Synthesis and optical and electronic properties of core-modified 21,23-dithiaporphyrins

Article abstract of DOI:10.1021/jo302707f

Core-modified 21,23-dithiaporphyrins, meso-substituted with both electron-withdrawing 4-phenylcarboxylic acids and related butyl esters, and electron-donating phenyldodecyl ethers were synthesized. The porphyrins displayed broad absorbance profiles that s

Full text of DOI:10.1021/jo302707f

Article
pubs.acs.org/joc
Synthesis and Optical and Electronic Properties of Core-Modified
21,23-Dithiaporphyrins
Ashley D. Bromby, Ryan P. Jansonius, and Todd C. Sutherland*
Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada
S
* Supporting Information
ABSTRACT: Core-modified 21,23-dithiaporphyrins, meso-
substituted with both electron-withdrawing 4-phenylcarboxylic
acids and related butyl esters, and electron-donating phenyl-
dodecyl ethers were synthesized. The porphyrins displayed
broad absorbance profiles that spanned from 400 to 800 nm
with molar absorptivities ranging from 2500 to 200000 M−1
cm−1. Electrochemical experiments showed the dithiaporphyr-
ins undergo two consecutive, one-electron, quasi-reversible
oxidations and reductions at −1.78, −1.43, 0.63, and 0.91 V
versus a ferrocene/ferrocenium internal standard. Spectroelec-
trochemistry and cyclic voltammetry revealed the dithiapor-
phyrins are stable and can endure many cycles of oxidation and reduction without signs of decomposition. The electronics of the
two dithiaporphyrins were similar, and DFT calculations showed the HOMO−LUMO energy difference was smaller than
tetrapyrrolic porphyrin analogues. Overall, the combination of desirable electronics, namely: quasi-reversible oxidations and
reductions as well as broad absorbance profiles, combined with stability, imply that these core-modified 21,23-dithiaporphyirns
could be potentially used as an ambipolar material for organic electronic applications.
separation are large obstacles to overcome in physical blends.
For these reasons, molecular entities that can be both reversibly
and readily oxidized and reduced are highly sought as
candidates for ambipolar materials.
INTRODUCTION
■
There has been much attention awarded to organic semi-
conducting materials as flexible, cost-effective, and processable
alternatives to inorganic devices for applications including
organic light emitting diodes (OLEDs), organic field effect
transistors (OFETs), and organic photovoltaic devices
(OPVs).1−4 There are two main approaches to organic
electronic materials, namely polymeric and small molecule.
Each approach has their own advantages and disadvantages
with respect to solution processability, purity, film morphology,
and deposition technique. A critical feature common to all of
the organic electronic applications is the ability to conduct
charges, whether they are holes, electrons, or both, efficiently
over large distances (micrometers). Among the families of
organic semiconductors, p-type, or hole-conducting materials
are more numerous, while their counterpart, n-type, or
electron-conducting materials are fewer.5−10 There is a clear
need to synthesize new compounds that can conduct electrons.
Furthermore, ambipolar materials, capable of both electron and
hole conduction, have the fewest examples in literature to
date.11−14 Ambipolar materials15 have advantages over unipolar
materials in both OFET designs for certain types of logic
circuits and OLED applications.16−20 The typical strategy used
to design ambipolar materials is to create blends of both p- and
n-type polymers or to covalently link p- and n-type molecules,
termed the donor−acceptor approach. Although the blending
strategy has shown success, it would be highly advantageous
and simpler to create a single molecule capable of both hole
and electron transport since microcrystallinity and phase
Porphyrins are highly conjugated, aromatic macrocycles that
possess broad absorbance profiles with high molar absorptiv-
ities and desirable redox properties and are easily functionalized
for electronic tuning. Porphyrin photochemical and electro-
chemical properties have garnered much interest16,21−23
regarding a variety of organic device applications. Porphyrins,
and particularly core-modified porphyrins, show reversible
oxidation and reduction reactions, which is an indicator of
ambipolar characteristics, and they have also shown promising
charge-transport properties in the solid state.16,24−31 This
contribution reports the synthesis and electronic properties of
two core-modified 21,23-dithiaporphyrins which have been
nonsymmetrically meso-functionalized. The optical properties
were assessed by UV−vis absorption and fluorescence experi-
ments, and the ground-state redox properties were assessed by
cyclic voltammetry (CV), which were compared to DFT-
calculated MO energy levels. Finally, spectroelectrochemistry
was carried out to assess the optical properties of both the
radical anion and cations.
Received: December 14, 2012
Published: February 5, 2013
© 2013 American Chemical Society
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a
Scheme 1. Synthesis of 21,23-Dithiaporphyrins
a
Key: (i) (1) 2.2 equiv of n-BuLi and TMEDA; (2) 2.2 equiv of benzaldehyde a/b; (ii) excess pyrrole and BF3·OEt2 (cat); (iii) (1) thiophenediol 1b
with thiatripyrrane 2a and BF3·OEt2 (cat.) (note: 2b does not react with 1a to form 3 cleanly under these conditions); (2) DDQ (iv) LiOH.
quantitative yields of 98%. Hence, a versatile route to
nonsymmetric meso-substituted dithiaporphyrins was achieved
using electron-deficient thiatripyrranes.
Electronic Absorption and Emission. The absorption
spectrum of 4 in DMF is shown in Figure 1 and displays a
RESULTS AND DISCUSSION
■
Synthesis. The synthesis began with diol formation, 1a or
1b, as shown in Scheme 1 by adding 2 equiv of n-BuLi and
TMEDA to thiophene to deprotonate the α positions forming
the dilithiated intermediate.32 The dilithiated thiophene was
then quenched by the addition of 2 equiv of the desired meso-
position aldehyde, resulting in diols 1a and 1b isolated as a
mixture of diastereomers in 63% and 22% yield, respectively.
The diastereomeric mixture was not separated as the chiral
centers become the final sp2-hybridized meso-positions upon
aromatization to the porphyrin. The alkoxyphenyl aldehyde was
obtained using standard Williamson ether conditions beginning
with 4-hydroxybenzaldehyde following our previous work,32
and the butyl 4-formylbenzoate was synthesized by reacting 4-
formylbenzoic acid in basic conditions with n-bromobutane.33
It is feasible to synthesize non-symmetric porphyrins by
reacting diols 1a and 1b with 1 equiv of pyrrole; however,
yields suffer and scrambling of the meso-positions can occur,
leading to challenging separations. To achieve a reasonable
porphyrin yield, an additional synthetic step was taken to avoid
scrambling of the dithiaporphyrin core. Based on the procedure
by Bhat et al.,34 either thiophene diol 1a or 1b was condensed
with excess pyrrole in the presence of BF3·OEt2 to form the
thiatripyrrane 2a or 2b in 40% and 93% yield, respectively, as a
diastereomeric mixture. To afford the nonsymmetrically
substituted meso-positions there are two approaches that
could result in dithiaporphyrin 3, namely reacting 2a with 1b
or reacting 2b with 1a. Interestingly, only the former reaction
sequence, 2a with 1b in the presence of BF3 etherate and
DDQ,33,36 resulted in a reasonable yield of 3 (16%).32,35,36 By
starting with 2b, en route to dithiaporphyrin 3, the Lewis acid
ring-closing step with diol 1a caused meso-position scrambling.
Figure 1. UV−vis absorption spectrum of 4 (1 × 105 M) in DMF
(black line). Inset: the expanded Q-band region (black line) and the
normalized emission spectrum of 4 (blue line) in DMF.
typical porphyrin profile.32,35−37 The Soret band for 4 is at 440
nm (ε = 2.4 × 105 M−1 cm−1) with less intense Q-bands, shown
in Figure 1 inset, at 518 nm, 554 nm, 637 and 701 nm with
molar absorptivities of 2.5 × 104 M−1 cm−1, 1.4 × 104 M−1
cm−1, 1.4 × 103 M−1 cm−1, and 6.9 × 103 M−1 cm−1,
respectively. Expectedly, the absorption spectrum of 3 was
nearly identical to 4, and the optical data are summarized Table
1. Furthermore, both porphyrins 3 and 4 presented Q-bands
that were linear in absorbance for concentrations ranging from
micro- to millimolar implying minimal aggregation in DMF,
and 3 showed linear absorbance behavior in methylene chloride
and insignificant solvatochromic effects. Compared to
tetrapyrrolic porphyrin analogues,36 dithiaporphyrins 3 and 4
display red-shifted absorbance maxima, and this observation is
consistent with the incorporation of thiophene moieties into
the aromatic core.32 The parent, tetraphenyldithiaporphyrin
(TP-N2S2), shows similar molar absorptivity trends with peak
1
The H NMR following the reaction of 2b with diol 1a after
aromatization showed minimal desired porphyrin 3 and
appreciable quantities of the symmetric dithiaporphyrin core
functionalized with four dodecyloxyphenyl chains. Presumably,
the thiophenes functionalized with electron-donating groups
are more able to stabilize the carbocation-like intermediates
resulting from the loss of pyrrole from the thiatripyrrane.
Finally, saponification of dithiaporphyrin 3 using LiOH in a 1:1
(water/THF) solvent mixture afforded dicarboxylate 4 at near-
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a
Table 1. UV-Vis Absorption and Fluorescence Data for 3 and 4
d
em
Soret
Q IV
Q III
Q II
Q I
λmax
Stokes shift
Egopt (eV)
b
c
3
3
4
440 (192)
441 (193)
440 (242)
517 (20)
518 (20)
518 (25)
553 (10)
554 (11)
554 (14)
638 (2.5)
637 (2.3)
637 (1.4)
702 (5.1)
701 (5.5)
701 (6.9)
714
714
715
239
259
279
1.75
1.75
1.75
c
a
Note: Absorption data are shown in nanometers followed by molar absorptivities in parentheses with units of 103 L·mol−1·cm−1. Emission data
b
c
given in nanometers and Stokes shift data in wavenumbers. Spectrum recorded in methylene chloride. Spectrum recorded in N, N-
dimethylformamide. Determined by the intersection of the absorption the emission spectra in respective solvent.
d
maxima at the following wavelengths in chloroform:37 435 nm
(Soret), 515 nm (Q IV), 548 nm (Q III), 635 nm (Q II), and
699 nm (Q I). The small changes in peak maxima of either 3 or
4 compared to TP-N2S2 is attributed to the small electronic
contribution that the meso-phenyl groups exert on the aromatic
macrocyclic N2S2 core.
The expanded Q-band region and normalized emission
spectrum of dithiaporphyrin 4 is shown in the inset of Figure 1,
and the optical data are summarized in Table 1. Excitation at
the Soret-band (S0 → S2) or the Q-bands (S0 → S1) II to IV
resulted in the same emission profile, indicating efficient
relaxation to the lowest energy S1 state. Both dithiaporphyrins 3
and 4 displayed a small Stokes shift (∼260 cm−1) with an
emission maximum at 714 nm that was insensitive to solvent
changes. Expectedly, the optically determined HOMO−LUMO
energy transition was very similar between the related
dithiaporphyrins at 1.75 eV (706 nm), as determined by the
intersection of the absorption and the emission spectra.
The parent compound, TP-N2S2, has similar emission
properties, which demonstrates the FMOs are only slightly
modified by the noncoplanar meso-phenyl substituents.
Electrochemistry. Nonmetalated, free-base, tetrapyrrolic
porphyrins exhibit two stepwise, one-electron, reversible
oxidations36,38 and reduction reactions.39 In methylene
chloride, dithiaporphyrin 3 displays four sets of quasi-reversible
redox peaks, two one-electron oxidations, and two one-electron
reductions as shown in Figure 2. The half-wave reduction and
oxidation potentials, summarized in Table 2, are −1.78, −1.43,
0.63, and 0.91 V versus a ferrocene/ferrocenium (Fc/Fc+)
internal standard. Two tetrasubstituted 21,23-dithiaporphyrins
bearing symmetric alkoxyphenyl groups were synthesized
previously by our group,32 and the oxidation reactions were
studied under similar electrochemical conditions leading to half
Figure 2. Cyclic voltammograms of (a) reduction region and (b)
oxidation region of 3 (2 mM) in methylene chloride containing 0.1 M
potentials of 0.47 and 0.67 V versus Fc/Fc+. Compared to our
previous symmetric dithiaporphyrins, porphyrin 3 requires a
more anodic potential to oxidize by 0.16 V, presumably
attributed to the installation of two electron-withdrawing
phenyl ester functional groups that replaced the phenyl ether
groups. In addition, the difference between half-wave oxidation
potentials of the radical cation and dication in our previous
work was 0.20 V compared to 0.28 V for 3, suggesting a
destabilization of the dication caused by the introduction of the
two pendant diester functionalities. The reported two, one-
electron oxidation half-potentials for metal-free, tetraphenyl
porphyrins (H2TPP) are 0.4 and 0.7 V versus Fc/Fc+,39 which
is 230 mV less anodic for the first oxidation then 3, which
shows the replacement of two nitrogen heteroatoms with
sulfurs within the aromatic core causes a stabilization of the
HOMO energy. Dithiaporphyrin 3 was also shown to have two
consecutive, one-electron reduction reactions at values similar
to the analogous TP-N2S2 (−1.44 V and −1.71 V vs versus Fc/
Fc+) measured under similar electrochemical conditions.40
Metal-free tetraphenylporphyrins (H2TPP) have a reported
(nBu)4NPF6 at a glassy carbon electrode at scan rates of 50, 100, 200,
500, and 1000 mVs−1 vs Fc/Fc+. Linear plots show peak current
dependence on the square root of the scan rate.
two-electron reduction with half-potentials of −2.17 and −1.86
V versus Fc/Fc+,39 which shows dithiaporphyrin 3 is easier to
reduce by approximately 0.4 V than its tetrapyrrolic analogue.
The incorporation of two sulfur heteroatoms into the aromatic
macrocyclic core and the addition of two ester functionalities
cause a lowering of the LUMO energy compared to a similarly
substituted all-nitrogen porphyrin and highlight these com-
pounds as potential ambipolar materials. The reason the sulfur
incorporation causes a lowering of the LUMO energy arises
from an trans-annular S···S bonding interaction, which allows
the sulfurs to act as an “electron drain”40 on the macrocyclic
aromatic π-system.41 Single-crystal measurements of TP-N2S2
show an intramolecular S···S distance of 3.05 Å42 as compared
to the sum of van der Waals radii at 3.7 Å.
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a
Table 2. Electrochemical Properties for 3 and 4
e
e
f
Ered2
Ered1
Eox1
Eox2
1/2
HOMO (eV)
LUMO (eV)
EgCV( eV)
1/2
1/2
1/2
a
b
b
3
3
4
−1.78
−1.43
−1.33
−0.92
0.63
0.60
0.50
0.91
−5.43
−5.40
−5.30
−3.37
−3.47
−3.88
1.9
1.9
1.8
d
d
c
c
a
b
Note: Electrochemical experiments were carried out in methylene chloride. Note: Electrochemical experiments were carried out in N,N-
dimethylformamide containing 0.1 M (nBu)4NPF6 with a glassy carbon working electrode, a Ag wire reference electrode, and a Pt wire counter
c
electrode. All potentials are referenced to Fc/Fc+. Half potentials are given in units of V. Irreversible oxidation, half potential given as estimated
d
e
f
onset. Quasi-reversible half potential. Calculated based on the Fc/Fc+ EHOMO = −4.8 eV.48,49 Calculated by the difference between HOMO and
LUMO.
The peak currents for all of the voltammograms (see the
Supporting Information for additional voltammograms of 3 and
4) show a linear relationship with the square root of the scan
rate, which is consistent with a diffusion-controlled reaction,
indicating that 3 and 4 do not physisorb to the glassy carbon
electrode. Using Nicholson’s method,43 the electron-transfer
rate constants for the quasi-reversible reduction and oxidation
peaks for 3 were determined to be 1.8 × 10−4 cm s−1 (Ered2),
4.0 × 10−4 cm s−1 (Ered1), 4.6 × 10−4 cm s−1 (Eox1), and 3.9 ×
10−4 cm s−1 (Eox2), respectively, and are similar to other
dithiaporphyrins and porphyrins.32,36 Additional electrochem-
ical properties for 3 and 4 are summarized in Table 2 and Table
S1 in the Supporting Information.
Interestingly, the two-electron reversible oxidation of 3 in
CH2Cl2 became irreversible for both dithiaporphyrins 3 and 4
when evaluated in DMF and the reduction of 3 in DMF
exhibited peak separations nearing 800 mV, negating electron-
transfer rate determinations. Porphyrins are known to undergo
chemical reactions once oxidized to the dicationic state, in the
presence of nucleophiles, to form isoporphyrins;44−47 however,
the cyclic voltammograms of both 3 and 4 do not support
formation of an isoporphyrin but do suggest that a chemical
reaction occurs following oxidation in the presence of DMF,
leading to an electrochemically silent compound on the reverse
scan. At faster scan rates (Figure S6, Supporting Information)
there is evidence of a small anodic peak (return to neutral
porphyrin) suggesting a slow chemical reaction follows the
oxidation to the dication. At slower scan rates, this return wave
disappears as the chemical reaction out-competes the return
electrochemical oxidation reaction to the neutral porphyrin. No
efforts were made to identify the oxidative decomposition
products in DMF.
The HOMO−LUMO energy difference for 3 was
determined both optically and electrochemically using the
onset of absorption and the onsets of the first reduction and
oxidation potentials, respectively. Optical and electrochemical
methods yielded similar HOMO−LUMO energy gaps of 1.75
and 1.9 eV, respectively, and compare favorably with TP-N2S2
at 1.7 eV and H2TPP at 1.8 eV.41
Spectroelectrochemistry. Spectroelectrochemical experi-
ments were carried out to understand the electronic transitions
involving both the single-electron, oxidized 3 and the single-
electron, reduced 3 in methylene chloride. Figure 3 shows the
evolution of the absorption spectra of 3 in a thin-layer optically
transparent electrochemical cell50 during oxidation to the
radical cation and during reduction to the radical anion. During
the one-electron oxidation to the radical cation (Figure 3a),
dithiaporphyrin 3 shows a marked decrease in the Soret band at
440 nm concomitant with a large new band growing in at 468
nm. In addition, Figure 3a shows several isosbestic points,
which supports a clean conversion to the radical cation with few
Figure 3. Spectroelectrochemistry spectra of neutral 3 (black line), (a)
oxidized cation 3+ (blue line), and (b) reduced anion 3− (red line) in
methylene chloride containing 0.1 M (nBu)4NPF6.
side products. In the Q-band region, 3 shows the disappearance
of Q III and Q IV bands upon oxidation and the growth of
three spectral features at 621 nm, 715 nm and a shoulder at 778
nm. Importantly, the spectrum for neutral porphyrin 3 can be
regenerated upon application of 0 V, indicating a stable cationic
species.
Upon the one-electron reduction of dithiaporphyrin 3 to its
radical anion (Figure 3b), a large decrease in Soret band
intensity is observed flanked by two small features that grow in
at 383 and 474 nm. Upon reduction, Q-bands I, II, and IV show
decreases in intensity concomitant with intensity increases of
two broad features at 600 and 850 nm and a small peak at 750
nm. The inset of Figure 3b shows an expansion of the Q-band
region and the neutral spectrum of 3 appears to have a peak
∼820 nm, which is an experimental artifact due to the diameter
of wires used in the Pt mesh electrode. Nevertheless, upon
reduction there is clear evidence for an increase in absorption
∼850 nm, which then returns to baseline at ∼930 nm. Again,
the neutral absorption spectrum of 3 can be regenerated by
applying 0 V to the cell.
Both the cationic and anionic absorption spectra show red-
shifted features compared to the neutral dithiaporphyrin 3.
There is a wealth of literature that has examined the
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Table 3. DFT Calculations and Molecular Orbital Energies for Cationic, Anionic and Neutral 3 and 4 Obtained at the B3LYP/6-
31G+(d) Level
calcd MO energies
3 (eV)
4 (eV)
3+ (eV)
4+ (eV)
3− (eV)
4− (eV)
LUMO+1
LUMO
−2.78
−2.85
−5.36
−5.71
−6.34
−2.82
−2.91
−5.42
−5.77
−6.38
−5.53
−5.70
−7.93
−8.36
−8.78
−5.60
−5.77
−7.99
−8.43
−8.84
+0.43
+0.31
−0.15
−1.09
−0.24
−1.17
−2.88
−3.24
a
a
a
a
HOMO
HOMO-1
HOMO-2
−2.81
−3.17
a
SOMO orbital.
spectroelectrochemical features of H2TPP51−56 and limited
reports on dithiaporphyrins.32 There are many similarities in
the spectral features between the porphyrins and dithiapor-
phyrins, and we have borrowed several of the ideas presented in
the H2TPP literature to explain similar phenomena in the
dithiaporphyrins. For example, the absorption spectrum of
diprotonated porphyrins (H4TPP2+) is very similar to the
electrochemically generated radical cation absorption spec-
trum.41 Similarly, the diprotonated dithiaporphyrin absorption
spectrum32 is similar to the radical cation spectrum. Drawing
conclusions pertaining to the structure of the porphyrin and its
absorption spectrum is difficult, as there are no crystal
structures of a radical cation porphyrin. However, the crystal
structure of H4TPP2+ was solved57 and shows a large deviation
in the macrocyclic aromatic core accompanied by a twist of the
four meso-phenyl groups to become more planar40 with the
methine meso-positions.37 We propose that the electrochemi-
cally generated radical cation of 3 exhibits a significant
deformation of the macrocyclic plane, and thus a break in the
FMO degeneracy, leading to the observed spectral features
shown in Figure 3. Beyond the break in orbital degeneracy, it is
postulated that oxidation of the dithiaporphyrin cores result in
a decrease in the S···S interaction, which holds the transannular
heteroatoms close, resulting in deforming the macrocyclic
aromatic plane of the neutral dithiaporphyrin. More discussion
on the structural features of the cation and anion is presented in
the calculations section. The reversibility of these redox species
implies that the compounds could address longevity issues
associated with organic materials applications.
4 illustrates the FMO energy levels and depicts the HOMO and
LUMO Kohn−Sham orbital shapes for dithiaporphyrin 4 (3 is
Figure 4. (a) Normalized energies of cationic, neutral and anionic
dithiaporphyrin 4 and (b) frontier molecular orbitals of neutral
porphyrin 4 using DFT methods at the B3LYP/6-31G+(d) level.
indistinguishable and included in Figure S10 the Supporting
Information). The 18 π-electron aromatic core sits nearly
planar and the meso-substituted phenyl rings are at dihedral
angles of 65° to the macrocyclic core, which compares well with
calculated structures of tetrapyrrolic porphyrin analogues.64
DFT calculations do reveal a break in the degeneracy of the two
highest occupied MOs resulting from a change in symmetry by
the incorporation of thiophene into the core. Interestingly,
compared to our previously synthesized dithiaporphyrins,32 the
HOMO energies of 3 and 4 have remained relatively
unchanged while the LUMO energies have decreased from
about −3.2 to −2.8 eV, resulting in a smaller HOMO−LUMO
energy gap overall, which is attributed to the addition of the
electron-withdrawing diester groups. Furthermore, Figure S13
in the Supporting Information shows significant overlap of
electron density between the trans-annular sulfurs in HOMO-6,
leading to the nearly planar macrocycle and the ambipolar
electrochemical behavior.
The DFT-calculated HOMO−LUMO gap is 2.51 eV for
both dithiaporphyrins 3 and 4, which corresponds to an optical
transition at 494 nm, clearly a large overestimation of the
experimental energy gap at 706 nm (1.75 eV). The failure of
TD-DFT calculations based on the B3LYP functional to
examine porphyrin charge-transfer excited states has been
documented previously65 and dithiaporphyrins seem to suffer
the same limitation. Note that long-range corrected (LC)
functionals were assessed, as deployed in Gaussian09, and did
not result in appreciable differences in the FMOs. The DFT-
calculated EHOMO of −5.4 eV matches the electrochemically
derived data and the ELUMO was calculated to be −2.9 eV, which
is slightly higher than the experimentally estimated value of
−3.4 eV by cyclic voltammetry. These discrepancies are most
likely attributed to limitations in DFT methods used and lack of
DFT Calculations. Optimized geometries of reduced model
compounds generated by replacing alkyl fragments on meso-
phenyl substituents with methyl groups for porphyrins 3 and 4
were calculated using DFT methods at the B3LYP/6-31G+(d)
level with the Gaussian09 suite of programs.58 The DFT-
calculated structures match reasonably well with the six crystal
structures of dithiaporphyrins42,59−62 in the Cambridge
Crystallographic Data Centre (updated February 2012),
especially considering the transannular S···S distance in the
calculated structures was 3.07 Å. The crystallographically
42
determined structure of TP-N2S2 was overlaid with the
calculated structure by removing the p-phenyl groups and
replacing them with hydrogen, and the resulting overlay (Figure
S14 in the Supporting Information) shows an average root-
mean-square deviation of 0.11 Å with a maximum deviation of
0.28 Å as determined by Mercury 3.0 CSD,63 which strongly
supports the basis set selection for the ground-state geometry.
The most significant structural difference arises from the
pyrrolic rings being out of macrocyclic plane by 0.16 Å in the
crystal structure, whereas the calculated structure shows the
pyrrolic rings coplanar with the macrocyclic ring.
The calculated FMO energies for 3 and 4 and both the
radical cations and anions data are compiled in Table 3. Figure
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solvent parameters.65 The calculated MO energies for 3 and 4
are very similar, which is consistent with their nearly identical
optical and electrochemical data. Normalized FMO energies of
4, its radical cation 4•+ and its radical anion 4•− are shown in
Figure 4. Note that a normalized energy scale was used by
gHSQC and gCOSY NMR spectra on a 600 MHz spectrometer at 330
K. Residual solvent peaks were as follows: chloroform, δ 7.26 ppm;
ethyl acetate, δ 2.05, 4.12 1.26 ppm; N,N-dimethylformamide, δ 8.02,
3.30, 2.96, 2.88 ppm. Accurate mass spectra were recorded using either
ESI or MALDI-TOF techniques. UV−vis spectra were recorded using
a spectrophotometer in dual beam mode. Cyclic voltammetry (CV)
experiments were carried out on a potentiostat that was controlled by
a PC in a temperature controlled, three-electrode cell (15 mL). The
working electrode was a glassy carbon disc with area of 0.28 cm2,
which was polished after each use with 0.05 μm diamond slurry in an
automated polisher. The reference electrode was a Ag wire and the
counter electrode was a Pt wire that was flame annealed prior to each
use. All potentials were referenced to the ferrocene/ferrocenium redox
couple. Each CV experiment consisted of approximately 1−3 mM
redox active species dissolved in 0.1 M tetrabutylammonium
hexafluorophosphate in deoxygenated methylene chloride, by bubbling
with Ar for 10 min prior to dissolving the redox active species and an
Ar blanket was maintained during the experiment. Spectroelectro-
chemical spectra were generated with a spectrophotometer linked with
a potentiostat using an optically transparent thin layer electrochemical
(OTTLE) cell.50 The working electrode in the beam path was a Pt
mesh, the counter electrode was another Pt mesh, and a silver wire
acted as a pseudoreference electrode using the same solvent and
electrolyte as above.
1
setting the Fermi level, /2(ELUMO−EHOMO), of the neutral
dithiaporphyrin at 0 eV. The complete FMO energies of
cationic, anionic and neutral 3 and 4 are summarized in Figures
S11 and S12 in the Supporting Information. The optimized
structures of the radical cations and anions provide insight into
the spectroelectrochemical features observed in Figure 3. First,
the optimized radical cation structure shows significant
deviations in planarity of the macrocycle from the neutral
nearly planar dithiaporphyrin. The pyrrolic rings remain in
plane and the thiophene sulfurs are 0.28 and 0.41 Å out of the
macrocycle plane, disrupting conjugation. Furthermore, to
stabilize the cationic charge, the two alkoxyphenyl meso-
substituents have rotated into conjugation with dihedral angles
of 49.5° compared to 65° in the neutral dithiaporphyrin. The
two diester substituted meso-phenyl substituents are nearly
unchanged with dihedral angles of 64°. The net result is an
extension of conjugation though the meso-position and
weakened conjugation in the macrocyclic core leading to an
overall modest red-shift in the absorption properties, as shown
in Figure 3a. Second, the optimized radical anion structure
shows a nearly planar macrocyclic structure with sulfurs
deviating from the macrocyclic plane by only 0.14 and 0.17
Å, which is similar to the calculated neutral dithiaporphyrin.
Similarly to the cation, the meso-phenyl substituents show a
change compared to the neutral dithiaporphyrin. The electron
deficient meso-phenyl esters show dihedral angles of 49.5°, and
the alkoxyphenyl meso-substituents show dihedral angles of
60.6° relative to the macrocyclic core. Clearly, the rotations of
the meso-phenyl groups play a role in stabilizing the anionic
charge on the macrocyclic ring. The net results of these
stabilizations are an extension of the aromatic character beyond
the macrocyclic core leading a significant red-shifted
absorbance as observed in Figure 3b for the radical anion.
Butyl 4-Formylbenzoate. 4-Formylbenzoic acid (1.0 g, 6.7
mmol), KI (1.1 g, 6.7 mmol), and K2CO3 (0.9 g, 13.4 mmol) were
dissolved in DMF (15 mL) and heated to 100 °C for 2 h. To the hot
solution was added 1-bromobutane (0.9 mL, 8.1 mmol), the resulting
yellow solution was left at 100 °C for an additional 12 h, and then
DMF was removed under reduced pressure. The residue was dissolved
in CHCl3 (50 mL), washed with water (100 mL) and saturated
NaHCO3 (50 mL), and then dried with Na2SO4. The crude yellow oil
was purified by column chromatography using hexanes and ethyl
1
acetate (4.5:0.5) to afford a clear oil (1.2 g, 90%): H NMR (400
MHz, CDCl3) δ 10.10 (s, 1H, CHO), 8.19 (d, J = 8.4 Hz, 2H, aryl),
7.94 (d, J = 8.2 Hz, 2H, aryl), 4.36 (t, J = 6.6 Hz, 2H, (CO)OCH2),
1.86−1.65 (m, 2H, (CO)OCH2CH2), 1.58−1.40 (m, 2H, (C
O)OCH2CH2CH2), 0.99 (t, J = 7.4 Hz, 3H, CH3); 13C NMR (101
MHz, CDCl3) δ 191.7, 165.8, 139.3, 135.7, 130.3, 129.6, 65.6, 30.8,
19.4, 13.9. Spectroscopic data are consistent with the reported
synthesis by Abreu and co-workers.33
4-(Dodecyloxy)benzaldehyde. 4-Hydroxybenzaldehyde (3.66 g,
30.0 mmol), KI (4.98 g, 30.0 mmol), and K2CO3 (8.28 g, 60.0 mmol)
were added to acetone (70 mL) and refluxed for 2 h. To the hot
solution was added slowly 1-bromododecane (7.9 mL, 33 mmol) and
the solution refluxed for 24 h. Acetone was then removed under
reduced pressure, and the residue was suspended in water (100 mL),
extracted with dichloromethane, and washed with NaOH (150 mL,
2%). The organic layer was dried with Na2SO4, and the solvent was
removed under reduced pressure to afford the crude product. The
crude mixture was subjected to silica gel column chromatography
using a mixture of hexanes and ethyl acetate (4.5:0.5) to afford the
CONCLUSION
■
In summary, two 21,23-dithiaporphyrins have been synthesized
that exhibit absorbance profiles with high molar absorptivities
over a wide wavelength range. The redox properties show that
these dithiaporphyrins are reversibly reduced and oxidized at
modest potentials. DFT calculations confirm that the
electronics of both dithiaporphyrins are similar and have been
electronically tuned to possess a smaller HOMO−LUMO
energy gap compared to tetraphenylporphyrins. Furthermore,
spectroelectrochemical experiments confirm the reversible
conversion to either the radical anion or radical cation, and
the combination of reversible redox properties with a small
HOMO−LUMO energy difference suggests these dithiapor-
phyrins could behave as ambipolar materials for organic
electronic devices.
1
pure compound as an off-white oil (5.77 g, 67%): H NMR (400
MHz, CDCl3) δ 9.84 (s, 1H, Ar-CHO), 7.78 (d, J = 8.8 Hz, 2H, aryl),
6.95 (d, J = 8.7 Hz, 2H, aryl), 3.99 (t, J = 6.6 Hz, 2H, Ar-OCH2),
1.82−1.74 (m, 2H, Ar-OCH2CH2), 1.48−1.39 (m, 2H, Ar-
OCH2CH2CH2), 1.33−1.21 (m, 16H, aliphatic), 0.86 (t, J = 6.8 Hz,
3H, CH3); 13C NMR (101 MHz, CDCl3) δ 190.7, 164.3, 132.0, 129.8,
114.77, 68.4, 32.0, 29.7, 29.7, 29.6, 29.6, 29.4, 29.1, 26.0, 22.7, 14.2.
Spectroscopic data are consistent with the reported synthesis by
Bromby and co-workers.32
2,5-Bis[(4-butoxycarbonylphenyl)hydroxymethyl]thiophene
(1a). To a solution of hexanes (30 mL) were added n-BuLi (5.2 mL of
2.3 M solution in hexane, 12 mmol) and distilled TMEDA (1.8 mL, 12
mmol) and the mixture stirred under nitrogen. Thiophene (0.44 mL,
5.5 mmol) was added, and the solution was refluxed for 1 h. The
dilithiated thiophene was cooled to room temperature and then
transferred to a 0 °C solution of butyl 4-formylbenzoate (2.44 g, 12
mmol). The reaction was quenched with water (30 mL) and the
aqueous layer was extracted with diethyl ether (100 mL). The organic
EXPERIMENTAL SECTION
■
Column chromatography was performed on P60 silica gel (230−400
mesh). Thin-layer chromatography was carried out on silica gel F-254
glass-backed TLC plates. Solvents were used as received or dried using
a solvent purification system and reactions were performed under
ambient atmosphere unless otherwise stated.
NMR spectra were recorded on either a 300 or 400 MHz
spectrometer. 1H and 13C NMR assignments were assisted by 2D
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The Journal of Organic Chemistry
Article
layer was dried with Na2SO4, and the solvent was removed under
reduced pressure to afford the crude product. Silica gel column
chromatography using hexanes and ethyl acetate (4:1) afforded a pure
yellow oil 1a (1.7 g, 63%): 1H NMR (400 MHz, CDCl3) δ 7.96 (d, J =
8.3 Hz, 2H, aryl), 7.44 (d, J = 6.7 Hz, 2H, aryl), 6.66 (s, 1H, CHOH),
5.95 (s, 1H, β-thiophene), 4.28 (t, J = 6.6 Hz, 2H, (CO)OCH2),
1.82−1.64 (m, 2H, (CO)OCH2CH2), 1.55−1.37 (m, 2H, (C
O)OCH2CH2CH2), 0.97 (t, J = 7.4 Hz, 3H, CH3); 13C NMR (101
MHz, CDCl3) δ 166.6, 148.0, 147.9, 147.8, 129.9, 126.2, 124.8, 72.0,
65.1, 30.8, 19.4, 13.9; HR-MS (ESI) calcd for C28H32SO6Na (M +
Na)+ 519.1812, found 519.1800.
(m, 16H, aliphatic), 0.90 (t, J = 6.8 Hz, 3H, CH3); 13C NMR (101
MHz, CDCl3) δ 158.3, 146.4, 134.8, 133.6, 129.5, 125.2, 117.2, 114.6,
108.4, 107.5, 68.1, 66.0, 45.4, 32.1, 31.7, 29.8, 29.8, 29.6, 29.5, 29.5,
26.2, 22.8, 14.2; HR-MS (MALDI-TOF) calcd for C50H70N2SO2Na
(M + Na)+ 785.5050, found 785.5015.
5, 10-Bis(4-butoxycarbonylphenyl)-15,20-bis(4-dodecyloxy-
phenyl)-21,23-dithiaporphyrin (3). Thiatripyrrane 2a (0.54 g, 0.9
mmol) and dicarbinol 1b (0.61 g, 0.9 mmol) were dissolved in
methylene chloride and degassed with N2 for 15 min. The solution was
foil wrapped, BF3·OEt2 (0.06 mL, 0.5 mmol) was added, and the
solution immediately turned dark. After the solution was stirred for 2
h, DDQ (0.20 g, 0.9 mmol) was added, and the solution was stirred in
air for an additional 3 h. The solution was passed through a short
alumina column using methylene chloride as the eluent. After removal
of solvent under reduced pressure, a black-purple crude compound
was obtained. Column chromatography was used using methylene
chloride and hexanes (4:1) to afford pure purple crystals of 3 (173 mg,
16%): 1H NMR (300 MHz, CDCl3) δ 9.83 (s, 1H, β-thiophene), 9.73
(s, 1H, β-thiophene), 8.82 (d, J = 4.6 Hz, 1H, β-pyrrole), 8.72 (d, J =
4.6 Hz, 1H, β-pyrrole), 8.59 (d, J = 8.3 Hz, 2H, aryl), 8.42 (d, J = 8.3
Hz, 2H, aryl), 8.20 (d, J = 8.6 Hz, 2H, aryl), 7.36 (d, J = 8.6 Hz, 2H,
aryl), 4.62 (t, J = 6.6 Hz, 2H, (CO)OCH2), 4.23 (t, J = 6.5 Hz, 2H,
O−CH2), 2.08−1.92 (m, 4H, aliphatic), 1.81−1.62 (m, 4H, aliphatic),
1.62−1.29 (m, 16H, aliphatic), 1.18 (t, J = 7.4 Hz, 3H, CH3), 1.01 (t, J
= 6.6 Hz, 3H, CH3); 13C NMR (75 MHz, CHCl3) δ 166.8, 159.5,
156.9, 156.0, 148.6, 147.2, 145.9, 135.9, 135.5, 135.1, 134.9, 134.2,
134.1, 133.2, 132.4, 130.3, 128.7, 113.7, 104.6, 68.3, 65.3, 32.1, 31.0,
30.9, 29.8, 29.8, 29.6, 29.5, 26.3, 22.8, 19.5, 19.4, 14.2, 14.0, 13.8; HR-
MS (MALDI-TOF) calcd for C78H93N2O6S2 (M + H)+ 1217.6470,
found 1217.6468.
5, 10-Bis(4-carboxyphenyl)-15,20-bis(4-dodecyloxyphenyl)-
21,23-dithiaporphyrin (4). Dithiaporphyrin 3 (173 mg, 0.14 mmol)
and LiOH (0.6 g, 25 mmol) were added to THF and water (20 mL,
1:1) and the solution heated to 70 °C and stirred for 24 h. The
solution was acidified to ∼pH 4 with concd HCl followed by removal
of THF under reduced pressure. The precipitate was filtered to afford
purple crystals of 4 (151 mg, 98%) which were not further purified: 1H
NMR (600 MHz, DMF) δ 9.86 (s, 1H), 9.79 (s, 1H), 8.76 (d, J = 4.4
Hz, 1H), 8.71 (d, J = 4.4 Hz, 1H), 8.56 (d, J = 8.2 Hz, 2H), 8.45 (d, J
= 7.9 Hz, 2H), 8.26 (d, J = 8.5 Hz, 2H), 7.52 (d, J = 8.6 Hz, 2H), 4.37
(t, J = 6.4 Hz, 3H), 2.07−1.94 (m, 3H), 1.67 (m, 2H), 1.56−1.47 (m,
2H), 1.47−1.30 (m, 14H), 0.91 (t, J = 7.1 Hz, 3H); 13C NMR (151
MHz, DMF) δ 169.0, 160.9, 157.7, 157.1, 149.4, 148.4, 145.9, 145.1,
137.0, 136.8, 136.6, 136.0, 135.9, 135.2, 135.2, 134.1, 134.1, 129.8,
115.2; 13C DEPT 135 NMR (151 MHz, DMF) 68.5, 31.8, 29.6, 29.6,
29.5, 29.4, 29.4, 29.2, 26.1, 22.5; HR-MS (MALDI-TOF) calcd for
C70H77N2O6S2 (M + H)+ 1105.5218, found 1105.5242.
2,5-Bis[(4-dodecyloxyphenyl)hydroxymethyl]thiophene
(1b). To a solution of hexanes (30 mL) were added n-BuLi (3.9 mL of
1.4 M solution in hexane, 5.5 mmol) and distilled TMEDA (0.8 mL,
5.4 mmol) and the mixture stirred under nitrogen atmosphere.
Thiophene (0.18 mL, 2.2 mmol) was added, and the solution was
refluxed for 1 h. The dilithiated thiophene was cooled to room
temperature and then transferred to a 0 °C solution of 4-
(dodecyloxy)benzaldehyde (1.58 g, 5.4 mmol). The reaction was
quenched with water (30 mL), and the aqueous layer was extracted
with diethyl ether (100 mL). The organic layer was dried with Na2SO4,
and the solvent was removed under reduced pressure to afford the
crude product. Silica gel column chromatography using hexanes and
1
ethyl acetate (4:1) afforded a pure yellow oil 1b (0.32 g, 22%): H
NMR (400 MHz, CDCl3) δ 7.32 (d, J = 8.5 Hz, 2H, aryl), 6.86 (d, J =
8.6 Hz, 2H, aryl), 6.68 (d, J = 2.7 Hz, 1H, CHOH), 5.90 (s, 1H, β-
thiophene), 3.94 (t, J = 6.6 Hz, 2H, ArOCH2), 2.38 (s, 1H), 1.85−1.68
(m, 2H, ArOCH2CH2), 1.55−1.38 (m, 2H, ArOCH2CH2CH2), 1.29−
1.21 (m, 16H, aliphatic), 0.89 (t, J = 6.8 Hz, 3H, CH3); 13C NMR
(101 MHz, CDCl3) δ 159.1, 148.5, 135.1, 127.7, 124.3, 114.6, 72.4,
68.2, 32.1, 29.8, 29.8, 29.8, 29.7, 29.6, 29.5, 29.4, 26.2, 22.8, 14.3; HR-
MS (ESI) calcd for C42H63O3S (M − OH)+ 647.4493, found
647.4492.
5,10-Bis(4-butoxycarbonylphenyl)-16-thiatripyrrane (2a).
Dialcohol 1a (180 mg, 0.4 mmol) was added to freshly distilled
pyrrole (5 mL, 72 mmol) and purged with N2 for 10 min. The solution
was foil wrapped, and 1 drop of BF3·OEt2 (∼0.2 mL, 1.63 mmol) was
added resulting in an immediate dark solution. After the solution was
stirred for 48 h, NaOH (30 mL, 40%) was added and the solution
extracted with methylene chloride (100 mL) and dried with Na2SO4.
The solvent was evaporated under reduced pressure, and the
remaining pyrrole was recovered by distillation. The crude brown oil
was purified by column chromatography using hexanes and ethyl
acetate (3.5:1.5) to afford the pure compound as a brown-pink oil 2a
(86 mg, 40%): 1H NMR (300 MHz, CDCl3) δ 7.98 (d, J = 8.2 Hz, 2H,
aryl), 7.30 (d, J = 8.1 Hz, 2H, aryl), 6.71 (s, 1H, α-pyrrole), 6.62 (s,
1H, β-thiophene), 6.14 (dd, J = 5.6, 2.7 Hz, 1H, β-pyrrole), 5.90 (s,
1H, β-pyrrole), 5.61 (s, 1H, meso-CH), 4.31 (t, J = 6.6 Hz, 2H, (C
O)OCH2), 1.82−1.66 (m, 2H, (CO)OCH2CH2), 1.55−1.37 (m,
2H, (CO)OCH2CH2CH2), 0.98 (t, J = 7.3 Hz, 3H, CH3); 13C
NMR (75 MHz, CDCl3) δ 166.6, 147.7, 145.4, 132.2, 130.0, 129.5,
128.5, 125.8, 117.8, 108.6, 108.0, 65.0, 46.1, 30.9, 19.4, 13.9; HR-MS
(MALDI-TOF) calcd for C36H38N2O4SNa (M + Na)+ 617.2444,
found 617.2429.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H and 13C NMR spectra; absorption and emission properties
and spectra of 3 and 4; cyclic voltammograms, diffusion
coefficients, and electron-transfer rates of 3 and 4; DFT
calculations and summarized energies of 3 and 4; crystallo-
graphic data overlaid with calculated structure. Cartesian
coordinates of DFT-calculated structures. This material is
available free of charge via the Internet at http://pubs.acs.org.
5,10-Bis(4-dodecyloxyphenyl)-16-thiatripyrrane (2b). Dicar-
binol 1b (0.31 g, 0.5 mmol) was added to freshly distilled pyrrole (5
mL, 72 mmol) and purged with N2 for 10 min. The solution was foil
wrapped, and 1 drop of BF3·OEt2 (∼0.2 mL, 1.63 mmol) was added
resulting in an immediate dark solution. After the solution was stirred
for 1 h, NaOH (30 mL, 40%) was added and the solution extracted
with methylene chloride (100 mL) and dried with Na2SO4. The
solvent was evaporated under reduced pressure, and the remaining
pyrrole was recovered by distillation. The crude brown oil was purified
by column chromatography using hexanes and ethyl acetate (4:1) to
AUTHOR INFORMATION
■
1
Corresponding Author
afford the pure compound as a green-brown oil 2b (0.36 g, 93%): H
NMR (400 MHz, CDCl3) δ 7.87 (s, 1H, NH), 7.14 (d, J = 8.6 Hz, 2H,
aryl), 6.83 (d, J = 8.6 Hz, 2H, aryl), 6.68 (s, 1H, α-pyrrole), 6.60 (s,
1H, β-thiophene), 6.14 (s, 1H, β-pyrrole), 5.92 (s, 1H, β-pyrrole), 5.51
(s, 1H, meso-CH), 3.94 (t, J = 6.5 Hz, 2H, PhO-CH2), 1.84−1.73 (m,
2H, PhOCH2CH2), 1.52−1.41 (m, 2H, PhOCH2CH2CH2), 1.39−1.23
*Tel: +1 403 220 7559. Fax: +1 403 289 9488. E-mail: todd.
sutherland@ucalgary.ca.
Notes
The authors declare no competing financial interest.
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The Journal of Organic Chemistry
Article
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ACKNOWLEDGMENTS
■
A.D.B. thanks Dr. A. Scott Hinman for helpful discussions,
Douglas Brown for fluorescence assistance, and the Govern-
ment of Alberta for a scholarship. We thank the Canada School
of Energy for a proof-of-principle grant and NSERC of Canada
Discovery Grant to carry out this work. The computational
research has been enabled by the use of computing resources
provided by WestGrid and Compute/Calcul Canada.
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