Comparative charge transfer studies in nonmetallated and metallated porphyrin fullerene dyads

Article abstract of DOI:10.1002/poc.3685

Single material organic solar cells become an interesting area of research to overcome the challenges with efficient charge separation efficiencies in conventional organic solar cells. In this article, we have synthesized nonmetallated and metallated porphyrin-fullerene dyad materials (H2P-C60 and ZnP-C60, respectively) with simple structure, comprehensively studied their charge transfer mechanism, and established a proof of concept that nonmetallated porphyrin-fullerene dyads are better candidates to be used in organic solar cells compared with metallated dyads. Absorption and electrochemical analysis revealed the ground state electronic interactions between donor-acceptor moieties in both types of dyads. Driving force (?ΔG^o _ET) for intramolecular electron transfer process was calculated by first oxidation and reduction potentials of dyads. The excited state electronic interactions were characterized by time-resolved fluorescence and pump-probe transient absorption experiments. Strong fluorescence quenching of porphyrin along with reduced lifetimes in dyads due to deactivation of singlet excited states by photoinduced charge transfer process between porphyrin/Zn-porphyrin core and fullerene in different polarity solvents was observed. Transient absorption spectroscopy was also applied to identify the transient spectral features, ie, cationic (H2P⁺/ZnP⁺) and anionic (C₆₀ ^?) radicals formed because of the charge separation in both types of dyads. Finally, organic solar cell device was also fabricated using the dyads. We obtained higher V_oc, J_sc, and fill factor in single material organic solar cell using H2P-C60 compared to previous reports.

Full text of DOI:10.1002/poc.3685

Received: 12 August 2016
DOI 10.1002/poc.3685
Revised: 22 December 2016
Accepted: 5 January 2017
R E S E A R C H A R T I C L E
Comparative charge transfer studies in nonmetallated and
metallated porphyrin fullerene dyads
Neha Gupta1 | Samya Naqvi1 | Mukesh Jewariya1,2,3 | Suresh Chand1 | Rachana Kumar1
1 CSIR‐National Institute of Solar Energy, Organic
and Hybrid Solar Cells Group, Physics of Energy
Abstract
Harvesting Division, CSIR‐National Physical
Laboratory, New Delhi, India
Single material organic solar cells become an interesting area of research to
overcome the challenges with efficient charge separation efficiencies in conventional
organic solar cells. In this article, we have synthesized nonmetallated and metallated
porphyrin‐fullerene dyad materials (H2P‐C60 and ZnP‐C60, respectively) with
simple structure, comprehensively studied their charge transfer mechanism, and
established a proof of concept that nonmetallated porphyrin‐fullerene dyads are
better candidates to be used in organic solar cells compared with metallated dyads.
Absorption and electrochemical analysis revealed the ground state electronic
interactions between donor‐acceptor moieties in both types of dyads. Driving force
(−ΔGoET) for intramolecular electron transfer process was calculated by first
oxidation and reduction potentials of dyads. The excited state electronic interactions
were characterized by time‐resolved fluorescence and pump‐probe transient
absorption experiments. Strong fluorescence quenching of porphyrin along with
reduced lifetimes in dyads due to deactivation of singlet excited states by photoin-
duced charge transfer process between porphyrin/Zn‐porphyrin core and fullerene
in different polarity solvents was observed. Transient absorption spectroscopy was
also applied to identify the transient spectral features, ie, cationic (H2P+/ZnP+)
and anionic (C60−) radicals formed because of the charge separation in both types
of dyads. Finally, organic solar cell device was also fabricated using the dyads.
We obtained higher Voc, Jsc, and fill factor in single material organic solar cell using
H2P‐C60 compared to previous reports.
2 Center for Quantum‐Beam‐based Radiation
Research, Korea Atomic Energy Research Institute
(KAERI), South Korea
3 Ultrafast Optoelectronics and Terahertz
Photonics Lab, Physics of Energy Harvesting
Division, National Physical Laboratory, New
Delhi, India
Correspondence
Rachana Kumar, Organic and Hybrid Solar Cells
Group, Physics of Energy Harvesting Division,
National Physical Laboratory, New Delhi‐110012
India.
Email: rachanak.npl@gov.in;
rachanak@nplindia.org
KEYWORDS
charge transfer, fullerene, porphyrin dyads, solar cells
1
|
INTRODUCTION
cells (OSCs), absorbed photon generates excitons that are dis-
sociated into free charge carriers at the heterojunction formed
Table S1. Absorption maxima (λmax, nm) and absorption
coefficient (ε, M−1 cm−1) of H2P, H2P‐C60, and ZnP‐C60
in different polarity solvents
Study of photoinduced electron transfer processes in
covalently linked fullerene‐porphyrin‐based donor‐acceptor
dyad single materials is one of the potential areas of research
owing to their future applications in artificial photosynthesis
and organic photovoltaics.[1–4] In conventional organic solar
by electron donor and acceptor materials interface. The diffu-
sion length of these excitons in organic semiconductors
ranges between 10 and 20 nm, and therefore, exciton diffu-
sion to interface and its dissociation into free carriers are
the main challenges in OSCs.[5] Electron‐hole recombination
before reaching the interface due to short exciton diffusion
length is one of the main limiting factors for low‐organic
solar cell efficiency than the theoretically calculated effi-
ciency.[6] Bulk heterojunction (BHJ) solar cell structure is
the most promising geometry for OSCs forming electron
donor‐acceptor interface within the exciton diffusion length,
Neha Gupta, Samya Naqvi, and Rachana Kumar are also at the Academy of
Scientific and Innovative Research, New Delhi.
J Phys Org Chem. 2017;e3685.
wileyonlinelibrary.com/journal/poc
Copyright © 2017 John Wiley & Sons, Ltd.
1 of 11
https://doi.org/10.1002/poc.3685

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GUPTA ET AL.
but still, the realization of solution‐processed BHJ solar cells
possess several unsolved problems, like critical optimization
of donor‐acceptor material ratio, layer thickness, and fabrica-
tion condition, and above all is to obtain ideal nanoscale
morphology of active layer. Thermodynamic stability of
donor‐acceptor phases in active layer is very low and form
clusters with time, reducing the overall stability of the device.
As a solution to all these problems, single material OSCs
(SMOSCs), like covalently linked porphyrin‐fullerene
donor‐acceptor dyads, have been developed showing
efficient light harvesting, exciton dissociation, and charge
transport.[7–11] Such dyad materials have also found several
other applications like photoinduced hydrogen evolution,[12]
sensing,[13] catalysis,[14] photodynamic therapy,[15] etc.
Porphyrin and metalloporphyrin represent the efficient
light harvesting molecules and offer promising potential in
artificial solar energy capture.[16–18] On the other hand, fuller-
ene molecules with spherical structure are known to generate
long‐lived charge separated states.[19–24] Exploiting the proper-
ties of these 2 materials, we found that several dyad systems
have been theoretically designed[25,26] and synthesized. The
main emphasis was to study the effect of donor‐acceptor
spacers and the type of attachment on fullerene ball on the
process of charge separation and charge recombination.[27,28]
Linker between porphyrin donor and fullerene acceptor in
dyads plays an important role and shows profound influence
on the rate of photoinduced electron transfer.[29–32] Several
types of conjugate linkers, like oligomers and π‐extended
spacers, have been used and studied for their effect on charge
transfer.[33–38] In this present article, we have discussed in
detail about ground state interactions via electrochemical
analysis and ground state absorption spectroscopy of 2 por-
phyrin‐fullerene dyads (metallated [ZnP‐C60] and
nonmetallated [H2P‐C60], respectively), where porphyrin
donor is linked to fullerene acceptor by cyclopropane ring
through an aliphatic spacer as shown in Scheme 1.
We have also focused on the charge separation and recom-
bination behaviour of these metallated and nonmetallated
porphyrin‐linked fullerene dyads and established that
nonmetallated porphyrin‐fullerene dyads are better for porphy-
rin‐fullerene‐based SMOSCs because of their faster charge sep-
aration and slower recombination compared with metallated
porphyrin‐fullerene dyad and showed in OSCs device. As the
distribution of electronic charges is directly influenced by the
surrounding environment,[39] we have also studied the molecu-
lar level charge transfer processes in the presence of different
polarity solvents for porphyrin and dyads via time‐resolved
fluorescence measurements. Photophysics of charge‐separated
states is compared by steady‐state fluorescence and transient
absorption (TA) spectroscopy. Finally, solar cell device has
been fabricated using dyad material in active layer, and approx-
imately 0.01% efficiency has been obtained with H2P‐C60
compared with no light effect in ZnP‐C60.
2
| EXPERIMENTAL SECTION
2.1 | Synthesis of A3B porphyrin, 5‐(4′‐hydroxyphenyl)‐
10,15,20‐triphenylporphyrin (H2P) (Scheme 1)
4‐Hydroxy benzaldehyde (6.245mM) was added in a 50‐mL
propionic acid and dissolved completely. Benzaldehyde
SCHEME 1 Synthesis scheme of porphyrin H2P,
dyad H2P‐C60, and ZnP‐C60. A, Propionic acid,
reflux, 30 minutes; B, 4‐dimethylaminopyridine,
N,N′‐dicyclohexylcarbodiimide, 0°C to r.t.,
24 hours; C, Zn(OCOCH3)2.2H2O, 60°C, 18 hours

GUPTA ET AL.
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(18.735mM) and freshly distilled pyrrole (24.98mM) were
added drop wise, and reaction mixture was refluxed with stir-
ring for 30 minutes. Reaction mixture was cooled to room
temperature (r.t.) and precipitated with sodium bicarbonate
solution. Crude product was collected by filtration and
washed several times with methanol followed by purification
by column chromatography on silica gel using dichlorometh-
ane as eluent. Two fractions were collected. The first one was
tetraphenyporphyrin, and the second fraction was the desired
product. Evaporation of solvent of fraction 2 left violet crys-
tals that were further recrystallized with dichloromethane
yielding violet crystals of porphyrin 1 (H2P). Yield: 12%.
Fourier transform infrared spectroscopy (FTIR) (ν, cm−1):
3518, 3321, 2958, 2921, 2851, 1581, 1468, 1258, 1016,
2.4 | Synthesis of ZnP‐C60 dyad (Scheme 1)
H2P‐C60 dyad (6.62μM) was dissolved in 20‐mL chloro-
form, and thereafter, 19.86μM of zinc acetate dihydrate was
added to the solution. The reaction mixture was stirred and
heated at 60°C for 18 hours resulting in a rosy pink solution.
Reaction mixture was washed with water several times, and
organic layer was dried over anhydrous sodium sulphate.
The solvent was removed on rotary‐evaporator, and
compound was isolated as violet‐coloured crystals. Fourier
transform infrared spectroscopy (ν, cm−1): 2917, 2849,
1734, 1573, 1425, 1258, 1012, 794, and 698.1H NMR
(400 MHz, CDCl3, δ) (ppm): 2.52 (2H), 2.9 (2H), 3.05
(2H), 7.4 (3H), 7.5 (2H), 7.9 (9H), 8.1 (8H), 8.4 (2H), and
8.67 (8H); UV‐vis (CHCl3, nm): 328, 419, 509, 547, 584,
and 752; HR‐MS (MALDI‐TOF): m/z[M‐1] for
C115H40N4O2Zn, calculated 1572.9176; found 1571.9117.
1
794, 964, and 697; H NMR (400 MHz, CDCl3, δ) (ppm):
−2.8 (2H), 7.9 (9H), 8.0 (2H), 8.15 (8H), and 8.76 (8H);
ultraviolet‐visible (UV‐vis) (CHCl3, nm): 412, 515, 550,
591, and 650; HR‐MS (MALDI‐TOF): m/z[M + 1] for
C44H30N4O, calculated 630.2520; found 631.2540.
2.5 | Characterization techniques
All chemicals and reagents were purchased from Sigma‐
Aldrich. Pyrrole was freshly distilled before use. Solvents were
purified by distillation before use. Poly(3‐hexyl)thiophene was
purchased from Sigma‐Aldrich. All the products were charac-
terized by FTIR using KBr pallets on Perkin Elmer FTIR
Spectrum 2. Fourier transform infrared spectroscopy spectra
were collected over a range from 3500 to 500 cm−1. A back-
ground in air was done before scanning the samples. Ultravio-
let‐visible spectroscopy measurement was performed on a
Shimadzu UV‐vis spectrophotometer (UV‐1800) in different
polarity solvents. 1H and 13C NMR spectra were recorded on
Jeol 400 MHz spectrometer in CDCl3 using tetramethylsilane
as internal standard. Molecular weights of the products were
confirmed from MALDI‐TOF mass spectrometry on AB
SCIEX using α‐cyano 4‐hydroxy cinnamic acid matrix. Micro-
structure of materials has been characterized by scanning elec-
tron microscope (SEM), EVO MA10, and high‐resolution
transmission electron microscope analysis on Technai G2
F30, HV‐300.0 kV using. Samples were prepared on glass
substrate for SEM analysis by spin‐coating dilute solution of
dyads in chloroform. Same solution was used for preparation
of high‐resolution transmission electron microscope sample
on 200 mesh copper grids by drop cast. Cyclic voltammetry
measurements were performed using a 3‐electrode standard
configuration with a platinum wire as counter electrode and
Ag wire as reference electrode and Pt‐disc as working
electrode in a 0.1M TBAPF6 (tetra‐n‐butylammonium
hexafluorophosphate) in chlorobenzene solution as electrolyte.
Current vs voltage was measured on an Autolab potentiostat.
Emission fluorescence measurements were performed on
varian (CARY eclipse) fluorescence spectrophotometer in dif-
ferent polarity solvents using 12.5μM solution. Horiba Jobin
Yvon (Fluorohub) was used to record time‐resolved fluores-
cence. To perform ultrafast optical pump‐probe spectroscopy,
we split a train of optical pulse from a Ti:Sapphire laser ampli-
fier (35 fs, 4 mJ/pulse, 1 KHz, 800 nm) into 2 beams with a
2.2 | Synthesis of phenyl‐C61‐butyric acid (PCBA)
For the 50 mg of PCBA methyl ester (PC61BM), 10 mL of
toluene was added and stirred well to dissolve. Ten‐mililiter
glacial acetic acid and 3‐mL conc. HCl were added. The
reaction mixture was stirred and refluxed for 36 hours.
Phenyl‐C61‐butyric acid being insoluble comes out of reac-
tion mixture and collected by centrifugation. Crude solid is
washed several times with toluene to remove minor impuri-
ties of unhydrolyzed PC61BM. Dark brown solid is dried at
120°C. Yield: 100%; FTIR (ν, cm−1): 3429, 2921, 2850,
1702, and 1124.
2.3 | Synthesis of H2P‐C60 dyad (Scheme 1)
Phenyl‐C61‐butyric acid (0.0477mM) was sonicated in a
mixture 15‐mL o‐Dichlorobenzene and 5‐mL CS2 for
1 hour followed by addition of H2P (0.0477mM) and fur-
ther sonicated for an additional 30 minutes to completely
dissolve all the components. Reaction mixture was cooled
to 0°C, and N,N′‐dicyclohexylcarbodiimide (0.3816mM)
and 4‐dimethylaminopyridine (0.3816mM) were added.
Temperature of reaction mixture was raised to r.t. and
stirred at this temperature overnight. Solvents were
removed on rotary‐evaporator, and product (H2P‐C60)
was isolated by column chromatography (on silica gel
with toluene as eluent). Yield: 75%; FTIR (ν, cm−1):
3314, 3057, 1748, 1259, 1163, 1124, 965, 797, and 525;
1H NMR (400 MHz, CDCl3, δ) (ppm): −2.87 (2H), 2.51
(2H), 2.89 (2H), 3.06 (2H), 7.4 (3H), 7.5 (2H), 7.7
(9H), 7.9 (2H), 8.1 (8H), and 8.77 (8H); UV‐vis (CHCl3,
nm): 328, 418, 508, 549, 590, and 650; HR‐MS (MALDI‐
TOF): m/z[M‐1] for C115H42N4O2, calculated 1511.3341;
found 1510.0508.

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GUPTA ET AL.
beam splitter. One with high intensity was used as a pump, and
an optical parametric amplifier (TOPAS, light conversion) was
used to vary the wavelength of this pump beam from 190 to
2600 nm. The other beam with weak intensity was propagated
through a CaF2 crystal to generate white light continuum cov-
ering the whole spectrum of visible light to be used as a probe
beam. The probe beam was optically delayed with respect to
pump beam using a computer‐controlled delay stage. The
intrinsic temporal resolution of delay stage is 7 fs. Here, we
have performed ultrafast pump‐probe spectroscopy using
420 nm as a pump beam at normal incidence, and the changes
in absorption was detected by using a gated CMOS detector.
The time‐resolved study was performed using HELIOS
(ultrafast systems) spectrometer.
synthesis processes are reported for A3B‐type porphyrin
synthesis, and among all Adler process is the most exploited
one to obtain porphyrin in good yield.[42] In this work, por-
phyrin H2P (Scheme 1) is synthesized by Adler process
where the 2 aldehydes, benzaldehyde, and 4‐hydroxy benz-
aldehyde are used in 1:3 molar ratio. Propionic acid is used
to act as catalyst for the reaction and also as reaction
medium. For the propionic acid solution of 4‐hydroxy benz-
aldehyde, benzaldehyde and freshly distilled pyrrole are
added simultaneously drop wise. Reaction mixture is
refluxed for approximately 30 minutes while protecting
from sunlight. On neutralization of reaction mixture with
saturated sodium bicarbonate solution, the crude material
precipitates out. Column chromatography is used to isolate
the desired A3B porphyrin (H2P) that comes as second
band after meso‐tetraphenylpophyrin in dichloromethane.
Porphyrin H2P is formed in good yield (~12% on average)
via this process. On the other hand, fullerene acceptor part
is prepared by hydrolyzing PC61BM synthesized by our
reported process.[43] Phenyl‐C61‐butyric acid methyl ester
is hydrolyzed in acidic medium to form PCBA. H2P and
PCBA are linked together by Steglich esterification reaction
in the presence of N,N′‐dicyclohexylcarbodiimide and 4‐
dimethylaminopyridine. Product H2P‐C60 dyad is collected
by column chromatography using toluene as eluent. For
synthesis of metallated dyad ZnP‐C60, reaction of H2P‐
C60 with zinc acetate dihydrate is performed in chloroform.
All the products are characterized by different spectroscopic
techniques for the establishment of their structure (see
Section 2 for details), and electron microscopic studies have
been performed to ascertain their self‐assembling proper-
ties. In looking at the SEM and TEM images of H2P‐C60
and ZnP‐C60 dyads (Figure 1), we can see a clear difference
2.6 | Bulk heterojunction device fabrication
The BHJ OPVs were fabricated with the basic diode config-
uration of ITO (anode)/PEDOT:PSS (hole transport layer)/
Dyad:P3HT (3:1)/Al (cathode). Glass plate coated with trans-
parent ITO electrode and ultrasonically cleaned with deter-
gent, distilled water, acetone, and isopropyl alcohol was
dried and used as anode. A thin (10 nm) layer of PEDOT:
PSS was spin coated on these glass substrates to act as hole
transport layer. Chlorobenzene: chloroform solutions (4:1)
of Dyad:P3HT (3:1) (20 mg/mL) were spin coated on top
of PEDOT:PSS (~100 nm) while filtering from 0.45 micron
PTFE filters in air followed by annealing at 100°C for
20 minutes. Al (100 nm) cathode was deposited on top of
active layer under vacuum to yield an area of 6 mm2 per
pixel. The performance of BHJ PSCs were measured using
a calibrated AM1.5 solar simulator with a light intensity of
100 mW/cm2 adjusted using a standard PV reference cell
and computer controlled Keithley 236 source measure unit.
All fabrication steps and characterizations were performed
in ambient without a protective atmosphere.
in self‐assembling behaviour of the
2 dyads. The
nonmetallated dyad, H2P‐C60, forms more fibrous structure
while metallated dyad, ZnP‐C60, remains as globules. TEM
image of H2P‐C60 shows the formation of layer by layer
stacked self‐assembly with a separation of approximately
2 nm in height while no such behaviour is seen for ZnP‐
C60 dyad.[44] We further analyzed the dyads for ground
state and excited state interaction between donor and accep-
tor components by different techniques as discussed below.
3
| RESULTS AND DISCUSSION
In this work, we have synthesized simple dyad molecules,
ie, H2P‐C60 and ZnP‐C60, to avoid the complexity in
understanding the charge transfer behaviour in porphyrin‐
fullerene dyad systems to find better candidate for
SMOSCs. Several types of covalently linked dyads, triads,
tetrads, etc have been synthesized and studied for their
photochemical and electrochemical properties where the
formation of large well‐defined arrays (self‐assembly) of
such molecules is highly desirable for ordered architectures
and efficient light harvesting, charge generation, and sepa-
ration.[40] Even the simple donor‐acceptor systems with per-
fect geometries and donor‐acceptor distances allow efficient
electron transfer.[41] In this work, we have also analyzed the
packing behaviour of both the dyads by microscopy to find
out their self‐assembling properties. Several types of
4
| GROUND STATE INTERACTION STUDY
4.1 | Electrochemical analysis and electron transfer
driving force
The cyclic voltammetry was performed in dry chlorobenzene
and using 0.1M supporting electrolyte (n‐Bu4NPF6). Three
electrode systems were used with a platinum disc as working
electrode and a silver wire and a platinum wire as reference
and counter electrodes, respectively. Ferrocene/ferricenium
couple was an internal reference to calibrate the redox

GUPTA ET AL.
5 of 11
FIGURE 1 Scanning electron microscope images of, A, H2P‐C60 and, B, ZnP‐C60 and TEM images of, C, H2P‐C60 and, D, ZnP‐C60
potentials. The redox potentials of H2P, H2P‐C60, and ZnP‐
C60 are summarized in Table 1.
shows higher potential for porphyrin ring oxidation com-
pared with H2P‐C60, suggesting easier oxidation of porphy-
rin ring in nonmetallated system that will help in fast charge
generation and separation. ZnP‐C60 dyad also shows 5
reversible reduction potential akin to H2P‐C60 but at higher
voltages.With the help of electrochemical data, the driving
force (−ΔGoET) for intramolecular electron transfer process
was calculated for dyads (Table 1).[45] The driving forces
(−ΔGoET(CR) in eV) for intramolecular charge recombination
processes from C60 radical anion (C60−) to porphyrin radical
cation (H2.+) or C60− to ZnP+are calculated by Weller
Equation 1,[46] where e stands for elementary charge,
Nonmetallated dyad H2P‐C60 shows 1 reversible oxida-
tion and 5 reversible reduction potentials in chlorobenzene
(Figure 2D). While metallated dyad ZnP‐C60 shows 2 revers-
ible oxidation potentials at 0.40 and 0.63 V for metal and por-
phyrin ring oxidation, respectively (Figure 2E). ZnP‐C60
TABLE 1 Redox potentials (Eo) of H2P, dyad H2P‐C60, and ZnP‐C60 in
various solvents (v/sFc/Fc+)
Chlorobenzene (ε = 5.62)
−ΔGo (eV) CR CS
ET
Eoox/V
Eored/V
Chlorobenzene
H2P
0.54
−0.85
−1.92
−2.26
1.39
0.50
ꢂ
ꢀ
ꢁ
ꢀ
ꢁꢃ
D:þ
D
A
A−
−ΔGoETðCRÞ ¼ e Eoox
−Eored
………:
(1)
H2P‐C60
0.55
−1.21
−1.58
−1.80
−1.94
−2.11
1.76
1.67
0.10
Eoox(D+/D) is the first oxidation potential of donor (H2P or
ZnP) moiety while Eored(A/A−) refers to first reduction
potential of acceptor (C60) moiety in chloroform, chloroben-
ZnP‐C60
0.40
0.63
−1.27
−1.60
−2.02
−2.19
−2.37
0.19
zene, and THF. Furthermore, the driving force (−ΔGo
ET(CR)
in eV) for intramolecular charge separation process was
determined by following Equation 2:
−ΔGoETðCSÞ ¼ ΔE0−0 þ ΔGoðCRÞ…………:
(2)
The redox potentials were measured by cyclic voltammetry in chlorobenzene with
0.1M n‐Bu4NPF6 as a supporting electrolyte with a scan rate of 10 mV s−1
−ΔGo and −ΔGo
are the driving force for intramolecular electron
transfer for charge recombination and charge separation processes, respectively,
calculated by Equations 1 and 2.
.
ET(CR)
ET(CS)
where the ΔEo‐o is the energy of the o‐o transition energy
gap between lowest singlet excited state and the ground state.

6 of 11
GUPTA ET AL.
first singlet excited state from ground state. These transitions
are quasi allowed and hence called Q‐bands. Another allowed
transition to second singlet excited state (S2) appears very
strong in the range of 400 to 420 nm and known as Soret
band (or B‐band). Ultraviolet‐visible absorption spectra of
H2P‐C60 and ZnP‐C60 dyads were recorded in chloroform
for qualitative analysis and compared with H2P and PC61BM
as shown in Figure 3. H2P porphyrin shows strong Soret
band at 412 nm followed by Q‐bands at 515, 550, 591, and
650 nm. H2P‐C60 dyad features absorption maxima at 328
(fullerene absorption), 418 (Soret band), 508, 549, 590, and
650 nm (Q‐bands). ZnP‐C60 dyad shows absorption maxima
at 328 (fullerene absorption), 419 (Soret band), 509, 547,
584, and 752 nm (very low‐intensity Q‐bands). Additional
absorption band at 330 nm in dyads confirms the formation
of fullerene‐linked porphyrin dyad system (Figure 3).
However, absorption band at 430 nm for monofunctionalized
fullerene derivatives could not be seen here because of
overlapping with strong porphyrin Soret band absorption.
The absorption spectra of dyads shows slight red shift in
Soret bands (~6 nm). An absorption at approximately
900 nm (inset Figure 3) in dyads for charge transfer band also
ascertains the effective charge transfer between porphyrin
donor and fullerene acceptor in dyads in ground state.
FIGURE 2 Cyclic voltammograms of, A, H2P, B, H2P‐C60, and, C,
ZnP‐C60 in chlorobenzene (v/s Fc/Fc+) with 0.1M n‐Bu4NPF6 as a
supporting electrolyte with a scan rate of 10 mV s−1
Base‐free porphyrin dyad (H2P‐C60) shows much lower
driving force for charge separation and higher for charge
recombination (0.10 eV and 1.76 eV, respectively) compared
with ZnP‐C60 dyad (0.19 eV and 1.67 eV, respectively)
resulting in more efficient and long‐lived charge separated
state in base‐free porphyrin dyads.
To further understand the absorption behaviour of dyads,
we perform absorption study in different polarity solvents
and compared with H2P porphyrin reference for same
concentration (12.5μM) in hexane, toluene, DCM,
dichloroethylene (DCE), chloroform, ethyl acetate, dimethyl
formamide, and dimethyl sulfoxide with increasing polarity
(0.0 to 7.2) (Figure 4). Peak positions and absorption coeffi-
cients are summarized in Table S1. Dyads show decreased
absorption coefficient compared with porphyrin precursor
4.2 | Steady‐state absorption spectroscopy
Ground state absorption spectra of porphyrin shows several
absorption bands corresponding to transition to first or higher
singlet excited states.[47] Two or more weak transitions occur
in 500 to 600 nm region corresponding to S0‐S1 transition, ie,
FIGURE 3 Normalized ultraviolet absorption and photoluminescence emission spectra of H2P, H2P‐C60, and ZnP‐C60 in chloroform. Inset shows charge
transfer band in dyads. Absorption spectra of phenyl‐C61‐butyric acid methyl ester (PC61BM) are also shown for comparison

GUPTA ET AL.
7 of 11
intramolecular electron transfer between donor and
acceptor components on excitation in dyads. Figure 5
shows the comparative fluorescence spectra for 12.5μM
solutions of H2P, H2P‐C60, and ZnP‐C60 dyads (same
solutions used in absorption measurement) in toluene,
dichloromethane, DCE, chloroform, ethyl acetate,
dimethylformamide, and dimethyl sulfoxide. Intensity of
emission for H2P increases with increase in solvent polar-
ity; however, fluorescence intensity is quenched more effi-
ciently in dyads with increasing solvent polarity because of
more efficient charge transfer from donor to acceptor and
better solvation for charged species. Transition 1 shows
an increase in intensity on increasing polarity of solvents
similar to reference porphyrin H2P in both the dyads
while intensity of transition 2 quenches with solvent polar-
ity (Figure 5).
While in H2P‐C60, transition 1 red shifts on varying
solvent polarity from toluene (2.4) to ethyl acetate (4.4)
and again highly blue shift in DMF (6.4) and DMSO
(7.2). Transition 2 remains in the same position except in
ethyl acetate where it is red shifted by approximately
6 nm. In contrast to H2P‐C60, in ZnP‐C60, no significant
shift is observed in transition 1 on varying solvent
polarity from toluene (2.4) to ethyl acetate (4.4) while it
red shifts in DMF (6.4) and DMSO (7.2). Red shifts of 4,
10, and 12 nm are observed for transition 2 in ethyl acetate,
DMF, and DMSO, respectively, compared with toluene in
ZnP‐C60. Thus, significantly different photoluminescence
behaviours are shown by the 2 types of dyads. The
emission peak of fullerene[48] is highly quenched in both
the dyads in all the solvents suggesting very weak energy
transfer from singlet excited porphyrin unit to fullerene on
excitation.
FIGURE 4 Change in absorption coefficient for Soret band with change in
solvent polarity for 12.5μM solution of H2P, H2P‐C60, and ZnP‐C60. DCE
indicates dichloroethylene
because of ground state intramolecular interactions. The
maximum absorption of Soret band in porphyrin was
obtained in ethyl acetate while dyads show maximum
absorption in chlorinated solvents (dichloromethane and
chloroform). Completely different absorption behaviours
were observed in dichloroethylene. Highly red shifted Soret
band was recorded in nonmetallated H2P‐C60 and porphy-
rin reference H2P. Such behaviour was not detected in
ZnP‐C60, suggesting a strong interaction between unsatu-
rated solvent and nonmetallated porphyrin core. ZnP‐C60
shows stronger absorbance for all the solvents compared
with H2P‐C60 dyad except DCE. Red shifted bands are
observed in ZnP‐C60 on increasing polarity while no signif-
icant shift is noticed in case of H2P‐C60 dyad. Shifts in
absorption bands along with change in absorption coeffi-
cient compared with porphyrin suggest significant ground
state electronic interaction in dyad materials. Also, the
low‐absorption intensity suggests the formation of aggrega-
tion for dyads in nonpolar solvent (hexane) and highly polar
solvent (DMSO).
5
| EXCITED STATE INTERACTION STUDY
5.1 | Steady‐state fluorescence
Steady‐state fluorescence from S1 state, ie, S1[0] to S0[0]
(transition 1) and S1[0] to S0[1] (transition 2) in porphyrin
H2P, is observed at 660 and 718 nm on excitation
with Soret band wavelength (Figures 3 and 5). Similarly
for H2P‐C60 and ZnP‐C60 dyads, the emission bands
appear at 670 and 716 nm and 667 and 714 nm,
respectively (Figure 3). H2P‐C60 shows approximately 7
to 10 nm red shift for transition 1 compared with
porphyrin precursor while ZnP‐C60 dyad shows apprecia-
ble shift compared with precursor porphyrin because of
FIGURE 5 Plot of fluorescence spectra in different polarity solvents for
equimolar solutions of, A, H2P‐C60, B, ZnP‐C60, C, H2P, and, D, ZnP.
CHL indicates chloroform; DCE, dichloroethylene; EA, ethyl acetate

8 of 11
GUPTA ET AL.
TABLE 2 Time‐resolved fluorescence data recorded in different polarity solvents for 12.5μM solutions of H2P, H2P‐C60, ZnP, and ZnP‐C60
H2P
H2P*1‐C60
λem, nm
τ, ns
λ
em, nm
τ, ns
kET(CS), s−1
ϕCS
Toluene
DCM
DCE
654
653
704
655
653
654
654
8.2
7.3
2.3
6.6
8.2
9.8
10.1
668
667
693
670
671
652
651
2.2
0.33 × 109
0.21 × 109
1.1 × 109
0.3 × 109
0.38 × 109
0.49 × 109
5.1 × 109
0.73
0.60
0.71
0.67
0.76
0.83
0.98
2.8
0.63
2.2
CHL
EA
2.0
DMF
DMSO
1.7
0.19
ZnP
ZnP*1‐C60
λ
em, nm
τ, ns
3.0
3.1
2.3
3.4
2.8
2.5
1.7
λ
em, nm
τ, ns
0.82
0.93
1.5
kET(CS), s−1
1.1 × 109
0.93 × 109
0.23 × 109
0.1 × 109
0.33 × 109
0.35 × 109
0.9 × 109
ϕCS
0.90
0.87
0.35
0.40
0.73
0.77
0.89
Toluene
DCM
DCE
600
600
700
598
602
608
610
669
668
692
667
671
676
678
CHL
1.2
EA
2.2
DMF
DMSO
2.2
1.0
Abbreviation: CHL indicates chloroform; DCE, dichloroethylene; EA, ethyl acetate.
Time‐resolved fluorescence data recorded in different polarity solvents for transitions 1 and 2. τ is the lifetime of singlet excited states in nanosecond; kET(CS) is rate of
charge separation per second; ϕCS is efficiency of charge separation.
5.2 | Dependence of lifetime on solvent polarity
decay component. Lifetime of singlet excited states is highly
decreased in dyads compared with porphyrin precursor H2P
and ZnP because of charge transfer from porphyrin‐excited
state to fullerene moiety for the formation of charge‐sepa-
rated states.On the basis of fluorescence lifetimes for por-
phyrin and dyads in different polarity solvents, electron
transfer rate constant for charge separation (kET(CS)) was cal-
We also calculated the singlet excited state lifetimes (τ) of
dyads and porphyrin H2P with a time‐correlated single
photon counting apparatus by using 460‐nm excitation in
different polarity solvents for equimolar concentrations
(Table 2 and Figure 6). The fluorescence decay was moni-
tored at transition 1 for porphyrin, H2P‐C60, and ZnP‐C60
dyads for 12.5μM concentration in hexane, toluene, DCM,
DCE, chloroform, ethyl acetate, dimethyl formamide, and
dimethyl sulfoxide with increasing polarity. The fluores-
cence decay curves were well fitted by monoexponential
culated using following Equation 3, and the efficiency (ϕCS
)
was calculated from Equation 4 for H2P+‐C60− formation.
1
1
kETðCSÞ
¼
−
…
(3)
(4)
τðdyadÞ τðporphyrinÞ
1
1
−
À
Á
τðdyadÞ
ϕ
H2Pꢀ1
¼
τðporphyrinÞ ……
ðCSÞ
1
τðdyadÞ
Lifetime of singlet excited state of porphyrin H2P*1 or
ZnP*1 shows different behaviours on increasing solvent
polarity. H2P*1 species is long lived in less polar solvents,
ie, toluene and DCM, whereas ZnP*1 decays rapidly in these
solvents. On the other hand, H2P*1 species is short lived and
decays into charge‐separated states ultrafast in polar solvents.
Shortest lifetime of H2P*1 and ZnP*1 was recorded in DMSO
and toluene, respectively. Efficiency of charge separation
(ϕCS) is greater than 60% in all the solvents for H2P‐C60
dyad and approximately 98% in DMSO that means maximum
H2P*1 state is converting into charge‐separated state. On the
other hand, ZnP‐C60 dyad shows ϕCS as low as 35% and
highest approximately 90% charge separation efficiency is
obtained in toluene, DCM, and DMSO that is comparatively
very low to H2P‐C60 dyad.
FIGURE 6 Bar diagram showing singlet excited state lifetime (τ) of H2P,
H2P‐C60, ZnP, and ZnP‐C60 in different polarity solvents for transition 1.
DCE indicates dichloroethylene

GUPTA ET AL.
9 of 11
FIGURE 8 Transient absorption spectra in NIR region for H2P‐C60 dyad
singlet excited state appeared at 610 and 630 nm and 686
and 727 nm, respectively, within 1 to 2 ps. After 16‐ps delay
time, new broad transition band appears in H2P‐C60 dyad at
697 nm ascribed to H2P+ radical cation[49] evolving from
photoinduced electron transfer from H2P*1 donor to C60
acceptor forming H2P+ and C60‐radicals.
−
Simultaneously after 16 ps, transient absorption of C60
radical anion appears at approximately[50] 1045 nm (Figure 8)
and H2P*3 broad absorption at 845 nm.[51] In case of
metallated ZnP‐C60 dyad, visible range spectra only shows
absorption and emission bleaching at 630, 663, 684, and
727 nm, and the transient species generates comparatively
slowly after 100 ps (Figure 7B). We can clearly see ZnP+
specie at 755 nm. The transient for ZnP*3 appears at
830 nm after 50 ps of excitation. Similarly after 50 ps,
transient absorption of C60− radical anion appears at approx-
imately 1035 nm in ZnP‐C60 dyad (Figure 9).[48,49] As the
decay lasted beyond the monitoring time of our femtosecond
transient spectrometer, the lifetime of cation and anion radi-
cals for both the dyads could not be calculated and requires
nanosecond transient measurements.
FIGURE 7 Transient absorption spectra for, A, H2P‐C60 and, B, ZnP‐C60
dyads in chloroform. Upward arrows show the absorption and emission
bleaching and downward arrows show transient porphyrin radical cation
5.3 | Transient absorption spectroscopy study
Ultrafast TA spectra of H2P, H2P‐C60, and ZnP‐C60 dyads
were measured by femto and pico second pump‐probe analy-
sis in chloroform to evidence the occurrence of photoinduced
charge separation from singlet excited state of H2P and ZnP
in both the dyads. The recorded transient absorption spectra
for H2P‐C60 and ZnP‐C60 dyads are shown in Figure 7.
Transient absorption spectra recorded in solution give direct
information regarding the excited states involved and also
specify the spectral regions for further electron transport
process. Transient absorption spectra gives the information
about ground state absorption and singlet excited state emis-
sion bleaching, singlet excited state absorption, and triplet
excited state absorptions. In this study, we have recorded
the TA spectra of porphyrin precursor (H2P) (Table S1), por-
phyrin‐fullerene dyad (H2P‐C60), and metalloporphyrin
dyad (ZnP‐C60) in chloroform solution at several delay times
after excitation with 530 nm wavelength.
Finally, we tested the materials in single material organic
photovoltaic devices using P3HT as matrix. The fabricated
Immediately after the pump, porphyrin as well as dyads
shows bleaching for porphyrin Q‐bands at 571, 595, and
650 nm (Figure 7A). In H2P‐C60 dyad, singlet excited state
absorption and stimulated emission bleaching for H2P*1
FIGURE 9 Transient absorption spectra in NIR region for ZnP‐C60 dyad

10 of 11
GUPTA ET AL.
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OPV device consisted of ITO (anode)/PEDOT:PSS (hole
transport layer)/Dyad:P3HT (3:1)/Al (cathode). Device with
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H2P‐C60 dyad (nonencapsulated and unoptimized) showed
promising photovoltaic property. From OPV device J‐V char-
acteristics, we calculated the open circuit voltage (Voc)
approximately 0.49 V, short circuit current (Jsc) of approxi-
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| CONCLUSION
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In conclusion, we have extensively studied a very simple sys-
tem of covalently bound donor‐acceptor dyad and compared
the photophysical properties between nonmetallated (H2P‐
C60) and metallated porphyrin‐fullerene (ZnP‐C60) dyads
to ascertain the better candidate to be used in single material
organic solar cell. H2P‐C60 shows much better properties
than ZnP‐C60 for faster oxidation of porphyrin ring at lower
voltage for the formation of charged species, high efficiency
of charge separation in variety of solvents, and formation of
long‐lived charge‐separated state. H2P‐C60 also forms stable
and defined self‐assembly that is one of the essential criterion
for efficient light harvesting, charge generation, and charge
separation. Further study of optimization of parameters for
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ACKNOWLEDGEMENTS
[34] A. M. Ontoria, M. Wielopolski, J. Gebhardt, A. Gouloumis, T. Clark, D. M.
Authors acknowledge DST (SB/FT/CS‐038/2014) and CSIR‐
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TAPSUN for funding, and NG thanks CSIR for fellowship.
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How to cite this article: Gupta N, Naqvi S, Jewariya
M, Chand S, Kumar R. Comparative charge transfer
studies in nonmetallated and metallated porphyrin
fullerene dyads. J Phys Org Chem. 2017;e3685.
https://doi.org/10.1002/poc.3685
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