Ultrafast singlet - singlet energy transfer in self-assembled via metal - ligand axial coordination of free-base porphyrin - zinc phthalocyanine and free-base porphyrin - zinc naphthalocyanine Dyads

Article abstract of DOI:10.1021/jp908115e

Singlet - singlet energy transfer in self-assembled via axial coordination of imidazole-appended (at different positions of one of the meso-phenyl entities) free-base tetraphenylporphyrin, H₂PIm, to either zinc phthalocyanine, ZnPc, or zinc nap

Full text of DOI:10.1021/jp908115e

2
68
J. Phys. Chem. A 2010, 114, 268–277
Ultrafast Singlet-Singlet Energy Transfer in Self-Assembled via Metal-Ligand Axial
Coordination of Free-Base Porphyrin-Zinc Phthalocyanine and Free-Base Porphyrin-Zinc
Naphthalocyanine Dyads
†
‡
‡
,‡
Eranda Maligaspe, Tatu Kumpulainen, Helge Lemmetyinen, Nikolai V. Tkachenko,*
†
†
,†
Navaneetha K. Subbaiyan, Melvin E. Zandler, and Francis D’Souza*
Department of Chemistry, Wichita State UniVersity, 1845 Fairmount, Wichita, Kansas 67260-0051, and
Department of Chemistry and Bioengineering, Tampere UniVersity of Technology,
P.O. Box 541, 33101 Tampere, Finland
ReceiVed: August 22, 2009; ReVised Manuscript ReceiVed: October 17, 2009
Singlet-singlet energy transfer in self-assembled via axial coordination of imidazole-appended (at different
positions of one of the meso-phenyl entities) free-base tetraphenylporphyrin, H PIm, to either zinc
2
phthalocyanine, ZnPc, or zinc naphthalocyanine, ZnNc, dyads is investigated in noncoordinating solvents,
o-dichlorobenzene and toluene, using both steady-state and time-resolved transient absorption techniques.
The newly formed supramolecular dyads were fully characterized by spectroscopic, computational, and
electrochemical methods. The binding constants measured from optical absorption spectral data were found
4
5
-1
to be in the range of 10 -10 M for the 1:1 dyads, suggesting fairly stable complex formation. Electrochemical
and computational studies suggested that photoinduced electron transfer is a thermodynamically unfavorable
process when free-base porphyrin is excited in these dyads. Selective excitation of the donor free-base porphyrin
entity was possible in both types of dyads formed by either of the ZnPc or ZnNc energy acceptors. Efficient
singlet-singlet energy transfer was observed in these dyads, and the position of imidazole linkage on the
free-base porphyrin entity, although flexible, seems to have some control over the overall efficiency of excited
energy transfer process. Kinetics of energy transfer was monitored by performing transient absorption
measurements using both up-conversion and pump-probe techniques. Such studies revealed ultrafast
singlet-singlet energy transfer in the studied dyads with time constants on the order of 2-25 ps depending
upon the type of the dyad.
Introduction
to mimic the natural energy transfer process, including covalently
13-16
17
18
linked dyads and polyads,
polymers, dendrimers, and self-
The amazing features of Nature to tap available resources
for the benefit of living organisms have intrigued mankind over
centuries. One such classic example is photosynthesis which
harvests solar energy and converts it to chemical energy in the
form of transmembrane charge separation via a multistep
assembled systems.19 In these model compounds, successful
excitation energy transfer from donor to the acceptor entities has
20
been demonstrated. In the majority of these studies, porphyrins
21
and phthalocyanines have been used as a energy/electron donor/
acceptor due to their close resemblance to the photosynthetic
pigment, chlorophyll, and the established synthetic methodologies.
Both macrocycles are ideal photoactive units with outstanding
electronic properties, namely, strong absorption in the visible region
and the possibility of fine-tuning the redox potentials. Further, their
absorption can easily be extended into near-IR region by increasing
macrocycle π-conjugation.2
1
electron transfer reaction. Chlorophylls and carotenoids are the
primary pigments of natural light harvesting systems responsible
for converting the energy of an absorbed photon into an electron
excitation. The supramolecular organization of these pigments
(
antenna) allows the unidirectional excitation energy transfer
toward a reaction center, the final destination of the collected
0,21
energy.1,2 Inspired by this natural phenomenon, researchers have
been attempting to mimic such complex processes with the help
of synthetic molecular architectures, often termed as artificial
photosynthesis.3 Research in this area holds promise not only
to improve the fundamental understanding but also to advance
technologically in building light energy harvesting photovoltaic
devices, to construct molecular optoelectronics and to develop
Self-assembly via metal-ligand axial coordination is one of
the successful approaches developed to study photoinduced
-11
11
electron transfer in donor-acceptor dyads. However, utiliza-
tion of this strategy to build dyads composed of different donor
and acceptor fluorophores to mimic the natural energy transfer
process has not been fully explored. This has been accomplished
in the present study by constructing donor-acceptor dyads using
free-base porphyrin as energy donor and zinc phthalocyanine
as energy acceptor via axial ligand coordination.22 Further, zinc
naphthalocyanine, a phthalocyanine structural analogue, having
absorption and emission well into the near-IR region, has been
utilized to verify energy transfer from singlet excited porphyrin
to a near-IR emitting fluorophore. To achieve axial coordination,
free-base porphyrin has been functionalized with an imidazole
entity at the ortho, meta, or para positions of one of the meso-
1
2
photocatalysts capable of producing hydrogen.
In artificial photosynthesis, two photoinduced events are
mainly targeted, viz., excitation energy transfer to mimic the
antenna functionality, and electron transfer to mimic reaction
center functionality.3 Several strategies have been employed
-11
*
Corresponding authors. E-mail: Francis.DSouza@wichita.edu (F.D.);
nikolai.tkachenko@tut.fi (N.V.T.).
†
Wichita State University.
Tampere University of Technology.
‡
1
0.1021/jp908115e  2010 American Chemical Society
Published on Web 11/23/2009

Ultrafast Singlet-Singlet Energy Transfer
J. Phys. Chem. A, Vol. 114, No. 1, 2010 269
CHART 1: Structure of the Donors and Acceptors Employed in the Present Study To Probe Excitation Energy Transfer
aryl groups (see Chart 1). The different substitutions are
expected to result in dyads of different orientations. Photo-
chemical studies using both steady-state and time-resolved
transient absorption techniques have been performed to probe
efficiency and kinetics of excitation energy transfer in the newly
formed dyads.
Figure 2a and 2b show absorption spectral changes recorded
during increasing addition of H Im to the solutions of ZnPc
2 p
P
and ZnNc, respectively. Similar spectral changes were observed
for H P Im and H P Im binding to the acceptor zinc macro-
2
o
2 m
cycles (see Supporting Information Figures S1 and S2). The
binding of H P Im to ZnPc was characterized by diminished
2
p
intensity of 614 and 681 nm bands with isosbestic points at
09, 663, and 672 nm, indicating existence of only one
equilibrium process in solution. Similarly the binding of H Im
Results and Discussion
6
2 p
P
Optical Absorption and Binding Constant Studies. Figure
to ZnNc was characterized by diminished intensity of 690, 737,
and 777 nm bands with 2-4 nm blue shifts (Figure 2b).
Isosbestic points were also observed at 665, 686, 731, and 750
nm, indicating existence of only one equilibrium process in
solution. Plots of method of continuous variation confirmed 1:1
complex formation between the donor and acceptor entities. The
1
2 o
shows the optical absorption spectra of H P Im, ZnPc, and
ZnNc derivatives in o-dichlorobenzene (DCB), normalized to
their most intense absorption bands. The absorption spectrum
of H
Im with an intense Soret at 424 and four visible bands at
18, 553, 594, and 652 nm. The spectrum of ZnPc exhibited
2 m 2 p
P Im and H P Im were found to be similar to that of
2 o
H P
5
formation constants, K, for H
2
PIm binding to ZnPc and ZnNc
bands at 348, 614, 653, and 681 nm while the spectrum of ZnNc
revealed peaks at 335, 690, 737, and 777 nm, respectively; that
is, the spectrum of ZnNc is stretched well into the near-IR
were obtained from the absorption spectral data using the
23
Benesi-Hildebrand method (Figure 2a and 2b insets) and are
listed in Table 1. The magnitude of the K values suggests stable
complex formation. The K values follow the trend: para ∼ meta
2
region. Importantly, the H PIm band at 518 nm had no overlap
with the absorption bands of either ZnPc or ZnNc, providing
the possibility for selective excitation of the donor, free-base
porphyrin.
>
ortho of imidazole substitution on the phenyl ring of porphyrin
macrocycle for a given zinc macrocycle binding, a trend that
could be easily attributed to the steric constraints of the ortho-
substituted porphyrin derivative. Additionally, binding constants
for ZnNc were found to be 2-3 times higher than that obtained
for the corresponding ZnPc binding. This could be attributed
to the electron-rich ZnNc macrocycle compared to ZnPc
macrocycle as revealed by their electrochemical oxidation
potentials, discussed in the next section.
Electrochemical Studies. Differential pulse voltammetric
studies (DPV) were performed to evaluate the oxidation and
reduction potential of the investigated donor-acceptor entities.
The first reversible oxidation and first two reversible reductions
2 o
of H P Im were located at 0.55 V, and -1.67 and -2.02 V vs
+
4 2 m 2 p
Fc/Fc in 0.1 (TBA)ClO , respectively. For H P Im and H P Im,
the first reduction was shifted in the negative direction by 50-60
mV while the first oxidation was anodically shifted by 40 mV
(
see Figure 3). The first oxidation of ZnPc and ZnNc were
overlapping two one-electron processes. The peak potential were
+
located at -0.05 V vs -0.38 V vs Fc/Fc , respectively, for
2 o
Figure 1. Absorption spectra of (i) H P Im, (ii) ZnPc, and (iii) ZnNc
in DCB, normalized to their most intense bands. The concentrations
are in the range of 5-10 µM.
ZnPc and ZnNc. That is, these compounds revealed easier
oxidations compared to the free-base porphyrins used in the

2
70 J. Phys. Chem. A, Vol. 114, No. 1, 2010
Maligaspe et al.
Figure 2. UV-visible-near IR spectral changes observed during increasing addition of H
2
P
p
Im (0.1 equiv) to a solution of (a) ZnPc (0.11 mM)
and (b) ZnNc (0.11 mM) in DCB. The figure insets show Benesi-Hildebrand plots constructed to obtain the binding constants. The absorption
changes of the 681 nm band of ZnPc and 776 nm band of ZnNc were utilized.
TABLE 1: Binding Constants, K, and B3LYP/3-21G(*) Computed Results for Dyads Formed by Binding Imidazole-Appended
Free-Base Porphyrin to Zinc Phthalocyanine or Zinc Naphthalocyanine
dyada
K, mol-1 b center-to-center distance, Å edge-to-edge distance, Å angle between rings, deg HOMO-LUMO gap, eV
4
H
H
H
H
H
H
2
2
2
2
2
2
P
P
P
P
P
P
o
Im:ZnPc
4.20 × 10
7.4
10.7
13.0
8.06
12.1
13.0
5.75
6.11
8.83
5.75
9.96
9.22
46
35
56
35
55
55
2.09
2.11
2.08
1.79
1.78
1.82
4
4
4
5
5
m
Im:ZnPc 3.50 × 10
p
o
Im:ZnPc
8.23 × 10
Im:ZnNc 6.60 × 10
m
Im:ZnNc 1.48 × 10
p
Im:ZnNc 1.53 × 10
a
b
See Chart 1 for the structure of different donor and acceptor entities; in DCB at room temperature. Error ) (10%.
the supramolecular dyads were visualized by performing
computational studies at the B3LYP/3-21G(*) level.2 Figure
4,25
4
shows the structures of the dyad optimized on a Born-
Oppenheimer potential energy surface. The two macrocyclic
rings of the H Im:ZnPc and H Im:ZnNc dyads were found
to be in a skipped coplanar arrangement while for the meta and
para imidazole derivatized dyads, H PIm:ZnPc and H PIm:
2
P
o
2 o
P
2
2
ZnNc, they were positioned at an angle less than 90°. The edge-
to-edge distances, center-to-center distances, and angles between
the two macrocycle planes of the donor-acceptor entities are
listed in Table 1. Generally, the distances varied as follows:
ortho < meta < para imidazole-substituted porphyrins. That is,
a closer distance and skipped coplanar configuration for the ortho
derivatives, and a relatively longer distance and near-orthogonal
configuration for the meta and para substituted derivatives, were
observed.
The frontier orbitals, HOMO and LUMO, were also evaluated
for both types of dyads, and the representative orbitals for
selected dyads are shown in Figure 4. It is important to note
that the HOMO for all of the studied dyads was fully localized
on the ZnPc or ZnNc macrocycles while the LUMO was located
on the porphyrin macrocycle. In conjunction with the earlier
discussed electrochemical results, these results point out that
electron transfer from the excited free-base porphyrin to either
the coordinated ZnPc or ZnNc is less likely to take place. The
gas-phase HOMO-LUMO gap is found to be smaller by ∼300
Figure 3. Differential pulse voltammograms of the investigated
compounds (∼0.5 mM) in DCB containing 0.1 M (TBAP)ClO . DPV
conditions: scan rate ) 20 mV/s, pulse width ) 50 ms, step time )
00 ms, and pulse height ) 0.025 V.
4
1
present study. The first reduction peaks of ZnPc and ZnNc were
+
located at -1.65 and -1.70 V vs Fc/Fc , indicating these to
be poor electron acceptors (vide infra).
Energy Optimization by DFT Calculations. Since the
relative orientation of the donor and acceptor dipoles in the
dyads is crucial for energy transfer efficiency, the structures of
mV for the H
PImZnPc series of dyads, a result that readily agrees with
the electrochemical results.
2
PIm:ZnNc series of dyads compared to the
H
2

Ultrafast Singlet-Singlet Energy Transfer
J. Phys. Chem. A, Vol. 114, No. 1, 2010 271
Figure 4. B3LYP/3-21G(*) optimized structures of the dyads formed via axial coordination of H
2
PIm (ortho, meta, or para) to (a) ZnPc and (b)
ZnNc. The HOMO and LUMO of H Im bound to ZnPc and ZnNc are shown in the lower panels.
2 m
P
Using the electrochemical, computational, and excited energy
Information for the spectral data, Figures S3-S6). Further, the
excitation spectra of the dyads were recorded by holding the
excitation wavelength at 758 for ZnPc and 782 for ZnNc. Such
spectra revealed absorption bands corresponding to both donor
and acceptor entities (see Supporting Information for the spectral
data, Figures S7). In a control experiment, free-base meso-
data, the free energies of charge-separation (∆GCS) were
2
6
calculated using eq 1 by Weller’s approach.
-
∆GCS ) ∆E0-0 - e(Eox - E ) + ∆G
(1)
red
S
2
tetraphenylporphyrin, H TPP, was also titrated with the acceptor
molecules. Under these conditions, only a slight increase of the
acceptor emission in the near-IR region was observed. These
results confirm occurrence of singlet-singlet energy transfer
in the self-assembled dyads.
Figure 5c and 5d shows the extent of energy transfer for each
of the porphyrin derivatives upon increasing addition of ZnPc
and ZnNc, respectively. It is clear from these plots that after
addition of about 3-4 equiv of the acceptor entities, the energy
transfer has attained its maximum value. Further, the extent of
where ∆E0-0 is the energy of the lowest excited state of H
2
P
2
(
1.89 eV), ∆G
S
) -e /(4πε
0
ε
R
R
Ct-Ct), and ε
0
R
and ε refer to
vacuum permittivity and dielectric constant of DCB.
The calculations revealed ∆GCS values to be endothermic by
.1-0.2 eV for electron transfer from the singlet excited free-
0
base porphyrin to either ZnPc or ZnNc, suggesting the less likely
occurrence of such reactions in the studied dyads.
Steady-State Fluorescence Studies: Singlet-Singlet En-
ergy Transfer. As pointed out earlier, excitation of the donor,
H
2
PIm, at 518 nm to a large extent selectively excites the free-
energy transfer followed the order: H
. H TPP according to their binding constants. The latter plots
for H TPP interactions being virtually horizontal imply occur-
2 p 2 m 2 o
P Im > H P Im > H P Im
base porphyrin, thus allowing us to monitor energy transfer to
the acceptor, ZnPc or ZnNc entities, without them being directly
getting excited. Figure 5a and 5b show the spectral changes
2
2
rence of little or no energy transfer.
observed for H
ZnPc and ZnNc in DCB. The emission band of H
at 655 nm revealed quenching with simultaneous appearance
of new emission bands at 694 and 758 nm corresponding to
ZnPc (Figure 5a), and 782 and 820 nm corresponding to ZnNc
2
P
p
Im emission during increasing addition of
Im located
Excited Energy Transfer: Theoretical Considerations. The
observed excitation energy transfer (EET) could occur either
via Dexter’s exchange mechanism or Forster’s dipole-dipole
mechanism. The former mechanism requires the presence of
electronic communication between the donor and acceptor
species (via orbital overlap).27 However, the frontier orbitals
from the DFT studies (Figure 4) in conjunction with the
2 p
P
(
Figure 5b). Similar results were obtained when H
2 m
P Im and
2 o
H P Im were titrated with either ZnPc or ZnNc (see Supporting

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72 J. Phys. Chem. A, Vol. 114, No. 1, 2010
Maligaspe et al.
-
4
Figure 5. Steady-state fluorescence spectra of H
excited at 518 nm. (c and d) Emission intensities of ZnPc monitored at 758 nm and of ZnNc monitored at 781 nm for the titration involving (i)
Im, (ii) H Im, (iii) H Im, and (iv) H TPP, demonstrating the extent of energy transfer efficiency.
2
P
p
Im (1.2 × 10 M) upon increasing addition of (a) ZnPc and (b) ZnNc, respectively, in DCB,
H
2
P
p
2
P
m
2
P
o
2
spectroscopic studies reveal that such intramolecular interactions
are almost nonexistent. Therefore, the results of the present study
have been analyzed according to Forster’s mechanism. Accord-
ing to this mechanism, the rate of excitation energy transfer,
dipoles. The transition dipoles of tetrapyrroles are known to lie
28
along a line joining two opposing pyrrole nitrogens. Depending
upon the relative orientation of the donor and acceptor, the value
2
of κ could range from 0 to 4. For head-to-tail parallel transition
2
2
kForster, is given by eq 2.
dipoles, κ ) 4, for parallel dipoles, κ ) 1, and for dipoles
2
oriented perpendicular to each other, κ ) 0. An analysis of
the results presented in Figure 4 shows different orientations
for the ortho, meta, and para substituted derivatives. However,
considering the flexible nature of the connecting axial bond, a
value of κ ) 2/3, generally used for randomly oriented dipoles,
is employed.
-
25
2
4
6
kForster ) [8.8 × 10 κ Φ J
]/[n τ R ]
(2)
D
Forster
D
2
where n is the solvent refractive index, Φ
D
and τ
D
are the
fluorescence quantum yield () 0.12) and the fluorescence
lifetime of the isolated donor (free-base porphyrin), JForster is
the Forster overlap integral representing the emission of the
donor and absorption of the acceptor ZnPc or ZnNc, and R is
the donor-acceptor center-to-center distance (Table 1). The τ
values measured using a strobe technique were found to be
The JForster spectral overlap integral representing the emission
of the donor and absorption of the acceptor is given by eq 4.
4
JForster
)
∫
F (λ)ε (λ)λ dλ
(4)
D
A
1
1.50, 9.95, and 9.45 ns, respectively, for the ortho, meta, and
para imidazole-derivatized free-base porphyrins, values close
to those reported for free-base tetraphenylporphyrins in the
literature.11 In eq 2, κ is the orientation factor as described in
eq 3, often playing a key role in determining the directionality
of excitation energy transfer.
where F (λ) is the fluorescence intensity of the donor with total
D
intensity normalized to unity, and ε (λ) is the molar extinction
A
2
-1
-1
coefficient of the acceptor expressed in units of M cm and
λ in nanometers. Figure 6 shows the spectral overlap of the
donor and acceptor absorption and emission bands for both types
of dyads. Analysis of the data according to eq 4 resulted in J
-
12
-1
3
-12
-1
3
values of 3.52 × 10
M
cm and 4.21 × 10
M
cm ,
2
2
κ ) [cos ν - 3 cos R cos ꢀ]
(3)
respectively, for the H
2
2
PIm:ZnPc and H PIm:ZnNc dyads.
Equation 1 can be further simplified in terms of Forster
distance, R , where half the donor molecules decay by energy
transfer and half decay by the usual radiative and nonradiative
mechanisms.
where R and ꢀ are the angles made by the transition dipoles of
the donor and acceptor entities with the line joining the centers
of the transitions, and ν is the angle between the two transition
o

Ultrafast Singlet-Singlet Energy Transfer
J. Phys. Chem. A, Vol. 114, No. 1, 2010 273
6
TABLE 2: Estimated (kForster) and Experimentally
Determined (kENT) Rates of Energy Transfer for the Dyads
Formed by Coordination of Imidazole-Appended Free-Base
Porphyrin to Zinc Phthalocyanine or Zinc Naphthalocyanine
in DCB
kForster ) 1/τ (R /R)
(5)
D
o
Further, the EET efficiency can be measured by using relative
fluorescence intensity or lifetime of the donor in the absence
and presence of acceptor, according to eq 6.
dyada
k
Forster, s-1 b
k
ENT, s-1 c
1
1
1
1
1
1
1
1
1
1
1
1
11
H
H
H
H
H
H
2
2
2
2
2
2
P
P
P
P
P
P
o
Im:ZnPc
3.94 × 10
1.32 × 10
2.30 × 10
3.88 × 10
1.41 × 10
1.50 × 10
1.4 × 10
1.7 × 10
3.3 × 10
2.0 × 10
9.0 × 10
4.0 × 10
10
10
11
10
10
m
Im:ZnPc
Im:ZnPc
Im:ZnNc
efficiency, E ) 1 - IDA/ID ) 1 - τDA/τD
(6)
p
o
m
Im:ZnNc
Im:ZnNc
p
From the steady-state data, values of E obtained were found
to be 0.55, 0.70, and 0.74, for the H PIm:ZnPc (o, m, and p)
dyads, and 0.40, 0.50, and 0.56 for the H PIm:ZnNc (o, m, and
p) dyads by monitoring the emission intensity of H PIm at 656
a
2
See Chart 1 for the structure of different donor and acceptor
b
entities. Estimated according to eqs 1-5; error
Determined from the pump-probe technique.
)
(10%
2
c
2
nm. The efficiency for ortho derivatives is found to be generally
smaller, a trend that readily follows their K values. It is
interesting to note that both of the acceptors have good spectral
overlap and transfer efficiency in the studied series of dyads.
The kForster values were estimated by using the parameters
measured using pump-probe and up-conversion techniques
agree well with the predictions of ultrafast energy transfer in
the dyads.
Pump-Probe and Up-Conversion Spectral Studies. The
photodynamics of the energy transfer of the newly formed dyads
was studied by transient spectroscopy techniques. Transient
absorption spectrum of H P Im in toluene obtained from the
2 m
pump-probe technique is shown in Figure S8, Supporting
Information. At the excitation wavelength of 410 nm, the internal
10
11
described in eqs 1- 5 and found to be on the order of 10 -10
-
1
s , revealing ultrafast energy transfer (Table 2). As explained
in the subsequent paragraphs, the rate of energy transfer
2 1
conversion (S f S ) was faster than the time resolution of the
pump-probe instrument, 200 fs; hence, the signal showed
instant formation of the lowest excited state. The transient
absorption response of ZnPc was relatively more complex and
at least biexponential fit had to be used to achieve a reasonable
fit of the results as presented in Figure S9, Supporting
Information. Similar results indicating the presence of a minor
transient absorption perturbation in 2-20 ps time domain have
been already reported,2 and a few explanations were proposed,
but the origin of this signal remains unclear. However, after
instant formation of a typical transient absorption spectrum of
the singlet excited ZnPc after an excitation pulse at 410 nm,
there was a minor (<20%) increase in Q-band bleaching with a
relatively slow time constant (15 ps). Apparently this slow
component had something to do with photodynamics of the
ZnPc, although it had only a minor effect on the transient
absorption spectrum of the compound. A similar component
was then observed for the porphyrin-phthlocyanine dyads. To
distinguish this component from the energy transfer processes
taking place in the dyad, one can notice that it has virtually
zero intensity in 500-650 nm wavelength range (Figure S9),
whereas the porphyrin excited-state specific feature are well
pronounced in this range (Figure S8).
0d,e
Figure 7a shows transient absorption decay component spectra
of the H
spectrum just after excitation (at 0 ps delay time), while Figure
b reveals the decay profiles of the 455 and 680 nm bands.
2 p
P Im:ZnPc dyad together with the time-resolved
7
The spectrum of the dyad immediately after excitation (at 0
ps) had features typical of the excited singlet state of the
porphyrin except for a band around 680 nm which was due to
a small portion of directly excited zinc phthalocyanine. This
band increased in intensity with a time constant of 30 ps.
Additionally, the 30 ps component showed the disappearance
of the porphyrin excited-state features and conversion of the
spectrum to one corresponding to the excited singlet state of
ZnPc. Thus, the 30 ps time constant has been attributed to the
Figure 6. Spectral overlap (highlighted area) for (a) H
2
PIm:ZnPc and
2 p
energy transfer from H P Im to ZnPc. The minor component
(
b) H PIm:ZnNc type dyads. The donor absorption and emission, and
2
with a 0.5 ps lifetime was similar to the one observed for
reference ZnPc and can be attributed to photodynamics of
directly excited phthalocyanines.
acceptor absorption and emission (all normalized), on an increasing
wavelength scale. The schematic above each spectrum shows the energy
transfer path for each type of dyad.

2
74 J. Phys. Chem. A, Vol. 114, No. 1, 2010
Maligaspe et al.
Figure 7. (a) Transient absorption decay component spectra of the H
2
P
p
Im:ZnPc dyad in toluene. The dashed line shows the differential absorption
spectrum just after excitation (at 0 ps delay time). (b) Transient absorption decays of the dyad monitored at 680 and 455 nm. The solid lines
represent the global fits at these wavelengths.
Figure 8. Time profiles showing (a) the porphyrin emission decay monitored at 656 nm and (b) rise of fluorescence of the phthalocyanine entity
at 690 nm of the H P Im:ZnPc dyad in toluene.
2 p
The transient absorption spectra of the H
2
P
o
Im:ZnPc dyad
was assigned for this dyad. No reliable results were obtained
for the H Im:ZnPc dyad from the up conversion technique.
shown in Figure S10 (Supporting Information) revealed similar
spectral features. The component corresponding to energy
transfer had a 7 ps time constant. The free-base porphyrin
2 m
P
The transient absorption component spectra for ZnNc from
the pump-probe technique are shown in Figure S13, Supporting
Information. The results are essentially the same as for the ZnPc
reference compound shown in Figure S9. There is some
photodynamics of the excited state with a time constant of 2.3
ps. Otherwise the transient spectrum (longer-lived component)
showed bleaching of the ground-state absorption bands at 770
and 685 nm and broadband absorption through the visible part
of the spectrum.
2 m
features were also observed for the H P Im:ZnPc dyad (Figure
S11, Supporting Information) even at the longest available delay
time of 1 ns. However, the 600 ps component had a shape
similar to the energy transfer component of the ortho and para
derivatives. From these studies, the energy transfer time constant
was found to follows the order: ortho > para > meta substitution
of the free-base porphyrin.
The dyads were also investigated using a complementary up-
conversion technique. Figure 8 shows the fluorescence decay of
the free-base porphyrin emission at 650 nm and the rise of the
2 p
The transient absorption component spectra of H P Im:ZnNc
dyad in toluene are presented in Figure 9. The spectrum of the
dyad just after excitation (at 0 ps) had features typical for the
excited singlet state of the porphyrin except for a band around
770 nm which was due to a small portion of directly excited
ZnNc. This band increased in intensity with a time constant of
25 ps. Additionally, the 25 ps component showed a disappear-
ance of the porphyrin excited-state features and conversion of
the spectrum to the one corresponding to excited singlet state
of ZnNc.
2 p
zinc phthalocyanine emission at 690 nm for the H P Im:ZnPc dyad
in toluene. The time constant for the fast decay at 650 nm was
8
1
0 ( 100 ps while for the rising component at 690 nm it was
8 ( 5 ps. These values are slightly higher than that observed
from the pump-probe measurements. Among these three time
constants, the one from the pump-probe experiments is the most
reliable because of less spectral interference; hence, the energy
transfer time constant for this dyad was concluded to be 30 ps.
As shown in Figure S12, the fast decay and rise in
2 o
Similar results were also obtained for the H P Im:ZnNc dyad
(Figure S14, Supporting Information). The energy transfer time
fluorescence can also be seen for the H
2
P
o
Im:ZnPc dyad. For
constant was 5 ps ((1.6 ps), somewhat faster than that obtained
this sample, a common fit of emission at two wavelengths gave
a time constant of 2.2 ( 0.5 ps for the energy transfer which
agreed well with the corresponding value from the pump-probe
measurements of 7 ps. As pointed out earlier, because the
interference of the photodynamics of phthalocyanine to some
extent makes the pump-probe results less reliable, an energy
transfer time constant of 2 ps from the pump-probe technique
2 p
for the H P Im:ZnPc dyad of 25 ps. As shown in Figure S15,
Supporting Information, somewhat uncertain results were
obtained for the H Im:ZnNc dyad. The component with a
2 m
P
lifetime of 11 ps ((4 ps) seems to be one coming from the
energy transfer (there are few deeps corresponding to porphyrin
Q-bands), which did not agree with the results for the H
2 m
P Im:
ZnPc dyad. However, for both dyads formed with H Im, the
2 m
P

Ultrafast Singlet-Singlet Energy Transfer
J. Phys. Chem. A, Vol. 114, No. 1, 2010 275
Experimental Section
Chemicals. Zinc 2,11,20,29-tetra-tert-butylphthalocyanine,
zinc 2,11,20,29-tetra-tert-butyl-2,3-naphthalocyanine, o-dichlo-
robenzene, and toluene (in sure seal bottles under nitrogen) were
from Aldrich Chemicals (Milwaukee, WI). Tetra-n-butylam-
monium perchlorate, (TBA)ClO , was from Fluka Chemicals.
4
All the chromatographic materials and solvents were procured
from Fisher Scientific and were used as received.
Synthesis of Imidazole-Appended Free-Base Porphy-
rins. 2-[3-(1H-Imidazol-1-yl)phenyl]-1,3-dioxolane (1a). Imi-
dazole (0.01 mol) was added to a suspension of sodium hydride
(0.01 mol) in 10 mL of DMF. The resulting solution was stirred
at room temperature for 20 min. Next, 2-(3-bromophenyl)-1,3-
dioxolane (0.01 mol) and copper powder (0.001 mol) were
added to the reaction mixture which was refluxed for 5 h. The
mixture was cooled to room temperature, chloroform and water
were added, and the mixture was stirred for 2 h and filtered.
The filtrate was washed with water and dried over sodium
sulfate. The solvent was evaporated under reduced pressure. The
compound was purified over a flash silica gel column using
2 p
Figure 9. Transient absorption component spectra of the H P Im:ZnNc
dyad in toluene. The dashed line shows the differential absorption
spectrum just after excitation (at 0 ps delay time).
interpretation of the pump-probe spectra is rather speculative,
as the intensity of the component responsible for the energy
transfer was found to be not well-defined.
2 2
Cl
/MeOH (95:5 v/v) as eluent. 1H NMR (400 MHz,
CH
CDCl
3
): δ 3.80-4.05 (m, 4 H), 5.68 (s, 1H), 7.05-7.38 (m, 6
2 2
H), 7.72 (s, 1H). Mass (APCI mode in CH Cl ): calcd, 184.3;
The data from the transient spectroscopic studies confirm the
occurrence of ultrafast excitation energy transfer in the studied
dyads. The magnitude of measured rates of energy transfer
agrees well with the estimated rate based on Forster’s mecha-
nism. Further, the energy transfer rates for dyads originated from
ortho imidazole-substituted free-base porphyrin are generally
higher compared to meta and para imidazole-substituted por-
phyrin likely because of closer proximity and favorable
orientations.
found [M + 1] 185.1.
3-(1H-Imidazol-1-yl)benzaldehyde (1b). 2-[3-(1H-Imidazol-
1
3
-yl)phenyl-1,3-dioxolane was stirred at room temperature for
h in 20 mL of 1 M HCl. Aqueous NaHCO was added to the
3
reaction and extracted with ethyl acetate. The organic layer was
washed with aqueous NaCl, dried over Na SO , and evaporated
under reduced pressure to obtain the product as a pale yellow
2
4
1
solid. H NMR (400 MHz, CDCl
H), 7.67-7.72 (m, 2H), 7.87-7.97 (m, 3H), 10.08 (s, 1H).
Mass (APCI mode in CH Cl ): calcd, 172.18; found 173.0.
-[3′-(1H-Imidazol-1-yl)phenyl]-10,15,20-tritolylporphy-
3
): δ 7.25 (s, 1H), 7.36 (s,
1
2
2
Conclusions
5
Supramolecular free-base porphyrin-zinc phthalocyanine and
free-base porphyrin-zinc naphthalocyanine dyads via a metal-
ligand axial coordination approach have been formed in
noncoordinating solvents and characterized by spectroscopic,
computational, and electrochemical methods. The formation of
a 1:1 complex was evident from an analysis of spectral data,
and the measured binding constants were found to be on the
rin (1c). Compound 1c was synthesized by reacting 1b (450
mg, 2.6 mmol), tolualdehyde (7.8 mmol), and pyrrole (10.5
mmol) in refluxing propionic acid. The crude product was
purified on a basic alumina column with chloroform/methanol
1
(95:5 v/v) as eluent. H NMR (400 MHz, CDCl
3
): δ -2.79 (s,
2H), 2.78 (s, 9H), 7.05 (d, 1H), 7.37 (s, 1H), 7.55 (d, 6H),
7.78-7.85 (m, 3H), 8.15 (d, 6H), 8.18 (s, 1H), 8.30 (d, 1H),
4
5
-1
order of 10 -10 mol depending upon the nature of the free-
base porphyrin and zinc macrocycle, suggesting moderate-to-
stable complex formation. The structures of the dyads were
deduced from computational studies using the B3LYP/3-21G(*)
method, and such studies, although highly flexible, revealed
structures depending upon the nature of imidazole-appended
free-base porphyrins. The redox potential of the donor and
acceptor entities were measured using differential pulse tech-
nique, and the performed free-energy calculations suggested
electron transfer to be an inefficient process when the free-base
porphyrin is selectively excited in these dyads. Selective
excitation of the donor free-base porphyrin entity was possible
in both types of dyads formed by either of the ZnPc or ZnNc
energy acceptors. Efficient singlet-singlet energy transfer was
observed in the studied dyads, and the position of imidazole
linkage on the free-base porphyrin entity seems to have little
control over the overall efficiency, perhaps because of the
flexible nature of the single-point axial linkage. Kinetics of
energy transfer was monitored by performing transient absorp-
tion measurements using both up-conversion and pump-probe
techniques. Such studies revealed ultrafast excitation energy
transfer in the studied dyads with time constants on the order
of 2-25 ps depending upon the type of the dyad.
8.80-8.91 (m, 8H). Mass (APCI mode in CH
722.88; found [M + 1] 723.6.
2 2
Cl ): calcd,
2-[2-(1H-Imidazol-1-yl)phenyl]-1,3-dioxolane (2a). Imida-
zole (0.01 mol) was added to a suspension of sodium hydride
(0.01 mol) in 10 mL of DMF. The resulting solution was stirred
at room temperature for 20 min. Next, 2-(3-bromophenyl)-1,3-
dioxolane (0.01 mol) and copper powder (0.001 mol) were
added to the reaction mixture which was refluxed for 4 h. The
mixture was cooled to room temperature, chloroform and water
were added, and the mixture was stirred for 2 h and filtered.
The filtrate was washed with water and dried over sodium
sulfate. The solvent was evaporated under reduced pressure. The
compound was purified over flash silica gel column using
CH
CDCl
2
Cl
3
2
/MeOH (96:4 v/v) as eluent. 1H NMR (400 MHz,
): δ 3.75-4.00 (m, 4 H), 5.35 (s, 1H), 7.01 (s, 1 H),
7.10-7.60 (m, 6H). Mass (APCI mode in CH
2 2
Cl ): calcd, 184.3;
found [M + 1] 185.1.
2-(1H-Imidazol-1-yl)benzaldehyde (2b).29 2-[3-(1H-Imida-
zol-1-yl)phenyl-1,3-dioxolane was stirred at room temperature
for 3 h in 20 mL of 1 M HCl. Aqueous NaHCO
the reaction and extracted with ethyl acetate. The organic layer
was washed with aqueous NaCl, dried over Na SO , and
evaporated under reduced pressure to obtain the product as a
3
was added to
2
4

2
76 J. Phys. Chem. A, Vol. 114, No. 1, 2010
Maligaspe et al.
1
pale yellow solid. H NMR (400 MHz, CDCl
(
3
): δ 7.00-7.45
The instrument and the used data analysis procedure have been
described earlier.30
m, 3H), 7.55 (m, 2H), 7.81 (dd, 1H), 9.58 (s, 1H). Mass (APCI
mode in CH Cl ): calcd, 172.18; found [M + 1] 173.1.
-[2′-(1H-Imidazol-1-yl)phenyl]-10,15,20-tritolylporphy-
rin (2c). Compound 2c was synthesized by reacting 2b (452
mg, 2.6 mmol), tolualdehyde (7.8 mmol), and pyrrole (10.5
mmol) in refluxing propionic acid. The crude product was
purified on a basic alumina column with chloroform/methanol
95:5 v/v) as eluent. H NMR (400 MHz, CDCl
H), 2.72 (s, 9H), 7.01 (d, 1H), 7.55 (d, 6H), 7.70-7.89 (m,
H), 8.15 (d, 6H), 8.18 (s, 1H), 8.30 (d, 1H), 8.80-8.91 (m,
H). Mass (APCI mode in CH
] 723.8.
2
2
Acknowledgment. The authors thank the National Science
Foundation (Grant 0804015 to F.D.), NSF-EPSCoR, and
Academy of Finland for support of this work.
5
Supporting Information Available: Absorption and fluo-
rescence changes observed during the formation of dyads,
Benesi-Hildebrand plots constructed to evaluate the binding
constants, excitation spectra of the dyads, transient absorptions
spectra of the donor, acceptor, and dyads in DCB. This material
is available free of charge via the Internet at http://pubs.acs.org.
1
(
3
): δ -2.79 (s,
2
4
8
1
2 2
Cl ): calcd, 722.8; found [M +
5
-[4′-(1H-Imidazol-1-yl)phenyl]-10,15,20-tritolylporphy-
References and Notes
rin (3c). Compound 3c was synthesized by reacting 4-(1H-
imidazol-1-yl)benzaldehyde (450 mg, 2.6 mmol), tolualdehyde
7.8 mmol), and pyrrole (10.5 mmol) in refluxing propionic acid.
The crude product was purified on a basic alumina column with
(
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