Spectroscopic study of electron and energy transfer in novel silicon phthalocyanine - Boron dipyrromethene triads

Article abstract of DOI:10.1039/b823432a

Phthalocyanines (Pcs) and boron dipyrromethenes (BDPs) are two versatile classes of functional dyes suitable for design of artificial light harvesting and charge separation systems. In the present work, we report the results of photophysical investigation

Full text of DOI:10.1039/b823432a

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PAPER
www.rsc.org/pccp | Physical Chemistry Chemical Physics
Spectroscopic study of electron and energy transfer in novel silicon
phthalocyanine—boron dipyrromethene triadsw
b
b
a
Eugeny A. Ermilov,*ac Jian-Yong Liu, Dennis K. P. Ng and Beate Roder
¨
Received 9th January 2009, Accepted 5th May 2009
First published as an Advance Article on the web 29th May 2009
DOI: 10.1039/b823432a
Phthalocyanines (Pcs) and boron dipyrromethenes (BDPs) are two versatile classes of functional dyes
suitable for design of artificial light harvesting and charge separation systems. In the present work, we
report the results of photophysical investigations of two novel non-sandwich-type BDP-Pc
heterotriads, in which two BDP or mono-styryl BDP moieties are linked to the central atom of a
silicon(IV) phthalocyanine core (triad 4 and 5, respectively). It was found that the photophysical
properties of the triads in toluene and N,N-dimethylformamide (DMF) are strongly affected by two
different types of interaction between the BDP and the Pc parts, namely excitation energy transfer
(EET) and photoinduced charge transfer (CT). The first process delivers the excitation to the first
excited singlet state of the Pc-part upon initial BDP-part excitation. The probability of EET
supersedes that of CT in toluene, whereas the latter transfer process dominates in BDP-part
depopulation when triads are dissolved in polar DMF. The direct or indirect (via EET) population of
Pc moiety is followed by the hole transfer to the charge-separated state in 4 (in DMF) and in 5 (in
DMF and toluene). At the same time, it was found that CT is energetically unfavorable for the triad
4 in toluene upon excitation of the Pc-part. The charge-recombination in DMF occurs very fast with
a decay time of 40 and 30 ps for 4 and 5, respectively, whereas toluene stabilizes the charge-separated
state, prolonging the lifetime to 4.5 and 1.7 ns, respectively.
Naturally occurring photosystems consist of chlorophylls,
1. Introduction
carotenes and quinones.2 In the past, great efforts have been
made to synthesize artificial systems mimicking the arrangement
of chlorophylls and quinones. Various classes of compounds
exhibiting similar redox and photophysical properties have
been studied extensively leading to an enormous variety of
photosynthetic model compounds. Examples are porphyrin–
quinone, porphyrin–fullerene, phthalocyanine–quinone and
phthalocyanine–fullerene systems.3 Also chlorophylls, carotenes,
perylenes, and viologens were used as parts of such molecular
systems.
One of the most important strategies to provide clean and
affordable solar energy consists of mimicking nature, in
which efficient harvesting and conversion of sunlight occurs.
Conversion of light to chemical potential energy in photo-
synthesis involves two basic photophysical processes: sunlight
is absorbed by ensembles of light-harvesting chromophores
and then the excitation energy is transported to a reaction
center where a charge-separated state is generated by charge
transfer (CT).1 The transport mechanism is a singlet–singlet
excitation energy transfer (EET) among the chromophores
within the antenna system. The antenna is efficiently coupled
to the reaction center which converts electronic excitation
energy to chemical energy in the form of long-lived trans-
membrane charge separation via multistep CT processes. The
functionality of the light-harvesting chromophores is determined
by their strong absorption throughout the visible spectrum (in
order to maximize sun light absorption) as well as high
fluorescence quantum yields (maximizing the probability of
excitation energy transfer to a reaction center).
The design of artificial light harvesting and charge separation
systems requires the components to have high chemical stability,
tuneable electronic and electrochemical properties, and a
facile synthetic route. Phthalocyanines (Pcs)4 and boron
dipyrromethenes (BDPs)5 are two versatile classes of functional
dyes exhibiting these desired qualities. Efficient strategies for
synthesis of both chromophore classes are well established
and the photophysical properties of these compounds can
also be tuned readily through chemical modification of
their structures. Another advantage is that many of these
chromophores are characterized by large extinction coefficients,
high fluorescence quantum yields, reasonably long first excited
singlet state lifetimes, and good solubility and stability in
many solvent systems. These properties have led to different
applications of Pcs and BDPs, for example, in optoelectronic
devices6 and as photosensitizes for photodynamic therapy,7
molecular labels for proteins and DNA8 and laser dyes.9
Pcs show strong absorptions in the UV-Vis region
(at ca. 350 and 670 nm), whereas BDPs exhibit large extinction
a Institut fur Physik, Photobiophysik, Humboldt-Universitat zu Berlin,
¨
¨
Newtonstr. 15, D-12489 Berlin, Germany.
E-mail: eugeny.ermilov@charite.de
b Department of Chemistry and Center of Novel Functional Molecules,
The Chinese University of Hong Kong, Shatin, N.T., Hong Kong, China
c Institut fur Physiologie, Campus Benjamin Franklin,
¨
Charite – Universitatsmedizin Berlin, Arnimallee 22, D-14195 Berlin,
¨
´
Germany
w Electronic supplementary information (ESI) available: UV-Vis,
DAF and transient absorption spectra; energy level schemes. See
DOI: 10.1039/b823432a
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Fig. 1 Structural formulas of BDP–SiPc triads (4 and 5) and the reference compounds 1–3.
coefficients at ca. 500 nm. Thus, a broad range of the visible
spectrum could be covered when these functional dyes are
coupled together. While a substantial number of conjugates of
these chromophores with other photo and redox-active units
such as porphyrins, fullerenes, carotenoids, anthraquinones,
perylenediimides and ruthenium polypyridine complexes
have been studied extensively,4,5,10 to our knowledge,
covalently-linked BDP-Pc arrays have not been reported so far.
Recently, we have prepared two novel BDP-Pc hetero-triads in
which a silicon(IV) phthalocyanine (SiPc) core is substituted
axially with two BDP or mono-styryl BDP (MS-BDP) moieties.11
The structures of these triads as well as the model compounds are
shown in Fig. 1. By means of steady-state and time-resolved
optical spectroscopy, it was found that the photophysical proper-
ties of these triads in toluene are strongly affected by two different
types of interactions between their parts, namely photoinduced
CT and EET, and it was shown that the probability of these
transfer processes depends on the axial substituents.11 In the
present paper, we report our further photophysical study of these
triads in polar and nonpolar environments.
The ground state absorption spectra were recorded using
commercial spectrophotometer Shimadzu UV-2501PC.
a
Steady-state fluorescence spectra were measured in 1 cm ꢁ 1 cm
quartz cells using a combination of a cw-Xenon lamp
(XBO 150) and a monochromator (Lot-Oriel, bandwidth
10 nm) for excitation and a polychromator with a cooled
CCD matrix as detector system (Lot-Oriel, Instaspec IV).12 To
obtain fluorescence quantum yields of compounds under
investigation, the following substances were used as standards—
solutions of tetraphenylporphyrin (H2TPP) in DMF,
Rhodamine 110 and Rhodamine 6G both in ethanol
H2TPP
(F
respectively)—and values were calculated according following
¼ 0:11,13 FflR110 = 0.9414 and FflR6G = 0.95,15
fl
expression (1):
ꢀ
ꢁ
2
Sfxl ODref ðlexc
Þ
nx
ref
fl
Fxfl ¼ F
;
ð1Þ
ref
ODxðlexc
Þ
nref
S
fl
where Fflx, Sflx, ODx(lexc) and Fflref, Srflef, ODref (lexc) are the
fluorescence quantum yield, the integrated fluorescence
intensity and the optical density at the excitation wavelength,
lexc, of a sample and a reference substance, respectively, nx
and nref are the refractive index of the solvent used for the
2. Experimental
2.1 General
sample and the reference, respectively.
All the reactions were performed under an atmosphere of
nitrogen. Tetrahydrofuran (THF), toluene and pyridine were
distilled from sodium benzophenone ketyl, sodium and KOH,
respectively. Toluene and dimethylformamide (DMF) used for
photophysical measurements were of spectroscopic grade
(Aldrich) and used without prior purification. All other
solvents and reagents were of reagent grade and used as
received. Chromatographic purifications were performed on
silica gel (Macherey-Nagel, 70-230 mesh) columns with the
indicated eluents.
2.3 Time-resolved fluorescence spectroscopy
Time correlated single photon counting (TCSPC) technique in
combination with scanning of the detection wavelength was
used to acquire decay associated fluorescence (DAF) spectra.16
The experimental setup was previously described.17 A pulsed,
frequency doubled, linear polarized radiation of a Nd:VO4
laser (Cougar, Time Bandwidth Products) with a wavelength
of 532 nm, a pulse width of 12 ps, and a repetition rate of
60 MHz was used directly for excitation of the samples or
to synchronously pump a dye laser (Model 599, Coherent)
tunable in the range from 610 to 670 nm. For excitation of the
samples in the blue spectral region the frequency doubled
pulses of a Ti:sapphire laser (Coherent Mira 900, 405 nm,
FWHM 200 fs, a repetition rate of 76 MHz) were used. Optical
2.2 Absorption and steady-state fluorescence
All photophysical measurements were performed in DMF and
in toluene of spectroscopic grade (Aldrich), which were used
without further purification.
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density (OD) of the samples was less than 0.15 at the
absorption band of lowest energy. Fluorescence was detected
under a ‘‘magic’’ polarization angle18 relative to excitation
with a thermo electrical cooled micro channel plate (R3809-01,
Hamamatsu). Detection wavelength was chosen by a computer-
controlled monochromator (77200, Lot-Oriel). Electrical
signals were processed by a PCI TCSPC controller card
(SPC630, Becker & Hickl). The instrument response function
was 42 ps, as measured at an excitation wavelength with
Ludox. Data were analyzed by a home-made program by
applying a variable projection algorithm19 to the global
fitting problem. The Nelder–Mead simplex algorithm20 was
used for optimization of the nonlinear parameters, and the
support plane approach18 to compute error estimates of the
decay times. Decay times and time shift were linked through
all measurements of one scan sampled every 3 nm.
Table 1 Electrochemical properties of the reference compounds
Compound
Eoxd/V
Ered/V
1
2
3
1.06 (irrev)
1.05 (irrev)
0.88 (irrev)
ꢂ0.57
ꢂ1.21
ꢂ1.05
4-hydroxybenzoate in the presence of pyridine. The experi-
mental details including full characterization data of these
compounds are given in ref. 11.
The electrochemical properties of the reference compounds
1–3 were studied by cyclic voltammetry and differential pulse
voltammetry in DMF. All of them exhibited a quasi-reversible
reduction and an irreversible oxidation, of which the potentials
are summarized in Table 1. The conjugates 4 and 5 had limited
solubility in DMF. Hence, their electrochemical data were not
measured.
2.4 Picosecond transient absorption spectroscopy
Fig. 2 shows the ground state absorption spectra of
compounds 1–5 in DMF. The corresponding spectra in
toluene are shown in the ESI.w The absorption maxima for
the triads 4 and 5 as well as the reference compounds 1, 2, and
3 in both DMF and toluene are given in Table 2. The
absorption spectra of the triads are essentially the superposition
of the absorption bands of the corresponding components and
have three pronounced peaks: one with a maximum at 683 nm
(belongs to Q-absorption band of SiPc 1), another one at
500 nm for the triad 4 (belongs to absorption of BDP 2) or
576 nm for the triad 5 (corresponds to the absorption of
MS-BDP monomer 3), and the third band, located in the blue
spectral region at around 350 nm, is a superposition of the
B-band absorption of 1 and 2 (for the triad 4) or 1 and 3 (for
the triad 5), respectively. Due to the absence of spectral shifts
of the absorption bands of the triads compared with those of
the corresponding reference compounds, the BDP and Pc
parts of the triads in the ground state are considered as
electronically decoupled.
To measure transient absorption spectra, a white light con-
tinuum was generated as a test beam in a cell with D2O/H2O
mixture using intense 25 ps pulses from a Nd3+:YAG laser
(PL 2143A, Ekspla) at 1064 nm. Before passing through the
sample, the continuum radiation was split to get a reference
spectrum. The transmitted as well as the reference beams
were focused into two optical fibers and were recorded
simultaneously at different traces on a CCD-matrix (Lot-Oriel,
Instaspec IV). Tunable radiation from an OPG/OPA (Ekspla
PG 401/SH, tuning range 200–2300 nm) pumped by third
harmonic of the same laser was used as an excitation beam.
The mechanical delay line allowed the measurement of
light-induced changes of the absorption spectrum at different
delays up to 15 ns after excitation. The OD of all samples
was 1.0 at the maximum of the absorption band of lowest
energy. Analysis of experimental data was performed using the
compensation method.21
2.5 Electrochemical study
Steady-state fluorescence spectra of the reference compounds
1–3 are shown in Fig. 3. The fluorescence maximum of 1 lies at
688 nm in toluene and undergoes a small bathochromic shift
to 689 nm in polar DMF (Table 2). The fluorescence quantum
yield of 1 has a value of 0.47 in DMF and 0.60 in toluene, as
Electrochemical measurements were carried out with a BAS
CV-50W voltammetric analyzer. The cell comprised inlets
for
a platinum-sphere working electrode, a platinum-
plate counter electrode, and a silver-wire pseudo-reference
electrode. Typically, a 0.1 M solution of [Bu4N][PF6] in
DMF containing the sample was purged with nitrogen for
15 min, then the voltammograms were recorded at ambient
temperature. Potentials were referenced to saturated calomel
electrode (SCE) using ferrocene as an internal standard
(E1/2 =+0.38 V vs. SCE).
measured using a solution of H2TPP in DMF as the reference
H2TPP
(F
¼ 0:11)13. The fluorescence bands of compounds 2
fl
and 3 are located at 510 and 580 nm in DMF and at 512 and
583 nm in toluene, respectively. The fluorescence quantum
yield of 2 was calculated to be 0.32 (DMF) and 0.55 (toluene)
using a solution of Rhodamine 110 in ethanol as the reference
3. Results
Treatment of the BDP-appended phenol 222 with the
commercially available silicon(IV) phthalocyanine dichloride
(SiPcCl2) in the presence of pyridine gave the bis(BDP)
substituted phthalocyanine 4 in 34% yield. The MS-BDP
analogue 5 was synthesized in a similar manner using phenol
3 as a starting material, which was prepared by Knoevenagel
condensation of
2
with 3,4,5-trimethoxybenzaldehyde
according to a previously described procedure.23 The reference
Fig. 2 UV/Vis absorption spectra of 1, 2 and 4 (a), and 1, 3 and 5 (b)
compound 1 was prepared by treating SiPcCl2 with methyl
in DMF.
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Table 2 Absorption and fluorescence maxima, labs and lfl, and fluorescence quantum yields, Ffl, of 1–5 in DMF and in toluene
lfl/nm
Ffl
BDP-part excitation
Pc-part excitation
BDP-part excitation
Pc-part excitation
Compound
Solvent
labs/nm
1
Toluene
DMF
Toluene
DMF
Toluene
DMF
Toluene
DMF
358, 683
355, 683
503
500
572
—
—
512
510
583
580
688
689
—
—
—
—
—
0.55
0.32
0.78
0.48
0.60
0.47
—
—
—
2
3
4
5
567
—
—
358, 500, 683
354, 500, 683
352, 570, 683
355, 570, 683
N.d.,a 687b
510,a 689b
588,a 686b
580,a 689b
687
689
686
689
N.d.,a 0.35b
0.005,a 0.006b
0.003,a 0.002b
0.004,a 0.003b
0.60
0.016
0.003
0.012
Toluene
DMF
a
b
BDP-part emission. Pc-part emission.
With excitation at 670 nm, where only the Pc-part absorbs
light, the fluorescence of the triads in both solvents has only
one band similar to that of the reference SiPc 1 (as an example,
the normalized fluorescence spectra of 1 and 5 in toluene are
shown in Fig. 3d). In DMF, the fluorescence of both triads is
much weaker compared to that of 1: the fluorescence quantum
yields were estimated to be only 0.016 and 0.012 for the triads
4 and 5, respectively (see Table 2). It should be mentioned that
these values are higher than those determined for the Pc-part
fluorescence at BDP-part excitation. Interestingly, no reduction
of the fluorescence quantum yield of the triad 4 in toluene was
observed when the Pc-part was selectively excited (Ffl = 0.60,
similar to that of the reference 1), whereas a 200-times lower
value was measured for the triad 5 (Ffl = 0.003, see Table 2) in
the same solvent.
According to the above results, one can make the following
preliminary conclusions: (i) the appearance of the Pc-part
fluorescence with selective excitation of the BDP-part of the
triads in both solvents is an indicator of EET from an initially
photoexcited energy donor (BDP) to an energy acceptor (SiPc
in its ground state). This conclusion is also supported by
the strong quenching of the BDP-part fluorescence; (ii) the
fluorescence quantum yield of the Pc-part emission was lower
when the Pc moiety was excited indirectly (via energy transfer)
compared to the case of direct excitation (see Table 2). This
indicates a competition between energy transfer and an
additional efficient deactivation process of the first excited
singlet state of the BDP moieties in the triads; (iii) for both
triads in DMF as well as the triad 5 in toluene, there is a
quenching process of the first excited singlet state of the Pc
moiety resulting in a strong reduction of its fluorescence
quantum yield compared to that of the reference SiPc 1.
The time evolution of fluorescence was investigated by
TCSPC technique. The fluorescence decays of all reference
compounds 1–3 were found to be mono-exponential in
both solvents and the estimated fluorescence lifetimes are
summarized in Table 3. The solvent polarity has a negligible
influence on the lifetimes of the first excited singlet state of
compounds 1 and 3, whereas the fluorescence of 2 decays two
times faster in DMF than in toluene.
Fig. 3 Steady-state fluorescence spectra: a—1, 2 and 4 in DMF,
excitation at 500 nm; b—1 and 4 in toluene, excitation at 500 nm; c—1,
3 and 5 in DMF, excitation at 567 nm; d—1 and 5 in DMF, excitation
at 670 nm.
(FRfl 110 = 0.94).14 The values are lower than those of 3
(0.48 and 0.78, respectively, see Table 2), which were measured
with respect to Rhodamine 6G in ethanol (FRfl 6G = 0.95).15
Due to the fact that the absorption bands of the Pc-
and BDP-parts of the triads are well separated, both the
steady-state fluorescence spectra with selective excitation of
the BDP or Pc moieties were recorded. Upon excitation of the
triads 4 and 5 at the BDP-part absorption, two effects were
observed in both solvents: fluorescence is strongly reduced
compared to the reference compounds 2 and 3, respectively
(for the triad 4 in toluene no fluorescence even of BDP
was detected, Fig. 3b), and peak of SiPc fluorescence with
maximum at around 689 nm appears (see Fig. 3a–c). The
calculated fluorescence quantum yields are summarized in
Table 2. It is seen that for both triads in polar DMF, the
fluorescence quantum yields of the BDP- as well as Pc-part
emission are much lower compared to those of the reference
compounds. While in nonpolar toluene similar results were
observed also for the triad 5 (see Table 2), a strong peak of the
Pc-part fluorescence with a fluorescence quantum yield of
0.35 was detected upon excitation of the triad 4 at 500 nm.
The fluorescence decay of the triads was measured upon
selective excitation of the BDP- or Pc-part. Excitation of the
triad 4 in DMF at 400 nm and monitoring the fluorescence
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Table 3 Fluorescence lifetimes of the triads 4 and 5 as well as the
reference compounds 1, 2 and 3 in DMF and toluene
tfl/ns
For BDP-part
emission
For Pc-part
emission
Compound
Solvent
1
DMF
Toluene
DMF
Toluene
DMF
—
—
5.35 ꢃ 0.01
5.07 ꢃ 0.01
—
—
—
2
3
4
1.69 ꢃ 0.01
3.03 ꢃ 0.01
3.55 ꢃ 0.01
3.46 ꢃ 0.01
(20 ꢃ 2)ꢁ10ꢂ3
Toluene
DMF
—
0.14 ꢃ 0.01
0.40 ꢃ 0.01
5.20 ꢃ 0.01
(65 ꢃ 5)ꢁ10ꢂ3
0.54 ꢃ 0.02
(15 ꢃ 2)ꢁ10ꢂ3
Toluene
DMF
N.d.
(30 ꢃ 5)ꢁ10ꢂ3
o1ꢁ10ꢂ2
5
Toluene
signal at 510 nm (maximum of the BDP-part fluorescence,
see Fig. 3a) resolves fast mono-exponential decay with a
fluorescence lifetime, tfl, of 20 ꢃ 2 ps. At the same time
fluorescence was detected at 690 nm, where the Pc-part gives
dominant contribution (Fig. 3a) and decays bi-exponentially
with decay times of 140 and 400 ps, both having practically
equal amplitude. Similar decay times were resolved when
the triad 4 was selectively excited at Pc-part absorption
(lexc = 614 nm). It should be mentioned that both decay
times belong to the fluorescence from the first excited singlet
state of the Pc moiety, which was supported by the decay-
associated fluorescence (DAF) spectra of the triad 4 in DMF
(see Fig. 4a).
Fig. 4 Decay-associated fluorescence spectra: a—triad 4 in DMF,
excitation at 614 nm; b—triad 5 in DMF, excitation 532 nm.
Normalized amplitudes are given in insert.
decay of the Pc-part (excited directly at 615 nm, or indirectly
via EET from the BDP-part) was fitted monoexponentially
with a decay time of 15 ꢃ 5 ps, contrary to the long decay time
measured for the triad 4 in toluene (5.2 ns, Table 3).
In good agreement with the results of steady-state
measurements, no fluorescence signal due to BDP was
detected when the triad 4 in nonpolar toluene was excited at
the BDP-part absorption. However, an emission at 690 nm
(maximum of the Pc-part fluorescence, see Fig. 3a) was seen
which has a monoexponential decay with a lifetime of
5.20 ꢃ 0.01 ns. This decay time is similar to that of the
reference compound 1, supporting the conclusion that no
quenching of the Pc-part of the triad 4 occurs in toluene.
The same fluorescence decay time of 5.20 ꢃ 0.01 ns was
resolved upon excitation of the triad 4 at 615 nm (Pc-part
excitation) and monitoring at 690 nm.
To obtain information about the non-fluorescent intermediates
and the recovery of the ground state population after photo-
excitation, the transient absorption spectra of these compounds
were also recorded. As expected, the recovery of the ground
state for all the reference compounds was found to be
monoexponential with characteristic times similar to the above
measured fluorescence decay times. Using the compensation
method,21 the intersystem crossing (ISC) S1 - T1 quantum
yields, FISC, were estimated to be 0.34 ꢃ 0.03 (in DMF) and
0.32 ꢃ 0.03 (in toluene) for SiPc 1, and 0 and 0.02 for 2 and 3,
respectively, in both solvents.
The DAF spectra of the triad 5 in DMF excited at 532 nm
(BDP-part excitation) are shown in Fig. 4b. Analysis of the
fluorescence decay data reveals one decay component of
the BDP-part with a decay time of 30 ꢃ 5 ps, whereas the
population of the first excited singlet state of Pc moiety decays
bi-exponentially: two DAF spectra with decay times of
65 ꢃ 5 ps and 540 ꢃ 20 ps were resolved, and the latter has
a 2.5 times lower amplitude compared to that of the fast
component (Fig. 4b). In the case of Pc-part selective excitation
(lexc = 615 nm), the bi-exponential character of the time-
resolved fluorescence decay was measured with lifetimes of
58 ꢃ 5 ps and 540 ꢃ 10 ps (the corresponding DAF spectra are
shown in the ESIw).
The transient absorption spectra of the triads 4 and 5 in
DMF at BDP- as well as Pc-part excitation are shown in
Fig. 5. Both triads show strong ground state bleaching of the
Pc moiety at around 690 nm independent of the excitation
wavelength used. Furthermore, even at Pc-part excitation, the
negative DOD signal with maximum at 500 and 560 nm for 4
and 5, respectively, was observed (see Fig. 5). The ground state
bleaching originates from the BDP-part of the triads, apparent
from its spectral position and shape. It is worth noting, that
EET from the initially excited Pc moiety to the BDP part of
the triads is energetically unfavorable (the energy of S1 - S0
transition for SiPc is 1.81 eV, whereas S0–S1 energy gap
has a value of 2.45 and 2.16 eV for BDP-part of 4 and 5,
respectively). Moreover, a newly induced absorption band
appears at around 580 nm, which is assigned to the absorption
For the triad 5 in toluene, the decay of the BDP-part
fluorescence was found to be very fast (tfl o 10 ps, the exact
value of the decay time was not determined due to the limited
time resolution of the experimental setup). The fluorescence
ꢂ
of the radical anion SiPcꢄ of the charge separated state.17,24
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Fig. 5 Transient absorption spectra of the triads 4 (a) and 5 (b) in DMF at BDP- (line) and Pc- (points) part excitation.
Control experiments verified that none of the reference
compounds exhibits this feature alone.
compensation of DOD signal was done at the Pc-part absorption.
Moreover, an additional component of 600 ꢃ 100 ps with a
much lower contribution was also resolved for Pc-part ground
state bleaching.
For the triad 4, it was found that at BDP-part excitation the
recovery of the ground state population of the BDP moiety
occurs monoexponentially with a characteristic time of
40 ꢃ 5 ps. This value is two times longer compared to the
lifetime of the BDP’s first excited singlet state as found in
time-resolved fluorescence measurements. The compensation of
the DOD signal in the spectral range of the Pc-part absorption
shows biexponential decay of the ground state bleaching with
characteristic times of 40 ꢃ 5 and 500 ꢃ 100 ps. The former,
which has a higher contribution, is much faster than the
fluorescence decay times for 4 being excited at 614 nm.
At Pc-part excitation, the recovery of the Pc ground state
population is much slower compared to the above-discussed
case of BDP-part excitation (see Fig. 6). It was fitted by
a biexponential function with the characteristic times of
130 ꢃ 20 and 410 ꢃ 100 ps having practically equal amplitudes.
Nevertheless, no remaining ground state bleaching signal was
observed at delay time of 15 ns resulting in an ISC quantum
yield value, FISC = 0 (the same result was obtained at
BDP-part excitation, too).
Upon excitation at 480 nm (BDP part), the ground state
bleaching of both BDP- and Pc- parts of the triad 4 solved in
nonpolar toluene was observed (Fig. 7a), but one should
remember that depopulation of the first excited singlet state
of the BDP moiety in the triad 4 occurs extremely fast (no
fluorescence was seen from the initially excited BDP). At the
same time, the bleaching of BDP ground state is visible even at
a delay time of 4 ns after photoexcitation. Moreover, we
should emphasize that only EET from the initially photo-
excited BDP part to the SiPc moiety in its ground state will
result in fast recovery of the BDP ground state. Thus one can
conclude that before returning to the BDP ground state after
photoexcitation, energy is ‘‘stored’’ in a long-lived dark state,
which could be a charge-separated (CS) state.
Excitation of the triad 4 at 630 nm showed that no bleaching
of the BDP ground state occurs (see Fig. 7b). The recovery of
the Pc ground state as well as decay of the transient amplification
signal (its intensity is proportional to the population of
the first excited singlet state of the Pc moiety) follow a
monoexponential manner with a characteristic time of 5.2 ns
at BDP- as well as Pc-part excitation. The same decay time
was found for the Pc-part fluorescence of the triad 4 in toluene
in time-resolved fluorescence experiments.
It was found that for the triad 5 in polar DMF, the recovery
of BDP as well as Pc ground state population occurs
monoexponentially with a characteristic time of 35 ꢃ 5 ps
upon excitation at 530 nm. This value is close to the lifetime of
the BDP-part fluorescence (tfl = 30 ꢃ 5 ps, see Table 3), but
shorter than the fast decay component of the Pc-part emission
(65 ꢃ 5 ps). With excitation at the Pc-part, the ground state
population of BDP-part recovers monoexponentially with a
lifetime of 50 ꢃ 20 ps. The same value was obtained when the
The ISC quantum yield, FISC, was calculated to be 0.32 at
BDP- as well as at Pc-part excitation. The same value was
estimated for the reference SiPc compound 1 in toluene.
The transient absorption spectra of the triad 5 in toluene are
similar to those observed in DMF (the spectra are shown in
the ESIw). Independent of the excitation spectral position, a
strong induced absorption band with maximum at around
580 nm (this band belongs to the SiPcꢄꢂ radical anion) as well
as a negative DOD signal with maximum at 560 nm (bleaching
of the ground state absorption of the BDP moiety) were
observed.
It was found that the ground state depletion of the SiPc
moiety recovers monoexponentially with a characteristic time
of 1.7 ꢃ 0.1 ns in both cases of excitation. At the same time, no
transient amplification of probe light was observed in the
spectral region of Pc-part. It means that depopulation of the
Pc first excited singlet state occurs very fast (as it was found by
time-resolved fluorescence measurements), but recovery of its
ground state population is relatively slow due to the fact that
excitation energy is stored in a ‘‘long-lived’’ intermediate state.
Fig. 6 Ground state bleaching (Pc-part) of the triad 4 in DMF at
BDP- and Pc-part excitation.
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Fig. 7 Transient absorption spectra of the triad 4 in toluene at BDP- (a) and Pc- (b) part excitation at different delay times.
singlet state of the BDP-part and excitation of the Pc-part of
the triads, but according to above arguments, we believe that
the FRET is a major mechanism of energy transfer in BDP-Pc
triads.
4. Discussion
The absorption and fluorescence spectra of both the triads 4
and 5 are well described as a superposition of the spectra of the
corresponding components in polar as well as nonpolar
solvents (see Fig. 2 and 3). These observations indicate that
the BDP and Pc moieties within the triads do not couple in the
ground state as well as in the first excited singlet state.
The edge-to-face linkage also hinders planar stacking of the
chromophores in the triads. Therefore, the solvent polarity
only has a minor influence on the molecular geometry,
thus allowing a better comparison of the transfer coupling
coefficients.
In order to estimate the probability of EET between
the initially photoexcited BDP-part and the Pc moiety in
its ground state, the Forster radii, R , for the triads were
¨
0
calculated using the well-known expression:26,27
Z
9000ðln 10ÞFfl
128p5n4Na
dn~
n~4
R60 ¼
Ifnlðn~Þeðn~Þ
;
ð2Þ
where n is the refractive index of used solvent (1.43 for DMF
and 1.49 for toluene); Ffl is the fluorescence quantum yield of a
donor in absence of an acceptor (i.e. the value of the reference
compound 2 in case of the triad 4 or the value of the reference
compound 3 in case of the triad 5); Na is the Avogadro’s
number; e(n~) is the molar absorbance of the Pc-part
at wavenumber n~ and Infl(n~) is the normalized fluorescence
spectrum of the BDP-part. It is noted that the orientation
To explain the results obtained, two different types of
interaction between the BDP and Pc moieties should be taken
into consideration, namely EET and photoinduced CT. It has
been shown that upon excitation at the BDP-part of the triads,
the fluorescence of the Pc-part appears. Hence, the existence of
EET between the photoexcited BDP moiety and the Pc-part in
its ground state is expected.
factor was excluded from the Forster radius calculation. For
¨
A Forster mechanism of EET between the initially photo-
¨
excited BDP-part and the Pc moiety in its ground state was
assumed based on the following reasons: (i) it is well known
the triad 4, the value of R0 was estimated to be 34 A in DMF
and 33 A in toluene, whereas a better overlap between the
fluorescence spectrum of an energy donor (MS-BDP) and the
absorption spectrum of an energy acceptor (SiPc) results in
that exchange mechanism of EET dominates over Forster one
¨
if the Coulomb part of the matrix element of perturbation to
the Hamiltonian is much lower compared to the exchange
term. This is possible, for example, when EET involves
spin-flip transitions of the donor and acceptor molecules
(one example is triplet–triplet EET). In case of BDP-Pc triads
spin-allowed singlet–singlet EET takes place between BDP-
and Pc-parts. (ii) If the exchange mechanism of EET is
dominant, there is usually a small spectral overlap between
the donor’s fluorescence emission and the acceptor’s absorption.
However, this is not the case for the triads. (iii) For efficient
exchange energy transfer, an indispensable condition is an
overlap of the electron clouds of the participating molecules.
Otherwise, the probability of exchange EET falls rapidly with
increasing distance between the donor and acceptor molecules.
The separation between BDP and Pc moieties was found to be
quite short (6 A estimated by the HyperChem program
package).25 However, the covalent linkage prevents a close
contact or staking of the planar chromophores within the
triads, allowing them to orient in an almost orthogonal
manner. Thus it is expected that the probability of exchange
EET in the case of BDP-Pc triads is small. Of course, the
exchange EET could also be an additional mechanism (besides
much higher values of Forster radius for the triad 5 (67 A and
¨
65 A in DMF and toluene, respectively). These values are
much larger than the center-to-center distance between the
donor and acceptor suggesting the presence of an efficient
energy transfer channel from the BDP- to the Pc-part.
As it was shown above, a strong induced absorption band
with maximum at around 580 nm appears during picosecond
pump–probe investigations of the triads (see Fig. 5 and 7a).
According to the data reported in literature,17,24 this band
could be assigned to the induced absorption of SiPcꢄꢂ radical
anion. In order to prove it, the free energy of charge separation
was calculated using the Rehm–Weller28,29 eqn (3):
e2
ꢂ
ꢃ
DGD0 ðAÞ ¼ e EoxdðD=DþÞꢂEredðA=AꢂÞ ꢂE0D;0ðAÞ ꢂ
4pe0esRDA
ꢄ
ꢅꢄ
ꢅ
e2
1
1
1
1
þ
þ
ꢂ
;
ꢂ
rA
þ
8pe0 rD
es eec
ð3Þ
where DGD0 (A) is the free energy of charge-separation (in
electron volts) with selective excitation of an electron donor
D(A)
D (BDP), or an electron acceptor A (SiPc); E
0,0
Forster EET) involved in depopulation of the first excited
¨
is the energy
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of S0 - S1 transition of the donor or acceptor; RDA is the
separation distance between the donor and acceptor moieties
of a triad. Electrostatic interaction of the uprising ions is
considered by the separation distance R of the ions, and es is
the dielectric constant of the solvent in which spectroscopic
measurements were performed (37 for DMF and 2.4 for
toluene). An additional term accounts for the different
dielectric constant eec of the solvent used for the electrochemical
studies (DMF) compared to that for the spectroscopic
measurements. It also accounts the effective radii of the cation
(rD+) and the anion (rAꢂ) radical. As usual, e0 is the vacuum
permittivity and e is the charge of the transferred electron.
transient absorption spectroscopy (ps-TAS). It should be
mentioned that this time is two times longer compared to
the depopulation time of the BDP first excited singlet state,
and was therefore associated with the lifetime of the created
CS state (the rate of charge recombination, kCR, was found to
be 2.5 ꢁ 1010
s
ꢂ1).
At the same time, the decay of the ground state bleaching
signal in the spectral range of the Pc-part absorption has
a biexponential character with lifetimes of 40 ꢃ 5 ps and
500 ꢃ 100 ps, and the latter decay component has a much
lower contribution. It is worth noting that the depopulation of
the Pc first excited singlet state occurs much slower compared
to the above mentioned 40 ps. It means that the ground state
of the Pc-part is recovered mostly due to charge recombination
(CR) followed by the photoinduced CT process:
Triad 4
+
BDP*-SiPc-BDP - BDPꢄ -SiPcꢄꢂ-BDP - BDP-SiPc-BDP,
For the triad 4, the following parameters were used in the
calculations: Eoxd = 1.05 V, Ered = ꢂ0.57 V, ED0,0 = 2.45 eV,
EA0,0 = 1.81 eV, RDA = 6 A, rD+ = 3 A, rAꢂ = 4.6 A. In polar
DMF, the value of DGD0 was found to be ꢂ0.83 eV for BDP-part
excitation, whereas for Pc-part excitation DGA0 = ꢂ0.19 eV.
As DG exhibits a negative value, electron transfer occurs when
either the BDP- or Pc-part of the triad is excited. A charge-
separated state has energy of 1.62 eV and an energy diagram of
the triad 4 in DMF is presented in Fig. 8a.
without touching ‘‘long-lived’’ first excited singlet state
of the Pc-part (e.g. BDP*-SiPc-BDP - BDP-SiPc*-BDP -
BDP-SiPc-BDP, or BDP*-SiPc-BDP - BDP-SiPc*-BDP -
+
BDPꢄ -SiPcꢄꢂ-BDP - BDP-SiPc-BDP). Thus, the photo-
induced CT is the dominant depopulation process of the first
excited singlet state of the BDP-part of the triad 4 in
polar DMF.
This conclusion is supported by steady-state fluorescence
measurements. In fact, one can carry out simple estimations:
at Pc-part excitation, the fluorescence quantum yield of the
The recovery of the ground state population of the BDP-part
after it was selectively excited occurs monoexponentially with
a characteristic time of 40 ꢃ 5 ps, as found by picosecond
Pc
fl
k
Pc
P
triad could be expressed as Ffl ðPc-part excitationÞ ¼
,
ki
i
P
where kPflc is the fluorescence rate,
ki is the sum of all
i
deactivation processes of the first excited singlet state of the
Pc-part (one should remember that the BDP-part of the triad 4
does not have fluorescence when the Pc-part is selectively
excited). The fluorescence quantum yield of the Pc-part
emission at BDP-part excitation could be calculated as
Pc
fl
k
Pc
fl
P
F
ðBDP-part excitationÞ ¼ FEET
ki
i
¼ FEETFPflcðPc-part excitationÞ;
ð4Þ
kEET
kEETþkBDPþ1=t0BDP
where FEET
BDP
¼ is the quantum yield of EET,
are the rates of the energy and charge transfer,
CT
k
EET, k
CT
respectively. Using the experimental data in Tables 2 and 3, it
BDP
can be seen readily that FEET E 0.38, kEET E 0.6k
, and
CT
BDP
CT
k
= 3.1 ꢁ 1010 sꢂ1
.
Excitation of the triad 4 at Pc-part absorption and
compensation of the ground state bleaching has shown that
the recovery of the ground state population follows the kinetic
of the Pc first excited singlet state depopulation. If one takes
into account that the above estimated lifetime of the CS state
is only 40 ps, and CT is a dominant deactivation process
of S1 state population of the Pc moiety, this result is clearly
understandable.
For the triad 4 in nonpolar toluene, the estimation of
the free energy of charge separation gives negative value at
BDP-part excitation (DGD0 = ꢂ0.29 eV), whereas for Pc-part
excitation DGA0 = 0.35 eV. Thus, photoinduced CT is an
energetically favorable process only at BDP-part excitation.
Fig. 8 Scheme of energy levels and transitions between them for the
triad 4 in DMF (a) and in toluene (b).
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A charge-separated state has energy of 2.16 eV, which lies high
above the first excited singlet state of SiPc moiety, and an
energy diagram of the triad 4 in toluene is presented in Fig. 8b.
It should be mentioned that for nonpolar solvents, the influence
of the radii of ions and the center-to-center distance of the
electron donor and acceptor on a calculated value of DG0
increases dramatically due to the small values of the dielectric
constants. Since determination of rD+ and rAꢂ as well as RDA
is quite erroneous and contains a lot of insecurities, the values
determined by eqn (3) are just a very rough estimation.
Nevertheless, the experimental data support the above
estimations, since no quenching of the first excited singlet state
Analysis of the ps-TAS data has shown that the recovery of
the BDP-part ground state population of the triad 4 in
nonpolar toluene occurs very slow compared to the extremely
fast depopulation of its first excited singlet state due to the
slow rate of charge recombination. The lifetime of the CS state
formed was estimated to be 4.5 ns (kCR = 2.2 ꢁ 108 sꢂ1).
Triad 5
Using the Rehm–Weller approach with the following
parameters: Eoxd = 0.88 V, Ered = ꢂ0.57 V, ED0,0 = 2.16 eV,
EA0,0 = 1.81 eV, RDA = 6 A, rD+ = 4 A, rAꢂ = 4.6 A, the
values of free energy of charge-separation at BDP- and Pc-part
excitation, DGD0 and DG0A, of 5 in DMF were estimated to be
ꢂ0.71 and ꢂ0.36 eV, respectively, whereas in toluene these
values were found to be ꢂ0.40 and ꢂ0.05 eV, respectively. As
DG0 exhibits a negative value, CT is energetically favorable
process in both solvents when either the BDP- or Pc-part of
the triad 5 is excited. A CS state has energy of 1.45 eV in DMF
and 1.76 eV in toluene, which lies below the first excited singlet
state of the Pc moiety in polar as well as nonpolar solvents. In
general, the energy diagrams of the triad 5 in DMF and
toluene are similar to those of the triad 4 in polar DMF (see
Fig. 8a), and due to this reason they are given in the ESI.w It
should be emphasized that these estimations are strongly
supported by the experimental observations, since strong
fluorescence quenching and fast depopulation of the first
excited singlet state of the BDP- and Pc-parts of the triad 5
took place in both solvents.
ꢂ
of the Pc moiety was observed, but SiPcꢄ induced absorption
band appears upon BDP-part excitation (see Fig. 7).
Comparing the fluorescence quantum yield values of the
Pc-part fluorescence at BDP- and Pc-part excitation, and
carrying out similar calculations described above, it is easy
to get that FEET E 0.57, and the rate of the photoinduced CT
BDP
CT
k
E 0.76kEET. Interestingly, in nonpolar solvent the
energy transfer becomes a dominant process in depopulation
of the first excited singlet state of the BDP moiety, which is in
contrast to the situation in polar DMF where probability of
EET is lower compared to CT. Due to the fact that the time
resolution of our experimental setups was not sufficient
to resolve ultra fast decay of BDP-part fluorescence, the
estimation of EET and CT rates was not done, but they are
surely faster in DMF. The highest limit for EET rate could be
obtained using the calculated Forster radius, R , value and the
¨
0
simple expression:
Combining the experimental data obtained by steady-state
and time-resolved fluorescence spectroscopy, the efficiency and
ꢀ
ꢁ
6
1
tB0 DP
R0
kEET
¼
k2
;
ð5Þ
RDA
rate of CT and EET processes were calculated to be
BDP
CT
FBCDTPE 0.75, FEET E 0.25, k
= 2.5 ꢁ 1010 sꢂ1 and
where tB0 DP is the lifetime of the BDP moiety in the absence
of an energy acceptor, RDA is the center-to-center
distance between the donor (BDP) and the acceptor (Pc),
k2 = (cos WDA ꢂ 3 cos WD cos WA)2 is the orientation factor,
WDA is the angle between the transition dipole moments of
donor and acceptor, WD and WA are the angles between these
respective vectors and the line joining the centers of the
transitions. It should be emphasized that the accurate calculation
of the orientation factor is a very difficult task lying far from
the aims of the present manuscript. Nevertheless, we were able
to do some reasonable assumptions of the k2 value. For a
related metal-free porphyrin–(Si)–phthalocyanine triad, it was
found that the orientation factor k2 E 0.817. Hence, taking
into account the difference between BDP and porphyrin
chromophores and according to the results of molecular
dynamic simulations, an orientation factor value of 1 was
chosen for further analysis. With tB0 DP = 3.03 ns, RDA = 6 A,
and R0 = 33 A, the EET rate, kEET, is estimated to be
kEET = 8.3 ꢁ 109 sꢂ1 for the 5 in DMF. It was found that
photoinduced CT is a dominant deactivation process of
the first excited singlet state of the BDP moiety. The same
conclusion is valid for the Pc-part of the triad 5 as well.
The lifetime of the CS state generated is shorter than 30 ps,
since the recovery of the ground state population of the
BDP-part at 530 nm excitation follows the decay of its first
excited singlet state as found in the ps-TAS experiments.
For both the triads in polar DMF, the decay of the
first excited singlet state of Pc moiety has a biexponential
character. The results of DAF spectroscopic study have shown
that both decay components (fast and slow) belong to the
same energetic state (both DAF spectra have the same spectral
position of their maximum as well as shapes of the bands, see
Fig. 4 and the ESI.w). One explanation could be that not only
BDP-Pc-BDP triads, but BDP-Pc dyads with only one BDP
chromophore also exist in solution. By comparing the
extinction coefficients of the reference compounds 2 and 3
with those of the BDP-part absorption of the triads 4 and 5, it
can be found that the latter are two times higher,11 clearly
showing that two BDP chromophores are covalently linked to
one Pc moiety. Moreover, as shown by NMR spectroscopy,
the samples are pure, and the signals due to the dyads were
not seen.11
9ꢁ1012
s
ꢂ1. One should take into account that the orientation
factor has a strong influence on the rate of EET (especially in
covalently-linked donor–acceptor molecular systems, in which
its value could strongly differ compared to commonly used 2/3
for homogeneous donor–acceptor mixtures in solution), and in
turn its value is affected by the environment. Due to this
reason the observed EET rate in the triad 4 in DMF is lower
compared to the above estimated one.
It is believed that there are two stable conformations for the
triads, which are stabilized by the solvent. To prove this
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hypothesis DAF experiments for triad 5 in acetonitrile (ACN),
which has the same dielectric constant as DMF (e = 37), were
carried out. The results are presented in Fig. S6 (ESIw). At
Pc-part excitation, two DAF spectra were resolved, but the
fast component has about two orders of magnitude higher in
amplitude compared to the slow one. Thus, not only the
polarity of the solvents, but also their chemical structure
affects the photophysical properties of the triads. Each
conformation could be characterized by its own orientation
and center-to-center distance between BDP- and Pc-parts of
the triads. As a result, the probability of charge transfer from
the initially photoexcited Pc-part is different.
toluene upon Pc-part excitation. It was found that CT is
energetically unfavorable for 4, whereas this process leads to
efficient charge separation in the case of the triad 5.
The charge-recombination of the CS state generated occurs
very fast directly to the ground state with a lifetime of 40 and
30 ps for the triad 4 and 5 in DMF, respectively. A lower
polarity of the solvent stabilizes the CS state leading to a
longer lifetime. The corresponding values in toluene were
determined to be 4.5 and 1.7 ns for the triad 4 and 5,
respectively. The triad 5 in toluene exhibits a broad absorption
region and can effectively generate long-lived CS state via
photoexcitation. This system is therefore a potentially useful
artificial photosynthetic system.
In nonpolar toluene, the behavior of the triad 5 after
photoexcitation is greatly different from that of 4. The
fluorescence of BDP- as well as Pc-part of the triad 5 is
strongly quenched (see Table 2). The fast depopulation of
the first excited singlet state of BDP moiety occurs via
two competitive processes, namely EET and photoinduced
CT. According to the results of steady-state fluorescence
measurements, the efficiency of the EET, FEET, was calculated
to be 0.67. Unfortunately, the decay time of BDP-part
Acknowledgements
The authors would like to thank the DFG (E. A. E. and B. R.:
grant No. ER 588/1-1) and The Chinese University of Hong
Kong (J. Y. L. and D. K. P. N.: a strategic investments
scheme) for financial support.
fluorescence was faster than the time-resolution of the TCSPC
BDP
References
setup, thus a better estimation of kEET and k
could not be
CT
1 J. Barber and B. Andersson, Nature, 1994, 370, 31–34; R. van
¨
Grondelle, J. P. Dekker, T. Gillbro and V. Sundstrom, Biochim.
Biophys. Acta–Bioenerg., 1994, 1187, 1–65; S. E. Bradforth,
R. Jimenez, F. van Mourik, R. van Grondelle and
G. R. Fleming, J. Phys. Chem., 1995, 99, 16179–16191;
made. The rough calculations could be done using the
measured quantum yield of BDP-part fluorescence, FflBDP
(0.002, see Table 2), and taking into account the fluorescence
lifetime, tfl, and the fluorescence quantum yield, Ffl, of the
reference compound 3 in toluene (3.46 ns and 0.78, respectively,
,
G.
McDermott,
S.
M.
Prince,
A.
A.
Freer,
A. M. Hawthornthwaite-Lawless, M. Z. Papiz, R. J. Cogdell and
N. W. Isaacs, Nature, 1995, 374, 517–521; R. K. Clayton,
Photosynthesis: Physical Mechanisms and Chemical Patterns,
Cambridge University Press, New York, 1980; B. P. Krueger,
S. S. Lampoura, I. H. M. van Stokkum, E. Papagiannakis,
J. M. Salverda, C. C. Gradinaru, D. Rutkauskas, R. G. Hiller
and R. van Grondelle, Biophys. J., 2001, 80, 2843–2855.
2 The Photosynthetic Reaction Center, ed. J. Deisenhofer and
J. Norris, Academic Press, San Diego, CA, 1993, vols. 1 and 2.
3 M. Wasielewski, Chem. Rev., 1992, 92, 435–461; D. M. Guldi,
Ffl
1
see Tables 2 and 3): kEET þ kBCDTP ꢅ t
¼ 1:1 ꢁ 1011 sꢂ1
.
FBDP
fl
fl
Then the rates of EET and CT were found to be 7.4 ꢁ 1011 sꢂ1
and 3.6 ꢁ 1010
s
ꢂ1, respectively.
The photoinduced CT is a dominant process of the Pc first
excited singlet state depopulation. Its rate was calculated to be
3.2 ꢁ 1010 sꢂ1 according to the results of fluorescence TCSPC
spectroscopy. The lifetime of the CS state was determined by
ps-TAS to be 1.7 ns, which is much longer compared to that
determined in polar DMF solution, but shorter than the
lifetime of the CS state in the triad 4 in toluene.
Chem. Soc. Rev., 2002, 31, 22–36; L. Sa
N. Martın, A. J. Myles, T. J. Dale, J. Rebek Jr, W. Seitz and
´
nchez, M. Sierra,
´
D. M. Guldi, Angew. Chem., Int. Ed., 2006, 45, 4637–4641;
J. L. Sessler, J. Jayawickramarajah, A. Gouloumis, T. Torres,
D. M. Guldi, St. Maldonado and K. J. Stevenson, Chem.
Commun., 2005, 1892–1894; H. J. Kim, K.-M. Park, T. K. Ahn,
S. K. Kim, K. S. Kim, D. Kim and H.-J. Kim, Chem. Commun.,
2004, 2594–2595; M. E. El-Khouly, O. Ito, P. M. Smith and
F. D’Souza, J. Photochem. Photobiol., C, 2004, 5, 79–104;
5. Conclusion
Phthalocyanine-boron dipyrromethene heteromers are good
candidates for light-harvesting and charge separation
complexes. We were able to demonstrate the potential of
two non-sandwich-type heterotriads as both light-harvesting
complex and charge separation unit in one molecule.
E. A. Ermilov, M. Helmreich, N. Jux and B. Roder, in Chemical
¨
Physics Research Trends Horizons in World Physics, ed.
B. V. Arnold, Nova Science Publishers, 2007, vol. 252,
pp. 215–246.
4 Phthalocyanines-Properties and Applications, ed. C. C. Leznoff and
A. P. B. Lever, VCH-Publishers, New York, 1989, vol. 1;
Phthalocyanines-Properties and Applications, ed. C. C. Leznoff
and A. P. B. Lever, VCH-Publishers, New York, 1989,
Upon BDP-part excitation, two types of transfer processes,
namely energy and charge transfer, were found to be in
competition for both triads independent of the solvent
polarity. The first process delivers the excitation to the first
excited singlet state of the Pc-part, whereas the latter one
populates the CS state directly. It has been found that CT
strongly supersedes EET for both triads in polar DMF.
However, the probability of EET becomes higher than that
of CT in nonpolar toluene.
vols.
2 and 3; Phthalocyanines-Properties and Applications,
ed. C. C. Leznoff and A. P. B. Lever, VCH-Publishers, New York,
1989, vol. 4; N. McKeown, Phthalocyanine Materials: Synthesis,
Structure and Function, Cambridge University Press, Cambridge,
1998; G. de la Torre, C. G. Claessens and T. Torres, Chem.
Commun., 2007, 2000–2015; C. G. Claessens, U. Hahn and
T. Torres, Chem. Rec., 2008, 8, 75–97; S. Makarov,
Ch. Litwinski, E. A. Ermilov, O. Suvorova, B. Roder and
D. Wohrle, Chem.–Eur. J., 2006, 12, 1468–1474.
¨
¨
For both triads in DMF, the direct or indirect (via EET)
population of S1 state of the Pc-part is followed by hole
transfer to the CS state. While the photophysical properties
of both triads are similar in polar DMF, they differ strongly in
5 A. Loudet and K. Burgess, Chem. Rev., 2007, 107, 4891–4932;
G. Ulrich, R. Ziessel and A. Harriman, Angew. Chem., Int. Ed.,
2008, 47, 1184–1201; Y.-H. Yu, Z. Shen, H.-Y. Xu, Y.-W. Wang,
T. Okujima, N. Ono, Y.-Z. Li and X.-Z. You, J. Mol. Struct.,
ꢀc
This journal is the Owner Societies 2009
Phys. Chem. Chem. Phys., 2009, 11, 6430–6440 | 6439

View Article Online
2007, 827, 130–136; M. Wada, S. Ito, H. Uno, T. Murashima,
N. Ono, T. Urano and Y. Uranod, Tetrahedron Lett., 2001, 42,
6711–6713.
D. M. Guldi, J. Am. Chem. Soc., 2006, 128, 15145–15154;
R. W. Wagner, J. S. Lindsey, J. Seth, V. Palaniappan and
D. F. Bocian, J. Am. Chem. Soc., 1996, 118, 3996–3997;
D. Kumaresan, N. Agarwal, I. Gupta and M. Ravikanth, Tetra-
hedron, 2002, 58, 5347–5356; T.-H. Tran-Thi, C. Desforge and
C. Thiec, J. Phys. Chem., 1989, 93, 1226–1233; Z. Zhao,
T. Nyokong and M. D. Maree, Dalton Trans., 2005, 3732–3737;
Z. Zhao, A. O. Ogunsipe, M. D. Maree and T. Nyokong,
J. Porphyrins Phthalocyanines, 2005, 9, 186–197.
6 G. Winter, H. Heckmann, P. Haisch, W. Eberhardt, M. Hanack,
L. Luer, H.-J. Egelhaaf and D. Oelkrug, J. Am. Chem. Soc., 1998,
¨
120, 11663–11673; V. Montes, C. Perez-Bolıvar, N. Agarwal,
´ ´
J. Shinar and P. Anzenbacher, J. Am. Chem. Soc., 2006, 128,
12436–12438; D. Dini, M. Barthel and M. Hanack, Eur. J. Org.
Chem., 2001, 3759–3769; R. A. Wagner and J. S. Lindsey, J. Am.
Chem. Soc., 1994, 116, 9759–9760; T. Akasaka, J. Otsuki and
K. Araki, Chem.–Eur. J., 2002, 8, 130–136; M. Drobizhev,
N. S. Makarov, A. Rebane, G. de la Torre and T. Torres,
J. Phys. Chem. C, 2008, 112, 848–858; P. Liljeroth, J. Repp and
G. Meyer, Science, 2007, 317, 1203–1206.
7 E. Ben-Hur and W. S. Chan, in The Porphyrin Handbook,
ed. K. M. Kadish, K. M. Smith and R. Guilard, Academic Press,
San Diego, 2003, pp. 1–36; Photodynamic Tumor Therapy,
ed. G. Moser, Harwood Academic Publishers, Amsterdam, 1998.
8 J. Karolin, L. B.-A. Johansson, L. Strandberg and T. Ny, J. Am.
Chem. Soc., 1994, 116, 7801–7806; R. P. Haugland, Handbook of
Fluorescent Probes and Research Chemicals, Molecular Probes,
Eugene, OR, 6th edn, 1996; K. Tan, L. Jaquinod, R. Paolesse,
S. Nardis, C. Di Natale, A. Di Carlo, L. Prodi, M. Montalti,
N. Zaccheroni and K. M. Smith, Tetrahedron, 2004, 60,
1099–1106; M.-C. Yee, S. C. Fas, M. M. Stohlmeyer,
T. J. Wandless and K. A. Cimprich, J. Biol. Chem., 2005, 280,
29053–29059; R. W. Wagner and J. S. Lindsey, Pure Appl. Chem.,
1996, 68, 1373–1380.
11 J.-Y. Liu, E. A. Ermilov, B. Roder and D. K. P. Ng, Chem.
Commun., 2009, 1517–1519.
¨
12 O. Korth, T. Hanke, I. Ruckmann and B. Roder, Exp. Tech. Phys.,
¨
¨
1995, 41, 25–36.
13 P. Seybold and M. Gouterman, J. Mol. Spectrosc., 1969,
31, 1–13.
14 R. Sens, PhD Thesis, University of Siegen, 1984.
15 R. F. Kubin and A. N. Fletcher, J. Lumin., 1982, 27, 455–462.
16 J. R. Knutson, D. G. Walbridge and L. Brand, Biochemistry, 1982,
21, 4671–4679.
17 S. Tannert, E. A. Ermilov, J.-O. Vogel, M. T. M. Choi,
D. K. P. Ng and B. Roder, J. Phys. Chem. B, 2007, 111, 8053–8062.
¨
18 J. Lakowicz, Principles of Fluorescence Spectroscopy, Plenum
Press, New York, 1999.
19 G. H. Golub and V. Pereyra, SIAM J. Num. Anal., 1973, 10,
413–432.
20 J. A. Nelder and R. Mead, Comput. J., 1965, 7, 308–313.
21 I. Ruckmann, A. Zeug, R. Herter and B. Roder, Photochem.
¨
Photobiol., 1997, 66, 576–584.
¨
9 A. Costela, I. Garsı
M. Liras, R. Sastre, F. Lo
I. Lopez Arbeloa, J. Photochem. Photobiol., A, 2008, 198, 192–199;
A. Costela, I. Garsıa-Moreno and R. Sastre, in Handbook of
´
a-Moreno, M. Pintado-Sierra, F. Amat-Guerri,
22 J.-Y. Liu, H.-S. Yeung, W. Xu, X. Li and D. K. P. Ng, Org. Lett.,
2008, 10, 5421–5424.
23 Z. Dost, S. Atilgan and E. U. Akkaya, Tetrahedron, 2006, 62,
8484–8488.
´
pez Arbeloa, J. Banuelos Prieto and
´
´
Advanced Electronic and Photonic Materials and Devices,
ed. H. S. Nalwa, Academic Press, San Diego, 2001, vol. 7,
pp. 161–208; S. C. Guggenheimer, J. H. Boyer, K. Thangaraj,
M.-L. Soong and T. G. Pavlopoulos, Appl. Opt., 1993, 32,
3942–3943.
24 G. Kodis, C. Herrero, R. Palacios, E. Marino Ochoa, S. Goul,
L. de la Garza, R. van Grondelle, D. Gust, T. A. Moore,
A. L. Moore and J. T. M. Kennis, J. Phys. Chem. B, 2004, 108,
414–425.
25 Hyperchem, 5.02 evaluation release. HyperCube, Inc., Gainesville,
FL, USA, 1997.
26 T. Forster, Ann. Phys., 1948, 2, 55–75.
¨
10 E. A. Ermilov, S. Tannert, Th. Werncke, M. T. M. Choi,
D. K. P. Ng and B. Roder, Chem. Phys., 2006, 328, 428–437;
¨
F. D’Souza and O. Ito, Coord. Chem. Rev., 2005, 249, 1410–1422,
27 T. Forster, Z. Naturforsch., 1949, 4a, 321–327.
¨
and references therein; A. de la Escosura, M. V. Martınez-Dıaz,
´
D. M. Guldi and T. Torres, J. Am. Chem. Soc., 2006, 128, 4112–4118;
M. S. Rodriguez-Morgade, T. Torres, C. Atienza-Castellanos and
´
28 D. Rehm and A. Weller, Ber. Bunsen-Ges. Phys. Chem., 1969, 73,
834–839.
29 A. Weller, Z. Phys. Chem., 1982, 133, 93–98.
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