Studies on porphyrin-quinhydrone complexes: Molecular recognition of quinone and hydroquinone in solution

Article abstract of DOI:10.1021/jo0100547

Free-base and zinc(II) porphyrins bearing either one, two, or four hydroquinone entities at the meso positions are shown to bind quinones in solutions via a quinhydrone pairing mechanism. Electrochemical studies reveal that the quinhydrone complexes are s

Full text of DOI:10.1021/jo0100547

J . Org. Chem. 2001, 66, 4601-4609
4601
Stu d ies on P or p h yr in -Qu in h yd r on e Com p lexes: Molecu la r
Recogn ition of Qu in on e a n d Hyd r oqu in on e in Solu tion
Francis D’Souza* and Gollapalli R. Deviprasad
Department of Chemistry, Wichita State University, 1845 Fairmount, Wichita, Kansas 67260-0051
Francis.DSouza@wichita.edu
Received J anuary 16, 2001 (Revised Manuscript Received April 23, 2001)
Free-base and zinc(II) porphyrins bearing either one, two, or four hydroquinone entities at the
meso positions are shown to bind quinones in solutions via a quinhydrone pairing mechanism.
Electrochemical studies reveal that the quinhydrone complexes are stabilized by charge-transfer
interactions between the donor (hydroquinone) and the acceptor (quinone). The redox potentials of
the quinhydrone complexes are governed by the potentials of the quinones utilized to form
quinhydrone. The 1H NMR studies reveal that the quinhydrone complexes are stabilized by
H-bonding in addition to the charge-transfer interactions. Singlet emission studies have shown
that the fluorescence quenching of the porphyrin increases with an increase in the number of
receptors, i.e., hydroquinone entities on the porphyrin macrocycle. Control experiments performed
by using zinc porphyrin bearing a dimethoxyphenyl group, i.e., a receptor entity with no H-bonding
ability, indicate that the H-bonding plays an important role in quinhydrone formation. Porphyrin-
quinhydrone formed by using covalently linked porphyrin-quinone and hydroquinone present in
solution shows fluorescence enhancement. The measured fluorescence quantum yields, φf, are found
to depend on the metal ion in the porphyrin cavity and the oxidation potential of the employed
hydroquinones. The present studies also reveal that the measured φf values depend on how the
quinhydrone is linked to the porphyrin macrocycle, i.e., either through quinone or hydroquinone.
Generally, porphyrin-quinhydrone formed by hydroquinone-appended porphyrins shows decreased
φf values as compared to porphyrin-quinhydrone formed by quinone-appended porphyrins.
In tr od u ction
standing of the naturally occurring photosynthetic elec-
tron-transfer reactions.1-3 Though porphyrin-quinone
dyads stabilized by noncovalent interactions, such as
hydrogen bonds, ion pairing, cation complexation, or van
der Waals interactions, are realized to be essential,
studies on such models with well-defined binding mech-
anisms remain limited.1-3 In addition, these nonco-
valently linked donor-acceptor dyads are highly useful
for the development of new molecular electronic devices
and optical sensors.6
There has been considerable research interest in the
synthesis of porphyrins bearing receptor entities on the
ring periphery for the development of new biomimetic
model systems.1-5 Among these, noncovalently linked
electron-transfer donor-acceptor dyads featuring por-
phyrin and quinone are important to further our under-
(1) (a) Harriman, A.; Kubo, Y.; Sessler, J . L. J . Am. Chem. Soc. 1992,
114, 388. (b) Sessler, J . L.; Wang, B.; Harriman, A. J . Am. Chem. Soc.
1995, 117, 704. (c) Sessler, J . L.; Wang, B.; Harriman, A. J . Am. Chem.
Soc. 1993, 115, 10418. (d) Harriman, A.; Magda, D. J .; Sessler, J . L.
J . Phys. Chem. 1991, 95, 1530. (e) Harriman, A.; Magda, D. J .; Sessler,
J . L. J . Chem. Soc., Chem. Commun. 1991, 345.
(2) (a) Turro, C.; Chang, C. K.; Leroi, G. E.; Cukier, R. I.; Nocera,
D. G. J . Am. Chem. Soc. 1992, 114, 4013. (b) Arimura, T.; Brown, C.
T.; Spring, S. L.; Sessler, J . L. Chem. Commun. 1996, 2293. (c) Hayashi,
T.; Asai, T.; Hokazono, H.; Ogoshi, H. J . Am. Chem. Soc. 1993, 115,
12210. (d) Hayashi, T.; Miyahara, T.; Kumazaki, S.; Ogoshi, H.;
Yoshibara, K. Angew. Chem., Int. Ed. Engl. 1996, 35, 1964. (e) Hunter,
C. A.; Hyde, R. K. Angew. Chem., Int. Ed. Engl. 1996, 35, 1936. (f)
Mizutani, T.; Kurahashi, T.; Murakami, T.; Matsumi, N.; Ogoshi, H.
J . Am. Chem. Soc. 1997, 119, 8991.
(3) (a) Ward, M. D. Chem. Soc. Rev. 1997, 26, 365. (b) Hayashi, T.;
Ogoshi, H. Chem. Soc. Rev. 1997, 26, 355. (c) Stang, P. J .; Fan, J .;
Olenyuk, B. Chem. Commun. 1997, 1453. (d) de Rege, P. J . F.;
Williams, S. A.; Therien, M. J . Science 1995, 269, 1409. (e) Piotrowiak,
P. Chem. Soc. Rev. 1999, 28, 143. (f) Blanco, M.-J .; J imenez, M. C.;
Chambron, J .-C.; Heitz, V.; Linke, M.; Sauvage, J .-P. Chem. Soc. Rev.
1999, 28, 293.
(4) For covalently linked porphyrin-quinone systems, see (a) Con-
nolly, J . S.; Bolton, J . R. In Photoinduced Electron Transfer; Fox, M.
A., Channon, M., Eds.; Elsevier: Amsterdam, The Netherlands, 1988;
Part D. (b) Gust, D. Nature (London) 1997, 386, 21. (c) Gust, D.; Moore,
T. A.; Moore, A. L. Acc. Chem. Res. 1993, 26, 198. (d) Wasielewski, M.
R. Chem. Rev. 1992, 92, 435. (e) Kurreck, H.; Huber, M. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 849. (f) Gust, D.; Moore, T. A. In The Porphyrin
Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic
Press: New York, 2000; Vol. 8, Chapter 57.
Quinhydrone, a 1:1 quinone-hydroquinone, is a well-
known molecular charge-transfer complex (Scheme 1).7,8
In this complex, the charge-transfer interactions between
the electron donor (hydroquinone) and the electron ac-
ceptor (quinone) primarily stabilize the complex while the
(5) For general reviews, see (a) Linton, B.; Hamilton, A. D. Chem.
Rev. 1997, 97, 1669. (b) Conn, M. M.; Rebek, J ., J r. Chem. Rev. 1997,
97, 1647. (c) Zeng, F.; Zimmerman, S. C. Chem. Rev. 1997, 97, 1681.
(d) Lehn, J .-M. Angew. Chem., Int. Ed. Engl. 1990, 29, 1304. (e) Lehn,
J .-M. In Supramolecular Chemistry: Concepts and Perspectives;
VCH: Weinheim, 1995.
(6) (a) Feldheim D. L.; Keating, C. D. Chem. Soc. Rev. 1998, 27, 1.
(b) Introduction of Molecular Electronics; Petty, M. C., Bryce, M. R.,
Bloor, D., Eds.; Oxford University Press: New York, 1995. (c) Emme-
lius, M.; Pawlowski, G.; Vollmann, H. W. Angew. Chem., Int. Ed. Engl.
1989, 28, 1445. (d) Hopfield, J . J .; Onuchic, J . N.; Beratan, D. N. J .
Phys. Chem. 1989, 93, 6350. (e) Lakowica, J . R. Principles of Fluores-
cence Spectroscopy, 2nd ed.; Kluwer/Plenum: New York, 1999. (f) de
Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A. J . M.;
McCoy, C. P.; Rademacher, J . T.; Rice, T. E. Chem. Rev. 1997, 97, 1515.
(g) Deviprasad, G. R.; Keshavan, B.; D’Souza, F. J . Chem. Soc., Perkin
Trans 1 1998, 3133. (h) Deviprasad, G. R.; D’Souza, F. Chem. Commun.
2000, 1915.
(7) (a) Eggins, B. R.; Chambers, J . Q. J . Electrochem. Soc. 1970,
117, 186. (b) Moser, R. E.; Cassidy, H. G. J . Am. Chem. Soc. 1965, 87,
3463. (c) Parker, V. D. Chem. Commun. 1969, 716.
10.1021/jo0100547 CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/07/2001

4602 J . Org. Chem., Vol. 66, No. 13, 2001
D’Souza and Deviprasad
Sch em e 1
ing no receptor hydroquinone or dimethoxyphenyl group,
is utilized to quantitate the intermolecular interactions
and the associated bimolecular quenching process.
The recognition of hydroquinone in solution is inves-
tigated by using covalently linked free-base porphyrin-
quinone (2a ) and zinc porphyrin-quinone (2b) dyads.
Here, the quinone substituent on the porphyrin ring
binds hydroquinone to form quinhydrone. Measurement
of the fluorescence quantum yields of the porphyrin-
quinhydrone complexes formed by various hydroquinones
monitors the molecular recognition process.
hydrogen bonds provide additional stability both in the
solid state and in solution.7,8 The formation and impor-
tance of such quinhydrone type charge-transfer com-
plexes involving electron-acceptor quinones and electron-
donor biomolecules, such as purines, pyrimidines, amino
acids, and proteins, is well-documented.8b,c Staab and co-
workers have reported on the orientation dependence of
charge-transfer interactions for intramolecular-type quin-
hydrone complexes in diastereomeric [2,2]paracyclophane
and [3,3]paracyclophane molecular systems.9
Resu lts a n d Discu ssion
Ch a r a cter iza tion of th e Solid P or p h yr in -Qu in -
h yd r on e Com p lexes. It is found that the porphyrin-
quinhydrone complexes slowly precipitate out in chloro-
form/hexane (1:1 v/v) solutions at -5 °C containing any
of 1a -d and quinones. The number of quinhydrone
entities depends on the number of hydroquinone sub-
stituents on the porphyrin periphery. That is, 1a , 1c, and
1d bearing one, two, and four hydroquinone substit-
uents, respectively, result in the formation of porphyrins
bearing one, two, and four quinhydrone entities. The
positive ion electrospray mass spectrum of the isolated
porphyrin-quinhydrone complexes in either acetonitrile
or dichloromethane matrix under mild conditions exhib-
ited the theoretically predicted molecular ion peaks.
Under the same experimental conditions, the starting
materials, i.e., 1a -d and benzoquinone, revealed molec-
ular ion peaks corresponding only to the mass of indi-
vidual compounds.
Figure 1 shows the FT-IR spectrum of 1a and 1a -
benzoquinone complex in a KBr matrix. The hydro-
quinone -OH groups of 1a appear at 3430 cm-1. After
complexation of 1a with benzoquinone, i.e., in the solid
zinc porphyrin-quinhydrone, the -OH stretching fre-
quency is lowered by about 200 cm-1 and appears at 3232
cm-1. Similarly, the carbonyl stretching frequency of the
bound quinone in zinc porphyrin-quinhydrone appears
at 1632 cm-1 (Figure 1b). That is, it is lowered by 30 cm-1
as compared to the carbonyl stretching frequency of
either benzoquinone or quinone-appended porphyrin, 2a .
Similar spectral trends have been observed for porphy-
rin-quinhydrone complexes formed by either 1c or 1d
and hydroquinone as well as by 2a and hydroquinone.
These results indicate formation of quinhydrone that is
quinone-hydroquinone complex12 both in hydroquinone-
appended porphyrins as well as quinone-appended por-
phyrins.
Recently, we reported a novel molecular recognition
route by using the quinhydrone pairing approach to form
noncovalently linked porphyrin-acceptor complexes and
fluorescent chemosensors.10,11 In our approach, porphy-
rin-quinhydrone involving either hydroquinone-append-
ed porphyrin and quinone present in solution10 or quinone-
appended porphyrin and hydroquinone present in solution
were formed.11 Quenching of singlet excited porphyrin
was observed in a hydroquinone-linked zinc porphyrin
after formation of hydroquinone-quinone complex.10
Moreover, it was possible to control the extent of excited-
state quenching by using a quinone-appended free-base
porphyrin and hydroquinone present in solution.11 In the
present study, we have performed extensive studies on
this molecular recognition approach by using porphyrins
bearing one, two, or four hydroquinone or quinone
entities (Schemes 2 and 3) and report on the mechanistic
details of quinhydrone formation in porphyrin systems.
The role of various quinones and the importance of
hydrogen bonding in stabilizing the quinhydrone are also
investigated. In contrast to the earlier reports,8b,c the
present study demonstrates that the hydrogen bonding
is a major factor in stabilizing quinhydrone in solution.
The rationale for the different compounds (Schemes 2
and 3) employed in the present study is as follows:
Compounds 1a and 1b are employed to probe the effect
of the metal ion in the porphyrin cavity on the free-energy
change and the resulting fluorescence quenching of the
excited porphyrin. The porphyrins bearing two and four
hydroquinone receptors, 1c and 1d , are employed to
visualize the effect of multiple receptors on a single
porphyrin ring and its effect on fluorescence quenching.
Compound 1e, bearing a dimethoxyphenyl substituent,
i.e., a substituent capable of forming quinhydrone sta-
bilized only by charge-transfer interactions and not by
H-bonding, is used to understand the role of H-bonding.
Compound 1f, meso-tetraphenylporphyrinatozinc(II), bear-
1H NMR Stu d ies. The 1H NMR studies have been
performed to verify the existence of H-bonding between
the hydroquinone -OH groups of 1a and the carbonyl
groups of the quinone entity in solution. Representative
spectral changes observed on addition of 2-methyl an-
thraquinone to a solution of 1a in benzene-d6 are shown
in Figure 2. The solvent benzene-d6 is used here to avoid
any intermolecular coordination between the -OH groups
and the zinc metal center.13 In the studied solvent, the
resonance peaks corresponding to the -OH protons of 1a
(8) (a) Foster, F. Organic Charge-Transfer Complexes; Academic
Press: New York, 1969. (b) Slifkin, M. A. Charge-Transfer Interactions
of Biomolecules; Academic Press: New York, 1971. (c) Charge-Transfer
Complexes in Biological Systems; Gutmann, F., J ohnson, C., Keyzer,
H., Molnar, J ., Eds.; Marcel Dekker: New York, 1997.
(9) (a) Rebafka, W.; Staab, H. A. Angew. Chem., Int. Ed. Engl. 1973,
12, 776; 1974, 13, 203. (b) Hausser, K. H.; Taglieber, V.; Staab, H. A.
Chem. Phys. 1976, 14, 183. (c) Staab, H. A.; Herz, C. P. Angew. Chem.,
Int. Ed. Engl. 1977, 16, 799. (d) Staab, H. A.; Zapf, U.; Gurke, A.
Angew. Chem., Int. Ed. Engl. 1977, 16, 801.
(12) (a) Slifkin, M. A.; Walmsley, R. H. Spectrochim. Acta 1970, 26A,
1237. (b) Kruk, J .; Strzalka, K.; Leblanc, R. M. Biophys. Chem. 1993,
45, 235. (c) Kubinyi, M.; Keresztury, G. Spectrochim. Acta 1989, 45A,
421. (d) Patil, A. O.; Pennington, W. T.; Desiraju, G. R.; Curtin, D. Y.;
Paul, I. C. Mol. Cryst. Liq. Cryst. 1986, 134, 279.
(10) D’Souza, F. J . Am. Chem. Soc. 1996, 118, 923.
(11) D’Souza, F.; Deviprasad, G. R.; Hsieh, Y.-Y. Chem. Commun.
1997, 533.
(13) Deviprasad, G. R.; Eichhorn, D. M.; D’Souza, F. J . Chem.
Crystallogr. 1999, 29, 849.

Studies on Porphyrin-Quinhydrone Complexes
J . Org. Chem., Vol. 66, No. 13, 2001 4603
Sch em e 2
Sch em e 3
of both the -OH proton peaks and negligibly small shift
of the porphyrin ring and the 2-methyl anthraquinone
proton peaks suggests quinhydrone formation, most
likely by forming two hydrogen bonds with the carbonyl
groups of quinone.
Interestingly, for 1c and 1d , bearing two and four
hydroquinone receptors, at least two types of binding are
possible as shown in Scheme 4 for 1c. The first type
involves quinhydrone formation like in 1a while the
second involves porphyrin-quinone π-π type stabilized
by H-bonding between the quinone carbonyl groups and
the hydroquinone ortho OH groups located at the opposite
sides of the porphyrin ring (Scheme 4b). The latter type
of binding is similar to that reported by Ogoshi et al. for
2-hydroxynaphthyl-derived porphyrins.2c If the second
type of binding were to occur, then one would expect
shielding of quinone protons due to porphyrin ring
current effects2c. The optical absorption bands of the
porphyrin are also expected to reveal spectral shifts
under these conditions. The optical absorption and 1H
NMR studies involving 1c or 1d and quinones revealed
spectral changes similar to that observed for titration of
1a with quinones, indicating a quinhydrone type of
are located at 4.00 and 4.85 ppm, respectively. The
positions of the resonance peaks corresponding to the
three phenyl and the â-pyrrole protons of 1a are located
at positions not much different from that of meso-
tetraphenylporphyrinatozinc(II).10,11 After addition of
quinone, the -OH peaks undergo a low field shift without
perturbing appreciably the peak positions of the other
porphyrin ring protons. Additionally, the resonance peak
positions of 2-methyl anthraquinone protons reveal a
small deshielding (<0.1 ppm). The larger downfield shift

4604 J . Org. Chem., Vol. 66, No. 13, 2001
D’Souza and Deviprasad
Sch em e 4
F igu r e 1. FT-IR spectrum of (a) 1a and (b) isolated 1a -
benzoquinone complex in a KBr matrix.
Sch em e 5
of the employed quinones varies between 0.56 V for
dichlorodicyanobenzoquinone (DDQ) and -0.97 V vs SCE
for 2-ethyl anthraquinone, covering a wide potential
range of 1.53 V (Table 1). The hydroquinones used to form
quinhydrone with quinone-appended porphyrins undergo
a two-electron irreversible electrooxidation process.14 The
values of the electrooxidation peak potential vary be-
tween 0.71 V for 2-methoxyhydroquinone and 1.0 V vs
Ag/AgCl for tetrachlorohydroquinone (Table 2).
Figure 3 shows cyclic voltammograms of 1a , 1a in the
presence of DDQ (1.2 equiv), and 2a in benzonitrile
containing 0.1 M (TBA)PF6. The two ring-centered elec-
trooxidations of the porphyrin macrocycle of 1a are
located at E1/2 ) 0.89 and 1.21 V vs SCE along with a
third one at Epa ) 0.83 V, corresponding to the electrooxi-
dation of the hydroquinone substituent of 1a .10 Addition
of an easily reducible quinone, DDQ, to the solution of
1a decreases the anodic peak current of the hydroquinone
oxidation accompanied by a new anodic process located
at E1/2 ) 1.40 V vs SCE. During the negative potential
scan, two new redox processes located at Epa ) -0.31 and
-0.49 V vs SCE, corresponding to electroreduction of the
quinhydrone, are also observed. To confirm that these
processes are due to the quinhydrone reduction and not
to quinone formed by chemical oxidation of the hydro-
quinone of 1a by DDQ, control experiments involving 2a ,
i.e., quinone-appended zinc porphyrin, are performed. As
shown in Figure 3c, the voltammograms of 2a are
different from those seen in Figure 3a,b. That is, only
two reversible anodic processes corresponding to the
F igu r e 2. 1H NMR spectrum of 1a (a) in the absence and (b)
in the presence of 1 equiv of 2-methyl anthraquinone in
benzene-d6. The origin of the spikelike peak at 3.9 ppm is not
clear.
binding. The observed deshielding of both the -OH
protons of the hydroquinone entities further supports the
quinhydrone-type binding as shown in Scheme 4a.
E lect r och em ist r y of P or p h yr in -Qu in h yd r on e
Com p lexes. The redox potentials of the charge-transfer
stabilized quinhydrone govern the overall stability and
energetics of the porphyrin-quinhydrone complexes.
Therefore, electrochemical study of the porphyrin-quin-
hydrone complexes leading to determination of the redox
potentials is important. Quinhydrone undergoes a two-
electron, two-proton electrochemical reduction ultimately
resulting in the formation of hydroquinone (Scheme 5).7
A complete characterization of the intermediates involved
in the electrochemical process is difficult due to the
associated electrochemical and chemical complications.7
In the present investigation, however, it has been possible
to determine the formal redox potentials of a few quin-
hydrone complexes by performing a systematic study
involving control experiments.
The quinones used to form quinhydrone with hydro-
quinone-appended porphyrins are listed in Table 1. They
undergo two one-electron reversible electroreductions in
nonaqueous solvent solutions.14 In benzonitrile contain-
ing 0.1 M (TBA)ClO4, values of the first redox potential
(14) Chambers, J . Q. In The Chemistry of Quinonoid Compounds;
Patai, S., Ed.; Wiley: New York, 1974; Chapter 14.

Studies on Porphyrin-Quinhydrone Complexes
J . Org. Chem., Vol. 66, No. 13, 2001 4605
Ta ble 1. Stea d y-Sta te F lu or escen ce-Qu en ch in g Da ta for F u n ction a lized P or p h yr in s by Va r iou s Qu in on esa
reduction potential
V vs SCEb
[Q1/2]c, mM
quinone
Q/Q•-
Q•-/Q2-
[QQ′H2]/[QQ′H2]-
1a
1b
1c
1d
1e
1f
19
62
68
241
288
350
540
585
800
1050
1180
dichlorodicyanobenzoquinone
tetrafluorobenzoquinone
tetrachlorobenzoquinone
dichloronaphthaquinone
1,4-benzoquinone
methyl-1,4-benzoquinone
naphthaquinone
2-methyl-1,4-naphthaquinone
duroquinone
0.56
-0.03
-0.05
-0.47
-0.55
-0.56
-0.72
-0.73
-0.84
-0.92
-0.97
0.25
-0.72
-0.73
-1.09
-1.12
-1.10
-1.20
-1.16
-1.42
-1.43
-1.45
-0.31
-0.49
-0.47
-0.82
-1.26
-1.33
0.026
0.078
0.082
0.255
0.302
0.333
0.534
0.566
0.776
1.018
1.100
0.198
1.15
1.16
1.60
2.37
2.55
6.06
6.82
8.11
0.023
0.092
0.099
0.013
0.134
0.148
0.365
0.417
0.521
0.659
0.735
0.015
0.028
0.032
0.035
0.046
0.060
0.185
0.212
0.266
0.354
0.478
1.86
5.57
5.85
18.20
21.56
23.77
38.12
40.40
55.39
72.67
78.52
2-methyl anthraquinone
2-ethyl anthraquinone
10.65
11.47
a
b
20 µM porphyrin in deaerated benzonitrile, λmax ) 556 nm. In benzonitrile; 0.1 M (TBA)PF6; 0.1 V s-1. c [Q1/2] represents the
concentration of quinone required to decrease the initial emission intensity of porphyrin by 50% according to the Stern-Volmer method;
error ) +6%.
Ta ble 2. Em ission P r op er ties of
) 1.40 V has tentatively been assigned to the oxidation
P or p h yr in -Qu in h yd r on e Com p lexes F or m ed betw een
of the newly formed quinhydrone, although it is difficult
Ben zoqu in on e-Ap p en d ed P or p h yr in sa a n d Hyd r oqu in on e
to obtain spectral evidences because of the occurrence of
Epab, V vs
φf for
φf for
electrochemical processes prior to this redox process.
Similar electrochemical behavior has been observed
during the titration of 1a in the presence of other
quinones. However, voltammetric peaks corresponding
to the quinhydrone reduction are not well-developed,
especially for quinhydrones formed by quinones of higher
reduction potentials. Additional difficulties are also
encountered due to the overlapping porphyrin ring
reductions at more negative potentials and large amounts
of quinone needed to produce measurable quantities of
quinhydrone. It may also be mentioned here that in the
presence of excess amounts of easily reducible quinones
(such as DDQ, tetrafluorobenzoquinone, or tetrachlo-
robenzoquinones), a slow oxidation of the hydroquinone
of 1a occurs to yield 2a . Therefore, determination of the
potential corresponding to the reduction of quinhydrone
has been possible only for the first six complexes (Table
1). The measured potentials exhibit a direct correlation
with the reduction potential of the employed quinones.
Generally, the quinhydrones are difficult to reduce by
200-500 mV as compared to the redox potentials of the
corresponding free quinones.
hydroquinone, H2Q
Ag/AgCl
2a :H2Q
2b:H2Q
2-methoxy hydroquinone
2,3-dimethyl hydroquinone
2-methyl hydroquinone
hydroquinone
2-chlorohydroquinone
2-phenyl hydroquinone
tetrachlorohydroquinone
ZnTPP
0.71
0.80
0.82
0.92
0.96
0.98
1.0
0.053
0.051
0.046
0.041
0.022
0.026
0.019
0.103c
0.266
0.239
0.256
0.249
0.129
0.110
0.096
H2TPP
0.130c
a
Porphyrin concentration ) 20.0 µM in deaerated benzonitrile.
c From ref
In 0.1M (TBA)PF6 benzonitrile; scan rate ) 0.1 V s-1
.
b
19.
Excited -Sta te Em ission Stu d ies. Quenching of the
porphyrin emission by quinones under inter- and in-
tramolecular reaction conditions is very well-known.4,15
Both steady-state and time-resolved emission studies
have confirmed that the quenching mainly involves
electron transfer from photoexcited porphyrin to qui-
nones.1-4,15 By using a variety of covalently linked
porphyrin-quinone dyads, mutual orientation of the
donor-acceptor entities and relevant geometrical factors
necessary for efficient electron transfer have been evalu-
ated by several research groups.4
The fluorescence quenching of 1f (a donor bearing no
receptor site) in the presence of quinones follows an
intermolecular bimolecular quenching process.15 The data
of such quenching can be analyzed by using the Stern-
Volmer equation.16
F igu r e 3. Cyclic voltammograms of (a) 1a , (b) 1a + DDQ (1.2
equiv), and (c) 2a in 0.1 M (TBA)ClO4 containing benzonitrile.
porphyrin ring oxidations and a reversible reduction
process located at E1/2 ) -0.51 V vs SCE corresponding
to the reduction of the appended quinone are observed.
These results suggest that the electrode processes located
at Epa ) -0.31 and -0.49 V in Figure 3b correspond to
the reduction of the newly formed quinhydrone. On the
basis of electrochemical redox potentials of simple quin-
hydrone in benzonitrile,10 the redox couple located at E1/2
Io/I ) 1 + Ksv[Q]
(1)
where Io and I represent fluorescence emission intensities
(15) Harriman, A.; Porter, G.; Searle, N. J . Chem. Soc., Faraday
Trans 2 1979, 75, 1515.
(16) Natarajan, L. V.; Blankenship, R. E. Photochem. Photobiol.
1983, 37, 329.

4606 J . Org. Chem., Vol. 66, No. 13, 2001
D’Souza and Deviprasad
F igu r e 5. Dependence of ln[Q1/2] on the first reduction
potential of the quinones for (a) 1a , (b) 1c, and (c) 1d .
the number of hydroquinone receptors on the porphyrin
ring. Plots of ln[Q1/2] vs E1/2 of the quenchers are found
to be linear (Figure 5). Generally, porphyrins bearing a
higher number of receptor hydroquinones undergo ef-
ficient quenching, and this has been ascribed to the
available higher number of binding sites. The dependence
of ln[Q1/2] on E1/2 also suggests that the primary quench-
ing mechanism involves photoinduced electron transfer
(PET).10,17 The PET quenching mechanism is also sup-
ported by a comparison between the [Q1/2] values obtained
for 1a and 1b. Here, the zinc porphyrin in 1a is a better
donor as compared to the free-base porphyrin in 1b
because of its lower oxidation potential18 and higher
energy of the singlet-singlet emission.19 The free-energy
change, ∆G, for PET, calculated by using the oxidation
potential of the donor, the reduction potential of the
acceptor, and the energy corresponding to the singlet-
singlet emission of the donor porphyrin, according to the
Rehm and Weller method,17 is found to be more exergonic
for reactions involving 1a than that involving 1b. Hence,
lower values of [Q1/2] are expected for 1a as compared to
1b. The calculated [Q1/2] values in Table 1 show such a
trend and support the PET-quenching mechanism.
Role of Hyd r ogen Bon d in g. The importance of
hydrogen bonding in quinhydrone formation and its role
in governing the fluorescence-quenching process has been
examined in the present study by using 1e bearing a
dimethoxyphenyl receptor entity. The results of fluores-
cence quenching, performed by using 1e, are given in
Table 1. The determined [Q1/2] values for 1e are 1 order
of magnitude smaller than that obtained for 1f (which
involves bimolecular quenching) and are 1 order of
magnitude higher than that of 1a . These results clearly
indicate that employing 1a , possessing both charge-
transfer and H-bonding abilities, markedly increases the
quenching ability of the porphyrin. Hence, the H-bonding
ability of the hydroquinone receptor is borne out to be
an important factor for efficient quenching of the por-
phyrin singlet state in the studied self-assembled via
quinhydrone pairing, donor-acceptor dyads.
F igu r e 4. Stern-Volmer plots for the fluorescence quenching
of 1a in benzonitrile by different quinones: (1) DDQ, (2)
tetrafluorobenzoquinone, (3) tetrachlorobenzoquinone, (4) dichlo-
ronaphthaquinone, (5) benzoquinone, (6) methyl benzoquinone,
(7) naphthaquinone, (8) 2-methyl naphthaquinone, (9) duro-
quinone, (10) 2-methyl anthraquinone, and (11) 2-ethyl an-
thraquinone.
of porphyrin in the absence and the presence of the
quencher, [Q], respectively; and Ksv is the Stern-Volmer
quenching constant. Ksv is equal to the product of the
bimolecular quenching constant, kq, and the excited-state
lifetime, τ, of the donor porphyrin in the absence of a
quencher. The lifetime of 1f in deareated benzonitrile
solutions by using time-correlated singlet-photon count-
ing technique is found to be 2.4 ns.10 The calculated kq
values for the bimolecular reactions involving 1f and
quinones, determined from the slopes of the Stern-
Volmer plots, are found to depend on the redox potentials
of the quenchers. The magnitude of kq varies between
21.9 × 109 M-1 for DDQ and 3.5 × 108 M-1 for 2-ethyl-
anthraquinone and are close to that expected for diffu-
sion-controlled processes.6e
Interestingly, an efficient quenching is observed when
porphyrins bearing hydroquinone receptors are em-
ployed. The Stern-Volmer plots constructed for the
fluorescence quenching of 1a -d by quinones are linear,
indicating absence of appreciable amounts of ground-
state complexation between the porphyrin π-ring and the
1
quinone, a result consistent with the H NMR results.
Representative Stern-Volmer plots for the donor 1a and
various quinones are shown in Figure 4. The quencher
concentration necessary to reduce the fluorescence in-
tensity to a minimum value is found to be nearly 2 orders
of magnitude smaller than that needed for 1f, suggesting
an intramolecular-type quenching process. To quantitate
this, we have calculated the concentration of quinones
required to quench 50% of the original fluorescence
intensity of the donor porphyrins, [Q1/2], from the Stern-
Volmer plots. The calculated values are listed in Table 1
for all the porphyrins shown in Scheme 1.
F or m a tion of Qu in h yd r on e in Qu in on e-Ap p en d -
ed P or p h yr in s. The covalently linked dyads (2a and 2b)
(17) Rehm, D.; Weller, A. Isr. J . Chem. 1970, 8, 529.
(18) Kadish, K. M.; Van Caemelbecke, E.; Royal, G. In The Porphyrin
Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic
Press: New York, 2000; Vol. 55, Chapter 1.
(19) (a) Seybold, P. G.; Gouterman, M. J . Mol. Spectrosc. 1969, 31,
1. (b) Quimby, D. J .; Longo, F. R. J . Am. Chem. Soc. 1975, 97, 5111.
From Table 1, it follows that the [Q1/2] values depend
on the redox potential of the acceptor, the quinone, and

Studies on Porphyrin-Quinhydrone Complexes
J . Org. Chem., Vol. 66, No. 13, 2001 4607
sion behavior. That is, the φf for 1a -quinone complexes
is 20-30% smaller than that of 2a -hydroquinone com-
plexes even though both have a zinc porphyrin and
quinhydrone donor-acceptor entities. Similar trends are
also observed for the free-base porphyrin containing 1b-
quinone and 2b-hydroquinone complexes.23 One possible
explanation for this behavior could be the formation of
intermediate charge-transfer states. It is likely that the
interactions between the charge-transfer stabilized quin-
hydrone [Qδ-:H2Qδ+] and porphyrin π-system alters the
porphyrin emission behavior. In the case of 2a -hydro-
quinone, the partial negative charge located on the
quinone unit of the quinhydrone entity interacts with the
porphyrin π-system thereby increasing the fluorescence
intensity.11 Similarly, when the linkage is through hyd-
roquinone, like in the case of 1a -quinone, the partial
positive charge located on the hydroquinone of the
quinhydrone entity interacts with the porphyrin π-system
thereby decreasing the fluorescence intensity. These
observations indicate that the mode of linking of quin-
hydrone to the porphyrin ring plays an important role
in controlling the photophysical properties of the por-
phyrin complex.
F igu r e 6. Dependence of φf on the oxidation potential, Epa
of the hydroquinones for (a) 2a and (b) 2b.
,
employed to investigate the quinhydrone formation are
found to be weakly fluorescent due to the occurrence of
PET from the singlet excited porphyrin to the covalently
linked quinone.11,20 Free-energy calculations have indi-
cated that the electron-transfer quenching is more fa-
vored in 2a than in 2b by about 0.42 eV. Addition of
hydroquinone to solutions of either 2a or 2b increases
the porphyrin fluorescence. Table 2 lists the fluorescence
quantum yield,21 φf, determined for 2a and 2b in the
presence of different hydroquinones at their saturation
point. For comparison, the fluorescence quantum yields
for meso-tetraphenylporphyrin, H2TPP, and ZnTPP, 1f,19
are also listed. From Table 2 and Figure 6, it follows that
the φf depends on the type of porphyrin (free-base or zinc
porphyrin) and on the oxidation potential, Epa, of the
employed hydroquinones. The higher scattering in the
plots may be due to the steric effects caused by the
hydroquinone substituents and the irreversible oxidation
potentials of hydroquinones. For 2b-hydroquinone, the
φf value is twice as much as H2TPP. However, the
maximum φf for 2a -hydroquinone is about 50% of that
of ZnTPP. It may also be mentioned here that the
addition of 1,4-dimethoxybenzene to a solution of 2a or
2b does not increase the φf appreciably.
F or m a t ion of Qu in h yd r on e-P a ir ed P or p h yr in
Dim er . Another interesting aspect of the present study
is to verify whether the quinhydrone pairing mechanism
can be utilized to form self-assembled supramolecular
porphyrins. With this in mind, we have attempted to form
a hetero porphyrin dimer containing porphyrins 1a and
2b. It is observed that the self-assembled dimer precipi-
tates out in solution (in about 6 h) when stoichiometric
amounts of 1a and 2b were allowed to interact in CHCl3/
hexane (1:1 v/v) solution at -5 °C. Positive ion electro-
spray mass spectrum of the dimer in CH3CN matrix
1
revealed the expected molecular ion peak. The H NMR
spectrum of 1a revealed a net deshielding of the hydro-
quinone -OH protons on addition of increasing amounts
of 2b, a result similar to that observed for 1a and quinone
pairing (Figure 2), indicating the formation of a quinhy-
drone-paired porphyrin dimer (see Supporting Informa-
1
tion for mass and H NMR spectral details). Presently,
we are engaged in the synthesis and physicochemical
studies of other self-assembled via quinhydrone-pairing
porphyrin dimers, trimers, and pentamers involving
porphyrins 1a , 1c, and 1d and porphyrins 2a and 2b.
The above observations can be rationalized as follows:
The charge-transfer interactions of the quinone-hydro-
quinone complexes [Qδ-:H2Qδ+] increase in the presence
of easily reducible quinones or easily oxidizable hydro-
quinones or both. The quinhydrone is a weak electron
acceptor as compared to the quinone employed to form
the quinhydrone (Table 1). As a result, the φf of 2a -
hydroquinone and 2b-hydroquinone is expected to be
larger than that of the starting materials, 2a and 2b. In
addition, the free-energy calculations17 suggest that the
electron-transfer quenching is more exergonic in the 2a -
hydroquinone complex than in the 2b-hydroquinone
complex. Hence, a lower φf is obtained for 2a -hydro-
quinone as compared to 2b-hydroquinone (Table 2).
Con clu sion
It has been shown that both the free-base and the
zinc(II) porphyrins bearing either one, two, or four
hydroquinone receptors bind quinones via quinhydrone
pairing. As shown by means of the electrochemical and
1H NMR studies, the porphyrin-quinhydrone complexes
are primarily stabilized by charge-transfer interactions
between the electron donor (hydroquinone) and the
electron acceptor (quinone) as well as by H-bonding
interactions. The redox potentials of the quinhydrone are
governed by the potential of the quinone utilized to form
the quinhydrone. The fluorescence emission studies have
indicated that the quenching ability of the donor, i.e.,
porphyrin, is larger the larger the number of receptors,
i.e., hydroquinone substituents, on the macrocycle. The
values of quenching efficiency, determined in terms of
A comparison of the emission behavior between the two
types of porphyrin-quinhydrone complexes investigated
here indicates that the mode of attachment of quinhy-
drone to the porphyrin macrocycle also affects the emis-
(20) (a) Dalton, J .; Milgrom, L. R. J . Chem. Soc., Chem. Commun.
1979, 609. (b) Chan, A. C.; Dalton, J .; Milgrom, L. R. J . Chem. Soc.,
Perkin Trans 2 1982, 707. (c) Bergkamp, M. A.; Dalton, J .; Netzel, T.
L. J . Am. Chem. Soc. 1982, 104, 253.
(21) The φf values were calculated according to the method described
in ref 22.
(22) Austin, A.; Gouterman, M. Bioinorg. Chem. 1978, 9, 281.
(23) This effect is seen in solutions of isolated solid porphyrin-
quinhydrone complexes as well as in situ generated complexes.

4608 J . Org. Chem., Vol. 66, No. 13, 2001
D’Souza and Deviprasad
of saturated sodium bicarbonate solution. This combined
solution was stirred for 0.5 h, followed by evaporation of the
combined solution under reduced pressure. The solid was
washed with water, dried, and then washed with chloroform.
The solid thus obtained was purified on a basic alumina
column using CHCl3/MeOH (95:5 v/v) as eluent to yield 1b.
Yield 70.1%. 1H NMR (CDCl3): δ 8.81 (m, 8H, â-pyrrole), 8.19
(m, 6H, o-phenyl), 7.75 (m, 9H, m- and p-phenyl), 7.32-7.01
(s, d, d, 3H, hydroquinone ring), 4.65 (s, br, -OH), -2.74 (s,
br, 2H, imino). UV-vis (benzonitrile) λ, nm (log ꢀ): 416.5
(5.36), 513 (4.13), 546 (3.79), 588 (3.68), 646(3.58). FAB-mass
spectroscopy (m/z): calcd for CH3CN, 646.7; found, 646.9.
[Q1/2], follow the E1/2 values of the quinones suggesting
that electron transfer from the singlet excited state is
the main quenching mechanism. Experiments performed
by using dimethoxyphenyl-bearing zinc porphyrin have
revealed the significance of the hydrogen bonding. Quin-
hydrone formed by using porphyrin covalently linked to
a quinone and hydroquinone present in solution shows
fluorescence enhancement. The measured φf values de-
pend on the nature of the porphyrin and the redox
potentials of hydroquinones. The mode of attachment of
the quinhydrone to the porphyrin ring is also shown to
affect the emission behavior of the porphyrin. Generally,
porphyrin-quinhydrone complexes formed by hydro-
quinone-linked porphyrins show decreased φf values as
compared to a porphyrin-quinhydrone complex formed
by quinone-linked porphyrins. Finally, the present quin-
hydrone-pairing approach is shown to be a useful ap-
proach to obtain self-assembled supramolecular porphy-
rins.
Compound 1a was synthesized by metalation of 1b with zinc
acetate. To a solution of 1b (0.2 g, 0.31 mM) in CHCl3, excess
of zinc acetate in methanol was added. This solution was
stirred for 30 min. The solvent was removed under reduced
pressure, and the crude product was dissolved in CH2Cl2,
washed with water, and dried over sodium sulfate. The
solution was concentrated and loaded on to a basic alumina
column. The pure 1a was eluted with 90:10 (v/v) CHCl3:MeOH.
1
Yield 94%. H NMR (CDCl3): δ 8.93-8.78 (m, 8H, â-pyrrole),
8.18-8.12 (m, 6H, o-phenyl), 7.77-7.66 (m, 9H, m- and
p-phenyl), 6.89-6.65 (d, d, s, 3H, hydroquinone), 4.60 (s, br,
2H, -OH). UV-vis (benzonitrile) λ, nm (log ꢀ): 422.5 (5.35),
553 (3.94), 592 (3.45). FAB-mass spectroscopy (m/z): calcd for
CH3CN 710.10; found, 709.9.
Exp er im en ta l Section
Gen er a l In for m a tion . Benzonitrile (Aldrich) for spectral
and electrochemical experiments was distilled over P2O5 under
vacuum. Tetra-n-butylammonium perchloride, (TBA)ClO4
(Kodak), and tetra-n-butylammonium hexafluorophosphate,
(TBA)PF6 (Aldrich), were recrystallized from ethyl alcohol. All
other reagents were commercial chemicals of analytical grade.
They were used without further purification, unless otherwise
indicated. The newly synthesized compounds were freshly
purified by column chromatography, and their purity was
tested by TLC prior to spectral measurements.
5,15-Dip h en yl-10,20-bis(2,5-d ih yd r oxyp h en yl)p or p h y-
r in a tozin c(II), 1c. This was synthesized according to Lindsey
and Lee24 with a few modifications. For this first, meso-
phenyldipyrromethane was synthesized according to the fol-
lowing procedure. A solution of benzaldehyde (0.2 mL, 2 mM)
and pyrrole (5.6 mL, 80 mM) was treated with trifluoroacetic
acid (0.016 mL, 0.1 mM) at rt for 15 min. The mixture was
diluted with CH2Cl2 (50 mL), washed with 0.1 M NaOH
solution and water, and dried over sodium sulfate. The solvent
and the unreacted pyrrole were removed by vacuum distilla-
tion at rt. The resulting yellow amorphous solid was dissolved
in a minimum amount of ether and purified by flash chroma-
tography on silica gel column with cyclohexane:ethyl acetate:
In str u m en ta tion . The UV-visible spectral measurements
were carried out with a Shimadzu model 1600 UV-visible
spectrophotometer. The fluorescence was monitored by using
a Spex Fluorolog spectrometer. A right angle detection method
1
was used. The H NMR studies were carried out on a Varian
1
400 MHz spectrometer. Tetramethylsilane (TMS) was used as
an internal standard. Cyclic voltammograms were obtained
by using a conventional three-electrode system on a EG&G
model 263A potentiostat. A platinum button electrode was
used as the working electrode. A platinum wire served as the
counter electrode. A KCl saturated calomel electrode (SCE)
or an Ag/AgCl electrode, separated from the test solution by a
fritted supporting electrolyte/solvent bridge, was used as the
reference electrodes.
triethylamine (80:19:1) as eluent. Yield 0.2 g, 45%. H NMR
(CDCl3): δ 7.82 (s, br, 2H, NH), 7.31-7.12 (m, 5H, phenyl),
6.62 (q, 2H, pyrrole), 6.09 (q, 2H, pyrrole), 5.87 (m, 2H,
pyrrole), 5.44 (s, 1H, m-H).
Next, 5,15-diphenyl-10,20-bis(2,5-demethoxyphenyl)porphy-
rin was synthesized by reacting 2,5-dimethoxybenzaldehyde
(0.15 g, 0.9 mM) and meso-phenyldipyrromethane (0.2 g, 0.9
mM) in 90 mL of CHCl3 under argon for 10 min. A solution of
BF3-O(Et)2 (120 µL of 2.5 M stock solution in CHCl3) was
added, and the solution was stirred for 1 h at rt. At the end,
DDQ (155 mg, 0.68 mM) was added, and the stirring was
continued at rt for an additional 1 h. The solvent was removed
under reduced pressure, and the crude porphyrin was dis-
solved in CHCl3 and purified over a basic alumina column with
CHCl3:hexane (1:1 v/v) as eluent. Yield 0.2 g, 30%. 1H NMR
(CDCl3): δ 8.79 (m, 8H, â-pyrrole), 8.19 (m, 4H, o-phenyl), 7.67
(m, 6H, m- and p-phenyl), 7.59-7.39 (d, d, s, 6H, bis-
dimethoxyphenyl), 4.02-3.65 (s, s, 12H, -OCH3), -2.78 (s, br,
2H, imino). UV-vis (benzonitrile) λ, nm (log ꢀ): 415 (5.39),
512.4 (4.12), 546.2 (3.76), 587 (3.65), 645 (3.53). FAB-mass
spectroscopy (m/z): calcd for CH3CN 734.85; found, 734.8.
The 5,15-diphenyl-10,20-bis(2,5-dihydroxyphenyl)porphyrin
was synthesized as follows. To a 5 mL solution of BBr3/
CH2Cl2 (1 M) at -78 °C, 0.2 g (0.27 mM) of 5,15-diphenyl-
10,20-bis(2,5-dimethoxyphenyl)porphyrin in a minimum amount
of CH2Cl2 was added. The solution was maintained at this
temperature for 1 h during which the addition was completed.
This mixture was then allowed to attain rt, and stirring was
continued for another 12 h. At the end of this 12 h period, the
mixture was again cooled to -5 °C, taking care that the
temperature was maintained below 5 °C, and 5 mL of cold
water was added to quench the reaction, followed by addition
of saturated sodium bicarbonate solution. This combined
5,10,15-Tr ip h en yl-20-(2,5-d ih yd r oxyp h en yl)p or p h yr i-
n a t ozin c, 1a , a n d 5,10,15-Tr ip h en yl-20-(2,5-d ih yd r oxy-
p h en yl)p or p h yr in , 1b. These were synthesized according to
the earlier published methods with few modifications.10,13 First,
5,10,15-triphenyl-20-(2,5-dimethoxyphenyl)porphyrin was syn-
thesized by reacting 2,5-dimethoxybenzaldehyde (1 mM),
pyrrole (4 mM), and benzaldehyde (3 mM) in 450 mL of
propionic acid for 45 min. The propionic acid was removed
under reduced pressure, and the solid mixture was washed
with methanol and purified on a basic alumina column using
toluene:hexane (1:1 v/v) as eluent. Yield 5.02%. 1H NMR
(CDCl3): δ 8.83 (s, 8H, â-pyrrole), 8.21 (m, 6H, o-phenyl), 7.77-
7.70 (m, 9H, m- and p-phenyl), 7.61-7.26 (d, d, s, 3H,
dimethoxy phenyl), 3.91-3.52 (s, s, 6H, -OCH3), -2.84 (s br,
2H, imino). UV-vis (benzonitrile) λ, nm (log ꢀ): 415 (5.40),
512 (4.13), 546 (3.79), 587 (3.68), 645 (3.58). FAB-mass
spectroscopy (m/z): calcd for CH3CN, 674.76; found, 674.8.
Compound 1b was synthesized by drop by drop addition of
5 mL of BBr3/CH2Cl2 (1 M) to a solution of 0.2 g (0.3 mM) of
5,10,15-triphenyl-20-(2,5-dimethoxyphenyl)porphyrin dissolved
in a minimum amount of CH2Cl2 at -78 °C. The solution was
maintained at -78 °C for 1 h during which the addition was
completed. This mixture was then allowed to attain rt and was
stirred for another 12 h. At the end of this 12 h period, the
mixture was again cooled to -5 °C, taking care that the
temperature was maintained below 5 °C, and 5 mL of cold
water was added to quench the reaction, followed by addition
(24) Lee, C.-H.; Lindsey, J . S. Tetrahedron 1994, 50, 11427.

Studies on Porphyrin-Quinhydrone Complexes
J . Org. Chem., Vol. 66, No. 13, 2001 4609
solution was stirred for 0.5 h, followed by evaporation of the
combined solution under reduced pressure. The solid was
washed with water, dried, and then washed with chloroform.
The solid thus obtained was purified on a basic alumina
column using CHCl3/MeOH (90:10 v/v) as eluent. Yield 76.3%.
1H NMR (CDCl3): δ 8.81 (m, 8H, â-pyrrole), 8.16 (m, 4H,
o-phenyl), 7.72 (m, 6H, m- and p-phenyl), 7.63-7.44 (d, d, s,
6H, bis-dihydroxyphenyl), 4.65 (s, br, 4H, -OH), -2.74 (s, 2H,
imino). UV-vis (benzonitrile) λ, nm (log ꢀ): 415 (5.40), 512
(4.13), 546 (3.79), 590 (3.68), 647 (3.58). FAB-mass spectros-
copy (m/z): calcd for CH3CN 678.7; found, 678.9.
(3.76), 592 (3.62), 648 (3.56). FAB-mass spectroscopy (m/z):
calcd for CH3CN 742.74; found, 742.7.
Finally, 1d was synthesized by metalation of 5,10,15,20-
tetrakis(2,5-dihydroxyphenyl)porphyrin with zinc acetate.25 To
a solution of 5,10,15,20-tetrakis(2,5-dihydroxyphenyl)porphy-
rin (0.2 g, 0.3 mM) in methanol, an excess of zinc acetate in
methanol was added. This solution was stirred for 30 min. The
solvent was removed under reduced pressure, and the crude
product was suspended in water and filtered. The product was
dissolved in acetone, adsorbed on silica gel, and loaded on to
a silica gel column. The pure 1d was eluted with CHCl3:MeOH
(85:15 v/v). Yield 94%. 1H NMR (CDCl3): δ 8.95-8.89 (m, 8H,
â-pyrrole), 8.19-8.17 (m, 4H, o-phenyl), 7.75-7.73 (m, 8H, m-
and p-phenyl), 5.74 (s, br, 8H, -OH). UV-vis (benzonitrile) λ,
nm (log ꢀ): 424 (5.20), 553 (3.95), 592 (3.47). FAB-mass
spectroscopy (m/z): calcd for CH3CN 806.13; found, 805.3.
5,10,15-Tr ip h en yl-20-(3,6-d ioxocycloh exa -1,4-d en yl)-
p or p h yr in , 2b. This was synthesized by chemical oxidation
of 1b. To a solution of 0.2 g (0.31 mmol) of 1b in 50 mL of
CHCl3, 0.176 g (0.774 mmol) of DDQ was added, and the
solution was stirred for 2 h. The solution was then concen-
trated and loaded on a basic alumina column. Using a mixture
of chloroform/hexane, pure 2b was eluted. Yield 68%. 1H NMR
(CDCl3): δ 8.90 (m, 8H, â-pyrrole), 8.20 (s, 6H, o-phenyl), 7.75
(m, 9H, m- and p-phenyl), 7.29, 7.60, 8.38 (d,d,s, 3H, quinone),
-2.78 (s, 2H, imino). UV-Vis (benzonitrile) λ, nm (log ꢀ): 413
(5.29), 511 (3.89), 545 (3.44), 588 (3.44), 646 (3.32). FAB-mass
spectroscopy (m/z): calcd for CH3CN 644.74; found, 644.1.
Finally, 1c was synthesized by metalation of 5,15-diphenyl-
10,20-bis(2,5-dihydroxyphenyl)porphyrin with zinc acetate.25
To a solution of 5,15-diphenyl-10,20-bis(2,5-dihydroxyphenyl)-
porphyrin (0.2 g, 0.3 mM) in CHCl3, excess zinc acetate in
methanol was added, and the solution was stirred for 30 min.
The solvent was removed under reduced pressure, and the
crude product was dissolved in CH2Cl2, washed with water,
and dried over sodium sulfate. The solution was concentrated
and loaded on to a basic alumina column. Pure 1c was eluted
1
with CHCl3:hexane (90:10 v/v). Yield 94%. H NMR (CDCl3):
δ 8.93-8.88 (m, 8H, â-pyrrole), 8.17-8.16 (m, 6H, o-phenyl),
7.74-7.72 (m, 6H, m- and p-phenyl), 7.44-7.09 (d, d, s, 6H,
dihydroxyphenyl), 4.57 (s, 4H, -OH). UV-vis (benzonitrile) λ,
nm (log ꢀ): 422 (5.35), 553 (3.92), 592 (3.42). FAB-mass
spectroscopy (m/z): calcd for CH3CN 742.13; found, 742.3.
5,10,15,20-Tetr a k is(2,5-d ih yd r oxyp h en yl)p or p h yr in a -
tozin c(II), 1d . For this, first 5,10,15,20-tetrakis(2,5-dimeth-
oxyphenyl)porphyrin was synthesized by reacting 2 g (30
mM) of pyrrole and 4.95 g (30 mM) of 2,5-dimethoxy benzal-
dehyde in 500 mL of propionic acid for 1 h. The propionic acid
was evaporated under reduced pressure. The solid was then
dissolved in a minimum amount of chloroform and purified
on a basic alumina column using CHCl3/hexane (75:25) as
eluent. Yield 22%. 1H NMR (CDCl3): δ 8.76 (s, 8H, â-pyrrole),
8.07 (d, 4H, m-phenyl), 7.62 (d, 4H, p-phenyl), 7.41 (s, 4H,
o-phenyl), 4.12-3.74 (s, s, 24H, -OCH3), -2.89 (s, br, 2H,
imino). UV-vis (benzonitrile) λ, nm (log ꢀ): 415 (5.36), 512
(4.12), 546 (3.76), 587 (3.62), 645 (3.58). FAB-mass spectros-
copy (m/z): calcd for CH3CN 854.95; found, 854.9.
5,10,15-Tr ip h en yl-20-(3,6-d ioxocycloh exa -1,4-d en yl)-
p or p h yr in a tozin c(II), 2a . To 0.1 g (0.14 mM) of 1a in 50
mL of CH2Cl2, 0.08 g (0.352 mM) of DDQ was added, and the
solution was stirred for 2 h. The solution was then concen-
trated under reduced pressure, loaded on a basic alumina
1
column, and eluted with CHCl3. Yield 50%. H NMR (CDCl3):
δ 8.99 (d, 8H, â-pyrrole), 8.17 (m, 6H, o-phenyl), 7.74 (m, 9H,
m- and p-phenyl), 7.43-7.41 (m, 3H, quinone). UV-vis (ben-
zonitrile) λ, nm (log ꢀ): 418 (5.38), 549 (3.92), 618 (3.42). FAB-
mass spectroscopy (m/z): calcd for CH3CN 708.13; found, 709.0.
Next, 0.2 g (0.23 mM) of 5,10,15,20-tetrakis(2,5-dimethoxy-
phenyl)porphyrin in a minimum amount of CH2Cl2 was treated
with 5 mL of BBr3/CH2Cl2 (1 M) at -78 °C follwed by stirring
for 12 h at rt. The mixture was cooled again to -5 °C, and 5
mL of cold water was added to quench the reaction, followed
by addition of saturated sodium bicarbonate solution. This
combined solution was stirred for 0.5 h, followed by evapora-
tion of the combined solution under reduced pressure. The solid
was washed with water, dried, and then washed with chloro-
form. The solid thus obtained was purified over a basic
alumina column using CHCl3/MeOH (80:20) as eluent to yield
5,10,15,20-tetrakis(2,5-dihydroxyphenyl)porphyrin. Yield 64%.
1H NMR (CDCl3:DMSO, 90:10 v/v): δ 8.96 (s, 8H, â-pyrrole),
8.17 (d, 4H, m-phenyl), 7.81 (d, 4H, p-phenyl), 7.62 (s, 4H,
o-phenyl), 4.74 (s, br, 8H, -OH), -2.72 (s, br, 2H, imino). UV-
vis (benzonitrile) λ, nm (log ꢀ): 415 (5.40), 512 (4.12), 549
Ack n ow led gm en t. We are thankful to Professor
David Smith, University of NebraskasLincoln; Dr. Tod
Williams, Mass Spectrometry Laboratory, University of
Kansas; and Dr. Mike van Stipdonk for mass spectral
results. The National Science Foundation through Grant
EPS-9550487 and a matching support from the State
of Kansas supported this work. The authors are also
thankful to the donors of the Petroleum Research Fund,
administered by the American Chemical Society, for
support of this work.
Su p p or tin g In for m a tion Ava ila ble: The mass spectra
of selected porphyrin-quinhydrone complexes and the por-
phyrin dimer as well as the 1H NMR spectral data of the
porphyrin dimer. This material is available free of charge via
the Internet at http://pubs.acs.org.
(25) Smith, K. M. Porphyrin and Metalloporphyrins; Elsevier: New
York, 1977.
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