Synthesis and spectroelectrochemical investigation of two tetraarylporphyrins

Article abstract of DOI:10.1139/V10-098

A free-base tetraarylporphyrin was synthesized by the [2+2] macrocyclization of a dipyrromethane derivative with 3,4,5-tris(dodecyloxy) benzaldehyde in 61% yield. The free-base porphyrin was metallated with zinc acetate in 94% conversion. The free-base an

Full text of DOI:10.1139/V10-098

214
Synthesis and spectroelectrochemical
investigation of two tetraarylporphyrins
Robin A. Kru¨ ger, Andrea S. Terpstra, and Todd C. Sutherland
Abstract: A free-base tetraarylporphyrin was synthesized by the [2+2] macrocyclization of a dipyrromethane derivative
with 3,4,5-tris(dodecyloxy)benzaldehyde in 61% yield. The free-base porphyrin was metallated with zinc acetate in 94%
conversion. The free-base and metallated porphyrins show typical intense Soret bands at 426 and 425 nm, respectively,
along with the expected number and intensity of Q-bands. Both porphyrins are also fluorescent and display small Stokes
shifts between 13 and 18 nm. Cyclic voltammetry established that each porphyrin underwent two reversible one-electron
oxidations at 492 and 725 mV (vs Fc/Fc+ (ferrocene reference)) for the free-base porphyrin and at 329 and 589 mV (vs
Fc/Fc+) for the Zn-metalloporphyrin. Electron-transfer rates were also determined to fall between 1.2 ꢀ 10–3 and 2.4 ꢀ
10–3 cm s–1. In addition, spectroelectrochemistry and density functional theory calculations of the oxidized products were
carried out to confirm the macrocyclic ring oxidations.
Key words: porphyrinoids, electrochemistry, redox chemistry, photochemistry, density functional calculations.
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Resume : On a realise la synthese d’une tetraaraylporphyrine sous la forme de base libre avec un rendement de 61 % par
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macrocyclisation [2+2] d’un derive de dipyrromethane avec du 3,4,5-tris(docecyloxo)benzaldehyde. On a effectue la metal-
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lation de la porphyrine a l’etat de base libre, avec un rendement de 94 %, par traitement avec de l’acetate de zinc. La base
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libre et la porphyrine metallee presentent respectivement les bandes typiques intenses de Soret a 426 et a 425 nm ainsi
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que le nombre et l’intensite attendu des bandes Q. Les deux porphyrines sont aussi fluorescentes et elles presentent de fai-
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bles deplacements de Stokes, entre 13 et 18 nm. La voltamperometrie cyclique a permis d’etablir que chaque porphyrine
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subit deux oxydations reversibles a un electron, a 492 et 725 mV (vs Fc/Fc ) pour la base libre et a 329 et 589 mV (vs
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Fc/Fc ) pour la Zn-metalloporphyrine. On a aussi determine que les vitesses de transfert d’electron se situent entre 1,2 ꢀ
10–3 et 2,4 ꢀ 10–3 cm s . De plus, de la spectroelectrochimie et des calculs d’apres la theorie de la fonctionnelle de la
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densite effectues sur les produits oxydes pour confirmer les oxydations des noyaux macrocycliques.
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Mots-cles : porphyrinoıdes, electrochimie, chimie d’oxydoreduction, photochimie, calculs de la fonctionnelle de la densite.
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[Traduit par la Redaction]
ing and oxidizing strength of these excited-state reagents is
crucial to understanding the driving forces that are operating
in the eventual applications. Generally, porphyrin ring sys-
tems can be easily oxidized to radical cations and sometimes
dications.20–24 Importantly, these cations are stable at room
temperature and can be used as reactive intermediates that
could be used in organic materials applications.
This contribution explores the electronic properties of two
porphyrins and compares them with two well-characterized
porphyrins, namely free-base tetraphenylporphyrin (TPP)
and Zn-TPP. In addition, full electrochemical and density
functional theory (DFT) studies were carried out to assess
the charge-transport properties of the oxidized porphyrin
ring systems. The goal of this work is to install electron-
rich substituents to the meso positions of porphyrins to
make stabilized cations for use as light-harvesting chromo-
phores in solar cell applications. Important parameters for a
candidate material are high molar absorptivity and excited-
Introduction
Porphyrinoids are commonly found compounds in both
the animal and plant kingdoms, as they play important roles
in the key steps of photosynthesis that ultimately convert
sunlight into chemical energy.1,2 Iron porphyrins play crit-
ical roles in oxygen transport,3,4 peroxide reduction,5–7 drug
metabolism (cytochrome P450),8,9 and the mitochondrial
electron-transport chain.10,11 Because of their prevalence in
nature, exploration of their potential applications in organic
materials is warranted. Photoinitiated charge-transfer reac-
tions are key steps in solar energy conversion,2,12,13 device
applications,14 and catalysis.15,16 Progress in these applica-
tions necessitates a critical understanding of the thermody-
namic factors that direct the charge-transfer reactions.
Excited-state reactants are versatile intermediates because
they are both better electron donors and better electron ac-
ceptors than their ground-state counterparts.17–19 The reduc-
Received 26 February 2010. Accepted 20 May 2010. Published on the NRC Research Press Web site at canjchem.nrc.ca on 25 January
2011.
This article is part of a Special Issue dedicated to Professor J. C. Scaiano.
R.A. Kru¨ger, A.S. Terpstra, and T.C. Sutherland.1 Department of Chemistry, University of Calgary, 2500 University Drive NW,
Calgary, AB T2N 1N4, Canada.
1Corresponding author (e-mail: todd.sutherland@ucalgary.ca).
Can. J. Chem. 89: 214–220 (2011)
doi:10.1139/V10-098
Published by NRC Research Press

Kru¨ ger et al.
215
state redox potential that are compatible with a typical n-type
material, such as phenyl-C61-butyric acid methyl ester
(PCBM), and ground-state stability of the radical cation.
Furthermore, these compounds were synthesized to explore
whether supramolecular structures, such as liquid crystals,
could form using meso-aryl substituted porphyrins. A com-
mon motif, the trisalkoxyphenyl group, has been employed
in a variety of functional materials25–29 to impart supramo-
lecular long-range ordering that may enhance charge trans-
port in solar applications.
Scheme 1. Synthesis of Zn-porphyrin.
Experimental
Zinc-porphyrin (4)
Free-base porphyrin (30 mg, 10.6 mmol), NaHCO3
(100 mg, 450 mmol) and zinc acetate dihydrate (100 mg,
450 mmol) were stirred at ambient temperature in CHCl3
for 4 h in the dark. The red solution was then poured on di-
luted aqueous NaHCO3 solution (50 mL). After phase sepa-
ration, the aqueous layer was extracted with CH2Cl2 (1 ꢀ
20 mL) and the combined organic layers were dried with
MgSO4 and evaporated. The crude red oil was purified by
column chromatography using CH2Cl2 as the eluent on alu-
mina (N, III) and collection of the first red fraction. The
yield was 29 mg (10 mmol, 94%). 1H NMR (CD2Cl2,
400 MHz) d: 9.10 (s, 8 H, aryl-H), 7.45 (s, 8 H, pyr-H),
4.29 (t, J = 6.6 Hz, 8 H, OCH2), 4.11 (t, J = 6.5 Hz, 16 H,
OCH2), 2.01–1.94 (m, 8 H, CH2), 1.93–1.86 (m, 16 H, CH2),
1.72–1.65 (m, 8 H, CH2), 1.56–1.25 (m, 208 H, CH2), 0.93
(t, J = 7.0 Hz, 12 H, CH3), 0.87 (t, J = 7.0 Hz, 24 H, CH3).
UV–vis (CH2Cl2) labs (nm) [3 (L mol–1 cm–1)] = 425
[130 000], 550 [6600], 589 [1300]. lem (nm) (relative inten-
sity) (lex = 425 nm) = 607 (0.96), 649 (1). HR–MS
(MALDI-TOF) calcd. for
found: 2886.365.
C188H316N4O12Zn: 2886.353;
bands of free-base tetraphenylporphyrin (H2-TPP) and Zn-
tetraphenylporphyrin (Zn-TPP) because of the electron-
donating tris(alkoxy) meso-substituents.
Results and discussion
The synthesis of metal-free porphyrin 3 and Zn-porphyrin
4 is shown in Scheme 1. The synthesis of 1 was carried out
using literature methods.30,31 Condensation of freshly dis-
tilled excess pyrrole and TFA with 1 resulted in the forma-
tion of the dipyrromethane 2 in 57% yield. Note that 2 was
moderately stable at room temperature under N2 atmosphere
and was immediately used in the ring-closing condensation
with 1 to produce the free-base porphyrin 3 in 61% yield.32
Porphyrin 3 was easily metallated with Zn-acetate at room
temperature in the dark to give Zn-porphyrin 4 in 94% yield.
The electronic absorption and emission properties of por-
phyrins 3 and 4 are summarized in Table 1, and the spectra
are included in the Supplementary data. Both 3 and 4 ex-
hibit strong near-UV absorptions (Soret) attributed to the
S0 ? S2 transition and sets of weaker visible transitions (Q-
bands) attributed to the S0 ? S1 transitions, typical of por-
phyrins.33 The absorption and emission properties of 3 and 4
are consistent with other reports, which have been exten-
sively reviewed.34
The emission spectra of 3 and 4 are similar to the emis-
sion spectra of H2-TPP and Zn-TPP, respectively. Note the
excitation wavelength used in each compound was the peak
of the Soret band. Free-base porphyrin 3 has two emission
peaks at 661 nm and 722 nm with peak height ratios of 1 to
0.44. The Q(0,0) peak position of 3 is red-shifted by 11 nm
compared with H2-TPP, whereas the vibronic sideband of 3
is red-shifted by only 5 nm, accompanied by a *50% de-
crease in peak intensity compared with H2-TPP. The emis-
sion spectrum of Zn-porphyrin 4 also exhibits two red-
shifted peaks compared with Zn-TPP at 607 nm and
649 nm. The emission peaks are typical for porphyrinoid
compounds and are consistent with emission from singlet
excited states. Triplet emissions typically show at 800 nm
and greater wavelengths.
Charge-transfer dynamics were assessed by cyclic voltam-
metry (CV) for the oxidation reaction. Figure 1a shows the
cyclic voltammagrams of 3 at various scan rates. Each vol-
tammagram shows two quasi-reversible oxidation waves,
which are assigned to two, one-electron oxidation steps,
consistent with other reports of related porphyrins.35–40 The
The Soret peak, B(0,0), for 3 is positioned at 426 nm with
a full width at half maximum (FWHM) of 20 nm, which is
similar to the Soret band of 4 at 425 nm with a narrower
FWHM of 14 nm (see Supplementary data). The Soret bands
of 3 and 4 are slightly red-shifted compared with Soret
ox
1=2
half-wave potentials (E ) were calculated from the average
of the anodic and cathodic peaks of each step, which results
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216
Can. J. Chem. Vol. 89, 2011
Table 1. Absorption and luminescence peaks of 3 and 4 in CH2Cl2 at room temperature.
Absorption peaksa
Luminescence peaksb
B(0,0)
Qy(1,0)
514(19.4)
518(7.1)
Qy(0,0)
547(8.2)
555(3.4)
Qx(1,0)
Qx(0,0)
Q(0,0)
650(1)
661(1)
595(1)
607(1)
Q(0,1)
Stokes shift (nm)
H2-TPPc
419(413)
426(151)
422(574)
425(130)
592(6.6)
592(2.2)
548(29.9)
550(6.6)
646(4.6)
648(1.7)
588(5.1)
589(1.3)
717(0.84)
722(0.44)
645(2.1)
649(1.1)
4
13
7
3
Zn-TPPc
4
18
Note: H2-TPP, free-base tetraphenylporphyrin; Zn-TPP, zinc-tetraphenylporphyrin.
aAll absorption peaks are shown in nanometres followed by molar absorptivities in parentheses with units of 103 L mol–1 cm–1.
bAll luminescence peaks are shown in nanometres followed by ratio of intensities. Compounds were excited at B bands.
cThese data were taken from PhotoChemCAD.
ox
1=2
ox
1=2
in E (1) of 492 mV and E (2) of 725 mV vs Fc/Fc+
Fig. 1. (a) Cyclic voltammagram of 3 (4.5 ꢀ 10–4 mol L–1) in
CH2Cl2 containing 0.1 mol L–1 tetrabutylammonium hexafluoro-
phosphate (TBAPF6) at various scan rates using a glassy carbon
electrode, a Ag|AgCl|KCl3M reference electrode, and a Pt wire
counter electrode at 25 8C. (b) Peak heights of the two oxidation
and reduction peaks as a function of the square root of the scan rate
for porphyrin 3.
(ferrocene reference). The difference between the two oxida-
tion half-wave potentials is 233 mV, which is similar to H2-
TPP.
The CV at slow scan rate (20 mV s–1) provides a good
indication of reversibility with peak separations of 78 mV
between anodic and cathodic redox couples, where peak sep-
arations of 57 mV are considered ideal reversible Nernstian
behaviour. The capacitive current, which is quantified as the
difference between anodic and cathodic currents in a region
that does not contain Faradic responses, is unexpectedly
large. There are reports of alkyl-derivatized porphyrins that
form liquid crystals41–44 or films on electrodes45–48 and this
is possible with compound 3; however, porphyrin 3 is very
soluble in CH2Cl2, which should minimize film formation.
In addition, the linear plot in Fig. 1b indicates the redox re-
action is diffusion controlled. Peak current is related to scan
rate according to eq. [1]49
½1ꢁ
ip ¼ ð2:69 ꢀ 105Þn3=2ADO1=2COꢂ y1=2
where ip is the peak current in amperes, n is the number of
electrons involved in the redox reaction, A is the area of the
electrode surface in cm2, DO is the diffusion constant of the
oxidized species in cm2 s–1, CO is the concentration of redox
species in mol cm–3, and n is the scan rate in V s–1. From
eq. [1], plots in Fig. 1b can be used to determine the diffu-
sion coefficient, assuming the diffusion coefficients of the
oxidized and reduced porphyrins are equal (DO & DR). The
diffusion coefficients for 3+ and 32+ were found to be 3.3 ꢀ
10–7 cm2 s–1 and 5.6 ꢀ 10–7 cm2 s–1, respectively. Using the
Stokes–Einstein equation and assuming spherical com-
pounds, these diffusion coefficients correspond to radii of
16 nm and 9 nm for 3+ and 32+, respectively. Clearly, these
hydration radii are much larger than the molecular dimen-
sions and could indicate aggregates are forming in solution
under CV conditions that are not present at the concentra-
tions used for optical studies. The proposal of aggregate for-
mation is also consistent with the large non-Faradic currents
and broad peaks due to different sizes of aggregates. The es-
timate of the diffusion coefficient enables the use of Nichol-
son’s approach50 to calculating the electron-transfer rate
constant from the difference in peak potentials at each scan
rate. The standard rate constants (k0) for the first and second
one-electron oxidations, shown in eq. [2], are 2.4 ꢀ
10–3 cm s–1 and 1.4 ꢀ 10–3 cm s–1, respectively, for 3. The
electron-transfer rate constants for 3 are typical for numer-
ous porphyrin systems.51
ꢃeꢃ
ꢃeꢃ
þ
þ
2
3
!
!
½2ꢁ
3
3
k01
k02
The CVs at increasing scan rates of Zn-porphyrin 4 are
included in the Supplementary data because they are similar
to those of the free-base porphyrin 3, exhibiting two quasi-
reversible, one-electron oxidation steps. The two redox
waves are typical for several Zn-porphyrinoid compounds52
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Kru¨ ger et al.
217
Table 2. Summary of electrochemical parameters.
ox
1=2
(mV)
ox
E
(1)
E (2)
1=2
DE
(mV)
DO(1)
DO(2)
k0(1)
k0(2)
rH(1)
(nm)
rH(2)
(nm)
Compound
(mV)
(10–6 cm2 s–1)
(10–6 cm2 s–1)
(10–3 cm s–1)
(10–3 cm s–1)
3
4
492
329
725
589
233
260
0.33
1.4
0.56
2.8
2.4
1.2
1.4
1.9
16
4
9
2
Note: All potentials are referenced to Fc/Fc+. Diffusion coefficients of oxidized and reduced species were assumed to be the same. Rate constants were
calculated using Nicholson’s method. Hydration radii were calculated using solvent viscosity of CH2Cl2 at 0.413 mPa s.
and the peaks are also easier to oxidize than 3. The two
Fig. 2. Spectroelectrochemistry of 3. (a) UV–vis spectra of com-
pound 3 at an applied potential of +1.0 V. Each spectrum is re-
corded at 2 min intervals and arrows indicate the direction of peak
change. (b) The absorbance difference spectra of the data shown in
Fig. 2a. Positive peaks indicate peak growth compared with neutral
porphyrin.
ox
ox
half-wave potentials of 4, E (1) and E (2), were 329 mV
1=2
1=2
and 589 mV vs Fc/Fc+, respectively. The difference between
the two oxidation half-wave potentials is 260 mV, which is
similar to the difference in the case of Zn-TPP. The peak
separation at the slowest scan rate (10 mV s–1) is 88 mV
and 81 mV for the first and second oxidation peaks, respec-
tively, consistent with quasi-reversible redox reactions.
The capacitive currents in Zn-porphyrin 4 are much less
than in the CVs for free-base porphyrin 3, even though the
concentration of 4 in the electrochemical cell was higher
than that of 3. In comparing the CVs of 3 and 4, it is clear
that the CV of 3 displays unique diffusion effects. Employ-
ing eq. [1], the diffusion coefficients were calculated to be
1.4 ꢀ 10–6 cm2 s–1 and 2.8 ꢀ 10–6 cm2 s–1 for 4+ and 42+,
respectively, resulting in hydration radii of 4 nm and 2 nm
for 4+ and 42+, respectively. Although the diffusion coeffi-
cients are much higher in the Zn-porphyrin case, the stand-
ard electron-transfer rates constants for the first and second
oxidation reactions are 1.2 ꢀ 10–3 cm s–1 and 1.9 ꢀ
10–3 cm s–1, respectively, which are similar to those of com-
pound 3. A summary of all electrochemical results is in-
cluded in Table 2.
The electrochemical oxidation potentials in conjunction
with the highest-energy fluorescent band, corresponding to
the lowest-lying singlet state, allow for the estimation of the
excited-state oxidation potential, first described by Rehm
and Weller53,54 in eq. [3].
ꢂox
ox
¼ E ꢃ E00 þ ur
½3ꢁ
E
1=2
1=2
ꢂox
ox
1=2
where E is the excited-state oxidation potential, E is the
1=2
ground-state redox potential, E00 is the energy gap between
the zeroth vibrational levels of the ground and excited states,
and ur is the electrostatic work term. For this work, it is as-
sumed that the electrostatic work term has been adequately
accounted for within the measurement of the ground-state
redox potentials. The excited-state oxidation potentials for 3
and 4 are –1.39 V and –1.71 V vs Fc/Fc+, respectively. For
comparison, H2-TPP is estimated to have a singlet excited-
state redox potential of –1.32 V vs Fc/Fc+.55 The second
oxidation potential was not calculated because the fluores-
cence spectrum of the radical cation is unknown.
The absorption spectrum of 3 oxidized to 3+ is shown in
Fig. 2a. Figure 2a shows three main features consistent with
the ring oxidation.56–60 The first is the decrease in the Soret
band intensity. Concomitantly, there are two new features
increasing in intensity at 470 nm and 696 nm. The isosbestic
points at 400 nm and 453 nm suggest the UV–vis spectra
represent two species that are in equilibrium, which is con-
sistent with the reversible nature of the CV results for 3.
Figure 2b shows the absorbance difference spectra of the
data shown in Fig. 2a. The resulting plot shows more of the
smaller transitions and clearly indicates isosbestic points as
the data cross the y = 0 line. The growth of the largest peak
at 470 nm is similar to other spectroelectrochemical results
of other porphyrins and is tentatively assigned to the Soret
band of the radical cation, 3+.
The smaller features that are visible in the difference
spectra (Fig. 2b) at 536, 574, and 634 nm are assigned to
the Q-bands of the oxidized porphyrin. After exhaustive ox-
idation of 3 to 3+, the potential was stepped to 1.2 V, where
the conversion of 3+ to 32+ is expected to occur. The absorp-
tion spectrum of 32+ is not shown; however, no significant
changes were observed with the exception of a slight in-
crease in the peak at 696 nm. The lack of visible changes is
due to two factors. First, the electronic transitions of 32+
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218
Can. J. Chem. Vol. 89, 2011
could be weak, and second, the cell design uses a longer
path length (1 mm) such that the resultant spectrum is a lin-
ear combination of all three species diffusing into the light
path. In addition, the oxidized form may be more reactive
and decompose. Although CV evidence supports a stable re-
dox couple, the spectroelectrochemical cell design does not
permit exclusion of O2 from the cell.
Fig. 3. DFT-calculated (B3LYP/6-31+G) energy levels of the fron-
tier orbitals of the neutral, radical cation, and dication of com-
pounds 3 and 4. The HOMO–LUMO energy differences are shown
over the arrows.
Spectroelectrochemical experiments were carried out with
4 to investigate the electronic structure of the oxidized form
of 4, which is included in the Supplementary data. The oxi-
dation of 4 to 4+ shows a decrease in the Soret band with an
applied potential of 0.8 V (first oxidation potential 4/4+).
Upon oxidation, a new Soret band is seen evolving at
442 nm, which is blue-shifted from the 3+ radical cation spe-
cies. New Q-bands of the radical cation (4+) appear at 523,
577, 643, and 680 nm. In addition, a new broad transition
occurs at 897 nm. Spectroelectrochemical experiments were
also carried out at applied potentials equal to the second ox-
ox
1=2
idation. However, at the E (2), the spectral features were
nearly the same as the spectrum of 4+. After spectral sub-
traction of the oxidized spectrum (4+), a broad peak at
*900 nm was observed, but considering the long path
length (1 mm), it is clear that the dication (42+) spectrum
consists of neutral 4, radical cation 4+, and the dication 42+,
which precludes accurate spectral deconvolution.
Fig. 4. FMOs of the radical cations 3+ and 4+ using DFT with the
B3LYP basis set and 6-31G+ functional.
Structures 3, 4, 3+, 4+, 32+, and 42+ were energy optimized
using DFT methods with the B3LYP basis set and 6-31+G
functional using the Gaussian03 software package.61 The
summaries of the frontier orbital energy levels are shown in
Fig. 3. Based on the gas-phase calculations, the HOMO en-
ergy levels of the free-base porphyrin 3 and the Zn-metallo-
porphyrin 4 are very similar in energy. This HOMO energy
level trend was not observed in the experimentally deter-
mined oxidation potential. Experimental results show the
Zn-porphyrin is easier to oxidize, suggesting it has a
higher-lying HOMO. This inconsistency could arise from
solvent effects that are not accounted for in the calculations.
The two highest occupied FMOs of 3 and 4 are nearly de-
generate and both have nearly degenerate LUMOs, consis-
tent with the four-orbit model.33 The HOMO–LUMO
energy gap is similar for 3 and 4, which is consistent with
the observed electronic absorption spectroscopy. Both radi-
cal cations, 3+ and 4+, show red-shifted adsorptions com-
pared with their neutral parent compounds. The SOMO of
4+ is nearly triply degenerate, whereas the SOMO of 3+
does not follow the same pattern. The FMOs of 3+ and 4+
are shown in Fig. 4. The triply degenerate occupied orbitals
of 4+ are confined to the electron-rich trisalkoxy phenyl
rings, in contrast to 3+, where the SOMO orbital is delocal-
ized over the macrocyclic ring and the four trisalkoxy phe-
nyl rings. The dication species 32+ and 42+ show the lowest
HOMO–LUMO energy difference of 0.8 eV, corresponding
to 1550 nm, which was not observed in the absorption spec-
tra. In addition, the HOMOs of both dications are triply de-
generate and the LUMOs are non-degenerate.
little electronic perturbations can occur from a meso-phenyl
substitution pattern. The monocations 3+ and 4+ have differ-
ent minimized structures. The free base 3+ has a SOMO that
is delocalized over the macrocyclic ring and the four phenyl
meso-substituents and a twist in the meso-phenyl groups,
such that contributions from the phenyl rings can participate
in delocalization. In contrast, the Zn-porphyrin cation, 4+,
maintains the same geometry as neutral 4 and the SOMO is
limited to the four meso-substituted phenyl rings. Both
LUMO and LUMO+1 of both monocations (3+ and 4+) are
centered on the macrocyclic ring system. The removal of an
additional electron to the dications results in HOMOs that
reside on the four phenyl substituents. In addition, both dica-
tions 32+ and 42+ show a twist in the meso-phenyl substitu-
ents allowing for stabilization of the dicationic macrocycle.
The LUMOs of the dications show coefficients on both the
phenyl substituents and the macrocycle. The calculations in-
dicate extensive electronic tuning is possible by modifying
the b-pyrrole positions, and limited HOMO energy level tun-
FMOs for all compounds are included in the Supplemen-
tary data. Starting with the neutral porphyrins 3 and 4, the
doubly degenerate HOMOs and LUMOs show electron den-
sity confined to the porphyrin macrocyclic ring. In addition,
the energy-minimized structures have the meso-substituted
phenyl rings perpendicular to the macrocyclic ring, implying
Published by NRC Research Press

Kru¨ ger et al.
219
ing is possible using phenyl-substituted meso positions.
However, the mono- and dications can be strongly stabilized
by electron-rich meso-derivatives.
and the Institute for Sustainable Energy, Environment and
Economy, for funding.
References
(1) Wasielewski, M. R. Chem. Rev. 1992, 92 (3), 435. doi:10.
1021/cr00011a005.
Conclusions
The optical measurements of the tetraarylporphyrins dis-
play typical behaviour with intense Soret and Q-band ab-
sorptions and fluorescence spectra with small Stokes shifts.
Electrochemical oxidations of both the free-base porphyrin
3 and Zn-metalloporphyrin 4 show two reversible one-elec-
tron redox waves by cyclic voltammetry methods. Further
analysis of the cyclic voltammetry data revealed an unusu-
ally large diffusion coefficient for 3, suggesting aggregate
formation. Considering the size and number of alkoxy
groups, aggregation is feasible, as others have used such mo-
tifs to build long-range order in liquid crystals. The electron-
transfer rate constants were 1.2 ꢀ 10–3 cm s–1 and 2.4 ꢀ
10–3 cm s–1 for compounds 3 and 4, respectively, which falls
within the range of other porphyrins. Spectroelectrochemical
experiments suggest that the electron removed to form the
radical cations (3+ and 4+) is part of the porphyrin macro-
cycle. In addition, the spectroelectrochemical results are
supported by the DFT-calculated energy-optimized struc-
tures of 3 and 4 in their neutral and radical cation states.
The reversibility and stability of the oxidized forms of both
3 and 4 support the use of these compounds in organic devi-
ces as light harvesters or supramolecular charge-transfer as-
semblies. Of particular utility could be the excited-state
energy of the oxidized porphyrin ring systems to generate
charge-separated components used in solar energy conver-
sion. The electrochemically determined HOMO energy lev-
els of 3 and 4 are –5.3 eV and –5.1 eV (assuming the
HOMO energy level of Fc/Fc+ is –4.8 eV), respectively,
which is comparable to the ubiquitous poly-3-hexyl-
thiophene (P3HT) at –5.0 eV and LUMO energy of –3.3 eV.
The Rehm and Weller equation gives an estimate of LUMO
energy levels of 3 and 4 at –3.4 eV and –3.1 eV, respec-
tively, which is similar to P3HT. The similar energy levels
of 3 and 4 with P3HT indicate porphyrins with their superior
molar absorptivities could be used as alternative light-
harvesting compounds (p-type material) in organic photovol-
taic cells. In addition, the LUMO level of PCBM, at –3.7 eV,
is lower than the excited-state energy of either 3 or 4, allow-
ing for a thermodynamic driving force for charge separation.
Unfortunately, the tetraaryl substituents did not impart
supramolecular properties, which would lead to organized
structures that could carry charge more efficiently over long
distances. Nevertheless, the electron-rich, trisalkoxy meso-
substituted porphyrins are remarkably air and electrochemi-
cally stable, which is a design necessity for devices.
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Acknowledgements
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Government, Canadian Foundation for Innovation (CFI),
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