A novel porphyrin based fluorescent chemosensor using a molecular recognition approach

Article abstract of DOI:10.1039/a607384k

Free-base porphyrin covalently linked to quinone exhibits enhanced fluorescence on coupling the quinone with hydroquinone, primarily due to the unfavourable conditions for an electron transfer reaction between the singlet excited porphyrin and the newly f

Full text of DOI:10.1039/a607384k

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A novel porphyrin based fluorescent chemosensor using a molecular
recognition approach
Francis D’Souza,*† Golapalli R. Deviprasad and Yi-Ying Hsieh
Department of Chemistry, The Wichita State University, Wichita, KS 67260-0051, USA
Free-base porphyrin covalently linked to quinone exhibits
enhanced fluorescence on coupling the quinone with hydro-
quinone, primarily due to the unfavourable conditions for an
electron transfer reaction between the singlet excited
porphyrin and the newly formed quinone–hydroquinone
entity.
R
n
Fluorescent chemosensors are compounds with a fluorophore,
binding site and a well-defined mechanism for communication.
In general, these molecular systems possess binding ability
coupled with photophysical–photochemical properties.1 Two
types of metal ion sensing chemosensors are known in
literature:1,2 (i) heterocycles bearing chelating arrays of donor
atoms, the intrinsic chemosensors and (ii) systems in which the
donor atoms remain insulated from the fluorophore p-system,
the conjugate chemosensors. Some studies have also reported
anion sensing fluorophores which often involve the inherent
quenching property of the analyte.1,3 On the other hand, neutral
analyte sensing devices are found to be rare due to the limited
availability of systems possessing complementary binding sites
with an electrostatic handle on them.4 Here we report a
porphyrin based fluorescent chemosensor, where H-bonds and
charge-transfer interactions are primarily responsible for stabi-
lizing the quinone–hydroquinone complex (Fig. 1) and demon-
strate enhanced fluorescence emission of porphyrin upon
coupling the quinone with hydroquinone.
The quinone appended porphyrin, 5,10,15-triphenyl-
20-(3,6-dioxocyclohexa-1,4-dienyl)porphyrin‡ 1a was synthe-
sized by the chemical oxidation of 5,10,15-triphenyl-
20-(2,5-dihydroxyphenyl)porphyrin5 using an excess of
2,6-dichloro-3,5-dicyano-1,4-benzoquinone (DDQ). The UV–
VIS spectrum of 1a in benzonitrile revealed spectral features
similar to that reported for porphyrin–quinone systems in
literature.6 Addition of hydroquinone (10 equiv.) to the solution
of 1a revealed a small red shift of ca. 2 nm without any dramatic
changes in the e values, indicating the absence of any
interactions between the added hydroquinone and the porphyrin
ring. 1H NMR studies revealed quinone–hydroquinone cou-
pling. The singlet peak corresponding to the aromatic protons of
the free hydroquinone exhibited a multiplet pattern with a net
deshielding of nearly 0.1 ppm on forming the quinone–
hydroquinone complex, while the porphyrin ring protons
experience a small shielding. The binding constant calculated
from the 1H NMR data is found to be 13.1 dm3 mol21 in CDCl3
at 25 °C.
Compound 1a is found to be weakly fluorescent (ff < 0.01)
and this has been ascribed to the photoinduced intramolecular
electron transfer from the excited singlet-state porphyrin, the
donor, to the appended quinone, the acceptor.7 Interestingly, as
shown in Fig. 2, addition of 1,4-hydroquinone to the solution of
1a exhibits a drastic enhancement of the porphyrin fluorescence
intensity§ and the two bands at 652 and 720 nm correspond to
the free-base porphyrin emission.8 After addition of 8 equiv. of
1,4-hydroquinone, the fluorescence emission reaches a max-
imum and further addition results in no appreciable changes. At
the saturation point, the calculated fluorescence quantum
yield,¶ ff is found to be 0.25 and is nearly twice that of
R
= alkyl groups
n
Fig. 1
the unsubstituted meso-tetraphenylporphyrin,8 H2TPP
(ff = 0.13).∑
Recently, we demonstrated a hydroquinone–quinone com-
plexation in hydroquinone appended zinc porphyrin with
intermolecularly added quinones.5 Efficient fluorescence
quenching of the zinc porphyrin was observed upon coupling
the hydroquinone with substituted quinones. Electrochemical
studies demonstrated that the reduction potential of the
hydroquinone–quinone complexes are negatively shifted by
500–800 mV compared to that of the free quinones used for
pairing. Based on the electrochemical reduction potentials of
the newly formed hydroquinone–quinone redox couple, the
optimum concentration of quenchers needed to saturate the
fluorescence intensity and the calculated free-energy change, an
excited-state electron transfer was suggested to be the quench-
ing mechanism.5 The present system, however, differs from the
earlier one studied, in that the quinone entity is directly attached
to the porphyrin p-system. In addition, it possesses a quinone–
hydroquinone pair with a low reduction potential,** a free-base
porphyrin with high oxidation potential†† and low energy of
singlet-singlet emission compared to that of zinc porphyrin.8
The net result of this redox tuning and altered photophysical
property is the inhibition of fluorescence quenching due to
intramolecular electron transfer. The calculated free energy
change for an electron transfer from the excited free-base
porphyrin to the quinone–hydroquinone complex, using the
Rehm and Weller equation,10 is found to be endergonic by ca.
0.33 V. As a result, the added hydroquinone upon complexing
with quinone, inhibits the excited-state electron transfer in 1a
and consequently enhances the porphyrin fluorescence emis-
sion.11
Under normal conditions, one would expect the ff of 1a in
presence of hydroquinone to be close to that of the unsubstituted
free-base porphyrin, H2TPP; however, in the present study, the
calculated ff is nearly two times that of H2TPP [see Fig. 2(h)].
This additional fluorescence enhancement could be attributed to
the mode of attachment of the quinone–hydroquinone to the
porphyrin macrocycle and the nature of the charge-transfer
interactions. Spectroscopic studies on the hydroquinone–
quinone complexes have shown substantial charge-transfer
interactions between the hydroquinone as a donor and benzo-
Chem. Commun., 1997
533

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quinone as an acceptor.12 This suggests that when the linkage is
through quinone, the partial negative charge residing on the
quinone would interact with the porphyrin p-system and might
be responsible for fluorescence enhancement. If this is correct,
then one would expect higher ff on fully reducing the quinone
of 1a instead of the partial negative charge of the quinone–
hydroquinone complex. The following electrochemical studies
suggest that this is more likely the case.
This work was supported by the National Science Foundation
under Grant No. EPS-9550487 and matching support from the
State of Kansas.
Footnotes
† E-mail: dsouza@wsuhub.uc.twsu.edu
‡ dH(CDCl3) 8.90 (m, 8 H, pyrrole-H), 8.20 (s, 6 H, ortho-H of the triphenyl
entity), 7.75 (m, 9 H, meta- and para-H), 7.29, 7.60, 8.38 (d, d and s, 3 H,
quinone-H) and 22.78 (s, 2 H, imino-H). lmax (PhCN)/nm (log e) 414.6
(5.85), 511.6 (4.31), 542.2 (3.77), 590.3 (3.75) and 649.8 (3.66).
§ Addition of excess of hydroquinone to a solution of H2TPP resulted in no
appreciable changes in the fluorescence emission.
¶ Fluorescence quantum yields were calculated using the method described
in ref. 9.
∑ Use of other alkyl substituted hydroquinones resulted in a similar
fluorescence enhancement.
** Reduction of the quinone–hydroquinone complex in PhCN occurs at
E1/2 = 21.26 V vs. SCE.
†† The first reversible oxidation of 1a in PhCN, 0.1 m TBAP occurs at
E1/2 = 1.01 V vs. SCE.
‡‡ Cyclic voltammograms were obtained on a EG & G Model 263 A
potentiostat using a three-electrode system. A glassy carbon electrode was
used as the working electrode. A platinum wire served as the counter
electrode, and Ag/AgCl was used as the reference electrode.
The cyclic voltammogram†† of 1a in PhCN exhibits four
well separated one-electron reductions (Fig. 3). The first two
processes corresponding to the reduction of the appended
benzoquinone are located at E1/2 = 20.51 and 20.93 V
respectively13 while the latter two processes corresponding to
the porphyrin ring reductions14 are located at E1/2 = 21.41 and
Epc = 21.83 V vs. SCE respectively at a scan rate of 0.1 V s21
.
In order to generate a neutral free-base porphyrin linked to
quinone anion radical, bulk electrolysis of 1a was carried out at
an applied potential of 20.75 V, a potential slightly more
negative than the first reduction potential of the appended
benzoquinone. After completion of the electrolysis, the fluores-
cence spectrum of the one-electron reduced product was
recorded [see Fig. 2(i)]; the emission intensity was nearly 10%
more than that observed for 1a in the presence of excess of
hydroquinone, suggesting that the interactions between the
partially or fully reduced quinone and the porphyrin macrocycle
could be responsible for additional fluorescence enhancement.
However, further studies are needed to fully understand the
mechanistic details of fluorescence enhancement in this novel
system and these studies are in progress.
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Received, 29th October 1996; Com. 6/07384K
534
Chem. Commun., 1997