Deepak Asthana, Sonam Chorol,
Yogendra Kumar, Prof. Dr. Pritam Mukhopadhyay
Supramolecular and Material Chemistry Lab, School of Physical
Sciences, Jawaharlal Nehru University, New Delhi 110 067, India
E-mail: m_pritam@mail.jnu.ac.in
Supporting information for this article is given via a link at the end of
the document.
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Scheme 1. Unified synthetic schemes of three classes of TTF/DTF-appended D–A chromophores (1–12), the grey coloured dashed enclosure show the
molecules reported earlier from our group.14
systems is a primary characteristic that can be finely modulated,
e.g., by molecular engineering to evolve a molecular material
having new generation of optical and electronic properties that
remain out of bound with the current molecular scaffolds. In this
work, we present a systematic study on the influence of the π-
acceptors and number of donors in TTF/DTF-fused D–A
chromophores. To this end, we present three classes of
TTF/DTF-appended D–A chromophores 1–12, in which the π-
acceptors and the number of donor moiety (TTF/DTF) vary
systematically from small π-acceptors like the benzene
derivatives (PMDA/PMDI) and for the first time to very large π-
acceptors like the benzoperylene and coronene derivatives and
the number of donors (TTF/DTF) from one to four units (Scheme
1). We have studied the optical and multistate redox properties,
solvatochromism and the mixed valence states. We have
explored the formation of radical cations (D•+) and radical anions
(A•−) simultaneously by chemical and electrochemical methods
in these systems. Notably, the TTF-fused molecules containing
a pair of the redox centers form radical cation with a mixed
valence state having characteristic IV-CT band in the mid
infrared (MIR) region. Remarkably, our design enables to tune
the IV-CT band of mixed valence state from 5130 to 4000 cm-1.
The π-acceptors having anhydride functionality allow easier
electron uptake compared to the imide groups, while the
increasing number of donor, TTF/DTF units make them easily
oxidizable. The imide groups can also affect the planarity of the
D-A system resulting in considerable changes in the IV-CT
bands.
Results and Discussion
Design and Synthesis. Scheme
1 represents a unified
synthetic scheme for the TTF/DTF appended D–A
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chromophores. Recently, we reported PMDA and NDA π-
acceptors appended with one or two TTF/DTF units which were
restricted to anhydride-based systems.14 Herein, we have
expanded the scope of this molecular design and incorporate
different π-acceptors, alter the anhydride to imide by pre- and
post-functionalization and understand its structural and
electronic effect and also increase the number of donor units.
Herein we report the synthesis of new chromophores belonging
to the class 1–3, namely 3a, 3b, 4, 7 and 9–12. The design of
chromophores (Scheme 1) is based on the investigation of
multifaceted D–A chromophores by integrating TTF/DTF units
with various π–acceptors. The π surface-area of the acceptors
have been progressively increased e.g. from the PMDA to NDA,
PMDI to NDI and benzoperylene anhydride and extremely large
corronene anhydride. These molecules were synthesized by
phosphite mediated cross coupling reactions in good yields. The
DTF precursors comprising of n-butyl, n-octyl or p-CF3 benzyl
groups were synthesized following standard reaction protocol.15
In the class 1, benzene-, naphthalene- and benzoperylene-
based anhydrides have been introduced as the π-acceptors
(Scheme 1). The molecules, 1, 2, 5 and 6 were synthesized by
trimethylphosphite [P(OMe)3] mediated cross coupling reactions
in toluene whereas 3, 4 and 7 were synthesized by neat
triethylphosphite [P(OEt)3] mediated cross coupling reaction
(Scheme 1).10b,11b Interestingly, with this strategy, dendritic D–A
molecular architectures with pendant three/four DTF units
integrated on a PMDA (3 and 4) can be synthesized. However,
these molecules (3 and 4) owing to their strong oxidation
properties were found to be unstable under ambient conditions.
On the other hand, the D–A chromophores of class 2 are
based on the mixed anhydride-imide π-acceptors in the form of
naphthalene- and the π-extended coronene derivatives. The
chromophore, 8 was synthesized by imidization of 5 in DMF14
whereas 9 was synthesized by neat P(OEt)3 mediated cross
coupling reaction of coronene imide-anhydride derivative.
Therefore, in the case of 8 and 9 the order of DTF fusion and
imidization has been reversed. This modulation shows the
versatility of our synthetic strategy. The post-imidization of 5
directs the imidization site far apart to the DTF moiety. The high
insolubility of coronene dianhydride in common solvents and the
subsequent challenge to condense the DTF unit was overcome
by pre-imidization of the coronene dianhydride (Scheme 2).
The class 3 of D–A chromophores describes the PMDI and
NDI as the π-acceptors coupled with the TTF/DTF scaffold. The
compounds of this class, 10, 11 and 12 were synthesized by
neat P(OEt)3 mediated cross coupling reaction. The presence of
the long alkyl chain in the π-acceptor imide or the donor
TTF/DTF unit aids solubility of these chromophores in common
organic solvents such as dichloromethane (DCM), CHCl3,
tetrahydrofuran (THF), etc. The TTF/DTF-fused π-acceptor-
based bridging systems allow the combination of energetically
high-lying HOMO (highest occupied molecular orbital)
delocalized over the D (donor) through the π-bridge and low-
lying LUMO (lowest unoccupied molecular orbital), which is
delocalized over the A (acceptor), supported by theoretical
calculation.
The π-acceptors 15–18 were synthesized by following the
literature protocols (Scheme 2). The acceptor 15 was
synthesized by Diels-Alder reaction from 19 and maleic
anhydride in the presence of oxidizing agent, p-chloranil.16 The
condensation of 19 with dodecylamine followed by second Diels-
Alder reaction resulted in 16. The molecules 17 and 18 were
synthesized by condensation of 21 and 22 respectively with
hexylamine in DMF (Scheme 2).4c
Optical Properties. The optical properties of the chromophores
in class 1–3 are explored by UV-Vis and fluorescence
spectroscopies (Figure 1 a-c). All the chromophores feature a
broad and low energy transition, assigned as intramolecular
charge-transfer (ICT) transitions, resulting from the excitation of
the TTF/DTF donor to the anhydride or imide π-acceptor. In the
class 1, the absorption spectrum of trans-TTF-fused D–A
molecule, 1 shows a two-band absorption profile with the
푎푏푠
푚푎푥
longest-wavelength absorption maximum (휆
) at 530 nm. The
absorption spectrum of cis-TTF-fused D–A molecule, 2, is very
similar to 1, but the two-band absorption profile is not
푎푏푠
푚푎푥
pronounced and the
휆
is blue-shifted by 15 nm.
Consequently, the optical onset band gap increases from 1.99 to
2.17 eV in 1–2. Increasing the number of donor DTF units in the
푎푏푠
푚푎푥
molecule 3b results in a broad band with 휆
at 450 nm, while
the onset absorption is indistinguishable to 1, and therefore, the
optical onset band gap is not noticeably changed. The optical
studies of molecule 4 could not be performed due to its
푎푏푠
푚푎푥
pronounced self-oxidation. Therefore, the 휆
in class 1 could
be tuned dependent on the number of donor TTF/DTF units,
acceptor anhydride carbonyl groups and the conjugation length
(consider the para- and meta- orientation of the TTF/DTF units
Scheme 2. Representative schemes for the synthesis of π-acceptors 15–18.16, 4c
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) of TTF/DTF D–A chromophores (1–12; 5 x 10-5 M) in DCM.
푎푏푠
푚푎푥
Figure 1. Normalized absorption spectra (a–c) and emission spectra (d–f) ( 휆푒푥 = 휆
in 1 and 2, respectively). The NDA-fused mono- and bis-DTF
at 515 nm, where the optical band gap increases from 1.99 eV in
1 to 2.19 eV in 11. In case of 11, the HOMO energy level
푎푏푠
molecules 5 and 6 show 휆
at 603 and 608 nm, respectively.
푚푎푥
The molecule 5 exhibits a broad band whereas 6 show a two-
band absorption profile. A 50 nm red-shift in optical onset band
in 5 compared to 6 is observed, which can be ascribed to the
increasing HOMO and LUMO energy levels (determined
experimentally) in 6 (Table 2). Interestingly, 7 show a multi-band
increases
(experimental/theoretical)
(experimental/theoretical) in 1 (Table 2) and hence the low band
gap. In general, the 휆
the PMDA/PMDI π-acceptors. The NDA/NDI π-acceptors have
low-lying LUMO energy levels compared to PMDA/PMDI.
Imidization increases the HOMO energy levels in PMDI/NDI vis-
à-vis PMDA/NDA π-acceptors. Furthermore, increasing the
number of TTF/DTF units increases the HOMO energy levels.
substantially
to
-5.10/-5.17
to -5.23/-5.44
eV
eV
compared
푎푏푠
increases in NDA/NDI as compared to
푚푎푥
absorption profile covering from UV to most of part of visible
푎푏푠
푚푎푥
range with 휆
at 510 nm. The characteristic vibronic fine
structure of the strongly allowed benzoperylene-core π-π∗
transitions is known to manifest between 410-470 nm. In the
푎푏푠
푚푎푥
class 2, the molecule 8 shows 휆
at 585 nm. The optical band
gap slightly increases from 1.71 eV in 5 to 1.77 eV in 8, as can
be expected due to the imidization in 8 increasing the LUMO
energy level, while the HOMO energy levels remain almost
unaltered. On the other hand, the absorption spectrum of 9 is
quite similar to 7, covering the visible range up to 625 nm with
푎푏푠
푚푎푥
휆
at 534 nm. Thus, the optical band gap decreases from 2.27
in 7 to 2.06 eV in 9, which can be attributed to the greater π-
conjugation in the coronene ring vis-à-vis the benzoperylene ring.
Furthermore, in the class 3, the absorption spectra of mono-
푎푏푠
푚푎푥
DTF-fused D–A 10 and 12, show a broad band with 휆
at 503
and 553 nm, respectively. This red-shift in 12 is attributed to the
greater π-conjugation provided by the NDI versus the PMDI
scaffold as well as the low-lying LUMO energy levels as
determined experimentally and theoretical calculations. Notably,
imidization increases the HOMO energy level as can be seen in
12 compared to the bis-anhydride-based molecule
5 as
Figure 2. Solvatochromic effect: Chromogenic changes of 10 and 12 in
determined from the experimental and theoretical results. On the
cyclohexane, THF, MeCN, MeOH, DMF and CHCl3.
푎푏푠
other hand, molecule 11 shows two-band absorption with 휆
푚푎푥
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Table 1. Solvation effects on photophysical properties of 10 and 12.
Entry
Solvent
휆
(10)
휆
(12)
휆
(10)
휆 (12)
푚푎푥
∆푣 ퟏퟎ
∆푣 ퟏퟐ
∆f
ET (10)
ET (12)
ET (30)[a]
푎푏푠
푚푎푥
푎푏푠
푚푎푥
푒푚
푚푎푥
푒푚
(
)
(
)
(cm-1)
(cm-1)
(1x10-2)
(kcal/mol)
(kcal/mol)
(kcal/mol)
(nm)
(nm)
(nm)
(nm)
1
2
3
4
5
CHCl3
507
484
496
497
493
499
557
530
542
547
540
547
644
590
686
665
650
686
711
655
725
725
713
725
4196
3712
5586
5083
4899
5463
3889
3600
4589
4488
4493
4488
4.344
0.158
56.39
59.07
57.64
57.53
57.99
57.30
51.33
53.94
52.75
52.26
52.94
52.36
39.1
32.9
45.6
54.4
37.4
43.2
Cyclohexane
MeCN
MeOH
THF
21.804
30.867
20.939
27.454
6
DMF
푎푏푠
푚푎푥
푎푏푠
푚푎푥
휆
: absorption wavelength maximum, 휆
: emission wavelength maximum, ∆푣 : Stokes shift, ∆f : solvent parameter, ET : Dimroth and Reichardt polarity
scale, and [a] Data from ref.17.
All these factors aid in modulating the optical properties of these
diversely TTF/DTF-fused π-acceptors.
developed into a powerful tool for the investigation of the polarity
of the medium and determine the environment of a reaction
pathway. We chose two representative molecules 10 and 12
from class 3 due to their favorable solubility in nonpolar and
polar solvents to explore their solvatochromic characteristics
(Figure 2 and Table 1). In 10 and 12, the alkyl groups appended
through the imide N atoms aids good solubility in a diverse
The emission spectra follow the same trend as that of the
absorption spectra with few exceptions (Figure 1 d-f). In class 1,
푒푚
푚푎푥
the emission longest-wavelength maximum (휆
) was found to
be 650 nm for trans-TTF-fused D–A molecule 1 with Stokes shift
of 3480 cm-1, which is maximally red-shifted in the PMDA-
TTF/DTF fused derivatives. The cis-TTF-fused D–A molecule 2
range of solvents. The photo physical properties of 10 and 12 in
푒푚
푎푏푠
is blue-shifted by 40 nm with 휆
of 610 nm and Stokes shift of
different solvents are summarized in Table 1. The 휆
was
푚푎푥
푚푎푥
3025 cm-1. Molecule 3b comprising of three DTF units shows
found to be 484 and 507 nm for 10 in cyclohexane and CHCl3,
푒푚
푎푏푠
휆
of 640 nm, which is 30 nm red-shifted compared to 2 with
respectively. 12 exhibited
휆
at 530 and 557 nm in
푚푎푥
푚푎푥
large Stokes shift of 6600 cm-1. Importantly, molecule 5 shows
cyclohexane and CHCl3, respectively (Figure 3a-b). On the
contrary, a substantial hypsochromic shift of the ICT emission
band in 10 and 12 was observed with increased solvent polarity
(Table 1 and Figure S1). Such ambiguous solvatochromism
behavior has been observed with few other D–A molecules.18
푒푚
the most red-shifted 휆
at 737 nm in the NIR region amongst
푚푎푥
the three classes of TTF/DTF-fused D–A molecules with Stokes
shift of 3015 cm-1. The trans-bis-DTF-NDA, 6 and mono-DTF, 7
푒푚
푚푎푥
display 휆
at 674 and 555 nm, respectively.
푒푚
푚푎푥
In the class 2, molecules 8 and 9 exhibit 휆
at 720 and
637 nm, respectively and the Stokes shifts were found to be
3205 and 3030 cm-1 in 8 and 9, respectively. The D–A
chromophores of class 3 covers the Vis to the NIR frontier and
푒푚
the 휆
increases from 643 nm to 705 nm on increasing the π-
푚푎푥
푒푚
surface of acceptors from 10 to 12, while the 휆
was found to
푚푎푥
be at 575 nm for trans-TTF-fused 11. In this class, molecule 10
showed the maximum Stokes shift of 4330 cm-1. The emission
spectra and large Stokes shifts in these D-A systems are
characteristic of photoinduced intramolecular charge transfer
(ICT) based systems. The emission spectra follow the same
trend as that of the absorption spectra on traversing from
푒푚
푚푎푥
PMDA/PMDI to the NDA/NDI π-acceptors. The 휆
decreases
with an increase in the number of TTF/DTF donor units. For
example, the mono-DTF substituted and 10 exhibits
as compared to symmetrically
5
푒푚
푚푎푥
substantial red-shifts in the 휆
bis-DTF substituted 6 and 11. The later can be ascribed to the
large changes in excited state dipole moment (∆µ) in the
asymmetrically substituted mono-DTF compared to the
symmetric bis-substituted DTF molecules.
It is well known that the electronic absorption and emission
spectra of D–A molecules show solvatochromic effects in the
solvents of different polarities. Solvatochromism17 has
Figure 3. a) Normalized absorption spectra of 10 and b) 12, c) and d) Lippert
Mataga plot of 10 and 12, respectively. The best fit result for 10 were: slope
5.83 ± 1.03 x 103 cm-1, intercept 3.60 ± 0.24 x 10-3 cm-1 and for 12 were: slope
3.25 ± 0.57 x 103 cm-1, intercept 3.57 ± 0.13 x 10-3 cm-1.
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Table 2. Optical and redox properties of TTF/DTF-appended molecules 1–12.
퐴푏푠.
퐸푚.
Compound
퐸표1푥
퐸표2푥
퐸푟1푒푑
퐸푟2푒푑
휆
휆
푚푎푥
HOMO/LUMO
(eV)[a]
HOMO/LUMO
(eV)[b]
ΔE
(eV)[a]
ΔE
(eV)[b]
ΔE
(eV)[c]
푚푎푥
(V)
(V)
(V)
(V)
(nm)
(nm)
1
2
0.83
0.98
0.47
0.93
0.72
0.83
0.95
0.86
0.86
0.70
0.80
0.93
–
-1.33
-1.23
-0.75
-0.81
-1.22
-0.53
-0.91
-0.82
-0.92
-0.91
-0.63
-1.66
–
530
515
450
603
608
510
585
534
503
515
553
650
610
640
737
674
555
720
637
643
575
705
-5.23/-3.07
-5.38/-3.17
-4.87/-3.65
-5.33/-3.59
-5.12/-3.18
-5.23/-3.87
-5.35/-3.49
-5.26/-3.58
-5.26/-3.48
-5.10/-3.49
-5.20/-3.77
-5.44/-2.74
-5.75/-2.70
-4.86/-2.39
-5.98/-3.55
-5.29/-2.92
-5.38/-2.37
-5.75/-3.28
5.55/-3.02
-5.64/-2.86
-5.17/-2.31
-5.73/-3.09
2.16
2.21
1.22
1.74
1.94
1.36
1.86
1.68
1.78
1.61
1.43
2.70
3.05
2.47
2.43
2.37
3.01
2.47
2.53
2.78
2.86
2.64
1.99
2.17
3b
5
0.70
–
-1.22
-1.25
-1.53
-1.04
-1.35
-1.12
-1.23
-1.27
-1.01
2.02
1.71
6
0.84
1.14
–
1.84
7
2.27
8
1.77
9
1.00
1.14
0.84
1.26
2.06
10
11
12
2.17
2.19
1.94
퐴푏푠.
퐸푚.
Redox potentials (Eox, Ered) reported in DCM with 0.1 M n-Bu4NPF6 as the electrolyte and saturated calomel reference electrode; 휆
and 휆
represents
푚푎푥
푚푎푥
absorption and emission maximum, respectively. [a] Calculated from the first oxidation and first reduction potential: EHOMO/LUMO = –(4.4 + E1(ox/red)). [b] Determined
from theoretical calculation (DFT/B3LYP/DGDZVP) and [c] optical spectroscopy onset absorption in DCM; ΔE = HOMO/LUMO energy gap calculated from
methods described in [a], [b] or [c].
푒푚
The 휆
for 10 in cyclohexane, CHCl3 and MeCN/DMF were
the plots in which ∆ 풗 is plotted against the solvent polarity
parameter Δf. A linear relationship is expected, according to the
Lippert Mataga relationship.19
푚푎푥
푒푚
recorded at 590, 644 and 686 nm, respectively. While the 휆
푚푎푥
for 12 in cyclohexane, CHCl3 and MeCN or DMF was found to
be at 655, 711 and 725 nm, respectively. Thus, for molecules 10
and 12 large Stokes shift of 5586 and 4589 cm-1, respectively,
was observed in MeCN, while considerably smaller Stokes shift
of 3712 and 3600 cm-1, respectively, was observed in non-polar
cyclohexane. The quite pronounced red-shift in fluorescence
spectra, as a function of the solvent polarity strongly indicates
that the solvent rearrangement increasingly stabilizes the
excited-states in polar solvents as expected for the ICT state in
these molecules.
Next, we also expressed the solvatochromic effect in the
terms of ET(30) scale of Dimroth and Reichardt,17 which is based
on the negative solvatochromism of betaine dye and defined by
the position of the maxima of the first absorption band of this dye
in kcal/mol. The solvent polarity scale can be defined by ET(D–
A).20 The ET(10) and ET(12) in different solvents are listed in the
Table 1. In contrast to the betaine dye, ET(10) was found to be
59.07 and 56.39 kcal/mol in cyclohexane and CHCl3,
respectively, whereas ET(12) was found to be 53.94 and 51.33
kcal/mol in cyclohexane and CHCl3, respectively. This suggests
positive solvatochromism in 10 and 12.
Moreover, we have demonstrated the solvatochromic
effect as a function of Stokes shift, ∆푣 (solvent polarity) and
orientation polarizability (∆f) by the well-known Lippert-Mataga
equation.19
Electrochemistry. The multistate electrochemical properties
and electronic energy levels of the TTF/DTF-fused molecules
were evaluated by cyclic voltammetry (CV) and differential pulse
voltammetry (DPV) in DCM and summarized in Table 2 and
shown in Figure S2. The electron rich TTF/DTF unit is capable
of donating electrons, while the electron deficient imide scaffolds
can accept electrons. The first single electron oxidation wave is
reversible for the most of the TTF/DTF-fused molecules, which
indicates that the cation radical states are stable within the time-
scale of the electrochemical experiments, while the first single
electron reduction wave is quasi-reversible or irreversible. In
class 1, the trans-TTF-fused molecule 1, shows the first
oxidation potential (퐸표1푥 ) at +830 mV and the first reduction
potential (퐸푟1푒푑) at –1330 mV, which is shifted to +980 mV and -
∆푣 = [2∆푓/ℎ푐푎]∆휇2 + 푐표푛푠푡.
∆푓 = (∈ −1)/(2 ∈ +1) − (푛2 − 1)/(2푛2 + 1)
Here, Δf is a function of the dielectric constant, ε and the
refractive index n, h represents the Planck constant, c the
velocity of light, a the Onsager cavity radius and Δµ the
difference between ground state and excited state dipole
moment. The ∆풗 of D–A chromophores is proportional to the
dielectric constant of the solvents which increases with
increasing the dielectric constant of the solvent. We have
noticed, the ∆풗 for 10, are 3712 and 5586 cm-1 in cyclohexane
and acetonitrile, while the ∆풗 (ퟏퟐ) are 3600 and 4589 cm-1 in
cyclohexane and acetonitrile, respectively. Figure 3c–d display
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1230 mV, respectively in the cis-TTF-fused 2. Molecule 3b
shows the 퐸표1푥 at +470 mV and the 퐸푟1푒푑 at –750 mV. On
traversing from the mono- to bis-DTF substituted molecules
increasing the number of the TTF/DTF donor substituents
increases the HOMO energy level as well as the LUMO energy
level. The molecule 3b exhibited 퐸표1푥 of +470 mV and has the
highest HOMO energy level, which is attributed to the increased
number of the TTF/DTF donor substituents. For instance, the
퐸표1푥 of mono-DTF fused NDA, 5, was found to be at +930 mV
and the 퐸푟1푒푑 at –810 mV, which shifted to +720 and –1220 mV,
respectively, in the trans-TTF-NDA fused molecule 6. The
molecule 7 of this class shows 퐸표1푥 and 퐸푟1푒푑 at +830 and –530
mV, respectively, and has the lowest LUMO at –3.87 eV
(determined experimentally) amongst the TTF/DTF-fused D–A
molecules. In the class 2, molecule 8 shows HOMO at –5.35 eV,
while the LUMO was found to be at –3.49 eV (LUMO of 5, –3.59
eV). Therefore, incorporation of single imide functionality in
molecules of class 2, a negative shift in the reduction potential of
molecule 8 is reported, while the oxidation potential remains
mostly unchanged. On the other hand, the 퐸표1푥 and 퐸푟1푒푑 of 9 are
+860 and –820 mV, respectively. In the class 3, similar to
molecules 5 and 6, the 퐸표1푥 decreases from +860 mV to +700
mV in 10 and 11, respectively, while 퐸푟1푒푑 was found to be almost
unchanged at –920 and –910 mV, respectively. As compared to
10, the 퐸표1푥 of molecule 12 is cathodically-shifted by 60 mV at
+800 mV, while the 퐸표1푥 is significantly anodically-shifted by 290
mV at –630 mV. Therefore, increasing the number of TTF/DTF
units shifts the 퐸표1푥 cathodically and the 퐸푟1푒푑 and 퐸푟2푒푑 values
towards more negative potentials. Increasing the acceptor
strength through the anhydride functionality results in 퐸푟1푒푑 being
anodically-shifted. These results clearly indicate that 퐸표1푥 and
퐸푟1푒푑 and the frontier orbital energies can be precisely modulated
depending on the number of D units and the A strength of the π-
bridge and also the number of the anhydride and the imide rings.
Oxidation and Reduction. The presence of favorable redox
window of the TTF/DTF-appended molecules allowed us to
pursue the oxidation/reduction reactions and generation of the
corresponding radical cation (Figure 4) and radical anion (Figure
5). In the class 1, on gradual addition of FeCl3 in a DCM solution
of trans-TTF-fused 1, led to a steady decrease in the 530 nm
band and emergence of the characteristic 765 nm band
corresponding to the radical cation 1•+ and a low energy IV-CT
band at 1950 nm (5130 cm-1), corresponding to the MV states.
Under similar condition, formation of the 2•+ was observed with
the appearance of the signature band at 780 nm. In contrast to
the 1•+, the cis-TTF-fused, 2•+, didn’t show any IV-CT band due
to the absence of electronic communication between the DTF
units. In the corresponding NDA scaffold, the trans-TTF-fused
6•+ exhibits characteristic peak at 930 nm and its IV-CT band at
2160 nm (4630 cm-1) upon oxidation with FeCl3 in DCM.
Similarly, the mono-DTF-fused molecules 5 and 7 formed the
corresponding 5•+ and 7•+ with FeCl3 in DCM.
Figure 4. UV-Vis-NIR spectra of TTF/DTF-fused molecules 7, 9, 10 and 11 (5 x 10-5 M) with addition of FeCl3 in DCM, showing the formation of the radical cations.
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Figure 5. Time-resolved absorption spectra of 5, 8, and 12, showing a reductive electrolysis at potential of -1.0 V in DCM solution of chromophores containing
0.15 M of n-Bu4NPF6 as the supporting electrolyte. a) Formation of radical anion, 5•−, b) 8•− and c) 12•−. The initial spectrum is shown by black bold trace whereas
the red bold trace is the final spectrum.
The radical cation, 7•+ was characterized by the signature peaks
at 611, 875 and 1235 nm (Figure 4a). In the class 2, the
chromophores, 8 and 9 formed radical cations 8•+ and 9•+ with
FeCl3 in DCM. In class 3, similar to the other mono DTF-fused
chromophores, when the molecule 10 was treated with FeCl3 in
DCM, a new set of bands evolved at 362, 629 and 875 nm with
a concomitant decrease of bands at 505 nm due to the
generation of 10•+ (Figure 4c). Likewise, molecule 11 led to the
formation of 11•+ on the addition of FeCl3 with the characteristic
bands at 314, 362 and 740 nm along with the formation of
mixed-valence state at 2500 nm (4000 cm-1). Importantly, the
corresponding radical cations could also be generated in the
presence of common oxidizing agent like Cu(ClO4)⋅ xH2O in
MeCN. The resulting absorption peaks matched with that of the
signature peaks of the radical cations generated from FeCl3 in
DCM.
functionality along with the naphthalene ring. The stability of
radical cations/anions of the representative chromophores have
been
demonstrated
by
time-dependent
UV-Vis-NIR
spectroscopy and were found to be persistently stable only for
few minutes (Figure S4). The formation of radical cations and
radical anions were also confirmed by electron paramagnetic
resonance (EPR) spectroscopy (Figure S5 and S6). The NDI
centered radical anion showed the characteristic multiline
spectra, while the TTF/DTF centered radical cations show a
single line spectra possibly due to spin-exchange phenomenon.
Mixed Valence State. The organic MV compounds are
characterized by the optically induced IV-CT bands, which
usually appear in the near-infrared (NIR) region. The electronic
coupling element, Hab can be estimated from the analysis of the
IV-CT transition.21-23 The molecules 1, 2, 6 and 11 comprising of
two DTF redox units are conjugated with π-acceptor bridge
PMDA, NDA or PMDI and readily affords the mixed-valence
states by chemical oxidation of corresponding molecules under
ambient condition. The oxidation of the trans-TTF-fused D-A
molecules, 1, 6 and 11 show interesting electron coupling
interaction in which the electron is transferred from neutral DTF
moiety to the radical cation containing DTF moiety.
Consequently, 1, 6 and 11 exhibits the IV-CT band due to the
On the other hand, the NDA/NDI appended mono-DTF-
fused molecules formed radical anions upon reductive
electrolysis in DCM in
a
thin-layer quartz glass
spectroelectrochemical cell with 0.5 mm path length (Figure 5)
or by addition of Na2S reductant in THF (Figure S3). We focused
on molecules 5, 8 and 12 as representative molecules from
class 1, 2 and 3, respectively. Molecule 5 formed the radical
anion 5•− upon reductive electrolysis at potential of -1.0 V (Figure
5a). The radical anion 5•− shows the characteristic new bands at
742, 511 and 425 nm due to the NDA centered radical anion
with subsequent disappearance of the ICT band at 603 nm. This
signature absorption characteristic is a manifestation of the NDA
centered D0→Dn transitions. The formation of the radical anion
8•− in the class 2, was characterized by the appearance of new
bands at 750, 610, and 480 nm along with the disappearance of
the ICT band at 585 nm (Figure 5b). Likewise, in class 3,
molecule 12 formed radical anions with an emergence of new
bands at 750, 675, 610, and 480 nm (Figure 5c). The formation
of these radical anions was also established chemically by the
addition of Na2S in THF (Figure S3). Therefore, the bis-
anhydride, mono-imide and bis-imide functionality in 5•−, 8•− and
12•−, respectively, can modulate the D0→Dn transitions in these
radical anions. This is due to the fact that the spin density is
centered predominantly over the anhydride or the imide
Table 3. IV-CT band analysis of 1.+, 6.+ and 11.+ in DCM.24
Hab (cm-1)
1438
푽 max (cm-1)
5130
ΔG* (cm-1)
248
kET (1010 s-1)
1.+
30.0
3.7
6.+
535
4627
683
11.+
433
4000
613
5.2
Hab: electron coupling, 푉 max: reorganization energy, ΔG*: free energy of
activation, and kET: rate constant of electron transfer.
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Figure 6. Electronic spectra of mixed-valence radical cation 1.+ (1x10-5M), 6.+ (1x10-5 M) and 11.+ (5x10-5 M) generated by FeCl3 in DCM, showing a low energy IV-
CT band.
mixed valence state at 1950, 2160 and 2500 nm (5130, 4630
and 4000 cm-1), respectively. Thus, we could tune the position
of the IV-CT state in the mid infrared (MIR) region from 1950 nm
Therefore, the bis-DTF-fused benzene-based π-acceptor bridge
systems (1 and 11) can be attractive platform to investigate the
effect of the geometry and its effects on the reorganization
(5130 cm-1) to 2500 nm (4000 cm-1) by changing the π-acceptors, energy, ET rate and its tunability of the opto- electronic property.
from PMDA to PMDI (Figure 6). Hab is inversely proportional to
the distance between the donor centers. The DTF groups in
molecule 6 is separated by naphthalene-based π-acceptor
bridge, which decreases the electronic coupling compared to the
benzene-based bridge in 1. The imide substitution in 11 results
in a planarity disruption of the redox centers with the central
benzene ring compared to that the redox centers with the central
benzene ring compared to that of 1, which affects the
delocalization in the radical cation 11•+. Consequently, electronic
coupling parameter, Hab derived from the IV-CT band of 11•+ (Hab
= 433 cm-1) is significantly reduced compared to 1•+ (Hab = 1438
cm-1) and the electron transfer in 1•+ (kET = 30 x 1010 s-1) is six-
fold faster than 11•+ (kET = 5.2 x 1010 s-1) (Table 3). As a
consequence of the small reorganization energy, 푉max = 4000
cm-1 for 11, the IV-CT band shifts to lower energy compared to
corresponding bands in 1, (푉max = 5130 cm-1).9 The radical cation
6.+ shows Hab of 535 cm-1 and 푉max of 4627 cm-1 with the rate of
electron transfer, kET of 3.7 x 1010 s-1. The overall phenomena
can be attributed to the increased free energy of activation (ΔG*)
on moving from 1.+ to 11.+, which is also reflected in the electron
coupling and reorganization energy of the D-A dyads.
Notably, the IV-CT band was not observed for 2•+ due to the
absence of electronic communication between the redox active
centers. The mixed valence state requires communicating redox
centers and in the case of 2•+ the DTF units are located in the
meta- positions of the π-acceptor bridge.9,24 Furthermore, the
lowest gap of HOMO/HOMO-1 was obtained in 1, and the
highest gap in 11, which also supported the large electronic
coupling in 1 and the smaller electronic coupling in 11 (Table 4).
Theoretical Calculation. The Density Function Theory (DFT)
calculations were performed at the B3LYP/DGDZVP basis set
using the pre-optimized structure at HF/6-31g** and by allowing
the relaxation of all the parameters, the calculations converged
to optimized geometries. To simplify the calculation, the long
alkyl chains were replaced by the methyl groups. The MOs were
symmetrically distributed at the both sides of the nodal planes in
the HOMO as well as the LUMO (Figure S7). In class 1, the
HOMOs in 1 and 6 are symmetrically delocalized over the
TTF/DTF moiety through the π-linker whereas the LUMO
surfaces are delocalized over the π-acceptor bridge. The HOMO
and LUMO in 2 are mostly localized over the TTF/DTF and
PMDA, respectively. The HOMO of 3 is mostly delocalized over
a pair of DTF through the one of the connecting anhydride
moiety of the acceptor, whereas, the LUMO is localized over
acceptor and to some extent over the DTFs. In addition, the
HOMOs of 5 and 7 are delocalized over the DTF and to a
significantly lesser extent on the acceptor, while the LUMOs are
delocalized over the entire π-acceptor surface. In class 2, the
HOMOs of 8 and 9 are mostly localized over DTFs whereas the
LUMOs are spread over the imide moiety of the π-acceptors. In
class 3, the HOMOs of 10 and 12 are localized over the DTFs
whereas the LUMOs are delocalized over the π-acceptor surface.
Moreover, the HOMO and LUMO in 11 are identically distributed
as in 1. In molecule 12, the steric repulsion between the N-
substituted alkyl group and the DTF unit is sufficient to twist the
molecule (Figure S7). The steric repulsion causes asymmetric
Table 4. Energy level of 1, 6 and 11 in different states in DCM.
[a]
[b]
[c]
Compound
HOMO (eV)
HOMO-1 (eV)
ΔE (meV)
1
6
-5.23
-5.33
100
-5.12
-5.24
120
11
-5.10
-5.24
140
[a] Calculated from the first oxidation potential, [b] calculated from the second
oxidation potential and [c] ΔE = HOMO – (HOMO-1).
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distribution of the electron densities in the HOMO/LUMO
surfaces. Next, in order to gain insight into the electronic
communication between the TTF/DTF units and MV states of 1.+,
2.+, 6.+ and 11.+, DFT/B3LYP/DGDZVP calculations were
performed (Figure 7).
TTF/DTF units or as imide functionality impart high solubility.
The donor units allow the formation of radical cations (D•+)
whereas the π-extended acceptors (A) allow the formation for
radical anions (A•−) by chemical oxidation/reduction under
ambient conditions. The large π-surface of the TTF/DTF-
appended systems is able to offer persistent stability to the
radical ions. The radical ions feature UV-Vis-NIR optical
absorption properties. Notably, some of the chromophores
푒푚
푚푎푥
showed NIR fluorescence with 휆
extending up to ~740 nm. In
addition, few of the TTF/DTF-appended chromophores showed
excited-state solvatochromism. The π-acceptors having
anhydride functionality allowed easier electron reduction
compared to the imide groups, while the increasing number of
donor units make them easily oxidizable. Interestingly, some of
the trans-TTF-fused molecules exhibited mixed-valence state in
the mid-infrared (MIR) region 5130-4000 cm-1. The multistate
redox activity along with panchromism and NIR to MIR optical
absorption of these systems can be attractive for application
towards multi-stimuli switchable molecular materials.24
Experimental Section
General procedure for synthesis of TTF/DTF-fused D-A
chromophores (1-12): The precursors typically anhydride or imide and
the DTF were added to double neck round bottom flask connecting to a
condenser in inert atmosphere. Toluene was added by a syringe and
trialkyl phosphite was also added by a syringe. The reaction mixture was
stirred at 120-135 oC under inert atmosphere. Note: In case of 3, 4, 7,
and 9-12, reaction was performed in neat i.e. toluene was not required.
However, the compound, 8 was synthesized by stirring a mixture of 5 and
hexyl amine in DMF at 110 oC under inert atmosphere.
Figure 7. Spin density distribution and the SOMO surfaces of the TTF
appended π-conjugated acceptors.
Acknowledgements
The SOMO surface and spin densities of the TTF-fused radical
species were calculated using their optimized geometries. The
SOMO and the spin density delocalization demonstrate the
moderate interactions between the two redox units. The SOMO
surface are delocalized uniformly over 1.+, 6.+ and 11.+ whereas
the delocalization is disrupted in 2.+. In 2.+, the spin density is
localized over the TTF units and the electronic communication is
interrupted. The shapes of these IV-CT bands are same and the
Hab < λ/2 for the IV-CT bands. The moderate electronic coupling,
Hab therefore endorse a Robin-Day class II type of mixed
valence molecules.
We thank DST-PURSE, UPE-II and FIST for the financial
support. PM thanks DST, GOI for the SwarnaJayanti Fellowship
(DST/SJF-02/CSA-02/2013-14). We also thank AIRF, JNU, for
500 MHz NMR and FT-IR and MALDI-TOF mass spectra. We
acknowledge council of scientific & industrial research (CSIR),
New Delhi for the financial support to Sudhir Kumar Keshri and
Yogendra Kumar.
Keywords: mixed-valence state • Mid IR absorption • radical
cation • radical anion • solvatochromism
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10.1002/chem.201704604
Chemistry - A European Journal
FULL PAPER
Entry for the Table of Contents
Sudhir Kumar Keshri, Deepak Asthana,
Sonam Chorol, Yogendra Kumar, Pritam
Mukhopadhyay*
Page No. – Page No.
Appending Diverse π-Extended
Acceptors with TTF/DTF Donors:
Multistate Redox, Radical Ion Generation
and Mid-IR Absorbing Mixed-Valence
States
Three classes of donor-acceptor π-extended chromophores (1–12) were synthesized via
phosphite-mediated cross-coupling reaction in which the anhydride or imide-based π-acceptor
(A) and the number of tetrathiafulvalene (TTF)/dithiafulvalene (DTF) donors (D) were
systematically changed. The donor (D) moieties allow the formation of radical cations (D•+)
whereas the π-extended acceptors (A) allow the formation for radical anions (A•−) by chemical
oxidation/reduction under ambient conditions. Interestingly, the trans-TTF-fused D–A molecules
(1, 6 and 11) exhibited mixed-valence state in the mid infrared (MIR) region (5130-4000 cm-1).
The multistate redox activity along with panchromism and NIR/MIR optical absorption property of
the radical ion systems can be attractive enablers for application towards switchable molecular
materials.
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