Meso-tetrathienylporphyrins: Electrochemical and axial ligation properties

Article abstract of DOI:10.1016/S0009-2614(01)01189-7

5,10,15,20-Tetrakis(2^′- and 3^′-thienyl)porphyrins [H₂T(2^′-Thio)P and H₂T(3^′-Thio)P] and their metal [Zn(II) and Cu(II)] complexes were examined by electrochemical redox and axial ligation properties. A large anodic shift (100-150 mV) in reduction potentials was observed for MT(2^′-Thio)Ps relative to MTPPs or MT(3^′-Thio)Ps. MT(2^′-Thio)Ps exhibited unusual redox behaviour and follows the trend in oxidation; CuT(2^′-Thio)P>H₂T(2 ^′-Thio)P≥ZnT(2^′-Thio)P in contrast to their corresponding MTPPs, H₂TPP>CuTPP>ZnTPP. It is reflected in the enhanced equilibrium constants (60%) for the ligation of nitrogenous bases with ZnT(2^′-Thio)P when compared to ZnTPP. This is interpreted in terms of increased electron deficient nature of the ZnT(2^′-Thio)P in contrast to Zn(3^′-Thio)P or ZnTPP.

Full text of DOI:10.1016/S0009-2614(01)01189-7

7 December 2001
Chemical Physics Letters 349 (2001) 399±404
www.elsevier.com/locate/cplett
Meso-tetrathienylporphyrins: electrochemical
and axial ligation properties
*
P. Bhyrappa , P. Bhavana
Department of Chemistry, Indian Institute of Technology, Chennai 600 036, India
Received 4 September 2001; in ®nal form 4 October 2001
Abstract
5; 10; 15; 20-Tetrakis(20- and 30-thienyl)porphyrins [H2Tꢀ20-Thio†P and H2Tꢀ30-Thio†P] and their metal [Zn(II) and
Cu(II)] complexes were examined by electrochemical redox and axial ligation properties. A large anodic shift (100±150
mV) in reduction potentials was observed for MT(20-Thio)Ps relative to MTPPs or MT(30-Thio)Ps. MT(20-Thio)Ps
exhibited unusual redox behaviour and follows the trend in oxidation; CuTꢀ20-Thio†P > H2Tꢀ20-Thio†P P ZnT
ꢀ20-Thio†P in contrast to their corresponding MTPPs, H2TPP > CuTPP > ZnTPP. It is re¯ected in the enhanced
equilibrium constants (>60%) for the ligation of nitrogenous bases with ZnT(20-Thio)P when compared to ZnTPP. This
is interpreted in terms of increased electron de®cient nature of the ZnT(20-Thio)P in contrast to Zn(30-Thio)P or
ZnTPP. Ó 2001Elsevier Science B.V. All rights reserved.
1. Introduction
substituents (electron de®cient or rich) at the pe-
ripheral positions, they serve as materials or
compounds with unusual properties [6]. Further,
high-valent metallotetraphenylporphyrins with
bulky ortho-substitutents on the phenyl rings serve
as robust catalysts for the oxidation of organic
substrates [7,8].
The meso-tetraarylporphyrins were used as
model compounds extensively owing to their ease
of synthesis and facile functionalisation. In recent
years, the meso-tetrathienylporphyrins are of
growing interest due to their use as models for
energy transfer reactions [9]. In addition, these
oligomeric thienylporphyrins showed ®lm-forming
[10] and conductivity behaviour [11]. The physico-
chemical studies on meso-thienylporphyrins have
been largely unexamined. To delineate the e€ect of
meso-thienyl groups on the electronic property of
The functionalised porphyrins are of consider-
able importance owing to their use as model
compounds [1,2] and also for their interesting
physico-chemical properties [3,4]. These properties
can be altered by the introduction of appropriate
substituents at the periphery of the porphyrin. The
past decade has witnessed numerous reports on the
synthesis and properties of highly substituted
porphyrins [5]. The introduction of substituents at
the meso- or at the b-pyrrole positions has dra-
matic in¯uence over the electronic properties of
the macrocycle [3,5]. By appending appropriate
* Corresponding author. Fax: +91-44-235-0509.
E-mail address: bhyrappa@iitm.ac.in (P. Bhyrappa).
0009-2614/01/$ - see front matter Ó 2001Elsevier Science B.V. All rights reserved.
PII: S0009-2614(01)01189-7

400
P.Bhyrappa, P.Bhavana / Chemical Physics Letters 349 %2001) 399±404
cool to room temperature and ®ltered. The purple
crystals were washed with methanol and recrys-
tallised from chloroform/methanol mixture. The
yield of the porphyrin was found to be 0.73 g
(14%). The synthesised porphyrins were charac-
1
terised by H NMR, UV±visible and mass spec-
troscopic methods. 1H NMR of H2T(30-Thio)P
(CDCl3, 400 MHz): d 8.97 (s, 8H, pyrrole b-H),
8.02 (d, 4H, thienyl 20-H), 7.99 (d, 4H, thienyl 50-
H), 7.74 (q, 4H, thienyl 40-H), )2.72 (s, 2H, imino-
H). FAB mass spectrum (m/z): 639 (calcd. 638.84).
MT(30-Thio)Ps (M ˆ CuꢀII† and Zn(II)) were
prepared using the reported procedures [16]. The
yield of the metal complexes was almost quanti-
tative.
The 1H NMR measurements were performed on
a 400 MHz Brucker FT-NMR spectrometer using
CDCl3 as the solvent at ambient temperature. UV±
visible spectral measurements were carried out on a
JASCO V-550 series spectrophotometer using
matched quartz cells of 1cm path length. Cyclic
voltammograms were obtained from Bioanalytical
System (BAS 100A, USA) provided with a three-
electrode cell assembly, videometer and electro-
chemical analyzer. The cell assembly consists of a
platinum button (2 mm), Ag/AgCl and platinum
wire as working, reference and counter electrodes,
respectively. All the electrochemical studies were
performed in CH2Cl2 solvent under N2 atmosphere
using 0.1M tetrabutylammonium hexa¯uoro-
phosphate, TBAHFP as the supporting electrolyte.
The concentrations of the porphyrins employed
were ꢁ0.4 mM. The axial ligation of nitrogenous
bases to Zn(II)-porphyrins were carried out in tol-
uene at 25 °C using a thermostat cell holder. The
equilibrium constants, Keq were evaluated using
literature procedure [17,18] and calculated at four
di€erent wavelengths. The average of four Keq val-
ues was taken for data analysis. Keq values at dif-
ferent wavelengths were found to be within 13%
error.
Fig. 1. Chemical structure of meso-tetraarylporphyrins.
the porphyrin p-system, electrochemical redox and
axial ligation studies were performed on a series of
thienylporphyrins (Fig. 1). These porphyrins ex-
hibit interesting electrochemical and axial ligation
properties.
2. Experimental
5; 10; 15; 20-Tetraphenylporphyrin, H2TPP [12]
and 5; 10; 15; 20-tetrakis(20-thienyl)porphyrin and
its metal [Cu(II) and Zn(II)] complexes were pre-
pared using reported procedures [13,14]. The
nitrogenous bases, pyridine, piperidine and mor-
pholine were procured from Rankem (India) and
puri®ed before use [15]. Imidazole, 3-hydroxy-
pyridine, 4-(dimethylamino)pyridine and 3-pyri-
dinecarboxaldehyde purchased from Fluka
(Switzerland) were used as received. Majority of
the solvents employed were of reagent grade and
puri®ed [15] prior to use.
2.1. Synthesis of 5; 10; 15; 20-tetrakis%30-thie-
nyl)porphyrin, H2T%30-Thio)P
3. Results and discussion
To a re¯uxing propionic acid (200 ml), thioph-
ene-3-carboxaldehyde (3 ml, 34 mM) and pyrrole
(2.3 ml, 34 mM) were added. The reaction mixture
was re¯uxed for 45 min. Then it was allowed to
The various thienylporphyrins were examined
to determine the e€ect of position of the meso-
thienyl-S substituent on the electrochemical and

P.Bhyrappa, P.Bhavana / Chemical Physics Letters 349 %2001) 399±404
401
Fig. 3. Cyclic voltammograms of ZnT(20-Thio)P in CH2Cl2
with 0.1M TBAHFP as the supporting electrolyte at a scan rate
of 100 mV/s at ambient temperature.
Fig. 2. Electronic absorption spectra of ZnT(20-Thio)P and
ZnTPP in CH2Cl2 at ambient temperature.
and the thienyl ring in ZnT(20-Thio)P (67±70°) [14]
is found to be greater than that observed for
ZnTPP (60±63°) [20]. This indicates that the extent
of conjugation of the thienyl group with the por-
phyrin ring is less favourable. An increase in red
shift of the absorption spectral features is possibly
due to inductive e€ect of the thienyl groups.
The electrochemical studies were performed on
thienylporphyrins to elucidate the e€ect of thienyl
groups on the electron de®ciency of the porphyrin
macrocycle. A representative cyclic voltammo-
gram of ZnT(20-Thio)P is shown in Fig. 3 and the
redox data of the complexes are shown in Table 2.
All the porphyrins showed two successive one-
electron oxidation and two one-electron reduction
potentials. For comparison, MTPPs were also ex-
amined under similar conditions and the data are
axial ligation properties. Fig. 2 shows the optical
absorption spectra of representative ZnT(20-
Thio)P along with ZnTPP and the spectral data
are summarised in Table 1. In general, thienyl-
porphyrins show a red shift of absorption bands
relative to their corresponding MTPPs. The ob-
served number of bands and spectral pattern of
MT(20 or 30-Thio)Ps are quite similar to MTPPs.
Further, the molar absorption coecients of thie-
nylporphyrins are not signi®cantly di€erent from
that of MTPPs [19]. For a smaller thienyl ring, one
would anticipate a possible co-planarity of the ring
with the porphyrin macrocycle. However, the di-
hedral angle formed between the porphyrin plane
Table 1
UV±visible absorption spectral dataa of tetrathienylporphyrins in CH2Cl2 at 25 °C
Porphyrin
B band
(nm)
Q bands
(nm)
H2T(20-Thio)P
H2T(30-Thio)P
H2TPP
CuT(20-Thio)P
CuT(30-Thio)P
CuTPP
ZnT(20-Thio)P
ZnT(30-Thio)P
ZnTPP
426(5.59)
421(5.67)
417(5.72)
421(5.62)
418(5.67)
415(5.80)
427(5.67)
422(5.71)
422(5.79)
523(4.30), 560(4.02), 597(3.88), 661(3.86)
519(4.29), 555(4.09), 595(3.85), 653(3.85)
514(4.36), 548(4.03), 590(3.90), 647(3.86)
546(4.37), 582(sh)
542(4.31), 577(sh)
539(4.45), 571(sh)
555(4.40), 595(3.79)
551(4.36), 591(3.92)
553(4.41), 595(4.00)
a The values in parentheses refer to log e values, e in dm3=mol=cm.

402
P.Bhyrappa, P.Bhavana / Chemical Physics Letters 349 %2001) 399±404
Table 2
Half-wave redox potentialsa;b (mV) of thienylporphyrins in CH2Cl2 using 0.1M TBAHFP at 25 °C
Porphyrin
Reduction
Oxidation
I
II
I
II
H2T(20-Thio)P
H2T(30-Thio)P
H2TPP
CuT(20-Thio)P
CuT(30-Thio)P
CuTPP
ZnT(20-Thio)P
ZnT(30-Thio)P
ZnTPP
890
950
1000
990
920
970
885
805
825
1130
1205
1335
1350
1300
1350
1085
1090
1235
)1090
)1225
)1230
)1190
)1345
)1325
)1225
)1385
)1360
)1420
)1525
)1540
)1577
)1750c
±
)1425
)1695
)1710c
a Vs Ag/AgCl electrode.
b Error in redox potentials is within Æ15 mV.
c Data from di€erential pulse voltammetry.
summarised in Table 2. The observed potentials of
MTPPs are quite similar to the literature values
[4]. MTPPs were chosen to delineate the e€ect of
meso-phenyl groups versus the thienyl groups at
the meso-carbons on the electronic properties of
the porphyrin p-system.
H2TPP > CuTPP > ZnTPP. However, the ob-
served trend in reduction is quite similar to that of
MTPPs. MT(30-Thio)P compounds follow similar
redox behaviour to that of MTPPs. The anodic
shift in redox potentials of MT(20-Thio)Ps relative
to MT(30-Thio)Ps can be ascribed to the enhanced
electron de®cient nature of the porphyrin mac-
rocycle induced by the 2-thienyl groups relative to
3-thienyl groups. As reported earlier, the ortho-
phenyl substitutents in MTPPs have signi®cant
Notably, H2T(20-Thio)P shows a cathodic shift
in ®rst oxidation (60±100 mV) and anodic shift
in ®rst reduction (>100 mV) relative to H2T
ꢀ30-Thio†P or H2TPP. From Table 2 it can be seen
that the redox potentials of thienylporphyrins in-
dicates an ease of oxidation and reduction in
H2T(20-Thio)P relative to H2T(30-Thio)P or
H2TPP. The free-base porphyrins examined in this
study follows the trend, H2T(20-Thio)P < H2T(30-
Thio)P < H2TPP in ®rst oxidation and H2T(20-
Thio)P < H2T(30-Thio)P ꢂ H2TPP in reduction.
For Zn(II)-porphyrins, ZnT(20-Thio)P showed al-
most similar ®rst oxidation potential relative to
H2T(20-Thio)P where as its ®rst reduction poten-
tial is cathodic by about 130 mV relative to
H2T(20-Thio)P. The redox potentials of CuT(20-
Thio)P complex are more anodic relative to
CuT(30-Thio)P or CuTPP. The shift is more pro-
nounced in reduction potentials (>120 mV) in
contrast to oxidation potentials. The observed re-
dox potentials of CuT(20-Thio)P are consistent
with the literature values [13]. The general trend in
®rst oxidation potentials for thienylporphyrins
follow the order: CuT(20-Thio)P > H2T(20-
Thio)P P ZnT(20-Thio)P in contrast to MTPPs,
Fig. 4. Optical absorption spectral features for the Soret region
of ZnT(20-Thio)P in presence of increasing addition of piperi-
dine in toluene at 25 °C. Inset shows the plot of concentration
of piperidine [Pip] against [Pip]/DA. For explanation of terms,
see [25].

P.Bhyrappa, P.Bhavana / Chemical Physics Letters 349 %2001) 399±404
403
Table 3
Equilibrium constants (Keq
a
)
for the ligation of various nitrogeneous bases with Zn(II)-thienylporphyrins in toluene at 25 °C
Keq  10À3
Base
ZnT(20-Thio)P
ZnT(30-Thio)P
ZnTPP
pKa
Piperidine
87.10
159.41
34.22
56.72
5.42
55.38
113.43
28.68
33.62
4.16
53.57
90.26
25.02
29.43
3.26
11.22
9.70
8.33
6.95
5.23
4-(Dimethylamino)pyridine
Morpholine
Imidazole
Pyridine
3-Hydroxypyridine
3-Pyridinecarboxaldehyde
4.86
1.69
3.712.84
1.36
4.86
1.08
4.77
a Within the error Æ13%.
about > 60% higher binding constants relative to
ZnT(30-Thio)P or ZnTPP. However, ZnT(30-
Thio)P showed a marginal increment of about
20±30% in Keq values relative to ZnTPP. The
enhanced Keq values of ZnT(20-Thio)P relative to
ZnT(30-Thio)P can be ascribed to greater inductive
e€ect of the 2-thienyl-S group.
in¯uence over the redox potentials relative to
meta- or para-substitution [4,21]. The greater an-
odic shift in redox potentials is possibly due to the
inductive e€ect in 2-thienyl relative to 3-thienyl
group.
In an e€ort to understand the unusual electro-
chemical behaviour of thienyl porphyrins, axial
ligation of various nitrogenous bases with ZnT(20-
and 30-Thio)P were carried out in toluene to elu-
cidate the e€ect of position of the thienyl-S group
on the extent of electron de®cient nature of the
porphyrin p-system. Zn(II)-porphyrins were cho-
sen for their simple well established ®ve coordi-
nation behaviour. A typical absorption spectral
features of ZnT(20-Thio)P with the increasing ad-
dition of piperidine is shown in Fig. 4. The ob-
served optical absorption spectral behaviour of
Zn(II)-thienylporphyrins in presence of bases is
similar to that reported for Zn(II)-porphyrins
[22,23]. The stoichiometry of the complexation
was analysed by Hill method [24]. In all cases, li-
gation of Zn(II)-thienylporphyrin with base (L)
showed 1:1 complexation Eq. (1).
4. Conclusions
The present work demonstrates the interesting
electrochemical redox and axial ligation properties
of meso-tetrathienylporphyrins. Upon variation of
S-group from 2- to 3-position on the thienyl
groups, thienylporphyrins showed an unusual shift
of redox potentials of MT(20-Thio)Ps relative to
the corresponding MTPPs or MT(30-Thio)Ps. Of
the two porphyrins, MT(20-Thio)Ps showed a
marginal anodic shift of oxidation potential and a
pronounced anodic shift in reduction potentials
were observed in contrast to the corresponding
MT(30-Thio)Ps. This was further supported by the
increased equilibrium constants for the axial
ligation of bases with Zn(II)-porphyrins and
thus follow the order: ZnTꢀ20-Thio†P > ZnTꢀ30-
Thio†P P ZnTPP.
Znꢀ20- or 30-Thio†P ‡ L
ꢀ1†
ꢀ ZnTꢀ20- or 30-Thio†PꢀL†
The data were analysed using available reported
procedures [17,18,25]. Keq values for the thienyl-
porphyrins are given in Table 3. For comparison,
ZnTPP complexation with all the bases were also
carried out under similar conditions and the data
are shown in Table 3. The observed Keq values for
ZnTPP are quite similar to that of reported values
[26]. It can be seen that the ZnT(20-Thio)P shows
Acknowledgements
We thank the Department of Science and
Technology, DST (Grant no. SP/S1/F09/98) and
Council of Scienti®c and Industrial Research,

404
P.Bhyrappa, P.Bhavana / Chemical Physics Letters 349 %2001) 399±404
[12] A.D. Adler, F.R. Longo, J. Goldmacher, J. Assour, L.
Korsako€, J. Org. Chem. 32 (1967) 476.
CSIR (Grant no. 01(1550)/98/EMR), New Delhi,
India for support.
[13] N. Ono, H. Miyagawa, T. Ueta, T. Ogawa, H. Tani, J.
Chem. Soc. Perkin Trans. I (1998) 1595.
[14] P. Bhavana, B. Verghese, P. Bhyrappa, Acta Cryst. C 57
(2001) 252.
References
[15] D.D. Perrin, W.L.F. Armarago, in: Puri®cation of Organic
Solvents, Pergamon Press, Oxford, 1988.
[1] B. Morgan, D. Dolphin, Struct. Bonding (Berlin) 64 (1987)
115.
[16] A.D. Adler, R.R. Longo, F. Kampas, J. Kim, J. Inorg.
Nucl. Chem. 32 (1970) 2443.
[2] M. Momenteau, C.A. Reed, Chem. Rev. 94 (1994) 659
(and references therein).
[17] J.P. Collman, J.I. Brauman, K.M. Doxsee, T.R. Halbert,
S.E. Hayes, K.S. Suslick, J. Am. Chem. Soc. 100 (1978)
2761.
[3] M. Gouterman, in: D. Dolphin (Ed.), The Porphyrins, vol.
3, Academic Press, New York, 1978.
[18] K.S. Suslick, M.M. Fox, T. Reinert, J. Am. Chem. Soc. 106
(1984) 4522.
[4] R.H. Felton, in: D. Dolphin (Ed.), The Porphyrins, vol. V,
Academic Press, New York, 1978, p. 127.
[19] M. Meot-Ner, A.D. Adler, J. Am. Chem. Soc. 97 (1975)
5107.
[5] M.O. Senge, in: K. Kadish, R. Guilard, K.M. Smith (Eds.),
The Porphyrin Handbook, vol. 1, Academic Press, San
Diego, 2000, p. 239 (and references therein).
[6] J.-H. Chou, M.E. Kosal, H.S. Nalwa, N.A. Rakow, K.S.
Suslick, in: K.M. Kadish, K.M. Smith, R. Guilard (Eds.),
The Porphyrin Handbook, vol. 6, Academic Press, New
York, 2000, p. 43.
[20] W.R. Scheidt, J.U. Mondal, C.W. Eigenbrot, A. Adler,
L.J. Radonovich, J.L. Hoard, Inorg. Chem. 25 (1986) 795.
[21] D.T. Sawyer, A. Sobkowiak, J.L. Roberts Jr., in: Electro-
chemistry for Chemists, Wiley Interscience, New York,
1995, p. 470.
[22] J.R. Miller, G.D. Dorough, J. Am. Chem. Soc. 74 (1952)
1277.
[7] R.A. Sheldon (Ed.), Metallopophyrins in Catalytic Oxida-
tions, Marcel Dekker, New York, 1994.
[23] G.C. Vogel, L.A. Searby, Inorg. Chem. 12 (1973) 936.
[24] A.V. Hill, J. Physiol. (London) 40 (1910) IV.
[25] [Pip] denotes the concentration of the piperidine added for
a given concentration of ZnT(20-Thio)P in toluene. DA
represents the di€erence in absorbance between ZnT(20-
Thio)P and ZnT(20-Thio)P(Pip) complexes in toluene
solution at a particular wavelength. The binding constants
were evaluated using procedures from [17] and [18].
[26] G. Szintay, A. Horvath, Inorg. Chim. Acta. 310 (2000) 175.
[8] B. Meunier, Chem. Rev. 92 (1992) 1411.
[9] M.S. Vollmer, F. Wurthner, F. E€enberger, P. Emele,
D.U. Meyer, T. Stump®g, H. Port, H.C. Wolf, Chem. Eur.
J. 4 (1998) 260.
[10] T. Simidzu, H. Segawa, F. Wu, N. Nakayama, J. Photo-
chem. Photobiol. A: Chemistry 92 (1995) 121.
[11] H. Maruyama, H. Segewa, S. Sotoda, T. Sato, N. Kosai, S.
Sagisaka, T. Shimidzu, K. Tanaka, Synth. Met. 96 (1996)
141.