Effect of five membered versus six membered meso-substituents on structure and electronic properties of Mg(II) porphyrins: A combined experimental and theoretical study

Article abstract of DOI:10.1021/ic1008522

Meso-tetrasubstituted Mg(II) porphyrins containing six membered phenyl groups (MgTPP) and five membered thienyl (MgTThP) and furyl groups (MgTFP) were synthesized and structurally characterized, and the effects of meso-substituents on electronic properties were studied using NMR, absorption, fluorescence spectroscopy, and electrochemical studies. Density functional theory (DFT) calculations were carried out to correlate with experimental observations. The three Mg(II) porphyrins MgTPP, MgTThP, and MgTFP were crystallized as hexa-coordinate systems with Mg(II) ion in the center of the porphyrin plane and having two tetrahydrofuran molecules as axial ligands. The X-ray studies clearly showed that the meso-furyl groups adopt a conformation in which they are more in-plane with the porphyrin plane whereas the thienyl and phenyl groups prefer an orthogonal arrangement with respect to the porphyrin plane. This arrangement of meso-substituents with the porphyrin plane helps in the enhancement of porphyrin π-delocalization in MgTFP compared to MgTThP and MgTPP. The differences in their structures are clearly reflected in their spectral and electrochemical properties. The absorption and fluorescence bands experienced bathochromic shifts on moving from six membered phenyls to five membered thienyl and furyl group, and the maximum effects were observed for meso-tetrafuryl Mg(II) porphyrin. The electrochemical studies indicated that the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) decreases as we move from six membered phenyl groups to five membered thienyl and furyl groups, which explains the bathochromic shifts observed in absorption and fluorescence bands. Results on structure and electronic properties based on DFT studies are in agreement with experimental observations.

Full text of DOI:10.1021/ic1008522

Inorg. Chem. 2010, 49, 8287–8297 8287
DOI: 10.1021/ic1008522
Effect of Five Membered Versus Six Membered Meso-Substituents on Structure
and Electronic Properties of Mg(II) Porphyrins: A Combined Experimental and
Theoretical Study
†
†
‡
§
,||
,†
Avijit Ghosh, Shaikh M. Mobin, Roland Fr €o hlich, Ray J. Butcher, Dilip K. Maity,* and Mangalampalli Ravikanth*
†
Department of Chemistry, Indian Institute of Technology, Bombay, Powai, Mumbai 400 076, India,
Organisch-Chemisches Institut, Universit a€ t M u€ nster, Corrensstrasse 40, 48149 M u€ nster, Germany,
‡
§
Department of Chemistry, Howard University, 525 College Street Nw, Washington, D.C. 20059, and
||
Theoretical Chemistry Section, Chemistry Group, Bhabha Atomic Research Centre, Mumbai 400085, India
Received April 29, 2010
Meso-tetrasubstituted Mg(II) porphyrins containing six membered phenyl groups (MgTPP) and five membered thienyl
MgTThP) and furyl groups (MgTFP) were synthesized and structurally characterized, and the effects of meso-
(
substituents on electronic properties were studied using NMR, absorption, fluorescence spectroscopy, and
electrochemical studies. Density functional theory (DFT) calculations were carried out to correlate with experimental
observations. The three Mg(II) porphyrins MgTPP, MgTThP, and MgTFP were crystallized as hexa-coordinate
systems with Mg(II) ion in the center of the porphyrin plane and having two tetrahydrofuran molecules as axial ligands.
The X-ray studies clearly showed that the meso-furyl groups adopt a conformation in which they are more in-plane with
the porphyrin plane whereas the thienyl and phenyl groups prefer an orthogonal arrangement with respect to the
porphyrin plane. This arrangement of meso-substituents with the porphyrin plane helps in the enhancement of
porphyrin π-delocalization in MgTFP compared to MgTThP and MgTPP. The differences in their structures are clearly
reflected in their spectral and electrochemical properties. The absorption and fluorescence bands experienced
bathochromic shifts on moving from six membered phenyls to five membered thienyl and furyl group, and the maximum
effects were observed for meso-tetrafuryl Mg(II) porphyrin. The electrochemical studies indicated that the gap
between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)
decreases as we move from six membered phenyl groups to five membered thienyl and furyl groups, which explains
the bathochromic shifts observed in absorption and fluorescence bands. Results on structure and electronic properties
based on DFT studies are in agreement with experimental observations.
Introduction
cence quantum yields and longer excited state lifetimes
3
(
8-10 ns) compared to zinc porphyrins (2-2.5 ns). Impor-
Magnesium(II) ion is present as centrally coordinated
metal in chlorophyll and bacteriochlorophyll and plays a
very important role in photosynthesis. Because of their
importance in biology, the study of magnesium containing
porphyrinic compounds has been of long-standing interest.
Magnesium porphyrins are fluorescent compounds bearing a
diamagnetic metal ion of ionic radius about 0.72 A in the
porphyrin cavity. Although both zinc and magnesium
porphyrins display quite similar absorption and emission
properties, magnesium porphyrins exhibit higher fluores-
tantly, magnesium porphyrins are easier to oxidize by over
250 mV because of the less electronegative magnesium.
4
1
These interesting electrochemical and photophysical proper-
ties of Mg(II) porphyrins have been utilized recently to pre-
5
pare novel porphyrin dyads and materials for optoelectronic
˚
2
(3) (a) Dorough, G. D.; Miller, J. R.; Huennekens, F. M. J. Am. Chem.
Soc. 1951, 73, 4315–4320. (b) Allison, J. B.; Becker, R. S. J. Chem. Phys. 1960,
3
2, 1410–1417.
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Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press:
New York, 2000; Vol. 8, Chapter 55, pp 1-114.
*
To whom correspondence should be addressed. E-mail: dkmaity@barc.
gov.in (D.K.M.), ravikanth@chem.iitb.ac.in (M.R.). Fax: þ91-22-25767152
M.R.). Phone: þ91-22 25767176 (M.R.).
1) (a) The Photosynthetic Bacteria; Clayton, R. K., Sistrom, W. R., Eds.;
Plenum Press: New York, 1978. (b) Deisenhofer, J.; Michel, H. Science 1989,
45, 1463–1473.
2) (a) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements;
(
5) (a) Zaleski, J. M.; Chang, C. K.; Leroi, G. E.; Cukier, R. I.; Nocera,
D. G. J. Am. Chem. Soc. 1992, 114, 3564–3465. (b) Wynne, K.; LeCours, S. J.;
Galli, C.; Therien, M. J.; Hochstrasser, R. M. J. Am. Chem. Soc. 1995, 117,
3749–3753. (c) Li, J.; Diers, J. R.; Seth, J.; Yang, S. I.; Bocian, D. F.; Holten, D.;
Lindsey, J. S. J. Org. Chem. 1999, 64, 9090–9100. (d) Natan, M. J.; Hoffman,
B. M. J. Am. Chem. Soc. 1989, 111, 6468–6470. (e) Hascoat, P.; Yang, S. I.;
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(
(
2
(
Pergamon: Oxford, 1984. (b) Li, F.; Gentemann, S.; Kalsbeck, W. A.; Seth, J.;
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r 2010 American Chemical Society
Published on Web 08/20/2010
pubs.acs.org/IC

8
288 Inorganic Chemistry, Vol. 49, No. 18, 2010
Ghosh et al.
6
device applications. Generally, the magnesium(II) porphyr-
ins were prepared under harsh conditions by treating por-
structure and electronic properties of tetraryl substituted
metalloporphyrins having six membered aryl groups versus
five membered aryl groups as meso-aryl substituents. In this
paper, we report the synthesis, structure, optical, electroche-
mical, and fluorescence properties of Mg(II) porphyrins
having six membered phenyl groups as well as Mg(II)
porphyrins having five membered thienyl and furyl groups
at meso-positions. The study clearly shows that the structure
and electronic properties of Mg(II) porphyrins are greatly
altered on changing six membered meso-phenyls with five
membered meso-thienyls or meso-furyls, and maximum ef-
fects were observed with Mg(II) porphyrin having five
membered furyl groups at meso-positions. To support the
inferences drawn based on experimental results and to pro-
vide more insight into the effect of substituents on electronic
properties of these Mg(II) porphyrins, quantum chemical
calculations were carried out applying density functional
theory (DFT). Despite their immense importance in biology,
only a few theoretical studies on Mg-porphyrin systems have
been made to study electronic absorption properties of these
macrocyclic systems (which are rather large for ab initio
phyrin with MgX (X = Cl and Br) in refluxing dimethyl-
2
7
8
formamide (DMF). Lindsey and co-workers recently
developed simple homogeneous and heterogeneous methods
at room temperature for insertion of Mg(II) ion into the
porphyrin and related macrocycles. There are several reports
on structurally characterized Mg(II) porphyrins where their
ground and excited state dynamics have been studied, but
these reports are restricted to Mg(II) porphyrins having six
membered aryl groups such as phenyls or substituted phenyls
or pyridyls at meso-positions. Recently, we and others have
synthesized meso-aryl porphyrins having five membered aryl
groups such as furyls, thienyls, pyrazolyls, imidazoles, pyr-
rolyls, or azulenyls at meso-positions. The limited studies
available in the literature clearly showed that the electronic
properties of the porphyrin ring are greatly altered when six
membered meso-aryl groups are replaced with five membered
aromatic heterocycles. A perusal of the literature reveals that
metalloporphyrin chemistry with five membered meso-sub-
9
10
11
stituents has remained largely unexplored, and to the best
of our knowledge, there is no systematic study on the
12,13
calculations).
In the present study, DFT based equilib-
rium structures in the ground (S ) state and electronic
0
properties in the ground as well as excited states of these
three Mg(II) porphyrins are reported. UV-vis spectra of
these systems are simulated based on calculated excited state
properties following Time Dependent DFT (TD-DFT). A
direct correlation is observed in the shift of absorption
maxima (λmax) between the experimental and theoretical
values especially in the Soret band region. Contour plots of
frontier orbitals are generated to assign the type of electronic
transitions involved. Energy gaps between the highest occu-
pied molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO) are also calculated to see the
effectof substituents intuning the energy gaps inthese Mg(II)
porphyrinic systems and correlated with the experimental
results based on electrochemical studies. Again, a very good
correlation is achieved between the experimental and the
theoretical values of energy gaps.
(
6) (a) Ambroise, A.; Wagner, R. W.; Rao, P. D.; Riggs, J. A.; Hascoat,
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(
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Arif, A. M.; Botoshansky, M.; Kaftory, M.; Miller, J. S. Inorg. Chem. 2003, 42,
Results and Discussion
There are several methods available to synthesize Mg(II)
porphyrins. However the approach adopted by Lindsey et
2
4
311–2322. (m) McKee, V.; Ong, C. C.; Rodley, G. A. Inorg. Chem. 1984, 23,
242–4248. (n) Szulbinski, W. S. Inorg. Chim. Acta 1995, 228, 243–250. (o)
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994, 230, 144–148. (p) Loewe, R. S.; Lammi, R. K.; Diers, J. R.; Kirmaier, C.;
Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Mater. Chem. 2002, 12, 1530–1552.
q) El-Khouly, M. E.; Araki, Y.; Ito, O.; Gadde, S.; McCarty, A. L.; Karr, P. A.;
al., is very simple and gives Mg(II) porphyrins in high yield.
1
8a
We adopted Lindsey’s homogeneous approach to synthe-
size MgTPP and MgTFP (Scheme 1). According to Lindsey’s
approach, we reacted the corresponding free base porphyrin
in CH Cl with MgBr O(Et) for 15 min at room tempera-
(
Zandlerb, M. E.; D'Souza, F. Phys. Chem. Chem. Phys. 2005, 7, 3163–3171. (r)
Bonnett, R.; Hursthouse, M. B.; Malik, K. M. A.; Mateen, B. J. Chem. Soc.,
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Dalton Trans. 2007, 2163–2170.
2
2
2
3
2
ture. The reaction progress was monitored by UV-visible
spectroscopy. Interestingly, the method worked well only for
the synthesis of MgTPP and MgTFP (Scheme 1) but did not
work for MgTThP. We used the conventional approach to
synthesize MgTThP by refluxing H TThP with excess MgCl
(
10) (a) Gupta, I.; Ravikanth, M. Tetrahedron 2003, 59, 6131–6139. (b)
Guo, C. C.; Ren, T. G.; Wang, J.; Li, C. Y.; Song, J. X. J. Porphyrins
Phthalocyanines 2005, 9, 430–436. (c) Bhaumik, J.; Yao, Z.; Borbas, K. E.;
Taniguchi, M.; Lindsey, J. S. J. Org. Chem. 2006, 71, 8807–8817. (d) Gupta, I.;
Ravikanth, M. J. Photochem. Photobiol. A: (Chem). 2006, 177, 156–163. (e)
Kurotobi, K.; Osuka, A. Org. Lett. 2005, 7, 1055–1058. (f) Furuta, H.; Maeda,
H.; Osuka, A. Org. Lett. 2000, 2, 187–189. (g) Brueckner, C.; Foss, P. C. D.;
Sullivan, J. O.; Pelto, R.; Zeller, M.; Birge, R. R.; Crundwell, G. Phys. Chem.
Chem. Phys. 2006, 8, 2402–2412. (h) Santosh, G.; Ravikanth, M. Chem. Phys.
Lett. 2007, 448, 248–252. (i) Santosh, G.; Ravikanth, M. Tetrahedron 2007, 63,
2
2
7
in DMF (Scheme 1). The compounds MgTPP, MgTThP,
and MgTFP were purified by alumina column chromatog-
raphy and recrystallized from tetrahydrofuran (THF). The
formation of Mg(II) porphyrins was confirmed by observa-
tion of a molecular ion peak in the mass spectra. The
structures of Mg(II) porphyrins were determined by X-ray
7
833–7844.
11) (a) Rochford, J.; Botchway, S.; McGarvey, J. J.; Rooney, A. D.; Pryce,
(
M. T. J. Phys.Chem. A 2008, 112, 11611–11618. (b) Boyle, N. M.; Rochford, J.;
Pryce, M. T. Coord. Chem. Rev. 2010, 254, 77–102. (c) Purushothaman, B.;
Varghese, B.; Bhyrappa, P. Acta Crystallogr. Sect. C: Cryst. Struct. Commun.
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(13) Rubio, M.; Roos, B. O. J. Chem. Phys. 1999, 110, 7202–7209.
2
001, C57, 252–253.

Article
Inorganic Chemistry, Vol. 49, No. 18, 2010 8289
Scheme 1. Synthesis of Mg(II) Porphyrins [MgTPP, MgTThP, and
MgTFP]
out of plane displacement of the central Mg(II) ion and
porphyrin nonplanarity is also neligible in all three cases.
However, the four pyrrole rings are not coplanar and
deviate alternately up and down from the mean plane giving
a wave-like conformation. The deviation of pyrrole rings
from the mean plane of the four meso-carbons of porphyrin
helps in the decrease of dihedral angle between porphyrin
plane and meso-substituents which in turn leads to strong
electronic interaction between the porphyrin ring and meso-
substituents. In Mg(II)TPP(THF) , Mg(II)TThP(THF) ,
2
2
and Mg(II)TFP(THF) , pyrroles A/C and B/D deviate by
2
8
.01°/5.05°, 7.16°/5.51°, and 9.74°/5.10°, respectively.
A close inspection of structures of Mg(II) porphyrins
indicates that in the case of MgTPP and MgTThP, one set
of trans meso-aryl groups are almost perpendicular to the
plane of the porphyrin (99.3° and 87.5°, respectively)
whereas the dihedral angle for the other set of meso-aryl
groups is considerably reduced (57.1° and 58.2°, re-
spectively). For MgTFP, on the other hand, not only is
the average dihedral angle much smaller than those for
both MgTPP and MgTThP but both dihedral angles have
similar values (46.3° and 50.8°). This reduced dihedral
angle indicates that there is more coplanarity between the
meso-furyl groups and the porphyrin plane in Mg(II)-
TFP(THF) which helps in the enhancement of π-con-
2
crystallography, and the electronic properties were studied by
spectroscopic and electrochemical techniques.
jugation resulting in the observed alteration of its electro-
nic properties. This is also clearly reflected in several bond
distances such as C -C , C -C , C -Cma, and so forth
Crystallographic Characterization of MgTPP, MgTThP,
and MgTFP. All three molecules were crystallized via slow
diffusion of n-hexane into the THF solutions of Mg(II)
porphyrins over a period of one week. The compounds
MgTPPandMgTThPwerecrystallizedinthetriclinicspace
group P1 whereas MgTFP was crystallized in the mono-
R
β
β
β
m
(
(
Figure 2 and Table 2), which are shorter for Mg(II)TFP-
THF) as compared with the other two Mg(II) porphyr-
2
ins (Table 2). Furthermore, the axial THF ligands are
involved in steric interaction with meso-substituents in
Mg(II)TPP(THF) and Mg(II)TThP(THF) ; hence the
2
2
clinic space group P2 /c. The crystal data and data collec-
1
axial ligands in these two Mg(II) porphyrins are distorted
unlike in Mg(II)TFP(THF) . This can be due to the ortho-
tion parameters are given in Table 1, and comparisons
of selected bond distances and bond angles of MgTPP,
MgTThP, and MgTFP are given in Table 2. A comparison
of perspective views of MgTPP, MgTThP, and MgTFP is
shown in Figure 1. All three Mg(II) porphyrin complexes
exhibit hexa-coordinated octahedral geometry with two
tetrahydrofuran molecules bonded to Mg(II) ion as fifth
and sixth ligands (hence these complexes are represented as
2
gonal arrangement of meso-aryl groups with respect to
porphyrin plane in MgTPP and MgTThP which causes
the steric hindrance leading to distortion of THF ligands.
However, in MgTFP, the meso-furyl groups are more
coplanar with the porphyrin leading to less steric hindrance
between axial THF ligands and meso-furyl groups. This is
also reflected in the bond distance between Mg(II) ion and
axial “O” of tetrahydrofuran which is longer for Mg(II)-
Mg(II)P(THF) , where P=TPP, TThP, and TFP). Mag-
2
nesium porphyrins may crystallize either in five-coordinated
or six-coordinated geometric fashion depending on the
nature of the solvent used for the crystallization.
˚
TPP(THF) and Mg(II)TThP(THF) (by ∼0.05 A) and
2
2
9a,j,k,m,14
shorter for Mg(II)TFP(THF) supporting the observation
2
that the axial ligands can approach Mg(II) ion more closely
in Mg(II)TFP(THF) because of the equivalent conforma-
In five-coordinated entities, the magnesium atom is dis-
placed out of the mean plane of the four pyrrole nitogen
2
tion adopted by its meso-furyl substituents. The binding
properties of central metal ion in these Mg(II) porphyrin
complexes are quite different compared to transition metal
atoms toward the axial ligands, by values ranging from
˚
0.027 to 0.496 A, whereas this out-of-plane displacement of
central magnesium atom is negligible in six coordinated
cases. In all three Mg(II) porphyrins the central Mg(II) ion
is perfectly in the plane defined by four meso-carbons as it is
located on a crystallographic center of inversion. A compar-
ison of the structures and geometrical features of all com-
plexes is shown in Table 3, and it is clear that there is no
porphyrin complexes. For the Fe(II)TPP(THF) com-
2
15
plex, which is quite similar to Mg(II)TPP(THF) , the
2
average M;N bond lengths are quite comparable with
Mg(II)TPP(THF) but the M;O distance is considerably
2
˚
longer by ∼0.11 A. This indicates that the axial THF
molecules are loosely bound to Fe(II)TPP(THF) complex
2
compared to Mg(II)TFP(THF) where the M;O distance
2
is shorter andindicatesthattheaxial THF ligand is strongly
coordinated to the oxo-philic Mg(II) ion.
(
14) (a) Fischer, M. S.; Templeton, D. H.; Zalkin, A.; Calvin, M. J. Am.
Chem. Soc. 1971, 93, 2622–2628. (b) Chow, H. C.; Serlin, R.; Strouse, C. E.
J. Am. Chem. Soc. 1975, 97, 7230–7237. (c) Serlin, R.; Chow, H. C.; Strouse,
C. E. J. Am. Chem. Soc. 1975, 97, 7237–7242. (d) Kratky, C.; Dunitz, J. D. Acta
Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1977, B33, 545–547.
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Crystallogr. Cryst. Chem. 1977, B33, 547–549.
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Varret, F.; Battioni, P.; Mansuy, D.; Fajer, J. Inorg. Chem. 2002, 41, 5647–
5649.
(

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290 Inorganic Chemistry, Vol. 49, No. 18, 2010
Ghosh et al.
Table 1. Crystallographic Data for MgTPP, MgTThP, and MgTFP
parameters
mol formula
fw
MgTPP
40MgN
MgTThP
36MgN
MgTFP
C
928.89
48
H
4
O
2
C
805.32
44
H
4
O
2
S
4
44 4 6
C H36MgN O
741.08
monoclinic
cryst sym
space group
triclinic
P1
triclinic
P1
1
P2 /c
˚
a (A)
12.3262(18)
12.326(3)
13.9882(16)
98.702(13)
105.311(11)
95.727(14)
2004.4(6)
2
0.091
1.267
944
3.08-49.96
6996/0.0327
9.5942(7)
10.1952(7)
11.7484(8)
66.492(3)
76.053(3)
65.476(6)
954.80(12)
1
2.804
1.401
420
4.12-67.82
10709/0.036
9.882(7)
20.508(3)
9.566(4)
90
115.24
90
1753.7(14)
2
0.110
1.403
776
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
3
˚
V (A )
Z
-
1
)
μ (mm
-3
Dcalcd (g cm
)
F(000)
θ range (deg)
e data (Rint
2
3.02-25.00
10800/0.0311
)
R1, wR2 [I > 2σ(I )]
R1, wR2 (all data)
GOF
0.0481, 0.1085
0.1292, 0.1307
1.013
0.0451, 0.1207
0.0487, 0.1245
1.041
0.0353, 0.0905
0.0496, 0.0935
1.030
-
3
)
˚
largest diff. peak/hole, (e A
0.823 /-0.570
0.499/-0.333
0.709/-0.264
˚
systems in case of MgTFP supramolecular assembly
resulting in all the dihedral angles between meso-groups
and porphyrin plane being similar as shown in Figure 4
since each unit feels equal interactions.
Table 2. Comparison of Some Selected Bond Length [A] and Bond Angles [deg]
Obtained from Both X-ray Crystal Structure and Optimized Structure
X-ray structure
optimized structure
parameter
Mg-N1
MgTPP MgTThP MgTFP MgTPP MgTThP MgTFP
Spectral and Electrochemical Properties. The effect of
six membered phenyl groups versus five membered thie-
nyl and furyl groups is clearly reflected in their spectral
and electrochemical properties, and these are in agree-
ment with their crystal structures. The comparison of H
NMR spectra of MgTPP, MgTThP, and MgTFP re-
2.073(1) 2.073(1) 2.084(2) 2.09
2.070(1) 2.073(1) 2.077(2) 2.09
2.073(3) 2.073(1) 2.084(2) 2.09
2.070(1) 2.073(1) 2.077(2) 2.09
2.241(1) 2.267(2) 2.210(2) 2.28
2.09
2.09
2.09
2.09
2.27
1.48
1.36
1.44
70.3
2.09
2.09
2.09
2.09
2.27
1.47
1.36
1.44
52.9
Mg-N2
Mg-N3
Mg-N4
Mg-O
1
a
C
C
C
m
-C ma a
1.493
1.349
1.442
1.488
1.352
1.444
72.8
1.473
1.346
1.444
48.5
1.49
1.36
1.44
65.9
corded in CDCl is presented in Figure 5. In all three
3
β
-C
β
a
Mg(II) porphyrins, the eight β-pyrrole protons appeared
as singlets. However, the β-pyrrole protons experien-
ced ∼0.2 ppm downfield shift on changing the meso-
substituent from six membered phenyl group to five
membered thienyl and furyl groups, and maximum down-
field shift was observed for MgTFP. This indicates that
there is an alteration in ring current on changing the meso-
substituent from six membered phenyl group to five
membered thienyl and furyl groups. From crystal struc-
tures, it is clear that among Mg(II) porphyrins, the meso-
substituents are more coplanar with the porphyrin plane
only in MgTFP, leading to strong interaction between
porphyrin macrocycle and meso-furyl groups. This inter-
action results in the alteration of π-conjugation which
results in the more downfield shift of β-pyrrole protons in
MgTFP compared to MgTPP and MgTThP.
β
-C
R
b
dihedral angle 78.2
a
b
Average bond distance (in A). Average dihedral angle (deg)
˚
between porphyrin ring and meso-substituents. C -Cma is the average
m
˚
distance (A) between meso-carbon atom and the carbon atom through
which respective meso-aryl group is attached to meso-positions.
In addition to the above-mentioned differences be-
tween the structures of Mg(II) TFP(THF) and Mg(II)-
TPP(THF) , a novel feature of Mg(II)TFP(THF) is the
2
presence of intramolecular and intermolecular hydrogen
bonding interactions. The intramolecular hydrogen
bonding as shown in the packing diagram (Figure 3) is
present between meso-furyl “O” and β-pyrrole “H” which
may be assisting in the decrease of dihedral angle between
meso-furyl groups and plane of the porphyrin. In addi-
2
2
tion, the Mg(II)TFP(THF) also shows intermolecular
2
Similar observations were made in the absorption
spectroscopy. The Q- and Soret band absorption spectra
of MgTPP, MgTThP, and MgTFP are shown in Figure 6,
and absorption data are presented in Table 4. Inspection
of Figure 6 and Table 4 reveals the following: (1) In
general, all three Mg(II) porphyrins showed two Q-bands
and one strong Soret band; (2) both Soret and Q-bands
experienced bathochromic shifts with slight alteration in
absorption coefficients on changing six membered phenyl
groups with five membered thienyl and furyl groups
at meso-positions and (3) the maximum red shifts in
absorption bands were observed for MgTFP compared
to MgTThP. Thus, the absorption study supported the
alteration of electronic properties of the Mg(II) porphyr-
in on changing six membered phenyl groups with five
hydrogen bonding (O2
H-C14 and C14;H
O2)
between meso-furyl “O” (O2) of one porphyrin unit with
3
3 3 3
3 3 3 3
“
H” (H14) of meso-furyl group of second porphyrin and
vice versa leading to the formation of the supramolecular
assembly (Figure 4). A weak hydrogen bonding is also
observed between O2 of meso-furyl group and H-C8
of meso-furyl group from other porphyrin unit. Thus,
MgTFP(THF) exhibits different structural properties
2
than other two Mg(II) porphyrins. Note that crystal
structures have produced two different sets of dihedral
angles between meso-groups and porphyrin plane in case
of MgTPP and MgTThP as the meso-groups are not
involved in H-bonding. However, as it was just discussed,
there are H-bonds between two macrocyclic porphyrin

Article
Inorganic Chemistry, Vol. 49, No. 18, 2010 8291
Table 3. Selected Geometrical Parameters
MgTPP
MgTFP
MgTThP
a,b
Mg-N
p
2.072
2.241(12)
0.00
0.00
0.07
0.13
0.36
2.072
2.080
2.210(16)
0.00
0.00
0.08
0.13
0.35
2.080
2.073
2.267(16)
0.00
0.00
0.03
0.08
0.31
2.073
a,b
Mg-Oax
Mgc
24
Δ
Δ
Δ
C
C
Mgd
4N
e
24
f
m
f
β
b
core size
a
b
c
Average value in A. Value in A. Displacement (in A) of magne-
˚
˚
˚
d
sium from the 24-atom mean plane of the porphyrin core. Displace-
˚
ment (in A) of magnesium from the mean plane of the four pyrrole
e
nitrogen atoms. Average displacement (in A) of the 24 atoms of the
˚
porphyrin macrocycle from the least-squares plane of the 24 atoms of the
f
porphyrin ring. Average displacement (in A) from four porphyrinic
˚
nitrogen atoms.
Figure 2. Formal diagram of the Mg(II) porphyrin core displaying the
common symbol for MgTPP, MgTThP, and MgTFP; the corresponding
independent bond lengths are summarized in Table 3.
in our DFT studies of Mg(II) porphyrins which supports
more orbital overlap of meso-substituents with porphyrin
ring in MgTFP. The trends observed in the absorption
properties are in agreement with the X-ray structures of
MgTPP, MgTThP, and MgTFP.
The redox properties of three Mg(II) porphyrins were
studied by cyclic voltammetric and differential pulse
voltammetric techniques using tetrabutylammonium per-
chlorate as supporting electrolyte (0.1 M) in dichloro-
methane as solvent. A comparison of oxidation and
reduction waves of cyclic voltammograms is shown in
Figure 7, and data for all three Mg(II) porphyrin deriva-
tives are presented in Table 5. In general, metallopor-
phyrins exhibit one or two ring centered oxidations and
one or two ring centered reductions along with metal
centered oxidation and reduction in case of redox active
metal center present in the porphyrin cavity. But each of
these three compounds exhibited only one ring-centered
reduction (-1.40 V to -1.60 V) and one or two ring-
centered oxidations (0.65 to 0.80 V). MgTPP and
MgTThP exhibited two ring centered oxidations whereas
MgTFP showed only one oxidation. The central metal
Mg(II) being redox inactive did not show any metal
centered oxidation or reduction. The oxidations and
Figure 1. ORTEP diagram of compounds (a) MgTPP, (b) MgTThP,
and (c) MgTFP. Ellipsoids are drawn at 50% probability level.
membered thienyl and furyl groups at meso-positions.
These effects are attributed to the orientation of meso-
substituents with respect to the mean plane of the por-
phyrin. In MgTPP and MgTThP, the meso-substituents
are arranged in orthogonal fashion with respect to the
mean plane of the porphyrin resulting in the prevention of
significant π-conjugation between porphyrin and meso-
substituents. However, in MgTFP, the five membered
meso-furyl groups are more in-plane with the porphyrin
ring resulting in efficient extension of the porphyrin
π-conjugation into meso-substituents. This is also reflected
reductions were reversible or quasi-reversible (ΔE =
p
60-120 mV) except MgTFP which showed one irrever-
sible oxidation. Inspection of the redox data (Table 5)
reveals that the first oxidation potentials were shifted to
more positive potential when six membered meso-phenyls
were replaced with five membered meso-thienyl groups
suggesting that MgTThP is harder to oxidize than
MgTPP. However, the reduction potentials were shifted
to less negative potential on replacement of meso-phenyl
groups with meso-thienyl and meso-furyl groups indicat-
ing that the MgTThP and MgTFP are easier to reduce

8
292 Inorganic Chemistry, Vol. 49, No. 18, 2010
Ghosh et al.
Figure 3. Molecular packing for compound MgTFP viewed down the a axis. The weak C-H
ring and O of meso-furyl group are shown by dashed lines.
O intramolecular interactions between C-H of pyrrole
3 3 3
3
Figure 4. Supramolecular assembly through intermolecular-hydrogen bonding network (between O2
group from two different porphyrin unit) in MgTFP complex.
H-C14 and C14;H 3 3 3 3 O2 of meso-furyl
3
3 3 3
compared to MgTPP. Furthermore, the maximum ano-
dic shifts for reduction potentials were observed for
MgTFP suggesting that the MgTFP is easier to reduce
than MgTThP.
The Δredox value which gives an estimate of HOMO-
LUMO gap calculated from the difference of the first
oxidation and first reduction potential is shown for
MgTThP in Figure 8. The Δredox is decreased on changing
meso-substituents from six membered phenyl groups to
five membered thienyl groups (Table 5). The Δredox was
not calculated for MgTFP since it exhibited an irrever-
sible oxidation. The reduction of HOMO-LUMO gap
explains the large red shifts of absorption bands of
MgTThP compared to MgTPP. It is assumed that the
HOMO-LUMO is further reduced in MgTFP which
showed maximum red shifts in absorption bands com-
pared to the other two Mg(II) porphyrins.
1
The fluorescence properties were studied by both
steady state and time-resolved fluorescence techniques
Figure 5. Comparison of H NMR of compounds MgTPP, MgTThP,
and MgTFP (* = residual CHCl in CDCl ).
3
3

Article
Inorganic Chemistry, Vol. 49, No. 18, 2010 8293
to understand the effect of meso-substituents on electro-
nic properties of Mg(II) porphyrins in the singlet excited
state. The comparison of fluorescence spectra is shown in
Figure 9a, and the data are presented in Table 5. The
MgTPP and MgTThP showed two fluorescence bands
and MgTFP showed one broad fluorescence band. The
inspection of Figure 9a and data presented in Table 5
reveal that the fluorescence bands experienced red shifts
on changing meso-phenyl substituents to meso-thienyl
and meso-furyl substituents, and maximum red shifts
were observed for MgTFP. These observations are in
line with the absorption studies and support the effi-
cient π-delocalization between porphyrin and meso-
substituents in MgTFP compared to MgTThP and
MgTPP. The quantum yields calculated by comparative
method showed the following trend: MgTPP > MgTFP
>MgTThP. The low quantum yield of MgTThP was due
to the presence of heavy sulfur atoms of meso-thienyl
groups which enhances the non-radiative pathways and
reduces the fluorescence yield. The singlet excited state
lifetime was measured by single-photon counting techni-
que. The Mg(II) porphyrins were excited at 406 nm, and
emissions were detected at emission peak maxima of the
corresponding Mg(II) porphyrin. The fluorescence decay
of Mg(II) porphyrins was fitted to a single exponential,
and the representative fluorescence decay profiles of
MgTPP, MgTThP, and MgTFP along with residual fits
are shown in Figure 9b and 9c, respectively. The singlet
excited state lifetime τ of Mg(II) porphyrins is in agree-
f
ment with their corresponding quantum yield, and the
low singlet state lifetime observed for MgTThP is due to
the presence of heavy sulfur atoms. Thus, the singlet state
excited properties support the alteration of electronic
properties on replacement of six membered meso-phenyl
groups with five membered meso-thienyl and meso-furyl
groups in Mg(II) porphyrins.
Quantum Chemical Studies. Minimum energy struc-
tures of three metalloporphyrins MgTPP, MgTThP,
and MgTFP calculated at B3LYP/6-31G(d) level of
theory are displayed in Figure 10. This choice of DFT
functional and basis set should be adequate for metallo-
porphyrin systems. Selected bond lengths and torsion
angles have been provided in Table 2 along with the
X-ray crystallographic data for comparison. To bring
about the direct correlation between calculated and
experimentally observed properties, the equilibrium
Figure 6. Comparison of normalized absorption spectra of MgTPP
(dotted lines), MgTThP (dashed lines) and MgTFP (solid lines), recorded
in toluene, (a) Soret band and (b) Q-band.
1
Table 4. H NMR Data and Absorption Data
1
a
b
H NMR
absorption data
δ in ppm
Soret band
compound β-pyrrole λ (nm) (logε)
Q-bands λ (nm) (log ε)
MgTPP 8.85 (s, 8 H) 426 (5.75)
MgTThP 9.02 (s, 8 H) 433 (5.74)
524 (sh), 564 (4.25), 604 (3.95)
530 (sh), 570 (4.28), 613 (3.79)
532 (sh), 572 (4.15), 620 (3.87)
MgTFP
9.10 (s, 8 H) 437 (5.56)
Figure 7. Comparison of oxidation and reduction waves of cyclic
voltammograms (scan rate =50 mV s ) of MgTPP, MgTThP, and
a
b
. All spectra recorded in toluene; the
-1
All spectra recorded in CDCl
3
-
5
concentration used for Q-band spectra was ∼5 ꢀ 10 M and for Soret
2 2
MgTFP in CH Cl containing 0.1 M tetrabutylammonium perchlorate
-7
band was ∼5 ꢀ 10 M, sh = shoulder.
(TBAP) as supporting electrolyte.
Table 5. Redox Potential Data and Emission Data
a
redox data (V)
reduction
b
oxidation
emission data
compound
I
II
I
HOMO-LUMO gap (Δredox
)
Q (0, 0) (nm)
Q (0, 1) (nm)
Φ
f
f
τ (ns)
MgTPP
MgTThP
MgTFP
0.65
0.78
0.68
0.94
0.98
-1.57
-1.42
-1.40
2.22
2.20
609
626
649
660
669
0.16
0.003
0.024
8.90
0.37
3.28
c
a
-1 b
2 2
In CHc Cl solvent with 0.1 M tetrabutylammonium perchlorate (TBAP) as supporting electrolyte; scan rate =50 mV s . All data were recorded in
toluene. Irreversible.

8
294 Inorganic Chemistry, Vol. 49, No. 18, 2010
Ghosh et al.
Figure 8. HOMO-LUMO gap of MgTThP.
Figure 9. (a) Comparison of normalized emission spectra of MgTPP
(solid lines), MgTThP (dotted lines), and MgTFP (dashed lines) recorded
in toluene, (b) fluorescence decay profile, and (c) the weighted residuals
distribution fit of fluorescence decays of MgTPP, MgTThP, and MgTFP.
The excitation wavelength used was 406 nm, and emissions were detected
at different wavelengths depending on the emission peak maxima of the
corresponding meso-tetrasubstituted Mg(II) porphyrins.
Figure 10. Fully optimized structures of (a) MgTPP, (b) MgTThP, and
c) MgTFP applying B3LYP/6-31G(d) level of theory (numbering of
(
structures are optimized keeping two THF solvent mole-
cules in the axial positions similar to crystal structures
obtained for these Mg(II) porphyrins. It is observed that
such geometrical arrangement leads to the most stable
atoms for each complex has been shown following the corresponding
crystal structure).
In fact, a few optimizations are carried out taking such
possible structures as the starting geometry which leads to
a less stable structure. The cavity size of MgTPP and
structure in the ground (S ) state when full geometry
0
optimization is carried out applying the Newton-
Raphson procedure. The possibility of other conformers
does exist, for example, in the case of MgTFP, the meso-
furyl group can be arranged in all possible orientations
with respect to the heteroatom oxygen because of possible
allowed rotation through the single bond between the
meso-carbon atom and the carbon atom through which
meso-furyl group is attached to the porphyrin unit.
˚
˚
MgTThP is ∼4.177 A, and for MgTFP it is 4.188 A. These
values are close to the size obtained from crystal structure
determination. The other parameters are also in line with
crystallographicallyobtaineddataasdisplayed in Table2.
It is worth mentioning that theoretical calculations are
carried out under isolated condition, and this leads to
only one set of dihedral angles between meso-substituents

Article
Inorganic Chemistry, Vol. 49, No. 18, 2010 8295
and porphyrin plane in each case as there are no inter-
molecular interactions unlike in crystals. It is to be noted
that the average dihedral angle between the meso-substit-
uents and mean porphyrin ring are ∼70° in MgTPP, and
in MgTThP and ∼53° in MgTFP. This indicates that
meso-furyl group is more inclined toward porphyrin ring
compared to meso-phenyl and meso-thienyl groups.
Frontier molecular orbitals (FMO) are also calculated
and displayed in Figure 11. This figure suggests more
electronic delocalization in case of MgTFP, and the meso-
furyl groups are involved in FMOs (HOMOs and LUMOs)
to bring down the orbital energy through extensive
π conjugation. In the case of MgTPP, there is least invol-
vement of meso-substituent orbitals in forming FMOs. The
energies of HOMOs follow the order as MgTPP >
MgTThP > MgTFP, and a similar trend is also observed
in the case of the LUMO energy. This feature is clearly seen
from the orbital correlation diagram as shown in Figure 11
with FMOs and corresponding energies of their respective
porphyrin units. It is to be noted that neither the axial
coordinating solvent molecule nor the central metal parti-
cipates in either of the FMOs.
tively. By scrutinizing the energy values of the HOMO
and LUMO of all three complexes, it is inferred that in
the case of MgTFP, the HOMO and LUMO are stabi-
lized by 0.115 and 0.258 eV, respectively, as compared to
MgTThP. Similarly, in case of MgTThP, HOMO and
LUMO are stabilized by 0.157 and 0.180 eV respectively
relative to MgTPP. The HOMO-LUMO gaps calculated
from these HOMO-LUMO energies follow the same
trend (MgTPP >MgTThP >MgTFP) as obtained from
cyclic voltammetry redox potential values. The substan-
tial decrease of energy gap between HOMO and LUMO
from MgTPP to MgTFP via MgTThP results in red shift
in UV-vis spectra.
Vertical electronic transitions are calculated for these
three metalloporphyrins applying the B3LYP functional
under the TD-DFT formalism with the ground state (S0)
optimized structures predicted at the B3LYP level of
theory. The COSMO model is also applied for accounting
for the macroscopic solvent effect in the dichloromethane
solvent. Results on a few low lying electronic transtions in
the Soret band region are supplied in Table 6 showing the
major electronic transitions involved in the observed
Soret bands of these systems. As it is shown in Table 6,
the strong electronic transitions having oscillator strength
f >1.2 are initiated from HOMO-1 orbital in all the three
cases. Note that mainly pyrrole π electrons have partici-
pated in the HOMO-1 orbital. The major electronic
transitions occur to either LUMO or LUMOþ1 orbital
in all these systems. In both these molecular orbitals,
mainly porphyrin ring π* electrons participate with a
small contribution from one set of trans meso-substitu-
tents. UV-vis spectra of these three metalloporphyrins in
the Soret band region are simulated following a Gaussian
model, and the spectra are depicted in Figure 12. The
actual absorption maxima (λmax) do not agree closely
with calculated values. However, it is the order of values
that is comparable.
The FMO correlation diagram indicates that meso-
substituents constitute a part of conjugated macrocycle
showing π overlap with the respective meso-substituents
in both HOMOs and LUMOs. The energies of HOMOs
for MgTPP, MgTThP, and MgTFP are -4.492 eV,
-
4.649 eV, and -4.764 eV, respectively. Similarly, the
energies of LUMOs are -1.799 eV, -1.979 eV, and
2.237 eV for MgTPP, MgTThP, and MgTFP, respec-
-
Conclusions
The influence of six membered versus five memberedmeso-
substituents on structure, spectral, and redox properties
of Mg(II) derivatives of meso-tetrasubstituted porphyrins
was described using both experimental and theoretical ap-
proaches. The crystallographic study showed that the meso-
furyl groups in MgTFP adopt a more in-plane arrangement
with the porphyrin whereas the meso-phenyls in MgTPP and
meso-thienyls in MgTThP are in out-of-plane arrangement
with the porphyrin. This indicates that in MgTFP, there is a
greater resonance interaction between meso-furyl groups and
porphyrin ring leading to extension of π-conjugation whereas
Figure 11. Orbital-correlationdiagram ofHOMO and LUMO(FMOs)
for MgTPP, MgTThP, and MgTFP.
Table 6. Vertical Transition Maxima (in nm) and Oscillator Strengths of Electronic Transitions in the Soret Band Region As Predicted by TD-DFT (B3LYP) Method
Employing 6-31G(d) Basis Set in MgTPP, MgTThP and MgTFP
system
dominant transitions
transition maxima (nm) oscillator strengths
MgTPP
HOMO-1fLUMOþ1 (49%), HOMOfLUMO (15%)
402.5
401.9
349.4
412.8
411.6
364.6
421.4
420.0
388.3
1.3137
1.3419
0.0001
1.2487
1.2656
0.0001
1.2205
1.2204
0.0000
HOMO-1fLUMO (47%), HOMOfLUMOþ1 (17%)
HOMO-3fLUMO (49%), HOMO-4fLUMOþ1 (21%), HOMO-3fLUMOþ1 (14%)
MgTThP HOMO-1fLUMOþ1 (45%), HOMOfLUMO (17%)
HOMO-1fLUMO (44%), HOMOfLUMOþ1 (18%)
HOMO-2fLUMO (51%), HOMO-2fLUMOþ1 (33%)
MgTFP
HOMO-1fLUMOþ1 (36%), HOMO-1fLUMO (17%)
HOMO-1fLUMO (35%), HOMO-1fLUMOþ1 (18%)
HOMO-2fLUMO (46%), HOMO-2fLUMOþ1 (28%), HOMO-3fLUMO (13%)

8
296 Inorganic Chemistry, Vol. 49, No. 18, 2010
Ghosh et al.
X-ray Crystallography. Suitable single crystals were grown by
slow diffusion of n-hexane into the THF solution of correspond-
ing Mg(II)porphyrins. Single crystal X-ray structural studies
of MgTPP and MgTFP were performed on a CCD Oxford
Diffraction XCALIBUR-S diffractometer equipped with an
Oxford Instruments low-temperature attachment. Data were
collected at 293(2) K (in case of MgTPP) and 120(2) K (in case of
MgTFP) using graphite-monochromoated Mo KR radiation
˚
=0.71073 A). The strategy for the data collection was eva-
(
λ
R
luated by using the CrysAlisPro CCD software. The data were
collected by the standard j-ω scan techniques, and were scaled
and reduced using the CrysAlisPro RED software. The struc-
1
7
tures were solved by the direct method using SHELXS-97 and
refined by full matrix least-squares with SHELXL-97, refining
on F . The positions of all the atoms were obtained by direct
2
method. All non-hydrogen atoms were refined anisotropically.
The hydrogen atoms were placed in geometrically constrained
positions and refined with isotropic temperature factor, gener-
ally 1.2Ueq of their parent atoms. The data set for complex
MgTThP was collected with a Nonius KappaCCD diffract-
ometer, equipped with a rotating anode generator at 223(2) K.
Programs used: data collection COLLECT (Nonius B.V., 1998),
Figure 12. Simulated UV-vis spectra of MgTPP, MgTThP, and
MgTFP in the Soret band region. Simulation of these absorption profiles
was carried out from10 low lying vertical excited states with Gaussian line
shape.
in MgTPP and MgTThP, the interaction between meso-
substituents and porphyrin ring is less. The DFT studies also
support strong interaction between meso-substituents and
porphyrin ring in MgTFP. The spectral and electrochemical
studies are in agreement with the crystallographic studies.
The absorption and fluorescence bands are shifted to the red
on changing the meso-substituent from six membered phenyl
groups to five membered thienyl and furyl groups, and
maximum effects were observed for meso-tetrafuryl Mg(II)
porphyrins. The electrochemical studies indicated a decrease
of HOMO-LUMO gap on changing meso-substituents from
six membered phenyl groups to five membered thienyl and
furyl groups, and maximum decrease was noted for MgTFP
which is in agreement with the large red shifts in absorption
and fluorescence bands observed for MgTFP. Thus, it is
concluded that by changing the meso-substituent from six
membered aryl group to five membered aryl group, the
electronic and structural properties of Mg(II) porphyrins
can be tuned.
1
8
19
data reduction Denzo-SMN, absorption correction Denzo,
structure solution SHELXS-97, structure refinement SHELXL-
20
1
7
9
7, graphics SCHAKAL (E. Keller, 1997).
In the case of MgTPP one of the carbon atoms (C3) of the
axial THF was disordered over two conformations with occu-
pancy factors of 0.64(1) and 0.36(1). In MgTThP both indepen-
dent thiophene molecules in the asymmetric unit are disordered,
they were refined with split positions to resulting occupancies of
0.896(3):0.104(3) and 0.646(2):0.354(2), respectively. Geometri-
cal (SAME = same geometry, SAME is used as a relative weak
restraint making all bond lengths in two parts of a molecule
equal within a given standard deviation), and thermal (EADP =
equal anisotropic displacement parameters) restraints were
applied for the minor components.
Theoretical Methods. DFT calculations were carried out on
three Mg(II) porphyrin systems, namely, MgTPP, MgTThP,
and MgTFP to find out minimum energy structures and to study
their electronic properties. Searches for these structures were
0
performed in the ground electronic state (S ) applying Becke’s
three parameters correlated hybrid density functional, B3LYP
adopting 6-31G(d) atomic basis functions which includes at
least 3200 primitive Gaussian functions in total. Equilibrium
structures of these systems were calculated based on full geo-
metry optimization following the Newton-Raphson proce-
dure. Excited state calculations were carried out applying the
TD-DFT procedure with the B3LYP functional for 10 low lying
vertical electronic transitions on the S state optimized structure
0
using the COSMO solvation model to account for the solvent
2 2
(CH Cl ) effect with the same set of basis functions. Simulation
of absorption profiles was carried out from 10 low lying excited
states with Gaussian line shape, that is,
Experimental Section
All general chemicals and solvents were procured from
S.D. Fine Chemicals, India. Column chromatography was
performed using silica gel and basic alumina obtained from
2 2
Sisco Research Laboratories, India. MgBr O(Et) was pur-
3
chased from Acros and used without further purifications.
Tetrabutylammonium perchlorate was purchased from Fluka
and used without further purifications.
1
H NMR spectra were recorded with Varian 400 MHz
instrument using tetramethylsilane as an internal standard.
All NMR measurements were carried out at room tempera-
ture in deuterochloroform. Absorption and steady state
fluorescence spectra were obtained with Perkin-Elmer
Lambda-35 and Lambda-55 instruments, respectively. The
!
2
X
fi
ðE - EiÞ
εðEÞ ¼ A
exp B
2
Δi
Δ
i
i
fluorescence quantum yields (Φ
f
) of Mg(II) porphyrins were
where, ε, Ε, f, Δ are molar extinction coefficient, transition
energy, oscillator strength, and half bandwidth, respectively.
UV-vis spectra were generated by considering the full-width at
half-maximum (fwhm) as 100 nm. The total integrated intensity
under the absorption profile was kept equal to the sum of all the
estimated from the emission and absorption spectra by com-
16
parative method. ES-MS spectra were recorded with a
Q-Tof Micromass spectrometer. Cyclic voltammetric (CV)
and differential pulse voltammetric (DPV) studies were car-
ried out with BAS electrochemical system utilizing the three
electrode configuration consisting of a glassy carbon (working
electrode), platinum wire (auxiliary electrode), and saturated
calomel (reference electrode) electrodes in dry dichloro-
methane using 0.1 M tetrabutylammonium perchlorate as
supporting electrolyte.
(
17) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112-122; SHELX,
Program for Crystal Structure Solution and Refinement, University of Goettingen,
Germany, 1997.
(
(
18) Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307–326.
19) Otwinowski, Z.; Borek, D.; Majewski, W.; Minor, W. Acta Crystal-
logr. 2003, A59, 228–234.
(20) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467–473.
(
16) Gupta, I.; Ravikanth, M. Inorg. Chem. Acta 2007, 360, 1731–1742.

Article
Inorganic Chemistry, Vol. 49, No. 18, 2010 8297
to remove the residual H TThP. Elution with CH Cl /acetone
(1: 1) afforded the desired MgTThP as dark blue solid in 91%
yield (94 mg). Mp >300 °C. H NMR (400 MHz, CDCl , 298
3
K): δ 9.02 (s, 8H, β-pyrrole), 7.85 (br s, 4H, R-H, meso-thienyl),
7.80 (d, 4H, β-H, meso-thienyl), 7.44-7.49 (m, 4H, β-H, meso-
oscillator strengths. All the electronic structure calculations
were carried out applying GAMESS suite of ab initio pro-
2
2
2
2
1
1
gram. Visualization of molecular structures and orbital con-
tour plots were carried out using the Molden visualization
software.
2
2
General Synthesis. The magnesium complexes of porphyrins
were prepared according to literature procedure. There are
several methods available for magnesium insertion into the
porphyrin cavity. We followed the most useful Lindsey’s
method which produces the desired complexes in very good
yield in case of MgTPP and MgTFP. But to synthesize MgTThP,
we had to follow conventional route where H TThP and MgCl
thienyl) ppm. ES-MS: C36
obsd m/z 661.3 [Mþ1] .
H
20
N
4
S
4
Mg calcd avg mass 660.1,
þ
MgTFP Complex. A 100 mg portion (0.174 mmol) of H TFP
was dissolved in 8 mL of CH C1 in a one-neck round-bottom
2
2
2
8
a
flask. Then 0.485 mL (3.48 mmol) of triethylamine was added
followed by 449 mg (1.74 mmol) of MgBr O(Et) . The reaction
2
3
2
mixture was stirred magnetically at room temperature for 15
min. The progress of complete metalation was judged by
2
2
7
were refluxed in DMF.
II
Synthesis of Mg Derivatives of TPP, TThP and TFP.
MgTPP Complex. MgTPP was synthesized by following Lind-
absorption spectroscopy. Then about 25 mL of CH
added into the reaction mixture, washed with 5% NaHCO
dried (over Na SO ), and filtered, and the filtrate was concen-
trated to 4-5 mL. Chromatography on an alumina column
(packed in CH Cl containing 0.1% triethylamine) with
CH C1 elution yielded unreacted H TFP. Repeat column
chromatography always yields some free base H TFP. Desired
compound was collected as a purple-green fraction by elution
with CH Cl /acetone (1: 1). These fractions were evaporated to
2 2
C1 was
3
,
8
a
1
sey’s method reported in literature. Mp>300 °C. H NMR
2
4
(
400 MHz, CDCl , 298 K): δ 8.85 (s, 8H, β-pyrrole), 8.21 (br s,
3
8
H, o-H, meso-phenyl), 7.74 (br s, 12 H, m-, p-H, meso-phenyl)
2
2
ppm. ES-MS: C44
H
28
N
4
Mg calcd avg mass 636.2, obsd m/z
2
2
2
þ
6
37.2 [Mþ1] .
2
MgTThP Complex. A sample of 100 mg (0.157 mmol) of
H TThP was dissolved in 100 mL of DMF in a one-neck round-
2
2
2
dryness, and the crude solid compound was recrystallized with
n-hexanes affording dark green solid. Yields: 96 mg (93%). Mp
2 2
bottom flask. Then 320 mg (1.57 mmol) of MgCl 6H O was
3
added into that solution. The mixture was then refluxed over-
night. The solution became greenish pink in color. The solvent
was removed completely to dryness under reduced pressure. The
solid obtained was loaded onto an alumina column packed in
1
>
300 °C. H NMR (400 MHz, CDCl
3
, 298 K): δ 9.10 (s, 8H, β-
pyrrole), 8.05(s, 4H, R-H, meso-furyl), 7.20 (br s, 4H, β-H, meso-
furyl), 7.00 (br s, 4H, β-H, meso-furyl) ppm. ES-MS: C H -
36 20
þ
N
4
O
4
Mg calcd avg mass 596.1, obsd m/z 597.2 [Mþ1] .
2 2 2 2
CH Cl containing 0.1% triethylamine and eluted with CH C1
(
21) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.;
Acknowledgment. M.R. thanks CSIR and DST for financial
Gordon, M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.;
Su, S. J.; Windus, T. L.; Dupuis, M.; Montgomery, J. A. J. Comput. Chem.
support, and A.G. thanks IIT-Bombay for a fellowship. Com-
puter Centre, BARC is gratefully acknowledged for generous
CPU time in ANUPAM parallel computer systems. Authors
also thank Mr. Ranjan Patra for his help.
1
993, 14, 1347–1363.
22) Schaftenaar, G.; Noordik, J. H. J. Comput.-Aided Mol. Des. 2000, 14,
23–134.
(
1