Spectroscopy of protonated tetraphenylporphyrins with amino/carbomethoxy substituents: Hyperporphyrin effects and evidence for a monoprotonated porphyrin

Article abstract of DOI:10.1021/jo400742f

Spectrophotometric titrations for a full series of para-amino/carbomethoxy- substituted tetraphenylporphyrins were carried out using methanesulfonic acid in DMSO to study the hyperporphyrin effect across different substitution patterns. The series included zero, one, two (cis and trans), three, and four amino groups, with the remaining para substituents carbomethoxy groups. With increasing numbers of aminophenyl groups, the relative basicity increased and the hyperporphyrin effect increased, marked by a strong red band and a split Soret band. The cis diamino derivative showed a stronger hyperporphyrin effect compared to the trans isomer, which can be explained based on simple resonance forms. For the monoamino derivative, an initial increase in the Soret band upon acid titration along with well-defined isosbestic points provided evidence for a monoprotonated porphyrin, distinct from the diprotonated and triprotonated states. The relative stability of this unusual intermediate is proposed to be due to charge delocalization of the first cation to the single amino group and destabilization of the second protonation by the electron-withdrawing carbomethoxy substituents.

Full text of DOI:10.1021/jo400742f

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Article
Spectroscopy of Protonated Tetraphenylporphyrins with
Amino/Carbomethoxy Substituents: Hyperporphyrin
Effects and Evidence for a Monoprotonated Porphyrin
Alexander B. Rudine, Brian D. DelFatti, and Carl C. Wamser
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/jo400742f • Publication Date (Web): 10 May 2013
Downloaded from http://pubs.acs.org on May 16, 2013
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Spectroscopy of Protonated
Tetraphenylporphyrins with
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Amino/Carbomethoxy Substituents:
Hyperporphyrin Effects and Evidence for a
Monoprotonated Porphyrin
Alexander B. Rudine, Brian D. DelFatti, and Carl C. Wamser*
Department of Chemistry, Portland State University, Portland, OR 97207ꢀ0751
*wamserc@pdx.edu
RECEIVED DATE (to be automatically inserted after your manuscript is accepted
if required according to the journal that you are submitting your paper to)
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Table of Contents Graphic
Abstract
Spectrophotometric titrations for a full series of paraꢀamino/carbomethoxyꢀsubstituted
tetraphenylporphyrins were carried out using methanesulfonic acid in DMSO to study the
hyperporphyrin effect across different substitution patterns. The series included zero,
one, two (cis and trans), three, and four amino groups, with the remaining para
substituents carbomethoxy groups. With increasing numbers of aminophenyl groups, the
relative basicity increased and the hyperporphyrin effect increased, marked by a strong
red band and a split Soret band. The cis diamino derivative showed a stronger
hyperporphyrin effect compared to the trans isomer, which can be explained based on
simple resonance forms. For the monoamino derivative, an initial increase in the Soret
band upon acid titration along with wellꢀdefined isosbestic points provided evidence for a
monoprotonated porphyrin, distinct from the diprotonated and triprotonated states. The
relative stability of this unusual intermediate is proposed to be due to charge
delocalization of the first cation to the single amino group and destabilization of the
second protonation by the electronꢀwithdrawing carbomethoxy substituents.
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Introduction
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Porphyrins and their analogs are ubiquitous in nature, being found in systems as
diverse as the oxygenꢀbinding protein hemoglobin, the oxidizing enzyme class of
cytochrome P450 proteins, and the porphyrinoid molecule chlorophyll, the light absorber
in photosynthesis. Porphyrins are of synthetic interest for applications such as
photodynamic therapy,1 lightꢀemitting diodes,2 and for use in artificial photosynthetic
systems.3,4 In each of these applications there is a great deal of interest in understanding
and modulating porphyrin photophysical properties. In particular, for all the applications
mentioned and more, it is of special interest to understand and enhance the red and nearꢀ
infrared absorption of porphyrins.
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The typical porphyrin UVꢀvisible spectrum is characterized by a strong absorption
around 400 nm (Soret band) and two (metalloporphyrin) to four (freeꢀbase porphyrin)
less intense bands in the region between 450 and 700 nm (Q bands). These absorptions
are well explained by the Gouterman fourꢀorbital model, which describes the Soret and Q
bands as resulting from π ꢀ π* transitions from nearly degenerate HOMOs [a1u(π) and
a2u(π)] and two degenerate LUMOs [eg(π*)].5 Under various conditions the UVꢀvisible
spectrum of a porphyrin can show a broadened and/or split Soret band along with a
greatly enhanced, redꢀshifted absorption beyond where the Qꢀbands usually end. This
type of spectrum is characteristic of a hyperporphyrin, which is defined as a porphyrin
that exhibits prominent extra absorption bands in the region λ > 320 nm which are not
attributed to porphyrin π ꢀ π* transitions.5 Hyperporphyrinꢀtype spectra have been
reported in metalꢀtoꢀporphyrin charge transfer (CT) transitions6 and when appropriately
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substituted porphyrins are subjected to acidic7ꢀ11 or basic12ꢀ15 conditions. Oxidation of
porphyrins with electronꢀrich phenyl substituents also gives hyperporphyrin spectra.16 In
general, the extra absorption bands are attributed to π (substituted phenyl) ꢀ π*
(porphyrin) CT transitions.
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The protonation chemistry of the porphyrin ring has been well studied, and the
macrocycle exists in acidic media in three main forms: free base (P), monocation (PH+),
and dication (PH2+2).12,17ꢀ33 However, evidence for the monocation species has been
minimal.17,25,27ꢀ29,32,33 It has been suggested that the monocation is hard to observe
because the first protonation of the porphyrin ring requires a loss of planarity that makes
the second protonation even more favorable, such that little of the monoprotonated
species is present at any given time.27,32,34 In several cases UVꢀvisible absorption data
have indicated the existence of a monoprotonated species,22,35,36 and it has been
characterized as having three absorptions in the Q band region.19,22,36 NMR spectroscopy
has indicated the presence of a monocation in the case of a hydrocarbonꢀcapped
porphyrin.37 The crystal structures of the octaethylporphyrin and dodecaphenylporphyrin
monocations have also been presented.34,38
In the two reported cases of
tetraphenylporphyrin monocation species,34,35 either bulky, weakly coordinating anions
were needed to stabilize the monocation,35 or a saddleꢀdistorted porphyrin was generated
such that the planarity of the four interior ring nitrogens is broken, thus relieving the
steric destabilization of having three coplanar hydrogen atoms inside the macrocyclic
ring.34
Hyperporphyrins derived from acidic conditions (porphyrin dications) have been
studied
extensively
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tetrakis(pꢀaminophenyl)porphyrin (TAPP),11 a series of one, two (cis) and four
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dimethylaminophenyl/phenyl
porphyrins,8,9,39
and series
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trans disubstituted
tris(pentafluorophenyl)porphyrin,40
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of
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arylethynylporphyrins.7 However, to the best of our knowledge, a study of the
hyperporphyrin behavior for a full series of mixedꢀsubstituent porphyrins with both
electronꢀdonating and electronꢀwithdrawing groups has yet to be realized.
In this study we present the hyperporphyrin spectra for a full series of paraꢀsubstituted
tetraphenylporphyrins (Figure 1), having zero, one, two (cis and trans), three, and four
electronꢀdonating amino groups, with the other substituents being electronꢀwithdrawing
carbomethoxy groups. We will also provide spectroscopic evidence for the existence of a
monoprotonated hyperporphyrin intermediate and discuss reasons for the stability of the
monocation in this case.
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Figure 1. Structure of paraꢀsubstituted tetraphenylporphyrins with abbreviations and
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substitution patterns for the amino/carbomethoxyphenylporphyrins studied.
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Results
Porphyrin Absorption Spectroscopy. The freeꢀbase mesoꢀtetraarylporphyrin (P)
absorption spectrum is characterized by an intense Soret band at about 400 nm and four
smaller Q bands between 450 and 700 nm. Upon titration with acid to the dication
(PH2+2), the absorption spectrum of most mesoꢀtetraarylporphyrins typically show a redꢀ
shifted Soret band and a collapse from four Q bands to two, indicating an increase in
symmetry from twofold to fourfold. As a reference, the acid titration spectrum for
TCMPP is shown in Figure 2. On the other hand, if the mesoꢀtetraarylporphyrin is
substituted with strong electronꢀdonating substituents, when it is protonated to its dication
a hyperporphyrin spectrum is observed.
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Figure 2. Spectrophotometric titration of 3 ꢁM TCMPP with 0ꢀ5 mmol of MSA in
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DMSO. Addition of two protons gives the diprotonated porphyrin.
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1. TACM3PP → TACM3PPH3+3. The spectrophotometric titration of TACM3PP
(Figure 3) shows three distinct protonation stages, which we interpret to represent
formation of the monocation, dication, and trication (A, B, and C in Figure 3,
respectively). The first phase to form the monocation shows a wellꢀdefined isosbestic
point at 550 nm (inset of Figure 3A), an increase in the first Q band at 515 nm, and a new
broad hyperporphyrin absorption at 720 nm. Further evidence of a distinct transition
during this initial phase is the sharpening and strengthening of the Soret absorbance at
420 nm. To the best of our knowledge this is the first example of a Soret band increase in
the transition from a neutral porphyrin to its hyperporphyrin form. However, this
behavior has been observed in the oxidation of a neutral porphyrin to its monocation
form.36 This is the opposite of the typical changes observed as a hyperporphyrin is
formed, in which the Soret band diminishes as it splits to form a new lowꢀenergy
absorbance.
During the second phase of the titration (monocation to dication, Figure 3B) the Soret
absorbance decreases (420 nm) and the absorbance at 515 nm continues to increase
(characteristic of a hyperporphyrin split Soret absorbance), while the absorbance at 720
nm also steadily increases. The isosbestic point at 550 nm disappears. During the third
phase of the titration (dication to trication, Figure 3C) the Soret band shifts from 420 to
447 nm while the split Soret absorbance at 515 nm diminishes. The hyperporphyrin band
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at 720 nm disappears and is replaced by an absorbance at 660 nm that is characteristic of
a symmetric fully protonated porphyrin.
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The acid titrations were followed spectrophotometrically at three or more distinct
wavelengths to distinguish the relative concentrations of different species at different
points in the titrations, using the method of Splan7 as described in the Experimental
Methods section and illustrated in Figure 4. Apparent pKa values were obtained from the
xꢀintercepts at which the acid concentration was sufficient to generate equal
concentrations of both the acid and conjugate base forms. These are considered apparent
pKa values because of the nonaqueous solvent (DMSO) and the unidentified state of
ionization of MSA. Nevertheless, they are useful indicators of the relative basicity of the
different derivatives, which are tabulated in Table 2. The specific data for the three
protonation stages of TACM3PP are presented in detail in Figure 4 because of the special
interest in documenting the novel monoprotonated state.
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Figure 3. Spectrophotometric titration of 3 ꢁM TACM3PP with MSA in DMSO.
(A) addition of the first proton (0ꢀ24 ꢁmol MSA), (B) addition of the second proton (24ꢀ
150 ꢁmol MSA), and (C) addition of the third proton to give the fully protonated
porphyrin (0.15ꢀ5.0 mmol MSA).
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+n
Figure 4. Log([PHn+1+(n+1)]/[PHn ]) versus log [MSA] for mono, di, and triprotonated
TACM3PP (A, B, and C respectively). Data taken from the titrations shown in Figures 3
A, B, and C, respectively. Apparent pKa values are determined from the log[MSA]
+n
intercept where [PHn+1+(n+1)]/[PHn ] = 1.
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2. Trans → TransH4+4. The titration of the trans disubstituted derivative goes
through two distinct spectroscopic stages from neutral porphyrin to dication and from
dication to the fully protonated species TransH4+4 (Figures 5A and 5B, respectively).
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Figure 5. Spectrophotometric titration of 3 ꢁM Trans with MSA in DMSO.
(A) addition of two protons to form the hyperporphyrin (0ꢀ50 ꢁmol MSA), (B) addition
of the final two protons to give the fully protonated porphyrin (0.05ꢀ8.0 mmol MSA).
The original broad Soret band at 424 nm increases in intensity, sharpens, and blue
shifts to 420 nm, while the hyperporphyrin bands at 500 and 760 nm continue to increase
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with added acid. The Soret band at 420 nm begins to decline after 0.15 mmol of MSA has
been added. Once again the initial increase in the Soret band and the incomplete
correlation with other hyperporphyrin bands may signal the presence of an intermediate
species such as the monoprotonated porphyrin; the evidence here is less clear than in the
case of TACM3PP. During the second stage the split Soret peaks coalesce into a single
peak at 446 nm and the hyperporphyrin absorbance at 760 nm decreases while the new
absorbance grows in at 660 nm, characteristic of a fully protonated porphyrin.
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+4
3. Cis → CisH4 . Upon acidification of the cis disubstituted derivative to form the
dication species (Figure 6A), the Soret absorbance initially sharpens and blue shifts from
425 to 420 then decreases in intensity as a new weaker band appears at 487 nm. The new
broad hyperporphyrin absorption is observed at 763 nm. It is interesting to note the
marked difference between the Cis and Trans dication spectra. Compared to Trans the
Cis hyperporphyrin has a much more prominent split (lowꢀenergy) Soret band (487 nm)
and a weaker normal Soret band (428 nm). This difference makes the ratio of the split
Soret absorbance (low energy absorbance : high energy absorbance)8 for Cis much larger
than that of Trans (0.55 versus 0.31, respectively). As the titration continues (Figure 6B,
dication to fully protonated CisH4+4) the split Soret absorbance coalesces into a new
Soret absorbance at 448 nm. The hyperporphyrin band at 763 nm blue shifts and
decreases as the absorbance characteristic of the fully protonated porphyrin appears at
660 nm.
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Figure 6.
Spectrophotometric titration of 3 ꢁM Cis with MSA in DMSO.
(A) addition of two protons to form the hyperporphyrin (0ꢀ50 ꢁmol MSA), (B) addition
of the final two protons to give the fully protonated porphyrin (0.05ꢀ8.0 mmol MSA).
+5
4. TA3CMPP → TA3CMPPH5
TA3CMPP are shown in Figure 7.
.
The various stages of the acid titration of
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Figure 7. Spectrophotometric titration of 3 ꢁM TA3CMPP with MSA in DMSO.
(A) addition of two protons to form the hyperporphyrin (0ꢀ25 ꢁmol MSA), (B) addition
of the final three protons to give the fully protonated porphyrin (0.025ꢀ12.0 mmol MSA).
In the titration of TA3CMPP to the dication (Figure 7A), the Soret band splits with the
initial band steadily decreasing and blue shifting from 433 to 420 nm, with the new peak
growing in at 480 nm. The ratio of the split Soret peaks is now reversed, with the higherꢀ
energy peak less intense than the lowerꢀenergy peak at 480 nm. The Q bands disappear
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in favor of a strong, broad hyperporphyrin absorbance at 780 nm. As the titration is
continued past the dication phase to the fully protonated species (Figure 7B), the typical
collapse of the split Soret band and red hyperporphyrin band is observed, with the bands
characteristic of the fully protonated porphyrin appearing at 445 nm and 665 nm. It was
not possible to distinguish protonation stages between the +2 and +5 states.
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5. TAPP → TAPPH4+6. The spectrophotometric titration of TAPP has been carried
out in this lab and the absorption spectra can be found elsewhere,11 but a brief summary
is presented here for comparison purposes. At the dication stage the Soret absorbance
splits to a relatively weak absorbance at 392 nm and a broad, strong absorbance at 466
nm. The hyperporphyrin absorbance is very strong, centered at 811 nm and covering
over 200 nm. Upon adding the third through sixth protons, the spectrum gradually
reverts to a typical symmetric fully protonated porphyrin spectrum with a Soret band at
443 nm and a Q band absorbance at 660 nm.
Absorbance peak positions and extinction coefficients for the free base (P) and cationic
(PH+, PH2+2, and PHn+2+n+2) forms of all six derivatives studied are listed in Table 1.
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Table 1. Porphyrin peak absorbance wavelengths and extinction coefficient values
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+2
+n+2
Porphyrin
P
PH+1
PH2
λ
PHn+2
λ
max (nm)
λ
max (nm)
max (nm)
λmax (nm)
(ε, x 103 Mꢀ1cmꢀ1
421 (380)
515 (20)
550 (11)
590 (8)
)
(ε, x 103 Mꢀ1cmꢀ1
)
(ε, x 103 Mꢀ1cmꢀ1
)
(ε, x 103 Mꢀ1cmꢀ1
)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
448 (360)
657 (45)
ꢀꢀꢀ
ꢀꢀꢀ
TCMPP
645 (7)
420 (360)
515 (22)
550 (10)
589 (6)
645 (6)
720 (8)
421 (240)
519 (14)
560 (10)
653 (5)
420 (230)
513 (32)
716 (26)
447 (330)
659 (44)
TACM3PP
423 (140)
521 (13)
570 (14)
660 (8)
425 (150)
522 (11)
571 (14)
660 (7)
433 (110)
523 (10)
576 (14)
663 (7)
438 (160)
528 (10)
579 (20)
669 (10)
420 (170)
500 (53)
756 (41)
446 (280)
660 (36)
ꢀꢀꢀ
ꢀꢀꢀ
ꢀꢀꢀ
ꢀꢀꢀ
Trans
420 (130)
487 (71)
763 (60)
446 (310)
660 (39)
Cis
420 (56)
480 (68)
784 (53)
445 (200)
664 (30)
TA3CMPP
TAPPa
392 (40)
466 (92)
811 (70)
443 (220)
660 (31)
a TAPP data from previous work.11
Relative Basicities. The molar concentrations of MSA required to reach the halfꢀ
equivalence point and apparent pKa values for each stage (PH+, PH2+2, and PHn+2
)
+n+2
are listed in Table 2.
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Table 2. Concentrations of MSA (mM) required to reach approximate halfꢀequivalence
7
8
points and apparent pKa values (pKa,app)a.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
+2
+n+2
P ꢀ PH2
PH2+2 ꢀ PHn+2
P ꢀ PH+
Porphyrinb
[MSA]
(mM)
pKa,app
[MSA]
(mM)
ꢀꢀꢀ
pKa,app
[MSA]
(mM)
pKa,app
ꢀꢀꢀ
ꢀꢀꢀ
2.9
ꢀꢀꢀ
ꢀꢀꢀ
ꢀꢀꢀ
ꢀꢀꢀ
ꢀꢀꢀ
1.6
3.1
3.1
3.8
5
150
0.8
0.5
0.5
0.5
0.5
0.5
TCMPP
TACM3PP
Trans
1.3
ꢀꢀꢀ
ꢀꢀꢀ
ꢀꢀꢀ
ꢀꢀꢀ
23
0.8
0.7
0.2
0.01
350
290
290
300
290
Cis
TA3CMPP
TAPPc
apKa,app = ꢀ log[MSA]1/2 eq pt. b3 mL of a 3.0 ꢁM solution 3 mL of a 6.0 ꢁM solution ꢀ
c
data from previous work11
Discussion
The acid titrations for this full series of amino/carbomethoxyphenyl porphyrins
illustrate clear trends in the series from monoamino to transꢀdiamino to cisꢀdiamino to
triamino to tetraamino. The trends are consistently in the same order in terms of relative
basicities, the wavelength and intensity of the new broad red peak (hyperporphyrin band),
and the wavelength and intensity of the new split Soret band (Figures 3ꢀ7 and Tables 1ꢀ
2). In every case, the trends may be explained by the generation of a diprotonated
porphyrin with increasing opportunities for conjugation between the peripheral
aminophenyl substituents and the porphyrin core (a hyperporphyrin). For each trend the
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driving force is a greater ability to donate electron density from the peripheral amino
groups to the porphyrin core, increasing with the number of electronꢀdonating
aminophenyl substituents. However, the disubstituted isomers show that the cis
derivative has stronger hyperporphyrin effects than the trans derivative, indicating that
molecular geometry is also important (via resonance forms) in determining the extent of
the hyperporphyrin effect.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Relative basicities are determined by the amount of acid titrant needed to reach halfꢀ
equivalence points. Optical and NMR studies have confirmed that the initial protonation
in aminophenylꢀsubstituted porphyrins is on the pyrrole nitrogens within the porphyrin
core, rather than on the peripheral amino groups.8,9,40 Each amino substituent (replacing
an electronꢀwithdrawing carbomethoxy substituent) increases the relative basicity of the
pyrrole nitrogens, indicted by the significant decrease in acid required to generate the
hyperporphyrin (PH2+2) as the number of amino substituents increase. This effect is
readily explained by the availability of resonance forms that delocalize the positive
charge of the conjugate acid to the peripheral amino groups. Upon addition of excess
acid, complete protonation converts the amino groups to ammonium ions, and spectra of
the fully protonated porphyrins appear as expected for tetraphenylporphyrins with nonꢀ
interacting substituents (i.e., like diprotonated TCMPP, Figure 2). Interestingly, the
amount of acid required to continue the protonation of the hyperporphyrins is
approximately equivalent throughout the whole series; about 300 mM MSA is required
for full protonation, regardless of whether the final protonation state is +3 or +6.
The red hyperporphyrin band appears farther into the red and increases in breadth and
intensity as the number of amino groups increases, with peak wavelengths moving from
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716 to 811 nm (Figure 8). TA3CMPP does not follow the increasing intensity pattern
seen in the other four cases; we believe this may be due to a concentration lower than
calculated brought on by sample degradation. However in every other metric the trends
are consistent. The red hyperporphyrin peak has been assigned as a chargeꢀtransfer
transition from an aminophenyl group to a porphyrin π* LUMO.41,42 With more
aminophenyl groups available, the transition is distinctly stronger and moves to lower
energy (longer wavelength).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 8. Comparison of the diprotonated hyperporphyrin spectra for the
amino/carbomethoxyphenylporphyrin series with one to four aminophenyl substituents.
The inset compares the dication spectra for the cis and trans diaminophenyl derivatives.
Earlier we examined substituent effects on hyperporphyrin spectra using Hammett
correlations and found that the position and intensity of the longꢀwavelength absorption
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8
9
peak can be correlated with the Hammett sigma constant; although any electronꢀdonating
substituent can give rise to a detectable hyperporphyrin effect, it is the strong electron
donors such as amino or alkoxy that give the obvious examples.11,43 For example, a cisꢀ
dimethylaminophenylporphyrin was studied by Ojadi et al.,8 showing a strong
hyperporphyrin spectrum comparable to our cisꢀdiaminophenylporphyrin. The Ojadi
study did not examine trans or trisubstituted cases, but did study a monosubstituted
dimethylaminophenylporphyrin without identifying a monoprotonated form of the
porphyrin.8 In this case, the range of acidities used appears to have stepped over the
range that would have been necessary to see the initial monoprotonation.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Splan and coworkers7 recently reported a series of trans disubstituted porphyrins
comparing the effects of substitution on phenylethynyl groups to those on phenyl groups.
The phenylethynyl linker was clearly more effective at transmitting the strong electronꢀ
donating capability of pꢀdimethylamino groups to a protonated porphyrin, with a stronger
and longer wavelength red band than the corresponding transꢀdisubstituted
phenylporphyrin. This effect is attributed to the difficulty in achieving coplanarity
between the phenyl and porphyrin rings, which is relieved when the alkynyl spacer is
used. The coplanarity that is achievable with the alkynyl spacer is clearly more important
than the greater distance in transmitting the favorable effects of conjugation.
The Soret band split also becomes more pronounced with greater numbers of
aminophenyl substituents. As the number of aminophenyl substituents increase, the lowꢀ
energy component of the transition becomes increasingly stronger at the expense of the
highꢀenergy component, and it also significantly shifts to the blue (Figure 7). Examining
the relative intensity of the highꢀenergy to the lowꢀenergy component of the split Soret
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band, we observe in order of increasing aminophenyl substituents intensity ratios as
follows: 0.14, 0.31, 0.55, 1.21, and 2.3 (for TACM3PP, Trans, Cis, TA3CMPP, and
TAPP, respectively). Roughly there is a doubling that occurs with each step of increased
favorability of a hyperporphyrin charge delocalization.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
This trend holds with literature comparisons as well. Comparing reported work on
pꢀdimethylamino derivatives, the trans derivative gives a Soret ratio of about 0.6 (based
on the extinction coefficients reported)7 and the cis derivative gives a ratio of about 1.8 It
is
also
notable
that
the
split
Soret
ratio
for
the
transꢀbis[(4ꢀdimethylamino)phenylethynyl]porphyrin derivative reported by Splan and
coworkers shows a split ratio Soret of 0.31,7 on par with what we report here for Trans.
The intensity of the red hyperporphyrin band relative to the blue Soret band mirrors the
relationship of the split Soret bands. An approximate doubling of the ratio is seen for
every step by which hyperporphyrin formation favorability increases, as summarized in
Table 3.
Resonance effects can be used to correlate most of the observed trends for this series.
As would be expected, increasing the number of amino substituents increases the total
number of available resonance forms. However, beyond the number of amino
substituents, there are geometric effects that significantly influence the number and types
of resonance forms that can be drawn for the different cases. For TACM3PP (Figure 9)
and Trans (Figure 10), none of the resonance forms written can support delocalization of
both positive charges from the interior pyrrole nitrogens to the aminophenyl substituents.
For the former it is a purely statistical effect, i.e., only one good donor group is available
to delocalize the positive charge. In Trans, however, there are two donor groups but the
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arrangement of the π system does not allow for both charges to be delocalized at the same
time. For Cis (Figure 11) the arrangement of the donor groups is such that delocalization
of both charges can occur. For TA3CMPP (Figure 12), there are two pairs of cis donor
groups that can similarly delocalize both charges. TAPP (resonance structures not
shown) has four amino substituents and four unique cis orientations that can delocalize
both positive charges. The increase in the number of resonance forms, especially those
forms that delocalize both charges, manifests itself as a stronger hyperporphyrin effect
and gives rise to all the trends discussed previously.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 9. Selected resonance forms for TACM3PP for addition of the first proton (top)
and the second proton (bottom), illustrating the sites for delocalization of the positive
charge. Type A and B forms are discussed in the text.
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37
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39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 10. Selected resonance forms for diprotonated Trans, illustrating the sites for
delocalization of the positive charge. Both charges cannot be placed on the transꢀdiamino
groups simultaneously. Type A and B forms are discussed in the text.
Figure 11. Selected resonance forms for diprotonated Cis, illustrating the sites for
delocalization of the positive charge. Note that both charges can be placed on the cisꢀ
diamino groups simultaneously. Type A, B, and C forms are discussed in the text.
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40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 12. Selected resonance forms for diprotonated TA3CMPP, illustrating the sites
for delocalization of the positive charge. Note that both charges can be placed on cisꢀ
diamino groups simultaneously. Type A, B, and C forms are discussed in the text.
Hyperporphyrin Resonance Forms. The spectroscopy of porphyrins has long been
understood in the framework presented by Gouterman.44 He has also presented an
organizational framework for specific cases of anomalous spectroscopy such as
hyperporphyrins.8 Basically, A type spectra are normal porphyrin spectra, in which the
four meso substituents do not interact significantly with the porphyrin ring; there are both
neutral and diprotonated A type spectra, as illustrated in Figure 2 where all substituents
are carbomethoxy. When meso substituents are capable of strong interaction with the
porphyrin π system, however, the chromophore can be extended significantly beyond the
porphyrin core and anomalous spectra result. In a type B spectrum, one such substituent
interacts, while in a type C spectrum, two such substituents interact. Careful
manipulation of the available resonance forms show that for diprotonated porphyrins
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(where the two protons are added at the central nitrogens), delocalization requires that
donor groups be available at cis meso positions, leading to the C type resonance forms
shown in Figures 11 and 12. The trans orientation does not allow for a C type resonance
form. Continued protonation leads all of our porphyrins back to a type A spectrum, in
which all meso substituents are noninteracting, i.e., either carbomethoxy groups or
ammonium ions.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 3 summarizes the effects of increasing numbers of the different resonance forms
available on what we call the hyperporphyrin effect, i.e., a stronger and more redꢀshifted
hyperporphyrin band (H) and a split Soret band with increasing strength at the longerꢀ
wavelength component (S2/S1). The numbers of resonance forms of each type cited in
Table 3 are not the total number of resonance forms possible (which would be very
numerous), rather they represent the number of unique peripheral positions to which the
positive charges may be delocalized.
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Table 3. Porphyrin peak absorbance wavelengths (Soret S1 and S2 and hyperporphyrin
5
6
H bands) with extinction coefficient values, correlated with resonance forms available.
7
8
9
+2
+2
+2
PH2
max (nm)
PH2
PH2
Type B
forms
Type C
forms
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Porphyrin
λ
ε(S2):ε(S1)a ε(H):ε(S1)b
(ε, x 103 Mꢀ1cmꢀ1
420 (230)
513 (32)
716 (26)
420 (170)
500 (53)
756 (41)
420 (130)
487 (71)
763 (60)
420 (56)
480 (68)
784 (53)
392 (40)
466 (92)
811 (70)
)
0.14
0.31
0.55
1.21
2.3
0.11
0.24
0.46
0.95
1.75
1
2
2
3
4
0
0
1
2
4
TACM3PP
Trans
Cis
TA3CMPP
TAPPe
a Split Soret intensity ratio8 Hyperporphyrin : Soret S1 intensity ratio e TAPP data
b
from previous work.11
Evidence for a Monoprotonated Porphyrin.
Evidence for the elusive
monoprotonated porphyrin is scant in the literature.27ꢀ29 Formation of the monocation is
proposed to cause significant deformation of the porphyrin ring in order to counteract the
steric destabilization of having three protons confined to the interior of the
macrocycle.27,32,34 However, reports of monocation porphyrins have appeared; in these
cases various means for stabilization of a single protonation have allowed their
observation and characterization.34,35 In this study, careful observation of the early stages
of the spectrophotometric acid titration of TACM3PP indicate a significant absorbance
increase in the Soret band (from about 0.7 to 1.1 AU), as well as a wellꢀdefined isosbestic
point at 550 nm as a distinct initial phase of the titration (up until 25 ꢁmoles of MSA
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added). After the Soret band reached its maximum absorbance of 1.1 (at 420 nm), its
intensity began to decrease and the isosbestic point at 550 nm disappeared. As the Soret
band diminished, other typical signs of dication formation became more pronounced; the
hyperporphyrin band (about 720 nm) increased and the lowꢀenergy absorbance of the
split Soret band (515 nm) became more intense as the original Q bands continued to
disappear. The observation of two distinct phases during the initial titration is taken as
evidence that the monoprotonated porphyrin can be the majority species early in the
titration. The driving force in this case is proposed to be resonance stabilization of the
first protonation by electron donation from the lone amino group to the porphyrin core,
creating a unique thermodynamic favorability for the first protonation. With only one
amino group available, generation of the dication on the subsequent protonation step does
not generate any further stabilization to the system. In fact with electronꢀwithdrawing
carbomethoxy groups on the three remaining phenyl rings, resonance destabilization
should decrease the favorability of the second protonation. Thus, three factors may be
instrumental in stabilizing the monocation intermediate of TACM3PP: 1) effective
charge delocalization of the first positive charge to the single amino substituent, 2)
unavailability of comparable charge delocalization for the second positive charge, and 3)
relative destabilization of subsequent positive charges by the three carbomethoxyphenyl
groups.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The Trans derivative shows characteristics intermediate between TACM3PP and Cis,
consistent with resonance forms that do not allow the ability to delocalize both positive
charges to the two trans aminophenyl groups. It is also possible to observe subtle
evidence for the monoprotonated species in the Trans spectrophotometric acid titration.
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In this case, possible evidence for the monocation is a small initial increase in the Soret
absorbance for Trans, analogous to the increase in the Soret band for TACM3PP. This
initial increase of the Soret band is smaller than that observed for TACM3PP (0.12
versus 0.38 absorbance units), and it is not possible to detect isosbestic points or stable
spectra between protonation steps, suggesting that the monocation in this case is much
less abundant. This is understandable for Trans, for which diprotonation becomes
significantly more favorable than it would be for TACM3PP. Even though both charges
are unable to delocalize to the two trans amino groups simultaneously, the charges can be
delocalized to each of the two trans amino groups independently. In TACM3PP, the
second charge must remain localized in the porphyrin core.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Conclusions
Acid titrations of aminophenylꢀsubstituted porphyrins show that the positive charges
generated upon protonation can be effectively delocalized to the aminophenyl groups.
Such delocalization shows distinctive spectra characteristic of hyperporphyrins, with a
strong absorption in the far red as well as a split Soret band. Hyperporphyrin spectra are
described as resulting from structures in which there is increased planarity and
conjugation between the peripheral aryl groups and the porphyrin core. The extent of the
hyperporphyrin effect increases with the number of aminophenyl groups, and cis is
distinctly more effective than trans, which is consistent with the resonance forms
available. Detection of the relatively rare monoprotonated porphyrin was observed in the
case of the monoamino derivative at early stages of the acid titration.
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Experimental Methods
5
6
Materials. Solvents and reagents were the highest commercial grade and used as
7
8
received. DMSO (spectrophotometric grade) was from SigmaꢀAldrich. Methanesulfonic
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
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27
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acid
(MSA)
(99%
extra
pure)
was
from
Acros
Organics.
Tetrakis(4ꢀcarboxyphenyl)porphyrin (TCPP) and TAPP were from Frontier Scientific.
TCMPP was synthesized using the AdlerꢀLongo method.45
Synthesis.
General
synthetic
procedures
for
the
mixedꢀsubstituent
(4ꢀamino/carbomethoxyphenyl)porphyrins (TAxCMyPP, Figure 1).46 A solution of
TCPP and polyphosphoric acid was warmed to 100°. To this, 2 eq of NH2OH•HCl was
added and the mixture was slowly heated to 160° over 3 h. Upon cooling to room
temperature, a solution of conc. H2SO4 and CH3OH (1:10) was slowly added and the
resulting mixture was heated at reflux for 36 h. The reaction mixture was neutralized
with concentrated NH4OH to a pH of 5.5. The dark solution was extracted with
chloroform, washed with water, and dried over Na2SO4. The solution was evaporated to
dryness and dried in vacuo overnight affording a dark purple solid. Isolation via column
chromatography (silica gel 230ꢀ400 mesh), eluting with 4:1 CH2Cl2:ethyl acetate and 1%
triethylamine, yielded TCMPP, TACM3PP, Trans, Cis, TA3CMPP, and TAPP as six
purple bands in that order.46
Spectroscopy. Typical spectra UVꢀvisible spectroscopy was performed from 300 to
900 nm using a single visible light source and single detector setting. Sample speed was
medium with sample interval of 0.2 nm and a slit width corresponding to 3 nm.
Titrations. Porphyrin titrations were carried out by adding diluted (DMSO) or pure
MSA in 2ꢀ5 ꢁL increments with a micropipet (2ꢀ20 ꢁL) to a 3 ꢁM porphyrin solution in
DMSO in a 3 mL cuvette. At each addition, the resulting solution was mixed with a
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