Photoinduced Electron Transfer Reactions of Water Soluble Porphyrins in Zeolite Environment

Article abstract of DOI:10.1007/s10895-021-02747-8

Excited state interactions of zeolite adsorbed porphyrins have been investigated by steady state luminescence quenching technique with certain antioxidants such as reduced glutathione, ascorbic acid and L-cysteine. The zeolite supported porphyrins, meso-tetra (N-methyl-4-pyridyl) porphyrin (H₂TMPyP⁴⁺) and zinc tetra(N-methyl-4-pyridyl) porphyrin (ZnTMPyP⁴⁺) were prepared and characterized by various techniques such as Diffuse Reflectance Spectra (DRS), Scanning Electron Microscope (SEM), powder X-Ray Diffraction (XRD) and BET surface area. The interaction of zeolites with porphyrins are shown to increase the lifetime of the singlet excited state of porphyrins and decays are biphasic in nature. The splitting of the emission band of porphyrins occurs in 1:1 glycerol: water solution due to the changes in the dielectric of the solvation sphere associated with porphyrin. The Stern-Volmer plots of I₀/I vs quencher concentration [Q] were linear in the whole range of [Q] used. These studies revealed the effective quenching for zinc porphyrin compared to free base porphyrin. The effect of quenchers and zeolite acidity has also been studied and the quenching rate constant (k_q) is found in the order of 10⁹?M^?1?s^?1. The quenching reaction obeys Rehm-Weller Equation and is shown to be due to thermodynamically favoured electron transfer from quenchers to the excited singlet state of porphyrins (reductive quenching).

Full text of DOI:10.1007/s10895-021-02747-8

Journal of Fluorescence
https://doi.org/10.1007/s10895-021-02747-8
ORIGINAL ARTICLE
Photoinduced Electron Transfer Reactions of Water
Soluble Porphyrins in Zeolite Environment
1
1
2
Kandavelu Velappan & Renganathan Rajalingam & Anbazhagan Venkattappan
Received: 5 February 2021 /Accepted: 4 May 2021
#
The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2021
Abstract
Excited state interactions of zeolite adsorbed porphyrins have been investigated by steady state luminescence quenching tech-
nique with certain antioxidants such as reduced glutathione, ascorbic acid and L-cysteine. The zeolite supported porphyrins,
4
+
4+
meso-tetra (N-methyl-4-pyridyl) porphyrin (H TMPyP ) and zinc tetra(N-methyl-4-pyridyl) porphyrin (ZnTMPyP ) were
2
prepared and characterized by various techniques such as Diffuse Reflectance Spectra (DRS), Scanning Electron Microscope
(
SEM), powder X-Ray Diffraction (XRD) and BET surface area. The interaction of zeolites with porphyrins are shown to
increase the lifetime of the singlet excited state of porphyrins and decays are biphasic in nature. The splitting of the emission
band of porphyrins occurs in 1:1 glycerol: water solution due to the changes in the dielectric of the solvation sphere associated
with porphyrin. The Stern-Volmer plots of I /I vs quencher concentration [Q] were linear in the whole range of [Q] used. These
0
studies revealed the effective quenching for zinc porphyrin compared to free base porphyrin. The effect of quenchers and zeolite
9
−1 −1
acidity has also been studied and the quenching rate constant (k ) is found in the order of 10 M s . The quenching reaction
q
obeys Rehm-Weller Equation and is shown to be due to thermodynamically favoured electron transfer from quenchers to the
excited singlet state of porphyrins (reductive quenching).
.
.
.
Keywords Zeolite/Porphyrins Electron transfer Fluorescence Antioxidants
Introduction
and their potential use in photogalvanic cells [11]. The mech-
anistic understanding of the transfer of electrons, protons and
hydrogen atoms is of greater importance since many photo-
chemical and photobiological processes are based on these
primary events. The investigations of the interactions of pho-
toexcited singlet states of the porphyrins by artificial
photosensitized electron-transfer reactions [12–16] are very
important to mimic photosynthesis as a means of solar energy
conversion and storage.
In homogeneous solution, the excited states of porphyrins
are not sufficiently long-lived for electron transfer quenching.
There are several studies on fluorescence quenching of the
porphyrins by quinones and nitroaromatics in organic solvents
Photochemical studies of metalloporphyrins have been of
great interest in fields extending from chemistry, biology,
medicine to optoelectronics [1–3]. Metalloporphyrins are the
active sites of numerous proteins whose functions range from
oxygen transport and storage (hemoglobin, myoglobin) to
electron transport (cytochrome c, cytochrome oxidase) to en-
ergy conversion (chlorophyll) [4, 5]. They are also used as
photosensitizers for photodynamic therapy (PDT) of cancer
[
6, 7]. Water soluble porphyrins are also of much interest
because of their ability to bind with DNA [8, 9], act as pho-
tosensitizers in photocatalytic water reduction, oxidation [10]
[17, 18]. The oppositely charged energetic intermediates of
the porphyrin and quinones, due to their electrostatic attrac-
tion, were found to result in considerable decrease in the effi-
ciency of the photoprocess. Many different host systems have
been successfully applied as a means of controlling the effi-
ciency of energy storage by preventing rapid back electron
transfer. The net photoionization efficiency in organic assem-
blies such as cyclodextrins [19], polyelectrolytes [20], colloids
*
Anbazhagan Venkattappan
anbu80@gmail.com
1
2
School of Chemistry, Bharathidasan University,
Tiruchirappalli, Tamil Nadu 620 024, India
Department of Chemistry, Vinayaka Mission’s Kirupananda Variyar
Arts and Science College, Vinayaka Missions Research Foundation
[21], charged micelles [22, 23], vesicles [24] and hydrophilic-
(Deemed to be University), Salem, Tamil Nadu 636 308, India

J Fluoresc
hydrophobic environments such as water-in-oil
microemulsions [25] are typically higher than that in homo-
geneous solution, but the photoinduced radicals are not stable
at room temperature. In some cases, the photoyields in organic
assemblies are limited by the polarity and solubility of the
photoactive molecules. Other investigators have explored the
design of microheterogeneous assemblies such as zeolites [26]
and silica gels [27] to achieve charge separation. Their cages
and pores provide an appropriate microenvironment to retard
back electron transfer and increase the lifetime of the
photogenerated radicals ion intermediates. This has been ac-
complished in several cases where reactions between an un-
charged substrate and/or quencher result in formation of
charged products, one of which is selectively expelled or se-
questered by the assembly. Picosecond transient absorption
techniques were performed to probe the excited-state dynam-
ics, revealing ultrafast charge separation (∼4 ps) occurring
from the donor segment to acceptor. Ultrafast transient ab-
sorption experiments allowed to identify the process of
quenching of the Zn-porphyrin fluorescence as an efficient
photoinduced electron transfer reaction between the cage por-
phyrin and the included NDI guest. The process occurs on fast
and ultrafast time scales in the two complexes (1.5 ps and ≤
Experimental Section
The tetra(4-pyridyl) porphyrin (TPyP), methyl-p-toluene sul-
fonate, dimethyl formamide, reduced glutathione, ascorbic ac-
id, L-cysteine and glycerol, zinc acetate, dowex and sephadex
were of high purity from Fluka or Aldrich and were used as
received. The zeolite-Y (Si/Al: 2.3) was obtained from Mid-
Century chemicals and the zeolite Y (Si/Al: 1.6) was prepared
by the reported procedure [32]. X-Ray powder diffraction
(XRD) pattern of the zeolite has excellent agreement with
the calculated simulation of the XRD pattern for faujasite
zeolite [33].
Preparation of Water Soluble Porphyrins
4
+
4+
H TMPyP and ZnTMPyP were synthesized by reported
2
4
+
procedure [34]. H TMPyP was prepared by methylation of
2
TPyP (100 mg) with methyl-p-toluene sulfonate (500 mg) by
refluxing overnight in dimethylformamide. Most of the
dimethylformamide was removed by vacuum distillation and
the solution was cooled until the precipitation of tosylate salt
4
+
of H TMPyP occurred. The solid was filtered, washed spar-
2
ingly with cold acetone and air dried. The metal (Zn) insertion
was carried out by dissolving H TMPyP in water together
4
+
3
00 fs) leading to a short-lived charge separated state (charge
2
recombination lifetimes in the order of 30–40 ps) [28, 29].
Photoinduced electron transfer (PET) is one of the most
important mechanisms for developing fluorescent probes
and biosensors for antioxidants. Quantitative prediction of
the quantum yields of these probes and sensors is crucial to
accelerate the rational development of novel PET-based func-
tional materials [30].
Antioxidants such as glutathione, ascorbic acid and L-
cysteine play a vital role in medicine, biology, polymer
chemistry, cosmetics and in food industry. By intercepting
oxidizing species, predominantly reactive radicals, they
prevent cellular damage and polymer or food degradation.
It is very important to understand the bimolecular reaction
kinetics by which antioxidants intercept reactive oxidizing
species, as studied herein, is of utmost importance for
modeling their actual activity and understanding the
mechanism by which antioxidants act. The important
aim in such research areas is the quantification of the
reactivity of antioxidants [31].
In this paper, influence of the antioxidants on the fluores-
cence behaviour of excited state of porphyrins adsorbed in
zeolites in 1:1 glycerol water medium are reported. The effect
of sensitizers, quenchers and zeolite acidity has also been
studied. The structures of porphyrins and quenchers used in
this study are given in Scheme 1. The mechanism of
quenching and the pathways have been analyzed in terms of
singlet excited state porphyrin-quencher interaction. The for-
mation of radical ion pairs will be a reason for fluorescence
quenching.
with 5 fold excess of zinc acetate and stirring the solution
overnight. The anion was exchanged by passing the solution
through chloride ion enriched resin, subsequently purified
with dowex or sephadex columns.
Preparation of Zeolite Adsorbed Porphyrins
4
+
4+
The porphyrin sensitizers H TMPyP and ZnTMPyP were
2
adsorbed onto zeolite Y with varying Si/Al ratio (i.e., 2.3 and
1.6) by suspending 1 g of latter in 25 mL of water containing
−5
2 × 10 M solution of porphyrin. After stirring overnight, the
suspension was filtered and washed with water until the wash-
ings were colourless. The filtered porphyrin exchanged zeolite
powder was washed in a soxhlet extractor with methanol to
remove unbound porphyrin, if any and allowed to dry in air.
Comparison of the filtered bathing solutions before and after
equilibration, by UV-Vis spectroscopy, showed that the load-
4
+
4+
ing levels of H TMPyP and ZnTMPyP were 0.5 to 1 ×
2
−5
10 mol/g zeolite. The observed porphyrin loadings corre-
−
10
−2
spond to approximately monolayer (10 mol cm ) cover-
age [35] of the particle external surface. The electrostatic in-
teractions between the anionic aluminosilicate support and the
cationic porphyrin molecules contribute to the successful ad-
sorption of the complexes.
Analyses
Absorbance spectra were recorded using Hewlett Packard
8452A spectrophotometer and Diffuse Reflectance Spectra

J Fluoresc
Scheme 1 Structures of
porphyrins and quenchers used in
this study
CH3
SH
N
O
O H
H
OH
+
H
N
HO
N
O
H
NH2
O
N
N
N
Reduced Glutathione
H C
N
+
M
+
N CH
3
3
N
CH OH
2
SH
H
C
OH
CH2
O
O
H N
2
C
H
COOH
+
N
OH
HO
CH3
TMPyP4
+
Ascorbic acid
L-cysteine
M = 2H
M = Zn ZnTMPyP
H
2
4
+
(
DRS) measurements were performed on CARY 5 UV-
showing the Soret band at 421 nm and 434 nm respectively.
The shift of the electronic absorption spectra (Soret band) for
metal porphyrin was closely connected to the electronegativ-
ity of the incorporated metal which strongly affects the π-
electron system [36]. In 1:1 glycerol: water solutions, the three
major absorption bands are red shifted approximately to 4 nm;
adsorption onto the zeolite surface results in a further red shift
of 2 nm in the Soret band and 2–4 nm in the Q bands. In terms
of a band profile or shape, the Soret band is broadened by
interaction with the zeolite. The shape of the spectra and band
positions indicate the existence of the monomeric dye and
non-occurrence of aggregation. The absorption spectra of ze-
olite adsorbed porphyrins were unaltered in the presence of
quenchers. Kano et al. [37] reported the ground state complex
Visible spectrometer with an integrating sphere reflectance
accessory. The X-Ray Diffraction (XRD) patterns of all the
catalysts in this study were obtained on a Siemens D5000 X-
ray diffractometer using Ni filtered Cu Kα radiation (λ =
1
.5406) from 2θ = 20 to 60°. A conventional all glass volu-
metric high vacuum system was employed for the surface area
measurements of the catalysts by nitrogen adsorption at liquid
nitrogen temperature. Philips XL 30 SFEG instrument was
employed for the Scanning Electron Microscope (SEM)
measurement.
Fluorescence quenching experiments were carried out
using a Perkin Elmer LS 50B spectrofluorimeter. The excita-
tion wavelength was in all cases at or near the maximum of the
Q(1,O) band of the porphyrins. The samples were dispersed
by sonication in the desired medium. In the steady-state fluo-
rescence measurements, a known amount of each porphyrin/
zeolite sample was dispersed in 3 mL of the 1:1 glycerol:
water solution with various concentrations of quenchers under
nitrogen atmosphere and measurements taken at right angles
in a 1 cm cuvette. The concentration of the quencher was
4
+
formation for the H TMPyP in the presence of AQDS by
2
observing isosbestic point. The oxidation potential of the
quencher used in our case ranges from −0.45 V to 0.35 V
and if there is any ground state charge transfer (CT) complex
formation, a new absorption peak should appear from the tail
4
+
4+
end of the H TMPyP or ZnTMPyP absorption spectrum.
2
This absorption shall be blue or red shifted depending on the
oxidation potential of the quencher. The absence of any such
absorption eludes the formation of the ground state complex.
−3
normally of the order of 10 M to 0.1 M. Fluorescence life-
times were measured with a PRA time-correlated single-pho-
ton counting system. Precautions were taken to eliminate spu-
rious single photon counting data arising from scattering of
the incident laser flash.
Emission Studies
4
+
4+
Emission spectrum of H TMPyP and ZnTMPyP in water
2
were identical with reported literature [34, 35]. As the porphy-
rins studied were free-base or closed shell metal porphyrin
complexes, the fluorescent emission is attributed to the radia-
tive deactivation of the electronic excited state associated with
the porphyrin macrocycle. The emission of free-base porphy-
rins, like the optical absorption spectra, differ from metallic
porphyrin emission due to the differences in their symmetries,
Results and Discussion
Absorption Studies
4
+
4+
Absorption spectrum of H TMPyP and ZnTMPyP in wa-
2
ter are identical with those reported in the literature [34]

J Fluoresc
the former being significantly red shifted, i.e., the λ
emis-
Ground State Reflectance Spectra
max
4
+
sion is 634 nm for ZnTMPyP whereas 685 nm for
4
+
H TMPyP . The change in the medium i.e., 1:1 glycerol:
The diffuse reflectance spectra (DRS) were examined to un-
derstand the presence and interaction of porphyrin with the
zeolite. The UV-Vis DRS of zeolite-Y (Si/Al: 2.3) adsorbed
2
water medium, results in marked changes in the fluorescence
4
+
4+
emission spectrum of H TMPyP and ZnTMPyP . A blue
2
4
+
4+
shift of about 3–5 nm was observed for both porphyrins and
H TMPyP , ZnTMPyP are shown in Fig. 1. The presence
2
the broad fluorescence band of porphyrins gives rise to two
of porphyrins was confirmed by the characteristic Soret band
4
+
4+
4+
distinct bands, i.e., 680 nm and 725 nm for H TMPyP from
of H TMPyP and ZnTMPyP . Zeolite adsorbed porphyrins
2
2
4
+
6
85 nm, 630 nm and 672 nm for ZnTMPyP from 634 nm.
showed a red shift of about 6 nm (427 nm from 421 nm for
4
+
4+
4+
The splitting of the emission band of H TMPyP has been
H TMPyP and 440 nm from 434 nm for ZnTMPyP ) in the
2
2
reported in methanol [34]. This effect is attributed to the
changes in the dielectric of the solvation sphere associated
with porphyrin. Adsorption onto the zeolite surface results
further changes in the fluorescence excitation and emission
Soret band when compared to the free porphyrins (not shown
in figure). Broadening in the Soret band is expected due to
interaction with zeolites. A similar observation was already
observed in the case of ZnTMPyP and ZnTAPP adsorbed
on zeolite–L [35]. The effects of Si/Al ratio and of the nature
of the balancing cation seem to follow the acid-base properties
of zeolites, indicating that the acid and basic sites are respon-
sible for porphyrin adsorption.
4
+
4+
4
+
4+
spectrum of H TMPyP and ZnTMPyP . A red shift of
2
about 6 nm in the excitation spectrum was observed for
porphyrin-adsorbed zeolites when compared to the excitation
spectrum of the free porphyrins. The shift in the emission
maxima of 7–25 nm to lower wavelength (blue shift) was
observed for both porphyrins adsorbed in zeolite. The broad
4
+
fluorescence band of H TMPyP shifts 20 nm and
BET Surface Area Measurements
2
4
+
ZnTMPyP shifts 3 nm to lower wavelength from the fluo-
rescence band observed in 1:1 glycerol: water. The observed
broadening in the spectrum is expected due to the interaction
with zeolites, which was observed in the case of ZnTMPyP
and ZnTAPP on zeolite-L [31, 35]. The photophysical prop-
Surface area measurements for the zeolite adsorbed porphy-
rins and bare zeolites give the information about the presence
of porphyrin on the surface. The BET Surface area measure-
ment for the zeolite adsorbed porphyrin catalysts is given in
Table 2. The zeolite adsorbed porphyrins shows reduced sur-
face area compared to bare zeolites. It could be due to the
presence of porphyrins on the surface of zeolites.
4
+
4
+
4
+
4+
erties of H TMPyP , ZnTMPyP in homogeneous solution
2
and on zeolite with varying Si/Al ratio are summarized in
Table 1.
4+
4+
d
2
Table 1 Physical properties of H TMPyP and ZnTMPyP in water, 1:1 glycerol: water and on the zeolite-Y surface
Property
2
H O
Glycerol
Zeolite-Y
Si/Al: 1.6
Si/Al: 2.3
4+
2
H TMPyP
Electronic absorption wavelength maximum, nm
421
425
525
565
589
427
528
567
591
520
560
585
Fluorescence emission wavelength maximum, nm
685
680
660
720
7
25
a
b
c
b
c
F
τ /ns, fluorescence lifetime
5.3
5.52
5.49 (77.32)
5.43 (60.90)
8.75 (39.10)
9
.54 (32.68)
4+
ZnTMPyP
Electronic absorption wavelength maximum, nm
434
438
562
602
440
566
606
560
00
6
Fluorescence emission wavelength maximum, nm
634
630
627
670
6
72
a
b
c
b
F
τ /ns, fluorescence lifetime
1.3
1.53
1.45 (83.50)
1.41 (68.47)
c
4
.78 (16.50)
6.37 (31.53)
a
b
c
d
from ref. [3, 8], fast component, slow component, in 1:1 glycerol: water, biphasic decay, Error limits: τ
F
, ± 20%

J Fluoresc
Lifetime Measurements
To understand the role of zeolite and the solvent system, the
singlet state lifetime (τ) of porphyrins in water, 1:1 glycerol
water and zeolite adsorbed porphyrins system were measured
and is shown in Table 1. The singlet state lifetime of
4
+
4+
H TMPyP and ZnTMPyP in homogeneous condition is
2
5
.3 ns and 1.3 ns respectively [34]. For 1:1 glycerol water
system, the lifetime of the porphyrins increased by about
0.2 ns. Fluorescence decays of porphyrins adsorbed on zeo-
~
lites were biphasic in nature. The observed difference is due to
the presence of zeolite environment and solvent used, which
stabilizes the singlet lifetime of porphyrin and it varies with
varying Si/Al ratio. The acid and basic sites present in zeolites
are possibly responsible for the difference observed in life-
time. We can reason that the fast decay comes from a porphy-
rin ion which interacts strongly with the zeolite surface. This
result suggests that singlet-state quenching of porphyrins by
the quenchers may occur because of close positioning of the
methyl pyridinium group of the two molecules.
Fig. 1 UV-Vis diffuse reflectance spectra of zeolite-Y (Si/Al:2.3),
adsorbed ZnTMPyP and H TMPyP
2
4+
4+
X-Ray Diffraction Studies
X-ray powder diffraction permits one to ensure that the zeolite
host) crystallinity remains unaltered after its chemical modi-
fication. The X-ray powder diffraction patterns of
(
Fluorescence Quenching
4
+
4+
H TMPyP and ZnTMPyP loaded zeolites (Si/Al: 2.3 and
2
Fluorescence quenching of zeolite adsorbed porphyrin by var-
ious quenchers such as ascorbic acid, reduced glutathione and
L-cysteine was carried out in 1:1 glycerol: water. The addition
of quenchers did not change the porphyrin shape of fluores-
cence spectra and the absence of any new peak appearance
eliminates the possibility of the ground state complex forma-
tion. Since there is no complexation between quencher and
sensitizer, the water molecules from the solvation shell of the
porphyrin remains as such. This, in effect, increases the pos-
sibility of collisional quenching of singlet excited state of por-
phyrins. The absence of CT complex absorption was already
reported [38, 39] for the quenching of ionic porphyrin by
neutral quenchers such as guanine and nitrobenzene. The con-
centration of quenchers used in this study is enough to quench
the fluorescence of porphyrins. The emission intensity de-
clines with the increase in the quencher concentration, which
clearly indicates the quenching of porphyrin. Fluorescence
quenching spectrum of zeolite-Y (Si/Al: 1.6) adsorbed
1
.6) are presented in Fig. 2. None of the solids showed reduc-
tion in the intensity of peaks, suggesting the crystallinity re-
tention of zeolite matrix. The main framework of the zeolite
was not damaged. No variation was observed in the zeolite
lattice parameters after the adsorption procedures.
SEM Analysis
The SEM image of porphyrin-zeolite-Y is similar to those
observed for zeolite-Y, indicating that they possess the same
morphology, i.e., the framework around the guest molecule
porphyrin is zeolite-Y. A typical micrographs of zeolite (Si/
4
+
Al: 1.6) adsorbed H TMPyP were shown in Figs. 3 and 4. It
2
suggests that the slid support is structurally unchanged and
porphyrins should be dispersed molecularly on the external
surface.
4
+
H TMPyP with various concentrations of ascorbic acid in
2
Table 2 Surface area of various zeolite-Y adsorbed porphyrins
1:1 glycerol: water is shown in Fig. 5. The Stern-Volmer
relationship was used for the analysis of fluorescence
quenching [40].
2
−1
Zeolite adsorbed porphyrin
Surface area (m g )
Zeolite-Y (Si/Al: 2.3)
652
640
642
660
645
650
I0=I ¼ 1 þ KSV ½QŠ
ð1Þ
4+
2
H TMPyP / Zeolite -Y (Si/Al: 2.3)
4+
ZnTMPyP / Zeolite -Y (Si/Al: 2.3)
where I and I are the intensities of the fluorophore in the
0
Zeolite -Y (Si/Al: 1.6)
absence and the presence of quencher, respectively. KSV is
Stern-Volmer constant and [Q] is the concentration of the
quencher.
4+
2
H TMPyP / Zeolite -Y (Si/Al: 1.6)
4+
ZnTMPyP / Zeolite -Y (Si/Al: 1.6)

J Fluoresc
Fig. 2 Powder X-ray diffraction
pattern of zeolite-Y (Si/Al:2.3),
zeolite-Y (Si/Al:2.3) adsorbed
4+
H
2
TMPyP , zeolite-Y (Si/Al:
1
.6) and zeolite-Y (Si/Al:2.3)
4+
adsorbed ZnTMPyP
The bimolecular quenching rate constant, k was calculated
using Eq. 2.
process involves purely a dynamic quenching, i.e., quenching
solely by collisional deactivation of the singlet excited state.
The quenching rate constants for various zeolite adsorbed
porphyrins with quenchers such as ascorbic acid, reduced glu-
tathione and L-cysteine are shown in Table 3.
q
kq ¼ KSV=τ0
ð2Þ
where τ is the fluorescence lifetime of the porphyrin in the
0
4
+
The k values observed for zeolite adsorbed H TMPyP
q
2
absence of quencher.
indicates that the quenching efficiency is more for reduced
The Stern-Volmer plot of I /I versus [Q] yields a straight
0
glutathione followed by ascorbic acid and L-cysteine. The
line for the porphyrin fluorescence quenching. In all the cases
the correlation coefficient (r) is in the range of 0.99–0.97. A
typical plot for the zeolite-Y (Si/Al: 1.6) adsorbed
4
+
same trend is observed for zeolite adsorbed ZnTMPyP .
This effect is due to better electron donating tendency of glu-
tathione than other quenchers such as ascorbic acid and L-
4
+
H TMPyP with ascorbic acid is shown in Fig. 6. The
2
Fig. 3 Scanning electron
microscope image of zeolite-Y
(
H
Si/Al: 1.6) adsorbed
2
TMPyP
4+

J Fluoresc
Fig. 4 Scanning electron
microscope image of zeolite-Y
(
H
Si/Al: 1.6) adsorbed
2
TMPyP
4+
4
+
cysteine. The quenching of H TMPyP is slower than its zinc
Lewis acid strength is increased. Higher the acid strength,
higher will be the quenching efficiency [43]. Due to the dif-
ference in the above properties, the quenching efficiency dif-
fers in the following order:
2
complex, it is due to the fact that zinc complex is better oxi-
dant than its free base porphyrin.
The dependence of k on zeolites reflects the influence of
q
the difference in the structural features of the two different
zeolites (Si/Al: 2.3 and 1.6) employed. The effects of Si/Al
ratio and of the nature of the balancing cation, seem to follow
the acid-base properties of zeolites, indicating that the acid and
basic sites are possibly responsible for porphyrin adsorption
and quenching. The strength of acid and basic sites is mainly
determined by its cation nature, framework composition and
the crystalline nature [41, 42]. For zeolite having Si/Al: 2.3
and 1.6, the difference is the framework composition. A richer
aluminium zeolite (Si/Al: 1.6) will present a higher negative
framework charge and a stronger basicity when the frame-
work composition is concerned. So the strength of base sites
is decreased in the order Si/Al: 1.6 > Si/Al: 2.3, while the
Porphyrin/zeolite − Y (Si/Al : 2.3) > Porphyrin/zeolite −
Y (Si/Al : 1.6)
Quenching Mechanism
The quenching of porphyrins through electron transfer pro-
cesses has been extensively studied. In number of systems
net formation of products is observed when they possess the
same charge. On the other hand, when an electron transfer
process produces products with opposite charges, quenching
of the excited state of the porphyrins leads to no net product
formation.
Fig. 5 Fluorescence quenching of
zeolite-Y (Si/Al: 1.6) adsorbed
4+
H
2
TMPyP with various con-
centrations of ascorbic acid (0–
.75 × 10− M) in 1:1 glycerol:
3
9
water

J Fluoresc
Table 3 Free energy change for electron transfer and quenching rate constant for the porphyrins in zeolites with various quenchers
4
+
4+
Quencher
2
H TMPyP
ZnTMPyP
9
−1 −1
9
−1 −1
ΔGet (eV)
k
q
(10 M s )
ΔGet (eV)
q
k (10 M s )
Z-Y(2.3)
Z-Y(1.6)
Z-Y(2.3)
Z-Y(1.6)
Reduced glutathione
Ascorbic acid
L-Cysteine
−2.12
−1.42
−2.22
16.99
15.30
2.96
14.20
11.76
2.55
−1.48
−0.78
−1.58
52.05
32.12
9.14
48.27
29.03
5.17
In the present system, only the porphyrin is being excited
and the emission spectrum of porphyrin does not overlap with
the absorption spectrum of the used quenchers. Therefore,
The oxidation potential of reduced glutathione, L-cysteine
and ascorbic acid are −0.35, −0.45 (vs HMDE) and 0.35 V (vs
SCE) respectively [46–48]. The reduction potential of
4
+
4+
4+
4+
energy transfer from H TMPyP and ZnTMPyP to the
H TMPyP and ZnTMPyP are - 0.23 V [49] and −
2
2
quenchers cannot be acceptable mechanism for the quenching.
Examination of redox potentials of the quenchers and the por-
phyrins demonstrate that the mechanism of singlet quenching
most likely involves electron transfer between excited singlet
of porphyrin and the quencher.
The nature of the electron transfer pathway (i.e., oxidative
or reductive quenching of the porphyrin singlet state) can be
understood by examining the free energy of the corresponding
electron transfer reactions. The thermodynamics of electron
transfer from the quencher to the porphyrin can be calculated
by the well known Rehm-Weller equation [44].
1.04 V [35] respectively, whereas the reduction potential
changed significantly when they are exchanged into zeolite-
Y showing a shift of both the metal and ring waves to more
4
+
positive values by ca. 200 mV, i.e., − 0.06 V for H TMPyP
2
4
+
and - 0.85 V for ZnTMPyP [35]. The excited singlet state
4
+
energy (E(
) for H TMPyP is 1.83 eV and for
0,0) 2
ZnTMPyP is 1.98 eV. Thus, a thermodynamically favorable
electron transfer from the quencher to the excited porphyrin
will be anticipated only when E (DH/DH ) is less than 1.77
and 1.13 V for H TMPyP and ZnTMPyP respectively. All
4
+
•
+
1
/2
4
+
4+
2
the quenchers used have less than that oxidation potential
value, are expected to reduce the singlet excited state of por-
phyrins. The possibility of oxidative quenching (i.e., electron
transfer from porphyrins to quenchers) can be eliminated due
to the high electron donating ability and absence of electron
accepting ability of quenchers. Thus it is suggested that ob-
served quenching reaction involves electron transfer from
ΔGet ¼ E1=2ðoxid:Þ–E1=2ðred:Þ–Eð0;0Þ þ C
ð3Þ
where E1/2(oxid.) is the oxidation potential of the donor,
E1/2(red.) is the reduction potential of the acceptor, E(0,0) is
the singlet state energy of the sensitizer and C is the coulombic
term. Since one of the species is neutral and the solvent used is
polar in nature the coulombic term in the above expression is
neglected [45].
4
+
quencher to porphyrin. The reduction of ZnTMPyP like that
4
+
for H TMPyP involve the formation of π-radical species,
2
Fig. 6 Stern-Volmer plot for the
fluorescence quenching of
zeolite-Y (Si/Al: 1.6) adsorbed
4+
H
2
TMPyP with various con-
centrations of ascorbic acid (0–
.75 × 10− M) in 1:1 glycerol:
3
9
water

J Fluoresc
4+
Scheme 2 Mechanism for the luminescence quenching of P * with DH
i.e., the metal ions are not readily reduced from the +2 state.
Scheme 2 is proposed for electron transfer reaction in the
present system.
sphere associated with porphyrin. In addition, the inter-
actions cause the broadening of the spectrum. The data
presented here demonstrate that the quenching of the
4+
4+
In Scheme 2, P and DH are porphyrins and quenchers
singlet excited state of porphyrins, H TMPyP and
2
4
+
respectively. k and k are the rate constants of diffusion and
ZnTMPyP occurs in the presence of quenchers such
as reduced glutathione, ascorbic acid and L-cysteine.
Quenching studies revealed effective quenching for zinc
porphyrin compared to free base porphyrin. Also
quenching efficiency is more for glutathione than ascor-
bic acid and L-cysteine and zeolite-Y (Si/Al: 2.3) have
high efficiency than zeolite-Y (Si/Al: 1.6). The decrease
in fluorescence intensity of porphyrins with quenchers is
now shown to be due to thermodynamically favoured
electron transfer from quenchers to the excited singlet
state of porphyrins (reductive quenching). The
quenching reaction is shown to be with electron transfer
d
-d
dissociation coefficients of the encounter complex, respective-
ly. k is the rate constant for the formation of radical ion pair
et
and k-et is the rate constant for the recombination. kesc is the
rate constant for the separation of the separated radical ion pair
and k is the rate constant for the recombination of the sepa-
R
rated radical ion pair. k is the rate constant for the charge
b
recombination reaction producing the acceptor molecule in
the ground state.
The radical ion pair either undergoes back electron transfer
leading to regeneration of reactants or escape from the solvent
cage to give the redox products (Eqs. 4 & 5).
9
−1 −1
rate on the order of 10 M s . Thus, zeolite-Y are
found to be promising heterogeneous hosts for long
lived photoinduced charge separation of adsorbed
porphyrins.
•
þ
•
þ
•
DH →D þ H ; 2D →D−D
ð4Þ
where DH = L-cysteine, reduced glutathione and ascorbic ac-
id.
P3 →disproportionation or protonation
þ•
ð5Þ
Authors’ Contributions AV and KV has carried out the experimental
parts and prepared the draft of the manuscript. RR has contributed in
the revision of the manuscript.
4
+
4+
4+
where P = H TMPyP and ZnTMPyP .
2
The quencher (DH) donates proton after the electron loss
and it may be taken by the zeolite since it has the great affinity
to take up the proton produced, by preventing the back elec-
tron transfer in heterogeneous system.
Funding No funding was received for conducting this study.
Data Availability All data generated or analysed during this study are
included in this published article.
Code Availability Not applicable.
Conclusions
Declarations
The presence of porphyrins on the surface and the crys-
tallinity of zeolite were confirmed by DRS, BET surface
area, XRD and SEM analysis. The interaction of zeo-
lites with porphyrins increases the lifetime of the singlet
excited state of porphyrins and decays are found to be
biphasic. The splitting of the emission band of porphy-
rins occurs in 1:1 glycerol: water solution, similar to
that observed for porphyrins in methanol which is at-
tributed to the changes in the dielectric of the solvation
Ethics Approval Not applicable.
Consent to Participate Not applicable.
Consent for Publications Not applicable.
Conflicts of Interest/Competing Interests Authors have no conflict of
interest.

J Fluoresc
A Chem 388:112167. https://doi.org/10.1016/j.jphotochem.2019.
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