Synthesis, excited-state dynamics, and reactivity of a directly-linked pyromellitimide - (Porphinato)zinc(II) complex

Article abstract of DOI:10.1021/ic0108641

N-[5-(10,20-Diphenylporphinato)zinc(II)]-N-(octyl)pyromellitic diimide (PZn-PI), a meso-pyromellitimide-substituted (porphinato)zinc(II) compound, has been fabricated from the reaction of (5-amino-10,20-diphenylporphinato)zinc(II) with pyromellitic dianhydride in the presence of octylamine. Interrogation of the photoinduced charge separation (CS) and thermal charge recombination (CR) electron-transfer (ET) dynamics for PZn-PI in CH₂CI₂ via pump-probe transient absorption spectroscopic methods shows that τ_CS and τ_CR are 770 and 5200 fs, respectively. These ET dynamics differ from those elucidated previously for closely related 5-quinonyl-substituted (porphinato)-metal compounds, and derive from the fact that the low-lying excited states for PZn-PI are porphyrin-localized, possessing little charge-transfer character. The synthesis of N-{5-[15-(2-(triisopropylsilyl)ethynyl)-10,20-diphenylporphinato]zinc(II)} -N-(octyl)pyromellitic diimide demonstrates that PZn-PI can be halogenated at its 15-meso-position and used subsequently as a substrate in metal-catalyzed cross-coupling reactions; the reactivity of PZn-PI is unusual with respect to many directly linked donor - acceptor compounds in that it is stable to these oxidizing and reducing reaction conditions.

Full text of DOI:10.1021/ic0108641

Inorg. Chem. 2002, 41, 566−570
Synthesis, Excited-State Dynamics, and Reactivity of a Directly-Linked
Pyromellitimide−(Porphinato)zinc(II) Complex
Naomi P. Redmore, Igor V. Rubtsov, and Michael J. Therien*
Department of Chemistry, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104-6323
Received August 9, 2001
N-[5-(10,20-Diphenylporphinato)zinc(II)]−N′-(octyl)pyromellitic diimide (PZn−PI), a meso-pyromellitimide-substituted
(porphinato)zinc(II) compound, has been fabricated from the reaction of (5-amino-10,20-diphenylporphinato)zinc(II)
with pyromellitic dianhydride in the presence of octylamine. Interrogation of the photoinduced charge separation
(CS) and thermal charge recombination (CR) electron-transfer (ET) dynamics for PZn−PI in CH Cl2 via pump−
2
probe transient absorption spectroscopic methods shows that τCS and τCR are 770 and 5200 fs, respectively.
These ET dynamics differ from those elucidated previously for closely related 5-quinonyl-substituted (porphinato)-
metal compounds, and derive from the fact that the low-lying excited states for PZn−PI are porphyrin-localized,
possessing little charge-transfer character. The synthesis of N-{5-[15-(2-(triisopropylsilyl)ethynyl)-10,20-diphenylpor-
phinato]zinc(II)}−N′-(octyl)pyromellitic diimide demonstrates that PZn−PI can be halogenated at its 15-meso-position
and used subsequently as a substrate in metal-catalyzed cross-coupling reactions; the reactivity of PZn−PI is
unusual with respect to many directly linked donor−acceptor compounds in that it is stable to these oxidizing and
reducing reaction conditions.
Introduction
We report the synthesis of a closely related D-A system
(PZn-PI) in which a pyromellitimide (PI) acceptor is directly
linked to the meso-carbon of a (porphinato)zinc(II) (PZn)
chromophore, and contrast its ET dynamics with those
established previously for its P-Q counterpart. We also show
that this complex is amenable to subsequent functionalization,
enabling this D-A unit to serve as a key building block for
the synthesis of more complex ET assemblies.
Ultrafast electron-transfer (ET) events play important roles
in nature. During photosynthesis, fast, multistep ET reactions
transform the energy of absorbed light into chemical potential
energy with near unit efficiency. Porphyrin-quinone (P-
Q) compounds, in which the quinone is directly fused to the
porphyrin macrocycle, have served as benchmark systems
with which to probe biologically relevant, photoinduced
charge separation (CS) and thermal charge recombination
(CR) reactions that involve strong donor-acceptor (D-A)
electronic coupling.1-10
Experimental Section
Materials. All manipulations were carried out under nitrogen
prepurified by passage though an O2 scrubbing tower (Schweizerhall
R3-11 catalyst) and a drying tower (Linde 3 Å molecular sieves)
unless otherwise stated. Air-sensitive solids were handled in a Braun
150-M glovebox. Standard Schlenk techniques were employed to
manipulate air-sensitive solutions, while solvents utilized in this
work were obtained from Fisher Scientific (HPLC grade). Tetrahy-
drofuran (THF) was distilled from K/benzoylbiphenyl. Pd(PPh3)4
(Strem or Aldrich) was used as received. Chromatographic purifica-
tion (silica gel 60, 230-400 mesh, EM Science, and Bio-Beads
S-X1, 200-400 mesh, BioRad) of all newly synthesized compounds
* To whom correspondence should be addressed. E-mail: therien@
a.chem.upenn.edu.
(1) Wynne, K.; LeCours, S. M.; Galli, C.; Therien, M. J.; Hochstrasser,
R. M. J. Am. Chem. Soc. 1995, 117, 3749-3753.
(2) Wasielewski, M. R. Chem. ReV. 1992, 92, 435-461.
(3) Gust, D.; Moore, T. A. Top. Curr. Chem. 1991, 159, 103-151.
(4) Sumida, J. P.; Liddell, P. A.; Lin, S.; Macpherson, A. N.; Seely, G.
R.; Moore, A. L.; Moore, T. A.; Gust, D. J. Phys. Chem. A 1998,
102, 5512-5519.
(5) Rodriguez, J.; Kirmaier, C.; Johnson, M. R.; Friesner, R. A.; Holten,
D.; Sessler, J. L. J. Am. Chem. Soc. 1991, 113, 1652-1659.
(6) Sessler, J. L.; Capuano, V. L.; Harriman, A. J. Am. Chem. Soc. 1993,
115, 4618-4628.
(7) Dalton, J.; Milgrom, L. R. J. Chem. Soc., Chem. Commun. 1979, 609-
610.
1
was accomplished on the benchtop. Chemical shifts for H NMR
spectra are relative to residual protium in the deuterated solvents
(CDCl3, δ ) 7.24 ppm). All coupling constants are reported in
(8) Bergkamp, M. A.; Dalton, J.; Netzel, T. L. J. Am. Chem. Soc. 1982,
104, 253-259.
(9) Cormier, R. A.; Posey, M. R.; Bell, W. L.; Fonda, H. N.; Connolly,
J. S. Tetrahedron 1989, 45, 4831-4843.
(10) D’Souza, F.; Deviprasad, G. R.; Hsieh, Y.-Y. Chem. Commun. 1997,
533-534.
566 Inorganic Chemistry, Vol. 41, No. 3, 2002
10.1021/ic0108641 CCC: $22.00 © 2002 American Chemical Society
Published on Web 01/17/2002

Synthesis of a PI-PZn Complex
hertz. Mass spectra were obtained at the University of Pennsylvania
Mass Spectrometry Laboratory.
washed with water, and dried over sodium sulfate. Removal of
volatiles yielded a purple solid which was chromatographed on silica
gel (CHCl3) to yield 3 (24 mg, 99% based on 5 mg of N-
Instrumentation. NMR spectra were recorded on a 250 MHz
AC-Bruker spectrometer. Electronic spectra were obtained on an
OLIS UV/vis/near-IR spectrophotometry system that is based on
the optics of a Cary 14 spectrophotometer. Uncorrected static
fluorescence emission spectra were recorded on a Perkin-Elmer LS-
50 luminescence spectrometer.
Cyclic voltammetric responses were obtained using an EG&G
Princeton Applied Research model 273A potentiostat/galvanostat.
The components of the electrochemical cell included a glassy carbon
working electrode, a Pt wire counter electrode, and a saturated
calomel (SCE) reference electrode. The potential of the ferrocene/
ferrocenium redox couple (0.4 V vs SCE) was used as an internal
redox standard.
Pump-Probe Transient Absorption Spectroscopic Measure-
ments. Transient absorption spectra were obtained using standard
pump-probe methods. Optical pulses, centered at 775 nm, were
generated using a Ti:sapphire laser (Clark-MXR, CPA-2001).
Optical parametric amplifiers (near-IR and visible OPAs, Clark-
MXR) generate excitation pulses tunable in wavelength from the
UV through the near-IR region; a white light continuum served as
the probe beam. After passing through the sample, the probe light
was focused onto the entrance slit of the computer-controlled image
spectrometer (SpectraPro-150, Acton Research Corp.). The noise
level in these transient absorption experiments corresponded to ∼0.2
mOD/s of signal accumulation. The time resolution is probe
wavelength dependent; in these experiments the fwhm of the
instrument response function varied between 140 and 200 fs. A
detailed description of the transient optical apparatus will appear
in an upcoming paper.11 All experiments were carried out at room
temperature (23 ( 1 °C).
1
bromosuccinimide). H NMR (250 MHz, CDCl3/pyr-d5): δ 0.86
(5 H, m, CH3, CH2), 1.3 (8 H, m, CH2), 1.7 (2 H, m, CH2), 3.7 (2
H, t, CH2), 7.7 (6 H, m, Ar H), 8.1 (4 H, m, Ar H), 8.6 (2 H, s,
Arpyrom H), 8.9 (6 H, m, â-H), 9.7 (2H, d, â-H). Vis (CH2Cl2):
λmax (nm) 422, 552, 592. ESMS: m/z 928.1364 (calcd 928.1351).
N-{5-[15-(2-(Triisopropylsilyl)ethynyl)-10,20-diphenylporphi-
nato]zinc(II)}-N′-(Octyl)pyromellitic Diimide (4). A solution of
(trimethylsilyl)acetylene (70 µL, 520 µmol) in dry, degassed THF
(50 mL) was cooled to -78 °C under N2. n-Butyllithium (570 µmol,
1.6 M in hexanes, 360 µL) was added dropwise with stirring, and
the resultant solution was warmed to room temperature. Zinc
chloride (180 mg, 1.29 mmol) was added under a rapid flow of
N2, and the mixture was stirred for 10 min. A portion (5 mL, 50
µmol, 1.0 µM) of the resultant solution was removed with a gastight
syringe and added to a 25 mL Schlenk tube charged with 3 (24
mg, 25.8 µmol) and Pd(Ph3)4 (1.0 mg, 1.0 µmol, 5%) in dry,
degassed THF (5 mL). After the mixture was stirred under nitrogen
for 4 h, TLC (CHCl3) indicated that the reaction was complete.
The reaction mixture was then diluted with ethyl acetate (25 mL),
washed with water, dried over calcium chloride, and evaporated.
The recovered residue was purified by column chromatography
1
(CHCl3) to give 4 (22 mg, 89% based on 24 mg of 3). H NMR
(250 MHz, CDCl3): δ 0.59 (9 H, s, SiCH3), 0.88 (5 H, m, CH3-
CH2), 1.3 (6 H, m, CH2), 1.8 (2 H, m, CH2), 3.8 (2 H, t, CH2), 7.7
(6 H, m, Ar H), 8.1 (4 H, m, Ar H), 8.6 (2 H, s, Arpyrom H), 8.9 (6
H, m, â-H), 9.7 (2 H, d, J ) 4.5 Hz, â-H). Vis (EtOAc): λmax
(nm) 430, 564, 612. ESMS: m/z 947.2680 (calcd 947.2720).
Results and Discussion
N-[5-(10,20-Diphenylporphinato)zinc(II)]-N′-(Octyl)pyrom-
ellitic Diimide (PZn-PI). (5-Amino-10,20-diphenylporphinato)-
zinc(II) (210 mg, 400 µmol), octylamine (130 µL, 800 µmol), and
pyromellitic dianhydride (131 mg, 600 µmol) were added to a 100
mL Schlenk tube charged with anhydrous dimethylformamide (50
mL). The resultant solution was stirred under N2 at reflux for 13 h.
After being cooled to room temperature, the reaction mixture was
diluted with chloroform (100 mL) and washed with deionized water
(3 × 50 mL). After the organic solution was dried over sodium
sulfate, the solvent was removed by distillation under vacuum, and
the residue chromatographed on silica gel (1:1 hexanes/THF). The
first fraction eluted as a mixture of PZn-PI and (10,20-diphe-
nylporphinato)zinc(II). Gel permeation chromatography (THF) was
required to separate these compounds; the first eluted fraction
corresponded to PZn-PI (41 mg, 12% based on 400 mmol of (5-
amino-10,20-diphenylporphinato)zinc(II)). 1H NMR (250 MHz,
CDCl3/pyr-d5): δ 0.83 (5 H, m, CH3, CH2), 1.3 (8 H, m, CH2), 1.8
(2 H, m, CH2), 3.7 (2 H, t, CH2), 7.7 (6 H, m, Ar H), 8.1 (4 H, m,
Ar H), 8.6 (2 H, s, Arpyrom H), 8.9 (6 H, m, â-H), 9.3 (2 H, d, J )
4.5 Hz, â-H), 10.2 (1 H, s, meso-H). Vis (CH2Cl2): λmax (nm) (log
ꢀ (M-1 cm-1)): 412 (5.58), 540 (4.15). ESMS: m/z 850.2232 (calcd
850.2246).
The D-A dyad N-[5-(10,20-diphenylporphinato)zinc(II)]-
N′-(octyl)pyromellitic diimide (PZn-PI) was prepared in
three steps from (10,20-diphenylporphinato)zinc(II) (1).
Nitration of 1 was accomplished regioselectively with iodine
and silver nitrite, affording (5-nitro-10,20-diphenylporphi-
nato)zinc(II) in high yield.12,13 This (meso-nitroporphinato)-
zinc(II) complex was then reduced to the corresponding
amino derivative (5-amino-10,20-diphenylporphinato)zinc-
(II) (2) with sodium borohydride and 10% Pd on carbon
catalyst.12,13 PZn-PI was prepared via the one-pot cyclization
reaction of 2 and octylamine with pyromellitic dianhydride,
as shown in Scheme 1.
Early work by Sanders and Osuka14,15 established the utility
of the PI acceptor in porphyrin-based donor-spacer-
acceptor (D-Sp-A) assemblies. This species is a strong
electron acceptor [E1/2(PI-/PI) ) -0.8 V vs SCE]16 that
features a radical anion absorbance with significant oscillator
strength (λ ) 710 nm). The PI moiety thus provides a
spectral signature for monitoring the growth and decay of
ET intermediates independent of the porphyrin ground-,
N-[5-(15-Bromo-10,20-diphenylporphinato)zinc(II)]-N′-(Oc-
tyl)pyromellitic Diimide (3). N-Bromosuccinimide (5 mg, 26 µmol)
in anhydrous methylene chloride (5 mL) was added dropwise under
N2 to a solution of PZn-PI (24 mg, 28 µmol) in anhydrous
methylene chloride (10 mL). The mixture was stirred at room
temperature for 5 min, quenched by the addition of acetone (1 mL),
(12) Baldwin, J. E.; DeBernardis, J. F. J. Org. Chem. 1977, 42, 3986-
3987.
(13) Baldwin, J. E.; Crossley, M. J.; DeBernardis, J. Tetrahedron 1982,
38, 685-692.
(14) Hunter, C. A.; Sanders, J. K. M.; Beddard, G. S.; Evans, S. J. Chem.
Soc., Chem. Commun. 1989, 1765-1767.
(15) Osuka, A.; Nakajima, S.; Maruyama, K.; Mataga, N.; Asahi, T. Chem.
Lett. 1991, 1003-1006.
(11) Rubtsov, I. V.; Kang, Y. K.; Rubtsov, G. I.; Therien, M. J. Manuscript
in preparation.
(16) Wiederrecht, G. P.; Svec, W. A.; Niemczyk, M. P.; Wasielewski, M.
R. J. Phys. Chem. 1995, 99, 8918-8926.
Inorganic Chemistry, Vol. 41, No. 3, 2002 567

Redmore et al.
Scheme 1a
a Conditions: (a) refluxing DMF.
excited-, and cation-radical-state spectral features. Numerous
studies have demonstrated the value of the PI acceptor for
probing multistep charge-transfer processes in a variety of
synthetic ET assemblies.16-20
The electronic spectrum of PZn-PI corresponds to that
observed for 1 (see the Supporting Information), demonstrat-
ing that the perturbation to the porphyrin S0, S1, and S2
electronic states upon appendage of the PI acceptor to the
(porphinato)zinc(II) chromophore is not severe. This obser-
vation is in marked contrast to analogous optical spectra
obtained for P-Q systems in which a quinonyl unit is fused
directly to the porphyrin macrocycle.1
Figure 1. (A) Transient absorbance spectra obtained following excitation
of PZn-PI at 549 nm measured at the labeled time delays. Experimental
conditions: solvent, methylene chloride; temperature, 23 °C. (B) Exemplary
wavelength-dependent transient decays. A global fit of the data, using three
exponential functions, gives τCS (charge separation) ) 770 fs, τCR ) 5.2
ps, τvibrational cooling (of the hot ground state formed following charge
recombination) ) 17 ps.
The photoinduced and thermal charge recombination ET
dynamics of PZn-PI in CH2Cl2 were investigated by pump-
probe transient absorption spectroscopy (Figure 1). PZn-
PI was excited by a 120 fs pulse (λ ) 549 nm), resulting in
the formation of a porphyrin-based doubly degenerate singlet
excited state [1(PZn)*-PI, Figure 2]. The subsequent charge
separation reaction can be fit as a monoexponential process
with a time constant τCS of 770 fs. The transient absorption
spectrum corresponding to a time delay of 2.0 ps (Figure 1)
displays the classic spectral signatures for both the PI anion
radical15 and the PZn cation radical;21 note that the charac-
teristic respective absorbances (710 and 470 nm) for these
two species are clearly evident. This charge-separated state
undergoes a rapid charge recombination reaction (τCR ) 5.2
ps), producing a vibrationally hot ground state. The hot
ground state cools with a time constant of 17 ps in methylene
chloride, similar to that observed for strongly coupled PQ
benchmarks.5 It is important to note that similar transient
dynamics are observed for PZn-PI irrespective of the
excitation wavelength within the Q-state absorption manifold.
Figure 2. Summary of the proposed electron-transfer dynamics for (A)
PMg-Q and (B) PZn-PI.
It is instructive to compare the ET dynamics manifest by
PZn-PI with that displayed by a related P-Q compound,
(5-quinonyl-10,15,20-triphenylporphinato)magnesium(II)
(PMg-Q). Ultrafast time-resolved spectroscopy demonstrates
extensive mixing between the quinone moiety and the
(porphinato)metal x-polarized Q state.1 Excitation within the
Qx absorption band (hν′, Figure 2A) produces a charge-
transfer (CT) excited state, [P•+Mg-Q•-]*, which decays via
rapid internal conversion to a vibrationally relaxed CT state
(P•+Mg-Q•-, Figure 2A) within a subpicosecond time
domain.1 In contrast, electronic excitation within the Qy
absorption envelope (hν, Figure 2A) affords a porphyrin-
localized excited state [1(PMg)*-Q], which decays via a
solvent-independent ET process to produce the charge-
separated state (P•+Mg-Q•-) on a time scale (τCS ) 350 fs)
likely rate-limited by electronic population equilibration
between the (porphinato)metal Qx and Qy levels.1 Notably,
the ET dynamics evinced for PZn-PI are pump wavelength
independent within the Q-band spectral region (Figure 2B),
indicating that no direct CT states are accessed over these
excitation energies. These data suggest that a combination
of diminished D-A electronic coupling and increased
activation free energy in PZn-PI may be responsible for
(17) Osuka, A.; Marumo, S.; Mataga, N.; Taniguchi, S.; Okada, T.;
Yamazaki, I.; Nishimura, Y.; Ohno, T.; Nozaki, K. J. Am. Chem. Soc.
1996, 118, 155-168.
(18) Imahori, H.; Yamada, K.; Hasegawa, M.; Taniguchi, S.; Okada, T.;
Sakata, Y. Angew. Chem., Int. Ed. Engl. 1997, 36, 2626-2629.
(19) Davis, W. B.; Svec, W. A.; Ratner, M. A.; Wasielewski, M. R. Nature
1998, 396, 60-63.
(20) Staab, H. A.; Nikolic, S.; Krieger, C. Eur. J. Org. Chem. 1999, 1459-
1470.
(21) Fajer, J.; Borg, D. C.; Forman, A.; Dolphin, D.; Felton, R. H. J. Am.
Chem. Soc. 1970, 92, 3451-3459.
568 Inorganic Chemistry, Vol. 41, No. 3, 2002

Synthesis of a PI-PZn Complex
the observed differences in the excited-state dynamics relative
to those elucidated for PMg-Q.
Computation of the driving force for charge separation
(∆GCS) utilizing the Weller expression to estimate the ion
pair destabilization energy of the CS state,22,23 and ap-
proximation of the outer-sphere reorganization energy (λS)
using the classic Marcus expression,24,25 provides some
insight into the origin of the disparate CS dynamics observed
for PMg-Q and PZn-PI. For PMg-Q, photoinduced ET
is predicted to be nearly activationless (∆GCS + λΤ ) -0.07
eV), congruent with the fact that the optical spectra for this
species display a prominent low-energy CT transition.1 In
contrast, the analogous computed value of ∆GCS + λΤ for
PZn-PI is 0.3 eV, consistent with the lack of an observable
CT transition in its absorbance spectrum. Note also that the
value of ∆GCR + λΤ ≈ -0.60 eV, indicating that inverted
region effects play a role in diminishing the magnitude of
kCR with respect to kCS.
Reduced D-A π-overlap relative to that evident for
PMg-Q is an additional factor responsible for the observed
porphyrin-localized nature of the PZn-PI Q states. MOPAC-
determined dihedral angle-dependent relative heats of forma-
tion (Figure 3) reflect the enhanced steric barrier to libration
about the PZn-to-PI bond with respect to analogous motions
involving the PMg-to-quinonyl linkage. The energetic barrier
to complete rotation about the D-A bond in PZn-PI was
calculated26 to be 41.0 kcal/mol (1.8 eV), while the analogous
rotational barrier for PMg-Q was determined to be 33.5 kcal/
mol (1.5 eV). Note that the surface describing the dihedral-
Figure 3. A comparison of computed relative heats of formation
determined as a function of the twist angle (θ) between the (porphinato)-
metal and acceptor least-squares planes for the PZn-PI (solid line) and
PMg-Q (dotted line) D-A complexes. The heats of formation at angle θ
were calculated with the CAChe MOPAC application, version 94, using
AM1 semiempirical Hamiltonians.
angle-dependent heat of formation for PMg-Q is shallow
with respect to that determined for PZn-PI. PZn-PI
structural conformers with D-A dihedral angles of 90 (
15° are computed to have heats of formation within 1 kcal/
mol of that determined for the minimum-energy structure.
In contrast, the structural heterogeneity of PMg-Q conform-
ers possessing similar stability is more extensive; conformers
having D-A dihedral angles of 90 ( 35° are calculated to
possess ∆Hf values within 1 kcal of that of the minimum-
energy structure. Because the extent of the π-conjugation
between D and A affects the magnitude of the electronic
coupling matrix element (HDA), augmented steric factors in
the PZn-PI system would be expected to diminish D-A
electronic communication and limit the degree of delocal-
ization in the PZn-PI singlet excited state.
In contrast to many directly linked D-A compounds,
further functionalization of PZn-PI is uncomplicated. meso-
Halogenation of the porphyrin macrocycle is carried out
regioselectively, in high yield and under facile conditions
(Scheme 2). The halogenation product N-[5-(15-bromo-10,-
20-diphenylporphinato)zinc(II)]-N′-(octyl)pyromellitic di-
imide (3) can thus serve as a building block in the synthesis
of more elaborate donor-spacer-acceptor assemblies that
incorporate multiple light-absorbing D or A units. Because
halogenated porphyrins are reactive substrates in Pd-
catalyzed cross-coupling reactions,27-37 3 can be elaborated
(22) The Gibbs energy for charge separation (∆GCS) was estimated using
the equations -(∆GCS) ) E(0,0)(MP) - E1/2(MP/MP+) + E1/2(A-/A)
+ ∆G(ꢀ) and ∆G(ꢀ) ) e2[(1/ꢀs){1/(2RD) + 1/(2RA) - 1/RDA} - (1/
ꢀT){1/(2RD) + 1/2RA)}], where E(0,0) is the energy of the lowest excited
singlet state (PZn-PI, 2.15 eV; PMg-Q, 2.05 eV), E1/2(MP/MP+) is
the one-electron oxidation potential of the (porphinato)metal com-
pound, E1/2(A-/A) is the one-electron reduction potential of the
acceptor, and ꢀT is the static dielectric constant for the high dielectric
solvent in which the potentiometric measurements were obtained. E1/2
-
(PI-/PI) and E1/2(ZnP/ZnP+) were determined by cyclic voltammetric
experiments to be -0.88 and 0.85 V vs SCE, respectively (see the
Supporting Information). E1/2(Q-/Q) and E1/2(MgP/MgP+) are respec-
tively -0.6 and 0.74 V vs SCE; these data are available from the
literature (see: Carnieri, N.; Harriman, A. Inorg. Chim. Acta 1982,
62, 103-107. Furhop, J.-H.; Mauzerall, D. J. Am. Chem. Soc. 1969,
91, 4174-4181. Furhop, J.-H. Z. Naturforsch. 1970, 25B, 255-
265).
(23) Weller, A. Z. Phys. Chem. N. F. 1983, 133, 93-98.
(24) Marcus, R. A. J. Chem. Phys. 1965, 43, 679-701.
(27) DiMagno, S. G.; Lin, V. S.-Y.; Therien, M. J. J. Am. Chem. Soc. 1993,
115, 2513-2515.
(25) The solvent reorganization energy (λS) and the total reorganization
energy (λΤ) were calculated using the equations λS ) e2/(4πꢀo)({1/
(2RD) + 1/(2RA) - 1/RDA})[(1/ꢀOP) - (1/ꢀS)] and λT ) λS + λi, where
RD is the radius of PZn and PMg (5.5 Å), RA is the acceptor radius
(PI, 4 Å; Q, 3.2 Å), RDA is the center-to-center distance between the
porphyrin and acceptor units (PMg-Q, 6.4 Å; PZn-PI, 8.5 Å), and
ꢀOP and ꢀS are the optical and static dielectric constants, respectively.
The inner-sphere reorganization energy (λi) was calculated to be 0.3
eV, a value typical for D-A systems of this size (see, for example:
Rubtsov, I. V.; Shirota, H.; Yoshihara, K. J. Phys. Chem. A 1999,
103, 1801-1808).
(26) Rotational barriers were calculated with the CAChe MOPAC applica-
tion, version 94, using the AM1 semiempirical Hamiltonians. To check
the accuracy of this computational study, a test calculation was
performed on (10,20-diphenyl-15-cyclohexylporphinato)zinc(II); the
theoretically predicted rotational barrier of the cyclohexyl group (14
kcal/mol) compared favorably with that evaluated experimentally (12
( 0.5 kcal/mol). See: Veyrat, M.; Ramasseul, R.; Turowska-Tyrk,
I.; Scheidt, W. R.; Autret, M.; Kadish, K. M.; Marchon, J.-C. Inorg.
Chem. 1999, 38, 1772-1779.
(28) DiMagno, S. G.; Lin, V. S.-Y.; Therien, M. J. J. Org. Chem. 1993,
58, 5983-5993.
(29) Lin, V. S.-Y.; DiMagno, S. G.; Therien, M. J. Science 1994, 264,
1105-1111.
(30) LeCours, S. M.; DiMagno, S. G.; Therien, M. J. J. Am. Chem. Soc.
1996, 118, 11854-11864.
(31) Hyslop, A. G.; Kellett, M. A.; Iovine, P. M.; Therien, M. J. J. Am.
Chem. Soc. 1998, 120, 12676-12677.
(32) Gauler, R.; Risch, N. Eur. J. Org. Chem. 1998, 1193-2000.
(33) Blake, I. M.; Rees, L. H.; Claridge, T. D. W.; Anderson, H. L. Angew.
Chem., Int. Ed. 2000, 39, 1818-1821.
(34) Deng, Y.; Chang, C. K.; Nocera, D. G. Angew. Chem., Int. Ed. 2000,
39, 1066-1068.
(35) Tse, M. K.; Zhou, Z.-Y.; Mak, T. C. W.; Chan, K. S. Tetrahedron
2000, 56, 7779-7783.
(36) Shi, X. L.; Amin, S. R.; Liebeskind, L. S. J. Org. Chem. 2000, 65,
1650-1664.
(37) Shanmugathasan, S.; Edwards, C.; Boyle, R. W. Tetrahedron 2000,
56, 1025-1046.
Inorganic Chemistry, Vol. 41, No. 3, 2002 569

Redmore et al.
Scheme 2a
recombination ET reactions. The ultrafast ET dynamics
observed for PZn-PI in methylene chloride differ markedly
from those delineated previously for closely related 5-quinonyl-
elaborated porphyrin compounds, such as PMg-Q.1 In
PMg-Q, excitation into an x-polarized Q state produces
directly a CT excited state, while excitation into a Qy
absorption band produces a porphyrin-localized excited state
which decays to the charge-separated state on a time scale
(τCS ) 350 fs) controlled by population exchange between
the Qx and Qy levels.1 Due to diminished D-A electronic
coupling and increased activation free energy relative to
PMg-Q, low-lying CT states do not mix effectively with
porphyrin-localized B and Q electronically excited states in
PZn-PI. Charge separation dynamics in PZn-PI are thus
pump wavelength independent, with the porphyrin-localized
excited-state decaying to the charge-separated state on a time
scale of 770 fs. Notably, the PZn-PI D-A complex can be
further functionalized via halogenation and subsequent metal-
catalyzed cross-coupling at the porphyrin meso-position anti
to the PI moiety; we thus expect that this species will be a
useful synthon in the preparation of more elaborate multi-
chromophoric donor-acceptor arrays.
a Conditions: (a) NBS, CH2Cl2, 0 °C; (b) n-buLi, THF, -78 °C; ZnCl2,
THF; (c) Pd(PPh3)4, THF, 65 °C.
Acknowledgment. This work was supported by a grant
from the Division of Chemical Sciences, Office of Basic
Energy Research, U.S. Department of Energy (DE-FG02-
94ER14494). M.J.T. thanks the Office of Naval Research
(N00014-97-0317) and the MRSEC Program of the National
Science Foundation (DMR-00-79909) for equipment grants
for transient optical instrumentation.
straightforwardly at the 15-position. The conversion of 3 to
N-{5-[15-(2-(triisopropylsilyl)ethynyl)-10,20-diphenylporphi-
nato]zinc(II)}-N′-(octyl)pyromellitic diimide (4) (Scheme
2) illustrates this point.
Conclusion
Supporting Information Available: Absorption and emission
spectra as well as cyclic voltammetric data. This material is
available free of charge via the Internet at http://pubs.acs.org.
In summary, we have prepared a meso-pyromellitimide-
substituted (porphinato)zinc(II) complex which undergoes
ultrafast, photoinduced charge separation and thermal charge
IC0108641
570 Inorganic Chemistry, Vol. 41, No. 3, 2002