Charge separation and singlet fission in covalently linked diketopyrrolopyrrole derivatives and triphenylamine triad in solution

Article abstract of DOI:10.1016/j.jphotochem.2020.113017

A covalently linked push-pull type triad containing two unsymmetrical electron donors, triphenylamine (TPA) and benzothiophene (BT), and diketopyrrolopyrrole (DPP), an electron acceptor, through thiophene spacer has been synthesized (BT-DPP-TPA(7)) and ultrafast relaxation dynamics of the triad has been explored in solution phase by various spectroscopic methods. Steady-state and time-resolved emission studies show the efficient fluorescence quenching of the DPP entity of as prepared BT-DPP-TPA(7) triad. The negative free energy values comprising the redox potentials and singlet state energy of BT-DPP-TPA(7) revealed the probability of electron transfer from the singlet ground state of TPA to the excited singlet state of DPP. Femtosecond transient absorption (fsTA) spectroscopic studies confirmed the formation of charge separation state by detecting triphenylamine radical cation as electron-transfer transients. The rate of charge separation, k_CS, is (10⁹?10⁸ s^?1) observed to be increasing from nonpolar to polar solvents and the rate of charge recombination, k_CR, was found to be slower (μs time scale) in polar solvents like DMF and chloroform, than that in hexane, non-polar solvent (ns time scale). This novel aspect could be due to asymmetrically designed push-pull type triad, a feature that was not evident in push-pull triad constructed using symmetric TPA as electron donors. Furthermore, fsTA studies also demonstrate, for the first time, that the DPP derivatives, TDPP(4) and BT-DPP-TPA(7), both undergo singlet fission (SF) event (S→TT) in 100?200 ps time scale in solution phase for the solution of concentration above ～100 μM. These results may pave the new avenue for device design comprising DPP derivatives.

Full text of DOI:10.1016/j.jphotochem.2020.113017

Journal Pre-proof
Charge Separation and Singlet Fission in Covalently Linked
Diketopyrrolopyrrole Derivatives and Triphenylamine Triad in Solution
Madhu Chakali, Haraprasad Mandal, Munisamy Venkatesan,
Bharath Dyaga, V. Jayathirtha Rao, Prakriti Ranjan Bangal
PII:
S1010-6030(20)30814-5
DOI:
https://doi.org/10.1016/j.jphotochem.2020.113017
JPC 113017
Reference:
To appear in:
Journal of Photochemistry & Photobiology, A: Chemistry
Received Date:
Revised Date:
Accepted Date:
3 July 2020
26 October 2020
29 October 2020
Please cite this article as: Chakali M, Mandal H, Venkatesan M, Dyaga B, Rao VJ, Bangal PR,
Charge Separation and Singlet Fission in Covalently Linked Diketopyrrolopyrrole Derivatives
and Triphenylamine Triad in Solution, Journal of Photochemistry and amp; Photobiology, A:
Chemistry (2020), doi: https://doi.org/10.1016/j.jphotochem.2020.113017
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Charge Separation and Singlet Fission in Covalently Linked
Diketopyrrolopyrrole Derivatives and Triphenylamine Triad in
Solution
a,b
a,b
a,b
§
Madhu Chakali , Haraprasad Mandal , Munisamy Venkatesan , Bharath Dyaga , V.
*§
a,b*
Jayathirtha Rao , Prakriti Ranjan Bangal
a
Department of Analytical and Structural Chemistry, CSIR-Indian Institute of Chemical
Technology, Uppal Road, Tarnaka, Hyderabad. 500007, India.
Email: prakriti@iict.res.in
bAcademy of Scientific and Innovative Research (AcSIR), Ghaziabad - 201002, India.
§
Fluoro-Agrochemical Division, CSIR-Indian Institute of Chemical Technology, Uppal Road.
Email: vaidya.opv@gmail.com
Graphical Abstract
1

Highlight
•
We have designed and synthesized a diketopyrrolopyrrole (DPP) based asymmetric push-pull
triad system (BT-DPP-TPA(7)) comprising diketopyrrolopyrrole, triphenylamine, benzothiophene
and thiophene.
•
Ultrafast relaxation dynamics of the central DPP derivative, TDPP(4) and BT-DPP-TPA(7) triad
system have been explored in solution phase by femtosecond transient absorption spectroscopy.
Upon photoexcitation to DPP moiety of BT-DPP-TPA(7) photoinduced reductive electron
transfer occurs from ground state TPA to excited singlet state of DPP leading to long-lived CS state.
Apart from this event, we demonstrate for the first time that TDPP(4) as well as BT-DPP-TPA(7)
•
•
take part in SF in the solution phase in 100-200 ps time scale in the solution of the concentration
on and above 100 µM.
•
These combined rare features in BT-DPP-TPA(7) may have implications for improving the
efficiency of photovoltaic devices using DPP derivatives.
Abstract
A covalently linked push-pull type triad containing two unsymmetrical electron donors,
triphenylamine (TPA) and benzothiophene (BT), and diketopyrrolopyrrole (DPP), an electron
acceptor, through thiophene spacer has been synthesized (BT-DPP-TPA(7)) and ultrafast
relaxation dynamics of the triad has been explored in solution phase by various
spectroscopic methods. Steady-state and time-resolved emission studies show the efficient
fluorescence quenching of the DPP entity of as prepared BT-DPP-TPA(7) triad. The negative
2

free energy values comprising the redox potentials and singlet state energy of BT-DPP-
TPA(7) revealed the probability of electron transfer from the singlet ground state of TPA to
the excited singlet state of DPP. Femtosecond transient absorption (fsTA) spectroscopic
studies confirmed the formation of charge separation state by detecting triphenylamine
radical cation as electron-transfer transients. The rate of charge separation, kCS, is (10 −108
9
−1
s ) observed to be increasing from nonpolar to polar solvents and the rate of charge
recombination, kCR, was found to be slower (s time scale) in polar solvents like DMF and
chloroform, than that in hexane, non-polar solvent (ns time scale). This novel aspect could
be due to asymmetrically designed push-pull type triad, a feature that was not evident in
push-pull triad constructed using symmetric TPA as electron donors. Furthermore, fsTA
studies also demonstrate, for the first time, that the DPP derivatives, TDPP(4) and BT-DPP-
TPA(7), both undergo singlet fission (SF) event (STT) in 100-200 ps time scale in solution
phase for the solution of concentration above ~100 M. These results may pave the new
avenue for device design comprising DPP derivatives.
Keywords: Diketopyrrolopyrrole (DPP), Charge Sseperation, Singlet Fission, Concentration,
Transient absorption
1. Introduction
In the last few decades, 3,6-diaryl-2,5-dihydro-1,4-diketopyrrolo[3,4-c] pyrrole (DPP)
derivatives have emerged as one of the important pigment in many different fields of
3

applications, such as inks, paints, plastic [1-4 ], and laser dyes [5-6], due to their excellent
thermal as well as photo-stabilities and glaring photophysical properties. However,
currently, DPP derivatives are attracting considerable research efforts in the field of organic
electronic materials due to its ambipolar nature and electron accepting as well as electron
donating abilities along with the aforementioned properties. In view of replacing
semiconductor solar cells, metal-free dye-sensitized solar cell [7-17] (DSSC) has emerged as
a promising alternative source to fulfill the ever increasing demand for green and clean
energy for a sustainable civilization. In the quest to have good sensitizer for DSSC in terms of
high solar energy conversion efficiency and low cost of production, the design and
development of a large array of π-conjugated organic materials with DPP as core unit and
testing of their device performances have rejuvenated the importance of DPP molecules in
DSSC research community [18-20]. Very recent DPP derivatives are accredited as singlet
fission (SF) chromospheres,21-23 where a singlet excited state energetically down-converts in
spin-allowed process yielding two triplet excitons on adjacent molecules (Scheme 1) of
proper orientation when energy of the singlet state is greater than or equal to two times
that of the triplet state, E(S
1
)≥ 2E(T ). Till date, triplet quantum yield up to 200% has been
1
achieved in single crystal, solid film, nano-aggregate, assembly of two or more
chromophores in solution by SF [24]. As a result, it is predicted that SF can lead a probable
way to cross-over 32% Shockley-Queisser limit of the overall efficiency of solar photovoltaics
[25-27]. Interestingly, DPP derivatives are also recognized as triplet fusion chromospheres
or triplet-triplet annihilator, a complimentary system of SF, where two or more low-energy
triplet states are up-converted to one high energy singlet state [28]. Likewise the SF, the TF
process is energetically allowed spin-conserved process so long as it satisfies E(S
1
)≥ 2E(T ).
1
However, TF eventually needs triplet sensitization by suitable sensitizer via the triplet-triplet
4

energy transfer process. Depending upon the requirement and device design, both SF and
TF are extensively exploited to the photovoltaic applications,[18-20, 29-31] among other
areas of applications [32-37]. Even though the quest for new SF chromospheres has arisen
as a focused research field, detailed understanding of SF mechanism, electronic couplings
between adjacent SF chromospheres with proper geometric orientations, and intermediate
excited states involved in the process are still lacking.
Scheme 1. Schematic energy diagram of singlet fission.
For a donor-acceptor (D-A) based efficient organic photovoltaic devices free charge
generation is created upon photoexcitation yielding very fast charge-separated (CS) states
and excitation decay to the triplet state is considered as an unwanted energy loss process.
On the contrary, a direct photoexcitation to triplet state or triplet sensitization is
advantageous for multiexciton harvesting in singlet fission photovoltaics and the formation
of CS state is considered as an obstructing factor for multiple triplet generation. When these
5

two extreme events are present in a D-A based system then it is expected that such a
system can provide important insights into the mechanism of free charge generation and
charge recombination (CR) in OPVs. Fortunately, D-A based system containing the DPP as
core unit is highly potent to those extreme events and yields fast CS state and slow CR along
with the efficient spin-conversion process, SF and TF [20]. In spite of immense literature
available on DPP where it has been considered as an important organic building block to
construct high performance materials for photovoltaic applications,[20-31] the
photophysical relaxation dynamics and the dynamics of charge separation and charge
recombination in DPP based systems are relatively spare [38-41]. In this connection, we
have designed and synthesized a covalently linked push-pull type triad containing two
unsymmetrical donors, triphenylamine (TPA) and benzothiophene (BT), and
diketopyrrolopyrrole (DPP) as an acceptor with thiophene as a spacer and explore the CS, CR
and SF dynamics of as prepared triad employing steady state and femtosecond transient
absorption spectroscopic measurements. Global and target analysis of TA data provides the
Spectro temporal information which can reveal the structure−property relationship of
widely used moieties in the design of efficient optoelectronic materials. This study may be
relevant to a wide range of fundamental research and applications pertaining to DPP
derivatives.
2. Experimental Section
2
.1. Materials and instruments
All the solvents and analytical grade reagents were obtained from commercial
sources, Sigma-Aldrich (USA), and used without further purification. UV−Vis absorption and
fluorescence emission spectra were recorded at room temperature on a Hitachi U-2910
(Japan) spectrophotometer and a Fluorolog-3 spectrofluorimeter of Horiba Jobin Yvon
6

(France), respectively. The absorption and fluorescence measurements were carried out in
different solvents with increasing solvent polarity from hexane to methanol. To ascertain
the reproducibility of data, absorption and emission experiments were repeated in three
different days with freshly prepared samples and every time data are found to be identical.
2
.2. Cyclic Voltammetry
The electrochemical experiments were performed using a PC-controlled CHI 620C
electrochemical analyzer (CH instruments) in 1 mM solution of degassed dry
−
1
dichloromethane at a scan rate of 100 mV s using 0.1 M, Tetrabutylammonium
perchlorate (TBAP) as the supporting electrolyte. The glassy carbon, Ag/AgCl, and platinum
wire were used as the working electrode, reference electrode, and counter electrode
respectively.
2
.3. Time-resolved Fluorescence Measurements
The fluorescence lifetime measurements were carried out using TCSPC method in
nanosecond time scale. The ~100 fs pulses with an 8 MHz repetition rate are obtained
Ti:sapphire oscillator (Spectra Physics, MaiTai, 100fs, 80MHz) output using the femtosecond
Pulse Selector (3980-5S, Spectra Physics). The sample was excited at 390 nm, which were
generated by frequency doubling of 780 nm Ti: sapphire output with a 0.5 mm BBO crystal.
The time distribution of fluorescence intensity data was recorded on an SPC-130 TCSPC
module (Becker and Hickl). The IRF (Instrument Response Function) value of this time
correlated single photon counting was about 200 ps.
2
.4. Transient Absorption/Pump−Probe
Transient absorption studies were performed using a pump−probe set up (ExciPro)
of CDP Systems Corporation (Moscow) and described elsewhere [42]. In brief, the output of
7

the optical parametric amplifier (TOPAS prime) was used as a pump source at the required
wavelength and fed into the spectrometer through a synchronized chopper at 1kHz
repetition rate. A lens (f = 200 mm) was used to adjust the pump diameter, while an iris and
neutral density filter combination were used to adjust the pump energy. A Berek’s variable
wave plate was placed in the pump beam, and polarization was fixed at magic angle with
respect to probe pulse. A part of the output of Ti:sapphire (f = 150 mm) was used to focus
both probe and reference beams to the rotating sample cell. Two lenses (f = 60 mm) made
probe and reference images at the entrance surfaces of two optical fibers, which are
connected to the entrance slit of the imaging spectrometer (CDP-2022i). This spectrometer
consists of UV−vis photodiode (Si linear photodiode) arrays and IR photodiode (GaAs linear
photodiode) array with Spectral response range 200−1000 and 900−1700 nm, respectively.
Quartz cells of 1 mm sample path length were used for all studies, and IRF was estimated to
be ≤150 fs. However, to minimize the solvent signal, pump pulse energy was kept below
3
μJ/s and probe pulse energy was from 0.1 to 0.5 μJ at the sample. For transient absorption
spectra, the group velocity dispersion compensation of the white light continuum (probe
beam) was done using the studied solvent’s two photon absorption data for a few
picoseconds delay.
2
.5. Global and Target Analysis of Transient Data
In order to estimate precise rate constants and species-related spectral signatures,
the transient data reported in this paper were analyzed using a combination of global and
target analysis [43,44]. With the aim of predicting the minimum number of components
involved in the evolution of the transient data, firstly, global analysis was performed via two
different approaches based on the principle of superposition of the least number of
independent exponential components, which provided a straightforward description of the
8

data at all measured wavelengths and all time points simultaneously. The number of
independent components fitted to all the data was determined by gradually increasing the
number of exponential components until the residuals were effectively zero. The simplest
description in global analysis used a parallel kinetic model, in which a number of
monoexponentially decaying independent components, each of which was represented by a
single rate constant (reciprocal of the lifetime) and amplitude at each recorded wavelength,
yielded decay-associated difference spectra. The decay-associated difference spectra
represented the growth and decay of the components with their corresponding decay
constants and lifetime values. A second sequential kinetic model, namely, an unbranched
unidirectional model, consisted of successive monoexponential decays with increasing time
constants and estimated the gross spectral evolution of the data to generate evolution
associated difference spectra. Finally, the data were fitted to a full kinetic model
(compartmental scheme) in target analysis by a combination of the parallel and sequential
kinetic models of global analysis, which included all possible branching routes and equilibria
between compartments to specify the microscopic rate constants that described the decay
of the compartments, as well as the transfer of excitation between the compartments. This
analysis estimated the real spectra of each compartment (excited species), which are
termed species-associated difference spectra (SADS). In target analysis of ET systems,
equality between the total rates of CS and CR is constrained to fit the proposed scheme
andindividual rates are free to be optimized to generate the spectra and lifetimes of the
respective states. The entire global and target analysis was performed with the R package
TIMP and its graphicaluser interface Glotaran (for details, see the literature [45–47].
2.6. Synthesis
9

The synthesis of 3-(5-(benzo[b]thiophen-2-yl) thiophen-2-yl)-6-(5-(4-(diphenylamino)
phenyl) thiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (BT-DPP-
TPA (7) is done following the synthetic route as shown in Scheme-2. First we have
synthesizes 4-(4,4,5,5- tetramethyl-1,3,2- dioxaborolan-2-yl)triphenylamine (1), 2-
(benzo[b]thiophen-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2), 3,6-Di(thiophen-2-
yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (3), 2,5-Bis(2-ethylhexyl)-3,6-di(thiophen-2-
yl)pyrrolo[3,4c]pyrrole-1,4(2H,5H)-dione(TDPP-2) (4), 3,6-bis(5-bromothien-2-yl)-2,5-bis(2-
ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (5) according to the literature method
describe elsewhere [48].
Scheme 2. Schematic diagram of synthetic route to prepare BT-DPP-TPA (7)
1
0

3-(5-(benzo[b]thiophen-2-yl)thiophen-2-yl)-6-(5-bromothiophen-2-yl)-
2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (6). The compound
(
6) was synthesized following Suzuki coupling reaction. In a 250 ml round bottom flask, the
mixture of 3,6-bis(5-bromothiophen-2-yl)-2,5-bis(2-ethylhexyl) pyrrolo[3,4-c] pyrrole-
1
,4(2H,5H)-dione (5), (2g, 2.93 mmol) and 2-(benzo[b]thiophen-2-yl)-4,4,5,5-tetramethyl-
,3,2-dioxaborolane (0.76g, 2.93 mmol), Pd(PPh )4 (67 mg), Na CO (4.95 g, 460 mmol) in
for 30 min. Then H O + EtOH (44.5mL 2:1 ratio
degassed for 30 min.) added to the reaction mixture. The reaction mixture was again
degassed with N for 30 min and heated to 90 °C for 14 hours. Reaction was monitored
using TLC, after completion of reaction, the solution was washed with CHCl and H O, the
organic extract with CHCl , dried over Na SO , the solvent was removed under reduced
1
3
2
3
toluene (70 ml), was degassed with N
2
2
2
3
2
3
2
4
pressure. The residue was purified by column chromatography using hexane: EtOAc (98: 2)
1
as the eluent. H NMR (300 MHz, CDCl
3
): δ 8.95 (s, 1H), 8.65 (s, 1H), 7.90-7.71 (m, 2H), 7.56
(s, 1H), 7.48-7.32 (m, 3H), 7.23 (s, 1H), 4.12-3.93 (m, 4H), 1.99-1.83 (m, 2H), 1.46-1.25 (m
1
8H), 0.99-0.84 (m, 12H) (Fig. S14).
3-(5-(benzo[b]thiophen-2-yl)thiophen-2-yl)-6-(5-(4-(diphenylamino)
phenyl) thiophen-2-yl)-2,5-bis(2-ethylhexyl) pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-
dione, {BT-DPP-TPA (7)}: The final compound BT-DPP-TPA (7) was again prepare from (6)
using Suzuki coupling reaction as described. 3-(5-(benzo[b]thiophen-2-yl)thiophen-2-yl)-6-
(
5-bromothiophen-2-yl)-2,5-bis(2-ethylhexyl) pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione, (6),
0.27 mmol) and equivalent borolane derivative (0.32 mmol) and Pd(PPh )4 (5mol %) were
gas for 15 min. To this 2M
gas for 30 min and refluxed (120 °C) for 14 hours.
(
3
dissolved in toluene (20 mL). The mixture was purged with N
CO was added and purged with N
2
K
2
3
2
1
1

Reaction was monitored using TLC, after completion of reaction, the reaction mixture was
washed with CHCl and H O, the organic extract with CHCl , dried over Na SO , and the
solvent was removed under rotary vacuum. The residue was washed with MeOH several
times and recrystallization in toluene. 1H NMR (300 MHz, CDCl ): δ 9.04 (d, J = 4.27 Hz, 1H),
.90 (d, J = 4.12 Hz, 1H), 7.83-7.80 (m, 1H), 7.79-7.76 (m, 1H), 7.55-7.53 (m, 2H), 7.53 (s, 1H),
.44-7.42 (d, J = 4.27 Hz, (Fig. S15).
3
2
3
2
4
3
8
7
+
+
MALDI-MS (positive mode, m/z): found (M H ) 900.69, calc. for C56
H
57
N
3
O
2
S , is 899.3613.
3
(
Fig. S16).
TDPP(4)
BT-DPP-TPA (7)
1.0
0.8
0.6
0.4
0.2
0.0
300
400
500
600
700
Wavelength(nm)
Fig. 1. Normalized Uv-Vis absorption spectra of TDPP (4) and BT-DPP-TPA (7) in Chloroform.
3.RESULTS
3
.1 Steady-State Absorption and Emission
The ground state absorption spectra of reference compound thiophene substituted
TDPP (4) and donor-acceptor-donor (D-A-D) type triad BT-DPP-TPA (7) are recorded in
1
2

different solvents with varying polarities (Fig. S1) and the peak positions are summarized in
Table 1. Fig. 1 shows typical normalized ground state absorption spectra of TDPP (4) and BT-
DPP-TPA (7) triad in chloroform solution (10µM) respectively. As shown in Fig. 1, the
characteristic absorption spectra of TDPP (4) consist of three distinct bands: a strong high
energy band at ~290 nm (S ), which arises due to n* transitions and the relatively weak
3
second absorption maxima appears in the range of 310 to 405 nm region with two
vibrational peaks at 341, 357 nm and one shoulder peak at ~383 nm which is assigned to S
2
state and it is attributed to ππ* transition of core DPP [49] moiety. Finally, the third broad
and intense low energy band emerges around 450-600 nm region which shows two distinct
peaks at ~513 and ~547 nm. The origin of this characteristic long wavelength absorption
band of DPP derivatives is consistent with the ππ* transitions and it is due to
intramolecular charge transfer (ICT) from thiophene moiety to DPP lactam unit [49,50]. This
low energy band is attributed to S (ICT) state. Ongoing from nonpolar to polar solvent a
1
slight blue shift for all the absorption peaks of TDPP(4) is observed along with the reduction
of structure of the spectra corresponds to low energy band. The absorption spectra of BT-
DPP-TPA(7) triad show a similar type of band patterning as observed in TDPP(4). Relatively
weak higher energy bands (S
3
and S
2
) of BT-DPP-TPA(7) are found to be peaking at ~309 and
3
6
1
6
50 nm respectively and intense low energy broad band is observed to be in between 500-
00 nm region with large bathochromic shift (~80 nm) with respect to that of TDPP(4) (Fig.
). The peak position of two characteristic vibronic structures are recorded to be at 592 and
30 nm respectively. The vibrational fine structures for both high energy and low energy
bands are reduced significantly in BT-DPP-TPA(7) triad with respect to TDPP(4) in all the
solvents studied. These vibrational fine structures reduce further on increasing the solvent
polarity along with 3-4 nm of blue shift of all absorption peaks. These results can be
1
3

associated with the charge transfer behavior between TPA/BT donors and DPP acceptors,
thus resulting in delocalization of charge in the ground state over the backbone between
donor and acceptor moieties. Furthermore, in higher energy side i.e. below 325 nm region,
a modulation of absorption spectra is observed which is due to the overlapping of TPA/BT
absorption to DPP absorption and this modulation of absorption spectra again confirms the
undergoing electronic interaction between TPA/BT and DPP moieties. Note, concentration
dependent absorption studies reveal that neither TDPP (4) nor BT-DPP-TPA(7) shows any
noticeable change in the shape of absorption spectra on increasing concentration up to 50-
70 µM. However, upon increasing further concentration (from 100 µM to 1 mM) a slight
change in the shape of absorption spectra of both the compounds is observed. The intensity
ratio of two prominent vibronic peaks at lower energy absorption band of respective
molecules starts changing along with increase in full width at half maxima(FWHM) of the
absorption band. Although, due to very high molar absorption coefficient, absorption
spectra get saturated below 1 mM concentration when studied in 10 mm cuvette( for detail
see Fig S2 and Fig.S3.). However, these results can be combined to the fact that in higher
concentration, approximately above 60-70 µM of concentration, both the molecules could
start forming aggregation in the ground state.
1
4

1
0
0
0
0
.0
.8
.6
.4
.2
5
5
5
5
5
4
4
2
M
0 M
3.0x10
2.5x10
2.0x10
1.5x10
1.0x10
5.0x10
30 M
50 M
7
1
200 mM
0.500 mM
0 M
00 M
0.0
5
50
600
650
700
Wavelength (nm)
0.0
550
600
650
700
750
Wavelength(nm)
Fig.2. Concentration dependent fluorescence emission spectra of TDPP(4) in Chloroform
solution λEX= 450 nm. Inset shows normalized fluorescence emission spectra as a function of
concentration.
-
5
The dilute solutions (~10 µM) of TDPP(4) exhibits very strong fluorescence
emission in different solvents with a characteristics peak at around 550 nm region
and a shoulder band at around 600 nm (Fig. S4) when excited at any wavelength of
the absorption band. All these fluorescence emission peaks show a systematic
redshift on increasing solvent polarities and they are summarizing in table 1. The
fluorescence quantum yield (fl) of TDPP(4) is estimated to be ~70% in the non-polar
solvents and it slightly reduces in polar solvents confirming intramolecular interaction
between thiophene and DPP moiety [51,52]. Fig. 2 shows the concentration
dependent fluorescence spectra TDPP(4) in chloroform solution when excitation
wavelength is selected to a low absorption reagion of 450 nm to reduce the inner
1
5

filter effect for concentrated. As shown in Fig.2, the intensity of fluorescence
emission increases initially on increasing concentration but after 60-70 µM of
concentration TDPP (4) the fluorescence intensity starts decreasing. More
significantly, the higher energy fluorescence emission peak of TDPP (4) shows gradual
redshift on increasing concentration and it becomes more prominent after 60-70 µM
concentration. These redshift of emission peak and prominent decrease of
fluorescence intensity after 60-70 µM can be attributed to the effect of the formation
of aggregation in higher concentration. It is important to note that the rate of the
intrinsic intersystem crossing of TDPP is very slow and triplet state yield is much less
than 1% [53,54].
1
6

Hex
DMF
MeOH
1
Chl
ACN
30k
20k
10k
.0
0.5
0.0
6
00
650
700
750
Wavelength (nm)
600
630
660
690
720
750
780
Wavelength (nm)
-
5
Fig. 3. Steady state fluorescence emission spectra of BT-DPP-TPA (7) (~10 M) in different solvents.
Inset shows normalized emission spectra; λEX=580 nm.
-
5
The fluorescence emission spectra of equimolar solutions (~10 M) of BT-DPP-
TPA(7) are recorded in different solvents of increasing polarity (Fig. 3 ) upon 580 nm
excitation and the observed fluorescence emission peak positions and fluorescence
quantum yields are listed in table 1. Likewise ground state absorption spectra, the
fluorescence emission maxima of BT-DPP-TPA(7) show over 100 nm redshift with
respect to that of TDPP(4). The fluorescence emission maxima of BT-DPP-TPA(7)
show further bathochromic shift on going from nonpolar to polar solvents and the
structured emission spectra consisting of two peaks reduce to single band emission
spectra in the polar solvent. More importantly, the fluorescence emission of BT-DPP-
TPA(7) is quenched enormously and the magnitude of quenching again enhanced in
1
7

polar solvents (Table 1). Note, the fluorescence quenching of BT-DPP-TPA(7) in
methanol, a protic solvent, is found to be very high with respect to its polarity. The
fluorescence quantum yield of BT-DPP-TPA(7) in ACN is calculated to be 0.03 (
whereas in methanol, relatively less polar solvent than ACN, it is observed to be
.006( fl). This discrepancy in solvent polarity dependent fluorescence quantum yield

fl
)
0

ongoing from aprotic solvent to protic solvent can be rationalized by probable
intermolecular hydrogen bonding between methanol and triad system. However, as
shown in Fig. 3, in hexane, a nonpolar solvent, BT-DPP-TPA(7) exhibits two well-
resolved fluorescence emission peaks at 650 and 700 nm respectively which are
consistent with emission profile of DPP derivatives although fluorescence yield
quenched extensively. In polar solvents, along with large fluorescence quenching the
fluorescence emission profile of BT-DPP-TPA(7) loses its fine structures which
indicates the non-planer structure of BT-DPP-TPA(7) in its excited states. The electron
donor units, TPA/BT, play a vital role in distorting the molecular backbone from
planarity and it can be due to charge separation upon excitation to DPP unit. Hence,
this quenching of fluorescence in BT-DPP-TPA(7) is attributed to photoinduced
electron transfer from TPA/BT to excited DPP moiety. Furthermore, an energy
transfer from the excited DPP moiety to BT or TPA is ruled out owing no overlap
between the emission spectra of DPP and the absorption spectra of neither BT nor
TPA.
Likewise TDPP(4), we measure fluorescence emission as a function of BT-DPP-
TPA(7) concentration and observe similar kinds of results. As shown in Fig.4, the
fluorescence emission intensity clearly increases on increasing BT-DPP-TPA(7)
1
8

concentration up to 70 µM, and thereafter it decreases sharply on increasing
concentration. Furthermore, the fluorescence emission peak shows prominent
redshift after 70 µM of concentration. These results confirm that likewise TDPP (4),
BT-TDPP-TPA(7) forms aggregation presumably above 70 µM of concentration.
1
.0
.8
1
0 M
5
4
4
4
4
1.0x10
8.0x10
6.0x10
4.0x10
2.0x10
20 M
0
3
5
0 M
0 M
0.6
70 M
0
.4
.2
1
1
2
00 M
50 M
00 M
0
250 M
5
1
0.0
6
50
700
750
800
00 M
mM
Wavelength (nm)
0.0
650
675
700
725
750
775
800
Wavelength(nm)
Fig.4. Concentration dependent fluorescence emission spectra of BT-DPP-TPA(7) in
Chloroform solution, λEX= 530 nm. Inset shows normalized fluorescence emission spectra as
a function of concentration.
-
5
The singlet state lifetimes of DPP moiety in dilute solution (10 M) TDPP(4)
and BT-DPP-TPA(7) were measured in different solvents (Fig. 5) using time correlated
single photon counting system(TCSPC) and they are listed in Table 1. The lowest
singlet state (S
1
(ICT) state) lifetime of TDPP(4) is found to be ~6.5 ns and it is almost
independent of the solvent polarity. However, as shown in Fig. 5, the S
1
(ICT) state
lifetime of DPP moiety in BT-DPP-TPA(7) is quenched significantly and the extent of
quenching of lifetime increases further on going from nonpolar to polar solvents.
These results again suggest that fluorescence quenching of BT-DPP-TPA(7) is a
1
9

dynamic process which occurs in the excited state of BT-DPP-TPA(7) and it can be
associated with the photoinduced electron transfer from ground state TPA/BT to
excited state DPP moiety. Assuming the quenching of the fluorescence decay is due
to electron transfer, the rate constant for charge separation is determined according
to eq 1, and solvent dependent rate constants (kCS) are listed in Table 2.
k
CS= (1 ⁄ τ
f
)triad-(1 ⁄ τ
)
f ref
(1)
Fig. 5. Fluorescence emission decay of TDPP(4) and BT-DPP-TPA(7) in different
solvents (concentration~10µM); λEX=400 nm and λEM=565 nm and 665 nm for
TDPP(4) and BT-DPP-TPA (7) respectively.
In accordance with the steady-state fluorescence quenching, the quantum
yield of charge separated state, ΦCS, is calculated following eq 2 and it was found to
be >59% in any of the studied solvents. The values of kCS, are found to be higher in
more polar solvents in comparison to those in the nonpolar solvents (Table 2).
2
0

Φ
CS=[(1 ⁄ τ
f
)
triad-(1⁄τ
f
)ref] ⁄ (1 ⁄ τ
f
)
triad
(2)
In view to observe the effect of concentration on the singlet state lifetime, we
measure the fluorescence emission decay of TDPP (4) and BT-DPP-TPA (7) as a function of
concentration in the range of from 4 µM to 1 mM of respective molecules by employing
TCSPC with IRF 200 ps. Fig. 6 displays the typical change in fluorescence decays with time at
around second vibronic peak of the emission band at 607 (see Fig.S5. & Fig.S6. for
fluorescence decay at other wavelengths) and 670 nm for TDPP(4) and BT-DPP-TPA (7)
respectively in varying concentration of 10 to 1000 µM. Both TDPP(4) and BT-DPP-TPA (7)
exhibits predominantly monoexponential fluorescence decay. As shown in Fig.6A, singlet
lifetime of TDPP(4) remains constant (~6.5 ns) till 50-60 µM of concentration and thereafter
it starts increasing on increasing concentration. The singlet lifetime of TDPP(4) enhances to
about 9.0 ns at 1 mM concentration.This increase of singlet lifetime can be attributed to
static excimer formation of TDPP(4) in higher concentration regime. For BT-DPP-TPA (7) the
change of singlet lifetime as a function of concentration is relatively smaller than that of
TDPP(4) (Fig.6B). The singlet lifetime changes from 2.3 ns to 3.5 ns ongoing from 10 µM to
5
00 µM. These results reveal that due geometric constrains excimer formation is relatively
weak in the triad system.
2
1

Fig.6. Fluorescence decays of TDPP(4) (A) and BT-DPP-TPA(7) (B) in different concentrations
in chloroform solvent, λEx= 400 nm.
3
.2. Electrochemical Properties
The electrochemical properties of TDPP (4), TPA and BT-DPP-TPA(7) were
investigated in DCM solvent using cyclic voltammetry (CV) to estimate the oxidation and
reduction potentials of the triad and therefore to explore the thermodynamic feasibility of
photoinduced electron transfer from TPA to singlet excited state of DPP in the triad. Fig. 7
represents the CV plots for TDPP(4), TPA, and BT-DPP-TPA(7). As shown in Fig. 7, TDPP(4)
exhibits two quasi reversible oxidation processes and one reversible reduction process. The
2
2

first oxidation potential of TDPP(4) is observed to 0.91V and the reduction potential is of -
.3V and TPA shows an oxidation potential 0.73 V. Upon attachment of TPA in BT-DPP-
1
TPA(7), the oxidation process in triad becomes reversible in nature and shifts towards a
negative potential (Fig. 7) whereas no noticeable change is observed in reduction process.
This result indicates the direct interaction of TPA on the TDPP(4) derivatives and the
influence of TPA to stabilize the radical cation and or radical anion during a redox reaction.
The first oxidation potential of BT-DPP-TPA(7) is found to be 0.6 V. Based on observed Eox
and Ered potentials of the triad, thermodynamic free energy, the driving force of electron
transfer from TPA moiety to exited DPP moiety for charge-separation (-GCS) as well as
charge-recombination (-GCR) are estimated in all solvents using the eqs [55] as follow
∆
퐺퐶푆 = 푒(퐸푂푋 −퐸푟ꢀ푑)−∆퐸푆 +∆퐺푠
(3)
(4)
0,0
∆퐺퐶푅 = −푒(퐸푂푋 −퐸푟ꢀ푑)−∆퐺푠
ꢀ2
1
1
1
1
ꢀ2
1
1
1
∆퐺푠 = 4
ꢂ
+
−푅 ꢆ휖
−
ꢂ
+
ꢆ휖
(5)
휋휖ꢁ ꢃ푅ꢄ
ꢃ푅ꢅ
4휋휖ꢁ ꢃ푅ꢄ
ꢃ푅ꢅ
푐푐
ꢇ
ꢈ
TPA
TDPP
BT-DPP-TPA
-
1.8
-1.2
-0.6
0.0
0.6
1.2
1.8
Potential (V)
Fig. 7. Cyclic voltammogram of TDPP(4), BT-DPP-TPA(7) (1mM) in a DCM solution containing
electrolyte 0.1 M, Tetrabutylammonium perchlorate (TBAP).
2
3

Table 1. Spectroscopic parameters of TDPP(4) and BT-DPP-TPA(7) ( ~10 µM) in different solvents
Absorption
Emission
TDPP(4)
BT-DPP-TPA(7)
TDPP(4)
BT-DPP-TPA(7)
4
maxx104

max

max x10

(nm)
max


(nm)
max
 (ns)

(nm)
max

(ns)

fl
-
1
-1
-1
-1
(nm) M cm
M cm
Hexane
2)
340
356
2.0 ± 0.5
1.70 ± 0.5
4.2 ± 1.0
5.2 ±1.0
1.9 ±0.5
1.7 ±0.5
3.6 ± 0.6
4.1 ±1.0
3.6 ±0.6
3.2 ± 0.6
7.3 ± 1.0
8.0 ± 1.0
5.3 ± 1.0
4.8 ± 1.0
10.0 ± 1.0
10.2 ±1.0
4.2 ± 1.0
3.8 ± 1.0
8.5 ± 1.0
8.6 ±1.0
346 ±2
581 ±2
624 ± 2
-----
1.9 ±0.5
3.5± 0.5
3.8±0.5
---
560 ±2
603 ±2
(
6.5± 0.3
6.5 ± 0.3
6.5± 0.3
650 ± 2
702± 2
2.6 ±0.3 0.1 ±
.02
0
5
5
08
47
Chl
4.81)
341
357
349 ±2
592 ±2
630 ±
2.2 ±0.5
3.7 ± 1.0
4.0 ± 1.0
------
(
565 ± 2
606 ± 2
662 ± 2
670 ± 2
673 ± 2
675 ± 2
2.3 ±0.3 0.08 ±
.01
0
5
5
12
48
------
DMF
36.7)
341
354
346 ± 2
584 ± 2
624 ± 2
----
2.3 ± 0.5
4.0 ± 1.0
4.3 ± 1.0
-----
(
560 ± 2
600 ± 2
1.8 ± 0.3 0.06 ±
.01
0
5
5
10
44
ACN
37.5)
341
353
345 ± 2
585 ± 2
620 ± 2
------
2.2 ± 0.6
3.8 ± 0.6
4.0 ± 1.0
-----
560 ± 2
600 ± 2
(
---
---
0.03 ±
.01
0
5
5
08
41
MeOH
32.7)
342
355
346 ±22 2.3 ± 0.6
560 ± 2
600 ± 2
(
586 ± 2
616 ± 2
-----
3.9 ± 0.6
4.0 ± 0.6
----
0.006 ±
.002
0
5
5
08
36
where ∆퐸 0푆 is the onset cross over point of absorption and fluorescence emission of
,0
TDPP(4) moiety in the triad and R
+
, R-, and RCC are radii of TPA and DPP and centre-to-centre
0
distance between donor and acceptor respectively, and e,  , are the electronic charge,
vacuum permittivity, S and E dielectric constant of solvent used for spectroscopic
measurement and electrochemical measurements respectively. Accounting the value of R ,
+
-
R and Rcc obtained from optimized geometry employing DFT calculation (see ESI for more,
Fig. S7.), which are of the order~5Å and ~7Å respectively, the GCS and GCR are calculated
and they are listed in Table 2. The GCS values are found to be negative in all solvents
whereas it is near to zero in hexane. These results suggest that the formation of
photoinduced charge-separated (CS) state upon excitation to DPP moiety in the triad is
exergonic in nature indicating CS reaction is an energetically downhill and spontaneous
2
4

process in reductive pathways. Note, BT moiety of BT-DPP-TPA(7) is also considered to be a
donor with very weak electron donating ability. The oxidation potential of BT derivatives is
expected to be very high (Eox=1.8 V vs SCE) [56] and no oxidation peak was appeared in
voltammogram of BT-DPP-TPA(7) in the observation window of 2 V. Hence, the possibility of
photoinduced electron transfer process from BT moiety to excited DPP moiety in BT-DPP-
TPA(7) is ruled out.
3
.3. Femtosecond Transient Absorption Spectra
To understand the thorough insight of relaxation dynamics of BT-DPP-TPA(7) and
probable photoinduced reaction dynamics we extend our quest by carrying out femtosecond
transient absorption (fsTA) studies for TDPP(4) and BT-DPP-TPA(7) in nonpolar and polar
solvents. First, we carried out fsTA studies of TDPP(4) upon S excitation at 340 nm in
2
different solvents probing in the range from 520 to 765 nm (the extreme limit of our
spectrometer) and explored the spectrotemporal information of relaxation mechanism.
Then we performed exactly similar types of fsTA studies for BT-DPP-TPA(7) and reveal the
role of TPA in BT-DPP-TPA(7) as an electron donor. Fig. (8) illustrates a typical profile diagram
of fsTA studies of TDPP(4) in chloroform which consists of elite OD heat map, TA spectra at
particular delay times, and time traces at different wavelengths. TA spectra are well
positioned corresponding to the ground state bleaching (GSB), stimulated emission (SE), and
excited state absorption (ESA). Immediate after excitation, a negative signal pertaining to
GSB, SE (blue color in Fig. 8a
these strong negative signals, a spontaneous rise of very broad positive signal (ESA) (red
color, Fig. 8a ) above 650 nm appears with no specific peak in the studied spectral wndow.
1
) appeared over the range of 520−650 nm region. In addition to
1
This positive signal observed at the very initial stage is assigned to be S
However, within a few ps of delay
n
S transition.
2
2
5

(
A)
Time (ps)
75
1.0
1
0
0
0
.0
.8
.6
.4
IRF
SADS
, (S )
1 2
0
2
4
10
500
5000
1
00
SADS , (hot S )
2
1
560 nm
760 nm
/
50
560 nm
0.8
0.6
(C)
SADS , (S (ICT))
3 1
760 nm
(
a )
(a4)
3
0.2
.0
SADS , (S (ICT))
4 1
0
0
0
1 2 3 10
250 5000
Time (ps)
SADS (T )
5 1
-
50
mOD
(
a )
2
1
00
0.4
50
(
a )
1
0
-30
69
5
ns
-
15 ps
0.2
1.5 ps
0.3 ps
0 ps
0.0
-1 0
2
5
10
75
1000
0
2
4
10
75
500
5000
-50
0
50 100
Time (ps)
Time (ps)
mOD
1
S2
1
=262fs
0.21
SADS , (S )
1
2
SADS , ( hot S )
(
B)
2
1
0
.14
.10
(D)
k12
/
SADS , (S (ICT))
3
1
0
SADS (S (ICT))
2=12 ps
4
1
hot S1
2
SADS , (T )
5 1
k23
0.05
0.00
ꢊ
(
ICT)  =90 ps
3
1
3
4
k34
(ICT)
1
4
6.2 ns k25
-0.05
k45
k40
5
T1
k  1.2 µs
550
600
650
700
750
5
0
5
S
Wavelength(nm)
0
Fig. 8. Pictorial summary of fsTA study of TDPP(4) upon 340 nm excitation in Chloroform solution
0.1 mM). (A) Profile picture of fsTA: ∆OD heat map, blue colour indicates negative signals (GSB/SE),
red indicates positive(ESA) and greenish blue to green indicates zero level (a ), TA spectra at selected
time delays (a ), time traces at two different wavelengths (a ) and normalised time traces of GSB, SE
and ESA at 560 and 760 nm respectively (a ). Time trace at 560 nm is inverted to positive values for
easy comparison with positive signal at 760 nm. (B) Five levels (states) kinetic target mode used for
(
1
2
3
4
-
1
global fitting. Rate of depopulation of S
2
1
, k12= (262fs) , rate of IC; rate of depopulation of hot S
1
-
-1

consists of rate of IC, k23= 0.90x(12 ps) ; rate of SF k25= 0.10x(12ps) ; rate of IC of ꢉ (ICT), k34=(90
ps) ; rate of depopulation of S (ICT), k40=0.95x(6.2 ns) ; rate of ISC, k45=0.05x(6.2 ns) . (C) & (D)
1
-1
-
1
-1
1
represent the spectrotemporal picture resulted from target based global analysis. Normalized
population profiles of corresponding to five states (C), SADSs spectra of corresponding to five states
(D). Time axis is linear till 5ps and thereafter it is logarithmic scale.
time, this positive signal becomes narrow and gets blue shifted to have a peak position
apparently at 760 nm. Furthermore, the intensity of this positive signal increases with delay
time for a few ps. This later time positive signal is assigned to S
n
S transition of TDPP(4)
1
2
6

[
40,41]. The spectral shape and the time evolution are shown in Fig. 8a
respectively. Fig. 8a shows the normalized time traces of GSB/SE signals and ESA signals at
60 and 760 nm respectively. For the easy comparison of the time evolution of GSB/SE signal
to that of ESA signal, GSB/SE signal is inverted. As shown in Fig. 8a , a distinct recovery of
GSB/SE signal is closely associated with the rise of ESA at 760 nm in the very early time of
spectral evolution. These results are primarily linked to the deactivation of S state to hot S
2
and Fig 8a
3
4
5
4
2
1
state. Followed by this, nearly similar kinds of rise of GSB/SE and ESA signals are observed in
few ps time domain at 560 and 760 nm respectively. This can be rationalized to subsequent
thermal and structural relaxation of hot S
1
state to S state. Finally, to our surprise, a
1
relatively slow rise both in GSB/SE signal at 560 and ESA signal at 760 nm are observed in
few tens of ps time scale followed by recovery dynamics of GSB/SE which is similar to the
decay dynamics of ESA signals at 760 nm (Fig. 8a ). Therefore, five exponential functions
4
with 200-300fs, 10-50 ps, 50-200 ps, 5000-6000 ps and a very slow component with large
time constants are essential to fit these time traces satisfactorily. Since the molecule was
excited at S
2
state, the first component could associate with the lifetime of S
2
state and the
second component can be assigned as the lifetime of hot S
1
state. In accordance with the
literature [22,57,58] the third component is assigned to the unrelaxed intramolecular charge

transfer state S (ICT)which structurally relaxes to S
1
(ICT) by virtue of planarization of the
1
thiophene donor to the DPP core acceptor [58]. The fifth long-lived component is attributed
to the lifetime of the triplet state. It is important to note here that the observed increase of
the GSB signal in 50-200 ps time domain is associated with the fact of depletion of the
ground state population and it can be correlated with the increase of triplet state population
due to singlet fission, SF (STT), in the hot S
1
state. In the same way, the increase of ESA
T absorption resulting from
signal at 50-200 ps time scale above 750 nm corresponds to T
n
1
2
7

SF. Note, the strong signal pertaining to T
be in the region of 750 to 1000 nm [59]. However, the increase of GSB signal with delay time
accompanied by an increase of ESA (T T ) signal above 750 nm is eventually assigned as
the signature of singlet fission. Note, Wasielewski and co-workers reported singlet fission
phenomena in DPP derivatives in solid-state exciting at 515 nm, a blue end of S (ICT)
absorption of TDPP derivatives [22,58]. However, they did not observe SF in DPP derivatives
in the solution phase upon 550 nm excitation, at the peak of the S absorption band. Indeed,
initially, we also failed to observe this increase of GSB on increase in delay time and increase
of ESA signal above 750 nm when TDPP (4) ([TDPP]<100 µM) was excited in the vicinity of S
n
T
1
transitions of TDPP is reported elsewhere to
n
1
1
1
1
absorption maxima at around 500nm. It is important to mention here that we perform fsTA
studies with a quartz cell of 1 mm optical path length and adjust the concentration of the
solution in such a way that optical density at the wavelength of excitation becomes less than
0
.5. Further to be noted, the molar extinction coefficient of the S
is more than twice time higher than that of the S band at 340 nm. Hence, fsTA studies
exciting at S band (340 nm) are performed in higher concentrations than fsTA studies
1
band of TDPP(4) at 500 nm
2
2
performed upon excitation at 500 nm. Therefore, the minimum concentration of TDPP(4)
employed for fsTA study upon 340 nm excitation is calculated to be ~0.10 mM, whereas the
minimum concentration employed for fsTA study upon 500 nm is <0.10 mM and we
observed neither the increase in GSB signal nor the rise of ESA signal on the delay time in 50-
200 time window. This result clearly indicates that concentration plays a role in observing
the enhancement of GSB on the delay time. Furthermore, concentration dependent fsTA
studies (Fig. S8a) confirm that the traceable increase of GSB signal in 50-200 ps (delay) time
window started appearing on 0.1 mM concentration of TDPP(4) and above 0.10 mM of
concentration it becomes more prominent (Fig. S8b). These results clearly indicate that SF
2
8

(
STT) occurs in a higher concentration regime when aggregation occurs in the ground
state.
Concentration dependent studies of steady state absorption, fluorescence emission
and time resolved fluorescence emission showed the propensity of aggregate formation in
the ground state of TDPP(4) in higher concentration (preferably above 60-70 µM). Hence
photo-excitation of TDPP(4) solution above 100 µM (0.1 mM) concentration could lead to
form static excimer like an intermediate between one photoexcited and one ground state
1
molecule which may produce correlated multiexciton (T
1
1
T ) that leads formation of the
uncorrelated triplet excitons. In addition to this, in higher concentration possibility of
formation of excimer like intermediate state (dynamic excimer) via diffusive encounters of
ground state molecules to the photo-excited molecules generating two triplets excitations
can not be completely ruled out. For both the instances (static and dynamic excimers), the
intermediate disappears when two triplets separate into two TDDP(4) molecules which in
turn increases the GSB on the delay time. Hence, we attribute this increase of GSB signal at
50-200 ps time window to SF and it occurs only in a relatively higher concentration regime of
TDDP(4). However, a more planned and systematic studies are essential to unveil the exact
mechanism of SF formation of TDPP(4) and its derivatives in solution phase and we plan it
soon.
Table 2. Free energy for charge separation, charge recombination, rate constants for charge
separation (kCS), rate constants charge recombination (kCR), and quantum efficiency of charge
separation in different solvents
2
9