Solvent and substituent effects on UV-vis spectra and redox properties of zinc p-hydroxylphenylporphyrins

Article abstract of DOI:10.1142/S1088424617500456

A series of zincp-hydroxylphenylporphyrins was synthesized and characterized by spectroscopic and electrochemical methods in four different nonaqueous solvents. The investigated compounds are represented as [(p-HOPh)_n(ptBuPh)_4-nP]Zn,

Full text of DOI:10.1142/S1088424617500456

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2017; 21: 1–11
DOI: 10.1142/S1088424617500456
Published at http://www.worldscinet.com/jpp/
Solvent and substituent effects on UV-vis spectra
and redox properties of zinc p-hydroxylphenylporphyrins
Guifen Lu*a,b, Xiaoqin Jiangb, Zhongping Oua,b, SenYana and Karl M. Kadish*b
aSchool of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, P. R. China
bDepartment of Chemistry, University of Houston, Houston, TX 77204-5003, USA
Received 20 March 2017
Accepted 18 April 2017
ABSTRACT: A series of zinc p-hydroxylphenylporphyrins was synthesized and characterized by
spectroscopic and electrochemical methods in four different nonaqueous solvents. The investigated
compounds are represented as [(p-HOPh)n(p-tBuPh)4-nP]Zn, where P represents the dianion of a
porphyrin, Ph represents a phenyl group, HO and tBu are para substituents on the meso-phenyl rings
of the macrocycle and n = 0–4. The four utilized nonaqueous solvents were dichloromethane (CH2Cl2),
N,N-dimethylformamide (DMF), dimethylsulfoxide (DMSO) and pyridine (Py) which were selected on
the basis of their coordinating capabilities. The UV-visible spectra and redox potentials of each porphyrin
were analyzed both as a function of Hammett substituent constants for groups at the para-positions of
the meso-phenyl rings and as a function of the Gutmann solvent donor number which is related to the
coordinating ability of the solvent. Each porphyrin exhibits two reductions in CH2Cl2, DMSO and Py while
three reductions are observed in DMF, the additional reaction being due to a phlorin product generated
in solution after formation of the porphyrin dianion. Two or three reversible oxidations were seen in
CH2Cl2, the exact number depending upon the specific porphyrin and the presence of p-OH substituents
at the meso-phenyl rings of the compound. The first two oxidations were assigned as involving the
conjugated macrocycle and the third is associated with oxidation of the meso-HOPh group(s).
KEYWORDS: zinc p-hydroxylphenylporphyrins, synthesis, electrochemistry, solvent effect, substituent
effect.
p-hydroxyphenyl substituted zinc porphyrins with 0 to 4
INTRODUCTION
hydroxyphenyl groups in different nonaqueous solvents.
Hydroxyphenyl substituted porphyrins have been
This is done in the present paper where a series of
used as photosensitizers in model systems for photo-
p-hydroxyphenyl-substituted zinc porphyrins have been
synthesis and photodynamic cancer therapy [1–4], as
synthesized and characterized as to their electrochemical
fluorescence probes [5, 6] or as organic functional
and UV-visible absorption properties in four different
materials [7–13] in the area of nonlinear optical materials
nonaqueous solvents. The investigated compounds are
[13] and supramolecular materials [10]. The structural
shown in Chart 1 and represented by the formula
[14–16], spectral [17–19] and electrochemical properties
[20–22] for these compounds have been reported, but to
the best of our knowledge no laboratory has investigated
the UV-visible spectroscopy and electrochemistry of
[(p-HOPh)n(p-tBuPh)4-nP]Zn, where n = 0–4, P represents
the dianion of a porphyrin, HOPh represents a meso-
hydroxyphenyl group and tBuPh, a meso-tbutylphenyl-
group on the macrocycle. The four utilized solvents
varied from the low-donor and nonbinding solvent
CH2Cl2 (DN = 0.0) to the higher or stronger coordinating
*Correspondence to: Guifen Lu, email: luguifen@ujs.edu.
solvents DMF (DN = 26.6), DMSO (DN = 29.8) and
pyridine (DN = 33.1) [23]. The binding constants (logK)
for addition of one solvent molecule to the Zn(II) center
cn, tel: +86 511-88791800, fax: +86 0511-88791800. Karl M.
Kadish, email: kkadish@uh.edu, tel: (713) 743-2740, fax: (713)
743-2745
Copyright © 2017 World Scientific Publishing Company

2
G. LU ET AL.
R1
R4
electrode. Potentials were applied and monitored with
an EG&G model 173 potentiostat. Time-resolved
UV-visible spectra were recorded with a Hewlett-
Packard model 8453 diode array spectrophotometer.
High purity N2 was used to deoxygenate the solution,
and nitrogen was kept over the solution during each
electrochemical and spectroelectrochemical experiment.
N
Zn
N
N
N
Determination of binding constants
R3
R1-R4 = tButyl (1)
R1 = OH, R2-R4 = tButyl (2)
R1, R2 = OH, R3, R4 = tButyl (3)
R1-R3 = OH, R4 = tButyl (4)
R1-R4 = OH (5)
R2
A series of solutions containing CH2Cl2 and different
concentrations of DMF, DMSO or Py was prepared
and used as a standard titrant. Microliter quantities of
the above standard solutions were added gradually to
a specially constructed UV-visible cell (path length =
1.0 cm) containing the metalloporphyrin in 6.0 ml
CH2Cl2. After each addition, the solution was thorou-
ghly mixed and the spectrum was recorded. The
changes in spectra were analyzed as a function of the
concentration of added solvent, and the Hill equation
was used to calculate equilibrium constants (logK)
as described in the literature [17, 24]. All UV-visible
spectral measurements were carried out at room
temperature (298 K).
Chart 1. Structures of investigated zinc porphyrins 1–5
of the porphyrin were determined using standard equations
during specroscopically monitored titrations which were
carried out in CH2Cl2. Relationships are examined between
the solvent binding constants (logK) and the Gutmann
donor number (DN) of the solvent (S) or the sum of the
Hammett substituents constants (Ss) of groups at the
para-position of the meso-phenyl rings. The effect of the
solvent and number of meso-hydroxyphenyl substituents
on the electrochemical and spectroscopic properties of the
porphyrins is also discussed.
Chemicals
Absolute dichloromethane (CH2Cl2), N,N-dimethylfor-
mamide (DMF), dimethyl sulfoxide (DMSO) and pyridine
(Py) were received from Aldrich Co. and used as received.
High-puritydinitrogenfromTrigaswasusedtodeoxygenate
the solution before each electrochemical experiment. Tetra-
n-butylammonium perchlorate (TBAP) was purchased from
Fluka Chemika Co. and used without further purification.
The investigated zinc porphyrins [(p-HOPh)n(p-tBuPh)4-nP]
Zn 1–5 were prepared according to literature procedures as
described below.
EXPERIMENTAL
Materials and equipment
1H NMR spectra were recorded in a DMSO-d6
solution at 400 MHz using a Bruker Advance 400
spectrometer at 298 K. Chemical shifts (ppm) were
determined with TMS as the internal reference. MALDI-
TOF mass spectra were carried out on a Bruker BIFLEX
III ultrahigh resolution. Elemental analyses were
performed on an Element analyzer FLASH1112A. IR
spectra (KBr pellets) were recorded on an AVATAR-370
spectrometer.
Cyclic voltammetry was carried out with an EG&G
Model 173 potentiostat or ElectrochemicalWorkstation.
A home-made three-electrode cell was used and
consisted of a glassy carbon working electrode, a
platinum counter electrode and a homemade saturated
calomel reference electrode (SCE). The SCE was
separated from the bulk of the solution by a fritted
glass bridge of low porosity which contained the
solvent/supporting electrolyte mixture. All potentials
are referenced to the SCE.
Preparation of [(p-HOPh)n(p-tBuPh)4-nP]Zn (1–5)
A mixture of Zn(CH3COO)2 ·2H2O (0.10 mmol) and
the free-base porphyrin [(p-HOPh)n(p-tBuPh)4-nP]H2
(0.05 mmol) was added to DMF (6 ml) and heated to
140°C for 1 h. After cooling the solution to room
temperature, the volatiles were removed under reduced
pressure, and the residue was chromatographed on a silica
gel column with CHCl3 as eluent. A small amount of
unreacted free-base porphyrin [(p-HOPh)n(p-tBuPh)4-nP]
H2 was eluted as the first fraction, and the target metal
porphyrin product was collected as the second fraction.
Repeated chromatography followed by recrystallization
from CHCl3 and n-hexane gave pure purple solid sample
of [(p-HOPh)n(p-tBuPh)4-nP]Zn (1–5).
1
[(p-tBuPh)4P]Zn 1. Yield: 38.7 mg (86%); H NMR
Thin-layer UV-visible spectroelectrochemical experi-
ments were performed with a home-built thin-layer cell
which has a light transparent platinum-net working
(400 MHz, DMSO-d6): d 8.78 (s, 8H, b), 8.13 (d, 8H,
o-Ph), 7.83 (d, 8H, m-Ph), 1.58 (s, 36H, tbu); MALDI-
TOF mass: calcd 902.5, found 901.8. Elemental analysis
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SOLVENT AND SUBSTITUENT EFFECTS ON UV-VIS SPECTRA AND REDOX PROPERTIES
3
calcd (%) for ZnC60H60N4: C 79.84, H 6.70, N 6.21;
found: C 79.47, H 6.75, N 6.15.
porphyrins 1–5 [25]. The reaction workup was facile and
afforded the products in a yield of 83–96%.
[(p-HOPh)(p-tBuPh)3P]Zn 2. Yield: 38.8 mg (90%);
1H NMR (400 MHz, DMSO-d6): d 9.86 (s, 1H, OH), 8.84
(d, 4H, b), 8.79 (m, 4H, b), 8.11 (d, 6H, o-Ph), 7.97 (d,
2H, o-Ph), 7.81 (d, 6H, m-Ph), 7.18 (d, 2H, m-Ph), 1.57
(s, 27H, tbu); MALDI-TOF mass: calcd 862.4, found
861.6. Elemental analysis calcd (%) for ZnC56H52N4O:
C 77.99, H 6.08, N 6.50; found: C 77.25, H 6.19, N 6.38.
Main IR band (cm-1): 3435 (n O–H).
The composition of each synthesized porphyrin was
confirmed by MALDI-TOF mass spectrometry. The
molecular ion [M]+ peak was observed in the spectrum of
each compound, and satisfactory elemental analyses were
also obtained for compounds 1–5 after repeated column
chromatographic purification and recrystallization. In
addition, well-resolved 1H NMR spectra (see Figs S1–S5)
were recorded in DMSO-d6 and the peak assignments
could easily be made on the basis of integration and
multiplicity of the signals.
[cis-(p-HOPh)2(p-tBuPh)2P]Zn 3. Yield: 39.4 mg
1
(96%); H NMR (400 MHz, DMSO-d6): d 9.86 (s, 2H,
OH), 8.84 (d, 4H, b), 8.77 (m, 4H, b), 8.11 (d, 4H,
o-Ph), 7.97 (d, 4H, o-Ph), 7.82 (d, 4H, m-Ph), 7.19 (d,
4H, m-Ph), 1.58 (s, 18H, tbu); MALDI-TOF mass: calcd
822.3, found 821.1. Elemental analysis calcd (%) for
ZnC52H44N4O2: C 75.95, H 5.39, N 6.81; found: C 75.78,
H 5.39, N 7.07; Main IR band (cm-1): 3453 (n O–H).
[(p-HOPh)3(p-tBuPh)P]Zn 4. Yield: 33.6 mg (86%);
1H NMR (400 MHz, DMSO-d6): d 9.85 (s, 3H, OH),
8.83 (d, 4H, b), 8.76 (d, 4H, b), 8.12 (d, 2H, o-Ph), 7.97
(d, 6H, o-Ph), 7.83 (d, 2H, m-Ph), 7.18 (d, 6H, m-Ph),
1.58 (s, 9H, tbu); MALDI-TOF mass: calcd 782.2, found
781.3. Elemental analysis calcd (%) for ZnC48H36N4O3:
C 73.70, H 4.64, N 7.16; found: C 73.55, H 4.37, N 7.12;
Main IR band (cm-1): 3457 (n O–H).
UV-Vis spectra
Previously characterized zinc hydroxyphenylpor-
phyrins were shown to exhibit a single Soret band and
two Q bands [26], and this was also the case for the
currently investigated compounds 1–5 in each solvent.
Examples of spectra for compound 4 in CH2Cl2, DMF,
DMSO or Py are given in Fig. 1 and a summary of the
absorption peak maxima is given in Table 1.
Compound 4 exhibits a Soret band at 423 nm and two
Q bands at 551 and 592 nm in the non-binding solvent
CH2Cl2, and all three bands are red-shifted by 6–17 nm
when the solvent is changed to DMF, DMSO or Py. As
seen in the table, the Soret band is located at 429 nm in
DMF, 432 nm in DMSO and 434 nm in Py while the Q
bands are located at 562 and 604 nm in DMF, 564 and
605 nm in DMSO and 568 and 609 nm in Py, respectively.
The relative intensity of the two Q bands varies with the
solvent, but similar values are obtained in DMF, DMSO
and Py as shown in Fig. 1 for the case of 4, where the
1
[(p-HOPh)4P]Zn 5. Yield: 30.8 mg (83%); H NMR
(400 MHz, DMSO-d6): d 9.85 (s, 4H, OH), 8.80 (s, 8H,
b), 7.96 (d, 8H, o-Ph), 7.18 (d, 8H, m-Ph); MALDI-TOF
mass: calcd 741.3, found 742.1. Elemental analysis calcd
(%) for ZnC44H28N4O4: C 71.21, H 3.80, N 7.55; found: C
71.55, H 3.73, N 7.39; Main IR band (cm-1): 3298 (n O–H).
ratio of the absorbance of the two Q bands, AQ(1,0)/AQ(0,0)
,
is the value of 3.30 in the non-binding solvent CH2Cl2.
As compared to 1.30 in DMF, 1.26 in DMSO and 1.11
in Py, where [(p-HOPh)3(p-tBuPh)P]Zn(S) is generated
as shown in Eq 1.
RESULTS AND DISCUSSION
Synthesis
[(p-HOPh)n(p-tBuPh)4-nP]Zn + S
 [(p-HOPh)n(p-tBuPh)4-nP]Zn(S)
Standard procedures were employed to metalate each
free-base porphyrin with a Zn(II) salt to give the zinc
(1)
Fig. 1. UV-visible spectra of [(p-HOPh)3(p-tBuPh)P]Zn 4 in different solvents
Copyright © 2017 World Scientific Publishing Company
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G. LU ET AL.
Table 1. UV-vis spectral data (λmax, nm) of compounds 1–5 in different
solvents
Solvent
DCM
Compound
[(p-tBuPh)4P]Zn 1
Soret band
Q-band
421
422
423
423
424
549
589
590
591
592
593
[(p-HOPh)(p-tBuPh)3P]Zn 2
[cis-(p-HOPh)2(p-tBuPh)2P]Zn 3
[(p-HOPh)3(p-tBuPh)P]Zn 4
[(p-HOPh)4P]Zn 5
550
551
551
552
DMF
DMSO
Py
[(p-tBuPh)4P]Zn 1
426
427
428
429
430
560
560
561
562
563
601
602
603
604
605
[(p-HOPh)(p-tBuPh)3P]Zn 2
[cis-(p-HOPh)2(p-tBuPh)2P]Zn 3
[(p-HOPh)3(p-tBuPh)P]Zn 4
[(p-HOPh)4P]Zn 5
[(p-tBuPh)4P]Zn 1
429
430
431
432
433
562
563
564
564
565
602
603
604
605
606
[(p-HOPh)(p-tBuPh)3P]Zn 2
[cis-(p-HOPh)2(p-tBuPh)2P]Zn 3
[(p-HOPh)3(p-tBuPh)P]Zn 4
[(p-HOPh)4P]Zn 5
[(p-tBuPh)4P]Zn 1
431
432
433
434
435
565
566
567
568
569
606
607
608
609
610
[(p-HOPh)(p-tBuPh)3P]Zn 2
[cis-(p-HOPh)2(p-tBuPh)2P]Zn 3
[(p-HOPh)3(p-tBuPh)P]Zn 4
[(p-HOPh)4P]Zn 5
the Soret band maxima and the relative intensity of the
two Q bands in a given solvent both depend upon the
presence or absence of solvent axial ligand binding to
the Zn(II) center [24, 27, 28].
The position of the Soret absorption bands and the
ratio of absorbances for the two Q bands of the porphyrin
are also both dependent upon the type and position of
substituents on the macrocycle. As seen in Table 1 and
Fig. 3, the porphyrin Soret bands in DMSO (at 429 nm
for 1 and 433 nm for 5) undergo a systematic shift
towards larger wavelengths with sequential increases in
the sum of the Hammett substituent constants (Ss) for
groups at the para-positions of the meso-phenyl rings.
Fig. 4 shows the plot of the Soret band energy (Eg, eV)
vs. the sum of the Hammett substituent constants for
compounds 1–5 in DMSO. The ratio of the Q(1,0)/Q(0,0)
band intensity is 1.63 in DMSO for the porphyrin lacking
OH substituents and decreases to 1.59, 1.34, 1.26 and
1.17 upon a stepwise increase in the number of the HO
groups on the meso-phenyl rings of the macrocycle.
Fig. 2. Plot of the Soret band energy (Eg, eV) for [(p-HOPh)3(p-
tBuPh)P]Zn 4 vs. the Gutmann Donor Number (DN) of solvents
The changes in Soret band position with changes in
the solvent can be quantitatively described by correlations
between the energy (Eg, eV) of the Soret band and the
Gutmann donor number (DN) of the solvent [23]. As
shown in Fig. 2, a linear correlation is observed for
compound 4 between the Soret band energy (E) and the
DN of the three binding solvents. Similar linear
correlations were observed for the other four investigated
compounds, consistent with the fact that the red shift of
Solvent binding reactions
Spectral titration methods were utilized to
characterize solvent binding to the Zn(II) center of the
porphyrin, and examples of the UV-vis spectral changes
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SOLVENT AND SUBSTITUENT EFFECTS ON UV-VIS SPECTRA AND REDOX PROPERTIES
5
Fig. 3. UV-visible spectra of compounds 1–5 in DMSO
was added stepwise to a CH2Cl2 solution of the
compound. The logK values were calculated using the
standard equations, and a summary of the solvent
binding constants is given in Table 2. In each case the
final porphyrin products of the titration are characterized
by a red-shifted Soret band and two red-shifted Q-bands
as compared to the initial porphyrin in CH2Cl2.
These types of spectral changes are typical for formation
of five-coordinate Zn(II) porphyrins in nonaqueous
media [24].
The magnitude of the solvent binding constants for
(OEP)Zn, (OEP)FeCl and (OEP)MnCl were previously
Fig. 4. Plot of the Soret band energy (Eg, eV) vs. the sum of
the Hammett substituent constants (Ss) for compounds 1–5 in
DMSO
shown to correlate with the Gutmann solvent donor
numbers [24], and this was also the case for the currently
investigated zinc porphyrins, an example of which is
shown in Fig. 6 for compound 2.
The magnitude of the binding constant, logK,
should also be related to the sum of the Hammett
constants (Ss) for groups at the para-positions of the
four phenyl rings. For example, in a previously
investigated series of (p-X)TPPCo derivatives (X =
OCH3, CH3, H, F, Cl, CN and NO2) [29], a larger solvent
binding constant was observed in solvents with larger
Hammett constants, while smaller solvent binding
constants were observed for compounds whose sub-
stituents had smaller Hammett constants. Thus, the logK
for solvent binding constant was expected to decrease
upon going from compound 1 to 5 in each investi-
gated solvent. This is indeed the case, as shown in
Table 2 and Fig. 7.
which occurred for compound 2 during titration with
DMF, DMSO or Py in CH2Cl2, are shown in Fig. 5.
These spectral changes were analyzed as a function of
the solvent concentration at each step of the titration and
this enabled calculation of the logK for solvent binding
along with the number of axially bound solvent molecules
using the Hill equation. For example, a diagnostic
analysis plot of the measured absorption at 427 nm is
shown in Fig. 5a, where a plot of log[(Ai–Ao)/(Af–Ai)]
vs. log[DMF] is linear with a slope of 1.0, indicating
that a single DMF molecule is added in a one-step
process to give the five-coordinate [(p-HOPh)
(p-tBuPh)3P]Zn(DMF) as the final porphyrin product. A
solvent binding constant of logK = 1.63 was then
calculated from the zero intercept of this plot. Similar
spectral changes were obtained during the titration of 2
with DMSO (Fig. 5b) or pyridine (Fig. 5c) and a five-
coordinate species is formed in each case, with logK =
2.46 for [(p-HOPh)(p-tBuPh)3P]Zn(DMSO) and 3.71
for [(p-HOPh)-(p-tBuPh)3P]Zn(Py).
Electrochemistry
The electrochemistry of each porphyrin was
examined in four nonaqueous solvents containing
0.1 M TBAP as supporting electrolyte. Half-wave
potentials for oxidation and reduction in each solvent are
summarized in Table 3 and examples of cyclic
voltammograms are illustrated in Figs 8 and 9.
Similar changes in the UV-vis spectra were observed
for the four other porphyrins when DMF, DMSO or Py
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G. LU ET AL.
λ
λ
λ
Fig. 5. UV-visible spectral changes during a titration of [(p-HOPh)(p-tBuPh)3P]Zn 2 with (a) DMF, (b) DMSO and (c) Py in CH2Cl2.
The insets are the Hill plots used to calculate the binding constants
Table 2. Solvent binding constants (logK) of compounds 1–5 in CH2Cl2
Compound
Ss
Binding constants (logK)
DMF
DMSO
Py
[(p-tBuPh)4P]Zn 1
-0.80
-0.97
-1.14
-1.31
-1.48
1.65
1.63
1.60
1.57
1.55
2.49
2.46
2.43
2.40
2.36
3.73
3.71
3.69
3.67
3.65
[(p-HOPh)(p-tBuPh)3P]Zn 2
[cis-(p-HOPh)2(p-tBuPh)2P]Zn 3
[(p-HOPh)3(p-tBuPh)P]Zn 4
[(p-HOPh)4P]Zn 5
The zinc metal center is electroinactive in the
investigated compounds and each porphyrin undergoes
two ring-centered reductions in CH2Cl2, DMSO and Py.
However, an additional reduction is observed in DMF at
much more negative potentials. The first reduction of
1–5 to give a p-anion radical is reversible in each solvent,
but the second one-electron reduction is followed by a
fast chemical reaction providing a species which can be
reversibly reduced by one electron at more negative
potentials. A third reduction was not observed for
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SOLVENT AND SUBSTITUENT EFFECTS ON UV-VIS SPECTRA AND REDOX PROPERTIES
7
Fig. 6. Plot of the binding constants (logK) vs. the Gutmann
Donor Number (DN) of the solvents (DMF = 26.6, DMSO =
29.8, Py = 33.1) for compound 2 in CH2Cl2
Fig. 7. Plots of the binding constants (logK) vs. the sum of the
Hammett substituent constants (Ss) for the addition of DMF,
DMSO and pyridine to zinc porphyrins 1–5 according to Equation 1
Fig. 8 illustrates the cyclic voltammograms
obtained for compound 3 in the four utilized solvents.
The first reduction is located at E1/2 = -1.40 V in CH2Cl2,
compounds 1–5 in other solvents but it should be noted
that compound 5 in DMSO also exhibits three one-
electron reduction processes.
Table 3. Half-wave potentials (V vs. SCE) of compounds 1–5 in different solvents containing 0.1 M TBAP
Compound
Solvent
Oxidation
2nd
1.04 0.71
Reduction
2nd
-1.37 -1.86a
3rd
1st
1st
3rd
[(p-tBuPh)4P]Zn 1
CH2Cl2
DMF
DMSO
Py
0.88b -1.39 -1.82
0.90b -1.37 -1.75
0.88b -1.41 -1.90
-1.97
[(p-HOPh)(p-tBuPh)3P]Zn 2
[cis-(p-HOPh)2(p-tBuPh)2P]Zn 3
[(p-HOPh)3(p-tBuPh)P]Zn 4
[(p-HOPh)4P]Zn 5
CH2Cl2
DMF
DMSO
Py
1.56
1.51
0.99 0.69
-1.40 -1.80a
0.82b -1.42 -1.81
0.85b -1.38 -1.78
0.81b -1.42 -1.90
-2.02
-2.06
-2.09
CH2Cl2
DMF
DMSO
Py
0.97 0.67
-1.40 -1.73a
0.81b -1.41 -1.82
0.82b -1.39 -1.82
0.82b -1.43 -1.90
CH2Cl2
DMF
DMSO
Py
1.58b 0.97 0.67
-1.49 -1.84a
0.78b -1.43 -1.83
0.80b -1.42 -1.82
0.82b -1.46 -1.95a
CH2Cl2
DMF
DMSO
Py
1.60b 0.93 0.65
-1.49 -1.95a
0.78b -1.42 -1.79
0.78b -1.43 -1.82
0.70b -1.47 -1.93
-2.15a
-2.13a
a The irreversible reduction peak potentials, Epc.
b The irreversible oxidation peak potentials, Epa.
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(p-HOPh)x
(a) [(p-tBuPh)4P]Zn
x = 0
0.71
1.04
(HOPh)x ox
1.54
(b) Phenol
x = 1
x = 1
(c) [(p-HOPh)(p-tBuPh)3P]Zn
0.69
0.99
µ
5.9
A
1.56
µ
5.0
A
(d) [cis-[(p-HOPh)2(p-tBuPh)2P]Zn
x = 2
0.67
0.97
µ
6.5
A
1.51
8.0
µ
A
(e) [(p-HOPh)4P]Zn
x = 4
Fig. 8. Cyclic voltammograms showing reductions of
compound 3 in CH2Cl2, DMSO, pyridine and DMF containing
0.1 M TBAP
0.65
0.93
µ
A
3.5
1.60
16.0
-1.41 V in DMF, -1.39 V in DMSO and -1.43 V in
pyridine. Thus, almost identical E1/2 values are obtained
for the first reversible one-electron addition, indicating
that solvent coordination has only a small effect on half-
wave potentials for first ring-centered reduction under
the given solution conditions. The second reduction of
compound 3 is associated with a coupled chemical
reaction and a reoxidation at Epa = -0.56 to -0.58 V in
CH2Cl2, DMSO or DMF but at -0.92 V in Py. The
irreversibility of the second reduction is associated with
formation of a phlorin anion from the porphyrin dianion
as earlier reported in the literature for (TPP)Zn in DMF
[30, 31]. The assignment was confirmed in the present
study by spectroelectrochemistry which is described
below. As shown in Fig. 8, the product of the chemical
reaction for compound 3 in DMF is reversibly reduced
at -2.06 V.
µ
A
2.0
1.6
1.2
0.8
0.4
0.0
Potential ( V vs SCE)
Fig. 9. Cyclic voltammograms showing oxidations of com-
pounds 1, 2, 3 and 5 in CH2Cl2 containing 0.1 M TBAP
Similar oxidations are seen for compounds 2–5
but with the potentials shifted negatively due to the
electron-donating hydroxyphenyl groups. A linear
relationship is observed between the half-wave potential
for the first or second oxidation and the sum of the
Hammett substituents constant (Ss), as shown in Fig. 10.
In addition, one new oxidation process is observed for
compounds 2–5 at potentials of 1.51 to 1.60 V. These
processes are shown within a dotted box in Fig. 9 and
involve the electroactive hydroxyphenyl groups on the
meso-positions of the porphyrin.
A single irreversible oxidation of compounds 1–5 is
observed in DMF, DMSO or Py, while two or three
reversible oxidations are seen in CH2Cl2, the exact
number depending upon the presence of OH substituents
on the meso-phenyl positions of the porphyrin. Compound
1, which lacks a hydroxyphenyl substituent, undergoes
The oxidation of phenol in CH2Cl2 is also shown in
two reversible macrocycle-centered oxidations at E1/2
=
Fig. 9. This quasi-reversible process is located at E1/2
=
0.71 and 1.04 V in CH2Cl2 containing 0.1 M TBAP
(Fig. 9). These oxidations have been shown to generate
the porphyrin p-cation radical and dication.
1.54 V and is at an almost identical half-wave potential
as for oxidation of the hydroxyphenyl groups on
compounds 2 (E1/2 = 1.56 V), 3 (E1/2 = 1.51 V) and 5
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SOLVENT AND SUBSTITUENT EFFECTS ON UV-VIS SPECTRA AND REDOX PROPERTIES
9
second ring-centered oxidation of the same compound at
E1/2 = 0.67 and 0.97 V. Likewise, the current for oxidation
of the hydroxyphenyl groups in compound 5 (x = 4) is
four times higher than the current for the first and second
ring-centered oxidations at E1/2 = 0.65 and 0.93 V. These
results are consistent with a two-electron and four-
electron addition to compounds 3 and 5, respectively, in
the third oxidation step.
Detailed electrochemical studies of meso- and/or
b-substituted porphyrins and corroles have shown that a
larger substituent effect is often observed for reductions
than for oxidations at the p-ring system of the compound
[32, 33]. The first oxidation and first reduction potentials
of [(p-HOPh)n(p-tBuPh)4-nP]Zn also appear to follow this
Fig. 10. Half-wave potentials of the first and second
oxidations for compounds 1–5 in CH2Cl2 containing 0.1 M
TBAP vs. the sum of the Hammett substituent constants (Ss)
trend (see Table 3). For example, the half-wave potential
for the first reduction of [(p-HOPh)4P]Zn 5 (E1/2
=
-1.49 V) in CH2C12 is negatively shifted by 120 mV as
compared to the first reduction of [(p-tBuPh)4P]Zn 1
(E1/2 = -1.37 V) in the same solvent, while the negative
shift in E1/2 for the first oxidation is only 60 mV upon
going from compound 1 to compound 5 in CH2C12.
(Epa = 1.60 V) under the same solution conditions. The
peak current for oxidation of the one hydroxyphenyl
group of compound 2 at 1.56 V is the same as that for the
first or second ligand-based oxidations at 0.69 and
0.99 V. This is consistent with a one-electron transfer
process in each step. In contrast, the peak current for
oxidation of the two hydroxyphenyl groups of compound
3 (x = 2) at 1.51 V is twice as high as that for the first or
Spectroelectrochemistry
Examples of the thin-layer UV-visible spectra
obtained during the first and second reductions of [cis-
(p-HOPh)2(p-tBuPh)2P]Zn 3 in DMF containing 0.1 M
Fig. 11. UV-vis spectral changes of [cis-(p-HOPh)2(p-tBuPh)2P]Zn 3 during the (a) first and (b) second reductions in DMF, 0.1 M TBAP
Copyright © 2017 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2017; 21: 9–11

10
G. LU ET AL.
TBAP are shown in Fig. 11. As seen from this figure, the
Soret band at 428 nm decreases significantly in intensity
Supporting information
1H NMR spectra in DMSO-d6 for compounds 1–5
is given in the supplementary material. This material
is available free of change via the Internet at http://
worldscinet.com/jpp/jpp.shtml.
upon applying a controlled reducing potential of Eapp
=
-1.60 V, while a new band grows in at 465 nm (Fig. 11a).
At the same time the two bands at 561 and 603 nm
completely disappear and a new broad band characteristic
of a porphyrin p-anion radical appears at 662 nm. This
reduction is reversible, and the original electronic
absorption spectrum of the neutral compound could be
regenerated upon reoxidation of the singly reduced
species at a position potential.
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CONCLUSIONS
The electrochemical and spectroelectrochemical
properties of four zinc p-hydroxylphenyl porphyrins
were investigated in CH2Cl2, DMF, DMSO and
pyridine. The effects of the solvent and meso-phenyl
ring substituents on the UV-vis spectra are quantitatively
described by shifts in the energy of the Soret band vs.
the Gutmann solvent donor number (DN) or the sum of
the Hammett constants of the substituents (Ss) at the
para-positions of the macrocycle. Solvent binding
constants (logK) were also determined for the in situ
generated five-coordinate complex [(p-HOPh)n
(p-tBuPh)4-nP]Zn(S).
The investigated compounds undergo two ring-
centered reductions, the second of which is followed by
a chemical reaction of the porphyrin dianion to give an
anionic phlorin product. The phlorin anion is reversibly
reduced by one electron at a more negative potential to
give a phlorin dianion in DMF. Each porphyrin also
undergoes two ring-centered one-electron oxidations,
leading to formation of a p-cation radical and dication in
CH2Cl2 containing 0.1 M TBAP. An additional oxidation
process is also observed for compounds 2–5 which
involves the redox active hydroxyphenyl groups on the
meso-positions of the porphyrin.
Acknowledgments
We gratefully acknowledge support from the National
Natural Science Foundation of China (Grant No.
21001054), the Foundation of Jiangsu University (Grant
No. FCJJ2015020, 05JDG051) and the Robert A. Welch
Foundation (K.M.K., Grant E-680).
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