Synthesis and spectroscopy of a series of substituted N-confused tetraphenylporphyrins

Article abstract of DOI:10.1021/jo049199e

A series of N-confused tetraphenylporphyrins (H₂NCTPPs) with substituents on either the paraor the 3,5-positions of the meso phenyl rings were prepared using Lindsey conditions. Both electron-withdrawing and electron-donating groups were chosen in order to probe the effects of peripheral substitution on the properties of the macrocycles. The series includes 5,10,15,20-tetra-(4-R-phenyl) N-confused porphyrins (where R = bromo (1), iodo (2), cyano (3), methoxy (4), 2′,5′-dimethoxyphenyl (5), or ethynyl (6)) and 5,10,15,20-(3,5-di-ieri-butylphenyl) N-confused porphyrin (7). Absorption and steady-state fluorescence measurements were carried out, and quantum yields were measured for all compounds in both dichloromethane (CH ₂Cl₂) and dimethylacetamide (DMAc).

Full text of DOI:10.1021/jo049199e

Syn th esis a n d Sp ectr oscop y of a Ser ies of Su bstitu ted N-Con fu sed
Tetr a p h en ylp or p h yr in s
†
†,‡
†,‡
,†
J anet L. Shaw, Shana A. Garrison, Elvin A. Alem a´ n, Christopher J . Ziegler,* and
David A. Modarelli*,
†,‡
Department of Chemistry and The Center for Laser and Optical Spectroscopy,
Knight Chemical Laboratory, The University of Akron, Akron, Ohio 44325-3601
dam@chemistry.uakron.edu; ziegler@uakron.edu
Received May 12, 2004
2
A series of N-confused tetraphenylporphyrins (H NCTPPs) with substituents on either the para-
or the 3,5-positions of the meso phenyl rings were prepared using Lindsey conditions. Both electron-
withdrawing and electron-donating groups were chosen in order to probe the effects of peripheral
substitution on the properties of the macrocycles. The series includes 5,10,15,20-tetra-(4-R-phenyl)
N-confused porphyrins (where R ) bromo (1), iodo (2), cyano (3), methoxy (4), 2′,5′-dimethoxyphenyl
(
5), or ethynyl (6)) and 5,10,15,20-(3,5-di-tert-butylphenyl) N-confused porphyrin (7). Absorption
and steady-state fluorescence measurements were carried out, and quantum yields were measured
for all compounds in both dichloromethane (CH Cl ) and dimethylacetamide (DMAc).
2
2
In tr od u ction
continues to be crucial to understanding their role in
biological systems.
Porphyrinic macrocycles are found in a variety of
natural systems, including the light harvesting com-
plexes1 and photosynthetic reaction centers in green
plants, where they assist in light harvesting and initiate
the charge separation process, and heme-based metallo-
enzymes such as hemoglobin and myoglobin, where they
Unlike normal porphyrins, the chemistry of porphyrin
isomers such as N-confused tetraphenylporphyrins
-3
(H
2
NCTPPs) (also called 2-aza-21-carba-5,10,15,20-tetra-
arylporphyrin and inverted tetraphenylporphyrin) has
yet to be fully explored. H NCTPP differs structurally
from tetraphenylporphyrin (H TPP) by inversion of one
2
2
4
bind and transport oxygen for metabolic needs. Free-
pyrrole ring, resulting in three nitrogens and one carbon
at the macrocycle core, and the inverted nitrogen at a
â-position at the periphery of the macrocycle. N-Confused
porphyrins can exist as two tautomers in solution de-
pending on the polarity of the solvent. In polar solvents
such as DMF or dimethylacetamide (DMAc), the exter-
nally protonated tautomer is more stable, presumably
due to hydrogen bonding or dipole-dipole interactions
between the external N-H bond and the solvent. How-
base and metalloporphyrin derivatives have been exten-
sively studied in order to understand and reproduce these
natural processes outside of the protein environment.5
The effects of substitution at the macrocycle periphery
on the photophysical properties of porphyrins have also
been thoroughly investigated. Deviations from planarity
caused by bulky substituents,6 changes in electronic
interactions due to the donor/acceptor properties of the
substrate, and the position of the substituent have all
ever, in nonpolar solvents such as chloroform or CH
the internally protonated tautomer is more stable. Figure
shows both N-confused porphyrin tautomers in com-
2 2
Cl ,
7
been shown to affect properties such as redox potentials,
absorption spectra8 and fluorescence quantum yields.9
Investigating the fundamental chemistry and photo-
physical processes of porphyrinic macrocycles, therefore,
1
parison to porphyrin.
The structural changes in the macrocycle leads to a
decrease in symmetry and a reduction in aromaticity as
†
Department of Chemistry.
The Center for Laser and Optical Spectroscopy.
‡
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3326-3328. (c) Kadish, K. M.; Lin, M.; Caemelbecke, E. V.; De Stefano,
G.; Medforth, C. J .; Nurco, D. J .; Nelson, N. Y.; Krattinger, B.; Muzzi,
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1
0.1021/jo049199e CCC: $27.50 © 2004 American Chemical Society
Published on Web 09/30/2004
J . Org. Chem. 2004, 69, 7423-7427
7423

Shaw et al.
F IGURE 1. Structures i and e are the internally (nonpolar
solvents) and externally (polar solvents) protonated tautomers
of N-confused porphyrin; freebase porphyrin (H
shown for comparison.
2
P, right) is
F IGURE 2. Structures of the N-confused porphyrins exam-
ined in this paper. Only the externally protonated form of each
tautomer is shown.
compared to H
that compare favorably to chlorins
Both tautomeric forms have absorption spectra that are
TPP,10 as well as photophysical properties
2
1
1,12
13
and porphyrins.
the para-position or 3,5-positions of the meso phenyl rings
2
different from that of tetraphenylporphyrin (H TPP) and
(Figure 2). The role of the substituent on purification and
are characterized by red-shifted Soret and Q-bands.1
4,15
final synthetic yield will be discussed. We have also
probed the effects of the substituent in both tautomers
of the macrocycle on properties such as absorption and
fluorescence and compared the results to unsubstituted
The red-shifted absorption maxima are attributed to a
break in the degeneracy of the e orbitals that results
g
from a decrease in symmetry within the macrocycle.1
The excited state properties of each tautomer are differ-
ent from one another, with the internally protonated
tautomer exhibiting a lower excited state energy and
fluorescence quantum yield than the externally pro-
5
H
2
NCTPP.
Resu lts a n d Discu ssion
1
5,16
tonated tautomer.
The fluorescence lifetimes of both
tautomers are comparable.15 These properties indicate
that N-confused porphyrins are viable alternatives to
Syn th esis. While the synthesis of H NCTPP is known
2
to be acid catalyzed,22 the mechanism of the reaction has
porphyrins and the difficult to prepare chlorins1
7,18
yet to be fully elucidated. Possible mechanisms in the
literature include anion templating23 and the presence
of multiple orientations of tetrapyrromethane link-
ages that allow for ring closure by electrophilic attack at
either the R- or â-pyrrole positions.24 Typical yields
in
artificial light-harvesting systems and photonic arrays.1
9-21
In this paper, we report the synthesis of a series of
seven H NCTPPs, where both electron-donating and
electron-withdrawing groups have been substituted at
2
(
2
isolated) for the synthesis of unsubstituted H NCTPP
2
5
using Lindsey conditions are around 35%. For the seven
compounds synthesized here, isolated yields ranged from
(
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966; pp 119-143.
7
424 J . Org. Chem., Vol. 69, No. 22, 2004

Substituted N-Confused Tetraphenylporphyrins
TABLE 1. Su m m a r y of th e Syn th etic Da ta for
N-Con fu sed P or p h yr in s 1-7 a n d H2NCTP P , In clu d in g %
Yield of N-Con fu sed P or p h yr in a n d % Yield of Regu la r
P or p h yr in
those of H
DMAc (Table 2). Similarly, the Q-band absorptions
Figure 3) are red-shifted to varying degrees, with the
amount depending upon both the substitution and Q-
band transition. Interestingly, the magnitude and range
of red-shifts in the Q-band region varies significantly
more in CH Cl than in DMAc, consistent with the out
2 2
of plane distortion of the inverted pyrrole ring having
an important impact on the interaction of the substituent
NCTPP by 3-7 nm in CH
2 2
Cl and 3-6 nm in
2
(
%
yield of
% yield of
regular
porphyrin
N-confused
porphyrin
%
N-confused
porphyrin
isolated yield
1
2
3
4
5
6
7
13
8
15
11
12
15
30
12
28
11
8.0
6.9
1.5
4.8
10
6.7
13
26
9.2
10
2
with the ring. Minor deviations from H NCTPP are also
observed for the extinction coefficient values, although
most differences are on the order of a factor of 2.
3.2
a
a
<10a
Notable exceptions include compound 3 where the
presence of the cyano group results in a bathochromic
shift of 18 nm of the Soret band in DMAc (460 nm) from
H2NCTPP
35
40
a
Taken from ref 25.
that of H
2
NCTPP (442 nm), whereas in CH
2
Cl
2
the band
NCTPP).
is only shifted to 444 nm (438 nm for H
2
1
0% (Table 1). No obvious trends between yield (crude
Similarly, the Soret band of 5 is significantly shifted in
DMAc (459 nm) but only 445 nm in CH Cl . In addition,
the extinction coefficients of the Soret and Q-bands for 5
and 6 are also unusually large compared to H NCTPP.
In CH Cl , 5 has a Soret band extinction coefficient of
or isolated) and electronic structure are found, although
it is tempting to cite the bulk of the para substituent as
a factor in the N-confused porphyrin yield. For example,
N-confused porphyrins 5 and 7, which have 2,5-dimethoxy-
benzene and 3,5-di-tert-butyl substituents respectively,
have larger crude yields by a factor of 2-3 than 1-4 and
2
2
2
2
2
5
4
3
.60 × 10 and a Q
x
(0,0) coefficient of 3.31 × 10 , and 6
5
has a Soret band extinction coefficient of 2.81 × 10 and
6
. These yields are nonetheless lower than those observed
4
NCTPP,25 and it is difficult from the synthetic data
a Q (0,0) coefficient of 2.40 × 10 , all significantly higher
for H
2
x
2
than those observed for H NCTPP.
presented here to provide a definitive comment on the
structure-yield relationship.
F lu or escen ce Sp ect r oscop y. The fluorescence
spectra15 of N-confused porphyrins H
NCTPP(i) and
NCTPP(e) are characterized by emission bands red-
shifted from those of H TPP (Figure 4). The fluores-
cence quantum yield (ΦFl) and fluorescence lifetime (τFl
also differ from each other and from H TPP, with the
NCTPP(e) greater than that of
Absor p tion Sp ectr oscop y. The absorption spectra
2
NCTPP are tautomer-dependent.1
4,15
In DMAc,
H
2
of H
2
H
2
NCTPP(e) is observed to have a Soret band red-shifted
to 442 nm from that of H TPP (417 nm). The Q-bands
are also observed to be red-shifted, with the Q (0,0) band
at 699 nm. Similarly, the Soret band for H NCTPP(i) in
CHCl absorbs at 438 nm and the low-energy Q (0,0)
band is at 724 nm. Because the MOs for H NCTPP are
2
2
)
x
2
2
quantum yield for H
2
3
x
H
2
NCTPP(i) by a factor of 2 (ΦFl ) 0.0460 vs ΦFl
)
1
6
2
0.0227, respectively), and a slightly longer-lived fluo-
rescence lifetime (i.e., 1.98 vs 1.60 ns,15 respectively).
These minor differences have been attributed1 to a more
rapid rate of internal conversion or intersystem crossing
slightly distorted from the normal Goutermann four-
electron four-orbital model by the decrease in symmetry
in the macrocycle,15 we anticipated observing absorption
bands in 1-7 that were shifted further still from both
5
2
in H NCTPP(i) that results from the presence of the extra
H
2
TPP and H
In general, the Soret band absorptions for the substi-
tuted H NCTPPs prepared here are red-shifted from
2
NCTPP.
hydrogen atom in the interior of the A ring in the
macrocycle that leads to a nonplanar geometry and
favorable steric conditions for deformation.
2
TABLE 2. Su m m a r y of Absor p tion Da ta Obta in ed for Com p ou n d s 1-7, H2NCTP P , a n d H2TP P
Soret (nm)
Q-bands (nm)
5
-1
(ꢀ × 10 M cm-1)
4
-1
compound
solvent
(ꢀ × 10 M cm-1
)
1
1
2
2
3
3
4
4
5
5
6
6
7
7
i
e
i
e
i
e
i
e
i
e
i
e
i
e
CH2Cl2
DMAc
CH2Cl2
DMAc
CH2Cl2
DMAc
CH2Cl2
DMAc
CH2Cl2
DMAc
CH2Cl2
DMAc
CH2Cl2
DMAc
441 (1.21)
447 (1.33)
442 (1.16)
447 (1.06)
444 (1.06)
460 (0.807)
441 (1.52)
445 (1.08)
445 (3.60)
459 (1.92)
444 (2.81)
448 (1.33)
441 (2.13)
445 (0.832)
438 (1.59)
442 (1.19)
419 (4.11)
417 (4.85)
542 (0.971)
552 (0.302)
543 (1.08)
553 (0.295)
543 (1.00)
550 (0.0800)
548 (0.819)
550 (0.214)
546 (1.54)
556 (0.467)
546 (1.41)
554 (0.338)
543 (0.976)
547 (0.302)
539 (0.78)
550 (0.2)
584 (1.35)
597 (0.659)
586 (1.23)
596 (0.536)
587 (1.25)
599 (0.267)
592 (1.53)
599 (0.548)
591 (3.92)
601 (0.928)
589 (2.89)
601 (0.660)
585 (1.53)
599 (0.632)
580 (1.08)
595 (0.61)
550 (0.81)
548 (0.967)
668 (0.0516)
646 (1.06)
667 (0.265)
646 (0.978)
658 (0.183)
646 (0.387)
682 (0.0504)
651 (0.774)
677 (0.512)
651 (1.30)
675 (0.455)
650 (1.09)
672 (0.270)
650 (1.04)
665 (0.27)
644 (0.95)
590 (0.64)
591 (0.669)
728 (1.12)
699 (1.36)
729 (0.804)
701 (1.24)
726 (0.993)
699 (0.600)
736 (1.29)
709 (1.01)
732 (3.31)
706 (1.57)
731 (2.40)
705 (1.41)
728(1.34)
704 (1.36)
724 (1.04)
699 (1.24)
645 (0.62)
646 (0.605)
a
H2NCTPP
H2NCTPP
H2TPP
CHCl3
a
DMAc
b
CHCl3
515 (1.73)
513 (2.08)
c
H2TPP
DMAc
a
Taken from ref 15. b Taken from: Dudic, M.; Lhot a´ k, P.; Kr a´ l, V.; Long, K.: Stibor, I. Tetrahedron Lett. 1999, 40, 5949-5952. c Taken
from: Datta-Gupta, N.; Malakar, D.; Rice, L.; Rivers, S. J . Heterocycl. Chem. 1987, 24, 629-632.
J . Org. Chem, Vol. 69, No. 22, 2004 7425

Shaw et al.
2 2 2
F IGURE 3. Expanded Q-band region in the absorption spectra of 1-7 and H NCTPP in CH Cl (left) and DMAc (right).
2 2
F IGURE 4. Fluorescence spectra of 1-7 in CH Cl (left) and DMAc (right).
TABLE 3. Su m m a r y of F lu or escen ce Da ta for
N-Con fu sed P or p h yr in s 1-7, H2NCTP P , a n d
Tetr a p h en ylp or p h yr in s H2TP P
The steady-state fluorescence spectra of 1-7 have
significantly red-shifted emission bands relative to those
2 2
of both H TPP and H NCTPP, consistent with the red-
shifted Soret and Q-bands observed in the absorption
spectra (Figure 4, Table 3). The fluorescence spectra of
fluorescence
Stokes
maxima (nm)a
ΦFLb
shift (cm-1)
compound
solvent
H2NCTPPic
CH2Cl2
DMAc
CH2Cl2
DMAc
CH2Cl2
DMAc
CH2Cl2
DMAc
CH2Cl2
DMAc
CH2Cl2
DMAc
CH2Cl2
DMAc
CH2Cl2
DMAc
CH2Cl2
DMAc
744, 815
713, 783
744, 818
713, 795
747, 809
718, 784
744, 823
717, 777
754, 827
730, 789
751, 825
726, 789
749, 822
722, 788
745, 823
720, 783
649, 713
649, 714
0.023
0.046
0.0050
0.020
0.0010
0.0073
0.020
0.035
0.026
0.031
0.026
0.042
0.023
0.049
0.033
0.065
0.11
371
281
295
281
331
337
333
400
324
406
346
410
329
334
313
316
143
95
1
∼
-7 are dominated by large Q
741-756 nm (in CH Cl
) and ∼715-728 nm (in DMAc)
and low-energy Q
(0,1) shoulders at ∼809-827 nm
CH Cl
) and ∼777-789 nm (DMAc). The ratios of the
areas of the Q (0,0) and Q (0,1) emission bands are
significantly larger than those of H TPP and are similar
to the ratios for the H NCTPP Q (0,0) and Q (1,0)
x
(0,0) emission bands at
c
H2NCTPPe
2
2
c
1
i
1ec
x
(
2
2
2i
2
3
3
4
e
i
e
i
x
x
2
2
x
x
absorption bands, a result that has been attributed to
4e
5i
5e
changes in the ground state T Q-state excited-state
transitions.15
6
6
7
i
e
i
Differences between the two tautomers are also ob-
served in the fluorescence quantum yield data obtained
for H
2
NCTPPs 1-7, with the ΦFl values smaller for
7e
H2TPP
H2TPP
tautomers 1i-7i. The presence of the extra hydrogen
atom at the macrocycle core of these tautomers leads to
distortion of the inverted pyrrole ring out of the plane of
the porphyrin macrocycle. Such a distortion provides a
mechanism for enhanced nonradiative decay by either
0.15
a
Excited at the Soret bands to avoid aggregation due to
concentration effects. b Relative to the fluorescence of H2TPP.
c
Quantum yields were calculated using standard methods. Taken
from ref 16.
1
5
internal conversion or intersystem crossing. This effect
appears most pronounced for the halogen-containing
compounds 1 and 2, where intersystem crossing is further
enhanced by the heavy atom effect resulting in a signifi-
cant decrease in ΦFl for these H
2 2 2
NCTPPs in CH Cl
relative to the ΦFl values in DMAc.
7
426 J . Org. Chem., Vol. 69, No. 22, 2004

Substituted N-Confused Tetraphenylporphyrins
Substituent effects on the fluorescence quantum yield
values of the para-substituted compounds 3-6 are not
pronounced except for the aforementioned bromo- and
steady-state optical properties (absorption and fluores-
cence). Absorption spectra showed Soret and Q-bands for
1-7 that were red-shifted from those of H
CH Cl and DMAc. The magnitude of these red-shifts in
the Q-band region varied more in CH Cl , probably as a
2
NCTPP in both
iodo-containing H
these compounds in CH
.020-0.025 and increase to 0.031-0.049 in DMAc. For
the 3,5-di-tert-butyl-substituted H NCTPP 7, the ΦFl
2
NCTPPs. In general, the ΦFl values for
2
2
2
2
Cl range from approximately
2
2
0
result of the distortion from planarity in the macrocycle
that results from the inverted pyrrole ring. The fluores-
cence spectra also revealed red-shifted bands. The fluo-
rescence quantum yields were found to be smaller for 1-7
in CH Cl as a result of the ring distortion caused by the
2 2
extra hydrogen atom in the macrocycle core that leads
to enhanced rates of nonradiative decay.
2
values in both solvents are ∼50% larger than the para-
substituted compounds, indicative of the decrease in
torsional motion about the meso-phenyl C-C bond brought
about by the sterically demanding tert-butyl groups that
decreases nonradiative decay.
Substituent effects on the photophysical properties of
regular porphyrins are well-known.13 The data presented
here demonstrate that the photophysical properties of
N-confused porphyrins are not only dependent upon
substituent but also highly sensitive to the particular
tautomer in solution. This unique solvent dependence
makes them potentially very useful for incorporation into
devices and arrays, where changing the tautomer can
result in a significant change in the absorption energy
and/or oscillator strength, and in the case of fluorescence,
the fluorescence quantum yield.
Ack n ow led gm en t. J .L.S., S.A.G., and E.A.A. thank
the U.S. Department of Education for GAANN fellow-
ships. We thank D. Savant for collecting the 400 MHz
NMR data. The authors also acknowledge the National
Science Foundation (CHE-9977144) for funds used to
purchase the 400 MHz NMR instrument used in this
work. The mass spectrometers used by the Ohio State
Mass Spectrometry and Proteomics Facility were pur-
chased by a grant from the Hayes Investment Fund of
the Ohio Board of Regents.
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
cedures and spectroscopic details for the characterization ( H,
C and TOF-MS data) of N-confused tetraphenylporphyrins
-7. This material is available free of charge via the Internet
at http://pubs.acs.org.
Con clu sion
1
1
3
A series of N-confused tetraphenylporphyrins (1-7)
containing electron-withdrawing or electron-donating
substituents on the meso phenyl rings were prepared in
order to study substituent effects on synthetic yield and
1
J O049199E
J . Org. Chem, Vol. 69, No. 22, 2004 7427