Energy and electron transfer in β-alkynyl-linked porphyrin-[60] fullerene dyads

Article abstract of DOI:10.1021/jp061844t

Three porphyrin-fullerene dyads, in which a diyne bridge links C ₆₀ with a β-position on a tetraarylporphyrin, have been synthesized. The free-base dyad was prepared, as well as the corresponding Zn(II) and Ni(II) materials. These represent the first examples of a new class of conjugatively linked electron donor-acceptor systems in which π-conjugation extends from the porphyrin ring system directly to the fullerene surface. The processes that occur following photoexcitation of these dyads were examined using fluorescence and transient absorption techniques on the femtosecond, picosecond, and nanosecond time scales. In sharp contrast to the photodynamics associated with singlet excited-state decay of reference tetraphenylporphyrins (ZnTPP, NiTPP, and H₂TPP), the diyne-linked dyads undergo ultrafast (<10 ps) singlet excited-state deactivation in toluene, tetrahydrofuran (THF), and benzonitrile (PhCN). Transient absorption techniques with the ZnP-C₆₀ dyad clearly show that in toluene intramolecular energy transfer (EnT) to ultimately generate C₆₀ triplet excited states is the dominant singlet decay mechanism, while intramolecular electron transfer (ET) dominates in THF and PhCN to give the ZnP^.+/C₆₀^.- charge-separated radical ion pair (CSRP). Electrochemical studies indicate that there is no significant charge transfer in the ground states of these systems. The lifetime of ZnP ^.+/C₆₀^.- in PhCN was ～40 ps, determined by two different types of transient absorption measurement in two different laboratories. Thus, in this system, the ratio of the rates for charge separation (k_CS) to rates for charge recombination (k_CR), k _CS/k_CR, is quite small, ～7. The fact that charge separation (CS) rates increase with increasing solvent polarity is consistent with this process occurring in the normal region of the Marcus curve, while the slower charge recombination (CR) rates in less polar solvents indicate that the CR process occurs in the Marcus inverted region. While photoinduced ET occurs on a similar time scale in a related dyad 15 in which a diethynyl bridge connects C₆₀ to the para position of a meso phenyl moiety of a tetrarylporphyrin, CR occurs much more slowly; i.e., k_CS/k _CR ≈ 7400. Thus, the position at which the conjugative linker is attached to the porphyrin moiety has a dramatic influence on k_CR but not on k_CS. On the basis of electron density calculations, we tentatively conclude that unfavorable orbital symmetries inhibit charge recombination in 15 vis a vis the β-linked dyads.

Full text of DOI:10.1021/jp061844t

J. Phys. Chem. B 2006, 110, 14155-14166
14155
Energy and Electron Transfer in â-Alkynyl-Linked Porphyrin-[60]Fullerene Dyads
Sean A. Vail,†, David I. Schuster,*,† Dirk M. Guldi,‡ Marja Isosomppi,§ Nikolai Tkachenko,§
Helge Lemmetyinen,§ Amit Palkar,| Luis Echegoyen,| Xihua Chen,† and John Z. H. Zhang†
Department of Chemistry, New York UniVersity, New York, New York 10003, Institute for Physical and
Theoretical Chemistry, UniVersity of Erlangen, 91058 Erlangen, Germany, Institute of Materials Chemistry,
Tampere UniVersity of Technology, 33101 Tampere, Finland, and Department of Chemistry,
Clemson UniVersity, Clemson, South Carolina 29634
ReceiVed: March 24, 2006; In Final Form: May 26, 2006
Three porphyrin-fullerene dyads, in which a diyne bridge links C60 with a â-position on a tetraarylporphyrin,
have been synthesized. The free-base dyad was prepared, as well as the corresponding Zn(II) and Ni(II)
materials. These represent the first examples of a new class of conjugatively linked electron donor-acceptor
systems in which π-conjugation extends from the porphyrin ring system directly to the fullerene surface. The
processes that occur following photoexcitation of these dyads were examined using fluorescence and transient
absorption techniques on the femtosecond, picosecond, and nanosecond time scales. In sharp contrast to the
photodynamics associated with singlet excited-state decay of reference tetraphenylporphyrins (ZnTPP, NiTPP,
and H2TPP), the diyne-linked dyads undergo ultrafast (<10 ps) singlet excited-state deactivation in toluene,
tetrahydrofuran (THF), and benzonitrile (PhCN). Transient absorption techniques with the ZnP-C60 dyad
clearly show that in toluene intramolecular energy transfer (EnT) to ultimately generate C60 triplet excited
states is the dominant singlet decay mechanism, while intramolecular electron transfer (ET) dominates in
•-
THF and PhCN to give the ZnP•+/C60 charge-separated radical ion pair (CSRP). Electrochemical studies
indicate that there is no significant charge transfer in the ground states of these systems. The lifetime of
ZnP•+/C60•- in PhCN was ∼40 ps, determined by two different types of transient absorption measurement in
two different laboratories. Thus, in this system, the ratio of the rates for charge separation (kCS) to rates for
charge recombination (kCR), kCS/kCR, is quite small, ∼7. The fact that charge separation (CS) rates increase
with increasing solvent polarity is consistent with this process occurring in the normal region of the Marcus
curve, while the slower charge recombination (CR) rates in less polar solvents indicate that the CR process
occurs in the Marcus inVerted region. While photoinduced ET occurs on a similar time scale in a related
dyad 15 in which a diethynyl bridge connects C60 to the para position of a meso phenyl moiety of a
tetrarylporphyrin, CR occurs much more slowly; i.e., kCS/kCR ≈ 7400. Thus, the position at which the conjugative
linker is attached to the porphyrin moiety has a dramatic influence on kCR but not on kCS. On the basis of
electron density calculations, we tentatively conclude that unfavorable orbital symmetries inhibit charge
recombination in 15 vis a vis the â-linked dyads.
Introduction
presence of electron-releasing or electron-withdrawing aryl rings
at the meso positions.1 These results indicate that the intrinsic
A major consideration in the design of molecular photonic
devices involves the factors that mediate electronic communica-
tion between photoactive moieties. Some of these include the
intrinsic properties of the individual chromophores, system
energetics, and overall molecular topology, including interchro-
mophoric distances and spatial orientation. Lindsey et al. showed
that energy transfer (EnT) and electron transfer (ET) processes,
which were operative in a series of covalent donor-acceptor
systems consisting of zinc and free-base porphyrins, were
strongly dependent on the location of the diarylethyne linker
(meso vs â linkages) and were significantly influenced by the
properties of the linking element, as well as the site of
connectivity between the porphyrin chromophores in covalently
linked porphyrin arrays, play a crucial role in the system’s
overall molecular electronics.
The synthesis and study of a series of polyalkynyl zinc
porphyrin-C60 dyads covalently linked to the para position of
a phenyl ring on a zinc tetraaryl porphyrin have previously been
reported.2 These systems exhibited values for the ratio of charge
separation to charge recombination, kCS/kCR, on the order of
3900-6100 and 4100-7400 in tetrahydrofuran (THF) and
benzonitrile (PhCN), respectively. To further investigate EnT
and ET processes in highly conjugated donor-acceptor systems
involving porphyrins and fullerenes, a new series of porphyrin-
C60 dyads has been synthesized. These dyads, in which the
ligand at the porphyrin core is either Ni, Zn, or H2, exhibit
homoconjugation through the butadiynyl bridge that links the
â-position of the porphyrin donor to the surface of the fullerene.
* Author to whom correspondence should be addressed. E-mail:
david.schuster@nyu.edu.
† New York University.
‡ University of Erlangen.
§ Tampere University of Technology.
|| Clemson University.
Present Address: Department of Chemistry and Biochemistry, Arizona
State University Tempe, AZ 85287.
10.1021/jp061844t CCC: $33.50 © 2006 American Chemical Society
Published on Web 07/06/2006

14156 J. Phys. Chem. B, Vol. 110, No. 29, 2006
Vail et al.
eluent. Free-base porphyrin 10 was obtained by demetalation
of porphyrin 9 using trifluoroacetic acid (TFA) in CH2Cl2 at
room temperature. A pure sample of nickel alkynylporphyrin
11 could not be isolated due to a nearly identical Rf value to
that of nickel bromoporphyrin 8 in a variety of solvent systems.
â-Ethynylporphyrins 12 and 14 were obtained in 98% and
97% yields, respectively, by deprotection of porphyrins 9 and
11 with tetrabutylammonium fluoride (TBAF) in THF at room
temperature. The corresponding free-base porphyrin 13 was
isolated in nearly quantitative yield by treatment of zinc
porphyrin 12 with TFA in CH2Cl2 at room temperature.
Dyads 1 and 3 were synthesized via oxidative heterocoupling
reactions between ethynyl fullerene derivative 4 (1 equiv) and
an excess of nickel and zinc porphyrins 12 and 14 (3 equiv).
This procedure, which involves in situ generation of the Hay
catalyst CuCl/tetramethylethylenediamine (TMEDA)/O2)7 in
chlorobenzene under a dry oxygen atmosphere, afforded non-
fluorescent dyads 1 and 3 in 27% and 31% yields, along with
homodimers (â-â-butadiynyl-linked NiTPP-NiTPP, ZnTPP-
ZnTPP, and C60-C60) formed under the reaction conditions.
Dyads 1 and 3 were effectively isolated via preparative TLC
on silica gel using CS2 as the eluent. Finally, treatment of 1
with TFA in CH2Cl2 furnished H2P-C60 dyad 2, which was
further purified by preparative TLC on silica gel using CS2 as
the eluent.
Electrochemistry. The potentials obtained by cyclic volta-
mmetry (CV) for both NiTPP and ZnTPP exhibit two reduction
and two oxidation peaks, respectively. Both oxidation peaks
appear to be reversible while the second reduction is irreversible.
A comparison of these data with those previously reported for
H2TPP8 leads to the following trend in the order of first
reduction potentials: ZnTPP > NiTPP > H2TPP. However, an
opposite trend is observed for the corresponding first and second
oxidation potentials along the series, namely, H2TPP > NiTPP
> ZnTPP.
As shown by potentials obtained from CV measurements for
bromoporphyrins 6, 7, and 8, incorporation of the electronegative
bromine atom on the porphyrin periphery caused a pronounced
effect on the voltammetric response. The electron-withdrawing
bromine atom lowers electron density on the porphyrin, making
porphyrins 6, 7, and 8 easier to reduce and more difficult to
oxidize.
In the cyclic voltammograms for porphyrins 5, 9, and 10,
two oxidations and two reductions are observed, the second
reduction being reversible. Extension of the alkyne chain of
porphyrin 9 by one acetylene unit, which affords TMS-terminal
porphyrin 5, has no significant effect on the reduction potentials.
The respective oxidation and reduction potentials for TMS-
terminal porphyrins 9 and 10 are shifted anodically with respect
to porphyrins 12 and 13, which possess an H terminus, and can
be rationalized in terms of the electropositive character of the
trimethysilyl group.
Figure 1. Molecular structures of porphyrin-C60 dyads 1-3 and
reference compounds 4, 5, and 12.
Results and Discussion
Synthesis. Tetraphenylporphyrin (H2TPP) was quantitatively
metalated with Zn(OAc)2‚2H2O in refluxing chloroform/
methanol (2:1) for 1 h, according to thin-layer chromatography
(TLC). Similarly, treatment of H2TPP with Ni(OAc)2‚4H2O in
refluxing chloroform/methanol (2:1) for 16 h showed quantita-
tive conversion to nickel tetraphenylporphyrin (NiTPP) by TLC.
The synthesis of 1-ethynyl-2-methyl[60]fullerene 4 was
carried out according to a procedure previously reported by us.3
In this procedure, nucleophilic addition of lithium (trimethylsilyl,
TMS)acetylide to C60, followed by quenching with excess
iodomethane, furnished a TMS-protected compound that was
subsequently deprotected with potassium carbonate in refluxing
THF/CH3OH (5:1), affording fullerene derivative 4 in 23%
overall yield. Zinc porphyrin 5 was prepared using a modified
procedure for oxidative heterocoupling of terminal alkynes as
previously reported in the literature (see Experimental Section
for details).2â-Butadiyne-linked porphyrin-C60 dyads 1-3 were
synthesized according to the pathway outlined in Scheme 1.
â-Bromotetraphenylporphyrin 6 was prepared using a procedure
previously reported for â-bromination of free-base porphyrins.4
Commercial H2TPP was treated with N-bromosuccinimide in
refluxing CHCl3, and the reaction progress was carefully
monitored by TLC. Due to similar Rf values for â(2)-bromo-
H2TPP 6 and 2,12(13)-dibromo-H2TPP, the reaction was
terminated when an increase in the formation of the dibro-
moporphyrin was demonstrated by TLC. The low yield of the
undesired dibromo product facilitated isolation of pure mono-
bromoporphyrin 6, which was isolated in 64% yield by column
chromatography on silica gel using toluene/cyclohexane (1:1)
as the eluent. The order of elution was: dibromo-TPP, mono-
bromo-TPP, followed by unreacted H2TPP. Bromoporphyrin 6
was metalated by treatment with Zn(OAc)2‚2H2O in refluxing
chloroform/methanol (2:1) for 1 h, quantitatively affording zinc
porphyrin 7. Analogously, nickel porphyrin 8 was prepared in
near quantitative yield by treating 6 with Ni(OAc)2‚4H2O in
refluxing chloroform/methanol (2:1) for 16 h.
The consequences of â-alkynyl substitution become apparent
from the reduction potentials of porphyrins 12, 13, and 14. The
50 mV anodic shifts observed for the first reduction potentials
of porphyrins 12-14, relative to ZnTPP, H2TPP, and NiTPP,
can be attributed to conjugation between the porphyrin and the
acetylene moieties, which enhances the electron accepting ability
of the porphyrin, thereby facilitating reduction. Porphyrins 12,
13, and 14, all of which contain an H terminus, exhibit no
additional peaks as compared to ZnTPP, H2TPP, and NiTPP.
Therefore, it can be concluded that both oxidation and reduction
of the acetylene moiety are beyond the operative potential
window.
â-(Trimethylsilyl)ethynyl porphyrins 9 and 11 were prepared
from 7 and 8 via a modified palladium-catalyzed Heck coupling
reaction, as previously reported in the literature.4-6 Zinc
porphyrin 9 was isolated in 63% yield from a long silica gel
column using cyclohexane/methylene chloride (1:1) as the

â-Alkynyl-Linked Porphyrin-C60 Dyads
J. Phys. Chem. B, Vol. 110, No. 29, 2006 14157
SCHEME 1: Synthetic Route to Porphyrin-C60 Dyads 1-3
TABLE 1: Oxidation and Reduction Potentials Obtained by
Cyclic Voltammetry and Differential Pulse Voltammetry for
ZnTPP, NiTPP, Compounds 1-10 and 12-14 in CH2Cl2 (V
The CV measurements for â-alkynyl-linked porphyrin-C60
dyads 1-3 show overlapping potentials for both the porphyrin
and the fullerene moieties, with each dyad exhibiting a total of
four reduction waves and two oxidation waves. The correspond-
ing oxidation potential for ZnP-C60 1 could not be determined
accurately due to limited sample availability. Since ethynyl-
fullerene derivative 4 does not show any oxidation within this
potential window,2 the two peaks are attributed to the oxidation
of the porphyrin moieties. Anodic shifts of approximately 30
and 10 mV, respectively, are observed in the potentials obtained
from CV measurements for dyads 2 and 3 as compared to
precursor alkynyl porphyrins 13 and 14. With regard to the
cathodic region of the scan, the cyclic voltammograms show
four reduction processes, which were confirmed by differential
pulse voltammetry (DPV) experiments. Since no reductions were
observed in the earlier cyclic voltammograms for the porphyrin
derivatives below -1.5 V, the first two reductions can be
attributed to fullerene derivative 4, while the remaining peaks
appear to be overlaps between porphyrin and fullerene reduction
peaks.
versus Fc/Fc+ a
)
compound
oxd1
0.39
0.59
NDc
oxd 2
red 1
red 2
red 3
red 4
ZnTPP
NiTPP
1
2
3
4
5
6
7
8
9
10
12
13
14
0.67
0.88
-1.77 -2.20b
-1.67 -2.07b
NDc
-1.03d -1.44d -1.68d -1.95d
0.6
0.63
0.84 -1.05 -1.45 -1.59d -1.83d
0.90 -1.04d -1.46d -1.66d -1.96d
-1.04 -1.42
-1.94
-1.69 -2.01
-1.55 -1.85b
-1.69 -2.07b
-1.62 -2.14b
-1.68 -2.04
-1.55 -1.85
-1.70 -2.06b
-1.58 -1.87b
-1.62 -2.07b
0.43
0.59
0.44
0.63
0.43
0.57
0.44
0.57
0.62
0.68
0.80
0.68
0.88
0.67
0.81
0.69
0.83
0.89
a All processes are reversible unless otherwise noted. b Irreversible.
c Not determined due to lack of sample availability. d Obtained from
DPV.
The first reduction peak potential obtained from DPV for
ethynylfullerene 4 is almost the same as that for ZnP-C60 dyad
1; H2P-C60 dyad 2 and NiP-C60 dyad 3 behave in a similar
manner. Therefore, data obtained from DPV measurements
support the notion that the porphyrin and fullerene moieties do
not exhibit any significant ground-state electronic communica-
tion in dyads 1-3. Table 1contains a summary of electrochemi-
cal potentials obtained from CV and DPV experiments.
Steady-State Absorption Studies. The steady-state absorp-
tion spectra of ZnTPP, H2TPP, and NiTPP references were
consistent with previously reported data.9,10 For comparison
purposes, the concentrations of all samples were normalized to
optical densities of 1.45 at the Soret band of ZnTPP (422 nm).
The data for ZnTPP, H2TPP, NiTPP, â-alkynyl-linked porphy-
rin-C60 dyads 1-3, and porphyrins 5-10 and 12-14 in CHCl3
are summarized in Table 2.
As indicated in Table 2, a small bathochromic shift (3-4
nm) is observed in the absorption spectra of the free-base and
zinc bromoporphyrins 6 and 7 relative to H2TPP and ZnTPP.

14158 J. Phys. Chem. B, Vol. 110, No. 29, 2006
Vail et al.
TABLE 2: Ground-State Absorption Maxima for ZnTPP,
H2TPP, NiTPP, Porphyrin-C60 Dyads 1-3, and Porphyrins
5-10 and 12-14 in CHCl3
λ
max (nm)
(Soret band)
compound
absorbance λmax (nm)
551, 593
515, 551, 589, 647
535
257 (C60), 563, 603
256 (C60), 526, 563, 603, 659
257 (C60), 541, 579
562, 603
518, 552, 594, 649
555, 595
533
559, 597
522, 558, 598, 655
558, 598
521, 556, 598, 654
536
ZnTPP
H2TPP
NiTPP
1
2
3
5
6
7
8
9
10
12
13
14
422
418
422
437
432
430
435
421
426
419
431
427
429
425
423
Figure 2. UV/visible absorption spectra of zinc porphyrins 5 and 9
and ZnP-C60 dyad 1 in chloroform.
Conversely, the spectrum of nickel bromoporphyrin 8 shows a
small hypsochromic shift (3 nm) relative to that of NiTPP.
A more significant change is apparent in the absorption
spectra of TMS-terminal alkynylporphyrins 9 and 10. The
bathchromic shifts (9 nm) for the porphyrin Soret bands of 9
and 10, relative to those of ZnTPP and H2TPP, respectively,
are consistent with previous observations for other â-alkyn-
ylporphyrin derivatives.4
The absorption spectra of porphyrins 12 and 13, with an H
terminus, exhibit a slightly smaller red shift in the Soret band
(7 nm) than corresponding TMS-terminal porphyrins 9 and 10,
relative to the bands in ZnTPP and H2TPP, respectively.
The absorption spectra of porphyrin-C60 dyads 1-3 all show
a significant bathochromic shift, accompanied by substantial
broadening of the porphyrin Soret band. In all cases, the spectra
show characteristic absorptions for both the porphyrin and the
fullerene moieties.
A considerable bathochromic shift of 7 nm is observed for
the porphyrin Soret band of NiP-C60 dyad 3 relative to its
porphyrin counterpart 14. The most significant red shift in
porphyrin Soret band absorption among these dyads is observed
in the case of ZnP-C60 dyad 1, which exhibits bathochromic
shifts of approximately 8 and 15 nm relative to zinc alkynylpor-
phyrin 12 and ZnTPP, respectively. A similar trend is evident
in the absorption spectrum of H2P-C60 dyad 2, which exhibits
bathochromic shifts of 7 and 14 nm relative to free-base
porphyrin 13 and H2TPP, respectively.
The significant bathochromic shifts for the porphyrin Soret
bands in porphryin-C60 dyads 1-3, relative to H-terminal
alkynylporphyrin precursors 12-14, are similarly observed in
the absorption spectrum of â-butadiynylporphyrin 5. Thus,
lengthening the â-alkyne chain on the porphyrin periphery
causes both an increased bathochromic shift and substantial
broadening of the porphyrin Soret band, arising from extension
of the porphyrin π-system. Indeed, this phenomenon has been
observed in other porphyrins with extended conjugation orig-
inating from the â-position.4
Since significant red-shifted and broadened absorption for
the porphyrin Soret band is observed for zinc porphyrin 5, which
lacks C60, as well as for ZnP-C60 dyad 1, the spectral changes
for dyads 1-3 cannot reasonably be attributed to significant
electronic interactions between the porphyrin and C60 moieties
in the ground state (Figure 2).
Figure 3. Normalized room-temperature fluorescence spectra of ZnP
12 and ZnP-C60 dyad 1, using identical optical densities at the 426
nm excitation wavelength (360 nm for C60) in toluene. The spectra of
ZnP 12 and C60 are scaled to fit the dyad emissions at 650 and 710
nm.
photoexcitation, efficient quenching of ZnP fluorescence by the
attached C60 moiety is observed in toluene. In addition to ZnP
emissions at 600 and 650 nm, a new band corresponding to
C60 fluorescence develops at 710 nm,11,12 which evolves with
quantum yields of 5 × 10-4, and is indicative of rapid
intramolecular transduction of singlet excitation energy from
the porphyrin to the fullerene (Figure 3).
Since measurements were performed using excitation wave-
lengths corresponding to the absorption maximum (λmax) for
the ZnP component, at wavelengths where absorption by the
C60 moiety composes less than 5% of the total ZnP-C60
absorption, these observations are consistent with the occurrence
of intramolecular transduction of singlet excitation.12 Further
support is derived from the excitation spectrum of the fullerene
emission above 700 nm, which exhibits features resembling the
ZnP ground state, with maxima at 430, 555, and 595 nm,
respectively.13 Although it can be assumed that photoexcited
H2P-C60 dyad 2 behaves similarly to dyad 1, overlapping H2P
(Φf 0.11) and C60 fluorescence prevented a quantitative deter-
mination of the fullerene fluorescence yield, typically ∼6.0 ×
10-4 11,12
.
In THF, the steady-state fluorescence spectra of porphyrin-
C60 dyads 1-3 were measured using normalized optical densities
of 0.5 at the 425 nm excitation wavelength. Quantum yields of
porphyrin fluorescence in THF of 6.0 × 10-4 for 1, 1.5 × 10-3
for 2, and <1.0 × 10-5 for 3 are indicative of ultrafast
deactivation of the porphyrin singlet excited states.
As shown in Figure 4, the fluorescence spectrum of ZnP-
C60 dyad 1 in PhCN exhibits the characteristic ZnP emissions
at 600 and 650 nm but lacks C60 fluorescence at 710 nm
(compare with Figure 3), suggesting that EnT processes
Steady-State Fluorescence Studies. Steady-state fluores-
cence studies on ZnP-C60 1 were carried out using identical
optical densities at the 426 nm excitation wavelength. After

â-Alkynyl-Linked Porphyrin-C60 Dyads
J. Phys. Chem. B, Vol. 110, No. 29, 2006 14159
TABLE 3: Fluorescence Quantum Yields (Φf), Rate
Constants for Charge Separation (kCS), Rate Constants for
Charge Recombination (kCR), and CSRP State Lifetimes
(τCS) for ZnP-C60 1, H2P-C60 2, and NiP-C60
3
a
b
Φf
kCS
(s-1
kCS
(s-1
kCR
(s-1
τCS
(ps)
solvent (porpyrin)
)
)
)
1 ZnPC60 toluene 9.0 × 10-4 2.2 × 1010 c
THF
PhCN
6.0 × 10-4 3.3 × 1010 9.0 × 1010 9.8 × 109 100
4.4 × 10-4 4.5 × 1010 1.1 × 1011 1.6 × 1010 60
2 H2PC60 toluene 2.1 × 10-3 7.0 × 109 c
THF
PhCN
1.5 × 10-3 9.9 × 109 6.6 × 1010 2.8 × 109 360
9.0 × 10-4 1.7 × 1010 7.6 × 1010 6.2 × 109 160
3 NiPC60 toluene <1.0 × 10-5
<1.0 × 10-5
5.2 × 1010
Figure 4. Room-temperature fluorescence spectrum of ZnP-C60 dyad
1 following excitation at 433 nm in PhCN.
THF
PhCN <1.0 × 10-5
6.2 × 1010 4.7 × 109 210
a Extrapolated from fluorescence experiments. b Determined from
femtosecond/picosecond transient absorption measurements. c Energy
transfer.
observed for 1 in toluene are not operative in PhCN. The same
observation was made for dyad 1 in THF; i.e., no C60
fluorescence at ∼710 nm was detected.
In the more polar solvent PhCN, the fluorescence quantum
yields decrease to 4.4 × 10-4 for 1 and 9.0 × 10-4 for 2.
Acceleration of porphyrin fluorescence quenching rates with
increasing solvent polarity is consistent with intramolecular ET
directly from 1P* to C60 to afford P•+/C60•- 12-19 This observa-
.
tion can be rationalized in terms of the ability of polar solvents
(i.e, THF or PhCN) to stabilize the charge-separated radical pair
(CSRP) state through effective solvation of ions, thereby
lowering its energy relative to the ground state, or viewed
alternatively by increasing the energy gap between excited and
CSRP states. Similar conclusions could not be made in the case
of NiP-C60 dyad 3 due to the characteristically low fluorescence
quantum yields (10-5) of nickel porphyrins.20
The alternative mechanism for the formation of the CSRP
state in THF and PhCN, namely, rapid intramolecular EnT
followed by ET evolving from the fullerene singlet excited state,
seems much less likely based on a comparison of rate constants
(see below) and consideration of the energy levels of the various
states. Furthermore, we have no spectroscopic evidence to
support such a sequence of intramolecular events.
Figure 5. Room-temperature emission decay of ZnP-C60 dyad 1 at
600 nm following femtosecond excitation at 410 nm in toluene. The
lifetimes (and amplitudes) of the decay components are indicated in
the figure.
From the fluorescence quantum yields (Φf) and lifetimes (τ)
of photoexcited 1, 2, and 3 and ZnTPP and H2TPP (X),21,22 the
rate constants (k) for intramolecular EnT (kEnT in toluene) and
ET (kET in THF and PhCN) for porphyrin-C60 dyads (Y) can
be calculated according to the following equation
Figure 6. Room-temperature emission decay of ZnP-C60 dyad 1 at
610 nm following femtosecond excitation at 410 nm in PhCN. The
lifetimes (and amplitudes) of the decay components are indicated in
the figure.
k(Y) ) [Φ(X) - Φ(1, 2, 3)]/[τ(X)Φ(1, 2, 3)]
in which Φ(X) and τ(X) represent the literature values for the
fluorescence quantum yields and fluorescence lifetimes of
ZnTPP (Φ ) 0.04, τ ) 2.1 ( 0.2 ns) and H2TPP (Φ ) 0.15,
τ ) 10.1 ( 0.5 ns).21 The corresponding rate constants obtained
from the fluorescence experiments are included in Table 3.
Interestingly, the rates for deactivation of the porphyrin S1
excited state are greater for ZnP-C60 (1) than those for H2P-
C60 (2). At least in toluene, where there is a weaker overlap
between the porphyrin fluorescence and the fullerene absorption,
singlet-singlet energy transfer dominates.
Time-Resolved Fluorescence Up-Conversion. Femtosecond
time-resolved up-conversion fluorescence experiments on ZnP-
C60 1 were carried out to elucidate the processes occurring on
extremely short time scales in this system. In the up-conversion
studies, excitation at 410 nm using 50 fs pulses was performed,
and emission was monitored at 600 or 610 nm, corresponding
to the first emission maximum of ZnP in toluene and PhCN,
respectively. The 410 nm excitation promotes ZnP to the S2
excited state, which rapidly decays via internal conversion to
the S1 state, whose decay is monitored in these experiments.
As seen in Figures 5 and 6, the major decay components in
toluene and PhCN have lifetimes of 5.3 and 3.2 ps, while the
longer-lived minor components have lifetimes of 25 and 41 ps.
In toluene, the second decay component is attributed to an
equilibrium between porphyrin and fullerene singlet excited
states (1ZnP*-C60 T ZnP-1C60*). In PhCN, the presence of a
second decay component can be rationalized in terms of
equilibrium between 1ZnP* and the CSRP state. It is also
possible that the biexponential decay arises from two competing
processes from 1ZnP*, namely, direct ET to give the CSRP state,
1
and singlet-singlet energy transfer to give C60* followed by
ET. Since the direct ET process in PhCN is very rapid and no
evidence for C60 emission in PhCN was obtained in this study,
we favor equilibration between ZnP* and the CSRP state.
Time-Resolved Transient Absorption Spectra. Transient
absorption spectroscopy was used to investigate the fate of
porphyrin excited states in dyads 1-3 and porphyrin reference
compounds ZnTPP, H2TPP, and NiTPP. The course of the

14160 J. Phys. Chem. B, Vol. 110, No. 29, 2006
Vail et al.
Figure 7. Room-temperature differential absorption spectrum obtained
following femtosecond flash excitation (387 nm) of ZnTPP in nitrogen-
saturated PhCN with a time delay of 5 ps.
Figure 8. Room-temperature differential absorption spectra obtained
following femtosecond flash excitation (387 nm) of ZnP-C60 1 in
nitrogen-saturated PhCN with time delays between 0 and 20 ps. Up
arrows indicate regions of increasing absorption with time, and down
arrows decreasing absorption.
excited-state processes was monitored through differential
absorption changes following irradiation (387 nm) of dilute
solutions (∼10-5 M) with femtosecond laser pulses.
The singlet-singlet features for photoexcited ZnTPP include
transient bleaching of the porphyrin Soret and Q-band transi-
tions, which is accompanied by a new, albeit weak, transition
in the region above 600 nm (Figure 7).13,23 The intrinsic decays
for ZnTPP and H2TPP, 2.1 ( 0.2 and 10.1 ( 0.5 ns,
respectively, were found to be independent of solvent (toluene,
THF, or PhCN).
In contrast to ZnTPP and H2TPP, the kinetics for NiTPP
suggest that formation of excited states proceeds through a two-
step mechanism. Initially, a short-lived singlet excited state,
which kinetically resembles those of ZnTPP and H2TPP, is
observed. A transient decay (0.89 ps), which involves trans-
formation to the d-d excited state,24 is observed prior to the
completion of singlet excited-state formation. This observation
is consistent with the rapid deactivation (<1 ps) of nickel
porphyrin singlet excited states.24 Both of these intermediates
can be differentiated spectroscopically (Figure S1).
The initial intermediate formed from NiTPP has a spectral
resemblance to the nickel porphyrin singlet excited state (1NiP*),
including a maximum at 500 nm, while the features of the d-d
photoexcited state, which has a measured lifetime of 220 ps,
lack the maximum at ∼500 nm. These findings are consistent
with previous observations associated with the decay of 1NiP*
and d-d excited states.24
Time-resolved transient absorption studies confirmed forma-
tion of 1P* upon photoexcitation of the porphyrin chromophores
in 1, 2, and 3. In the case of NiP-C60 3, an additional pathway
followed, leading to the d-d photoexcited state. Instead of the
slow decay dynamics typical of porphyrin excited states, a rapid
deactivation of singlet excited energy was observed for 1 and
2 (∼1010 s-1) and of the d-d state in the case of 3. Figures 8
and S2 show the typical decay dynamics for ZnP-C60 1,
measured in PhCN at room temperature after the 387 nm
femtosecond laser pulse, with time delays between 0 and 20 ps
and between 5 and 50 ps, respectively.
The differential absorption changes that accompany the decay
of ZnP singlet excited states in ZnP-C60 1 in THF and PhCN,
for H2P in H2P-C60 2, and NiP in NiP-C60 3, all display the
spectral features associated with one-electron-oxidized π-radical
cations, as shown by strong and broad absorptions in the 600-
800 nm range.13,23 In contrast, fullerene singlet-singlet absorp-
tion at 880 nm evolves in toluene, which on a longer time scale
(ns) leads to population of the triplet manifold through efficient
Figure 9. Time absorption profiles for the spectrum depicted in Figure
8 at 440 nm (black circles) and 640 nm (red circles), monitoring the
•-
formation and subsequent decay of ZnP•+/C60 in PhCN.
intersystem crossing, as shown by growth of the characteristic
C60 triplet-triplet absorption maximum at 700 nm (data not
shown).11,12 These observations support the conclusion that
intramolecular ET from the porphyrin to C60 in 1, 2, and 3,
which leads to the formation of P•+/C60•-, is the dominant
pathway for deactivation of porphyrin excited states in 1-3 in
both THF and PhCN, while singlet-singlet EnT is the dominant
if not exclusive decay pathway in toluene.
The corresponding time absorption profiles at 440 and 640
nm for ZnP-C60 1 in PhCN are depicted in Figure 9. As shown,
ZnP•+/C60 begins to decay on the picosecond time scale.
•-
Singlet ground states for dyads 1-3 evolve as a product of
charge recombination processes in all cases.
As shown in Table 3, the lifetimes of the CSRP states for 1
and 2 were longer in THF than in the more polar PhCN: 1,
100 ps in THF, 60 ps in PhCN; 2, 360 ps in THF, 160 in PhCN.
In PhCN, the CSRP state lifetime for 3 was determined to be
210 ps. This trend in rates as a function of solvent polarity
indicates charge recombination is occurring in the Marcus
inverted region.
Pump-Probe Transient Absorption Measurements on
ZnP-C60 1. Femtosecond pump-probe transient absorption
experiments were performed to elucidate the processes occurring
on shorter time scales in ZnP-C60 1. These studies confirmed

â-Alkynyl-Linked Porphyrin-C60 Dyads
J. Phys. Chem. B, Vol. 110, No. 29, 2006 14161
The decay component spectrum of ZnP-C60 1, as determined
from pump-probe measurements, indicates that the CSRP state
is formed in ∼10 ps in PhCN. The formation of ZnP•+ and
•-
C60 is demonstrated by the negative amplitudes at 650 and
1020 nm; the formation lifetimes are 11 and 6 ps, respectively
(Figures S5 and S7). Noteworthy is the fact that the lifetimes
in the two wavelength ranges are in good agreement. However,
up-conversion (fluorescence) measurements yielded a lifetime
1
of 3.2 ps for the decay for ZnP*. Since the solvent gives rise
to its own signal in the pump-probe measurements, the lifetime
obtained from the fluorescence measurements is probably more
reliable. All the data point to formation of the CSRP state in
less than 10 ps.
Figure 10. Room-temperature transient absorption spectra (decay
components) of ZnP-C60 dyad 1 (460-760 nm) following femtosecond
excitation at 413 nm in toluene.
•-
The lifetime of ZnP•+/C60 in PhCN was determined to be
∼40 ps from pump-probe transient absorption measurements.
The longest-lived component (680 ps) resolved in the measure-
ments does not exhibit spectral features associated with a CSRP
state and is reasonably attributed to solvent relaxation, with
lifetimes of 8 and 600 ps, respectively. The broad band observed
at 650 nm for the 39 ps component, which has a mirror image
corresponding to an 11 ps component, is attributed to ZnP•+.13,23
The time profile for the 650 nm signal shows that at least 90%
of the molecules in the CSRP state (ZnP•+/C60•-) decay in less
than 1 ns. The lifetime of the main component is 48 ps, which
is consistent with the global fitting (39 ps). Noteworthy is the
fact that rates for charge separation and charge recombination,
which were determined independently through two entirely
different experimental methods for ZnP-C60 dyad 1 in PhCN,
are entirely consistent. Due to a lack of sample availability,
analogous femtosecond experiments could not be performed on
dyads 2 and 3.
Figure 11. Room-temperature transient absorption spectra (decay
components) of ZnP-C60 dyad 1 (850-1070 nm) following femto-
second excitation at 413 nm in toluene.
previous results obtained for photoexcited 1, namely, the
occurrence of intramolecular EnT (toluene) and ultrafast in-
tramolecular ET (PhCN) producing ZnP•+/C60 within ap-
•-
Molecular Structure Calculations. Theoretical calculations
on ZnP-C60 dyad 1 were carried out with the Gaussian 03
package40 using a fully optimized ground-state structure. Results
are shown in Figure 12.
proximately 10 ps.
Time-resolved absorption experiments were performed using
413 nm excitation, which is located at the blue shoulder of the
porphyrin Soret band. The measurements were carried out in
three wavelength ranges: 460-680, 540-760, and 850-1070
nm. The data in the visible region of the spectrum (460-680
and 540-760 nm) were combined for global fitting, while those
in the near-infrared (NIR) were fitted separately. The decay
component spectra for ZnP-C60 1 in toluene (visible and NIR)
are presented in Figures 10 and 11. The corresponding calculated
time-resolved spectra derived from Figures 10 and 11, at selected
delay times, are shown in Figures S3 and S4.
As seen in Figure 10, strong bleaching of the zinc porphyrin
Q-band (∼550 nm) and the appearance of broad absorption
between 580 and 750 nm, following photoexcitation in toluene
at 413 nm, attests to the formation of 1ZnP*.13 The decay
component spectra show no evidence of ZnP•+ formation, which
is normally detected by its absorption around 650 nm.13,23 In
addition, the absence of spectral features in the NIR region
characteristic of C60•-, which would lead to absorption around
1000 nm, corresponding to a negative amplitude of the decay-
associated spectra in this region, indicates that CS does not take
place in toluene (Figure 11).12-19,23,25-39 Rather, the strong
quenching of ZnP fluorescence in toluene is attributed to rapid
As shown in Figure 12, the phenyl rings at positions 5, 10,
and 15 are staggered by approximately 65° with respect to the
plane of the porphyrin. Due to its proximity to the butadiynyl
linker at position 2, the phenyl ring at meso position 20 shows
increased staggering, from 65° to 82°. The butadiyne group is
covalently bonded to one of the two pyrrole units coordinated
to the cental zinc atom. The zinc porphyrin core shows little
elasticity, due to the presence of the dialkynyl group, which
essentially “pulls” on the pyrrole nitrogen ring, increasing the
N-Zn bond length by about 0.01 Å.
The calculated interchromophoric center-to-center distance
(Rcc) in the optimized structure was determined to be 14.75 Å.
With respect to the porphyrin, the bond angle between the
â-carbon and the first two carbon atoms of the alkyne moiety
(â-C-CtC) is slightly bent due to the influence of the phenyl
ring on the adjacent meso position. The methyl group adjacent
to C1 on the fullerene exerts a similar effect, causing a slight
bending of the C1-CtC bond angle. Therefore, it can be
concluded that the butadiyne bridging element is both rigid and
planar, although not completely linear.
The two sp3 carbon atoms on the fullerene, designated C1
and C2 in Figure 12, constitute a bridge that disrupts electronic
conjugation within the dyad. The C1-C2 bond (1.638 Å) is
considerably longer than the other C-C bonds between
hexagons on C60 (∼1.41 Å). In addition, the two C1-C (1.561
Å) and two C2-C bonds (1.550 Å) on C60 are longer than the
C-C bonds between other hexagons and pentagons of C60
(∼1.46 Å).
1
intramolecular EnT from ZnP* to C60.
The decay component spectra and calculated time-resolved
spectra for ZnP-C60 dyad 1 in PhCN are shown in Figures S5-
S8, respectively. The spectra in Figure S5 possess characteristic
features of ZnP•+, as shown by an absorption band at 650 nm
for the 39 ps component.13,23 In Figure S7, the absorption band
at 1020 nm observed for the 31 ps component is attributed to
C60•- 12-19,23,25-39 and is consistent with intramolecular ET from
•-
1ZnP* to C60 to afford ZnP•+/C60 in PhCN.
Although Kohn-Sham density functional orbitals are of little

14162 J. Phys. Chem. B, Vol. 110, No. 29, 2006
Vail et al.
Figure 12. Optimized structure of zinc porphyrin-C60 dyad 1 showing selected bond lengths (in Å). The angles (in deg) between each of the four
phenyl groups and the porphyrin plane are indicated by numbers in boxes. The arrows leading to the number of 81.9° illustrate how dihedral angles
were measured.
physical significance, they are consistent with the ground-state
electron density and can be qualitatively analyzed.41 An
examination of the molecular orbitals (MOs) reveals that the
two highest occupied MOs (HOMO + 1 and HOMO) are
centered on the dialkynyl zinc porphyrin while the three lowest
unoccupied MOs (LUMO, LUMO + 1 and LUMO + 2) are
centered almost exclusively on the fullerene, as illustrated in
Figure 13. This suggests a strong propensity for intramolecular
charge transfer from the zinc porphyrin to the fullerene following
photoexcitation. The properties of the frontier orbitals and the
HOMO-LUMO energy gap (calculated by B3LYP) have
previously been shown to be in good agreement with experi-
mental data.42
In ZnP-C60 dyad 1, the HOMO-LUMO energy gap was
determined to be ∼1.6 eV, corresponding to a wavelength of
∼770 nm. However, more extensive computational analysis of
the ground- and excited-state electronic structures are required
to elucidate the detailed optical transition features.
Conclusion
A series of P-C60 dyads with covalent linkages originating
from the â-position of a nickel, zinc, or free-base porphyrin to
C60 was synthesized to assess photophysical and electrochemical
properties of such dyads as a function of molecular structure.
Dyads 1-3 represent a new class of conjugatively linked
electron donor-acceptor systems in which the π conjugation
extends from the porphyrin to the surface of the fullerene
through a butadiynyl bridge.
Significant bathochromic shifts for the porphyrin Soret bands
in the ground-state absorption spectra of P-C60 dyads 1-3 can
be attributed to extended conjugation of the porphyrin π system
and not to ground-state interactions between the chromophores.
Cyclic voltammetry and differential pulse voltammetry mea-
surements confirm that the porphyrin and fullerene moieties in
dyads 1-3 are not electronically coupled to any detectable extent
in the dyad ground states.
Figure 13. The two highest occupied MOs (HOMO + 1 and HOMO)
and three lowest unoccupied MOs (LUMO, LUMO + 1 and LUMO
+ 2) of zinc porphyrin-C60 dyad 1, accompanied by the corresponding
calculated orbital energies. Computations were performed with B3LYP/
LANL2DZ using an optimized geometrical structure.
In nonpolar solvents, these systems exhibit singlet-singlet
EnT from the photoexcited porphyrin to C60, as shown by the
formation of 3C60*. In polar solvents, rapid formation of a CSRP
state is followed by somewhat slower charge recombination
dynamics leading back to the singlet ground state. Deactivation
of porphyrin excited states via EnT (in nonpolar solvents) and

â-Alkynyl-Linked Porphyrin-C60 Dyads
J. Phys. Chem. B, Vol. 110, No. 29, 2006 14163
ET (in polar solvents) is similar to the behavior observed in a
number of other photoexcited porphyrin-C60 systems.13,38,39,43-47
An increase in ET rates with increasing solvent polarity is
consistent with charge separation occurring in the normal region
of the Marcus parabola. Slower charge recombination rates in
less polar solvents, namely, THF as compared with PhCN, are
indicative of charge recombination events operating in the
Marcus inVerted region.
of 15, which would retard charge recombination to produce the
ground state. Alternatively, the longer CSRP lifetime for 15
may be due, at least in part, to unfavorable orbital interactions
between the ZnP and the C60 moieties, which could inhibit
charge recombination processes from the CSRP state.31
Experimental Section
Electrochemistry (Clemson University). The electrochemi-
cal measurements for ZnTPP, NiTPP, ethynylfullerene 4, and
porphyrins 5-10 and 12-14 were performed using a CHI 440
electrochemical workstation (CH Instruments, Inc., Austin, TX).
Tetrabutylammonium hexafluorophosphate (Fluka) in redistilled
CH2Cl2 (0.1 M) was used as the supporting electrolyte (degassed
with argon and saturated with CH2Cl2 vapors). A platinum wire
was employed as the counter electrode, and a nonaqueous Ag/
AgCl electrode was used as the reference. Ferrocene (Fc) was
added as an internal reference, and all the potentials were
referenced relative to the Fc/Fc+ couple. A glassy carbon
electrode (CHI, 1.5 mm in diameter), polished with 0.3 µm
aluminum paste and ultrasonicated in a deionized water and
CH2Cl2 bath, was used as the working electrode. The scan rates
for cyclic voltammetry were 100 mV while the pulse rate was
set at 0.05 s at increments of 4 mV and an amplitude of 50 mV
for differential pulse voltammetry measurements. All experi-
ments were performed at room temperature (20 ( 2 °C).
Dyads 1-3 were studied in a special cell similar to one
described earlier.48,49 This assembly consists of a glass tube (1.5
mm diameter) fitted with a Vycor tip (working volume of 100
µL) and contains the test solution (compound + TBAPF6 in
CH2Cl2). This cell was immersed in a glass vial containing the
electrolyte solution. A platinum wire was employed as the
counter electrode, and a nonaqueous Ag/AgCl electrode was
used as the reference. This electrode was similarly immersed
in the electrolyte contained in the outside glass vial.
Photophysical Measurements (University of Erlangen).
Picosecond laser flash excitation experiments were performed
using 355 or 532 nm laser pulses from a mode-locked,
Q-switched Quantel YG-501 DP Nd:YAG laser system (18 ps
pulse width, 2-3 mJ/pulse). Nanosecond laser flash excitation
experiments were performed with 355 or 532 nm laser pulses
from a Quanta-Ray CDR Nd:YAG system (6 ns pulse width)
in a front-face excitation geometry. The photomultiplier output
was digitized with a Tektronix 7912 AD programmable digitizer.
A typical experiment consisted of 5-10 replicate pulses per
measurement. The averaged signal was processed with a LSI-
11 microprocessor interfaced with a VAX-370 computer. The
details of the experimental setups and their operation can be
found elsewhere.50
Finally, it is interesting to compare the photophysical
behaviors of â-alkynyl ZnP-C60 dyad 1 and para-phenylalkynyl
ZnP-C60 dyad 15, which has been previously described.2
1
In toluene, fast intramolecular EnT from ZnP* to C60 is
observed for both ZnP-C60 dyads 1 and 15, as shown by high
quantum yields for the formation of 3C60*. In THF and PhCN,
both dyads 1 and 15 exhibit rapid and efficient intramolecular
ET leading to a CSRP state. Increasingly fast deactivation of
porphyrin fluorescence for both 1 and 15 with increasing solvent
polarity is consistent with intramolecular ET operating in the
Marcus normal region. However, increasingly faster rates for
charge recombination with a decrease in thermodynamic driving
force, resulting in shorter CSRP state lifetimes in more polar
solvents, proves that charge recombination processes for both
1 and 15 are occurring in the Marcus inVerted region.
In THF and PhCN, photoinduced electron transfer from ZnP
to C60 affords a CSRP state (ZnP•+/C60•-) with kCS on the order
of 1011 s-1 for 1 and 1010 s-1 for 15. For dyad 1, CSRP state
lifetimes were 100 and 60 ps in THF and PhCN, respectively.
In contrast, substantially longer CSRP state lifetimes were found
in the case of 15, namely, 550 ns in THF and 320 ns in PhCN.
It is particularly interesting that the kCS/kCR ratios derived from
charge separation and charge recombination data in PhCN were
more than 3 orders of magnitude greater for 15 (kCS/kCR ≈ 7400)
than those for 1 (kCS/kCR ≈ 7).
Fluorescence lifetimes were measured with a laser strobe
fluorescence lifetime spectrometer (Photon Technology Inter-
national) with 337 nm laser pulses from a nitrogen laser fiber
coupled to a lens-based T-formal sample compartment equipped
with a stroboscopic detector. Details of the systems are described
on the manufacturer’s Web site, http://www.pti-nj.com.
Emission spectra, all measured at room temperature, were
recorded on a SLM 8100 spectrofluorometer. Each spectrum
represents an average of at least five individual scans, with
appropriate corrections when necessary. Quantum yields were
measured as before relative to that of H2TPP (0.11).38
Photophysical Measurements (Tampere University of
Technology). Steady-state fluorescence spectra were measured
on a Fluorolog 3 spectrofluorimeter (ISA, Inc.) equipped with
a cooled IR-sensitive photomultiplier (R2658, Hamamatsu, Inc.).
The excitation wavelength was at the maximum of the Soret
It has been shown that porphyrins containing electron-
releasing aryl groups at the nonlinking meso positions have an
a2u HOMO,1 as is the case for both 1 and 15. Despite an
electronic distribution favoring through-bond communication
via a linking element extending from the meso position of the
porphyrin to C60, kET values are similar for 1 and 15. Since
dyads 1 and 15 exhibit CS rates of approximately the same order
of magnitude, the significantly higher kCS/kCR values for 15 are
obviously a consequence of much slower charge recombination
rates in this dyad. A possible explanation for these results is
significantly greater stabilization of the CSRP state in the case

14164 J. Phys. Chem. B, Vol. 110, No. 29, 2006
Vail et al.
band (426 nm). The emission spectra were recorded in the
wavelength range from 600 to 1000 nm.
Zn(II)-Tetraphenylporphyrin. H2-Tetraphenylporphyrin (300
mg) and Zn(OAc)2‚2H2O (800 mg) were heated at reflux in
300 mL of chloroform/methanol (2:1) for 1 h. The mixture was
cooled, and the solvent was removed under reduced pressure.
The product was redissolved in chloroform and eluted through
a short silica column with chloroform to furnish ZnTPP in near
quantitative yield. UV/vis (CHCl3): λmax 422, 551, 593 nm. 1H
NMR (400 MHz, CDCl3): δ 7.78 (d, 12H), 8.23 (d, 8H), 8.94
(s, 8 â-H) ppm. MALDI-TOF: m/z 678.38 [M+].
Ni(II)-Tetraphenylporphyrin. H2-Tetraphenylporphyrin (300
mg) and Ni(OAc)2‚4H2O (1 g) were heated at reflux in 300
mL of chloroform/methanol (2:1) for 16 h. The mixture was
cooled, and the solvent was removed under reduced pressure.
The product was redissolved in chloroform and eluted through
a short silica column with chloroform to furnish NiTPP in near
quantitative yield. UV/vis (CHCl3): λmax 422, 535 nm. 1H NMR
(400 MHz, CDCl3): δ 7.68 (d, 12H), 8.02 (d, 8H), 8.74 (s, 8
â-H) ppm. MALDI-TOF: m/z 671.71 [M+].
Ultrafast fluorescence decays were measured by an up-
conversion method as described previously.34 The instrument
(FOG100, CDP, Moscow, Russia) utilizes the second harmonic
(420 nm) of a 50 fs pulsed Ti:sapphire laser (TiF50, CDP,
Moscow, Russia) pumped by an Ar ion laser (Innova 316P,
Coherent). The samples were placed into rotating disk-shaped
1 mm cuvettes. Typical time resolution for the instrument was
150 fs (fwhm). Subnanosecond emission decays were studied
using a time-correlated single-photon-counting instrument de-
scribed elsewhere.51 Femtosecond to picosecond time-resolved
absorption spectra were collected using a pump-probe tech-
nique as described elsewhere.34 The femtosecond pulses of the
Ti:sapphire generator were amplified by using a multipass
amplifier (CDP-Avesta, Moscow, Russia) pumped by a second
harmonic of the Nd:YAG Q-switched laser (model LF114, Solar
TII, Minsk, Belarus). The amplified pulses were used to generate
the second harmonic (420 nm) for sample excitation (pump
beam) and white continuum for time-resolved spectrum detec-
tion (probe beam).
1-Ethynyl-2-methyl[60]fullerene 4. Ethynylfullerene 4 was
synthesized according to the procedure previously reported for
the preparation of the same compound.3 1H NMR (200 MHz,
CDCl3): δ 0.44 (s, 9H, TMS), 3.43 (s, 3H, CH3). MALDI-
TOF: m/z 832.99 [M+], 817.92 [M+ - CH3], 720.64 [C60].
Molecular Modeling Calculations. (X.C. and J.Z.H.Z.) All
computations were performed with the Gaussian 03 package,40
using Becke’s three-parameter hybrid exchange functional with
the Lee-Yang-Parr correlation functional (B3LYP)52-55 and
a LANL2DZ basis set. LANL2DZ, with effective core potentials
(ECPs), is a double-ú basis set containing ECP presentations
of near-nuclei electrons.56-59
Zn(II)-â-(Trimethylsilyl)butadiynyltetraphenylporphyrin 5.
To a solution of Zn(II)-â-ethynyltetraphenylporphyrin 12 (25
mg, 35.6 µmol) in dry CH2Cl2 (25 mL) were added (trimeth-
ylsilyl)acetylene (35 mg, 0.36 mmol) and CuCl (75 mg). The
mixture was stirred vigorously under a dry oxygen atmosphere
for 2 h followed by dropwise addition of TMEDA (0.8 mL).
The reaction mixture was vigorously stirred for 2 h under dry
oxygen and was filtered through a silica funnel to remove any
insoluble residue. The solvent was removed under reduced
pressure, and the crude porphyrin was redissolved in chloroform
and transferred onto a silica funnel. The porphyrin was washed
thoroughly on the funnel with hexanes and then eluted with
chloroform. The solvent was removed under reduced pressure
to afford 5 as a purple crystalline solid (26 mg, 92%). UV/vis
(CHCl3): λmax 435, 562, 603 nm. 1H NMR (400 MHz,
CDCl3): δ 1.30 (s, 9H Si-CH3), 7.68-7.72 (m, 12H), 8.11 (d,
2H), 8.18 (m, 6H), 8.84-8.97 (m, 6 â-H), 9.22 (s, 1 â-H) ppm.
MALDI-TOF: m/z 798.85 [M+].
Synthesis. General Information. All commercially available
reagents were used as received unless otherwise noted. Anhy-
drous toluene (containing <0.001% water) was purchased from
Aldrich and was used as received. Tetrahydrofuran was freshly
distilled over potassium, with benzophenone as an indicator,
prior to use. Methylene chloride was dried by distillation over
sodium metal and was used immediately following distillation.
All moisture-sensitive reagents were transferred via syringe or
double-tipped cannulae through rubber septa under an argon
atmosphere. Oxidative coupling reactions were performed in
one-neck round-bottom flasks equipped with a calcium sulfate
drying tube, protected from light. Whenever possible, the
progress of the reactions was monitored by TLC using precoated
glass plates (Silica gel 60, 0.25 mm thickness) with a fluorescent
indicator (254 nm). All spots were visualized using a shortwave
ultraviolet lamp (λ ) 254 nm). Column chromatography was
performed using Silica 60 from Sorbent Technology, Inc.
Preparative thin-layer chromatography was performed using
TLC standard grade silica gel (Aldrich), with gypsum binder
(particle size 2-25 µm, Brunauer-Emmett-Teller surface area
∼500 m2/g, pore volume 0.75 cm3/g, average pore diameter 60
Å).
H2-â-Bromotetraphenylporphyrin 6. To a solution of TPP
(425 mg, 0.69 mmol) in chloroform (350 mL) at reflux was
added N-bromosuccinimide (135 mg, 0.76 mmol). The course
of the reaction was monitored by TLC and was terminated when
the quantity of undesired 2,12(13)-dibromo-TPP began to
increase. The mixture was cooled and washed with a saturated
sodium bicarbonate solution until the wash water was slightly
basic. The crude product was dried with sodium sulfate,
concentrated, and chromatographed on a long silica column
using toluene/cyclohexane (1:1) as the eluent, affording H2-
â-bromoTPP 6 (306 mg, 64%). UV/vis (CHCl3): λmax 421, 518,
1H NMR spectra for 4 were acquired using a 200 MHz Varian
1
NMR spectrometer. H NMR spectra for ZnTPP, NiTPP, and
1
compounds 1-3, 5-10, and 12-14 were obtained using a 400
MHz Bruker-AV spectrometer. All spectra were measured in
CDCl3 and reported relative to a trimethylsilane internal standard
(0.1% v/v).
552, 594, 649 nm. H NMR (400 MHz, CDCl3): δ -2.81 (s,
2H), 7.7-7.82 (m, 12H), 8.10 (m, 2H), 8.21 (m, 6H), 8.78 (m,
2 â-H), 8.8-8.92 (m, 5 â-H) ppm. MALDI-TOF: m/z 694.17
[M+].
Mass spectra for all samples were obtained using a Bruker
Daltronics matrix-assisted laser desorption ionization time-of
flight (MALDI-TOF) mass spectrometer without a matrix. Mass
spectra for compounds 1-4 were obtained using pristine C60
as standard. Mass calibrations for ZnTPP, NiTPP, and com-
pounds 5-10 and 12-14 were performed using commercial
H2TPP. All absorption spectra were measured on a Hewlett-
Packard 8453 UV/vis spectrophotometer.
Zn(II)-â-Bromotetraphenylporphyrin 7. A solution of 6 (240
mg) and Zn(OAc)2‚4H2O (750 mg) in 2:1 chloroform/methanol
(240 mL) was heated at reflux for 1 h. The mixture was cooled,
and the solvent was removed under reduced pressure. The
product was redissolved in chloroform and eluted through a short
silica column with chloroform to furnish Zn(II)-â-bromoTPP
7 in quantitative yield. UV/vis (CHCl3): λmax 426, 555, 595
nm. 1H NMR (400 MHz, CDCl3): δ 7.66-7.84 (m, 12H), 8.02-

â-Alkynyl-Linked Porphyrin-C60 Dyads
J. Phys. Chem. B, Vol. 110, No. 29, 2006 14165
8.11 (m, 2H), 8.13-8.28 (m, 6H), 8.83-8.98 (m, 7 â-H) ppm.
carbonate, and dried with sodium sulfate. The solvent was
removed under reduced pressure to furnish 13 in near quantita-
tive yield. UV/vis (CHCl3): λmax 425, 521, 556, 598, 654 nm.
1H NMR (400 MHz, CDCl3): δ -2.82 (s, 2H), 3.28 (s, 1H CC-
H), 7.64 (d, 2H), 7.68-7.83 (m, 10H), 8.09 (d, 2H), 8.18 (m,
6H), 8.88 (m, 2 â-H), 8.91 (m, 4 â-H), 9.22 (s, 1 â-H) ppm.
MALDI-TOF: m/z 639.39 [M+].
MALDI-TOF: 757.59 [M+].
Ni(II)-2-Bromotetraphenylporphyrin 8. A solution of 6 (240
mg) and Ni(OAc)2‚4H2O (800 mg) in 2:1 chloroform/methanol
(240 mL) was heated at reflux for 16 h. The mixture was cooled,
and the solvent was removed under reduced pressure. The
product was redissolved in chloroform and eluted through a short
silica column with chloroform to furnish Ni(II)-â-bromoTPP
8 in near quantitative yield. UV/vis (CHCl3): λmax 419, 533
nm. 1H NMR (400 MHz, CDCl3): δ 7.58-7.71 (m, 12H), 7.86
(m, 2H), 7.98 (m, 6H), 8.63-8.76 (m, 6 â-H), 8.83 (s, 1 â-H)
ppm. MALDI-TOF: m/z 750.76 [M+].
Zn(II)-â-(Trimethylsilylethynyl)tetraphenylporphyrin 9. To
a deoxygenated solution of 7 (200 mg, 0.26 mmol) in dry
triethylamine (300 mL) and dimethylformamide (DMF, 30 mL)
were added bis(triphenylphosphine) palladium(II) chloride (150
mg), cuprous iodide (75 mg), and (trimethylsilyl)acetylene (128
mg, 1.3 mmol). The reaction mixture was heated at reflux for
16 h, cooled, and filtered through a short silica column using
chloroform as the eluent. The solvent was removed under
reduced pressure, and the residue was chromatographed over
silica gel using cyclohexane/CH2Cl2 (1:1) as theeluent, affording
9 (126 mg, 63%). UV/vis (CHCl3): λmax 431, 559, 597 nm. 1H
NMR (400 MHz, CDCl3): δ 1.29 (s, 9H Si-CH3), 7.67 (d,
2H), 7.70-7.82 (m, 10H), 8.12 (d, 2H), 8.21 (m, 6H), 8.78 (s,
1 â-H), 8.88 (m, 1 â-H), 8.91 (m, 4 â-H), 9.21 (s, 1 â-H) ppm.
MALDI-TOF: m/z 774.70 [M+].
Ni(II)-â-Ethynyltetraphenylporphyrin 14. To a solution of
11 (100 mg, 0.13 mmol) in THF (20 mL) was added TBAF
(410 mg, 1.56 mmol) with vigorous stirring. After 15 min,
glacial acetic acid (1 mL) was added, and the reaction mixture
was filtered through a silica funnel using CH2Cl2 as the eluent
to afford 34 (88 mg, 97%) as a purple crystalline solid. UV/vis
1
(CHCl3): λmax 423, 536 nm. H NMR (400 MHz, CDCl3): δ
3.20 (s, 1H CC-H), 7.58 (d, 2H), 7.64-7.72 (m, 10H), 7.89
(d, 2H), 7.98 (m, 6H), 8.68 (m, 1 â-H), 8.71 (m, 4 â-H), 8.80
(m, 1 â-H), 8.98 (s, 1 â-H) ppm. MALDI-TOF: m/z 695.94
[M+].
Zn(II)-â-Butadiynylporphyrin-C60 Dyad 1. To a solution
of 4 (20 mg, 26.2 µmol) and 12 (55 mg, 78.3 µmol) in
chlorobenzene (70 mL) at room temperature were added CuCl
(250 mg, 2.5 mmol) and TMEDA (0.4 mL) under a dry oxygen
atmosphere. The reaction mixture was stirred at room temper-
ature for 2 h and filtered through a silica funnel using CS2 the
as eluent. The solvent was removed under reduced pressure,
and the crude product was dissolved in CS2 and transferred onto
a silica funnel. Separation of the zinc porphyrin-C60 dyad from
the zinc porphyrin homodimer was accomplished by eluting the
silica funnel with CS2. The dyad was further purified by
preparative TLC on silica gel using CS2 the as eluent to furnish
dyad 1 (10.3 mg, 27%). UV/vis (CHCl3): λmax 257, 437, 563,
H2-â-(Trimethylsilyl)ethynyltetraphenylporphyrin 10. To a
solution of 9 (25 mg) in CH2Cl2 (15 mL) was added TFA (0.5
mL). The reaction mixture was stirred vigorously at room
temperature for 15 min, washed with water and saturated sodium
hydrogen carbonate, and dried with sodium sulfate. The solvent
was removed under reduced pressure to furnish 10 in near
quantitative yield. UV/vis (CHCl3): λmax 427, 522, 558, 598,
1
603 nm. H NMR (400 MHz, CDCl3): δ 3.42 (s, 3H, C60-
CH3), 7.67-7.88 (m, 12H), 7.98-8.24 (m, 8H), 8.81-8.97 (m,
6 â-H), 9.33 (s, 1 â-H) ppm.MALDI-TOF: m/z 1461.13 [M+].
H2-â-Butadiynylporphyrin-C60 Dyad 2. To a solution of 1
(6 mg) in CH2Cl2 (10 mL) was added TFA (0.2 mL). The
reaction mixture was stirred vigorously at room temperature for
10 min, washed with water, and dried with sodium sulfate. The
solvent was removed under reduced pressure, and the dyad was
further purified by preparative TLC on silica gel using CS2,
affording 2 (4.4 mg). UV/vis (CHCl3): λmax 256, 432, 526, 563,
1
655 nm. H NMR (400 MHz, CDCl3): δ -2.70 (s, 2H), 1.29
(s, 9H Si-CH3), 7.68 (d, 2H), 7.71-7.84 (m, 10H), 8.13 (d,
2H), 8.18-8.22 (m, 6H), 8.74 (m, 3 â-H), 8.82 (m, 1 â-H),
8.86 (m, 2 â-H), 9.06 (s, 1 â-H) ppm. MALDI-TOF: m/z 711.54
[M+].
Ni(II)-â-(Trimethylsilylethynyl)tetraphenylporphyrin 11. To
a deoxygenated solution of 8 (200 mg, 0.27 mmol) in dry
triethylamine (300 mL) and DMF (30 mL) were added bis-
(triphenylphosphine) palladium(II) chloride (150 mg), cuprous
iodide (75 mg), and (trimethylsilyl)acetylene (132 mg, 1.35
mmol). The reaction mixture was heated at reflux for 12 h,
cooled, and filtered through a short silica column using
chloroform as the eluent. The solvent was removed under
reduced pressure, and the residue was chromatographed over
silica gel using cyclohexane/CH2Cl2 (2:1) as the eluent, affording
impure 11, which was deprotected prior to further purification.
Zn(II)-â-Ethynyltetraphenylporphyrin 12. To a solution of
9 (100 mg, 0.13 mmol) in THF (20 mL) was added TBAF (410
mg, 1.56 mmol) with vigorous stirring. After 15 min, glacial
acetic acid (1 mL) was added, and the reaction mixture was
filtered through a silica funnel using CH2Cl2 as the eluent to
afford 12 (89 mg, 98%) as a purple crystalline solid. UV/vis
(CHCl3): λmax 429, 558, 598 nm. 1H NMR (400 MHz, CDCl3):
δ 3.28 (s, 1H CC-H), 7.64 (d, 2H), 7.68-7.83 (m, 10H), 8.09
(d, 2H), 8.18 (m, 6H), 8.88 (m, 2 â-H), 8.91 (m, 4 â-H), 9.22
(s, 1 â-H) ppm. MALDI-TOF: m/z 702.65 [M+].
1
603, 659 nm. H NMR (400 MHz, CDCl3): δ -2.62 (s, 2H),
3.51 (s, 3H, C60-CH3), 7.64-7.86 (m, 12H), 8.08-8.29 (m,
8H), 8.72-8.98 (m, 6 â-H), 9.21 (m, 1 â-H) ppm.MALDI-
TOF: m/z 1397.98 [M+].
Ni(II)-â-Butadiynylporphyrin-C60 Dyad 3. To a dry, oxy-
genated solution of 4 (15 mg, 19.7 µm) and 14 (41 mg, 59 µm)
in chlorobenzene (50 mL) were added CuCl (200 mg, 2 mmol)
and TMEDA (0.3 mL) at room temperature. The reaction
mixture was stirred at room temperature for 90 min and filtered
through a silica funnel using CS2 as the eluent. The solvent
was removed under reduced pressure, and the crude product
was dissolved in CS2 and transferred onto a silica funnel.
Separation of the nickel porphyrin-C60 dyad from the nickel
porphyrin homodimer was accomplished by eluting the silica
funnel with CS2. The dyad was further purified by preparative
TLC on silica gel using CS2 as the eluent to furnish dyad 3 (9
1
mg, 31%). UV/vis (CHCl3): λmax 257, 430, 541, 579 nm. H
NMR (400 MHz, CDCl3): δ 3.48 (s, 3H, C60-CH3), 7.63-
7.81 (m, 12H), 7.96-8.08 (m, 8H), 8.68-8.74 (m, 5 â-H), 8.82
(m, 1 â-H), 9.13(s, 1 â-H) ppm. MALDI-TOF: m/z 1454.56
[M+].
H2-â-Ethynyltetraphenylporphyrin 13. To a solution of 12
(25 mg) in CH2Cl2 (15 mL) was added TFA (0.5 mL). The
reaction mixture was stirred vigorously at room temperature for
15 min, washed with water and saturated sodium hydrogen
Acknowledgment. The work at New York University
(NYU) was supported by grants from the National Science

14166 J. Phys. Chem. B, Vol. 110, No. 29, 2006
Vail et al.
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Foundation (NSF), the American Chemical Society Petroleum
Research Fund, and a fund established by former members of
the Schuster laboratory. The studies at Clemson were supported
by grants from the NSF. This investigation at NYU was
conducted in part in a shared instrument facility constructed
with support from Research Facilities Improvement Grant No.
C06 RR-16572-01 from the National Center for Research
Resources, National Institutes of Health. The Bruker NMR
spectrometers were purchased using funds provided by Grant
No. CHE-0116222 under the NSF major instrument acquisition
program. D.M.G. gratefully acknowledges support from the
U.S. Department of Energy and Fonds der Chemischen Industrie.
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Supporting Information Available: UV/vis absorption
spectra for ZnTPP, zinc porphyrin 5, and zinc porphyrin 9,
differential absorption spectra for NiTPP and ZnP-C60 dyad
1, pump-probe decay component and time-resolved transient
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