UV-vis spectrophotometric titrations and vibrational spectroscopic characterization of meso-(p-Hydroxyphenyl)porphyrins

Article abstract of DOI:10.1021/jp048981f

Sequentially spectrophotometric titrations by sodium hydroxide of meso-tetraphenylporphyrin derivatives bearing one, two, three, or four p-hydroxyl groups result in new types of spectra. The strong new bands appear in the visible region with splitting or broadening of the Soret band and its significant loss of oscillator strength. To understand the molecular origin of these phenomena, the Resonance Raman (RR) and Fourier Transform Infrared (FTIR) experiments are carried out. The results demonstrate that the charges of the deprotonated para-hydroxy substituted meso-tetraphenylporphyrins are localized on the substituents, not delocalized into the π system of the porphyrin macrocycles and that the ground states of the macrocycles remain essentially unperturbed. Both the related behavior of diprotonated tetrakis(p-(dimethylamino)phenyl) porphyrin and protonated Schiff base porphyrins show that the new bands considered as hyperporphyrin spectra are due to π(phenoxide anion) → π* (porphyrin) transitions, where π is an orbital on the phenoxide anion substitutent and π* is a LUMO on the porphyrin.

Full text of DOI:10.1021/jp048981f

J. Phys. Chem. B 2004, 108, 10185-10191
10185
UV-Vis Spectrophotometric Titrations and Vibrational Spectroscopic Characterization of
meso-(p-Hydroxyphenyl)porphyrins
†
†
‡
†
†
Hongwei Guo, Junguang Jiang, Yingyan Shi, Yuling Wang, Jiannan Liu, and
Shaojun Dong*,†
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun, Jilin, 130022, P. R. China,
and Department of Chemistry, Jilin UniVersity, Changchun, Jilin, 130023, P. R. China
ReceiVed: March 6, 2004; In Final Form: April 21, 2004
Sequentially spectrophotometric titrations by sodium hydroxide of meso-tetraphenylporphyrin derivatives
bearing one, two, three, or four p-hydroxyl groups result in new types of spectra. The strong new bands
appear in the visible region with splitting or broadening of the Soret band and its significant loss of oscillator
strength. To understand the molecular origin of these phenomena, the Resonance Raman (RR) and Fourier
Transform Infrared (FTIR) experiments are carried out. The results demonstrate that the charges of the
deprotonated para-hydroxy substituted meso-tetraphenylporphyrins are localized on the substituents, not
delocalized into the π system of the porphyrin macrocycles and that the ground states of the macrocycles
remain essentially unperturbed. Both the related behavior of diprotonated tetrakis(p-(dimethylamino)phenyl)
porphyrin and protonated Schiff base porphyrins show that the new bands considered as hyperporphyrin
spectra are due to π(phenoxide anion) f π*(porphyrin) transitions, where π is an orbital on the phenoxide
anion substitutent and π* is a LUMO on the porphyrin.
1
. Introduction
even “split”, Soret bands and strongly enhanced and red-shifted
5-8
absorption in the visible region. For example, the diprotonated
form of tetrakis(p-(dimethylamino)phenyl)porphyrin shows a
sharply split Soret band and a unusually intense red band. Such
molecules have been described as hyperporphyrins.
1
The importance of porphyrins in nature and in technical
applications has led to electronic absorption and other spectro-
scopic studies of these compounds. In the ultraviolet-visible
region, the characteristic spectra of porphyrins including the
intense Soret band around 400 nm and a series of bands
Hyperporphyrins have been defined as porphyrins that exhibit
-
1
-1
prominent extra absorption bands (ꢀ > 1000 M cm ) in the
region λ > 320 nm which are not porphyrin π-π* transitions.3
These extra bands are often due to charge transfer (CT)
interactions of porphyrins with either metal orbitals or substit-
uents. In diprotonated meso-tetrakis(p-(dimethylamino)phenyl)-
(Q-bands) through the visible region are usually well-described
2,3
by the so-called four-orbital model. This ascribes the ground-
state absorptions of porphyrins as transitions to (π, π*) states
derived from [a1u(π), eg(π*)] and [a2u(π), eg(π*)] configurations,
where a1u(π) and a2u(π) are the nearly degenerate porphyrin
ring highest occupied molecular orbital (HOMOs) and eg(π*)
are the degenerate ring lowest unoccupied molecular orbital
1
9
porphyrin, molecular orbital calculations suggest that the
hyperporphyrin transitions are of π(phenylamine)
f
π*(porphyrin) origin, where π(phenylamine) is an orbital on
the phenylamine substitutent and π*(porphyrin) is a LUMO on
the porphyrin. To better extend these observations, in the present
study, sequentially spectrophotometric titrations of mono-, di-,
tri-, and tetra-p-hydroxy substituted meso-tetraphenylporphyrins
by sodium hydroxide are carried out. RR and FTIR data
presented here provide essential evidence necessary for further
understanding of the molecular origins of the spectral changes
upon deprotonation.
(LUMOs), respectively. Mixing of these configurations gives
rise to the strongly allowed, near-UV Soret band and the quasi-
allowed, less-intense, lower-energy Q-bands. The effects of
peripheral or central substitutions on the optical properties of
porphyrins can generally be explained by slight energy shifts
of these transitions.
However, some substitutions of the central metal or on the
ring of porphyrins have led to unusually-spectroscopic effects
that require additional interpretation. Some metalloporphyrins
have been know to show some extra absorption bands in the
region λ > 320 nm. Certain para-hydroxy TPP derivatives such
as meso-tetrakis(3,5-di-tert-butyl-4-hydroxyphenyl)porphrin in
basic solution have shown numerous spectral and structural
features, complicated by accompanying redox processes leading
to quinonelike structures. The partial protonation of para-amino-
substituted meso-tetraphenylporphyrins leads to broadened, or
3
2. Experimental Section
4
Materials. The porphyrins were prepared and characterized
10
by previously described methods as follows. For abbreviations,
we use the notation (OH)m(ONa)nPH2, where m is the number of
para-hydroxy-substituted phenyls and n is the number of depro-
tonated para-hydroxy-substituted phenyls. The notation and sub-
stitution patterns of these porphyrins are shown in Figure 1.
Analytical grade N, N-dimethylformamide (DMF) was dried
over activated molecular sieves (4 Å) for 24 h and then was
distilled under reduced pressure prior to use. Doubly-distilled
*
Corresponding author. Fax: +86-431-5689-711. E-mail: dongsj@
ns.ciac.jl.cn.
†
‡
Chinese Academy of Sciences.
Department of Chemistry, Jilin University.
1
0.1021/jp048981f CCC: $27.50 © 2004 American Chemical Society
Published on Web 06/08/2004

10186 J. Phys. Chem. B, Vol. 108, No. 28, 2004
Guo et al.
Figure 1. Structure of substituted free-base porphyrins. Porphyrin
substitution patterns (I) (OH) PH A)OH BdC)D)H; (II) (OH) PH
A)BdOH C)D)H; (III) (OH) PH A)BdCdOH D)H; (IV)
OH) PH A)BdC)D)OH.
Figure 2. Spectrophotometric titration of (OH)
1
PH
2
with NaOH. 0 f
1
2
2
2
1
indicates the neutralization of one proton. The × 11 is the scale factor
3
2
used for Q-bands.
(
4
2
water was used in all sample preparations. All other chemicals
used were analytical grade and were used as received.
A Series of Meso-(p-hydroxyphenyl)porphyrins. Samples of
p-hydroxylbenzaldehyde (5.3 g, 43.4 mmol, 0.174 M) and
benzaldehyde (11.5 mL, 0.115 mol, 0.46 M) were dissolved in
instrument. Laser excitation at 514.5 nm was obtained with a
Spectra Physics 2017 argon ion laser. All spectra were collected
at backscattering geometry at room temperature. Incident powers
were 20-50 mW. Optical spectra were recorded before and after
each RR experiment on a Cary 500 spectrophotometer to ensure
that the samples had not decomposed. Baseline corrections were
carried out using Labspec (J-Y) software (ver 3.03T). FT-IR
spectra were obtained with a Bruker IFS-66v/S (Germany) FT-
IR spectrometer. The spectral resolution was set at 2 cm . The
average scan times were 100. For all samples, the AquaSpec
Flow Cell (Micro-biolytics Company, Germany) with calcium
fluoride windows was used, and the light length was 6 µm.
UV-Vis Spectrophotometric Titration. Solutions of por-
phyrins in DMF + H2O (V: V ) 1:1) mixture were titrated
directly in 1-cm absorption cells by successive additions of
sodium hydroxide while monitoring the spectra between 350
and 750 nm. Increasingly concentrated NaOH (0.1-10 M)
solution was added in 2 to 10 µL aliquots using a microliter
syringe. The original porphyrin solution was ∼ 4 µM in a total
volume of 3.0 mL. The total volume change during the titration
was negligible. After addition of each aliquot, the cell was
capped and mixed by inversion and the spectrum was retaken.
In all cases, the reversibility of the deprotonation was demon-
strated by the addition of 4.0 M HCl.
250 mL of propionic acid, and then the solution was vigorously
stirred and heated to reflux. Pyrrole (10.6 mL, 0.153 mol, 0.612
M) was then added dropwise. After completion of addition, the
mixture was refluxed a further 1.5 h. The reaction mixture was
then stirred and cooled to room temperature. The cooled solution
was left overnight, and then the mixture was neutralized with
NaOH and filtered at reduced pressure to give a dark purple
powder. This powder was placed on top of a silica gel column
-1
(Si 60-100 mesh) poured with chloroform. The porphyrins were
eluted using chloroform followed by ethanol/chloroform in
different ratios. Further separation via thin-layer chromatography
afforded meso-tetraphenylporphyrin (TPPH2)(OH)0PH2),
5
5
,10,15-triphenyl-20-(4-hydroxyphenyl)porphyrin (I)(OH)1PH2),
,15-diphenyl-10,20-bis(4-hydroxyphenyl)porphyrin (trans-
(OH)2PH2), 5,10-diphenyl-15,20-bis(4-hydroxyphenyl) porphy-
rin (II)cis-(OH)2PH2), 5-Phenyl-10,15,20-tri(4-hydroxyphenyl)-
porphyrin (III)(OH)3PH2), and meso-tetrakis(4-hydroxyphenyl)-
1
porphyrin (IV)(OH)4PH2): H NMR (DMSO-d6) (I) δ -2.79
(
(
s, 2H, NH), 7.32 ∼ 7.34 (d, 2H, m-C6H4OH), 7.91 ∼ 7.96
m, 9H, m, p-C6H5), 8.12 ∼ 8.14 (d, 2H, o-C6H4OH), 8.32
∼
8.34 (t, 6H, o-C6H5), 8.93 ∼ 9.04 (m, 8H, CH2), 10.13 (s,
3
. Results
1
H, OH); (II) δ -2.80 (s, 2H, NH), 7.32 ∼ 7.34 (d, 4H,
m-C6H4OH), 7.94 ∼ 7.96 (d, 6H, m, p-C6H5), 8.12 ∼ 8.14
d, 4H, o-C6H4OH), 8.33 ∼ 8.34 (t, 4H, o-C6H5), 8.93 (s,
H, CH2), 9.02 (s, 4H, CH2), 10.12 (s, 2H, OH); (III) δ -2.79
3.1 UV-Vis Spectrophotometric Titration. Figures 2-5
(
4
present spectrophotometric titrations for (OH)1PH2, (OH)2PH2,
(OH)3PH2, and (OH)4PH2. Deprotonation of the para-hydrox-
yphenyl substituents of these compounds has been shown to
induce dramatic change in the optical spectra. Comparing the
spectral changes of these four species during the titration, we
can find that species with the same number of peripheral
phenoxide anion substituents have similar spectra. Hence, these
titration procedures can be roughly resolved into four steps due
to somewhat overlapping deprotonation. The initial products,
assigned to these species with one phenoxide anion substituent
(0 f 1; solid lines in Figure 2, 3, 4A, and 5A), show a markedly
reduced, broadened, and slightly blue-shifted Soret peak, the
reduced height of the Oy(1,0) band and two new bands, a very
broad canopy-like strong band at around 570 nm, and a strong
red band at around 656 nm. The second step (1 f 2; dashed
lines in Figure 3, 4A, and 5A) at higher concentrated NaOH
deprotonates the second peripheral hydroxyl, giving a “split
(
(
s, 2H, NH), 7.32 ∼ 7.35 (m, 6H, m-C6H4OH), 7.93 ∼ 7.95
d, 3H, m, p-C6H5), 8.12 ∼ 8.15 (m, 6H, o-C6H4OH), 8.32
∼
8.34 (t, 2H, o-C6H5), 8.92 (d, 2H, CH2), 9.01 (s, 6H, CH2),
1
7
8
0.11 ∼ 10.12 (d, 3H, OH); (IV) δ -2.78 (s, 2H, NH), 7.32 ∼
.34 (d, 8H, m-C6H4OH), 8.11 ∼ 8.13 (d, 8H, o-C6H4OH),
.99 (s, 8H, CH2), 10.10 (s, 4H, OH); absorbance (CH2Cl2):
(
I) 418.40, 515.68, 547.68, 589.92, 647.52 nm; (II) 419.68,
5
5
15.68, 550.24, 592.48, 648.80 nm; (III) 418.40, 515.68, 552.80,
92.48, 648.80 nm; (IV) 419.68, 518.24, 554.08, 593.76, 648.80
nm;
Spectroscopy. UV-Visible absorption spectra were taken
on a Cary 500 Scan UV-Vis-NIR spectrophotometer (Varian,
United States) using a 2 mm slit width. Data acquisition was
via a GPIB card and a PC using Cary Win UV software. RR
spectra were recorded with a J-Y T64000 laser Raman

UV-Vis Spectrophotometric Titrations
J. Phys. Chem. B, Vol. 108, No. 28, 2004 10187
Figure 3. Spectrophotometric titration of (OH)
2 2
PH with NaOH. 0 f
1
(solid lines) and 1 f 2 (dashed lines) indicate the neutralization of
the first proton and the second proton. The × 11 is the scale factor
used for Q-bands.
Figure 5. Spectrophotometric titration of (OH)
f 1 (solid lines) and 1 f 2 (dashed lines) indicate the neutralization
of the first proton and the second proton. (B) 2 f 3 (solid lines) and
f 4 (dashed lines) indicate the neutralization of the third proton and
4 2
PH with NaOH. (A)
0
3
the fourth proton. The × 11 and × 4 are the scale factors used for
Q-bands.
change of (OH)2PH2 slightly differs from that of (OH)3PH2 and
(OH)4PH2. The main reason may be that (OH)3PH2 and (OH)4-
PH2 are apt to form the two “ trans” (i.e., on opposite sides of
the macrocyclic skeleton), not “cis” (i.e., on adjacent sides of
the macrocyclic skeleton), phenoxide anion substituents in
4
,11
(OH)2PH2 (Scheme 1).
In the third step, (2 f 3; solid lines
in Figure 4B and 5B), the short-wave component of the “split
Soret” disappears, the low-energy component moves to 443 nm
and increases slightly in height, and the two new bands move
to the red (592 nm, 676 nm in Figure 4B and 594 nm, 680 nm
in Figure 5B) and continue to increase in height. The final
deprotonation
(3 f 4; dashed lines in Figure 5B) forms a three-band spectrum
similar to that in the acidic solution, with a Soret peak (at 445
nm) and two red-shifted, markedly-enhanced new bands at 597
and 682 nm.
Figure 4. Spectrophotometric titration of (OH)
3 2
PH with NaOH. (A)
0
f 1 (solid lines) and 1 f 2 (dashed lines) indicate the neutralization
3
.2 The Resonance Raman and FTIR Spectra. Figure 6 shows
of the first proton and the second proton. (B) 2 f 3 (solid lines)
indicates the neutralization of the third proton. The × 11 and × 4 are
the scale factors used for Q-bands.
-
1
the high-frequency region (1000-1700 cm ) RR spectra
obtained for (OH)1PH2, (OH)2PH2, (OH)3PH2, and (OH)4PH2
in neutral and basic DMF + H2O (V:V ) 1:1) mixture using a
514.5 nm excitation. In this region, it is well-known that the
RR spectra of TPP derivatives are usually dominated with
porphyrin skeletal modes due to a resonant effect, though the
Soret” structure in which two components have about equal
height. The reduced Oy(1,0) band disappears, and two new bands
at 579 nm and at 666 nm in Figure 3 move to 587 and 672 nm
in Figure 4A, move further to the red in Figure 5A (to 588 and
12
phenyl modes may be observed occasionally. Using the normal
coordinate analysis of meso-tetraphenylporphyrinato Ni (II), We
6
74 nm), and increase in height. During this stage, the spectral

10188 J. Phys. Chem. B, Vol. 108, No. 28, 2004
Guo et al.
SCHEME 1
-
1
have collected several of the prominent vibrations of these
species in Table 1 and assigned these in analogy with the results
of Li et al.13 The atom labeling is defined in the structural
diagram (Figure 1).
In resonance with the Q-band (514.5 nm excitation) the RR
spectrum is dominated by depolarized (dp) and anomalously
polarized (ap) bands. They arise from B1g or B2g (dp) and A2g
Upon deprotonation, the O-H bending at 1273 cm disappears
-
1
-1
and the C-O stretching at 1234 cm shifts up to 1271 cm
(Figure 7). The phenomena closely related to those described
here occur in the case of metal complexes of Schiff base
17
1
8
porphyrins (SB), as follows (Scheme 2). RR measurements
showed that protonation had very little effect on ring vibration
frequencies but that protonation of the Schiff’s base shifted up
the NdC stretching frequency by 11 cm . All the phenomena
provide enough evidence that the negative center is not
thoroughly delocalized throughout the π system, but rather
deprotonation has the effect of generating an exceptionally
strong electron-donating group.
-
1
(ap) modes which are vibronically active in Q-B mixing and
are enhanced via the B (vibronic) term. In Figure 6 the RR
spectra of (OH)1PH2, (OH)2PH2, (OH)3PH2, and (OH)4PH2 in
neutral DMF + H2O (1:1) show strong enhancement of non-
-
1
totally symmetric modes at 1232-1238 cm (ν26) and 1543-
-
1
1
548 cm (ν19). Upon deprotonation the porphyrin skeletal
vibrations are nearly unaffected; the band positions and the
relative intensity of (OH)1PH2, (OH)2PH2, (OH)3PH2, and (OH)4-
PH2 are nearly identical under neutral versus basic conditions
4
. Discussions
The P-OHTPPH2 has ionizable protons (Hs) at the two
-
1
centers, the comparatively acidic phenolic-Hs on the meso-
hydroxyphenyl substituents in the peripheral region and the inner
core pyrrolic-Hs on the two dNH groups. The imino (dNH)
groups are very weakly acidic (pK > 15). Therefore, on titration
with strong alkali only the P-hydroxyphenyl group is expected
(about a 5 cm change in ν11, upon formation of peripheral
phenoxide anion substituents, most likely arises from a slight
porphyrin core expansion accompanying the deprotonation
3
14
reaction ). We do not observe clearly identifiable phenyl modes
that are internal to phenyl rings in all cases. To determine the
changes of phenyl rings upon deprotonation, we carry out
analogous FTIR experiments of (OH)4PH2 (only FTIR experi-
ments of (OH)4PH2 are carried out due to the lower solubility
of (OH)1PH2, (OH)2PH2, and (OH)3PH2 in neutral solution) in
neutral and basic DMF + H2O (V:V ) 1:1) mixture (shown in
Figure 7). Due to the low solubility of (OH)4PH2 in a neutral
solution leading to a low signal-to-noise ratio, we only assign
1
9
to ionize in the high pH range. This has been inferred from
2
0
the experiment of deprotonation of free-base meso-tetrakis-
p-hydroxyphenyl)porphyrin ((OH)4PH2) and nickel (II) meso-
(
tetrakis-(p-hydroxyphenyl)porphyrin (Ni(OH)4PH2). The same
effect observed for both ((OH)4PH2) and (Ni(OH)4PH2) indicates
that the spectral changes resulted from the deprotonation of the
phenolic protons and not the dNH protons of the free-base
porphyrin.
As shown in Figures 2-5, for all four macrocycles the
deprotonation results in the markedly red-shifted, strong new
bands in the visible region, and increasing the number of
peripheral phenoxide anion substituents increases the extent of
the red shift of all bands. These dramatically spectral changes
have been observed by Manna et al. in a DMF + H2O (1:1)
mixture, for which they suggested the possibility of delocal-
ization of these peripheral charges in solution along the
conjugative pathways into the core region, causing an ac-
cumulation of negative charges only on the pyridine-type Ns.
This new charge distribution due to the resonance effect will
create a dipole with the positive pole on the phenolic oxygen
in the meso-positions and the negative pole on the N in the
center. Thus, a charge transfer state is created. But our RR and
FTIR data demonstrate that the charges of the deprotonated para-
hydroxy substituted meso-tetraphenylporphyrins are localized
on the substituents, not delocalized into the π system of the
porphyrin macrocycles and that the ground states of the
1
5
the main IR bands. Upon deprotonation the frequency of the
-
1
band at 1349 cm assigned to the Cm-phenyl stretching
vibration does not nearly change. The major changes are the
-1
phenyl CdC stretching frequency at 1611 cm that shifts down
-1
by 19 cm , and the disappearance of the p-hydroxyphenyl C-O
-1 10
stretching frequency at 1234 cm . Presumably, the formation
of phenoxide anion substituents should lead to the C-O
stretching frequency shifted up due to delocalization of the
negative charge onto the phenyl ring, which makes the bond
length of the phenoxide anion normalized. To determine its exact
peak position under basic conditions, FTIR spectra of a simple
model system (phenol/phenolate) in DMF + H2O (1:1) mixture
are shown in Figure 8. Comparing Figure 7 with 8, very similar
phenomena are observed. Upon deprotonation the CdC stretch-
1
9
-
1
-1
ing vibration at ∼ 1600 cm shifts down to 1589 cm and
-
1
the two peaks at 1238 and 1271 cm disappear and combine
into one peak (Figure 8). Hence, related to the band assignment
-
1
of phenol/phenolate, the two peaks at 1234 and 1273 cm are
ascribed to C-O stretching and O-H bending,16 respectively.

UV-Vis Spectrophotometric Titrations
J. Phys. Chem. B, Vol. 108, No. 28, 2004 10189
1 2 1 2 2 2 2 2 3 2 3 2 4 2
Figure 6. Resonance Raman spectra of (A) (OH) PH and (ONa) PH ; (B) (OH) PH and (ONa) PH ; (C) (OH) PH and (ONa) PH ; (D) (OH) PH
and (ONa)
4
PH
2
2
in a DMF + H O (1:1) mixture with 514.5 nm laser excitation. The small negative peaks are due to solvent subtraction or noise.
*
)solvent band.

10190 J. Phys. Chem. B, Vol. 108, No. 28, 2004
Guo et al.
TABLE 1: Raman Frequency (cm-1) of (OH)
PH
, (OH)
PH
, (OH)
PH
, and (OH)
PH
1
2
2
2
3
2
4
2
2
in Neutral and Basic DMF + H O
(
V:V ) 1:1) Mixturea
OH) PH (ONa) PH
(
1
2
1
2
(OH)
2
PH
2
(ONa)
2
PH
2
(OH)
3
PH
2
(ONa)
3
PH (OH)
2
4
PH
2
(ONa) PH
4
2
assignmentb
1546
1492
1358
1324
1234
1077
1548
1495
1356
1324
1236
1079
1547
1486
1358
1326
1238
1077
1546
1485
1358
1325
1235
1077
1543
1545
1485
1357
1325
1233
1074
1545
1489
1356
1325
1237
1077
1544
1484
1358
1323
1234
1074
ν(C
ν(C
ν(C
ν(C
R
C
m
) + δ(C
R m Ph
C C )
1489
1354
1323
1232
1073
â
C
â
) + δ(C H)
â
â
R
C
C
) + δ(C
) + δ(C
â
R â
H) + δ(C C )
H)
R
â
â
R R â R â â R m
ν(NC ) + ν(C C ) + δ(C C C ) + δ(C C )
δ(C H) + ν(C C )
â
â â
a
O (1:1) mixture is 5 × 10 M. b Assignments
-4
The concentration of (OH)
1
PH
, (OH) PH , (OH) PH
2 2 2 3 2
and (OH) PH
4 2
in neutral and basic DMF+H
2
1
3
are based on those for NiTPP.
Figure 7. FT-IR spectra of (OH)
1:1) mixture.
PH
4 2
and (ONa) PH
4 2
2
in DMF + H O
Figure 8. FT-IR spectra of phenol and sodium phenolate in a DMF
O (1:1) mixture.
(
+
H
2
macrocycles remain essentially unperturbed. Therefore, the extra
absorption bands can not possibly originate from Manna’s
SCHEME 2
“
charge transfer state” but correspond to characteristics of
hyperporphyrin spectra, which, based on different HOMOs or
LUMOs due to charge localization, are generally considered to
originate from charge-transfer transitions. In the p-type hyper-
porphyrins found with main-group metals in lower oxidation
states, an extra orbital appears in the region of the HOMOs,
and the extra bands are due to charge-transfer transitions a2u-
(
npz) (metal) f eg(π*) (ring); in the d-type hyperporphyrins
m
found with transition metals in configurations d (1 e m e 6),
extra orbitals appear among the LUMOs, and the extra bands
are attributed to charge-transfer transitions a1u(π), a2u(π) (ring)
f eg(dπ) (metal).3 Recent molecular orbital calculations on
meso-tetrakis-(p-(dimethylamino)phenyl)porphyrin and its pro-
tonated forms suggest that the effect originates from π orbitals
that are primarily localized on the aminophenyl substituent.
These orbitals extend over the amino group as well as the phenyl
π system and are calculated to correspond to HOMO and
HOMO-1 for diprotonated meso-tetrakis-(p-(dimethylamino)-
,7
phenyl)porphyrin. The corresponding LUMO in these cases is
a π* orbital extending only over the porphyrin core. Thus, the
lowest energy (hyperporphyrin) transition is assigned to π
(aminophenyl) f π*(porphyrin), which can be interpreted as a
charge-transfer transition. The reason that the π(aminophenyl)
orbital appeared as the HOMO in these cases is a combination
of the electron-donating effect of the amino group and the

UV-Vis Spectrophotometric Titrations
J. Phys. Chem. B, Vol. 108, No. 28, 2004 10191
depression of core porphyrin π and π* orbitals upon diproto-
nation.8 In these sequentially deprotonated p-hydroxy substi-
tuted meso-tetraphenylporphyrins, we suggest that the negative
charges on the phenoxide anion groups increase all the orbital
energies, which has been demonstrated that meso-tetrakis-(3,5-
di-tert-butyl-4-hydroxyphenyl)porphyrin undergoes facile aerial
oxidation in basic solution to give novel diphenoquinoid
tetrapyrrolic macrocycles. Hence, the (ONa)nPH2 complexes
ought to be more difficult to reduce and easier to oxidize.
However, deprotonation increases the energy of π orbitals
localized on the phenoxide anion substituents proportionally
more than that of the macrocycle π orbitals. Hence the
phenoxide anion π orbital crosses over the porphyrin π orbital,
creating a different HOMO and thereby a charge-transfer
transition (π(phenoxide anion) f π*(porphyrin)) at lower
energy and in higher oscillator strength. This results in red-
shifted, enhanced new bands in the visible region. Increasing
deprotonation of the para-hydroxyphenyl increases the energy
of π(phenoxide anion) orbital causing the charge-transfer
transition (s) to lower energy. Therefore, the more phenoxide
anions on the para-hydroxy substituted meso-tetraphenylpor-
phyrins, the more the new bands in the optical spectra are red-
shifted.
The RR and FTIR data clearly indicate that for this porphyrin
series the ground states of the macrocycles are not very much
perturbed and that the charges are mainly localized on the
substituents. This behavior of meso-(p-hydroxyphenyl)porphy-
rins is similar to that for protonated Schiff base porphyrins.
Therefore, the unusual red shift of absorption maxima in the
visible region and the Soret band splitting or broadening
observed for all four porphyrins upon deprotonation is consid-
ered to be characteristic of hyperporphyrin spectra which are
due to π(phenoxide anion) f π*(porphyrin) transitions. To
further fully understand the basified oxyphenylporphrins, de-
tailed calculation is needed.
,9
Acknowledgment. This work was supported by Special
Funds for Major State Basic Research of China (No 2002CB-
713803).
References and Notes
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In the visible-near region, the splitting or broadening of the
Soret band and its significant loss of oscillator strength upon
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(
4) Milgrom, L. R. Tetrahedron 1983, 39, 3895-3898.
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(
6) Gunter, M. J.; Robinson, B. C. Aust. J. Chem. 1989, 42, 1897-
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1
905.
,21
(
SB). Hanson et al.,9 through the extensive calculations and
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analysis, related the spectral change to decrease in the energy
of the SB π* orbital caused by protonation. This leads to
extensive mixing with one of the π*(porphyrin) orbitals. Thus,
the visible-near spectrum arises from transitions from two
HOMO(π) orbitals to three LUMO(π*) orbitals thereby quali-
tatively accounting for the observed split or broadened Soret
bands. Here we give a similar interpretation that the spectral
change in the visible-near region may originate from charge-
transfer transitions π(phenoxide anion) f π*(porphyrin) coming
into the energy range of the normal π(porphyrin) f
π*(porphyrin) of the four-orbital model due to the HOMO (s)
localized on the phenoxide anion substituent (s) close to
π(porphyrin) in energy.
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. Summary
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Meso-(p-hydroxyphenyl)porphyrins are a series of very
7
871.
important porphyrins because of their wide use for photosen-
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reduction catalysis
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3-25
It is fundamental, therefore, to understand the
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molecular origin bringing about red-shifted optical spectra in
this porphyrin series under basic conditions. Despite several
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spectra of (OH)4PH2 in a basic solution,8,19,20 the direct evidence
that assign (OH)4PH2 as a hyperporphyrin has not been reported
as yet. The present study provides RR and FTIR data that
support (OH)4PH2 as a hyperporphyrin and extends this result
to the whole meso-(p-hydroxyphenyl)porphyrin series.
(
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(
8
(
(