Molecular recognition via hydroquinone-quinone pairing: Electrochemical and singlet emission behavior of [5,10,15-triphenyl-20-(2,5-dihydroxyphenyl)porphyrinato]zinc(II)-quinone complexes

Full text of DOI:10.1021/ja9526271

J. Am. Chem. Soc. 1996, 118, 923-924
923
Molecular Recognition via Hydroquinone-Quinone
Pairing: Electrochemical and Singlet Emission
Behavior of [5,10,15-Triphenyl-20-(2,5-dihydroxy-
phenyl)porphyrinato]zinc(II)-Quinone Complexes
Francis D’Souza
Department of Chemistry
The Wichita State UniVersity
Wichita, Kansas 67260-0051
ReceiVed August 3, 1995
A popular methodology in the line of biomimetic chemistry
for constructing porphyrin-quinones to study photoinduced
electron transfer (PET) reactions involves the formation of
noncovalently linked molecular systems. However, very few
investigations have appeared in the literature,1 mainly due to
the associated synthetic difficulties. Ogoshi and co-workers1f-h
utilized two- and four-point H-bonding strategy, wherein the
phenolic groups of the aromatic substituents on the porphyrin
ring were used for the construction of noncovalently bound
cofacial porphyrin-quinones. More recently, Sessler and co-
workers1b-d reported a sidewise-linked porphyrin-quinone
system with a donor and acceptor separation of nearly 12 Å
via base pairing. In this communication, we report yet another
novel approach for forming noncovalently linked donor-
acceptor complexes by pairing the “hydroquinone-quinone”
entities, as shown in Chart 1.
The hydroquinone-quinone redox couple, which is involved
in biological electron transport,2 is also of special interest in
the present study, since this strongly interacting pair would result
in the formation of an altogether different redox couple, thus
altering the energetics of the PET reaction originating from the
photoexcited porphyrin. Moreover, in the present model, a small
separation between the donor porphyrin and the acceptor
hydroquinone-quinone has been achieved3 for this sidewise-
linked system, thus facilitating a strong electronic coupling
between them.
Figure 1. Cyclic voltammograms of (a) 1, (b) 1 + tetraflurobenzo-
quinone (10 equiv), and (c) 1 + DDQ (1.5 equiv) in benzonitrile
containing 0.1 M TBAP. A strong one-electron reduction in the potential
range 0.4 to -0.4 V is seen in (b), corresponding to the presence of 10
equiv of tetrafluorobenzoquinone in solution (see Table 1).
Chart 1
The hydroquinone-appended porphyrin, [5,10,15-triphenyl-
20-(2,5-dihydroxyphenyl)porphyrinato]zinc(II) (1), was prepared
according to the method of Rothemund and Mennotti4 by
condensing pyrrole (4 mM), 2,5-dihydroxybenzaldehyde (1
mM), and benzaldehyde (3 mM) in propionic acid, followed
by zinc(II) insertion.5 The compound 1 thus obtained was
purified several times on basic alumina and by thin-layer
chromatography.6 It is observed that addition of 10 equiv of
easily reducible quinones, such as 2,6-dichloro-3,5-dicyanao-
1,4-benzoquinone (DDQ), to the solution of 1 results in a small
blue shift of 2-3 nm for both the Soret and the visible bands
without any dramatic changes in the ꢀ values, thus indicating
the absence of π-π interactions.1f The absence of π-π
interactions is further confirmed by the 1H NMR spectrum of 1
in the presence of 5 equiv of DDQ in CDCl3.1fg,7 The aromatic
protons of the â-pyrrole and the triphenyl entities experience a
downfield shift of <0.1 ppm, while the resonance corresponding
to the OH protons of the hydroquinone entity are broadened in
addition to experiencing a greater (∼0.5 ppm) downfield shift,
thus confirming the hydroquinone-quinone pairing.8
Electrochemical studies also provide evidence for the forma-
tion of a stable hydroquinone-quinone complex, and it is
possible to evaluate its redox potentials within the molecule.
Figure 1a shows the cyclic voltammogram9 of 1 in benzonitrile
containing 0.1 M TBAP. The expected two ring-centered
oxidations of porphyrins are easily detectable in 1. These two
oxidations are located at E1/2 ) 0.89 and 1.21 V vs SCE
(processes I and II), with a potential separation, ∆E1/2, of 310
mV. This ∆E1/2 agrees well with a 320 mV difference reported
(1) (a) Tecilla, P.; Dixon, R. P.; Slobodkin, G.; Alavi, D. S.; Waldeck,
D. H. J. Am Chem. Soc. 1990, 112, 9408. (b) Harriman, A.; Magda, D. J.;
Sessler, J. L. J. Chem. Soc., Chem. Commun. 1991, 345. (c) Harriman, A.;
Magda, D. J.; Sessler, J. L. J. Phys. Chem. 1991, 95, 1530. (d) Harriman,
A.; Kubo, Y.; Sessler, J. A. J. Am. Chem. Soc. 1992, 114, 388. (e) Turro,
C.; Chang, C. K.; Leroi, G. E.; Cukier, R. I.; Nocera, D. G. J. Am. Chem.
Soc. 1992, 114, 4013. (f) Aoyama, Y.; Asakawa, M.; Matsui, Y.; Ogoshi,
H. J. Am. Chem. Soc. 1991, 113, 6233. (g) Hayashi, T.; Miyahara, T.;
Hashizume, N.; Ogoshi, H. J. Am. Chem. Soc. 1993, 115, 2049. (h) Sessler,
J. L.; Wang, B.; Harriman, A. J. Am. Chem. Soc. 1995, 117, 704.
(2) (a) Vervoort, J. Curr. Opin. Struct. Biol. 1991, 1, 889. (b) Walker, J.
E. Q. ReV. Biophys. 1992, 25, 253.
(3) The center-to-center distance estimated from the CPK models for
the complex in Chart 1 is about 6 Å, while the edge-to-edge distance is
<1.5 Å.
(4) Rothemund, P.; Mennotti, A. R. J. Am. Chem. Soc. 1941, 63, 267.
(5) The free base porphyrin was converted to its zinc(II) complex by
treatment with Zn(OAc)2‚H2O.
(6) 1H NMR in CDCl3: δ 8.84 (m, 8H, pyrrole-H), 8.20 (m, 6H, meta-H
of the triphenyl entity), 7.75 (m, 9H, ortho- and para-H of the triphenyl
entity), 7-7.3 (m, 4H, substituted phenyl-H), 4.75 (s, 2H, phenolic-OH).
UV-vis in PhCN: λ (nm) (log ꢀ) 428 (5.81), 556 (4.32), and 598. The
FAB mass spectrum revealed the molecular ion peak at m/z ) 709 (calcd
709.4).
(7) (a) Fulton, G. P.; LaMar, G. N. J. Am. Chem. Soc. 1976, 98, 2119;
2124. (b) Hill, H. A. O.; Sadler, P. J.; Williams, R. J. P. J. Chem. Soc.,
Dalton Trans. 1973, 1663.
(8) The formation constant for hydroquinone-quinone is reported to be
0.083 mol-1 (see Eggins, B. R.; Chambers, J. Q. J. Electrochem. Soc. 1970,
117, 186).
0002-7863/96/1518-0923$12.00/0 © 1996 American Chemical Society

924 J. Am. Chem. Soc., Vol. 118, No. 4, 1996
Communications to the Editor
Table 1. Reduction Potentials for the Quinone and
the reduction potential for the hydroquinone-quinone entity
formed by complexing the hydroquinone entity of 1 with
different quinones, along with the reduction potentials of the
free quinones. As expected, the observed reduction potentials
for the hydroquinone-quinone entity are found to be dependent
on the reduction potential of the free quinone used for pairing.
Hydroquinone-Quinone Entitiesa and the Optimum Concentration
of Quinone Required To Saturate the Fluorescence Intensity of 1b
reduction potentialc
[QQ′H2]/
optimum
quinone
Q/Q•- Q•-/Q2- [QQ′H2]- concn (mM)
Dramatic changes in the fluorescence spectrum of 1 have been
observed on addition of quinones. In benzonitrile, compound
1 fluoresces at 612 and 652 nm. Addition of DDQ (5 equiv)
to a solution of 1 quenches the fluorescence intensity to over
90% of its original value; further addition leads to a point where
the fluorescence intensity does not change significantly. Control
experiments using (TPP)Zn show that addition of DDQ (10
equiv) decreases the fluorescence intensity by <3%. Further,
a series of quinones which differ in their reduction potentials
have also been employed, and Table 1 lists the optimum
concentration of quinone required to saturate the fluorescence
intensity. At the saturation point, the fluorescence intensity of
1 is around 10-15% and does not show any specific trend. An
examination of Table 1 indicates that the concentration of the
quinones required to reach the saturation is governed by the
reduction potential of the hydroquinone-quinone entity, which
in turn depends on the reduction potential of the quinone. This
observation is suggestive of an electron transfer quenching
mechanism, since it is known that the concentration of quencher
needed depends on its reduction potential for an electron transfer
mechanism.14 Alternatively, the different equilibrium processes
involved may also be responsible for the observed behavior.
However, as discussed below, the results on lifetime measure-
ments indicate that the electron transfer process is the most likely
quenching mechanism.
Lifetime measurements15 using single photon counting tech-
nique revealed that the photoexcited 1 decays monoexponen-
tially, with a lifetime of 2.4 ns in deaerated benzonitrile. The
quinone-hydroquinone complex formed by treating 1 with
either DDQ or tetrafluorobenzoquinone just above their optimum
concentrations needed to saturate the fluorescence intensity
exhibited dramatic quenching of excited 1. The decay of 1
bound to DDQ could be explained in terms of a three-
exponential decay, with lifetimes of 0.10 (amplitude, 81%), 0.74
(0.16%), and 2.43 ns (3%), while a biexponential decay with
lifetimes of 3.06 (24%) and 0.08 ns (76%) gave a satisfactory
fit in the case of tetrafluorobenzoquinone-bound complex. The
average lifetime of 1 complexed to DDQ is 0.25 ns, which is
89% smaller than the lifetime of the uncomplexed 1. Similarly,
the average lifetime of 1 is reduced by 67% on pairing it with
tetraflurobenzoquinone. A comparison of these results with the
measured reduction potentials for the hydroquinone-quinone
entity (Table 1) indicates that the efficiency of lifetime quench-
ing depends on the reduction potentials (reduction of the
hydroquinone-quinone entity in DDQ-derived complex is 180
mV easier than in the tetrafluorobenzoquinone-derived com-
plex). These results are indicative of an electron transfer
quenching mechanism. In general, the fluorescence lifetime of
1 on complexing with either DDQ or tetraflurorobenzoquinone
is dramatically reduced, indicating an efficient dynamic quench-
ing process in a porphyrin-bearing hydroquinone-quinone redox
couple.
DDQ
0.56
0.25
-0.31d
-0.49d
-0.47d
-0.82
-1.26
-1.33
e
0.15
0.86
0.87
1.20
1.21
1.30
3.03
3.44
tetrafluorobenzoquinone -0.03 -0.72
tetrachlorobenzoquinone -0.05 -0.73
dichloronaphthaquinone -0.47 -1.09
1,4-benzoquinone
-0.55 -1.12
methyl-1,4-benzoquinone -0.56 -1.10
naphthaquinone
2-methyl-1,4-
-0.72 -1.20
-0.73 -1.16
e
naphthaquinone
duroquinone
2-methylanthraquinone
2-ethylanthraquinone
-0.84 -1.42c
-0.92 -1.43
-0.97 -1.45
e
e
e
4.08
5.35
5.76
a Formed by complexing the quinone with 1. b The concentration of
1 employed for the fluorescence measurements was 20 µM. c In
benzonitrile containing 0.1 M tetra-n-butylammonium perchloride. The
potential values for quinone reductions agree with the literature values
(see ref 12e). Epc at 0.1 V/s. e Reduction waves could not be isolated
d
due to the overlap of porphyrin ring reductions.
for (meso-tetraphenylporphyrinato)zinc(II) [(TPP)Zn].10 A third
oxidation process is also observed at Epa ) 0.83 V vs SCE
(process III in Figure 1a), and this wave has been ascribed to
the oxidation of the appended hydroquinone entity of 1.11
Addition of quinone to the solution of 1 decreases the current
for oxidation corresponding to the hydroquinone, and a new
oxidation process is observed in the potential range of 1.30-
1.40 V, depending upon the type of quinone used for pairing.
Figure 1b,c illustrates the cyclic voltammograms obtained in
the presence of 10 equiv of tetrafluorobenzoquinone and 1.5
equiv of DDQ in a solution of 1 in benzonitrile. In the latter
case, process III, corresponding to the oxidation of the hydro-
quinone, has completely vanished, and a new quasi-reversible
wave, corresponding to the oxidation of the hydroquinone-
quinone entity, is observed at E1/2 ) 1.40 V vs SCE (process
IV in Figure 1c).12,13 On complexing with quinone, the
porphyrin ring oxidations (process I and II) experience a small
anodic shift of 20 mV due to the electron-withdrawing nature
of the neighboring hydroquinone-quinone. Similar observa-
tions have also been made with tetrafluorobenzoquinone, but
in this case, a complete conversion using 10 equiv of quinone
was not possible; however, the overall redox behavior remains
the same. Changing the direction of the anodic scan shows
additional waves (process VII) due to the secondary chemical
reactions of the species produced at the electrode surface.10,12
It has also been possible to obtain the reduction potential of
the hydroquinone-quinone unit during the cathodic scan of the
potential.12c,d These processes are located at Epa ) -0.49 and
-0.31 V vs SCE for tetraflurobenzoquinone and DDQ com-
plexes respectively (process V in Figure 1b,c). Table 1 lists
(9) Cyclic voltammograms were obtained with an IBM Model EC 225
voltammetric analyzer or on a EG&G Model 263A potentiostat using a
three-electrode system. A platinum button or glassy carbon electrode was
used as the working electrode. A platinum wire served as the counter
electrode, and a saturated calomel electrode (SCE) was used as the reference
electrode. The ferrocenium/ferrocene redox couple was used as the internal
standard for potentials, and in 0.1 M TBAP-benzonitrile, its E1/2 value
was 0.45 V vs SCE.
Acknowledgment. The author is thankful to Dr. Karl M. Kadish
for helpful discussions and also to the Wichita State University for
startup grants. Thanks are also due to the Center for Fast Kinetic
Research, Austin, TX, permitting the use of the picosecond laser facility.
(10) (a) Kadish, K. M. Prog. Inorg. Chem. 1986, 34, 435. (b) Davis, D.
G. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978;
Vol. V, Chapter 4.
(11) Hydroquinone in PhCN, 0.1 M TBAP, undergoes a two-electron
irreversible oxidation at Epa ) 1.08 V vs SCE (see refs 12a-d).
(12) (a) Hammerich, O.; Parker, V. D. Acta Chem. Scand. 1982, B36,
63. (b) Eggins, B. R. Discuss. Faraday Soc. 1974, 56, 276. (c) Parker, V.
D. Electrochim. Acta 1973, 18, 519. (d) Eggins, B. R.; Chambers, J. Q. J.
Electrochem. Soc. 1970, 117, 186. (e) Chambers, J. Q. In The Chemistry of
Quinonoid Compounds; Patai, S., Ed.; John Wiley and Sons: New York,
1974; Chapter 14.
JA9526271
(14) (a) Rehm, D.; Weller, A. Isr. J. Chem. 1970, 8, 529. (b) Natarajan,
L. V.; Blankenship, R. E. Photochem. Photobiol. 1983, 37, 329.
(15) A mode-locked, synchronously-pumped, cavity-dumped Rhodamine
6G dye laser was used. The samples were excited at 578 nm, and the
emission was monitored at 660 nm. Solutions containing 1 (50 µM) in the
absence and presence of quinones in degassed benzonitrile were employed.
(13) The DDQ-hydroquinone complex formed by treating an equimolar
mixture of DDQ and hydroquinone undergoes oxidation at Epa ) 1.48 V
vs SCE in benzonitrile containing 0.1 M TBAP.