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