UV–visible and fluorescence spectroscopic assessment of meso?tetrakis?(4?halophenyl) porphyrin; H2TXPP (X = F, Cl, Br, I) in THF and THF-water system: Effect of pH and aggregation behaviour

Article abstract of DOI:10.1016/j.saa.2020.118570

The current study determines optical and fluorescence response of halogen substituted series of meso?tetrakis?(4?halophenyl) porphyrin; H2TXPP (Halo = F, Cl, Br, I) dye in tetrahydrofuran; THF and THF-water system at changing pH in relationship with changing medium of allure. Effects produced by varying the pH and medium, over spectral and aggregation were discussed in detail. Results show sequential protonation and deprotonation of H2TXPP series in acidic (pH = 4) and (pH =10) basic medium. Specific structural changes of monomeric absorption band were put in evidence on lowering pH, which includes broadening and splitting of soret or B band. Other changes include increasing in intensity and red-shifting of Q1 band indicating some degree of aggregation. The side-by-side aggregation and formation of J-aggregate were quite evident. The red shift of B band featured self-aggregation through head-to-tail molecular ordering which is consonant with absorption-emission data.

Full text of DOI:10.1016/j.saa.2020.118570

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UV–visible and fluorescence spectroscopic assessment of
meso‑tetrakis‑(4‑halophenyl) porphyrin; H2TXPP (X=F, Cl, Br,
I) in THF and THF-water system: Effect of pH and aggregation
behaviour
Umar Ali Dar, Shakeel A. Shah
PII:
S1386-1425(20)30549-7
https://doi.org/10.1016/j.saa.2020.118570
SAA 118570
DOI:
Reference:
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
To appear in:
Received date: 28 April 2020
Revised date:
28 May 2020
1 May 2020
Accepted
date:
3
Please cite this article as: U.A. Dar and S.A. Shah, UV–visible and fluorescence
spectroscopic assessment of meso‑tetrakis‑(4‑halophenyl) porphyrin; H2TXPP (X=F, Cl,
Br, I) in THF and THF-water system: Effect of pH and aggregation behaviour,
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2020),
https://doi.org/10.1016/j.saa.2020.118570
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UV-Visible and Fluorescence Spectroscopic Assessment of Meso-Tetrakis-(4-
halophenyl) Porphyrin; H TXPP (X=F, Cl, Br, I) in THF and THF-Water System:
2
Effect of pH and Aggregation behaviour
*, 1
**, 1
Umar Ali Dar
Department of Chemistry, National Institute of Technology Srinagar, Hazratbal, Srinagar-
90006, J&K India.
Shakeel A. Shah
,
1
1
Abstract
The current study determines optical and fluorescence response of halogen substituted series
of meso-tetrakis-(4-halophenyl) porphyrin; H TXPP, (Halo= F, Cl, Br, I) dye in
2
tetrahydrofuran; THF and THF-water system at changing pH in relationship with changing
medium of allure. Effects produced by varying the pH and medium, over spectral and
aggregation were discussed in detail. Results show sequential protonation and deprotonation
of H TXPP series in acidic (pH = 4) and (pH =10) basic medium. Specific structural changes
2
of monomeric absorption band were put in evidence on lowering pH, which includes
broadening and splitting of soret or B band. Other changes include increasing in intensity and
red-shifting of Q1 band indicating some degree of aggregation. The side-by- side aggregation
and formation of J-aggregate were quite evident. The red shift of B band featured self
aggregation through head-to-tail molecular ordering which is consonant with absorption-
emission data.
Key words: H TXPP, Halogen derivatives, pH sensor, Aggregation
2
*
Corresponding author. Department of Chemistry, National Institute of Technology Srinagar,
Hazratbal, Srinagar- 190006, J&K India.
*Corresponding author.
*
E-mail address:drshakeelshah@gmail.com (S.A. Shah),
umar74202@gmail.com (U.A. Dar).

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Introduction
Porphyrin compounds are important model systems for chemosensors and photosensitizers
[
[
1], widely used in industrial application. Numerous industrial applications such as in the dye
2,3], photography industries [4], and in the new growing fields of molecular electronics [2],
photodynamic therapy; (PDT) [5] magnetic resonance imaging; (MRI) [6] and solar energy
sensing; (SES) [7] etc . Optical properties and photophysical properties are affected by the
presence of substituents at β-pyrrole or at meso-aryl positions of tetraphenylporphyrins; TPP
[
8–11]. High absorption coefficient and long life time in triplet excited state efficiently
produce singlet molecular oxygen [12]. Photophysical properties of porphyrin sensitizers
might get affected by change in their aggregation state which could bring changes in
absorption spectra [10,13]. Derivatives of substituted porphyrin compounds synthesized are
highly sensitive towards electronic absorption spectra on pH variation. The sensitivity is
because they have two ionisable protons, Pyrrolic−H on two opposite N−H groups which
under goes protonation at around (pH= 4), and deprotonation at (pH= 10). Spectral properties
have been monitored by UV-Visible and fluorescence spectroscopy under (a) acidic, (b) basic
and neutral condition in aqueous and mixture of aqueous, non-aqueous system. On addition
of analyte to the sensitive dye shows significant interference in colour and absorption
behaviour. However results displays typical monomeric spectral behaviour in pure organic
solvents with typical single soret band and four Q bands of lower intensity [14,15]. Less
attention has been received on fluorescent compound which are highly sensitive towards pH
variation and particularly for synthetic porphyrin compounds. Porphyrin ring is hydrophobic
in nature and highly fluorescent optical pH sensor and could be best alternative for sol-gel
silica and polymeric membranes [16]. Due to high immobilization and strong limitation of
leaching, this could prove higher potential of covalent immobilization of dye in solid matrix.
The urgency and need for monitoring the effect of pH of strongly acidic and basic media, is
because they facilitates and responds to various chemical and biological systems such as
human body, industrial used waters, acid rain water, medical monitoring [17,18] etc. which
involves significant pH changes. Experimental data has proven that medium in the cancerous
tissues are acidic, similarly hydrophilic molecules in lysosomes are characteristic of acidic
medium [19,20]. Synthetic porphyrin framework with different hetero-atoms at peripheral or
at β postion are sensitive sensing compounds for optochemical pH sensors in organic matrix.
Therefore it would be worth to explore and could prove effective photosensitizes model
compounds of porphyrin in order to understand and tune the chemical recognition and

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aggregation behaviour. Therefore current study aimed to develop florescent and synthesize
chemosensor based on halogen derivative of meso-tetrakis-(4-phenyl) porphyrin; TPP for pH
sensing and study their absorption, emission and aggregation behaviour in acidic, basic and
neutral medium under THF and THF+ H O system.
2
Experimental
Material and Chemicals
Analytical grade chemicals were used for the synthesis of H TXPP. AR grade pyrrole, 4-
2
floro-benzaldehyde, 4-chloro-benzaldehyde, 4-bromobenzaldehyde and 4-iodobenzaldehyde,
were purchased from sigma aldrich. However tetrahydrofuran, chloroform and propionic
acid, was purchased from Merck and was used as such.
Measurements and Apparatus
-1
-1
FT-IR spectra of these compounds were recorded between 4000 cm to 400 cm
using KBr pellets on Perkin Elmer Spectrum. The elemental analysis was performed on
1
Elementar Vario EL II. HNMR of the synthesized compounds was recorded in CDCl on
3
Bruker spectrometer, model AV 400 N (400 MHZ) with TMS (tetramethylsilane) used as
internal standard. UV-Visible spectra were recorded on Perin Elmier (Lamda 360)
spectrophotometer in chloroform; (CHCl ), THF, THF+H O system ranging from 200 to 800
3
2
nm at different pH variations. However fluorescence spectra were recorded on Shimadzu
RF-5301PC) spectrophotometer in THF and THF+ H O system. pH was maintained by hand
(
2
held digital pH-meter (Cole-parmer, P100)
Spectroscopic studies
Absorption and fluorescence spectra were recorded at room temperature using 1 cm path
length cell. To introduce acidic and basic media in analyate solution 0.1 M stock aqueous
solution of HCl and NaOH were added and stirred vigorously. The pH was adjusted by digital
pH meter. Florescence quantum chemical yield; Φf was calculated by preciously reported
equation using TPP area gradient against H TXPP [21].
2
Synthetic procedure used for preparation of H TXPP [22–25]
2
Adler method has been employed using direct condensation reaction of equimolar amounts
of freshly distilled pyrrole (20 mmol) and 4-halobenzaldehyde; halo = floro, chloro, bromo
and iodo (20 mmol) added to propionic acid (20 ml) and heated to a refluxing temperature.

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The solutions were allowed to reflux and put on constant stirring for about 30 minutes. The
°
reaction mixture was allowed to cool at room temperature (30 C). Later reaction mixtures
were filtered and the residues were washed with methanol until the filtrates was colourless.
The courses of reaction were monitored and purity of these corresponding compounds has
been achieved by column chromatography on a basic alumina. However CHCl was used as
3
eluent in all cases. Compounds were re-crystallized from CHCl on vaccum further subjected
3
to various spectroscopic studies.
H TFPP was obtained as shiny purple crystals. Yield 0.46 g (27 %), Anal. data calc.
2
for C H N F : C, 76.96; H, 3.82; N, 8.16, found: C, 77.12; H, 3.90; N, 7.96. FT-IR (KBr,
4
4
26
4 4
-1
cm ): ν (N-H) 3287, ν (C-H) 3044, ν (C-N) 1378, ν (C=C) 1577, ν (C=N) 1538, (C-H; β-
1
pyrole) 968, ν(C-F) 1199. H NMR (400 MHz, CDCl ) δ: 8.84 (s, 8H, porphyrin), 8.17 (d,
3
8
H, Ph), 7.47 (d, 8H, Ph), –2.85 (s, 2H, NHimino). UV-Vis [λmax (nm) in THF/ CHCl ]: 417
3
+
(
Soret), 514, 549, 590, 649 (Q bands). GC-MS (EI); m/z: 687.40 (M +H).
H TClPP was obtained as shiny blue crystals. Yield 0.62 g (41 %). Anal. data calc. for
2
-
C H N Cl : C, 70.23; H, 3.48; N, 7.45, found: C, 71.12; H, 3.09; N, 7.96. FT-IR (KBr, cm
44
26
4
4
1
): ν (N-H) 3317, ν (C-H) 3070, ν (C-N) 1393, ν (C=C) 1589, ν (C=N) 1556, (C-H; β-pyrole)
1
9
7
5
66, ν(C-Cl) 1089. H NMR (400 MHz, CDCl ) δ: 8.86 (s, 8H, Porphyrine), 8.16 (d, 8H, Ph),
3
.77 (d, 8H, Ph), –2.84 (s, 2H, NHimino). UV-Vis [λmax (nm) in THF/ CHCl ]: 418 (soret),
3
+
14, 550, 590, 646 (Q bands). GC-MS (EI); m/z: 753.40 (M +H).
H TBrPP was obtained as shiny blue crystals. Yield 0.62 g (41 %). Anal. data calc.
2
for C H N Br : C, 56.81; H, 2.82; N, 6.02, found: C, 56.12; H, 2.60; N, 6.16. FT-IR (KBr,
4
4
26
4
4
-1
cm ): ν (N-H) 3312, ν (C-H) 3023, ν (C-N) 1389, ν (C=C) 1586, ν (C=N) 1554, (C-H; β-
1
pyrole) 966, ν(C-Br) 1177. H NMR (400 MHz, CDCl ) δ: 8.08 (s, 8H, Porphyrin), 8.26 (d,
3
8
H, Ph), 7.792 (d, 8H, Ph), –2.84 (s, 2H, NHimino). UV-Vis [λmax (nm) in CHCl ]: 418
3
+
(
soret), 514, 550, 590, 646 (Q bands). GC-MS (EI); m/z: 931.20 (M +H).
H TIPP was obtained as shiny dark blue crystals. Yield 0.38 g (21 %). Anal. data calc.
2
for C H N I : C, 47.26; H, 2.34; N, 5.01, found: C, 47.23; H, 2.38; N, 5.06. FT-IR (KBr,
4
4
26 4 4
-1
cm ): ν (N-H) 3323, ν (C-H) 3084, ν (C-N) 1388, ν (C=C) 1559, ν (C=N) 1539, ν (C-H; β-
1
pyrole) 972, ν(C-I) 1186. H NMR (400 MHz, CDCl ) δ: 8.88 (s, 8H, Porphyrin), 8.18 (d,
3
8
5
H, Ph), 7.86 (d, 8H, Ph), –2.86 (s, 2H, NHimino). UV-Vis [λmax (nm) in CHCl ]: 418 (soret),
3
+
14, 550, 590, 646 (Q bands). GC-MS (EI); m/z: 1118.82 (M +H).

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Results and discussion
Characterization studies
Synthesized compounds and the reaction scheme employed are shown in Scheme 1. After
purification all corresponding compounds were subjected for Elemental analysis, FTIR, GC-
1
MS, HNMR. UV-Visible, fluorescence spectroscopy was employed to study the influence of
pH and aggregation behaviour on porphyrin derivative in detail.
FT-IR well versed technique to identify the characteristic functional group is presented in
Fig. 1 and various frequencies for important functional group are detailed in Table 1. Among
all characteristic functional groups, N-H stretching band can be well observed from the
-1
spectrum and exhibit N-H stretching vibration at ~3287 cm for H TFPP, it appears at ~3323
2
-1
-1
cm for H TIPP with decreases in frequency by (±5) cm for remaining derivatives. A band
2
-1
observed at ~3044 cm was attributed to aromatic  stretching frequency for H TFPP and
C-H
2
-1
-1
3
070 cm , 3020 cm for Cl and H TBrPP. However H TIPP appears sharp intense spikes at
2 2
-1
-1
-1
~
3323 cm . ν(C=C/ C=N) appears in the range of ~1589 cm to ~1468 cm . νC-H (β-
-1
-1
pyrole) was assigned ~1330 cm with increase in frequency by (±15) cm for all other
-1
-1
-1
derivatives. The C-N frequency is observed at ~1330 cm , 1344 cm and 1346 cm for F, Cl,
-1
Br and H TIPP. The important  was respectively assigned ~1199 cm with decreases in
2
C-X
1
frequency by (±15) for remaining derivatives [26,27] . HNMR of H TXPP series were
2
recorded in CDCl to identify characteristic resonating proton analogues of free base
3
porphyrin. All corresponding halo series showed similar pattern with slight or marginal
change in chemical shift by (±0.5) ppm. β-pyrole protons and phenyl protons (ortho and
meta) resonates in the range of δ = ~7.75-8.55. Most prominent (N−H) proton among all free
base porphyrin, which is highly shielded appears at δ= -2.75 as singlet [10,28,29]. UV-visible
and emission spectrum was recorded between 200 and 800 nm in pure organic and aqueous

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electrolyte solution and are discussed in detail in coming section which are in good
agreement with literature data [14,15,30].
90
60
30
N-H C-H
90
60
H TFPP
2
H TClPP
2
H TBrPP
2
(C=C/ C=N)
1
30
500
Wavenumber (Cm
H TIPP
-1
)
2
3500
3000
2500
Wavenumber (Cm )
2000
1500
1
1000
500
-
Fig. 1: FT-IR spectrum of H TXPP series.
2
-1
Table 1: Characteristic FT-IR frequencies in cm for H TXPP series.
2
R
H TFPP
2
H TClPP
2
H TBrPP
2
H TIPP
2
ν N-H
3287
3317
3312
3323
ν=C-H (phenyl) 3044
νC=C/ C=N 1577, 1538, 1480
νC-N 1330
νC-H (β-pyrole) 968
3070
1589, 1556, 1468
1344
966
3023
3084
1586, 1554, 1471 1559, 1539, 1472
1346
966
1346
972
νC-X
1199
1089
1177
1186
Absorption spectroscopy
Porphyrin dye is recognised as essential photosensitizer, highly sensitive towards electronic
processes concerning acidic and basic influence [14,31]. Optical absorption and emission
spectrum of purified compounds were performed in pure organic solvent i.e. (THF/ CHCl3)
for bare halogen derivatives represented in Fig. (2a-2b). The initial pH of halo-derivative in
pure solvents was in range of 7-6. However acidic and basic medium was established by
addition of aqueous solution of 0.1 M (HCl for Acidic and NaOH for basic) in (1:1, v/v).

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THF, water was used because they are highly miscible and form saturated aqueous
electrolytic solution. Reports published previously brought acidic and basic medium in
CHCl , which is completely immiscible with aqueous medium. For better optical properties
3
to study, the electrolytic solution must be clear and miscible to overcome inadequate and
misleading spectral data. Effect of solvent, variation in polarity and concentration was
checked, keeping in view the sensitive structural character of porphyrin, but no significant
spectral, either marginal change was observed. Therefore we restricted our observation for
pH variation with single solvent of higher solubility concerning halo derivatives. The
solubility chart of pure organic solvent in water is represented in Table 2. However the
absorption spectra of bare halogen derivatives were recorded between 200 and 800 nm in
THF and CHCl which displayed two important characteristic bands very important and
3
known among porphyrin compounds. Among these the high intensity band near UV region at
λmax ~417 nm originates from Soret or B band for H TFPP in both the solvents for Cl, Br, and
2
H TIPP it appears at slightly higher wavelength at ~421 nm. Other low intensity band which
2
is further distributed among four weaker bands originated from Q band represented as Q1-
Q4. B band are ascribed to π-π* (a → e *) electronic transition and arises from ground state
1
u
g
(
S ) to second existed state (S ), when compared with the unsubstituted TPP or TTP. The
0 2
band shifts toward lower wavelength for floro derivative as expected by (±4) nm because of
I effect exerted by fluorine, suggesting poor π conjugation. However later shifted toward
–
higher wavelength by ~(±4) nm, which is due to increased π conjugation and decrease in
HOMO-LUMO energy gap exerted by the electron releasing halogen functional. Indeed Q
bands appears in the visible range at λ ~480-650 nm with lower intensity consistent with π-π*
(
a → e ) transition from ground state (S ) to singlet excited state (S ). Beside above two
2u g 0 1
prominent (B and Q) bands an additional low intensity band appears as shoulder to soret band
at around ~365 (±5) nm is assigned to charge transfer transition from lone pair P orbital of
peripheral halogen to empty π* orbital for H TFPP and H TClPP. However for bromo and
2
2
iodo the charge transfer would be either from n→π*, d→π*, or π→d transitions at higher
ionic strength, that appears blue shifted by (±4) nm [10]. and Detailed absorption bands with
intensity in parenthesis are presented in Table 3. Overall it was argued and observed that
there was marginal or no appreciable change in intensity and position of the characteristic
bands on variable solvation.

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3
2
2
1
1
0
0
.0
.5
.0
.5
.0
.5
.0
3.0
Soret or B band
Soret or B band
H TFPP (CHCl )
H TFPP (THF)
2
3
2
2.5
2.0
H TClPP
H TClPP
2
2
H TBrPP
H TBrPP
2
2
H TIPP
H TIPP
2
2
1.5
1.0
0.5
0.0
Q bands
QIV
Q bands
QI
QIV
QIII
QII
QIII
QII
QI
3
00
400
500
600
700
3
00
400
500
Wavelength (nm)
600
700
Wavelength (nm)
(a)
(b)
Fig. 2: (a) UV-Visible absorption spectrum of monomer in THF, (b) CHCl at pH=7 (C =
3
M
5
3
.0×10 M)
Table 2: Solubility chart of H TXPP series in various solvents and their nature in H O.
2
2
H TXPP CHCl
Acetone MeOH
CH CN DCM
Toluene
partial
partial
partial
partial
THF
DMSO
2
3
3
H TFPP partial
partial
insoluble insoluble partial
soluble insoluble
soluble partial
soluble partial
soluble partial
2
H TClPP soluble
soluble partial
soluble partial
soluble partial
partial
partial
partial
soluble
soluble
soluble
2
H TBrPP soluble
2
H TIPP
2
soluble
Water
immiscible miscible miscible miscible immiscible immiscible miscible miscible
Table 3: Characteristic (Soret or B and Q) bands in nm for H TXPP in THF and CHCl .
2
3
THF/ PH=7 ( Neutral) Sorte or B
Q band
QII
Ref.
[9]
Sample Codes
Band
420
419 (3.58)
420 (2.95)
417 (2.80)
QIV
516
QIII
552
QI
644
(
DCM)
TTP
TPP
TMP
592
(
THF)
THF)
480 (0.05)
516 (0.16)
513 (0.36)
514 (0.50)
513 (0.12)
515 (0.40)
513 (0.22)
516 (0.45)
514 (0.26)
516 (0.45)
513( 0.18)
552 (0.13)
546 (0.18)
549 (0.26)
548 (0.04)
550 (0.17)
548 (0.10)
549 (0.22)
549 (0.13)
551 (0.24)
546(0.08)
594 (0.06)
589 (0.14)
590 (0.21)
590 (0.02)
589 (0.12)
590 (0.06)
592 (0.13)
590 (0.08)
591 (0.16)
648 (0.05)
651 (0.07)
649 (0.12)
649 (0.23)
646 (0.01)
646 (0.09)
649 (0.05)
651 (0.33)
647 (0.07)
647 (0.12)
[15]
(
(
THF/ CHCl3)
H TFPP
2
4
17 (2.89)
420 (2.60)
21 (2.77)
421 (2.70)
22 (2.82)
419 (2.70)
21 (2.89)
This
Work
H TClPP
2
4
H TBrPP
2
4
H TIPP
2
4
Influence of pH
The absorption spectra was recorded under basic (pH=10), and acidic (pH= 5-2) in THF+
H O system. The initial colour of H TXPP was dark pink and pH was in range of 7-6 at (3.0×
2
2
5
1
0 C ) i.e. approximately neutral. Upon increasing the acidity by addition of 0.1 M, HCl to
M
the analyate, sudden fascinating colour change was recognized depicted in Fig. 3 with change

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in absorption spectra. The usual soret band splits into two individual lorentzian bands at ~417
and ~444 nm for floro and 417/ 450, 416/ 448, 417/ 453 nm for Cl, Br and I derivatives
consistent with the literature reported [12], [13]. Besides splitting of soret band, the Q bands
starts diminishing leaving behind the high intensity Q1 band at ~660 (±10) nm which is
otherwise forbidden, accompanied by broadening of soret band at higher ionic strength. This
deviation in absorption spectra is attributed to variation in ionic strength developed by the
acidic medium in the allure, which in turn results in dication formation. The two amino
nitrogen in porphyrin core picks up two additional hydrogen ions and undergoes protonation
which inturn induces positive charge in central core of porphyrin molecule thereby by
increasing HOMO-LUMO energy gap. This decrease in no of Q bands to single sharp band is
ascribed to increase of D h symmetry to D h by protonation and dication formation. Further
2
4
extension of pronounced shoulder to B band which is very low in intensity at around ~365
±5) nm present among all corresponding compounds is assigned to chare transfer transition
(
from nonbonding lone pair P orbital of peripheral halogen atoms to empty π* orbital of
porphyrin. For bromo and iodo derivative the charge transfer could diversify from n→π*,
d→π*, or π→d transitions at higher ionic strength [10]. Observation from spectral splitting
and broadening of soret band suggesting aggregation character in acidic medium. In basic
medium at pH= 10, the colour and absorption spectra remain marginally same as that of
neutral environment. There was no any abrupt change in colour, it appears light pink, either
the pattern of absorption spectra coincide with absorption spectra of pH= 7, with increasing
intensity of soret and Q-bands. However the porphyrin core undergoes deprotonation and
remains in dianionion form, leaving aside hyperpophyrin effect. From absorption spectral
behaviour it can be clearly noticed that at higher pH values, absorption spectra show regular
pattern as that of neutral environment indicting structural stability of porphyrin ring.
Structure of H TXPP at various pH is represented in Fig. 4.
2
Fig. 3: Effect and change on colour of H TXPP analyate on pH variations.
2

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3.0
3
2
2
1
1
0
0
.0
.5
.0
.5
.0
.5
.0
H TClPP
H TFPP
2.5
2.0
2
2
pH7
pH5
pH4
pH3
pH2
pH10
pH7
pH5
pH4
pH3
pH2
pH10
1.5
1.0
0.5
0.0
300
3
00
400
500
600
700
800
400
500
600
700
800
Wavenumber (nm)
Wavenumber (nm)
(a)
(b)
3
2
2
1
1
0
0
.0
3.0
H TIPP
H TBrPP
2
2
pH7
pH5
pH4
pH3
pH2
pH10
pH7
pH5
pH4
pH3
pH2
pH10
.5
.0
.5
.0
.5
.0
2.5
2.0
1.5
1.0
0.5
0.0
400
500
600
700
800
400
500
600
700
800
Wavelength (nm)
Wavenumber (nm)
(c)
(d)
Fig. 5a: UV-Visible absorption spectra of H TXPP; (a) H TFPP, (b) H TClPP, (c) H TBrPP
2
2
2
2
and (d) H TIPP at various pH scales in THF+H O system.
2
2

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3.0
2.5
2.0
1.5
1.0
0.5
0.0
Soret or B band
H TFPP (pH=10)
2
H TClPP
2
H TBrPP
2
H TIPP
2
Q bands
Q1
Q1
Q1
Q1
400
500
600
700
800
Wavenumber(nm)
Fig. 5b: UV-Visible absorption spectra of H TXPP at pH= 10, in THF+H O system.
2
2
Emission Spectroscopy
Emission spectra of porphyrin i.e. H TXPP series was recorded at various pH ranges from
2
acidic (pH=5-2), basic (pH=10) in THF+ H O medium at single concentration C =
2
M
5
3
.0×10 M with excitation wavelength 415 nm. The emission spectra for corresponding series
are presented in Fig. (6a-6b). Featured characterises emission peaks of H TXPP series are
2
presented in Table 4. All the spectra of corresponding series exhibit two potential bands. A
high intensity band at around ~654 (±3) nm assigned to Q(0,0) and weak band at ~712 (±5)
to Q(0,1) electronic transition. These two bands in visible region reflect usual S emission.
1
However the less common B(0,0) band in UV region appears at ~475 (±5) nm reflects S2
emission. By continuously monitoring emission spectra induced by the exposure to acidic and
basic media, we observed linearity and decreases in intensity with lowering pH values.
However in basic medium the emission spectrum coincides with neutral medium, however
there was decrease in intensity in the later. Previously reported fluorescence characterization
and less common S -S emission is parallel to our results and are in close relationship with
2
0
absorption spectra [14,15,32]. It has been argued and reported that on absorption of UV-
Visible radiation, π electrons of conjugated system undergoes excitation from (S ) → (S or
0
1
S2). Which in turn return to ground state in very short span of time and emit fluorescence at
470 and 650-720 nm respectively. However the fluorescence quantum yields (Φ ) depends
~
f
on the peripheral phenyl functional groups. The values obtained are as (Φf = 0.052, 0.050,
.054, 0.046 for F, Cl, Br and H TIPP). Heavier halogen atoms decrease the quantum yield
0
2
with least for H TIPP.
2

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1
60
8
6
4
2
0
pH7
H TFPP
pH10
2
140
120
100
80
H TFPP
2
H TClPP
2
H TClPP
2
H TBrPP
2
H TBrPP
2
H TIPP
2
H TIPP
2
6
0
40
2
0
0
450
500
550
600
650
700
750
450
500
550
600
650
700
750
Wavelength (nm)
Wavelength (nm)
(a)
(b)
Fig. 6(a): (a) Emission spectrum of H TXPP monomer in THF at pH=7 and (b) pH=10 (C
2
M
5
=
3.0×10 M)
1
1
1
1
60
40
20
00
1
40
H TFPP
H TClPP
2
2
pH7
pH5
pH4
pH3
pH2
pH10
120
100
80
pH7
pH5
pH4
pH3
pH2
PH10
8
6
4
2
0
0
0
0
0
60
4
0
0
0
2
4
50
500
550
600
650
700
750
4
50
500
550
600
650
700
750
Wavelength nm
Wavelength (nm)
(c)
(d)
6
5
4
3
2
1
0
H TIPP
2
H TBrPP
2
2
0
0
0
pH7
pH7
pH5
pH4
pH3
pH2
pH10
PH5
PH4
PH3
PH2
PH10
1
450
500
550
600
650
700
750
450
500
550
600
650
700
Wavelength (nm)
Wavelength (nm)
(e)
(f)

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Fig. 6b: (c) Emission spectra of H TFPP, (d) H TClPP, (e) H TBrPP and (f) H TIPP at
2
2
2
2
various pH scales in THF+H O system.
2
Table 4: Featured characterises emission peaks of H TXPP series
2
Compound
TPP
B(0,0)
-
470
475
475
475
Q(0,0)
654
652
651
654/ 654
652
Q(0,1)
712
712
711
720/ 718
713
Φf
Solvent
DCM
CHCl3
CHCl3
Ref.
[21,33]
this work
this work
0.110
0.052
0.050
0.054
0.046
H TFPP
2
H TClPP
2
H TBrPP
DCM/ CHCl3 [34]/ this work
CHCl3 this work
2
H TIPP
2
Aggregation behaviour
From the combined results from absorption and absorption-emission spectra for
H TXPP series it is clearly evident that in pure organic solvents. The spectrum sows typical
2
non aggregated monomeric complex. Indeed we measured the spectrum in THF and CHCl3
solvent separately, the sharp absorption bands were observed in all the corresponding halo
series indicating no aggregation of porphyrin. Further the absorption spectra in distinct
organic solvent and at lowered concentrations show no any significant spectral changes.
Upon introducing the pH variation in aqueous electrolyte solution, absorption spectra in
acidic medium at around pH=4. Soret band splits into two individual lorentzains bands at
~444 (±10) and ~417 (±3) nm (known as j-band), accompanied by continuously broadening
and red shift of soret band at 417 (±3) nm significant within all H TXPP series, which is
2
attributed to side-by- side aggregation and formation of J-aggregate. The red shift of B band
featured self aggregation through head-to-tail molecular ordering which is consonant with
literature data [10,35]. The red shift observed is due to increased conjugation exerted by para
substituted halogen group’s characteristic of electron donating tendency, which in turn
reduces the HOMO-LUMO energy gap. Other spectral changes include reduction of Q bands
to only single Q1 band, which is otherwise forbidden. The Q1 band is red shifted towards 647
(±5) nm with significant increase of the intensity. These changes in spectrum are attributed to
increases of symmetry followed by dication formation at higher ionic strength. Absorption
spectral pattern for electrically neutral and basic allure at pH=10 coincides, the modest
differences consists only in enhancement of intensity of soret band. Indeed the
hyperporphyrin effect is missing and exhibits dianionic form.

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Conclusion
In conclusion, H TXPP series shows some variable electro-optic spectral changes on
2
varying pH from acidic, basic and neutral medium in pure organic and aqueous electrolyte
solution. UV-Visible and fluorescence spectroscopy were employed to study aggregation
behaviour indeed.
Acknowdelegment
U.A.D is great full to NIT Srinagar for financial support in the form of research stipend and
research facilities.
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Graphical Abstract

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Highlights

Optical and fluorescence response of halogen derivatives of meso-tetrakis-(4-phenyl)
porphyrin; TPP.

Effect on absorption and emission spectrum of meso-tetrakis-(4-halophenyl)
porphyrin;
H TXPP on pH variation.
2

Specific structural changes and aggregation behavior.