Structure and photophysics of β-octafluoro-meso-tetraarylporphyrins

Article abstract of DOI:10.1021/ic001116z

The structure of THF-coordinated [2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetraphenylporphinato]zinc, Zn(F₈TPP)· THF, and photophysical studies of 2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetraphenylporphyrin, F₈TPP, Zn(F₈TPP), 2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetrakis(pentafluorophenyl) porphyrin, F₂₈TPP, and [2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetrakis(pentafluorophenyl) porphinato]zinc, Zn(F₂₈TPP), in benzonitrile, are reported. A key point from these studies is that the octafiuorinated F₈TPP and perfluorinated F₂₈TPP porphyrins possess similar absorption spectra, but dissimilar X-ray crystal structures and disparate photophysical characteristics. These data cannot be easily accommodated within currently accepted theories which relate macrocycle distortion and optoelectronic properties.

Full text of DOI:10.1021/ic001116z

2614
Inorg. Chem. 2001, 40, 2614-2619
Structure and Photophysics of â-Octafluoro-meso-tetraarylporphyrins§
Valeriy V. Smirnov,† Eric K. Woller,† Dereck Tatman,‡ and Stephen G. DiMagno*,†
Department of Chemistry, University of Nebraska-Lincoln, Lincoln, Nebraska 68588-0304, and
Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287-1604
ReceiVed October 10, 2000
The structure of THF-coordinated [2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetraphenylporphinato]zinc, Zn(F8-
TPP)‚THF, and photophysical studies of 2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetraphenylporphyrin, F8TPP,
Zn(F8TPP), 2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetrakis(pentafluorophenyl)porphyrin, F28TPP, and [2,3,7,8,-
12,13,17,18-octafluoro-5,10,15,20-tetrakis(pentafluorophenyl)porphinato]zinc, Zn(F28TPP), in benzonitrile, are
reported. A key point from these studies is that the octafluorinated F8TPP and perfluorinated F28TPP porphyrins
possess similar absorption spectra, but dissimilar X-ray crystal structures and disparate photophysical characteristics.
These data cannot be easily accommodated within currently accepted theories which relate macrocycle distortion
and optoelectronic properties.
Introduction
distortion, and comprehensive reviews have been published
recently.7-9 Several of these nonplanar porphyrins have been
subjected to detailed photophysical studies.10-17 Studies of
electron-rich, sterically encumbered nonplanar porphyrins18-22
demonstrate that these compounds exhibit bathochromically
shifted absorption spectra, reduced S1 lifetimes (τf), small
quantum yields of fluorescence (φf), broad emission bands with
large Stokes shifts, and increased internal conversion rate
constants (kic) relative to standard planar porphyrins, such as
5,10,15,20-tetraphenylporphyrin (TPP) and 2,3,7,8,12,13,17,-
Recent X-ray crystal structure determinations of light harvest-
ing antenna and reaction center proteins show porphyrinic
pigments constrained to adopt nonplanar geometries by polypep-
tide matrices.1-4 It is an unresolved issue whether the observed
pigment conformational diversity is integral to the function of
these proteins or if it is simply a product of evolutionary
happenstance. In principle, nonplanar distortion of porphyrin
rings could serve as a means by which nature modulates the
redox and photophysical properties of these important pig-
ments.5,6
To understand the functional importance, if any, of the
observed landscape of pigment conformational diversity in
natural systems, comparative electrochemical, spectroscopic, and
photophysical studies of structurally characterized planar and
nonplanar porphyrins are required. Many conformationally
constrained model compounds have been synthesized in an effort
to delineate the electronic consequences of porphyrin ring
(7) Ravikanth, M.; Chandrashekar, T. K. Struct. Bonding 1995, 82, 105-
188.
(8) Senge, M. O. In The Porphyrin Handbook; Kadish, K. M., Smith, K.
M., Guilard, R., Eds.; Academic Press: New York, 2000; Vol. 1; pp
239-347.
(9) Shelnutt, J. A. In The Porphyrin Handbook; Kadish, K. M., Smith,
K. M., Guilard, R., Eds.; Academic Press: New York, 2000; Vol. 7;
pp 167-223.
(10) Ravikanth, M.; Chandrashekar, T. K. J. Photochem. Photobiol. A.,
Chem. 1993, 74, 181-187.
(11) Maiti, N. C.; Ravikanth, M. J. Chem. Soc., Faraday Trans. 1996, 92,
1095-1100.
* To whom correspondence should be addressed.
† University of Nebraska.
(12) Gentemann, S.; Medforth, C. J.; Forsyth, T. P.; Nurco, D. J.; Smith,
K. M.; Fajer, J.; Holten, D. J. Am. Chem. Soc. 1994, 116, 7363-
7368.
‡ Arizona State University.
§ Abbreviations used in this manuscript are as follows: DPP, 2,3,5,7,8,-
10,12,13,15,17,18,20-dodecaphenylporphyrin; TPP, 5,10,15,20-tetraphe-
nylporphyrin; OEP, 2,3,7,8,12,13,17,18-octaethylporphyrin; T(t-Bu)P, 5,-
10,15,20-tetrakis(tert-butyl)porphyrin; OETPP, 5,10,15,20-tetraphenyl-2,3,
7,8,12,13,17,18-octaethylporphyrin; TRfP, 5,10,15,20-tetrakis(perfluoro-
alkyl)porphyrin; TC3F7P, 5,10,15,20-tetrakis(heptafluoropropyl)porphyrin;
F8TPP, 5,10,15,20-tetraphenyl-2,3,7,8,12,13,17,18-octafluoroporphyrin; F28-
TPP, 5,10,15,20-tetrakis(pentafluorophenyl)-2,3,7,8,12,13,17,18-octafluo-
roporphyrin; TFPP, 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin; Cl8-
TFPP, 5,10,15,20-tetrakis(pentafluorophenyl)-2,3,7,8,12,13,17,18-octachlor-
oporphyrin; Br8TFPP, 5,10,15,20-tetrakis(pentafluorophenyl)-2,3,7,8,12,-
13,17,18-octabromoporphyrin.
(13) Gentemann, S.; Medforth, C. J.; Ema, T.; Nelson, N. Y.; Smith, K.
M.; Fajer, J.; Holten, D. Chem. Phys. Lett. 1995, 245, 441-447.
(14) Gentemann, S.; Nelson, N. Y.; Jaquinod, L.; Nurco, D. J.; Leung, S.
H.; Medforth, C. J.; Smith, K. M.; Fajer, J.; Holten, D. J. Phys. Chem.
B 1997, 101, 1247-1254.
(15) Goll, J. G.; Moore, K. T.; Ghosh, A.; Therien, M. J. J. Am. Chem.
Soc. 1996, 118, 8344-8354.
(16) Charlesworth, P.; Truscott, T. G.; Kessel, D.; Medforth, C. J.; Smith,
K. M. J. Chem. Soc., Faraday Trans. 1994, 90, 1073-1076.
(17) Drain, C. M.; Kirmaier, C.; Medforth, C. J.; Nurco, D. J.; Smith, K.
M.; Holten, D. J. Phys. Chem. 1996, 100, 11984-11993.
(18) Takeda, J.; Ohya, T.; Sato, M. Chem. Phys. Lett. 1991, 183, 384-
386.
(1) Deisenhofer, J.; Michel, H. Science 1989, 245, 1463-1473.
(2) Deisenhofer, J.; Michel, H. Angew. Chem., Int. Ed. Engl. 1989, 28,
829-847.
(3) Freer, A.; Prince, S.; Sauer, K.; Papiz, M.; Hawthornwaite-Lawless,
A.; McDermott, G.; Cogdell, R.; Isaacs, N. W. Structure 1996, 4, 449-
462.
(19) Medforth, C. J.; Senge, M. O.; Smith, K. M.; Sparks, L. D.; Shelnutt,
J. A. J. Am. Chem. Soc. 1992, 114, 9859-9869.
(20) Barkigia, K. M.; Berber, M. D.; Fajer, J.; Medforth, C. J.; Renner, M.
W.; Smith, K. M. J. Am. Chem. Soc. 1990, 112, 8851-8857.
(21) Ema, T.; Senge, M. O.; Nelson, N. Y.; Ogoshi, H.; Smith, K. M.
Angew. Chem., Int. Ed. Engl. 1994, 33, 1879-1881.
(22) Jentzen, W.; Simpson, M. C.; Hobbs, J. D.; Song, X.; Ema, T.; Nelson,
N. Y.; Medforth, C. J.; Smith, K. M.; Veyrat, M.; Mazzanti, M.;
Ramasseul, R.; Marchon, J.-C.; Takeuchi, T.; Goddard, W. A., III;
Shelnutt, J. A. J. Am. Chem. Soc. 1995, 117, 11085-11097.
(4) Tronrud, D. E.; Schmid, M. F.; Matthews, B. W. J. Mol. Biol. 1986,
188, 443-454.
(5) Barkigia, K. M.; Chantranupong, L.; Smith, K. M.; Fajer, J. J. Am.
Chem. Soc. 1988, 110, 7566-7567.
(6) Shelnutt, J. A.; X.-Z., S.; Ma, J.-G.; Jia, S.-L.; Jentzen, W.; Medforth,
C. J. Chem. Soc. ReV. 1998, 27, 31-41.
10.1021/ic001116z CCC: $20.00 © 2001 American Chemical Society
Published on Web 04/26/2001

â-Octafluoro-meso-tetraarylporphyrins
Inorganic Chemistry, Vol. 40, No. 11, 2001 2615
18-octaethylporphyrin (OEP).12-14,23 Representative compounds
in this class are shown below. The metalated complexes Zn-
(T(t-Bu)P), Zn(DPP), and Zn(OETPP) also exhibit photophysical
properties which differ markedly with respect to Zn(TPP).14
Thus, potential excited-state relaxation mechanisms involving
tautomerism or proton transfer in the free bases were discounted.
Moreover, at low temperature and in frozen matrices, the
emission behavior for each of the sterically encumbered
nonplanar porphyrins approaches that observed for planar
porphyrins.12,13 These considerations led to the proposal that
conformational dynamics in the 1(π,π*) state involving thermal
population of low-frequency out-of-plane vibrational modes of
the macrocycle underlie excited-state relaxation.14
porphyrin periphery (C-F contacts ∼2.8 Å). Consequently,
these compounds are useful and sensitive probes of excited-
state conformational dynamics.
Here, we present the structural characterization of Zn(F8TPP)
and photophysical studies of two sterically encumbered â-oc-
tafluoroporphyrins and their zinc complexes.26,27 A key point
from these studies is that remarkably disparate excited-state
behavior is observed for the octafluorinated (F8TPP) and
perfluorinated (F28TPP) porphyrins, possessing similar absorp-
tion spectra. This observation indicates that current theory,
relating porphyrin ring conformation to their optoelectronic
properties, may need to be expanded upon or revisited.
Experimental Section
Materials. Methylene chloride and benzonitrile were distilled from
calcium hydride, the latter under reduced pressure. F8TPP and F28TPP,
and their zinc complexes were prepared as described previously.26
Photophysical Studies. Absorption spectra were obtained using an
OLIS-14 modification of a Carey-14 UV-vis-NIR spectrometer.
Steady-state fluorescence spectra were measured on a SPEX fluorolog
using optically dilute samples. Optical densities at excitation wave-
lengths were 0.03. Fluorescence decay measurements were performed
on ∼1 × 10-5 M solutions by the time-correlated single photon counting
method. The excitation source was a cavity-dumped Coherent 700 dye
laser pumped by a frequency doubled Coherent Antares 76s Nd:YAG
laser.28 The instrument response function was 35 ps, as measured at
the excitation wavelength for each decay experiment with Ludox AS-
40. Nanosecond transient absorption measurements were made with
excitation from an Opotek optical parametric oscillator pumped by the
third harmonic of a Continuum Surelite Nd:YAG laser. The pulse width
was ∼5 ns, and the repetition rate was 5 Hz. The detection portion of
this system has been described elsewhere.29 Detection of singlet oxygen
phosphorescence was via a North Coast Scientific Ge detector (EO-
817PP). All samples were saturated with oxygen, and disparities in
ground-state absorption at excitation wavelengths were corrected.
Steady-state fluorescence and singlet oxygen quantum yields were
calculated using the comparative method.30 The calculation was as
follows:
In stark contrast to the above studies, electron-deficient 5,-
10,15,20-tetrakis(perfluoroalkyl)porphyrins (TRfP)24 are non-
planar, but exhibit hypsochromic absorption spectra, sharp
emission bands, and small Stokes shifts typical of normal planar
porphyrins, such as TPP.15 The fluorescence quantum yields of
TRfPs are reduced by a factor of 3 compared to TPP, and the
S1 lifetimes are only reduced by a factor of 7. The differences
between the metalated complexes Zn(TC3F7P) (φf ) 0.013, τf
) 1.2 ns, THF) and Zn(TPP) (φf ) 0.033, τf ) 1.86 ns, THF)
are smaller still.15
Thus, the photophysics of two classes of sterically encum-
bered, nonplanar porphyrins, the electron-rich T(t-Bu)P, DPP,
and OETPP and the electron-poor TRfPs, are dramatically
different: the former class exhibits highly “perturbed” opto-
electronic behavior, while the latter does not. The current
understanding of the stereoelectronic consequences of porphyrin
ring distortion is unable to explain the disparity in these
spectroscopic studies. It is important to be able to reconcile these
data if one is to understand the photophysics of protein-bound
porphyrin-derived pigments and how macrocycle structure
modulates the electronic properties in the panoply of heme
protein active sites in biology.
â-Octafluoro-meso-tetraarylporphyrins (F8TPP and F28TPP)
are structurally intriguing compounds. It is clear from semiem-
pirical calculations (AM1),25 which predict relatively minor
endotherms for small excursions from planarity, that the planar
conformation resides on a shallow potential energy surface.26
The ease with which â-octafluoro-meso-tetraarylporphyrins can
be deformed has its origins in the substantial steric interactions
between the â-fluorine substituents and the aryl residues at the
Xu/Xr ) (ΦuODu)/(ΦrODr)
where X is the integrated fluorescence areas for the fluorescence
quantum yields and the excited-state luminescence counts in the singlet
oxygen calculations. The unknown is denoted by “u” and the reference
(27) Leroy, J.; Bondon, A.; Toupet, L.; Rolando, C. Chem. Eur. J. 1997,
3, 1890-1893.
(28) Gust, D.; Moore, T. A.; Luttrull, D. K.; Seely, G. R.; Bittersman, E.;
Bensasson, R. V.; Rougee, M.; Land, E. J.; de Schryver, F. C.; Van
der Auweraer, M. Photochem. Photobiol. 1990, 51, 419-426.
(29) Davis, F. S.; Nemeth, G. A.; Anjo, D. M.; Makings, L. R.; Gust, D.;
Moore, T. A. ReV. Sci. Instrum. 1987, 58, 1629-1631.
(30) Bensasson, R. V.; Land, E. J.; Truscott, T. G. Photolysis and Pulse
Radiolysis: Contributions to the Chemistry of Biology and Medicine;
Pergamon: Oxford, 1983.
(23) Retsek, J. L.; Gentemann, S.; Medforth, C. J.; Smith, K. M.; Chirvony,
V. S.; Fajer, J.; Holten, D. J. Phys. Chem. B 2000, 104, 6690-6693.
(24) DiMagno, S. G.; Williams, R. A.; Therien, M. J. J. Org. Chem. 1994,
59, 6943-6948.
(25) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J. Am.
Chem. Soc. 1985, 107, 3902-3909.
(26) Woller, E. K.; DiMagno, S. G. J. Org. Chem. 1997, 62, 1588-1593.

2616 Inorganic Chemistry, Vol. 40, No. 11, 2001
Smirnov et al.
worthy aspect of the structure of Zn(F8TPP)‚THF is the large
degree of nonplanarity of the porphyrin ring. The porphyrin ring
is extensively saddled; the largest deviations of carbon atoms
from the porphyrin mean plane (0.43 Å average) occur at the
â-pyrrole positions. Superimposed upon this distortion mode
is a second, smaller ruffling of the porphyrin core. These gross
features of the ring distortion in Zn(F8TPP)‚THF are very similar
to those observed for other â-octahalotetraarylporphyrins;
however, the structures of Zn(Cl8TFPP) and Zn(Br8TFPP) show
larger displacements of the â-pyrrole carbon atoms from the
porphyrin mean planes.35,36 The smaller degree of nonplanar
distortion observed in Zn(F8TPP)‚THF stems from the mitigated
aryl-halogen steric interactions at the porphyrin periphery;
fluorine has a substantially reduced van der Waals radius
compared to the heavier halogens.37
Figure 1. Perspective drawing from the X-ray crystal structure
determination of Zn(F8TPP)‚THF with thermal ellipsoids at 25 %
probability.
A comparison of the structure of Zn(F8TPP)‚THF to that
recently reported for Zn(F8TPP)‚H2O27 reveals that the axial
ligand plays an extremely modest role in modulating the solid-
state conformation of the porphyrin core. The aquo complex of
Zn(F8TPP) exhibits a more pronounced saddle distortion (0.59
Å average displacement of the pyrrole â-positions) and a shorter
Zn-O contact (2.092 Å) than Zn(F8TPP)‚THF. The remaining
structural parameters (Table 1) are identical, within experimental
error. The small changes induced in the structure of Zn(F8TPP)
by altering (or removing) the axial ligand are also consistent
with recent theoretical predictions.38
Although the structure of Zn(F8TPP)‚THF follows the trends
observed for Zn(Cl8TFPP) and Zn(Br8TFPP), it is quite dis-
similar to the previously reported structure of five-coordinate
Zn(F28TPP), in which the porphyrin ring is essentially flat.26
(Planar structures are also obtained for the free base and the
Co(II) and Rh(III) complexes of F28TPP.)39-41 It is curious that
Zn(F28TPP) and Zn(F8TPP) should show such different solid-
state structures. It is clear that these fluorinated porphyrins reside
on shallow conformational potential energy surfaces, that crystal
packing forces are different for Zn(F8TPP)‚THF and Zn(F28-
TPP), and that the ability of crystal packing forces to distort
even sterically unencumbered porphyrins is well documented.42
Thus, crystal packing forces cannot be discounted as the origin
for the structural dissimilarities of Zn(F8TPP)‚THF and Zn(F28-
TPP).
compound is “r”. OD is the ground-state absorption at the excitation
wavelengths for both experiments, and Φ is the singlet oxygen
fluorescence quantum yield. Luminescence signals were linear with
respect to the excitation energy for all singlet oxygen calculations.
X-ray Diffraction Studies of Zn(F8TPP)‚THF. Crystals of Zn(F8-
TPP)‚THF were obtained by slow evaporation (3 weeks) of a THF:
octane 1:1 solution (30 mg of Zn(F8TPP) in 80 mL) at room
temperature. A deep red, flat block crystal, of approximate dimensions
0.4 × 0.4 × 0.2 mm, was sealed in a glass capillary (to prevent loss of
solvent) for collection of diffraction data. All measurements were
performed with graphite monochromated Ag KR radiation (λ ) 0.5608
Å) produced by a Siemens rotating-anode X-ray source. Data were
collected at room temperature by the oscillation method using a
MARResearch (18 cm) imaging plate area detector. The data were
processed using the MARXDS program.31 Data were corrected for
Lorentz and polarization factors, but not for absorption. A total of 19662
reflections were measured and averaged to 5298 unique reflections (Rint
) 0.0438). Structure solution and refinement were carried out using
the SHELX-97 package of programs.32 The structure was solved in
the monoclinic space group P21/n by Patterson methods. After
determination of the location of the zinc ion, subsequent iterations of
difference Fourier synthesis followed by determination of the heavy
atom positions led to the location of all heavy atoms. All non-hydrogen
atoms were refined anisotropically. Hydrogen atom positions were
determined by idealized geometries and treated as riding atoms. At
convergence, R1 ) 0.0693 (for 4700 reflections with Fo > 4σ(Fo)),
wR2 ) 0.1755, and GOF ) 1.185. A final difference Fourier map was
judged to be significantly free of features, with the largest positive
peak having a height of 1.11 e/Å3 and the lowest negative peak having
a height of 0.30 e/Å3.
A second potential explanation for the structural differences
between Zn(F8TPP)‚THF and Zn(F28TPP) can be found in the
details of â-fluorine-aryl interactions in the two fluorinated zinc
complexes. In the structure of Zn(F8TPP)‚THF, the nonbonded
C-F contacts (e.g., F3-C21) average 2.82 Å, as compared to
2.77 Å for the corresponding nonbonded contacts in Zn(F28-
TPP). For F8TPP and Zn(F8TPP), the electrostatic contribution
to the interaction of the â-fluorines with the (negatively charged)
face of a meso-phenyl substituent should be quite unfavorable.
In contrast, the same â-fluorine-aryl interaction with a meso-
Results
X-ray Crystal Structure of Zn(F8TPP)‚THF. Crystals of
Zn(F8TPP)‚THF suitable for X-ray diffraction were grown by
the slow evaporation of a THF/octane solution; the compound
obtained from this process was a five-coordinate THF adduct.
A perspective drawing from the crystal structure determination
is shown in Figure 1, and average bond lengths and angles are
given in Table 1. The zinc atom is displaced by 0.17 Å from
the plane of the porphyrin ring in the direction of the coordinated
THF molecule. The Zn-O bond is short, (2.147 Å), accurately
reporting the electron deficient nature of the porphyrin. The
Zn-N bond lengths (2.063 Å average) are in the range expected
for nearly planar zinc complexes, but are long in comparison
to those reported for the saddled compounds Zn(Cl8TFPP),
(2.032 Å), and Zn(Br8TFPP), (2.040 Å).35,36 The most note-
(35) Birnbaum, E. R.; Hodge, J. A.; Grinstaff, M. W.; Schaefer, W. P.;
Henling, L.; Labinger, J. A.; Gray, H. B. Inorg. Chem. 1995, 34,
3625-3632.
(36) Marsh, R. E.; Schaefer, W. P.; Hodge, J. A.; Hughes, M. E.; Gray, H.
B.; Lyons, J. E.; Ellis, P. E. J. Acta Crystallogr. 1993, C49, 1339-
1342.
(37) Bondi, A. J. Phys. Chem. 1964, 68, 441-451.
(38) Nguyen, K. A.; Day, P. N.; Pachter, R. J. Phys. Chem. A 1999, 103,
9378-9382.
(39) Leroy, J.; Bondon, A.; Toupet, L. Acta Crystallogr. 1999, C55, 464-
466.
(31) Kabsch, W. J. Appl. Crystallogr. 1988, 21, 916.
(32) Sheldrick, G. M. SHELX-97; University of Gottingen, 1997.
(33) Schauer, C. K.; Anderson, O. P.; Eaton, S. S.; Eaton, G. R. Inorg.
Chem. 1985, 24, 4082-4086.
(34) Scheidt, W. R.; Kastner, M. E.; Hatano, K. Inorg. Chem. 1978, 17,
706-710.
(40) Smirnov, V. V.; Woller, E. K.; DiMagno, S. G. Inorg. Chem. 1998,
37, 4971-4978.
(41) Nelson, A. P.; DiMagno, S. G. J. Am. Chem. Soc. 2000, 122, 8569-
8570.
(42) Scheidt, W. R.; Lee, Y. J. Struct. Bond. 1987, 64, 1-70.

â-Octafluoro-meso-tetraarylporphyrins
Inorganic Chemistry, Vol. 40, No. 11, 2001 2617
Table 1. Average Bond Lengths (Å) and Bond Angles (Deg) for Zn(F8TPP)‚THF and Selected Zinc Porphyrins
parameter
Zn(F8TPP)‚THF Zn(F28TPP)‚THF Zn(F8TPP)‚H2O Zn(TPP)‚2THF Zn(TPP) Zn(TFPP) Zn(Cl8TFPP) Zn(Br8TFPP)
N-Zn
2.063(4)
1.381(7)
1.440(8)
1.328(8)
1.395(8)
108.2(4)
107.5(5)
126.1(5)
108.4(5)
124.5(5)
this work
2.059(6)
1.382(10)
1.439(12)
1.337(12)
1.397(12)
107.4(6)
108.3(7)
125.5(7)
108.0(8)
126.0(8)
26
2.068(5)
1.372(8)
1.429(9)
1.320(10)
1.393(9)
108.5(5)
107.3(5)
126.1(6)
108.4(6)
124.5(6)
27
2.057(2)
1.371(4)
1.444(5)
1.349(4)
1.403(4)
107.0(3)
109.3(3)
125.8(3)
107.2(3)
125.0(3)
33
2.036(2)
1.375(2)
1.443(3)
1.351(3)
1.399(3)
106.5(1)
109.6(2)
125.7(2)
107.2(2)
125.0(2)
34
2.036(2)
1.371(3)
1.438(4)
1.339(4)
1.398(3)
106.2(2)
109.6(2)
125.1(2)
107.3(2)
126.1(2)
35
2.032(11)
1.380(13)
1.427(16)
1.337(15)
1.403(15)
106.9(8)
107.4(9)
124.2(9)
108.8(10)
126.5(10)
35
2.038(21)
1.375(23)
1.450(26)
1.352(28)
1.392(26)
109.1(16)
107.6(14)
124.6(16)
107.7(17)
124.0(17)
36
a
N-CR
b
CR-Câ
Câ-Câ
c
CR-Cm
CR-N-CR
N-CR-Câ
N-CR-Cm
CR-Câ-Câ
CR-Cm-CR
reference
a Carbon atoms in R-positions of pyrrole rings. b Carbon atoms in â-positions of pyrrole rings. c Carbon atoms in meso-positions of porphyrin
ring.
Table 2. Optical Spectroscopy, Fluorescence, and Photophysics of â-Octafluorinated Porphyrins in Benzonitrile, with Data for Selected
Porphyrins
λmax
λmax
(fluor)
(nm)
Stokes
Shift
S1
(absorbance) Q(0,0)/
lifetime
kobs
kf
kic
kisc
τT
compd
F8TPP
(nm)
Q(1,0)a
(cm-1
)
Φfb
0.06
(ns)c
(108 s-1
)
(107 s-1
)
(108s-1
)
(108 s-1
)
ΦT (O2)d (µs)e
409, 502, 535, 1.29
584, 639
399, 498, 534, 0.07
579, 632
422, 516, 552, 0.80
592, 649
417, 547, 585 0.45
662 (br) 544
640, 702 198
652, 716 71
610, 639 701
2.97
6.26
3.37
1.60
0.94
2.02
0.48
1.04
0.408
0.400
0.21
2.76
1.15
0.63
0.82
0.72
0.67
26
775
120
F28TPP
TPP
0.03
0.11f
10.6g
Zn(F8TPP)
Zn(F28TPP)
Zn(TPP)
0.018
0.445
1.09
2.2h
0.83
0.046
0.5
22.5
9.17
4.54
12.05
217
20
4.04
1.74
1.43
0.72
0.63
2
5.20
2.21
0.397
<5.88
>333
<9.8
16.9
6.76
4.0
0.75
0.74
0.88
63
2150
416, 544, 585 0.003 592, 637 202
429, 560, 600 0.42
0.019
607, 658 192
772 (br) 975
720
0.032h
0.006
DPPi
718
696
>5.88
>0.5
<0.2
>0.5
T(t-Bu)Pj
Zn(DPP)k
Zn(T(t-Bu)P)k
TC3F7Pm
478
1360
1300
2 × 10-4
0.010
>83.3
>10
3 × 10-5
0.028
0.007 1428
1.4
0.43
2
-
l
405, 510, 544, 2.19
593, 647
651, 717 126
7.14
Zn(TC3F7P)n
409, 554, 593 1.91
607, 652 384
0.013
1.2
8.33
1.08
a Defined as the extinction coefficient of the lowest energy absorption band divided by the extinction coefficient for the lowest energy first
vibrational overtone, as discussed in ref 47. b Fluorescence quantum yields measured in the steady state using hematoporphyrin dimethyl ester
(HPDME) as a standard (Φf)0.09). HPDME was in methanol. Samples were not purged with Ar. c Samples were not purged with Ar. d Triplet
yields determined assuming 100% singlet oxygen sensitization. Quantum yields of singlet oxygen sensitization were measured by emission at 1270
nm. Linear dependence with respect to laser energy was verified, and samples were in oxygen saturated benzonitrile. Reference for F8TPP and
F28TPP was TPP, and for Zn(F8TPP) and Zn(F28TPP) was HPDME. e Triplet lifetimes measured in argon purged benzonitrile. f In benzene, ref 48.
g In methanol, ref 49. h In toluene, ref 50. i In toluene, ref 12. j In 3-methylpentane/isopentane ) 1/1, ref 13. k In 2-methyltetrahydrofuran, ref 14.
l Very low, ref 14. m In CH2Cl2, ref 15. n In THF, ref 15.
pentafluorophenyl group should have a less unfavorable (and
perhaps attractive) electrostatic contribution in F28TPP and Zn-
(F28TPP). Thus, the reversed quadrupolar charge distributions
of the phenyl and perfluorophenyl substituents (as in hexafluo-
robenzene and benzene43-46) may be responsible for the differing
solid-state structures of Zn(F8TPP)‚THF and Zn(F28TPP). This
argument is congruent with the observation that the structures
of F28TPP and its metal chelates reported to date show more
planar macrocycle conformations than the corresponding F8-
TPP derivatives.
Optical Spectroscopy. The absorption data for the fluorinated
porphyrin derivatives are given in Table 2. The Soret (B) bands
are hypsochromically shifted compared to all other tetraarylpor-
phyrins, but are still lower in energy than the B-band of
porphine. The lowest energy (Q) bands are also hypsochromi-
cally shifted compared to TPP, and these blue shifts are similar
to that of planar octaalkyltetraarylporphyrins.51 The shifts of
the absorption bands to higher energy may arise from the
π-electron donation from the â-fluorines, which disproportion-
ately destabilizes the eg LUMO orbitals in Gouterman’s four
orbital model.52 A similar effect is responsible for the extremely
high energy Soret band noted for 2,3,7,8,12,13,17,18-oc-
tamethoxyporphyrin.53 Additionally, the increased orthogonality
of the aryl rings, particularly seen in the structure of Zn(F28-
TPP), may contribute to the blue shifting in the optical spectra
by minimizing conjugative interactions at the meso-positions.54
The relative intensities of the Q(0,0) and Q(1,0) absorption
transitions are strikingly different for the octafluorinated and
perfluorinated porphyrins (Figure 2). Gouterman has related the
(47) Spellane, P. J.; Gouterman, M.; Antipas, A.; Kim, S.; Liu, Y. C. Inorg.
Chem. 1980, 19, 386-391.
(48) Seybold, P. S.; Gouterman, M. J. Mol. Spectrosc. 1969, 31, 1-13.
(49) Bonnett, R.; McGarvey, D. J.; Harriman, A.; Land, E. J.; Truscott, T.
G.; Winfield, U. J. Photochem. Photobiol. 1988, 48, 271-276.
(50) Grayushko, A. T.; Tsvirko, M. P. Opt. Spectrosc. 1971, 31, 291-
295.
(51) Senge, M. O.; Medforth, C. J.; Sparks, L. D.; Shelnutt, J. A.; Smith,
K. M. Inorg. Chem. 1993, 32, 1716-1723.
(43) Coates, G. W.; Dunn, A. R.; Henling, L. M.; Dougherty, D. A.; Grubbs,
R. H. Angew. Chem., Int. Ed. Engl. 1997, 36, 248-251.
(44) Williams, J. H. Acc. Chem. Res. 1993, 26, 593-598.
(45) Patrick, C. R.; Prosser, G. S. Nature 1960, 187, 1021.
(46) Dahl, T. Acta Chem. Scand. 1994, 48, 95-106.
(52) Gouterman, M. J. Mol. Spectrosc. 1961, 6, 138-163.
(53) Merz, A.; Schropp, R.; Lex, J. Angew. Chem., Int. Ed. Engl. 1993,
32, 291-293.
(54) DiMagno, S. G.; Wertsching, A. K.; Ross, C. R., II J. Am. Chem.
Soc. 1995, 117, 8279-8280.

2618 Inorganic Chemistry, Vol. 40, No. 11, 2001
Smirnov et al.
1
a2ufeg) transitions according to eq 1, where E(a2ufeg) and
1E(a1ufeg) are the energies of the two single transitions.47 Thus,
A[Q(0,0)]/A[Q(1,0)] ) (const)[1E(a2ufeg) - 1E(a1ufeg)]2
(1)
the small Q(0,0) extinction coefficients for F28TPP and Zn(F28-
TPP) indicate energetically degenerate a1u and a2u filled frontier
orbitals. In contrast, the Q(0,0) bands for F8TPP and Zn(F8-
TPP) have significant oscillator strengths and Q(0,0)/Q(1,0)
intensity ratios similar to that of the TPP derivatives. Elementary
substituent effect arguments (phenyl is a significantly better
electron-donating group than pentafluorophenyl)55 predict HO-
MOs for F8TPP and Zn(F8TPP) that are predominantly a2u in
character.
Although there are small differences in the energies of the
absorption bands for the free bases F8TPP and F28TPP, the
positions of the absorption maxima for the zinc derivatives are
virtually superimposable. Using conventional theories of por-
phyrin stereoelectronic effects, in which nonplanar distortions
are correlated with dramatic red shifts in the optical spectra,
these data would suggest that the two fluorinated porphyrins
are nearly isostructural (and planar) in benzonitrile. According
to this analysis, one would expect no dramatic differences in
the excited-state properties of the octafluorinated and perflu-
orinated derivatives. To test whether there were discernible
differences between the solid-state and solution structures of
Zn(F8TPP), we compared the optical spectra of crystalline F8-
TPP, Zn(F8TPP), F28TPP, Zn(F28TPP), TPP, and Zn(TPP) in
the solid state (in a KBr pellet)54 to the spectra of the same
compounds in CH2Cl2 and benzonitrile (see Supporting Infor-
mation). Transfer of F8TPP and Zn(F8TPP) to solution resulted
in shifts of the optical absorption bands that were not discernibly
different from those observed for transfer of F28TPP and Zn-
(F28TPP) from the solid state to solution. Were optical spectra
correlated with porphyrin nonplanarity, the results of the solid-
state and solution spectroscopic studies would lead to the
conclusion that the nonplanar solid-state structure of Zn(F8TPP)
is maintained in solution. However, the same posited correlation
between porphyrin nonplanarity and optical absorption spec-
troscopy leads to the opposite conclusion that Zn(F8TPP) and
Zn(F28TPP) should have similar structures. Clearly, these two
conclusions cannot be simultaneously true, since there are clear
structural differences between Zn(F8TPP) and Zn(F28TPP) in
the solid state. Thus, the anticipated red shifts in optical spectra
associated with ring distortion in porphyrins must be absent in
these electron-deficient derivatives, and no structural information
may be inferred from a comparison of optical absorption spectra
alone. In this situation, excited-state properties could provide
valuable information regarding the solution conformations of
these fluorinated porphyrins.
Fluorescence Spectroscopy. Despite the similarities in the
ground-state absorption spectra of the octafluorinated and
perfluorinated porphyrins, the steady-state emission data are
remarkably different (Table 2). TPP, Zn(TPP), F28TPP, and Zn-
(F28TPP) exhibit emission spectra with small Stokes shifts and
fine structures that mirror the absorption spectra in intensity
and spacing. These aspects of the porphyrin emissions indicate
that the compounds do not undergo significant reorganization
of nuclear coordinates in the excited state. Compounds F8TPP
and Zn(F8TPP) show broad, relatively featureless emission
spectra and substantial Stokes shifts. These data are reminiscent
Figure 2. Normalized optical absorption spectra of TPP, Zn(TPP),
F8TPP, Zn(F8TPP), F28TPP, and Zn(F28TPP) in benzonitrile: (a) B-band
region for free base porphyrins, (b) Q-band region for free base
porphyrins, (c) B-band region for zinc porphyrins, (d) Q-band region
for zinc porphyrins.
intensities of these absorption bands {A[Q(0,0)]/A[Q(1,0)]} to
the degree of degeneracy in the highest lying (a1ufeg and
(55) Hansch, C.; Leo, A.; Taft, R. W. Chem. ReV. 1991, 91, 165-195.

â-Octafluoro-meso-tetraarylporphyrins
Inorganic Chemistry, Vol. 40, No. 11, 2001 2619
of the emission spectra of distorted porphyrins with a2u ground
states, such as T(t-Bu)P and OETPP,12-14 and indicate that there
is substantial excited-state conformational reorganization. The
excited-state conformational reorganization for F8TPP and Zn-
(F8TPP) appears to be intrinsic to the ring system and
independent of the solvent, since comparable Stokes shifts are
also observed in the emission spectra of methylene chloride
solutions (see Supporting Information).
Table 2 show. Particularly noteworthy is the lack of a significant
Stokes shift observed for TC3F7P or Zn(TC3F7P); why these
compounds differ significantly from the other nonplanar por-
phyrins listed in Table 2 remains an open question. The
compounds listed in Table 2 exhibit different nonplanar distor-
tion modes (DPP and Zn(DPP) are saddled, T(t-BuP), ZnT(t-
BuP), TC3F7P, and Zn(TC3F7P) are ruffled), so the distortion
mode can be eliminated as a possible explanation for the unusual
photophysical properties of TC3F7P and Zn(TC3F7P). Similarly,
the redox potentials of the compounds in Table 2 span a wide
range, but the fact that the nonplanar electron-deficient por-
phyrins F8TPP and Zn(F8TPP) exhibit excited-state behavior
that mirrors other nonplanar porphyrins, while TC3F7P and Zn-
(TC3F7P) do not, seems to rule out porphyrin ring electron
density as a cause for the unusual properties of the latter
compounds. One feature that distinguishes TC3F7P and Zn-
(TC3F7P) from the remaining compounds in Table 2 is that their
ground-state electronic structure is different; they are predicted
to possess a1u HOMOs rather than a2u HOMOs. Were ground-
state electronic structure correlated with the type and degree of
excited-state conformational change, the dilemma concerning
the divergent photophysical behavior of nonplanar porphyrins
would be resolved. Such a hypothesis could provide a general
explanation for the interesting differences observed in the
excited-state behavior of nonplanar TRfPs and other highly
substituted model compounds. Moreover, such proposals are
attractive targets for theoretical inquiry.
The quantum yields of fluorescence of the â-octafluorinated
compounds are slightly diminished from those of TPP and Zn-
(TPP), but F8TPP, Zn(F8TPP), F28TPP, and Zn(F28TPP) still
possess impressive yields for extremely electron-deficient
porphyrins, resembling meso-tetrakis(perfluoroalkyl)porphyrins
in this regard. In contrast, chlorinated and brominated tet-
raarylporphyrin derivatives exhibit sharply reduced fluorescence
quantum yields.56,57 Within the set of â-octafluorinated por-
phyrins, it is interesting to note that the fluorescence quantum
yield of F8TPP exceeds that of F28TPP. This result is consistent
with the reduced integrated oscillator strength of F28TPP in the
Q-band region compared to that of F8TPP (Figure 2). The similar
quantum yields of fluorescence for Zn(F8TPP) and Zn(F28TPP)
result from a combination of factors, which are discussed below.
The fluorescence lifetimes of the fluorinated compounds are
reduced from those of TPP, with F8TPP and Zn(F8TPP)
exhibiting the most precipitous drop. Nevertheless, the radiative
lifetimes are quite long in comparison to other extremely
electron-deficient porphyrins.15,58 The radiative rates of F8TPP,
Zn(F8TPP), F28TPP, and Zn(F28TPP) were calculated from the
S1 lifetimes and the quantum yield of fluorescence (kf ) φf/τf,
Table 2). The larger radiative rate constants measured for F8-
TPP and Zn(F8TPP) relative to F28TPP and Zn(F28TPP) are
consistent with the larger integrated oscillator strength for the
Q-bands of the octafluorinated derivatives.
Conclusion
The sterically encumbered porphyrins F8TPP and F28TPP
occupy an interesting position between nonplanar porphyrins,
featuring electron-donating meso-substituents and a2u HOMOs,
and nonplanar meso-tetrakis(perfluoroalkyl)porphyrins, for which
empirical and theoretical studies indicate a1u HOMOs.15,54 The
steric interactions at the periphery of the octafluorinated
porphyrins are balanced by the countervailing electronic prefer-
ence for planarity. Thus, these compounds are exquisite models
to probe the sensitivity of porphyrin excited states to steric and
electronic effects. As the studies reported herein show, minimal
changes in the substituent steric parameters lead to surprisingly
large differences in the excited-state properties of these pig-
ments, given their similar optical spectra. These studies highlight
that it is impossible to predict the optoelectronic behavior of
porphyrins within the current theoretical framework.
The magnitudes of the remaining excited-state deactivation
rate constants (kisc and kic) were determined from the quantum
yields for triplet formation and nonradiative events, respectively.
Increased rates of intersystem crossing and internal conversion
are characteristics generally observed for nonplanar porphyrins
possessing a2u HOMOs,12 and this relationship also seems to
hold for â-octafluorinated porphyrins. The increased nonpla-
narity of F8TPP and Zn(F8TPP) is manifested in slightly faster
rates of intersystem crossing and internal conversion, in
comparison to F28TPP and Zn(F28TPP).
Discussion
The data presented here indicate that F8TPP and Zn(F8TPP)
bear the photophysical signatures of nonplanar porphyrins, while
F28TPP and Zn(F28TPP) do not. The available structural data
and the disparate excited-state behavior are consistent with
nonplanar ground-state solution conformations for F8TPP and
Zn(F8TPP) and planar solution conformations for F28TPP and
Zn(F28TPP). As discussed above, such behavior is not altogether
consistent with the current quasitheoretical treatments relating
optical absorption spectra to the ground-state conformations of
porphyrins, and suggests that excited-state properties might be
more sensitive and reliable diagnostic indicators of the porphyrin
ground-state conformation than absorption spectra.
Acknowledgment. S.G.D. acknowledges the donors of the
Petroleum Research Fund, administered by the American
Chemical Society, NSF (CHE-9817247), and the Office of Naval
Research (N00014-00-1-0283) for the support of this research.
D.T. acknowledges support from the Division of Chemical
Sciences, Office of Basic Energy Sciences, Office of Energy
Research, and U.S. Department of Energy (DE-FG03-
93ER14404).
Supporting Information Available: Listing of X-ray structural
information including crystal data, atomic coordinates, and bond lengths
and angles for Zn(F8TPP)‚THF, emission spectra for the F8TPP, Zn-
(F8TPP), F28TPP, Zn(F28TPP), TPP, and Zn(TPP) in CH2Cl2 and
benzonitrile, and absorption data for F8TPP, Zn(F8TPP), F28TPP, and
Zn(F28TPP) in CH2Cl2, benzonitrile, and in the solid state. This material
is available free of charge via the Internet at http://pubs.acs.org.
The correlation of porphyrin ring conformations with excited-
state properties is obviously not 100% reliable, as the data in
(56) Bonnett, R.; Harriman, A.; Kozyrev, A. N. J. Chem. Soc., Faraday
Trans. 1992, 88, 763-769.
(57) Quimby, D. J.; Longo, F. R. J. Am. Chem. Soc. 1975, 97, 5111-
5117.
(58) Bhyrappa, P.; Krishnan, V. Inorg. Chem. 1991, 30, 239-245.
IC001116Z