Femtosecond fluorescence dynamics of porphyrin in solution and solid films: The effects of aggregation and interfacial electron transfer between porphyrin and TiO2

Article abstract of DOI:10.1021/jp055365q

The excited-state relaxation dynamics of a synthetic porphyrin, ZnCAPEBPP, in solution, coated on a glass substrate as solid films, mixed with PMMA and coated on a glass substrate as solid films, and sensitized on nanocrystalline TiO₂ films were investigated by using femtosecond fluorescence up-conversion spectroscopy with excitation in the Soret band, S₂. We found that the S₂→S₁ electronic relaxation of ZnCAPEBPP in solution and on PMMA films occurs- in 910 and 690 fs, respectively, but it becomes extremely rapid, <100 fs, in solid films and TiO₂ films due to formation of porphyrin aggregates. When probed in the Si state of porphyrin, the fluorescence transients of the solid films show a biphasic kinetic feature with the rapid and slow components decaying in 1.9-2.4 and 19-26 ps, respectively. The transients in ZnCAPEBPP/TiO₂ films also feature two relaxation processes but they occur on different time scales, 100-300 fs and 0.8-4.1 ps, and contain a small offset. According to the variation of relaxation period as a function of molecular density on a TiO ₂ surface, we assigned the femtosecond component of the TiO ₂ films as due to indirect interfacial electron transfer through a phenylethynyl bridge attached to one of four meso positions of the porphyrin ring, and the picosecond component arising from intermolecular energy transfer among porphyrins. The observed variation of aggregate-induced relaxation periods between solid and TiO₂ films is due mainly to aggregation of two types: J-type aggregation is dominant in the former case whereas H-type aggregation prevails in the latter case.

Full text of DOI:10.1021/jp055365q

410
J. Phys. Chem. B 2006, 110, 410-419
Femtosecond Fluorescence Dynamics of Porphyrin in Solution and Solid Films: The Effects
of Aggregation and Interfacial Electron Transfer between Porphyrin and TiO2
Liyang Luo,† Chen-Fu Lo,‡ Ching-Yao Lin,*,‡ I-Jy Chang,§ and Eric Wei-Guang Diau*,†
Department of Applied Chemistry, Institute for Molecular Science and Center for Interdisciplinary Molecular
Science, National Chiao Tung UniVersity, Hsinchu, Taiwan 300, Department of Applied Chemistry, National
Chi Nan UniVersity, Puli, Nantou Hsien, Taiwan 545, and Department of Chemistry, National Taiwan Normal
UniVersity, Taipei, Taiwan 116
ReceiVed: September 21, 2005; In Final Form: NoVember 4, 2005
The excited-state relaxation dynamics of a synthetic porphyrin, ZnCAPEBPP, in solution, coated on a glass
substrate as solid films, mixed with PMMA and coated on a glass substrate as solid films, and sensitized on
nanocrystalline TiO2 films were investigated by using femtosecond fluorescence up-conversion spectroscopy
with excitation in the Soret band, S2. We found that the S2 f S1 electronic relaxation of ZnCAPEBPP in
solution and on PMMA films occurs in 910 and 690 fs, respectively, but it becomes extremely rapid, <100
fs, in solid films and TiO2 films due to formation of porphyrin aggregates. When probed in the S1 state of
porphyrin, the fluorescence transients of the solid films show a biphasic kinetic feature with the rapid and
slow components decaying in 1.9-2.4 and 19-26 ps, respectively. The transients in ZnCAPEBPP/TiO2 films
also feature two relaxation processes but they occur on different time scales, 100-300 fs and 0.8-4.1 ps,
and contain a small offset. According to the variation of relaxation period as a function of molecular density
on a TiO2 surface, we assigned the femtosecond component of the TiO2 films as due to indirect interfacial
electron transfer through a phenylethynyl bridge attached to one of four meso positions of the porphyrin ring,
and the picosecond component arising from intermolecular energy transfer among porphyrins. The observed
variation of aggregate-induced relaxation periods between solid and TiO2 films is due mainly to aggregation
of two types: J-type aggregation is dominant in the former case whereas H-type aggregation prevails in the
latter case.
1. Introduction
component decays in <150 fs and the slow one in 1.2 ps. Later,
Lian and co-workers5 employed fs transient IR spectroscopy to
study the IET dynamics of dye-sensitized nanocrystalline TiO2
thin-film systems; their research focused mainly on Gra¨tzel’s
N3/TiO2 system, and several analogous dye-semiconductor
systems were also investigated.5d,6 These authors found that the
forward IET process was characterized by an ultrarapid com-
ponent with a time constant smaller than 100 fs, and a slow
component that is sensitive to sample conditions. Furthermore,
the rate of electron injection from the N3 dye to various
semiconductors obeyed a trend TiO2 > SnO2 > ZnO, indicating
a strong dependence on the nature of the semiconductor; the
rate of injection depended on the redox potential of the
photosensitizer, pH of the solution, and excitation energy. A
two-state injection model was proposed: the rapid, <100 fs,
component was attributed to injection from a nonthermalized
excited state and the slow component to injection from the
thermalized excited state of the dye.
Sundstro¨m and co-workers discussed the issue of forward IET
rates for N3/TiO2 and other related systems based on measure-
ments of fs UV/visible transient absorption spectra.7 Their results
are similar to those of others who found the interfacial dynamical
process of the N3/TiO2 family to involve two distinct time
scales. The assignment made by the former group is also
similar: the sub-100-fs component corresponds to electron
injection from the singlet excited state, 1MLCT, of a sensitizer
to the conducting band of TiO2 nanoparticles, whereas the 1-10-
ps component relates to electron injection from the triplet excited
state, 3MLCT, to the conducting band of the TiO2 nanoparticles.
The most efficient dye-sensitized solar cells (DSC) with use
of ruthenium polypyridyl complexes as photosensitizers and
nanocrystalline TiO2 films as semiconductor substrates have
achieved energy conversion efficiency greater than 10%.1-3 In
addition to the enhanced surface area due to the nanoporous
nature of TiO2 films, which greatly increases the light-harvesting
capability,1a there are two interfacial electron-transfer (IET)
processes, forward and backward IET, believed to be critical
to the overall efficiency of a DSC.1b The forward IET ensures
electron injection from the electronically excited state of the
molecular dyes into the conduction band of TiO2 through the
carboxylate group that transports electrons effectively.2 The
electrons injected into the conduction band might recombine
with the dye cation through the backward IET channel, thus
reducing the dye molecule to the ground state.1,2 Understanding
the mechanistic features and the corresponding dynamics of
these IET processes would help to improve further the efficiency
of the photovoltaic effect in solar energy conversion and storage.
The electron-injection dynamics of dye/TiO2 systems based
on femtosecond (fs) transient absorption measurements were
first reported from the pioneering work of Tachibana et al.;4
they observed a biphasic temporal profile of which the rapid
* Address correspondence to this author. Fax: (886)-03-572-3764.
E-mail: diau@mail.ac.nctu.edu.tw.
† National Chiao Tung University.
‡ National Chi Nan University.
§ National Taiwan Normal University.
10.1021/jp055365q CCC: $33.50 © 2006 American Chemical Society
Published on Web 12/07/2005

Femtosecond Fluorescence Dynamics of Porphyrin
J. Phys. Chem. B, Vol. 110, No. 1, 2006 411
The work of Kuciauskas et al.8 using ruthenium and osmium
polypyridyl complexes as photosensitizers supported his two-
state mechanism. Furthermore, because the excited state is
localized on a ligand of the sensitizer that is not attached to the
semiconductor, the rate of so-called “interligand” electron
transfer was found also to occur on a ps time scale.7e Kallioninen
et al.7f compared all recent temporally resolved experiments
under varied conditions of sample preparation and concluded
that a large photon energy of the pump and exposure of the
sensitized film to air considerably altered the slower (ps)
kinetics; these effects were attributed mostly to degradation of
the dye. In addition, the observed nonstatistical behavior of
electron injection implies that interfacial electron injection might
occur from a photosensitizer with an excited-state redox potential
below the conduction band edge of the semiconductor, indicating
that the forward IET following an “upstream” potential-energy
pathway is possible.7a
CHART 1: Chemical Structure of ZnCAPEBPP
unique detection window to monitor only the relaxation process
occurring in the excited state of the dye without involving the
complicated ionic states of the dye and the semiconductor after
charge separation. Thus, interpretation of kinetic data obtained
from these fluorescence decay methods should be simpler and
more straightforward than that obtained from transient absorp-
tion methods. In the present work, we have employed a fs
fluorescence up-conversion technique to interrogate the forward
IET dynamics of the ZnCAPEBPP-sensitized TiO2 nanocrys-
talline films upon excitation into the S2 state of the porphyrin
(λex ) 420-430 nm). Because aggregation of porphyrins might
be a drawback for photoenergy conversion, we have assessed
the effect of aggregation through comparison of the relaxation
dynamics obtained from ZnCAPEBPP/THF solution, pure
ZnCAPEBPP deposited on a plate-glass substrate, and the
ZnCAPEBPP/PMMA mixture deposited on a plate-glass sub-
strate. We found that S2 f S1 internal conversion (IC) of
ZnCAPEBPP becomes extremely rapid when the porphyrin
molecules stack together to form J-type aggregates; the S1
species are produced within that time scale, <100 fs, but
disappear with a biphasic dynamical nature: the rapid compo-
nent decays in ∼2 ps whereas the slow one decays in ∼20 ps.
The fluorescence transients of the ZnCAPEBPP/TiO2 system
observed in the S1 state also feature two major decay compo-
nents, but the decay coefficients are on much shorter time
scales: 100-300 fs and 1-4 ps for the rapid and slow decays,
respectively. We assigned the rapid component to electron
injection from porphyrin to TiO2; the slow component, of which
the decay coefficients are sensitive to the molecular density of
porphyrins on TiO2 films, are due mainly to intermolecular
energy transfer among porphyrins.
Ultrarapid IET dynamics of many other systems with use of
organic dyes as photosensitizers have been investigated.9-17
A
common feature of the forward IET dynamics of all dye/TiO2
systems is the biphasic nature of decay of the transients with
rapid and slow processes occurring on fs and ps time scales,
respectively; the rapid process has been coherently assigned to
involve electron injection directly from dye to TiO2 whereas
the assignment of the slow process remains under discussion.
For example, Wenger et al.18 reported that the slow component
of the N3/TiO2 system is due to the aggregated state of the dye,
which is against the two-state mechanism aforementioned.
Another ultrarapid investigation19 indicates, however, that the
electron-injection dynamics of the porphyrin/TiO2 system are
similar to those of the N3/TiO2 system, making the former
system a prospective candidate for DSC applications.20-24
The capability of porphyrins to absorb visible light in the
400-450 nm region (B or Soret band) and in the 500-700 nm
region (Q bands) has been used in the design of artificial light-
harvesting systems.25 As examples, multiporphyrin arrays have
been constructed with several linkers that are suitable for the
preparation of linear or extended architectures via attachment
at the meso position.26,27 Recent advances in organic synthesis
have allowed the preparation of linear and semirigid harvesting
arrays with use of porphyrin pigments linked by phenylethyne
(PE) spacers.28 On the basis of a similar idea, we have designed
zinc porphyrin pigments to serve as photosensitizers in our
investigation of interfacial electron-transfer dynamics between
porphyrins and TiO2 in a systematic way. The molecular
structure of the sensitizer system is comprised of three parts: a
zinc biphenylporphyrin (ZnBPP) unit serves as light-harvesting
center to provide photoinduced electrons, the PE group is
connected to the meso position of the porphyrin to transfer the
electrons efficiently, and the carboxylic acid (CA) attached to
the end of the spacer serves as an anchoring group to bond
tightly onto the surface of TiO2 nanoparticles. To vary system-
atically the distance between the electron donor (porphyrin) and
the electron acceptor (TiO2), the π-conjugated linker has been
extended from one to four PE spacers with only one CA end
group; hence we denote the porphyrin pigments as ZnCA-
(PE)xBPP, with x ) 1-4 for the series; the chemical structure
of ZnCAPEBPP is shown in Chart 1.
2. Experimental Section
2.1. Materials. A standard Sonogashira cross-coupling
method was used to synthesize porphyrin ZnCAPEBPP.29 The
reactions were monitored by TLC and UV/visible spectra. Cross-
coupling mono-brominated biphenylporphinato zinc with 3 equiv
of 4-ethynylbenzoic acid typically afforded ZnCAPEBPP in 80%
yield after purification on silica gel with MeOH/CH2Cl2 eluants
and crystallization from THF/hexane. 4-Ethynylbenzoic acid was
prepared from trimethylsilylethyne and 4-iodobenzoic acid in
65% yield. A glovebox (MBraun Uni-lab), a vacuum line, and
standard Schlenk techniques were employed to process all
materials sensitive to air. Solvents (Merck, Darmstadt, Ger-
many), Pd(PPh3)4 catalyst (Strem Chemical Inc, MA), and other
chemicals (Acros Organics, NJ) were obtained from the
indicated suppliers. Solvents used in cross-coupling experiments
were purified according to literature methods.30 Chromato-
graphic purification was performed with silica gel 60 (230-
Most recent results on IET dynamics of dye/semiconductor
systems were obtained from fs transient absorption measure-
ments, but there are few reports on such systems based on using
the fs fluorescence decay methods (e.g., ref 9) because of the
minuscule quantum yield of fluorescence in the presence of the
IET process. Fluorescence decay methods offer, however, a

412 J. Phys. Chem. B, Vol. 110, No. 1, 2006
Luo et al.
Figure 2. Femtosecond fluorescence up-conversion spectrometer
setup: A, aperture or iris; BS1-BS3, beam splitters; F1-F2, color
filters; L1-L4, focusing lenses; M1-M6, broadband dielectric mirrors;
NC, nonlinear crystal; P1-P2, parabolic mirrors; RS, rotary sample
cell; PMT, photomultiplier tube; SHG, optical set for second harmonic
generation.
Figure 1. SEM photomicrographs of an unsensitized TiO2 film. The
average size of particles is ∼15 nm; the thickness of the film is ∼2
µm.
C-440, PicoQuant) coupled with a laser-diode driver (PDL-
800B, PicoQuant) that produces excitation pulses at 435 nm
with an average power of ∼300 µW.
400 mesh, Merck). All NMR solvents (Cambridge Isotope Lab,
Inc.) were used as received. Data for characterization of
ZnCAPEBPP follow: 1H NMR (d6-DMSO at 2.50 ppm) 10.36
(s, 1H), 9.84 (d, J ) 5 Hz, 2H), 9.46 (d, J ) 5 Hz, 2H), 8.92
(d, J ) 5 Hz, 2H), 8.84 (d, J ) 5 Hz, 2H), 8.25 (m, 8H), 7.86
(m, 6H). Elemental analysis of C41H24N4O2Zn‚THF, calculated
C 72.83, H 4.35, N 7.55, found C 72.35, H 4.60, N 7.27.
2.2. Sample Preparation. Samples were prepared in both
solution and thin films. For solutions, ZnCAPEBPP was
dissolved in THF (Merck, spectral grade) to form a homoge-
neous solution of concentration 1.0 × 10-5 M. The pure
ZnCAPEBPP solid films were obtained by spin-coating the
ZnCAPEBPP/THF solution onto a plate-glass substrate and
allowing the solvent to evaporate slowly in a vacuum hood. To
make thin films in solid solution, we mixed ZnCAPEBPP with
a transparent polymer, poly(methyl methacrylate) (PMMA), in
a 1:5000 ratio by mass. The ZnCAPEBPP/PMMA mixture was
dissolved in trichloromethane (Merck, pro analysi) to form a
viscous solution and then smoothly deposited onto a glass plate
via a standard spin-coating procedure.
The procedure for synthesis of TiO2 nanoparticles is described
elsewhere.31 Briefly, a quantity of titanium isopropoxide was
added, dropwise at 25 °C, to nitric acid (0.1 M, 750 mL) under
vigorous stirring. Immediately after this hydrolysis, the white
slurry was heated at 80 °C and the solution was stirred
vigorously for 8 h to achieve peptization. The nanoparticle thin
films of TiO2 were prepared with 15 layers by spin-coating on
glass and heating at 450 °C in an oven for 30 min. The electron
micrograph of the TiO2 nanocrystalline thin film (Figure 1)
shows that the size of TiO2 particles is about 15 nm and the
film thickness is ∼2 µm. Porphyrin molecules were adsorbed
onto TiO2 nanocrystalline thin films from ZnCAPEBPP solu-
tions in THF/CH2Cl2 (1:14) for 4 h. To examine the effect of
aggregation affecting the observed IET dynamics, we prepared
three porphyrin-sensitized TiO2 thin-film samples with initial
concentrations 37 (Film 1), 73 (Film 2), and 150 µM (Film 3)
of porphyrin solutions.
2.4. Time-Resolved Fluorescence Measurements. Figure 2
shows the experimental setup for femtosecond (fs) time-resolved
fluorescence measurements. Basically, a fluorescence optically
gated (up-conversion) system (FOG100, CDP) is implemented
in combination with a mode-locked Ti-sapphire laser (Mira
900D, Coherent) pumped with a 10-W Nd:YVO4 laser (Verdi-
V10, Coherent). The femtosecond laser system generates output
pulses at 860 nm (or 840 nm) with duration ∼120 fs at a
repetition rate of 76 MHz. The frequency of the laser pulse was
doubled for excitation (λex ) 430 or 420 nm). The residual
fundamental pulse was used as an optical gate; a dichroic beam
splitter served to separate excitation and gate beams. The
excitation beam intensity was appropriately attenuated, and then
focused onto a 1-mm rotating cell containing either the solution
or the solid thin-film samples. The fs system includes two optical
paths for excitation: path one is designed for measurements of
thin-film samples via collection of emission from the front face;
path two is designed for measurements of solution samples via
collection of emission from the back face.32 The emission was
collected with two parabolic mirrors and focused onto a crystal
(BBO type-I); the gate pulse was also focused on that BBO
crystal for sum-frequency generation. The latter signal was
collected with a lens, and separated from interference light with
an iris, a band-pass filter, and a double monochromator (DH10,
Jobin Yvon) in combination, then detected with a photomulti-
plier (R1527P, Hamamatsu) connected to a computer-controlled
photon-counting system. On varying the temporal delay between
gate and excitation pulses via a stepping-motor translational
stage, we obtained a temporal profile (transient). The polariza-
tion between pump and probe pulses was fixed at the magic-
angle condition, 54.7°.
2.5. Data Analysis and Kinetic Model. The fluorescence
transient signal, I(t), was fitted through convolution of the
instrument-response function g(t) with an appropriate molecular-
response function f(t),33,34
2.3. Steady-State Spectral Measurements. UV/visible ab-
sorption spectra of ZnCAPEBPP in solution and in solid thin
films were recorded with a standard spectrophotometer (Cary
50, Varian). Emission spectra were measured with a composite
CCD spectrometer (USB2000FLG, Ocean Optics) via a Y-shape
fiber (R600-UV, Ocean Optics) for front-face probes. The
excitation source contains a pulsed diode-laser head (LDH-P-
I(t) ) ∞ g(t - s)f(s) ds
(1)
∫
0
in which g(t) is a Gaussian function; the choice of f(t) relies on
a proper kinetic model. When a rising feature appears in the
transients, a consecutive kinetic model is employed to fit

Femtosecond Fluorescence Dynamics of Porphyrin
J. Phys. Chem. B, Vol. 110, No. 1, 2006 413
absorption spectra of ZnCAPEBPP on solid films of various
types. The absorption spectrum in solution consists of two
branches, the B (or Soret) band and the Q bands, traditionally
identified as the S2 r S0 and S1 r S0 transitions, respectively.
Maxima of these bands at 435, 560, and 615 nm are assigned
as B(0,0), Q(1,0), and Q(0,0), respectively; the number of quanta
shown in parentheses represents the dominant Franck-Condon
active vibrational mode. The fluorescence spectrum in solution
(dashed curves) clearly shows both S2 f S0 and S1 f S0
emissions with the corresponding maxima located at 447 and
621 (and 670) nm, respectively.
Because of aggregation, the sharp Soret band in solution
becomes substantially broadened when porphyrin molecules are
coated on a plate-glass substrate as a thin film or on a TiO2-
nanocrystalline thin film, shown as a dotted curve or dashed
curve in the inset of Figure 3, respectively. Furthermore, the
absorption maxima of the molecule on a glass film are red-
shifted with respect to those in solution, indicating that the head-
to-tail (J-type) aggregation might be involved upon formation
of porphyrin aggregates in films. In contrast, the Soret band of
the TiO2 film is shifted toward the blue side of the spectrum,
indicating that formation of porphyrin aggregates inside the
nanoenvironment of a TiO2 thin film is mainly of H type. When
the porphyrin molecules were evenly distributed within a
PMMA matrix, the aggregation effect was minimized and the
absorption spectrum of a PMMA film (solid curve shown in
the inset of Figure 3) is identical with that of solution. To
improve our understanding of the effects of aggregation and
interfacial energy transfer between porphyrins and TiO2 nano-
particles, we performed a time-resolved investigation on Zn-
CAPEBPP in the above environments using a femtosecond
fluorescence up-conversion method.
Figure 3. Absorption (solid curves) and fluorescence (dashed curves)
spectra of ZnCAPEBPP in THF with B (Soret) and Q bands (enlarged
by a factor 5) as indicated. The sample concentration was 5 × 10-6
M. The inset shows relative absorption spectra of ZnCAPEBPP
deposited as a pure film on a glass substrate (dotted curve) mixed with
PMMA (1:5000) to form a uniform thin film on a glass substrate (solid
curve) and sensitized on a nanocrystalline TiO2 thin film (dashed curve).
The absorbance of each spectrum in the inset is normalized at the
maximum of the S2 band for comparison.
appropriately the transients,
A τ
9
18B
9
τ28C
(2)
in which component A is described with a single-exponential
function with a decay coefficient τ1 and component B is
described with a biexponential function with a rise coefficient
τ1 and a decay coefficient τ2. The molecular response function
f(t) is expressed accordingly as
3.2. Relaxation Dynamics of ZnCAPEBPP in Solution.
Porphyrin molecules behave as if effectively separate in THF
solvent.35 Therefore, time-resolved spectral data in ZnCAPEBPP/
THF solution provide pertinent information about the dynamical
behavior of porphyrin monomers in free solvents. Figure 4A
shows three fluorescence transients of a ZnCAPEBPP/THF
solution with excitation at λex ) 420 nm and emission observed
at λem ) 470, 620, and 675 nm. The transient at λem ) 470 nm
(circles) is well fitted with a single-exponential decay function
with a decay coefficient τ ) 910 ( 10 fs; the uncertainty
represents two standard errors. The transients observed at λem
) 620 (squares) and 675 nm (triangles) differ remarkably from
the transient observed at λem ) 470 nm: the former show a
rising feature with a rise coefficient comparable to the decay
coefficient of the latter in the short-time region; the transient
signals of the former increase gradually to an asymptotic level
in the long-time region, as shown in parts B and C of Figure 4
for λem ) 620 and 675 nm, respectively. According to the
steady-state spectral measurements shown in Figure 3, the
transients observed at λem ) 470 and 620 nm (and 675 nm)
reflect the dynamical behavior occurring in the S2 and S1
electronic states, respectively. We therefore conclude that the
S2 f S1 internal conversion of ZnCAPEBPP in solution takes
∼1 ps and the vibrationally hot S1 species are produced on that
time scale.
t
τ1
t
τ2
t
τ1
f(t) ) a exp -
+ b exp -
- exp -
(3)
(
)
(
)
(
)
]
[
in which the preexponential factors a and b represent the relative
weights (or amplitudes) of components A and B, respectively.
Alternatively, a parallel kinetic model is employed to account
for the observed temporal profiles showing a biphasic dynamical
feature
A τ
9
18B
(4)
(5)
A′
9
τ28 B′
in which components A and A′ are described with a single-
exponential function and decay coefficients τ1 and τ2, respec-
tively. The molecular response function f(t) is expressed
accordingly as
t
τ1
t
τ2
f(t) ) a exp -
+ b exp -
(6)
(
)
(
)
in which the preexponential factors a and b represent the relative
weights (or amplitudes) of components A and A′, respectively.
Because of the rising character of the transients shown in
Figure 4, the transients observed at λem ) 620 and 675 nm are
described satisfactorily according to a consecutive kinetic model
with two components convoluted with the laser pulse: the first
component (olive curves) rises in τ1 ) 910 fs and then decays
in τ2 ∼ 9 ps whereas the second component (magenta curves)
appears following the decay of the first component but persists
3. Results and Discussion
3.1. Steady-State Spectra of ZnCAPEBPP in Various
Environments. Figure 3 shows the UV/visible absorption (solid
curves) and fluorescence (dashed curves) spectra of Zn-
CAPEBPP in THF solution; the inset shows the normalized

414 J. Phys. Chem. B, Vol. 110, No. 1, 2006
Luo et al.
Figure 4. Femtosecond fluorescence transients of ZnCAPEBPP in THF
solution (1 × 10-5 M) obtained at λem/nm ) (A) 470 (open circles),
(B) 620 (open squares), and (C) 675 (open triangles), with excitation
at λex ) 420 nm for λem ) 470 nm and λex ) 430 nm for λem ) 620
and 675 nm. Open symbols denote raw data; solid black curves
represent theoretical fits with residuals shown as green traces; the olive
and magenta curves under each transient are deconvoluted components.
Figure 5. Femtosecond fluorescence transients of ZnCAPEBPP mixed
with PMMA and spin-coated on a glass substrate obtained at λex
)
420 or 430 nm with fluorescence probed at λem/nm ) (A) 460 (open
circles), (B) 620 (open squares), and (C) 675 (open triangles). Excitation
was performed at λex ) 420 nm for λem ) 460 nm and λex ) 430 nm
for λem ) 620 and 680 nm. Open symbols denote raw data; solid black
curves represent theoretical fits with residuals shown as green traces;
the olive and magenta curves under each transient are deconvoluted
components.
on the observed 100-ps scale (τ3 ∼ 2 ns). As the transients
observed at λem ) 620 and 675 nm represent the relaxation
dynamics of ZnCAPEBPP in the S1 state, the observed two
consecutive components might be assigned to the hot and cold
S1 species, respectively. Accordingly, the hot S1 species were
produced from the S2 state in ∼1 ps. The excess vibrational
energy in the hot S1 species is released to the solvent molecules
via vibrational relaxation, and this process yields the cold S1
species being produced in ∼9 ps. The cold S1 species are
relatively enduring because S1 f T1 intersystem crossing of
ZnCAPEBPP in solution occurs in ∼2 ns, as is confirmed by
our ps measurements. Our real-time observation on the Zn-
CAPEBPP system is consistent with femtosecond results for
the zinc tetraphenylporphyrin (ZnTPP) system, for which the
lifetime of the S2 state of ZnTPP is reported to be in a range
1.45-2.35 ps, and vibrational cooling from the nonrelaxed
vibronic state of the hot S1 species takes place on a time scale
of ∼10 ps.36-38
time coefficients τ1, τ2, and τ3 evaluated to be 690 fs, 12 ps,
and ∼2 ns, respectively. Our assignment for the observed
excited-state dynamics of a ZnCAPEBPP/PMMA film is similar
to the case in THF solution: the observed relaxation time τ1 )
690 fs is due to efficient S2 f S1 IC; the observed 12-ps
component represents energy transfer from the hot S1 species
to the PMMA environment; the offset component (τ3 ∼ 2 ns)
is due to S1 f T1 ISC from the cold S1 species.
The observed relaxation period τ1 ) 690 fs in the transient
of the ZnCAPEBPP/PMMA film is similar to that in THF
solution (τ1 ) 910 fs). The former at λem ) 460 nm involves,
however, an “offset” component that is completely absent in
the solution sample (Figure 5A vs Figure 4A), implying that
some ZnCAPEBPP molecules in the PMMA film do not
undergo S2 f S1 IC to the S1 state, probably due to the
restriction of nuclear motions in the solid state. The observation
of a dominating fast-decay component in the transient indicates
that the S2 f S1 IC is still a major process to be considered for
ZnCAPEBPP isolated inside the PMMA matrix. Because large-
amplitude nuclear motions of porphyrin molecules are restricted
in the solid state whereas motions are relatively free in liquid
solution, we expect that the observed offset signal in the former
is due to the contribution of intramolecular motions constrained
in the PMMA matrix that impedes the S2 f S1 IC. We speculate
that the in-plane expansion and compression motions of the
porphyrin ring might be involved for such an efficient electronic
relaxation to occur.
3.3. Relaxation Dynamics of ZnCAPEBPP/PMMA Solid
Films. When the ZnCAPEBPP molecules are evenly mixed with
PMMA on a solid film, the observed fluorescence transients
are similar to those in a homogeneous liquid solution. Figure
5A shows the corresponding transients of a ZnCAPEBPP/
PMMA film with excitation performed at λex ) 420 nm and
emissions observed at λem ) 460, 620, and 680 nm. The transient
at 460 nm was fitted with two components: the major part of
the transient decays in 690 ( 20 fs but there exists a minor
offset component that is persistent on a 10-ps scale. The
transients observed at both 620 and 680 nm exhibit a rising
feature that is comparable to the decay of the transient observed
at 460 nm. According to the same fitting procedure employed
for the solution data aforementioned, both transients were fitted
according to a consecutive kinetic model with the corresponding
As mentioned earlier, the hot S1 species were produced after
IC from the S2 state. In solution, the excess vibrational energy
of ZnCAPEBPP in the S1 state would be carried away in ∼9
ps by the solvent molecules (Figure 4B,C). For the PMMA film,

Femtosecond Fluorescence Dynamics of Porphyrin
J. Phys. Chem. B, Vol. 110, No. 1, 2006 415
Figure 6. Femtosecond fluorescence transients of ZnCAPEBPP as a
pure solid film on a glass substrate (open circles) obtained at λex
)
420 nm with fluorescence probed at λem ) 470 nm (fwhm ) 200 fs).
The solid curve is the theoretical fit with residuals shown in the lower
panel. The dashed curve shows a theoretical fit of the transient of a
ZnCAPEBPP/PMMA film for comparison.
Figure 7. Femtosecond fluorescence transients of ZnCAPEBPP as a
we observed that the vibrationally hot S1 species transfer their
excess internal energy into the surrounding PMMA in ∼12 ps
(Figure 5B,C). The similarity for the UV/visible spectra and
the relaxation dynamics between the PMMA film and the
solution sample indicates that the ZnCAPEBPP molecules were
perfectly isolated inside the PMMA matrix in the solid state
and that intermolecular interactions between porphyrin mol-
ecules become negligible.
pure solid film on a glass substrate (open circles) obtained at λex
)
430 nm with fluorescence probed at λem/nm ) (A) 620, (B) 640, (C)
660, and (D) 680. Solid black curves represent theoretical fits with
residuals shown as green traces; the blue and olive curves under each
transient are deconvoluted components.
aggregation. Because the intermolecular energy transfer between
porphyrins is so rapid, the relatively slow S1 f T1 ISC process
is unlikely to compete, which causes the disappearance of the
ns component in the transients and the quenching of the
fluorescent intensity.
3.4. Relaxation Dynamics of Pure ZnCAPEBPP Solid
Films. When pure ZnCAPEBPP molecules were spin-coated
as a thin film on a glass substrate without mixing with PMMA,
two peculiar spectral features appear. First, the absorption bands
of the sold film become broadened and the maxima are red-
shifted as mentioned above (Figure 3). Second, the ZnCAPEBPP
film shows no appreciable emission with use of a conventional
spectrofluorometer, i.e., aggregation between porphyrins would
produce strong intermolecular interactions, which would cause
significant fluorescence quenching. Figure 6 shows the fluo-
rescence transient of a ZnCAPEBPP film recorded at λex ) 420
nm and λem ) 470 nm. The transient exhibits a spike-like
temporal profile with the decay coefficient indeterminate
because of limited time resolution of the instrument (τ <100
fs). That is, the S2 f S1 IC process of ZnCAPEBPP becomes
extremely rapid when the porphyrin molecules stack together
to form aggregates on a plate-glass substrate and the hot S1
species are produced within that time scale (<100 fs). To probe
the S1 dynamics of the ZnCAPEBPP thin film, the detection
window must move to the longer wavelength region.
As our instrument cannot resolve the decay coefficient of
the transient at λem ) 470 nm, we observed no rising character
of transients taken at λem ) 620, 640, 660, and 680 nm, as shown
from top to bottom in Figure 7, parts A-D, respectively. The
transients shown in Figure 7 display a biexponential decay
feature, which was fitted with two components: the rapid
component decays in 1.9-2.4 ps whereas the slow component
decays in 19-26 ps. The ns component was completely absent
in all four transients. This evidence is important for us to
rationalize quenching of fluorescence from porphyrins upon
3.5. Relaxation Dynamics of ZnCAPEBPP on Nanocrys-
talline TiO2 Films. We investigated the relaxation dynamics
of ZnCAPEBPP/TiO2 thin films to gain dynamical information
for an understanding of the rate of interfacial electron transfer
between porphyrin and a nanocrystalline semiconductor film.
From results discussed in section 3.4, we understand that
aggregation of porphyrins plays an important role in the
observed relaxation dynamics. As a result, we prepared thin-
film samples by controlling the initial immersion concentration
of ZnCAPEBPP solutions to vary the adsorbed molecular
density on three identical nanocrystalline TiO2 films. Figure 8
shows absorption spectra of ZnCAPEBPP/TiO2 thin-film samples
from top to bottom with decreasing absorbance of the Q bands,
i.e., ZnCAPEBPP is more concentrated on Film 3 but it is less
condensed on Film 1. Note that the shapes and the positions of
the Q bands shown in Figure 8 are very similar irrespective of
their large discrepancies in optical density. This might indicate
that there are no significant differences in the degree of
aggregation for the samples with different optical densities, as
was previously observed by Kamat and co-workers for cresyl
violet dye adsorbed on metal oxide surfaces.39 However, in our
case the formation of H-aggregates of porphyrins on TiO2
surfaces was very sensitive to the shape and position of the
Soret band but it has very little influence for the Q bands (Figure
3). Therefore, the degree of aggregation might be different as
the optical densities of ZnCAPEBPP on TiO2 films were

416 J. Phys. Chem. B, Vol. 110, No. 1, 2006
Luo et al.
Figure 8. Absorption spectra of ZnCAPEBPP-sensitized TiO2 films
immersed in THF/CH2Cl2 (1:14) co-solvents at three concentrations
of ZnCAPEBPP: 3.7 × 10-5 M (Film 1); 7.3 × 10-5 M (Film 2); and
1.5 × 10-4 M (Film 3). The immersion duration is 4 h for all three
TiO2 films.
Figure 10. Femtosecond fluorescence transients of ZnCAPEBPP-
sensitized TiO2 films under three conditions: (A) Film 1, (B) Film 2,
and (C) Film 3, obtained at λex ) 430 nm with fluorescence probed at
λem ) 620 nm (fwhm ) 220 fs). All transients are fitted with a parallel
model with a small offset as indicated. Open circles denote raw data;
solid black curves represent theoretical fits with residuals shown as
green traces; the blue, olive and magenta curves under each transient
are deconvoluted components.
three parts the fluorescence transients of Film 1-Film 3 at λem
) 620 nm; Figure 11A-C shows the fluorescence transients
of Film 1-Film 3 at λem ) 680 nm. All transients were fitted
with two decay components with a tiny offset. At λem ) 620
nm, the rapid-decay coefficient τ1 was observed to be 200 (
20 fs in Film 1 but decreased to 130 ( 10 fs in Film 2 and
further to 100 ( 10 fs in Film 3; the slow-decay coefficient τ2
decreased significantly from 4.1 ( 1.1 ps in Film 1 to 0.8 (
0.1 ps in Film 3; at λem ) 680 nm, τ1 is almost constant (∼300
fs) for Film 1-Film 3 whereas τ2 decreased from 3.7 ( 0.7 ps
in Film 1 to 1.9 ( 0.6 ps in Film 3. The above real-time
observations indicate three major points. First, due to the effect
of aggregation, the S2 f S1 IC was still ultrarapid because we
observed no rising character from all transients shown in Figures
10 and 11. Second, the values of τ1 in the ZnCAPEBPP-
sensitized TiO2 films are less than one tenth those of Zn-
CAPEBPP solid films (Figure 7), indicating that relaxation of
the former is much more efficient than that of the latter; values
of τ2 in the former are comparable to the values of τ1 in the
latter. This comparison implies that one must consider the effect
of aggregation of porphyrin to account for the observed slow
relaxation dynamics of the former via intermolecular energy
transfer.26,33,40,41
Figure 9. Femtosecond fluorescence transients of a ZnCAPEBPP-
sensitized TiO2 film (open circles) and an unsensitized TiO2 film
(dashed curves) obtained at λex ) 420 nm with fluorescence probed at
λem ) 470 nm (fwhm ) 200 fs). The solid curve is the theoretical fit
with residuals shown in the lower panel.
increased by a factor of 4; the corresponding dynamics will be
discussed in the following.
Figure 9 shows the S2 dynamics of a ZnCAPEBPP/TiO2 film;
the spike-like transient signal of the ZnCAPEBPP-sensitized
TiO2 film was observed at λex ) 420 nm and λem ) 470 nm
and no transient signal was observed for the unsensitized TiO2
film (dashed curve shown in Figure 9). This dynamical feature
is similar to that of ZnCAPEBPP thin-film samples that involved
no TiO2 nanoparticles: the observed ultrarapid relaxation
dynamics (<100 fs) reflect not only the aggregate-induced
energy transfer among porphyrins but also the interfacial electron
transfer from porphyrin to TiO2. The limited temporal resolution
of our instrument precluded resolution of the IET dynamics from
the transient.
Third, although the slow relaxation coefficient τ2 decreased
significantly from the sample condition varying from small
density (Film 1, OD ) 0.2) to larger density (Film 3, OD )
0.8), the rapid decay periods are generally less sensitive (at λem
) 620 nm) or insensitive (at λem ) 680 nm) to the large variation
of molecular density on the TiO2 surface. The trend of variation
of both τ1 and τ2 values at λem ) 620 nm is consistent with
involvement of an efficient energy transfer among isoenergetic
porphyrin pigments: the fs component (τ1) results from both
The relaxation dynamics of ZnCAPEBPP-sensitized TiO2
films occurring in the S1 state of porphyrin were studied with
excitation at λex ) 430 nm and detection in the emission
wavelength region 600-700 nm. Figure 10A-C displays in

Femtosecond Fluorescence Dynamics of Porphyrin
J. Phys. Chem. B, Vol. 110, No. 1, 2006 417
because the detection window at the former wavelength would
probe more “hot” species than that at the latter wavelength. We
therefore conclude that the internal energy in the S1 state of the
former (ZnCAPEBPP) is much greater than that of the latter
(ZnTCPP), which leads to an increased electron injection
fraction (fs component) being observed for the former.
Moreover, we found that the molecular structure of the former
is planar whereas that of the latter is nonplanar with the linker
(phenyl group) twisted with respect to the porphine plane by
∼60°. We thus expect that conjugation of the π-electrons in
the former should be better than that in the latter. However, the
IET rate of the former is slower than that of the latter; we think
that this condition might reflect the number of anchoring
carboxylate groups in porphyrin: there are four carboxylate
groups in ZnTCPP but there is only one in ZnCAPEBPP. Hence,
when ZnTCPP molecules are excited to the S1 state, excited-
state populations to a certain proportion (37%) undergo direct
electron injection into the conducting band of TiO2 within 100
fs; the other populations might be trapped in the excited state
and undergo much slower relaxation via either indirect electron
transfer or intermolecular energy transfer if aggregation is taken
into account. For ZnCAPEBPP, there is only one-fourth the
possibility for the S1 species to undergo direct electron injection;
hence this ultrarapid direct IET process (τ < 100 fs) is minor
and unresolvable in our experiments. As a result, we assign the
300-fs relaxation period observed at 680 nm to be due to the
majority of excited-state population (76%) undergoing indirect
IET: the excited electrons in ZnCAPEBPP might be initially
trapped in the porphine ring and seek an entrance to the electron-
transfer bridge for indirect IET to occur. Because the phenyl-
ethynyl group in ZnCAPEBPP is apparently better as an
electron-transfer bridge than the phenyl group in ZnTCPP, we
expect that searching for the entrance of the bridge is a
bottleneck for the slower dynamics observed for the former.
Therefore, to be consistent with the assignment of the dynamics,
the observed ∼300-fs relaxation period in the ZnCAPEBPP/
TiO2 system might be compared with the ∼1-ps relaxation
period in the ZnTCPP/ TiO2 system for the indirect IET process
and the ∼4-ps relaxation period of the former to be compared
with the ∼10-ps relaxation period of the latter for intermolecular
energy transfer among porphyrins.
Figure 11. Femtosecond fluorescence transients of ZnCAPEBPP-
sensitized TiO2 films under three conditions: (A) Film 1, (B) Film 2,
and (C) Film 3, obtained at λex ) 430 nm with fluorescence probed at
λem ) 680 nm (fwhm ) 220 fs). All transients are fitted with a parallel
model with a small offset as indicated. Open circles denote raw data;
solid black curves represent theoretical fits with residuals shown as
green traces; the blue, olive and magenta curves under each transient
are deconvoluted components.
forward IET and energy transfer at smaller intermolecular
distance whereas the ps component (τ2) arose mainly from
energy transfer at a larger distance. At λem ) 680 nm, resonance
energy transfer in porphyrin aggregates is less efficient and
occurs on a ps time scale to affect only the ps dynamics. The
invariance of τ1 values observed in all three sample conditions
indicates that the fs component is due not to aggregation but to
interfacial electron transfer between ZnCAPEBPP and TiO2
nanostructure.42
Tachibana et al.19 studied the electron-injection kinetics for
a zinc porphyrin dye (ZnTCPP) sensitized on TiO2 films using
fs transient absorption; they found that adsorption of the
porphyrin dye on TiO2 films results in quenching of the
stimulated emission that produces three relaxations on the time
scales <100 fs, ∼1 ps, and ∼10 ps. As the optical density of a
ZnTCPP-sensitized TiO2 film (OD ∼0.3) is similar to that of
our low-density sample (OD ∼0.2 in Film 1), the observed
relaxation dynamics might be comparable. We seem to observe
that electron injection from ZnCAPEBPP to TiO2 at 680 nm is
much slower than that from ZnTCPP to TiO2: τ1 ∼290 fs (76%)
vs <100 fs (37%); τ2 ∼4 ps (19%) vs ∼1 ps (12%) and ∼10
ps (51%). A small offset seems to be common to both cases.
The amplitude of the fs component in our case is much larger
than that of Tachibana et al., which might be due to the internal
energy available from different excitations: the excitation of
the former occurs in the S2 state (λex ) 430 nm), whereas the
excitation of the latter takes place in the S1 state (λex ) 560
nm). We have shown that S2 f S1 electronic relaxation is
ultrarapid, which would result in substantial amounts of hot S1
species being produced in the former case. This condition is
consistent with our results that the relaxation dynamics observed
at 620 nm are much more rapid than those observed at 680 nm
On the basis of the ultrarapid nature of the S2 f S1 IC
observed for ZnCAPEBPP/TiO2 films, we expect aggregation
to be significant. Important dynamical evidence related to the
aggregation that occurs in the S1 state is discussed in the
following. At λem ) 620 nm (Figure 10), the relative amplitude
of the ps component increases from 14% in Film 1 to 19% in
Film 3; at λem ) 680 nm (Figure 11), the relative amplitude of
the ps component increases from 19% in Film 1 to 22% in Film
3. Moreover, the offset component becomes more pronounced
in Film 1 than in Film 3. These observations indicate that the
increased adsorption of porphyrin molecules on TiO2 films
enhances the contribution of the ps component. Hence both
interfacial electron transfer and intermolecular energy transfer
processes compete: when aggregation is less significant as in
Film 1, the contribution of the fs component (due to the IET
process) and the offset component (due to the monomer) would
become more pronounced, and vice versa. Due to the red-shift
spectral nature of aggregation in emission, we expect that the
ps component would be more significant for transients observed
at greater emission wavelengths; the relative amplitudes of the
ps component observed at λem
larger than those observed at λem ) 620 nm (Figure 10).
) 680 nm (Figure 11) are indeed
Therefore, it supports the assignment of the observed slower

418 J. Phys. Chem. B, Vol. 110, No. 1, 2006
Luo et al.
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Forward interfacial electron-transfer dynamics of a synthetic
porphyrin sensitizer adsorbed on nanocrystalline TiO2 films have
been investigated by using femtosecond fluorescence up-
conversion with S2 excitation. The S2 f S1 electronic relaxation
was found to be ultrarapid (<100 fs) because of aggregation of
porphyrin molecules. The observed fluorescence transients in
the S1 state show a biphasic dynamical feature with two distinct
time scales. Depending on the detection window, the rapid
component decayed with τ1 ) 100-300 fs and the slow
component with τ2 ) 0.8-4.1 ps. The fs component is assigned
to indirect IET through a phenylethynyl bridge attached to one
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a TiO2 surface, the ps component arose from intermolecular
energy transfer among porphyrins. This evidence supports recent
fs transient absorption results for ruthenium-complex-sensitized
TiO2 films in that the ps component was found to be due to the
dye in an aggregated state.18 The results obtained from time-
resolved investigations of porphyrin in solution, directly de-
posited on solid films, and mixed with PMMA on solid films
have also demonstrated the significance of aggregation affecting
the observed relaxation dynamics in the solid phase.
Acknowledgment. We thank Dr. M. C. Lin for many helpful
discussions and acknowledge financial support from the National
Science Council of the Republic of China (project contracts 94-
2113-M-009-016 and 94-2120-M-009-014 for E.W.G.D. and 94-
2113-M-260-005 for C.Y.L.).
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(42) The observed discrepancies in the relaxation dynamics might arise
from the differences in the state of aggregation of the dye, i.e., the
femtosecond components shown in Figures 10 and 11 are due to H-
aggregation of ZnCAPEBPP on TiO2 films while this dynamical feature is
absent as the dye forming J-aggregates on glass plate (Figure 7). However,
previous study on J- and H-aggregates of porphyrin-surfactant complexes
(J. Phys. Chem. B 1998, 102, 1528) indicates that the fluorescence decay
of the H-aggregate (1.1 and 8.9 ns) is much slower than that of the
J-aggregate (∼0.1 ns), which supports our assignment for the femtosecond
component being due to interfacial electron transfer between porphyrin and
TiO2.