Ultrafast Charge-Separation in Triphenylamine-BODIPY-Derived Triads Carrying Centrally Positioned, Highly Electron-Deficient, Dicyanoquinodimethane or Tetracyanobutadiene Electron-Acceptors

Article abstract of DOI:10.1002/chem.201701604

A series of new triphenylamine (TPA)-substituted BODIPYs 1–3 have been designed and synthesized through the Pd-catalysed Sonogashira cross-coupling and [2+2] cycloaddition-retroelectrocyclization reactions in good yields. This procedure yielded highly ele

Full text of DOI:10.1002/chem.201701604

A Journal of
Accepted Article
Title: Ultrafast charge separation in triphenylamine-BODIPY derived
triads carrying centrally positioned, highly electron deficient,
dicyanoquinodimethane or tetracyanobutadiene acceptors
Authors: Francis D'Souza, Prabhat Gautam, Rajneesh Misra, and
Michael Thomas
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To be cited as: Chem. Eur. J. 10.1002/chem.201701604
Link to VoR: http://dx.doi.org/10.1002/chem.201701604
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FULL PAPER
DOI: 10.1002/chem.200((……))
Ultrafast charge separation in triphenylamine-BODIPY derived triads carrying
centrally positioned, highly electron deficient, dicyanoquinodimethane or
tetracyanobutadiene electron acceptors
[a]
[a]
[b]
*[b]
Prabhat Gautam, Rajneesh Misra,* Michael B. Thomas, Francis D’Souza
Abstract:
A series of triphenylamine
charge transfer type absorption in the case
of 3. The electrochemical studies revealed
multi-redox processes involving the TPA,
TCBD or DCNQ and BODIPY entities. The
computational studies were performed at the
B3LYP/6-31G** level to elucidate the
geometry and electronic structures. An
energy level diagram established for triads 2
time-resolved emission and femtosecond
transient absorption studies in solvents of
(
TPA) substituted BODIPYs 1–3 have been
newly designed and synthesized by the
Pd-catalysed Sonogashira cross-coupling
and
retroelectrocyclization reaction in good
yields. This procedure yielded highly
electron
varying polarity.
Ultrafast charge
separation has been witnessed in these
closely spaced, strongly interacting triads.
The charge separated state returned to the
ground state without populating the
[2+2]
cycloaddition–
3
deficient
or
tetracyanobutadiene
dicyanoquinodimethane
BODIPY*.
(
(
TCBD)
and 3 revealed that the photoinduced charge
1
DCNQ) electron acceptor units centrally
separation from the
BODIPY* is
Keywords: charge separation . BF2
located at the TPA-BODIPY system. As a
consequence, significant perturbation of the
photonic and electronic properties was
observed. The triads 2 and 3 showed red-
shifted absorption in addition to a strong
thermodynamically possible. In addition,
charge transfer from TPA to TCBD in 2 and
DCNQ in 3 were also possible. These
charge transfer mechanisms was confirmed
by photochemical studies performed using
chelated
dicyanoquinodimethane
tetracyanobutadiene . triphenylamine
azadipyrromethene
.
.
Introduction
The ability to harvest and convert sunlight into chemical energy
in the process of photosynthesis has motivated the chemists for the
development of artificial photosynthetic systems, which can
efficiently absorb and transform sunlight into useful forms of energy
to meet the increasing demand for clean and renewable energy
[1-7]
sources.
A wide variety of artificial photosynthetic model
systems have been designed and synthesized mimicking natural
photosynthesis, and exhibiting strong absorption in the visible and
[8]
near-infrared (NIR) regions. The donor-acceptor (D-A) approach
is the most common pathway to design artificial molecular systems
as the light harvesting properties can be tuned significantly by
[9,10]
altering the D/A units or the π-linker connecting them.
In recent years, BF -chelated dipyrromethenes (BODIPY) have
gained attention as building blocks of antenna systems and as
2
Figure 1. Structure of the BODIPY derived strongly interacting donor-
acceptor triads.
[11]
electron donor or acceptor entities.
BODIPY dyes exhibit high
In continuation of our work in this area of research, in the
present study we have synthesized D–π–A dyad, abbreviated as
BODIPY 1 in Figure 1 by substitution of the donor triphenylamine
absorption coefficients, high emission quantum yields and tunable
[12]
redox potentials.
Their photonic properties can be tuned by
altering the strength of D/A units or by varying the π-linker at the
(
TPA) at the meso-position of the BODIPY acceptor via an
[12,13]
meso-position, as well as at the pyrrolic position.
These features
acetylenic linker. TPA is an electron donor and frequently used for
[15]
make the BODIPY dyes an attractive chromophore, since their
absorption can be extended over a wide spectral range. Our groups
have explored various antenna-donor-acceptor models possessing
the design of D–A molecular systems. Next, the acetylene linker
of the D–π–A BODIPY 1 was systematically replaced by strong
electron acceptor, 1,1,4,4-tetracyanobutadiene (TCBD) and
[12b, 14]
substituted BODIPYs in recent years.
dicyanoquinodimethane (DCNQ) acceptor (BODIPY
2 and
BODIPY 3 in Figure 1) to probe the effect of acceptor strength on
[
a]
b]
Department of Chemistry, Indian Institute of Technology, Indore 453552, India.
E-mail: rajneeshmisra@iiti.ac.in; Fax: +91 731 2361 482
Department of Chemistry, University of North Texas, 1155 Union Circle,
[16]
the photonic properties. The substitution of the TCBD and DCNQ
units have been achieved via [2+2] cycloaddition–
retroelectrocyclization of BODIPY 1, which resulted BODIPYs 2
and 3. The TCBD and DCNQ substitution resulted in strong
intramolecular charge transfer and broadening of the absorption
[
#
305070, Denton, TX 76203-5017, USA, E-mail: Francis.DSouza@UNT.edu

Supporting information for this article, additional spectral and femtosecond spectra,
1
13
H and C NMR and MALDI-mass of triads.
1
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Chemistry - A European Journal
10.1002/chem.201701604
bands. We have explored the photophysical and electrochemical
properties of the BODIPYs 1–3 and performed the theoretical
calculations to see the effect of enhanced π-conjugation and
acceptor strength. Ultrafast charge separation has been witnessed in
these triads as revealed by studies involving steady-state and time-
resolved emission, and femtosecond transient absorption studies.
exhibited a strong absorption band between 500–510 nm,
corresponding to the S →S (π→π*) transition, and a weak
0 1
Results and Discussion
Syntheses: The synthetic methodology developed for BODIPYs 1–
3
donor-acceptor conjugates is shown in Scheme 1. The meso-4-
bromophenyl-BODIPY (Br-BODIPY) was synthesized in two
[17]
steps. The first step involved the reaction of pyrrole and 4-
bromobenzaldehyde in the presence of catalytic amounts of
trifluoroacetic acid (TFA), resulted in bromo-dipyrromethane,
which upon DDQ oxidation and complexation with BF
3
·OEt
2
,
Figure 2. (a) Normalized absorption spectrum of the indicated compounds in
toluene, the spectrum of BODIPY 3 in benzonitrile is also shown for
comparison purposes. (b) Fluorescence spectrum of BODIPY 1 in (i)
toluene (ex = 505 nm) and (ii) benzonitrile (ex = 507 nm).
resulted Br-BODIPYs in the final step. The Pd-catalyzed
Sonogashira cross-coupling of Br-BODIPY with 4-ethynyl-N,N-
[18]
diphenylaniline resulted BODIPY
1
in 74% yield.
The
tetracyanobutadiene (TCBD) and dicyanoquinodimethane (DCNQ)
substituted BODIPYs 2 and 3 were synthesized by the [2+2]
cycloaddition–retroelectrocyclization reaction of the BODIPY 1
0 2
absorption band between 300–400 nm due to the S →S (π→π*)
transition with contributions from the TPA entity. The
comparison of the absorption spectra of BODIPYs 1 and 2
indicated that the substitution of TCBD unit results in marginal
red-shift of ~3 nm but significant broadening of the absorption
with
tetracyanoethene
(TCNE)
and
7,7,8,8-
[
19]
tetracyanoquinodimethane (TCNQ). The reaction of BODIPY 1
with one equivalent of TCNE and TCNQ resulted BODIPYs 2 and 3
in 75% and 80% yields respectively. The BODIPYs 1–3 were
purified by column chromatography and were fully characterized by
band due to the S
BODIPY 3 exhibited a distinct charge transfer band at 621 nm
and a red-shift of ~6 nm for the S →S transition as compared to
0 1
→S transition. The DCNQ substituted
1
13
H, C NMR, and HRMS techniques. The BODIPYs 1–3 are
0
1
readily soluble in common organic solvents such as acetone,
chloroform, dichloromethane, toluene, tetrahydrofuran, etc.
BODIPY 1. These results clearly indicate that the electronic
absorption of the TPA-substituted BODIPYs is a function of the
acceptor strength. As expected for charge
transfer bands, changing the solvent to a
more polar benzonitrile, caused additional
red-shift of the charge transfer band to 662
[
21]
nm, as shown in Figure 2a.
As shown in Figure 2b(i), the steady-
state fluorescence spectrum of BODIPY 1
in toluene was found to be located at 612
nm. This peak was significantly red-
shifted from the fluorescence of pristine
BODIPY derivatives known to emit in the
5
20 nm range. This observation suggests
electronic coupling between the TPA and
BODIPY entities via the acetylenic linker
resulting into intramolecular exciplex
emission in nonpolar solvent. Changing
the solvent to a more polar benzonitrile
quenched the fluorescence quantitatively
(
Figure 2b(ii)) revealing complete electron
transfer from the singlet excited BODIPY
to TPA.[22]
As expected for strongly
coupled donor-acceptor systems, no
measurable fluorescence was observed for BODIPY 2 and
BODIPY 3 in both nonpolar and polar solvents. Fluorescence
lifetime of BODIPY 1 was measured in toluene using time-
correlated single photon counting (TCSPC) method using 495
Scheme 1. Syntheses of BODIPY derived dyad, 1 and triads 2 and 3.
Absorbance and fluorescence studies:
The electronic
absorption spectral studies of the BODIPYs 1–3 were performed
in toluene at room temperature and is shown in Figure 2a. The
BODIPYs are known to exhibit a strong absorption band around
nm nanoLED excitation source.
The decay was
monoexponential with the lifetime of 2.24 ns (see Figure S1 in
SI for decay curve).
5
00 nm along with a broad transition in the higher energy
[
11, 20]
region.
The present TPA functionalized BODIPYs 1–3
2
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Chemistry - A European Journal
10.1002/chem.201701604
Electrochemical properties: The electrochemical properties of
the BODIPYs 1–3 were explored by cyclic voltammetry (CV)
and differential pulse voltammetry (DPV). All the measurements
were performed in benzonitrile at room temperature using
Table 1. Redox potential corresponding to both oxidation and reduction
processes in benzonitrile, 0.1 M (TBA)ClO
4
(V vs. Ag/AgCl) and free-
energy change for charge recombination and charge separation from
1
BODIPY* of BODIPY 1-3. The site of electron transfer corresponding to
each redox couple is listed in the table footnote.
tetrabutylammonium perchlorate, (TBA)ClO
electrolyte. The electrochemical data are listed in Table
, and the representative cyclic voltammograms are
4
, as a supporting
Compound
3rd red, 2nd red, 1st red, 1st ox
2nd ox
GCRf,
eV
-1.74g
-1.49h
GCSj,
eV
1
V
V
V
shown in Figure 3. BODIPY 1 revealed the first
BODIPY 1
BODIPY 2
-0.68a
-0.91a
--
-0.56c
--
-0.20c
1.07b
1.36b,e
1.71a,e
1.71a,e
-0.59g
-0.84h
-0.44i
-1.19h
-0.80i
reduction at -0.68 V vs. Ag/AgCl corresponding to
BODIPY0/•– formation.
In addition, two oxidation
processes, the first one a quasi-reversible process at Epa
1.07 corresponding to oxidation of TPA entity and the
-
1.89i
-1.14h
1.53i
BODIPY 3
-0.78a
-0.18d
-0.05d
1.09b,e
1.72a,e
=
-
second one at Epa = 1.71 V corresponding to oxidation of
a
-
BODIPY centered oxidation and reduction processes
-TPA centered oxidation
TCBD centered reductions
DCNQ centered reductions
b
c
BODIPY entity were observed. Interestingly, for BODIPY 2 and
BODIPY 3, in addition to the expected reduction and oxidation
processes of BODIPY and TPA, additional reduction processes
were observed corresponding to the reduction of TCBD and
DCNQ entities. For BODIPY 2, two one-electron reductions
-
d
e
-
-
Epa at scan rate = 0.1 V/s
f
-calculated using equation 1
g
h
i
•–
•+
-
-
for BODIPY -TPA
for BODIPY-Acceptor -TPA (Acceptor = TCBD or DCNQ)
•-
•+
corresponding to TCBD and TCBD•–/2– were observed at -0.56
and -0.20 V, respectively. Similarly, for BODIPY 3, two one-
electron reductions corresponding to DCNQ0/•– and DCNQ•–/2–
were observed at -0.18 and -0.05 V, respectively. It is important
to note that the first reduction potential corresponding to TCBD
in BODIPY 2 and DCNQ in BODIPY 3 was lower than that
reported for traditional electron acceptors like quinone or
fullerene.[23] The BODIPY reduction in BODIPY 2 and BODIPY
0/•–
•+
•-
-
for BODIPY -Acceptor -TPA (Acceptor = TCBD or DCNQ)
j
1
-from BODIPY* (E0,0 = 2.33 eV, mid-point of absorption and emission peak
maxima.
Computational Studies: In order to explore the geometry and
electronic structures of triphenylamine functionalized BODIPYs,
DFT (density functional theory) calculations were performed at the
[24]
B3LYP/6-31G+g(d) level in the gas phase. The structures were
fully optimized on a Born-Oppenheimer potential energy surface
and establishment of true minima was confirmed by frequency
calculations. No steric constraints between the centrally located
electron acceptor, TCBD or DCNQ, with the BODIPY or TPA
entities were observed. The contours of the HOMOs and LUMOs of
the optimized structures of BODIPYs 1–3 along with the
electrostatic potential map is shown in Figure 4 while higher
LUMOs are shown in Figures S2 and S3. In all the structures, the
HOMO-1 was found to be localized over the BODIPY segment
while HOMO was delocalized mainly over the triphenylamine donor
entity with some contributions on -extended central entities. The
3
were cathodically shifted and appeared at -0.91 and -0.78 V,
respectively, owing to the presence of reduced TCBD and
DCNQ entities prior to the reduction of BODIPY. Such a trend
in oxidation potentials was also observed. In the case of
BODIPY 2, the oxidation processes corresponding to TPA and
BODIPY were located at Epa = 1.36 and 1.71 V while in the case
of BODIPY 3, these processes were located at Epa = 1.09 and
1
.72 V. The harder oxidations are due to the electronic effects
caused by the strong electron acceptor entities directly located
between the BODIPY and TPA entities (see Figure 1 for
structures).
Figure 3. Cyclic voltammogram of triphenylamine functionalized
Figure 4. Frontier molecular orbitals and electrostatic potential map of
triphenylamine functionalized BODIPYs 1–3.
BODIPYs 1–3 in 0.1M (TBA)ClO
rate of 100 mV s−1
4
in benzonitrile recorded at a scan
.
3
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Chemistry - A European Journal
10.1002/chem.201701604
LUMO was found to be localized mainly over BODIPY while
LUMO+1 on the TCBD and DCNQ acceptor entities in the case of 2
and 3. The LUMO on BODIPY and LUMO+1 on TCBD or DCNQ
due to level of calculations done in gas phase. The electrostatic
potential maps were consistent with the electron rich (TPA) and
deficient parts (TCBD and DCNQ) of the studied systems.
the case of BODIPY 2 and BODIPY 3 is shown in Figure 5. The
solid arrows show the major process while the dashed arrows show
1
minor process. In these compounds BODIPY* could undergo
3
intersystem crossing to populate the BODIPY* state in competition
with electron transfer to result in charge separated states. Due to
close distance and exothermic nature, electron transfer might prevail
over the intersystem crossing process. The charge recombination
process also deserves special mention. Similar to the charge
separation process, the charge recombination process could also be
predicted to occur faster due to close distal separation and lack of
molecular entities causing additional electron or hole transfer
processes. In addition, among all of the charge separated states,
The orbital location is helpful in arriving the electron transfer
path. For example, in BODIPY 1, the initial excitation of BODIPY
involves HOMO-1 to LUMO. For charge separation to occur, one
of the two electron on the HOMO located on TPA drops to the half-
•
-
•+
filled HOMO-1, giving rise to BODIPY -TPA charge separated
state. In addition, the ground state charge transfer band observed in
the case of BODIPY 3 (see Fig. 2a) corresponds to the transition
from the HOMO to LUMO+1 transition, resulting in a partial charge
transfer from TPA to DCNQ without much contribution from
BODIPY centered transition involving HOMO-1 and LUMO. This
is conceivable since the oxidation of TPA entity is much more facile
than BODIPY in these molecular systems (see Table 1) making the
charge transfer interactions between TPA-DCNQ much stronger
than BODIPY-DCNQ.
•
+
•-
only BODIPY - TCBD -TPA possess energy higher than that of
3
BODIPY*. In such a case, the charge separated state could
populate the BODIPY* prior returning to the ground state. In order
3
to unravel these mechanistic aspects, femtosecond transient
absorption studies were performed as discussed in the following
section.
Free-energy calculations and energy level diagram: Using the
spectral, computational and electrochemical data, Gibbs free-energy
change associated for charge recombination and charge separation
[25]
were estimated according to equations 1-3
-
GCR = Eox – Ered + G
S
(i)
-GCS = E00(-GCR
)
(ii)
1
where E00 corresponds to the energy of BODIPY* (= 2.33 eV,
mid-point of absorption and emission peak maxima). 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 of the electron donor (TPA or BODIPY) and the
first reduction potential of the electron acceptor (BODIPY in the
case of 1, TCBD in the case of 2 and DCNQ in the case of 3),
respectively.
2


1

Figure 5. Energy level diagram showing the different photochemical events
occurring in BODIPY 2 and BODIPY 3 benzonitrile. Energies of different
states were evaluated from spectral and electrochemical studies. Solid
arrows indicate major photo-processes, dashed arrow indicates minor photo-
process. ISC – intersystem crossing, CS = charge separation, CR = charge
recombination, and T = emission from triplet excited state.
e
 1
1
  1



G 



 
S




4 0
2R
2R
R
(iii)
 
R 
CC R
The symbols 
0 R
, and  represent vacuum permittivity and dielectric
constant of the solvent used for photochemical and electrochemical
studies, respectively. The symbols R
cation and anion, respectively.
+
and R
-
represent radii of the
R
CC are the center-to-center
Femtosecond transient absorption studies: Since strong excited
distances between donor and acceptor entities of the dyad from the
computed structures in Figure 4.
state charge transfer type interactions in the studied systems
especially in BODIPY
2 and BODIPY 3 were expected,
The GCR and GCS values estimated using the above
procedure are given in Table 1. The following observations were
made. (i) In the case of BODIPY 1, the only thermodynamically
femtosecond transient absorption studies were performed in solvents
of varying polarity. Four solvents, toluene, tetrahydrofuran (THF),
benzonitrile (PhCN) and dimethylformamide (DMF) were employed.
The samples were excited at 400 nm where BODIPY was mainly
excited and to a lesser extent TPA was also excited. Figures 6 and 7
show representative spectral features at different delay times in
toluene and benzonitrile while Figures S2 and S3 in SI show the
data in THF and DMF. Additional time profiles are shown in
1
•–
possible charge separated state from BODIPY* is BODIPY -
•+
TPA due to facile oxidation of TPA over BODIPY. In the case of
BODIPY 2 and BODIPY 3, at least two charge separated states were
•
+
•-
•-
possible, viz., BODIPY -Acceptor -TPA and BODIPY-Acceptor -
•+
TPA (Acceptor = TCBD or DCNQ). At the excitation wavelength
of 400 nm (operating wavelength of our femtosecond transient
spectrometer), it is possible that part of TPA might be directly
excited to yield BODIPY-Acceptor -TPA charge separated state.
Although not discussed in detail, such processes are also
thermodynamically feasible.
Figures S7-S9.
Steady-state fluorescence studies revealed
quantitative quenching of all three BODIPY derivatives in both
polar and nonpolar solvents except in the case of BODIPY 1 in
toluene. Figure 6a shows the transient absorption spectra of
•-
•+
BODIPY 1 in toluene at the indicated delay time.
instantaneously formed BODIPY* revealed peak at 508 nm due to
ground state depletion, and positive peaks at 461 and 540 nm along
The
1
The relative energy levels of the charge separated states with
respect to the energy of the singlet and triplet excited BODIPY in
4
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Chemistry - A European Journal
10.1002/chem.201701604
with a broad peak in the near-infrared region at 1200 nm. These
peaks have been assigned to the exciplex state. Figure 6a right hand
panel shows the time profiles of the 508 and 1200 nm peaks.
Slower recovery of the 508 nm peak and slower decay of the 1200
nm are consistent with the longer lifetime of the exciplex state of
BODIPY 1 (2.24 ns) discussed earlier.
the 520 nm range, and rise and decay of the near-IR peak is shown at the
right hand side of each figure.
In sharp contrast, both BODIPY 2 (Figure 6b) and BODIPY 3
(Figure 6c) revealed ultrafast photochemical events. In the case of
1
BODIPY 2, the instantly formed BODIPY* revealed a positive
peak at 465 nm and depleted peak at 522 nm due to
ground state bleaching. Decay and recovery of
these peaks were fast that revealed new peaks at
5
01 and 1350 nm that has been assigned to the
•+
•-
formation of BODIPY -TCBD -TPA charge
separated state with additional contributions from
•
-
•+
the BODIPY-TCBD -TPA charge separated state.
Decay of the charge separated signals was also
rapid as shown in Figure 6b right hand side panel
meaning ultrafast charge recombination. In the
case of BODIPY 3, the instantaneously formed
1
BODIPY* revealed positive peaks at 465 and 842
nm and negative peaks at 518 and 620 nm. These
•+
transient peaks have been attributed to BODIPY -
•-
•-
•+
DCNQ -TPA and BODIPY-DCNQ -TPA charge
separated states.
Changing the solvent to more polar solvents
would facilitate the charge separation process, this
indeed seems to be the case as shown in the
transient spectra of the investigated compounds in
benzonitrile in Figure 7 (see Figures S2 and S3 for
transient spectra in THF and DMF). In the case of
1
BODIPY 1 in PhCN, instantly formed BODIPY*
revealed positive peaks at 566 and 1055 nm
corresponding to TPA•+ and BODIPY -, respectively (Figure 7a)
implying occurrence of excited state electron transfer. The
formation and decay of these signals were much faster than that
observed in toluene, consistent with higher degree of quenching
observed for BODIPY 1 in benzonitrile (see time profiles at the
right hand panels). In the case of BODIPY 2 and
Figure 6. Femtosecond transient spectra at the indicated delay times of (a)
BODIPY 1, (b) BODIPY 2 and (c) BODIPY 3 in Ar-saturated toluene (400
nm, 100 fs pulses). Time profiles of the ground state bleaching peak in the
•–
5
20 nm range, and rise and decay of the near-IR peak is shown at the right
hand side of each figure.
BODIPY 3, the spectral features were similar to
that observed in toluene, however, with shorter
time constants for both growth and decay of the
signals, predicted for charge transfer interactions.
The spectral features were attributable to
•
+
•-
•-
BODIPY -TCBD -TPA and BODIPY-TCBD -
•+
TPA charge separated states in the case of
BODIPY 2, and BODIPY -DCNQ -TPA and
•+
•-
•-
•+
BODIPY-DCNQ -TPA charge separated states in
the case of BODIPY 3.The time constants for the
growth and decay of the signals from global
analysis for the transient spectral data is given in
Table 2.
From the data presented in Table 2 it is clear
that ultrafast charge separation and recombination
processes occur especially in BODIPY 2 and
BODIPY 3. In some cases the CS are within the
time resolution of the transient setup. To a large
extent, both CS and CR follow the expected
solvent polarity trend. The very close proximity of
BODIPY to the strong electron acceptors, TCBD
or DCNQ made such processes possible.
Figure 7. Femtosecond transient spectra at the indicated delay times of (a)
BODIPY 1, (b) BODIPY 2 and (c) BODIPY 3 in Ar-saturated benzonitrile
(400 nm, 100 fs pulses). Time profiles of the ground state bleaching peak in
5
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Chemistry - A European Journal
10.1002/chem.201701604
1
23.8, 122.0, 118.7, 115.2, 114.1, 93.0, 87.7; HRMS (ESI‐TOF) m/z calcd for
H24 BF N + Na:558.1930 [M + Na] , found 558.1931 [M+ Na] .
35 2 3
Table 2. Time constants for photoinduced charge separation, CS and charge
recombination, CR of the investigated donor-acceptor systems in solvents of
varying polarity (estimated error = +10%).
+
+
C
BODIPY 2. TCNE (26 mg, 0.20 mmol) was added to a solution of BODIPY
(107 mg; 0.20 mmol) in DCM (50 mL). The mixture was refluxed at 40 ºC
for 20 h. The solvent was removed in vacuo, and the product was purified by
1
Compound
Solvent
Dielectric
constant
2.38

CS, ps
CR, ps
SiO
2
column chromatography with DCM as the eluent to yield BODIPY 2 as
1
brown colored solid (100 mg, Yield: 75 %); H NMR (400 MHz, (CDCl
3
, δ in
BODIPY 1
Toluene
THF
18.4
2420
64.5
7.80
4.0
ppm): 7.99 (s, 2H), 7.87 (d, 2H, J = 8 Hz), 7.75-7.71 (m, 4H), 7.43 (t, 4H),
.31-7.24 (m, 3H), 7.24 (bs, 3H), 6.98 (d, 2H, J = 8 Hz), 6.88 (d, 2H, J = 4
Hz), 6.59 (d, 2H, J = 4 Hz); 13C NMR (100 MHz, CDCl
, δ in ppm): 167.5,
62.9, 154.3, 145.6, 144.4, 139.5, 134.6, 133.6, 132.1, 131.6, 131.5, 130.3,
7.58
3.73
7
PhCN
DMF
26.0
1.72
3
36.7
2.49
1
1
BODIPY 2
BODIPY 3
Toluene
THF
2.38
1.50
2.65
2.74
5.86
4.94
104
29.5, 127.2, 121.0, 119.5, 118.2, 113.5, 113.0, 111.7, 111.0, 89.2, 77.8;
7.58
1.12
+ Na: 686.2053 [M + Na]+,
HRMS (ESI-TOF) m/z calcd for C41H24BF N
found 686.2054 [M + Na]+.
2 7
PhCN
DMF
26.0
1.55
36.7
< 1ps
9.60
BODIPY 3. TCNQ (41 mg, 0.20 mmol) was added to a solution of BODIPY
(107 mg; 0.20 mmol) in DCE (50 mL). The mixture was refluxed at 100 ºC
for 40 h. The solvent was removed in vacuo, and the product was purified by
Toluene
THF
2.38
1
7.58
< 1 ps
< 1 ps
<1 ps
1.59
5.72
1.54
PhCN
DMF
26.0
SiO
2
column chromatography with DCM as the eluent to yield BODIPY 3 as
36.7
1
black colored solid (115 mg, Yield: 80 %); H NMR (400 MHz, (CDCl
3
, δ in
ppm): 7.99 (s, 2H), 7.81 (d, 2H, J = 8 Hz), 7.69 (d, 2H, J = 8 Hz), 7.56-7.53
Conclusion
(m, 1H), 7.40-7.32 (m, 5H), 7.25-7.19 (m, 9H), 7.06-6.99 (m, 3H), 6.84 (d,
2
1
1
1
H, J = 4 Hz), 6.59 (bs, 2H); 13C NMR (100 MHz, CDCl
3
, δ in ppm): 170.6,
In summary, triphenylamine functionalized donor–acceptor
BODIPYs 1–3 were newly synthesized to explore the effect of
substitution of the highly electron deficient tetracyanobutadiene
53.7, 151.7, 149.3, 145.4, 145.2, 144.0, 138.6, 136.2, 135.1, 134.5, 133.4,
33.1, 131.4, 131.2, 130.0, 129.5, 126.6, 126.5, 126.3, 126.1, 119.3, 113.9,
12.5, 111.8, 88.9, 75.4; HRMS (ESI-TOF) m/z calcd for C47
, found 762.2365 [M + Na]
2 7
H28BF N + Na:
(
TCBD) and dicyanoquinodimethane (DCNQ) acceptor units on
762.2367 [M + Na]
+
+
.
their physico-chemical properties. The synthesis involving
Pd-catalysed Sonogashira cross-coupling and [2+2] cycloaddition–
retroelectrocyclization reactions resulted in good yields. The
absorption studies showed strong intramolecular charge transfer and
bathochromic shift of the absorption bands upon increasing the
acceptor strength. The electron acceptor properties of the centrally
positioned DCNQ and TCBD in the BODIPY-TPA systems
revealed reversible reductions whose potentials were lower than the
traditional electron acceptors. Due to this strong electron acceptor
behavior and close proximity between the donor and acceptor
entities, strong intramolecular type interactions both in the ground
and excited state were observed. Excited state charge transfer from
singlet excited BODIPY and TPA to the electron to the electron
acceptors was established from femtosecond transient absorption
studies in solvents of varying polarity. Ultrafast kinetic events were
witnessed in the present series of compounds highlighting their
importance in solar energy harvesting and optoelectronic
applications.
Instruments: The UV-visible spectral measurements were carried out with a
Shimadzu Model 2550 double monochromator UV-visible spectrophotometer.
The fluorescence emission was monitored by using a Horiba Yvon Nanolog
coupled with time-correlated single photon counting with nanoLED
excitation sources. A right angle detection method was used. Differential
pulse and cyclic voltammograms were recorded on an EG&G PARSTAT
electrochemical analyzer using a three electrode system. A platinum button
electrode was used as the working electrode. A platinum wire served as the
counter electrode and an Ag/AgCl electrode was used as the reference
electrode. Ferrocene/ferrocenium redox couple was used as an internal
standard. All the solutions were purged prior to electrochemical and spectral
measurements using argon gas. The computational calculations were
performed with the use of GAUSSIAN 09 software package [24]. All the
oxygen or moisture sensitive reactions were carried out under argon/nitrogen
1
atmosphere. H NMR spectra were recorded using a 400 MHz spectrometer.
Chemical shifts are reported in delta (δ) units, expressed in parts per million
(
ppm) downfield from tetramethylsilane using residual protonated solvent as
, 7.26 ppm. 13C NMR spectra were recorded
an internal standard CDCl
3
using a 100 MHz spectrometer. Chemical shifts are reported in delta (δ) units,
expressed in parts per million (ppm) downfield from tetramethylsilane using
1
the solvent as internal standard CDCl
3
, 77.0 ppm. The H NMR splitting
Experimental Section
patterns have been described as “s, singlet; d, doublet; t, triplet and m,
Chemicals. All the reagents were from Aldrich Chemicals (Milwaukee, WI)
multiplet”. HRMS was recorded on TOF-Q mass spectrometer.
while the bulk solvents utilized in the syntheses were from Fischer
Chemicals. The tetra-n-butylammonium perchlorate (TBA)ClO used in
4
electrochemical studies was from Fluka Chemicals.
Femtosecond pump-probe transient spectroscopy. Femtosecond transient
absorption spectroscopy experiments were performed using an Ultrafast
Femtosecond Laser Source (Libra) by Coherent incorporating diode-pumped,
mode locked Ti:Sapphire laser (Vitesse) and diode-pumped intra cavity
doubled Nd:YLF laser (Evolution) to generate a compressed laser output of
Syntheses
BODIPY 1. To a stirred solution of Br-BODIPY (347 mg; 1 mmol) and 4-
ethynyl-N,N-diphenylaniline (296 mg; 1.1 mmol) in THF and TEA (1:1, v/v)
1
.45 W. For optical detection, a Helios transient absorption spectrometer
coupled with femtosecond harmonics generator both provided by Ultrafast
Systems LLC was used. The source for the pump and probe pulses were
derived from the fundamental output of Libra (Compressed output 1.45 W,
pulse width 100 fs) at a repetition rate of 1 kHz. 95% of the fundamental
output of the laser was introduced into harmonic generator, which produces
second and third harmonics of 400 and 267 nm besides the fundamental 800
nm for excitation, while the rest of the output was used for generation of
white light continuum. In the present study, the second harmonic 400 nm
excitation pump was used in all the experiments. Kinetic traces at appropriate
wavelengths were assembled from the time-resolved spectral data. Data
2 3 2
were added [PdCl (PPh ) ] (50 mg, 0.07 mmol) and CuI (10 mg, 0.05 mmol)
at room temperature under an argon flow. The reaction mixture was stirred
for 12 h at 70 °C, and then cooled to room temperature. The solvent was
evaporated under reduced pressure, and the mixture was purified by SiO
2
chromatography with DCM/hexane (1:1, v/v), to obtain BODIPY 1 as red
1
colored solids (396 mg, Yield: 74 %); H NMR (400 MHz, δ in ppm): 7.95 (s,
2
7
4
1
H), 7.65 (d, 2H, J = 8 Hz), 7.55 (d, 2H, J = 8 Hz), 7.40 (d, 2H, J = 8 Hz),
.31-7.27 (m, 4H), 7.14-7.08 (m, 6H), 7.02 (d, 2H, J = 8 Hz), 6.96 (d, 2H, J =
Hz), 6.57 (d, 2H, J = 4 Hz); 13C NMR (100 MHz, CDCl
47.1, 144.2, 139.3, 134.7, 133.1, 132.8, 131.4, 130.6, 129.5, 126.7, 125.2,
, δ in ppm): 148.5,
3
6
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Chemistry - A European Journal
10.1002/chem.201701604
analysis was performed using Surface Xplorer software supplied by Ultrafast
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[26] C. B. KC, G. N. Lim, V. N. Nesterov, P. A. Karr, F. D’Souza, Chem.
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Received: ((will be filled in by the editorial staff))
Revised: ((will be filled in by the editorial staff))
Published online: ((will be filled in by the editorial staff))
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Chemistry - A European Journal
10.1002/chem.201701604
In search of stronger electron
Donor-Acceptor Interactions
acceptors: Newly synthesized molecular
triads carrying centrally positioned
dicyanoquinodimethane or
tetracyanobutadiene electron acceptors
are shown to promote ultrafast charge
separation and recombination.
Ultrafast charge separation in
triphenylamine-BODIPY derived triads
carrying centrally positioned, highly
electron deficient, dicyanoquinodimethane
or tetracyanobutadiene electron acceptors
P. Gautam, R. Misra*, M. B. Thomas, F.
D’Souza*
9
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