Modulation of Energy Transfer into Sequential Electron Transfer upon Axial Coordination of Tetrathiafulvalene in an Aluminum(III) Porphyrin-Free-Base Porphyrin Dyad

Article abstract of DOI:10.1021/acs.inorgchem.5b01190

Axially assembled aluminum(III) porphyrin based dyads and triads have been constructed to investigate the factors that govern the energy and electron transfer processes in a perpendicular direction to the porphyrin plane. In the aluminum(III) porphyrin-fr

Full text of DOI:10.1021/acs.inorgchem.5b01190

Article
pubs.acs.org/IC
Modulation of Energy Transfer into Sequential Electron Transfer
upon Axial Coordination of Tetrathiafulvalene in an Aluminum(III)
Porphyrin−Free-Base Porphyrin Dyad
Prashanth K. Poddutoori,*,† Lucas P. Bregles,† Gary N. Lim,‡ Patricia Boland,† Russ G. Kerr,†
and Francis D’Souza*,‡
†Department of Chemistry, University of Prince Edward Island, 550 University Avenue, Charlottetown, PE C1A 4P3, Canada
‡Department of Chemistry, University of North Texas, 1155 Union Circle, #305070, Denton, Texas 76203-5017, United States
S
* Supporting Information
ABSTRACT: Axially assembled aluminum(III) porphyrin based dyads and
triads have been constructed to investigate the factors that govern the energy
and electron transfer processes in a perpendicular direction to the porphyrin
plane. In the aluminum(III) porphyrin−free-base porphyrin (AlPor-Ph-
H2Por) dyad, the AlPor occupies the basal plane, while the free-base
porphyrin (H2Por) with electron withdrawing groups resides in the axial
position through a benzoate spacer. The NMR, UV−visible absorption, and
steady-state fluorescence studies confirm that the coordination of pyridine
appended tetrathiafulvalene (TTF) derivative (TTF-py or TTF-Ph-py) to the
dyad in noncoordinating solvents afford vertically arranged supramolecular
self-assembled triads (TTF-py→AlPor-Ph-H2Por and TTF-Ph-py→AlPor-Ph-
H2Por). Time-resolved studies revealed that the AlPor in dyad and triads
undergoes photoinduced energy and/or electron transfer processes.
Interestingly, the energy and electron donating/accepting nature of AlPor can be modulated by changing the solvent polarity
or by stimulating a new competing process using a TTF molecule. In modest polar solvents (dichloromethane and o-
dichlorobenzene), excitation of AlPor leads singlet−singlet energy transfer from the excited singlet state of AlPor (1AlPor*) to
H2Por with a moderate rate constant (kEnT) of 1.78 × 108 s−1. In contrast, excitation of AlPor in the triad results in ultrafast
electron transfer from TTF to 1AlPor* with a rate constant (kET) of 8.33 × 109−1.25 × 1010 s−1, which outcompetes the energy
transfer from 1AlPor* to H2Por and yields the primary radical pair TTF+•-AlPor−•-H2Por. A subsequent electron shift to H2Por
generates a spatially well-separated TTF+•-AlPor-H2Por−• radical pair.
known about these processes in the perpendicular (axial)
direction to the porphyrin plane.26,29−41 One reason for this is
the difficulty associated with attaching different ligands to the
porphyrin in the axial position. In this respect, aluminum(III)
porphyrins (AlPors) are unique as they form (i) covalent bonds
with carboxylic acids42−49 or alcohols50−53 and (ii) coordina-
tion bonds with Lewis bases42,44,45,54,55 (e.g., pyridine or
imidazole). Since AlPor can form two different types of axial
bonds, it is possible to attach two different molecular
components to the opposite faces. These advantages of AlPor
provide an excellent opportunity to study the influence of
electronic coupling, orientation, and reorganization energy on
the energy and electron transfer processes in the perpendicular
direction to the porphyrin plane.42−45,56,57 Moreover, this
particular geometry not only keeps the axial components
spatially well separated from each other but also prevents
aggregation between the molecules, which is a most common
problem in porphyrin based D−A systems.
INTRODUCTION
■
Inspired by natural photosynthesis, many donor−acceptor (D−
A) systems have been synthesized to mimic the function of
light harvesting antenna1−5 and reaction center6−9 complexes.
These studies aim not only at a better understanding of the
photoinduced energy transfer (EnT) and electron transfer
(ET) processes but also at building artificial photosynthetic
systems for solar energy conversion and storage applications as
well as photo- and electronic devices.10−21 In most of these
artificial systems, porphyrin (Por) has emerged as a promising
building block and an attractive chromophore because they
have structural similarities with chlorophyll as they absorb
strongly in the visible region, are often highly fluorescent, and
have rich redox chemistry, and most importantly, their optical
and redox properties are easily tunable.22 In combination with
other photo and redox active species, porphyrins have been
used to produce a wide array of covalent and noncovalent
multicomponent systems to mimic the light harvesting and
reaction center complexes of photosynthetic systems.6,8,17,23−28
However, most of these model compounds deal with the
photoinduced processes along the porphyrin plane. Little is
Received: May 26, 2015
© XXXX American Chemical Society
A
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Chart 1. Structures of the Investigated Compounds in This Study
Figure 1. Formation of vertically arranged self-assembled supramolecular triads through Lewis acid−base interactions.
In a photosystem, the light harvesting antenna and the
reaction center complex work together to perform the
conversion of solar energy into chemical energy.58−62 One of
the similarities between the light harvesting antenna and
reaction center complex is that they both utilize a chlorophyll
molecule as the chromophore. However, chlorophyll’s role is
entirely different as it serves as an energy donor in the light
harvesting antenna complex, whereas in the reaction center
complex the “chlorophyll special pair” performs predominantly
the role of an electron donor. This is because the surrounding
protein alters the chlorophyll’s spectral and redox properties for
a desired role in each of these complexes.63,64 For artificial
systems, there is no surrounding protein, and therefore, other
ways of tuning the spectral and redox potential of the
photosensitizer must be required. To mimic the role of the
chlorophyll in photosynthesis, here we report a new vertical
AlPor based dyad aluminum(III) porphyrin−free-base porphyr-
in (AlPor-Ph-H2Por, see Chart 1) and corresponding two
axially arranged self-assembled triads tetrathiafulvalene−
aluminum(III) porphyrin−free-base porphyrin (TTF-py→
AlPor-Ph-H2Por and TTF-Ph-py→AlPor-Ph-H2Por, Figure
1). We have chosen tetrathiafulvalene (TTF) as the secondary
donor because of its strong electron donating ability, which
makes it an excellent candidate as a reductive electron quencher
in D−A systems.25,65,66 As an acceptor, we have selected free-
base porphyrin (H2Por) with electron withdrawing fluorinated
phenyl substituents in its meso positions. Its positive redox
potentials and strong spectral overlap with AlPor makes the
selected H2Por as an acceptor for both energy and electron
transfer processes.67,68 Time-resolved studies have shown that
B
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M appropriate for measuring the porphyrin Q bands. A solution
containing the acceptor (A = AlPor-Ph-H2Por or AlPor-Ph) was
placed in a cuvette and titrated by adding aliquots of a concentrated
solution of the donor (D = TTF-py, TTF-Ph-py, or py). The donor
solution also contained the acceptor at its initial concentration so that
the porphyrin concentration remained constant throughout the
titration. The binding constants were calculated using the Benesi−
Hildebrand equation,69 [A]/Abs = (1/[D])(1/εK) + (1/ε), where,
[A] is the total concentration of bound and unbound acceptor and is
kept fixed, Abs is the absorption of the complex at the wavelength λ,
[D] is the total concentration of the donor which is varied, K is the
binding constant, and ε is the molar absorptivity of the D−A complex.
In an analogous manner, steady-state fluorescence titrations were
carried out in dry dichloromethane using solutions at a constant
concentration of A and varying concentrations of D. The solutions
were excited at the isosbestic point wavelength, which was obtained
from the corresponding absorption titrations.
Time-Resolved Fluorescence Spectroscopy. A time-correlated
single-photon-counting apparatus utilizing a picosecond-pulsed diode
laser was used to measure the porphyrin fluorescence decay. Excitation
pulses were delivered at 406 nm by a picosecond diode laser
(PicoQuant, PDL 800-B), 54 ps fwhm, at a repetition rate of 10 MHz.
The porphyrin fluorescence was measured by a Hamamatsu R3809
microchannel plate photomultiplier screened by a double mono-
chromator. A single-photon-counting PC card (Becker & Hickl, SPC-
730) was used for data collection. The instrument response time of the
system was 80 ps.
Femtosecond Laser Flash Photolysis. Femtosecond transient
absorption spectroscopy experiments were performed using an
Ultrafast Femtosecond Laser Source (Libra) by Coherent incorporat-
ing a diode-pumped, mode locked Ti:sapphire laser (Vitesse) and a
diode-pumped intracavity 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 a femto-
second 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. Ninety-five percent of
the fundamental output of the laser was introduced into a harmonic
generator that 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 the generation of a white light continuum. In
the present study, the second harmonic 400 nm excitation pump was
used in all of the experiments. The absorbance of AlPor and H2Por are
in ∼1:2 ratio at this excitation wavelength. 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.
these compounds undergo photoinduced energy and/or
electron transfer processes. We will also show that the energy
and electron donating nature of AlPor can be modulated by
changing the solvent polarity or by stimulating a new
competing process using the TTF molecule. In the AlPor-Ph-
H2Por dyad, AlPor serves as an energy donor to the H2Por. In
contrast, the same AlPor in the triad acts as an electron
acceptor/donor in the presence of a secondary electron donor
TTF moiety.
EXPERIMENTAL SECTION
■
Synthesis. All chemicals and solvents used in this study were
purchased from either Sigma-Aldrich Chemical Co. or Alfa-Asear. The
synthesis of 5,10,15,20-tetra(phenyl)-porphyrinatoaluminum(III)-
hydroxide (AlPor-OH), reference compound AlPor-Ph, and the
pyridine appended tetrathiafulvalene derivatives (TTF-py and TTF-
Ph-py) have been previously reported.43,45 Caution:! AlMe3 reacts
violently with water and therefore should be handled with great care. The
reaction must be performed in anhydrous conditions. The synthesis details
of precursor porphyrins, 5-(4-methylcarboxyphenyl)-10,15,20-tri-
(pentafluorophenyl)porphyrin (H2Por-Ph−COOCH3, from now on
referred to as H2Por-Ph) and 5-(4-carboxyphenyl)-10,15,20-tri-
(pentafluorophenyl)porphyrin (H2Por-Ph-COOH), are given in
Supporting Information.
Preparation of AlPor-Ph-H2Por. AlPor-OH (50 mg, 0.076 mmol)
and H2Por-Ph-COOH (71 mg, 0.077 mmol) were dissolved in 15 mL
of dry dichloromethane. Anhydrous Na2SO4 (100 mg, 0.70 mmol) was
added, and the resulting solution was stirred at room temperature
under nitrogen atmosphere. After 12 h, the solution was passed
through Na2SO4, and the solvent was removed under reduced
pressure. The obtained product was washed with hexane to get the
pure dyad as a purple solid. Yield: 110 mg (92%). Mass (ESI): m/z
1
1567.3076 [M + H]+, calculated 1567.3085 for C89H43AlF15O2N8. H
NMR (300 MHz, CDCl3): ppm 9.16 (s, 8H), 8.83 (s, 4H), 8.59 (d,
2H, J = 4.8 Hz), 8.36 (d, 2H, J = 4.8 Hz), 8.25 (d, 8H, J = 6.0 Hz),
7.76 (m, 12H), 7.29 (d, 2H, J = 8.0 Hz), 5.51 (d, 2H, J = 8.0 Hz), −
3.05 (bs, 2H).
Physical Methods. NMR Spectroscopy and Mass Spectrometry.
1
1H NMR and H−1H COSY spectra were recorded with a Bruker
Avance 300 MHz NMR spectrometer using CDCl3 as the solvent.
High-resolution mass spectrometry analysis was performed on an LTQ
Orbitrap Velos mass spectrometer (ThermoScientific) using an ESI
ion source operating in positive mode with a resolution of 30,000,
monitoring a mass range from 150 to 2000 atomic mass units (amu).
Voltammetry. Cyclic voltammetric experiments (in dichloro-
methane with 0.1 M tetrabutylammonium perchlorate (TBAClO4))
were performed on the Potentiostat/Galvanostat Model 283 (EG & G
Instruments, Princeton Applied Research) electrochemical analyzer
(working electrode, platinum; auxiliary electrodes, Pt wire; reference
electrode, Ag/AgCl). The Fc+/Fc (Fc = ferrocene, E1/2(Fc+/Fc) =
0.48 V vs SCE in CH2Cl2, 0.1 M TBAClO4 under our experimental
conditions) redox couple was used to calibrate the potentials, which
were reported in V vs SCE. Spectroelectrochemical study was
performed by using a cell assembly (SEC-C) supplied by ALS Co.,
Ltd. (Tokyo, Japan). This assembly comprises a Pt counter electrode,
a 6 mm Pt 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.
Steady-State UV−Visible Absorption and Emission Spectroscopy.
The UV/vis spectra were recorded with a Varian Cary 50 Bio UV−vis
spectrometer. Concentration of the samples used for these measure-
RESULTS AND DISCUSSION
■
Synthesis. A porphyrin dyad was prepared by previously
established methods42,44,45,70 in quantitative yields. Complete
experimental details are shown in Schemes S1 and S2 (see
Supporting Information) as well as in the Experimental Section.
The dyad (AlPor-Ph-H2Por) was prepared by reacting equal
molar ratios of AlPor-OH and H2Por-Ph-COOH, and the
reaction was monitored by NMR spectroscopy. The obtained
dyad was stored on a freshly prepared CaCl2 desiccator prior to
optical studies. The triads shown in Figure 1 were assembled by
using the dyad (AlPor-Ph-H2Por) and TTF-py/TTF-Ph-py
derivatives in noncoordinating solvents. Lewis acid−base
interactions were utilized to build these vertically arranged
supramolecular self-assembled triads. NMR, UV−visible
absorption, and steady-state fluorescence titrations were
employed to monitor the formation of triads. However, the
formed self-assembled triads could not be isolated.
ments ranged from 1 × 10−6 M (porphyrin Soret band) to 5 × 10−5
M
(Q-bands) solutions. Steady-state fluorescence spectra were recorded
using a Photon Technologies International LS-100 luminescence
spectrometer (L-format), equipped with a 70 W xenon lamp, running
with Felix software.
Absorption and Fluorescence Titrations. Absorption titrations
were carried out in dry dichloromethane at concentration of 6 × 10−5
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1
Figure 2. H NMR (300 MHz) spectra of TTF-py (top), AlPor-Ph-H2Por (middle), and TTF-py→AlPor-Ph-H2Por (bottom) in CDCl3.
Structural Characterization. Preliminary characterization
of the dyad was carried out by ESI mass spectrometry. The
mass spectrum of the dyad AlPor-Ph-H2Por showed peaks at
1567 and 639, which corresponds to the mass (m/z) of [M +
dichloromethane, and the spectra are shown in Figure 3. The
band positions (Q-bands and B- or Soret bands) and their
1
H]+ and [M − axial H2Por]+, respectively. The H NMR
spectra of the dyad AlPor-Ph-H2Por and one of its monomers
from which it was prepared, H2Por-Ph-COOH, are shown in
Figure S4. Shielding effects are apparent for the protons on the
axial porphyrin subunit. For example, resonances due to the
phenyl protons b and a (Figure 1), which appear at 8.58 and
8.37 ppm in the isolated H2Por-Ph-COOH porphyrin, are
strongly shifted upfield to 7.29 and 5.51 ppm, respectively in
the dyad AlPor-Ph-H2Por due to the ring current effect of the
porphyrin macrocycle. Similarly, resonances due to the β-
protons (c, d, and e) as well as inner NH protons are shifted
upfield compared to the corresponding resonances in the
spectrum of compound H2Por-Ph-COOH. These chemical
shifts (δ) agree well with those of the axial bonding type
porphyrin systems.42,44,45,70 Hence, the Δδ values (i.e., δmonomer
− δdyad) are a function of their proximity to the porphyrin ring.
Figure 3. UV−visible absorption spectra of dyad AlPor-Ph-H2Por
(green) and reference compounds H2Por-Ph (blue) and AlPor-Ph
(red) in dichloromethane.
1
Figure 2 shows the H NMR spectrum of the triad (TTF-
py→AlPor-Ph-H2Por (bottom), that is, a 1:1 mixture of AlPor-
Ph-H2Por and TTF-py) along with the individual spectra of
AlPor-Ph-H2Por (middle) and TTF-py (top). In the triad
complex, TTF-py→AlPor-Ph-H2Por, shielding due to the
porphyrin ring causes an upfield shift of the TTF-py protons
on the pyridine unit (f and g) and the TTF moiety (j and k).
The magnitude of the shift depends on the distance of the
protons from the porphyrin ring; hence, the pyridinyl protons
(f and g) display a large shift indicating that coordination
occurs via the pyridinyl group. On the benzoate bridging group
to the H2Por, the protons (a) closest to the porphyrin ring
show an increased upfield shift upon coordination, suggesting
that the aluminum(III) center lies out of the porphyrin plane in
AlPor-Ph-H2Por and is pulled into the plane when TTF-py
coordinates. Analogous results were obtained from the triad,
TTF-Ph-py→AlPor-Ph-H2Por (Figures S5).
molar extinction coefficients are summarized in Table 1. As
shown in Figure 3 and Table 1, the absorption spectrum of the
dyad is essentially a linear combination of its reference
porphyrins. Furthermore, the band positions and molar
extinction coefficients (ε) of the dyad are similar to
corresponding monomer porphyrins. Therefore, the majority
of the absorbance at 510, 547, and 639 nm is attributed to the
H2Por (87%), AlPor (90%), and H2Por (60%), respectively.
The absorption spectrum of the dyad AlPor-Ph-H2Por, also
measured in acetonitrile, is shown in Figure S6. Noticeable
differences are observed when compared with the spectrum in
dichloromethane. The AlPor bands are red-shifted (∼10 nm)
due to the coordination of the solvent at the Al center of the
AlPor. However, the band positions and molar extinction
coefficients remain similar to its monomeric compounds.
Overall, the absorption studies suggest that there exist no or
weak interactions between basal porphyrin (AlPor) and axial
porphyrin (H2Por). The absorption bands of AlPor and H2Por
UV−Visible Absorption Spectroscopy. The UV−visible
spectra of the dyad AlPor-Ph-H2Por and its reference
compounds (AlPor-Ph and H2Por-Ph) were measured in
D
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Table 1. UV−Visible Absorption and Steady-State Fluorescence Data of Investigated Compounds
fluorescence
a
absorption λmax (nm) (log ε)
dichloromethane λem (nm) (%Q)
acetonitrile λem, (nm) (%Q)
sample
AlPor-Ph
H2Por-Ph
AlPor-Ph-H2Por 639 (3.25), 586 (3.95), 547 (4.37), 510 (4.33) 415 (5.85)
Q-bands/TTF
B-band
λex = 550 nm
595, 646
λex = 640 nm
λex = 560 nm
608, 662
λex = 640 nm
585 (3.42), 547 (4.32), 510 (3.44)
416 (5.74)
639 (2.96), 585 (3.78), 540 (3.46), 510 (4.27) 415 (5.47)
642, 708
708
706
640, 704
704
596, 642, 708 (90%)
608, 660, 704 (91%)
706 (56%)
TTF-py
435 (3.45), 324 (4.17), 285 (4.22)
428 (3.64), 298 (4.48)
TTF-Ph-py
a
Spectra were measured in dichloromethane.
Figure 4. (a) Absorption and (b) fluorescence titrations of AlPor-Ph-H2Por with TTF-py in dichloromethane. The inset shows the Benesi−
Hildebrand plot of the change of absorbance at 603 nm. TTF-py was added up to 1.88 × 10−3 M in 20 μL (2.22 × 10−4 M) increments to 1 mL (6 ×
10−5 M) solution of AlPor-Ph-H2Por. The excitation wavelength was chosen at the isosbestic point, 555 nm, which was obtained from absorption
titrations.
are weakly overlapping; therefore, by choosing the wavelengths
of 550/560 and 640 nm it is possible to excite mostly AlPor and
the axial H2Por units, respectively, for steady-state fluorescence
studies. The py-appended TTF derivatives (TTF-py and TTF-
Ph-py) have relatively weak and very broad absorption bands
(see Figure S7) at λ = 304 (average of 285 and 324 nm bands)
and λ = 435 nm for TTF-py, and at λ = 298 and 428 nm for
TTF-Ph-py.
with NMR and absorption titrations, we conclude the
formation of the triad (TTF-py→AlPor-Ph-H2Por or TTF-
Ph-py→AlPor-Ph-H2Por) in the solution.
Cyclic Voltammetry. Cyclic voltammetry of the newly
investigated dyad and reference monomers were measured in
0.1 M TBAClO4 dichloromethane with ferrocene as an internal
standard. Representative voltammograms are shown in Figure
S11, and the data are summarized in Table 2. The redox
Figure 4a shows absorption titrations of TTF-py vs AlPor-
Ph-H2Por in dichloromethane. Upon addition of TTF-py, the
Q bands of the porphyrin 547 and 586 nm are shifted to 563
and 603 nm. An isosbestic point is observed at 555 nm,
indicating the formation of TTF-py→AlPor-Ph-H2Por in
equilibrium, and the shifts in the porphyrin bands are typical
of axial coordination of nitrogen ligands to AlPors.42,44,45,54,55
Benesi−Hildebrand analysis69 (Figure 4A, inset) gives a linear
plot indicating that a 1:1 complex is formed, and the slope
yields a binding constant K = 1.8 × 103 M−1. In a similar
fashion, the binding constant K was calculated for the titrations
of TTF-Ph-py vs AlPor-Ph-H2Por (Figure S8a), py vs AlPor-
Ph-H2Por (Figure S10a), and the data are summarized in Table
S1. Titrations TTF-py vs AlPor-Ph and TTF-Ph-py vs AlPor-Ph
and corresponding binding constants have been reported
previously.45 Titrations of TTF-py/TTF-Ph-py with AlPor-
Ph-H2Por have shown greater binding (∼2.0 × 103 M−1) than
that of the AlPor-Ph (∼1.0 × 103 M−1), which suggests that as
the electron withdrawing nature (Ph < H2Por) of the axial
ligand increases, the Al center becomes a better Lewis acid;
hence, it binds stronger with the Lewis base pyridine. Together
Table 2. Redox Potential Data of Investigated Compounds
in Dichloromethane with 0.1 M TBAClO4 as a Supporting
Electrolyte
potential (V vs SCE)
sample
oxidation
reduction
AlPor-Ph-H2Por
AlPor-Ph
0.91, 1.12, 1.54, 1.70
0.91
−0.87, −1.20, −1.35
−1.21
H2Por-Ph
1.51, 1.77
−0.87, −1.30
TTF-py
0.48, 0.83
TTF-Ph-py
0.47. 0.87
processes of all of the compounds are found to be one-electron
reversible based on the peak-to-peak separation values, and the
cathodic-to-anodic peak current ratio. The voltammogram of
the dyad is essentially a sum of the voltammograms of its
reference monomers AlPor-Ph and H2Por-Ph. During the
cathodic scan, the dyad showed three reduction processes. On
the basis of the respective monomers, the observed first and
third processes are assigned to the first and second reduction of
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ΔGCS = ECS − E0−0
where E1/2(D•+/D) is the first oxidation potential of the donor,
the axial porphyrin (H2Por) unit, whereas the second process is
assigned to the first reduction of the AlPor unit. Conversely, the
anodic scan reveals four oxidation processes for the dyad. The
first two processes are assigned to the AlPor, and later
processes belong to the axial H2Por unit. As anticipated, the
dyad showed a combination of processes from their respective
monomeric porphyrin units without any perturbation in their
redox potentials. Therefore, the observed cyclic voltammo-
grams and redox data suggest that the components of the dyad
do not influence one another significantly. The TTF derivatives
(TTF-py and TTF-Ph-py) show two processes corresponding
to the first and second oxidations of the TTF moiety. These
results have been published elsewhere.45
(2)
E
1/2(A/A•−) is the first reduction potential of the acceptor. GS
is ion-pair stabilization and incorporates both the solvent-
dependent Coulomb energy change upon ion-pair formation
and the free energy of solvation of the ions,
e2
πε0
1
1
2R−
1
1
1
2R
1
2R−
1
⎢
⎢
⎥
⎥
⎛
⎜
⎞
⎟
⎠
⎛
− ⎜
⎝
⎞
⎟
⎠
GS =
+
−
+
4
⎢ 2R
RD−A
ε
ε
⎥
⎦
⎝
⎣
+
S
+
R
(3)
where R+, R−, and RD‑A are the donor radius, acceptor radius,
and center-to-center distance between the donor and acceptor,
respectively.73 εS is the dielectric constant of the solvent used
for the photophysical studies (9.1 and 37.5 for dichloromethane
and acetonitrile, respectively). εR is the dielectric constant of
the solvent used for measuring the redox potentials, in this case
The redox potentials are used in combination with optical
data to estimate the energetics of the energy and electron
transfer reactions in the investigated compounds. Figure 5
dichloromethane. The lowest excited singlet state energy (E0−0
)
is estimated from the crossing point of absorption and
fluorescence spectra and found to be 579 and 620 nm for
AlPor and H2Por, respectively (see Figure S12). Similarly, the
band position of the phosphorescence spectrum (see Figure
S13) at 768 and 770 nm of AlPor-Ph and H2Por-Ph,
respectively, is taken as the lowest excited triplet state. The
calculated energy levels suggest that energy transfer (EnT),
electron transfer (ET), and electron shift (ES) processes are
energetically feasible in the investigated dyad and its
corresponding self-assembled supramolecular triads. With
acetonitrile as the solvent, the radical ion-pairs are found to
be slightly lower in energy than in dichloromethane (Figure
S14).
Fluorescence Spectroscopy. Steady-state fluorescence
measurements were performed in dichloromethane and
acetonitrile. Figure 6a illustrates the fluorescence spectra of
the dyad and its corresponding monomers in dichloromethane,
and the data are summarized in Table 1. As shown in Figure 6a,
excitation of the dyad at 550 nm (where 90% of light is
absorbed by AlPor) results in the appearance of fluorescence
bands due to H2Por (bands II and III) and AlPor (bands I and
II) components. It was found that the fluorescence band
maxima of AlPor and H2Por components are quite close to
those of AlPor-Ph or H2Por-Ph, respectively. However, the
fluorescence intensity of the AlPor component is strongly
quenched (90%, emission monitored at the band I; see Table
1) in comparison with that of the reference compound AlPor-
Figure 5. Energy level diagram for the dyad and its corresponding self-
assembled triads in dichloromethane. Black solid and dashed lines
represent energy transfer and electron transfer, respectively.
summarizes the energy levels of the dyad and its corresponding
supramolecular triads. The energies of the radical ion pair states
(ECS) and free energy change for the charge separation (ΔGCS)
are estimated using the Weller equation:71,72
ECS = e[E1/2(D•+/D) − E1/2(A/A•−)] + GS
(1)
Figure 6. (a) Steady-state fluorescence spectra of AlPor-Ph-H2Por (green), AlPor-Ph (red), and H2Por-Ph (blue) with excitation at 550 nm in
dichloromethane. (b) Absorption (solid) and corrected fluorescence excitation (dashed) spectra of the dyad in dichloromethane. The spectra were
normalized between 610 and 650 nm. The excitation spectrum was collected at 720 nm corresponding to the emission of H2Por.
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Figure 7. (a) Steady-state fluorescence spectra of AlPor-Ph-H2Por (green), AlPor-Ph (red), and H2Por-Ph (blue) with excitation at 560 nm in
acetonitrile. (b) Absorption (solid) and corrected fluorescence excitation (dashed) spectra of the dyad in acetonitrile, and the spectra were
normalized between 610 and 650 nm. The excitation spectrum was collected at 720 nm corresponding to the emission of H2Por.
Ph. In contrast, band III intensity increased with respect to
H2Por-Ph. These results clearly suggest that the AlPor first
excited singlet state (1AlPor*) is being strongly quenched by
H2Por through the singlet−singlet energy transfer mechanism.
The possibility of energy transfer is supported by significant
spectral overlap between the fluorescence spectrum of AlPor
and absorption spectrum of H2Por (Figure S15a). This is
further investigated by using the fluorescence excitation
spectrum. Figure 6b shows the corrected excitation spectrum
of the dyad in dichloromethane collected at 720 nm (emission
monochromator) where only the H2Por emits. Overlap of the
corrected and normalized excitation spectra with the
corresponding absorption spectra revealed that the singlet−
singlet energy transfer (EnT) efficiency is ∼70% for the dyad in
dichloromethane. Thus, it appears that the major portion (70%
overlap, the fluorescence excitation spectrum (collected at 720
nm, Figure 7b) of the dyad did not reveal AlPor excitation
bands (at 560 and 610 nm) suggesting that the energy transfer
1
is a minor process or is negligible from the AlPor* to H2Por
unit in polar acetonitrile. Hence, the observed strong quenching
can be attributed solely to the electron transfer process from
1AlPor* to the H2Por unit. The Gibbs free energy change
(ΔGCS) for this process was found to be −0.54 eV.
Furthermore, excitation of the dyad at 640 nm (where 60%
light is absorbed by H2Por) also resulted in quantitative
quenching of the 720 nm band, which is exclusively coming
from H2Por (Figure S16b). A careful examination of the energy
level diagram suggests that this quenching could be attributed
to the electron transfer from AlPor to 1H2Por*, and the
corresponding ΔGCS was found to be exergonic by −0.39 eV.
Figure 4b shows the fluorescence spectra of the AlPor-Ph-
H2Por dyad with increasing amounts of pyridine-appended
tetrathiafulvalene (TTF-py) in dichloromethane. The excitation
wavelength was adjusted to the isosbestic point at 555 nm
(where 95% of absorption is due to AlPor). In the absence of
TTF-py, the AlPor-Ph-H2Por dyad shows AlPor bands (I and
II) and H2Por bands (II and III) similar to those of its
monomeric compounds AlPor-Ph and H2Por-Ph. However,
their intensities were strongly quenched due to the energy
1
out of 90%) of AlPor* state is quenched due to the energy
transfer process. The remaining 20% quenching could be
attributed to the electron transfer (ET) from 1AlPor* to H2Por
as the Gibbs free energy change (ΔGCS) for this process was
found to be exergonic (−0.49 eV) in dichloromethane.
Furthermore, excitation of the dyad at 640 nm (where 60%
of light is absorbed by H2Por) showed a band exclusively due to
the free-base component in the ∼720 nm (band III) region,
and its intensity was found to be similar to the sum of the
calculated spectrum of H2Por-Ph + AlPor-Ph (Figure S15b).
Thus, we conclude that electron transfer from AlPor to the
excited singlet state of free-base porphyrin (1H2Por*) is weak
or negligible, although the corresponding ΔGCS is estimated to
be −0.34 eV.
1
transfer from AlPor* to axial H2Por. Upon addition of TTF-
py, the fluorescence bands of AlPor (I and II) shifted to longer
wavelengths, and the relative fluorescence intensities of all three
bands (I, II and III) are strongly quenched. These notable
spectral changes indicate the formation of the TTF-py→AlPor-
Ph-H2Por triad in the solution. However, to explain the
possible quenching mechanism/s various control titrations were
carried out. Upon addition of py (without the appended TTF
unit (Figure S10b)) or TTF (without the appended py group
(Figure S9b)) to the dyad, no considerable change in
fluorescence intensity was revealed. However, addition of
TTF-py to the reference monomer AlPor-Ph revealed an
To investigate the quenching processes in high polar
solvents, fluorescence was measured in acetonitrile. Figure 7a
illustrates the fluorescence spectra of the dyad and correspond-
ing monomers in acetonitrile. Solutions were excited at 560 nm,
where AlPor absorbs 95% of light. The band positions of the
dyad are quite similar to its reference monomeric porphyrins
(Figure 7a). However, two changes can be seen: (1)
fluorescence bands of AlPor are red-shifted (∼10 nm) in
comparison with dichloromethane data, and (2) fluorescence
intensity is strongly quenched (∼91%). The red shift can be
explained by the coordination of acetonitrile to AlPor. To
examine the quenching mechanism, spectral overlap between
AlPor fluorescence and H2Por absorption was estimated
(Figure S16a). Significant spectral overlap has been found,
although it is slightly smaller compared to the overlap in
dichloromethane solution. Despite having a strong spectral
1
electron transfer process from TTF to the AlPor* unit; these
results have been reported previously.42,45 Therefore, the most
likely explanation for the quenching of fluorescence bands (I
and II) is the electron transfer from TTF to 1AlPor* upon triad
formation. This process is exergonic (−0.59 eV) and has been
observed in other systems in which TTF was attached axially to
a porphyrin macrocycle.42,45,70,74 Insertion of an additional
phenyl spacer between the TTF and py units is expected to
decrease the electronic coupling and to slow down the electron
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transfer rate. Consistent with this expectation, bands I and II
are weakly quenched in titrations of TTF-Ph-py vs AlPor-Ph-
H2Por (see Figure S8b), that is, the formation of the TTF-Ph-
py→AlPor-Ph-H2Por triad. Regardless of the fluorescence
trends in bands I and II, intriguingly, band III was quenched
in both the triads upon addition of py appended TTF
derivatives. This could be due to newly introduced electron
of pyridine coordinated hexavalent AlPor bands (560 and 614
nm) and H2Por bands (590 and 640 nm) as well as a minor
uncomplexed pentavalent AlPor (550 nm). Interestingly, the
bands at 560 and 614 nm are completely absent in the
excitation spectra (dashed line). These results evidently suggest
that the energy transfer is not participating in the quenching of
1
the AlPor* state in the triad molecule. Overall, two changes
1
1
transfer process between the TTF and AlPor* units. As a
occur in the AlPor* state due to the presence of the TTF
consequence of the electron transfer, the 1AlPor* state is being
redox center: (1) it initiates the electron transfer from TTF to
1AlPor*, and (2) it inhibits the energy transfer from 1AlPor* to
H2Por. Moreover, these results suggest that the electron
transfer from TTF to 1AlPor* is presumably faster than that of
quenched rapidly by the TTF unit; hence, the energy transfer
1
from AlPor* to H2Por is expected to be blocked. Thus, band
III, which is predominantly the result of the energy transfer
process in triads (TTF-py→AlPor-Ph-H2Por and TTF-Ph-py→
AlPor-Ph-H2Por), reduces significantly. To verify this scheme,
the fluorescence excitation spectrum was collected for the
triads, TTF-py→AlPor-Ph-H2Por (Figure 8) and TTF-Ph-py→
1
the energy transfer from AlPor* to H2Por.
Femtosecond Laser Flash Photolysis. Femtosecond
transient absorption studies were performed in 1,2-dichlor-
obenzene (o-DCB) (instead of low boiling dichloroemethane)
and acetonitrile to secure the evidence of energy transfer,
electron transfer, and electron shift processes in the dyad and
triads. Samples were excited using 400 nm wavelength light
where the absorbance ratio of AlPor and H2Por is
approximately 1:2.
(a). In o-Dichlorobenzene. First, transient spectra of the
precursor compounds, AlPor-Ph and H2Por-Ph, were inves-
tigated. As shown in Figure S18a, excitation of AlPor-Ph
instantly populated the S1 and S2 states with a maximum in the
∼450 nm region. At the higher wavelength region, a depleted
signal in the Q-band region of AlPor (550 nm), an opposite
mimic of the ground state absorption of the Q-band, was
observed. In addition, depleted bands at 592 and 648 nm,
corresponding to the stimulated emission of Al porphyrin, were
observed. Positive peak maxima at 450, 576, 614, and 681 nm
were also observed. Interestingly, in the near-IR region, an
additional peak at 1256 nm was observed. The decay rate of
this peak lasted over 3 ns (see Figure S18b), the monitoring
window of our instrument setup, suggesting that this could be
due to the singlet−singlet excited state of AlPor.42 Similarly, the
instantly formed S1 and S2 states of H2Por-Ph upon excitation
revealed depleted peaks at 509, 533, and 585 nm corresponding
to their Q-band absorption, and at 647 and 720 nm
corresponding to stimulated emission of free-base porphyrin
(Figure S19a). Positive peaks at 440, 530, 558, 616, and 680 nm
were observed. In the near-infrared region, a peak at 1067 nm
was also observed corresponding to the singlet excited state of
free-base porphyrin. The excited singlet state lifetime (τS) of
Figure 8. Absorption (solid) and corrected fluorescence excitation
(dashed) spectra of the triad TTF-py→AlPor-Ph-H2Por in dichloro-
methane. The spectra were normalized between 610 and 650 nm. The
excitation spectrum was collected at 720 nm corresponding to the
emission of H2Por. TTF-py and AlPor-Ph-H2Por are present in 1.88 ×
10−3 M and 6 × 10−5 M in solution, respectively.
AlPor-Ph-H2Por (Figure S17), at 720 nm where the emission is
exclusively from the H2Por unit. As shown in Figure 8, the solid
curve represents the absorption spectra of the triad consisting
Figure 9. (a) Femtosecond transient absorption spectra and (b) decay profile at 1240 nm of AlPor-Ph-H2Por in o-DCB.
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Figure 10. (a) Femtosecond transient absorption spectra and (b) decay profile at 1240 nm of TTF-py→AlPor-Ph (1:3 ratio) in o-DCB.
Figure 11. Femtosecond transient absorption spectra of (a) AlPor-Ph-H2Por (black), TTF-py→AlPor-Ph-H2Por (red), and TTF-Ph-py→AlPor-Ph-
H2Por (blue) at a delay time of 500 ps, (b) decay profile at 1244 nm of TTF-py→AlPor-Ph-H2Por, and (c) decay profile at 1244 nm of TTF-Ph-
py→AlPor-Ph-H2Por in o-DCB.
AlPor and H2Por is measured using the time-resolved
fluorescence method and found to be 5.80 and 9.00 ns,
respectively (see Figure S20 for decay curves) in dichloro-
methane.
were observed. However, it was possible to secure evidence for
singlet−singlet energy transfer from AlPor to H2Por. At the
early time scale, the depleted peaks of AlPor located at 545 and
585 nm recovered faster than that observed for pristine AlPor,
while the negative growth of stimulated emission of H2Por at
646 and 716 nm was much faster than that seen in pristine
H2Por. This was also the case for the singlet peaks of H2Por at
1064 nm and AlPor at 1240 nm; the former grew in intensity at
the expense of the latter. By taking the lifetime and decay rate
constant of the AlPor peak at 1240 nm (3.31 ns, 3.02 × 108 s−1)
and using a lifetime and rate constant of 1AlPor* (5.80 ns, 1.72
× 108 s−1), a rate constant for energy transfer process, kEnT, in
this dyad was estimated to be 1.78 × 108 s−1. As predicted, the
coordination of py to AlPor-Ph-H2Por (which forms a hexa-
valent Al center) had no noticeable changes in its spectral
features (Figure S24).
To help interpret the transient spectral data of charge
separation products, spectroelectrochemical studies were
performed on H2Por, AlPor, and TTF (Figures S21−S23).
The one-electron reduced product of H2Por-Ph (Figure S21)
revealed radical anion peaks at 443, 557, 588, and 632 nm. The
Soret band of H2Por located at 415 nm revealed a red-shift of
28 nm and appeared at 443 nm. The one-electron oxidized
product of AlPor-Ph (Figure S22, top spectra) showed peaks at
447, 600, and 685 nm along with broad peaks at 791 and 882
nm. The radical anion of AlPor-Ph (Figure S22, bottom
spectra) exhibited peaks at 432, 570, 610, and 645 nm. The
Soret band red-shifted from 418 to 432 nm. The radical cation
peaks of TTF were located at 450 and 610 nm (Figure S23).
As shown in Figure 9a, femtosecond transient spectra
obtained for the AlPor-Ph-H2Por dyad were rather complex,
revealing peaks corresponding to both AlPor and H2Por
entities. That is, negative peaks at 509, 545, 645, and 716, and
positive peaks at 448, 528, 565, 618, 671, 1064, and 1240 nm
From steady-state experiments, it was postulated that
1
quenching of the AlPor* state in the dyads TTF-Phn-py→
AlPor-Ph (n = 0, 1) is due to the electron transfer from TTF to
1
the AlPor* unit. To verify this claim, femtosecond transient
absorption studies were performed on these dyads. As shown in
Figure 10a, upon excitation of the dyad (TTF-py→AlPor-Ph),
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Figure 12. (a) Femtosecond transient absorption spectra of AlPor-Ph (red), H2Por-Ph (blue), and AlPor-Ph-H2Por (green) at 200 ps, and (b) time
profile at ∼1205 nm of AlPor-Ph-H2Por in acetonitrile.
absorbance with a maximum in the ∼450 nm region
triad, which indicates that the electron transfer is slightly slower
because of the extra phenyl ring between the TTF and AlPor
units. The formation of TTF+• and AlPor−•, expected to result
in absorption changes in the 450−500 nm and 600−650 nm
region, respectively, were also observed, although they were
overlapped with other peaks in this wavelength region. After 1
ps, the visible region is obscured by intense spectral features
from the dyad (unbound 75%). Hence, it was challenging to
track the time course of electron shift and charge recombina-
tion processes. However, further evidence for electron transfer
came from the near-IR peak of AlPor as it undergoes an
additional quenching (Figure 11b and c and Figure 9b).
Whereas, evidence for the electron shift from AlPor−• to H2Por
is inferred from the observed low efficiency of energy transfer
1
characteristic of the AlPor* state was observed. Subsequently,
the 1AlPor* state evolved into the TTF+•AlPor−• charge-
separated state with a rate constant (kET) of 1.25 × 1010 s−1
(see Figure 10b). This state can be readily identified by
distinctive peaks for TTF+• in the ∼496 nm region, and at
∼570 and ∼610 nm for the counterpart AlPor−•. Similarly, the
transient absorption spectra of the TTF-Ph-py→AlPor-Ph dyad
was measured, and the spectra are shown in Figure S25. As
anticipated, the charge-separated state TTF+•AlPor−• formed
with a rate constant of 8.33 × 109 s−1, which is slightly slower
than the rate constant in TTF-py→AlPor-Ph. This is because of
the increased distance between TTF and AlPor in TTF-Ph-
py→AlPor-Ph due to an extra phenyl spacer unit. Overall, these
results conclude the occurrence of electron transfer in TTF-
Phn-py→AlPor-Ph (n = 0, 1) dyads that are consistent with
steady-state fluorescence results.
1
from AlPor* to H2Por. As shown in Figure S27, the depleted
emission of the free-base porphyrin at ∼647 nm decreases,
1
which suggest the AlPor* state is involved in the electron
For transient absorption studies, triads (TTF-py→AlPor-Ph-
H2Por and TTF-Ph-py→AlPor-Ph-H2Por) were prepared by
adding the TTF-py or TTF-Ph-py (0.15 mM) to AlPor-Ph-
H2Por (0.05 mM) in o-DCB. The resultant solutions were
excited at 400 nm where the absorption ratio of AlPor and
H2Por is 1:2. As shown in Figure S26, the transient spectra of
the triads appeared to be even more complex, and identifying
spectral features was a challenging task. This is because of two
reasons: (i) due to the moderate binding constant between
TTF-py (or TTF-Ph-py) and AlPor-Ph-H2Por, the exper-
imental solutions comprised only ∼25% (0.013 mM) of the
triad and the remaining ∼75% of the sample existed as
individual components, and (ii) due to the strong spectral
presence of AlPor and H2Por in the monitoring visible region, it
was difficult to isolate the TTF+•, AlPor−•, and H2Por−• radical
ion peaks. However, the early spectral features (<1 ps after
excitation) to some extent suggest the occurrence of electron
transfer and electron shift processes in the presence of a TTF
unit. Figure 11 shows the absorption changes at 500 fs after
excitation of the sample. There are significant differences in
spectral features of the dyad and its corresponding triads. The
absorbance change (ΔA) diminished to half for the TTF-py→
AlPor-Ph-H2Por triad compared to its reference dyad (AlPor-
transfer process rather than energy transfer in the triad (TTF-
py→AlPor-Ph-H2Por or TTF-Ph-py→AlPor-Ph-H2Por) mole-
cule.
(b). In Acetonitrile. To establish evidence for charge-
separation in polar solvents, femtosecond transient absorption
studies were performed on the dyad (AlPor-Ph-H2Por) and its
reference monomers (AlPor-Ph and H2Por-Ph) in acetonitrile.
As shown in Figure S28a, for AlPor-Ph there were instantly
populated S1 and S2 states, bleaching due to both absorption
(558 nm) and emission (598 and 660 nm) peaks, and positive
peak maxima (450, 580, 627, and 704 nm), and a near-infrared
(1205 nm) peak was observed. The other reference compound,
H2Por-Ph (Figure S29a), showed instantaneous formation of
the S1 and S2 states, bleaching due to the absorption (505, 539,
and 581 nm), and stimulated emission (635 and 707 nm)
peaks, positive peaks (440, 530, 558, 616, and 680 nm), and
also the near-infrared (1023 nm) peak. The observed near-
infrared peaks for AlPor-Ph and H2Por-Ph correspond to
singlet−singlet transition, and they decayed slower than the 3
ns monitoring window of our instrument setup (Figures S28b
and S29b). Figure 12 shows the femtosecond transient
absorption spectra of AlPor-Ph-H2Por in acetonitrile at 200
ps (see Figure S30 for spectra at other delay times). The
spectra revealed peaks from both the AlPor and H2Por units.
However, the observed peaks decayed rapidly, while the
bleaching at 505 and 558 nm was recovered faster than their
pristine compounds. The formation of AlPor+• and H2Por−•,
1
Ph-H2Por) suggesting a rapid quenching of the AlPor* state
by the electron transfer from the TTF unit. In the case of the
TTF-Ph-py→AlPor-Ph-H2Por triad, the absorbance change was
found to be higher than that of the TTF-py→AlPor-Ph-H2Por
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Figure 13. Proposed modulation of photoinduced processes in the dyad and its corresponding triads.
1
anticipated to result in absorption changes in the 540−590 nm
and 600−650 nm region, respectively, was also observed
(Figure 12a, inset). By monitoring the near-infrared peak of
AlPor, the rate of charge separation was estimated to be 1.79 ×
108 s−1 (Figures 12b). Transient absorption studies on triad
solutions were not carried out because the experimental
solutions contain a very small amount of the triad molecule.
These lower concentrations are caused by the competition
between TTF-Phn-py and CH3CN binding to the Al center.
Modulation of Energy Transfer into Electron Transfer.
Figure 13 summarizes the photoinduced processes that are
involved in the dyad and its corresponding triads. Upon
photoexcitation of AlPor in the dyad in noncoordinating
solvents (dichloromethane or o-DCB), the AlPor moiety serves
as an antenna by transferring its singlet excitation to the H2Por
with a rate constant (kEnT) of 1.78 × 108 s−1. In contrast, the
same AlPor in the dyad acts as an electron acceptor in the
presence of TTF (i.e., formation of triad) and it initiates a
sequential electron transfer between TTF, AlPor, and H2Por
components. In triads, photoexcitation of the AlPor results in
rapid electron transfer from TTF to 1AlPor* with a rate
constant (kET) of 8.33 × 109 − 1.25 × 1010 s−1 to yield a
primary radial pair TTF+•-AlPor−•-H2Por. This is followed by
an electron shift to produce the final charge separated state
TTF+•-AlPor-H2Por−•. The electron transfer between TTF and
1AlPor* redox centers was found to be nearly two orders of
magnitude faster than the energy transfer between 1AlPor* and
H2Por entities. Consequently, the electron transfer outperforms
the energy transfer process and generates the primary radical
ion-pair, which ultimately undergoes an electron shift to yield
the final radical ion-pair. Therefore, these results represent the
modulation of energy transfer into sequential electron transfer
via axial coordination of the TTF moiety.
than the energy transfer from AlPor* to H2Por. These results
can be explained by using the electronic coupling between TTF
and AlPor units as well as the Gibbs free energy change. Our
recent DFT studies on TTF-Phn-py→AlPor-Ph (n = 0,1),
where the py appended TTF derivatives were bound to pristine
AlPor-Ph through a coordination bond in the axial direction,
revealed a significant delocalization of their molecular orbitals
(HOMO and LUMO) onto the bridging unit.42,45 As
anticipated, the HOMO and LUMO are localized primarily
on the TTF and AlPor units, respectively. However, in both
cases some delocalization onto the Ph and/or Py bridge occurs.
This results in a substantial electronic coupling between donor
(TTF) and acceptor (AlPor) units, which could potentially
favor the electron transfer process. Furthermore, Figure 5
reveals that the electron transfer process is more exergonic
(−0.59 eV) than the energy transfer (−0.15 eV) process.
Together with strong electronic coupling and large driving
1
force, it is reasonable to expect that the AlPor* state in the
triad would quench strongly by rapid electron transfer rather
than energy transfer.
CONCLUSIONS
■
The above presented results show that it is possible to
construct multicomponent “donor-AlPor-acceptor” systems in
the axial direction by exploiting the unique properties of AlPor.
Excited state properties revealed that energy transfer (major)
and electron transfer (minor) from 1AlPor* to H2Por coexist in
the dyad (AlPor-Ph-H2Por) molecule. However, the relative
ratio can be modulated (i) by changing the solvent polarity or
(ii) by converting the dyad (AlPor-Ph-H2Por) into a triad
(TTF-Phn-py→AlPor-Ph-H2Por, n = 0,1) using the secondary
electron donor (TTF) through axial linkage. These results
represent the first such example where the energy donor is
converted into an electron acceptor/donor through axial
coordination of the TTF moiety. However, the pyridine unit
was found to be less optimal for linking the TTF to the dyad
molecules. Because of its moderate binding constant, the
equilibrium in triad solutions always pushed more toward the
dyad in our experimental solutions (Figure 1). This hampered
the tracking of electron shift and charge recombination
processes in the triad molecule. Recent studies suggest that
As revealed by transient spectral studies, energy transfer is
1
the main quenching mechanism for AlPor* in the dyad and
AlPor-Ph-H2Por in nonpolar solvents. However, this is no
longer the case in the triads (TTF-Phn-py→AlPor-Ph-H2Por, n
= 0,1) where a new decay pathway for the 1AlPor* state opens
up in the presence of the electron rich TTF moiety. The optical
studies identified electron transfer as the new process from
TTF to 1AlPor*, and moreover, it was found to be much faster
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b01190.
Synthesis details, NMR, and absorption spectra, titrations
(absorption and florescence), binding constants, estima-
tion of E0−0 energies, cyclic voltammetry, phosphor-
escence spectra, steady-state optical data (spectral
overlap, fluorescence spectra, and excitation spectra),
spectroelectrochemistry, and transient absorption spectra
(PDF)
(29) Wilson, S. R.; MacMahon, S.; Tat, F. T.; Jarowski, P. D.;
Schuster, D. I. Chem. Commun. 2003, 226.
(30) Trabolsi, A.; Elhabiri, M.; Urbani, M.; de la Cruz, J. L. D.;
Ajamaa, F.; Solladie, N.; Albrecht-Gary, A. M.; Nierengarten, J. F.
Chem. Commun. 2005, 5736.
AUTHOR INFORMATION
Corresponding Authors
*(P.K.P.) E-mail: ppoddutoori@upei.ca.
*(F.D.) E-mail: Francis.DSouza@unt.edu.
■
(31) Stangel, C.; Schubert, C.; Kuhri, S.; Rotas, G.; Margraf, J. T.;
Regulska, E.; Clark, T.; Torres, T.; Tagmatarchis, N.; Coutsolelos, A.
G.; Guldi, D. M. Nanoscale 2015, 7, 2597.
Notes
The authors declare no competing financial interest.
(32) Schuster, D. I.; Cheng, P.; Jarowski, P. D.; Guldi, D. M.; Luo, C.
P.; Echegoyen, L.; Pyo, S.; Holzwarth, A. R.; Braslavsky, S. E.;
Williams, R. M.; Klihm, G. J. Am. Chem. Soc. 2004, 126, 7257.
(33) Kim, H. J.; Park, K. M.; Ahn, T. K.; Kim, S. K.; Kim, K. S.; Kim,
D. H.; Kim, H. J. Chem. Commun. 2004, 2594.
ACKNOWLEDGMENTS
■
This work was supported by the Department of Chemistry,
University of Prince Edward Island, Canada and the US-
National Science Foundation (Grant No. 1401188 to F.D.).
R.K. acknowledges financial support from the Natural Sciences
and Engineering Research Council (NSERC) and the Canada
Foundation for Innovation (CFI). Also, we thank Professor Art
van der Est and Professor Melanie Pilkington (Department of
Chemistry, Brock University) for providing resources for the
synthesis of TTF derivatives. Dr. Serguei Vassiliev is thanked
for the time-resolved fluorescence studies.
(34) Fukuzumi, S.; Honda, T.; Ohkubo, K.; Kojima, T. Dalton Trans.
2009, 3880.
(35) Fazio, M. A.; Durandin, A.; Tkachenko, N. V.; Niemi, M.;
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