Photoswitched singlet energy transfer in a porphyrin-spiropyran dyad

Article abstract of DOI:10.1021/ja010058t

A photochromic nitrospiropyran moiety (Sp) has been covalently linked to a zinc (P_Zn) and to a free-base (P_H2) porphyrin. In the resulting dyads (P_Zn-Sp_c and P_H2-Sp_c), the porphyrin first e

Full text of DOI:10.1021/ja010058t

7124
J. Am. Chem. Soc. 2001, 123, 7124-7133
Photoswitched Singlet Energy Transfer in a Porphyrin-Spiropyran
Dyad
Jeffrey L. Bahr, Gerdenis Kodis, Linda de la Garza, Su Lin, Ana L. Moore,*
Thomas A. Moore,* and Devens Gust*
Contribution from the Department of Chemistry and Biochemistry, Center for the Study of Early EVents in
Photosynthesis, Arizona State UniVersity, Tempe, Arizona 85287-1604
ReceiVed January 5, 2001. ReVised Manuscript ReceiVed May 10, 2001
Abstract: A photochromic nitrospiropyran moiety (Sp) has been covalently linked to a zinc (PZn) and to a
free-base (PH2) porphyrin. In the resulting dyads (PZn-Spc and PH2-Spc), the porphyrin first excited singlet
states are unperturbed by the closed form of the attached spiropyran. Excitation of the spiropyran moiety of
either dyad in the near-UV region results in ring opening to a merocyanine form (P-Spo) that absorbs at 600
nm. The open form re-closes thermally in 2-methyltetrahydrofuran with a time constant of 20 s, or following
irradiation into the 600 nm band. Excitation of the zinc porphyrin moiety in the merocyanine form of the dyad
1
yields PZn-Spo. The lifetime of the zinc porphyrin excited state is reduced from its usual value of 1.8 ns to
130 ps by singlet-singlet energy transfer to the merocyanine moiety to give PZn-1Spo. The quantum yield of
1
energy transfer is 0.93. Quenching is also observed in the free base dyad, where PH2-Spo and PH2-1Spo
exchange singlet excitation energy. This photoswitchable quenching phenomenon provides light-activated control
of the porphyrin excited states, and consequently control of any subsequent energy or electron-transfer processes
that might be initiated by these excited states in more complex molecular photonic or optoelectronic devices.
Introduction
process is initiated by a porphyrin excited singlet state. These
can serve as donors of singlet excitation energy in photonic and
light-gathering antenna systems or as electron donors or
acceptors, or undergo intersystem crossing to triplet states, which
can also initiate energy and electron-transfer processes. Thus,
control of the lifetime of the porphyrin first excited singlet state
provides a mechanism for modulation of various photonic and
optoelectronic processes. In principle, such control can be
exerted by quenchers that shorten the lifetime of the porphyrin
excited state, rendering it ineffective in other photochemical or
photophysical processes.
Energy transfer is an attractive mechanism for such quenching
because it does not directly produce redox-active ions that could
lead to photodamage or other undesirable processes. Quenching
of the porphyrin first excited singlet state by singlet-singlet
energy transfer requires an acceptor whose first excited singlet
state lies at a lower energy than that of the porphyrin. Thus,
one approach to a light-controlled photonic or optoelectronic
molecular switch would be to associate with a porphyrin a
second chromophore which could be switched with light
Porphyrins absorb light strongly in the visible and near-UV,
are often fluorescent with reasonably long excited singlet state
lifetimes, and undergo reversible one-electron redox reactions.
They are readily synthesized, and their properties are easily
tuned by substitution and metalation. A large number of
porphyrin-based multicomponent molecular systems that dem-
onstrate efficient singlet energy transfer, triplet energy transfer,
and photoinduced electron transfer to yield energetic charge-
separated states1-6 have been reported. These serve as mimics
of the primary energy conversion steps of photosynthesis, as
molecular photovoltaics that transduce light energy into elec-
trochemical energy, and as the basis for prototype molecular-
scale mechanical, photonic, and optoelectronic switches and
wires.7-20 In such systems, the photonic or optoelectronic
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10.1021/ja010058t CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/29/2001

Porphyrin-Spiropyran Dyad
J. Am. Chem. Soc., Vol. 123, No. 29, 2001 7125
between a short-wavelength-absorbing form (incapable of
quenching the porphyrin) and a long-wavelength-absorbing form
capable of accepting singlet excitation energy from the por-
phyrin. In a few reports, photochromic moieties including
dithienylethenes21,22 and fulgides23 have been linked to chro-
mophores, and switching of the photochromic unit has been
shown to affect the emission properties of the chromophore. In
these cases, the quenching mechanism is either unknown or a
process other than energy transfer to the photochromic moiety.
However, these studies suggest that the approach mentioned
above might be feasible.
In various classes of photochromic molecules, the difference
in conjugation between the two forms has been used as the basis
for control of electron-transfer rates, redox potentials, and
nonlinear optical properties.21,36-44
In the present application, the difference in energy of the first
excited singlet states of the two forms of a spiropyran is
exploited. In the closed form, the spiropyran first excited singlet
state is too high in energy to quench the porphyrin first excited
singlet state by energy transfer. The absorption and emission
spectra of the open form, however, overlap those of a typical
porphyrin, suggesting the possibility of energy transfer.
Herein, we report the synthesis and photophysical properties
of zinc porphyrin-spiropyran dyad 2, free base analogue 3, and
related model compounds. As described in detail below, the
spiropyran in the closed form has no effect on the photophysics
of the first excited singlet state of the attached porphyrin. In
the open, merocyanine form, however, efficient energy transfer
quenching of the porphyrin is observed, demonstrating the
desired photoswitching phenomenon.
Spiropyrans are promising candidates for such light-controlled
quenchers. Spiro[indoline-benzopyran] molecules such as 1 exist
in nonpolar environments in a “closed” spiro form (1c) that does
Results
Synthesis. Porphyrin-spiropyran 3 was prepared according
to the sequence depicted in Figure 1. Fischer’s base was
benzylated with methyl 4-(bromomethyl)benzoate. Base-induced
conversion of the intermediate salt gave 4. Condensation of 4
with 5-nitrosalicyaldehyde produced 5 in good yield. Reduction
of the methyl ester with lithium aluminum hydride and
reoxidation of the resulting alcohol 6 with manganese dioxide
yielded 1, which bears the requisite aldehyde for formation of
the porphyrin. Reaction of 1 under conditions developed by
Lindsey and co-workers45 gave the expected mixture of por-
phyrin-containing products, which were separated by column
chromatography to yield free-base porphyrin-spiropyran dyad
3. Insertion of zinc to give 2 was accomplished by stirring 3
overnight with zinc acetate in chloroform.
Cyclic Voltammetry. Electrochemical measurements were
undertaken to investigate the redox properties of the dyads. The
zinc porphyrin moiety of dyad 2 in benzonitrile solution
containing 0.1 M tetra-n-butylammonium hexafluorophosphate
and ferrocene as an internal redox standard was found to exhibit
a first oxidation potential of 0.34 V (0.74 V vs SCE) and a first
reduction potential of -1.85 V (-1.46 V vs SCE) in the closed
form. The spiropyran moiety in 2 showed a first oxidation
potential of 0.79 V (1.20 V vs SCE) and a first reduction
potential of -1.67 V (-1.25 V vs SCE). Irradiation of 2 with
ultraviolet light at wavelengths known to open the spiropyran
not absorb above 400 nm. When excited, this form undergoes
an intramolecular rearrangement to an “open”, merocyanine
form (1o), which absorbs in the visible in the 500-700 nm
region. As indicated in the structural drawing, the open form is
a resonance hybrid of a polar, zwitterionic structure and a less
polar, quinonoid form. Thus, the three bonds in the linkage
joining the indolino and phenolic rings have some double-bond
character. This suggests that 1o can exist as an equilibrating
mixture of 8 isomeric forms. Irradiation of the open form
induces ring closure, regenerating the closed spiro form. Thermal
reversion to the closed form is also observed, with time constants
as large as many minutes. The lifetime is a function of the
conditions and the substituents on the spiropyran skeleton.24 The
open form of the spiropyran is considerably more polar than
the closed form, and this polarity change has been exploited
for the photonic control of molecular interactions with electrode
surfaces25-30 and for various possible separation applications.31-35
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J. Phys. Chem. A 2000, 104, 6103-6107.
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Switches; New York Academy of Sciences: New York, 1998; pp 116-
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Soc 1982, 104, 5502-5504.

7126 J. Am. Chem. Soc., Vol. 123, No. 29, 2001
Bahr et al.
Figure 1. Synthetic scheme for preparation of the porphyrin-
spiropyran dyads. Details are given in the Experimental Section.
Figure 2. Absorption spectra of (a) zinc dyad 2 and (b) free base dyad
3 in deoxygenated 2-methyltetrahydrofuran: (s) samples kept in the
dark prior to spectra, (- -) samples irradiated at ∼360 nm prior to
spectra, converting the spiropyran to the open, merocyanine form.
moiety to the merocyanine form resulted in no observable
change in the redox behavior. It has been reported that for very
similar spiropyrans, partial conversion of the sample to the open
form with UV light does not reveal any more easily oxidized
or reduced species.46,47 Similar experiments with free base dyad
3 gave a first oxidation potential for the porphyrin of 0.55 V
(0.96 V vs SCE). The spiropyran moiety was oxidized at 0.79
V (1.20 V vs SCE). A reduction wave at -1.60 V (-1.20 V vs
SCE) corresponded to reduction of both the spiropyran and the
porphyrin at nearly the same potential. The values for both
compounds are close to those for model spiropyrans and
porphyrins, indicating little effect on redox potentials due to
linking the chromophores.
Absorption Spectra. The absorption spectrum of zinc dyad
2c (closed form) in 2-methyltetrahydrofuran is shown in Figure
2a. The maxima in the Q-band region are at 597, 557, and 518
(sh) nm, and the maximum of the Soret occurs at 420 nm. These
absorption bands are essentially identical to those observed for
zinc meso-tetraphenylporphyrin. There is an additional absorp-
tion band at 330 nm (not shown). This is ascribed to the closed
form of the spiropyran chromophore by reference to the
absorption spectrum of model spiropyran 1c, whose longest
wavelength absorption band is in this region (Figure 3).
Similarly, the absorption spectrum of free base dyad 3c is similar
to that of meso-tetraphenylporphyrin, with maxima at 648, 592,
549, 514, and 416 nm (Figure 2b). The spectrum also features
the 330 nm absorption characteristic of the closed spiropyran.
Thus, the absorption spectra of the dyads are essentially
superpositions of the spectra of the porphyrin and spiropyran
chromophores, and show no significant perturbations indicative
of strong interactions between the linked moieties.
Figure 3. Spectra from a 2-methyltetrahydrofuran solution of model
spiropyran 1: (- -) absorption spectrum of the closed spiropyran from
a sample kept in the dark, (s) absorption spectrum of the solution
after irradiation at 340 nm to generate the open, merocyanine form,
and (‚‚‚) emission of the irradiated solution containing the merocyanine
form, with excitation at 580 nm.
(Figure 2a). The main absorption bands of the zinc porphyrin
are still apparent, but the absorption is enhanced in the 600-nm
region. The reason for the enhancement is apparent from Figure
3, which shows the absorption spectrum of the model spiropyran
1 following similar UV excitation. The absorption in the 330-
nm region has decreased, and new absorption has appeared in
the 380 and 600 nm regions. The new absorption is due to the
open, merocyanine form of the spiropyran (1o), which features
significantly extended conjugation relative to the spiro form.
Clearly, UV irradiation of dyad 2c has led to opening of the
spiropyran moiety, and its visible absorption underlies the
porphyrin Q-bands, resulting in the observed enhancement in
A 2-methyltetrahydrofuran solution of dyad 2c was irradiated
in the near-UV at ∼360 nm, where the spiropyran moiety
absorbs, and the absorption spectrum was again measured
(46) Zhi, J.-F.; Baba, R.; Hashimoto, K.; Fujushima, A. Chem. Lett. 1994,
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Photobiol. A 1995, 92, 91-97.

Porphyrin-Spiropyran Dyad
J. Am. Chem. Soc., Vol. 123, No. 29, 2001 7127
Figure 4. Decay of the transient absorption of solutions of model
spiropyran 1 (0) and dyads 2 (b) and 3 (4) in 2-methyltetrahydrofuran
at 600 nm following irradiation at ∼360 nm to generate the merocyanine
form. The solid lines are calculated decays with time constants of 20
s for 2 and 3 and 20 (60%) and 70 s (40%) for 1. The ordinate is the
total absorbance at 600 nm minus any (invariant) absorbance due to
the porphyrin at that wavelength.
Figure 2a. The enhancement is maximal at 600 nm, which is
the maximum of the merocyanine absorption. A similar effect
is observed for free base dyad 3 (Figure 2b). After irradiation
at ∼360 nm, the four Q-bands are still apparent, but the spectrum
is strongly perturbed by the merocyanine absorption of 3o at
600 nm.
Figure 5. Corrected fluorescence spectra of dyads 2 (a) and 3 (b) in
2-methyltetrahydrofuran solution, with excitation into the porphyrin at
430 nm; (s) samples kept in the dark prior to spectrum, (- -) same
samples irradiated at 340 nm to generate the open, merocyanine form.
The spectral perturbations resulting from UV irradiation of
2 and 3 decay with time after irradiation ceases (Figure 4), and
the original spectrum is recovered. The decay is exponential,
and fitting yields a time constant of 20 s for each molecule.
This decay is ascribed to thermal closing of the merocyanine
to regenerate the spiro form. Closing is more rapid if the
molecule is irradiated in the 600-nm region. Repetitive cycling
of either 2 or 3 leads to the appearance of degradation products,
as is the case with most spiropyrans. The degradation rate was
not investigated quantitatively, but such degradation could
obviously limit the ultimate utility of practical devices employ-
ing this spiropyran. Model spiropyran 1 shows similar opening
and closing behavior (Figures 3 and 4), in common with related
spiropyrans. The thermal closing of merocyanine 1o can be fitted
with two exponential components with time constants of 20
(60%) and 70 s (40%). The decay of the merocyanine form of
many spiropyrans is not a single-exponential process.48
Emission Spectra. No significant fluorescence emission was
observed upon irradiation of the closed form of the model
spiropyran, 1c. During steady-state irradiation of the sample with
light from a 500 W xenon arc lamp passed through a 340-nm
filter with a 10-nm band-pass, which generates the open,
merocyanine form 1o, excitation at 580 nm produced readily
detectable fluorescence (Figure 3). The emission spectrum was
relatively broad and featureless, with a maximum at 657 nm.
Fluorescence experiments were also carried out with the
dyads. Figure 5a shows the emission of dyad 2c in 2-meth-
yltetrahydrofuran (with the spiropyran in the closed form).
Excitation was at 430 nm, where the porphyrin absorbs strongly
(Figure 2a), but the absorption of both the closed and open forms
of the spiropyran moiety is negligible (Figure 3). The emission
is essentially identical to that of zinc 5,10,15,20-tetrakis(4-
methylphenyl)porphyrin, with maxima at 607 and 657 nm.
Furthermore, the amplitude of the emission is essentially
identical to that of a solution of the model zinc porphyrin with
an equal absorbance at 430 nm. Thus, the closed form of the
spiropyran has no effect on the emission properties of the
porphyrin moiety.
The emission spectrum was measured again after irradiation
of the sample at 340 nm; the irradiation was stopped before the
spectrum was measured. The emission spectrum after irradiation
(Figure 5a) is significantly reduced in amplitude. This result
provides additional evidence that the spiropyran moiety under-
goes photoinduced ring opening even in the presence of the
porphyrin. In addition, it indicates that the zinc porphyrin excited
singlet state is quenched by the open form of the spiropyran. It
will be noted that the decrease in amplitude in the open form is
greater in the 610-nm region than in the 655-nm region. There
are two reasons for this effect. The spectrum was measured by
scanning from shorter to longer wavelengths after the 340-nm
radiation was turned off. Thus, thermal reversion to the closed
form 2c is occurring during the measurement, leading to
enhanced porphyrin emission at longer wavelengths. Second,
any emission from the merocyanine chromophore at 657 nm
will also enhance the total emission amplitude at these wave-
lengths.
Similar experiments were performed with free base dyad 3.
Emission from a 2-methyltetrahydrofuran solution of the closed
form 3c following irradiation at 430 nm was observed with
maxima at 653 and 720 nm (Figure 5b). The spectral shape
and emission intensity are essentially identical with those of a
solution of 5,10,15,20-tetrakis(4-methylphenyl)porphyrin having
the same absorbance at 430 nm. After partial conversion to the
open form 3o by irradiation at 340 nm, quenching of the
(48) Pimienta, V.; Lavabre, D.; Levy, G.; Samat, A.; Guglielmetti, R.;
Micheau, J. C. J. Phys. Chem. 1996, 100, 4485-4490.

7128 J. Am. Chem. Soc., Vol. 123, No. 29, 2001
Bahr et al.
porphyrin fluorescence is again observed. However, the quench-
ing is not as great as in the zinc analogue. The spectrum is
distorted for the reasons mentioned above.
These results show that the zinc and free base dyad porphyrin
first excited singlet states are quenched by more than 35% and
10%, respectively, by the open form of the spiropyran. However,
these estimates are not quantitative for several reasons. Because
both the open and closed forms of the spiropyran moiety absorb
at 340 nm, irradiation at this wavelength produces a photosta-
tionary state of the spiro and merocyanine forms, rather than
driving the system totally to one isomer. As mentioned above,
irradiation was not performed during collection of the emission
spectra, and thermal reversion to the closed form was occurring
before and during data collection. Finally, the fluorescence
quenching is partially offset by fluorescence from the open form
of the spiropyran in the same wavelength region.
To obtain more quantitative information concerning the
excited-state properties of the dyads, time-resolved emission
experiments were carried out using the single photon timing
technique. A ∼1 × 10-5 M solution of dyad 2c in 2-methyltet-
rahydrofuran was excited with 2 ps laser pulses at 430 nm
(porphyrin absorption), and the emission was monitored at 15
wavelengths in the 580-760 nm region. Global analysis of the
resulting data as a single-exponential decay gave a satisfactory
fit (ø2 ) 1.11) with a lifetime of 1.80 ns, which is a typical
lifetime for a zinc tetraarylporphyrin of this type.49 The lack of
lifetime quenching in 2c is consistent with the unquenched
steady-state emission spectra discussed above. Similar experi-
ments were carried out with some of the spiropyran in the open
form. The sample of 2 was irradiated for ∼1 min at 340 nm as
discussed above. The irradiation source was turned off, and time-
resolved emission data with excitation at 430 nm were collected
for ∼25 s. Data collection was stopped, the sample again
irradiated at 340 nm, and the cycle repeated until useable signal-
to-noise was obtained. Some sample degradation was noted after
a number of cycles. Data were collected at 14 wavelengths in
the 580-720 nm region, and analyzed globally. A satisfactory
fit (ø2 ) 1.12) was obtained with two exponential components
with lifetimes of 130 ps and 1.78 ns. These are shown as decay-
associated spectra in Figure 6a. The 1.78-ns component has the
emission spectrum and lifetime of the unperturbed zinc por-
phyrin first excited singlet state, and is assigned to the closed
form of the dyad 2c. The 130-ps component also has the general
shape of zinc porphyrin emission, perhaps distorted slightly by
a small contribution from the merocyanine emission band at
∼657 nm. The 130-ps component is associated with the open
form of the dyad, 2o, and the short lifetime is qualitatively
consistent with the quenched steady-state emission spectrum.
Similar time-resolved emission experiments were performed
on free base dyad 3. Irradiation of the closed form 3c at 430
nm with measurements at 15 wavelengths in the 580-760 nm
region gave decays that were analyzed globally to yield a single-
exponential process with a lifetime of 10.5 ns (ø2 ) 1.07). Model
chromophore 5,10,15,20-tetrakis(4-methylphenyl)porphyrin gave
a lifetime of 10 ns under the same conditions, indicating that
the attached closed spiropyran has no significant effect on the
lifetime of the free base porphyrin singlet state. Decays were
also obtained after irradiation of 3 at 340 nm to open the
spiropyran ring, as described above. Data from measurements
at 14 wavelengths in the 630-760 nm region were fitted
globally (ø2 ) 1.08) to yield three exponential components with
Figure 6. Decay-associated fluorescence spectra of dyads 2 and 3 in
2-methyltetrahydrofuran solution following irradiation at 340 nm to
generate the open, merocyanine form as described in the text. Excitation
was at 430 nm, where absorption is due to the porphyrin moiety. (a)
Zinc dyad 2 (ø2 ) 1.12): (b) 1.8 ns, (9) 130 ps. (b) Free base dyad
3 (ø2 ) 1.08): (2) 11 ns, (b) 1 ns, (9) 85 ps.
lifetimes of 11 ns, 1.0 ns, and 85 ps (Figure 6b). The 11-ns
component has the emission spectrum and lifetime of the free
base porphyrin, and is attributed to the closed form of the dyad,
3c. The other two components have spectral shapes suggesting
a mixture of emissions from both quenched porphyrin and
merocyanine chromophores.
Time-resolved emission studies of model spiropyran 1 were
undertaken to aid in the interpretation of the results for the dyads.
No emission was observed from the closed, spiro form of the
molecule, 1c. A ∼1 × 10-5 M solution of 1 in 2-methyltet-
rahydrofuran was irradiated with light from a 500-W xenon arc
lamp passed through a 340-nm interference filter with a 10-nm
band-pass, the light was turned off, and time-resolved fluores-
cence spectra of the resulting mixture of 1c and 1o were obtained
by the single photon timing method as described above.
Excitation was at 620 nm using ∼7 ps laser pulses. Global
analysis of data at 10 wavelengths in the 580-760-nm region
as a 4-exponential process (ø2 ) 1.26) gave significant lifetimes
of 15, 45, and 120 ps (Figure 7). A very minor component with
a lifetime of 2 ns was also noted. Although the 15-ps component
appears as a decay at 630 nm and below, it has a negative
amplitude at 640 nm and above. This represents a grow-in of
emission at these wavelengths with time. The 45-ps component
appears to have approximately the same spectral shape as the
15-ps component above 640 nm, whereas the 120-ps transient
is weak, and shows an emission maximum around 700 nm.
Transient Absorption Measurements. Additional informa-
tion on the merocyanine form of the model compound, 1o, was
obtained by the pump-probe transient absorption technique,
with the sample under continuous irradiation at 340 nm to
(49) Gust, D.; Moore, T. A.; Moore, A. L.; Macpherson, A. N.; Lopez,
A.; DeGraziano, J. M.; Gouni, I.; Bittersmann, E.; Seely, G. R.; Gao, F.;
Nieman, R. A.; Ma, X. C.; Demanche, L. J.; Luttrull, D. K.; Lee, S.-J.;
Kerrigan, P. K. J. Am. Chem. Soc. 1993, 115, 11141-11152.

Porphyrin-Spiropyran Dyad
J. Am. Chem. Soc., Vol. 123, No. 29, 2001 7129
above the ground states. The situation with the spiropyran
chromophores is somewhat more complex (vide infra), but the
absorption and emission spectra for the open, merocyanine form
of model compound 1o in Figure 3 give a value of 1.98 eV for
1Spo. The closed form of the spiropyran is not emissive, but
from the absorption spectrum, the energy of 1Spc may be roughly
approximated as ∼3.8 eV. These data suggest that in zinc dyad
2o, 1PZn-Spo lies 0.08 eV above PZn-1Spo, while 1PH2-Spo is
0.07 eV lower in energy than PH2-1Spo. The energy differences
between the ground states of the closed and open forms of the
dyads cannot be determined from the spectroscopic data, but
are not relevant to the discussion that follows.
Singlet state quenching by electron, as well as energy, transfer
•+
must be considered. We can estimate the energies of PZn
-
Spc and PH2•+-Spc (the lowest energy charge-separated
states) as 1.99 and 2.21 eV above the ground states, respectively,
in polar solvents based on the cyclic voltammetric data given
earlier. Thus, 1PH2 at 1.91 eV lies at significantly lower energy
than the charge-separated state, and its formation on the
picosecond time scale is unlikely. These estimates place 1PZn-
Spc ∼0.07 eV above PZn•+-Spc•-, and photoinduced electron
transfer is in principle possible. However, neither 1PZn nor 1PH2
are quenched in the closed forms of the dyads, so it may be
concluded that photoinduced electron transfer does not occur
in 2-methyltetrahydrofuran. As mentioned above, the open,
merocyanine form of the spiropyran does not display observable
reduction behavior in cyclic voltammetry at potentials less
negative than those of the closed form, so photoinduced redox
processes involving this moiety are also unlikely. The energy
•-
•-
Figure 7. Decay-associated fluorescence spectra of model spiropyran
1 in 2-methyltetrahydrofuran solution following irradiation at 340 nm
to generate the open, merocyanine form as described in the text.
Excitation was at 620 nm (ø2 ) 1.20): (9) 15 ps, (b) 45 ps, (2) 120
ps, (1) 2 ns).
1
of 1Spo is lower than that of PZn, so its involvement in
photoredox processes is also improbable. These arguments
suggest that photoinduced electron transfer is unlikely as a
quenching mechanism. Thus, singlet-singlet energy transfer is
the most probable cause of the quenching. This conclusion is
supported by the time-resolved emission spectra measured for
3 (Figure 6b), which have features characteristic of merocyanine
emission, even though only the porphyrin chromophore was
excited.
Figure 8. Decay-associated absorption spectra of model spiropyran 1
in 2-methyltetrahydrofuran solution following irradiation at 340 nm to
generate some of the open, merocyanine form as described in the text.
Excitation was at 600 nm:(b) 20 ps, (2) 45 ps, (0) 140 ps. Shown for
comparison are the absorption and emission spectra of the merocyanine
form, 1o, from Figure 3.
Model Spiropyran 1. Although the focus of this work is on
the excited-state properties of the porphyrin moieties of the
dyads, understanding these properties requires some consider-
ation of the photochemistry of the spiropyran. We will therefore
now discuss the spectroscopic results for the model spiropyran
1. The absorption and emission data in Figure 3 illustrate that
in common with other closely related spiropyrans, the closed,
spiro form of 1c absorbs only in the ultraviolet region, and has
no strong fluorescence. Upon irradiation in the near-UV region,
the molecule isomerizes to an open, merocyanine form with
strong absorption at 600 nm and emission at 657 nm. This form
slowly reverts thermally to the closed form. Irradiation of the
merocyanine form with visible light enhances the reversion rate,
as is true for other spiropyrans. The time-resolved fluorescence
results (Figure 7) indicate that the open molecule, 1o, exists in
at least three major isomeric forms, with lifetimes of 15, 45,
and 120 ps. This is confirmed by the transient absorption data
in Figure 8, where three components with similar lifetimes were
observed. The species with the 15 ps lifetime emits relatively
weakly in the 600-630-nm region, and its decay is apparent in
Figure 7. However, at longer wavelengths, where the main
spiropyran emission occurs, this component appears as a rise
in emission intensity with time. Thus, the species responsible
for this component is evidently converted into the species having
the 45-ps lifetime with a time constant of 15 ps. The 45-ps
component is responsible for most of the fluorescence emission,
produce some merocyanine form of the molecule. Excitation
of a ∼1 × 10-3 M solution in 2-methyltetrahydrofuran
employed ∼100 fs laser pulses at 600 nm, which excite only
the open form. Global analysis of these data shows absorption
difference decay components with lifetimes of 20, 45, and 140
ps (Figure 8). These correspond, within experimental error, to
the 15, 45, and 120 ps lifetimes determined by fluorescence
techniques. Although all three components display bleaching
of the merocyanine ground state in the 550-600-nm region,
only the 45-ps component shows significant stimulated emission
near the maximum of merocyanine fluorescence around 657 nm.
Note that a component with the same lifetime dominates the
time-resolved emission spectrum in Figure 7.
Discussion
Energetics. The energies of the first excited singlet states of
the various chromophores may be estimated as the frequency-
domain average of the long-wavelength absorption and short-
wavelength emission maxima. Thus, the porphyrin first excited
singlet states 1PZn and 1PH2 lie at 2.06 and 1.91 eV, respectively,

7130 J. Am. Chem. Soc., Vol. 123, No. 29, 2001
Bahr et al.
whereas the third component with the lifetime of 120 ps is only
weakly emissive around 700 nm.
These results might be taken to suggest that the component
with the 120-ps lifetime is of minor importance. However, the
transient absorption results in Figure 8 show that this is not
necessarily the case. All three components demonstrate signifi-
cant ground-state bleaching in the region of the merocyanine
absorption around 600 nm. If they all have comparable
extinction coefficients in this region, then all three are present
in considerable amounts. Only the component with a lifetime
of 45 ps shows significant stimulated emission around 657 nm,
and this is consistent with the time-resolved fluorescence results.
We interpret these results in terms of at least three confor-
mationally isomeric forms of 1o. There have been many reports
of conformational heterogeneity in the open form of spiropyrans
and related photochromic compounds.50-57 The conformers are
usually ascribed to slow rotation about the carbon-carbon bonds
in the linkage joining the two rings on the time scale of the
observation. The resonance forms of the merocyanine suggest
that all three of these bonds have double-bond character, which
would pose a significant barrier to rotation. As mentioned above,
8 stereoisomeric forms could exist in principle. In addition,
studies of related donor-acceptor substituted stilbene derivatives
have been interpreted to show that the excited states of these
molecules evolve into emissive twisted intramolecular charge
transfer states and nonemissive biradicaloid states via rotations
about the corresponding bonds. Thus, generating several tran-
sient states after excitation of the merocyanine is fully consistent
with previous findings. In addition, it must be remembered that
repeated cycling of the spiropyran between the open and closed
forms (necessary to obtain the time resolved data) eventually
leads to some degradation products that may contribute to the
spectra.
Figure 9. Energies and relevant interconversion pathways for high-
energy states of (a) zinc dyad 2 and (b) free base dyad 3. Excited-state
energies are calculated from spectroscopic data, as described in the
text.
Dyads 2 and 3. Because the spiropyran moiety absorbs at
shorter wavelengths than the porphyrin in the closed forms of
the dyads, energy transfer quenching of the spiropyran singlet
state by the porphyrin is thermodynamically possible. However,
it is obvious from the spectroscopic results that significant
energy transfer does not occur; the spiropyran opens in the usual
way to generate the merocyanine. This is consistent with the
lack of fluorescence emission from the closed spiropyran and
the short lifetimes for the excited singlet states of these and
similar photochromic molecules as observed in transient absorp-
tion experiments,58 as both factors would make the closed
spiropyran excited singlet state a poor energy donor.
ambient temperatures. Thus, the open form has a long lifetime
relative to any of the other processes of interest in this work,
and the ring closure process will be ignored in the kinetic
analyses that follow.
Dyad 2. Figure 9a shows the relevant ground- and excited-
state species for zinc dyad 2 and associated interconversion
pathways. The energies are derived from the spectroscopic
results given above. In the closed form of zinc dyad, 2c, the
fluorescence lifetime of the zinc porphyrin first excited singlet
state is 1.80 ns, which is essentially identical with that of a
1
model zinc tetraarylporphyrin. Thus, PZn-Spc decays by the
usual photophysical pathways of internal conversion, intersystem
crossing, and fluorescence.
In both 2 and 3, the merocyanine form closes thermally to
the spiro form in an exponential fashion with a rate constant of
3.5 × 10-2 s-1 in deoxygenated 2-methyltetrahydrofuran at
When the sample is irradiated in the UV where the spiropyran
absorbs, the steady-state absorption and emission results show
that the sample is partially converted to the merocyanine form,
PZn-Spo. Excitation of this open form at 430 nm, where only
the porphyrin absorbs, reveals a quenching of the porphyrin
emission, relative to PZn-Spc. As photoinduced electron transfer
seems unlikely for the reasons mentioned avove, the most
reasonable quenching mechanism is singlet-singlet energy
transfer to the merocyanine.
The time-resolved emission experiments provide more quan-
titative information. Continuous irradiation at 340 nm of a
sample of 2 in 2-methyltetrahydrofuran generated a mixture of
the closed and open forms of the spiropyran. Excitation of this
mixture at 430 nm after turning off the continuous light
necessarily generated only the porphyrin first excited singlet
states. The global analysis of the time-resolved fluorescence
emission data yielded only two components, with lifetimes of
1.78 ns and 130 ps (Figure 6a). The 1.78-ns component shows
(50) Heiligman-Rim; R.; Hirshberg, Y.; Fischer, E. J. Phys. Chem. 1962,
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(51) Yuzawa, T.; Ebihara, K.; Hiura, H.; Ohzeki, T.; Takahashi, H.
Spectochim. Acta 1994, 50A, 1487-1498.
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Phys. Lett. 1987, 140, 90-94.
(53) Hobley, J.; Malatesta, V. Phys. Chem. Chem. Phys. 2000, 2, 57-
59.
(54) Abe, Y.; Nakao, R.; Horii, T.; Okada, S.; Irie, M. J. Photochem.
Photobiol. A 1996, 95, 209-214.
(55) Cottone, G.; Noto, R.; La Manna, G.; Fornili, S. L. Chem. Phys.
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(56) Wilkinson, F.; Worrall, D. R.; Hobley, J.; Jansen, L.; Williams, S.
L.; Langley, A. J.; Matousek, P. J. Chem. Soc., Faraday Trans. 1996, 92,
1331-1336.
(57) Marevtsev, V. S.; Zaichenko, N. L. J. Photochem. Photobiol. A 1997,
104, 197-202.
(58) Malatesta, V. In Organic Photochromic and Thermochromic
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1999; Vol. 2, pp 68-75.

Porphyrin-Spiropyran Dyad
J. Am. Chem. Soc., Vol. 123, No. 29, 2001 7131
emission maxima characteristic of the zinc porphyrin first
excited singlet state, and a lifetime characteristic of the
unperturbed porphyrin excited state; it is ascribed to 1PZn-Spc.
This material is present in relatively large amount because some
is present in the photostationary state during irradiation with
the 340-nm light, and more is formed as the open form of the
spiropyran isomerizes to the closed form during time-resolved
data acquisition. The 130-ps component has the general emission
profile of the zinc porphyrin first excited singlet state, with
possibly some contribution from the open spiropyran moiety
around 650 nm. This component is assigned to molecules having
the open, merocyanine form of the spiropyran, PZn-Spo. The
substantial quenching of the zinc porphyrin emission relative
to the molecule in the closed form is attributed to singlet-singlet
energy transfer to the merocyanine (step 2 in Figure 9a). The
rate constant for energy transfer may be estimated from eq 1,
In the time-resolved emission data for 3o (Figure 6b), three
components were observed, with lifetimes of 11 ns, 1.0 ns, and
85 ps. The 11-ns component has the spectral features and typical
lifetime of the unperturbed free base porphyrin first excited
singlet state, and is attributed to PH2-Spc whose ground state
is present during steady-state UV excitation and forms by
thermal isomerization during data collection. The other two
1
components may be ascribed to the merocyanine form, PH2
Spo.
-
Turning to Figure 9b, it is evident that the lifetime of the
porphyrin first excited singlet state can be shorter than 11 ns
only if there is some quenching by the open form of the
spiropyran. Quenching by singlet-singlet energy transfer is an
endergonic process because PH2-1Spo lies at slightly higher
1
energy (1.98 eV) than PH2-Spo (1.91 eV), based on the
absorption and emission data reported above. The energy
1
1
where τ is the lifetime of PZn-Spo. Substituting τ ) 130 ps
difference between PH2-Spo and PH2-1Spo yields an equilib-
and k1 ) 5.6 × 108 s-2 (from the 1.80-ns lifetime of the
rium constant of 0.065. This relatively small energy difference,
coupled with the relatively long (11 ns) lifetime of the free base
porphyrin first excited singlet state, suggests that singlet-singlet
energy transfer will be significant for 3o.
unperturbed zinc porphyrin first excited singlet state) yields a
1/τ ) k1 + k2
(1)
In the general case of two excited states rapidly undergoing
exchange of singlet excitation, two fluorescence lifetimes will
be observed. The reciprocals of these lifetimes, γ1 and γ2, do
not correspond to any rate constants, but are related to the rate
constants in Figure 9b according to the following equations:
value of 7.1 × 109 s-1 for k2. The quantum yield of singlet-
singlet energy transfer is given by eq 2, and equals 0.93.
Φ ) k2/(k1 + k2)
(2)
The PZn-1Spo formed by the singlet-singlet energy transfer
process should also emit, as suggested by the fluorescence of
model spiropyran 1o (Figure 3). The time-resolved fluorescence
results for 1o indicate that the only strongly emissive form of
the merocyanine has an excited-state lifetime of 45 ps. However,
as this decay rate of PZn-1Spo is significantly faster than its
formation rate (130 ps), the concentration of PZn-1Spo is always
small, and no contribution from a short decay lifetime was
observed in the decay-associated spectra.
(X + Y) + (X - Y)2 + 4k k
x
1
3
γ1 )
γ2 )
(3)
(4)
2
(X + Y) - (X - Y)2 + 4k k
x
1
3
2
where
In principle, PZn-1Spo could decay not only to the ground
state and PZn-Spc but also by reverse energy transfer to the
X ) k1 + k2
Y ) k3 + k-2
(5)
(6)
1
zinc porphyrin, yielding PZn-Spo (step -2 in Figure 9a). If
this process occurred, additional lifetime components in the
decay-associated spectra would be expected (see below), but
none were observed. The reason is that k-2 is small, relative to
the rate constants for other processes. The energy difference
and the rate constants are defined as per Figure 9b. The time-
resolved fluorescence results for 3o in Figure 6b show that two
decay components (85 ps and 1.0 ns) are indeed observed, in
addition to the 11-ns component due to porphyrin emission from
1
between PZn-Spo and PZn-1Spo based on the absorption and
1
emission data given above is 0.08 eV, yielding an equilibrium
constant of 23 at ambient temperature. This and the value for
k2 estimated above give a nominal value for k-2 of 3 × 108
s-1. This value is less than 4% of the rate constant for even the
slowest decay process measured for model spiropyran 1 in the
open form. Thus, PZn-1Spo decays by other pathways, and
endergonic energy transfer by the reverse of step 2 may be
ignored.
the closed, spiro form PH2-Spc. If we assign the reciprocals
of γ1 and γ2 to these lifetimes, we can solve eqs 3-6 for k2
and k-2 if we have values for k1 and k3. The value of k1 is 9.5
× 107 s-1, based on the 10.5-ns lifetime of the unperturbed
free base porphyrin first excited singlet state in 3c. Choosing a
value for k3 is more problematic, as three lifetimes were found
for the mixture of isomeric merocyanine structures in the open
form of model spiropyran 1o: 15 ps, 45 ps, and ∼140 ps
(Figures 7 and 8). One or several of these could be involved in
the exchange of singlet excitation energy with the free base
porphyrin in 3o. As it turns out, solutions for eqs 3-6 could
only be found using the 140-ps lifetime (k3 ) 7.1 × 109 s-1).
Using this lifetime, k2 ) 9.7 × 108 s-1 and k-2 ) 4.6 × 109
s-1. These values in turn give an equilibrium constant K ) k2/
k-2 ) 0.2. This is larger than the nominal value of 0.065
calculated from the absorption and emission spectra of 3o and
model compounds, but still in relatively good agreement
considering that the absorption and emission spectra observed
for model 1o are actually composites of the spectra of at least
three isomeric forms of the merocyanine, each of which has its
own unique spectral properties. Some of the corresponding
Dyad 3. Turning our attention to free base dyad 3, the
energetics and interconversion pathways are shown in Figure
1
9b. When the spiropyran is in the closed form, the PH2-Spc
state has a lifetime of 10.5 ns, which is similar to that of a
porphyrin model compound. Thus, this state is not quenched
by the attached spiropyran. Irradiation of the compound in the
UV results in opening of some of the spiropyran to yield PH2
-
Spo, as shown by the absorption spectra in Figure 2b and the
emission spectra in Figure 5b. The emission spectrum with
excitation into the porphyrin (430 nm) from the solution
previously irradiated at 340 nm is less intense than that from
pure PH2-Spc, suggesting that the porphyrin first excited singlet
state is quenched by the open spiropyran.

7132 J. Am. Chem. Soc., Vol. 123, No. 29, 2001
Bahr et al.
was ca. 100 µW. The excitation beam was attenuated as needed to
obtain 1000-5000 fluorescence counts per second. For excitation at
380-440 nm, a Spectra Physics Tsunami laser was used. Its funda-
mental output was doubled by a Spectra Physics Model 3980 frequency
doubler and pulse selector. The pulse width was 2 ps, and the average
power was ca. 1 µW. Fluorescence emission was detected at the magic
angle using a single-grating monochromator and a microchannel plate
photomultiplier (Hamamatsu R2809U-11). The instrument response
time was ca. 35-50 ps, as verified by scattering from Ludox AS-40.
The spectrometer was controlled by software based on the LabWindows
program from National Instruments.
isomers in 3o may not be directly involved in the energy transfer
process. Thus, the energy of the merocyanine first excited singlet
state involved in the energy transfer process is somewhat
uncertain. On the basis of the rate constants reported above,
kinetic simulations show that singlet-singlet energy transfer
1
quenches the fluorescence emission of P-Spo to 14% of its
value in the closed form of the dyad, or a comparable model
porphyrin. Of course, the emission in Figure 5b is not quenched
to this degree because a substantial amount of the compound
in the sample is in the closed, unquenched form.
In both dyads, the open form of the photochromic moiety
closes to the spiropyran both thermally and photochemically.
Singlet-singlet energy transfer from the porphyrin to the
merocyanine would therefore enhance the overall rate of closing.
The degree to which this occurs depends on the yields of the
various photochemical and isomerization processes, and has not
been investigated.
The femtosecond transient absorption apparatus consists of a
kilohertz pulsed laser source and a pump-probe optical setup. The
laser pulse train was provided by a Ti:Sapphire regenerative amplifier
(Clark-MXR, Model CPA-1000) pumped by a diode-pumped CW solid-
state laser (Spectra Physics, Model Millennia V). The typical laser pulse
was 100 fs at 790 nm, with a pulse energy of 0.9 mJ at a repetition
rate of 1 kHz. Most of the laser energy (80%) was used to pump an
optical parametric amplifier (IR-OPA, Clark-MXR). The excitation
pulse was sent through a computer-controlled optical delay line. The
remaining laser output (20%) was focused into a 1.2 cm rotating quartz
plate to generate a white light continuum. The continuum beam was
further split into two identical parts and used as the probe and reference
beams, respectively. The probe and reference signals were focused onto
two separated optical fiber bundles coupled to a spectrograph (Acton
Research, Model SP275). The spectra were acquired on a dual diode
array detector (Princeton Instruments, Model DPDA-1024).
Conclusions
These experiments demonstrate that in both the zinc, 2, and
free base, 3, porphyrin-spiropyran dyads, the porphyrin first
excited singlet states are unperturbed by the attached spiropyran
when it is in the closed form. Conversely, even in the presence
of the porphyrin moiety, the excited state of the closed
spiropyran chromophore formed by UV excitation still opens
smoothly to yield the merocyanine, which in turn closes to the
spiro form thermally, with a time constant of 20 s, or upon
irradiation into its visible absorption band. In the merocyanine
form of the zinc dyad, 2o, the porphyrin first excited singlet
state is quenched by the merocyanine with a quantum yield of
0.93, reducing the lifetime from 1.8 ns to 130 ps. The quenching
is most reasonably assigned to singlet-singlet energy transfer.
In the free base dyad, strong quenching is also observed, even
though the energy transfer reaction is slightly endergonic. Thus,
in these dyads, the spiropyran is a light-activated “switch” that
controls the lifetime of the porphyrin first excited singlet states.
As such, it could be used to turn on or off photochemical
processes arising from the porphyrin excited singlet or triplet
states, including photoinduced electron transfer, singlet energy
transfer, and triplet energy transfer. Linking electron or energy
donors and/or acceptors to the porphyrin moiety would allow
realization of this potential.
Synthesis. All reactions were run under an argon atmosphere. The
4-tolyldipyrrolemethane was prepared according to a literature proce-
dure,59 and was used immediately after recrystallization.
1-(4-Carbomethoxybenzyl)-exo-methyleneindoline (4). A solution
of commercially obtained 2,3,3-trimethyl-3H-indole (7.26 g, 45.7 mmol)
and methyl 4-bromomethylbenzoate (10.0 g, 45.7 mmol) in 35 mL of
3-methyl-2-propanone was stirred at 85 °C for 24 h. Upon cooling the
solution to room temperature, a viscous sludge formed at the bottom
of the flask. Upon addition of ether to the supernatant, additional salt
precipitated. This material was dissolved in chloroform and washed
with aqueous sodium hydroxide (pH 10). After drying the chloroform
solution and removing the volatiles by distillation at reduced pressure,
chromatography (silica gel, dichloromethane) was employed to isolate
5.6 g of the product (40%): 1H NMR (300 MHz, CDCl3) δ 8.06 (2H,
d, J ) 8 Hz, Ar 3,5H), 7.36 (2H, d, J ) 8 Hz, Ar2,6H), 7.22 (1H, d,
J ) 7.2 Hz, indoline-ArH), 7.14 (1H, t, J ) 7 Hz, indoline-ArH), 6.87
(1H, t, J ) 7 Hz indoline-ArH), 6.54 (1H, d, J ) 7.2 Hz, indoline-
ArH), 4.79 (2H, s, CH2-Ar), 3.94 (2H, s, -CH2), 3.91 (3H, s, COOCH3),
1.49 (6H, s, indoline-CH3); MS (EI) m/z 307 (M+).
1′-(4-Carbomethoxybenzyl)-6-nitroindolinospirobenzopyran (5).
A solution of 4 (3.0 g, 9.8 mmol) and 5-nitrosalicyaldehyde (1.6 g, 9.8
mmol) in 60 mL of ethanol was refluxed for 2 h. After the mixture
was cooled to room temperature, the solvent was removed by distillation
at reduced pressure, and the residue was dissolved in dichloromethane.
Chromatography (silica gel, dichloromethane) yielded 3.5 g of 5
(79%): 1H NMR (CDCl3, 300 MHz) δ 8.04 (1H, d, J ) 8 Hz, 7H),
7.98 (1H, s, 5H), 7.95 (2H, d, J ) 8 Hz, Ar3,5H), 7.35 (2H, d, J ) 8
Hz, Ar2,6H), 7.13 (1H, d, J ) 8 Hz, 8H), 7.07 (1H t, J ) 7.5 Hz,
6′H), 6.90 (1H, t, J ) 7.5 Hz, 5′H), 6.90 (1H d, J ) 10 Hz, 4H), 6.78
(1H, d, J ) 7.5 Hz, 4′H), 6.29 (1H, d, J ) 7.5 Hz, 7′H), 5.90 (1H, d,
J ) 10 Hz, 3H), 4.54 (1H, d, J ) 16 Hz, -CH2-), 4.26 (1H, d, J )
16 Hz, -CH2-), 3.91 (3H, s, -OCH3), 1.35 (3H, s, 3′-CH3), 1.31 (3H,
s, 3′-CH3); MS (MALDI-TOF) m/z 456 (M+).
1′-(4-Hydroxymethylbenzyl)-6-nitroindolinospirobenzopyran (6).
A portion of 5 (3.0 g, 6.6 mmol) was dissolved in 50 mL of dry
tetrahydrofuran at 0 °C, and excess lithium aluminum hydride was
added slowly. After being stirred for 1 h the solution was poured into
dichloromethane and washed with dilute citric acid. The organic layer
was dried over sodium sulfate, filtered, and concentrated by distillation
of the solvent at reduced pressure. Chromatography (silica, dichlo-
romethane containing 2% methanol) was employed to isolate 2.27 g
Experimental Section
Instrumental Techniques. The 1H NMR spectra were recorded on
Varian Unity spectrometers at 300 or 500 MHz. Unless otherwise noted,
samples were dissolved in deuteriochloroform with tetramethylsilane
as an internal reference. Low-resolution mass spectra were obtained
on a Varian MAT 311 spectrometer operating in EI mode or a matrix-
assisted laser desorption/ionization time-of-flight spectrometer (MALDI-
TOF), as indicated. High-resolution mass spectra were determined on
a Kratos MS50 mass spectrometer operating at 8 eV in FAB mode.
Steady-state ultraviolet-visible spectra were measured on a Shimadzu
UV2100U UV-vis spectrometer.
Steady-state fluorescence emission spectra were measured using a
SPEX Fluorolog-2 and corrected. Excitation was produced by a 450-W
xenon lamp and single grating monochromator. Fluorescence was
detected at 90° to the excitation beam via a single grating monochro-
mator and an R928 photomultiplier tube having S-20 spectral response
operating in the single-photon-counting mode.
Fluorescence lifetime measurements were made using the single-
photon-timing method. Two different laser systems were used. For
excitation between 570 and 630 nm an ultrafast dye laser system was
used. It included a frequency-doubled, mode-locked Coherent Antares
Nd:YAG laser, which synchronously pumped a cavity-dumped Coherent
700 dye laser. The pulse width was ca. 7 ps, and the average power
(59) Littler, B. J.; Miller, M. A.; Hung, C.-S.; Wagner, R. W.; O’Shea,
D. F.; Boyd, P. D. W.; Lindsey, J. S. J. Org. Chem. 1999, 64, 1391-1396.

Porphyrin-Spiropyran Dyad
J. Am. Chem. Soc., Vol. 123, No. 29, 2001 7133
of 6 (81%): 1H NMR (CDCl3, 300 MHz) δ 8.02 (1H, d, J ) 7.2 Hz,
7H), 7.97 (1H, d, J ) 3 Hz, 5H), 7.29 to 7.25 (4H, m, ArH), 7.12 (1H,
d, J ) 7.2 Hz, 8H), 7.07 (1H, t, J ) 7.5 Hz, 6′H), 6.88 (1H t, J ) 7.5
Hz, 5′H), 6.86 (1H, d, J ) 10 Hz, 4H), 6.78 (1H, d, J ) 7.5 Hz, 4′H),
6.34 (1H, d, J ) 7.5 Hz, 7′H), 5.87 (1H, d, J ) 10 Hz, 3H), 4.67 (2H,
d, J ) 6 Hz, CH2OH), 4.50 (1H, d, J ) 16.5 Hz, -CH2-), 4.21 (1H,
d, J ) 16.5 Hz, -CH2-), 1.60 (1H, m, -OH), 1.34 (3H, s, 3′-CH3),
1.30 (3H, s, 3′-CH3); MS (MALDI-TOF) m/z 428 (M+).
1′-(4-Formylphenyl)-6-nitrospirobenzopyran (1). Excess manga-
nese oxide was added to a solution of 6 (2.0 g, 4.7 mmol) in 40 mL of
dichloromethane. After the mixture was stirred at room temperature
for 24 h, the suspension was filtered through Celite and concentrated
by distillation at reduced pressure. A short silica gel column (dichlo-
romethane containing 0.5% methanol) was employed to remove traces
of unreacted starting material. A total of 1.52 g of 1 was isolated
(76%): 1H NMR (CDCl3, 300 MHz) δ 9.98 (1H, s, -CHO), 8.04 to
7.99 (2H, m, 5H and 7H), 7.83 (2H, d, J ) 7.8 Hz, Ar3,5H), 7.47 (2H,
d, J ) 7.8 Hz, Ar2,6H), 7.15 (1H, d, J ) 7 Hz, 8H), 7.08 (1H, t, J )
7.6 Hz, 6′H), 6.91 (1H, t, J ) 7.6 Hz, 5′H), 6.91 (1H, d, J ) 10.5 Hz,
4H), 6.78 (1H, d, J ) 7.6 Hz, 4′H), 6.30 (1H, d, J ) 7.6 Hz, 7′H),
5.92 (1H, d, J ) 10.5 Hz, 3H), 4.56 (1H, d, J ) 16.8 Hz, -CH2-),
4.30 (1H, d, J ) 16.8 Hz, -CH2-), 1.37 (3H, s, 3′-CH3), 1.34 (3H, s,
3′-CH3); MS (MALDI-TOF) m/z 426 (M+).
stirred at room temperature in the dark for 1 h, and 2,3-dichloro-5,6-
dicyanobenzoquinone (0.98 g, 4.3 mmol) was added. After being stirred
for an additional 1 h, the solution was concentrated by distillation at
reduced pressure, and the residue was purified by flash chromatography
on a short silica column using chloroform as the eluant. All fluorescent
bands were collected. The porphyrin-containing products were then
separated by chromatography (silica gel, dichloromethane containing
35% petroleum ether). Free base dyad 3 was isolated as purple crystals
in 5% yield (138 mg). It was characterized after quantitative conversion
to zinc-containing dyad 2 by stirring a solution of 3 overnight with an
excess of Zn(OAc)2 in chloroform and chromatographing the product
on a short silica gel column: 1H NMR (300 MHz, CDCl3) δ 8.89 to
8.81 (8H, m, pyrrole-H), 8.17 to 8.10 (10 H, m, 7H, 5H, Ar3,5H, por-
10,15,20-Ar-2,6H), 7.67 (2H, d, J ) 7.8 Hz, Ar2,6H), 7.57 (6H, d, J
) 7.7 Hz, por-10,15,20-Ar-3H,5H), 7.31 to 7.22 (2H, m, 6′H and 8H),
7.03 (1H, t, J ) 7.6 Hz, 5′H), 7.03 (1H, d, J ) 10.5 Hz, 4H), 6.92
(1H, d, J ) 7.6 Hz, 4′H), 6.72 (1H, d, J ) 7.6 Hz, 7′H), 6.15 (1H, d,
J ) 10.5 Hz, 3H), 4.86 (1H, d, J ) 16 Hz, -CH2-), 4.54 (1H, d, J )
16 Hz, -CH2-), 2.72 (9H, s, por-CH3), 1.45 (3H, s, 3′-CH3), 1.43
(3H, s, 3′-CH3); MS (MALDI-TOF) m/z 974 (M+). Dyad 2: FAB-
HRMS, calcd 1038.3236, found 1038.3202 (M+).
Acknowledgment. This work was supported by a grant from
the National Science Foundation (CHE-0078835). We thank the
Nebraska Center for Mass Spectrometry for FAB high resolution
mass spectrometry experiments.
Porphyrin-Spiropyrans (2 and 3). A solution of 4 (1.20 g, 2.85
mmol), 4-methylbenzaldehyde (0.342 g, 2.85 mmol), and meso-(4-
methylphenyl)dipyrromethane (1.35 g, 5.70 mmol) was prepared in 570
mL of CHCl3. After the solution was purged with argon for 15 min,
BF3OEt2 (0.23 mL, 1.9 mmol) was added by syringe. The solution was
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