Photoswitchable sensitization of porphyrin excited states

Article abstract of DOI:10.1071/CH05310

Light-driven molecular switches consisting of a porphyrin covalently linked to a fulgimide photochrome antenna have been prepared. Light absorbed by the cyclic form of the fulgimide is transferred to the porphyrin with ? 95% efficiency, generating the por

Full text of DOI:10.1071/CH05310

RESEARCH FRONT
Rapid Communication
CSIRO PUBLISHING
Aust. J. Chem. 2006, 59, 170–174
www.publish.csiro.au/journals/ajc
Photoswitchable Sensitization of Porphyrin Excited States
Stephen D. Straight,A Yuichi Terazono,A Gerdenis Kodis,A Thomas A. Moore,A,B
Ana L. Moore,A,B and Devens GustA,B
A Department of Chemistry and Biochemistry, Center for the Study of Early Events in Photosynthesis,
Arizona State University, Tempe AZ 85287, USA.
B Corresponding authors. Email: gust@asu.edu; tom.moore@asu.edu; amoore@asu.edu
Light-driven molecular switches consisting of a porphyrin covalently linked to a fulgimide photochrome antenna
have been prepared. Light absorbed by the cyclic form of the fulgimide is transferred to the porphyrin with ≥95%
efficiency, generating the porphyrin first excited singlet state. Irradiation with visible light photoisomerizes the
fulgimide to the open form, which does not absorb above 450 nm, and antenna function is abolished. UV irradiation
converts the fulgimide back into the cyclic form, re-establishing efficient energy transfer. Photoisomerization of
the fulgimide thus controls generation of the porphyrin excited state, which may then be used to carry out a variety
of useful photochemical processes.
Manuscript received: 17 November 2005.
Final version: 12 January 2006.
Biology abounds with examples of molecular-scale switches
and other ‘devices’. An example is photosynthesis, which
employs molecular photonic devices, photovoltaics, pumps,
chemical reactors, and mechanical motors. Photosynthetic
N
N
N
N
reaction centres convert excitation energy into electrochem-
ical potential by means of photoinduced electron transfer.
Antenna systems gather light in spectral regions where reac-
tion centres are not efficient absorbers, and increase the
effective absorption cross-section of reaction centre chloro-
phylls. Under conditions of excessive sunlight, the excitation
energy flow to reaction centres can become too fast, leading
to photodamage. One of the photoprotective mechanisms
developed by photosynthetic organisms is the xanthophyll
cycle,[1–5] which has the effect of switching off antenna
function during periods of undesirably high light intensity,
and turning the antenna back on again when light levels
decrease. The basic chemistry and physics underlying pho-
tosynthesis may be applied to artificial molecular systems.
For example, a large variety of porphyrin-based ‘molecular
photovoltaics’ that convert light energy into charge separa-
tion have been reported.[6] We have recently coupled such
molecules to artificial antenna systems for enhanced solar
energy conversion,[7–9] and also used them as the basis for
a variety of molecule-based switches, Boolean logic gates,
and related ‘photonic devices’.[10–16] Several examples of
photochromes that, in one isomeric form, accept excitation
energy from attached fluorophores have been reported.[17–19]
Herein we report a molecular switch that controls the gen-
eration of a porphyrin excited state through singlet energy
transfer from an attached antenna — a covalently linked
photoisomerizable fulgimide moiety. The cyclic form of the
fulgimide photochrome in dyads 1 and 2 (Scheme 1) acts as
M
O
Br
O
N
N
O
O
N
1a: Mϭ Zn
2a: Mϭ 2H
N
5 (E-form)
366 nm
560 nm
N
N
N
N
Br
M
O
O
N
N
O
O
1b: Mϭ Zn
2b: Mϭ 2H
N
N
5 (C-form)
O
O
N
N
N
N
Br
M
O
NH
O
O
N
3a: Mϭ 2H
3b: Mϭ Zn
4
Scheme 1. Structures of the porphyrin–fulgimide dyads 1 and 2 in
their isomeric forms, and associated model compounds.
© CSIRO 2006
10.1071/CH05310
0004-9425/06/030170

RESEARCH FRONT
Photoswitchable Sensitization of Porphyrin Excited States
171
Wavelength (nm)
500
an efficient antenna for the porphyrin in the visible region.
Reversible photoisomerization of the fulgimide to the open
form eliminates this antenna function.
300
(a)
400
600
700
800
0.6
0.4
0.2
0.0
0.8
0.4
0.0
Fulgides are a well studied class of photochromes that
undergo reversible photochemical isomerization between
open, colourless forms (E/Z) and a cyclic, coloured form
(C).[20] The C-form typically has a long-wavelength absorp-
tion maximum in the 500–600 nm region, which suggests
that this isomer might be a good energy donor to porphyrins.
However, fulgides are typically non-fluorescent. This lim-
its the efficiency of singlet energy transfer by the Förster
dipole–dipole mechanism, as the energy transfer rate con-
stant is proportional to the fluorescence quantum yield of
the donor chromophore. Rentzepis and co-workers have
found that 2-indoylfulgimides in the C-form fluoresce around
600 nm,[21,22] and used them to manipulate the fluorescence
quantum yield of an attached fluorophore by altering the
polarity of the local environment.[23] Fulgimides have also
been used to manipulate energy transfer between a cova-
lently linked anthracene donor and a coumarin acceptor, with
the cyclic form of the fulgimide acting as an excited state
quencher.[24]
400
600
800
(b)
150
100
50
In order to prepare dyads 1 and 2, porphyrin precursor 3a
(Scheme 1) was synthesized by condensation of 5-(2,4,6-
trimethylphenyl)dipyrromethane, 4-bromobenzaldehyde, and
[(4-formylphenyl)methyl]carbamic acid phenylmethyl ester
using BF3·Et2O. The porphyrin was deprotected with acid
to give the free benzylamine, which was in turn metallated
with zinc acetate and allowed to react with 2-indolylfulgide 4.
The resulting amic acid was cyclized to the imide using zinc
chloride and hexamethyldisilazane to give dyad 1, which
was demetallated using dilute trifluoroacetic acid to yield 2.
Model fulgimide 5 was prepared by following the procedure
of Rentzepis.[21,22] Porphyrin 3a was metallated with zinc
acetate to give 3b. Synthetic details as well as descriptions of
the spectroscopic apparatus used below are available in the
Accessory Materials.
The use of the fulgimide as a sensitization switch for the
porphyrin necessitates reversible photoisomerization even
when the fulgimide is linked to the porphyrin. This requires
that the electronic interaction between the moieties be weak
enough that the fulgimide and porphyrin still act as discrete
chromophores, and that fulgimide excited state quenching,
for example by energy transfer to the porphyrin, not be so
great as to prevent photoisomerization. Absorption spec-
troscopy was employed to investigate these points. Model
fulgimide 5 in its open form consists of a mixture of
E- and Z-isomers that absorb maximally around 380 nm in
2-methyltetrahydrofuran (Fig. 1a, inset). Irradiation with UV
light (366 nm) leads to isomerization between the E- and
Z-forms and cyclization of the E-form, ultimately convert-
ing the sample to a photostationary state greatly enriched
in the C-form, which has an absorption maximum at about
510 nm. The absorption spectrum of free base dyad 2a,
which features the open form of the fulgimide (Fig. 1a),
shows unperturbed porphyrin Q-bands at 648, 591, 547,
and 513 nm and the Soret band at 418 nm. UV irradiation
was found to isomerize the fulgimide to its C-form, giving
0
450 500
700
550 600 650
Wavelength (nm)
750
Fig. 1. (a) Absorption spectra of dyad 2a, with the fulgimide in the
open, E/Z-form (solid line) and dyad 2b with the fulgimide in the cyclic
form (dotted line). Inset: Absorption spectra of model fulgimide 5 in
the open (solid line) and cyclic (dotted line) forms, and the emission
spectrum of the cyclic form on an arbitrary linear scale (dashed line).
(b) Emission spectra of 2a (dashed line) and the same sample after UV
irradiation to form 2b (dot-dashed line) with excitation at 470 nm, and
corrected fluorescence excitation spectra of 2a (solid line) and the same
sample after UV irradiation to form 2b (dotted line), with detection at
720 nm.
dyad 2b (as a racemic mixture), which has increased absorp-
tion between 430 and 600 nm and decreased absorbption
between 320 and 400 nm. Zinc dyad 1a, which displays typ-
ical porphyrin absorption bands at 424, 556, and 596 nm,
was also found to undergo fulgimide isomerization with UV
light, forming 1b. In both 1b and 2b the reverse isomer-
ization of the fulgimide C-form to the open form does not
occur thermally at room temperature but is readily achieved
using visible light (450 < λ < 650 nm). Linkage of the por-
phyrin to the fulgimide reduces the relative quantum yield
of this isomerization by a factor of approximately fifty
in both dyads relative to fulgimide 5, as determined by
measuring isomerization rates under identical illumination
conditions. The absorption spectra of all four dyads are
essentially linear combinations of the spectra of appropri-
ate model porphyrins and fulgimides, suggesting that even
though the moieties have been linked they still act as discrete
chromophores.
Having established reversible photoisomerization of the
fulgimide in the dyads, we examined singlet energy trans-
fer. Dyads 2a and 2b both show normal free-base porphyrin
fluorescence, with maxima at 650 and 720 nm (Fig. 1b).

RESEARCH FRONT
172
S. D. Straight et al.
Time (ps)
20
Similar results were obtained for zinc dyads 1a and 1b,
which emit with maxima at 600 and 650 nm. Although the
cyclic form of model fulgimide 5 emits with a maximum at
600 nm (Fig. 1a, inset), no fulgimide emission was observed
in either 1b or 2b. The corrected fluorescence excitation
spectrum of 2a was obtained by monitoring the porphyrin
emission at 720 nm as a function of the excitation wavelength
(Fig. 1b). The excitation spectrum was normalized to the
absorptionspectruminthe650 nmregion, whereneitherform
of the fulgimide absorbs, whereupon it was observed that the
absorption and excitation spectra were coincident through-
out the 430–650 nm region. This is expected, as only the
porphyrin moiety of 2a absorbs significantly in this spectral
region. Similar experiments with a solution highly enriched
in the cyclic form of the dyad, 2b, yielded excitation spec-
tra (Fig. 1b) that were also coincident with the absorption
spectrum, within experimental error. Because the cyclic form
of the fulgimide moiety absorbs in the 430–650 nm spec-
tral region, this result indicates efficient (∼100%) energy
transfer from the fulgimide to the porphyrin. This energy
transfer explains the lack of fulgimide fluorescence and the
reduced quantum yield for photoisomerization. The effect of
this energy transfer on the porphyrin fluorescence intensity is
readily apparent in Fig. 1b.Although 2a and 2b have identical
fluorescence intensities with excitation at 650 nm, emission
from 2b is much enhanced with excitation at wavelengths
where the fulgimide absorbs strongly. For example, with exci-
tation at 470 nm, emission from 2b is about 2.5 times more
intense than that from 2a. Energy transfer was also observed
in zinc dyad 1b. With excitation at 470 nm, conversion of
1a to 1b with UV irradiation led to a four-fold increase in
porphyrin fluorescence intensity. The fluorescence excita-
tion spectrum of 1b indicated essentially quantitative energy
transfer.
0
10
30
40
50
(a)
0.009
0.006
0.003
0.000
Ϫ0.003
(b)
200
150
100
50
0
Cycle number
Fig. 2. (a) Transient absorption kinetics at 630 nm, with excitation
at 450 nm and continuous UV irradiation, for model fulgimide 5 (ꢀ),
dyad 2b (♦), and dyad 1b (ꢁ). The solid lines are multiexponential
fits to the data, as discussed in the text. (b) Photochemical cycling of
free base dyad 2 between the open E/Z-form and the cyclic C-form.
The porphyrin emission intensity at 650 nm with excitation at 470 nm is
shown following each photoisomerization. The low emission intensity
arises from the dyad with the fulgimide in the open form, and the higher
intensity from the dyad with the fulgimide in the cyclic form, which
transfers excitation energy to the porphyrin.
Time-resolved fluorescence experiments gave additional
information concerning energy transfer. Excitation of model
porphyrin 3a at 568 nm engendered a single-exponential flu-
orescencedecayat720 nmwithalifetimeof5.75 ns(χ2 1.13).
This excited state lifetime is roughly half that of a compara-
ble porphyrin lacking the bromine atom, due presumably to
heavy-atom-induced intersystem crossing. A similar experi-
ment with dyad 2a yielded similar results (τ 6.26 ns, χ2 1.23).
Thus, the porphyrin first excited singlet state is not perturbed
by the presence of the open form of the fulgimide. Excitation
of a solution of fulgimide 5 at 568 nm yielded an emis-
sion decay (λem 610 nm) with a lifetime of 135 ps (99.8%,
χ2 1.19), corresponding to the decay of the fulgimide’s first
excited singlet state. A similar experiment with 2b yielded
a decay that was fitted with three exponential components
(χ2 1.20) of 5 ps (98%), 881 ps (1.8%), and 5.93 ns (0.2%).
Themajor, 5-pscomponentisassignedtothehighlyquenched
fulgimide excited state. Porphyrin emission at 720 nm is
unquenched in dyad 2b with the fulgimide in the cyclic
form; a single exponential decay of 6.41 ns (χ2 1.15) was
observed. The excited state lifetime of model zinc porphyrin
3b was 905 ps (λex 532 nm, λem 650 nm), that of the zinc
porphyrin in 1a was 1.02 ns (98.8%, χ2 1.18), and that in
1b was 901 ps (98.1%, χ2 1.15), indicating no quenching by
the photochrome in either the open or the cyclic form. It was
not possible to observe fulgimide emission in 1b due to the
low quantum yield and short singlet state lifetime resulting
from energy transfer (see below), coupled with overlap by
zinc porphyrin fluorescence.
The fluorescence results are consistent with very strong
quenching of the fulgimide emission in the cyclic form due
to energy transfer to the free base or zinc porphyrin, but
quantitative data were difficult to obtain due to the low quan-
tum yield of fulgimide emission and the very rapid transfer,
which is close to the detection limits of the spectrometer.
Thus, transient absorption experiments were performed using
the pump–probe technique and excitation at 450 nm with
∼100-fs laser pulses. Representative results are shown in
Fig. 2a. Decays were followed for about 3 ns, but only the
data for the first 50 ps are shown. Results for fulgimide 5,
obtained with continuous irradiation at 366 nm to maintain
a high population of the cyclic form, included a prompt
bleach at 630 nm whose decay was fitted with exponential
components having time constants of 135 ps (70%), 45 ps
(25%), and non-decaying on this time scale (5%). The 135 ps
component is assigned to the fulgimide excited singlet state

RESEARCH FRONT
Photoswitchable Sensitization of Porphyrin Excited States
173
(consistent with the fluorescence results), the 45-ps compo-
nent to ground-state equilibration of the cyclic form,[25] and
the non-decaying portion to light-induced conversion of some
of the cyclic form to the open form. At the same wavelength,
dyad 2b gave a rising component with a time constant of
5.6 ps followed by a decay of 6.4 ns. The rise is ascribed
to the disappearance of the fulgimide stimulated emission
and corresponding rise of the porphyrin transient absorption
resulting from singlet–singlet energy transfer. Zinc dyad 1b
also gave a rise (4.0 ps, energy transfer from the fulgimide to
the porphyrin) followed by a decay (900 ps, decay of the zinc
porphyrin excited state).
the presence of the energy transfer pathway for deactivation
of the excited state. Because a small fraction of the photons
absorbed by the cyclic fulgimide lead to photoisomerization
rather than energy transfer, continued visible irradiation will
eventually lead to a reduction and eventual elimination of the
antenna effect, whereupon UV excitation would be necessary
to reset the device to its ‘on’ state.
Accessory Materials
Descriptions of syntheses and instrumental techniques are
availablefromtheauthorsor, untilMarch2011, theAustralian
Journal of Chemistry.
Thus, the spectroscopic data indicate very rapid singlet–
singlet energy transfer from the excited state of the cyclic
fulgimide to the attached porphyrin in both the zinc and
free base dyads. No antenna function is observed when the
fulgimides are in the open form. From the transient data,
the energy transfer rate constants (kent) may be calculated
from the expression kent = τs−1 − kd, where τs is the lifetime
of the excited singlet state of the cyclic fulgimide in the
presence of energy transfer and kd is the rate constant for
decay of this state in the absence of energy transfer (taken
as 1/135 ps from model 5). For free base dyad 2b kent is
Acknowledgments
This work was supported by the US National Science Founda-
tion (CHE-0352599). This is publication 649 from the ASU
Center for the Study of Early Events in Photosynthesis.
References
[1] H. A. Frank, A. Cua, V. Chynwat, A. Young, D. J. Gosztola,
M. R. Wasielewski, Photosynth. Res. 1994, 41, 389. doi:10.1007/
BF02183041
[2] B. Robert, P. Horton, A. A. Pascal, A. V. Ruban, Trends Plant Sci.
2004, 9, 385. doi:10.1016/J.TPLANTS.2004.06.006
[3] N. E. Holt, G. R. Fleming, K. K. Niyogi, Biochemistry 2004, 43,
8281. doi:10.1021/BI0494020
1.7 × 1011 s−1 and for zinc dyad 1b kent is 2.4 × 1011 s−1
.
The quantum yields of energy transfer, calculated from these
rate data, are 95% and 96% for the free base and zinc dyads,
respectively.
[4] H. A. Frank, J. A. Bautista, J. S. Josue, A. J. Young, Biochemistry
2000, 39, 2831. doi:10.1021/BI9924664
In order to examine the reversibility of the system, the
porphyrin emission intensity at 650 nm (λex 470 nm) was
measured as a function of cycling the photochrome in
dyad 2 through its isomeric forms (Fig. 2b). The exper-
iments were carried out in 2-methyltetrahydrofuran after
deoxygenation by argon bubbling for 15 min. When the
fulgimide is in the open form, a low-emission intensity is
observed, due to the small porphyrin extinction coefficient
at 470 nm. Irradiation at 366 nm (1.5 mW cm−2) for 2 min
converts the fulgimide into the visible-absorbing C-form,
giving rise to more intense porphyrin emission due to
highly efficient energy transfer. Irradiation with visible light
(450 < λ < 650 nm, ∼10.5 mW cm−2) for 3 min converts the
fulgimide back to the open form and the emission inten-
sity again drops. After twelve switching cycles only minimal
degradation was observed.
These experiments show that light can be used to switch
thefulgimideofdyads1and 2betweenanopenformthatdoes
not act as a visible-light-absorbing antenna for the attached
porphyrin and a cyclic form that transfers excitation with
efficiencies ≥95%. Thus, photoisomerization of the photo-
chrome may be used to efficiently switch sensitization of
the porphyrin excited states on or off. Because porphyrin
excited states can perform many useful photochemical func-
tions, including photoinduced electron transfer, this switch
could be useful in the design of porphyrin-based molecu-
lar switches and logic gates. It could also provide a control
mechanism for light harvesting in artificial antenna–reaction
centre complexes. The photoinduced ring opening of the
cyclic fulgimide in 1b and 2b occurs, but the quantum yield
is reduced greatly, relative to the model compound, due to
[5] C. Kulheim, J. Ågren, S. Jansson, Science 2002, 297, 91.
doi:10.1126/SCIENCE.1072359
[6] D. Gust, T. A. Moore, in The Porphyrin Handbook (Eds
K. M. Kadish, K. M. Smith, R. Guilard) 2000, Vol. 8, Ch. 57,
pp. 153–190 (Academic Press: New York, NY).
[7] G. Kodis, P.A. Liddell, L. de la Garza, P. C. Clausen, J. S. Lindsey,
A. L. Moore, T. A. Moore, D. Gust, J. Phys. Chem. A 2002, 106,
2036. doi:10.1021/JP012133S
[8] D. Kuciauskas, P. A. Liddell, S. Lin, T. E. Johnson, S. J. Weghorn,
J. S. Lindsey, A. L. Moore, T. A. Moore, D. Gust, J. Am. Chem.
Soc. 1999, 121, 8604. doi:10.1021/JA991255J
[9] G. Kodis, Y. Terazono, P. A. Liddell, J. Andréasson, V. Garg,
M. Hambourger, T.A. Moore,A. L. Moore, D. Gust, J.Am. Chem.
Soc. 2006, 129, in press. doi:10.1021/JA055903C
[10] J. Andréasson, G. Kodis, Y. Terazono, P. A. Liddell,
S. Bandyopadhyay, R. H. Mitchell, T. A. Moore, A. L. Moore,
D. Gust, J. Am. Chem. Soc. 2004, 126, 15926. doi:10.1021/
JA045577L
[11] J. L. Bahr, G. Kodis, L. de la Garza, S. Lin, A. L. Moore,
T. A. Moore, D. Gust, J. Am. Chem. Soc. 2001, 123, 7124.
doi:10.1021/JA010058T
[12] P. A. Liddell, G. Kodis, A. L. Moore, T. A. Moore, D. Gust,
J. Am. Chem. Soc. 2002, 124, 7668. doi:10.1021/JA026327C
[13] P. A. Liddell, G. Kodis, J. Andréasson, L. de la Garza,
S. Bandyopadhyay, R. H. Mitchell, T. A. Moore, A. L. Moore,
D. Gust, J. Am. Chem. Soc. 2004, 126, 4803. doi:10.1021/
JA039800A
[14] S. D. Straight, J.Andréasson, G. Kodis,A. L. Moore,T.A. Moore,
D. Gust, J. Am. Chem. Soc. 2005, 127, 2717. doi:10.1021/
JA044128I
[15] S. D. Straight, J. Andréasson, G. Kodis, S. Bandyopadhyay,
R. H. Mitchell, T. A. Moore, A. L. Moore, D. Gust, J. Am. Chem.
Soc. 2005, 127, 9403. doi:10.1021/JA051218U
[16] Y. Terazono, G. Kodis, J. Andréasson, G. Jeong, A. Brune,
T. Hartmann, H. Dürr, A. L. Moore, T. A. Moore, D. Gust, J. Phys.
Chem. B 2004, 108, 1812. doi:10.1021/JP037005D

RESEARCH FRONT
174
S. D. Straight et al.
[17] L. Giordano, T. M. Jovin, M. Irie, E. A. Jares-Erijman, J. Am.
Chem. Soc. 2002, 124, 7481. doi:10.1021/JA016969K
[18] X. Guo, D. Zhang,Y. Zhou, D. Zhu, J. Org. Chem. 2003, 68, 5681.
doi:10.1021/JO034243W
[22] Y. Liang, A. S. Dvornikov, P. M. Rentzepis, J. Mater. Chem. 2000,
10, 2477. doi:10.1039/B002374O
[23] A. S. Dvornikov,Y. Liang, P. M. Rentzepis, J. Mater. Chem. 2005,
15, 1072. doi:10.1039/B414772C
[19] M. Irie, T. Fukaminato, T. Sasaki, N. Tamai, T. Kawai, Nature
2002, 420, 759. doi:10.1038/420759A
[20] Y. Yokoyama, Chem. Rev. 2000, 100, 1717. doi:10.1021/
CR980070C
[21] Y. Liang, A. S. Dvornikov, P. M. Rentzepis, J. Photochem.
Photobiol.A1999, 125, 79. doi:10.1016/S1010-6030(99)00079-9
[24] J. Walz, K. Ulrich, H. Port, H. C. Wolf, J. Wonner, F. Effenberger,
Chem. Phys. Lett. 1993, 213, 321. doi:10.1016/0009-2614(93)
85139-F
[25] V. Gulbinas, G. Kodis, S. Jursenas, L. Valkunas, A. Gruodis,
J.-C. Mialocq, S. Pommeret, T. Gustavsson, J. Phys. Chem. A
1999, 103, 3969. doi:10.1021/JP9845468