Triplet photophysics of gold(III) porphyrins

Article abstract of DOI:10.1021/jp0449399

Gold porphyrins are often used as electron-accepting chromophores in donor-acceptor complexes for the study of photoinduced electron transfer, and they can also be involved in triplet-triplet energy-transfer interactions with other chromophores. Since the lowest excited singlet state is very short-lived (240 fs), the triplet state is usually the starting point for the transfer reactions, and it is therefore crucial to understand its photophysics. The triplet state of various gold porphyrins has been reported to have a lifetime of around 1.5 ns at room temperature and to have a biexponential decay both in emission and in transient absorption with decay times of around 10 and 100 μs at 80 K. In this paper, the triplet photophysics of two gold porphyrins (Au^III 5,15-bis(3,5-di-terf-butylphenyl)-2,8,12,18-tetraethyl-3,7,13, 17-tetramethylporphyrin and Au^III 5,10,15,20-tetra(3,5-di-fert- butylphenyl)porphyrin) are studied by steady-state and time-resolved absorption and emission spectroscopy over a wide temperature range (4-300 K). The study reveals the existence of a dark state with an approximate lifetime of 50 ns, which was not previously observed. This state acts as an intermediate between the short-lived singlet and the triplet state manifold. In addition, we present DFT calculations, in which the core electrons of the central metal were replaced by a pseudopotential to account for the relativistic effects, which suggest that the lowest excited singlet state is an optically forbidden ligand-to-metal charge-transfer (LMCT) state. This LMCT state is an obvious candidate for the experimentally observed dark state, and it is shown to dictate the photophysical properties of gold porphyrins by acting as a gate for triplet state formation versus direct return to the ground state.

Full text of DOI:10.1021/jp0449399

1776
J. Phys. Chem. A 2005, 109, 1776-1784
Triplet Photophysics of Gold(III) Porphyrins
Mattias P. Eng, Thomas Ljungdahl, Joakim Andre´asson,† Jerker Mårtensson, and
Bo Albinsson*
Department of Chemistry and Bioscience, Chalmers UniVersity of Technology, SE-412 96 Go¨teborg, Sweden
ReceiVed: NoVember 5, 2004
Gold porphyrins are often used as electron-accepting chromophores in donor-acceptor complexes for the
study of photoinduced electron transfer, and they can also be involved in triplet-triplet energy-transfer
interactions with other chromophores. Since the lowest excited singlet state is very short-lived (240 fs), the
triplet state is usually the starting point for the transfer reactions, and it is therefore crucial to understand its
photophysics. The triplet state of various gold porphyrins has been reported to have a lifetime of around 1.5
ns at room temperature and to have a biexponential decay both in emission and in transient absorption with
decay times of around 10 and 100 µs at 80 K. In this paper, the triplet photophysics of two gold porphyrins
(AuIII 5,15-bis(3,5-di-tert-butylphenyl)-2,8,12,18-tetraethyl-3,7,13,17-tetramethylporphyrin and AuIII 5,10,-
15,20-tetra(3,5-di-tert-butylphenyl)porphyrin) are studied by steady-state and time-resolved absorption and
emission spectroscopy over a wide temperature range (4-300 K). The study reveals the existence of a dark
state with an approximate lifetime of 50 ns, which was not previously observed. This state acts as an
intermediate between the short-lived singlet and the triplet state manifold. In addition, we present DFT
calculations, in which the core electrons of the central metal were replaced by a pseudopotential to account
for the relativistic effects, which suggest that the lowest excited singlet state is an optically forbidden ligand-
to-metal charge-transfer (LMCT) state. This LMCT state is an obvious candidate for the experimentally
observed dark state, and it is shown to dictate the photophysical properties of gold porphyrins by acting as
a gate for triplet state formation versus direct return to the ground state.
Introduction
behavior has been suggested to originate from slow relaxation
between triplet sublevels,27 different conformations frozen in
The study of porphyrins and porphyrin-related compounds
has grown into an extensive research area largely due to their
presence in systems involved in natural photosynthesis, where
chlorophylls absorb and shuttle energy to a reaction center.1
Porphyrins are often used instead of chlorophylls in model
systems designed to mimic the structures involved in the transfer
processes of photosynthesis. Studies using such model systems
have involved electron- and excitation energy-transfer reactions2-22
as well as photoinduced redox reactions as a means to store
light energy.23 The model systems often consist of two moieties,
acceptor and donor of the energy and/or electrons, kept at a
fixed distance from each other by a spatial separator that either
mediates the transfer reactions or not. In some systems, gold(III)
porphyrins have been used due to their good stability24 and
suitable redox properties,4,8,9,25,26 making them excellent as
electron acceptors in electron-transfer reactions. The gold(III)
porphyrins can act as electron acceptors either in the ground
state or in the first excited triplet state (electron hole donor).
To quantitatively analyze the photoinduced electron or energy-
transfer processes that emanate from the triplet excited gold(III)
porphyrins, the intrinsic triplet relaxation processes must be
thoroughly understood.
the rigid media,10 or the presence of two different ion pairs.13
In previous studies of gold(III) porphyrins, it has also been
tentatively suggested that the short lifetime at room temperature
might be due to thermal population of a dark state situated
between S1 and T1,28 and there have also been discussions con-
cerning the existence of charge-transfer states.27,29 Observations
of weakly resolved charge-transfer (CT) bands in the absorption
spectrum have also been reported,27,28 and there have been dis-
cussions on whether the first reduction takes place at the por-
phyrin ring8,28 or at the metal.25,26 In measurements on a gold(III)
5,15-diphenylporphyrin, it was observed that in addition to the
two previously reported microsecond lifetimes the compound
also exhibited a lifetime on the nanosecond time scale.22 This
raises interesting questions about the complex photophysics of
this class of porphyrin chromophores that might have important
implications on their use in donor-acceptor complexes.
In this work, the deactivation of the excited triplet state of
gold porphyrins with a varying substitution pattern, AuIII 5,15-
bis(3,5-di-tert-butylphenyl)-2,8,12,18-tetraethyl-3,7,13,17-tetra-
methylporphyrin (AuP), AuIII 5,10,15,20-tetra(3,5-di-tert-butyl-
phenyl)-porphyrin (AuT(DtBP)P), and AuIII 2,3,7,8,12,13,17,18-
octaethylporphyrin (AuOEP) has been studied (Figure 1). The
study is closely related to ongoing projects in which photoin-
duced transfer of electrons and excitation energy between zinc
porphyrin and AuP separated by various rigid spacers is
investigated.20,22 As mentioned earlier, a prerequisite for un-
derstanding the influence of the transfer reactions on the
deactivation of the gold porphyrin triplet state in such model
systems is that the intrinsic deactivation of the gold porphyrin
It was observed early on that luminescence from excited
gold(III) tetraphenylporphyrin decays biexponentially on the
microsecond time scale in organic glasses at 80 K.27 This
* To whom correspondence should be addressed. Phone: +46317723044.
Fax: +46317723858. E-mail: balb@chembio.chalmers.se.
† Current address: Department of Chemistry and Biochemistry, Arizona
State University, Tempe, AZ 85287.
10.1021/jp0449399 CCC: $30.25 © 2005 American Chemical Society
Published on Web 02/15/2005

Triplet Photophysics of Gold(III) Porphyrins
J. Phys. Chem. A, Vol. 109, No. 9, 2005 1777
either a temperature-controlled liquid nitrogen cryostat (Oxford
LN2) or, for temperatures below 77 K, a liquid helium cryostat
(Oxford Optistat).
The nanosecond to microsecond transient absorption
measurements were performed on a setup described previ-
ously21 where the exciting light was provided by a pulsed
Nd:YAG laser (Continuum Surelite II-10, pulse width < 7 ns)
pumping an OPO, giving a tunable light source in the
wavelength region between 400 and 700 nm. Alternatively, the
frequency-doubled fundamental output of the laser system (λ
) 532 nm) was used. The intensity of the exciting laser was
kept below 20 mJ/pulse to prevent photodegradation of the
samples. The probe light, penetrating the sample at an angle of
90° relative to the excitation light, was provided by a xenon
arc lamp. After passing through the sample, the probe light was
passed through a monochromator (symmetrical Czerny-Turner
arrangement) and detected by a five-stage Hamamatsu R928
photomultiplier tube. In general, 16-64 transient signals were
collected and averaged by a 200 MHz digital oscilloscope
(Tektronix TDS2200 2 Gs/s) and stored by a homemade
LabView program controlling the whole system.
Figure 1. Gold(III) porphyrins used in this study.
monomer is thoroughly characterized. To this end, time-resolved
as well as steady-state emission and absorption spectroscopy
measurements were performed on the three porphyrins in
different media and at temperatures between 4 and 300 K.
Materials and Methods
-
Materials. The synthesis and purification of AuP+BF4 is
described elsewhere.30 AuOEP+BF4 was prepared by gold
-
insertion into purchased OEP (Aldrich) using the procedure
-
developed by Sauvage et al.31 AuT(DtBP)P+BF4 was made
-
from AuT(DtBP)P+AuCl4 by ion exchange with 10 equiv of
The picosecond transient absorption measurements were
performed using the pump-probe technique on a setup de-
scribed previously21 with some modifications on the probe and
detector part. In short, the sample was excited at 522 nm with
the second harmonic of the signal from a TOPAS OPA (Light
Conversion Ltd.). The TOPAS was pumped by a Ti:sapphire
regenerative amplifier (Spitfire, Spectra Physics) at 1 kHz
repetition rate. The amplifier was in turn pumped by a
frequency-doubled diode-pumped Nd:YLF laser (Evolution-X,
Spectra Physics) and seeded by a mode-locked femtosecond
Ti:sapphire laser (Tsunami, Spectra Physics). The seed laser
was pumped by a continuous-wave frequency-doubled diode-
pumped Nd:YVO4 laser (Millennia Vs, Spectra Physics). The
output of the amplifier was split in two parts by a beam splitter
(70/30) and used as the pump and the probe light, respectively.
Instead of the previously described photodiode/monochromator
setup, a CCD spectrograph (Avantes) was used, and a white
light continuum, for use as the probe light, was generated by
letting the probe beam pass through a slowly rotating 4 mm
thick CaF2 disk (Harrick). The samples were kept in continu-
ously moving 2 mm quarts cuvettes. The absorption was
approximately 1 at the excitation wavelength (522 nm), and the
pulse energy was kept low (approximately 1.4 µJ/pulse) to
prevent sample degradation.
Quantum mechanical calculations were performed with the
Gaussian 03 program suite.35 For all atoms except the central
metal, the 6-31G(d)36 basis set was used. For the central metals,
the CEP (Stevens/Basch/Krauss ECP)37-39 basis set was used.
In this basis set, the core electrons are replaced with an effective
core potential (ECP) to account for relativistic effects. The
nonsubstituted metal-porphines were chosen as models in order
to reduce calculation time. Each metal-porphine was first
geometry-optimized with D4h symmetry constraints using the
B3LYP functional.40-42 The symmetry constraints were con-
sidered to be justified as optimizations with and without con-
straints, resulting in very similar structures for both metalation
states. The resulting D4h optimized geometries were then used
for time-dependent density functional theory (TD-DFT)43-45
calculations of the vertical excited states using the same
functional and basis sets as for the geometry optimizations.
AgBF4 in dichloromethane. AuT(DtBP)P+AuCl4- was synthe-
sized from the free base porphyrin T(DtBP)P according to
literature procedures.32 T(DtBP)P was synthesized from 3,5-
di-tert-butylbenzaldehyde33 and pyrrole using the reaction
conditions for porphyrin synthesis described by Lindsey et al.34
All H1 NMR data were in accordance with those reported in
-
the literature, although AuOEP+BF4 showed some contami-
nants in the aliphatic region. This should however be of
negligible importance to the determination of absorption and
emission maxima in the UV-vis region.
Solvents. The solvents ethanol (Kemethyl), diethyl ether
(Sigma-Aldrich), isopentane (Merck), toluene (Merck), and
methyl cyclohexane (Fluka) were used directly as delivered.
The 2-methyl tetrahydrofuran (2-MTHF) (Merck) and methyl
methacrylate (MMA) (Merck) were distilled prior to use to
remove stabilizers.
To minimize the variations in viscosity and inhibit bimo-
lecular quenching in the whole temperature interval and to get
samples more easily handled at low temperatures, the porphyrin
samples were immobilized in a solid matrix of poly(methyl
methacrylate) (PMMA). The PMMA immobilization was per-
formed by dissolving the porphyrin sample in a glass test tube
containing the distilled methyl methacrylate monomer. A minute
amount of the initiator azobisisobutyronitrile (AIBN) was added,
and the tube was submerged in a water bath with a temperature
of 80 °C for approximately 24 h. Subsequently, the glass tube
was shattered, and the PMMA “rod” was cut and polished into
a square 1 cm × 1 cm rod, mimicking the dimensions of a
regular sample cell. The absorption spectra of the rods were
compared to those of the respective unpolymerized samples to
ensure that no degradation had taken place during the process.
At low temperatures, measurements were also performed on
samples in a diethyl ether/isopentane/ethanol (5:5:2) (EPA)
glass.
Absorption spectra were recorded at room temperature using
a normal 1 cm quartz sample cells in a Cary 4B UV-vis
spectrophotometer. A baseline of the pure solvent was also
recorded for every spectrum. For the steady-state emission
measurements, a SPEX Fluorolog-3 spectrofluorimeter was used,
and for the time-gated phosphorescence measurements the same
setup was used but with a pulsed xenon lamp as the excitation
source and a SPEX 1934D3 phosphorimeter as a time-gated
detector. Low-temperature measurements were performed using
Results
Steady-State Measurements. Figure 2 shows the absorption
and emission spectra of the studied gold(III) porphyrins. They

1778 J. Phys. Chem. A, Vol. 109, No. 9, 2005
Eng et al.
Figure 2. Absorption spectra (left) at room temperature and emission
spectra (right) at 80 K for AuP (s), AuT(DtBP)P (- -), and AuOEP
(- ‚ -) in EPA. The excitation wavelengths were 518, 524, and 547 nm
for AuP, AuT(DtBP)P, and AuOEP, respectively.
Figure 4. Transient absorption (positive), probed at 450 nm, and
ground-state bleaching (negative), probed at 412 nm, of AuT(DtBP)P
in PMMA at 298, 200, and 80 K. The excitation wavelength was 532
nm.
observed6,7,10,12,13,27 lifetimes of about 10 and 100 µs at 80 K
for AuT(DtBP)P in PMMA. However, measurements at higher
time resolution reveal (Figure 4) an additional significant and
short-lived component with a lifetime of about 20-50 ns. In
EPA, this short lifetime (as well as the two longer lifetimes) is
only observed between 80 and 140 K (not shown), because at
higher temperatures the lifetimes become too short to detect
with the experimental setup used. In PMMA, however, three
lifetimes are observed for all studied gold(III) porphyrins in
the entire temperature interval (4-300 K), and the short lifetime
is approximately temperature-independent. The fact that all three
lifetimes are present in EPA at temperatures when the solvent
is still fluid (120-140 K) indicates that the phenomenon does
not originate from the presence of different conformations of
the molecules frozen in the solid matrix but is an inherent
characteristic of gold(III) porphyrins. To identify the properties
of the short-lived component, transient absorption measurements
were performed at wavelengths where either the ground state
(412 nm) or the excited state (450 nm) dominates the absorption.
At room temperature, the fitting of both the transient absorption
decay and the ground state recovery requires three exponential
functions. At lower temperatures, though, the lifetimes of the
short-lived and the two long-lived components deviate too much
from each other, making it experimentally impossible to fit all
three in one decay trace. This makes obtaining quantitative
information (i.e., accurate rate constants) about the short-lived
state difficult. However, it is possible to come to the following
qualitative conclusions.
The observation of the same three lifetimes in the ground-
state recovery and in the transient absorption decay indicates
that all three excited states are independently coupled to the
ground state. However, from a careful comparison of the decay
traces in Figure 4, it can be seen that the relative contributions
of the short-lived and the two long-lived states are not the same
in the ground-state recovery as in the transient absorption decay.
This is most obvious at low temperatures where the relative
contribution of the short-lived component is clearly larger in
the ground-state recovery (negative) than in the transient
absorption decay (positive). This could either mean that the
short-lived state has the same shape of its transient absorption
spectrum as the two long-lived states but a smaller absorption
coefficient or a different shape of its transient absorption
spectrum. On one hand, if all species have the same shape of
their transient absorption spectra, then no difference between
Figure 3. Quantum yields of phosphorescence vs temperature for AuP
in PMMA (9) and EPA (O) and for AuT(DtBP)P in PMMA (2). The
excitation wavelengths were 518 and 524 nm for AuP and AuT(DtBP)P,
respectively.
display the usual characteristics of hypso-porphyrins,46 with the
Soret or B-band around 400 nm and a Q-band centered at 530
nm. The studied compounds do not emit in fluid media but
phosphoresce in rigidified media at low temperatures.
Figure 3 shows the quantum yield of phosphorescence as a
function of temperature for AuP and AuT(DtBP)P in a PMMA
matrix. In PMMA, the phosphorescence of all the studied
compounds is virtually undetectable at temperatures above 200
K, but at lower temperatures the quantum yields increase, in a
sigmoidal fashion, to level out at 4 K to a maximum value of
about 8% and 4% for AuT(DtBP)P and AuP, respectively. In
EPA, there is virtually no emission until the glass setting
temperature (at around 95 K) is approached and the viscosity
is increased. At lower temperature, there is a sharp increase in
the phosphorescence quantum yield (Figure 3), probably caused
by slower bimolecular deactivation due to the large increase in
viscosity. Since we are primarily interested in the intrinsic
photophysics of the gold porphyrins, and studying its variations
over larger temperature intervals, we will mostly focus on the
measurements done in PMMA since this media is a glass
throughout the whole temperature range of interest, 4-300 K,
and will efficiently prevent any bimolecular processes.
Time-Resolved Measurements. Transient absorption mea-
surements at low temperatures show the two previously

Triplet Photophysics of Gold(III) Porphyrins
J. Phys. Chem. A, Vol. 109, No. 9, 2005 1779
Figure 5. Transient absorption spectra of ZnP (top) at 1.5 ns in EPA
(- -) and 100 ns in 2-MTHF at 150 K (s) and of AuT(DtBP)P (bottom)
at 1.5 ns in EPA at room temperature (- -) and at 100 ns in EPA at 80
K (s).
Figure 6. Steady-state emission spectrum (s) and the time-gated (10
µs-1.3 ms) emission spectrum (- -) of AuP in EPA at 80 K. The
excitation wavelength was 518 nm.
spectra. Also, their relative contributions in transient absorption
(probed at 450 nm) and the ground-state recovery (probed at
412 nm) are the same, indicating similar absorption coefficients
of the two states. To further investigate the spectral properties
of the emission from the two long-lived species, a time-gated
phosphorescence spectrum of AuP in EPA at 80 K, collecting
the emission between 10 µs and 1.3 ms, was compared to the
steady-state spectrum. This comparison is displayed in Figure
6 and shows the invariance of the shape of the phosphorescence
spectrum after time gating, which indicates that the two long-
lived states have the same emission profile and thus are
degenerate. The invariance of the phosphorescence spectrum
together with the early spectral evolution of the transient
absorption spectrum also suggests that the short-lived state is
nonemissive.
spectra recorded at short and longer times is expected. If, on
the other hand, the short-lived species have different shape of
their transient absorption spectra, then a spectral evolution on
the nanosecond time scale would be expected.
To distinguish between the two possibilities, transient absorp-
tion spectra recorded at two delay times are expected to be
informative. The short-lived state dominates the transient
absorption spectra at times shorter than 20 ns and the two long-
lived states at times longer than 100 ns. Ideally, these spectra
would be measured with the same experimental setup, but due
to experimental difficulties with scattering and impurities the
time resolution of our nanosecond setup was not good enough
to record the transient spectrum of the short-lived species. The
transient absorption spectrum of the initially formed species was
therefore recorded with a pump-probe system having picosec-
ond time resolution. For this measurement, room temperature
and fluid media were necessary because in the highly focused
laser beams the use of rigid media led to severe photodegra-
dation of the solid samples. In addition, measurements of spectra
for the long-lived species required high viscosity (glass) and
low temperature. Since the measurements had to be done with
different experimental setups for the two delay times, measure-
ments were also performed on ZnP, a well-characterized
porphyrin not expected to display spectral evolution in this time
frame. Figure 5 (bottom) shows the transient absorption
spectrum for AuT(DtBP)P after 1.5 ns at room temperature and
after 100 ns (when the short-lived component has decayed) at
80 K. It can be seen that there is a spectral evolution between
the two delay times, which gives strength to the earlier indication
of different transient absorption spectral profiles of the short-
lived and the two longer-lived transient states. It can also be
seen in Figure 5 (top) that there is good general correlation
between measurements performed on the two different setups
for ZnP. Thus, the spectral evolution displayed for AuT(DtBP)P
in the time window between the two measurements cannot be
explained by experimental differences.
A summary of the results from the time-resolved measure-
ments is that the short-lived state is most probably populated
with unit efficiency from the excited singlet state since deactiva-
tion of the singlet is very fast ((240 fs)-1 22
and it is most
)
improbable that several processes operate in parallel on this time
scale. If the short-lived state is populated from the excited singlet
with unit efficiency, then there is no other possible route for
populating the two long-lived states than from the short-lived
state. The short-lived state is thus directly coupled to the ground
state and to the two long-lived states.
Quantum Chemical Calculations. To gain insight into the
character of the excited states, we investigated the differences
between the two metalation states, ZnP and AuP, in a set of
DFT calculations performed on the corresponding metal-
porphines. As was the case in the time-resolved transient absorp-
tion study (see above), ZnP was used for comparison because
of its regular and well-characterized photophysics. The resulting
frontier orbitals of the geometry-optimized molecules are shown
in Figure 7 and reveal important differences for AuP (left)
relative to ZnP (right). The calculated frontier orbitals of ZnP
are the normal highest occupied molecular orbitals (HOMOs)
and lowest unoccupied molecular orbitals (LUMOs) of regular
2
2
Having established that the short-lived component has a
different transient absorption spectrum than the two long-lived
components, resulting in a spectral evolution that takes place
within the first 100 ns, we now turn to investigate the properties
of the two remaining states. After the first 100 ns, there is no
observed spectral evolution, which indicates that the two long-
lived states have the same shape of their transient absorption
porphyrins,46 but AuP has an orbital with pronounced dx -y
character raised in energy, relative to ZnP, so that it becomes
the LUMO of the system. This orbital ordering is different from
earlier results from iterative extended Hu¨ckel (IEH)27 and XR
multiple scattering47 calculations, which predicted a higher
energy for this orbital resulting in an orbital ordering with this
metal-centered orbital aboVe the ring-centered eg orbitals. It

1780 J. Phys. Chem. A, Vol. 109, No. 9, 2005
Eng et al.
Figure 7. Frontier orbital diagram for gold(III) porphine (left) and zinc(II) porphine (right).
TABLE 1: Calculated Vertical Excitation Energies and
Oscillator Strengths (f) of Gold(III) and Zinc(II) Porphinef
could be suspected that the orbital ordering might be sensitive
to the substitution pattern of the porphyrin ring, but geometry
optimization of gold(III) diphenyl-octamethyl-porphyrin did not
change the orbital ordering and even lowered the energy of the
metal-centered orbital. Our result, placing the metal-centered
orbital below the unoccupied ring-centered orbitals, goes hand
in hand with recent studies showing that the first reduction25,26
as well as the accepting of an electron in electron-transfer
reactions2 is metal-centered in gold(III) porphyrins. To inves-
tigate the effects on the excited electronic states caused by the
difference in orbital ordering between the two metalation states,
a set of TD-DFT calculations were performed. This set of
calculations, not surprisingly, resulted in several low-lying states
with pronounced ligand-to-metal charge-transfer (LMCT) char-
acter for AuP (Table 1). Transition from the ground state to
one of these is weakly allowed and has also been observed in
experiments as a small shoulder between the Soret- and
Q-bands.27,28 It can bee seen from Table 1 that the TD-DFT
calculations consistently overestimate the excitation energies for
both AuP and ZnP with approximately 0.3 eV, but the energy
separation between the Soret- and Q-bands is accurately
described. Thus, so far, the calculations give results that are
consistent with experimental observations. Having established
that the calculations give results that can be verified by
experiments, we now turn to the calculated differences between
AuP and ZnP that are hard to directly detect experimentally.
The most striking difference is that for AuP the two lowest
excited singlet states are of LMCT character. For ZnP, however,
the lowest excited singlet states are the normal ligand-localized
states associated with the Q-band absorption. Thus, from the
calculations, the different photophysics of AuP could be
explained by the presence of LMCT states located between the
lowest singlet and triplet states. The triplet versions of the low-
lying CT states in AuP are nearly degenerate with the normal
ligand-localized triplets.
AuP ZnP
E (eV) E (eV)
character
f
f
Singlets
Eu, B-band
A2u, CT
3.52 (3.18)a 0.7062 3.54 (3.22)c 0.8907
3.39 (2.76)b 0.0001
Eg, CT
Eu, Q-band
B2u, CT
3.05
3.43
2.53 (2.27)a 0.0038 2.44 (2.17)c 0.0023
2.02
1.93
B1u, CT
Triplets
Eu
2.16
2.08
B2u, CT
B1u, CT
Eu
1.92
1.90
1.88 (1.80)d
1.77 (1.72)e
a Value for AuOEP in MMA (this study). b Value for AuT(DtBP)P
from refs 27 and 28. c Value for ZnOEP vapor from ref 46. d Value
from phosphorescence maximum of AuOEP in PMMA (this study).
e Value for Zn-porphine from ref 49. f Experimental values (when
available) are given in parentheses.
Also, if one compares the ground-state absorption spectra of
AuP and ZnP (Figure 1S in the Supporting Information), then
one can see clear differences that can be explained by the
calculations. The absorption spectra of the two metalation states
are very similar in that they have the same number of peaks
with roughly the same relative intensity. The most obvious
difference is that for AuP the absorption tails out into the far
red of the spectrum whereas for ZnP the absorption decreases
abruptly at the absorption edge. This might be explained by
transition to the LMCT state between the B- and Q-bands and
transitions to the formally forbidden LMCT states below the
Q-band for AuP. For ZnP, these states are not present, and the
porphyrin displays a normal absorption spectrum.
Solvent Polarity Dependence. To get experimental indica-
tions for the presence of a low-lying CT state, we studied AuT-

Triplet Photophysics of Gold(III) Porphyrins
J. Phys. Chem. A, Vol. 109, No. 9, 2005 1781
TABLE 2: Quantum Yield of Phosphorescence (Φp), the
Lifetimes (τ), and Their Corresponding Preexponential
Factors (r) from Transient Absorption Decaysb
τ1
R1
τ2
R2
τ3
R3
solvent
EPA
2-MTHF
toluene/MCHa lp
Φp
(ns) (%) (µs) (%) (µs) (%)
mp 0.0167 40
29 11
64
71
18 124 53
lp
0.0086 48
0.0074 34
3.7 25 83 11
3.4 17
82 12
a A mixture of toluene/methyl cyclohexane, 1:6. b The presented data
are for AuT(DtBP)P in more polar (mp) and less polar (lp) solvents at
80 K. The excitation wavelength was 532 nm.
Figure 9. Lifetimes of the most long-lived (circles) and middle-lived
(triangles) components for AuP (O and 4) and AuT(DtBP)P (b and
2) from the global fitting procedure.
temperature-dependent rate constants (kA,kB) and preexponential
factors (RA,RB)
∆A(t,T) ) RA(T) e-tk (T) + RB(T) e-tk (T)
(1)
A
B
The effect of temperature on the preexponential factors in eq 1
was not a priori settled, but instead the values were allowed to
vary at all temperatures, and thus, the temperature variation was
revealed from the fitting results. The temperature dependence
of the rate constants was assumed to be described by Arrhenius
type expressions, giving the rate of deactivation from the two
triplets as a function of temperature as
Figure 8. Proposed model for deactivation of excited gold(III)
porphyrins.
(DtBP)P in different solvents (Table 2). When switching from
more polar solvents to solvents of lower polarity, the energy of
a CT state would be expected to increase more than the energy
of the less polar triplet states. This would increase the CT-T1
energy gap and lead to less formation of the triplets from the
CT state and, hence, a larger contribution from the short-lived
state in the ground-state recovery. The results show that in the
more polar EPA the phosphorescence quantum yield and the
relative contribution of the three components to the total
transient absorption are clearly different from the other, less
polar solvents (2-MTHF and a mixture of toluene and methyl-
cyclohexane). The less polar solvents increase the contribution
of the short-lived state to the decay and also decrease the
phosphorescence quantum yield in the same proportion, again
indicating that the short-lived state is nonemitting. The results
from the solvent study, thus, suggest that the dark state has CT
character.
aA/RT)
aB/RT)
kA(T) ) kPA + AA e(-E
kB(T) ) kPB + AB e(-E
(2)
Here, kPA and kPB are the rate constants for phosphorescence,
and EaA and EaB are the activation energies for nonradiative
deactivation (intersystem crossing to the ground state) of states
A and B, respectively. The parameters kPA, AA, EaA, RA, kPB
,
AB, EaB, and RB were varied to give as good a fit as possible of
eq 1 to all of the individual decay traces in the whole
temperature interval at the same time. This procedure gives a
good fit to all of the individual decays, and the resulting lifetimes
are within 10% of the ones attained when fitting the transient
decays individually. The temperature intervals were 4-160 K
for AuP and 4-220 K for AuT(DtBP)P, with a total of 11 and
22 decay traces in the global fitting procedures, respectively.
Figure 9 shows the lifetimes at different temperatures of the
two long-lived components for AuP and AuT(DtBP)P in PMMA
from the global fitting procedure. The lifetimes show similar
temperature dependences as that of the phosphorescence quan-
tum yield (Figure 3). The fitting procedure also yields the
activation energies for nonradiative deactivation from the most
long-lived (A) and middle-lived (B) states as EaA ) 2.7 kJ/mol
Discussion
The results above give support for the model depicted in
Figure 8, with the excited singlet state populating a dark CT
state that in turn populates two degenerate triplet states in
parallel. The two triplet states are subsequently deactivated to
the ground state independently. The lowest excited singlet is
only directly coupled to the CT state since the rate constant for
depopulation of the excited singlet is very large, (240 fs)-1 for
AuP,48 and it is improbable that another deactivating process
(internal conversion to the ground state or intersystem crossing
to the triplet manifold) operates in parallel on this time scale.
The decay kinetics based on the proposed model were used to
derive a method to fit the transient absorption decay data
globally, as follows. The decay of the transient absorption, after
the short-lived component has decayed, at a given temperature
can be expressed as a sum of two exponential decays with
and EaB ) 2.5 kJ/mol for AuP and EaA ) 3.1 kJ/mol and EaB
3.7 kJ/mol for AuT(DtBP)P.
)
As a check of the consistency of the model, it is possible to
compare the results from the phosphorescence quantum yield
measurements to the average relative populations (fi) of the two
emitting states. The average relative population of an emitting
state, fi ) Riτi ) Riki-1, should be proportional to the quantum
yield of emission (Φi) from this state. For this relation to hold,
it is important to note that the states have to be populated and
depopulated independently. Figure 10 shows f as a function of

1782 J. Phys. Chem. A, Vol. 109, No. 9, 2005
Eng et al.
TABLE 3: Quantum Yield of Phosphorescence and Rate
Constants, Calculated According to Appendix A, with τCT
Assumed to Have a Constant Value of 50 nsa
AuT(DtBP)P
4 K 200 K
AuP
4 K
150 K
Φp (%)
kS/s-1
8
0.55
4
0.35
1.8 × 107
1.1 × 106
5.4 × 105
0
1.9 × 107
2.3 × 105
1.2 × 106
1 × 104
7 × 103
2.4 × 105
4.8 × 104
1.9 × 107
4.4 × 105
3.6 × 105
0
2 × 107
kTA/s-1
kTB/s-1
knrA/s-1
kPA/s-1
knrB/s-1
kPB/s-1
1 × 105
1.6 × 105
7.4 × 103
6.7 × 103
1.7 × 105
2.8 × 10y
7 × 103
0
6.7 × 103
0
4.8 × 104
2.8 × 104
a See Figure 8 for details.
quantum yield for both AuP and AuT(DtBP)P, giving further
strength to the proposed model.
The good correlation between experiments and global fittings
to the model gives strength to the assumption that the two triplet
states decay in parallel with no or little interconversion in the
temperature intervals given above. As the temperature increases
above the interval though, the observed lifetimes become shorter
than those predicted by the globally fitted parameters. This
indicates thermal activation of additional deactivation processes
that are not included in the model. Examples of such processes
are the back reaction from the triplets to the CT state or
interconversion between the triplets. It can be seen from the
DFT calculations that to reach the lowest excited singlet state
from the lowest triplet state in a stepwise fashion a thermal
energy of about 230 K (0.02 eV) is required. This is in
agreement with the finding that the simple model depicted in
Figure 8 holds for temperatures below 200 K but fails at higher
temperatures. The failure of the model at higher temperatures
could then be explained by thermal back reaction to the lowest
excited singlet state, which subsequently is deactivated to the
ground state.
Figure 10. Average relative population of the most long-lived (circles)
and middle-lived (triangles) components for AuP (O and 4) and AuT-
(DtBP)P (b and 2) from the global fitting procedure.
When making estimations of the rate constants given in Figure
8, one can use the fact that at 4 K the nonradiative processes
from the triplet states are turned off. This can be seen in Figures
3 and 9 as a leveling out of the quantum yield of phosphores-
cence and the lifetimes, at lower temperatures, to a maximum
value. The quantum yield of phosphorescence and a reasonable
value of the lifetime of the short-lived component (50 ns) then
yields the rate constants given in Table 3. To more easily see
the temperature effect, the rate constants are calculated at two
extreme temperatures, the lowest, 4 K, and near the limit where
the phosphorescence quantum yield was too low to be detected,
150 and 200 K for AuP and AuT(DtBP)P, respectively. The
rate constants are derived by solving the differential equations
for the population of the states according to the model in
combination with the phosphorescence quantum yield measure-
ments as described more thoroughly in Appendix A. One can
see from Table 3 that as the temperature is increased less of
the triplet states are formed and the phosphorescence is also
quenched by the temperature-dependent intersystem crossing
processes. The attentive observer may note that the rate constants
for formation of the two triplet states (kTA and kTB) decrease
slightly with increasing temperature. This would indicate
negative activation energies, but since the calculations were
highly approximate we estimate this to be within the error of
uncertainty.
Figure 11. Quantum yields of phosphorescence (filled symbols) and
the sums of the average relative populations (fA + fB) of the two long-
lived components (open symbols) from the global fitting procedure for
AuP ([ and ]) and AuT(DtBP)P (9 and 0). The properties are
normalized for comparison.
temperature of the most long-lived (fA) and middle-lived (fB)
states, for AuP and AuT(DtBP)P in PMMA. The value of f gives
an indication of the relative contribution to the total quantum
yield from the individual states. It can be seen that the most
long-lived state gives the largest contribution to the total
quantum yield and that the temperature dependence is similar
to that of the phosphorescence quantum yield (Figure 3). The
difference in phosphorescence quantum yield between AuP and
AuT(DtBP)P is also reflected in Figure 10 in that for AuT-
(DtBP)P the sum of fA and fB is approximately twice as high as
that for AuP, which is also the case for the phosphorescence
quantum yields. This shows that the phosphorescence quantum
yields of the two porphyrins are related in the same way to the
average relative populations. It was previously established that
the two emitting states have the same emission spectra (Figure
3), and thus the sum of fA and fB should be compared to the
total steady-state phosphorescence quantum yield. Figure 11
shows the temperature dependence of the quantum yield of
phosphorescence and of the sum of fA and fB, both normalized
at 4 K. It can be seen that fitting the data from the time-resolved
measurements according to the model with the global fitting
procedure gives a very good correlation to the steady-state
Conclusions
We have investigated the photophysics of gold(III) porphyrins
with steady-state and time-resolved spectroscopic methods as

Triplet Photophysics of Gold(III) Porphyrins
J. Phys. Chem. A, Vol. 109, No. 9, 2005 1783
well as DFT calculations. It was found that the deactivation of
the excited gold(III) porphyrins can be described by the model
presented in Figure 8, at low temperatures in rigid media. We
have observed a short-lived state, with a lifetime of about 20-
50 ns in rigid media, that is an intermediate between the first
excited singlet state and the triplet manifold and acts as a
gatekeeper state for formation of the triplets. We show, by
calculations and solvent studies, that this short-lived state is of
pronounced LMCT character with a small splitting between its
singlet and triplet versions. Once the lowest triplets have been
formed from this CT state, they are deactivated to the ground
state in parallel, without any interconversion at temperatures
below 200 K. Furthermore, as the temperature is raised, the
triplets are deactivated by additional pathways, presumably
through thermal excitations to the CT state.
based on the fact that the lifetime of the two long-lived states
are about 1000 times larger than that of the short-lived state,
i.e., τCT , τT1A ≈ τT1B
.
(1) At intermediate times, when the short-lived state has
decayed and the two long-lived states have just been formed,
the two parentheses containing the exponentials can each be
approximated by
(e-0 - e-∞) ) 1
(2) The parentheses containing the lifetimes can be ap-
proximated by τ-1 ) kCT
.
CT
For the two long-lived states, this intermediate time is
negligible and can be regarded as time zero, and thus the relative
concentrations of the two states at intermediate times can be
approximated by RA/RB, the preexponential factors from the
global fitting procedure.
Acknowledgment. This work was supported by grants from
the Swedish Research Council, the Knut and Alice Wallenberg
Foundation, and the Hasselblad Foundation.
This reduces eq 3A to
RA kTA
RB kTB
)
(4A)
Supporting Information Available: Comparison of the
Q-band region of the absorption spectra of AuP and ZnP. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Equation 4A contains two unknowns, and thus we need
another relationship between kTA and kTB. For this, we can turn
to the phosphorescence quantum yield. The total quantum yield
of phosphorescence from the two triplet states can be expressed
in rate constants and lifetimes according to
Appendix A
The differential equations for the population of the states
according to the model described in Figure 8, if one assumes
that the CT state is formed immediately with a concentration
of [CT]0, can be written as
kTA
kPA
ΦP ) ΦPA + ΦPB
)
+
kS + kTA + kTB knrA + kPA
kTB
kPB
) kTA
τ
CT kPAτA + kTBτCT kPBτB
d[CT(t)]
kS + kTA + kTB knrB + kPB
-
) [CT(t)](kS + kTA + kTB)
dt
(5A)
d[T1A(t)]
dt
In eq 5A, everything but kTA and kTB is known from the global
fitting procedure, and thus if we combine eqs 4A and 5A we
have enough information to estimate the rate constants kTA and
kTB at different temperatures.
-
-
) [T1A(t)](knrA + kPA) - [CT(t)]kTA
) [T1B(t)](knrB + kPB) - [CT(t)]kTB (1A)
d[T1B(t)]
dt
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