Push–Pull Porphyrins via β-Pyrrole Functionalization: Evidence of Excited State Events Leading to High-Potential Charge-Separated States

Article abstract of DOI:10.1002/chem.201902286

A new set of free-base and zinc(II)-metallated, β-pyrrole-functionalized unsymmetrical push–pull porphyrins were designed and synthesized via β-mono- and dibrominated tetraphenylporphyrins using Sonogashira cross-coupling reactions. The ability of donors

Full text of DOI:10.1002/chem.201902286

A Journal of
Accepted Article
Title: Push-Pull Porphyrins via β -Pyrrole Functionalization: Evidence
of Excited State Events Leading to High-Potential Charge-
Separated States
Authors: Francis D'Souza, S. Bijesh, Youngwoo Jang, and Rajneesh
Misra
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Push-Pull Porphyrins via -Pyrrole Functionalization: Evidence of Excited
State Events Leading to High-Potential Charge-Separated States
S. Bijesh,a‡ Youngwoo Jang,b‡ Rajneesh Misra,*a Francis D’Souza*b
aDepartment of Chemistry, Indian Institute of Technology, Indore 453552, India. E-mail:
rajneeshmisra@iiti.ac.in ;
bDepartment of Chemistry, University of North Texas, 1155 Union Circle, #305070, Denton, TX
76203-5017, USA, E-mail: Francis.DSouza@UNT.edu
‡Equal contribution
Abstract. A new set of free-base and zinc(II) metallated, β-pyrrole functionalized unsymmetrical
push-pull porphyrins were designed and synthesized via β-mono- and dibrominated
tetraphenylporphyrins by the Sonogashira cross-coupling reaction. The ability of donor and
acceptor on the push-pull porphyrins to produce high-potential charge separated states was
investigated. The porphyrins were functionalized at the opposite β,β’- pyrrole positions of
porphyrin ring carrying triphenylamine push groups and naphthalimide pull group. Systematic
studies involving optical absorption, steady-state and time-resolved emission revealed existence
of intramolecular type interactions both in the ground and excited states. The push-pull nature of
the molecular systems was supported by frontier orbitals generated on optimized structures,
wherein delocalization of HOMO over the push group and LUMO over the pull group connecting
the porphyrin -system was witnessed. Electrochemical studies were performed to visualize the
effect of push and pull groups on the overall redox potentials of the porphyrins.
Spectroelectrochemical studies combined with frontier orbitals helped in characterizing the one-
electron oxidized and reduced porphyrins. Finally, by performing transient absorption studies in
polar benzonitrile, the ability of push-pull porphyrins to produce charge-separated states upon
photoexcitation was confirmed and the measured rates were in the range of ~109 s-1. The lifetime
of the final charge separated state was ~ 5 ns. The present study ascertains the importance of push-
pull porphyrins in solar energy conversion and diverse optoelectronic applications, where high-
potential charge-separated states are warranted.
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Introduction
Solar energy harvesting technology, basically derived from natural photosynthetic reaction center,
has provided major driving force for building and studying a new generation of molecular
materials for light energy conversion.[1-14] The energy-transfer and electron-transfer processes in
the light-harvesting capability of porphyrin chromophores have been well studied for organic
photovoltaics and photodynamic therapy.[15] Structurally close to naturally occurring
chlorophylls,[16] porphyrins, have served as perfect building blocks for electron donor-acceptor
conjugates.[17] Porphyrins can be peripherally modified with diverse electron donor or acceptor
entities using established synthetic protocols, which would also allow one to fine-tune their
photophysical and redox properties to a desired level. In recent years, push-pull porphyrins have
attracted intense attention in the field of dye-sensitized solar cells (DSSCs).[18] The introduction
of electron donating push-groups and electron withdrawing pull-groups on the porphyrin periphery
have made outstanding impact on light energy conversion efficiencies due to the possibility of
promoting vectorial electron transfer, when immobilized on semiconducting nanoparticles.[19-21]
In fact, the best performing DSSCs to-date have been derived from this novel concept. Generally,
the push and pull groups are relatively weaker electron donor and acceptor groups, which is in
contrast to stronger electron donor and acceptor groups used in building multi-modular donor-
acceptor systems to observe excited state electron transfer leading to charge separated states.[7-9]
The stronger electron donor and acceptor groups are characterized by their redox potentials, that
is, they are easier to oxidize and reduce compared to photosensitizing primary electron donor,
porphyrin.
For the molecular design, both meso- and -pyrrole functionalized porphyrins have been
successfully adopted, although -pyrrole functionalized porphyrins are better in terms of
promoting intramolecular interactions due to direct attachment of the addend groups to the
porphyrin -system. However, it may be mentioned here that studies on -pyrrole functionalized
push-pull porphyrins are limited in number due to associated synthetic difficulties.[21-22] The
synthesis of di- and tri-substitution on the β-pyrrole ring was first reported by Callot in 1974.[23]
Although when connected with push and pull groups at the opposite side of the porphyrin ring,
intramolecular charge transfer type interactions both in the ground and the excited state are
expected to occur; evidence for generation of charge separated states upon photoexcitation in such
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closely positioned molecular systems has been limited,[24] due primarily the weak electron donating
and accepting nature of the push and pull groups.
The synthesis and regiochemistry of β-disubstituted porphyrins and metalloporphyrins are
largely unexamined due to the formation of mixture of products. Bhyrappa et. al. have reported
the synthesis of partial β-pyrrole brominated TPP with isomers and the crystal structures of the
complexes.[25] Recently Pizzotti et al. synthesized β-disubstituted push–pull tetraaryl ZnII
porphyrinates and explored its DSSC applications.[26] Sankar et. al. have reported the synthesis of
mixed antipodal β-substituted porphyrins and studied their electronic and the redox properties.[27]
Tagliatesta et al. have synthesized 2,12 positions of β-substituted porphyrin and its electron-
transfer processes were investigated.[28]
In the present study, we have designed and synthesized push−pull porphyrins 1–6 via -
pyrrole functionalization wherein the push group, triphenylamine (TPA) and the pull group,
naphthalimide (NI) units have been incorporated in the porphyrin -system (Figure 1). Upon
complete characterization, the ability to promote intramolecular interactions leading to charge
separated states especially in electron transfer favoring polar solvents are investigated.
Figure 1. Structure of the push-pull porphyrins, 5-6, and the control compounds, 1-4, used in the
present study.
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Results and Discussion
Synthesis
The synthesis of push-pull porphyrin and its zinc derivatives 1–6 are shown in Scheme 1. The
tetraphenylporphyrin (TPP) and monobrominated TPPBr, 10 were synthesized as per the reported
procedures.[29] The synthesis of β-dibrominated TPP, 11 was carried out using controlled amount
of N-bromosuccinimide (NBS) (1.8 equiv) in CHCl3 at room temperature for 24 hours, which
resulted in H2TPPBr2, 11 as mixture of products (Scheme 1). [25b] The control compounds 13 and
14 were synthesized according to the reported procedures (Figure S5).[30] The Pd-catalyzed
Sonogashira cross-coupling reaction using phenylacetylene 7 with TPPBr in the presence of
Pd(PPh3)4 at 70 °C in THF solvent resulted in porphyrin 1 with 53% yield (Scheme 1). The
Sonogashira cross-coupling reaction of β-monobrominated TPPBr with 1 equiv of 2-butyl-6-
ethynyl-1H-benzo[de]isoquinoline-1,3(2H)-dione 8 resulted in porphyrin 3 with 78% yield. The
H2TPPBr2 11 mixture was subjected to Sonogashira cross-coupling reaction with 4-ethynyl-N, N-
diphenyl aniline 9 resulted in porphyrin 12 (12a and 12b) isomers in 79% yield. The porphyrin
12a and 12b were separated by silica gel column chromatography. The separation of the porphyrin
1
12a was evidenced from H NMR characterization. However, it may be mentioned here that we
were unable to discriminate the positions of bromination, whether it was at the 2,12- or 2,13
1
positions. Although we have clear H NMR evidence that only one dibrominated product was
isolated.[26] The Sonogashira cross-coupling reaction was further carried out on porphyrin 12a with
1 equiv of 2-butyl-6-ethynyl-1H-benzo[de]isoquinoline-1,3(2H)-dione 8 resulted in porphyrin 5
with 47% yield. The zinc insertion on porphyrins 1, 3 and 5 were carried out with Zn(OAc)2 in
MeOH/CHCl3 (3:1) mixture, which resulted in zinc porphyrin 2, 4 and 6 with 87%, 86% and 81%
yields, respectively. The porphyrin and its zinc derivatives 1–6 were purified and well
1
characterized by H and 13C NMR, HR-MS and MALDI-TOF mass spectra. (see supporting
information for spectral details).
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Scheme 1. Synthesis of porphyrins 1−6.
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Absorption and Emission Studies
Figure 2a and b show the normalized absorption and fluorescence spectra of the free-base
and zinc(II) derived push-pull porphyrins along with their respective control compounds. Free-
base porphyrins were characterized by an intense broad Soret and four visible bands, whereas the
zinc derivatives were characterized by a broad Soret and two visible bands. Mono- and bis-pyrrole
functionalization had marked changes in the absorption spectral behavior. Compared to pristine
meso-tetraphenylporphyrin, H2TPP and its zinc analog, ZnTPP, compounds 1 and 2 having a
phenylacetylene entity at the β-pyrrole position revealed broadening of the Soret and visible bands
with a small red-shift. Replacing the phenyl entity, with an electron deficient, naphthalimide entity
in 3 and 4 caused additional red-shift and spectral broadening. Upon appending the push group,
triphenylamine and the pull group naphthalimide at the opposite β, β’- pyrrole positions of
porphyrin ring (5 and 6) had much more pronounced effects, that is, substantial broadening with
red-shift approaching the near-IR region of the spectrum was witnessed (see Table 1). There was
a gradual increase in the calculated full width at half maxima (FWHM) values for the Soret band
(Table 1) for a given series, affirming the existence of ground state interactions between the
porphyrin -system and the appended entities in both sets of porphyrin derivatives. Additionally,
the Q-bands gained in intensity for both push-pull porphyrins, 5 and 6, a result that is consistent
with our earlier observation on this type of systems.[24]
Noticeable changes in the emission behavior was also observed for the investigated push-
pull porphyrins. As shown in Figure 2c and d, progressive red-shift in the fluorescence emission
maxima for both free-base and zinc porphyrin series (1 > 3> 5 and 2 > 4 > 6) was observed (Table
1). Interestingly, fluorescence quenching in free-base porphyrin derivative 3 was minimal (< 5%)
which was in contrast to zinc porphyrin derivative 4 where about 65% of the original fluorescence
intensity was quenched, although both were appended with the electron acceptor, NI. In the case
of free-base porphyrin 5 having both NI and TPA entities, substantial quenching (80%) was
observed. For the zinc derivative 6 this quenching was about 33%. These results reveal the
significance of push-pull effects caused by the presence of both electron donor and acceptor
entities on their excited state properties such as electron and energy transfer.[31]
The ascertain excited state interactions in the push-pull porphyrins, fluorescence lifetimes
were also measured using time correlated single photon counting (TCSPC) method (Figure 3).
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The decay could be satisfactorily fitted to one to three exponentials. The average lifetime of free-
base porphyrin derivatives, 1, 3 and 5 were found to be 8.73, 7.27 and 1.72 ns while that for the
zinc porphyrin derivatives, 2, 4 and 6, they were 1.72, 1.08 and 0.66 ns. The lifetimes of
compounds 1 and 2 were close to those of H2TPP (9.6 ns) and ZnTPP (1.82 ns), and they tracked
well with the earlier discussed steady-state measurements. Response: Figure 3 has been revised
according to referee suggestion. The appearance of multiexponential decays could be due to
aggregation, existence of different solution conformers, or return of the charge separated state to
locally excited S1 state due to high energy of charge-separated state (vide infra).
Figure 2. Normalized to the Soret band absorption (a and b) and fluorescence (c and d) of free-
base porphyrin (a and c) and zinc porphyrin (b and d) derived push-pull porphyrins and their
control compounds in benzonitrile. The samples were excited at the Soret band maxima.
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Figure 3. Fluorescence decay curves of (a) free-base porphyrin and (b) zinc porphyrin derivatives
in benzonitrile. The samples were excited using nanoLED excitation sources and the emission
was collected at the emission peak maxima.
Electrochemistry, Frontier Orbitals, and Spectroelectrochemistry
In order to understand the electronic effects caused by the push-pull entities on the
porphyrin -system, electrochemical studies were performed using cyclic voltammetry (CV)
technique. Figure S1 show the CVs of free-base and zinc porphyrins in benzonitrile containing
0.1 M (TBA)ClO4 while the data are summarized in Table 1. Relatively difficult oxidation and
easier reduction for free-base porphyrin derivatives compared to related zinc(II) porphyrin
derivatives was witnessed. The presence of push-pull groups shrunk the electrochemical HOMO-
LUMO gap (i.e., difference between the first oxidation and reduction potentials) by lowering the
reduction potentials while such changes in oxidation potentials were minimal. In order to realize
the redox potentials of push and pull groups, electrochemical studies on two control compounds,
viz., phenylacetylene functionalized TPA 13 and phenylacetylene functionalized NI 14 were
performed (Figure S2). The first reduction of the former compound was located at -1.48 V vs.
Fc/Fc+ while the first oxidation of the latter compound was located at 0.56 V vs. Fc/Fc+. These
results suggest that the TPA oxidation occurs only after the first oxidation of free-base and zinc
porphyrin entities in the push-pull porphyrins. However, the presence of directly linked electron
rich TPA entity could make the oxidation process of porphyrin entity easier and could involve both
TPA and porphyrin entities. Due to easier reduction of NI compared to ZnP, the first reduction in
compounds 4 and 6 could be ascribed to the reduction of NI entity, however, for free-base
porphyrins the first reduction of NI and H2P were close and was difficult to assign the site of
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electron transfer. However, it may be noted that the presence of NI would make porphyrin
reduction relatively easier involving the directly connected NI and porphyrin entities, a trend that
is obvious from the electrochemical data shown in Table 1.
Table 1. Absorption maxima, fluorescence maxima and singlet excited lifetime, and
electrochemical redox potentials of the investigated push-pull porphyrins in benzonitrile.
Compound
fwhm, cm-1
Potential V vs. Fc/Fc+
Ered Eox

max, nm
flu, nm
, ns
1.57x103
2.13x103
2.96x103
430, 520, 561, 600, 658
433, 526, 567, 605, 660
435, 533, 583, 617, 671
664, 727
669, 736
679, 749
8.73 -1.86, -1.59 0.48, 0.72
7.27 -1.69, -1.50 0.48
1
3
5
1.72 -1.65, -1.41 0.43, 0.59
1.047x103
1.76x103
3.36x103
440, 566, 607
620, 670
633, 677
636, 689
1.78
-1.79 0.30, 0.62
2
4
438, 474, 569, 612
340, 444, 475, 580, 621
1.08 -1.77, -1.65 0.33
0.66 -1.69, -1.42 0.32, 0.49
6
0.53
13
14
-1.48
To gain further support for the above rationale additional studies involving computational
and spectroelectrochemical studies were performed. The frontier orbitals were generated on
B3LYP/6-31G* optimized structures [32] as shown in Figure 4. While majority of the HOMO and
LUMO of 1 and 2 were located on the porphyrin -system with little contributions on the
phenylacetylene group, results for other porphyrin systems were significantly different. In the case
of 3 and 5, the HOMO was located mainly on the porphyrin -system while the LUMO were on
both porphyrin and NI entities. In the case of 5 and 6 push-pull systems, majority of HOMO on
TPA with some contributions on the porphyrin entity while the LUMO was on the porphyrin-NI
entity. These results suggest the push and pull groups directly interacting with the porphyrin -
system, rather than behaving as individual entities. It may be mentioned here that position of NI
entity either at the 12a or 12b positions of porphyrin ring in the triads is expected not to change
the geometry and electronic structures significantly, as demonstrated earlier by Parsa et. al. in
push-pull porphyrins.[33]
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Figure 4. Frontier HOMO and LUMO on B3LYP/6-31G* optimized structures of the investigated
compounds.
Spectroelectrochemical results shown in Figure S3 were supportive of oxidation involving
the porphyrin -system. Peaks corresponding to porphyrin -radical cation covering the 600–800
nm range in the case of 3 and 5, and 650–950 nm range in the case of 6 were clear. One or more
isosbestic points were also observed. Due to -pyrrole functionalization of the push-pull groups,
the radical cation spectra were much more red-shifted compared to those of pristine samples.
During reduction, the changes were not so clear. An overall increase in optical density was
observed during first reduction of 5 and 6 suggesting that the NI entity is mainly involved in
reduction (lacking absorption peaks in the visible range) and the porphyrin to a lesser extent.
Free-Energy Calculations
From the optical and electrochemical data, the driving forces for charge recombination (-
GCR) and charge separation (-GCS) were calculated according to eqns. (1) and (2):[34]
-GCR = Eox - Ered + Gs
-GCS = E0,0 – (-GCR)
(1)
(2)
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where Eox is the first oxidation potential of the porphyrin, Ered is the first reduction potential of the
porphyrin, E0,0 is the energy of the 0-0 transition between the lowest excited state of the
porphyrin, evaluated as the mid-point of 0,0 transitions of absorption and fluorescence peaks.
These values were respectively, 1.87, 1.99, 1.85 and 1.97 eV for 3, 4, 5, and 6. The GS refers to
the static energy, calculated by using the ‘Dielectric Continuum Model’ according to eqn. (3).
GS = -e2/(4π0S RCt-Ct
)
(3)
The symbols 0 and s represent vacuum permittivity and dielectric constant of benzonitrile (8.85
x 10-12 C2/Nm and 26), respectively. The calculated GS value for 3 and 4 was 0.06 eV, and for 5
and 6 it was 0.03 eV. These values and the redox potentials were used to evaluate -GCR and were
found to be 1.92, 1.92, 1.82 and 1.71 eV, respectively, for 3, 4, 5, and 6. These values directly
represent the energy stored in the charge-separated states. The magnitude of which suggests
possibility of generation of high-potential charge separated states. Finally, the -GCR values were
combined with E0,0 values which led to GCS values of 0.05, -0.07, -0.03 and -0.26 eV,
respectively, for 3, 4, 5, and 6. These free-energy values suggest moderate thermodynamic
feasibility of excited state charge separation in compounds 4, and 5, and exergonic reaction in 6,
however, such calculations suggested thermodynamic unfeasibility of excited state charge transfer
in the case of 3.
Femtosecond Transient Absorption (fs-TA) Spectral Studies
The fs-TA spectra at indicated delay times for compounds 1 and 2 are shown in Figures
1
S4a and b, respectively. For 1, immediately after excitation, the instantaneously formed 1*
revealed positive peaks at 484, 544, 623, 860, 1102 and 1280 nm. In addition, negative peaks at
522, 560, 600, 662 and 736 nm were observed. By comparison with the earlier discussed
absorption and fluorescence spectra, the first three negative peaks were attributed to ground state
bleaching while the latter two peaks were attributed to stimulated emission. The near-IR peak at
1280 nm was attributed to singlet-singlet transition as such a trend was observed earlier for pristine
porphyrins.[35] With time, decay of the positive signals were noticed, however, within the 3 ns time
window of our instrument most of the signal persisted which is in agreement with longer lifetime
of 1 (8.73 ns). In the case of 2, the instantaneously form 12* had positive peaks at 480, 586, 630,
732, 836 and 1320 nm. Negative peaks at 565, 609 and 673 nm were also observed. The first
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peak was due to ground state bleaching while the latter two peaks were due to stimulated emission.
Decay of the positive peaks was accompanied by a strong near-IR peak at 858 nm corresponding
to 32*.
The fs-TA spectral characterization of porphyrins, 3 and 4, bearing an electron
withdrawing group, NI was subsequently performed. The earlier discussed free-energy
calculations were suggestive of lack of charge separation in 3 while possibility for such a process
in 4 upon photoexcitation in polar benzonitrile. Figure 5a and b show the fs-TA spectra at the
indicated delay times for 3 and 4 in benzonitrile. For 3, the spectral features were close to that
1
observed for 1, that is, the instantaneously formed 3* revealed decay/recovery of positive and
negative peaks without providing strong evidence of charge separated state. If there was charge
Figure 5. Fs-TA spectra at the indicated delay times for (a) 3 and (b) 4 in benzonitrile. The first
sample was excited at 433 nm while the second one was excited at 438 nm. The right panel shows
the decay curves for the near-IR bands of 1 (1325 nm) and 3 (1420 nm) (blue and red, upper panel),
and 2 (1320 nm) and 4 (1308 nm) (blue and red, lower panel).
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separation then one would expected to see a new transient peak covering the 600-800 nm range
corresponding to free-base porphyrin cation radical. The decay curve of the near-IR band shown
on the right-hand panel of Figure 5a for 1 and 3, revealed only a moderate level of accelerated
relaxation supporting lack of charge separation in 3.
On the contrary, spectral features observed for 4 were indicative of charge separation in
this molecular system (Figure 5b). In this case, the relatively rapid decay of the 14* transient peaks
as shown in Figure 5b lower panel for the decay of near-IR peaks of 2 and 4. This was
accompanied by new transient peaks having spectral signature of zinc porphyrin radical cation as
shown in Figure S3, suggesting ZnP.+-NI.- to be the charge separated state.
Figure 6. Fs-TA spectra at the indicated delay times for (a) 5 and (b) 6 in benzonitrile. The first
sample was excited at 435 nm while the second one was excited at 444 nm. The right panel shows
the decay curves for the near-IR bands of 1 (1325 nm) and 5 (1287 nm) (blue and red, upper panel)
and 2 (1320 nm) and 6 (1378 nm) (blue and red, lower panel).
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Finally, fs-TA spectral characterization of push-pull porphyrins, 5 and 6 were performed.
As discussed earlier, free-energy calculations suggested that the excited state charge separation is
indeed possible in these systems in polar solvent generating high-energy charge separated states.
Figure 6a and b show the transient spectra at the indicated delay times for 5 and 6, respectively.
In both cases, rapid decay/recovery of the corresponding singlet excited state generated new
transient spectra characteristic of charge separated states. As shown in the right-hand panel of
each figure, the decay curves corresponding to the singlet excited state revealed rapid decay
compared to the control 1 and 3.
In order to estimate the rate of charge separation, kCS, the earlier discussed fluorescence
lifetimes were used according to equation 4.[36]
kCS = 1/DA – 1/probe
(4)
where DA is the fluorescence lifetime of the donor-acceptor systems, 3–6 and probe is the lifetime
of probe molecules, 1 and 2 having no push or pull groups. The kCS thus calculated were found to
be 4.67 x 108 s-1 for 4, 3.6 x 108 s-1 for 5 and 9.4 x 108 s-1 for 6 in benzonitrile indicating moderate
rates of charge separation. In all of the cases, the charge-separated states lasted for few nanosecond
(3–5 ns) although they were all high-energy states. The relatively faster recombination could be
attributed to closely spaced push-pull groups on the porphyrin macrocycle and low-lying triplet
excited states of porphyrins (ET = 1.4 – 1.55 eV). Nonetheless, generating high-potential, charge
separation in these push-pull porphyrins has been successful.
Summary
The push-pull porphyrins, newly synthesized in the present study, revealed several
interesting results and insights. First, appending the electron push and pull groups to the -pyrrole
positions, instead of commonly utilized meso-positions, is shown to promote significant
intramolecular interactions both in the ground and excited states due to their proximity to
porphyrin -system. Delocalization of HOMO over the push group and the central porphyrin -
system, and LUMO over the pull group and porphyrin -system were key in visualizing the push-
pull effects in the case of both free-base and zinc porphyrin derived systems. Spectral broadening
accompanied by reduction in fluorescence intensity and lifetime provided first evidence of
intramolecular push-pull effects. The important question of whether push-pull effects could
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promote excited state reactions to generate high-potential charge-separated states was answered
by studies performed using fs-TA studies. Compounds 4, 5 and 6, were able to generate charge-
separated states that lasted for few ns prior returning to ground state by populating their individual,
low-laying triplet excited states. These findings suggest usefulness of push-pull porphyrins in
relevant optoelectronic applications.
Experimental Section
General: All chemicals were used as received unless otherwise noted. All moisture
sensitive reactions were performed under argon/nitrogen atmosphere. The NMR spectra were
recorded at room temperature (298 K). Chemical shifts are given in ppm with respect to
1
tetramethylsilane as internal standard (CDCl3, 7.26 ppm, 77.0 ppm). H NMR and 13C NMR
spectra were recorded using a 400 MHz and 100 MHz spectrometer. Tetra-n-butylammonium
perchlorate, n-Bu4NClO4, used in electrochemical studies was from Fluka Chemicals. The density
functional theory (DFT) calculation were carried out at the B3LYP/6-31G** level for C, N, H in
the Gaussian 09 program. HRMS was recorded on TOF-Q mass spectrometer.
Instrumentation
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 Varian Eclipse spectrometer. A right angle detection method was used. Fluorescence
lifetimes were determined using time correlated single photon counting using nanoLED excitation
sources.
Cyclic voltammograms were recorded on an EG&G 263A potentiostat/galvanostat 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 nitrogen gas.
Spectroelectrochemical study was performed by using a cell assembly (SEC-C) supplied
by ALS Co., Ltd. (Tokyo, Japan). This assembly comprised of a Pt counter electrode, a 6 mm Pt
15
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Chemistry - A European Journal
Gauze working electrode, and an Ag/AgCl reference electrode in a 1.0 mm path length quartz cell.
The optical transmission was limited to 6 mm covering the Pt Gauze working electrode.
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 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 a TOPAS-Prime-
OPA system with 290-2600 nm tuning range from Altos Photonics Inc., (Bozeman, MT), 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 analysis was
performed using Surface Xplorer software supplied by Ultrafast Systems. All measurements were
conducted in degassed solutions at 298 K.
Synthesis
Synthesis of 5,10,15,20-tetraphenyl-7-(phenylethynyl) porphyrin (1).
7-bromo-5,10,15,20-tetraphenylporphyrin 10 (0.150 g, 0.21 mmol), ethynylbenzene 7
(0.033 g, 0.32 mmol) in THF: TEA (1: 1, v/v), palladium(0)-tetrakis(triphenylphosphine) (0.070
g, 0.06 mmol), and CuI (0.010 g, 0.05 mmol) were added under argon atm at room temperature.
The reaction mixture was stirred for 12 h at 70 °C, and then cooled to room temperature. The
solvent was then evaporated under reduced pressure and the resultant mixture was diluted with
DCM and the organic layer was collected, dried over anhydrous Na2SO4 and evaporated under
vacuum. The solid was adsorbed on silica gel and purified by column chromatography, using a
DCM: hexane (20: 80) mixture to produce (0.080 g, 53%) of compound 1 as purple solid. 1H NMR
(400 MHz, CDCl3): δ ppm= -2.66 (s, 2 H) 7.37 (m, 4 H) 7.64 - 7.82 (m, 13 H) 8.23 (s, 8 H) 8.76 -
8.89 (m, 6 H) 9.09 (s, 1 H). 13C NMR (100 MHz, CDCl3): δ ppm= 85.58, 98.48, 119.58, 119.69,
119.74, 120.04, 123.47, 126.28, 126.37, 127.43, 128.25, 131.64, 134.12, 134.16, 140.80, 141.43,
16
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Chemistry - A European Journal
141.67, 141.72, 141.77. HRMS (ESI-TOF): m/z calculated for C52H34N4 [M+nH] + 715.2856,
found 715.2834.
Synthesis of 5,10,15,20-tetraphenyl-7-(phenylethynyl)porphyrin, zinc (2).
5,10,15,20-tetraphenyl-7-(phenylethynyl) porphyrin 1 (0.100 g, 0.14 mmol), Zn (OAc)2,
(0.289 g, 1.32 mmol) in MeOH: CHCl3 (3 : 1, v/v), were added and the reaction mixture was stirred
for 1 h at room temperature. The solvent was then evaporated under reduced pressure and the
resultant mixture was diluted with DCM/water and the organic layer was collected, dried over
anhydrous Na2SO4 and evaporated under vacuum. The solid was adsorbed on silica gel and
purified by column chromatography, using a DCM: hexane (40: 60) mixture to produce (0.095 g,
87%) of compound 2 as red solid. 1H NMR (400 MHz, CDCl3): δ ppm= 7.35-7.40 (m, 4 H), 7.60
13
- 7.78 (m, 13 H) 8.23 (s, 8 H) 8.89-8.95 (m, 6 H) 9.26 (s, 1 H). C NMR (100 MHz, CDCl3): δ
ppm= 86.41, 99.28, 121.38, 121.74, 124.22, 126.22, 126.76, 126.85, 128.07, 128.72, 132.30,
134.41, 134.62, 139.17, 142.21, 142.83, 148.49, 150.36, 150.90, 151.01, 151.32. HRMS (ESI-
TOF): m/z calculated for C52H32N4Zn [M+nH] + 777.1991, found 777.1996.
Synthesis of 2-butyl-6-((5,10,15,20-tetraphenylporphyrin-7-yl) ethynyl)-1H-benzo[de]
isoquinoline-1,3(2H)-dione (3).
7-bromo-5,10,15,20-tetraphenylporphyrin 10 (0.200 g, 0.28 mmol), 2-butyl-6-ethynyl-1H-
benzo[de]isoquinoline-1,3(2H)-dione 8 (0.104 g, 0.37 mmol) in THF: TEA (1: 1, v/v),
palladium(0)-tetrakis(triphenylphosphine) (0.070 g, 0.06 mmol), and CuI (0.010 g, 0.05 mmol)
were added under argon atm at room temperature. The reaction mixture was stirred for 12 h at 70
°C, and then cooled to room temperature. The solvent was then evaporated under reduced pressure
and the resultant mixture was diluted with DCM and the organic layer was collected, dried over
anhydrous Na2SO4 and evaporated under vacuum. The solid was adsorbed on silica gel and
purified by column chromatography, using a DCM: hexane (30: 70) mixture to produce (0.195 g,
78%) of compound 3 as black solid. 1H NMR (400 MHz, CDCl3): δ ppm= -2.66 (s, 2 H), 1.02 (t,
J=7.28 Hz, 3 H), 1.46 - 1.54 (m, 2 H), 1.79 (quin, J=7.53 Hz, 2 H), 4.22 (m, 2 H), 7.28 - 7.35 (m,
3 H), 7.52 (t, J=7.65 Hz, 2 H), 7.69 (d, J=7.78 Hz, 1 H), 7.72 - 7.85 (m, 10 H), 8.20 - 8.27 (m, 7
H), 8.53 (d, J=7.53 Hz, 1 H), 8.61 (dd, J=10.79, 7.78 Hz, 2 H), 8.75 - 8.79 (m, 2 H), 8.83 - 8.88
(m, 1 H), 8.92 (s, 2 H), 9.21 (s, 1 H). 13C NMR (100 MHz, CDCl3): δ ppm= 13.90, 20.45, 29.71,
40.33, 95.09, 96.02, 120.06, 120.38, 120.42, 120.58, 121.53, 122.78, 126.77, 126.84, 126.87,
17
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126.95, 127.88, 128.05, 128.55, 128.72, 129.09, 130.07, 130.90, 131.25, 131.41, 133.25, 134.44,
134.55, 134.59, 134.63, 141.23, 141.73, 141.92, 141.97, 163.90, 164.15. HRMS (ESI-TOF): m/z
calculated for C62H43N5O2 [M+nH] + 890.3490, found 890.3493.
Synthesis
of
2-butyl-6-((5,10,15,20-tetraphenylporphyrin-7-yl)ethynyl)-1H-benzo[de]
isoquinoline-1,3(2H)-dione, Zinc (4).
2-butyl-6-((5,10,15,20-tetraphenylporphyrin-7-yl) ethynyl)-1H-benzo[de]isoquinoline-
1,3(2H)-dione 3 (0.100 g, 0.11 mmol), Zn (OAc)2, (0.270 g, 1.23 mmol) in MeOH: CHCl3 (3 : 1,
v/v), were added and the reaction mixture was stirred for 1 h at room temperature. The solvent was
then evaporated under reduced pressure and the resultant mixture was diluted with DCM/water
and the organic layer was collected, dried over anhydrous Na2SO4 and evaporated under vacuum.
The solid was adsorbed on silica gel and purified by column chromatography, using a DCM:
hexane (50: 50) mixture to produce (0.090 g, 86%) of compound 4 as black solid. 1H NMR (400
MHz, CDCl3): δ ppm= 0.81 (t, J=7.28 Hz, 3 H) 1.11 - 1.19 (m, 2 H) 1.26 - 1.32 (m, 2 H) 3.30 (t,
J=7.40 Hz, 2 H) 7.20 - 7.25 (m, 1 H) 7.46 (t, J=7.65 Hz, 3 H) 7.52 - 7.56 (m, 1 H) 7.64 (dt, J=15.62,
7.62 Hz, 2 H) 7.73 - 7.82 (m, 9 H) 8.09 - 8.14 (m, 3 H) 8.19 - 8.25 (m, 6 H) 8.52 (d, J=8.28 Hz, 1
H) 8.72 (d, J=5.02 Hz, 1 H) 8.86 (d, J=4.77 Hz, 1 H) 8.90 - 8.95 (m, 3 H) 9.28 (s, 1 H). 13C NMR
(100 MHz, CDCl3): δ ppm= 13.62, 20.01, 29.70, 39.32, 95.17, 96.48, 120.30, 120.77, 121.22,
121.32, 121.50, 121.92, 124.21, 126.52, 126.65, 126.74, 127.58, 127.74, 128.36, 128.55, 129.80,
130.82, 131.17, 131.98, 132.38, 132.56, 132.89, 133.15, 134.23, 134.47, 138.90, 142.04, 142.57,
142.59, 163.95, 164.10. HRMS (ESI-TOF): m/z calculated for C62H41N5O2Zn [M+nH]+ 952.2624,
found 952.2644.
Synthesis of 4-((17-bromo-5,10,15,20-tetraphenylporphyrin-7-yl)ethynyl)-N,N-diphenyl
aniline (12a).
4-ethynyl-N, N-diphenyl aniline 9 (0.078 g, 0.28 mmol), TPPBr2 mixture 11 (0.45 g, 0.58
mmol) in THF: TEA (1: 1, v/v), palladium(0)-tetrakis(triphenylphosphine) (0.100 g, 0.08 mmol),
and CuI (0.010 g, 0.05 mmol) were added under argon atm at room temperature. The reaction
mixture was stirred for 12 h at 70 °C, and then cooled to room temperature. The solvent was then
evaporated under reduced pressure and the resultant mixture was diluted with DCM and the
organic layer was collected, dried over anhydrous Na2SO4 and evaporated under vacuum. The
solid was adsorbed on silica gel and purified by column chromatography, using a DCM: hexane
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(20: 80) mixture to produce (0.211 g, 79%) compound 12 mixture with slight separation of
substituted TPPBr2 mixture on TLC Figure S13. However, compound 12 isomer were separated
by silica gel column chromatography. The separation of the porphyrin 12a isomer was evidenced
from the 1H NMR characterization Figure S14. However, it has to be mentioned that we have clear
1H NMR evidence that only one dibrominated product was isolated as black solid. 1H NMR of 12
isomers (400 MHz, CDCl3): δ ppm= -2.79, -2.67 (s,s, 4 H) 6.95 - 7.02 (m, 4 H) 7.05 - 7.12 (m, 4
H) 7.13 - 7.22 (m, 12 H) 7.29 - 7.35 (m, 8 H) 7.69 - 7.80 (m, 23 H) 8.04 - 8.11 (m, 3 H) 8.15 - 8.25
(m, 13 H) 8.67 - 8.78 (m, 4 H) 8.78 - 8.92 (m, 7 H) 8.95 - 8.99 (d, 1 H) 9.03 (s, 1 H). 1H NMR of
12a (400 MHz, CDCl3): δ ppm= -2.65 (s, 2 H) 7.01 (d, J=8.55 Hz, 2 H) 7.10 (t, J=7.32 Hz, 2 H)
7.15-7.25 (m, J=8.54 Hz, 6 H) 7.30 - 7.37 (m, 4 H) 7.55 (d, J=8.55 Hz, 1 H) 7.70 - 7.80 (m, 11 H)
8.19 - 8.24 (m, 7 H) 8.72 (d, J=4.88 Hz, 1 H) 8.78 (s, 2 H) 8.82 (d, J=4.88 Hz, 1 H) 8.87 (s, 2 H)
9.04 (s, 1 H). 13C NMR (100 MHz, CDCl3): δ ppm= 85.5, 99.6, 119.9, 120.1, 120.5, 121.9, 123.5,
125.0, 126.7, 126.8, 127.8, 127.9, 128.6, 129.4, 133.1, 134.6, 141.9, 142.1, 147.3, 147.6. HRMS
(ESI-TOF): m/z calculated for C64H42N5Br [M+2] + 962.2681, found 962.2686.
Synthesis of 2-butyl-6-((17-((4-(diphenylamino)phenyl)ethynyl)-5,10,15,20-tetraphenyl
porphyrin-7-yl)ethynyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (5).
4-((17-bromo-5,10,15,20-tetraphenylporphyrin-7-yl)ethynyl)-N,N-diphenylaniline
12a
(0.100 g, 0.10 mmol), 2-butyl-6-ethynyl-1H-benzo[de]isoquinoline-1,3(2H)-dione 8 (0.034 g,
0.12 mmol) in THF: TEA (1: 1, v/v), palladium(0)-tetrakis(triphenylphosphine) (0.070 g, 0.06
mmol), and CuI (0.010 g, 0.05 mmol) were added under argon atm at room temperature. The
reaction mixture was stirred for 12 h at 70 °C, and then cooled to room temperature. The solvent
was then evaporated under reduced pressure and the resultant mixture was diluted with DCM and
the organic layer was collected, dried over anhydrous Na2SO4 and evaporated under vacuum. The
solid was adsorbed on silica gel and purified by column chromatography, using a DCM: hexane
1
(25: 75) mixture to produce (0.054 g, 47%) compound 5 as black solid. H NMR (400 MHz,
CDCl3): δ ppm= -2.66 (s, 2 H) 1.01 (t, J=7.63 Hz, 3 H) 1.48 (dd, J=14.95, 7.63 Hz, 2 H) 1.69 -
1.79 (m, 2 H) 4.19 (t, J=7.63 Hz, 2 H) 6.99 (d, J=7.93 Hz, 2 H) 7.08 (t, 2 H) 7.15 - 7.21 (m, 5 H)
7.26 - 7.35 (m, 5 H) 7.50 (td, J=7.63, 3.66 Hz, 2 H) 7.60 - 7.66 (m, 1 H) 7.69 - 7.80 (m, 11 H) 8.19
- 8.26 (m, 8 H) 8.47 (t, J=7.32 Hz, 1 H) 8.52 - 8.61 (m, 2 H) 8.69 (s, 1 H) 8.74 (s., 1 H) 8.84 (d,
J=7.93 Hz, 1 H) 8.91 (s, 1 H) 8.98 (d, J=4.27 Hz, 1 H) 9.16 (d, J=3.05 Hz, 1 H). 13C NMR (100
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MHz, CDCl3): δ ppm= 13.9, 20.4, 22.7, 29.70, 40.3, 85.5, 95.2, 96.0, 100.0, 121.8, 123.5, 125.0,
126.8, 126.9, 128.7, 129.4, 130.9, 131.4, 133.1, 134.4, 134.6, 140.9, 141.1, 141.7, 141.9, 147.3,
147.6, 163.9, 164.1. HRMS (ESI-TOF): m/z calculated for C82H56N6O2 [M+nH] + 1157.4538,
found 1157.4544.
Synthesis of 2-butyl-6-((17-((4-(diphenylamino)phenyl)ethynyl)-5,10,15,20-tetraphenyl
porphyrin -7-yl)ethynyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione, Zinc (6).
2-butyl-6-((17-((4-(diphenylamino)phenyl)ethynyl)-5,10,15,20-tetraphenylporphyrin-7-
yl)ethynyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione 5 (0.050 g, 0.04 mmol), Zn (OAc)2, (0.104g,
0.47 mmol) in MeOH: CHCl3 (3 : 1, v/v), were added and the reaction mixture was stirred for 1 h
at room temperature. The solvent was then evaporated under reduced pressure and the resultant
mixture was diluted with DCM/water and the organic layer was collected, dried over anhydrous
Na2SO4 and evaporated under vacuum. The solid was adsorbed on silica gel and purified by
column chromatography, using a DCM: hexane (40: 60) mixture to produce (0.039 g, 81%) of
compound 6 as green solid. 1H NMR (400 MHz, CDCl3): δ 9.28 (s, 1H), 9.15 (s, 1H), 8.91 (s, 1H),
8.84 (dd, J = 4.7, 1.6 Hz, 1H), 8.74 (dd, J = 4.7, 3.2 Hz, 1H), 8.67 (s, 1H), 8.60 (d, J = 8.9 Hz, 1H),
8.48 (d, J = 7.2 Hz, 1H), 8.14 – 8.24 (m, 8H), 7.68 – 7.78 (m, 11H), 7.47 – 7.51 (m, 3H), 7.32 (t,
J = 12.9, 4.8 Hz, 5H), 7.21 – 7.24 (dd, 2H), 7.16 (d, J = 8.4 Hz, 4H), 7.09 (t, J = 7.3 Hz, 2H), 7.00
(m, 2H), 3.91 (t, 2H), 2.33 (t, J = 7.5 Hz, 2H), 1.35 – 1.41 (m, 2H), 0.95 (t, 3H). MS (MALDI-
TOF) m/z: calculated for C82H54N6O2Zn [M] + 1220.75, found 1220.60.
Acknowledgements
Support from the DST, (DST/TMD/SERI/D05 (C)), INSA (SP/YSP/139/ 2017/2293),
Govt. of India, New Delhi, and US-National Science Foundation (1401188 to FD) is gratefully
acknowledged.
Conflict of interest
The authors declare no conflict of interest.
Keywords: push-pull porphyrins, excited state charge separation, fluorescence quenching,
spectroelectrochemistry, cyclic voltammetry.
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Table of contents
Push-Pull Porphyrins via -Pyrrole Functionalization: Evidence of Excited
State Events Leading to High-Potential Charge-Separated States
S. Bijesh, Youngwoo Jang, Rajneesh Misra,* Francis D’Souza*
Push-Pull for Charge Separation: Creation of high-potential charge separated
states in a series of push-pull porphyrins fearing triphenylamine push groups and
naphthalene imide pull groups is reported.
25
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