Molecular tools for the self-assembly of bisporphyrin photodyads: A comprehensive physicochemical and photophysical study

Article abstract of DOI:10.1021/ic802407x

Accessible and hindered phenanthroline-strapped Zn(II) porphyrin receptors exhibited suitable topography tailored to strongly and selectively bind N ₁-unsubstituted imidazoles and imidazoles appended to free-base porphyrins. Distal binding was

Full text of DOI:10.1021/ic802407x

Inorg. Chem. 2009, 48, 3743-3754
Molecular Tools for the Self-Assembly of Bisporphyrin Photodyads: A
Comprehensive Physicochemical and Photophysical Study
Je´re´my Brandel,† Ali Trabolsi,† Hassan Traboulsi,† Fre´de´ric Melin,‡ Matthieu Koepf,‡
Jennifer A. Wytko,‡ Mourad Elhabiri,*,† Jean Weiss,*,‡ and Anne-Marie Albrecht-Gary*,†
Laboratoire de Physico-Chimie Bioinorganique, ULP-CNRS (UMR 7177), Institut de Chimie,
ECPM, 25 Rue Becquerel, 67200 Strasbourg, France, and Laboratoire de Chimie des Ligands a`
Architecture Controˆle´e, ULP-CNRS (UMR 7177), Institut de Chimie, 4 Rue Blaise Pascal,
67000 Strasbourg, France
Received December 19, 2008
Accessible and hindered phenanthroline-strapped Zn(II) porphyrin receptors exhibited suitable topography tailored
to strongly and selectively bind N1-unsubstituted imidazoles and imidazoles appended to free-base porphyrins.
Distal binding was clearly driven by the formation of strong bifurcated hydrogen bonds with the phenanthroline unit
of the receptors. An extensive physicochemical study emphasized the influence of bulkiness of the substrate and
of the porphyrin receptor on the binding and self-assembly mechanism. Knowledge of the corresponding spectroscopic,
thermodynamic, and kinetic data were of fundamental importance to elucidate and to model the photoinduced
properties which occur within the self-assembled porphyrin dyads.
Introduction
is essential to the design of functional materials.4-10 A large
number of covalently associated donor-acceptor porphyrin
dyads or extended assemblies have been designed to model
In the search for photo- and electroactive materials,
porphyrin assemblies exhibiting well-defined geometries and
interactions, dyads, triads, or higher oligomers,1,2 have been
the focus of much attention over the past decades. Synthetic
efficiency and processability requirements call for efficient
but tunable self-assembly processes. Because the formation
of discrete species is generally entropy favored in solution,3
control of the assembly process during deposition on surfaces
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Sunderland, C. J. In The Porphyrin Handbook; Kadish, K. M., Smith,
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* To whom correspondence should be addressed. E-mail: elhabiri@
chimie.u-strasbg.fr (M.E.), jweiss@chimie.u-strasbg.fr (J.W.), amalbre@
chimie.u-strasbg.fr (A.-M.A.-G.).
†
Laboratoire de Physico-Chimie Bioinorganique.
Laboratoire de Chimie des Ligands a` Architecture Controˆle´e.
‡
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10.1021/ic802407x CCC: $40.75  2009 American Chemical Society
Inorganic Chemistry, Vol. 48, No. 8, 2009 3743
Published on Web 03/18/2009

Brandel et al.
Figure 2. Chemical structures of imidazoles and benzimidazoles S1-S8.
dazole recognition results from a strong N-imidazolyl-Zn(II)
axial coordination6,7,8a,17 and bifurcated hydrogen bonds
between the substrate and the nitrogen atoms of the 1,10-
phenanthroline (noted phen) strap of the receptor. Moreover,
introduction of secondary interactions (π-π stacking, CH-π,
or hydrophobic interactions) may contribute to strengthen
the final assemblies. This self-assembly approach was
successfully implemented in photodyads18 and linear por-
phyrin arrays.19 Very recently, investigation of the self-
assembly behavior of self-complementary building blocks
showed that self-assembly guided via preferential weak
interactions developed with a surface may compete with the
classical entropy governed self-assembly in solution. As a
result, drastic changes in the morphology of self-assembled
species were observed, depending on a solution or a surface
assembly mode.20
In the prospect of utilizing self-assembled porphyrin wires
in prototypal devices, it is essential to fully control the
parameters of the assembly process. Such control requires a
careful assessment of all energetic parameters of the recogni-
tion process associated with the programmed assembly. We
present herein an extensive physicochemical study of the
recognition processes21 of various substrates (Figures 2 and
3) by phen-strapped zinc(II) porphyrins ZnL1-ZnL4 (Figure
1). In these receptors, meso-substitution by xylyl (ZnL2 and
ZnL3) or resorcinol-type (ZnL4)22 groups offers the op-
portunity to study the influence of steric hindrance around
the distal cavity. Imidazoles S1-S5, benzimidazoles S6-S8,
and imidazole free-base porphyrins H2L5 and H2L6 were used
to identify the key structural parameters controlling the
Figure 1. Chemical structures of the zinc porphyrin receptors.
elementary recognition or communication events between
building blocks.9-11 In noncovalent assemblies of functional
building blocks, the components are held together by multiple
weak interactions such as metal-ligand coordination bonds,
hydrogen bonds, electrostatic interactions, or hydrophobic
forces. The efficiency and success of self-assembly are
conditioned by the careful programming of the recognition
patterns between building blocks. Like other applications of
various strategies designed for efficient linear self-as-
sembly,12 multiporphyrins are mostly attained by multiple
H-bonding, metal coordination, or a combination of both.13,14
Inspired by the imidazole binding on zinc porphyrins as an
assembling tool, we have taken advantage of both the
selective recognition of imidazoles (Figure 1)15,16 and the
very efficient synthesis of phenanthroline superstructured
porphyrins to design self-assembling supramolecular building
blocks. We demonstrated that a careful ligand design based
on the induced-fit principle led to the selective and strong
recognition of imidazoles by the receptor (Figure 1). Imi-
(11) (a) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2001, 34,
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(14) For recent examples of self-assembled porphyrin devices, see: (a)
Nomoto, A.; Mitsuoka, H.; Oseki, H.; Kobuke, Y. Chem. Commun.
2002, 1074–1075. (b) Nomoto, A.; Kobuke, Y. Chem. Commun. 2002,
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Chem. Soc. 2004, 126, 8668–8669. (e) Takahashi, R.; Kobuke, Y. J.
Org. Chem. 2005, 70, 2745–2753. (f) Furutsu, D.; Satake, A.; Kobuke,
Y. Inorg. Chem. 2005, 44, 4460–4462. (g) Hwang, I. W.; Park, M.;
Ahn, T. K.; Yoon, Z. S.; Ko, D. M.; Kim, D.; Ito, F.; Ishibashi, Y.;
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Ogawa, K.; Satake, A.; Kobuke, Y. Chem.-Eur. J. 2005, 11, 3753–
3761. (h) Shoji, O.; Tanaka, H.; Kawai, T.; Kobuke, Y. J. Am. Chem.
Soc. 2005, 127, 8598–8599. (i) Hajjaj, F.; Yoon, Z. S.; Yoon, M. C.;
Park, J.; Satake, A.; Kim, D.; Kobuke, Y. J. Am. Chem. Soc. 2006,
128, 4612–4623. (j) Wytko, J. A.; Weiss, J. In N4-Macrocyclic metal
complexes; Zagal, J., Bedioui, F., Dodelet, J. P., Eds.; Springer: New
York, 2006; Vol. 18, pp 63-250. (k) Satakea, A.; Shojia, O.; Kobuke,
Y. J. Organomet. Chem. 2007, 692, 635–644.
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J. P. The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard,
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Boudon, C.; Weiss, J. Photochem. Photobiol. Sci. 2005, 4, 280–286.
(19) Koepf, M.; Trabolsi, A.; Elhabiri, M.; Wytko, J. A.; Albrecht-Gary,
A. M.; Weiss, J. Org. Lett. 2005, 7, 1279–1282.
(20) Koepf, M.; Wytko, J. A.; Bucher, J.-P.; Weiss, J. J. Am. Chem. Soc.
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A. M.; Lehn, J. M. HelV. Chim. Acta 2001, 84, 1694–1711. (b)
Hamacek, J.; Blanc, S.; Elhabiri, M.; Leize, E.; Van Dorsselaer, A.;
Piguet, C.; Albrecht-Gary, A. M. J. Am. Chem. Soc. 2003, 125, 1541–
1550. (c) Kalny, D.; Elhabiri, M.; Moav, T.; Vaskevich, A.; Rubinstein,
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(16) (a) Dharam, P.; Wytko, J. A.; Koepf, M.; Weiss, J. Inorg. Chem. 2002,
41, 3699–3704. (b) Dharam, P.; Melin, F.; Hirtz, C.; Wytko, J. A.;
Ochsenbein, P.; Bonin, M.; Schenk, K.; Maltese, P.; Weiss, J. Inorg.
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3744 Inorganic Chemistry, Vol. 48, No. 8, 2009

Self-Assembled Porphyrin Dyads
Sn (n ) 1-8), H2L5, or H2L6 with microliter Hamilton syringes
(#710 and #750). The excitation wavelength corresponds to an
isosbestic point (Soret band) between the free receptors and the
pentacoordinated complexes or to the smallest absorbance ampli-
tudes measured along the absorption spectrophotometric titrations.
Besides, spectrofluorimetric variations of the S1 f S0 transitions
were monitored at absorbances <0.1 to minimize reabsorption
processes. The zinc(II) porphyrin-centered luminescence spectra
were recorded from 400 to 800 nm on a Perkin-Elmer LS-50B
maintained at 25.0(2) °C by the flow of a Haake FJ thermostat.
The light source was a pulsed xenon flash lamp with a pulse width
at half-peak height <10 µs and power equivalent to 20 kW. The
slit width was set at 15 nm for both the excitation and the emission.
Formation Kinetics. Formation kinetics of the pentacoordinated
complexes (ZnLi-Sn, ZnLi-H2L5, and ZnLi-H2L5, i ) 1-4, n
) 3-8) were investigated using an Applied Photophysics SX-18MV
stopped-flow spectrophotometer with absorption and emission
detection. The reactants were thermostated at 25.0(2) °C (Lauda
M12 thermostat) and mixed in a 1 cm optical cell. Pseudo-first-
order conditions with respect to imidazoles were used with
imidazole concentrations at least 10 times higher than receptor
concentrations. With absorption detection, the reaction was moni-
tored at wavelengths corresponding to the largest amplitude (Soret
Figure 3. Chemical structures of the imidazole free-base porphyrins H2L5
and H2L6.
proximal/distal23 recognition of these hosts. Knowledge of
the spectroscopic, thermodynamic, structural, and kinetic data
are fundamental prerequisites to examine the photoinduced
processes of the dyads ZnLi (i ) 1-4) (Figure 1) and H2L5
or H2L6 (Figure 3).
Experimental Section
Solvents and Material. Zinc porphyrins, ZnL1, ZnL2, ZnL3,
ZnL4, and ZnTPP (TPP ) tetraphenylporphyrin), were synthesized
according to previously published procedures.15,16,19,22,24 Imidazolyl
free-base porphyrins H2L5 and H2L6 were synthesized according
to a previously published procedure.16a,19 The imidazole substrates
(1-methyl-1H-imidazole S1, 1H-imidazole S2, 2-methyl-1H-imi-
dazole S3, 2-phenyl-1H-imidazole S4, N-acetyl-L-histidine methyl
ester S5, 1H-benzimidazole S6, 2-methyl-1H-benzimidazole S7, 5,6-
dimethyl-1H-benzimidazole, S8) were purchased from commercial
sources and used without further purification. All analyses were
carried out with spectroscopic grade 1,2-dichloroethane (Merck,
99.8% or Carlo Erba, 99.8% for spectroscopy), which was further
purified by distillation over CaH2 to remove traces of HCl. All
solutions were protected from daylight to avoid any photochemical
degradation. All stock solutions were prepared using a AG 245
Mettler Toledo analytical balance (precision 0.01 mg), and the
complete dissolution in 1,2-dichloroethane was achieved using an
ultrasonic bath. The concentrations of the stock solutions of the
receptors and substrates (≈10-4 M) were calculated by quantitative
dissolution of solid samples in 1,2-dichloroethane.
UV-Visible Titrations. The spectrophotometric titration of the
receptors ZnLi (i ) 1-4; ∼10-5-10-6 M) and ZnTPP (∼10-6 M)
with Sn (n ) 1-8), H2L5 and H2L6 were carried out in a Hellma
quartz optical cell (1 or 2 cm). Microvolumes of a concentrated
solution of Sn (n ) 1-8), H2L5, or H2L6 were added to 4 mL (for
l ) 2 cm) or 2 mL (for l ) 1 cm) of porphyrinic receptors with
microliter Hamilton syringes (#710 and #750). Special care was
taken to ensure that complete equilibration was attained. The
corresponding UV-visible spectra were recorded from 290 to 700
nm on Kontron Uvikon 941 or Varian Cary 300 spectrophotometers
maintained at 25.0(2) °C by the flow of a Haake NB 22 thermostat.
Luminescence Titrations. Luminescence titrations were carried
out on solutions of ZnLi (i ) 1-4) or ZnTPP with an absorbance
smaller than 0.1 at wavelengths gλexc in order to avoid any errors
due to the inner filter effect. The titration of 2 mL (4 mL) of the
metalloporphyrins was carried out in a 1 cm (2 cm) Hellma quartz
optical cell by addition of known microvolumes of a solution of
n
i
n
band for ZnLi
-S ). For fluorescence measurements with ZnL -S ,
the excitation monochromator slits were set at 3 nm, and the
excitation wavelength corresponded to an isosbestic point (Soret
or Q bands) between the free receptors and the pentacoordinated
complexes. A 550 nm band-pass plastic-glass filter (transparent to
light with λ > 550 nm, Schott KV550) was used for measuring the
emitted light. Due to the fact that H2L5 or H2L6 substrates are also
absorbing and light emitting species within the same spectral
windows of the zinc porphyrins, a pair of band-pass filters (λ >
550 nm; Schott KV550) and interference filters (λ < 660 nm; IF660)
is selected to isolate the emission bands of interest in the spectral
region that allows the kinetics formation of the dyads to be
monitored. The data sets, averaged out of at least three replicates,
were recorded and analyzed with the commercial software Biok-
ine.25 This program fits up to three exponential functions to the
experimental curves with the Simplex algorithm26 after initialization
with the Pade´-Laplace method.27 Variation of the pseudo-first-
order rate constants with the analytical concentrations of reagents
were processed using commercial programs (Origin 5.028).
Activation Parameters. The temperature dependence of the
formation kinetic was investigated for ZnLi-S3 and ZnLi-S4
complexes (i ) 1-4). The experimental protocol described above
was herein applied. Kinetic experiments were performed at tem-
peratures ranging from 21.0 to 38.0 (error ) (0.2 °C). A Lauda
cryostat (Ultra Kryomat TK30) was used as the cooling unit whereas
a Lauda M12 thermostat was employed as the heating system. The
poor solubility of the imidazoles at low temperature prevented the
decrease of the temperature below 20 °C. Enthalpies and entropies
of the reactions were calculated from conventional Eyring plots
(ln(k/T) ) f(1/T)) using the Origin 5.0 program.28
Photophysical Studies. Emission spectra were collected on a
Perkin-Elmer LS-50B luminescence spectrophotometer with solu-
tions of absorbance less than 0.05 at a wavelength >λex in order to
avoid any errors due to the inner filter effect. Absorption spectra
were also measured along the fluorescence titration by using a
Varian CARY 300 spectrophotometer. Fluorescence quantum yields
(23) For definition of distal and proximal binding sites, see: (a) Sage, J. T.;
Champion, P. M. In Small Substrate Recognition in Heme Proteins;
ComprehensiVe Supramolecular Chemistry; Suslick, K. S., Ed.;
Pergamon Press: Oxford, U.K., 1996; Vol. 5, pp 171-213. (b)
Momenteau, M.; Reed, C. A. Chem. ReV. 1994, 94, 659.
(24) Wytko, J. A.; Graf, J. E.; Weiss, J. J. Org. Chem. 1992, 57, 1015–
1018.
(25) Biokine; Bio-Logic Company: Echirolles, 1991.
(26) Nelder, J. A.; Mead, R. Comput. J. 1965, 7, 308–313.
(27) Yeramian, E.; Claverie, P. Nature 1987, 326, 169–174.
(28) Microcal Origin; Microcal Software, Inc.: Northampton, MA, 1997.
Inorganic Chemistry, Vol. 48, No. 8, 2009 3745

Brandel et al.
Table 1. Spectroscopic and Photophysical Parameters of Zinc(II) Strapped Porphyrins ZnLi (i ) 1-4)a
max
absorption λmax (nm) [ελ (×104 M-1 cm-1)]
S1 r S0
emission λmax (nm)
S1 f S0
abs
L
S2 r S0 B(0,0)
Q(1,0)
Q(0,0)
Q(0,0)
Q(0,1)
QL (%) [λexc (nm)]
3.3c
τ (ns)
∼2c
ZnTPP
ZnL1
ZnL2
ZnL3
ZnL4
419 (66)
419 (25)
424 (30)
430 (55)
434 (33)
548 (2.4)
585 (0.5)
599
594
601
609
611
647
646
648
659
664
549 (1.47)
550 (1.75)
559 (1.83)
564 (1.77)
586 (0.24)
585 (0.19)
601 (0.52)
604 (0.43)
1.6 (433d, 550e); 2.3b (430)
0.9 (428d, 550e)
2.2b
1.4 (422d, 550e)
1.5 (420d, 550e)
a Solvent ) 1,2-dichloroethane; T ) 25.0(2) °C. b Reference 18. Solvent ) dichloromethane; T ) 25 °C. c References 30e, 41, and 42. Solvent ) toluene;
T ) 25 °C. The uncertainties on the λmax, ελmax, and QLabs are 1 nm, 5%, and 30%, respectively. d λexc used with (Ru(Bpy)3)2+ as a reference. e λexc used with
ZnTPP as a reference.
in solution.33 In contrast, in the solid state the X-ray structure
for zinc and free-base porphyrins were determined relative to two
references, the fluorescent standard (Ru(bpy)3)2+ (tris[(2,2′-bipy-
ridyl)ruthenium(II)] chloride hexahydrate (Φ ) 0.028, air-
equilibrated water), Strem Chemicals 98%) and the fluorescent
ZnTPP (Φ ) 0.033, toluene), with the possibility of correcting
for differences between the refractive index of the reference nr and
the sample solutions ns using the following expression:
of ZnL1 revealed that no conformational restraints of the
tetrapyrrolic macrocycle were imposed by the rigid strap.15
The additional substitution of the two free meso-positions
by one xylyl (ZnL2), two xylyls (ZnL3), and two 2,6-
dimethoxyphenyl (ZnL4) groups induced red shifts of the
metalloporphyrin-centered transitions (Table 1). These shifts
were attributed to electronic effects and porphyrin ring
distortions caused by these substituents. Moreover, photo-
excitation of the Soret transitions of the phen-strapped Zn(II)
porphyrins resulted in two emission bands in the 600-700
nm spectral window. These emission bands were attributed
to the S1 f S0 (Q(0,0) and Q(0,1)) transitions (Table 1).30-32
Coordination of Imidazoles by ZnTPP. To evaluate the
influence of the phen strap on the recognition properties of
a series of imidazoles, it was essential to first consider a
reference metalloporphyrin, ZnTPP (Figure 1), that possesses
two identical faces. For ZnTPP complexes, the only interac-
tion with imidazole is the strong N-Zn coordination.34 Even
though the binding properties of ZnTPP with various
nitrogen bases have been reported,35 for the sake of
comparison, the stability of ZnTPP-Sn (n ) 1-8, Figure
2) and ZnTPP-H2L6 (Figure 3) were examined under our
experimental conditions (1,2-dichloroethane, T ) 25 °C).
Absorption and emission titrations were conducted, and the
corresponding thermodynamic parameters (Table 2) were
calculated by statistical processing36-40 of the data.
I (λ) × A × n 2
∫
s
r
s
Qf(s) ) Qf(r)
I (λ) × A × n 2
∫
r
s
r
The indices s and r denote sample and reference, respectively. The
integrals over I represent areas of the corrected emission spectra,
and A is the optical density at the excitation wavelength.
Processing of the Spectrophotometric Data. The spectropho-
tometric data were processed with the Specfit programs, which
adjust the stability constants and the corresponding extinction
coefficients of the species formed at equilibrium. Specfit36-38 uses
factor analysis to reduce the absorbance matrix and to extract the
eigenvalues prior to the multiwavelength fit of the reduced data
set according to the Marquardt algorithm.39,40 Distribution curves
of the various species were calculated using the Haltafall program.29
Results and Discussion
Spectrophotometric Properties of the Free Porphyrin
Receptors. The absorption spectra of free receptors
ZnL1-ZnL4 exhibit the usual set30 of bands for zinc(II)
metalloporphyrins with a Soret (or ꢀ) band centered at ∼420
nm (ε ≈ 105 M-1 cm-1) and two Q (or R/ꢁ) bands (ε ≈ 104
M-1 cm-1) between 500 and 650 nm.30a,d,31,32 The phen strap
in receptors ZnLi (i ) 1-4) gives rise to an additional
absorption band in the UV range between 260 and 375 nm.
In ZnL1, the 2,9-diphenyl-1,10-phenanthroline strap in-
duced significant hypochromic effects on the Soret and Q
bands with respect to ZnTPP. This hypochromism could be
due to classical electronic effects and geometrical constraints
The stability constants mainly reflect electronic effects of
the substituents on the N3-heteroatom (Figure 2), which binds
to zinc(II). The binding constants follow the order of the
pKa values that are tuned by substituent-induced electronic
effects.43,50 The similar values obtained for complexes
ZnTPP-S2 and ZnTPP-H2L5 confirm the absence of steric
(33) (a) Haddad, R. E.; Gazeau, S.; Pe´caut, J.; Marchon, J. C.; Medforth,
C. J.; Shelnutt, J. A. J. Am. Chem. Soc. 2003, 125, 1253–1268. (b)
Cramariuc, O.; Hukka, T. I.; Rantala, T. T. J. Phys. Chem. A 2004,
108, 9435–9441.
(34) Satake, A.; Kobuke, Y. Tetrahedron 2005, 61, 13–41.
(35) (a) Szintay, G.; Horvath, A. Inorg. Chim. Acta 2000, 310, 175–182.
(b) Bhyrappa, P.; Krishnan, V.; Nethaji, M. J. Chem. Soc., Dalton
Trans. 1993, 1901–1906. (c) Vogel, G. C.; Stahlbush, J. R. Inorg.
Chem. 1977, 16, 950–953. (d) Zaitseva, S. V.; Zdanovich, S. A.;
Ageeva, T. A.; Golubchikov, O. A.; Semeikin, A. S. Russ. J. Coord.
Chem. 2001, 27, 152–157.
(36) Gampp, H.; Maeder, M.; Meyer, C. J.; Zuberbu¨hler, A. D. Talanta
1985, 32, 95–101.
(37) Gampp, H.; Maeder, M.; Meyer, C. J.; Zuberbu¨hler, A. D. Talanta
1985, 32, 257–264.
(29) Ingri, N.; Kakolowicz, W.; Sillen, L. G.; Warnqvist, B. Talanta 1967,
14, 1261–1286.
(30) (a) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic
Press: New York, 1978; Vol. 3, pp 1-165. (b) Dorough, G. D.; Miller,
J. R.; Huennekens, F. M. J. Am. Chem. Soc. 1951, 73, 4315–4320.
(c) Thomas, D. W.; Martell, A. E. J. Am. Chem. Soc. 1956, 78, 1338–
1343. (d) Edwards, L.; Dolphin, D. H.; Gouterman, M.; Adler, A. D.
J. Mol. Spectrosc. 1971, 38, 16–32. (e) Seybold, P. G.; Gouterman,
M. J. Mol. Spectrosc. 1969, 31, 1–13.
(31) (a) Humphry-Baker, R.; Kalyanasundaram, K. J. Photochem. 1985,
31, 105–112. (b) Seely, G. R.; Calvin, M. J. Chem. Phys. 1955, 23,
1068–1078.
(38) Gampp, H.; Maeder, M.; Meyer, C. J.; Zuberbu¨hler, A. D. Talanta
1986, 33, 943–951.
(39) Marquardt, D. W. J. Soc. Ind. Appl. Math. 1963, 11, 431–441.
(32) Nappa, M.; Valentine, J. S. J. Am. Chem. Soc. 1978, 100, 5075–5080.
3746 Inorganic Chemistry, Vol. 48, No. 8, 2009

Self-Assembled Porphyrin Dyads
Table 2. Stability Constants of Imidazoles S1-S8, H2L5, and H2L6 with ZnL1-ZnL4 and ZnTPP Receptorsa
S1
S2
S3
S4
S5
S6
S7
S8
H2L5
H2L6
log KZnTPP-S
log KZnL1-S
log KZnL2-S
log KZnL3-S
log KZnL4-S
4.7(2)
4.3(3)
3.6(1)
3.4(2)
3.6(1)
4.5(1)
5.9(1)
6.10(8)
6.1(1)
7.0(2)
4.9(2)
7.6(4)
7.0(2)
7.0(1)
7.7(5)
2.54(6)
3.92(7)
4.25(9)
4.10(7)
2.3(3)
3.8(1)
5.3(4)
5.1(1)
4.9(2)
5.14(4)
4.34(6)
5.8(4)
5.5(3)
4.9(4)
6.0(3)
3.0(1)
5.3(4)
5.50(6)
5.6(1)
6.14(9)
4.6(1)
4.1(2)
6.0(4)
6.3(3)
6.1(4)
5.0(3)
5.69(9)
5.86(9)
5.69(7)
5.5(1)
5.9(5)
5.8(2)
6.0(4)
5.2(4)
a Solvent ) 1,2-dichloroethane; T ) 25.0(2) °C. Stability constants determined from UV-visible absorption and fluorescence spectrophotometric titrations
(see Supporting Information). Uncertainties ) 3σ, with σ the standard deviation.
interactions within the self-assembled bisporphyrin species
(Table 2).
The electronic and relative fluorescence emission spectra
of the Zn(II)-pentacoordinated species are given in Figures
S1-S16 in the Supporting Information. Upon formation of
the fifth coordination bond to the zinc atom, the S1 r S0
and S2 r S0 (Q and Soret bands, respectively)30,41,44 and S1
f S0 transitions41c,d showed significant and anticipated
bathochromic shifts.45-47 These characteristic spectral changes
are mainly due to (i) the polarizability of the substrate, (ii)
the strength of the N-axial coordination and the charge
Figure 4. Electronic spectra of ZnL1 and its pentacoordinated complex
transfer, which mainly rely on the basicity of the sub-
with S2. Solvent ) 1,2-dichloroethane; T ) 25.0(2) °C.
strate,32,47-49 and (iii) the induced out-of-plane displacement
Under our experimental conditions, the absorption band
of 2,9-diphenyl-1,10-phenanthroline underwent spectropho-
tometric changes upon protonation (Figure S33 in the
Supporting Information). Similar spectral changes were
observed for distal23 recognition of substrates S2-S8 by
ZnL1. In contrast, the titration of ZnL1 with N1-methylated
S1 led to very small bathochromic shifts of the Soret and Q
bands (∆λ ) 2 nm), and no clear spectral variation of the
phen-centered transitions was observed.22 These spectro-
photometric observations clearly demonstrate a proximal23
recognition of S1 by ZnL1. For substrates H2L5 or H2L6,
the highly intense Soret bands limited the spectral window
to the Q bands (Figure 5 and Figures S34-S37 in the
Supporting Information).
of the metal center and deformation of the metalloporphyrin
macrocycle.50,51
Coordination of Imidazoles by Strapped-Zn(II)-Por-
phyrin ZnL1. Significant bathochromic shifts of the Soret
and Q absorption bands (∆λ > 10 nm, Figure 4)22 were
observed for the axial binding of the N1-unsubstituted
substrates S2-S8 on ZnL1 (Figures S17-S32 in the Sup-
porting Information).
(40) Maeder, M.; Zuberbu¨hler, A. D. Anal. Chem. 1990, 62, 2220–2224.
(41) (a) Ohno, O.; Kaizu, Y.; Kobayashi, H. J. Chem. Phys. 1985, 82, 1779–
1787. (b) Gurzadyan, G. G.; Tran-Thi, T. H.; Gustavsson, T. J. Chem.
Phys. 1998, 108, 385–388. (c) Baskin, J. S.; Yu, H. Z.; Zewail, A. J.
Phys. Chem. A 2002, 106, 9837–9844. (d) Yu, H. Z.; Baskin, J. S.;
Zewail, A. J. Phys. Chem. A 2002, 106, 9845–9854.
(42) Strachan, J. P.; Gentemann, S.; Seth, J.; Kalsbeck, W. A.; Lindsey,
J. S.; Holten, D.; Bocian, D. F. J. Am. Chem. Soc. 1997, 119, 11191–
11201.
(43) Ogoshi, H.; Mizutani, T.; Hayashi, T.; Kuroda, Y. In The Porphyrin
Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic
Press: San Diego, CA, 2000; Vol. 6, pp 1-49.
(44) Gouterman, M. J. Chem. Phys. 1959, 30, 1139–1161.
(45) Wang, M. Y. R.; Hoffman, B. M. J. Am. Chem. Soc. 1984, 106, 4235–
4240.
(46) Kirskey, C. H.; Hambright, P.; Storm, C. B. Inorg. Chem. 1969, 8,
2141–2144.
(47) (a) Antipas, A.; Dolphin, D.; Gouterman, M.; Jonhson, E. C. J. Am.
Chem. Soc. 1978, 100, 7705–7709. (b) Quimby, D. J.; Longo, F. R.
J. Am. Chem. Soc. 1975, 97, 5111–5117. (c) Caughey, W. S.; Fujimoto,
W. Y.; Johnson, B. P. Biochemistry 1966, 5, 3830–3843.
(48) Wang, M. Y.; Rachel Hoffman, B. M. J. Am. Chem. Soc. 1984, 106,
4235–4240.
Figure 5. Electronic spectra of ZnL1, of H2L5, and of the ZnL1-H2L5
complex. Solvent ) 1,2-dichloroethane; T ) 25.0(2) °C.
(49) Kirskey, C. H.; Hambright, P.; Storm, C. B. Inorg. Chem. 1969, 8,
2141.
(50) (a) Mc Dermott, G. A.; Walker, F. A. Inorg. Chim. Acta 1984, 91,
95–102. (b) Kadish, K. M.; Shiue, L. R. Inorg. Chem. 1982, 21, 3623–
3630. (c) Kadish, K. M.; Shiue, L. R.; Rhodes, R. K.; Bottomley,
L. A. Inorg. Chem. 1981, 20, 1274–1277. (d) Kadish, K. M.; Rhodes,
R. K. Inorg. Chem. 1981, 20, 2961–2966. (e) Vogel, G. C.; Stahlbusch,
J. R. Inorg. Chem. 1977, 16, 950–953. (f) Valentine, J. S.; Tatsuno,
Y.; Nappa, M. J. Am. Chem. Soc. 1977, 99, 3522–3523. (g) Cole,
S. J.; Curthoys, G. C.; Magnusson, E. A.; Phillips, J. N. Inorg. Chem.
1972, 11, 1024–1028.
Statistical processing36-38 of the spectrophotometric data
supported the exclusive formation of 1:1 species correspond-
ing to the axial ligation of a guest on the metallic center of
ZnL1, which is in agreement with the observation of distinct
(52) (a) Miller, J. R.; Dorough, G. D. J. Am. Chem. Soc. 1952, 74, 3977–
3981. (b) Kirskey, C. H.; Hambright, P. Inorg. Chem. 1970, 9, 958–
960.
(51) Wertsching, A. K.; Koch, A. S.; DiMagno, S. G. J. Am. Chem. Soc.
2001, 123, 3932–3939.
Inorganic Chemistry, Vol. 48, No. 8, 2009 3747

Brandel et al.
isosbestic points.47,52 The corresponding stability constants
of the pentacoordinated complexes with ZnL1 are given in
Table 2. These thermodynamic data emphasized that N1-
unsubstituted substrates are stabilized by factors of 12 to 500
with respect to ZnTPP. In contrast, N1-methyl-imidazole (S1)
altered by the presence of the phen strap in ZnL1 (S3 > H2L5
> S2 > S6 > S8 > S7-S5 > S1 > S4) and the nature and
bulkiness of the substituents of the substrates. π-π stacking
interactions between the phenyl group of S4 and the
porphyrin plane of ZnL1 constitute a relevant example of
these secondary, weak interactions (Figure S38 in the
Supporting Information). CH-π interactions between the
methyl substituent of S3 or S7 and the phenyl spacers of
the strap (Figure S38 in the Supporting Information) or π-π
interactions between the benzimidazole planes (S6, S7, S8)
and the same phenyl spacers (Figure S39 in the Supporting
Information) also contribute to strengthening the correspond-
ing edifices.
Interestingly, a strong stabilization of approximately 2
orders of magnitude of the ZnL1-H2L6 bisporphyrin com-
plex with respect to ZnTPP-H2L6 was observed. This effect
was ascribed to π-π interactions between the phenylethynyl
spacer of H2L6 and the metalloporphyrin plane of ZnL1
(Figure 7). These data emphasized that stable self-assembled
bisporphyrin edifices could be generated in solution, and that
the modeled center-to-center distance of ∼13 Å is properly
tailored to minimize steric interactions between the two
porphyrins (Figure 7).
forms a less stable complex with ZnL1 (log KZnL1-S1
)
4.3(3)) than with unstrapped ZnTPP metalloporphyrin (log
KZnTPP-S1 ) 4.7(2)), in agreement with a proximal recogni-
tion process.
The stabilization for binding of substrates S2-S8, H2L5,
and H2L6 within ZnL1 originates from the presence of the
phen strap, which is able to interact with substrates via
bifurcated hydrogen bonds. Indeed, clear evidence for
hydrogen bonding between the imidazole substrates and the
two phen nitrogens was previously established by 1H NMR
1
and 2D H NMR data in solution and X-ray diffraction in
the solid state (ZnL1-S2, ZnL1-S3, ZnL1-S4).15,16b The
distances measured between the N1H pyrrolic proton, which
was accurately located by the presence of residual electronic
density, and the two phen nitrogen atoms varied from 2.12
to 2.40 Å, thus confirming the presence of unsymmetrical
bifurcated hydrogen bonds.15,16b
Our thermodynamic data revealed, in excellent agreement
with the available X-ray structures and 1H NMR studies,15,16
that the apparently rigid Zn(II) porphyrin ZnL1 was able to
accommodate on its distal face23 a series of bulky substrates
as a result of porphyrin-ring distortion and the large tilt15,16b
of the strap above the tetrapyrrolic plane (∼10°). Comple-
mentary weak interactions (hydrogen bonds, π-π, and
CH-π) between the substrates and ZnL1 aid in stabilization
compared with ZnTPP. In addition, these weak interactions
likely compensate for the steric interactions due to bulkiness
of the substrates and geometrical constraints of the porphyrin
receptor induced upon the distal binding (Figure 6).
Figure 7. Modeled structure of the bisporphyrin complex ZnL1-H2L6
(semiempirical method at the PM3 level). Hydrogen bonds are included as
dotted lines.
These thermodynamic parameters highlighted the recogni-
tion processes of imidazole substrates by a phen strapped
receptor. These parameters showed that the binding led to
the formation of two different types of pentacoordinated
complexes depending on whether the N1-pyrrolic nitrogen
was substituted or not. A “distal” coordination for the N1-
unsubstituted imidazoles (S2-S8, H2L5, and H2L6) and a
“proximal” coordination for the N1-substituted ones (S1) were
clearly demonstrated. Receptor ZnL1 thus exhibits a topog-
raphy that allows a specific recognition of N1-unsubstituted
imidazoles through a subtle set of a strong N-Zn coordination
bond and hydrogen bonds and a panoply of weak interactions
(CH-π and π-π stacking interactions).
Coordination of Imidazoles by Strapped-Zn(II)-
Porphyrins ZnL2, ZnL3, and ZnL4 with Meso-Substitu-
ents. After the characterization of the ZnL1 complexes with
various substrates, the influence of meso-substituents of the
strapped porphyrins ZnL2-ZnL4 on the recognition proper-
ties of imidazoles S1-S8 (Figure 2), H2L5, and H2L6 (Figure
3) was examined. The corresponding spectrophotometric
titrations are given in Figures S40-S99 in the Supporting
1
Figure 6. Difference in stability (log KZnL -S-log KZnTPP-S) of S1-S8
and H2L5 with ZnL1. Abscissa coordinates correspond to the log KZnL -S
sequence (Table 2). Solvent ) 1,2-dichloroethane; T ) 25.0(2) °C.
1
Destabilization of ZnL1-S1 with respect to ZnTPP-S1
results from the constraints exerted by the strap on the
tetrapyrrolic macrocycle as well as from the out-of-plane pull
of the metal center toward the distal face (∼0.3 Å).15,16 The
very small red shifts of the Soret and Q bands (∼2 nm) and
the unaltered phen absorption bands confirmed the proximal
binding of S1. The stability sequence, determined within
experimental errors, for ZnTPP and the substrates (S3 > S1
> S8 > S2 > S6 > H2L5 > S5 > S7 > S4) is significantly
3748 Inorganic Chemistry, Vol. 48, No. 8, 2009

Self-Assembled Porphyrin Dyads
Information.22 Regardless of the nature of the substrate and
of the receptor, 1:1 pentacoordinated complexes were
characterized, in agreement with available structural
data.15,16b The phen-centered spectroscopic probe was used
to establish the proximal or distal coordination. For the meso-
substituted receptors (ZnL2-ZnL4), the same recognition as
for ZnL1 was observed. These data confirm that the presence
of bulky groups (xylyl or 2,6-dimethoxyphenyl) do not alter
the distal insertion of the various N1-unsubstituted imidazoles
within the phen cavity. In all cases, except for S1, the sizable
increase of the stability constants compared with those with
ZnTPP arises from bifurcated hydrogen bonding between
the N1-H of the substrate and the two nitrogen atoms of the
phen moiety. The influence of the meso-substitution on the
stability constants was studied by comparing the values
obtained for ZnL2-ZnL4 and ZnL1 (Table 2). For
ZnL2-ZnL4, the complexes formed with N1-methylated S1
were further destabilized by a factor of 5-8 in comparison
with ZnL1-S1. This destabilization was due to electronic
effects and porphyrin ring distortions50 induced by these
meso-substituents.
The thermodynamic data (Table 2) clearly pointed out that
the presence of one (ZnL2) or two (ZnL3) meso-xylyl
substituents had only minor effects on the stability of the
corresponding complexes and unexpectedly did not increase
significantly the steric hindrance on the distal side as the
binding of S2-S8, H2L5, and H2L6 remained unchanged.
The extensive tilt15,16b,24 of the phen-strap over the porphyrin
plane, the important ruffling of the porphyrin macrocycle,33,53
and the partial rotation of the xylyl groups toward coplanarity
with the porphyrin plane combine to minimize the steric
interactions in the inclusion complexes. The main stabilizing
interactions (N-Zn coordination bond, bifurcated hydrogen
bonds, and π-π and CH-π interactions) are indeed
preserved.15,16b
loporphyrin macrocycle and the phenylethnyl spacers of H2L5
and H2L6.
Figure 8. CPK representation of the modeled structures of (a) ZnL3 and
(b) ZnL4 showing the more hindered binding cavity for ZnL4 compared
with ZnL3.
It is rather surprising that the steric hindrance contributes to
the stabilization of pentacoordinated complexes with small
substrates (S2 and S3) and ZnL4. The changes in the stability
within the ZnLi-S2 series of complexes (i ) 1-4) shows that
S2 is stabilized by about 1 order of magnitude upon inclusion
in ZnL4 with respect to its inclusion in receptors ZnL1-ZnL3.
This higher affinity most likely results from an enhanced
inertness due to the meso-resorcinol-type substituents of ZnL4,
which act as “stoppers” (Table 2). Similar increased ligand
affinities were previously reported for picket fence heme
models54 and for iron(II) tetramesitylporphyrin55 compared to
iron tetraphenylporphyrin. Traylor has called this type of
stabilization effect the “ortho effect”.55
Recognition Mechanism. Kinetic data on metalloporphy-
rins are extremely scarce in the literature.56 Nevertheless
these data are essential to both deciphering the assembly
mechanism21 and the understanding of the distal recognition
process. Therefore, a kinetic study (absorption and/or emis-
sion monitoring) was conducted to decrypt the formation of
pentacoordinated species with phen-strapped porphyrin re-
ceptors ZnL1-ZnL4. Kinetic measurements were carried out
under pseudo-first-order conditions using stopped-flow tech-
niques. For 1-methyl-1H-imidazole S1 (proximal recogni-
tion), the reactions were too fast to be measured under our
experimental conditions, in agreement with data reported,
for instance, for ZnTPP and S2 (log kf ) 9.5 ( 0.5 at 298
K in chlorobenzene).56,57 For substrates S3-S8, H2L5, and
H2L6, for which distal binding was clearly demonstrated, the
reaction times were slow enough to be recorded with
stopped-flow devices. The binding of the smallest substrate
S2 (distal recognition) by ZnL1-ZnL4 was too fast to be
measured, however.
In contrast, the presence of 2,6-dimethoxyphenyl meso-
substituents had drastic consequences on the association
constants (Table 2). The steric hindrance in ZnL4 forbids
the rotation of the 2,6-dimethoxy-phenyl groups toward the
porphyrin plane, and thus, the strapped receptor is more
sensitive to the bulkiness of the substrate (Figure 8). As a
result, the stability constant of ZnL4-S4 (log KZnL4-S4
2.3(3)) was significantly lowered in comparison to ZnL1-S4
(log KZnL1-S4 ) 3.92(7)) and ZnL3-S4 (log KZnL3-S4
)
)
4.10(7)) because of the strong steric interactions between
the phenyl substituent of S4 and the ZnL4 host. Similarly,
both bisporphyrin scaffolds ZnL4-H2L5 (log KZnL4-H2L
)
5
5.2(4)) and ZnL4-H2L6 (log KZnL4-H2L ) 5.0(3)) were about
1 order of magnitude less stable than the respective com-
plexes with ZnL1. Thus, the presence of the resorcinol-type
substituents inhibits π-π interactions between the metal-
The presence of the phen strap in ZnL1-ZnL4 significantly
slows down the formation reactions of the corresponding
pentacoordinated complexes, thus rendering the formation
kinetics accessible to stopped-flow techniques. A single rate-
limiting step was observed with no evidence of slowest or
fastest steps (see example provided in Figure 9a).
6
(53) Wondimagegn, T.; Ghosh, A. J. Phys. Chem. A 2000, 104, 4606–
4608.
(54) Collman, J. P.; Brauman, J. I.; Iverson, B. L.; Sessler, J. L.; Morris,
R. M.; Gibson, Q. H. J. Am. Chem. Soc. 1983, 105, 3052–3064.
(55) Portela, C. F.; Madge, D.; Traylor, T. G. Inorg. Chem. 1993, 32, 1313–
1320.
(56) Caldin, E.; Field, J. P. J. Chem. Soc., Faraday Trans. 1 1982, 78,
1923–1935.
(57) Ware, W. R.; Novros, J. S. J. Phys. Chem. 1966, 70, 3246–3253.
The pseudo-first-order rate constants kobs (s-1) vary linearly
with the concentration of the substrates (Figure 9b and
Figures S100-S127 and Tables S1-S28 in the Supporting
Information), and the ordinates at the origin were determined
with good accuracy. The bimolecular formation rate constants
Inorganic Chemistry, Vol. 48, No. 8, 2009 3749

Brandel et al.
efficient “stoppers”. Due to their rigid and planar structures,
the three benzimidazoles S6, S7, and S8 strongly distort the
receptors ZnL1-ZnL4 and consequently lead to more labile
species (Table 3). When the bulkiness of the meso-substit-
uents increases in ZnL4, the strong steric interactions between
2-phenyl-1H-imidazole S4 and the 2,6-dimethoxyphenyl
groups lead to more labile complexes (Table 3). For the
imidazolyl free-base porphyrins H2L5 and H2L6, no signifi-
cant variation of the dissociation rate constants kd was
observed in the ZnL1-ZnL4 series.
It was demonstrated that the sizable flexibility of the
porphyrin receptors favored the distal axial coordination of
a wide series of substrates without weakening the main
stabilizing interactions (N-Zn coordination and hydrogen
bonding). This distal binding, however, induces a significant
energetic cost and leads to severe deformations of the
porphyrin receptors, especially when bulky meso-substituents
are introduced. Kinetic data clearly exemplify this tendency
and highlight that when binding requires strong distortion
of the receptors because of bulky guests, the resulting
assemblies are more labile. The kd values hence result from
a delicate balance between secondary stabilizing interactions
(CH-π and π-π) and destabilizing steric interactions and
structural deformations of the porphyrin host exerted by
bulky substrates.
Activation Parameters. Substrate S4 exhibited the most
interesting kinetic behavior. Therefore, to better understand
the self-assembly mechanism, the activation enthalpies and
entropies of both the formation and dissociation steps were
determined for complexes ZnLi-S4 (i ) 1-4). As an
example, Eyring plots (kf and kd) are given in Figure 10 and
Figures S128-S135 in the Supporting Information.22 The
activation parameters determined for ZnLi-S4 (i ) 1-4)
are given in Table 4, and relevant data are provided in Tables
S29-S36 in the Supporting Information.
Among the four pentacoordinated metalloporphyrins (Table
4), complexes ZnL1-S4 and ZnL4-S4 represent two distinct
examples. ZnL4 bears bulky resorcinol-type meso-substituents
and exhibits similar activation enthalpies for the formation and
dissociation of ZnL4-S4 (∆Hqf ) 74(4) kJ mol-1 and ∆Hqd )
71(3) kJ mol-1), whereas the activation entropies values are
significantly different (∆Sqf ) 53(12) J mol-1 K-1 and ∆Sqd )
4(9) J mol-1 K-1). Consequently, the binding equilibrium is
dissociative and entropically driven with ∆∆Sq ) 49(15) J
mol-1 K-1. On the contrary, the unhindered complex ZnL1-S4
possesses activation enthalpies for formation and dissociation
(∆Hqf ) 42(2) kJ mol-1 and ∆Hqd ) 65(4) kJ mol-1) that
Figure 9. (a) Variation of the luminescence intensity versus time for the
formation of the ZnL1-H2L6 complex. Solvent ) 1,2-dichloroethane; T )
25.0(2) °C; l ) 1 cm; [ZnL1] ) 4.93 × 10-7 M; [H2L6] ) 1.24 × 10-5 M;
λexc ) 424 nm; set of band-pass filter (λ > 550 nm) and interference filter
(λ < 660 nm). (b) Variation of the pseudo-first-order constants kobs (s-1
)
for the formation of the ZnL1-H2L6 complex versus [H2L6]tot. Solvent )
1,2-dichloroethane; T ) 25.0(2) °C; [ZnL1] ) 4.93 × 10-7 M.
kf (M-1 s-1) and the monomolecular dissociation constants
kd (s-1) are given in Table 3.
Under our experimental conditions, except for S4, the
sequence of the second-order rate constants kf for all the
receptors examined (ZnL1-ZnL4) was as follows: benzimi-
dazoles > substituted imidazoles > porphyrin-imidazoles.
Substrate S4 stands in an interesting contrast, which could
be explained by the bulky phenyl group that is very sensitive
to meso-substitution of the receptors. Consequently, the
largest decrease in kf (>1.6 orders of magnitude) was
observed for ZnL4-S4 in comparison with ZnL1-S4 (Table
3). For substrates S3, S5-S8, H2L5, and H2L6, the substitu-
tion by one (ZnL2) or two (ZnL3) xylyl groups only
moderately influenced the kf values (Table 3). This kinetic
property is mainly due to the free rotating nature of the xylyl
substituents that keeps the steric constraints to a minimum.
The two resorcinol-type substituents bore by ZnL4 drastically
decrease the formation kinetics by about 1 order of magnitude
with respect to ZnL3, especially when steric bulk increases
at the 2-position of the imidazole. For the 2-methyl-
benzimidazole S7, no influence of the meso-substitution was
observed in the ZnL1-ZnL4 series, whereas an unusual
acceleration of the binding reaction rate occurred with S6
and S8. These results could be explained by the rigid and
planar structure of these benzimidazole substrates.
suggest an enthalpically controlled equilibrium (∆∆Hq
)
-23(2) kJ mol-1). These parameters are completed by ∆Sqf )
-26.0(9) J mol-1 K-1 and ∆Sqd ) -28(2) J mol-1 K-1. Thus,
in the absence of steric interactions between S4 and ZnL1, the
binding event proceeds through an associative mechanism with
an enthalpic contribution. ZnL2 binds S4 via a similar pathway.
The assembly mechanism of ZnL4 with S4 is dissociative and
The lability of the substituted imidazoles S3 and S5 was
very sensitive to the presence of steric bulk at the meso-
substitution of the zinc porphyrin receptors. For instance, a
significant decrease (1-3 orders of magnitude) in inertness
was observed for the corresponding ZnL4 complexes, sug-
gesting that the resorcinol-type meso-substituents act as
(58) (a) Espenson, J. H. In Chemical Kinetics and Reaction Mechanisms,
2nd ed.; McGraw-Hill: New York, 1995. (b) Poe¨, A. J. In Mechanisms
of Inorganic and Organometallic Reactions; Twigg, M. V., Ed.;
Plenum Press: New York, 1994, Vol. 8, Chapter 10.
3750 Inorganic Chemistry, Vol. 48, No. 8, 2009

Self-Assembled Porphyrin Dyads
a
Table 3. Averaged Rate Constants Determined by Kinetic Studies for the Formation Mechanism of Pentacoordinated Complexes with ZnL1-ZnL4
S3
S4
S5
S6
d
d
ZnL1
ZnL2
ZnL3
ZnL4
ZnL1
ZnL2
ZnL3
ZnL4
kf (M-1 s-1
)
2.2(9) × 107
1.7(3) × 107
1.7(7) × 107
3(2) × 106
16(8)
2.0(7) × 104
1.3(2) × 104
6.0(2) × 103
4.9(1) × 102
1.4(2)
0.5(1)
0.44(5)
4.3(3)
1.54(8) × 105
5(3) × 105
8(2) × 105
2.0(3) × 104
14(2)
11(7)
16(9)
0.65(9)
6.4(8) × 107
nd
nd
nd
d
kd (s-1
)
162(51)d
1.7(8)c
nd
nd
nd
1.7(8)c
0.06(6)c
S7
S8
H2L5
H2L6
d
b
b
ZnL1
ZnL2
ZnL3
ZnL4
ZnL1
ZnL2
ZnL3
ZnL4
kf (M-1 s-1
)
2.5(4) × 107
6.3(6) × 107
3.2(1.0) × 105
2.2(3) × 105
2.3(3) × 105
1.7(6) × 104
0.24(0.16)
2.3(7)
4.2(1.0) × 105
2.1(9) × 105
1.7(3) × 105
1.6(2) × 104
1.3(7)
3.0(1.3)
0.5(2)
0.22(7)
b
b
3.2(2) × 107
2.5(2) × 107
b
d
2.0(4) × 107
1.4(2) × 108
b
1.9(6) × 107
nd
d
kd (s-1
)
136(21)b
92(11)b
54(20)b
192(22)b
110(35)b
44(3)b
142(42)b
nd
0.5(3)
1.2(3)
a Solvent ) 1,2-dichloroethane; T ) 25.0(2) °C. UV-vis absorption and fluorescence kinetic measurements. Uncertainties ) 3σ. See ESI for detailed
experiments. b Fluorescence kinetic measurements. c Estimated values. nd: not accurately determined because of the fast reaction. kobs ) kf[Sn]tot + kd.
d
entry of substrate S4 within the receptor’s binding cavity;
however, they do play a key role in the release pathway.
Self-Assembled Photochemical Dyads. Regardless of the
nature of the meso-substituents, the strapped Zn(II) porphyrin
receptors (ZnL1-ZnL4) are able to self-assemble with the
two imidazole-appended free-base porphyrins (H2L5 and
H2L6). In the corresponding dyads, the length of the
phenylethynyl spacer (∼8 Å) is sufficiently long enough to
minimize steric hindrance between the two porphyrins. It is
noteworthy that in these photoactive dyads, the strapped
Zn(II) porphyrin-imidazole complexes constitute the effec-
tive energy donors (or photochemical antenna) and the
appended free-base porphyrins play the role of the energy
acceptors (Figure 11). The center-to-center distances between
the two porphyrins were estimated from molecular mechanics
to be ∼12.5-13 Å.
Figure 10. Eyring plots for (a) the formation and (b) the dissociation of
ZnL4-S4 complex. Solvent ) 1,2-dichloroethane.
Table 4. Enthalpies and Entropies for the Formation (∆Hfq, ∆Sfq) and
a
q
q
the Dissociation (∆Hd , ∆Sd ) of ZnLi-S4 Complexes (i ) 1-4)
formation
dissociation
q
q
∆Hfq
∆Sfq
∆Hd
∆Sd
(kJ mol-1
)
(J K-1 mol-1
)
(kJ mol-1
)
(J K-1 mol-1
)
Figure 11. Schematic representation of the bisporphyrin photodyad
ZnL1-H2L6.
ZnL1
ZnL2
ZnL3
ZnL4
42(2)
29(2)
35(2)
74(4)
-26(6)
-70(6)
-54(6)
53(12)
65(4)
72(4)
82(6)
71(3)
-28(12)
-4(9)
24(18)
4(9)
The spectrophotometric properties (Q bands) of the free
receptors, of the imidazolyl free-base porphyrins and of the
corresponding dyads are given in Table S37 in the Supporting
Information. For the sake of comparison, the spectrophoto-
metric data of complexes ZnLi-S2 (i ) 1-4) are also
provided (Table S37 in the Supporting Information), because
they correspond to the effective energy donors within the
corresponding photochemical dyads. Regardless of the nature
of the dyads, the ground-state electronic spectra of their Q
a Solvent ) 1,2-dichloroethane. Uncertainties ) 3σ with σ(∆Hq) ∼
σ(∆Sq) × 1/Tav.58
entropically driven, due to conformational changes of the
substrate and to distortions of the receptor. The presence of two
xylyl substituents in ZnL3 significantly alters the energetic
profile of both the formation and the dissociation reactions,
which are associative and dissociative, respectively. The two
meso-xylyl substituents in ZnL3 do not strongly influence the
Inorganic Chemistry, Vol. 48, No. 8, 2009 3751

Brandel et al.
bands correspond to the sum of the spectra of the component
porphyrins. This observation strongly suggests weak elec-
tronic coupling59 between the two chromophores (Figures
S136-S143 in the Supporting Information). These results
are also supported by the electrochemical properties obtained
for the ZnL1-H2L5 complex.18
excitation light will be absorbed by the Zn(II) porphyrin
(66.6% for ZnL3-H2L5 and 19.9% for free ZnL3), whereas
∼13.5% will be absorbed by free H2L5. These observations
are also valid for the other photodyads and emphasized that
our experimental conditions are well set up to monitor the dyad
formation by steady-state luminescence spectroscopy and to
conclude whether the energy transfer is efficient or not.
The photophysical properties of the receptors ZnLi (i )
1-4) and of the imidazolyl free-base porphyrins H2L5 and
H2L6 are given in Table 5. The data obtained for ZnLi-S2,
which mimic the energy donors in the photochemical dyads,
are also reported. The photophysical characteristics of these
porphyrins are dependent on the structure, symmetry, and
bulkiness of the tetrapyrrolic macrocycle.41 Indeed, deforma-
tion of the porphyrin imposed by the meso-substituents
decreases the HOMO and LUMO energy gap and hence
results in bathochromic shifts of the S1 f S0 emission bands
with respect to the unsubstituted ZnL1 reference. Further-
more, the constraints induced by the bulky meso-substituents
as well as the asymmetrical substitution, such as in ZnL2,
led to a significant decrease of luminescence quantum yield
compared with that of ZnL1 (Table 1).
Figure 12. Spectrofluorometric titration of ZnL3 by H2L5. Solvent ) 1,2-
dichloroethane; T ) 25.0(2)°C; [ZnL3]tot ) 1.11 × 10-5 M; (1) [H2L5]tot/
[ZnL3]tot ) 0; (2) [H2L5]tot/[ZnL3]tot ) 1.56; λexc ) 554 nm; emission and
excitation bandwidths ) 10 nm.
Table 5. Photophysical Parameters of ZnLi (i ) 1-4) Receptors, of
H2L5 and H2L6, and of ZnLi-S2 Pentacoordinated Complexesa
As an example, Figure 13 displays the steady-state
luminescence emission spectrum of ZnL3-H2L5 compared
to those of ZnL3-S2 and of H2L5. The emission spectrum
of the dyad closely resembles that of the free-base porphyrin
acceptor, thus substantiating that efficient energy migration
occurs within these photoactive edifices. Moreover, a residual
emission signal most likely arising from the donor (i.e., the
pentacoordinated Zn(II) strapped porphyrin) is also observed
and corresponds to the ZnL3-S2 model emission. About
80% of the excitation energy of ZnL3-S2 is transferred to
the appended free-base porphyrin H2L5 within the corre-
sponding dyad (Tables 5 and 6). The other self-assembled
dyads exhibit comparable behavior, with at least 4-10-fold
quenching of the zinc porphyrin’s emission (Table 6).
λem (nm)
max
abs
compounds
λexc (nm)
Q(0,0)
Q(0,1)
QL (%)
H2L5
421
421
420
422
428
433
424
429
435
436
653
650
594
601
609
611
604
613
622
620
716
712
646
648
659
664
657
667
676
674
3.5
2.1
1.6
0.9
1.4
1.5
1.2
0.7
0.9
1.3
H2L6
ZnL1
ZnL2
ZnL3
ZnL4
ZnL1-S2b
ZnL2-S2b
ZnL3-S2b
ZnL4-S2b
a Solvent ) 1,2-dichloroethane; T ) 25.0(2) °C. b Calculated from
emission titrations of the ZnLi receptors with the 1H-imidazole S2 substrate.
λ
exc corresponds either to the maximum of absorption of the Soret band for
ZnLi, H2L5, and H2L6 or to isosbestic point between ZnLi and ZnLi-S2.
abs
The uncertainties on the λem and QL are 1 nm and 30%, respectively.
max
The recognition of the imidazolyl free-base porphyrins
H2L5 and H2L6 by ZnLi (i ) 1-4) was therefore monitored
by spectrofluorimetry (Figure 12). The stability constants
obtained were in excellent agreement with those determined
in the ground state (Table S38), thus demonstrating that the
recognition properties were preserved in the excited state
(Table S38 in the Supporting Information). As a representa-
tive example, the spectrofluorimetric titration of ZnL3 by
H2L5 (Figure 12) clearly shows a drastic decrease of the
Zn(II) metalloporphyrin emission band (609 nm) with a
concomitant increase of the free-base porphyrin-centered
emission bands at 653 and 716 nm (Table 5). The absorption
spectra of the strapped zinc porphyrins severely overlapped
with those of the free-base porphyrins. This overlap pre-
vented selective excitation of the input unit (i.e., the ZnLi-S2
model of the dyads). In ZnL3-H2L5, for example, it is
calculated that for an equimolar mixture of both reactants
(Figure 12) and at λexc ) 554 nm, ∼86.5% of the incident
Figure 13. Steady-state fluorescence emission spectra of ZnL3-H2L5 (gray
line) compared to that of H2L5 (black line) and that of the antennae model
unit ZnL3-S2 (dotted line). ZnL3-S2 centered emission is quenched, while
H2L5 centered emission is exalted within the dyad ZnL3-H2L5. The area
filled in gray corresponds to the nontransmitted emission signal arising from
the pentacoordinated Zn(II) porphyrin within the dyad. Solvent ) dichlo-
roethane; T ) 25.0(2) °C; λex ) 543 nm.
Modeling Energy Transfer: Fo¨rster Approach. Our
spectrophotometric results, as well as electrochemical data,18
clearly indicate the absence of interporphyrin electronic
(59) Dexter, D. L. J. Chem. Phys. 1953, 21, 836–850.
3752 Inorganic Chemistry, Vol. 48, No. 8, 2009

Self-Assembled Porphyrin Dyads
interactions. Therefore, the possibility of dipole-dipole
Fo¨rster interactions60-62 was examined using a steady-state
approach. The efficiency of the energy transfer QET (eq 1)
was estimated from spectrophotometric and photophysical
data of the separate donor (ZnLi-S2) and acceptor (H2L5
or H2L6) entities, using the following equation:
6
6
QET ) R0 /(R0 + d6)
(1)
where d (∼13 Å) is the donor-acceptor distance that was
assumed to be comparable for all dyads, R0 is the Fo¨rster
critical radius that is defined as the center-to-center distance
between the two fluorophores for which QET ) 50%.60,61
The distance R0 (eq 3) varies with
Figure 14. Spectral overlap integral J(λ) for ZnL1-S2 and H2L6. Solvent
) 1,2-dichloroethane; T ) 25.0(2) °C.
spectral overlap between the emission and absorption spectra
of the donors and of the acceptors, respectively. The values of
the spectral overlap integrals J(λ) (1.27 × 10-13 M-1 cm3 <
J(λ) < 7.44 × 10-13 M-1 cm3 (Figures S144-S151 in the
Supporting Information) are consistent with literature values
for covalent porphyrin polyads.64,66-68 The Fo¨rster critical
distances (24.4 Å < R0 < 34.2 Å) indicated that the distance (d
∼ 13 Å)18 between the two chromophores is suitable for
quantitative energy transfer (QET . 95%).
R0 ) 9.78 × 103[κ2n-4QDJ(λ)]1/6
(2)
where QD is the donor’s fluorescence quantum yield (Table
5), n is the index of refraction of the medium (n25 ) 1.4448),
and J(λ) is the spectral overlap of the fluorescence bands of
the donor (ZnLi-S2; i ) 1-4) with the absorption bands
of the acceptor (H2L5 or H2L6). The mutual orientation of
the donor’s and the acceptor’s transition moments was
estimated63 to be κ2 ) 0.9 from the PM3 model of the
ZnL1-H2L6 dyad (Figure 7). This κ2 value agrees well with
values (2/3 < κ2 < 1.25) commonly used in the literature for
donor-acceptor pairs similar to those presented in this
work.18,64,65 The spectral overlap integral J(λ) (shown in
Figure 14 and Figures S144-S151 in the Supporting
Information) is calculated from the acceptor’s absorption
spectrum (ε(λ)) and the donor’s normalized emission spec-
trum (F(λ)) as follows:
Table 6. Spectral-Overlap Integrals J(λ), Fo¨rster Critical Radii (R0),
and Experimentalb and Estimatedc Efficiencies of the Energy Transfers
for the ZnLi f H2Lj Dyads (i ) 1-4; j ) 5, 6)a
b
c
J(λ) × 1013
QET
QET
donor
acceptor (M-1 cm3) R0 (Å) experimental estimated
ZnL1-S2
H2L5
H2L5
H2L6
H2L5
H2L6
H2L5
H2L6
H2L5
H2L6
7.4
34.2
24.7d
29.1
30.3
24.7
30.7
24.4
33.0
26.3
91
99.7
97.8d
99.3
99.5
98.3
99.5
98.1
99.7
98.8
2.8
6.1
1.8
5.2
1.3
5.5
1.4
89
86
92
80
80
88
78
b
ZnL2-S2
ZnL3-S2
ZnL4-S2
J ) ∞ ε(λ) F(λ)λ4 dλ
(3)
∫
0
The values of the spectral overlap integral J(λ), of the Fo¨rster
critical radius (R0), and of the estimated energy transfer
efficiency (QET) are given in Table 6.60,61 These data clearly
reflect the optimization of the photoinduced processes by a large
a Solvent ) 1,2-dichloroethane; T ) 25.0(2) °C. QET ) 1-QDA/QD,
where QDA is the quantum yield of the donor in the dyad and QD is the
quantum yield of complex ZnLi-S2. c Calculated from eq 2 with d ) 13
d
Å. From reference 18 with d ) 12.6 Å (see eq 1) and k2 ) 1 (see eq 2).
The uncertainty of the QET is 10%.
(60) Fo¨rster, T. Discuss. Faraday Soc. 1959, 27, 7–17.
(61) (a) Fo¨rster, T. Naturwissenchaften 1946, 33, 166–175. (b) Fo¨rster, T.
Ann. Phys. 1948, 2, 55–73.
(62) Clegg, R. Fluorescence Resonance Energy Transfer and Nucleic Acids.
In Methods in Enzymology; Lilley, D. M. J., Dahlberg, J. E., Eds.;
Academic Press: San Diego, CA, 1992; Vol. 211, pp 353-388.
(63) Martensson, J. Chem. Phys. Lett. 1994, 229, 449–456.
(64) Kilsa, K. J.; Kajanus, J.; Martensson, J.; Albinsson, B. J. Phys. Chem.
B 1999, 103, 7329–7339.
(65) (a) Hsiao, J. S. P.; Krueger, B.; Wagner, R. W.; Johnson, T. E.;
Delaney, J. K.; Mauzerall, D. C.; Fleming, G. R.; Lindsey, J. S.;
Bocian, D. F.; Donohoe, R. J. Am. Chem. Soc. 1996, 118, 11181–
11193. (b) Osuka, A.; Kobyaashi, F.; Maruyama, K.; Mataga, N.;
Asashi, T.; Okada, T.; Yamazaki, I.; Nishimura, Y. Chem. Phys. Lett.
1993, 201, 223–228. (c) Osuka, A.; Maruyama, K.; Yamazaki, I.;
Tamai, N. Chem. Phys. Lett. 1990, 165, 392–396.
(66) Hsio, J. S.; Krueger, B. P.; Wagner, R. W.; Thomas, E. J.; Delaney,
J. K.; Mauzerall, D. C.; Fleming, G. R.; Lindsey, J. S.; Bocian, D. F.;
Donohoe, R. J. J. Am. Chem. Soc. 1996, 118, 11181–11193.
(67) Kilsa, K. J.; van Berlekom, S. B.; Kajanus, J.; Martensson, J.;
Albinsson, B. J. Phys. Chem. A 1997, 101, 2218–2220.
(68) Hindin, E.; Kirmaier, C.; Diers, J. R.; Tomizaki, K. Y.; Taniguchi,
M.; Lindsey, J. S.; Bocian, D. F.; Holten, D. J. Phys. Chem. B 2004,
108, 8190–8200.
In dyad ZnL1-H2L5, the existence of energy back-transfer
from H2L5 to ZnL1 was previously demonstrated.18 To test the
feasibility of an energy back-transfer from H2L6 to the penta-
coordinated metalloporphyrin within the H2L6 f ZnL1 system,
the corresponding critical Fo¨rster radius R′0 (J′(λ) ) 1.23 ×
10-13 M-1 cm3 and R′0 ) 27.8 Å) was calculated. These data
emphasized that the back-transfer process is theoretically
efficient too (QET-1 ) 99.1%). This back-transfer may explain
the differences in the values of QET presented in Table 6. By
considering the ZnL1-H2L6 system and the lifetimes τH2L6
(8.99(5) ns)69 and τZnL1-S2 (1.80(3) ns),18 we determined kET
-1
and kET-1 values (kET ) (τZnL1-S2
)
× (R0/d)6 ) 6.99 × 1010
-1
s-1; kET-1 ) (τH2L6
)
× (R′0/d)6 ) 1.06 × 1010 s-1) (Figure
15). These dynamic parameters show that among the numerous
radiative and nonradiative routes possible in our photodyads,
the ZnL1 f H2L6 energy transfer dominates the photoinduced
processes.
(69) Our work: solvent ) 1,2-dichloroethane; T ) 25.0(2) °C; λex ) 590
nm; λem ) 650 nm.
Inorganic Chemistry, Vol. 48, No. 8, 2009 3753

Brandel et al.
assembly mechanism. These parameters revealed that bulky
ortho substituents in ZnL4 induced important steric constraints,
which were reflected by the associative and enthalpic character
of the binding process. For ZnL1 and ZnL2, the coordination
was dissociative, entropically driven and highlighted weak
conformational constraints.
The photoinduced energy migration pathways within the
photoactive dyads have been clearly established and quantified.
Dipole-dipole interactions between the two porphyrin cores
were demonstrated. Experimental data derived from lumines-
cence titrations confirmed that, upon excitation of the metal-
loporphyrin, excited-state energy migrations occurred efficiently,
regardless of the number and the nature of the meso-substituents.
However, contrary to models, QET values were attenuated
because of possible back-transfer processes. Dynamic param-
eters showed that energy transfer from the pentacoordinated
input to the free-base porphyrin output was strongly favored
and very efficient. This work provided a basis for the develop-
ment of multiporphyrin arrays with predictable electronic and
photophysical properties. The noncovalent molecular edifices
examined in this work revealed to be very efficient photoactive
systems, even compared with covalent analogues.
Figure 15. Dynamic processes in ZnL1-H2L6 photoactive self-assembled
dyad.
Conclusion
The phen-strapped Zn(II) porphyrin receptors ZnLi (i ) 1-4)
exhibit suitable topography tailored to strongly and selectively
bind various N1-unsubstituted imidazole-type substrates
(S2-S8) and imidazoles appended with free-base porphyrins
(H2L5 and H2L6). The distal discrimination of imidazole N-Zn
axial binding is imposed by the possibility of bifurcated
hydrogen bonds with the phenanthroline moiety of the receptors.
This property consequently stabilizes the corresponding com-
plexes and leads to an efficient multipoint host-guest system.
An extensive physicochemical approach indicated that the
introduction of xylyl substituents (ZnL2, ZnL3) has little
influence on the stability and kinetics of the targeted supramol-
ecules. Xylyl groups do not provide significant steric hindrance
around the distal binding of S2-S8, H2L5, and H2L6. Indeed,
the sizable flexibility of the receptors and the free rotation of
the xylyl substituents toward the porphyrin plane constitute
structural features which drastically diminish the steric interac-
tions in the recognition process. For the ZnL1-ZnL3 series,
the stabilizing interactions (N-Zn coordination bond, bifurcated
hydrogen bonds, and π-π and CH-π interactions) are
preserved. In contrast, meso-substitution with resorcinol-type
moieties (ZnL4) strikingly led to a host that was much more
sensitive to the bulkiness of the substrate due to the inhibited
rotation of these moieties. For ZnL4, the most remarkable
outcome concerns the stabilization of the complexes with the
smallest substrates (S2, S3).
The self-assembly mechanism was fully elucidated and
supported the conclusions deduced from the equilibrium studies.
The presence of the xylyl units does not significantly influence
the formation kinetics, whereas resorcinol-type substituents in
ZnL4 drastically slow down the formation kinetics, particularly
for 2-substituted substrates. The most important effect was
observed when bulkiness was introduced both at the imidazole
2-position (S4) and the receptor’s meso-positions (ZnL4). In
comparison with ZnL1-ZnL3, the inertness of the pentacoor-
dinated ZnL4 complexes was considerably increased for the
smallest substrates (“ortho effect”), whereas more labile species
were produced when substrates with bulky 2-substituents were
combined.
Acknowledgment. This work has been supported by the
Centre National de la Recherche Scientifique (UMR 7177
CNRS/ULP). J.B., A.T. and F.M. thank the French Ministry
of Research and Education for granting them Ph.D. fellowships.
M.K. thanks the Re´gion Alsace and Centre National de la
Recherche Scientifique for a Ph.D. fellowship. J.W. is grateful
to the CNRS (ACI-NX001) and Research Council of the
Universite´ Louis Pasteur for financial support. The authors
express their thanks to Dr. Martine Heinrich (Service d’analyses,
de mesures physiques et de spectroscopie optique, Institut de
Chimie, UMR 7177) for measuring the luminescence lifetime
of H2L6 and to Mohamad Hmadeh (Laboratoire de Physico-
Chimie Bioinorganique, UMR 7177 CNRS/ULP) for quantum
yields measurements.
Supporting Information Available: Spectrophotometric titrations
(absorption and emission) of the ZnTPP receptor by substrates S1-S8
(Figures S1-S16). Spectrophotometric titrations (absorption and
emission) of the ZnL1 receptor by substrates S1-S8, H2L5, and H2L6
(Figures S17-S36). Electronic spectra of 2,9-diphenyl-1,10-phenan-
throline and its protonated species (Figure S37). X-ray crystal structures
of pentacoordinated Zn(II)-porphyrin complexes (Figures S38-S39).
Spectrophotometric titrations (absorption and emission) of the ZnL2
receptor by substrates S1-S8, H2L5, and H2L6 (Figures S40-S99).
Kinetic studies of receptor ZnL1 (Tables S1-S8 and Figures
S100-S107), of receptor ZnL2 (Tables S9-15 and Figures S108-S114),
of receptor ZnL3 (Tables S16-S22 and Figures S115-S121), and of
receptor ZnL4 (Tables S23-S28 and Figures S122-S127). Activation
parameters for ZnL1-S4 (Tables S29-S30 and Figures S128-S129),
for ZnL2-S4 (Tables S31-S32 and Figures S130-S131), for
ZnL3-S4 (Tables S33 and S34 and Figures S132 and S133) and for
ZnL4-S4 (Tables S35 and S36 and Figures S134 and S135).
Spectrophotometric parameters of the porphyrin receptors, substrates,
and complexes (Table S37). Stability constants of the bisporphyrin
dyads (Table S38). Comparison of the electronic spectra of the dyads
with the sum of the electronic spectra of their components (Figures
S136-S143). Spectral overlap integrals (Figures S144-S151). This
material is available free of charge via the Internet at http://pubs.acs.org.
The determination of the activation parameters with the
representative substrate S4 provided deeper insight into the self-
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3754 Inorganic Chemistry, Vol. 48, No. 8, 2009