Controlling photoinduced electron transfer within a hydrogen-bonded porphyrin-phenoxynaphthacenequinone photochromic system [3]

Full text of DOI:10.1021/ja002733p

J. Am. Chem. Soc. 2001, 123, 177-178
177
Controlling Photoinduced Electron Transfer within a
Hydrogen-Bonded
Porphyrin-Phenoxynaphthacenequinone
Photochromic System
Andrew J. Myles and Neil R. Branda*
Department of Chemistry, UniVersity of Alberta
Edmonton, AB, Canada T6G 2G2
ReceiVed July 25, 2000
ReVised Manuscript ReceiVed NoVember 10, 2000
Photoswitchable photoinduced electron transfer (PET) systems
have potential applications in efficient and fast-responding
photonic devices,1 using the “ON/OFF” properties of the system
to store and/or transfer information in a nondestructive manner.
Possible mechanisms for regulating PET include varying the
oxidation potential of the electron donor species, altering the
conjugation pathway or through space orientation between the
donor and acceptor species, or changing the reduction potential
of the acceptor species. Although several examples of PET
regulation have been reported,2,3 few rely on the practical and
efficient properties of light energy as the regulatory stimulus,3
and none utilize porphyrins as photoexcited electron donors and
quinones as electron acceptors. This hybrid system is among those
most commonly found in natural and many elegant synthetic light-
harvesting systems.4
Our strategy to regulate PET involves reversibly changing the
reduction potential of the acceptor species by incorporating a
porphyrin and a photochromic phenoxynaphthacenequinone into
a supramolecular system. The photochrome is well-known for
its reversible photoisomerization between its trans and ana forms
(highlighted within the boxes in Figure 1), thermal irreversibility,
and fatigue resistance.5 By exploiting the expected difference in
reduction potentials between the trans and ana forms, a photo-
controlled PET system can be realized provided the photoexci-
tation of the porphyrin occurs at wavelengths at which both the
trans and ana forms are transparent. Not only is this report one
of few examples of photocontrolled PET, it represents the first
example of photoregulation of PET in a porphyrin-quinone
system by reversibly changing the electronic properties of the
electron acceptor.
Figure 1. Structures of hydrogen-bonded complexes used in this study.
component system in which the association of the porphyrin P1
and the phenoxynaphthacenequinone 1 relies on hydrogen bonding
as illustrated in Figure 1.7 Our design takes advantage of strong
hydrogen bonding between the carboxylate acceptor and the urea
donor. These molecular recognition elements were chosen to
ensure that the supramolecular structures are retained even at the
low concentrations required to conveniently evaluate electron-
transfer processes using luminescence spectroscopy without
having to add excessive quantities of the quenching component.
Cyclic voltammetry experiments clearly show that the ana
isomer 1a should act as a better electron acceptor than the trans
isomer 1t. When a CH2Cl2 solution of 1t was irradiated at 365
nm, the peak corresponding to the reduction of the trans isomer
was replaced by two new peaks representing the reduction of the
ana form (Figure 2). Isomer 1a was reduced at a significantly
less negative potential (-723 mV) than 1t (-1145 mV). The peak
for the reduction of 1t never fully disappearing in the cyclic
voltammogram and 1H NMR spectroscopy reveals that the
photostationary state generated at the wavelength used is made
up of a 5:1 ana:trans mixture. The solution can be irradiated with
light greater than 434 nm to reform 1t and regenerate the original
voltammogram.
We recently reported how the photoisomerization of the
phenoxynaphthacenequinone was shut down due to its intimacy
with a porphyrin within a covalently linked hybrid.6 This
inhibition can be circumvented by using a dynamic two-
(1) (a) Balzani, V.; Scandola, F. Supramolecular Photochemistry; Ellis
Horwood: New York, 1991; Chapter 12, pp 355-394. (b) Lehn, J.-M.
Supramolecular Chemistry; VCH: Weinheim, 1995; Chapter 8, pp 89-138.
(2) (a) Zahavy, E.; Fox, M. A. Chem. Eur. J. 1998, 4, 1647-1652. (b)
Gosztola, D.; Niemczyk, M. P.; Wasielewski, M. R. J. Am. Chem. Soc. 1998,
120, 5118-5119. (c) Harada, A.; Yamaguchi, H.; Okamoto, K.; Fukushima,
H.; Shiotsuki, K.; Kamachi, M. Photochem. Photobiol. 1999, 70, 298-302.
(d) Hatzidimitriou, A.; Gourdon, A.; Devillers, J.; Launay, J.-P.; Mena, E.;
Amouyal, E. Inorg. Chem. 1996, 35, 2212-2219.
(3) (a) Endtner, J. M.; Effenberger, F.; Hartschuh, A.; Port, H. J. Am. Chem.
Soc. 2000, 122, 3037-3046. (b) Daub, J.; Beck, M.; Knorr, A.; Spreitzer, H.
Pure Appl. Chem. 1996, 68, 1399-1404. (c) Fernandez-Acebes, A.; Lehn,
J.-M. Chem. Eur. J. 1999, 5, 3285-3292. (d) Tsuchiya, S. J. Am. Chem. Soc.
1999, 121, 48-53. (e) Tsivgoulis, G. M.; Lehn, J.-M. Chem. Eur. J. 1996, 2,
1399-1406. (f) Huck, N. P. M.; Feringa, B. L. J. Chem. Soc., Chem. Commun.
1995, 1095. (g) Archut, A.; Azzelini, G. C.; Balzani, V.; De Cola, L.; Vogtle,
F. J. Am. Chem. Soc. 1998, 120, 12187-12191. (h) Ranjit, K. T.; Marx-
Tibbon, S.; Ben-Dov, I.; Willner, B.; Willner, I. Isr. J. Chem. 1996, 36, 407-
419.
The negative values of the free energies for photoinduced
electron-transfer calculated8 for P1‚1t (-0.94 kcal mol-1) and
P1‚1a (-10.67 kcal mol-1) indicate that both reactions are
thermodynamically favorable with that for the ana isomer being
significantly more exergonic.
The free energies of association for both 1t and 1a with P1
were measured by 1H NMR titration experiments. The significant
(4) (a) Kavarnos, G. J. Fundamentals of Photoinduced Electron Transfer;
VCH: New York, 1993. (b) Ward, M. D. Chem. Soc. ReV. 1997, 26, 365-
375.
(5) Fang, Z.; Wang, S.; Yang, Z.; Chen, B.; Li, F.; Wang, J.; Xu, S.; Jiang,
Z.; Fang, T. J. Photochem. Photobiol., A 1995, 88, 23.
(6) Myles, A. J.; Branda, N. R. Tetrahedron Lett. 2000, 41, 3785-3788.
(7) The preparation and characterization of compounds 1 and P1 are
described in the Supporting Information.
(8) Calculated using the Rehm equation for photoinduced electron transfer
(see Supporting Information).
10.1021/ja002733p CCC: $20.00 © 2001 American Chemical Society
Published on Web 12/14/2000

178 J. Am. Chem. Soc., Vol. 123, No. 1, 2001
Communications to the Editor
Figure 2. Cyclic voltammograms of CH2Cl2 solutions of 1t (top) and
the photostationary state of 1a + 1t generated with 365-nm light (bottom).
Residual 1t is highlighted in the bottom trace.
Figure 3. Changes in the UV-vis absorption spectra of a CH2Cl2 solution
of P1‚1t (1 × 10-5 M) upon irradiation with 365-nm light. Irradiation
periods are 10, 20, 30, 40, 80, and 120 s. The inset shows the changes
for 1t under identical conditions.
downfield shift (∆δ greater than 5 ppm) observed for the urea
N-H protons of P1 (1.09 mM in CD2Cl2) as it was treated with
aliquots of a solution of either 1t or 1a (6.54 mM in the same
solvent) clearly indicates effective hydrogen bonding. For these
experiments, solutions of the ana isomer were prepared by
irradiating the trans form at 365 nm until its photostationary state
was reached, at which time the mixture contained 81% 1a. Data
from all titrations correlate well with calculated curves using 1:1
binding models9 and give values for the free energy of association
of -6.15 ( 0.12 kcal mol-1 for 1t and -5.15 kcal mol-1 for 1a.
The value measured for 1a greatly depends on the relative amount
of this isomer present in solution, and when the titration
experiments were repeated with solutions containing less of the
ana isomer, more negative free energy values were obtained
(-5.60 kcal‚mol-1 at 78% 1a and -5.99 kcal‚mol-1 at 74% 1a).
These observations indicate that the association of the trans form
is greater than that of the ana form.
Photophysical studies show that the absorption spectrum in the
UV-vis region for the hydrogen-bonded complex P1‚1t is
essentially the sum of the absorption spectra of its components.
The low-energy absorptions of the porphyrin Q-bands allow for
the porphyrin to be selectively irradiated at a region of the
spectrum where photochrome 1t and 1a are transparent. Upon
irradiation at 365 nm, 1t cleanly photoisomerizes to 1a as shown
by the appearance of a new absorption at 480 nm (Figure 3).
This trend mimics the changes that occur when phenoxynaph-
thacenequinone 1t is irradiated alone (see Figure 3, inset).
Irradiation of the ana isomer at wavelengths greater than 434
nm results in a return to the original absorption spectrum,
confirming the reversible photoisomerization of the phenoxynaph-
thacenequinone in the presence of porphyrin.
Figure 4. Inverse Stern-Volmer quenching of CH2Cl2 solutions (1 ×
10-5 M) of P1 (λexcit ) 560 nm, λem ) 652 nm) when treated with
tetrabutylammonium 4-methylbenzoate (4), 1t (b), 1a (9), methyl ester
of 1t + tetrabutylammonium 4-methylbenzoate (O), and methyl ester of
1a + tetrabutylammonium 4-methylbenzoate (0).
controlled quenching. The addition of 1t resulted in an even
greater extent of quenching due to its forming a hydrogen-bonded
complex with P1. The most dramatic decrease in the fluorescence
intensity was observed when 1a was titrated into P1. In this case,
the quenching process more than compensates for the hydrogen
bond-induced increase in emission. We attribute this quenching
to more favorable electron transfer from P1 to 1a. These results
are consistent with our hypothesis and clearly indicate that
photocontrolled PET can be achieved by photoisomerization of
phenoxynaphthacenequinone from the trans to the ana form.
Energy transfer is not a likely cause of the quenching as there
are only zero overlap integrals between the absorption and
emission spectra of the acceptor and donor and the calculations
of the free energy for electron-transfer argues our hypothesis. This
system represents a nondestructive read/write system in which
both reading and writing are photoinduced.
The steady-state emission properties of P1‚1 differ greatly from
those of the system’s building blocks (Figure 4). The effect of
hydrogen bonding on the fluorescence spectrum of the porphyrin
was assessed by titrating CH2Cl2 solutions of P1 with tetrabutyl-
ammonium 4-methylbenzoate dissolved in the same solvent.
Hydrogen bonding afforded an increase in the fluorescence
intensity and a slight bathochromic shift (4 nm). Analogous
titrations of P1 with equimolar solutions of tetrabutylammonium
4-methylbenzoate and the methyl ester of 1t or 1a show a similar
trend. We attribute the small decrease in porphyrin fluorescence
upon the addition of the esters of 1t and 1a to diffusionally
Acknowledgment. We thank the Natural Sciences and Engineering
Council of Canada and the University of Alberta for financial support of
this research.
Supporting Information Available: Synthetic details for compounds
1 and P1 and calculations of the free energies for photoinduced electron
transfer (PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
(9) The 1H NMR titration data were analyzed using Christopher A. Hunter’s
1:1 complexation model program (Krebs Institute for Biomolecular Science,
Department of Chemistry, University of Sheffield, UK). All titrations were
performed in triplicate.
JA002733P