Interaction between certain porphyrins and CdS colloids: A steady state and time resolved fluorescence quenching study

Article abstract of DOI:10.1016/j.saa.2008.05.011

The interaction between porphyrins namely, meso-tetrakis (4-methoxyphenyl)porphyrin (TMeOPP), protoporphyrin IX (PPIX) and Zinc(II) meso-tetraphenylporphyrin (ZnTPP) with colloidal CdS has been studied by using steady state and time resolved fluorescence quenching measurements. The porphyrins adsorbed on the surface of colloidal CdS due to electrostatic interaction. This adsorption leads to changes in the absorption spectra related to the complex formation. The apparent association constant (K_app) was in the order of 4.34-5.58 × 10⁵ M^-1 from the effect of colloidal CdS on the absorption spectra and 0.64-1.6 × 10⁵ M^-1 from fluorescence quenching data. Quenching is attributable mainly to static mechanism through ground state complex formation as confirmed by lifetime measurements.

Full text of DOI:10.1016/j.saa.2008.05.011

Spectrochimica Acta Part A 71 (2008) 1507–1511
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Interaction between certain porphyrins and CdS colloids: A steady
state and time resolved fluorescence quenching study
∗
M. Asha Jhonsi, A. Kathiravan, R. Renganathan
School of Chemistry, Bharathidasan University, Tiruchirappalli 620 024, Tamil Nadu, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The interaction between porphyrins namely, meso-tetrakis (4-methoxyphenyl)porphyrin (TMeOPP), pro-
toporphyrin IX (PPIX) and Zinc(II) meso-tetraphenylporphyrin (ZnTPP) with colloidal CdS has been
studied by using steady state and time resolved fluorescence quenching measurements. The porphyrins
adsorbed on the surface of colloidal CdS due to electrostatic interaction. This adsorption leads to changes
in the absorption spectra related to the complex formation. The apparent association constant (Kapp)
Received 14 March 2008
Received in revised form 29 April 2008
Accepted 12 May 2008
Keywords:
Porphyrins
Fluorescence quenching
Porphyrins with CdS colloids
5
−1
was in the order of 4.34–5.58 × 10 M from the effect of colloidal CdS on the absorption spectra and
5
−1
0
.64–1.6 × 10 M from fluorescence quenching data. Quenching is attributable mainly to static mecha-
nism through ground state complex formation as confirmed by lifetime measurements.
©
2008 Elsevier B.V. All rights reserved.
1
. Introduction
dictated by binding or interaction with another molecule and are
expected to alter the porphyrin spectrophotometric characteristics
[17]. Porphyrins are of great importance in the fields of catalytic
chemistry, biochemistry and photochemistry [18]. Several groups
have studied physical and chemical processes of the porphyrins
on colloid surfaces [19]. Electronic coupling of porphyrins with
the semiconductor is certainly one of the key parameters for the
design of efficient sensitizers [20]. In our previous work we studied
the fluorescence quenching of meso-tetrakis (4-sulfonatophenyl)
porphyrin by colloidal TiO2 [21]. Structure of porphyrins used are
shown below:
Bhamro et al. studied the photochemistry of ZnTPP induced by
CdS nanoparticles in 2-propanol [22]. In this present work, we have
studied the interaction between substituted porphyrins with CdS
colloids as a quencher. It is expected that this study might lead to
clues on how to use the colloidal CdS nanoparticles for widespread
applications in biochemical research.
Photosensitization of stable, large band-gap semiconductors
using dyes in the visible light has been the subject of active
investigation [1,2]. These types of reactions were successfully
studied by Hagfeldt and Gratzel [3]. Colloidal suspensions of the
semiconductor nanoparticles are luminescent, and the spectral
properties of these colloids depended on the size of colloids
[
4].
There are molecules, whose interaction with nanoparticles
yields surface-modified nanoparticles, which results in chang-
ing their optical properties [5]. Datta et al. reported the possible
interaction of cadmium-enriched CdS nanoparticles with certain
biomolecules like tyrosine [6] and tryptophan [7]. The band-gap
energy of CdS (Eg = 2.6 eV) [8] is lower than the band-gap of
TiO2 (3.2 eV) [9] corresponding to the absorption of CdS in the
visible region. Cadmium sulphide has applications as photocat-
alyst [10] and non-linear optical material [11] in solar cells [12]
and in display devices [13]. The interaction of CdS nanoparticles
with bovine serum albumin in reversed micelles has been studied
by Liang Tan et al. [14].Porphyrins are common photosensitizers
2. Experimental
2.1. Materials
[
15]. Chemical reactivity between a porphyrin and other molecule
involves either direct docking/binding of the two compounds or the
formation of a reactive intermediate (e.g., proton, activated oxy-
gen, etc.) that moves between the two non-complexed molecules
Protoporphyrin IX (PPIX), meso-tetrakis (4-methoxyphenyl)
porphyrin (TMeOPP) and Zinc(II) meso-tetraphenylporphyrin
ZnTPP) were obtained form Sigma and used without further
(
[
16]. The spectrophotometric characteristics of the porphyrin are
purification. Cadmium chloride (CdCl ) purchased from Fluka was
2
ꢀ
used as such. N,N -dimethylformamide (DMF) was distilled under
diminished pressure prior to use, and hydrogen sulphide gas
was generated in a Kipp’s apparatus from ferrous sulphide and
hydrochloric acid.
∗ Corresponding author. Tel.: +91 431 2407053; fax: +91 431 2407045.
E-mail address: rrengas@gmail.com (R. Renganathan).
1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2008.05.011

1508
M.A. Jhonsi et al. / Spectrochimica Acta Part A 71 (2008) 1507–1511
2.2. Instrumentation
2.3. Preparation of CdS colloids
2
.2.1. Steady state and time resolved measurements
The steady state fluorescence quenching measurements were
CdS colloids were prepared by the procedure reported by
Khanna et al. [23]. For the typical preparation, 0.02 M DMF solution
of CdCl2 was taken in a round bottom flask and hydrogen sulphide
gas was passed for a few seconds with continuous stirring at room
temperature in air. The reaction mixture was stirred at the same
temperature for a few minutes to obtain colorless (<5 min) or fluo-
rescent yellow (>5 min) solutions. The solutions were analyzed by
UV–vis spectroscopy to identify the presence of nanoparticles.
The diameter of the particles determined from the relationship
carried out with JASCO FP-6500 spectrofluorometer. The excita-
tion and the emission wavelengths for all porphyrins are shown
in Table 1. The excitation and emission slit width (each 5 nm) and
scan rate (500 nm/min) were kept constant for all the experiments.
Absorption spectra were recorded using Cary 300 UV–vis spec-
trophotometer. Fluorescence lifetime measurements were carried
out in a picosecond time correlated single photon counting (TCSPC)
technique.
between band-gap shift (ꢁE ) and radius (R) of quantum size par-
g
ticles using the following equation.
2
.2.2. Cyclic voltammetric measurements
In the present study, the oxidation potential of TMeOPP, ZnTPP
ꢀ
ꢁ
ꢂ2h2
1
me∗
1
mh∗
1.8e2
εR
Polarisation
terms
ꢁEg =
+
−
+
(1)
R2
2
and PPIX were measured in DMF with tetrabutylammonium per-
chlorate (TBAP, 0.1 M) as electrolyte. The experimental setup
consisted of a platinum working electrode, a glassy carbon counter
electrode and a silver reference electrode. Reversible peak poten-
tials were measured at different scan rates (0.05 V/s). All samples
were deaerated by bubbling with pure nitrogen gas for ca. 5 min at
room temperature.
where h is Planck’s constant, R is the radius of the particle, me* and
−
+
m * are the effective masses of the e and h , respectively, in the
h
semiconductor, e is the electron charge, ε is the relative permittivity
of the semiconductor.
We used this equation to estimate the size of colloidal CdS
nanoparticles. A value of 0.153 me was used for the reduced effec-
tive mass of the exciton (1/␮ = 1/me + 1/m ) of CdS, the coloumbic
and polarisation terms in the equation are neglected. The particle
size of the prepared colloidal CdS is 3.35 nm.
h
Table 1
Absorption (ꢀex) and emission (ꢀem) wavelengths of the porphyrins and appar-
ent association constant (Kapp) of porphyrins with colloidal CdS calculated from
absorption and fluorescence study
3. Results and discussion
Kapp × 105
M
−1
S. No.
Sensitizer
ꢀex (nm)
ꢀem (nm)
Absorbance
Fluorescence
3.1. Absorption characteristics
1
2
3
ZnTPP
PPIX
TMeOPP
425
407
421
606
634
657
5.58
4.34
4.51
1.60
0.67
0.64
Fig. 1 shows the normalized absorption and emission spectra
of the porphyrins (TMeOPP, PPIX and ZnTPP). The absorption and

M.A. Jhonsi et al. / Spectrochimica Acta Part A 71 (2008) 1507–1511
1509
Fig. 1. Normalized absorption and emission spectra of porphyrins in DMF (1, 2,
are absorption and 1a, 2a, 3a are emission spectra of PPIX, ZnTPP and TMeOPP,
respectively).
Fig. 4. Absorption spectrum of ZnTPP (ꢀex 421 nm) in the presence of colloidal CdS
−3
3
in the concentration range of 0–5 × 10 M in DMF. The insert is the straight line
dependence of 1/A –A0 on the reciprocal concentration of CdS colloid.
obs
We can express equilibrium between the adsorbed and unad-
sorbed porphyrins by the following equation,
Kapp
Porphyrin + CdS ꢀ Porphrin · · · CdS
(2)
(3)
[
Porphyrin · · · CdS]
Porphyrin] · [CdS]
Kapp =
[
By the well known Benesi and Hildebrand method [25] we can cal-
culate the apparent association constant Kapp using the following
equation:
Aobs = (1 − ˛)C0εporphyrinl + ˛C εcl
(4)
0
where Aobs is the observed absorbance of the solution containing
different concentrations of colloidal CdS at 421, 407 and 425 nm
(the absorbance band maximum of the porphyrins TMeOPP, PPIX
and ZnTPP, respectively); ˛ is the degree of association between
porphyrins and CdS; εporphyrin and εc are the molar extinction coef-
ficients at the defined wavelengths for porphyrins and the formed
complex, respectively. Eq. (4) can be expressed as Eq. (5), where
the A0 and Ac are the absorbances of porphyrins and the complex
at 421, 407 and 425 nm, respectively, with the concentration of C0:
Fig. 2. Absorption spectrum of TMeOPP (ꢀex 421 nm) in the presence of colloidal
CdS in the concentration range of 0–5 × 10 M in DMF. The insert is the straight
line dependence of 1/Aobs–A0 on the reciprocal concentration of CdS colloid.
−3
emission wavelengths are shown in Table 1. To study the dye-
sensitized electron transfer reaction in the excited state, it is very
important to know the type of interaction of the dye molecule when
they adsorb on the nanoparticle surface. Figs. 2–4 show the steady
state absorption spectra of porphyrins in DMF with colloidal CdS
solution at different concentrations. It has been observed that with
increasing CdS concentration, the absorbance increases, and there
is neither a spectral shift nor a new peak was noticed. These changes
in the absorption spectral features clearly indicate that the por-
phyrins adsorbed on the surface of colloidal CdS to form a ground
state complex. Similar adsorption of sensitizers on colloidal CdS
surface was reported earlier [24]. In the present system porphyrins
exists in two forms, unadsorbed porphyrin monomer and adsorbed
porphyrin · · · CdS.
Aobs = (1 − ˛)A + ˛Ac
(5)
0
At relatively high CdS concentrations, ˛ can be equated to (Kapp
[CdS])/(1 + Kapp[CdS]). In this case, Eq. (5) can be expressed as fol-
lows:
1
1
1
=
+
(6)
Aobs − A0
Ac − A0
Kapp(Ac − A )[CdS]
0
Therefore, if the enhancement of absorbance at 421, 407 and
25 nm was due to absorption of complex, one would expect a
4
linear relationship between 1/(Aobs − A ) and the reciprocal con-
0
centration of colloidal CdS with a slope equal to 1/Kapp(Ac − A )
0
and an intercept equal to 1/(Ac − A ). The inset of Figs. 2–4 shows
0
the plot for porphyrins, there is a good linear dependence of
1
/(Aobs − A ) on the reciprocal concentration of colloidal CdS. The
0
calculated apparent association constant (Kapp) values from the
straight line are shown in Table 1.
3.2. Fluorescence quenching of porphyrins by CdS colloids
The effects of CdS colloids on the fluorescence spectra of por-
phyrins were examined. Fig. 5 shows the effect of increasing
concentration of CdS colloids on the fluorescence spectrum of
ZnTPP. The other two porphyrins such as TMeOPP and PPIX also gave
the similar type of fluorescence behaviour (not shown here). As
seen, addition of CdS colloids to the solution of porphyrins results
in the quenching of fluorescence intensity. The Stern–Volmer plot
Fig. 3. Absorption spectrum of PPIX (ꢀex 407 nm) in the presence of colloidal CdS
−3
in the concentration range of 0–5 × 10 M in DMF. The insert is the straight line
dependence of 1/A –A0 on the reciprocal concentration of CdS colloid.
obs

1510
M.A. Jhonsi et al. / Spectrochimica Acta Part A 71 (2008) 1507–1511
−
6
Fig. 5. Fluorescence quenching of ZnTPP (1 × 10 M) with CdS colloids in the con-
Scheme 1. Diagram illustrating the energetics of porphyrins and colloidal CdS.
−3
centration range of 0–5 × 10 M in DMF. The insert is the straight line dependence
of 1/I0–I on the reciprocal concentration of CdS colloid.
This inference can be further defined by the difference between
the oxidation potential of the porphyrins and the energy level of the
conduction band potential of CdS. With the use of porphyrins oxi-
dation potential and the excited state energy (calculated from the
fluorescence maximum wavelength of the porphyrins [27]), accord-
ing to the equation, E
+
= E
+
− Es, one can obtain the oxidation
s∗/s
s/s
potential of excited singlet of porphyrins shown in Table 2. Since
these levels are energetically higher than the energy level of the
conduction band of CdS (−1.0 V vs NHE) [28], the electron-transfer
path in Eq. (7) is impossible thermodynamically (shown below in
Scheme 1).
So the fluorescence quenching may be due to the formation
of complex which has no fluorescence as shown in the following
equation.
Porphyrin + CdS ꢀ [Porphyrin· · ·CdS]
(8)
Fig. 6. Stern–Volmer plot for the quenching of porphyrins by colloidal CdS in the
concentration range of 0–5 × 10− M in DMF.
3
The observed fluorescence quenching is due to the non-fluorescent
nature of complexed porphyrins with colloidal CdS and the flu-
orescence only from the uncomplexed porphyrins. Based on the
fluorescence quenching data we can calculate the apparent associ-
ation constant (Kapp) for the equilibrium between unadsorbed and
adsorbed porphyrins using the method reported by Kamat and Fox
between I /I vs [CdS] gave upward curvature (Fig. 6) indicated that
0
the static nature of quenching occurred in these systems.
The quenching of fluorescence emission may be attributed to
the electron transfer, energy transfer or the formation of certain
complex which has no fluorescence. The band-gap energy of CdS
semiconductor (Eg = 2.6 eV) is greater than the excited state energy
of porphyrins (Es) in Table 2 [26,27] and no fluorescence emis-
sion of the porphyrins can be absorbed by the colloidal CdS. Thus,
energy transfer from the excited state of porphyrins to colloidal CdS
is impossible. It can therefore be concluded that the fluorescence
quenching shown in Fig. 5 should not be caused by energy trans-
fer.The other possible mechanism of quenching of porphyrins by
colloidal CdS is electron transfer between excited singlet of por-
phyrins and the conduction band of CdS shown in the following
equation.
[
28] and the calculated values are shown in Table 1.
3.3. Fluorescence lifetime measurements
Fluorescence lifetime measurement is useful in understanding
the interaction between the colloidal semiconductor–dye systems.
In general, the measurement of fluorescence lifetime is the most
definitive method to distinguish static and dynamic quenching [29].
In the present work we have studied the effect of CdS colloids on
the fluorescence lifetime of porphyrins, Table 3 shows the fluores-
cence lifetime of porphyrins with different concentrations of CdS
colloids. Fig. 7 shows the fluorescence decay of TMeOPP with and
without CdS colloids (the decay for other two porphyrins are as
same as TMeOPP, not shown here). The fluorescence of porphyrins
exhibit biexponential decay not only in the dilute solutions but also
in the presence of CdS colloids. While increasing the concentration
Porphyrin + CdS → Porphyrin+ + CdS(e−)
∗
(7)
Table 2
Photophysical properties of porphyrins such as fluorescence lifetime (ꢃ0), excited
state energy (Es), ground (Es/s
+
) and excited state oxidation potential (E
s∗/s+
)
S. No.
Porphyrins
ꢃ0 (ns)a
Es (eV)b
E
+
(V)c
E
s∗/s+
(V)d
s/s
1
2
3
ZnTPP
PPIX
TMeOPP
1.89
13.4
9.2
2.04
1.95
1.87
1.06
1.01
1.04
−0.98
−0.94
−0.83
Table 3
a
Fluorescence lifetime of porphyrins at different concentrations of CdS colloids
(0–5 × 10− M) in dimethyl formamide
3
a
[CdS] (×10−3 M)
Obtained from time-resolved measurements.
Singlet state energy of the porphyrins calculated from the fluorescence maxi-
ZnTPP
TMeOPP
PPIX
b
0
1
2
3
4
1.89
1.89
1.88
1.89
1.89
9.2
9.2
9.1
9.2
9.11
13.4
mum wavelength based on the reported method [27].
c
13.71
13.70
13.4
The oxidation potentials are in DMF vs NHE from cyclic voltammetric measure-
ments.
d
Calculated from the equation, E
+
= E
+
− Es, where E
s/s
+
is the oxidation
is the oxidation potential of the
s∗/s
s/s
13.33
potential of the ground state porphyrins and E
singlet excited state porphyrins and Es is the excitation energy.
+
s∗/s
a
Lifetime given in nanoseconds.

M.A. Jhonsi et al. / Spectrochimica Acta Part A 71 (2008) 1507–1511
1511
trofluorimeter facility in the School of Chemistry, Bharathidasan
University, Trichy, and Dr. S. Anandan (NIT, Trichy) and Dr. R. Rama-
raj (MKU, Madurai) for providing their CV facilities. We are thankful
to Prof. P. Ramamoorthy, NCUFP, University of Madras, Chennai, for
lifetime measurements.
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R.R. thank CSIR (Ref: 01(2217)/08/EMR-II, dt. 06-05-2008) for
the project. Authors also thank DST-FIST and UGC-SAP for spec-
1