Aryl ring rotation in porphyrins. A carbon-13 NMR spin-lattice relaxation time study

Article abstract of DOI:10.1021/jp962209y

Overall tumbling and internal rotational motions in porphyrins bearing meso aryl substituents and, in some cases, flanking alkyl groups at the β-pyrrolic positions have been determined using ¹³C spin-lattice relaxation time measurements. In deuteriochloroform solution at 303 K, the overall reorientation of all three porphyrins investigated occurs with diffusion coefficients of ～1 × 10⁹ s^-1. In porphyrins with only hydrogen at the β-pyrrolic positions, the meso phenyl rings undergo rotations about their single bonds to the porphyrin with diffusion coefficients of ～4 × 10⁹ s^-1. Introduction of methyl substituents at the β-pyrrolic positions adjacent to the phenyl rings reduces these motions, but only to ～1 ×10⁹ s^-1. Thus, significant internal motions are present in both types of molecules. These motions occur on the time scale of many photoinduced electron and energy transfer processes in porphyrins covalently linked to electron or energy donors or acceptors through meso aryl groups. Thus, the internal librational motions may affect rates of photoinduced electron and energy transfer, even in relatively "rigid" molecular constructs.

Full text of DOI:10.1021/jp962209y

458
J. Phys. Chem. B 1997, 101, 458-465
Aryl Ring Rotation in Porphyrins. A Carbon-13 NMR Spin-Lattice Relaxation Time
Study
Lori Noss, Paul A. Liddell, Ana L. Moore,* Thomas A. Moore,* and Devens Gust*
Center for the Study of Early EVents in Photosynthesis, Department of Chemistry and Biochemistry,
Arizona State UniVersity, Tempe, Arizona 85287-1604
ReceiVed: July 22, 1996; In Final Form: October 14, 1996X
Overall tumbling and internal rotational motions in porphyrins bearing meso aryl substituents and, in some
cases, flanking alkyl groups at the â-pyrrolic positions have been determined using 13C spin-lattice relaxation
time measurements. In deuteriochloroform solution at 303 K, the overall reorientation of all three porphyrins
investigated occurs with diffusion coefficients of ∼1 × 109 s-1. In porphyrins with only hydrogen at the
â-pyrrolic positions, the meso phenyl rings undergo rotations about their single bonds to the porphyrin with
diffusion coefficients of ∼4 × 109 s-1. Introduction of methyl substituents at the â-pyrrolic positions adjacent
to the phenyl rings reduces these motions, but only to ∼1 × 109 s-1. Thus, significant internal motions are
present in both types of molecules. These motions occur on the time scale of many photoinduced electron
and energy transfer processes in porphyrins covalently linked to electron or energy donors or acceptors through
meso aryl groups. Thus, the internal librational motions may affect rates of photoinduced electron and energy
transfer, even in relatively “rigid” molecular constructs.
Introduction
Synthetic meso-polyarylporphyrins are important constituents
of many molecular and supramolecular model systems for
photosynthetic electron and energy transfer, where they stand
in for naturally occurring porphyrins and chlorophylls found in
reaction centers and other components of the photosynthetic
apparatus.1-8 The aryl groups of such porphyrins serve as
convenient points of attachment for various electron or energy
donor and acceptor moieties. The aryl rings serve a structural
role by providing a linkage that separates the porphyrin
chromophore from the π-electron systems of attached donors
and acceptors. In most cases, they also mediate the actual
transfer of electrons, triplet excitation energy, and sometimes
singlet excitation energy9 through a superexchange mechanism,
which involves mixing of aryl bridge wave functions with those
of the adjacent moieties.
The role of π-π overlap between the porphyrin macrocyclic
ring and the attached meso aryl groups has always been
problematic. If the aryl rings of a meso-polyarylporphyrin such
as 1 were coplanar with the macrocycle, then conjugation would
be complete, and the aryl rings would be part of the porphyrin
chromophore. For steric reasons, this is not the case. In the
solid state, the dihedral angle between the plane of the porphyrin
and the plane of the aryl group is usually between 60° and
90°.10-14 Calculations15 and NMR studies in solution16,17 have
aryl carbon atoms ortho or meta to the point of attachment,
estimated the angle to be about 45°. At angles between 0° and
90°, conjugation between the two π-electron systems would be
partial. The angle of rotation of the aryl ring can affect energy
and photoinduced electron transfer rates in several ways. In
the equilibrium conformation, the angle will determine the
degree of orbital overlap of the π-electrons, which in turn will
affect the rate of transfer, because the rate is a function of the
orbital overlap between the donor and the acceptor. In addition,
rotational motions about the single bond joining the aryl group
to the macrocycle will modulate the orbital overlap and could
thus affect, or even determine, the rate of transfer. Finally, for
donor or acceptor groups joined to the porphyrin through the
rotations can alter both the dihedral angles and the distances
separating the various donor and acceptor moieties. Electron
transfer rate constants can also depend strongly on these
parameters. Thus, an understanding of the aryl rotational
motions is crucial to understanding the electron and energy
transfer processes.
Numerous investigators have prepared photosynthetic model
systems in which the meso aryl groups are flanked by methyl,
ethyl, or other alkyl groups at the “â-pyrrolic” positions on the
porphyrin periphery (cf. porphyrin 2).12-14,18-29 Molecular
modeling and some X-ray crystal structure determinations
suggest that the â-alkyl groups force the aryl rings into a position
perpendicular to the (idealized) porphyrin plane. If this were
the case, and if the rings do not deviate from that position, then
X Abstract published in AdVance ACS Abstracts, December 15, 1996.
S1089-5647(96)02209-2 CCC: $14.00 © 1997 American Chemical Society

Aryl Ring Rotation in Porphyrins
J. Phys. Chem. B, Vol. 101, No. 3, 1997 459
TABLE 1: Selected Carbon-13 Chemical Shifts and Spin-Lattice Relaxation Times for Porphyrins 1-3 in Deuteriochloroform
at 303 K
porphyrin 1a
porphyrin 2
porphyrin 3
carbon
δ (ppm)
T1 (s)
δ (ppm)
T1 (s)
δ (ppm)
T1 (s)
2, 8
3, 7
131.1
131.1
0.45 ( 0.03
0.45 ( 0.03
10, 20
12, 18
13, 17
5Ar2,6
15Ar2,6
5Ar3,5
15Ar3,5
5Ar4
96.5
0.52(0.01
101.9
131.9
130.8
133.4
134.6
127.6
126.6
128.3
127.4
0.38 ( 0.02
0.40 ( 0.01
0.40 ( 0.01
0.48 ( 0.01
0.61 ( 0.01
0.46 ( 0.01
0.60 ( 0.01
0.39 ( 0.01
0.41 ( 0.01
131.1
131.1
134.6
134.6
126.7
126.7
127.7
127.7
0.45 ( 0.03
0.45 ( 0.03
0.72 ( 0.01
0.72 ( 0.01
0.72 ( 0.01
0.72 ( 0.01
0.46 ( 0.01
0.46 ( 0.01
133.1
133.1
127.6
127.6
128.3
128.3
0.58 ( 0.01
0.58 ( 0.01
0.57 ( 0.01
0.57 ( 0.01
0.46 ( 0.01
0.46 ( 0.01
15Ar4
a For porphyrin 1, the chemical shifts and T1 values for the carbon atoms of the phenyl groups at the 10- and 20-positions are identical to those
reported for the corresponding carbon atoms of the phenyl rings at the 5- and 15-positions, for reasons of symmetry.
the porphyrin and its attached aryl rings would form a rigid
molecular framework with no π-π interaction between the rings
and the macrocycle, and the question of π-delocalization onto
the aryl rings would be settled.
Given the single bond joining an aryl ring to the macrocycle
and the notorious flexibility of molecular species, the assumption
of rigidity for porphyrins with â-alkyl groups is suspect. If
aryl rotation does occur, and occurs on a time scale comparable
to or faster than electron or energy transfer, then the ambiguities
discussed above in connection with porphyrins such as 1 are
also present in structures related to 2.
Carbon-13 nuclear magnetic resonance spectroscopy can yield
detailed information about overall and internal motions of
molecules in solution. As will be discussed in detail below,
dipole-dipole spin-lattice relaxation of a 13C nucleus by a
directly bonded hydrogen is a function of the time scale of
reorientation of the internuclear vector with respect to the
spectrometer magnetic field. Thus, motional information may
be extracted from experimentally determined dipole-dipole
spin-lattice relaxation times (T1DD). In this paper, we report
the results of such investigations on meso-tetraphenylporphyrin
1 and porphyrins 2 and 3, which feature alkyl groups that flank
the phenyl rings.
discussed in the literature, only a brief summary of the approach
used here will be given. Following excitation, 13C nuclei relax
by interaction with local magnetic fields which have motional
components at the Larmor frequency. Relaxation mechanisms
that may be of importance for molecules such as 1-3 include
dipole-dipole, spin rotation, chemical shift anisotropy, and
scalar coupling. In practice, the dipole-dipole mechanism often
dominates for carbon atoms directly bonded to hydrogen, and
this has been shown to be the case for porphyrins such as 1-3
under the experimental conditions used in this study.30 Relax-
ation arises from the interaction of the 13C nuclear magnetization
with that of an attached proton, modulated by molecular motions
that change the orientation of the C-H internuclear vector
relative to the external magnetic field provided by the spec-
trometer. Under these conditions, the dipole-dipole spin-
lattice relaxation time, T1DD, for a 13C nucleus is given31 by eq
1,
2
2
1/T1DD ) 0.1NγH γC p2r-6[J0(ωH - ωC) + 3J1(ωC) +
6J2(ωH + ωC)] (1)
where N is the number of directly attached hydrogens, γH and
γC are the magnetogyric ratios of hydrogen and carbon, r is the
internuclear distance, ωH and ωC are the Larmor frequencies
of hydrogen and carbon, and the J(ω) are spectral density
functions describing molecular reorientational motions.
For a rigid, isotropically rotating molecule, the spectral density
functions in eq 1 may be expressed31 according to eq 2. The
Results and Discussion
Synthesis. Porphyrin 1 is a known compound. Compounds
2 and 3 were synthesized by an application of the MacDonald
[2+2] condensation, as described in the Experimental Section.
Chemical Shift Assignments. The numbering system used
to identify the various carbon atoms in the porphyrins is
Ji(ω) ) τc/(1 + ω2τc2)
(2)
1
indicated on the structural drawing. The H and 13C NMR
molecular reorientation is given in terms of a single rotational
correlation time τc, which is sometimes more conveniently
expressed as the rotational diffusion constant D (τc ) 1/6D).
For this situation, eq 1 becomes
spectra were assigned by a combination of COSY, NOESY,
HMQC, and one-dimensional experiments. The relevant 13C
1
chemical shifts appear in Table 1, and the H NMR data are
given in the Experimental Section.
13C Spin-Lattice Relaxation Time Measurements. The
T1 measurements were made at 11.7 T in degassed deuterio-
chloroform solution at 303 ( 0.5 K. Measurements were
repeated at least six times for each molecule. Compounds 1
and 2 were measured at 0.024 and 0.0092 M, and identical
results were obtained at the two concentrations, within experi-
mental error. The concentration for 3 was 0.0082 M. The
inversion-recovery (D-180-t-90-FID) pulse sequence was
employed (see Experimental Section). The T1 values for the
monoprotonated carbon atoms of 1-3 are reported in Table 1,
along with the standard deviations in the measurements.
Theoretical Framework for Analysis. As the theoretical
principles used to interpret the T1 data have been thoroughly
τc
2
2
1/T1DD ) 0.1NγH γC p2r-6
+
2
2
[
1 + (ωH - ωC) τc
3τc
6τc
+
(3)
1 + ωc2τc2 1 + (ωH + ωC)2τc2
]
When the overall rotation of the molecule is isotropic, but
internal rotations such as those around carbon-carbon single
bonds also contribute to relaxation, the important motions
include not only the tumbling of the molecule about its center
of mass (D0) but also rotations about single bonds joining the
chain of atoms separating the carbon of interest from the center

460 J. Phys. Chem. B, Vol. 101, No. 3, 1997
Noss et al.
TABLE 2: Calculated Isotropic Diffusion Coefficients for Porphyrins 1-3 in Deuteriochloroform at 303 K
porphyrin 1a
porphyrin 2
porphyrin 3
b
b
b
carbon
D0 (s-1
)
D1 (s-1
)
D0 (s-1
)
D1 (s-1
)
D0 (s-1
)
D1 (s-1
)
2, 8
3, 7
1.4 × 109
1.4 × 109
10, 20
12, 18
13, 17
5Ar2,6
15Ar2,6
5Ar3,5
15Ar3,5
5Ar4
1.7 × 109
1.2 × 109
1.2 × 109
1.2 × 109
1.4 × 109
1.4 × 109
(1.5 × 109)
(1.5 × 109)
(1.5 × 109)
(1.5 × 109)
1.5 × 109
1.5 × 109
4.3 × 109
4.3 × 109
4.3 × 109
4.3 × 109
(1.5 × 109)
(1.5 × 109)
(1.5 × 109)
(1.5 × 109)
1.5 × 109
1.5 × 109
1.5 × 109
1.3 × 109
1.3 × 109
(1.2 × 109)
(1.2 × 109)
(1.2 × 109)
(1.2 × 109)
1.2 × 109
1.1 × 109
3.7 × 109
8.4 × 108
3.6 × 109
15Ar4
1.5 × 109
1.3 × 109
a For porphyrin 1, the diffusion coefficients for the carbon atoms of the phenyl groups at the 10- and 20-positions are identical to those for the
corresponding carbon atoms of the phenyl groups at the 5- and 15-positions, for reasons of symmetry. b D0 values in parentheses are those used to
calculate D1 values for rotation about the single bond joining the phenyl ring to the macrocycle.
τ2 ) (2D + 4D|)-1
τ1 ) (5D + D|)-1
τ0 ) (6D )-1
(8)
(9)
of mass. There are several models for treating the effects of
such motions on relaxation. In this analysis, internal rotations
will be considered as rotational diffusion motions about the bond
axes.31-34 In this case, the spectral densities are given by
eq 4,
(10)
2
2
2
2
Ji(ω) )
|d (â )| |d (â )| ... |d (â )| |d (â )| ×
∑
ab
a
bc
b
nx
n
x0
x
Porphyrins as Isotropic Rotors. The T1 data for 1-3 were
first interpreted in terms of eqs 3-6, which require that the
molecule rotate isotropically about its center of mass. Under
these conditions, the carbon-hydrogen bond vectors for all of
the hydrogen-bearing carbon atoms on the periphery of the
porphyrin macrocycle reorient with a diffusion coefficient D0,
the overall tumbling rate of the molecule, as the macrocyclic
framework is assumed to be rigid. The same motions will relax
the carbons at the 4-positions of the aryl rings attached to the
macrocycle (Ar4), as internal oscillations of the phenyl groups
about their single bonds will have no effect on this C-H bond
vector. Application of the equations requires knowledge of the
carbon-hydrogen bond lengths. We chose a value of 1.09 Å
for all bond lengths, although strictly speaking these lengths
cannot all be exactly equal. Equation 3 yielded the values for
D0 reported in Table 2.
Having obtained values for the rates of overall tumbling of
the molecules, we went on to calculate the diffusion coefficients
for rotations about the single bonds joining the phenyl rings to
the porphyrin macrocycle (D1) using eqs 3-6. These values
can be extracted from the T1 data for the Ar2,6 and Ar3,5 carbon
atoms (Table 1), because these relaxation times are affected not
only by overall tumbling of the molecule but also by internal
rotational motions about the single bond in question. For each
porphyrin, the D0 values used in these calculations were the
average of those found for the Ar4 carbons, because the carbon-
hydrogen bond lengths for these carbons are expected to most
closely approximate those for the Ar2,6 and Ar3,5 carbon atoms.
The results appear in Table 2.
Porphyrins as Anisotropic Rotors. Using 113Cd NMR
spectroscopy, Ellis and co-workers have shown that for the
pyridine complex of 1 metalated with cadmium the molecule
does not tumble completely isotropically.17 Rather, rotational
diffusion about an axis through the center of the porphyrin and
orthogonal to the porphyrin plane (D|) occurs 3.2 ( 0.8 times
faster than rotation perpendicular to this axis (D ). Although
this anisotropy is relatively small, we also analyzed the spin-
lattice relaxation time results for 1-3 under these conditions
using the equations discussed earlier. Values for D| ()3.2D )
were calculated from the T1 values for the Ar4 carbons and the
carbon atoms rigidly fixed to the porphyrin skeleton, and the
values obtained for the Ar4 carbons were used in the calculation
a,b,...x
[τ/(1 + ω2τ2)] (4)
where
τ ) (6D0 + a2Da + b2Db + ... + x2Dx)-1
(5)
The dij(âi) are the reduced second-order Wigner rotation
matrices,32-34 the âi are the angles between successive axes of
rotation (i.e., bonds), and âx is the angle between the final
rotational (bond) axis and the C-H internuclear vector for the
two nuclei in question. The Di values are the diffusion constants
for each of the single bonds about which internal rotation occurs,
and D0 is the diffusion coefficient for the overall isotropic
motion. Symmetry considerations allow the 5 × 5 matrices
that arise from a treatment using the Wigner rotation matrices
to be replaced by 3 × 3 matrices.32 The resulting spectral
density functions are given32-34 by
τ
Ji(ω) )
B Bbc ... BhxBx0
(6)
∑
ab
2
2
[
]
a,b,...x
1 + ω τ
where Bij are elements of a 3 × 3 matrix and the sum is run
from 0 to 2 over all the indexes.
An additional complication occurs when the motions of the
molecule are no longer isotropic. A special case involves
approximation of the overall molecular tumbling as that of an
axially symmetric body such as an ellipsoid of revolution, with
rotational diffusion constants D| (for rotation about the major
axis) and D (for rotation perpendicular to that axis). In this
case, the spectral density functions are given17 by eq 7, where
τ2
1 + ω2τ2
τ1
1 + ω2τ1
Ji(ω) ) sin4 â
2 + sin2 2â
+
2
τ0
1 + ω2τ0
1
3
(3 cos2 â-1)2
(7)
2
â is the angle between the major axis and the C-H vector. In
this case, the correlation times are related to the diffusion
constants by eqs 8-10.

Aryl Ring Rotation in Porphyrins
J. Phys. Chem. B, Vol. 101, No. 3, 1997 461
TABLE 3: Calculated Anisotropic Diffusion Coefficients for Porphyrins 1-3 in Deuteriochloroform at 303 K
porphyrin 1a
porphyrin 2
porphyrin 3
b
b
-1
b
b
b
b
carbon
D| (s-1
)
D (s-1
)
D1 (s
)
D| (s-1
)
D (s-1
)
D1 (s-1
)
D| (s-1
)
D (s-1
)
D1 (s-1
)
2, 8
3, 7
2.4 × 109
7.4 × 108
2.4 × 109
7.4 × 108
10, 20
12, 18
13, 17
5Ar2,6
15Ar2,6
5Ar3,5
15Ar3,5
5Ar4
2.9 × 109
8.9 × 108
1.9 × 109
2.0 × 109
2.0 × 109
(2.0 × 109)
(2.0 × 109)
(2.0 × 109)
(2.0 × 109)
2.0 × 109
2.0 × 109
5.9 × 108
6.3 × 108
6.2 × 108
(6.4 × 108)
(6.4 × 108)
(6.4 × 108)
(6.4 × 108)
6.1 × 108
6.6 × 108
2.4 × 109
2.4 × 109
(2.4 × 109)
(2.4 × 109)
(2.4 × 109)
(2.4 × 109)
2.4 × 109
2.4 × 109
7.4 × 108
7.4 × 108
(7.6 × 108)
(7.6 × 108)
(7.6 × 108)
(7.6 × 108)
7.6 × 108
7.6 × 108
3.8 × 109
3.8 × 109
3.8 × 109
3.8 × 109
(2.5 × 109)
(2.5 × 109)
(2.5 × 109)
(2.5 × 109)
2.5 × 109
(7.7 × 108)
(7.7 × 108)
(7.7 × 108)
(7.7 × 108)
7.7 × 108
1.9 × 109
1.9 × 109
1.8 × 109
1.8 × 109
1.4 × 109
3.2 × 109
1.2 × 109
3.0 × 109
15Ar4
2.5 × 109
7.7 × 108
a For porphyrin 1, the diffusion coefficients for the carbon atoms of the phenyl groups at the 10- and 20-positions are identical to those for the
corresponding carbon atoms of the phenyl groups at the 5- and 15-positions, for reasons of symmetry. b D| and D values in parentheses are those
used to calculate D1 values for rotation about the single bond joining the phenyl ring to the macrocycle.
of D1 values for the Ar2,6 and Ar3,5 carbon atoms. The results
appear in Table 3.
As mentioned above, in porphyrins such as 1 the phenyl rings
make angles of ∼45° or greater with the porphyrin macrocyclic
plane in solution. The equilibrium angle is that which best
balances repulsive forces due to steric interactions of the phenyl
groups with the hydrogen atoms at the â-pyrrolic positions
(which are minimized in the orthogonal conformation), attractive
forces between these two regions, and conjugative interactions
between the phenyl group and the macrocycle (which are
maximized in a planar conformation).
Rotation from the equilibrium position into the plane is a
high-energy process, for steric reasons. In metalated porphyrins,
barriers to rotation of phenyl groups through the plane in the
range 14-18 kcal/mol have been reported.35,36 Motions that
achieve this rotation are much too slow to affect carbon-13
relaxation. Thus, the librational motions of interest are those
that move the plane of the phenyl ring through an arc centered
at the orthogonal orientation. In the case of relatively unhin-
dered porphyrins such as 1, this arc must cover angles of roughly
(45° from the orthogonal conformation, and the calculated D1
value represents oscillatory motion through this arc.
For porphyrins such as 2 that feature flanking alkyl groups,
steric hindrance between these groups and the phenyl rings will
constrict the size of this arc and thus decrease the D1 value. In
the case of 2, this decrease is observed, but is relatively small.
The data do not allow us to discriminate between a situation
similar to 1, where two conformational minima are located on
either side of the orthogonal arrangement, and a single
conformation with a minimum at the orthogonal orientation that
undergoes rotational excursions on either side of this minimum.
An additional feature that may contribute to the facile rotation
of the phenyl rings in 2 is distortion of the macrocycle from
planarity. Porphyrins with large substituents at both the meso
and â positions tend to distort significantly,37-40 and this could
lower the rotational barrier for the phenyl rings. For example,
the macrocycle of the dication of 1, which bears four protons
instead of two on the central nitrogen atoms, is significantly
distorted, with the pyrrole rings tilted at angles of ∼33° relative
to the mean porphyrin plane.41,42 In a closely related porphyrin
dication, the barrier for rotation of the aryl rings through the
mean porphyrin plane is reduced by 2.9 kcal/mol, relative to
the free base form.43
Discussion
Isotropic Rotation. For porphyrin 1, the T1 values for the
carbons of the porphyrin macrocycle bearing one hydrogen atom
and for the carbons at the 4-positions of the aryl rings are
identical, within experimental error, as expected for an isotro-
pically rotating rigid body. The T1 value for the Ar4 carbons
yields an overall reorientation rate D0 of 1.5 × 109 s-1. In
porphyrin 2, the T1 values for the Ar4 carbons yielded a D0
value identical to that found for 1, whereas the 10,20 carbons
of 2 gave a slightly higher value (1.7 × 109 s-1). In zinc
porphyrin 3, the two different Ar4 carbons and the three different
sets of hydrogen-bearing carbons on the porphyrin macrocycle
all had the same T1 value within experimental error. The
average D0 value for 3 is 1.2 × 109 s-1. Thus, the overall
reorientational motions of porphyrins 1-3 are essentially
identical. The isotropic D0 values are very similar to that
reported previously for the zinc analog of 1 (1.3 × 109 s-1).30
Turning now to the librational motions about the single bonds
joining the phenyl groups to the macrocycle, it is evident from
the data in Table 1 that the T1 values for the Ar2,6 and Ar3,5
carbon atoms of 1 are significantly greater than those for the
Ar4 carbons and the proton-bearing carbons on the macrocycle.
This indicates that the carbon-hydrogen internuclear vectors
are reorienting relative to the external magnetic field not only
due to overall tumbling of the molecule but also due to librations
around the single bonds joining the phenyl groups to the
macrocycle. The overall more rapid motions of these carbon-
hydrogen bond vectors make dipole-dipole relaxation less
efficient. The D1 value for diffusion about the single bonds to
the phenyl groups of 1 is 4.3 × 109 s-1. This is very similar to
the value of 2.9 × 109 s-1 found for the zinc analog of 130 and
the value of 2.2 × 109 s-1 determined for the pyridine adduct
of the cadmium analog of 1.17
Porphyrin 2 differs from 1 in that the two phenyl groups are
flanked by methyl groups. Table 1 shows that even with the
steric hindrance imposed by these groups, the T1 values for the
Ar2,6 and Ar3,5 carbon atoms are still significantly greater than
those for the Ar4 and macrocyclic carbons. Thus, the flanking
alkyl groups do not eliminate rapid rotational motions. Quan-
titatively, the average D1 value for librations about the single
bond of the phenyl group is 1.4 × 109 s-1 (Table 2). Thus, the
steric hindrance reduces the librational motions by only a factor
of about 3. The molecule is only slightly more “rigid” than
the less hindered analog 1.
In the case of zinc porphyrin 3, it is clear from the T1 data in
Table 1 that the less hindered phenyl ring at position 15
undergoes librations about its bond to the macrocycle that are
large enough to significantly increase the relaxation times of
the Ar2,6 and Ar3,5 carbon atoms relative to those at Ar4 and
on the macrocycle itself. Table 2 shows that this increase gives
rise to a D1 value of about 3.7 × 109 s-1, which is slightly
smaller than the corresponding value for 1. In the case of the
In this connection, it is clear that the librational motions in
question cannot be complete 360° rotations of the phenyl rings.

462 J. Phys. Chem. B, Vol. 101, No. 3, 1997
Noss et al.
more hindered phenyl ring at position 5, the average T1 for the
Ar2,6 and Ar3,5 carbon atoms is 0.47 s, whereas the T1 for the
corresponding Ar4 carbon is 0.39 s. Thus, there is detectable
internal motion about the single bond linking the phenyl group
at the 5-position to the macrocycle. The D1 value for this
motion is ∼1 × 109 s-1, which is about one-quarter of the
corresponding value for the less hindered phenyl group at the
15-position. Thus, the effect of alkyl group hindrance is
comparable in zinc porphyrin 3 and free base porphyrins 1 and
2.
The average diffusion constants of both the less and more
hindered phenyl groups of 3 are slightly smaller for the zinc
porphyrin than for comparable phenyl groups in the free base
forms. This is likely due to a stiffening of the porphyrin
skeleton resulting from introduction of the metal and/or changes
in the degree of nonplanarity of the porphyrin skeleton.35-40
Anisotropic Rotation. Treating the porphyrins as anisotropic
rotors with internal motions has very little effect on the
calculated D1 values for rotation of the phenyl rings (Table 3).
Thus, the conclusions presented above for the isotropic case
apply equally well to the anisotropic analysis.
dichloro-5,6-dicyanobenzoquinone to remove any chlorin im-
purity, and purified by chromatography before use.
2,8,12,18-Tetraethyl-3,7,13,17-tetramethyl-5,15-diphenylpor-
phyrin (2) was prepared in 80% yield from benzaldehyde and
bis(4-ethyl-3-methyl-2-pyrrolyl)methane using the method of
Maruyama and co-workers:46 1H NMR (300 MHz, CDCl3) δ
-2.41 (1H, br s, 2NH), 1.77 (12H, t, J ) 8 Hz, 2-CH3, 8-CH3,
12-CH3, 18-CH3), 2.49 (12H, s, 3-CH3, 7-CH3, 13-CH3, 17-
CH3), 4.02 (8H, q, J ) 8 Hz, 2-CH2, 8-CH2, 12-CH2, 18-CH2),
7.73 (4H, m, 5Ar3,5-H, 15Ar3,5-H), 7.79 (2H, m, 5Ar4-H,
15Ar4-H), 8.07 (4H, d, J ) 8 Hz, 5Ar2,6-H, 15Ar2,6-H), 10.23
(2H, s, 10-CH, 20-CH); MS (FAB) m/z 631.3778 (calcd for
(M+H)+, 631.3722); UV/vis (CH2Cl2) 408, 508, 540, 576, 628
nm.
Benzyl Pyrrole-2-carboxylate (4). A mixture of pyrrole-2-
carboxylic acid (11.11 g, 0.10 mol), dimethylformamide (150
mL), and potassium carbonate (15.2 g, 0.11 mol) was stirred
under a nitrogen atmosphere for 15 min. Benzyl bromide (14.3
mL, 0.12 mol) was added to the resulting suspension, and the
mixture was stirred for 29 h. The reaction mixture was then
poured into 400 mL of diethyl ether, washed five times with
water, and washed once with aqueous sodium chloride. The
organic layer was separated, the solvent was removed by
distillation at reduced pressure, and the residue was recrystallized
from a mixture of dichloromethane and hexane to yield pure 4.
The crude material remaining in the mother liquor was chro-
matographed on silica gel (hexane/ethyl acetate, 4:1) and
combined with the recrystallized material to produce a total of
Conclusions
These results show that the introduction of methyl groups at
the â-pyrrolic positions flanking meso aryl substituents on
porphyrin macrocycles results in steric hindrance to librational
motions of the aryl groups around the single bonds joining them
to the macrocycle. However, the effect is relatively small, and
librational motions still occur at rates comparable to or greater
than overall reorientation of the macrocycle. Diffusion constants
for the oscillatory motions are ∼1 × 109 to 4 × 109 s-1. Thus,
the distribution of motional frequencies describing the librations
is comparable to the rate constants for photoinduced electron
transfer often observed in porphyrin-based electron donor-
acceptor systems.1-7 The observed D1 values are substantially
larger than rate constants for charge recombination in some of
these systems and larger than some rates of triplet-triplet energy
transfer from porphyrins to carotenoids or other acceptors.9,44
Because these librations necessarily modify the π-π overlap
between the macrocycle and the aryl rings, they may affect the
rates of photoinduced electron or energy transfer when such
transfer is mediated by the bonds of the covalent linkage joining
the donor and acceptor. In addition, donors or acceptors linked
to meso aryl rings through positions other than the 4-position
will undergo changes in their angular relationship to the
porphyrin macrocycle as a result of these oscillations and
possibly changes in the distances separating them from other
donor or acceptor moieties. Donor-acceptor systems based
upon porphyrins bearing meso aryl rings flanked by alkyl groups
are certainly more conformationally constrained than many other
donor-acceptor species, and rotations of the aryl rings are more
hindered than those of their counterparts that lack the flanking
alkyl groups. However, even these molecules are far from being
“rigid” in the same sense that rigidity is imparted by double
bonds or some bicyclic systems.
1
19.10 g (95% yield) of fibrous white crystalline 4: H NMR
(300 MHz, CDCl3) δ 5.31 (2H, s, CH2), 6.27 (1H, dd, J ) 6.3,
2.5 Hz, 4-H), 6.96 (2H, m, 5-H, 3-H), 7.38 (5H, m, Ar-H), 9.17
(1H, br s, NH); MS m/z 201 (M+).
Benzyl 5-Formylpyrrole-2-carboxylate (5). An 11.2 mL (0.12
mol) portion of POCl3 was added dropwise to 23 mL (0.3 mol)
of dimethylformamide at 5-10 °C under a nitrogen atmosphere.
Stirring for 30 min produced the Vilsmeier reagent. A 12.07 g
(0.06 mol) portion of 4 was dissolved in 250 mL of 1,2-
dichloroethane, the solution was cooled to -20 °C, and the
Vilsmeier reagent was added dropwise over 12-15 min. The
reaction mixture was stirred at 0 °C for 3 h and then at room
temperature for 27 h. Aqueous sodium bicarbonate was added,
and the mixture was stirred for an additional 16 h. The organic
layer was separated and washed with water, and the solvent
was evaporated under reduced pressure. The residue was
dissolved in 200 mL of diethyl ether and washed with water
(×5) and aqueous sodium chloride solution. Drying the solution
over MgSO4 and filtering gave a clear yellow solution that was
concentrated by evaporation at reduced pressure and chromato-
graphed on silica gel (hexane/ethyl acetate/dichloromethane, 70:
15:15) to yield 8.66 g of 5 (63% yield) and 4.54 g of benzyl
4-formylpyrrole-2-carboxylate (6) (33% yield): 1H NMR of 5
(300 MHz, CDCl3) δ 5.32 (2H, s, CH2), 6.93 (2H, m, 3-H, 4-H),
7.36 (5H, m, Ar-H), 9.63 (1H, s, CHO), 9.86 (1H, br s, NH);
1
MS m/z 229 (M+); H NMR of 6 (300 MHz, CDCl3) δ 5.34
(2H, s, CH2), 7.39 (5H, m, Ar-H), 7.56 (2H, m, 3-H, 5-H), 9.77
(1H, br s, NH), 9.85 (1H, s, CHO).
The magnitude of the effect of these internal motions on
electron and energy transfer rate constants will depend on the
details of the system. In a companion study, it was found that
introduction of methyl substituents at porphyrin â-pyrrolic
positions adjacent to aryl rings bearing electron donor or
acceptor groups reduces electron transfer rate constants by a
factor of ∼1/5.45
Benzyl 5-(Hydroxymethylphenyl)pyrrole-2-carboxylate (7). A
solution of 11.5 g (0.05 mol) of 6 in 200 mL of tetrahydrofuran
was cooled to 0 °C under a nitrogen atmosphere, and 50 mL
(0.10 mol) of phenylmagnesium chloride (2M) in tetrahydro-
furan was added dropwise over 10 min. The reaction mixture
was stirred for 15 min at 0 °C and poured into a mixture of ice,
citric acid, and dichloromethane. The mixture was stirred until
the ice melted, the organic layer was separated, and the aqueous
phase was extracted twice with dichloromethane. The combined
Experimental Section
Synthesis. Porphyrin 1 (5,10,15,20-tetraphenylporphyrin)
was purchased from Aldrich Chemical Co., treated with 2,3-

Aryl Ring Rotation in Porphyrins
J. Phys. Chem. B, Vol. 101, No. 3, 1997 463
extracts were washed with water and aqueous sodium bicarbon-
ate, and the resulting solution was dried over anhydrous sodium
sulfate and filtered. Distillation of the solvent under vacuum
gave 14.31 g of 7 as a pale yellow solid (93% yield): 1H NMR
(300 MHz, CDCl3) δ 2.77 (1H, d, J ) 4 Hz, OH), 5.27 (2H, s,
CH2), 5.87 (1H, d, J ) 4 Hz, 4-CH), 5.96 (1H, dd, J ) 3, 4 Hz,
CH), 6.88 (1H, dd, J ) 3, 4 Hz, 3-CH), 7.33 (10H, m, Ar2-H,
Ar5-H), 9.47 (1H, br s, NH); MS m/z 307 (M+).
2′-H), 6.21 (1H, dd, J ) 16.5, 1 Hz, 1′-CH), 8.68 (1H, br s,
NH); MS m/z 249 (M+).
Ethyl 3,5-Dimethyl-4-hexylpyrrole-2-carboxylate (11).
A
mixture of 10 (5.0 g, 20 mmol) and 0.50 g of 5% palladium on
carbon in 80 mL of ethyl acetate was stirred under 50 psi of
hydrogen for 24 h. The catalyst was removed by filtration and
washed with dichloromethane/methanol (9:1). The solvent was
distilled from the combined filtrates at reduced pressure to give
11 (5.0 g, 99% yield): 1H NMR (300 MHz, CDCl3) δ 0.88
(3H, t, J ) 7 Hz, 6′-CH3), 1.29 (3H, m, 2-CH3), 1.37 (8H, m,
2′-CH2 3′-CH2 4′-CH2 5′-CH2), 2.19 (3H, s, 5-CH3 or 3CH3),
2.26 (3H, s, 3-CH3 or 5-CH3), 2.34 (2H, t, J ) 7 Hz, 1′-H),
4.29 (2H, q, J ) 7 Hz, 2- CH2), 8.56 (1H, br s, NH); MS m/z
251 (M+).
1,9-Bis(carbobenzoxy)-5-phenyldipyrromethane (8). To a
stirred mixture of 7 (3.10 g, 10.0 mmol) and 4 (2.1 g, 10.0
mmol) in 60 mL of dichloromethane under a nitrogen atmo-
sphere was added 1.90 g (10.0 mmol) of p-toluenesulfonic acid.
After stirring the mixture for 30 min, it was diluted with
dichloromethane, washed with water, washed with aqueous
sodium bicarbonate, and dried over anhydrous sodium sulfate.
The solvent was removed from the resulting solution by
distillation at reduced pressure, and the residue was chromato-
graphed on silica gel (hexane/ethyl acetate, 83:17) to give 4.25
g of 8 (87% yield): 1H NMR (300 MHz, CDCl3) δ 5.19 (4H, s,
1-CO2CH2, 9-CO2CH2), 5.46 (1H, s, 5-CH), 5.88 (2H, m, 2-H,
8-H or 3-H, 7-H), 6.80 (2H, m, 3-H, 7-H or 2-H, 8-H), 7.34
(15H, m, Ar-H), 9.77 (2H, br s, NH); MS m/z 490 (M+).
Benzyl 3,5-Dimethyl-4-hexylpyrrole-2-carboxylate (12). A
mixture of benzyl alcohol (800 mL) and sodium metal (0.64 g,
28 mmol) was stirred under nitrogen until all the sodium had
reacted, and 70.0 g (0.278 mol) of 11 was then added. The
solution was stirred at 90 °C under a pressure of 16 mmHg for
19 h. The solution was cooled, and 24 mL of acetic acid was
added. Distillation of the solvent at reduced pressure gave a
white solid, which was dissolved in dichloromethane. The
solution was washed with water, dried over anhydrous sodium
sulfate, and filtered, and the solvent was removed by distillation
at reduced pressure. Recrystallization of the residue from
dichloromethane/hexane yielded 12. Additional material was
obtained by chromatography of the residue in the mother liquor
on silica gel (dichloromethane/hexane, 3:1) to yield a total of
84.1 g of 12 (96% yield): 1H NMR (300 MHz, CDCl3) δ 0.85
(3H, br t, J ) 7 Hz, 6′-CH3), 1.25 (6H, s, 3′-CH2 4′-CH2 5′-
CH2), 1.37 (2H, m, 2′-CH2), 2.15 (3H, s, 5-CH3 or 3-CH3), 2.25
(3H, s, 3-CH3 or 5-CH3), 2.31 (2H, t, J ) 7 Hz, 1′-CH2), 5.26
(2H, s, CO2CH2), 7.35 (5H, m, Ar-H), 8.52 (1H br s, NH); MS
m/z 313 (M+).
5-Phenyldipyrromethane (9). A mixture of 8 (5.00 g, 10.2
mmol) and 0.5 g of 10% palladium on carbon in 100 mL of
tetrahydrofuran was stirred under an atmosphere of hydrogen
for 14 h. The catalyst was removed by filtration through Celite,
and the filtrate was concentrated by evaporation of the solvent
under reduced pressure. The resulting solid was dissolved in a
solution of sodium hydroxide (8.2 g, 0.2 mol) in 100 mL of
ethylene glycol, and the solution was heated at 185 °C under a
nitrogen atmosphere for 1 h. The cooled solution was mixed
with toluene and water, and the aqueous phase was extracted
with toluene four times. The combined organic extracts were
washed with aqueous sodium chloride, and the toluene was
distilled at reduced pressure to produce 9, which was used in
Benzyl 4-Hexyl-5-carbomethoxy-3-methylpyrrole-2-carboxy-
late (13). A solution of 600 mL of carbon tetrachloride and
31.35 g (0.100 mol) of 12 was stirred and cooled to 0 °C under
a nitrogen atmosphere. A 32.1 mL portion (0.400 mol) of
sulfuryl chloride was added dropwise at a rate of 8.8 mL/h.
After completion of the addition, the reaction mixture was stirred
for 8 h, the solvent was distilled under vacuum, and residual
liquids were removed by codistillation with 200 mL of benzene.
The resulting yellow-orange oil was dissolved in 500 mL of
methanol, and 100 g of sodium acetate was added. The
suspension was warmed at 50 °C under a nitrogen atmosphere
for 5 h, and the reaction mixture was mixed with dichlo-
romethane and water and aqueous sodium bicarbonate. The
organic layer was separated, dried over anhydrous sodium
sulfate, and filtered, and the solvent was removed by distillation
at reduced pressure. Chromatography of the residue on silica
gel (hexane/ethyl acetate, 85:15) gave 24.75 g of 13 (69%
yield): 1H NMR (300 MHz, CDCl3) δ 0.88 (3H, t, J ) 7 Hz,
6′-CH3), 1.30 (6H, br s, 3′-CH2, 4′-CH2, 5′-CH2), 1.46 (2H, m,
2′-CH2), 2.28 (3H, s, 3-CH3), 2.71 (2H, t, J ) 7 Hz, 1′-CH2),
3.87 (3H, s, CO2CH3), 5.33 (2H, s, CO2CH2), 7.38 (5H, m, Ar-
H), 9.35 (1H, br s, NH); MS m/z 357 (M+).
1
later reactions without further purification: H NMR (300 MHz,
CDCl3) δ 5.47 (1H, s, 5-H), 5.93 (2H, m, 1-H, 9-H or 3-H,
7-H), 6.17 (2H, m, 2-H, 8-H), 6.70 (2H, m, 3-H, 7-H or 1-H,
9-H), 7.28 (5H, m, Ar-H), 7.91 (2H, br s, NH); MS m/z 222
(M+).
Ethyl 3,5-Dimethyl-4-(1-hexenyl)pyrrole-2-carboxylate (10).
A 120 mL portion (0.24 mol) of pentylmagnesium bromide in
diethyl ether was added dropwise to a solution of 20 g (0.10
mol) of ethyl 3,5-dimethyl-4-formylpyrrole-2-carboxylate in 250
mL of tetrahydrofuran under a nitrogen atmosphere and cooled
in an ice bath. When addition was complete, the mixture was
stirred at ambient temperature for 30 min and poured over ice.
The layers were separated, and the aqueous layer was extracted
four times with 120 mL portions of diethyl ether. The solvent
was distilled from the combined extracts at reduced pressure to
yield crude ethyl 3,5-dimethyl-4-(1-hydroxyhexyl)pyrrole-2-
carboxylate, which was dissolved in 250 mL of dichlo-
romethane. This solution was mixed with 150 mL of 3 M
hydrochloric acid and stirred for 25 min. The organic layer
was separated, and the aqueous phase was washed twice with
100 mL portions of dichloromethane. The combined organic
extracts were washed with aqueous sodium bicarbonate, dried
over anhydrous sodium sulfate, and filtered. Evaporation of
the solvent at reduced pressure yielded a pale yellow solid,
which was recrystallized from methanol to give 10 in 91%
yield: 1H NMR (300 MHz, CDCl3) δ 0.93 (3H, t, J ) 7 Hz,
6′-CH3), 1.35 (3H, t, J ) 7 Hz, 2-CH3), 1.42 (4H, m, 4′-CH2,
5′-CH2 or 3′-CH2), 2.19 (2H, m, 3′-CH2 or 4′-CH2 or 5′-CH2),
2.29 (3H, s, 5-CH3 or 3-CH3), 2.36 (3H, s, 3-CH3 or 5-CH3),
4.30 (2H, q, J ) 7 Hz, CO2CH2), 5.74 (1H, td, J ) 16.5, 7 Hz,
Methyl 3-Hexyl-4-methylpyrrole-2-carboxylate (14). A mix-
ture of 40.0 g (0.112 mol) of 13 and 2 g of 10% palladium on
charcoal in 400 mL of tetrahydrofuran was stirred under a
positive hydrogen atmosphere for 5 h. The catalyst was
removed by filtration through Celite and washed with a mixture
of tetrahydrofuran and methanol (4:1). Distillation of the solvent
from the combined filtrates at reduced pressure gave 4-hexyl-
5-carbomethoxy-3-methylpyrrole-2-carboxylic acid, which was
dissolved in 400 mL of water containing 18.23 g (0.217 mol)

464 J. Phys. Chem. B, Vol. 101, No. 3, 1997
Noss et al.
of sodium bicarbonate. The solution was heated to 70 °C, 350
mL of 1,2-dichloroethane was added, and the mixture was stirred
as 34.4 g (0.136 mol) of iodine was added in small portions.
When addition was complete, the solution was heated at reflux
for 40 min. After treating the cooled reaction mixture with
aqueous sodium bisulfite, the organic layer was separated and
the aqueous phase extracted with dichloromethane. The com-
bined organic extracts were washed with an aqueous solution
of sodium bicarbonate and sodium bisulfite, dried over anhy-
drous sodium sulfate, and filtered. The solvent was distilled
from the filtrate at reduced pressure to yield a yellow fibrous
material, which was added to a mixture of 500 mL of methanol,
75 g of sodium acetate, and 70 mg of PtO2. The suspension
was stirred under a positive pressure of hydrogen gas for 21 h.
The solvent was distilled under reduced pressure, and the residue
dissolved in a mixture of water and dichloromethane. The
organic layer was separated, dried over sodium sulfate, and
filtered, and the solvent was distilled under reduced pressure.
Chromatography of the residue on silica gel (hexane/diethyl
ether 85:15) gave 14 in quantitative yield based on 13: 1H NMR
(300 MHz, CDCl3) δ 0.89 (3H, br t, J ) 7 Hz, 6′-CH3), 1.32
(6H, br s, 3′-CH2, 4′-CH2, 5′-CH2), 1.50 (2H, m, 2′-CH2), 2.03
(3H, s, 4-CH3), 2.72 (2H, t, J ) 8 Hz, 1′-CH2), 3.8 (3H, m,
CO2CH3), 6.66 (1H, d, J ) 2 Hz, CH), 8.27 (1H, br s, NH);
MS m/z 223 (M+).
After stirring the mixture for 20 min, it was poured over a
mixture of ice, diethyl ether, and citric acid and stirred. The
organic layer was separated, and the aqueous phase extracted
twice with 80 mL of diethyl ether. The combined organic
extracts were washed with aqueous sodium bicarbonate and
aqueous sodium chloride, dried over sodium sulfate, and filtered,
and the solvent was distilled at reduced pressure to give 7.85 g
1
of 17 as a white solid (96% yield): H NMR (300 MHz, CDCl3)
δ 0.85 (3H, t, J ) 7 Hz, 6′-CH3), 1.22 (6H, m, 3′-CH2, 4′-CH2,
5′-CH2), 1.41 (2H, m, 2′-CH2), 1.88 (3H, s, 4-CH3), 2.38 (1H,
d, J ) 3 Hz, OH), 2.65 (2H, m, 1′-CH2), 5.25 (1H, d, J ) 8
Hz, CO2CHH), 5.26 (1H, d, J ) 8 Hz, CO2CHH), 5.90 (1H, d,
J ) 3 Hz, CH), 7.35 (10H, m, Ar-H), 8.97 (1H, br s, NH); MS
m/z 405 (M+).
1,9-(Dibenzyloxycarbonyl)-2,8-dihexyl-3,7-dimethyl-5-phenyl-
dipyrromethane (18). To a stirred solution of 3.00 g (10.0
mmol) of 15 and 4.06 g (10.0 mmol) of 17 in 60 mL of
dichloromethane was added 1.9 g (10.0 mmol) of p-toluene-
sulfonic acid. The pink solution was stirred for 30 min, diluted
with dichloromethane, washed with aqueous sodium bicarbonate
and aqueous sodium chloride, dried over sodium sulfate, and
filtered. The solvent was distilled at reduced pressure, and the
residue was chromatographed on silica gel (hexane/ethyl acetate,
85:15) to give 6.56 g of 18 (95% yield): 1H NMR (300 MHz,
CDCl3) δ 0.85 (6H, t, J ) 6 Hz, 6′′-CH3, 6′-CH3), 1.22 (12H,
m, 3′′-CH2, 3′-CH2, 4′′-CH2, 4′-CH2, 5′′-CH2, 5′-CH2), 1.43 (4H,
m, 2′′-CH2, 2′-CH2), 1.76 (6H, s, 3-CH3, 7-CH3, 2.66 (4H, m,
1′′-CH2, 1′-CH2), 5.23 (4H, s, 1-CO2CH2, 9-CO2CH2), 5.47 (1H,
s, 5-CH), 7.08-7.35 (15H, m, 1Ar-H, 5Ar-H, 9Ar-H), 8.25 (2H,
br s, NH); MS m/z 686 (M+).
Benzyl 3-Hexyl-4-methylpyrrole-2-carboxylate (15). Metallic
sodium (0.23 g, 0.010 mol) was dissolved in 500 mL of benzyl
alcohol, and 23 g (0.10 mol) of 14 was added. The solution
was heated at 90 °C for 15 h under the vacuum from an
aspirator. The solvent was distilled under reduced pressure and
the residue was chromatographed on silica gel (hexane/ethyl
acetate, 85:15). The product was recrystallized from dichlo-
romethane and hexane to give 29.11 g of 15 (94% yield): 1H
NMR (300 MHz, CDCl3) δ 0.86 (3H, m, 6′-CH3), 1.25 (6H,
m, 3′-CH2, 4′-CH2, 5′-CH2), 1.46 (2H, m, 2′-CH2), 2.02 (3H, s,
3-CH3), 2.70 (2H, t, J ) 8 Hz, 1′-CH2), 5.29 (2H, m, CO2-
CH2), 6.65 (1H, d, J ) 3 Hz, CH), 7.37 (5H, m, Ar-H), 8.72
(1H, br s, NH); MS m/z 299 (M+).
1,9-Diformyl-2,8-dihexyl-3,7-dimethyl-5-phenyldi-
pyrromethane (19). A 6.0 g (8.73 mmol) portion of 18 and 0.6
g of 10% palladium on carbon were added to 100 mL of
tetrahydrofuran, and the suspension was stirred under a positive
pressure of hydrogen for 16 h. The catalyst was removed by
filtration through Celite, and the residue was washed with
tetrahydrofuran/methanol (4:1). The solvent was removed by
distillation at reduced pressure. The residue was dissolved in
50 mL of dimethylformamide, purged of oxygen with nitrogen
gas, and heated at reflux for 4 h. The reaction mixture was
cooled to 0 °C under a nitrogen atmosphere, and a Vilsmeier
reagent prepared from 4.07 mL (43.7 mmol) of POCl3 and 7.3
of mL dimethylformamide was added. The resulting mixture
was stirred at 0 °C for 20 min and at room temperature for 1 h.
The solution was diluted with 100 mL of 1,2-dichloroethane,
and aqueous sodium bicarbonate was added. After stirring the
mixture for 16 h, the organic layer was separated and the
aqueous phase extracted twice with dichloromethane. The
combined extracts were washed with aqueous sodium bicarbon-
ate and aqueous sodium chloride to yield 19 (3.60 g, 87%): 1H
NMR (300 MHz, CDCl3) δ 0.88 (6H, t, J ) 7 Hz, 6′′-CH3,
6′-CH3), 1.31 (12H, br s, 3′′-CH2, 4′′-CH2, 5′′-CH2, 3′-CH2, 4′-
CH2, 5′-CH2), 1.54 (4H, m, 2′′-CH2, 2′-CH2), 1.85 (6H, s,
7-CH3, 3-CH3), 2.66 (4H, m, 1′′-CH2, 1′-CH2), 5.58 (1H, s,
5-CH), 7.07-7.30 (5H, m, Ar-H), 9.27 (2H, br s, NH), 9.44
(2H, s, CHO); MS m/z 474 (M+).
Benzyl 5-Formyl-3-hexyl-4-methylpyrrole-2-carboxylate (16).
A Vilsmeier reagent was formed by adding 10.6 mL (0.114 mol)
of POCl3 dropwise to 24 mL of dimethylformamide at 0 °C
and stirring the resulting solution for 30 min. The reagent was
added dropwise to a solution of 17.0 g (0.057 mol) of 15 in
300 mL of 1,2-dichloroethane at 0 °C under a nitrogen
atmosphere. The mixture was stirred for 10 min, warmed to
room temperature, and stirred for an additional 30 min. The
solution was cooled in an ice bath, aqueous sodium acetate was
added, and the resulting mixture was stirred for 14 h. The
organic layer was separated and washed with aqueous sodium
bicarbonate, and the solvent was distilled at reduced pressure.
The residue was redissolved in diethyl ether, washed four times
with water, washed with aqueous sodium chloride, dried over
anhydrous magnesium sulfate, and filtered. The solvent was
removed by distillation at reduced pressure, and the resulting
yellow solid recrystallized from dichloromethane and hexane
1
to give 17.86 g of 16 (96% yield): H NMR (300 MHz, CDCl3)
δ 0.87 (3H, m, 6′-CH3), 1.25 (6H, m, 3′-CH2, 4′-CH2, 5′-CH2),
1.45 (2H, m, 2′-CH2), 2.30 (3H, s, 4-CH3), 2.69 (2H, t, J ) 8
Hz, 1′-CH2), 5.32 (2H, m, CO2CH2), 7.40 (5H, m, Ar-H), 9.44
(1H, br s, NH), 9.77 (1H, s, CHO); MS m/z 327 (M+).
2,8-Dihexyl-3,7-dimethyl-5,15-diphenylporphyrin (20). To a
mixture of 0.48 g (1.00 mmol) of 19 and 0.23 g (1.00 mmol)
of 9 in 350 mL of dichloromethane under a nitrogen atmosphere
was added 8.0 g of p-toluenesulfonic acid (azeotropically dried
three times with 200 mL portions of toluene) in 20 mL of
methanol. The dark red reaction mixture was stirred at room
temperature for 19 h, and 20 mL of methanol and excess zinc
acetate were added. The mixture was refluxed for 5 h and
stirred at room temperature for 14 h. After addition of 2,3-
Benzyl 3-Hexyl-5-(hydroxymethylphenyl)-4-methylpyrrole-2-
carboxylate (17). A solution of 16 (6.55 g, 0.020 mol) in 150
mL of tetrahydrofuran was stirred at 0 °C under a nitrogen
atmosphere, and 20 mL of a 2.0 M solution of phenylmagnesium
chloride in tetrahydrofuran (0.040 mol) was added dropwise.

Aryl Ring Rotation in Porphyrins
J. Phys. Chem. B, Vol. 101, No. 3, 1997 465
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dichloro-5,6-dicyanobenzoquinone (1.0 g, excess) the mixture
was stirred for an additional 5 h. The mixture was washed with
water, 10% hydrochloric acid, aqueous sodium bicarbonate (×3),
and aqueous sodium chloride. The solvent was distilled at
reduced pressure, and the residue chromatographed on silica
gel (dichloromethane/hexane, 2:1) to give 0.210 g of 20 (32%
yield): 1H NMR (300 MHz, CDCl3) δ -3.03 (1H, s, NH),
-2.54 (1H, s, NH), 0.91 (6H, t, J ) 7 Hz, 6′-CH3, 6′′-CH3),
1.37 (4H, m, 5′-CH2, 5′′-CH2), 1.48 (4H, m, 4′-CH2, 4′′-CH2),
1.72 (4H, m, 3′-CH2, 3′′-CH2), 2.16 (4H, m, 2′-CH2, 2′′-CH2),
2.47 (6H, s, 8-CH3, 12-CH3), 3.97 (4H, t, J ) 8 Hz, 1′-CH2,
1′′-CH2), 7.78 (6H, m, 20Ar3,4,5-H, 10Ar3,4,5-H), 8.08 (2H,
d, J ) 7 Hz, 10Ar2,6-H), 8.28 (2H, dd, J ) 4, 7 Hz, 20Ar2,6-
H), 9.07 (2H, d, J ) 4 Hz, 18-CH, 2-CH), 9.37 (2H, d, J ) 4
Hz, 17-CH, 3-CH), 10.24 (2H, s, 5-CH, 15-CH); MS (FAB)
m/z 659.4120 (calcd for (M+H)+, 659.4035); UV/vis (CH2Cl2)
406, 506, 538, 576, 630 nm. Porphyrin 20 was quantitatively
converted to zinc 2,8-dihexyl-3,7-dimethyl-5,15-diphenyl-
porphyrin (3) by stirring a dichloromethane solution with zinc
acetate and purifying the crude product by chromatography on
silica gel (dichloromethane/hexane, 2:1).
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I ) A + Bexp(-t/T1)
(11)
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is the peak intensity for the spectrum obtained at time t. To
verify the 90° pulse tip angle and other parameters, T1
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dioxane in D2O. The results were consistent with literature
reports.47
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Acknowledgment. This research was supported by a grant
from the Division of Chemical Sciences, Office of Basic Energy
Sciences, Office of Energy Research, U.S. Department of Energy
(DE-FG03-93ER14404). FAB mass spectrometric studies were
performed by the Midwest Center for Mass Spectrometry, with
partial support by the National Science Foundation (DIR9017262).
This is publication number 312 from the ASU Center for the
Study of Early Events in Photosynthesis.
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