Electronic and conformational features of derivatives of meso-thien-2-ylporphyrins on protonation and perbromination

Article abstract of DOI:10.1016/j.molstruc.2014.09.060

The effect of substituents at the meso-position on the electronic and stereochemical properties of thienylporphyrins has been investigated by analyzing the spectra of series of porphyrins. The role of conformation in dictating the extent of electronic pro

Full text of DOI:10.1016/j.molstruc.2014.09.060

Journal of Molecular Structure 1079 (2015) 486–493
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Electronic and conformational features of derivatives
of meso-thien-2-ylporphyrins on protonation and perbromination
⇑
Rangaraj Prasath, Purushothaman Bhavana
Birla Institute of Technology and Science (BITS), Pilani, K.K. Birla Goa Campus, Goa 403726, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
ꢀ
Electronic effects of meso-
substituents on the properties of the
porphyrins.
Effect of orientation of meso-
substituents on the electronic
properties of porphyrins.
Electronic properties of protonated
and perbrominated
thienylporphyrins.
Structural aspects of
thienylporphyrins on protonation and
perbromination.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 August 2014
Received in revised form 21 September 2014
Accepted 21 September 2014
Available online 13 October 2014
The effect of substituents at the meso-position on the electronic and stereochemical properties of thienyl-
porphyrins has been investigated by analyzing the spectra of series of porphyrins. The role of conforma-
tion in dictating the extent of electronic properties have been analysed both by electronic spectroscopy
and 1H NMR spectroscopy. Changes in the electronic properties of the thienylporphyrins by bringing the
conformational changes by protonation at the core and perbromination at the periphery of the macrocy-
cle have been related to the properties of chlorophyll.
Keywords:
Ó 2014 Elsevier B.V. All rights reserved.
Thienylporphyrins
Conformation
Protonation
Extended conjugation
Sterics
Imino hydrogen
Introduction
[4,5] has made porphyrins always an interesting class of com-
pounds for research. Electronic properties of porphyrins are highly
The existence in different biological systems together with the
applications in mimicking biocatalysts [1,2], in photodynamic
therapy [3], in solar energy conversion and in optoelectronics
tunable owing to the presence of a very electron rich central aro-
matic -system and to the highly versatile structure on which var-
ious types of structurally and electronically different moieties can
be appended. Though studied for decades, tuning the electronic
properties of porphyrins is still attempted in research in order to
p
achieve the extension of the
the periphery. The significance of this type of work lies on the fact
that molecules with extended -electron cloud be potential
candidates for nonlinear optics. Also molecules with high absor-
p-electron cloud to the groups at
⇑
Corresponding author at: Department of Chemistry, Birla Institute of Technol-
ogy and Science (BITS), Pilani, K.K. Birla Goa Campus, Zuarinagar, Goa 403 726,
India. Tel.: +91 832 2580156 (O), +91 9421249883 (mobile); fax: +91 832 2557033.
E-mail addresses: prasad24487@yahoo.co.in (R. Prasath), juliebhavana@yahoo.
co.in (P. Bhavana).
p
http://dx.doi.org/10.1016/j.molstruc.2014.09.060
0022-2860/Ó 2014 Elsevier B.V. All rights reserved.

R. Prasath, P. Bhavana / Journal of Molecular Structure 1079 (2015) 486–493
487
bance in the visible region are vital for the solar energy conversion
and as photosensitisers in photodynamic therapy. Derivatives of
tetraphenylporphyrins have widely been studied as materials for
these applications. In these compounds, there is a limitation in
extending the ring conjugation effectively through the meso-posi-
tion as the phenyl group is bulky and is nearly at an orthogonal dis-
position to the central porphyrin macrocycle (hereafter called
core). For example, the electron withdrawing nature of aldehydic
group has more effect on redox potentials when it is attached
directly to the meso-position than on the phenyl ring at meso-posi-
tion [6]. This limitation is generally overcome either by appending
the groups of interest at the pyrrole b-position [7] or by using
spacer like ethenyl at the meso-position [8–11] which will favour
the conjugation between the aryl group and the core. Another
way to achieve the same is to use small aryl groups like thienyl
as spacers at the meso-position that can come in conjugation with
both parts of the molecule [12]. Brückner et al. [13] and Ghosh
et al. [14] have reported the effect of smaller size in influencing
the electronic interaction between the core and the meso-thienyl
rings.
Compared to the four Q bands that is generally observed for free
base porphyrins, chlorophyll (which consists of peripherally mod-
ified porphyrin ring) has less number of electronic spectral bands
in the same region [15]. The longest wavelength band in the visible
region of chlorophyll is relatively intense. Mimicking these mole-
cules is always an active area in research because of their crucial
role in photosynthesis. A type of porphyrin that has a ‘modified’
(at the centre) porphyrin skeleton having similar electronic spec-
R1
2Th
NO2
2Th5N
S
S
N
NH
N
CH3
Br
2Th5Me
2Th5Br
P
R2
R4
S
S
HN
Br
PBr
H3C
R3
2Th3Me
S
Compound
H2TaNThP
-R1
-R2
-R3
-R4
2Th5N
2Th5N
2Th5N
2Th5N
H2SThP (1a)
H2TThP(1b)
H2CThP(1c)
H2TiThP(1d)
2Th
2Th
2Th
2Th
2Th
2Th5N
2Th5N
2Th
2Th5N
2Th
2Th5N
2Th
2Th5N
2Th5N
2Th5N
2Th5N
2Th
2Th
H2TaThP(1e)
2Th
2Th
H2S5BrThP(2a) 2Th5Br
H2T5BrThP(2b) 2Th5Br
H2C5BrThP (2c) 2Th5Br
H2Ti5BrThP (2d) 2Th5Br
H2Ta5BrThP(2e) 2Th5Br
2Th5N
2Th5N
2Th5Br
2Th5Br
2Th5Br
2Th5N
2Th5Br
2Th5N
2Th5Br
2Th5Br
2Th5N
2Th5N
2Th5N
2Th5N
2Th5Br
H2S5MeThP(3a) 2Th5Me
H2T5MeThP(3b) 2Th5Me
H2C5MeThP(3c) 2Th5Me
H2Ti5MeThP (3d) 2Th5Me
H2Ta5MeThP(3e) 2Th5Me
2Th5N
2Th5N
2Th5Me
2Th5N
2Th5Me
2Th5Me
2Th5N
2Th5N
2Th5N
2Th5N
2Th5Me
2Th5N
2Th5Me
2Th5Me
2Th5Me
H2S3MeThP (4a) 2Th3Me
H2T3MeThP(4b) 2Th3Me
H2C3MeThP(4c) 2Th3Me
H2Ti3MeThP (4d) 2Th3Me
H2Ta3MeThP(4e) 2Th3Me
2Th5N
2Th5N
2Th3Me
2Th3Me
2Th3Me
2Th5N
2Th3Me
2Th5N
2Th3Me
2Th3Me
2Th5N
2Th5N
2Th5N
2Th5N
2Th3Me
H2SBrPhP (5a)
H2TBrPhP(5b)
H2CBrPhP(5c)
H2TiBrPhP(5d)
PBr
PBr
PBr
PBr
2Th5N
2Th5N
PBr
PBr
PBr
2Th5N
PBr
2Th5N
PBr
PBr
2Th5N
2Th5N
2Th5N
2Th5N
PBr
H2TaBrPhP(5e) PBr
H2SPhP(6a)
H2TPhP(6b)
H2CPhP(6c)
H2TiPhP(6d)
H2TaPhP (6e)
P
P
P
P
P
2Th5N
2Th5N
P
P
P
2Th5N
P
2Th5N
P
P
2Th5N
2Th5N
2Th5N
2Th5N
P
Fig. 1. Chemical structures of nitro group containing free base porphyrins.

488
R. Prasath, P. Bhavana / Journal of Molecular Structure 1079 (2015) 486–493
tral behaviour as that of chlorophyll is protonated porphyrins. The
incorporation of two more protons at the centre of the ring using
protic acids makes the central macrocycle generally nonplanar in
nature [16–18]. The radical changes in their electronic properties
(without reducing one of the double bond at the periphery as in
chlorin ring of chlorophyll) are attributed mainly to the conforma-
tional change of the molecule induced by the repulsion between
the four protons at the centre.
In our previous reports, we have explained the synthesis of a
few porphyrins that can be used for further functionalisation by
taking advantage of the synthetic versatility of the groups at the
periphery of the porphyrins. A few among those were nitro substi-
tuted tetrathien-2-ylporphyrins derivatives [19]. The synthesis of
these compounds was simple by nitrating the different thien-2-
ylporphyrins using the mild and easy-to-handle nitrating agent,
cupric nitrate trihydrate. It was previously observed that the site
of nitration using cupric nitrate mainly depend on the metal ion
at the centre of the phenylporphyrin and in the case of Cu(II) or
Ni(II) ions at the centre, the nitro group gets directed towards
the pyrrole b-position of tetraphenylporphyrin [20,21]. Further
study on thienylporphyrins showed that the conjugation between
aryl rings at the meso-position and the core plays a major role in
the site of nitration [22,23]. This was evident by the formation of
meso-5-nitrothien-2-ylporphyrin and meso-3-nitro-5-methylth-
ien-2-ylporphyrin on nitrating the respective parent compound
with Cu(NO3)2.
mixture of TFA and CDCl3 or in a mixture of deuteriated dimethyl-
sulfoxide (DMSO) and CDCl3 in the ratio 1:15 (v/v).
Synthesis of nitro group containing free base porphyrins
Free base form of all the nitro group containing porphyrins were
synthesized by following Lindsey’s approach [26] with minor
modification. In a typical procedure, to a mixture of 3-methylthi-
ophene-2-aldehyde (0.82 mL, 7.6 mmol) and 5-nitrothiophene-2-
aldehyde (1.2 g, 7.6 mmol) in 600 mL of dichloromethane, pyrrole
(1 mL, 15.2 mmol) was added under N2 atmosphere. To this, BF3
etherate (0.62 mL, 5.06 mmol) was added and the reaction mixture
was stirred for one hour at room temperature. At the end of the
period, p-chloranil (3.73 g, 15.2 mmol) was added and the stirring
was continued for another half an hour. Following this, triethyl-
amine (0.7 mL, 5.06 mmol) was added and further stirred for one
hour. The reaction mixture was then adsorbed on 200 g of silica
gel (230–400 mesh) and the mixture of porphyrins (H2TaNThP,
4a, 4b, 4c, 4d and 4e; Fig. 1) was eluted using chloroform. The solu-
tion was then concentrated by rotary evaporation and the porphy-
rins were separated by column chromatography using chloroform
as the eluent. The yield of all products is given in supporting infor-
mation. The compounds were characterized by UV visible, 1H NMR
and mass spectral techniques. UV visible spectral details are given
in Table 1. 1H NMR and mass spectral details of all the synthesized
compounds are given in supporting information.
To understand the electronic aspects of the heteroarylporphy-
rins better, porphyrins having two different meso-groups (nitroaryl
and aryl) have been synthesized and investigated spectroscopi-
cally. One of the meso-substituent in all the series of porpyhyrins
in this study is 5-nitrothien-2-yl ring (2Th5N, Fig. 1). Nitro group
was chosen as the substituent on the thienyl ring as it can give fur-
ther insight to the electrophilic nitration reactions of various types
of thienylporphyrins reported previously and also due to its rich
synthetic chemistry [24,25]. To assess on the role of conformation
of free base form of perbrominated thienylporphyrins in deciding
their electronic properties [19], spectral investigations have also
been carried out on conformationally similar protonated nitrothie-
nylporphyrins. The chemical structure of the nitro group contain-
ing free base porphyrins in the present study is shown in Fig. 1.
Results and discussion
Synthesis
All the porphyrins in the study are synthesized by following the
procedure by Lindsey et al. [26] using the mixture of aldehydes
(1:1 ratio). p-Chloranil was used as the oxidizing agent. Cis and
trans compounds were found to form in higher yield compared
to the other meso-‘tri + mono’ aryl and nitroaryl- and, tetra aryl/
nitroaryl-derivatives. Compounds were separated by column chro-
matography and characterized by UV visible, 1H NMR and mass
spectral techniques. All the compounds showed well defined Soret
and four Q bands of etio pattern (Table 1).
Experimental
Materials
Electronic spectroscopy
Pyrrole, p-chloranil, trifluoroacetic acid (TFA) and various thio-
phene-carboxaldehydes were procured from Sigma–Aldrich and
were used as received. Other required chemicals were procured
from sd fine chemicals, India. Details of the synthesis of perbromi-
nated thienylporphyrins is reported in literature [19].
UV visible spectroscopy is widely used to study the electronic
properties of the porphyrins as they show a very attractive and
unique absorption spectrum, which consists of Soret (B) band
and visible bands (generally 4 in free base and 2 in metallo deriv-
atives). The visible bands are also called Q bands. Gouterman’s four
orbital approach explain the presence of B and Q bands, their rela-
tive intensity, multiplicity and also the spectral change on altering
the porphyrin skeleton [27]. The shift in the spectral bands (both in
position as well as in intensity) on substitution as well as on con-
formational changes can be explained by four orbital model. In this
model, various spectral aspects were explained based on the one
electron transition between the accidentally degenerate HOMOs
(initially) and LUMOs.
In the present work, the extent of shift in the electronic spectral
bands on systematically varying the groups at the meso-position of
the porphyrin has been investigated. Investigation was done on
both the electronic effect (inductive and resonance) of substituents
and the effect due to the conformational changes on the spectral
properties of various free base porphyrins (different series) and
of their protonated forms. Analysis of spectral changes in porphy-
rin series in their neutral form is made initially followed by a
Measurements
1H NMR spectra were recorded on a Bruker AVANCE III 500 MHz
spectrometer using tetramethylsilane as the internal standard.
Optical absorption spectra were recorded on a JASCO V-570 model
UV visible/NIR spectrophotometer using quartz cells of 1 cm path
length. Electrospray mass spectra were recorded on a THERMO
Finnigan LCQ Advantage max ion trap mass spectrometer.
Protonated porphyrins were prepared by treating the free base
porphyrin with TFA in the ratio 1:2. Further increase in the concen-
tration of the acid in the mixture did not bring any further change
in the electronic spectrum of the protonated form. For recording
the NMR spectra, samples were prepared by dissolving porphyrin
in CDCl3. For the study of the difference in the conformation of free
base and protonated porphyrins, spectra were also recorded in

R. Prasath, P. Bhavana / Journal of Molecular Structure 1079 (2015) 486–493
489
Table 1
UV visible absorption spectral dataa (in nm) of various porphyrins in CH2Cl2.
Series
Porphyrin
Soret band
Q bands
fwhm
H2TaNThP
H2SThP
H2TThP
H2CThP
H2TiThP
H2TaThP
432 (5.50)
431 (5.42)
428 (5.43)
429 (5.43)
426 (5.51)
426 (5.59)
522 (4.45), 560 (4.12), 596 (4.05), 656 (3.46)
523 (4.34), 562 (4.04), 596 (3.95), 658 (3.39)
523 (4.30), 563 (4.05), 596 (3.90), 658 (3.55)
523 (4.35), 562 (4.08), 596 (3.99), 658 (3.54)
522 (4.30), 562 (4.02), 596 (3.90), 658 (3.48)
523 (4.30), 560 (4.02), 597 (3.88), 661 (3.86)
30.73
29.45
26.11
26.77
21.46
16.56
1a
1b
1c
1d
1e
2a
2b
2c
2d
2e
H2S5BrThP
H2T5BrThP
H2C5BrThP
H2Ti5BrThP
H2Ta5BrThP
432 (5.38)
431 (5.50)
431 (5.28)
430 (5.47)
429 (5.55)
523 (4.34), 561 (4.06), 597 (3.99), 657 (3.52)
523 (4.37), 563 (4.08), 596 (3.97), 658 (3.46)
524 (4.15), 563 (3.86), 597 (3.77), 658 (3.20)
524 (4.27), 564 (3.97), 597 (3.85), 658 (3.31)
524 (4.23), 562 (3.91), 597 (3.76), 658 (3.22)
29.30
25.75
26.32
22.05
17.80
3a
3b
3c
3d
3e
H2S5MeThP
H2T5MeThP
H2C5MeThP
H2Ti5MeThP
H2Ta5MeThP
432 (5.52)
433 (5.58)
434 (5.61)
432 (5.40)
431 (5.50)
522 (4.44), 560 (4.05), 596 (4.00), 658 (2.96)
524 (4.41), 566 (4.15), 597 (3.97), 661 (3.38)
525 (4.49), 565 (4.22), 596 (4.12), 659 (3.38)
524 (4.15), 567 (3.90), 596 (3.68), 661 (3.60)
526 (4.14), 567 (3.98), 597 (3.77), 662 (3.49)
29.20
28.40
28.35
21.68
18.49
4a
4b
4c
4d
4e
H2S3MeThP
H2T3MeThP
H2C3MeThP
H2Ti3MeThP
H2Ta3MeThP
431 (5.52)
429 (5.66)
429 (5.50)
427 (5.55)
425 (5.42)
523 (4.45), 561 (4.08), 596 (4.01), 656 (3.16)
522 (4.55), 562 (4.22), 596 (4.10), 656 (3.54)
523 (4.39), 561 (4.03), 596 (3.96), 656 (3.12)
522 (4.38), 560 (3.98), 596 (3.92), 655 (3.22)
520 (4.20), 556 (3.80), 596 (3.76), 661 (3.69)
29.45
26.43
26.86
21.17
15.87
5a
5b
5c
5d
5e
H2SBrPhP
H2TBrPhP
H2CBrPhP
H2TiBrPhP
H2TaBrPhP
428 (5.36)
423 (5.42)
424 (5.49)
420 (5.55)
419 (5.52)
521 (4.31), 559 (4.03), 594 (3.92), 652 (3.53)
518 (4.25), 557 (4.08), 593 (3.84), 651 (3.75)
519 (4.40), 557 (4.07), 592 (3.99), 650 (3.52)
517 (4.33), 554 (4.03), 591 (3.84), 648 (3.60)
515 (4.19), 551 (3.81), 590 (3.66), 646 (3.44)
29.10
24.37
25.03
18.20
12.78
6a
6b
6c
6d
6e
H2SPhP
H2TPhP
H2CPhP
H2TiPhP
H2TaPhP
428 (5.62)
422 (5.65)
423 (5.64)
419 (5.66)
417 (5.58)
521 (4.56), 560 (4.28), 594 (4.17), 652 (3.76)
518 (4.50), 558 (4.38), 593 (4.11), 651 (4.10)
520 (4.55), 558 (4.25), 592 (4.16), 650 (3.76)
517 (4.45), 554 (4.18), 591 (4.02), 647 (3.85)
515 (4.19), 550 (3.83), 591 (3.68), 647 (3.61)
30.84
25.75
26.98
18.28
14.22
a
The values in parenthesis refer to loge values, e .
in dm3 molꢁ1 cmꢁ1
discussion on the conformational aspects due to the protonation at
the porphyrin core.
the number of 2Th5Br groups at the meso-position in series 2. But
compared to series 1, the extent of hypsochromic shift is less (H2-
TaNThP ? 2e, 3 nm). This less magnitude in the shift in the Soret
band compared to that in series 1 can be due to the combined ꢁI
(inductive) and +M (mesomeric) effect of the bromo groups on
the thien-2-yl ring(s). It is known that the groups with electron
withdrawing effect (ꢁI) shifts the absorption spectral bands
hypsochromically.
By comparing series 2 and 5, the effect of substituents on the
aryl groups with different size on the electronic properties of the
porphyrins is analysed. When the number of bromoaryl group
decreases, the effect is higher in series 5 than that in series 2.
The conjugation is not much affected in series 2 as all the groups
at the meso-position are derivatives of thien-2-yl groups. Also
the +M effect of bromo substituent(s) is considerably felt in this
series.
Effect of the size of the meso-aryl group is also clear on compar-
ing the series 1 and 6. On going down the series, the number of
unsubstituted aryl groups (2Th and P, respectively) increases
(keeping 2Th5N as the other meso-substituent(s)) and the effect
on the electronic properties is prominent in series 6. The difference
in absorption maximum of Soret band of H2TaNThP and tetra-(6e)
phenyl compounds is 15 nm whereas the same between H2TaNThP
and tetra-(1e) thien-2-yl compounds is only 6 nm. The reason for
the lesser effect in series 1 than in series 6 can be similar to that
between series 2 and 5.
Effect due to the orientation of the groups at the meso-position
of the porphyrin on the electronic properties is investigated by
comparing two ‘isomeric’ series 3 and 4. The difference between
the analogous members of these series is in the position of the
methyl group(s). There is an interesting trend in the shift in Soret
band position on decreasing the number of 2Th5N groups on the
Comparison of UV visible spectral properties of free base porphyrins
In the following discussion, members of each series are abbrevi-
ated after the group other than 2Th5N group at the meso-position.
One of the substituents at the meso-position(s) in all series is
2Th5N group. Electronic spectral measurements were carried out
in dichloromethane and the details are summarized in Table 1.
In series 6, meso-positions are occupied by P and 2Th5N groups.
As the number of P groups increases in the series, it is observed
that the absorption maximum of B band decreases and for H2TaPhP
(commonly known as H2TPP in literature) the value is 417 nm. As
the number of P groups increases, the conjugation of
p-electrons
between the core and the P groups decreases and so a blue-shift
is observed. The requirement of coplanarity of the core with the
groups present at the meso-position in bringing the red-shift of
electronic spectral bands is further clear on analysing the absorp-
tion spectral maxima of molecules in series 5. In this series, PBr
group at the meso-position cannot come effectively in conjugation
with the core like the phenyl group in series 6. But the other group,
i.e., 2Th5N group can easily come in conjugation with the core. So
as expected and as in series 6, there is a blue-shift in the B band on
increasing the number of PBr groups. Considerable difference in
the spectral values of series 5 and 6 is not observed.
The Soret band positions of the members of series 1 show a
blue-shift on increasing the 2Th groups. As the number of 2Th5N
group increases, there is an enhanced conjugation due to the
increase in the number of electron withdrawing (mesomeric, -M)
nitro groups. The full width at half maximum (fwhm) also
increases with increase in the number of nitro groups on the mol-
ecule. There is a blue-shift in the Soret band position on increasing

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R. Prasath, P. Bhavana / Journal of Molecular Structure 1079 (2015) 486–493
molecules. In contrast to the series 1 and 2, as the number of
2Th5N groups on the molecule decreases, first Soret band got
shifted to red region and then blue shifted in series 3. So, in effect,
there is only one nanometer blue shift on moving from H2TaNThP
to H2Ta5MeThP (3e) in the series (Fig. 2). As the nitro group
decreases, the Soret band get blue shifted due to the lesser electron
delocalization. This result shows that resonance effect of nitro has
a prominent role over the inductive effect and mesomeric (through
no-bond resonance) effect of the methyl group in dictating the shift
of Soret band. Though methyl group(s) is present, the conformation
has the major role in deciding the energy of the molecular orbitals
of the members in series 4. The effect of electron donating induc-
tive nature of methyl group(s) on the energy of molecular orbitals
of porphyrin is expected to be higher in series 4 than in series 3
based on the lesser number of intervening bonds of methyl group
with the core. The position of the methyl group in series 4 makes
the corresponding thien-2-yl group of the molecule perpendicu-
lar/nearly perpendicular to the
p-system at the centre. There is
no drastic influence of 2Th3Me groups in dictating the electronic
properties of the compounds. The observed blue shift on moving
down the series may be due to the decrease in the number of
2Th5N groups. The initial increase in the Soret absorption in com-
pounds 3a to 3b (or 3c) shows the considerable effect of 2Th5Me
group on the properties of porphyrin. This effect is absent in series
4 only due to the completely off plane orientation of the 2Th3Me
ring with the core which in turn shows the importance of confor-
mation in influencing the electronic properties of porphyrins. The
absence of resonance effect from methyl group due to the orienta-
tion of 2Th3Me ring is clear on comparing series 1 and 4. Decrease
in absorption wavelength of Soret band due to decreasing number
of groups other than 2Th5N groups is more series 4 than in 1
(though the difference is very less). Though an electron donating
methyl group (no-bond resonance) is present, its effect on the ori-
entation of the 2Th3Me group brings out the difference in the
absorption maxima between the members H2TaNThP and 4e more
than that between the analogous members in series 1. This trend
confirmed the importance of resonance in the electronic spectrum
of porphyrins. The UV visible spectra of H2Ta3MeThP were found
to be same when recorded at 22 and 50 °C. This indicates that even
at 50 °C, the 2Th3Me group is unable to overcome the rotational
barrier (Fig. 3).
Fig. 3. Optical absorption spectra of H2Ta3MeThP at 22 °C and 50 °C recorded in
CHCl3.
of centre porphine, it is found that Q bands also undergo a batho-
chromic shift [16]. The red-shift of bands on protonation is
ascribed to the nonplanarity of the core which in turn allows the
aryl groups at the meso-position to come in plane with the core.
The nonplanarity of the core affect the energy level of a1u orbital
as well as the eg orbitals and this results in the red-shift of the
Soret band. Shift in Q bands (which arises due to the a2u to eg tran-
sition) is accounted by the electronic effect of the substituents at
the meso-position. In order to shed further light on the effect of
extended conjugation due to the coplanarity/near planarity of the
meso-substituents on the spectral properties (conformational
aspects), members of series 1 and 6 were protonated using TFA
and their electronic spectral shifts were monitored (Fig. 4, Table 2).
In series 6, one of the groups at the meso-position, P, is highly off-
plane with the core due to steric reason.
On increasing the amount of TFA in the solution of free base
porphyrin, there was a reduction in the intensity of its Soret band.
Intensity of the Soret band of protonated form increased gradually
Comparison of UV visible spectral properties of protonated porphyrins
In general, there is a high red-shift in the electronic spectral
bands of porphyrins on protonation of the core. In the case of para
substituted tetraphenylporphyrin, red-shifted Soret band and
blue-shifted Q bands were observed on protonation, but on cor-
recting the data by considering the structural and electronic factors
R1
2Th
P
NH HN
S
R2
R4
HN
NH
NO2
2Th5N
S
R3
433
series 3
Compound
H4TaNThP
-R1
-R2
-R3
-R4
2Th5N
431
429
2Th5N
2Th5N
2Th5N
H4SThP (7a)
H4TThP(7b)
H4CThP(7c)
H4TiThP(7d)
2Th
2Th
2Th
2Th
2Th
2Th5N
2Th5N
2Th
2Th
2Th
2Th5N
2Th
2Th5N
2Th
2Th5N
2Th5N
2Th5N
2Th5N
2Th
series 4
427
H4TaThP(7e)
2Th
H4SPhP(8a)
H4TPhP(8b)
H4CPhP(8c)
H4TiPhP(8d)
H4TaPhP (8e)
P
P
P
P
P
2Th5N
2Th5N
P
P
P
2Th5N
2Th5N
2Th5N
2Th5N
2Th5N
P
425
P
0
1
2
3
4
2Th5N
P
P
Number of 3- or 5- methylthien-2-yl groups
at meso posiꢀon
Fig. 2. Plot of the position of Soret band maximum with the number of 3- or 5-
methylthien-2-yl rings in series 3 and series 4.
Fig. 4. Chemical structures of protonated form of substituted meso-thien-2-
ylporphyrins.

R. Prasath, P. Bhavana / Journal of Molecular Structure 1079 (2015) 486–493
491
Table 2
Series Groups at meso-posi ons
Shi in nm
UV visible absorption spectral dataa (in nm) of various protonated porphyrins in
CH2Cl2.
1 and 7
2Th
2Th5N
Q
S
Porphyrin
Soret band
Q band
fwhm
H4TaNThP
H4SThP
H4TThP
H4CThP
H4TiThP
H4TaThP
474 (5.72)
473 (5.59)
470 (5.66)
470 (5.64)
465 (5.62)
459 (5.55)
704 (5.01)
712 (4.90)
719 (4.98)
718 (4.96)
719 (4.94)
720 (4.83)
31.56
30.30
29.40
29.39
25.84
20.77
7a
7b
7c
7d
7e
48 → 59
42 → 33
P
2Th5N
Q
S
6 and 8
8a
8b
8c
8d
8e
H4SPhP
H4TPhP
H4CPhP
H4TiPhP
H4TaPhP
466 (5.71)
457 (5.73)
458 (5.69)
447 (5.70)
437 (5.69)
692 (4.99)
688 (4.92)
679 (4.96)
667 (4.95)
654 (4.79)
32.05
27.54
32.28
26.93
20.66
48 → 7
42 → 20
Scheme 1. Difference in the position of Soret and of Qx(0,0) bands for the extreme
members in the respective free base and protonated series (": increasing; ;:
decreasing).
a
The values in parenthesis refer to log
e
values,
e
in dm3 molꢁ1 cmꢁ1
.
Table 3
UV visible absorption spectral shift (in nm) of protonated porphyrins from the
respective free base.
(Fig. 5). Increasing the concentration of TFA beyond 2-fold did not
bring any noticeable change in the electronic spectrum of the
compounds.
Porphyrin
Soret band
Lowest energy band
The difference in the positions of Soret as well as in Qx(0,0)
band of extreme members of each series on protonation were com-
pared (difference in the Soret band positions of H4TaNThP and
H2TaNThP is compared with that of 1e and 7e, for example) in
order to quantify the extent of nonplanarity induced in different
molecules. On protonation, Soret and Q bands of all compounds
has shifted to the red region. As the number of 2Th5N group
decreases, in series 1 and its corresponding protonated analogue,
series 7, the difference in the positions of Soret band decreases
[42 nm (H4TaNThP–H2TaNThP) ? 33 nm (7e–1e)] where as the
difference in the positions of lowest energy electronic band
increases [48 nm (H4TaNThP–H2TaNThP) ? 59 nm (7e–1e)]
(Scheme 1, Table 3). But for series 6 and its protonated series 8,
the difference in the positions of both Soret band [42 nm (H4TaNT-
hP–H2TaNThP) ? 20 nm (8e–6e)] and lowest energy electronic
band [48 nm (H4TaNThP–H2TaNThP) ? 7 nm (8e–6e)] decreases.
It is known that on protonation, the core become highly nonpla-
nar and this affects the energy of the a1u orbital that has electronic
H4TaNThP
H4SThP
H4TThP
H4CThP
H4TiThP
H4TaThP
42
42
42
41
39
33
48
54
61
60
61
59
7a
7b
7c
7d
7e
8a
8b
8c
8d
8e
H4SPhP
H4TPhP
H4CPhP
H4TiPhP
H4TaPhP
38
35
35
28
20
40
37
29
20
7
which in turn is more than that of H2TaPhP (20 nm). So the trend
in extent of induced nonplanarity on protonation, based solely on
the shift in Soret band, looks like H2TaNThP > H2TaThP > H2TaPhP.
A comparison of lowest energy electronic band on protonation
also has been done owing to the possibility in explaining the inter-
action between TFA/trifluoroacetate and imino hydrogens (at the
centre of the molecule), and also to shed light on the electronic
influences felt at the meso-carbons due to the substituents result-
ing from the induced nonplanarity in the molecule. The shift of
lowest energy band on protonation is in the order H2TaThP > H2-
TaNThP > H2TaPhP. In nonplanar protonated porphyrins, since the
hydrogen on all the four pyrrole nitrogens can bind to the TFA/tri-
fluoroacetate, the a2u orbital which has electronic contribution
from pyrrole nitrogen will get stabilized and so a blue shift is gen-
erally expected for lowest energy electronic band. Observed red
shift shows, therefore, that the effect due to the nonplanarity of
the core and in turn the coplanarity/near planarity of meso-aryl
ring with the mean porphyrin plane together decide the position
of the lowest energy electronic band of protonated porphyrins.
Even in the protonated state, the phenyl rings of H2TaPhP are not
in conjugation to the extent that (nitro)thienyl rings of H2TaNThP
and H2TaThP are with their respective core. Also since the induced
nonplanarity is less, the interaction of the imino hydrogens with
the TFA/trifluoroacetate is less in H2TaPhP. Both these result in a
relatively very less difference (only 7 nm) of lowest energy elec-
tronic band.
contribution from
a and b carbons of the pyrrole. The transition
responsible for the Soret band is a1u to eg orbitals. Comparing
the trends in the shift of Soret band on protonation given above,
it is understood that the effect of induced nonplanarity by proton-
ation is higher in H2TaNThP (42 nm) than in H2TaThP (33 nm)
A difference of 5 nm between the Qx(0,0) band of H2TaNThP and
H2TaThP in free base form itself was observed due to both ꢁI and
ꢁM effect of nitro group (Table 1). Comparing with trend of shift in
Soret band, difference in the shift of lowest energy band on proton-
ation indicates that as more saddled the structure in H4TaNThP
(saddle conformation is observed in most of the protonated por-
phyrin), more electron density is donated to TFA/trifluoroacetate.
Even when nitro groups are present to extent the conjugation
Fig. 5. Absorption changes of H2SThP with increasing concentration of TFA.

492
R. Prasath, P. Bhavana / Journal of Molecular Structure 1079 (2015) 486–493
indicated by electronic spectroscopy and supported by shift in
L
imino hydrogen signals of both the protonated molecules in the
1H NMR spectra), the conformational changes of the molecules
seems to be considerably different. Relatively enhanced conforma-
tional change (induced nonplanarity of the core) makes the coordi-
nation of all the Hs (of NH) of H4TaNThP with the TFA/
trifluroacetate moiety more effective compared to that in H4TaThP.
This resulted in the considerable downfield shift of NH signals of
H4TaNThP (signals at d 0.44 ppm and 0.69 ppm for 2H each due
to the two different interacting species TFA and trifluoroacetate).
The difference of the position of these two signals from the signal
in DMSO containing sample is 3.20 and 3.45 ppm, respectively and
that only in CDCl3 is 3.26 and 3.51 ppm, respectively. In the
1H NMR spectrum of H4TaThP, a broad signal appeared for NH near
to the signal of the reference, tetramethylsilane. Here the exact
integration of the signal was not possible, and the difference of
the position of this signal from that in DMSO containing sample
is approximately 2.71 ppm. These differences in the values clearly
indicate that the nonplanarity induced due to the protonation of
the core is higher in H2TaNThP.
H
H
N
N
N
N
2L
H+
HN
N
NH
H
N
N
H
L
Scheme 2. Representation of imino hydrogen interacting with two TFA/trifluoro-
acetate in nonplanar protonated forms (L = TFA or trifluoroacetate).
effectively in the molecule, in effect, only 48 nm shift is observed
after overcoming the effect due to the stabilization of a2u orbital
(Scheme 2). However, the 59 nm shift observed in the case of H2-
TaThP can be attributed to the in-plane orientation of the meso-
2Th ring. The stabilization of a2u in this molecule due to interac-
tion with TFA/trifluoroacetate will be less as this molecule is less
nonplanar than H2TaNThP.
The 1H NMR spectral difference of H2TaNThP and H2Ta3MeThP
in presence of TFA gives clear indication on the difference in the
planarity of these porphyrins on protonation. The difference in
the chemical shift of imino proton signal of H2Ta3MeThP is
2.13 ppm (d at ꢁ2.62 ppm in CDCl3, ꢁ0.49 ppm in TFA containing
CDCl3 solution of the sample) on protonation. This lower shift of
H2Ta3MeThP on protonation compared that of H2TaNThP shows
that though getting protonated, the extent of deformation is less
in H2Ta3MeThP compared to that in H2TaNThP. The presence of
methyl group at the third position of the thien-2-yl rings hinders
the core from being highly nonplanar. This can be attributed to
the repulsion between methyl group and b-H of the pyrrole ring
[30–33]. The imino protons of less deformed core has relatively
upfielded signal (compared to that of H4TaNThP).
Modifying the basic skeleton by reduction (as in chlorin) and
protonating the core are the two ways to alter the electronic prop-
erties of porphyrins considerably. Highly altered electronic proper-
ties can also be obtained for thienylporphyrins by perbromination
at the periphery (pyrrole b-position) [19]. By these methods, the
core is neither getting reduced at the double bond (Cb@Cb) nor
getting protonated at the core. Porphyrins (free base) with
Comparison of 1H NMR spectral properties of porphyrins
1H NMR spectral data was also analysed in order to delineate
the effect of various thien-2-yl groups at the meso-position in
extending the conjugation in porphyrin. Cases where there is no
interference by the group on the thieny-2-yl ring (2Th, 2Th5N,
2Th5Me and 2Th5Br) in dictating the orientation of the group at
the meso-position with respect to core, signal corresponding to
pyrrole b-proton is singlet and appeared at 9.07 ppm or downfield
to that. In H2Ta3MeThP, methyl groups make the pyrrole b-proton
signal multiplet in nature due to the possible atrop isomers (unable
to separate by column chromatography). Also, related to the above
mentioned molecules, the signal of the pyrrole b-proton is rela-
tively upfielded (8.91–9.09 ppm). This is due to the lesser elec-
tronic conjugation between the meso-groups with the core and in
turn the relatively less ring current (anisotropy). The almost simi-
lar chemical shift value for pyrrole b-protons (8.89–8.93 ppm) in
the 1H NMR spectrum recorded at 50 °C showed that the 2Th3Me
groups in H2Ta3MeThP could not overcome the rotational barrier.
This is further reflected in the position of signal for imino hydro-
gens which appeared at ꢁ2.62 ppm and ꢁ2.60 ppm in spectra
recorded at 22 °C and 50 °C respectively.
It is known that in cases where there is enhanced ring current as
a result of extended conjugation, generally the imino hydrogen sig-
nal get upfielded and pyrrole b-protons get downfielded [28,29].
The enhanced nonplanarity of H2TaNThP compared to H2TaThP
on protonation is also investigated by 1H NMR spectroscopy. Spec-
tra were recorded in CDCl3 in presence of DMSO (deuteriaed) and
TFA separately. DMSO is a highly coordinating solvent and can
coordinate with the imino hydrogens of the core. The chemical
shift of imino hydrogen signal in sample containing DMSO with
respect to that in CDCl3 alone is almost same for both H2TaThP
(ꢁ2.64 ppm in CDCl3, ꢁ2.65 ppm in mixture of CDCl3 and DMSO)
and H2TaNThP (ꢁ2.82 ppm in CDCl3, ꢁ2.76 ppm in mixture of
CDCl3 and DMSO). The nitro group on the H2TaNThP is not influ-
encing the conformation of the core of the molecule and so the
imino hydrogens behave almost similar to that in H2TaThP. Coordi-
nation of imino hydrogens in the planar form with oxygen of the
DMSO is not to a great extent, but is similar in both the molecules.
But the chemical shift values of imino hydrogens are very different
for the samples which contain TFA. On protonation using TFA (as
Fig. 6. Optical absorption spectra of 5,10,15,20-tetrakis(5-bromothien-2-yl)por-
phyrin (solid line) and 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(5-bro-
mothien-2-yl)porphyrin (dashed line) recorded in CH2Cl2.

R. Prasath, P. Bhavana / Journal of Molecular Structure 1079 (2015) 486–493
493
Table 4
UV visible absorption spectral dataa (in nm) of perbrominated tetrahalothien-2-ylporphyrins in CH2Cl2.
Porphyrin
Soret bands (nm)
Q bands (nm)
H25ClThOBPb
H25BrThOBPc
H2OBPd
366 (4.61), 484 (5.46)
365 (4.41), 484 (5.22)
369 (4.37), 469 (5.23)
417 (5.58)
654 (4.42), 760 (4.14)
654 (4.19), 761 (3.93)
571 (3.88), 627 (4.07), 746 (3.85)
515 (4.19), 550 (3.83), 591 (3.68), 647 (3.61)
H2TaPhPe
a
b
c
The values in parenthesis refer to loge values, e .
in dm3 molꢁ1 cmꢁ1
2,3,7,8,12,13,17,18-Octabromo-5,10,15,20-tetrakis(5-chlorothien-2-yl)porphyrin.
2,3,7,8,12,13,17,18-Octabromo-5,10,15,20-tetrakis(5-bromothien-2-yl)porphyrin.
2,3,7,8,12,13,17,18-Octabromo-5,10,15,20-tetraphenylporphyrin.
d
e
5,10,15,20-Tetraphenylporphyrin (commonly known as H2TPP in literature).
considerably modified electronic spectrum (like in protonated
cases) by bromination can be obtained if the groups at the meso-
position are smaller group like thienyl (compared to phenyl)
(Fig. 6). The reduced number of Q bands of these perbrominated
porphyrin shows that it behaves more like chlorins (Table 4). The
intensity gain of higher wavelength band on perbromination sup-
port this fact and indicates that it will be one of the methods to
achieve porphyrins that can be used for further functionalisation
to get molecules with high absorption in a higher wavelength
(without reducing the double bond or protonating the core).
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Supplementary data associated with this article can be found, in
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