Exceptional electronic modulation of porphyrins through meso-arylethynyl groups. Electronic spectroscopy, electronic structure, and electrochemistry of [5,15-bis](aryl)ethynyl]-10,20-diphenylporphinato]zinc(II) complexes

Article abstract of DOI:10.1021/ja962403y

A new class of porphyrins in which intervening ethynyl moieties connect aryl groups to the porphyrin 5 and 15-positions is described. The synthesis and optical spectroscopy of five new compounds are reported: [5,15-bis[(4'-dimethylaminophenyl)ethynyl]-10,20-diphenylporphinato]zi nc(II), [5,15-bis[(4'-methoxyphenyl)ethynyl]-10,20-diphenylporphinato]zinc(II) , [5,15-bis[(phenyl)ethynyl]-10,20-diphenylporphinato]zinc(II), [5, 15-bis[(4'-fluorophenyl)ethynyl]-10,20-diphenylporphinato]zinc(II), and [5,15-bis[(4'-nitrophenyl)ethynyl]-10,20-diphenylporphinato]zinc(II). The X-ray structure of one of these complexes, [5,15-bis[(4'-fluorophenyl)ethynyl]-10,20-diphenylporphinato]zinc(II), is unusual in that it shows the arylethynyl-phenyl rings to be coplanar with the porphyrin macrocycle. This arrangement of aryl rings with respect to the porphyrin macrocycle underscores the fact that the steric barrier to rotation between the porphyrin and arylethynyl-phenyl aromatic systems has been removed and highlights the enhanced π-conjugative interactions responsible for the unusual electronic characteristics of these molecules. Electronic spectroscopy and electrochemistry of these systems evince the ability of this new porphyrin structural motif to modulate extensively chromophore electronic structure. ZINDO-determined (INDO-1 semiempirical parameters; CI level = 20) frontier orbital energies for the [5,15-bis[(aryl)ethynyl]-10,20-diphenylporphinato]zinc(II) complexes demonstrate that these properties derive from energetic differences between the porphyrin HOMO and HOMO-1 and a substantial splitting of the formerly degenerate porphyrin LUMO; these significant electronic perturbations (relative to simple (porphinato)zinc(II) complexes) are manifest in the fact that the low lying singlet excited states of these compounds are highly polarized.

Full text of DOI:10.1021/ja962403y

11854
J. Am. Chem. Soc. 1996, 118, 11854-11864
Exceptional Electronic Modulation of Porphyrins through
meso-Arylethynyl Groups. Electronic Spectroscopy, Electronic
Structure, and Electrochemistry of [5,15-Bis[(aryl)ethynyl]-
10,20-diphenylporphinato]zinc(II) Complexes. X-ray Crystal
Structures of [5,15-Bis[(4′-fluorophenyl)ethynyl]-
10,20-diphenylporphinato]zinc(II) and
5,15-Bis[(4′-methoxyphenyl)ethynyl]-10,20-diphenylporphyrin
Steven M. LeCours, Stephen G. DiMagno,‡ and Michael J. Therien*
Contribution from the Department of Chemistry, UniVersity of PennsylVania,
Philadelphia, PennsylVania 19104-6323
ReceiVed July 12, 1996X
Abstract: A new class of porphyrins in which intervening ethynyl moieties connect aryl groups to the porphyrin 5-
and 15-positions is described. The synthesis and optical spectroscopy of five new compounds are reported: [5,15-
bis[(4′-dimethylaminophenyl)ethynyl]-10,20-diphenylporphinato]zinc(II), [5,15-bis[(4′-methoxyphenyl)ethynyl]-10,20-
diphenylporphinato]zinc(II), [5,15-bis[(phenyl)ethynyl]-10,20-diphenylporphinato]zinc(II), [5,15-bis[(4′-fluorophenyl)-
ethynyl]-10,20-diphenylporphinato]zinc(II), and [5,15-bis[(4′-nitrophenyl)ethynyl]-10,20-diphenylporphinato]zinc(II).
The X-ray structure of one of these complexes, [5,15-bis[(4′-fluorophenyl)ethynyl]-10,20-diphenylporphinato]zinc(II),
is unusual in that it shows the arylethynyl-phenyl rings to be coplanar with the porphyrin macrocycle. This arrangement
of aryl rings with respect to the porphyrin macrocycle underscores the fact that the steric barrier to rotation between
the porphyrin and arylethynyl-phenyl aromatic systems has been removed and highlights the enhanced π-conjugative
interactions responsible for the unusual electronic characteristics of these molecules. Electronic spectroscopy and
electrochemistry of these systems evince the ability of this new porphyrin structural motif to modulate extensively
chromophore electronic structure. ZINDO-determined (INDO-1 semiempirical parameters; CI level ) 20) frontier
orbital energies for the [5,15-bis[(aryl)ethynyl]-10,20-diphenylporphinato]zinc(II) complexes demonstrate that these
properties derive from energetic differences between the porphyrin HOMO and HOMO-1 and a substantial splitting
of the formerly degenerate porphyrin LUMO; these significant electronic perturbations (relative to simple (porphinato)-
zinc(II) complexes) are manifest in the fact that the low lying singlet excited states of these compounds are highly
polarized.
Introduction
control of porphyrin electronic properties has been accom-
plished, the scope of electronic modulation possible for a given
porphyrin parent structure is limited. For example, a survey of
the wide range of readily synthesized 5,10,15,20-tetrakis-
(substituted-aryl)porphyrin ligands shows that (por-
phinato)metal spectral features are only moderately perturbed
by aryl-group modification for a given metal ion, solvent, and
axial ligand environment.3 While single â-substitution of
5,10,15,20-tetra(aryl)porphyrins has a modest effect on the
energies of the ligand valence orbitals,4 tetra(meso-aryl)-
porphyrin systems having highly-elaborated â-carbon positions
exhibit considerably red-shifted electronic spectra;5 however,
since the optical and photophysical properties of such dodeca-
substituted porphyrins derive from a combination of electronic
effects and nonplanar distortions of the porphyrin ring, they often
display surprisingly similar electronic and emission spectra,
irrespective of whether their ring substituents are electron
releasing or electron withdrawing.6 Furthermore, many of the
The ability to tune the electronic properties of porphyrins is
crucial to catalyst design and for the precise control of the
spectroscopic properties of photophysical probes, sensitizers,
and the electron and energy transfer components of supra-
molecular systems.1,2 The fact that there are twelve sites on
the porphyrin (four meso- and eight â-positions) at which to
attach electron-releasing or electron-withdrawing groups sug-
gests that tremendous electronic modulation of the porphyrin
ligand should be possible. Despite the fact that considerable
‡ Address: Department of Chemistry, University of Nebraska-Lincoln,
Lincoln, Nebraska 68588-0304.
X Abstract published in AdVance ACS Abstracts, November 1, 1996.
(1) (a) O’Neil, M. P.; Niemczyk, M. P.; Svec, W. A.; Gosztola, D.;
Gaines, G. L., III; Wasielewski, M. R. Science (Washington, D.C.) 1992,
257, 63-65. (b) Gust, D.; Moore, T. A.; Moore, A. L.; Acc. Chem. Res.
1993, 26, 198-205. (c) Anderson, S.; Anderson, H. L.; Sanders, J. K. M.
Acc. Chem. Res. 1993, 26, 469-475. (d) Sessler, J. L.; Capuano, V. L.;
Harriman, A. J. Am. Chem. Soc. 1993, 115, 4618-4628. (e) Prathapan, S.;
Johnson, T. E.; Lindsey, J. S. J. Am. Chem. Soc. 1993, 115, 7519-7520.
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Yamada, H.; Maruyama, K. J. Am. Chem. Soc. 1993, 115, 12137-12143.
(g) Nishide, H.; Suzuki, T.; Nakagawa, R.; Tsuchida, E. J. Am. Chem. Soc.
1994, 116, 4503-4504.
(3) (a) Kim, J. B.; Leonard, J. J.; Longo, F. R. J. Am. Chem. Soc. 1972,
94, 3986-3992. (b) Meot-Nor, M.; Adler, A. D. J. Am. Chem. Soc. 1975,
97, 5107-1511. (c) Quimby, D. J.; Longo, F. R. J. Am. Chem. Soc. 1975,
97, 5111-5117. (d) Spellane, P. J.; Gouterman, M.; Antipas, A.; Kim, S.;
Liu, Y. C. Inorg. Chem. 1980, 19, 386-391. (e) McDermott, G. A.; Walker,
F. A. Inorg. Chim. Acta 1984, 91, 95-102.
(2) (a) Lin, V. S.-Y.; DiMagno, S. G.; Therien, M. J. Science (Wash-
ington, D.C.) 1994, 264, 1105-1111. (b) Angiolillo, P. J.; Lin, V. S.-Y.;
Vanderkooi, J. M.; Therien, M. J. J. Am. Chem. Soc. 1995, 117, 12514-
12527. (c) Lin, V. S.-Y.; Therien, M. J. Chem. Eur. J. 1995, 1, 645-651.
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S0002-7863(96)02403-1 CCC: $12.00 © 1996 American Chemical Society

Modulation of Porphyrins through meso-Arylethynyl Groups
J. Am. Chem. Soc., Vol. 118, No. 47, 1996 11855
14 spectrophotometer. Cyclic and square wave voltammetric responses
were recorded on an EG&G Princeton Applied Research Model 273A
Potentiostat/Galvanostat.
more electronically unusual porphyrins are substituted at the
macrocycle periphery with elements beyond the first row of
the periodic table;3c,6-8 this generally results in deactivation of
the porphyrin singlet excited state, a potential drawback if
modulation of porphyrin photophysical properties is desired.
We report herein the utility of porphyrin-pendant meso
arylethynyl moieties for extensively modulating the electronic
properties of the porphyrin π system. This ability stems from
the fact that the intervening ethynyl bridge assures a minimal
steric barrier to rotation of the arylethynyl phenyl group with
respect to the porphyrin core, thus providing enhanced conjuga-
tive interactions between their respective π symmetry orbitals.
Electronic Structure Calculations. Frontier orbital energies for
the [5,15-bis[(aryl)ethynyl]-10,20-diphenylporphinato]zinc(II) com-
plexes as well as the peripherally unsubstituted (porphinato)zinc(II)
metallomacrocycle were determined utilizing CAChe ZINDO with
standard INDO-1 semiempirical parameters at a configuration interac-
tion (CI) level 20.10 For all these complexes, the central zinc metal
atoms were assigned a dsp2 (square planar) geometry. (Porphinato)-
zinc(II) was constructed with D4h symmetry while the [5,15-bis[(4′-
X-phenyl)ethynyl]-10,20-diphenylporphinato]zinc(II) compounds were
fashioned with either C2h (X ) NMe2, OMe) or D2h (X ) H, F, NO2)
symmetries. All calculations were performed on ZINDO-optimized
geometrical structures in which the dihedral angles of the 5- and 15-
arylethynyl phenyl moieties with respect to the porphyrin least-squares
plane were adjusted to 0°; the 10- and 20-meso phenyl groups were
fixed at 90° with respect to the porphyrin plane. The convergence
criteria for these Restricted Hartree-Fock (RHF) self-consistent field
(SCF) calculations required the root mean square difference in the
elements of the density matrix to be below 0.000 001 on two successive
SCF cycles.
General Procedure for the Preparation of [5,15-Bis[(aryl)-
ethynyl]-10,20-diphenylporphinato]zinc(II) Complexes. [5,15-di-
bromo-10,20-diphenylporphinato]Zn(II) (250 mg, 370 µmol), CuI (15
mg, 78 µmol), Pd[PPh3]4 (35 mg, 30 µmol), diethylamine (5 mL), THF
(30 mL), and the desired phenylacetylene derivative (250 mg, ∼2 mmol)
were brought together in a 100 mL Schlenk tube under an N2
atmosphere. The resulting mixture turns green as the reaction proceeds
at room temperature over 12-24 h. At the reaction endpoint, the crude
product was purified by column chromatography on silica gel using
85:15 hexanes:THF as eluant. The green band was collected and
evaporated to give an essentially pure [5,15-bis[(aryl)ethynyl]-10,20-
diphenylporphinato]zinc(II) complex in high yield (> 85%). Recrys-
tallization from THF/hexanes provided samples suitable for elemental
analysis and X-ray crystallography.
Experimental Section
Materials. All manipulations were carried out under nitrogen
previously passed through 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
glove box. Standard Schlenk techniques were employed to manipulate
air-sensitive solutions. All solvents utilized in this work were obtained
from Fisher Scientific (HPLC Grade). Tetrahydrofuran (THF) was
distilled from Na/benzophenone under N2. Diethylamine was dried
over KOH pellets and distilled under vacuum. All NMR solvents were
used as received. 4-Iodoaniline, sodium cyanoborohydride, 4-iodo-
anisole, phenylacetylene, 1-fluoro-4-iodobenzene, 1-bromo-4-nitroben-
zene, trimethylsilylacetylene, and ethynylmagnesium bromide were used
as received (Aldrich). Formaldehyde solution (37% w/w) and anhy-
drous diethyl ether were used without further purification (Fisher), as
was the Pd[(PPh)3]4 catalyst (Strem). 2,2′-Dipyrrylmethane was
prepared according to the published procedure,9 and stored under inert
atmosphere at -40 °C. Chemical shifts for 1H NMR spectra are relative
to residual protium in the deuterated solvents (CDCl3, δ ) 7.24 ppm;
THF-d8, δ ) 3.58 ppm). Chemical shifts for 13C NMR spectra are
relative to deuteriochloroform solvent (CDCl3, δ ) 77.00 ppm). All J
values are reported in Hertz. The number of attached protons is found
in parentheses following the chemical shift value. Chromatographic
purification (silica gel 60, 230-400 mesh, EM Science) of all newly
synthesized 5,15-elaborated porphyrins was accomplished on the bench
top. Elemental analyses were performed at Robertson Microlit
Laboratories (Madison, NJ).
[5,15-Bis[(4′-dimethylaminophenyl)ethynyl]-10,20-diphenylpor-
phinato]zinc(II). Isolated yield ) 62.3 mg (99%, based on 53.0 mg
of the porphyrin starting material). Selected characterization data are
as follows. 1H NMR (250 MHz, 25:1 CDCl3:pyridine-d5): δ 9.63 (d,
4 H, J ) 4.6), 8.75 (d, 4 H, J ) 4.6), 8.10 (dd, 4 H, J ) 7.5, J
)
1
2
2.9), 7.83 (d, 4 H, J ) 8.6), 7.68 (m, 6 H), 6.80 (d, 4 H, J ) 8.7), 3.02
(s, 12 H). 13C NMR (60 MHz, 25:1 CDCl3:pyridine-d5): δ 151.65,
149.91, 149.35, 142.65, 135.47, 132.42, 131.83, 130.29, 126.97, 126.12,
118.60, 111.85, 111.28, 110.90, 97.46, 77.23, 40.00. Vis (THF): 466
(5.31), 675 (4.75). FAB MS: 810 (calcd 810). Anal. Calcd for
C52H38N6Zn: C, 76.89; H, 4.71; N, 10.35. Found: C, 76.63; H, 4.69;
N, 10.37.
Instrumentation. Electronic spectra were recorded on an OLIS UV/
vis/NIR spectrophotometry system that is based on the optics of a Cary
(5) (a) Traylor, T. G.; Tsuchiya, S. Inorg. Chem. 1987, 26, 1338-1339.
(b) 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. (c) Bhyrappa,
P.; Krishnan, V. Inorg. Chem. 1991, 30, 239-245. (d) Takeda, J.; Ohya,
T.; Sato, M. Chem. Phys. Lett. 1991, 183, 384-386. (e) Wu, G.-Z.; Gan,
W.-X.; Leung, H.-K. J. Chem. Soc., Faraday Trans. 1991, 87, 2933-2937.
(f) Mandon, D.; Ochsenbein, P.; Fischer, J.; Weiss, R.; Jayaraj, K.; Austin,
R. N.; Gold, A.; White, P. S.; Brigaud, O.; Battioni, P. Mansuy, D. Inorg.
Chem. 1992, 31, 2044-2049. (g) Medforth, C. J.; Senge, M. O.; Smith, K.
M.; Sparks, L. D.; Shelnutt, J. A. J. Am. Chem. Soc. 1992, 114, 9859-
9869. (h) Lyons, J. E.; Ellis, P. E., Jr.; Wagner, R. W.; Thompson, P. B.;
Gray, H. B.; Hughes, M. E.; Hodge, Jg. A. Prepr.sAm. Chem. Soc., DiV.
Pet. Chem. 1992, 37, 307-317. (i) Sparks, L. D.; Medforth, C. J.; Park,
M.-S.; Chamberlain, J. R.; Ondrias, M. R.; Senge, M. O.; Smith, K. M.;
Shelnutt, J. A. J. Am. Chem. Soc. 1993, 115, 581-592. (j) Senge, M. O.;
Medforth, C. J.; Sparks, L. D.; Shelnutt, J. A.; Smith, K. M. Inorg. Chem.
1993, 32, 1716-1723.
(6) A comparison of the Soret absorption wavelengths in a series of
substituted (porphinato)nickel complexes (solvent ) CH2Cl2) shows the
following: (5,10,15,20-tetraphenylporphinato)Ni(II) (λ ) 414 nm);
(5,10,15,20-tetraphenyl-2,3,7,8,12,13,17,18-octamethylporphinato)Ni(II) (λ
) 418 nm); (5,10,15,20-tetraperfluorophenyl-2,3,7,8,12,13,17,18-octa-
bromoporphinato)Ni(II) (λ ) 436 nm); (5,10,15,20-tetraphenyl-2,3,7,8,12,
13,17,18-octabromoporphinato)Ni(II) (λ ) 448 nm); (2,3,5,7,8,10,12,13,
15,17,18,20-dodecaphenylporphinato)Ni(II) (λ ) 448 nm). See refs 5b, e,
f, and g.
[5,15-Bis[(4′-methoxyphenyl)ethynyl]-10,20-diphenylporphinato]-
zinc(II). Isolated yield ) 285 mg (99%, based on 250 mg of the
porphyrin starting material). Selected characterization data are as
follows. 1H NMR (500 MHz, 25:1 CDCl3:pyridine-d5): δ 9.78 (dd, 4
H, J ) 4.2, J ) 2.2), 8.89 (dd, 4 H, J ) 4.4, J ) 1.4), 8.17 (d, 4
1
2
1
2
H, J ) 6.4), 7.95 (d, 4 H, J ) 8.5), 7.73 (m, 6 H), 7.06 (d, 4 H, J )
8.5), 3.84 (s, 6 H). 13C NMR (60 MHz, 25:1 CDCl3:pyridine-d5): δ
159.57, 151.72, 149.69,142.60, 134.32, 132.81, 132.16, 130.43, 127.17,
126.26, 122.20, 116.32, 114.16, 101.17, 96.03, 77.20, 55.18. Vis
(THF): 451 (5.60), 602 (4.05), 656 (4.79). FAB MS: 784 (calcd 784).
Anal. Calcd for C50H32N4O2Zn: C, 76.39; H, 4.10; N, 7.13. Found:
C, 76.04; H, 4.22; N, 7.01.
[5,15-Bis[(phenyl)ethynyl]-10,20-diphenylporphinato]zinc(II). Iso-
lated yield ) 47.5 mg (85%, based on 52.9 mg of the porphyrin starting
material). Selected characterization data are as follows. 1H NMR (250
MHz, 25:1 CDCl3:pyridine-d5): δ 9.67 (d, 4 H, J ) 4.66), 8.81 (d, 4
H, J ) 4.57), 8.12 (m, 4 H), 7.95 (d, 4 H, J ) 6.87), 7.69 (m, 6 H),
7.45 (m, 6 H). 13C NMR (60 MHz, 25:1 CDCl3:pyridine-d5): δ 151.81,
149.91, 134.39, 132.34, 131.39, 130.52, 128.54, 128.15, 127.27, 126.33,
124.22. Vis (THF): 446 (5.63), 593 (4.02), 650 (4.76). FAB MS:
724 (calcd 724). Anal. Calcd for C50H32N4O2Zn: C, 79.39; H, 3.89;
N, 7.72. Found: C, 79.16; H, 3.86; N, 7.57.
(7) Gouterman, M. In The Porphyrins, Dolphin, D., Ed.; Academic Press,
London, 1978; Vol. III, pp 1-165.
(8) Bonnett, R.; Harriman, A.; Kozyrev, A. N. J. Chem. Soc., Faraday
Trans. 1992, 88, 763-769.
(9) Chong, R.; Clezy, P. S.; Liepa, A. J.; Nichol, A. W. Aust. J. Chem.
1969, 22, 229-238.
(10) ZINDO Software provided by CAChe Scientific, Beaverton, OR.

11856 J. Am. Chem. Soc., Vol. 118, No. 47, 1996
LeCours et al.
Table 1. Summary of Structure Determination of [5,15-Bis(4′-fluorophenylethynyl)-10,20-diphenylporphinato]zinc(II) and
5,15-Bis(4′-methoxyphenyl)-10,20-diphenylporphyrin
formula
formula weight
crystal class
space group
C48H26N4F2Zn‚2(C4H8O)
C50H32N4O2‚2(CHCl3)
906.35
961.61
triclinic
P1h (no. 2)
1
hexagonal
R3h (no. 148)
9
Z
cell constants
a
b
c
28.595(2) Å
14.980(4) Å
14.980(4) Å
10.352(3) Å
10.975(2) Å
11.694(2) Å
78.51(2)°
107.14(2)°
113.92(2)°
R
â
γ
V
10608(5) Å3
5.84 cm-1
0.32 × 0.45 × 0.50
95.73, 99.97, 99.03
1.277 g/cm3
4230
1156(1) Å3
38.26 cm-1
0.08 × 0.40 × 0.42
µ
crystal size, mm
trans. (min, max, av, %)
Dcalc
1.381 g/cm3
494
F(000)
radiation
θ range
scan mode
h,k,l collected
no. reflcns measured
no. unique reflcns
no. reflcns used in refinement
no. parameters
data/parameter ratio
R1
Mo-KR (λ ) 0.710 73 Å)
2.0-25.0
ω-2θ
Cu-KR (λ ) 1.541 84 Å)
2.0-65.0
ω-2θ
+18, +18, (20
-12, (12, (13
4177
5144
4140
3938
3045 (F2 > 3.0σ)
295
2538 (F2 > 3.0σ)
289
10.3
0.057
8.8
0.088
R2
GOF
0.086
0.974
0.114
3.247
[5,15-Bis[(4′-fluorophenyl)ethynyl]-10,20-diphenylporphinato]-
zinc(II). Isolated yield ) 64.9 mg (93%, based on 62.9 mg of the
porphyrin starting material). Selected characterization data are as
follows. 1H NMR (250 MHz, CDCl3,): δ 9.66 (d, 4 H, J ) 4.6), 8.83
(d, 4 H, J ) 4.6), 8.16 (dd, 4 H, J ) 7.1, J ) 2.1), 7.96 (td, 4 H,
space group R3h with a ) 28.595(2) Å, b ) c ) 14.980(4) Å, V
) 10608(9) Å3, Z ) 9, and dcalc ) 1.277 g cm-3. The cell constants
were determined from a least-squares fit of the setting angles for 25
accurately centered reflections. X-ray intensity data were collected on
an Enraf-Nonius CAD4 diffractometer employing graphite-monochro-
mated Mo-KR radiation (λ ) 0.710 73 Å) and using the ω-2θ scan
technique. A total of 5144 reflections were measured over the ranges:
4 e 2θ e 50°, 0 e h e 18, 0 e k e 18, -20 e l e 20. Three standard
reflections measured every 3500 s of X-ray exposure showed an
intensity decay 15.1% over the course of the data collection.
1
2
J1) 7.1, J ) 3.2), 7.75 (m, 6 H), 7.22 (t, 4 H, J ) 8.7). 13C NMR
2
(60 MHz, 25:1 CDCl3:pyridine-d5): δ 171.34 (d, 2 C, J ) 275), 151.74,
149.91, 142.50, 134.35, 133.21 (d, 4 C, J ) 7.5), 132.38, 130.46,
127.27, 126.33, 122.5, 115.80 (d, 4 C, J ) 21.9), 100.54, 94.84, 92.80,
77.2. Vis (THF): 446 (5.57), 592 (4.02), 648 (4.70). FAB MS: 760
(calcd 760). Anal. Calcd for C48H26N4F2Zn: C, 75.65; H, 3.44; N,
7.35. Found: C, 75.39; H, 3.31; N, 7.32.
[5,15-Bis[(4′-nitrophenyl)ethynyl]-10,20-diphenylporphinato]-
zinc(II). Isolated yield ) 205 mg (93%, based on 184.5 mg of the
porphyrin starting material). Selected characterization data are as
follows. 1H NMR (500 MHz, pyridine-d5): δ 9.67 (d, 4 H, J ) 4.5),
8.91 (d, 4 H, J ) 4.5), 8.27 (d, 4 H, J ) 8.5), 8.16 (d, 4 H, J ) 6.4),
7.97 (d, 4 H, J ) 8.6), 7.75 (m, 6 H). 13C NMR (125 MHz, pyridine-
d5): δ 148.91, 134.49, 133.68, 132.32, 131.16, 130.21, 126.91, 125.89,
123.07. Vis (THF): 460 (5.45), 666 (4.82). Anal. Calcd for
C48H26N6O4Zn: C, 70.64; H, 3.21; N, 10.30. Found: C, 70.37; H,
3.27; N, 10.14.
The intensity data were corrected for Lorentz and polarization effects
and an empirical absorption correction was applied (minimum transmis-
sion 95.73%, max 99.97%, mean 99.03%). Of the reflections measured,
a total of 3045 unique reflections with F2 > 3σ(F2) were used during
subsequent structure refinement.
Refinement was by full-matrix least squares techniques based on F
to minimize the quantity ∑w(|Fo| - |Fc|)2 with w ) 1/σ2(F). Non-
hydrogen atoms were refined anisotropically and hydrogen atoms were
included as constant contributions to the structure factors and were
not refined. Refinement converged to R1 ) 0.057 and R2 ) 0.086.
[5,15-Bis[(4′-methoxyphenyl)ethynyl]-10,20-diphenylpor-
phyrin. Crystallization was induced by dissolving the compound in
CHCl3 and storing the crystallization vessel at -40 °C overnight to
give purple plates. The crystal dimensions were 0.08 × 0.40 × 0.42
mm3. [5,15-Bis[(4′-methoxyphenyl)ethynyl]-10,20-diphenylporphyrin
crystallizes in the triclinic space group P1h with a ) 10.352(3) Å,
b ) 10.975(2)Å, c ) 11.694(2) Å, a ) 78.51°, b ) 107.14(2)°, c )
113.92(2)°, V ) 1156(1) Å3, Z ) 1 and dcalc ) 1.381 g cm-3. The cell
constants were determined from a least squares fit of the setting angles
for 25 accurately centered reflections. A total of 4177 reflections were
measured over the ranges: 4 e 2θ e 130°, -12 e h e 0, -12 e k e
12, -13 e l e 13. Three standard reflections measured every 3500 s
of X-ray exposure showed an intensity decay of 17.7% over the course
of the data collection. A linear decay correction was applied.
X-ray Crystallography.11 The crystal structures for [5,15-bis[(4′-
fluorophenyl)ethynyl]-10,20-diphenylporphinato]zinc(II) and [5,15-bis-
[(4′-methoxyphenyl)ethynyl]-10,20-diphenylporphyrin were solved by
direct methods (SIR88).12 Table 1 contains details of the crystal and
data collection parameters. The structures were determined by Dr.
Patrick Carroll at the Chemistry Department’s X-ray facility at the
University of Pennsylvania.
[5,15-Bis[(4′-fluorophenyl)ethynyl]-10,20-diphenylporphinato]-
zinc(II). Crystallization was induced by layering hexane onto a
saturated THF solution of the compound; storing the mixture at 0 °C
under N2 for 3 days gave rectangular purple prisms. The crystal
dimensions were 0.32 × 0.45 × 0.50 mm3. [5,15-Bis[(4′-fluorophenyl)-
ethynyl]-10,20-diphenylporphinato]zinc(II) crystallizes in the hexagonal
The intensity data were corrected for Lorentz and polarization effects,
and an empirical absorption correction was (DIFABS) was performed.
Of the reflections measured, a total of 2538 unique reflections with F2
> 3σ(F2) were used during subsequent structure refinement.
(11) The atomic coordinates for these structures have been deposited
with the Cambridge Crystallographic Data Centre. The coordinates can be
obtained, on request, from the Director, Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge, CB2 1EZ UK.
(12) Burla, M. C.; Camalli, M.; Cascarano, G.; Giacovazzo, C.; Polidori,
G.; Spagna, R.; Viterbo, D. J. Appl. Crystallogr. 1989, 72, 389-93.
Refinement was by full-matrix least squares techniques based on F
to minimize the quantity ∑w(|Fo| - |Fc|)2 with w ) 1/σ2(F). Non-

Modulation of Porphyrins through meso-Arylethynyl Groups
J. Am. Chem. Soc., Vol. 118, No. 47, 1996 11857
Figure 1. ORTEP view of [5,15-bis[(4′-fluorophenyl)ethynyl]-10,20-diphenylporphinato]zinc(II) with thermal ellipsoids at 30% probability.
hydrogen atoms were refined anisotropically and hydrogen atoms were
included as constant contributions to the structure factors and were
not refined. Refinement converged to R1 ) 0.088 and R2 ) 0.114.
carbon-carbon triple bond length is 1.19 Å, and hence, is
unexceptional. (iii) The bond lengths observed in the porphyrin
framework resemble those observed in Schauer′s crystal struc-
ture of [5,10,15,20-tetraphenylporphinato]zinc(II).15 Since these
latter points indicate that there is no cumulenic character at the
acetylenic- or meso-carbon atoms, the near coplanarity of the
arylethynyl moiety with the porphyrin unit in the solid state
may derive in large part from crystal packing forces.
Results and Discussion
Synthesis, Solution Properties, and Structural Studies. In
5,10,15,20-tetrakis(substituted-aryl)porphyrins, steric crowding
from the interaction of the phenyl ortho protons with the
porphyrin â-hydrogens results in a large dihedral angle between
the planes of the rings, thereby limiting porphyrin electronic
tuning via π-conjugative interactions. In an effort to prepare
structurally simple porphyrins with unusual electronic charac-
teristics, we have sought to remove this steric barrier between
the two aromatic systems; this has been accomplished through
the utility of an intervening acetylenyl moiety which serves as
a cylindrically π-symmetric interconnect between the phenyl
and porphyryl rings.
A series of [5,15-bis[(4′-substituted-phenyl)ethynyl]-10,20-
diphenylporphinato]zinc(II) complexes that included [5,15-bis-
[(4′-dimethylaminophenyl)ethynyl]-10,20-diphenylporphinato]-
zinc(II), [5,15-bis[(4′-methoxyphenyl)ethynyl]-10,20-diphenyl-
porphinato]zinc(II), [5,15-bis[(phenyl)ethynyl]-10,20-diphen-
ylporphinato]zinc(II), [5,15-bis[(4′-fluorophenyl)ethynyl]-10,20-
diphenylporphinato]zinc(II), and [5,15-bis[(4′-nitrophenyl)-
ethynyl]-10,20-diphenylporphinato]zinc(II) were synthesized in
high yield (> 85%) through metal-mediated cross-coupling of
the substituted arylethynyl groups13 to [5,15-dibromo-10,20-
diphenylporphinato]zinc(II).2,14 An X-ray crystallographic struc-
ture was determined for one of these compounds, the bis(THF)
adduct of [5,15-bis[(4′-fluorophenyl)ethynyl]-10,20-diphenylpor-
phinato]zinc(II); an ORTEP view of this complex is shown in
Figure 1. Of particular importance are the following: (i) While
the 10,20-aryl groups show a typical orientation with respect
to the porphyrin ring (dihedral angle ) 79.7°), the arylethynyl
phenyl rings and the porphyrin macrocycle adopt a nearly
coplanar arrangement with a dihedral angle of 4.3°. (ii) The
The free base derivatives of these complexes can be prepared
from their (porphinato)zinc(II) precursors using standard ex-
perimental conditions; straightforward remetalation of such 5,15-
bis[(aryl)ethynyl]-10,20-diphenylporphyrin compounds provides
an entry into other [5,15-bis[(aryl)ethynyl]-10,20-diphenylpor-
phinato]metal complexes.16 In contrast to the [5,15-bis[(aryl)-
ethynyl]-10,20-diphenylporphinato]zinc(II) compounds, which
are appreciably soluble in coordinating solvents such as THF
and pyridine, as well as polar solvents such as CH2Cl2 and
benzonitrile that have small amounts of added pyridine, the free
ligand derivatives are less soluble, with ∼0.1-1 mM solutions
saturating CHCl3 and aromatic solvents (benzene, toluene,
benzonitrile).
The results of our single-crystal X-ray crystallographic study
of a 5,15-bis[(4′-methoxyphenyl)ethynyl]-10,20-diphenylpor-
phyrin are shown in Figure 2; note that this ORTEP diagram
emphasizes many features in common with the [5,15-bis[(4′-
fluorophenyl)ethynyl]-10,20-diphenylporphinato]zinc(II)‚
(THF)2 structure. Table 2 compiles selected bond lengths and
bond angles for these two compounds.
The porphyrin cores for both 5,15-bis[(4′-methoxyphenyl)-
ethynyl]-10,20-diphenylporphyrin and [5,15-bis[(4′-fluoro-
phenyl)ethynyl]-10,20-diphenylporphinato]zinc(II)‚(THF)2 are
flat, as indicated by the fact that the maximal deviation of any
atom from the porphyrin least squares plane is 0.04 Å and 0.05
Å, in these two respective structures. The C(5)meso-to-CR(ethynyl)
,
CR(ethynyl)-to-Câ(ethynyl), and Câ(ethynyl)-to-C(1′)(phenyl) bond lengths
in 5,15-bis[(4′-methoxyphenyl)ethynyl]-10,20-diphenylpor-
phyrin are similar to that observed for [5,15-bis[(4′-fluoro-
phenyl)ethynyl]-10,20-diphenylporphinato]zinc(II)‚(THF)2 (1.437,
1.187, and 1.432 Å, respectively) and likewise signal no ground-
(13) (a) Eastmond, R.; Walton, D. R. M. Tetrahedron 1972, 28, 4591-
4599. (b) Westmijze, H.; Vermeer, P. Synthesis 1979, 390-392. (c)
Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N. Synthesis 1980,
627-630. (d) Mesnard, D.; Bernadou, F.; Miginiac, L. J. Chem. Res. (S)
1981, 270-271. (e) Zhang, Y.; Wen, J. Synthesis 1990, 727-728. (f) Park,
K. M.; Schuster, G. B. J. Org. Chem. 1992, 57, 2502-2504.
(14) (a) DiMagno, S. G.; Lin, V. S.-Y.; Therien, M. J. J. Am Chem.
Soc. 1993, 115, 2513-2515. (b) DiMagno, S. G.; Lin, V. S.-Y.; Therien,
M. J. J. Org. Chem. 1993, 58, 5983-5993.
(15) Schauer, C. K.; Anderson, O. P.; Eaton, S. S.; Eaton, G. R. Inorg.
Chem. 1985, 24, 4082-4086.
(16) LeCours, S. M.; Therien, M. J. Manuscript in preparation.

11858 J. Am. Chem. Soc., Vol. 118, No. 47, 1996
LeCours et al.
Figure 2. ORTEP view of 5,15-bis[(4′-methoxyphenyl)ethynyl]-10,20-diphenylporphyrin with thermal ellipsoids at 30% probability.
state cumulenic character along the vector connecting the
porphyrin C(5)meso and the arylethynyl phenyl C(1′) carbon
atoms.
Table 3 summarizes the anodic cyclic voltammetric data for
these[5,15-bis[(4′-X-phenyl)ethynyl]-10,20-diphenylporphinato]-
zinc(II) species in benzonitrile solvent.17 The marginal solubility
of the X ) NO2 and X ) NMe2 derivatives in both CH2Cl2
and benzonitrile precluded a comprehensive evaluation of the
effect of the 4′-X substituent on the ZnP/ZnP+ and ZnP+/ZnP2+
potentials in the absence of the inherent experimental and data-
analysis complications that arise if exogenous coordinating
ligands are added to the solvent. Nevertheless, a clear trend is
discernible in the ZnP/ZnP+ redox potentials, which shows that
the E1/2 values of X ) OMe, H, and F derivatives extend over
a 118 mV range and track with substituent electronic properties.
These data are remarkable in that they demonstrate that these
electron releasing and withdrawing groups impact the stability
of the chromophore HOMO despite the fact that they lie at
remote distances from the porphyrin core. It is worth noting
in this regard that 2-bromo-5,10,15,20-tetraphenylporphyrin (2-
BrTPP) possesses an oxidation potential only 80 mV more
positive than TPP,18 and that the series of compounds [5,10,-
15,20-tetraphenylporphinato]copper(II) (TPPCu), 2-OMeTPPCu,
and 2-ClTPPCu, exhibit ring-centered first oxidation potentials
that span only a 102 mV regime.4
There are measurable differences between the two structures
in their respective C(5)meso-CR(ethynyl)-Câ(ethynyl) bond angles: this
angle is 174.4(5) in the 5,15-bis[(4′-methoxyphenyl)ethynyl]-
10,20-diphenylporphyrin structure and 178.3(4) for that of [5,15-
bis[(4′-fluorophenyl)ethynyl]-10,20-diphenylporphinato]-
zinc(II)‚(THF)2. It is interesting to note that the magnitudes of
the C(5)meso-CR(ethynyl)-Câ(ethynyl) angles parallel those observed
for CR(ethynyl)-Câ(ethynyl)-C(1′)(phenyl) in these two structures.
Because there is no obvious electronic genesis for this difference
in topology about the ethyne unit in these two compounds, these
variances may derive from dissimilar crystal packing forces for
these (porphinato)zinc(II) and porphyrin compounds; congruent
with this notion, the arylethynyl phenyl rings and the porphyrin
macrocycle are not as coplanar in the 5,15-bis[(4′-methoxy-
phenyl)ethynyl]-10,20-diphenylporphyrin (dihedral angle )
16.6° between the two aromatic systems) as they are in the [5,15-
bis[(4′-fluorophenyl)ethynyl]-10,20-diphenylporphinato]-
zinc(II))‚(THF)2 structure (Vide supra). If dissimilar crystal
packing forces are indeed responsible for these structural
differences, a possible origin could lie in the fact that two
chloroform molecules pack closely about each ethyne unit in
5,15-bis[(4′-methoxyphenyl)ethynyl]-10,20-diphenylporphyrin
(data not shown); three intermolecular contacts are made
between the porphyrin ligand and solvent that are close to the
sum of the Cl-H van der Waals radius of 2.95 Å.
Electrochemical Studies. To probe for evidence of ground-
state electronic communication between the arylethynyl-phenyl
rings and the porphyrin macrocycle, we examined the cathodic
and anodic electrochemistry of the [5,15-bis[(4′-X-phenyl)-
ethynyl]-10,20-diphenylporphinato]zinc(II) complexes. Figure
3A shows the cyclic voltammetric response obtained for a
benzonitrile solution of [5,15-bis[(4′-methoxyphenyl)ethynyl]-
10,20-diphenylporphinato]zinc(II) while Figure 3B shows the
observable redox couples for [5,15-bis[(phenyl)ethynyl]-10,20-
diphenylporphinato]zinc(II) recorded in THF. Due to the
magnitude of the potentiometric window defined by the ZnP+/
ZnP2+ and ZnP-/ZnP2- redox couples for this series of
compounds, no single solvent was suitable for examining all
four redox processes that characterize their oxidative and
reductive electrochemistry.
A comprehensive cathodic cyclic voltammetric data set can
be obtained for the [5,15-bis[(4′-X-phenyl)ethynyl]-10,20-
diphenylporphinato]zinc(II) complexes in THF solvent (Table
4). Consistent with the data shown in Table 3, the entries in
Table 4 underscore the conclusion that the 4′-X substituents
can indeed modulate the stability of the (porphinato)zinc(II)
frontier orbitals to a significant degree; note that the first
cathodic reduction wave (Epc) for these complexes varies from
-1333 mV for the X ) NMe2 derivative to -1088 mV for the
X ) NO2 complex, spanning a 245 mV potential domain. As
was observed for the ZnP+/ZnP2+ potentials with respect to their
respective ZnP/ZnP+ redox processes, the complexes that exhibit
(17) Electrochemical studies carried out in benzonitrile were performed
on analytically pure (porphinato)zinc(II) compounds. Note that the [5,15-
bis[(4′-X-phenyl)ethynyl]-10,20-diphenylporphinato]zinc(II) complexes (X
) NO2, F, OMe, and NMe2) reported in this study analyze as their axial-
ligand free derivatives (see Experimental Section). In general, if dried under
vacuum for 24 h at 120 °C, these compounds are isolated free of axial
THF ligands; if simply recrystallized from THF, they are isolated as their
bis(THF) adducts.
(18) Giraudeau, A.; Callot, H. J.; Gross, M. Inorg. Chem. 1979, 18, 201-
206.

Modulation of Porphyrins through meso-Arylethynyl Groups
J. Am. Chem. Soc., Vol. 118, No. 47, 1996 11859
Table 2. Selected Bond Distances and Angles
bond
distance, Å
angle
angle, deg
5,15-Bis(4′-methoxy-
phenylethynyl)-10,20-diphenylporphyrin
C1-C2
1.444(6)
1.410(6)
1.354(5)
1.349(6)
1.445(7)
1.412(6)
1.361(5)
1.503(6)
1.418(6)
1.360(5)
1.345(7)
1.435(6)
1.398(6)
1.358(6)
1.435(6)
1.185(6)
1.434(6)
1.392(7)
1.384(7)
1.397(7)
1.353(6)
1.396(6)
1.381(9)
1.383(8)
1.357(10)
C2-C1-C10
C2-C1-N1
123.8(4)
110.1(3)
126.0(4)
106.9(4)
106.3(4)
123.2(4)
110.2(4)
126.6(4)
117.5(4)
107.7(4)
108.0(4)
107.7(4)
125.9(4)
107.2(4)
127.0(4)
125.3(4)
116.4(4)
118.3(4)
174.4(5)
175.5(5)
122.5(4)
119.4(4)
118.1(4)
117.2(5)
106.4(3)
109.4(3)
118.5(4)
C1-C10
C1-N1
C10-C1-N1
C1-C2-C3
C2-C3
C3-C4
C2-C3-C4
C4-C5
C3-C4-C5
C4-N1
C3-C4-N1
C5-C20
C6-C7
C5-C4-N1
C4-C5-C20
C7-C6-N2
C6-N2
C7-C8
C6-C7-C8
C8-C9
C7-C8-C9
C9-C10
C9-N2
C8-C9-C10
C8-C9-N2
C10-C11
C11-C12
C12-C13
C13-C14
C14-C15
C15-C16
C16-O1
C19-O1
C20-C21
C21-C22
C22-C23
C10-C9-N2
C1-C10-C9
C1-C10-C11
C9-C10-C11
C10-C11-C12
C11-C12-C13
C12-C13-C14
C12-C13-C18
C14-C13-C18
C21-C20-C25
C1-N1-C4
C6-N2-C19
C10-O1-C19
Figure 3. (A) Cyclic voltammogram of [5,15-bis[(4′-methoxyphenyl)-
ethynyl]-10,20-diphenylporphinato]zinc(II) recorded in benzonitrile
solvent showing the two anodic redox couples (ZnP/ZnP+ and ZnP+/
ZnP2+) as well as the initial cathodic redox process (ZnP/ZnP-). (B)
Cyclic voltammogram of [5,15-bis[(phenyl)ethynyl]-10,20-diphenylpor-
phinato]zinc(II) recorded in THF solvent showing the two cathodic
redox couples (ZnP/ZnP- and ZnP-/ZnP2-) as well as the initial anodic
redox process (ZnP/ZnP+). Cyclic voltammetric experimental conditions
are described in Tables 3 and 4, respectively.
[5,15-Bis(4′-fluorophenylethynyl)-
10,20-diphenylporphinato]zinc(II)
Zn-O1
2.429(5)
1.366(9)
1.362(7)
1.332(9)
1.498(7)
1.378(8)
1.353(10)
1.367(10)
2.053(4)
1.360(7)
1.439(8)
1.431(7)
1.402(8)
1.439(9)
1.400(8)
1.189(9)
1.430(9)
N1-Zn-N2
Zn-N1-C1
90.5(2)
127.0(4)
109.2(5)
107.0(5)
125.5(5)
117.5(5)
125.1(5)
117.1(5)
178.3(4)
106.8(4)
109.4(5)
125.8(5)
121.1(6)
91.0(2)
126.1(3)
126.8(4)
107.6(5)
125.1(5)
120.5(5)
178.8(5)
118.4(6)
119.2(5)
126.6(5)
120.8(6)
F1-C22
N1-C1
N1-C1-C1
C1-C2-C3
C2-C3
C10-C11
C19-C20
C21-C22
C20-C21
Zn-N1
N1-C4-C5
C4-C5-C17
C8-C9-C10
C9-C10-C11
C5-C17-C18
C1-N1-C4
N1-C4-C3
N2-C9-C10
C19-C20-C21
O1-Zn-N1
Zn-N1-C4
Table 3. Oxidative Cyclic Voltammetric Dataa for
[5,15-Bis[(4′-X-aryl)ethynyl]-10,20-diphenylporphinato]zinc(II)
Compounds Recorded in Benzonitrile Solvent
N1-C4
C1-C2
ZnP/ZnP+
ZnP+/ZnP2+
C3-C4
C4-C5
Epa
(mV) (mV) (mV) (mV) (mV) (mV) (mV) (mV)
Epc E1/2 ∆Ep Epa
Epc E1/2 ∆Ep
C5-C17
C9-C10
C17-C18
C18-C19
compd
b
X ) NMe2
X ) OMe
X ) H
Zn-N2-C9
662 599 630
745 690 718
780 715 748
63 1122 1022 1072 100
55 1242 1112 1177 130
65 1222 1105 1164 117
C2-C3-C4
C3-C4-C5
X ) F
C10-C11-C12
C17-C18-C19
C20-C19-C24
F1-C22-C21
C4-C5-C6
b
X ) NO2
a Cyclic voltammetric experimental conditions: [Porphyrin] ) 1-3
mM; [TBAPF6] ) 0.15 M; scan rate ) 200 mV/s; reference electrode
) saturated calomel (SCE); working electrode ) platinum disk; the
ferrocene/ferrocenium (FeII/FeIII) redox couple (0.43 V vs SCE) was
used as an internal standard. The experimental uncertainty in these
reported potentials is (3 mV. b Very low solubility of these complexes
in benzonitrile and CH2Cl2 precluded investigation of the anodic
electrochemistry of these complexes.
C18-C19-C20
chemically reversible ZnP/ZnP- redox couples show consider-
able leveling of the effect of the 4′-X substituents on their
corresponding ZnP-/ZnP2- E1/2 values (Tables 3 and 4).
Optical Spectra of [5,15-Bis[(4′-X-phenyl)ethynyl]-10,20-
diphenylporphinato]zinc(II) Complexes. The electronic spec-
tra of the [5,15-bis[(4′-X-phenyl)ethynyl]-10,20-diphenylpor-
phinato]zinc(II) complexes shown in Figure 4 display a number
of interesting characteristics; Table 5 highlights the prominent
visible region spectral signatures of these complexes. In contrast
to the electronic spectrum of [5,15-bis(trimethysilylethynyl)-
10,20-diphenylporphinato]zinc(II),2a which shows a Soret (B)
band split into discernible Bx and By transitions, the optical
spectra of the [5,15-bis[(aryl)ethynyl]-10,20-diphenylporphinato]-
zinc(II) complexes show a single broad B band (FWHM ) 753
- 1157 cm-1; Table 5). Apparently, the addition of the auxiliary
conjugated groups to the ethynes further diminishes the relative
intensity of the By transition;2a,c the lion’s share of the observed
B band’s oscillator strength thus corresponds to a transition
considerably polarized along the C2 (x) axis defined by the
arylethynyl groups.
Another consequence of removing the degeneracy of the
porphyrin eg symmetry LUMO through the attachment of
arylethynyl moieties to the porphyrin meso position is manifest
in the Q band region of the spectrum; here the lowest energy
π-π* transition features an extinction coefficient of approxi-
mately 60 000 M-1 cm-1, displaying significantly enhanced
oscillator strength (Table 5) with respect to tetra(meso-aryl)-

11860 J. Am. Chem. Soc., Vol. 118, No. 47, 1996
LeCours et al.
Table 4. Reductive Cyclic Voltammetric Dataa for [5,15-Bis[(4′-X-phenyl)ethynyl]-10,20-diphenylporphinato]zinc(II) Compounds Recorded in
THF Solvent
ZnP/ZnP-
ZnP-/ZnP2-
compd
Epc (mV)
Epa (mV)
E1/2 (mV)
∆Ep (mV)
Epc (mV)
Epa (mV)
E1/2 (mV)
∆Ep (mV)
X ) NMe2
X ) OMe
X ) H
-1333
-1253
-1303
-1218
-1088b
-1246
-1168
-1188
-1163
-1290
-1210
-1246
-1190
87
85
115
55
-1818
-1783
-1820
-1843
-1361c
-1703
-1608
-1710
-1693
-1198c
-1760
-1696
-1765
-1768
-1280c
115
175
110
150
163c
X ) F
X ) NO2
a Cyclic voltammetric experimental conditions: [Porphyrin] ) 1-3 mM; [TBAPF6] ) 0.15 M; scan rate ) 200 mV/s; reference electrode )
saturated calomel (SCE); working electrode ) platinum disk; the ferrocene/ferrocenium (FeII/FeIII) redox couple (0.43 V vs SCE) was used as an
internal standard. The experimental uncertainty in these reported potentials is (3 mV. b The first electrode-coupled is irreversible and displays a
cyclic voltammetric response consistent with an EC process. Noticeable fouling of the electrode surface was apparent after this initial reduction
event irrespective of scan rate. c The nature of the second cathodic redox process exhibited by 5,15-bis[(4′-nitrophenyl)ethynyl]-10,20-
diphenylporphinato]zinc(II) is undetermined.
Table 5. Comparative Integrated Oscillator Strengths and Absorptive Domains of the Blue and Red Spectral Regions of
[5,15-Bis[(4′-X-phenyl)ethynyl]-10,20-diphenylporphinato]zinc(II) Complexes in THF Solvent
FWHMa, B-band
oscillator strength
B-band regionc
FWHMb,d Q-band
oscillator strength
Q-band regione
oscillator strength
(360 f 750 nm)
compd
region [cm-1, (nm)]
region [cm-1, (nm)]
X ) NMe2
X ) OMe
X ) H
1157 (466)
789 (451)
753 (446)
779 (446)
976 (460)
1.29
1.59
1.65
1.42
1.52
888 (676)
660 (656)
598 (650)
587 (648)
749 (666)
0.244
0.213
0.200
0.171
0.251
1.54
1.80
1.85
1.59
1.77
X ) F
X ) NO2
a Taken as the spectral width of the B-band region at half the height of λmax
electronic transition in parentheses. c Oscillator strengths calculated over the (360 f 530 nm) wavelength domain. d Taken as the spectral width of
the Q-band region at half the height of λmax
e Oscillator strengths calculated over the (530 f 750 nm) wavelength domain.
.
b Entries correspond to the spectral window centered about the
.
porphyrins, twice that observed for the Q(0,0) absorption of
[5,10,15,20-tetraphenylporphinato]zinc(II).2c The intense, low
energy Q bands of [5,15-bis[(4′-X-phenyl)ethynyl]-10,20-di-
phenylporphinato]zinc(II) species are on average 620 cm-1 more
red shifted than the analogous transition observed for [5,15-
bis(trimethysilylethynyl)-10,20-diphenylporphinato]zinc(II),2a and
exhibit extinction coefficients three times as great. Both theory
and experiment show that when porphyrin ring substituents
enhance the splitting between the key filled (a1u, a2u) or empty
(eg) orbitals in simple 4-fold symmetric porphyrins, the Q
transition gains in intensity; intensity enhancements of the type
shown in Figure 2, however, are unprecedented for simple meso-
substituted porphyrins.3d,7,19 A weaker transition (ꢀ ∼ 10 000
M-1 cm-1), slightly higher in energy with respect to the λmax
of the lowest energy Q-type transition is observed for these
species; in Figure 4, this absorption is clearly discernible for
the 4′-X ) F, H, and OMe [5,15-bis[(4′-X-phenyl)ethynyl]-
10,20-diphenylporphinato]zinc(II) derivatives, and lies ∼1400
cm-1 to the blue of the intense, low energy transition, consistent
with its assignment as a vibronic band.7 It should be emphasized
that the Q-band region exhibits considerable spectral hetero-
geneity for all the [5,15-bis[(4′-X-phenyl)ethynyl]-10,20-di-
phenylporphinato]zinc(II) derivatives.20 Simplex spectral fitting
shows that the x-polarized excited states possess augmented
oscillator strength and lie hundreds of wavenumbers to the red
of their respective Qy transitions.20 These spectral assign-
ments are consistent with the fact that conjugation is ex-
panded about the long axis of the molecule and our electronic
structure calculations (Vide infra), which indicate that the lowest
lying singlet excited states for these complexes is Qx(0,0) in
nature.20,21
The absolute energies of the B- and Q-type transitions in the
[5,15-bis[(aryl)ethynyl]-10,20-diphenylporphinato]zinc(II) com-
plexes are also noteworthy. As a family, these porphyrins
possess extensively red-shifted spectral features; the optical
spectrum of [5,15-bis[(4′-dimethylaminophenyl)ethynyl]-10,20-
diphenylporphinato]zinc(II) in fact displays the largest B- and
Q-band bathochromic shifts of any simple meso-substituted
(porphinato)zinc(II) complex yet reported. In general, the
porphyrin B bands in the [5,15-bis[(4′-X-phenyl)ethynyl]-10,20-
diphenylporphinato]zinc(II) series are red-shifted over a 2700
to 3700 cm-1 range relative to the B band of the unsubstituted
ring system, (porphinato)zinc (λmax,THF ) 398.0 nm). It is
important to note that the magnitude of these shifts greatly
exceeds that which can be achieved by modulation of the
porphyrin axial ligand environment and illustrates the efficiency
by which meso arylethynyl groups conjugate with the porphyrin
π system.22
Electronic Structure of the (Porphinato)zinc(II) Com-
plexes. In order to provide further insight into the origin of
the extensive electronic perturbations derived from meso-
arylethynyl group substitution at the porphyrin macrocycle, the
orbital energies for the [5,15-bis[(4′-X-phenyl)ethynyl]-10,20-
diphenylporphinato]zinc(II) complexes were determined using
semiemprical methods at a configuration interaction (CI) level
of 20 (see Experimental Section);10 an analogous calculation
was performed for the unsubstituted (porphinato)zinc(II) met-
(21) (a) The x-polarized nature of the intense, low energy Q band for
these complexes has been confirmed via ultrafast, singlet-excited-state
anisotropy measurements. LeCours, S. M.; Jahn, L.; Therien, M. J.
Manuscript in preparation. (b) Assignments of the π-π* electronic transitions
at low temperature have been accomplished using MCD spectroscopy. Kirk,
M. L.; Wall, M. H.; Helton, M.; Jones, R.; LeCours, S. M.; Therien, M. J.
Manuscript in preparation.
(22) (a) Nappa, M.; Valentine, J. S. J. Am. Chem. Soc. 1978, 100, 5075-
5080. (b) Chien, J. C. W. J. Am. Chem. Soc. 1978, 100, 1310-1312. (c)
Kolling, O. W. Inorg. Chem. 1979, 18, 1175-1176. (d) Walker, F. A.;
Balke, V. L.; McDermott, G. A. Inorg. Chem. 1982, 21, 3342-3348.
(19) (a) Gouterman, M. J. Chem. Phys. 1959, 30, 1139-1161. (b) Perrin,
M. H.; Gouterman, M.; Perrin, C. L. J. Chem. Phys. 1969, 50, 4137-4150.
(20) The lowest energy Q band for all the [5,15-bis[(4′-X-phenyl)-
ethynyl]-10,20-diphenylporphinato]zinc(II) complexes can be mathemati-
cally fit using simplex methods as the sum of Gaussians. The amplitudes
of each of these Gaussians depend on the nature of the 4′-X substituent
and likely reflect the relative ambient temperature population of the
maximally conjugated conformer that features coplanar ring systems with
respect to those in which the arylethynyl phenyl rings and the porphyrin
macrocycle assume torsional angles far removed from 0°.21

Modulation of Porphyrins through meso-Arylethynyl Groups
J. Am. Chem. Soc., Vol. 118, No. 47, 1996 11861
Figure 5. Evolution of the ZINDO-calculated frontier orbital sym-
metries and energies of [5,15-bis[(4′-X-phenyl)ethynyl]-10,20-di-
phenylporphinato]zinc(II) complexes relative to (porphinato)zinc(II) as
a function of the electronic properties of the (aryl)ethynyl moiety’s
4′-X substituent. See experimental section for computational details.
of [5,15-bis[(4′-nitrophenyl)ethynyl]-10,20-diphenylporphinato]-
zinc(II) differ significantly from the other [5,15-bis[(4′-X-
phenyl)ethynyl]-10,20-diphenylporphinato]zinc(II) complexes in
this series in that its nitro substituents electronically com-
municate with both the ethynyl moieties and the porphyrin core
via conjugative interactions; these FOs are shown in Figure 7.
The classic Gouterman-type FOs of (porphinato)zinc(II) are
displayed in Figure 8,7 underscoring the substantive electronic
differences that are manifest in the FOs of the meso-arylethynyl-
substituted porphyrins with respect to porphyrins bearing more
conventional substituents.
As seen in Figure 5, for all the [5,15-bis[(4′-X-phenyl)-
ethynyl]-10,20-diphenylporphinato]zinc(II) species, the interac-
tion of the cylindrically π-symmetric ethyne moiety with the
meso-carbon localized electron density at the 5- and 15-
porphyrin positions raises the energy of the a2u-derived bu and
b1u orbitals above the a1u-derived orbitals having au symmetry.
Note that the calculated energy gap (Table 6) between the
HOMO-1 (the au orbital) and HOMO (either the bu or b1u orbital)
decreases from 0.269 f 0.222 f 0.186 f 0.181 f 0.101 eV
as the arylethynyl moiety’s 4′-substituent is varied from NMe2
f OMe f H f F f NO2; thus, it is seen that as the arylethynyl
phenyl rings become progressively less electron rich, the
destabilizing influence of the 5- and 15-ethynyl moieties on the
bu and b1u orbitals is attenuated, resulting in smaller au-bu/b1u
electronic splitting.
Another manifestation of the cylindrically π-symmetric ethyne
moiety fused directly to the 5- and 15-porphyrin meso-carbon
positions is the considerable splitting of the formerly degenerate
eg-symmetry LUMO of (porphinato)zinc(II) (Figure 8). The
calculated energy gap (Table 3) between the eg-derived orbitals
of the [5,15-bis[(4′-X-phenyl)ethynyl]-10,20-diphenylpor-
phinato]zinc(II) complexes increases from 0.283 f 0.301 f
0.311 f 0.330 f 0.522 eV as the arylethynyl moiety’s
4′-substituent is varied from NMe2 f OMe f H f F f NO2;
the magnitude of these separations between the low-lying empty
orbitals is congruent with these complexes possessing disparate
Qx and Qy transition energies.7 While these calculated splittings
Figure 4. Electronic spectral dependence of [5,15-bis[(4′-X-phenyl)-
ethynyl]-10,20-diphenylporphinato]zinc(II) complexes in THF as a
function of the (aryl)ethynyl moiety’s 4′-X substituent.
allomacrocycle, in order to obtain an appropriate benchmark
for comparison. The results of this study are summarized in
Figure 5 and Table 6; Figure 5 displays the relative energies
and symmetries of the frontier orbitals (FOs) of these complexes
and Table 6 lists the calculated FO energies of these species.
The electron density distributions of the FOs of the [5,15-bis-
[(4′-X-phenyl)ethynyl]-10,20-diphenylporphinato]zinc(II) com-
plexes in which inductive effects dominate the electronic
interaction of the 4′-substituents with the aromatic (porphinato)-
zinc(II) core are similar (X ) NMe2, OMe, H, and F); the FOs
of [5,15-bis[(4′-dimethylaminophenyl)ethynyl]-10,20-diphe-
nylporphinato]zinc(II) are depicted in Figure 6 as a representa-
tive example of the members of this class of 5,15-bis[(4′-X-
phenyl)ethynyl]-10,20-diphenylporphyrins. In contrast, the FOs

11862 J. Am. Chem. Soc., Vol. 118, No. 47, 1996
LeCours et al.
Table 6. Calculated Frontier Orbital Energies (eV)a of [5,15-Bis[(4′-X-phenyl)ethynyl]-10,20-diphenylporphinato]zinc(II) Complexes Relative
to (porphinato)zinc(II)
[5,15-Bis[(4′-X-phenyl)ethynyl]-10,20-diphenylporphinato]zinc(II) complexes
reference
orbital
(porphin-ato)zinc(II)
X ) NMe2
X ) OMe
X ) H
X ) F
X ) NO2
LUMO+2
LUMO+1
LUMO
HOMO
HOMO-1
0.491
-1.215
-1.215
-6.344
-6.894
0.197
-1.109
-1.392
-6.141
-6.410
0.106
-1.176
-1.477
-6.257
-6.479
0.080
-1.190
-1.501
-6.307
-6.493
-0.065
-1.285
-1.615
-6.411
-6.592
-1.547
-1.551
-2.069
-6.764
-6.865
a See Figure 5 for relevant symmetry labels.
Figure 6. Frontier molecular orbitals of [5,15-bis[(4′-dimethylami-
nophenyl)ethynyl]-10,20-diphenylporphinato]zinc(II). The electron den-
sity centered on the 10- and 20-phenyl rings has been omitted for clarity.
Figure 7. Frontier molecular orbitals of [5,15-bis[(4′-nitrophenyl)-
ethynyl]-10,20-diphenylporphinato]zinc(II). The electron density cen-
tered on the 10- and 20-phenyl rings has been omitted for clarity.
between the ag/b2g-bg/b3g orbitals (Figures 5-7) are substantial
for all the [5,15-bis[(4′-X-phenyl)ethynyl]-10,20-diphenyl-
porphinato]zinc(II) compounds, those bearing dimethylamino,
methoxy, protio, and fluoro substituents possess similar LUMO-
LUMO+1 energy separations with their absolute magnitude
tracking with the Hammett σp value of the group attached to
the 4′-phenyl position. When 4′-nitro substituents are utilized,
more substantial splittings of the eg-derived LUMO (b2g) and
LUMO+1 (b3g) are realized; pendant arylethynyl moieties
featuring more drastically electron deficient groups would thus
be expected to both further stabilize the LUMO as well as lead
to increased energy differences between the eg-derived orbitals.
In contrast to the eg set of (porphinato)zinc(II) (Figure 8), in
which the electronic contribution of the four meso carbons is
equivalent in each of the degenerate orbitals, the ag/b2g and bg/
b3g orbitals of the [5,15-bis[(4′-X-phenyl)ethynyl]-10,20-di-
phenylporphinato]zinc(II) complexes evince asymmetric distri-
butions of meso-carbon electron density. As shown pictorially
in Figures 6 and 7, the LUMOs (ag/b2g orbitals) for these
complexes show an atomic contribution from only the meso-
carbon atoms lying along the molecular dimension of highest
conjugation, while the LUMO+1s (bg/b3g orbitals) display
appreciable contributions stemming from the meso-carbon atoms
lying orthogonal to the vector defined by the two arylethynyl
moieties; thus electronic excitations from filled levels into the
ag/b2g and bg/b3g FOs will generate excited states that are
polarized along the x- and y-molecular axes, respectively.
The LUMO+2 (bu/b1u) of the [5,15-bis[(4′-X-phenyl)eth-
ynyl]-10,20-diphenylporphinato]zinc(II) complexes in which the
4′-substituents are either NMe2, OMe, H, or F is calculated to
lie on average 1.28 eV higher in energy than the LUMO+1.
The atomic character of this FO is unusual however and merits
some comment. As can be seen in Figure 6, this orbital displays
significant electronic contributions from the porphyrin meso
carbons, the ethynyl groups, and the arylethynyl phenyl rings,
as well as the porphyrin â- and CR-carbon atoms. Focusing
our attention along the highly conjugated dimension of the
molecule, one sees that electronic population of the LUMO+2

Modulation of Porphyrins through meso-Arylethynyl Groups
J. Am. Chem. Soc., Vol. 118, No. 47, 1996 11863
While promotion of electrons into the empty bu/b1u levels from
the au and bu/b1u orbitals of the [5,15-bis[(4′-X-phenyl)ethynyl]-
10,20-diphenylporphinato]zinc(II) compounds will give rise to
lower oscillator strength transitions relative to those involving
the empty ag/b2g and bg/b3g sets due to their symmetry forbidden
nature, excitations involving the bu/b1u-derived levels in elec-
tronically asymmetric arylethynylporphyrins would be predicted
to significantly impact the nature of the excited states of such
complexes, particularly if at least one porphyrin meso substitu-
ent, such as a (4′-nitrophenyl)ethynyl moiety, served to stabilize
such an orbital.23,24
Table 7 compares the experimental electronic spectral data
for the [5,15-bis[(4′-X-phenyl)ethynyl]-10,20-diphenylpor-
phinato]zinc(II) species with the theoretically determined transi-
tion energies derived from ZINDO CI calculations. Note that
the calculated electronic spectral transitions mirror the salient
features displayed in Figure 4, namely that the energies of the
B- and Q bands progressively blue shift as the 4′-substituents
of the [5,15-bis[(4′-X-phenyl)ethynyl]-10,20-diphenylporphin-
ato]zinc(II) complexes become less electron donating throughout
the series X ) NMe2, OMe, H, F; moreover, these calculations
are consistent with experiment in that the energy separating the
most intense visible transitions increases in the order NMe2 <
OMe < H < F for the 4′-X substituents. When the 4′-
substituent is changed to the even more electron withdrawing
X ) NO2 group, theory and experiment evince abrupt spectral
red shifts of the B- and Q-band transitions coupled with a
decrease in the B-Q spectral splitting with respect to the 4′-X
) F derivative (Table 7). While this behavior for the 4′-X )
NO2 complex is predictable a priori since the porphyrin-centered
π-π* transitions should shift to lower energy as conjugation is
expanded out onto the nitro moieties, it is gratifying that these
electronic structure calculations correctly predict how each of
the 4′-X substituents perturbs the optical spectrum of their
respective [5,15-bis[(4′-X-phenyl)ethynyl]-10,20-diphenylpor-
phinato]zinc(II) complexes. Moreover, this dramatic reordering
of the FO energies and electron distribution is concordant with
the exceptional nonlinear optical properties reported for por-
phyrins bearing the 4′-NO2-phenyl)ethynyl electron acceptor.23-24
These electronic structure studies are consistent with the high
oscillator strength B- and Q bands having substantial Bx(0,0)
and Qx(0,0) character. The disparity in oscillator strength
between the x- and y-polarized absorptions has its genesis in
the removal of the degeneracy of the porphyrin LUMO which
gives rise to transition dipoles corresponding to the lowest
singlet excited configurations that differ substantially in mag-
nitude, consistent with the essential features of the classic
Gouterman four-orbital model.3d,7,18 Previously performed
INDO/SCI calculations which examined excited-state wave
functions of a donor-acceptor version of the 5,15-bis[(aryl)-
ethynyl]-10,20-diphenylporphyrin structural motif quantitatively
made a similar prediction, estimating the oscillator strength of
the Qx(0,0) transition to be approximately ten times that of the
Qy(0,0) absorption with the Bx(0,0) thrice that of the By(0,0).24
Figure 8. Frontier molecular orbitals of (porphinato)zinc(II).
will result in increased bonding interactions between the ethynyl
groups and the porphyrin 5- and 15-meso carbon positions as
well as between the arylethynyl phenyl ring C-1′ positions and
the ethyne moieties; reduced bond order would be manifest in
the ethyne bridge as well as in the arylethynyl phenyl rings.
Optical excitation into the LUMO+2 would therefore impart
significant cumulenic character to such a higher lying electroni-
cally excited state.
For [5,15-bis[(4′-nitrophenyl)ethynyl]-10,20-diphenylpor-
phinato]zinc(II), the highly conjugating and electron withdraw-
ing nitro group effects considerable stabilization of all the FOs
(Figure 5; Table 6). Most conspicuous is the precipitous drop
in energy of the empty b1u orbital, which falls below the b3g
level to become the LUMO+1 (Figure 5). As a consequence
of this extreme stabilization of the antibonding levels, the
LUMO, LUMO+1, and LUMO+2 orbitals of [5,15-bis[(4′-
nitrophenyl)ethynyl]-10,20-diphenylporphinato]zinc(II) (Figure
7) display a number of differences from those of the [5,15-bis-
[(4′-X-phenyl)ethynyl]-10,20-diphenylporphinato]zinc(II) com-
pounds that bear 4′-NMe2, OMe, H, or F substituents (Figure
6). Firstly, significant nitro-localized electron density is present
in the b2g-symmetric LUMO. Secondly, in contrast to the FOs
displayed for [5,15-bis[(4′-dimethylaminophenyl)ethynyl]-10,20-
diphenylporphinato]zinc(II) (Figure 6), it is the LUMO, not the
LUMO+2 of [5,15-bis[(4′-nitrophenyl)ethynyl]-10,20-di-
phenylporphinato]zinc(II) that evinces extensive cumulenic
character. Thirdly, the ungerade empty FO (Figure 7) is
essentially localized on the arylethynyl nitrophenyl moieties and
bears little porphyrinic electronic constitution. Finally, as seen
in Figure 7, the LUMO and LUMO+1 now possess augmented
x-polarized electron density; the formation of y-polarized excited
states now requires electronic population of the LUMO+2.
Summary and Conclusion
Removing the steric barrier to rotation between the porphyrin
core and its pendant aryl substituents through an intervening
ethynyl moiety allows considerable tuning of the porphyrin
frontier orbital energies. Our work demonstrates that porphyrin
meso-arylethynyl groups can be utilized to significantly regulate
(23) LeCours, S. M.; Guan, H.-W.; DiMagno, S. G.; Wang, C. H.;
Therien, M. J. J. Am. Chem. Soc. 1996, 118, 1497-1503.
(24) Priyadarshy, S.; Therien, M. J.; Beratan, D. N. J. Am. Chem. Soc.
1996, 118, 1504-1510.

11864 J. Am. Chem. Soc., Vol. 118, No. 47, 1996
LeCours et al.
Table 7. Comparison of Experimental and Calculated Electronic Spectral Features of [5,15-Bis[(4′-X-phenyl)ethynyl]-10,20
diphenylporphinato]zinc(II) Complexes
a
b
a
b
B band λmax
,
Q band λmax
(exp., eV)
spectral splittingc
(exp., eV)
B band λmax
,
Q band λmax
(calc., eV)
spectral splitting
(calc., eV)
compd
(exp., eV)
(calc., eV)
(porphinato)Zn
X ) NMe2
X ) OMe
X ) H
3.10
2.66
2.75
2.78
2.78
2.70
2.22
1.83
1.89
1.91
1.91
1.86
0.88
0.83
0.86
0.87
0.87
0.84
3.80
3.33
3.41
3.43
3.44
3.13
2.15
2.01
2.01
2.02
2.02
2.00
1.65
1.32
1.40
1.41
1.42
1.13
X ) F
X ) NO2
a Refers to the B(0,0) transition for (porphinato)zinc(II) and the intense, B transition for the [5,15-bis[(4′-X-phenyl)ethynyl]-10,20-
diphenylporphinato]zinc(II) complexes which has augmented Bx(0,0) in character. Experimental values determined in THF solvent. b Refers to the
Q(0,0) transition for (porphinato)zinc(II) and the Q transition for the [5,15-bis[(4′-X-phenyl)ethynyl]-10,20-diphenylporphinato]zinc(II) complexes
which has augmented Qx(0,0) in character. Experimental values determined in THF solvent. c Defined as the energy separation between the respective
B and Q bands.
chromophore spectroscopic properties as well as macrocycle
electrochemical properties. A unique aspect of these [5,15-bis-
[(4′-X-phenyl)ethynyl]-10,20-diphenylporphinato]zinc(II) sys-
tems lies in the fact that this extensive electronic modulation
was achieved at an undistorted, planar macrocycle through
substitution at only two points on the porphyrin periphery. Such
arylethynyl-substituted porphyrins will allow wide-ranging
substituent electronic effects to be evaluated independently of
porphyrin core structural perturbations. Moreover, this chem-
istry defines a general method for regulating the optoelectronic
properties of long wavelength absorbing dyes. Studies of the
photophysical, magnetic, electrochemical, ground- and excited-
state vibrational properties, as well as further work focused on
the details of the electronic structure of other metal derivatives
of these and similar systems will be the subjects of future
publications.
Acknowledgment. M.J.T. is indebted to the Searle Scholars
Program (Chicago Community Trust), the Arnold and Mabel
Beckman Foundation, E. I. du Pont de Nemours, and the
National Science Foundation for Young Investigator Awards
as well as the Alfred P. Sloan Foundation, the National Institutes
of Health (GM 48130-01A1.4), the U.S. Department of Energy
(DE-FGO2-94ER14494), the Office of Naval Research (96-1-
0725), and the Exxon Education Fund for their generous
financial support.
JA962403Y