Mediated electronic coupling: Singlet energy transfer in porphyrin dimers enhanced by the bridging chromophore

Article abstract of DOI:10.1021/jp9909098

We have studied singlet electronic energy transfer (EET) in two donor-bridge-acceptor series (D-B-A), in which the donor (zinc porphyrin or its pyridine complex) and the acceptor (free base porphyrin) were covalently connected by a geometrically well-defined bridging chromophore. We have investigated how the medium between a donor and an acceptor influences EET by separating the influence of the electronic structure of the bridging chromophore from other effects known to influence the energy transfer. The electronic structure of the bridging chromophore was varied by changing the central unit (bicyclo[2.2.2]octane, benzene, naphthalene, or anthracene) in the bridging chromophore. In all systems the excited state energy separation donor-bridge and bridge-acceptor is large enough to prevent stepwise singlet energy transfer. In addition, the systems were designed to minimize conjugation to preserve the identity of the separate chromophores (donor, bridge, acceptor). Compared with the rate constant expected from the Foì?rster theory, the bridging chromophore with bicyclo[2.2.2]octane as the central unit did not significantly enhance the energy transfer rate constant. However, the bridging chromophores with benzene and naphthalene as the central unit showed a moderate increase, whereas the bridging chromophore with anthracene as the central unit showed the largest increase in energy transfer rate constant. This increase is ascribed to a mediating effect of the bridging chromophore and it is proposed to be strongly correlated to the energy splitting between the singlet excited states of donor and bridging chromophores. ? 1999 American Chemical Society.

Full text of DOI:10.1021/jp9909098

J. Phys. Chem. B 1999, 103, 7329-7339
7329
Mediated Electronic Coupling: Singlet Energy Transfer in Porphyrin Dimers Enhanced by
the Bridging Chromophore
Kristine Kilså,† Johan Kajanus,‡ Jerker Mårtensson,‡ and Bo Albinsson*,†
Department of Physical Chemistry and Department of Organic Chemistry, Chalmers UniVersity of Technology,
SE-412 96 Go¨teborg Sweden
ReceiVed: March 15, 1999; In Final Form: June 2, 1999
We have studied singlet electronic energy transfer (EET) in two donor-bridge-acceptor series (D-B-A),
in which the donor (zinc porphyrin or its pyridine complex) and the acceptor (free base porphyrin) were
covalently connected by a geometrically well-defined bridging chromophore. We have investigated how the
medium between a donor and an acceptor influences EET by separating the influence of the electronic structure
of the bridging chromophore from other effects known to influence the energy transfer. The electronic structure
of the bridging chromophore was varied by changing the central unit (bicyclo[2.2.2]octane, benzene,
naphthalene, or anthracene) in the bridging chromophore. In all systems the excited state energy separation
donor-bridge and bridge-acceptor is large enough to prevent stepwise singlet energy transfer. In addition, the
systems were designed to minimize conjugation to preserve the identity of the separate chromophores (donor,
bridge, acceptor). Compared with the rate constant expected from the Fo¨rster theory, the bridging chromophore
with bicyclo[2.2.2]octane as the central unit did not significantly enhance the energy transfer rate constant.
However, the bridging chromophores with benzene and naphthalene as the central unit showed a moderate
increase, whereas the bridging chromophore with anthracene as the central unit showed the largest increase
in energy transfer rate constant. This increase is ascribed to a mediating effect of the bridging chromophore
and it is proposed to be strongly correlated to the energy splitting between the singlet excited states of donor
and bridging chromophores.
Introduction
Coulomb coupling, even though they act in conjunction. The
former short-range orbital-dependent coupling has been medi-
ated over relatively large D-A distances by the bonds in the
intervening medium, i.e., the bridge.5 Closs et al.6 first
demonstrated that a bridge with only σ-bonds could promote
triplet-triplet EET analogously to promotion of electron
transfer.7 A marked rate dependence on the conformations of
the σ-bonds was also found for the studied isomers, which
indicates through-bond electronic coupling.7,8 These observations
of EET mediated through σ-bonds have been confirmed and
similar results concerning singlet-singlet EET have been
reported in several recent publications.9 The theoretical treat-
ment10 of such systems is based on the through-bond mechanism
used to explain intramolecular electron transfer.11 Other issues
that have been addressed experimentally are the distance
dependence on the EET rate in D-B-A systems in which the
bridge has a large conjugated π-electronic system,12 and the
dependence on the point of attachment of the bridge at the D
and A.13
Knowledge of the factors that govern energy and electron-
transfer processes is crucial to our understanding of natural
photosynthesis and many other biological processes.1 It will also
be important for the future construction of artificial photosyn-
thetic complexes2 and molecular photonic or electronic devices.3
The electronic coupling between a donor and an acceptor that
underlies energy and electron transfer processes has been studied
intensively with respect to different factors that affect its
magnitude.4 The magnitude of the coupling determines the rate
of the transfer process. One of the factors that influence
electronic coupling is the electronic structure of the medium
between the donor and the acceptor. The intervening medium
may be a protein scaffold, as where the natural transfer processes
occur. It may also be solvent molecules or a bridge covalently
linking the donor and acceptor, as found in most artificial
systems. The effect of the medium between donor and acceptor
can be studied in donor-bridge-acceptor (D-B-A) systems
in which the three parts can be regarded as individual chro-
mophores. The bridging chromophore, in addition to acting as
an intervening medium, serves as a spacer between donor and
acceptor and provides geometrical constraints on the system.
Furthermore, the bridging chromophore can be changed to either
“insulate” or “conduct”, thus enabling its effect on the transfer
processes to be studied.
The Coulomb coupling may occur over large distances
without mediation because no orbital overlap is required.
However, it also has been suggested that this coupling may be
relayed by the intervening medium.9e,14 As the energy of the
lowest excited electronic state of the bridge approaches that of
the donor excited state it is expected to become increasingly
important as a mediating channel. Nevertheless, to the best of
our knowledge, a systematic study of the EET rate dependence
on the energy of the lowest excited state of such bridges has
not yet been reported. The diversity of features of the intervening
medium that influences the transfer processes makes it one of
Electronic energy transfer (EET) is often described as caused
by either an orbital overlap-dependent exchange coupling or a
* To whom correspondence should be addressed.
† Department of Physical Chemistry.
‡ Department of Organic Chemistry.
10.1021/jp9909098 CCC: $18.00 © 1999 American Chemical Society
Published on Web 08/05/1999

7330 J. Phys. Chem. B, Vol. 103, No. 34, 1999
Kilså et al.
Design. We have constructed the D-B-A systems to answer
questions about the influence of the medium between donor
and acceptor on the EET process. Therefore, the design of the
series of systems was such that variation in all other parameters
known to affect the EET process was minimized. The systems
were designed based on the following criteria: (i) The distance
between the donor and the acceptor should be constant
throughout the series. (ii) The relative orientation of the donor
and the acceptor should be independent of the bridging
chromophore. (iii) Simple π-conjugation through the system
should be avoided to preserve the identity of the donor, bridge,
and acceptor chromophores.
Structure. The design criteria are met in the investigated
systems. (i) Quantum mechanical calculations (Table 1) show
that the length of the bridging chromophore is, within the
accuracy of the methods used, equal for all four bridging
chromophores, 16.4 Å. Furthermore, the length of the bridging
chromophore does not depend on the orientation of the central
unit relative to the plane of the phenyl groups. Thus, the distance
between donor and acceptor is equal in all systems. From the
MM+16 optimized structure of ZnP-BB-H2P, a center-to-
center distance of 25.3 Å was estimated.15 (ii) The angular
distribution between the porphyrin planes and the central unit
are almost uniform because of the use of the triple bonds in the
bridging chromophore (Figure 1). The rotational barrier of the
central unit was investigated by keeping the phenyl groups in
one plane and rotating the central unit around the triple bonds,
thereby changing the dihedral angle (R) between the central unit
and the plane of the phenyl groups. The structures were
optimized and the energy calculated at fixed angles (0°, 15°...
90°). The largest rotational barrier (0.82 kcal/mol at R ) 90°)
was found for AB using PM317 (Table 1) whereas ab initio (HF/
3-21G*)18 calculations gave a smaller barrier for AB and the
largest for BB (0.96 kcal/mol). The low barriers show that at
room temperature all relative orientations of the porphyrin planes
are energetically attainable and that a rapid interconversion
between different conformations is to be expected. (iii) Con-
jugation through the system is minimized as judged by the
conservation of the absorption spectra of the D-B-A systems
compared with its components (vide infra). The methyl groups
in the â-position of the porphyrin moieties (next to the phenyl
groups) force the porphyrin and phenyl planes to be almost
orthogonal because of steric encumbrance. The angular deviation
from orthogonality at room temperature was estimated compu-
tationally by varying the dihedral angle between the porphyrin
and phenyl planes (90°, 85° ... 55°) in H2P, optimizing the
structure, and calculating the potential energy at these fixed
dihedral angles (PM3). In this way, the span of the dihedral
angle between the planes of the porphyrin and the adjacent
phenyl group was estimated to be 90° ( 22° at room temperature
(90% of the conformers according to the Boltzmann distribu-
tion).
Figure 1. Structure of the studied donor-bridge-acceptor systems.
The dimers are denoted ZnP-RB-H2P and the corresponding reference
compounds are denoted ZnP-RB and RB-H2P. With pyridine as an
axial ligand to zinc porphyrin the donor is changed to ZnPpy. RB is
the bridging chromophore, where R is the central unit, which is either
bicyclo[2.2.2]octane (O), benzene (B), naphthalene (N), or anthracene
(A).
the least understood factors controlling energy and electron
transfer, yet it is important because of its potential use in the
future design of supramolecular systems to control and optimize
the transfer processes.
Preliminary results15 have shown that in a series of porphyrin
dimers, the energy transfer rate constant, kEET, increased with
decreasing energy level of the lowest singlet excited state of
the aromatic bridging chromophore. This article presents full
experimental details for an extended number of D-B-A
systems accompanied by a qualitative analysis of the bridge
mediating effect. The systems consist of porphyrin moieties
covalently linked by a rigid bridging chromophore and the three
parts can be regarded as individual chromophores. In both series
the electronic structure of the bridging chromophore is varied,
whereas variations in all other parameters known to influence
EET have been minimized carefully.
Results
The general structure of the studied D-B-A systems is
shown in Figure 1. We have synthesized and investigated two
series of porphyrin dimers in which the central unit in the
bridging chromophore has been varied. The electronic structure
is hereby varied and hence the energy of the lowest singlet
excited state of the bridging chromophore. Three of the four
bridging chromophores are fully conjugated systems; bis-
(phenylethynyl)arylene where the central arylene is either 1,4-
phenylene (BB), 1,4-naphthylene (NB), or 9,10-anthrylene (AB).
In the fourth bridging chromophore the conjugation is broken
by substitution of the arylene by 1,4-bicyclo[2.2.2]octylene
(OB). In both series the acceptor is 5,15-diphenyl-R,â-octaalkyl
porphyrin (H2P), whereas the donor is either the corresponding
zinc porphyrin (ZnP) or its pyridine complex (ZnPpy) (Figure
1).
Synthesis. The D-B-A systems were assembled using a
building block approach with a palladium-catalyzed cross-
coupling reaction (Sonogashira coupling).19,20 The syntheses of
ZnP, H2P, the arene-linked porphyrin dimers, and corresponding
reference compounds are presented elsewhere.21
The formally nonconjugated porphyrin dimer ZnP-OB-H2P
was prepared from zinc iodoporphyrin 8,21 free base iodopor-
phyrin 11,21 and a bicyclo[2.2.2]octane building block 5
(Scheme 2). Diyne 5 was monosilylated, so that the two
porphyrins in different states of metalation could be introduced
in a stepwise manner. The reference compounds ZnP-OB and
OB-H2P, and the bridging chromophore 1,4-bis(phenylethynyl)-

Singlet EET Enhanced by Bridging Chromophore
J. Phys. Chem. B, Vol. 103, No. 34, 1999 7331
TABLE 1: Calculated Relative Energies and Distances for Different Dihedral Angles in the Bridging Chromophores
AM1
energy,c kcal mol-1
PM3
HF/3-21G*
energy,c kcal mol-1
bridging
chromophore
R,a deg
distance,b Å
distance,b Å
energy,c kcal mol-1
distance,b Å
OB
OB
BB
BB
NB
NB
AB
AB
0
90
0
90
0
90
0
90
16.3
16.3
16.4
16.4
16.4
16.4
16.5
16.5
0.00
0.00
0.00
0.45
0.00
0.36
0.00
0.34
16.3
16.3
16.4
16.4
16.4
16.4
16.4
16.4
0.00
0.00
0.00
0.48
0.00
0.58
0.00
0.82
16.3
16.3
16.5
16.5
16.5
16.5
16.5
16.5
0.00
0.00
0.00
0.96
0.00
0.66
0.00
0.35
a Orientation of the plane of the central unit relative to the plane of the adjacent phenyls. R ) 0° corresponds to both phenyl groups and the
central chromophore in the same plane. For bicyclo[2.2.2]octane, R ) 0° corresponds to both phenyls and one of the three vertical symmetry planes
of bicyclo[2.2.2]octance in the same plane. b Length of the bridging chromophore (between the para-carbons of the phenyl groups). c Energy of the
bridging chromophore relative to the global minimum at R ) 0°.
SCHEME 1a
SCHEME 2a
a (a) PPh3, Zn, CBr4, CH2Cl2; 89%; (b) BuLi, TIPSCl, THF; 52%;
(c) t-BuLi, N-formylpiperidine, ether; 70%; (d) See Step A; 94%; (e)
BuLi, THF; 92%; (f) PhI, Pd(PPh3)4, CuI, Piperidine; 96%; (g) Bu4NF,
THF, 79%; (h) See Step f; 94%; (i) 8, Pd2Dba3•CHCl3, PPh3, PhMe/
piperidine (1:1); 75%; (j) TFA, CH2Cl2; 93%.
a (a) 5, Pd2Dba3•CHCl3, PPh3, PhMe/Piperidine (1:1); 53%; (b)
Bu4NF, THF; 86%; Pd2dba3•CHCl3, PPh3, PhMe/piperidine (1:1); 40%.
bicyclo[2.2.2]octane (OB), were synthesized from building block
7 (Scheme 1).
workers,26 who showed that piperidine and pyrrolidine are far
more effective than triethylamine. Based on their findings the
triethylamine was replaced by piperidine. The porphyrin dimer
ZnP-OB-H2P could then be obtained in 17% yield in three
steps from 5, zinc iodoporphyrin 8, and free base iodoporphyrin
11 (Scheme 2). Despite the higher reactivity in piperidine, the
coupling reactions had to be performed at high temperature (70-
80 °C) for 2-3 days because of the lower reactivity of
alkylacetylenes compared with phenylacetylenes.19
The synthesis of the building block 5 went via 5-iodobicyclo-
[2.2.2]octane-1-carbaldehyde22 (Scheme 1), which was con-
verted to dibromoalkene 1 by using the Corey-Fuchs’ protocol.23
The anion formed by treatment of 1 with butyllithium was
reacted with chlorotriisopropylsilane (TIPS chloride) affording
the silylated alkyne 2. Aldehyde 3 was prepared from 2 by
treatment with tert-butyllithium and N-formylpiperidine using
the procedure of Olah et al.,24 and finally the Corey-Fuchs’
procedure was repeated to give the monosilylated diyne 5 via
dibromoalkene 4.
In the next step, bridge building block 5 was coupled with
zinc iodoporphyrin 8. The first attempt using the same conditions
as in the preparation of arene-linked compounds, i.e., treatment
with Pd2dba3•CHCl3, AsPh3 in toluene/triethylamine (3:1) at 40
°C,21 was not successful. Under those conditions porphyrin 8
reacted with two molecules of 5, probably forming an enyne-
linked porphyrin derivative.25 The effect of different amines on
the Sonogashira coupling was studied by Linstrumelle and co-
Building block 7 was prepared from 5 by palladium-catalyzed
coupling with iodobenzene under the conditions described by
Alami et al.26 followed by desilylation with tetrabutylammonium
fluoride. The coupling with iodobenzene was repeated to give
OB. The reference porphyrin ZnP-OB was prepared from 7
and zinc iodoporphyrin 8. The use of PPh3 instead of AsPh3 in
the preparation of ZnP-OB led to somewhat shorter reaction
time (22 h at 70 °C) and the yield was improved compared
with the coupling of zinc iodoporphyrin 8 with 5. Demetalation27
of ZnP-OB with trifluoroacetic acid gave OB-H2P.

7332 J. Phys. Chem. B, Vol. 103, No. 34, 1999
Kilså et al.
Figure 2. Absorption spectra of dimers (s) and the sum of absorption
spectra of reference compounds (- - -) in CHCl3, 20 °C. From top
to bottom: (a) ZnP-OB-H2P and ZnP-OB + H2P; (b) ZnP-BB-
H2P and ZnP-BB + H2P; (c) ZnP-NB-H2P and ZnP-NB + H2P;
(d) ZnP-AB-H2P and ZnP-AB + H2P.
Figure 3. Top: Absorption spectra in the region of the first transition
(S0 f S1) in the bridging chromophores: AB (s), NB (- - -), BB
(. . .), and OB (- ‚ -) in CHCl3, 20 °C. Bottom: Calculated S0 f S1
excitation energies and oscillator strengths of different dihedral
conformations of the bridging chromophores. See text for details.
Spectroscopic Properties. The identity of the separate
chromophores (donor, bridge, acceptor) is preserved in the
systems, which is seen by comparing the absorption spectra of
the systems with the sum of spectra of the separate chro-
mophores (Figure 2). The absorption spectra of ZnP-OB-H2P,
ZnP-BB-H2P, and ZnP-NB-H2P can be resolved into a 1:1:1
mixture of their components: ZnP/OB/H2P, ZnP/BB/H2P, and
ZnP/NB/H2P, respectively, or a 1:1 mixture of their reference
compounds, either ZnP-RB/H2P or ZnP/RB-H2P (R ) O, B,
N). Similarly, the absorption spectrum of ZnP-AB-H2P is the
spectral sum of its components or reference compounds in the
Q-band region (λ > 520 nm) but significant deviations are
observed in the Soret bands (λ ≈ 410 nm) of the porphyrins
(Figure 2d). In Figure 2 the spectral sum of ZnP-RB and H2P
is shown and compared with the spectra of ZnP-RB-H2P. The
major contribution to the absorption spectra of the D-B-A
systems is from the porphyrin moieties. The same behavior is
observed for the systems with ZnPpy as the donor (data not
shown), but the spectrum of ZnPpy is red-shifted compared with
that of ZnP, and the relative intensities of the Q-bands are
changed.
bridging chromophores should be compared with the lowest
singlet excitation energies of ZnP (17 400 cm-1), ZnPpy (17
000 cm-1), and H2P (16 000 cm-1). The excitation energy of
the bridging chromophore is large enough in all systems to
prevent stepwise energy transfer: DfBfA.
Fo1rster Theory. For donor-acceptor pairs in which the
electronic structure of the medium does not directly influence
the energy transfer, the Fo¨rster theory can be used to describe
long-range EET (D-A distances > 10 Å).29 According to the
Fo¨rster theory, the expected energy-transfer rate constant,
Fo¨rster
EET
k
, can be estimated from photophysical properties of the
separate donor (ZnP or ZnPpy) and acceptor (H2P) moieties
kFEoE¨rTster ) 8.79 × 10-25 (kDκ2J/n4R6) s-1
(1)
The orientation factor, κ2, is defined from unit vectors as30
κ2 ) ((µˆd‚µˆa) - 3(µˆd‚rˆ)(rˆ‚µˆa))2 )
(sin θ sin ω cos φ - 2 cos θ cos ω)2 (2)
The energies of the lowest singlet excited states are estimated
from absorption spectra (Figure 3, top panel) and fluorescence
spectra (not shown) of the separate chromophores. The energies
are 35 000 cm-1, 29 000 cm-1, 26 000 cm-1, and 21 300 cm-1
for the bicyclo[2.2.2]octane, benzene, naphthalene, and an-
thracene bridging chromophores, respectively. Calculations
using INDO/S28 on the geometry-optimized (PM3) bridging
chromophores with varying dihedral angles yielded fairly good
agreement with experimental data (Figure 3, lower panel). The
experimental energies of the lowest singlet excited states of the
where ω is the angle between the donor transition dipole
moment (represented by the unit vector µˆd) and the (unit) vector
(rˆ) connecting the donor and acceptor transition dipole moment;
θ is the angle between the acceptor transition dipole moment
(represented by the unit vector µˆa) and rˆ; and φ is the angle
between the two porphyrin planes. Zinc 5,10,15,20-tetraphenyl
porphyrin with D4h symmetry has the lowest transition moments
degenerated in the porphyrin plane.31 Fluorescence anisotropy
measurements on ZnP in a vitrified glass gave the anisotropy
value 0.1 for all excitation wavelengths indicating that zinc 5,15-

Singlet EET Enhanced by Bridging Chromophore
J. Phys. Chem. B, Vol. 103, No. 34, 1999 7333
diphenylporphyrin (and ZnPpy) also has the lowest transition
moments effectively degenerated in the porphyrin plane (0° e
ω e 360°). Semiempirical calculations (INDO/S) showed that
the first transition moment of the H2P is along its N-N-axis,
giving θ ) 45°, i.e., the angle expected for a D2h symmetric
free base porphyrin.31 The value of κ2 is then found to vary
between 2/3 and 1 for the different configurations of the
porphyrin planes and for the dynamic average κ2 ) 5/6.30
Because of the small variance in κ2, kEFoE¨rTster will change only
slightly for different conformations. For the present porphyrin
dimers the calculated energy-transfer efficiency is independent
of whether the dynamic or the static average is used.
As previously mentioned the donor-acceptor distance, R, is
the same for all D-B-A systems in the two series and is
estimated to be 25.3 Å. The refractive index of the solvent
(CHCl3), n, is 1.45.32 The radiative rate constant, kD, is calculated
from the fluorescence quantum yield (φD) and the fluorescence
lifetime (τD) of the donor in absence of acceptor; kD ) φD/τD
) 2.0 ( 0.1 × 107 s-1 for ZnP and kD ) 1.6 ( 0.1 × 107 s-1
for ZnPpy. The spectral overlap integral, J, is calculated from
the acceptor absorption spectrum (ꢀ(λ)) and the normalized
donor emission spectrum (F(λ)) as
J ) ∞ ꢀ(λ)F(λ)λ4dλ
(3)
∫
0
which gives J ) 2.6 ( 0.2 × 10-14 M-1 cm-3 for systems with
ZnP as the donor and J ) 0.9 ( 0.2 × 10-14 M-1 cm-3 for
systems with ZnPpy as the donor. The spectral overlap integral
with ZnPpy as the donor is significantly smaller than with ZnP,
because the emission spectrum of ZnPpy is red-shifted and the
intensities of the Q-bands are altered (Figure 4). Within the
Figure 4. Top: Absorption spectrum of H2P (. . .) and emission spectra
of ZnP (- - -) and ZnPpy (s) in CHCl3, 20 °C. On adding pyridine
to ZnP the relative emission intensity of Q(1,0):Q(0,0) is changed from
approximately 1:1 to 3:1. Bottom: The calculated spectral overlap
between ZnP and H2P (- - -, area //) and between ZnPpy and H2P
(s, area \\).
Fo¨rster approximation, the expected EET rate constants are equal
) 3.3 ( 0.1 × 108
Fo¨rster
for all ZnP-RB-H2P systems, kEET
Fo¨rster
s-1, and for all ZnPpy-RB-H2P systems, kEET
) 0.9 (
0.1 × 108 s-1
.
the acceptor emission increases in the covalently linked por-
phyrin dimers when compared with the 1:1 reference mixture.
The same trend of EET efficiencies: ZnP(py)-OB-H2P <
ZnP(py)-BB-H2P < ZnP(py)-NB-H2P < ZnP(py)-AB-
H2P is found in both series. However, the ZnPpy-RB-H2P
series shows less efficient EET than the ZnP-RB-H2P series,
which agrees with expectations from the Fo¨rster theory.
Fluorescence Emission Spectra and Lifetimes. The meas-
ured EET rate constants of the systems differ from the values
predicted by the Fo¨rster theory. The EET rate constants are
calculated as
E
kEET
)
(4)
(1 - E)τ0
The fluorescence lifetimes of the reference compounds (ZnP-
RB, ZnPpy-RB, and RB-H2P) are, within experimental error,
independent of the bridging chromophore connected to the
porphyrin moiety. The lifetimes of ZnPpy-RB are a little shorter
than that found for the corresponding ZnP-RB. In the D-B-A
systems, fluorescence from both donor and acceptor is observed.
In the analysis of the time-resolved data, the lifetime of the
acceptor is fixed to the lifetime measured for RB-H2P, and
the donor lifetime is found from the best fit to the data points.
The resulting lifetimes and fractions (f) of ZnP and ZnPpy
emission are shown in Table 2. The lifetime of the donor in the
reference compounds was compared with the donor lifetime in
the D-B-A systems to calculate E.
where E is the EET efficiency (see Table 2). The efficiency is
calculated from the steady-state or lifetime measurements as
I
I0
τ
τ0
E ) 1 - ) 1 -
(5)
where I and I0 are the donor emission intensities of the systems
and of the 1:1 reference mixtures, respectively, and τ and τ0
are the corresponding fluorescence lifetimes.
Figure 5a shows fluorescence emission spectra of ZnP-OB-
H2P, ZnP-BB-H2P, ZnP-NB-H2P, and ZnP-AB-H2P, and
of a 1:1 mixture of ZnP and H2P. The fluorescence spectrum
of ZnP is almost indistinguishable from the spectra of ZnP-
RB (not shown). Figure 5b shows the fluorescence emission
spectra of the corresponding ZnPpy-RB-H2P systems and
reference mixture. The donors dominate the emission between
550 and 600 nm (ZnP) or between 570 and 610 nm (ZnPpy),
whereas the acceptor (H2P) dominates the emission above 680
nm. The energy-transfer efficiencies were calculated from the
decrease in intensity of the donor emission. Similar results were
found by calculating E from the increase in acceptor emission.
Figure 5 clearly shows that the donor emission decreases and
Because the efficiencies from steady-state and time-resolved
measurements are slightly different, a weighted average E-value
was calculated. This E-value was used to calculate kEET. The
results clearly show that the rate constant for EET is highest in
the systems with anthracene in the bridging chromophore. The
systems with naphthalene or benzene in the bridging chro-
mophore give a slightly higher rate constant than in the systems
with bicyclo[2.2.2]octane in the bridging chromophore.

7334 J. Phys. Chem. B, Vol. 103, No. 34, 1999
Kilså et al.
TABLE 2: Observed Donor Fluorescence Intensities (I), Donor and Acceptor Lifetimes (τ), Fraction of Donor Fluorescence
Intensity in Lifetime Measurements (f), Calculated EET Efficiencies (E) and Rate Constants (kEET) (CHCl3 solutions at 20 °C)
compound
I, au
E
τ
XB-H P,a ns
fZnP, %
τ
ZnP, ns
E
Eb
kEET,c/s-1
2
ZnP-OB
41.3
26.6
41.7
24.7
40.6
23.3
37.6
13.5
14.4
12.3
14.2
10.6
14.3
10.2
12.3
5.6
1.29 ( 0.05
0.96 ( 0.05
1.27 ( 0.04
0.84 ( 0.06
1.26 ( 0.05
0.82 ( 0.05
1.24 ( 0.04
0.55 ( 0.05
1.21 ( 0.04
1.07 ( 0.06
1.20 ( 0.04
0.92 ( 0.06
1.20 ( 0.05
0.89 ( 0.05
1.19 ( 0.07
0.66 ( 0.13
ZnP-OB-H2P
ZnP-BB
0.36 ( 0.04
0.41 ( 0.04
0.43 ( 0.04
0.64 ( 0.06
0.15 ( 0.02
0.25 ( 0.04
0.29 ( 0.04
0.54 ( 0.07
8.52 ( 0.21
8.37 ( 0.23
8.48 ( 0.23
8.32 ( 0.25
8.52 ( 0.21
8.37 ( 0.23
8.48 ( 0.23
8.32 ( 0.25
49
43
40
26
60
46
41
19
0.26 ( 0.06
0.34 ( 0.06
0.35 ( 0.06
0.56 ( 0.06
0.12 ( 0.07
0.23 ( 0.07
0.26 ( 0.08
0.45 ( 0.13
0.32
0.38
0.40
0.60
0.15
0.25
0.29
0.53
3.7 × 108
4.8 × 108
5.3 × 108
12.1 × 108
1.5 × 108
2.8 × 108
3.4 × 108
9.5 × 108
ZnP-BB-H2P
ZnP-NB
ZnP-NB-H2P
ZnP-AB
ZnP-AB-H2P
ZnPpy-OB
ZnPpy-OB-H2P
ZnPpy-BB
ZnPpy-BB-H2P
ZnPpy-NB
ZnPpy-NB-H2P
ZnPpy-AB
ZnPpy-AB-H2P
a Lifetimes of acceptor reference compounds were fixed in the analysis of time-resolved dimer fluorescence. b Estimated efficiency based on the
weighted average between the values from steady-state and time-resolved measurements. c Rate constant calculated from the estimated efficiency.
The calculations were done on linear bridging chromophores.
From a high-field NMR study, Bothner-By et al. found that for
a porphyrin dimer linked with diphenylethyne the out-of-plane
bending could be as large as 26° for the entire dimer.33 Our
bridging chromophores have one more phenylethylene group
in the linker, and in the worst case we could expect this linker
to bend as much as the entire system of Bothner-By et al. For
a bridging chromophore bend of 28°, the end-to-end distance
was calculated to be 16.0 Å. The bending is assumed to be equal
for all bridging chromophores. This large bending corresponds
to an energy ≈2.7 kcal/mol higher than for the planar
conformation as calculated by PM3. It is therefore not likely to
be found at room temperature, and we believe that, even if the
systems are slightly bent, this does not influence the donor-
acceptor distance or the energy transfer significantly.
Calculations also showed that the dihedral angle between the
plane of the central unit and the plane of the phenyl groups
was distributed almost uniformly between 0° and 90° at room
temperature. For OB no rotational barrier was found. In the other
bridging chromophores the barriers for rotation of the central
unit from R ) 0° (central unit plane parallel to the plane of the
phenyl groups) to R ) 90° (central unit plane perpendicular to
the plane of the phenyl groups) were small enough (<1 kcal/
mol) to ensure the presence of all conformations at room
temperature.
Spectroscopic Properties. The absorption spectra of all
systems could be resolved numerically into equimolar contribu-
tions from the corresponding reference compounds. Figure 2
shows that the spectrum of ZnP-OB-H2P is resolved perfectly
in both the Soret and Q-band regions. Also, the Q-bands in the
Figure 5. Fluorescence emission spectra of the dimers and a numerical
1:1 mixture of the reference compounds in CHCl3, 20 °C. (a) ZnP:H2P
absorption spectra of ZnP-BB-H2P and ZnP-NB-H2P are
equal to the sum of reference compounds. However, a small
deviation is seen in the Soret bands: the deviation is larger for
ZnP-NB-H2P than for ZnP-BB-H2P which is probably due
to the first absorption band of NB being closer in energy to the
Soret bands than the absorption of BB. For ZnP-AB-H2P only
the Q-bands could be described as a sum of the reference
compounds. The significant deviation in the Soret band region
is expected because, in the spectrum of ZnP-AB-H2P, the
Soret bands of the porphyrins overlap with the first absorption
band of AB, making the situation for electronic coupling
perfect.34 A similar behavior was found for the absorption
spectra of the systems with ZnPpy as donor. Therefore, in all
fluorescence experiments an excitation wavelength in the Q-band
(- ‚‚ -), ZnP-OB-H2P (. . .), ZnP-BB-H2P (- - -), ZnP-NB-
H2P (s) and ZnP-AB-H2P (- ‚ -). (b) ZnPpy:H2P (- ‚‚ -), ZnPpy-
OB-H2P (. . .), ZnPpy-BB-H2P (- - -), ZnPpy-NB-H2P (s),
and ZnPpy-AB-H2P (- ‚ -).
Discussion
Structure. All systems studied are built from the same kind
of building blocks. We therefore expect all the systems to behave
similarly with respect to geometric parameters, and thus facilitate
immediate comparison. The calculated length of the bridging
chromophores (16.4 Å) was, within the expected error, neither
dependent on the method used nor on the central unit in the
bridging chromophore. Furthermore, the length was independent
of the orientation of the central unit relative to the phenyl groups.

Singlet EET Enhanced by Bridging Chromophore
J. Phys. Chem. B, Vol. 103, No. 34, 1999 7335
TABLE 3: Comparison of Measured Rate Constants for
Energy Transfer and Rate Constants Expected from the
Fo1rster Theory
essentially equal for each pair of porphyrin dimers with the same
bridging chromophore. The difference varies with the electronic
structure of the bridging chromophore, and is therefore a
mediating effect specific for each bridging chromophore. The
mediating
contribution,a
k
b
mediating effect is much higher for anthracene (8 × 108 s-1
)
∆EDB
,
Fo¨rster
EET
Med
EET
kEET , s-1
k
, s-1
, s-1
cm-1
than for benzene or naphthalene (1.5 and 2 × 108 s-1) in the
bridging chromophore, whereas bicyclo[2.2.2]octane showed the
smallest mediating effect (0.4 × 108 s-1).
compound
ZnP-OB-H2P
ZnPpy-OB-H2P
ZnP-BB-H2P
ZnPpy-BB-H2P
ZnP-NB-H2P
ZnPpy-NB-H2P
ZnP-AB-H2P
ZnPpy-AB-H2P
3.7 × 108 3.3 × 108
1.5 × 108 0.9 × 108
4.8 × 108 3.3 × 108
2.8 × 108 0.9 × 108
5.3 × 108 3.3 × 108
3.4 × 108 0.9 × 108
12.1 × 108 3.3 × 108
9.5 × 108 0.9 × 108
0.4 × 108
0.6 × 108
1.5 × 108
1.9 × 108
2.0 × 108
2.5 × 108
8.8 × 108
8.6 × 108
17 600
18 000
11 600
12 000
8 600
9 000
3 900
The increase in electronic coupling between the donor and
acceptor, observed as an increased rate of EET, seems to be
strongly correlated to the excitation energies of the bridging
chromophores. According to the Fermi golden rule35 the rate
constant for a nonradiative process is proportional to the square
of the total electronic coupling (HtDoAt ) between the initial and
final state
4 300
a kMEEeTd ) kEET - kFEoE¨rTster b ∆EDB ) Ebridge - Edonor
.
.
region was selected, where the identity as independent chro-
mophores of donor, bridge, and acceptor is preserved.
The energies of the lowest excited state of the bridging
chromophores are found from the absorption (Figure 3) and
fluorescence spectra (not shown). Figure 3 also shows the
calculated energy (INDO/S) of the first absorption band for
different rotations of the central unit in the bridging chro-
mophore (R ) 0°, 15° ... 90°). These calculated excitation
energies are in fair agreement with the observed spectra. They
are calculated to be approximately 4000 cm-1 higher in energy
than observed, but the relative energies agree well. The
calculated intensities also agree with the observed spectra. For
AB, NB, and BB the S0 f S1 transition is of high intensity. In
OB the first transition is due to exciton coupling34 of the
phenylethynyl groups, and it is calculated and observed to have
low intensity. According to the experimental data the lowest
excited singlet state of the bridging chromophores are ap-
proximately 4 000 cm-1 (AB), 8 000 cm-1 (NB), 12 000 cm-1
(BB), and 18 000 cm-1 (OB) higher in energy than the donor
porphyrins. The separation is large enough to exclude stepwise
energy transfer but the bridging chromophores do have some
nontrivial effect on the EET, according to the obtained rate
constants. Because AB is closest in energy to the donor and
acceptor, we found the largest mediating effect for this bridging
chromophore (vide infra).
The difference between the two series is related to the
properties of the donor. The absorption and emission of ZnPpy
is red-shifted approximately 10 nm, and the relative intensity
of the two emission peaks is altered compared with the spectra
of ZnP. This changes the spectral overlap significantly (Figure
4), and it is this change that makes the largest contribution to
the decrease in Fo¨rster rate constant for the ZnPpy-RB-H2P
systems compared with the ZnP-RB-H2P systems.
Mediation Contribution to Electronic Energy Transfer.
Our results show that with anthracene as the central unit in the
bridging chromophore the rate constant for singlet-singlet EET
between the donor and the acceptor in the porphyrin dimers is
significantly increased. Furthermore, the systems with benzene
and naphthalene in the bridging chromophore showed a moder-
ate increase in kEET, whereas the systems with bicyclo[2.2.2]-
octane as the central unit showed almost the kEET expected from
the Fo¨rster theory.
2π
p
tot
DA
2
kEET
)
F|H
|
(6)
where F is the density of final states. For eq 6 to be valid it
must be assumed that the electronic coupling is sufficiently
weak, i.e., the donor relax vibrationally before the EET process
takes place. Furthermore, the excitation energy must quickly
dissipate in the acceptor states to avoid repopulation of the donor
excited state.36 This is presumably the case in the studied
systems. The electronic coupling (HtDoAt ), relevant for EET, may
be divided arbitrarily into a Fo¨rster-type coupling (dipole-dipole
coupling) and a mediation contribution that involves the
electronic states of the bridging chromophore. The mediating
effect, observed as a contribution to the rate constant, is assumed
to be proportional to the square of the electronic coupling
associated with the mediation, HMDAed. This term may be sub-
divided into several different contributions as has been done
by Scholes, Ghiggino, Paddon-Row, and co-workers.9e,10b,c,37
However, the emphasis in their studies has been on non-π-
electron bridging subunits with large separation between the
donor and bridge excited states. The results therefore are not
immediately applicable to our systems. It should be noted that
the separation of the Fo¨rster and mediation contributions is not
suggested to be based on a theoretical model, but is merely the
simplest procedure to show how our empirical observa-
tions could be rationalized. A more thorough theoretical
treatment and explicit molecular orbital calculations for the
Med
DA
H
matrix element will be presented elsewhere,38 and we
will limit the discussion in this paper to a qualitative analysis
of the relative magnitude of H
Med
DA
for the different bridging
chromophores.
A simple approximation for the mediation contribution to the
electronic coupling is expected to be given by perturbation
theory35 which predicts that HDMAed is proportional to the inverse
energy splitting between the excited states of donor and bridging
chromophores (∆EDB), giving
1
Med 1/2
Med
DA
(k
)
H
(7)
EET
∆EDB
In Figure 6 the square root of the contribution to the observed
rate constant from the mediation effect, (k
Med
EET
1/2
)
is plotted
Table 3 shows the experimentally determined rate con-
stants compared with the rate constants predicted from the
Fo¨rster theory. In the fourth column of Table 3, the difference
against the inverse excitation energy difference for donor and
bridging chromophore, ∆EDB-1, for the present D-B-A
systems. A linear relation is indeed found, and the intercept is
close to zero as expected for the infinite energy difference (i.e.,
no mediation effect). It is also seen in Figure 6 that the effect
is much more pronounced for the bridging chromophore with
anthracene as the central unit than for any of the other bridging
between measured (kEET) and calculated (kFo¨rster) rate constants
EET
is shown. If the difference for a specific ZnP-RB-H2P system
is compared with the difference for the corresponding ZnPpy-
RB-H2P system, it can be seen that these differences are

7336 J. Phys. Chem. B, Vol. 103, No. 34, 1999
Kilså et al.
(redistilled 99.5+ %) and other commercially available reagents
were purchased from Aldrich and used without further purifica-
tion.
Methods. Column chromatography of zinc porphyrins and
porphyrin dimers was performed using silica gel (Merck, grade
60, 70-230 mesh). Flash chromatography was performed using
silica gel (Matrex LC 60 Å/35-70 micron). Size exclusion
chromatography (SEC) was performed using BioRad Bio-Beads
1
S-X3 with toluene as eluent. H (400 MHz) and 13C (100.6
MHz) NMR spectra were recorded at room temperature with
CDCl3 as solvent, using a Varian UNITY-400 NMR spectrom-
eter. Chemical shifts are reported relative to tetramethylsilane
δH 0 ppm and CDCl3 δC 77.0 ppm. Mass spectra were recorded
using a VG ZabSpec instrument. Porphyrin-containing sub-
stances were analyzed by positive FAB-MS, other substances
were analyzed by EI-MS (70 eV). Deaeration of reaction
mixtures was performed by bubbling argon through the solution
for 30 min. All palladium-catalyzed reactions were performed
under argon atmosphere, palladium-catalyzed reactions involving
porphyrins were performed in the dark.
Figure 6. Linear plot of the square root of the mediating contribution
Med
[(k
)
1/2] Versus the reciprocal energy separation between the donor
EET
and the bridging chromophore (∆EDB-1). Best linear fit (s), ZnP-
OB-H2P ([), ZnPpy-OB-H2P (]), ZnP-BB-H2P (2), ZnPpy-
BB-H2P (4), ZnP-NB-H2P (B), ZnPpy-NB-H2P (n), ZnP-AB-
H2P (9), and ZnPpy-AB-H2P (0).
1,1-Dibromo-2-(4-iodobicyclo[2.2.2]oct-1-yl)ethene (1). Tri-
phenylphosphine (7.67 g, 29.2 mmol) and zinc powder (1.94
g, 29.7 mmol) were suspended in 80 mL of CH2Cl2 under argon.
Carbon tetrabromide (9.69 g, 29.7 mmol) was added and the
suspension was stirred at room temperature for 24 h. A solution
of 4-iodobicyclo[2.2.2]octane-1-carbaldehyde22a in 20 mL of
CH2Cl2 was added, and the mixture was stirred for an additional
24 h at room temperature. Hexane, 50 mL, was added and a
brown precipitate was formed. The slurry was filtered through
a pad of silica to remove insoluble material. The filtrate was
evaporated to dryness and the residue was dissolved in 150 mL
of ether. The solution was washed with 10% aqueous sodium
bisulfite to remove traces of aldehyde, washed with water, dried
(Na2SO4), and filtered. Evaporation of the solvent gave 5.48 g
(89% yield) of a white solid. Recrystallization from pentane
chromophores. To clearly observe the mediating effect the
energy splitting should not be too large, preferably <6 000 cm-1
.
On the other hand, the energy difference between bridging
chromophore and donor excited states should not be smaller
than the average thermal energy to avoid stepwise energy
transfer (i.e., energy migration) becoming an important contri-
bution to the overall EET rate. In this limit it might be very
difficult to properly distinguish the mediation effect from simple
migration behavior. This is, however, a very important situation
because small energy gaps are an expected feature in most of
the multichromophoric arrays for artificial and natural light-
harvesting.
1
gave white crystals, mp 95-96 °C. H NMR δ 1.96 (m, 6 H,
sCH2CH2s), 2.47 (m, 6 H, sCH2CH2s), 6.32 (s, 1 H,
sCHCBr2); 13C NMR δ 33.0, 33.4, 40.3, 44.0, 86.2, 144.8;
HRMS Calcd for C10H13Br2I: 417.843. Found: 417.839.
Conclusion and Remarks
In this investigation of EET in geometrically well-defined
D-B-A systems the following conclusions were made: (1)
The design of the present D-B-A systems allows the separa-
tion of bridging chromophore effects on the EET rate from other
parameters known to govern energy transfer. (2) The electronic
structure of the bridging chromophore, representing a medium
between donor and acceptor, influences the EET rate. The
bridging chromophore mediates electronic coupling. (3) The
mediating contribution from the bridging chromophore can be
separated from the total (observed) EET rates by comparison
with the Fo¨rster theory. (4) The mediating contribution is
correlated to the inverse energy splitting between excitation
energies of the donor and bridging chromophores.
1-(Triisopropylsilyl)-2-(4-iodobicyclo[2.2.2]oct-1-yl)-
ethyne (2). A solution of 1 (3.00 g, 7.14 mmol) in 50 mL of
THF at -78 °C under argon was treated with n-butyllithium in
hexanes (16 mmol, 10.1 mL of a 1.55 M solution). After being
stirred for 1 h at -78 °C, the reaction mixture was allowed to
reach room temperature and maintained for 1 h at that
temperature. Chlorotriisopropylsilane (3.1 mL, 14 mmol) was
added, and the reaction mixture was heated under reflux for 2
h. The reaction mixture was cooled to room temperature and
water was added. The phases were separated and the aqueous
layer was extracted with pentane. The combined organic layers
were washed with water, dried (Na2SO4), filtered, and evapo-
rated. Flash chromatography (silica gel, hexane) followed by
bulb-to-bulb distillation (169-170 °C, 0.8 mbar) gave 1.54 g
Work is in progress to perform molecular orbital calculations
of the mediating matrix elements and to develop a theory for
the mediating contribution. Furthermore, the corresponding
ZnP-RB-Fe(III)P systems are currently being synthesized to
investigate mediation effects on electron transfer.
1
i
(52% yield) of a colorless oil. H NMR δ 1.01 (m, 21 H, Pr),
1.92 (m, 6 H, sCH2CH2s), 2.46 (m, 6 H, sCH2CH2s); 13C
NMR δ 11.1, 18.6, 25.0, 35.8, 40.2, 43.9, 80.1, 115.0; HRMS
Calcd for C19H33ISi: 416.140. Found: 416.138.
4-[1-(Triisopropylsilyl)ethynyl]bicyclo[2.2.2]octane-1-carb-
aldehyde (3). A solution of 2 (1.54 g, 3.70 mmol) in 25 mL of
ether at -100 °C under argon was treated dropwise with tert-
butyllithium in pentane (7.7 mmol, 4.4 mL of a 1.76 M solution).
The reaction mixture was warmed to -80 °C during 10 min
and maintained for 5 min at that temperature. The reaction
mixture was cooled to -100 °C, and N-formylpiperidine (5.7
Experimental Section
Synthesis. Materials. Diethyl ether, toluene, and tetrahydro-
furan (THF) were dried by distillation from sodium/benzophe-
none under nitrogen. Dichloromethane and triethylamine were
dried by distillation from calcium hydride under nitrogen. Dried
solvents were used immediately after distillation. Piperidine

Singlet EET Enhanced by Bridging Chromophore
J. Phys. Chem. B, Vol. 103, No. 34, 1999 7337
mL, 51 mmol) was added dropwise. The reaction mixture was
then allowed to reach room temperature and stirred for 30 min
at that temperature. The reaction was quenched by addition of
10 mL of 2 M HCl. The phases were separated, and the aqueous
layer was extracted with ether. The combined organic layers
were washed with water, saturated aqueous NaHCO3, and brine,
dried (Na2SO4), filtered, and evaporated. Flash chromatography
(silica, pentane/ether, 10:1) gave 0.82 g (70% yield) of a
colorless oil. H NMR δ 1.03 (m, 21 H, Pr), 1.64 (m, 6 H,
-CH2CH2-), 1.83 (m, 6 H, -CH2CH2-), 9.42 (s, 1 H, CHO);
13C NMR δ 11.1, 18.6, 25.3, 28.2, 31.3, 42.7, 79.6, 115.3, 205.6;
HRMS Calcd for C20H34OSi: 318.238. Found: 318.237.
the combined organic layers were washed with water, dried
(Na2SO4), filtered, and evaporated. Flash chromatography (silica,
pentane/ether, 100:1) followed by sublimation (50 °C, 1 mbar)
afforded 48 mg (79% yield) of white crystals, mp 89-90 °C.
1H NMR δ 1.84 (m, 12 H, -CH2CH2-), 2.10 (s, 1 H, CCH),
7.25 (m, 3 H, ArH), 7.35 (m, 2 H, ArH); 13C NMR δ 26.0,
26.7, 31.67, 31.70, 68.1, 80.6, 91.2, 96.2, 123.7, 127.5, 128.1,
131.5; HRMS Calcd for C18H18: 234.141. Found: 234.137;
Anal. Calcd for C18H18: C 92.26; H 7.74. Found: C 92.31; H
7.79.
1
i
1,4-Bis(phenylethynyl)bicyclo[2.2.2]octane (OB). Pd(PPh3)4
(6 mg, 5 mmol) and CuI (2 mg, 10 mmol) were added under
argon flushing to a deaerated solution of iodobenzene (29 mg,
0.14 mmol) and 7 (25 mg, 0.11 mmol) in 3 mL of piperidine.
The reaction mixture was stirred for 30 min at room temperature
and concentrated to dryness. The solid residue was suspended
in pentane/ether (100:1) and added to a chromatographic
column. Flash chromatography (silica, pentane/ether, 100:1)
gave 32 mg (94% yield) of white crystals, mp 165-166 °C. 1H
NMR δ 1.89 (s, 12 H, -CH2CH2-), 7.26 (m, 6 H, ArH), 7.36
(m, 4 H, ArH); 13C NMR δ 26.8, 31.8, 80.6, 96.4, 123.8, 127.5,
128.1, 131.5; HRMS Calcd for C24H22: 310.172. Found:
310.174; Anal. Calcd for C24H22: C 92.86; H 7.14. Found: C
92.70; H 7.10.
1,1-Dibromo-2-[4-(1-(triisopropylsilyl)ethynyl)bicyclo[2.2.2]-
oct-1-yl]-ethene (4). A solution of aldehyde 3 (0.75 g, 2.35
mmol) in 5 mL of CH2Cl2 was treated with PPh3 (1.23 g, 4.70
mmol), zinc powder (0.31 g, 4.70 mmol), and CBr4 (1.56 g,
4.70 mmol) suspended in 13 mL of CH2Cl2, using the same
conditions as in the preparation of 1. Hexane (20 mL) was
added, and the resulting slurry was filtered through a pad of
silica. The solvents were removed affording 1.05 g (94% yield)
of colorless oil, which was used in the next step without further
1
i
purification. H NMR δ 1.02 (m, 21 H, Pr), 1.80 (s, 12 H,
-CH2CH2-), 6.40 (s, 1 H, -CHCBr2); 13C NMR δ 11.1, 18.6,
27.5, 29.5, 32.1, 35.2, 79.2, 85.5, 116.0, 145.6; HRMS Calcd
for C21H34Br2Si: 472.080. Found: 472.077.
4-Phenylethynyl-1-{[zinc(II) 5-(3,5-di-tert-butylphenyl)-
2,8,12,18-tetraethyl-3,7,13,17-tetramethyl-15-porphyrinyl]-
phenylethynyl}bicyclo[2.2.2]octane (ZnP-OB). Pd2dba3•CHCl3
(6 mg, 6 mmol) and PPh3 (13 mg, 49 mmol) were added under
argon flushing to a deaerated solution of zinc iodoporphyrin
821 (38 mg, 41 mmol) and 7 (15 mg, 62 mmol) in 8 mL of
toluene/piperidine (1:1). The reaction mixture was stirred for
22 h at 70 °C and concentrated to dryness. Chromatography
(silica, pentane/CH2Cl2, 4:1 to pentane/CH2Cl2, 7:3) and SEC
[4-(1-(Triisopropylsilyl)ethynyl)bicyclo[2.2.2]oct-1-yl]-
ethyne. (5). A solution of 4 (1.00 g, 2.10 mmol) in 15 mL of
THF at -78 °C under argon was treated with n-butyllithium in
hexanes (4.9 mmol, 3.6 mL of a 1.36 M solution). After being
stirred at -78 °C for 1 h the reaction mixture was allowed to
reach room temperature and maintained for 1 h at that
temperature. Water was added and the phases were separated.
The aqueous layer was extracted with pentane and the combined
organic phases were washed with water, dried (Na2SO4), filtered,
and evaporated. Flash chromatography (silica, pentane/ether, 50:
1
gave 32 mg (75% yield) of a red solid. H NMR δ 1.51 (s, 18
H, t-Bu), 1.77 (m, 12 H, -CH2CH3), 2.01 (m, 6 H, -CH2CH2-
), 2.08 (m, 6 H, -CH2CH2-), 2.44 (s, 6 H, pyrrole-CH3), 2.49
(s, 6 H, pyrrole-CH3), 4.00 (m, 8 H, -CH2CH3), 7.28 (m, 3 H,
ArH), 7.41 (m, 2 H, ArH), 7.77 (dm, J ) 8 Hz, 2 H, ArH),
7.81 (t, J ) 1.8 Hz, 1 H, ArH), 7.93 (d, J ) 1.8 Hz, 2 H, ArH),
8.00 (dm, J ) 8 Hz, 2 H, ArH), 10.18 (s, 2 H, meso); HRMS
Calcd for C70H76N4Zn: 1036.536. Found: 1036.550.
1
1) gave 0.61 g (92% yield) of a colorless oil. H NMR δ 1.02
i
(m, 21 H, Pr), 1.78 (s, 12 H, -CH2CH2-), 2.08 (s, 1 H,
CCH);13C NMR δ 11.1, 18.6, 26.1, 26.9, 31.7, 31.8, 68.0, 79.4,
91.3, 115.7; HRMS Calcd for C21H34Si: 314.243. Found:
314.243; Anal. Calcd for C21H34Si: C 80.18; H 10.89. Found:
C 79.99; H 11.04.
1-Phenyl-2-[4-(triisopropylsilylethynyl)bicyclo[2.2.2]oct-1-
yl]ethyne (6). Pd(PPh3)4 (48 mg, 42 mmol) and CuI (16 mg,
84 mmol) were added under argon flushing to a deaerated
solution of iodobenzene (224 mg, 1.1 mmol) and 5 (262 mg,
0.83 mmol) in 10 mL of piperidine. The reaction mixture was
stirred for 30 min at room temperature. The solvent was removed
under reduced pressure. The solid residue was suspended in
pentane/ether (100:1) and added to a chromatographic column.
Flash chromatography (silica, pentane/ether, 100:1) afforded the
product containing traces of iodobenzene. The iodobenzene was
removed by bulb-to-bulb distillation (120 °C, 4 mbar) leaving
311 mg (96% yield) of a colorless oil that crystallized upon
cooling and scratching, mp 47-48 °C. 1H NMR δ 1.03 (m, 21
H, iPr), 1.83 (bs, 12 H, -CH2CH2-), 7.25 (m, 3 H, ArH), 7.35
(m, 2 H, ArH); 13C NMR δ 11.2, 18.6, 26.8, 27.0, 31.8, 32.0,
79.3, 80.5, 96.5, 116.0, 123.8, 128.1, 131.5; HRMS Calcd for
C27H38Si: 390.274. Found: 390.279.
4-Phenylethynyl-1-{free base porphyrinyl]phenylethynyl}-
bicyclo[2.2.2]octane (OB-H2P). The zinc porphyrin ZnP-
OB (16 mg, 15 mmol) was dissolved in 10 mL of CH2Cl2.
Trifluoroacetic acid (0.5 mL) was added, and the reaction
mixture was stirred at room temperature for 1 h in the dark.
The mixture was poured into ethyl acetate, washed with 5%
aqueous NaHCO3 until the reddish porphyrin color returned,
washed with water, and dried (Na2SO4). The free base porphyrin
was purified by chromatography on silica. Traces of zinc
porphyrin were removed using CH2Cl2 and the free base
porphyrin was then eluted by adding ether to the eluent.
Removal of the solvents and drying in vacuo gave 14 mg (93%
yield) of a brown solid. 1H NMR δ -2.43 (bs, 2 H, NH), 1.50
(s, 18 H, t-Bu), 1.77 (m, 12 H, -CH2CH3), 2.01 (m, 6 H,
-CH2CH2-), 2.07 (m, 6 H, -CH2CH2-), 2.46 (s, 6 H, pyrrole-
CH3), 2.53 (s, 6 H, pyrrole-CH3), 4.02 (m, 8 H, -CH2CH3),
7.29 (m, 3 H, ArH), 7.41 (m, 2 H, ArH), 7.77 (dm, J ) 8 Hz,
2 H, ArH), 7.80 (t, J ) 1.8 Hz, 1 H, ArH), 7.91 (d, J ) 1.8 Hz,
2 H, ArH), 8.00 (dm, J ) 8 Hz, 2 H, ArH), 10.23 (s, 2 H,
meso); FAB-MS Calcd for C70H78N4+H+: 974.6. Found: 974.5.
[4-(Phenylethynyl)bicyclo[2.2.2]oct-1-yl]ethyne (7). Tetra-
butylammonium fluoride (0.7 mmol, 0.6 mL of a 1.1 M solution
in THF) was added to a solution of 6 (102 mg, 0.26 mmol) in
5 mL of THF under argon. The reaction mixture was stirred at
room temperature for 90 min. Water was added and the phases
were separated. The aqueous layer was extracted with ether and
4-[(Triisopropylsilyl)ethynyl]-1-{[zinc(II) 5-(3,5-di-tert-bu-
tylphenyl)-2,8,12,18-tetraethyl-3,7,13,17-tetramethyl-15-
porphyrinyl]phenylethynyl}bicyclo[2.2.2]octane (9). Pd2dba3•

7338 J. Phys. Chem. B, Vol. 103, No. 34, 1999
Kilså et al.
CHCl3 (9 mg, 8 mmol) and AsPh3 (21 mg, 67 mmol) were added
under argon flushing to a deaerated solution of zinc iodopor-
phyrin 8 (52 mg, 56 mmol) and 5 (21 mg, 76 mmol) in 16 mL
of toluene/piperidine (3:1). The reaction mixture was stirred at
75 °C for 70 h and concentrated to dryness. Chromatography
(silica, pentane/CH2Cl2, 4:1 to pentane/CH2Cl2, 1:1) gave 33
mg (53% yield) of a red solid. 1H NMR δ 1.07 (m, 21 H, iPr),
1.51 (s, 18 H, t-Bu), 1.76 (m, 12 H, -CH2CH3), 1.94 (m, 6 H,
-CH2CH2-), 2.03 (m, 6 H, -CH2CH2-), 2.44 (s, 6 H, pyrrole-
CH3), 2.48 (s, 6 H, pyrrole-CH3), 4.00 (m, 8 H, -CH2CH3), 7.75
(dm, J ) 8 Hz, 2 H, ArH), 7.81 (t, J ) 1.8 Hz, 1 H, ArH), 7.93
(d, J ) 1.8 Hz, 2 H, ArH), 7.99 (dm, J ) 8 Hz, 2 H, ArH),
10.17 (s, 2 H, meso); FAB-MS Calcd for C73H92N4SiZn:
1116.638. Found: 1116.664.
recorded on a SPEX Fluorolog t2 spectrofluorimeter. To prevent
aggregation of the samples, the concentration of the chro-
mophores was kept at approximately 5 mM. The low concentra-
tion also excluded the possibility for intermolecular EET and
that inner filter effects were negligible. Quantum yields were
determined using rhodamine B in ethanol as reference (φ )
0.9740). The energy for the lowest singlet excited state was
estimated for all chromophores from the midpoint between the
maxima of the first transition observed in fluorescence and
absorption spectra. To facilitate immediate comparison of the
emission spectra of the D-B-A systems, the optical densities
of the samples were matched at the excitation wavelength: 538
nm for ZnP samples and 550 nm for ZnPpy samples. The
emission spectra of the reference mixtures were calculated from
the numerical 1:1 mixture of the separate reference compounds.
These “mixtures” were scaled to match the optical density of
the dimers. The energy-transfer efficiency was calculated from
the intensity of the donor emission at 580 nm for ZnP samples
and at 592 nm for ZnPpy samples.
4-Ethynyl-1-{[zinc(II) porphyrinyl]phenylethynyl}bicyclo-
[2.2.2]octane (10). Tetrabutylammonium fluoride (105 mmol,
95 mL of a 1.1 M solution in THF) was added to a solution of
9 (38 mg, 35 mmol) in 10 mL of THF under argon. The reaction
mixture was stirred for 1 h at room temperature, and the solvent
was evaporated. The residue was dissolved in CHCl3 and the
solution was washed with saturated aqueous NaHCO3, dried
(Na2SO4), filtered, and evaporated. TLC (silica, pentane/ether,
Fluorescence lifetimes were measured by the phase and
modulation technique using a SPEX Fluorolog τ spectrofluo-
rimeter. For the acceptor reference compounds (RB-H2P) 20
modulation frequencies were selected on a logarithmic scale
ranging from 2 to 200 MHz. The samples were excited at 507
nm and the emission was collected through a 550-nm cutoff
filter. No change in lifetime with excitation wavelength was
observed. For the donor reference compounds (ZnP-RB and
ZnPpy-RB) and for the porphyrin dimers 20 modulation
frequencies were selected on a logarithmic scale in ranging from
10 to 300 MHz. The emission was collected through a cutoff
filter (550 nm for ZnP samples and 580 nm for ZnPpy samples),
and the excitation wavelengths used were the same as in the
steady-state measurements. In all time-resolved measurements
a diluted silica sol scattering solution was used as the reference.
For all reference compounds the observed demodulations and
phase shifts could be fitted to a single-exponential model. For
the dimers the data were fitted to a biexponential model. One
component, corresponding to the relevant acceptor reference,
RB-H2P, was kept constant at the value determined from the
pure reference compound. The other component was interpreted
as the donor fluorescence decay. The goodness-of-fit was
evaluated by the value of ø2 and by visual inspection of the fit
to the data points.
1
10:1) and H NMR indicated that the reaction was essentially
complete. The product was purified by SEC to remove traces
of 9 and residues of the protecting group. Removal of the solvent
1
gave 24 mg (86% yield) of a red solid. H NMR δ 1.51 (s, 18
H, t-Bu), 1.76 (m, 12 H, -CH2CH3), 1.93 (m, 6 H, -CH2CH2-
), 2.04 (m, 6 H, -CH2CH2-), 2.14 (s, 1 H, CCH), 2.44 (s, 6
H, pyrrole-CH3), 2.49 (s, 6 H, pyrrole-CH3), 4.00 (m, 8 H,
-CH2CH3), 7.75 (dm, J ) 8 Hz, 2 H, ArH), 7.81 (t, J ) 1.8
Hz, 1 H, ArH), 7.93 (d, J ) 1.8 Hz, 2 H, ArH), 8.00 (dm, J )
8 Hz, 2 H, ArH), 10.18 (s, 2 H, meso); HRMS Calcd for
C64H72N4Zn: 960.505. Found: 960.508.
4-{5-(3,5-Di-tert-butylphenyl)-2,8,12,18-tetraethyl-3,7,13,
17-tetramethyl-15-porphyrinyl]phenylethynyl}-1-{[zinc(II)
5-(3,5-di-tert-butylphenyl)-2,8,12,18-tetraethyl-3,7,13,17-tet-
ramethyl-15-porphyrinyl]phenylethynyl}bicyclo[2.2.2]-
octanne (ZnP-OB-H2P). Pd2dba3•CHCl3 (5 mg, 5 mmol) and
AsPh3 (12 mg, 40 mmol) were added under argon flushing to
a deaerated solution of 10 (24 mg, 25 mmol) and free base
iodoporphyrin 1121 (29 mg, 33 mmol) in 12 mL of toluene/
piperidine (1:1). The reaction mixture was stirred at 80 °C for
60 h and concentrated to dryness. Chromatography (silica,
CH2Cl2 to CH2Cl2/MeOH, 100:1) and SEC gave 17 mg (40%
yield) of a red solid. 1H NMR δ -2.43 (bs, 2 H, NH), 1.51 (s,
18 H, t-Bu), 1,52 (s, 18 H, t-Bu), 1.78 (m, 24 H, -CH2CH3),
2.19 (s, 12 H, -CH2CH2-), 2.44 (s, 6 H, pyrrole-CH3), 2.46
(s, 6 H, pyrrole-CH3), 2.53 (s, 6 H, pyrrole-CH3), 2.55 (s, 6 H,
pyrrole-CH3), 4.02 (m, 16 H, -CH2CH3), 7.82 (m, 6 H, ArH),
7.92 (d, J ) 1.8 Hz, 2 H, ArH), 7.93 (d, J ) 1.8 Hz, 2 H,
ArH), 8.04 (m, 4 H, ArH), 10.20 (s, 2 H, meso), 10.24 (s, 2 H,
meso); HRMS Calcd for C116H132N8Zn: 1700.987. Found:
1700.975.
The fractions of donor fluorescence was calculated as f )
(RDτD)/(RDτD + RAτA) where the R’s are the preexponential
factors and τD and τA are the lifetimes of donor and acceptor,
respectively.
Quantum Mechanical Calculations. Because of the large
size of our D-B-A systems we chose molecular mechanics
calculations for estimation of the porphyrin center-to-center
distance. The calculation was done with the force-field MM+16
as implemented in the program package HyperChem.41
The semiempirical method PM317 was used to geometry
optimize and calculate the potential energy of H2P with different
dihedral angles between phenyl and porphyrin planes. The
calculations were performed in the program package MOPAC
6.0.42 The angle for the first transition moment of H2P was
estimated using an INDO/S28 calculation on the PM3 optimized
structure.
For the bridging chromophores both semiempirical (AM143
or PM3,17 HyperChem41) and ab initio (HF/3-21G*,18 Gaussian
9444) methods were used in geometry optimizations and
calculations of the potential energy surface for rotation of the
central unit.
Spectroscopic Measurements. All measurements were per-
formed at 20 °C with amylene-stabilized CHCl3 (Merck, HPLC
grade) as the solvent.39 Pyridine (LabScan) was distilled over
KOH (BDH Chemicals) before use. ZnPpy, ZnPpy-RB, and
ZnPpy-RB-H2P were prepared directly before spectroscopic
measurements by adding pyridine to the corresponding ZnP-
solution. At a final pyridine concentration of approximately 2
M, full conversion of ZnP to ZnPpy was obtained, as judged
from the changes in the absorption spectra.
Absorption spectra were recorded on a Cary 4 Bio spectro-
photometer. Fully corrected steady-state emission spectra were

Singlet EET Enhanced by Bridging Chromophore
J. Phys. Chem. B, Vol. 103, No. 34, 1999 7339
(14) Craig, D. P.; Thiruamchandran, T. Chem. Phys. 1989, 135, 37-
Acknowledgment. This work was supported by grants from
the Swedish Natural Science Research Council (NFR), the
Swedish Research Council for Engineering Sciences (TFR), and
the Carl Trygger Foundation. K.K. is grateful to the Danish
Research Academy for a fellowship. We thank Dr. Gunnar
Stenhagen for performing the mass spectrometry measurements.
48.
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