Charge stabilizationviaelectron exchange: excited charge separation in symmetric, central triphenylamine derived, dimethylaminophenyl-tetracyanobutadiene donor-acceptor conjugates

Article abstract of DOI:10.1039/d0sc04648e

Photoinduced charge separation in donor-acceptor conjugates plays a pivotal role in technology breakthroughs, especially in the areas of efficient conversion of solar energy into electrical energy and fuels. Extending the lifetime of the charge separated

Full text of DOI:10.1039/d0sc04648e

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Charge Stabilization via Electron Exchange: Excited Charge Separation in
Symmetric, Central Triphenylamine Derived, Dimethylaminophenyl-
Tetracyanobutadiene Donor-Acceptor Conjugates
a‡
b‡
a*
b*
Indresh S. Yadav, Ajyal Z. Alsaleh, Rajneesh Misra and Francis D’Souza
aDepartment of Chemistry, Indian Institute of Technology, Indore 453552, India. E-mail:
rajneeshmisra@iiti.ac.in
bDepartment of Chemistry, University of North Texas, 1155 Union Circle, #305070, Denton, TX
7
6203-5017, USA, E-mail: Francis.DSouza@UNT.edu
‡Equal contribution
(Abstract). Photoinduced charge separation in donor-acceptor conjugates play a pivotal role in
technology breakthroughs, especially in the areas of efficient conversion of solar energy into
electrical energy and fuels. Extending the lifetime of the charge separated species is a necessity
for their practical utilization, and this is often achieved by following the mechanism of natural
photosynthesis where the process of electron/hole migration occurs distantly separating the radical
ion-pairs. Here, we hypothesize and demonstrate a new mechanism to stabilize the charge
separated states via the process of electron exchange among the different acceptor entities in
multimodular donor-acceptor conjugates. For this, star-shaped, central triphenylamine derived,
dimethylamine-tetracyanobutadiene conjugates have been newly designed and characterized.
Electron exchange was witnessed upon electroreduction in conjugates having multiple numbers of
electron acceptors. Using ultrafast spectroscopy, occurrence of excited state charge separation, and
the effect of electron exchange in prolonging the lifetime of charge separated states in the
conjugates having multiple acceptors has been successfully demonstrated. This work constitutes
the first example of stabilizing charge-separated states via the process of electron exchange.
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Introduction
Excited state charge transfer in donor-acceptor conjugates is one of the widely investigated topics
in recent years due to their usage in building energy harvesting photonics devices.1-19
Understanding the principles governing the kinetics of charge transfer and separation, securing
high charge separation quantum yields, avoiding large energy losses, and prolonging the lifetime
of the radical ion-pairs by molecular engineering of the conjugates have been the main focus of
these studies.1-12 In simple donor-acceptor conjugates, charge separation from the excited singlet
state of the donor or acceptor can store the greatest amount of energy, however, since the process
originates from the singlet excited state, the charge separated states are generally short-lived. In
natural photosynthesis, lifetime of charge separation is prolonged by subsequent electron transfer
to secondary acceptors.20-22 This method of charge stabilization, optimizes the quantum yields but
comes at the cost of lowering the overall efficiency due to the energy losses encountered during
secondary electron transfer steps. Alternate approaches including electron/hole delocalization in
conjugates having closely interacting multiple donor or acceptor entities,23-24 and utilization of
high energy triplet sensitizers to promote charge transfer from long-lived triplet excited states to
prolong lifetime of charge separated states25-26 have also been proven to work.
In recent years, the design and synthesis of π-conjugated symmetrical and unsymmetrical
donor-acceptor chromophores have been extensively investigated due to their potential
applications in organic photovoltaics,27-30 molecular electronics and bioimaging. The star-
shaped π-conjugated molecular systems exhibit many advantages over linear conjugated molecular
systems including excellent solubility and lesser aggregation.33 Tuning of electronic and photonic
properties of these systems can be achieved by modulating the design of donor or acceptor units
and connecting π-spacer unit.34-36 The small organic π-conjugated donor-acceptor systems exhibit
low band gap, intense absorption, and strong intramolecular interactions.37-38 In several of these
studies, triphenylamine, a classical nonplanar propeller shaped optoelectronic molecule, has been
extensively used; extending their applications for developing field effect transistors, sensors, and
solid state fluorescent and smart fluorescent materials.39-40
31
32
2

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Chart 1.
Structure and abbreviation of star-shaped, central triphenylamine derived,
dimethylamine-tetracyanobutadiene conjugates, 1-4 and the control compounds, C1-C2 newly
designed, synthesized to demonstrate charge stabilization via electron exchange in the present
study.
Tetracyanoethylene (TCNE) is a strong electron acceptor due to presence of four cyano
groups, undergoes [2+2] cycloaddition reaction with electron rich alkynes to form cyclobutene
rings followed by retroelectrocyclization reaction to give 1,1,4,4-tetracyanobutadiene (TCBD)
4
1
derivatives. The donor-acceptor systems containing TCBD acceptor are potential candidates for
organic photovoltaics and non-linear optics due to strong intramolecular charge transfer (ICT) and
lower HOMO-LUMO gaps.42-48 Photochemical behavior of few donor-TCBD derived systems
have been reported in literature.49-56 Although with high quantum yields, due to close proximity
between the donor and acceptor entities, ultrafast charge separation and recombination was
observed in these systems. That is, no charge stabilization could be accomplished. In this regard,
developing higher analogs of donor-TDCB bearing systems that would exhibit novel
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photochemical properties including charge stabilization have been scarce due to the associated
synthetic challenges. A recent example involves C -symmetric central truxene-derived
3
5
4
phenothiazine-TCBD and its expanded molecular systems. Although elegant, the central truxene
made exclusively of saturated carbons played no role in stabilizing the charge separated states.
In the present study, we hypothesize that by choosing a redox/photoactive central unit
instead of truxene, we could modulate the properties that would lead to novel redox- and
photochemical discoveries. With this in mind, we have newly designed and synthesized star-
shaped symmetric compound (NND)
3
-TPA, 1 and their TCBD functionalized symmetric and
-TPA, 2–4 (see Chart 1 for structures; NND = N,N-
unsymmetric derivatives (NND-TCBD1-3
)
3
dimethylaminophenyl, TPA = triphenylamine and TCBD = 1,1,4,4-tetracyanobutadiene). These
novel systems show strong intramolecular charge transfer (ICT) with lowered HOMO-LUMO
gaps. Further, upon electroreduction of NND-TCBD entities in compounds 3 and 4 containing two
and three NND-TCBD entities, electron exchange among the NND-TCBD was witnessed.
Femtosecond transient absorption studies revealed occurrence of ultrafast charge transfer
processes in these systems. Importantly, charge stabilization in 3 and 4 was witnessed as a
consequence of electron exchange. These unpresented new findings provide a new mechanism of
stabilizing the charge separated states via electron exchange in multi-modular donor-acceptor
conjugates.
Results and Discussion
Scheme 1 shows the developed synthetic scheme for compounds 1-4 and their controls.
Briefly, the symmetric (NND)
Sonogashira cross coupling of tris-(4-iodo-phenyl)-amine and 4-ethynyl-N,N-dimethylaniline in
degassed THF:TEA (1:1) under argon atmosphere, in the presence of Pd(PPh and CuI. Next,
-TPA, 2–4 were synthesized via [2+2] cycloaddition-retroelectrocyclization
3
-TPA, 1 was synthesized in 60% yield by the Pd-catalyzed
)
3 4
(NND-TCBD1-3
)
3
reaction with the strong electron acceptor TCNE. The reaction of 1 with one equivalent of TCNE
in DCM at room temperature for 4 h resulted in an exclusive mono TCBD bearing (NND-
TCBD
1
)
3
-TPA, 2 in 63% yield. Similarly, the reaction of 1 with two equivalent of TCNE in DCM
o
solvent at 40 C for 12 h resulted in (NND-TCBD
2
3
) -TPA, 3 in 65% yield, whereas upon increasing
o
the reaction temperature to 80 C in DCE solvent for 24 h using four equivalent of TCNE with 1,
resulted in symmetrical (NND-TCBD
3
)
3
-TPA, 4 in 70% yield. The control compound C1 was
4

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synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction of 4-ethynyl-N,N-
dimethylaniline and iodobenzene in 60% yield. The acetylene linked control compound C1 further
subjected to [2+2] cycloaddition-retro-electrocyclization reaction with one equivalent of TCNE at
room temperature for 8 h, which resulted in TCBD substituted control compound C2 in 82% yield.
The newly synthesized compounds were purified over silica gel (100-200 mesh) column
1
13
chromatography using Hexane:DCM as solvent and fully characterized by H, C NMR and high-
resolution mass spectroscopy (HRMS) techniques (see SI for spectral details, Figures S1 – S18).
Scheme 1. Synthetic scheme of compounds (NND)
3
-TPA, 1 and (NND-TCBD1-3) -TPA, 2–4.
3
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Absorption spectrum of the investigated compounds is shown in Figure 1a. Control C1,
having only a NND entity without either TPA or TCBD entities revealed an absorption band at
3
42 nm. In case of control C2, having an electron acceptor, TCBD next to the electron donor,
NND entity, promoted charge transfer interactions between them. Consequently, two peaks, the
first one at 315 and a second broad peak corresponding to charge transfer absorption at 478 nm,
was observed. For compound 1, having a central TPA and three terminal NND entities revealed a
single absorption peak at 386 nm. As predicted, no charge transfer type peak was present.
However, in the case of compounds 2-4, having one, two and three TCBD entities between the
NND and TPA entities, the expected charge transfer peak in the 478-484 nm region was possible
to witness. In addition, a UV peak at 372 nm for 2, 350 nm for 3 and, <300 nm for 4, respectively,
was also observed. Intensity of the charge transfer band increased with increasing the number of
TCBD entities. Due to spectral similarities between C2 and compounds 2-4, and enhaneced
absorption of the charge transfer band with increase in TDCB, it was possible to conclude that the
origin of the charge transfer band is primarily due to interaction between NND and TCBD entities
with lesser contributions from TPA interaction with TCBD. Optical data is summarized in Table
1
.
Figure 1. (a) Absorption and (b) fluorescence spectra of indicated compounds in DCB.
Compound 1 was excited at 386 nm. No measurable emission was observed for compounds 2-4
upon exciting the samples either at the locally excited or charge transfer absorption peak positions.
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Table 1. Absorption and fluorescence, redox potentials (V vs. Ag/AgCl), and free-energy changes
for charge recombination (CR), charge separation (CS) and charge transfer (CT) for the
investigated central triphenylamine derived, dimethylamine-tetracyanobutadiene conjugates in
DCB.
5
-1
System
max
nm
,
Ε (10 M
cm )
 ,
nm
Red-2 Red-1 Ox-1 Ox-2
TCBD TCBD TPA NND
Ox-3
NND
--
-GCR -GCS -GCT
em
-
1
C1
C2
337
327
2.99
--
--
--
--
0.96
1.43
--
--
--
--
--
--
1.16
2.23
-0.73
-0.39
--
4
73
1
2
384
372
6.89
420
419
--
--
0.80
0.88
1.15
1.30
--
--
--
--
4.40
2.91
-0.69
-0.43
1.45
1.11
1.85
2.56
4
77
3
4
354
72
1.80
5.43
418
417
-0.73
-0.70
-0.33a 0.96
-0.48
1.20
1.63
1.50
--
1.01
1.35
1.95
1.61
2.59
2.59
4
316
76
1.63
6.56
-0.30a 1.35
-0.40
4
a-split peak
Among the investigated compounds, only compound 1 revealed fluorescence emission as
shown in Figure 1b. A broad peak with maxima at 420 nm and spectrum spanning the 400-575
nm range was observed (estimated quantum yield = 0.43). Lifetime from single photon counting
technique revealed a monoexponential decay with a lifetime of 1.16 ns. For compounds 2-4,
having 1-3 numbers of strong electron acceptor TCBD entities, no measurable emission, either at
the locally excited or charge transfer band positions, was observed; perhaps such emission were
too weak to detect. In any case, the strong quenching observed in the case of compounds 2-4
suggest occurrence of excited state events such as energy or electron transfer in the highly
interacting push-pull conjugates.
Next, in order to seek possible intramolecular interactions among the NND-TCBD entities
via central TPA in compounds 3 and 4, electrochemical studies using differential pulse (DPV) and
cyclic voltammetry (CV) were performed in DCB containing 0.1 M (TBA)ClO . The site of
4
electron transfer was arrived from control C1 and C2 and remainder of compounds, and is
summarized in Table 1 and representative voltammograms are shown in Figure 2. Complete CVs
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are shown in Figure S19 in SI.
Figure 2. DPVs (left panel) and CVs (right panel) of indicated compounds in DCB containing 0.1
M (TBA)ClO . For DPV: scan rate = 5 mV/s, pulse width = 0.25 s, pulse height = 0.025 V. For
4
CV: scan rate = 100 mV/s. The ‘*’ in left panel represents oxidation peak of ferrocene used as
internal standard. Note: the first reduction corresponding to TCBD in 3 and 4 is a split wave (see
text for details).
Key observations from electrochemical studies involved: (i) The first oxidation of C1
located at 0.96 V vs. Ag/AgCl was anodically shifted to 1.43 V in C2 due the presence of electron
deficient TCBD. The TCBD reductions, all one-electron reversible, were located at -0.39 and -
0
.73 V. (ii) Compound 1 revealed two oxidations, the first one at 0.80 V and a second one at 1.15
V. From the peak currents and by comparison with oxidation potential of C1, the first oxidation
to TPA and the second one to NND entities was possible to arrive. (iii) In the case of compound
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2
, having a single NND-TCBD entity, the TPA oxidation was shifted to 0.88 V while the NND
oxidations were split and appeared at 1.30 and 1.45 V owing to the presence of two types of NNDs
one linked to TCBD and another without TCBD). The TCBD reductions were located at -0.43
(
and -0.69 V. (iv) Introduction of a second TCBD entity in 3 and a third one in 4 revealed additional
interesting features. As predicted, oxidation peaks revealed further anodic shift especially for TPA
oxidation. Interestingly, the first reduction of TCBD in both compounds 3 and 4 were found to be
Figure 3. Frontier HOMO, LUMO and LUMO+1 of investigated compounds from the B3LYP/6-
3
1G** optimized structures (see SI for coordinates of computed structures).
split waves. The split reduction peaks for 3 were located at -0.33 and -0.48 V, that is, a 140 mV
potential difference while for 4, the split peaks were located at -0.30 and -0.40 V, that is, about
1
00 mV potential difference. The second reduction of TCBD in both 3 and 4 were one-electron
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reductions without noticeable splitting. The splitting of first reduction show electron exchange
among the NND-TCBD entities in 3 and 4 via the central TPA entity.
The electron exchange between the NND-TCBD entities upon first electroreduction of
compounds 3 and 4 motivated us to perform computational studies to probe their electronic
structures. Compounds 1-4 were fully optimized on a Born-Oppenheimer potential energy surface
at the B3LYP/6-31G* level calculations.57 The generated frontier orbitals on the optimized
structures are shown in Figure 3. The C type symmetry originating from the central TPA entity
3
was obvious in all these compounds. In the case of 1, the HOMO was distributed evenly on the
entire molecule while the LUMO coefficient was slightly more on one of the arms. In the case of
2
, the HOMO was localized NND-TPA arms while the LUMOs were on the TCBD entity with
some contributions extending into the NND and TPA entities. The energy difference between the
two LUMOs was 0.0211 hartrees. In the case of compound 3, the HOMO occupied only the NND-
TPA arm while the LUMOs occupied the two NND-TCBD with almost even distribution. The
energy difference between the two LUMOs was 0.00527 hartrees. This situation was also true for
compound 4, where the LUMO was distributed over two NND-TCBD entities while the LUMO+1
had contribution on all three NND-TDBD entities. The energy difference between the LUMOs
was 0.0047 hartrees. Splitting of the first reduction peak due to electron exchange in the case of
compounds 3 and 4 can now be attributed to energetically closely spaced LUMOs.
One of the approaches to visualize the spectrum of charge separation products is by
performing spectroelectrochemical studies followed by spectral interpretation. Here, by applying
appropriate potentials corresponding to oxidation or reduction, spectrum of the radical cation and
radical anion can be generated. Subsequently, the average of the radical cation and radical anion
spectrum will be digitally generated and subtracted from the spectrum of the neutral compound.
This represents the differential absorption spectrum of the charge separation product. Positive
peaks represent transitions associated with the electron transfer product while negative peaks
5
4
represent depletion of the absorption of the neutral compound. We have used this approach in
the present study as shown in Figure 4a-c. Spectral changes associated during first oxidation of 3
is shown in Figure 4a. A new peak during the process of oxidation was observed at 662 nm. The
spectral changes were minimal in the visible region as the main 478 nm peak was due to NND-
TDCB charge transfer transition whereas the oxidation was on the TPA entity (vide supra),
ascertaining the earlier discussed site of electron transfer. On the contrary, the spectral changes
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during first reduction (Figure 4b) revealed drastic decrease in intensity of the charge transfer band
with broad positive spectral features in the 600-800 nm range. It may be mentioned here that
during both oxidation and reduction, now new peaks beyond 800 nm was observed. The spectrum
generated for the charge transfer product using the above described procedure is shown in Figure
4
c. Such spectrum revealed a positive peak at 670 nm and depleted peak at 478 nm. Witnessing
such a spectrum in transient absorption spectral studies would provide direct proof of charge
transfer in these donor-acceptor conjugates. Similar spectra were derived for compounds 2 and 4
(see Figure S20 in SI).
Figure 4. Spectral changes observed during (a) first oxidation and (b) first reduction of 3 in DCB
containing 0.2 M (TBA)ClO . (c) Spectrum deduced for the charge separation state using
4
spectroelectrochemical data (see text for details, and Figure S20 in SI for complete results). (d)
Energy level diagram showing possible charge transfer and charge separation events upon
photoexcitation of the compounds 2-4. NND without linked TCBD in 2-3 is not shown in the
abbreviated formula for simplicity.
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An energy diagram was established to visualize the energetics of charge transfer and charge
separation states in these conjugates, as shown in Figure 4d. Energy of different states were
established from free-energy calculations,58-59 as listed in Table 1. From such a diagram, it was
clear that excitation of the conjugates, 2-4 either that the locally excited (near-UV) or charge
transfer (visible) peak positions would produce the respective excited states. The excited state
species, (NND-TCBD)1-3-1TPA* formed from the locally excited state would readily produce


initial charge transfer state, (NND-TCBD)1-3 -TPA involving one of the NND-TCBD entities
free NND is not abbreviated for simplicity). Such a state can also be produced by direct excitation
of the visible charge transfer band. The charge transfer state thus generated could further undergo
(



electron transfer to generate the (NND-TCBD1-3) -TPA charge separated species. Although
initial charge separation state would involve only one of the NND-TCBD entities, due to electron
exchange, the anion radical could spread over other NND-TCBD entities, as suggested by the
earlier discussed frontier LUMOs. Finally, the charge separated species could relax back to the
ground state.
In order to probe the anticipated photochemical events and to seek the effect of multiple
NND-TCBD entities in prolonging lifetime of charge separated states via the earlier discussed
electron exchange mechanism, femtosecond transient absorption studies (fs-TA) were performed.
Three solvents of varying polarity were used as the solvent polarity would influence lifetime of
charge separated states, and samples were excited at both locally excited (350 nm) and charge
transfer (500 nm) peak positions.
1
As shown in Figure 5a(i), singlet excited state of compound 1 ( TPA*) in benzonitrile was
formed instantaneously upon 350 nm laser excitation featuring excited state absorption (ESA)
maxima at 533, 601 and 1382 nm (see spectrum at 2.22 ps) as well as ground state bleach in 400-
4
50 nm range. To gather insight into the deactivation, global target analysis 60-61 was performed.
A kinetic model including three species was satisfactory. The species associated spectra (SAS)
and population kinetics of the three species is shown in Figure 5a, ii and iii, respectively. The first
species with a lifetime of ~200 fs was within the temporal resolution of our instrument that decayed
to develop the second component with singlet excited state features with a lifetime of 49 ps. The
third component which could be attributed to the decaying singlet to triplet state had a time
constant of about 2.30 ns. Due to lack of any TCBD entities in 1, no electron transfer could be
detected.
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In contrast to the spectral features of compound 1, the donor-acceptor conjugates 2-4
revealed the anticipated ultrafast charge transfer and charge separation processes. In the case of
2
, where only one NND-TCBD entity is present, occurrence of relatively simple photoinduced
Figure 5. Fs-TA spectra at the indicated delay times, (a-d, panel i), species associated spectra (a-
d, panel ii), and population kinetics (a-d, panel iii) of compounds 1-4 (a through d) in benzonitrile.
The samples were excited at 350 nm corresponding to locally excited state.
charge transfer could be envisioned. However, in the case of 3 and 4 featuring two and three
entities of NND-TCBD attached to TPA, a symmetry breaking charge transfer could be envisioned
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due to presence of multiple numbers of equally positioned acceptor entities. Presence of higher
number of acceptors could improve the charge transfer by the respective statistical factor or even
6
2
more by quantum coherence effects. The first panel in Figure 5b-d show transient spectra at the
indicated delay times for compounds 2-4. The ESA peak of the singlet excited state located in the
4
50-500 nm range revealed rapid decay with a new peaks in the 610-620 nm range and near-IR
reange. These spectra were subjected to target analysis that required three components for
satisfactory fit. These SAS are shown in Figure 5b-d, middle panel. In all these, the first spectrum
with characteristic features of the singlet excited state had a time constant of less than 1 ps, and as
predicted, the magnitude of these time constants further decreased with increase in the number of
NND-TCBD entities (statistical factor of quenching). The second SAS with time constants of
3
.48-5.64 ps has been attributed to the charge transfer state. Here, the depleted peak intensity in
the near-IR region has been tentatively assigned to stimulated emission of CT state. With the
decay of the second component, the third component was evolved that has been attributed to the
charge separated species as this spectrum resembled largely that derived for charge separation
product from the earlier discussed spectroelectrochemical studies (see Figure 4c). It may be
mentioned here that the SAS of third component is distinctly different from the triplet state SAS
shown in Figure 5a, panel ii. The time constants for the charge separated state were found to be
1
4.9, 32.68 and 75.1 ps, respectively, for compounds 2, 3, and 4. These results reveal persistence
of the charge separated state in compounds 3 and 4 compared to that in 2.
Intrigued with these findings, next, we changed the solvent to less polar DCB and nonpolar
toluene. In both of these solvents the spectral trends were almost the same (see Figures S21 and
S22 in SI). Further, target analysis was performed to evaluate the kinetic factors as listed in Table
2
. Such data confirmed persistence of charge separated states in both solvents. Changing the
excitation wavelength to 500 nm corresponding to the charge transfer also revealed excited state
charge separation (Figure 6 and Figures S23 and S24). In this case, the data could be satisfactorily
fitted to two components, one to the excited state charge transfer with time constants of few ps and
the second one for the charge separated state. It may be mentioned here that irrespective of the
excitation wavelengths (LE or CT), the SAS generated for charge transfer and charge separation
states revealed close resemblance.
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Table 2. Time constants evaluated from GloTarAn target analysis of fs-TA spectral data in
solvents of varying polarity and at different excitation wavelengths for the investigated central
triphenylamine derived, dimethylamine-tetracyanobutadiene conjugates.
Compound
Solvent
ex, nm
S1, ps
CT, ps
CS, ps
1
2
3
4
Toluene
350
842
2.36
1.56
0.54
--
4.17
--
17.02
39.32
93.20
10.01
25.37
2
3
4
500
350
--
--
--
3.59
7.29
16.8
14.92
24.10
56.30
1
2
3
4
DCB
111
1.99
1.14
0.35
--
--
3.87
8.12
13.6
16.40
37.59
87.70
2
3
4
500
350
--
--
--
2.49
5.78
8.76
13.93
20.45
60.41
1
2
3
4
PhCN
49
--
--
0.89
0.80
0.67
3.48
4.99
5.64
14.9
32.68
75.10
2
3
4
500
--
--
--
1.38
3.69
4.98
10.23
24.65
43.29
As pointed out earlier, in synthetic multimodular donor-acceptor systems, charge
stabilization is often achieved by following the mechanism of natural photosynthesis where
electron migration occurs across the system distantly separating the positive and negative ions thus
minimizing their electrostatic attraction.20-22 Electron/hole delocalization in multiple donor or
accepting bearing systems, and utilization of high-energy triplet sensitizers to promote electron
transfer from the long-lived triplet excited states are also some of the known mechanisms to extend
the lifetime of the charge separated species. The present multi-modular systems, 3 and 4, differ in
their design wherein the same acceptor unit, NND-TCBD is covalently linked to the central TPA.
Electron exchange has been witnessed upon first reduction of these compounds unlike that in 2
bearing a single NND-TCBD entity. It appears that such electron exchange is responsible for
extending the lifetime of the charge separated states in these novel donor-acceptor conjugates.
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Figure 6. Fs-TA spectra at the indicated delay times of compounds 2-4 in benzonitrile. The
samples were excited at 500 nm corresponding to charge transfer band. Right hand panel shows
the population kinetics. The dip at 500 nm is due to excitation laser.
Conclusions
In summary, we have developed exceptional molecular donor-acceptor systems consisting of C
3
symmetric central triphenylamine derived, dimethylamine-tetracyanobutadiene conjugates. In
these systems, the NND-TCBD promoted charge transfer extending the absorption covering the
visible region. Electrochemical studies revealed electron exchange in compounds 3 and 4 carrying
multiple numbers of NND-TCBD entities. Frontier LUMO energy levels and orbital coefficients
helped us in rationalizing such electron exchange. The spectrum of the charge transfer state was
possible to deduce from manipulation of spectroelectrochemical data. Finally, we have been able
to demonstrate the effect of electron exchange in prolonging the lifetime of charge separated states
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in compounds 3 and 4 from fs-TA spectral studies in solvents of varying polarity. To our
knowledge, this is the first report where such a charge stabilizing mechanism involving electron
exchange has been proposed and demonstrated experimentally. The present findings are very
important to further our understanding on the fundamentals of electron transfer in multi-modular
systems; strengthen our knowledge on the early events of natural photosynthesis, and seek novel
applications in optoelectronics. In this context, it may be pointed out here that in bacterial
photosynthesis, the primary electron donor is an electronically interacting bacteriochlorophyll
.63 The initial electron transfer species, [BChl]
.+
could slow down the charge
dimer, [BChl]
2
2
recombination via a similar electron exchange mechanism and could be a reason for natural choice
of a bacteriochlorophyll dimer instead of a monomer.
1
13
Electronic Supplemental Information. Experimental and synthetic details, H and C NMR and
MALDI-mass of synthesized compounds. Additional CVs, spectroelectrochemical and fs-TA
spectral data. Coordinates of optimized geometries.
Conflict of Interest. There are no conflicts to declare.
Acknowledgements. This research was supported by the US-National Science Foundation
(2000988 to FD), Council of Scientific and Industrial Research (Project No. CSIR
0
1(2934)/18/EMR-II), New Delhi and SERB (Project No. CRG/2018/000032) New Delhi, Govt.
of India. We gratefully acknowledge Sophisticated Instrumentation Centre (SIC), IIT Indore. IY
is thankful to CSIR, India for a research fellowship.
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GCR = Eox – Ered + G
S
(i)
-GCS = E00(-GCR
)
(ii)
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where ∆E00 corresponds to the singlet state energy of 1. The term ∆G
S
refers to electrostatic
energy calculated according to dielectric continuum model (see equation iii). The Eox and Ered
represent the oxidation potential and the first reduction potential, respectively.
2
G
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= e /4 
o
[ - 1/Rcc

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(iii)
The symbols 휀
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and 휀
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Table of content
Charge Stabilization via Electron Exchange: Excited Charge Separation in Symmetric, Central
Triphenylamine Derived, Dimethylaminophenyl-Tetracyanobutadiene Donor-Acceptor
Conjugates
I. S. Yadav, A. Z. Alsaleh, R. Misra and F. D’Souza*
*
Significance of electron exchange in stabilizing the charge-separated state is revealed in multi-
modular donor-acceptor conjugates.
22