Ground- and Excited-State Dynamics of Aluminum and Gallium Corroles

Article abstract of DOI:10.1021/ic900056n

The steady-state absorption and emission spectra and the temporal fluorescence decay profiles of two metallocorroles, Al(tpfc)(py)_n and Ga(tpfc)(py)_n (n ) 1,2), have been measured in a noncoordinating solvent, benzene, in a coordinat

Full text of DOI:10.1021/ic900056n

Inorg. Chem. 2009, 48, 2670-2676
Ground- and Excited-State Dynamics of Aluminum and Gallium Corroles
Dorota Kowalska,† Xia Liu,† Umakanta Tripathy,† Atif Mahammed,‡ Zeev Gross,*,‡ Satoshi Hirayama,†
and Ronald P. Steer*,†
Department of Chemistry, UniVersity of Saskatchewan, 110 Science Place, Saskatoon,
SK, Canada S7N 5C9, and Schulich Faculty of Chemistry, Technion - Israel Institute of
Technology, Haifa 32000, Israel
Received January 12, 2009
The steady-state absorption and emission spectra and the temporal fluorescence decay profiles of two metallocorroles,
Al(tpfc)(py)n and Ga(tpfc)(py)n (n ) 1,2), have been measured in a noncoordinating solvent, benzene, in a coordinating
solvent, pyridine, and in mixed benzene-pyridine solutions. The ground-state spectra reveal that an equilibrium
between the pentacoordinate corrole (n ) 1) and the hexacoordinate corrole (n ) 2) is established in the mixed
benzene-pyridine solutions. The ground-state equilibrium constants are 135 M-1 and 1.0 M-1 at 295 K for the Al
and Ga species, respectively. The excited-state radiative and nonradiative decay constants of the pentacoordinate
and the hexacoordinate species have been obtained from measurements of the fluorescence quantum yields and
monoexponential fluorescence decay times in pure benzene and pure pyridine. Temporal fluorescence decays of
the gallium system in a mixed benzene-pyridine solution are biexponential due to dissociation of the hexacoordinate
species in the excited state leading to the establishment of a dissociation-association equilibrium. The rate constants
for the pyridine association and dissociation processes for the gallium corrole in the excited state have been
measured, ka* ) 2.3 × 108 M-1 s-1 and kd* ) 2.9 × 108 s-1, respectively, leading to a value for the excited-state
association equilibrium constant of Ka* ) 0.78 M-1.
Introduction
porphyrins is absent in the corroles, so that the corrole
macrocycle in metal-chelated derivatives belongs to the C2V
point group (cf. D4h for the metalloporphyrins). The corrole
macrocycle thus possesses a smaller cavity for metal
coordination than that of the porphyrins and accommodates
trivalent group 13 metals (cf. zinc(II) and magnesium(II)
porphyrins) such as aluminum(III) and gallium(III) that can
bind up to two additional axial ligands.4-6
Recently, the emphasis in corrole research has been
devoted to understanding the unique electronic structures and
chemical properties of their transition metal complexes and
to using them as catalysts for a variety of processes.9-14
Metal-free (commonly called free-base) corroles and their
chelates with nontransition metal ions have received less
Research interest in the corroles, porphyrin-like tetrapy-
rrolic macrocycles, has been widespread since the first
practical methods of triarylcorrole synthesis were disclosed
in 1999.1-8 One of the four bridging methine groups in
* To whom correspondence should be addressed. Phone: (306) 966-
4667 (R.P.S.), (972) 4829-3954 (Z.G.). Fax: (306) 966-4730 (R.P.S.),
(972) 4829-5703 (Z.G.). E-mail: ron.steer@usask.ca (R.P.S.), chr10zg@
tx.technion.ac.il (Z.G.).
†
University of Saskatchewan.
Technion - Israel Institute of Technology.
‡
(1) Gross, Z.; Galili, N.; Saltsman, I. Angew. Chem., Int. Ed. 1999, 38,
1427.
(2) Paolesse, R.; Jaquinod, L.; Nurco, D. J.; Mini, S.; Sagone, F.; Boschi,
T.; Smith, K. M. Chem. Commun. 1999, 1307.
(3) Erlen, C.; Will, S.; Kadish, K. M. In The Porphyrin Handbook; Kadish,
K. M.; Smith, K. M.; Guilard, R., Eds.; Academic Press: New York,
2000; Vol. 2, pp 233-300.
(4) Bendix, J.; Dmochowski, I. J.; Gray, H. B.; Mahammed, A.; Simkhov-
ich, L.; Gross, Z. Angew. Chem., Int. Ed. 2000, 39, 4048.
(5) Mahammed, A.; Gross, Z. J. Inorg. Biochem. 2002, 88, 305.
(6) Sorasaenee, K.; Taqavi, P.; Heling, L. M.; Gray, H. B.; Tkachenko,
E.; Mahammed, A.; Gross, Z. J. Porphyrins Phthalocyanines 2007,
11, 189.
(9) Ye, S.; Tuttle, T.; Bill, E.; Simkhovich, L.; Gross, Z.; Thiel, W.; Neese,
F. Chem.sEur. J. 2008, 14, 10839.
(10) Simkhovich, L.; Mahammed, A.; Goldberg, I.; Gross, Z. Chem.sEur.
J. 2001, 7, 1041.
(11) Mahammed, A.; Gross, Z. J. Am. Chem. Soc. 2005, 127, 2883.
(12) Gross, Z.; Gray, H. B. AdV. Synth. Catal. 2004, 346, 165.
(13) Mahammed, A.; Gross, Z. Angew. Chem., Int. Ed. 2006, 45, 6544.
(14) Haber, A.; Mahammed, A.; Fuhrman, B.; Volkova, N.; Coleman, R.;
Hayek, T.; Aviram, M.; Gross, Z. Angew. Chem., Int. Ed. 2008, 47,
7896.
(7) Nardis, S.; Monti, D.; Paolesse, R. Min-ReV. Org. Chem. 2005, 2,
355.
(8) Gross, Z. J. Biol. Inorg. Chem. 2001, 6, 733.
2670 Inorganic Chemistry, Vol. 48, No. 6, 2009
10.1021/ic900056n CCC: $40.75  2009 American Chemical Society
Published on Web 02/10/2009

Aluminum and Gallium Corroles
Scheme 1
M(tpfc)(py) + py a M(tpfc)(py)2
(1)
whereas only the fully six-coordinate species is found in pure
pyridine. The effects on the absorption and fluorescence
emission spectra of adding incremental amounts of pyridine
to a benzene solution of Ga(tpfc)(py)n are illustrated in and
Figure 1b and d. Figure 1a and c show the distinct shift in
the transition energies of the absorption and emission spectra
of Al(tpfc)(py)n when pyridine is substituted for benzene as
the solvent. In both corroles, the six-coordinate species
absorbs and emits further to the red than the five-coordinate
one, leading to clear isosbestic points in the absorption
spectra near 602, 578, 572, and 566 nm in the Q bands and
near 400 and 425 nm in the Soret bands for Ga(tpfc)(py)n.
Isoemissive points appear near 608, 643, and 665 nm in its
S1 (Q-band) fluorescence spectra. For Al(tpfc)(py)n, the
corresponding isosbestic points are located at very similar
wavelengths, 602 and 426 nm, whereas the isoemissive points
are somewhat further red-shifted at 612, 647, and 669 nm.
The metalloporphyrins exhibit bathochromic shifts in their
UV-visible spectra that are linear functions of the Lorenz-
Lorentz polarizability function of the solvent, f1 ) (n2 - 1)/(n2
+ 2), indicating that dispersive solute-solvent interactions
dominate their solvatochromism.23 The refractive indices, n,
of pyridine and benzene are almost identical, leading to
values of f1 that differ only in the third significant digit. The
corroles and their metalloporphyrin analogs are expected to
exhibit similar solvatochromic behavior in solution, so any
solvatochromically induced shifts in their absorption spectra
will be essentially independent of the composition of the
benzene-pyridine mixtures. Thus, the red shifts in the
absorption and emission spectra observed when pyridine
replaces benzene as a solvent must be due exclusively to
the formation of the six-coordinate species. The five-
coordinate species is clearly stabilized by the second axial
pyridine ligand. Using the Q- and Soret-band spectral shifts
as a measure of the difference between the energies of the
ground and excited electronic states, we note that stabilization
produced by adding a second pyridine ligand is smaller for
the gallium corrole than for the aluminum corrole and is
smaller in the S2 states of both molecules than in their S1
states.
attention, despite the fact that some early studies of their
spectroscopic and photophysical properties revealed that they
would be attractive potential alternatives to the porphy-
rins15,16 used in dye-sensitized photovoltaic cells and pho-
todynamic therapy and proposed for use in photon-actuated
molecular logic devices. These properties include large
quantum yields of S1-S0 fluorescence,4,5 dual S2-S0 and
S1-S0 fluorescence,17 large quantum yields of singlet mo-
lecular oxygen in oxygenated solutions,18 and facile synthetic
access to water-soluble derivatives.19,20
The effects of substituents and their position on the
photophysical properties of free-base corroles and of some
corrole-containing dyads have been reported recently,18,21,22
but similar information about their metallated derivatives is
sparse. In particular, the effects of noncovalent axial ligand
binding on the spectroscopic and photophysical properties
of the metallocorroles have not yet been thoroughly inves-
tigated, even though such insight may be very important
given the increasing interest in applying corroles in biological
media.14,16 We have collaborated to fill this information gap
by examining the coordination dynamics of two metallocor-
roles, Al(tpfc)(py)n and Ga(tpfc)(py)n (n ) 1, 2) (cf. Scheme
1), in both the ground and excited states. The combination
of approaches adopted in this investigation has resulted in
the first measurements of ligand association-dissociation
reaction rate constants of metallocorroles in their electronic
excited states.
Results and Discussion
Al(tpfc)(py)n and Ga(tpfc)(py)n are both five-coordinate
(n ) 1) when dissolved in noncoordinating solvents such as
benzene and toluene, used here.17 A ground-state equilibrium
between the five-coordinate and the six-coordinate (n ) 2)
species is established when pyridine, py, is added to such a
solution, namely,
Quantitative analyses of the absorption spectra as a function
of pyridine concentration yield values of the equilibrium
constants for the ground-state association reactions, eq 1 above.
Under conditions where [py] . [M(tpfc)(py)1], plots of (∆A)-1
versus [py]-1 are expected to be linear,24,25 as shown in Figure
2 for Ga(tpfc)(py)n in benzene. Here, ∆A is the change in
absorbance of the Q-band absorption spectrum of Ga(tpfc)(py)n
in benzene with added pyridine (in Figure 2 for the Ga(tpfc)(py)2
band maximum at 612 nm), and [py] is the molar concentration
of pyridine. The ratio of the intercept to slope of such plots
equals the value of the ground-state association constant, Ka,
which equals 1.0 M-1 for Ga(tpfc)(py)n at 295 K. The
six-coordinate aluminum corrole is considerably more stable
(15) Tripathy, U.; Steer, R. P. J. Porphyrins Phthalocyanines 2007, 11,
228.
(16) Aviv, I.; Gross, Z. Chem. Commun. 2007, 1987.
(17) Liu, X.; Tripathy, U.; Mahammed, A.; Gross, Z.; Steer, R. P. Chem.
Phys. Lett. 2008, 459, 113.
(18) Ventura, B.; Esposti, A. D.; Koszarna, B.; Gryko, D. T.; Flamigni, L.
New J. Chem. 2005, 29, 1557.
(19) Aviezer, D.; Cotton, S.; David, M.; Segev, A.; Khaslev, N.; Galili,
N.; Gross, Z.; Yayon, A. Cancer Res. 2000, 60, 2973.
(20) Saltman, I.; Mahammed, A.; Goldberg, I.; Tkachenko, E.; Botoshansky,
M.; Gross, Z. J. Am. Chem. Soc. 2002, 12, 4–7411.
(21) Paolesse, R.; Sagnore, F.; Macagnano, A.; Boschi, T.; Prodi, L.;
Montalli, M.; Zacchereni, N.; Bolletta, F.; Smith, K. M. J. Porphyrins
Phthalocyanines 1999, 3, 364.
(23) Liu, X.; Tripathy, U.; Bhosale, S. V.; Langford, S. J.; Steer, R. P. J.
Phys. Chem. A 2008, 112, 8986.
(22) Ding, T.; Aleman, E. A.; Modarelli, D. A.; Ziegler, C. J. J. Phys.
Chem. A 2005, 109, 7411.
(24) Benesi, H. A.; Hildebrand, J. H. J. Am. Chem. Soc. 1949, 71, 2703.
(25) Novokov, E.; Boens, N. J. Phys. Chem. A 2007, 111, 6054.
Inorganic Chemistry, Vol. 48, No. 6, 2009 2671

Kowalska et al.
Figure 1. Absorption (a) and emission (c) spectra of Al(tpfc)(py)n in benzene and pyridine and absorption (b) and emission (d) spectra of Ga(tpfc)(py)n in
benzene as a function of added pyridine (increasing, as shown; 0, 1, 3, 5, 10, 30, and 100% by volume for d and 0, 2, 4, 6, 10, and 15% by volume for b).
The fluorescence lifetimes of the gallium corroles are
substantially shorter that those of the corresponding alumi-
num compounds, due almost exclusively to their larger S1
radiationless decay constants. This difference in the radia-
tionless decay rates of the two metallocorroles is consistent
with a faster rate of S1-T1 intersystem crossing in the gallium
compound due to its enhanced heavy-atom-induced spin-orbit
coupling. Similar heavy-atom effects have been reported
Figure 2. Benesi-Hildebrand plot of (∆A)-1 at 612 nm versus [py]-1 for
Ga(tpfc)(py)n in benzene at room temperature.
previously in the S1-T1 decay rates of the d0 and d10
metalloporphyrins.26 Curiously, however, again due primarily
to differences in their radiationless decay rates, the S1 lifetime
than its gallium counterpart; that is, aluminum(III) acts as a
of the six-coordinate aluminum corrole (6.60 ns in pyridine)
significantly stronger Lewis acid than gallium(III) in these
is somewhat shorter that that of the five-coordinate species
corroles. The corresponding value of Ka is 135 M-1 for the
(7.34 ns in benzene), whereas the lifetime of the S1 state of
Al(tpfc)(py)n system in benzene at 295 K.
the six-coordinate gallium compound (3.45 ns in pyridine)
Time-correlated single-photon counting measurements of
is slightly longer that its five-coordinate counterpart (3.04
the S1 fluorescence lifetimes of the two metallocorroles in
ns in benzene).
deoxygenated solutions of pure benzene and pure pyridine
In the Al(tpfc)(py)n system, the large value of the ground-
yielded the data presented in Figure 3a and Table 1. The
state association constant made it impossible to excite
temporal fluorescence decay profiles of each compound in
significant amounts of the five-coordinate species at pyridine
both pure solvents are well-modeled by single exponential
concentrations where the excited-state association reaction
rates and the excited-state decay rates were comparable and
the effect of added pyridine could be observed in the
functions (1.00 < ꢀ2 < 1.10) with lifetimes that are similar
in magnitude, but nevertheless significantly different, for the
five- and the six-coordinate species. The quantum yields of
fluorescence decay. However, this was not the case for the
Ga(tpfc)(py)n system in which Ka ) 1.0 M-1. Temporal S1
S1 f S0 fluorescence of these corroles in a variety of media
have been measured previously.5 The rate constants of the
fluorescence decay profiles of Ga(tpfc)(py)n were measured
radiative decay and the sums of the rate constants of the
nonradiative decay processes of these five- and six-coordinate
species in their S1 states may thus be calculated from kr )
φf/τ and Σknr ) (1 - φf)/τ. The data are compiled in Table 1.
as a function of the concentration of pyridine added to
benzene solutions of the (initially) five-coordinate corroles,
(26) Harriman, A. J. Chem. Soc., Faraday Trans. 2 1981, 77, 1281.
2672 Inorganic Chemistry, Vol. 48, No. 6, 2009

Aluminum and Gallium Corroles
Figure 3. (a) S1-fluorescence decays for Al(tpfc)(py)n in (1) benzene, τ ) 7.34 ns, ꢀ2 ) 1.07, and (2) in pyridine, τ ) 6.60 ns, ꢀ2 ) 1.08, and Ga(tpfc)(py)n
in (3) benzene, τ ) 3.04 ns, ꢀ2 ) 1.10, and (4) in pyridine, τ ) 3.45 ns, ꢀ2 ) 1.10, at room temperature. (b) S1-fluorescence decays for Ga(tpfc)(py)n in (1)
benzene + 3% pyridine, τ1 ) 3.15 ns, τ2 ) 1.55 ns, ꢀ2 ) 1.08, and (2) benzene + 30% pyridine, τ1 ) 3.41 ns, τ2 ) 0.68 ns, ꢀ2 ) 1.10, at room temperature.
Table 1. Photophysical Data for the S1 States of Al(tpfc)(py)n and
Ga(tpfc)(py)n in Deoxygenated Benzene and Pyridine Solutions at Room
Temperature
The fraction of the total fluorescence intensity associated with
each component of the double exponential was obtained from
the fitting parameters using the expression Fi ) aiτi/Σaiτi.
Data were obtained as a function of solution composition
and for several excitation and emission wavelengths; they
are collected in Table 2. No evidence of a fluorescent species
with a measurable rise time was obtained in these systems.
Note that the benzene-pyridine solutions were excited
exclusively at or near the two isosbestic points in the Soret
bands of the corroles, resulting in the initial population of
the S2 states of the five- or the six-coordinate species or of
both. Previous work17 has demonstrated that the lifetimes
of these initially excited S2 states lie in the 200-300 fs region
for both the aluminum and gallium corroles, that their
quantum yields of S2-S1 internal conversion are both ca.
0.8, and that the S1 emission spectra obtained for each corrole
are identical when exciting in either the Soret or the Q
absorption bands. Thus, the kinetic phenomena observed here
solvent λex, nm
λ
em, nm τ, ns φfa kr, × 108 s-1 Σknr, × 107 s-1
M ) Al
benzene
(n ) 1)
pyridine
(n ) 2)
400
400
596
624
7.34 0.76
6.60 0.76
1.03
1.15
3.27
3.64
M ) Ga
3.04 0.37
benzene
(n ) 1)
pyridine
(n ) 2)
400
400
599
617
1.22
1.36
20.7
15.4
3.45 0.47
a Data from ref 5; solvent for n ) 1 is toluene with λex in the Q band.
with results that are shown in Figure 3b. Iterative convolution
of the instrument response function with trial fluorescence
decay functions revealed that the fluorescence profiles of the
corroles in the benzene-pyridine mixtures are all biexponen-
tial, that is, of the form I(t) ) a1 exp(-t/τ1) + a2 exp(-t/τ2).
Inorganic Chemistry, Vol. 48, No. 6, 2009 2673

Kowalska et al.
Table 2. Photophysical Data for the S1 State of Ga(tpfc)(py)n (n ) 1,
2) in Deoxygenated Benzene-Pyridine Mixtures at Room Temperature
of each change with changing solution composition. The
mole fractions, x1 and x2, of the five- and six-coordinate
gallium corroles calculated from the measured ground-state
association constant are given in Table 2. If the species-
weighted average “lifetimes” of the S1 states of Ga(tpfc)(py)1
and Ga(tpfc)(py)2 are calculated using the same mole
fractions of the two species that exist in the ground state
(i.e., from τ ) Σxiτi using the data of Table 2), the resulting
weighted lifetimes are slightly longer than the observed ones.
That is, such a calculation systematically overestimates the
contribution from the longer-lived six-coordinate species.
This suggests that the equilibrium constant for the association-
dissociation reaction in the excited state of the Ga(tpfc)(py)n
system should be somewhat smaller, but not much smaller,
than the ground-state value of Ka ) 1.0 M-1. Moreover,
excitation of the system at equilibrium in the ground state
will produce an initially nonequilibrium distribution of
excited state five- and six-coordinate species whose approach
to equilibrium results in the observed biexponential fluores-
cence decay on a time scale similar to those of the lifetimes
of the excited species. The best fit for the excited-state
association equilibrium constant is found by independent
calculation to be Ka* ) 0.78 ( 0.07 M-1 (vide infra).
Values of the rate constants for the association reaction
of the five-coordinate species with pyridine and the dissocia-
tion of the six-coordinate species in the excited state were
determined from a kinetic model baed on Scheme 1. We
define ka* and kd* as the rate constants for the forward
(association) and reverse (dissociation) processes in the
excited state, eq 2:
a
b
solvent
[py], M λex, nm
λ
em, nm τ1, ns τ2, ns F1
x1
benzene
+1% py
+3% py
+5% py
0
400
400
400
400
400
400
400
425
425
400
400
400
605
605
605
605
620
660
680
605
620
620
620
620
3.04
2.98
3.15
3.24
3.29
3.30
3.30
3.24
3.31
3.41
3.41
3.45
1
1
0.124
0.372
0.621
1.74 0.88 0.99
1.55 0.57 0.97
1.49 0.52 0.95
1.34 0.94
1.42 0.91
1.42 0.96
1.50 0.53
1.38 0.93
1.09 0.94 0.89
0.68 0.98 0.68
+10% py
+30% py
pyridine
1.24
3.72
1
0
a Fraction of emission intensity due to component 1; F2 ) 1 - F1. b Mole
fraction of Ga(tpfc)(py)1; mole fraction of Ga(tpfc)(py)2 is x2 ) 1 - x1.
on a nanosecond time scale are clearly associated with
processes occurring in the vibrationally relaxed S1 states of
the molecular species present in solution.
The corrole concentrations were sufficiently dilute (c <
10-5 M) that aggregation of the solute was not significant;
differences in the photophysical behavior of the solute as a
function of solution composition were therefore attributed
solely to the effects of the ground-state equilibrium repre-
sented by eq 1 and by the corresponding excited-state
reactions leading to equilibrium in which the ligated corroles
are radiating from their S1 excited states (cf. Scheme 1). Any
effects due to ligand dissociation of the five-coordinate
species in the excited state were ruled out because the
temporal fluorescence decay profile of the five-coordinate
species excited in pure benzene is single exponential; that
is, there is no evidence of emission from an excited
M(tpfc)(py)n, n ) 0, species.
M(tpfc)(py)* + py a M(tpfc)(py)2*
(2)
Note the following from the data for the Ga(tpfc)(py)n
system in Tables 1 and 2: (i) The measured “lifetime” of
thelonger-livedfluorescentcomponentinthepyridine-benzene
mixtures is independent of observation wavelength and,
within experimental error, moves monotonically with in-
creasing pyridine concentration from the lifetime of excited
Ga(tpfc)(py) in benzene (3.04 ( 0.05 ns) to the lifetime of
excited Ga(tpfc)(py)2 in pyridine (3.45 ( 0.05 ns). (ii) The
lifetime of the shorter-lived component of the fluorescence
decreases monotonically with increasing pyridine concentra-
tion but is (within experimental error) independent of
observation wavelength, suggesting perhaps that it is associ-
ated with the kinetics of the ligand association-dissociation
reaction in the excited state (vide infra). (iii) The fraction of
the total emission due to the long-lived component decreases
with increasing pyridine content when observed at the
isoemissive wavelength and is much larger (approaching 1)
when observing emission at the band maxima of the emission
spectrum of the six-coordinate species. The fraction of the
total emission associated with the short-lived component of
the fluorescence decay varies in the opposite sense and
approaches zero at the largest pyridine concentrations when
observing in the emission bands of the six-coordinate species.
(iv) The values of τi and Fi are the same for excitation at
the two isosbestic wavelengths within the Soret-band system.
Two absorbing corrole species are present at equilibrium
in the benzene-pyridine mixtures, and the relative amounts
Thus, Ka* ) ka*/kd*, and kPC ) (1/τ1)PC ) 3.29 × 108 s-1 is
the overall (radiative plus nonradiative) first-order decay
constant for the pentacoordinate species in benzene, whereas
kHC ) (1/τ1)HC ) 2.90 × 108 s-1 is the overall first-order
decay constant for the hexacoordinate species in pyridine.
The model in Scheme 1 is very similar to that used to
describe the excited-state kinetics of radiative exciplexes,
except that in the present system, the two radiative species
may be excited directly from their ground states due to the
operation of the ground-state association equilibrium. The
well-established exciplex kinetics analysis of Hui and
Ware27,28 was therefore applied to this system, using the data
in Table 2 to construct simultaneous equations from which
the two unknown rate constants are calculated (cf. Supporting
Information). Iterative fitting of these data results in values
of ka* ) (2.2 ( 0.2) × 108 M-1 s-1, kd* ) (2.9 ( 0.1) ×
108 s-1, and Ka* ) 0.78 ( 0.07 M-1 for the Ga(tpfc)(py)n
system. Note that the value of Ka* ) 0.78 M-1 is slightly
smaller than the value of Ka ) 1.0 M-1, as predicted by the
qualitative argument presented above. Note also that the
value of ka* ) (2.2 ( 0.2) × 108 M-1 s-1 is considerably
smaller (by a factor of about 40-50) than the diffusion-
limited value of 1.0 × 1010 M-1 s-1 in benzene at 295 K.
(27) Hui, M.-H.; Ware, W. R. J. Am. Chem. Soc. 1976, 98, 4712.
(28) Hui, M.-H.; Ware, W. R. J. Am. Chem. Soc. 1976, 98, 4718.
2674 Inorganic Chemistry, Vol. 48, No. 6, 2009

Aluminum and Gallium Corroles
hexacoordinate species have been obtained from measure-
ments of the fluorescence quantum yields and fluorescence
decay times in pure benzene and pure pyridine, respectively.
The steady-state and time-resolved measurements all suggest
that coordination of the metal by a second pyridine molecule
stabilizes the corrole, with the lowest excited singlet state
(present work) stabilized more than the second excited singlet
(past work17) state and the aluminum corrole stabilized more
than the gallium species.
Temporal fluorescence decays of the gallium system in a
mixed benzene-pyridine solution are biexponential due to
dissociation of the hexacoordinate species in the excited state,
leading to establishment of an excited-state dissociation-
association equilibrium. The rate constants for the pyridine
association and dissociation processes for the gallium corrole
in the excited state have been extracted using a kinetic
scheme similar to that employed for excimer photophysics;
the values are ka* ) 2.3 × 108 M-1 s-1 and kd* ) 2.9 × 108
s-1, leading to a value for the excited-state association
equilibrium constant of Ka* ) 0.78 M-1. To our knowledge,
these are the first rate constants to be measured for excited-
state ligand association-dissociation reactions in metallo-
corrole systems. We expect that the information and meth-
odologies provided in this manuscript will be very useful in
the design of tailor-made corroles that can be used in various
applications.
Figure 4. Plot showing -(λ1 + λ2) ) (τ1)-1 + (τ2)-1 versus [py] for
Ga(tpfc)(py)n in benzene at room temperature. The slope is 2.3 × 108 M-1
s-1 ) ka*.
This is entirely reasonable, given that productive coordination
of the second ligand can only occur from the unligated side
of the five-coordinate species, and then only when the N
atom of the pyridine is appropriately oriented toward the
macrocycle.
The short-lived component of the measured biexponential
decays in the benzene-pyridine mixtures has a measured
lifetime that is independent of the observation wavelength
and that decreases with increasing pyridine concentration.
In addition, the fraction of the emission of the shorter-lived
component is considerably smaller when observing at
wavelengths where the six-coordinate species emits. This
suggests that this short-lived component is associated with
emission from the excited five-coordinate species that is
being removed by the pseudo-first-order ligand association
process described in eq 2. In the kinetic model employed,
-(λ1 + λ2) ) (1/τ1 + 1/τ2) is equal to the sum of the first-
order and pseudo-first-order rate constants of the parallel
processes by which the two excited species decay, that is,
(1/τ1 + 1/τ2) ) kHC + kd* + kPC + ka*[py]. The plot of (1/τ1
+ 1/τ2) versus [py] is shown in Figure 4. A good linear
relationship is obtained, and the slope yields a value of ka*
) (2.3 ( 0.1) × 108 M-1 s-1, which is in excellent agreement
with the value of ka* ) (2.2 ( 0.2) × 108 M-1 s-1 calculated
from the best fit to the kinetic model described above (cf.
Supporting Information).
Experimental Section
The Al and Ga corroles were synthesized as reported previ-
ously.4,5 All experiments were carried out in benzene, pyridine, or
mixed benzene-pyridine solutions at room temperature. Solvents
(Aldrich) were of the highest available purity and exhibited
negligible fluorescence at the excitation wavelengths employed.
Deoxygenation was effected by purging the solutions with dry, O2-
free nitrogen. No significant photodegradation of the corroles was
observed due to brief exposure to room light, to the narrow spectral
bandwidths of the Xe arc lamps in the steady-state spectrometers,
or to picosecond pulsed laser light during fluorescence lifetime
measurements.
Absorption spectra were recorded on a Varian (Cary) 500
UV-visible spectrophotometer. Steady-state fluorescence measure-
ments were performed using a Photon Technology International
QuantaMaster spectrofluorometer fitted with double monochroma-
tors on both the excitation and emission sides and a calibrated
photodiode for correction of the excitation spectra. The emission
spectra were corrected for detector sensitivity; reabsorption cor-
rections were negligible in the dilute solutions (c < 10-5 M)
employed.
Conclusions
The steady-state absorption and emission spectra and the
temporal fluorescence decay profiles of two metallocorroles,
Al(tpfc)(py)n and Ga(tpfc)(py)n (n ) 1, 2), have been
measured in a noncoordinating solvent, benzene, in a
coordinating solvent, pyridine, and in mixed benzene-pyridine
solutions. The ground-state spectra reveal that an equilibrium
between the pentacoordinate corrole (n ) 1) and the
hexacoordinate corrole (n ) 2) is established in the mixed
benzene-pyridine solutions. The ground-state equilibrium
constants are 135 M-1 and 1.0 M-1 at 295 K for the Al and
Ga species, respectively. The excited-state radiative and
nonradiative decay constants of the pentacoordinate and the
Time-correlated single-photon counting measurements of
nanosecond excited-state fluorescence lifetimes were measured
with a Coherent picosecond laser excitation source and a home-
built detection and data-processing system that has been
described previously.29 Typical measurements involved the
excitation of dilute solutions of the corroles (c < 10-5 M) within
their strong Soret (S2 r S0) absorption bands near 400 nm and
observing a narrow bandwidth of dispersed fluorescence from
the vibrationally relaxed lower-energy S1 state that is produced
in high quantum yield17 via S2-S1 internal conversion. Typically,
(29) Kowalska, D.; Steer, R. P. J. Photochem. Photobiol. A 2008, 195,
223.
Inorganic Chemistry, Vol. 48, No. 6, 2009 2675

Kowalska et al.
>104 counts were accumulated in the maximum channel, and
deconvolution of the measured fluorescence profile from the
instrument response function (ca. 100 ps width) was straight-
forward, yielding reduced ꢀ2 values between 1.00 and 1.10 and
a random distribution of weighted residuals for both single and
double exponential fits.
Time bases were carefully calibrated, and multiple replicate
measurements were made to ensure reproducibility. The resulting
fluorescence lifetimes reported are averages; those resulting from
single exponential fits have a 2σ error of ( 0.05 ns, whereas double
exponential fits produce lifetimes with a slightly larger estimated
error.
Engineering Research Council of Canada. A.M. and Z.G.
wish to acknowledge the Binational U.S.-Israel Science
foundation. The support of the Canada Foundation for
Innovation, the Saskatchewan Structural Sciences Centre, and
the assistance of Dr. Sophie Brunet with laser maintenance
and training are also gratefully acknowledged.
Supporting Information Available: Additional text giving the
set of processes used to construct the kinetic model for the system
under consideration. This material is available free of charge via
the Internet at http://pubs.acs.org.
Acknowledgment. The authors are grateful for the con-
tinuing support of this research by the Natural Sciences and
IC900056N
2676 Inorganic Chemistry, Vol. 48, No. 6, 2009