Porphyrinic phenoxynaphthacenequinones

Article abstract of DOI:10.1016/S0040-4039(00)00503-7

The photophysical properties of porphyrinic phenoxynaphthacenequinones 1 and 2 were investigated to assess their potential use as gated photoinduced electron transfer systems. Although the photophysical properties of the porphyrin chromophores remain unal

Full text of DOI:10.1016/S0040-4039(00)00503-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 3785–3788
Porphyrinic phenoxynaphthacenequinones
Andrew J. Myles and Neil R. Branda ∗
Department of Chemistry, University of Alberta, Edmonton, AB T6G 2G2, Canada
Received 23 February 2000; revised 16 March 2000; accepted 20 March 2000
Abstract
The photophysical properties of porphyrinic phenoxynaphthacenequinones 1 and 2 were investigated to assess
their potential use as gated photoinduced electron transfer systems. Although the photophysical properties of the
porphyrin chromophores remain unaltered upon derivatization, the photochemical isomerization of the naphthace-
nequinone fragment is shut down due to its intimacy with the porphyrin. © 2000 Elsevier Science Ltd. All rights
reserved.
Keywords: electron transfer; macrocycles; porphyrins and analogues; quinones.
Interest in harnessing light energy has resulted in the development of a large variety of elegant
synthetic photoinduced electron transfer (PET) systems incorporating porphyrins as the photoactivated
electron donors and quinones as electron acceptors.1 Our current interest in PET and the use of molecular
switches to control molecular-scale electronic devices prompted us to explore the possibility of regulating
photoinduced electron transfer by introducing a molecular switching component into porphyrinic arrays.
The regulation of PET can be potentially accessed using several approaches including the tailoring of: (1)
the through-space distance2 or the through-bond conjugation pathway3 between the electron donor and
acceptor groups; (2) the ability of the porphyrin to act as a reductant; and (3) the ability of the quinone
to act as an oxidant. In principle, all of these strategies can take advantage of intrinsic properties of
molecular switches.4 Here we describe two novel porphyrin–phenoxynaphthacenequinone hybrids that
target the third approach to regulate PET.
By taking advantage of the fact that phenoxynaphthacenequinones undergo rapid and reversible
interconversion between their trans- and ana-forms when irradiated with appropriate wavelengths of
light5 and by exploiting the expected difference in the reduction potential between the trans- and ana-
forms,6 a gated photoinduced electron transfer system can be realized. This concept is illustrated in Fig.
1.
The porphyrinic phenoxynaphthacenequinone 1a was conveniently prepared in one step by co-
upling mono-hydroxytoluoylporphyrin 37 with the known chloronaphthacenequinone 48 (Scheme 1).
∗
Corresponding author. Tel: 780 492-8187; fax: 780 492-8231; e-mail: neil.branda@ualberta.ca (N. R. Branda)
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00503-7
tetl 16782

3786
Fig. 1.
The amide-linked analogue 2 was synthesized from carboxyphenoxynaphthacenequinone 6 and 5-(4-
aminophenyl)10,15,20-(triphenyl)porphyrin9 (Scheme 2). All new compounds were characterized by 1H
and 13C NMR spectroscopy, UV–vis and FT-IR spectroscopy, and mass spectrometry.
Scheme 1.
Scheme 2.
The absorption spectrum of trans-1a in the UV–vis region is essentially equivalent to the sum of the
absorption spectra of the molecule’s components (trans-5 and tetratolylporphyrin, TTP) indicating there
is little change in the ground state of either chromophore upon covalent linking (Fig. 2a). Replacing
the phenoxy ring on trans-5 with a para-substituted carbomethoxyphenoxy ring and repeating all
experiments gave identical photophysical results showing that any inductive effect the porphyrin may
have on the system is negligible.

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Fig. 2. UV–vis absorption spectra of: (a) 1a, 5 and TTP; (b) an equimolar mixture of 5 and TTP showing the photochemical
isomerization upon irradiation at 254 nm. All spectra were run in dry degassed THF at 7.5×10−6
M
The excited state behavior of 1a, however, differs significantly from that of its constituents. When 1a
was irradiated with light of wavelengths varying from 240 to 280 nm and from 350 to 380 nm, there
was no observable change in the absorption spectrum even under prolonged exposure to high intensity
light at several wavelengths within these ranges. To illustrate the expected change in the absorption
spectrum, an equimolar mixture of trans-5 and TTP was irradiated at 254 or 365 nm (Fig. 2b) which
resulted in the rapid disappearance of the absorbance corresponding to the trans-isomer of 5 at λmax=240
(48 000), 292 (39 000) nm and the appearance of the absorbances corresponding to the ana-isomer at
λ
max=267 (58 000), 476 (16 000) nm. Changing the solvent and concentration, as well as using the
metalloporphyrin 1b,10 had no effect on the observed behavior. A similar porphyrin-induced inhibition
of the photoisomerization of the azobenzene chromophore has been previously reported by Hunter.11
Altering the through-bond communication of the porphyrin to the phenoxynaphthacenequinone was
explored using amide 2, which upon photoirradiation resulted in the same inhibition as was observed for
1a and 1b.
Clearly, there exists a pathway for the decay of the excited state of the phenoxynaphthacenequinone
component in 1 that is faster than the photochemical isomerization reaction. Alternatively, the excited
state of the molecular switch may be quenched through an electron/energy transfer process from the
phenoxynaphthacenequinone to the porphyrin. These results suggest that the quenching is due to a

3788
through-space and not a through-bond phenomenon. Further photophysical studies are underway to shed
light onto this issue.
Acknowledgements
We thank the University of Alberta and the Natural Sciences and Engineering Research Council of
Canada for financial support for this project.
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