Energy Transfer in Hydroporphyrins versus Porphyrins
A R T I C L E S
depending on array and medium. Thus, k(TS) increases in the
series Zn porphyrin-Zn porphyrin < Zn porphyrin-Fb por-
phyrin < Zn chlorin-Fb chlorin < Zn chlorin-Zn chlorin.
Evidently, the enhanced Fo¨rster contribution to energy transfer
in ZnFb-dyad and Oxo-ZnFb-dyad (Table 2) is not sufficient
to compensate for the attenuated TB contribution (relative to
ZnFbU), giving an overall 4-5-fold decrease in the energy-
transfer rate and a related drop in energy-transfer efficiency
(Table 2). The salient point is that a net enhancement in energy-
transfer rate derived from increased TS coupling does not
necessarily follow from a simple replacement of porphyrins with
hydroporphyrins for a given linker architecture. The results
obtained herein for one such linker motif indicates that additional
design criteria (e.g., shorter linkers, â-linkages) will need to be
explored for the ZnFb-chlorins and -oxochlorins to achieve
the same rates as the ZnFb porphyrins.
porphyrins. Instead, proper synthetic designs will need to take
into consideration the unwanted effects that we have uncovered
in terms of both energy transfer (reduced TB coupling for the
same linkage motif) and excited-state quenching processes.
Through proper molecular design, it is likely that these same
concepts can be exploited for chlorins and oxochlorins. In this
way, the desirable spectral coverage of the latter macrocycles
can be combined with proper tuning to yield large arrays that
have both superior light harvesting attributes and energy-transfer
rates and yields.
Experimental Section
General. All 1H NMR spectra (300 or 400 MHz) were
obtained in CDCl3 unless noted otherwise. Chlorin and oxochlo-
rin dyads were analyzed by laser desorption mass spectrometry
without a matrix (LD-MS) or with the matrix POPOP (MALDI-
MS).25 Fast atom bombardment mass spectrometry (FAB-MS)
data are reported for the molecule ion or protonated molecule
ion at greater than unit resolution. Column chromatography was
performed with flash silica (Baker). Toluene and triethylamine
for use in the Pd-mediated coupling process were distilled from
CaH2.
Noncommercial Compounds. Compounds Zn1,8 Zn2,8 and
all other chlorins and oxochlorins (1, 2, Cu1, Cu2, Oxo-1, Oxo-
2, Oxo-Zn1, Oxo-Zn2, Oxo-Cu1, Oxo-Cu2)9 were prepared
as described in the literature.
ZnFb-dyad. Following the refined Pd-mediated coupling
procedure,10 samples of Zn1 (26.5 mg, 38.3 µmol), 2 (28.0 mg,
38.3 µmol), Pd2(dba)3 (5.26 mg, 5.75 µmol), and P(o-tol)3 (14.0
mg, 46.0 µmol) were weighed into a 100-mL Schlenk flask
which was then pump-purged three times with argon. Toluene/
triethylamine (5:1, 15 mL) was added, and the flask was stirred
at 35 °C. Analytical SEC showed that the reaction had leveled
off after 5 h. The solvent was removed, and the residue was
chromatographed (silica, toluene) affording unreacted chlorin
monomers followed by the desired dyad and then high molecular
weight material (HMWM). The mixture of dyad and HMWM
was concentrated to dryness, dissolved in THF, and chromato-
graphed in four equal portions (SEC, THF) with gravity elution.
The dyad-containing fractions were combined and chromato-
graphed (silica, toluene), affording a bluish-purple solid (11.3
mg, 23%): 1H NMR (toluene-d8) δ -1.39 to -1.37 (br, 2H),
1.50 (s, 18H), 1.54 (s, 18H), 1.92 (s, 6H), 1.93 (s, 6H), 4.16 (s,
2H), 4.29 (s, 2H), 7.92-7.96 (m, 4H), 7.97-8.03 (m, 4H),
8.11-8.15 (m, 2H), 8.22 (d, J ) 2.0 Hz, 2H), 8.29 (d, J ) 2.0
Hz, 2H), 8.43 (1H, s), 8.45 (1H, s), 8.56-8.63 (m, 5H), 8.69-
8.76 (m, 4H), 8.79-8.81 (m, 1H), 8.83-8.86 (m, 2H), 8.94-
8.97 (m, 2H); LD-MS obsd 1293.18, calcd 1292.61 (C86H84N8-
Zn); λabs 415 (log ꢀ ) 5.44), 510 (4.24), 609 (4.55), 641 (4.47)
nm; λem 611, 642, 708 nm (Φf ) 0.22).
A potential undesirable side effect of the use of simple
chlorins or oxochlorins for light-harvesting applications is that
excited-state charge-transfer reactions are somewhat more facile
than for porphyrins (Figure 3). For ZnFb-dyad and Oxo-ZnFb-
dyad, ZnFb* f Zn+Fb- hole transfer that reduces the Fb*
lifetime and emission is modest (∼20% in toluene and ∼35%
in benzonitrile), while Zn*Fb f Zn+Fb- electron-transfer
competing with Zn*Fb f ZnFb* energy transfer is generally
minor (e10%). On the other hand, for the porphyrin dyad
ZnFbU these charge-transfer reactions have much lower yields
(generally e5% and only ∼15% hole transfer in benzonitrile).
A principal reason for the somewhat enhanced charge-transfer
processes for the chlorin and oxochlorin dyads is that the
Zn+Fb- charge-separated states lie 0.1-0.2 eV lower in energy
than for the porphyrin analogue due to the differences in redox
properties of the chromophores. Charge-transfer quenching of
the excited acceptor unit (Fb*), for example, compromises use
of the harvested energy for emission or for transfer to a
subsequent stage in a larger architecture. However, one should
be able to manipulate the redox properties of the chlorins or
oxochlorins to raise the energies of the charge-separated states
and minimize these unwanted quenching processes.
In addition to the increased red-region absorption and
enhanced TS energy transfer of ZnFb-dyad and Oxo-ZnFb-
dyad compared to ZnFbU, the TS transfer in Zn2-dyad and
Oxo-Zn2-dyad is increased by a factor of 10 in both nonpolar
and polar ligating solvents (entries 1 versus 2, 3 versus 5, 4
versus 6 in Table 2). [Such differences between the various Zn2
and analogous ZnFb dyads in a given solvent can be traced
largely to the spectral-overlap integral J listed in Table 2.] As
a result, the Zn2 chlorin and oxochlorin arrays have Fo¨rster rates
that are ∼170-fold faster than the Zn2 porphyrin array in toluene
and ∼20-fold faster in benzonitrile. Thus, properly designed
long linear rods and large branched architectures based on zinc
chlorin and oxochlorins that harvest light and transfer the
resulting excited-state energy between these units in route to a
trap site have advantages compared to the analogous porphyrin
systems.
Zn2-dyad. Following the procedure described for the prepa-
ration of ZnFb-dyad, samples of Zn1 (13.8 mg, 20.0 µmol)
and Zn2 (15.9 mg, 20.0 µmol) were coupled using Pd2(dba)3
(2.75 mg, 3.00 µmol) and P(o-tol) (7.31 mg, 24.0 µmol) in
toluene/triethylamine (5:1, 8 mL) at 35 °C under argon. After
Conclusions
(25) (a) Fenyo, D.; Chait, B. T.; Johnson, T. E.; Lindsey, J. S. J. Porphyrins
Phthalocyanines 1997, 1, 93-99. (b) Srinivasan, N.; Haney, C. A.; Lindsey,
J. S.; Zhang, W.; Chait, B. T. J. Porphyrins Phthalocyanines 1999, 3, 283-
291.
(26) Muthukumaran, K.; Loewe, R. S.; Kirmaier, C.; Hindin, E.; Schwartz, J.
K.; Sazanovich, I. V.; Diers, J. R.; Bocian, D. F.; Holten, D.; Lindsey, J.
S. J. Phys. Chem. B 2003, 107, 3431-3442.
The studies reported herein indicate that while hydroporphy-
rins display enhanced TS (Fo¨rster) electronic coupling, synthetic
light-harvesting arrays based on these constructs cannot be
implicitly assumed to be more efficient than those based on
9
J. AM. CHEM. SOC. VOL. 125, NO. 44, 2003 13469