Triazole bridges as versatile linkers in electron donor-acceptor conjugates

ARTICLE

pubs.acs.org/JACS

Triazole Bridges as Versatile Linkers in Electron DonorꢀAcceptor

Conjugates

Gustavo de Miguel,^‡Mateusz Wielopolski,^‡David I. Schuster,*^,†Michael A. Fazio,^†Olivia P. Lee,^†

Christopher K. Haley,^†Angy L. Ortiz,^||Luis Echegoyen,^{^}Timothy Clark,^§and Dirk M. Guldi*^,‡

^†Department of Chemistry, New York University, New York, New York 10003, United States

^‡Department of Chemistry and Pharmacy & Interdisciplinary Center for Molecular Materials, Friedrich-Alexander-Universit€at

Erlangen-N€urnberg, 91058 Erlangen, Germany

^§Computer-Chemie-Centrum, University of Erlangen, 91052 Erlangen, Germany

Department of Chemistry, Clemson University, Clemson, South Carolina 29634, United States

^{^}Department of Chemistry, University of Texas at El Paso, El Paso, Texas 79968-0519, United States

S Supporting Information

b

ABSTRACT: Aromatic triazoles have been frequently used as

π-conjugated linkers in intramolecular electron transfer pro-

cesses. To gain a deeper understanding of the electron-mediat-

ing function of triazoles, we have synthesized a family of new

triazole-based electron donorꢀacceptor conjugates. We have

connected zinc(II)porphyrins and fullerenes through a central

triazole moiety—(ZnPꢀTriꢀC₆₀)—each with a single change

in their connection through the linker. An extensive photophysical

and computational investigation reveals that the electron transfer

dynamics—charge separation and charge recombination—in the

diﬀerent ZnPꢀTriꢀC₆₀conjugates reﬂect a signiﬁcant inﬂuence of the connectivity at the triazole linker. Except for the m4m-

ZnPꢀTriꢀC₆₀17, the conjugates exhibit through-bond photoinduced electron transfer with varying rate constants. Since the

through-bond distance is nearly the same for all the synthesized ZnPꢀTriꢀC₆₀conjugates, the variation in charge separation and

charge recombination dynamics is mainly associated with the electronic properties of the conjugates, including orbital energies,

electron aﬃnity, and the energies of the excited states. The changes of the electronic couplings are, in turn, a consequence of the

diﬀerent connectivity patterns at the triazole moieties.

’ INTRODUCTION

most importantly low reduction potential and low reorganiza-

tion energy.⁹The third important unit in these systems is the

bridging unit, which provides the most signiﬁcant diﬀerences

between various artiﬁcial photosynthetic mimics.¹⁰While the

major emphasis of research in this area has involved covalently

linked donor-C₆₀systems,¹¹attention has been focused re-

cently on mechanically interlocked supramolecular donor-C₆₀

systems.¹²

While the donor and acceptor moieties of an artiﬁcial

photosynthetic system are absolutely crucial to the function of

the system, these two components alone are not suﬃcient to

achieve a long-lived charge-separated (CS) state. Fluorescence

spectroscopy shows that a mixture of donor and acceptor in

solution at micromolar concentrations does not signiﬁcantly lead

to quenching of the donor excited state by intermolecular

electronic interactions. Simply introducing a bridge between

the two units, however, has a drastic eﬀect on the electron

and energy transfer rates and eﬃciencies of the system.¹³

Natural photosynthesis has evolved to separate charges eﬃ-

ciently and use the potential of this charge imbalance to drive

reactions.^1,2Artiﬁcial photosynthetic mimics can imitate a charge

imbalance by manipulating the rates of charge separation (CS)

and charge recombination (CR).^3ꢀ6If the rate of intramolecular

CS (k_CS) is greater than the rate of intramolecular CR (k_CR), a

long-lived charge-separated state can be achieved. The Marcus

theory of electron transfer describes the parameters governing

electron transfer (ET) rates and provides the theoretical frame-

work for designing donorꢀacceptor systems capable of achieving

long-lived charge-separated states.⁷

Due to the complexity of the components of the ET chain in

natural photosynthesis, researchers have drastically simpliﬁed the

process using model systems. These artiﬁcial systems incorporate

an electron donor, an electron acceptor, and a bridging unit.³

The donor in many of these systems is a porphyrin or phthalocya-

ninedue to the relative ease of synthesis, stability, low ionization

potential, and excellent light absorption properties of these

materials.⁸The electron acceptor in many model systems

is [60]-fullerene (C₆₀) which has superior acceptor properties,

Received: March 18, 2011

Published: June 27, 2011

r

2011 American Chemical Society

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The bridging unit in artiﬁcial photosynthetic mimics performs

two functions: it acts as a conduit through which ET is facilitated,

and it serves to orient the two chromophores at set distances and

bond angles. These functions are seen in natural systems: the

donorꢀacceptor cofactors are embedded in the reaction center

and cell membrane at optimal distances from each other to carry

an electron down a potential gradient over a distance that makes

CR along the same path essentially impossible.¹Thus, the goal

of research into artiﬁcial photosynthetic systems is to transfer

the electron from the donor to the acceptor at a fast rate while

retarding the CR rate as much as possible, thus enhancing the

lifetime of the CS state. This is an important factor in applications

of organic materials in solar energy storage and photovoltaic

devices.^{2ꢀ6,8,10,12}

“Click chemistry” is a term widely used to deﬁne a particular

class of chemical reactions that have gained much attention from

the scientiﬁc community in the past decade.¹⁴The main attri-

butes of these reactions are reliability (high yield of intended

products), selectivity (little or no byproduct), functional group

tolerance, and simplicity of reaction conditions. Among the

chemical transformations that meet all of these requirements,

the copper-catalyzed azideꢀalkyne cycloaddition (CuAAC),

producing 1,2,3-triazoles, is considered to be a premier example.^12,15

As a result, the 1,2,3-triazole group has become a widely utilized

chemical linker that provides a straightforward way to covalently

connect precursor fragments containing azide and alkyne groups.¹⁶

The great versatility of the “click” cycloaddition reaction has led to

a wide range of applications in, for example, bioconjugation,¹⁷

materials science,¹⁸and drug discovery.¹⁹In particular, the CuAAC

has been used in the synthesis of dendronized linear polymers

containing photoactive molecules²⁰and in the functionalization of

the surface of single-wall carbon nanotubes with polystyrene to

solubilize them in a variety of organic solvents.²¹These examples

represent interesting applications in molecular electronics, which

justiﬁes our interest in determining the electronic properties of the

1,2,3-triazole linker in greater detail. In addition, aromatic triazoles

have recently been shown to act as conjugative π-linkers in intra-

molecular ET processes,^22,23a property we exploit in the present

study by exploring the use of 1,2,3-triazoles as the linkers in a new

series of ZnP-C₆₀electron donorꢀacceptor conjugates.

Figure 1. Structures of the triazole-linked ZnP-C₆₀conjugates high-

lighting the diﬀerent connectivity patterns.

’ RESULTS AND DISCUSSION

1. Synthesis. The Huisgen 1,3-dipolar cycloaddition between

an azide and a terminal alkyne results in formation of a 1,2,3-

triazole ring. The uncatalyzed reaction produces both 1,4- and

1,5-disubstituted isomers, while the copper(I)-catalyzed reac-

tion, a class of the so-called “click reactions”, is regioselective for

the 1,4-disubstituted isomer.^14,15The CuAAC reaction is synthe-

tically very useful due to its regioselectivity, the low amount of

side products, high yields, short reaction times (especially in a

microwave apparatus),²⁵high tolerance toward functional groups

on the reactants, and low toxicity of solvents used. The final step

in the projected synthesis of the target zinc(II)porphyrinꢀ

triazoleꢀfullerene (ZnPꢀTriꢀC₆₀) conjugates involves a second

1,3-dipolar cycloaddition reaction to C₆₀, the well-known Prato

reaction, via an intermediate azomethine ylide.²⁶

The series of conjugates depicted in Figure 2 were synthesized

incorporating a 1,4-disubstituted 1,2,3-triazole linker. In naming

these compounds, the ﬁrst letter designates the connection of

the alkyne or azide group to the phenyl ring on the porphyrin. The

number indicates whether the porphyrin is attached to the

triazole ring at the 1 or 4 position. The second letter denotes

the orientation of the substituents on the phenyl ring attached to

the fulleropyrrolidine.

The overall synthetic scheme is depicted in Figure 3. The

synthesis of the monoethynyl porphyrins was carried out by a

Lindsey mixed-aldehyde condensation²⁷using 3,5-di-tert-butyl-

benzaldehyde, tetraphenylphosphonium chloride, and either

3-(trimethylsilyl)ethynyl-benzaldehyde (1) or 4-(trimethylsilyl)-

ethynyl-benzaldehyde (2) in dichloromethane under an argon

atmosphere. Addition of pyrrole and boron triﬂuoride diethyl

etherate aﬀorded the porphyrinogen, which was then oxidized to the

corresponding porphyrin using 2,3-dichloro-5,6-dicyano-1,4-benzo-

quinone (DDQ). Insertion of zinc(II) into the porphyrin using

Zn(OAc)₂in methanol and subsequent removal of the trimethylsilyl

protecting group on the alkyne with tetrabutylammonium ﬂuoride

(TBAF) yielded alkynylporphyrins m-ZnP (3) and p-ZnP (4).

A series of triazole-linked zinc(II)porphyrinꢀfullerene

(ZnPꢀTriꢀC₆₀) conjugates was synthesized, each with one

change in mode of connection through the linker (Figure 1).

We report here on the eﬀects of the phenyl ring connectivities

and triazole orientation on the rates of CS and CR processes.

Diﬀerences have been observed in past work both in attachment

sites on phenyl rings and in orientation of a conjugated linker.²⁴

By combining the structural variations of these three moieties, we

hoped to be able to ﬁne-tune rates of CS and CR processes

and to extend the CS state lifetimes beyond that of fully

conjugated systems.¹⁰Copper-catalyzed Huisgen 1,3-dipolar

cycloaddition was used to selectively synthesize 1,4-disubstituted

1,2,3-triazoles, which then serve as covalent linkers between the

photo- and redoxactive porphyrins and fullerenes. Molecular model-

ing calculations were carried out to assess whether through-space

and/or though-bond electron transfer mechanisms are feasible in

these systems. Finally, photophysical studies using steady-state ab-

sorption and emission techniques and femtosecond and nanosecond

transient absorption spectroscopy were used to determine absolute

rate constants for the intramolecular processes occurring after

photoexcitation of the ZnPꢀTriꢀC₆₀conjugates.

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Figure 2. ZnPꢀTriꢀC₆₀conjugates: m4m (17), m4p (18), p4p (19), p1p (20), p1m (21), and p4m (22).

This mixed-aldehyde condensation reaction aﬀorded a number

of porphyrins whose ratios are controlled by statistics, steric

factors, and reaction conditions. The most prevalent porphyrins

formed from this reaction are A₄, A₃B, and A₂B₂, where A and B

represent the two starting aldehydes. Due to diﬃculties in

isolating the desired A₃B porphyrins on the scale required for

subsequent reactions, the unreactive nature of the tetraphenyl A₄

porphyrin toward azides, and the small amount of reactive A₂B₂

porphyrins present, puriﬁcation of m-alkynylꢀZnP 3 and

p-alkynylꢀZnP 4 by column chromatography was only carried

out on a small scale (see Experimental Section). The puriﬁed A₃B

alkynyl porphyrins were used for yield determinations, spectral

analysis, and photophysical studies.

Meta- and para-azidobenzaldehydes (5 and 6, respectively)

were synthesized by reacting the corresponding bromobenzal-

dehydes with sodium azide, copper iodide, sodium ascorbate,

and N,N⁰-dimethylethylenediamine according to a published

procedure.²⁸The progress of the reaction was monitored by

the appearance of the characteristic azide absorption band near

2100 cm^ꢀ1in the Fourier-transform IR (FTIR) spectrum (see

Figure S1, Supporting Information).

The CuAAC between alkynylporphyrins and azidobenzalde-

hydes was performed under microwave irradiation. An argon-

purged solution of alkynyl porphyrins 3 or 4, azides 5 or 6,

copper iodide, and sodium ascorbate in 9:1 dimethylsulfoxide/

water (DMSO/H₂O) was heated in a microwave reactor

(80 ꢀC, 50 W) for 30 min. Standard workup (see Experimental

Section) and column chromatography provided pure ZnPꢀTri

products—m4m (7), m4p (8), and p4p (9)—in yields ranging from

9% to quantitative. Since the yield of 9in the uncatalyzed reaction was

low (9%), a reaction was carried out between 4 and 6 in the presence

of 1 equiv of the additive tris(benzotriazolylmethyl)amine (TBTA),²⁹

whereupon the yield of 9 improved from 9% to 40%.

The ZnPꢀTri adducts were characterized by MALDI-TOF

1

mass spectrometry and H NMR. Mass spectra display peaks

for the molecular ion and M-28 resulting from loss of nitrogen

(Figure S2, Supporting Information). The most signiﬁcant

changes in the NMR spectra are the appearance of a singlet at

8.3ꢀ8.6 ppm (proton on triazole ring) and the disappearance of

the singlet at 3.1ꢀ3.3 ppm (alkynyl proton) (Figure 4).

The synthesis of p-azidophenylporphyrin³⁰(12) also utilized

the Lindsey mixed-aldehyde method, substituting p-acetamido-

benzaldehyde for ethynylbenzaldehyde (1). The resulting

p-acetamidophenylporphyrin³¹(10) was hydrolyzed³²to the

p-aminophenylporphyrin (11), which was then converted using

sodium nitrite and sodium azide, via the diazonium salt, to the

p-azidophenylporphyrin (12). This porphyrin and either

p- or m-ethynylbenzaldehyde³³(13 or 14) were then used in a

microwave-assisted CuAAC in the presence of TBTA²⁹to

aﬀord p1p- or p1m-ZnPꢀtri-benzaldehyde (15 or 16) in 84%

yields.

Finally, the ZnPꢀTriꢀC₆₀conjugates 17ꢀ21 were synthesized

by the Prato 1,3-dipolar cycloaddition to C₆₀of the azomethine

ylides derived from the respective porphyrin-triazole-benzaldehydes

and sarcosine. The ﬁrst Prato reactions attempted followed the

conditions of the standard references²⁶using 1 equiv of C₆₀for

every 2 equiv of aldehyde. Thin-layer chromatography (TLC)

showed complete consumption of C₆₀, but the conjugates showed

similar R_fas the ZnPꢀTri precursors, causing diﬃculties in

isolating the desired conjugates. Changing to 2 equiv of C₆₀for

every equivalent of the aldehyde provided two major beneﬁts: (1)

chromatographic puriﬁcation from the excess C₆₀was made easier

by requiring only one short column rather than multiple long

columns, saving time and resources, and (2) the product yield

was doubled as the less available ZnPꢀTri precursor became

the limiting reagent. The ZnPꢀtriazoleꢀfulleropyrrolidine products

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Figure 3. General scheme for the synthesis of m4m-, m4p-, p4p-, p1p-, p4m-, and p1m-ZnPꢀTriꢀC₆₀conjugates.

were characterized by MALDI-TOF mass spectrometry, which

showed peaks for the conjugates minus N₂and for the

The assignment of peaks in the ¹H NMR spectrum of m4m-

ZnPꢀTriꢀC₆₀(17) (Figure S6, Supporting Information) was

aided by its COSY NMR spectrum (Figure 5) by ﬁrst assigning

peaks to the simpler p4p-ZnPꢀTriꢀC₆₀(19) (Figures S9 and

S11, Supporting Information). It was then possible to make

assignments in the more complicated NMR spectrum of

34

conjugates minus C₆₀+ N₂(Figures S3, S4, and S5, Supporting

1

Information), H NMR (Figures S7, S8, and S9, Supporting

Information), and correlation spectroscopy (COSY) NMR

(Figure 5 and Figures S10 and S11, Supporting Information).

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Figure 4. Top: ¹H NMR spectrum of m-alkynylꢀZnP (3). The ethynyl proton appears at 3.1 ppm. Bottom: ¹H NMR spectrum of m4m-ZnPꢀTri (7).

The triazole proton appears at 8.35 ppm.

2. Theoretical Calculations. Molecular modeling provides

strong evidence favoring a through-bond electron transfer

(ET) mechanism via the triazole linker in all but one of the ZnPꢀ

TriꢀC₆₀conjugates. As a first step, it was important to ascertain

whether through-space interactions in 17ꢀ22 are feasible due

to their structural properties. Thus, the geometries of the six

different conjugates were optimized, and the center-to-center

donorꢀacceptor distances were determined. Since the structural

flexibility of the molecules rendered it difficult to obtain reason-

able starting geometries, the initial molecular models were

constructed with the shortest possible donorꢀacceptor distance

with respect to the van der Waals radii of the atoms. These

geometries were then relaxed using different quantum chemical

optimization methods involving semiempirical calculations and

density functional theory (DFT). For semiempirical calculations,

the PM3³⁵and AM1³⁶Hamiltonians were used. The DFT cal-

culations were performed using the M062X³⁷functional and the

6-31G*³⁸basis set with LANL2DZ³⁹pseudopotentials on the zinc

atoms. Superpositioning the geometries obtained by the different

computational methods resulted in root-mean-square deviation

(rmsd) values between 0.11 and 0.36 Å, which represents a very

good match between the PM3, AM1, and M062X structures. The

center-to-center distances obtained at the different levels of cal-

culations for all geometries varied in the range of e0.5 Å (Table 1).

In cases where the distance between donor and acceptor

1

Figure 5. COSY H NMR spectrum of m4m-ZnPꢀTriꢀC₆₀17 in

CDCl₃.

moieties is small enough to allow for direct overlap between

the frontier orbitals of the donor and acceptor, ET may occur by a

m4p-ZnPꢀTriꢀC₆₀(18) (Figures S8 and S10, Supporting

Information), aided by its COSY spectrum, in which similar

chemical shifts are observed for protons on the meta-substituted

phenyl ring on the porphyrin. The m4m-ZnPꢀTriꢀC₆₀¹H

NMR spectrum also shows better resolution of the β-porphyrinic

protons (8.8ꢀ9.1 ppm) than was observed in the other con-

jugates and their precursors.

through-space mechanism. When the donors and acceptors are

too far apart for direct orbital overlap to be signiﬁcant, ET may

occur via a superexchange or hopping mechanism governed by

through-bond interactions. Diﬀusionless through-space ET is

known to occur at distances of 4ꢀ10 Å since only then are the

electron donors and acceptors close enough to allow for eﬃcient

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Table 1. Calculated Center-to-Center Distances in Å between ZnP and C₆₀as Computed by Diﬀerent Computational Methods

ARTICLE

m4m-

ZnPꢀ

TriꢀC₆₀17

m4p-

ZnPꢀ

TriꢀC₆₀18

p1m-

ZnPꢀ

TriꢀC₆₀21

p1p-

ZnPꢀ

TriꢀC₆₀20

p4m-

ZnPꢀ

TriꢀC₆₀22

p4p-

ZnPꢀ

TriꢀC₆₀19

AM1

10.57

10.88

9.78

14.34

14.83

14.33

16.01

16.10

16.69

18.91

19.05

19.37

15.33

15.29

15.32

18.59

18.84

19.12

PM3

M062X

Table 2. Calculated Orbital Energies in eV (M062X/6-31G*//LANL2DZ) of the ZnPꢀTriꢀC₆₀Conjugates, ZnP and C₆₀

m4m-ZnPꢀ

TriꢀC₆₀17

m4p-ZnPꢀ

TriꢀC₆₀18

p1m-ZnPꢀ

TriꢀC₆₀21

p1p-ZnPꢀ

TriꢀC₆₀20

p4m-ZnPꢀ

TriꢀC₆₀22

p4p-ZnPꢀ

TriꢀC₆₀19

ZnP

C₆₀

HOMO-4

HOMO-3

HOMO-2

HOMO-1

HOMO

ꢀ8.0

ꢀ6.1

ꢀ6.0

ꢀ1.7

0.3

ꢀ7.5

ꢀ7.1

ꢀ7.0

ꢀ6.5

ꢀ3.0

ꢀ2.6

ꢀ2.3

ꢀ1.7

ꢀ1.2

ꢀ7.1

ꢀ6.6

ꢀ6.1

ꢀ5.9

ꢀ3.1

ꢀ2.7

ꢀ2.4

ꢀ1.8

ꢀ1.7

ꢀ7.1

ꢀ6.6

ꢀ6.1

ꢀ5.9

ꢀ3.0

ꢀ2.6

ꢀ2.4

ꢀ1.7

ꢀ7.0

ꢀ6.5

ꢀ6.2

ꢀ6.1

ꢀ3.0

ꢀ2.6

ꢀ2.3

ꢀ1.8

ꢀ7.0

ꢀ6.9

ꢀ6.4

ꢀ6.2

ꢀ6.1

ꢀ2.9

ꢀ2.5

ꢀ2.3

ꢀ1.8

ꢀ7.2

ꢀ7.1

ꢀ6.6

ꢀ6.1

ꢀ5.9

ꢀ3.1

ꢀ2.7

ꢀ2.5

ꢀ1.8

ꢀ1.6

ꢀ7.1

ꢀ6.6

ꢀ6.1

ꢀ5.9

ꢀ3.1

ꢀ2.7

ꢀ2.4

ꢀ1.8

ꢀ1.7

LUMO

LUMO+1

LUMO+2

LUMO+3

LUMO+4

0.8

N atom (ꢀ6.1 eV), e.g., p1m-ZnPꢀTriꢀC₆₀(21) vs p4m-ZnPꢀ

TriꢀC₆₀(22) and p1p-ZnPꢀTriꢀC₆₀(20) vs p4p-ZnPꢀTriꢀC₆₀

(19). On the other hand, higher LUMO energies (ꢀ2.9 eV) were

found for the conjugates in which the ZnP moiety is attached to the

N atom rather than the C atom (ꢀ3.1 eV) of the triazole. The varia-

tion of the HOMO/LUMO gap goes along with the diﬀerences in

ground-state conjugation imposed by the variation of the con-

nectivity pattern at the triazoles. Lowering the HOMO/LUMO

gap increases the electronic coupling, which, in turn, increases the

charge transfer in the conjugates. In general, the HOMOꢀLUMO

gaps scale linearly with the donorꢀacceptor distance.

As noted above, through-space orbital overlap in m4m-ZnPꢀ

TriꢀC₆₀17 is present due to the donorꢀacceptor proximity.

Thus, HOMO coeﬃcients are found not only on ZnP but also

on C₆₀(Figure 7).

Local electron aﬃnity (EA) and semiempirical excited state

conﬁguration interaction (CI) calculations were used to account

for the variation of ET rates in the ZnPꢀTriꢀC₆₀conjugates.

The photophysical studies to be discussed below conﬁrm the

formation of radical ion pairs in all six systems. Charge separation

and charge recombination rates in all but one of the conjugates

varied with the dielectric constant (molecular permittivity) of the

solvent, as expected for a through-bond ET process. Through-

space ET processes lack such a solvent dependence, as seen for

the m4m conjugate. The through-bond donorꢀacceptor dis-

tance, as determined from the bond lengths, is invariant in all of

the ZnPꢀTriꢀC₆₀conjugates. Hence, the variation of the

connectivity pattern on the triazole ring imposes diﬀerent

structural arrangements of the bridge moieties, which impact

the through-bond electronic coupling. In general, meta as

opposed to para connectivity imposes bending of the bridge

and favors a conformation in which ZnP is in closer proximity to

C₆₀. The variation of the nitrogen atom positions in the triazole

moiety, that is, whether ZnP is attached at position 1 (nitrogen)

Figure 6. Optimized geometry of the m4m-ZnPꢀTriꢀC₆₀conjugate

17 resulting from DFT calculations.

orbital overlap. Such a situation is feasible only in m4m-ZnPꢀ

TriꢀC₆₀17 with its face-to-face arrangement (see Figure 6). In all

the other conjugates, the conﬁguration of the bridge does not allow

donorꢀacceptor distances to be closer than 14 Å. Hence, through-

space ET is likely to occur only in 17, while in all other systems

through-bond ET dominates.

The analysis of molecular orbitals (MOs) conﬁrms the

donorꢀacceptor character of the ZnPꢀTriꢀC₆₀conjugates.

In all systems, the highest occupied MO (HOMO) is localized

on the ZnP donor and the lowest unoccupied MO (LUMO) on

the C₆₀acceptor. Importantly, the energies of the orbitals depend

on (1) the positions of nitrogen in the triazole ring and (2) the

substitution pattern at the bridge, that is, meta vs para (Table 2). In

particular, the HOMO energy is higher (ꢀ5.9 eV) when the ZnP

moiety is attached to the C atom of the triazole ring and not to the

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Figure 7. Coeﬃcients of the HOMO (bottom) and LUMO (top) of m4m-ZnPꢀTriꢀC₆₀(17) and p1p-ZnPꢀTriꢀC₆₀(20) as computed by DFT

methods, demonstrating through-space interactions in 17.

or position 4 (carbon), aﬀects the electronic properties of

the bridge. Local EA calculations provided insight into these

ﬁndings. EA maxima are found on all nitrogen atoms (Figure S12,

Supporting Information), and a homogeneous ET pathway

through the bridge is found in all of the conjugates. However,

the pronounced local EA maxima on the nitrogens suggest that

the triazoles are, to some extent, electronically isolated and may

function as intermediate electron acceptors. Nevertheless, C₆₀

remains the ultimate electron acceptor as seen from the color

code, and the triazoles may serve to lower the charge injection

barrier into the bridge.

A pathway of high EA values can only be realized in the

presence of homogeneous π-conjugation, which again favors

through-bond interactions (Figure S12, Supporting Information).

To analyze the structural diﬀerences in ZnPꢀTriꢀC₆₀conju-

gates 17ꢀ22 quantitatively, we have compared the local EA

values on the triazole rings in the six diﬀerent conﬁgurations.

Interestingly, with meta-connectivity and ZnP attachment to the

triazole carbon, the EA values on the triazole ring increase as

compared to para-connectivity or ZnP attachment to the triazole

nitrogen (Table S1, Supporting Information).

The results of the theoretical studies agree nicely with the

accelerated electron transfer dynamics found in the photophysi-

cal studies described below. With higher EA at the triazole sites,

lower energy states are available. These energy states may be

occupied by the electrons to be transferred, facilitating through-

bond ET by lowering the charge injection barrier. When the C₆₀

is connected to the triazole nitrogen, higher EA values appear on

the carbon atom directly connected to the ZnP, as compared to

the alternative pattern (see Figure S12, Supporting Information).

Such a situation provides favorable coupling between the triazole

and the adjacent phenyl ring on the porphyrin, resulting in

accelerated ET. Thus, the electron-accepting features may aﬀect

the lone pair electrons of the substituted nitrogen atom and

induce electron deﬁciency at the adjacent carbon. CI calculations

were performed to corroborate these ﬁndings. It was possible to

determine the energies of the charge-transfer (CT) states for all

ZnPꢀTriꢀC₆₀conjugates. Analysis of their heats of formation in

relation to the ground state as a function of donorꢀacceptor

distance results in a linear relationship (Figure S13, Supporting

Information). Higher dipole moments are induced at larger

through-bond donorꢀacceptor distances, leading to increased

activation barriers for the formation of the radical ion pair

and, hence, to higher values of ΔH_f. This underlying dependence

reﬂects the trends of the charge transfer rate constants deter-

mined in the photophysical studies.

Imaging the electrostatic potentials of the CT states on the van

der Waals surface of the molecules corroborates the ﬁndings of

the local EA investigations, namely, the electron-accepting

features of the triazole units. In all systems, the positive charge

resides on the ZnP moiety, while the negative charge is partially

distributed over the C₆₀framework and the triazole atoms.

Calculation of the partial charges on these residues indicates

that the ZnP accommodates +1.1 e^ꢀ, whereas ꢀ0.3 e^ꢀand ꢀ0.6 e^ꢀ

reside on the triazoles and C₆₀, respectively. Thus, the higher

the EA value on the nitrogen atoms, the more charge they can

accommodate. These results suggest that the triazole may serve

as a secondary electron acceptor in these systems, which is

further conﬁrmed by considering additional excited states. Bridge

charge transfer (BCT) states of true CT character were dis-

covered by calculations including the solvent (Figure 8). With

increasing solvent polarity, their energies decrease, as shown in

Figure S14 (Supporting Information). In all these BCT states,

the positive charge is located on both C₆₀and the triazole.

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Figure 8. Molecular electrostatic potentials (red to blue: positive to

negative) from CI calculations for the CT (left) and BCT (right) states

of m4m-ZnPꢀTriꢀC₆₀(17), p4m-ZnPꢀTriꢀC₆₀(22), and p1p-ZnPꢀ

TriꢀC₆₀(20).

Figure 10. Cyclic voltammograms of ZnPꢀTriꢀC₆₀conjugates

17ꢀ20 in dichloromethane containing TBAPF₆(0.1 mol dm^ꢀ3) with

a sweep rate of 100 mV s^ꢀ1

.

Because of the weak conjugation in the ground state, HOMO and

LUMO energies remain close to those of the individual ZnP and

C₆₀energies, as shown in Table 2. Correlation between these

ﬁndings and the EA and CI calculations suggests that the most

eﬃcient ET occurs when the electronic perturbation in the excited

state does not aﬀect the conjugation and the electronic coupling in

the ground state. Attaching ZnP directly to the triazole nitrogen

interrupts the conjugation because the CT resonance structure

has cationic character duetoformationof a quaternary ammonium

center.⁴⁰This break in conjugation is reﬂected by the mixing of

BCT and CT states. Despite the fact that para-connectivity

provides more eﬃcient electronic coupling in the ground state,

meta-connectivity may improve the charge separation features due

to enhanced through-space coupling induced by the signiﬁcantly

shorter donorꢀacceptor distances.

Figure 9. Energy diﬀerence between the CT and BCT states and the

ground state as a function of donorꢀacceptor distance, as determined

from CI calculations, shows increasing mixing between CT and BCT

states upon increasing the donorꢀacceptor distance.

3. Electrochemical Studies: Study of Ground State Inter-

actions. The redox properties of ZnPꢀTriꢀC₆₀conjugates

(17ꢀ20), ZnPꢀTri references (7, 8, 9, 15, and 16), and

reference compound alkynyl porphyrins 3 and 4 were studied

by means of differential pulse voltammetry (DPV) and cyclic

voltammetry (CV). The measurements were carried out in

anhydrous dichloromethane (DCM) using tetra-n-butylammo-

nium hexafluorophosphate (TBAPF₆) as the supporting electro-

lyte and ferrocene/ferrocinium (Fc/Fc⁺) as the internal

reference. The half-wave potentials, E_1/2, were determined as

(E_pa+ E_pc)/2, where E_paand E_pcare the anodic and cathodic

peak potentials determined from the CV measurements. All

potentials obtained from DPV were converted to E_1/2values

using the formula E_max= E_1/2ꢀ (ΔE/2), where ΔE is the pulse

amplitude (50 mV).⁴¹

Figure 10 and Figure S15 (Supporting Information) show the

CV and DPV data for ZnPꢀTriꢀC₆₀conjugates 17ꢀ20.⁴²

Each of these compounds shows two reversible one-electron

oxidation processes. The ﬁrst oxidation peak is attributed to

the ZnP group (7ꢀ9 and 15), which was shifted cathodically

(70, 10, 10, and 20 mV for compounds 17, 18, 19, and

20, respectively, compared to the ZnP references 3 and 4). On

the reduction side, the materials exhibit at least four peaks each.

Figure 9 shows that with increasing donorꢀacceptor distance

the energies of the BCT and CT states approach each other,

suggesting mixing of these states with increasing distance upon

photoexcitation. In this situation, BCT states may act as traps

and, depending on their energies, provide an alternative CT

pathway. When the BCT states are energetically well-separated

from the CT states, photoexcitation will result in direct popula-

tion of the CT state (Figure 8, left); however, when the energies

of the BCT and CT states approach each other, the “true” CT

state results from a mixing of the CT (Figure 8, left) and BCT

(Figure 8, right) electronic distributions. In such cases, the CT

mechanism involves localization of charge on the linker, which

decelerates the charge separation and also the charge recombina-

tion processes.

Thus, the CI calculations clearly demonstrate that ET occurs

quite eﬃciently but depends on the substitution pattern on the

triazole ring. Connecting the phenyl moieties at diﬀerent posi-

tions at the triazole impacts the ground-state conjugation,

as reﬂected by the variation of the HOMO and LUMO energies.

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Table 3. Electrochemical Potentials E/V vs Fc/Fc⁺Measured in Dichloromethane Containing TBAPF₆(0.1 mol dm^ꢀ3) as

Supporting Electrolyte

reduction potentials (V)

oxidation potentials (V)

compound

m-ZnP 3

E¹

E²

E³

E⁴

E⁵

E⁶

E⁷

E¹

E²

HOMOꢀLUMO^b(V)

ꢀ1.87^a

ꢀ1.98 ^a

ꢀ1.84^a

ꢀ1.86^a

ꢀ1.85^a

ꢀ1.81^a

ꢀ1.93

ꢀ2.22

0.32^a

0.29^a

0.68^a

0.69^a

2.19

2.16

p-ZnP 4

2-NMFP 23^43a

ꢀ1.07^a

ꢀ1.45^a

m4m-ZnPꢀTri 7

m4p-ZnPꢀTri 8

p4p-ZnPꢀTri 9

p1p-ZnPꢀTri 15

m4m-ZnPꢀTriꢀC₆₀17

m4p-ZnPꢀTriꢀC₆₀18

p4p-ZnPꢀTriꢀC₆₀19

p1p-ZnPꢀTriꢀC₆₀20

24^43b

ꢀ2.15

ꢀ2.10

ꢀ2.09

ꢀ2.18

ꢀ2.08

ꢀ2.02

ꢀ2.04

ꢀ2.07

ꢀ1.99

0.30^a

0.31^a

0.33^a

0.23 ^a

0.29 ^a

0.30 ^a

0.31 ^a

0.34 ^a

0.67^a

0.65^a

0.67^a

0.66^a

0.67 ^a

0.69 ^a

0.68 ^a

0.70 ^a

0.69 ^a

2.14

2.16

2.14

1.39

1.41

1.42

1.44

1.41

ꢀ2.24

ꢀ1.16 ^a

ꢀ1.12^a

ꢀ1.13^a

1.07^a

ꢀ1.54^a

ꢀ1.49^a

ꢀ1.54

ꢀ1.90

ꢀ1.67

ꢀ1.90

ꢀ1.02

ꢀ1.87^a

ꢀ1.82 ^a

ꢀ1.45^a

^aDenote E_1/2potentials. Range 60ꢀ92 mV of anodic-to-cathodic peak separation. ^bHOMOꢀLUMO: ΔE = E_{1 oxidation}ꢀ E₁

.

st

reduction

The ﬁrst and the second reversible one-electron reductions

correspond to the fulleropyrrolidine moiety, while the third

irreversible one-electron reduction appears to be centered on

ZnP, based on the values obtained for the ZnP and ZnPꢀTri

references (Figure S16, Supporting Information, and Table 3).

Considering the structure of compounds 17ꢀ20, in which the

triazole group is connected to the carbon of the pyrrolidine ring,

the ﬁrst reduction potential of each conjugate was compared with

the well-known N-methyl-2-pyridylfulleropyrrolidine 23 re-

ported previously (Figure S17, Supporting Information).^43a

The potential had shifted cathodically compared with the ﬁrst

reduction potential (ꢀ1.07 V) of 23 (90, 50, 50, and 60 mV for

compounds 17, 18, 19, and 20, respectively). This shift indicates

that the conjugates are slightly harder to reduce than 23.^43aThe

third reduction peak for 17, 18, 19, and 20 was shifted cath-

odically by 90, 40, 50, and 60 mV, respectively, with respect to the

corresponding ZnPꢀTri references. This cathodic shift can be

assigned to the electronic coupling between C₆₀and the por-

phyrin. The two extra reduction peaks in the DPV of 20 (Figure

S15, Supporting Information) are unassigned and may be due to

contamination with a small amount of pristine C₆₀. The CV and

DPV curves for 17ꢀ20 closely correspond to the sum of the

independent electrochemical features of the individual C₆₀and

ZnP moieties with some potential value shifts as described above.

Table 3 reports all the electrochemical potentials for this

series of ZnPꢀTriꢀC₆₀conjugates and the corresponding refer-

ence compounds. It can be observed that the HOMOꢀLUMO

gaps of all conjugates are similar (1.39ꢀ1.44 V) and in the range

(1.41 V) previously reported for azobenzene-linked porphyrinꢀ

fullerene dyad 24 (Figure S17, Supporting Information).^43b

4. Photophysical Properties: Study of Excited State Inter-

actions. 4.1. Absorption Spectroscopy. To provide insight into

possible ground state electronic interactions between the photo-

and redox-active moieties, namely, ZnP and C₆₀, the absorp-

tion spectra of the ZnPꢀTriꢀC₆₀conjugates 17ꢀ22 were

compared with those of the individual constituents. The dominating

Soret- and Q-bands of ZnP are seen in the 400ꢀ450 nm and

550ꢀ650 nm regions, respectively, while the strongest C₆₀bands

the exception of m4m-ZnPꢀTriꢀC₆₀17, the absorption spectra of

the conjugates are best described as simple superimpositions of the

component spectra, lacking notable perturbations or additional

charge transfer transitions (see Figure S18, Supporting Informa-

tion). These results indicate that no significant interchromophoric

interactions exist in the electronic ground state of these conjugates.

A close inspection of the Soret band of m4m-ZnPꢀTriꢀC₆₀17

in toluene shows 2 nm red-shift along with peak broadening

(Figure 11). These spectral changes are also evident in the more

polar solvents THF and PhCN. A comparison of the Q-band

regions of m4m conjugate 17 and m4m-ZnPꢀTri 15 revealed no

new absorption bands in the case of 17. Consequently, formationof

a charge transfer (CT) state between ZnP and C₆₀accounts for the

Soret band changes in the m4m conjugate, although the absence of

a detectable CT absorption band suggests rather weak electronic

coupling. The shorter distance between ZnP and C₆₀in m4m-

ZnPꢀTriꢀC₆₀17, ∼10 Å, compared to the other conjugates, >14

Å, facilitates electronic interaction in the ground state. This trend

resembles that of previously reported similar short-distance por-

phyrinꢀfullerene hybrids,^24a,44in which donorꢀacceptor separa-

tions shorter than 10 Å are crucial for CT state formation.

4.2. Fluorescence Spectroscopy. Fluorescence studies were

carried out with the ZnPꢀTriꢀC₆₀conjugates 17ꢀ22 in solvents

of different polarity (i.e., toluene, THF, and benzonitrile) using

solutions with identical optical density (0.2) at the excitation

wavelengths of 425 and 432 nm. The conjugates and the

ZnPꢀTri references exhibit fluorescence maxima at 599/

648 nm in toluene, 602/655 nm in THF, and 610/665 nm in

benzonitrile (see Figure 12 for the spectra of p1m conjugate 21).

The fluorescence quantum yields of the ZnPꢀTri references

(Φ_f= 0.04) are invariant in these three solvents. In the ZnPꢀ

TriꢀC₆₀conjugates, the ZnP fluorescence is significantly

quenched, with quantum yields ranging from 0.0017 to 0.0085

(for details see Table 4), which translates to quenching factors of

up to 23 compared to the ZnP reference compounds. Although

the ZnPꢀTriꢀC₆₀conjugate quantum yields decrease, the ZnP

emission patterns are not affected by the presence of the

electron-accepting C₆₀moiety (Figure 12, lower).

appear at 220, 265, and 330 nm and extend all the way to the

energetically lowest-lying transition at 690 nm (see Figure 12). With

Finally, the near-infrared region was probed in terms of possible

CT emissions. Only for m4m-ZnPꢀTriꢀC₆₀(17) and only in

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Figure 11. Absorption (left) spectra of m4m-ZnPꢀTri 7 (black line) and m4m-ZnPꢀTriꢀC₆₀17 (red line) and emission (right) spectra of 17 in

toluene.

Table 4. Photophysical Data of ZnPꢀTriꢀC₆₀Conjugates in Solution at Room Temperature

m4m-

m4p-

p1p-

p4p-

p4m-

ZnPꢀ

TriꢀC₆₀22

p1m-

ZnPꢀ

TriꢀC₆₀21

ZnPꢀ

TriꢀC₆₀17

ZnPꢀ

TriꢀC₆₀18

ZnPꢀ

TriꢀC₆₀20

ZnPꢀ

TriꢀC₆₀19

feature

solvent

Φ_fꢁ 10³

toluene

THF

2.6

3.0

7.5

8.5

6.6

4.0

4.1

2.2

1.7

4.7

4.9

BzCN

toluene

THF

3.2

8.5

3.7

2.4

1.7

4.7

τ_f[ns]

0.24

0.20

0.22

6.3

0.32

0.31

0.36

1.9

0.39

0.19

0.20

2.2

0.25

0.20

0.21

3.8

0.12

0.18

0.10

9.8

0.25

0.12

0.13

3.3

BzCN

toluene

THF

k_cs[s^ꢀ1] ꢁ 10^{9 a}

5.4

1.6

3.9

7.5

9.8

3.1

BzCN

toluene

THF

5.0

1.6

4.2

6.8

9.8

3.3

τ_s[ns]

0.25

0.20

0.09

3.6

0.34

0.24

0.08

2.5

0.42

0.17

0.09

1.9

0.20

0.10

0.09

4.6

0.08

0.05

12.1

19.6

419

405

2.4

0.11

0.06

0.07

8.7

BzCN

toluene

THF

k_cs[s^ꢀ1] ꢁ 10^{9 b}

4.6

3.7

5.5

9.6

16.2

13.9

656

485

1.5

BzCN

THF

10.7

418

420

2.4

12.1

522

440

2.2

10.7

844

646

1.2

10.7

645

560

1.5

τ_csrp[ns]

BzCN

THF

k_cr[s^ꢀ1] ꢁ 10^-6

BzCN

2.4

1.5

1.8

2.5

2.1

^ak_cswas determined from the ﬂuorescence quantum yields via

k_cs¼ ½Φ_{ðreferenceÞ}ꢀ Φ_{ðconjugateÞ}ꢂ=½τ

Φ

_{ðconjugateÞ}ꢂ

ð1Þ

ðreferenceÞ

^bk_cswas determined from the time decay curves via

k_cs¼ 1=τ_{ðconjugateÞ}ꢀ 1=τ

ð2Þ

ðreferenceÞ

toluene was a CT-type emission detected (see Figure 11). The

aforementioned results corroborate the absence of interactions

between ZnP and C₆₀in the ground state. For 17, the fact that the

CT emission band was only detected in toluene suggests very

weak coupling between the active units. CT emission has been

reported to occur in ZnPꢀC₆₀conjugates with similar do-

norꢀacceptor distances in a variety of solvents,^24a,44which

suggests that the particular orientation of the components in

our ZnPꢀTriꢀC₆₀conjugates might prevent the formation of

CT states.

Changing the solvent polarity from toluene (ε = 2.38) via THF

(ε = 7.6) to benzonitrile (ε = 24.8) leads to intensiﬁcation of the

ﬂuorescence quenching, particularly for p1p-ZnPꢀTriꢀC₆₀(20)

and p4p-ZnPꢀTriꢀC₆₀(19), while the remaining conjugates

exhibit much weaker dependence on solvent polarity. The sizable

ﬂuorescence quenching and its trend as a function of solvent

polarity for 20 and 19 are attributed to an intramolecular ET

process. Thus, the latter must occur through the connecting bridge

since the center-to-center distances between the ZnP and C₆₀

moieties in the most stable conformations of these conjugates

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deactivation by intramolecular energy transfer (EnT) competes

with charge separation (ET). Such an energy transfer scenario

leads to the most intense C₆₀ﬂuorescence in the case of p4m-

ZnPꢀTriꢀC₆₀(22), possibly due to a particularly favorable

orientation of the dipole moments since the center-to-center

distance is midrange.

Based upon the relationship between the ﬂuorescence quan-

tum yields in the conjugates (Φ_(conjugate)), the ﬂuorescence

quantum yields of ZnP (Φ_(reference)), and the ﬂuorescence life-

times of the ZnP's (τ_(reference)), one can determine the rate

constants for ﬂuorescence deactivation (Table 4). In general, the

rates are extremely fast, indicating strong electronic coupling

between the donors and acceptor moieties. The relationship

between charge separation rates and structure is interesting.

Larger k_csvalues were observed for the ZnPꢀTriꢀC₆₀conju-

gates with ZnP linked to the C atom of the triazole ring, namely,

p4m-ZnPꢀTriꢀC₆₀(22) and p4p-ZnPꢀTriꢀC₆₀(19), than in

the corresponding conjugates with ZnP linked to the N atom,

p1m-ZnPꢀTriꢀC₆₀(21) and p1p-ZnPꢀTriꢀC₆₀(20), respec-

tively. Thus, the connectivity pattern between ZnP and C₆₀

strongly aﬀects the rates of charge separation in these conjugates.

The magnitude of ET quenching can be independently

determined through ﬂuorescence lifetime measurements, by

monitoring the ZnP ﬂuorescence decays at 605 and 660 nm.

Importantly, the ﬂuorescence lifetime experiments allow separa-

tion of the contributions of ZnP from those of C₆₀. In all cases,

the ﬂuorescence decays were best ﬁt by a single exponential

function. For all ZnPꢀTri references, a lifetime of 2.1 ns was

observed, corresponding to a decay rate of ∼5 ꢁ 10⁸s^ꢀ1. As

shown in Table 4, all six conjugates exhibit ﬂuorescence lifetimes

between 0.1 and 0.4 ns, corresponding to decay rates of 2.5 ꢁ 10⁹

to 1 ꢁ 10¹⁰

s

^ꢀ1. The dependence of the ﬂuorescence lifetimes on

connectivity pattern and solvent agrees with the results from the

steady-state ﬂuorescence experiments.

In summary, both steady-state and time-resolved ﬂuorescence

data indicate that a very rapid decay of the ZnP singlet excited

state prevails in all six ZnPꢀTriꢀC₆₀conjugates.

4.3. Transient Absorption Spectroscopy. Insight into the

nature of the processes following photoexcitation of the ZnPꢀ

TriꢀC₆₀conjugates 17ꢀ22 was obtained using femtosecond

and nanosecond transient absorption spectroscopy. Photoexcita-

tion of the ZnPꢀTri references (7, 8, 9, 15, and 16) at 387 and

355 nm leads to the appearance of spectral features of ZnP singlet

excited states. In toluene, the ZnP singlet excited states are

characterized by the bleaching of the Q-band absorption at

550 nm and the appearance of broad absorption between 570

and 750 nm. In THF and benzonitrile, the bleaching of the

Q-band absorption shifts to 565 nm. The formation of the ZnP

singlet excited state (E_singlet= 2.00 eV) occurs on the picosecond

time scale (1 ꢁ 10¹²s^ꢀ1). The ZnP singlet is then deactivated

via a slow intersystem crossing process (4.0 ꢁ 10⁸s^ꢀ1) to the

energetically lower-lying triplet excited state (E_triplet= 1.53 eV)

(Figure S19, Supporting Information). The transient absorption

spectra of the ZnPꢀTri references revealed a component that

rises in ∼ 6 ps, which is slightly shorter in the less polar solvents

—THF and toluene. This component might be ascribed to an

excited state conformational change or intermolecular vibra-

tional cooling by dissipation of excess energy with the solvent.⁴⁵

For the ZnPꢀTriꢀC₆₀conjugates 17ꢀ22, the strong ZnP

singletꢀsinglet absorption (570ꢀ750 nm) grows in immediately

after laser excitation (k ∼ 1 ꢁ 10¹²s^ꢀ1), indicating the success-

ful formation of the ZnP singlet excited states in the presence

Figure 12. Top: absorption spectra at 298 K of p1m-ZnPꢀTri 16

(black line) and p1m-ZnPꢀTriꢀC₆₀21 (gray line) in benzonitrile.

Middle: ﬂuorescence spectra at 298 K of p1m-ZnPꢀTriꢀC₆₀21 in

toluene (light gray lines), THF (dark gray line), and benzonitrile (black

line), λ_exc= 425 nm. Bottom: ﬂuorescence spectra at 298 K of p4p-

ZnPꢀTri 9 (black line) and p4p-ZnPꢀTriꢀC₆₀19 (gray line) in

benzonitrile, λ_exc= 432 nm.

exceed 10 Å, preventing a direct through-space ET process. Only

in m4m-ZnPꢀTriꢀC₆₀(17), where the center-to-center distance

barely reaches 10 Å (vide supra), is a through-space ET process

possible. However, the possibility that a through-bond ET may

also occur to some extent in 17 cannot be excluded.

In toluene, all ZnPꢀTriꢀC₆₀conjugates feature a weak

ﬂuorescence maximum at 710 nm, corresponding to emission

from the C₆₀singlet excited state, which indicates transduction of

singlet excited state energy from ZnP to C₆₀. In this solvent,

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Figure 13. Top: transient absorption spectra (visible and near-infrared)

observed upon femotosecond excitation (387 nm, 180 mJ) of p1m-

ZnPꢀTriꢀC₆₀21 in benzonitrile at room temperature with time

delays between 0 and 3000 ps. Bottom: timeꢀabsorption proﬁles of the

spectra on the left at 620 nm, monitoring the kinetics of charge separation.

of C₆₀. In these systems, the singletꢀsinglet absorption decays

with accelerated dynamics instead of the slow intersystem cross-

ing dynamics exhibited by the ZnP reference materials. The

singlet excited state lifetimes τ_s, determined from an average of

ﬁrst-order ﬁts of the timeꢀabsorption proﬁles at various wave-

lengths, are listed in Table 4. The values agree well with the

lifetimes τ_fderived from the ﬂuorescence decay experiments.

However, the diﬀerential absorption spectra observed after

completion of the ZnP singlet excited state decay bear no resem-

blance to those of the ZnP triplet excited state.⁴⁶The new

1

features, which develop simultaneously with ZnP* decay, in-

Figure 14. Transient absorption spectra in the visible (top) and near-

infrared (middle) regions observed upon femtosecond ﬂash photolysis

(387 nm, 150 mJ) of m4p-ZnPꢀTriꢀC₆₀(18) in toluene at room

temperature with time delays between 0 and 3000 ps. Bottom: timeꢀ

absorption proﬁles of the spectra on the left at 480 nm, monitoring the

kinetics of charge separation.

clude (1) broad absorption in the 600ꢀ700 nm region and (2)

absorption in the near-infrared region with a maximum near

1020 nm (see Figures 13 and 14). The ﬁrst absorption band

corresponds to the transient absorption of the one-electron

oxidized form of ZnP (ZnP^•+),⁴⁷and the latter absorption is the

signature of the one-electron reduced form of C₆₀(C₆₀^•ꢀ).⁴⁸

These results indicate that the charge-separated radical ion-pair

•ꢀ

The spectral features of the ZnP^•+ꢀTriꢀC₆₀state persist

(CSRP) state ZnP^•+ꢀTriꢀC₆₀is generated from the ZnP

on the picosecond time scale. Determination of the CSRP

lifetimes required the use of nanosecond transient absorption

spectroscopy. Upon nanosecond ﬂash excitation of the conju-

gates at 355 nm in oxygenated solutions, it was possible to follow

the decay of the C₆₀^•ꢀabsorption at 1000 nm and that of ZnP^•+

in the 600ꢀ800 nm range (Figure 15).⁴⁹Fitting the decay

proﬁles with single exponential kinetics aﬀorded value for the

CSRP lifetimes τ_csrpin the submicrosecond time domain

•ꢀ

singlet excited state in all six ZnPꢀTriꢀC₆₀conjugates. However,

owing to the close proximity between ZnP and C₆₀in m4m-

ZnPꢀTriꢀC₆₀17, formation of a very short-lived exciplex inter-

mediate cannot be ruled out. Nevertheless, the initial rise in

C₆₀^•ꢀ, which is seen in all of the ZnPꢀTriꢀC₆₀conjugates,

prompts us to invoke evolution of charge separation from a

local excited state.

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Figure 16. Energy level diagram, proposed decay pathways, and rate

constants in s^ꢀ1for p1p-ZnPꢀTriꢀC₆₀20 following excitation at the

Soret band in THF. Energy values are derived from transient absorption

and electrochemical experiments. k_ET= rate constant for electron

transfer; k_CR= rate constant for charge recombination; and k_ISC= rate

constant for intersystem crossing.

derived from the crossing of the absorption and emission spectra,

is 2.05 eV.⁴⁶The energy of the CSRP state can be derived from

the theoretical calculations, namely, the HOMOꢀLUMO gaps

(1.5 eV), or from electrochemical measurements (1.4 eV), as the

diﬀerence between the ﬁrst ZnP oxidation potential and the ﬁrst

C₆₀reduction potential, with solvent correction, according to the

Weller equation.⁵⁰The reorganization energies λ are assumed

to be the same for all six ZnPꢀTriꢀC₆₀conjugates since, with

the exception of m4m-ZnPꢀTriꢀC₆₀(17), similar lowest energy

conformations for these isomeric conjugates were found by

theoretical calculations. These considerations leave the electronic

coupling as the only variable in evaluating changes in the CR and

CS dynamics. To determine the subtle inﬂuence of the connectivity

pattern on electronic coupling between ZnP and C₆₀, the detailed

theoretical analysis described earlier was undertaken.

For conjugates with para-connectivity, the dependence of

ﬂuorescence quantum yields and lifetimes of the CSRP states

on the solvent polarity is consistent with a through-bond ET

mechanism. However, for conjugates with meta-connectivity, the

solvent dependence is weaker and becomes negligible in the case

of 17. The shorter donorꢀacceptor separation in meta-linked

conjugates may induce additional through-space coupling.

Considering the linkage of ZnP to either the C or N atom of

the triazole, an increase of the charge separation rate is observed

when the electron-donating ZnP is linked to the carbon atom of

the triazole ring rather than to nitrogen. This result is in agree-

ment with the theoretical analysis, where it was found that linking

the ZnP to the carbon atoms lowers the HOMO/LUMO gap and

therefore facilitates charge injection into the bridge. It also

explains the accelerated charge separation dynamics. Note that

the eﬀective center-to-center distances in these conjugates are

nearly constant, 19.4 vs 19.1 Å for p4p-ZnPꢀTriꢀC₆₀(19) and

p1p-ZnPꢀTriꢀC₆₀(20) and 15.3 vs 16.7 Å for p4m-ZnPꢀTriꢀ

C₆₀(22) and p1m-ZnPꢀTriꢀC₆₀(21), respectively. Moreover,

a decrease in the charge recombination rate can be observed

when the electron-donating ZnP is linked to the nitrogen atom of

the triazole ring rather than to the triazole carbon, namely, 1.8 ꢁ

10^ꢀ6vs 1.5 ꢁ 10^ꢀ6s^ꢀ1for p4p 19 and p1p 20 and 2.5 ꢁ 10^ꢀ6vs

2.1 ꢁ 10^ꢀ6s^ꢀ1for p4m 22 and p1m 21 in benzonitrile,

respectively. The calculated EA values on the triazole rings for

each conjugate correlate well with the CR dynamics. Only the

behavior of conjugate 18 deviates from the theoretical trend, as

its k_crvalue is comparable to that of p4m conjugate 22, despite its

higher EA value.

Figure 15. Top: transient absorption spectra (visible and near-infrared)

observed upon nanosecond ﬂash photolysis (355 nm) of p1m-

ZnPꢀTriꢀC₆₀21 in benzonitrile at room temperature with time delays

of 250, 400, 800, and 1500 ns. Bottom: timeꢀabsorption proﬁles of the

spectra on the left at 1000 nm, monitoring the charge recombination process.

(400ꢀ850 ns) as given in Table 4. All ZnPꢀTriꢀC₆₀con-

jugates, with the signiﬁcant exception of m4m-ZnPꢀTriꢀC₆₀

(17), had shorter CSRP lifetimes in the more polar solvent

benzonitrile than in THF. Conjugate m4p-ZnPꢀTriꢀC₆₀

(18), for instance, shows CSRP lifetimes of 522 and 440 ns

in THF and benzonitrile, respectively. The longest lifetimes

were observed for p1p-ZnPꢀTriꢀC₆₀20 with values of 844

and 646 ns in benzonitrile and THF, respectively. Remark-

ably, for 17 there is no diﬀerence between the CSRP lifetimes

in THF and in benzonitrile, which indicates that through-

space ET is occurring due to the close proximity of the

ZnP and C₆₀moieties (vide supra). An energy level diagram

showing the decay pathways upon excitation of p1p-

ZnPꢀTriꢀC₆₀(20) at 425 nm is shown in Figure 16.

The thermodynamic driving force for charge recombination

(CR) ꢀΔGꢀ_etdecreases as the solvent polarity increases due to

stabilization of the CSRP state. Faster CR kinetics in solvents of

higher polarity is indicative of inverted Marcus behavior, i.e., to a

process occurring in the region of the Marcus parabola where the

ET rates decrease as the values of ꢀΔGꢀ_etincrease.⁷In this

context, there are three major parameters that govern the ET

kinetics at a given temperature: the Gibbs free energy for the ET

process (ΔGꢀ_et), the reorganization energy λ, and the electronic

coupling parameter V. For the present conjugates, ΔGꢀ_etcan be

determined from the relative energies of the ZnP singlet excited

state and of the CSRP state. The ZnP singlet excited state energy,

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Comparison between the para vs meta substitution pattern on

the triazole-linked phenyl rings shows that higher electron

aﬃnity (EA) values (see above) are found in the case of meta

substitution on both sides of the triazole ring, suggesting that

the ET process is facilitated in meta-linked conjugates due to

available lower-energy states. Focusing on the CR process,

the k_crvalues of the conjugates are larger for meta than para

connectivity, namely, 2.1 ꢁ 10^ꢀ6vs 1.5 ꢁ 10^ꢀ6s^ꢀ1for p1m-

ZnPꢀTriꢀC₆₀(21) and p1p-ZnPꢀTriꢀC₆₀(20) and 2.5 ꢁ

10^ꢀ6vs 1.8 ꢁ 10^ꢀ6s^ꢀ1for p4m-ZnPꢀTriꢀC₆₀(22) and p4p-

ZnPꢀTriꢀC₆₀(19) in benzonitrile, respectively. Thus, incor-

porating meta connectivity accelerates the charge recombination

process. This eﬀect is similar to the changes that are imposed by

the variation in the ZnP linkage to the triazole, i.e., whether it is

connected via a nitrogen or carbon atom.

The variation of connectivity at the triazole moieties signiﬁ-

cantly aﬀects the electronic properties of the conjugates, as

reﬂected by the electron aﬃnity calculations. Thus, we propose

that the values of EA at the triazole linker strongly aﬀect elec-

tronic coupling among the six ZnPꢀTriꢀC₆₀conjugates; i.e., the

higher the EA value, the stronger the coupling. On this basis, we

conclude that the scale of electronic coupling in the conjugates is

as follows: p1p-ZnPꢀTriꢀC₆₀(20) < p1m-ZnPꢀTriꢀC₆₀(21)

e p4p-ZnPꢀTriꢀC₆₀(19) < p4m-ZnPꢀTriꢀC₆₀(22).

as the lock. The spectra were collected at 25 ꢀC, and chemical

shifts (δ, ppm) were referenced to the tetramethylsilane internal

standard (0.1% v/v). In the assignments, the chemical shift

(in ppm) is given first, followed, in brackets, by multiplicity

(s, singlet; d, doublet; t, triplet; m, multiplet; br, broad), the value

of the coupling constants in Hz if applicable, the number of

protons implied, and finally the assignment. Mass spectra were

obtained on an Agilent 1100 Series Capillary LCMSD Trap XCT

spectrometer in positive- or negative-ion mode and a Thermo-

Finnigan PolarisQ ion-trap GCMS spectrometer. MALDI-TOF

mass spectra were recorded in a Bruker OmniFLEX MALDI-

TOF MS spectrometer. This instrument was operated at an

accelerating potential of 20 kV in linear mode. The mass spectra

represent an average over 256 consecutive laser shots. The mass

scale was calibrated using the matrix peaks and the calibration

software available from Bruker OmniFLEX. Values of m/z cor-

respond to monoisotopic masses. MALDI-TOF mass spectra

were obtained without using a matrix unless otherwise noted.

MALDI-TOF mass calibrations were performed using tetra[3,5-

di(tert-butyl)phenyl]porphyrin and pristine C₆₀. All chemicals

were purchased from Sigma-Aldrich and used without further

purification. The progress of the reactions was monitored by

thin-layer chromatography (TLC) whenever possible. TLC was

performed using precoated glass plates (Silica gel 60, 0.25 mm

thickness) containing a 254 nm fluorescent indicator. Column

chromatography was carried out using Merck Silica gel 60

(0.063ꢀ0.200 mm) and neutral alumina (Brockmann I, acti-

vated, 150 mesh, 58 Å). Infrared spectra were obtained using a

Thermo Nicolet Avatar 360 FTIR spectrophotometer.

2. Electrochemical Studies. The samples were all analyzed in

oxygen-free anhydrous dichloromethane (DCM). Tetrabutylam-

monium hexafluorophosphate (TBAPF₆) was added as support-

ing electrolyte. TBAPF₆was recrystallized two times from

ethanol and dried under vacuum for 24 h prior to use. Cyclic

voltammetry (CV) and differential pulse voltammetry (DPV)

were performed with a CH Instruments potentiostat under an

argon atmosphere at room temperature at a scan rate of 100 mV

s^ꢀ1and pulse rate of 0.05 s with increments of 4 mV and an

amplitude of 50 mV. A standard three-electrode setup was used,

consisting of a glassy carbon working electrode (Cypress, 1.0

mm), a platinum wire auxiliary electrode (Aldrich, 1.0 mm), and

a nonaqueous silver/silver nitrate (Ag/AgNO₃) reference elec-

trode (calibrated externally versus the ferrocene/ferrocenium

(Fc/Fc+) redox couple). The half-wave potential, E_1/2, was

determined as (E_pa+ E_pc)/2, where E_paand E_pcare the anodic and

cathodic peak potentials from the CV. Additionally, E_1/2was corrob-

’ CONCLUSIONS

A series of ZnPꢀTriꢀC₆₀conjugates have been synthesized

using “click chemistry” from electron donor and acceptor frag-

ments containing terminal alkyne and azide functionality. The

photophysical properties of these conjugates clearly demonstrate

that 1,2,3-triazoles represent excellent bridges for rapid and

eﬃcient photoinduced electron transfer (ET) between remote

electron donor and acceptor moieties, in the present case ZnP

and C₆₀, respectively. The photophysics of the ZnPꢀTriꢀC₆₀

conjugates also reﬂect a signiﬁcant inﬂuence of the substitution

pattern of the bridge, most prominently with respect to the

dynamics of charge recombination. Except for m4m-ZnPꢀ

TriꢀC₆₀(17), all the conjugates undergo through-bond ET pro-

cesses, for both charge separation and charge recombination. For

17, the proximity of the ZnP and C₆₀moieties allows for the

occurrence of through-space ET processes. The long lifetimes of

the charge-separated radical ion-pair (CSRP) states of these

conjugated materials, in the range of 415ꢀ850 ns, in THF and

benzonitrile, show that charge recombination occurs in the

Marcus inverted region.^3ꢀ7,9Similar submicrosecond CSRP

lifetimes have been observed for other types of covalently linked

ZnPꢀC₆₀hybrids studied by us, with polyether, alkyne, and azo

linkers.^10b,10c,43Since the through-bond distance is nearly equal

in all the conjugates, the variation in charge separation and charge

recombination dynamics in the ZnPꢀTriꢀC₆₀conjugates must

be ascribed to changes in the electronic properties of these

materials, including orbital energies, electron aﬃnity, and the

energies of the excited states. The changes in the electronic

couplings are, in turn, a consequence of the diﬀerent connectivity

patterns at the 1,4-diphenyl-1,2,3-triazole linker.

orated by DPV experiments using the equation: E_max(DPV)= E_1/2

(ΔE/2), where ΔE is the pulse amplitude (50 mV).

ꢀ

3. Photophysical Studies. All solvents used were spectro-

scopic grade (99.5%) and were purchased from Sigma-Aldrich.

Femtosecond transient absorption studies were performed with

387 and 420 nm laser pulses (1 kHz, 150 fs pulse width) from an

amplified Ti:Sapphire laser system (model CPA 2101, Clark-

MXR Inc.). Nanosecond Laser Flash Photolysis 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. Fluorescence lifetimes were measured using

a Fluorolog (Horiba Jobin Yvon). Steady-state fluorescence mea-

surement were performed using a Fluoromax 3 (Horiba Jobin

Yvon). The experiments were performed at room temperature.

4. Synthesis. Synthesis of 3-(Trimethylsilyl)ethynylbenzalde-

hyde (1). Bis(benzonitrile)palladium(II) chloride (0.325 mmol,

’ EXPERIMENTAL SECTION

1. General Information and Materials. NMR spectra were

obtained on either a Bruker AVANCE 400 (400 MHz) or

AVANCE 800 (800 MHz) spectrometer using CDCl₃solvents

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130 mg) and copper(I) iodide (0.216 mmol, 42 mg) were added

to a flask which was then sparged with argon and charged with

dioxane (11 mL). In a separate flask, tri-tert-butylphosphine

(0.60 g) was added to dioxane (11 mL) to create a 0.27 M

solution. A 2.2 mL aliquot of this solution was added to the flask

containing the palladium and copper catalysts, turning the

solution dark. Diisopropylamine (13 mmol, 1.8 mL), 3-bromo-

benzaldehyde (10.8 mmol, 1.26 mL), and trimethylsilylacetylene

(13 mmol, 1.8 mL) were all added to the reaction flask by syringe.

After ∼30 min the solution bacame warm, and a precipitate

formed, turning the reaction mixture nearly solid. The reaction

was run for 7 h before being diluted with ethyl acetate and filtered

through a short silica funnel. The product was purified on a silica

column using a 1:1 mixture of hexanes and methylene chloride as

the eluent to afford a yellow oil in quantitative yield. ¹H NMR

(400 MHz, CDCl₃): δ =10.0 ppm (s, 1H, CHO), 7.9 ppm (s, 1H,

ArH), 7.8 ppm (d, 1H, ArH), 7.7 ppm (d, 1H, ArH), 7.5 ppm

(t, 1H, ArH), 0.3 ppm (s, 9H, TMS).

Synthesis of Zinc(II)-5-(3-ethynylphenyl)-10,15,20-tris-(3,5-

di-tert-butylphenyl) Porphyrin (m-ZnP) (3) and Zinc(II)-5-(4-

ethynylphenyl)-10,15,20-tris-(3,5-di-tert-butylphenyl) Porphyr-

in (p-ZnP) (4). Tetraphenylphosphonium chloride (0.19 mmol,

71 mg), 3-TMS-ethynylbenzaldehyde (1), or 4-TMS-ethynyl-

benzaldehyde (2) (15.3 mmol, 3.08 g) and 3,5-di-tert-butylben-

zaldehyde (45.8 mmol, 10 g) were dissolved in a flask containing

dichloromethane (600 mL). The flask was sealed and purged

with argon. Freshly distilled pyrrole (61.1 mmol, 4.2 mL) and

boron trifluoride diethyl etherate (6 mmol, 1 mL of 1 M solution

in CH₂Cl₂made from 8.1 M stock solution) were added by

syringe. The reaction flask was protected from light using

aluminum foil and stirred at room temperature for 1 h. The

resulting porphyrinogen was oxidized by adding 2,3-dichloro-

5,6-dicyano-p-benzoquinone (DDQ) (45.9 mmol, 10.56 g) and

stirring overnight. Triethylamine (6 mmol, 0.84 mL) was added

to neutralize the solution which was then filtered through a pad of

silica using CH₂Cl₂as eluent. The solution was concentrated,

and the porphyrin was then metalated by adding an excess of

iodide (0.1 g, 0.54 mmol), and EtOH/H₂O (10 mL, 7:3 ratio)

were added to a flask which was then degassed with argon for

10 min. N,N⁰-Dimethylethylenediamine (0.9 mL, 0.81 mmol)

was added to the flask, and the solution turned blue. The reaction

was heated at reflux for 1.5 h and monitored by FTIR for the

appearance of the azide band near 2100 cm^ꢀ1. After cooling to rt,

water was added to the solution which was then extracted with

50 mL of methylene chloride. The product was purified on a silica

column using methylene chloride. Some unreacted starting

material remained which could not be easily separated due to

similar R_f. The presence of residual bromobenzaldehyde was not

a problem because it does not react in the following reaction

to form triazoles and can be separated afterward. The ratios of

product to starting material were obtained by ¹H NMR and used

in calculations of quantities of reagents needed for the triazole

reaction.

NOTE. In retrospect, the use of N,N⁰-dimethylethylenediamine

as a catalyst in this case is problematic, as it can form an imine

with the aldehyde, which was actually observed in later studies.

Hydrolysis after the reaction would increase the final product

yield. No aldehydes were tested with respect to imine formation

in the cited reference 28.

1

3-Azidobenzaldehyde (5). 88% purity by H NMR. 30%

1

overall yield. H NMR (400 MHz, CDCl₃): 10.0 ppm (s, 1H,

CHO), 7.6 ppm (d, 1H, ArH), 7.5 ppm (m, 2H, ArH), 7.2 ppm

(d, 1H, ArH).

1

4-Azidobenzaldehyde (6). 37% purity by H NMR. 30%

1

overall yield. H NMR (400 MHz, CDCl₃): 9.95 ppm (s, 1H,

CHO), 7.9 ppm (d, 2H, ArH), 7.15 ppm (d, 2H, ArH). Note: the

spectrum shows peaks for CH₂Cl₂(5.3 ppm), acetone (2.1

ppm), and toluene (2.3 ppm, 7.1 ppm, 7.2 ppm).

General Procedure for Synthesis of Zinc Porphyrin Triazoles

(7, 8, 9). In a small round-bottomed flask were added alkynyl-

porphyrin (3 or 4, 813 mg, 0.78 mmol), azidobenzaldehyde (5 or

6, 115 mg, 0.78 mmol), copper(I) iodide (15 mg, 78 μmol), and

sodium ascorbate (31 mg, 0.16 mmol). A 9:1 mixture of DMSO/

H₂O (30 mL) was added, and the flask was purged with argon.

The reactions were run in a CEM Discover LabMate microwave

reactor at a power setting of 50 W for 30 min. The reaction was

cooled constantly during the reaction with a stream of air blown

Zn(OAc)₂2H₂O (3.5 g) in MeOH (35 mL) and heating at

3

reflux for 1 h. Solvent was evaporated and the product dissolved

in CHCl₃/THF (1:1, 250 mL). Tetrabutylammonium fluoride

(TBAF) (1.0 M solution in THF, 90 mL) was added, and the

solution was stirred at rt for 1 h. Glacial acetic acid (25 mL) was

added, and the product was washed over a silica funnel with

CHCl₃. Yield was ∼5 g (31%).

ο

into the reaction chamber to maintain a temperature of 80 C.

Microwave power adjusts automatically to help maintain the set

reaction temperature. The product was extracted using CH₂Cl₂

and purified by column chromatography (silica gel, CH₂Cl₂).

The starting m-alkynyl-porphyrin was not pure but was a mixture

of the alkynyl porphyrin and the tetra-(3,5-di-tert-butylphenyl)

porphyrin that is a byproduct of its synthesis. Since the tetraaryl

porphyrin does not react in this step, it is easily separated out

during chromatography. Yields of m4m-ZnPꢀTri and m4p-

ZnPꢀTri were calculated from the estimated 50% of m-alkynylꢀ

porphyrin in the mixture of starting porphyrin.

m-ZnP (3). Purified by column chromatography (silica gel,

1

CH₂Cl₂). H NMR (400 MHz, CDCl₃): δ = 9.0 ppm (q, 6H,

βH), 8.9 ppm (t, 2H, βH), 8.4 ppm (s, 1H, ArH), 8.2 ppm (d,

1H, ArH), 8.0 ppm (m, 6H, 3,5-di-tert-Bu-Ar-meta-H), 7.9

ppm (d, 1H, ArH), 7.8 ppm (m, 3H, 3,5-di-tert-Bu-Ar-para-H),

7.7 ppm (t, 1H, ArH), 3.1 ppm (s, 1H, alkynyl-H), 1.5 (s, 54H,

tert-BuH). MALDI-TOF calculated for C₇₀H₇₆N₄Zn = 1036.54

[M⁺], found 1036.2 [M⁺].

p-ZnP (4). Purified by column chromatography (neutral

alumina, CH₂Cl₂/hexanes, 1:9). ¹H NMR (400 MHz, CDCl₃):

δ = 9.0 ppm (d, 6H, βH), 8.9 ppm (d, 2H, βH), 8.2 ppm (d, 2H,

ArH), 8.0 ppm (s, 6H, 3,5-di-tert-Bu-Ar-meta-H), 7.9 ppm (d,

2H, ArH), 7.8 ppm (s, 3H, 3,5-di-tert-Bu-Ar-para-H), 3.3 ppm

(s, 1H, alkyne-H), 1.5 ppm (s, 54H, tert-BuH). MALDI-TOF

calculated for C₇₀H₇₆N₄Zn = 1036.54 [M⁺], found 1036.9 [M⁺].

General Procedure for Synthesis of Azidobenzaldehydes (5,

6). Bromobenzaldehyde (1 g, 5.4 mmol), sodium azide (0.7 g,

10.8 mmol), sodium ascorbate (53 mg, 0.27 mmol), copper(I)

m,4,m-ZnPꢀTriazoleꢀBenzaldehyde (m4m-ZnPꢀTri) (7)

1

was obtained in 28.7% yield. H NMR (400 MHz, CDCl₃):

δ = 10.0 ppm (s, 1H, CHO), 9.0 ppm (m, 8H, βH), 8.7 ppm

(s, 1H, ArH), 8.4 ppm (d, 1H, ArH), 8.35 ppm (s, 1H, triazole-

H), 8.3 ppm (d, 1H, ArH), 8.15 ppm (s, 1H, CHO-ArH),

8.1ꢀ8.0 ppm (m, 6H, 3,5-di-tert-Bu-Ar-meta-H), 8.0 ppm (d,

1H, CHO-ArH), 7.9 ppm (t, 1H, ArH), 7.8 ppm (m, 4H, 3,5-

di-tert-Bu-Ar-para-H and CHO-ArH), 7.6 ppm (t, 1H, CHO-

ArH), 1.5 ppm (s, 54H, tert-BuH). MALDI-TOF calculated for

C₇₇H₈₁N₇OZn = 1183.58 [M⁺], 1155.57 [M ꢀ 28]. Found:

1183.6 [M⁺], 1155.1 [M ꢀ 28].

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m,4,p-ZnPꢀTriazoleꢀBenzaldehyde (m4p-ZnPꢀTri) (8)

was obtained in quantitative yield. ¹H NMR (400 MHz, CDCl₃):

δ = 9.9 ppm (s, 1H, CHO), 9.0 ppm (m, 8H, βH), 8.7 ppm

(s, 1H, ArH), 8.4 ppm (d, 1H, ArH), 8.35 ppm (s, 1H, triazole-

H), 8.3 ppm (d, 1H, ArH), 8.2 ppm (m, 6H, 3,5-di-tert-Bu-Ar-

meta-H), 7.9 ppm (m, 5H, ArH and CHO-ArH), 7.8 ppm (s, 3H,

3,5-di-tert-Bu-Ar-para-H), 1.5 ppm (s, 54H, tert-BuH). MALDI-

TOF calculated for C₇₇H₈₁N₇OZn = 1183.58 [M⁺], 1155.57

[M ꢀ 28] for loss of nitrogen from the triazole ring. Found:

1183.6 [M⁺], 1155.5 [M ꢀ 28].

TFA in a 10 mL flask and cooled to 0 ꢀC in an ice bath. Sodium

nitrite (0.0271 g, 0.3922 mmol) was dissolved in 0.75 mL of

distilled water and added to the reaction, which was then stirred

for 15 min at 0 ꢀC. Sodium azide (0.0382 g, 0.5883 mmol) was

dissolved in 0.75 mL of distilled water and added to the reaction

mixture. After the reaction was stirred on ice for 1 h, cold water

was added to the flask. The crude mixture was extracted with

CH₂Cl₂, and the green organic layer was washed with water until

it turned purple. The organic phase was dried over MgSO₄,

filtered, and concentrated under vacuum to about 150 mL. Zinc

acetate (0.50 g, 2.28 mmol) was dissolved in ∼15 mL of MeOH

and added to the porphyrin solution. The reaction was heated at

reflux for 1 h, and the crude mixture was washed with water three

times. The organic layer was dried over MgSO₄and filtered.

Flash column chromatography with silica gel was used to purify

the crude material using 2:1 CH₂Cl₂:hexanes as the eluent, and

the desired product was eluted with the solvent front. The yield

p,4,p-ZnP-Triazoleꢀbenzaldehyde (p4p-ZnPꢀTri) (9) was

obtained in 40% yield. ¹H NMR (400 MHz, CDCl₃): δ = 10.1

ppm (s, 1H, CHO), 9.0 ppm (m, 8H, βH), 8.5 ppm (s, 1H,

triazole-H), 8.4 ppm (d, 2H, ArH), 8.3 ppm (d, 2H, ArH), 8.14

ppm (s, 4H, CHO-ArH), 8.1 ppm (m, 6H, 3,5-di-tert-Bu-Ar-

meta-H), 7.8 ppm (m, 3H, 3,5-di-tert-Bu-Ar-para-H), 1.5 ppm (s,

54H, tert-BuH). MALDI-TOF calculated for C₇₇H₈₁N₇OZn =

1183.58 [M⁺], 1155.57 [M ꢀ 28] for loss of nitrogen from the

triazole ring. Found: 1182.6 [M⁺], 1154.8 [M ꢀ 28].

1

for this two-step reaction was ∼62%. H NMR (400 MHz,

CDCl₃): δ = 8.9 ppm (m, 6H, βH), 8.8 ppm (d, 2H, βH), 8.1

ppm (d, 2H, ArH), 8.0 ppm (s, 6H, Ar-meta-H), 7.7 ppm (s, 3H,

Ar-para-H), 7.1 ppm (d, 2H, ArH), 1.4 ppm (s, 54H, tert-BuH).

MALDI-TOF calculated for C₆₈H₇₅N₇Zn = 1053.54 [M⁺],

1025.54 [M ꢀ 28]. Found: 1027.43 [M ꢀ 28] and 2056.85

Synthesis of 5-(4-Acetamidophenyl)-10,15,20-tris-(3,5-di-

tert-butylphenyl) Porphyrin (10). 3,5-Di-tert-butylbenzaldehyde

(1.637 g, 7.5 mmol) and p-acetamidobenzaldehyde (0.410 g, 2.5

mmol) were dissolved in 1 L of CH₂Cl₂contained in a 2 L three-

neck flask. The flask was covered with aluminum foil and purged

with N₂for 10 min before freshly distilled pyrrole (700 μL, 10

mmol) was injected. The reaction was stirred for another 10 min,

and the reaction was initiated by the injection of TFA (1.4 mL, 20

mmol). After the reaction was stirred at room temperature for 1.5

h, DDQ (1.70 g, 7.5 mmol) was added, and the reaction was

stirred for 1 h at room temperature and was then neutralized with

triethylamine (2.8 mL, 20 mmol). The crude mixture was puri-

fied with silica gel flash chromatography; elution with CH₂Cl₂

removed all tetrarylporphyrin, and the desired A₃B porphyrin

was then eluted with 20% EtOAc in CH₂Cl₂. Solvent was

removed under vacuum, affording a purple solid in a yield

[2 (M ꢀ 28)]. IR: a strong azide peak was seen at 2120 cm^ꢀ1

.

NOTE. The mass at 2056.85 is attributed to dimerization of the

porphyrin in the MALDI instrument. When light from the laser

hits the azide, N₂is lost yielding a very reactive aryl nitrene. Two

aryl nitrenes then come together to form the dimer. There is also

a visible color change of this porphyrin from purple to green in

solution upon exposure to light.

Synthesis of p-Ethynylbenzaldehyde (13). p-(Trimethylsilyl)-

ethynylbenzaldehyde (0.202 g, 1 mmol) and K₂CO₃(0.014 g, 0.1

mmol) were dissolved in 11.7 mL of MeOH. The reaction was

stirred at room temperature for 3 h, and the solvent was removed

under vacuum. The solid was redissolved in CH₂Cl₂and washed

with aqueous NaHCO₃three times. The organic layer was dried

over Na₂SO₄and afforded a yellow solid in ∼96% yield after

1

of ∼18%. H NMR (400 MHz, CDCl₃): δ = 8.8 ppm (s, 4H,

βH), 8.8 ppm (d, 2H, βH), 8.7 ppm (d, 2H, βH), 8.0 ppm (m,

6H, Ar-meta-H), 7.9 ppm (d, 2H, ArH), 7.7 ppm (m, 3H, Ar-

para-H), 7.4 ppm (d, 2H, ArH), 7.1 ppm (s, 1H, NH), 2.0

ppm (s, 3H, CH₃), 1.4 ppm (s, 54H, tert-BuH), ꢀ2.8 ppm (s, 2H,

pyrrole H). MALDI-TOF calculated for C₇₀H₈₀N₅O = 1006.64

[M⁺]. Found: 1007.43 [M⁺].

1

solvent was removed. H NMR (400 MHz, CDCl₃): δ = 9.9

ppm (s, 1H, CHO), 7.7 ppm (d, 2H, ArH), 7.5 ppm (d, 2H,

ArH), 3.2 ppm (s, 1H, ethynyl H).

Synthesis ofm-Ethynylbenzaldehyde(14). m-(Trimethylsilyl)-

ethynylbenzaldehyde (0.340 g, 1.68 mmol) wasdissolved in 50 mL

of methylene chloride in a flask containing a stir bar. Tetra-

butylammonium fluoride (3.36 mL, 1.0 M in THF, 3.36 mmol)

was added to the reaction mixture, which was stirred at room

temperature for 5 min. Flash silica gel chromatography with

CH₂Cl₂as aneluentwasusedtopurify thecrudemixture. Fractions

were collected, and the progress of separation was monitored by

TLC. Solvent was removed under vacuum and afforded a yellow

solid in ∼65%. ¹H NMR (400 MHz, CDCl₃): δ = 10.0 ppm (s, 1H,

CHO), 8.0 ppm (s, 1H, ArH), 7.9 ppm (d, 1H, ArH), 7.7 ppm (d,

1H, ArH), 7.5 ppm (t, 1H, ArH), 3.2 ppm (s, 1H, ethynyl H).

Synthesis of p,1,p-ZnP-TriazoleꢀBenzaldehyde (15). p-Azi-

dophenyltriarylzinc(II)porphyrin (12) (0.0500 g, 0.0474 mmol),

p-ethynylbenzaldehyde (13) (0.0185 g, 0.1421 mmol), copper-

(I) iodide (0.0028 g, 0.0142 mmol), sodium ascorbate (0.0056 g,

0.0284 mmol), and TBTA (0.0075 g, 0.0142 mmol) were

suspended and sonicated in 1.78 mL of DMSO in a 10 mL

microwave reaction vessel. Distilled water (0.2 mL) was added to

the mixture, and the reaction vessel was capped with a Teflon

septum. The reaction mixture was stirred vigorously and purged

with N₂for 5 min before the vessel was placed into the

Synthesis of 5-(4-Aminophenyl)-10,15,20-tris-(3,5-di-tert-

butylphenyl) Porphyrin (11). p-Amidophenyltriarylporphyrin

(10) (0.2067 g, 0.2 mmol) was dissolved in 60 mL of EtOH in a

250 mL flask. Concentrated HCl (40 mL) was added to the

reaction mixture, which was heated at reflux for 17 h. The crude

mixture was diluted with water and extracted with CH₂Cl₂five

times. The organic layer was washed with H₂O three times and

with saturated NaHCO₃solution twice. The pooled organic layer

was dried over Na₂SO₄and filtered. The solvent was removed

under vacuum, and flash chromatography with silica gel and

CH₂Cl₂afforded the desired aminophenylporphyrin in quanti-

tative yield. ¹H NMR (400 MHz, CDCl₃): δ = 8.8 ppm (d, 2H,

βH), 8.8 ppm (m, 6H, βH), 8.0 ppm (m, 6H, Ar-meta-H), 7.9

ppm (d, 2H, ArH), 7.7 ppm (m, 3H, Ar-para-H), 6.9 ppm (d,

2H, ArH), 3.9 ppm (s, 2H, NH₂), 1.4 ppm (d, 54H, tert-BuH),

ꢀ2.7 ppm (s, 2H, pyrrole H). MALDI-TOF calculated for

C₆₈H₇₉N₅= 965.63 [M⁺]. Found: 966.64 [M⁺].

Synthesis of Zinc(II)-5-(4-azidophenyl)-10,15,20-tris-(3,5-di-

tert-butylphenyl) Porphyrin (12). p-Aminophenyltriarylporphyr-

in (11) (0.1896 g, 0.1961 mmol) was dissolved in 3.17 mL of

13051

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Journal of the American Chemical Society

ARTICLE

microwave reactor. The reaction was run at 50 W microwave

power and maintained at 80 ꢀC for 45 min. The reaction mixture

was then sonicated and irradiated with microwave for another

15 min under the same power and temperature settings, and this

step was repeated until the total reaction time reached 2 h. The

reaction progress was monitored by TLC. The crude mixture was

first diluted with CH₂Cl₂and washed twice with aqueous

NH₄OH and twice with water. The aqueous layer was extracted

with CH₂Cl₂four times, and the pooled organic layer was washed

with water until it turned purple. The organic layer was dried over

MgSO₄and filtered, and the solvent was removed under vacuum.

Flash column chromatography with silica gel was used to purify

the crude product, and eluting the column with 100% CH₂Cl₂

and then with 3% ethyl acetate in CH₂Cl₂affords the desired

porphyrin-triazole-benzaldehyde in ∼84% yield. ¹H NMR (400

MHz, CDCl₃): δ = 10.0 ppm (s, 1H, CHO), 9.0 ppm (m, 6H,

βH), 8.9 ppm (d, 2H, βH), 8.5 ppm (s, 1H, triazole-H), 8.4 ppm

(d, 2H, ArH), 8.1 ppm (d, 4H, CHO-ArH), 8.0 ppm (m, 6H, 3,5-

di-tert-Bu-Ar-meta-H), 7.9 ppm (d, 2H, ArH), 7.7 ppm (m, 3H,

3,5-di-tert-Bu-Ar-para-H), 1.4 ppm (d, 54H, tert-BuH). MALDI-

TOF mass calculated C₇₇H₈₁N₇OZn = 1183.58 [M⁺], 1155.58

[M ꢀ 28]. Found: 1156.29 [M ꢀ 28].

mmol) were dissolved in toluene (10 mL) and heated at reﬂux

overnight. The product was puriﬁed by column chromatography

(silica gel, toluene/ethyl acetate 99:1)

m4m-ZnPꢀTriazoleꢀC₆₀(17) was obtained in ∼30% yield

by method 1 and ∼70% yield by method 2. ¹H NMR (400 MHz,

CDCl₃): δ = 8.8ꢀ9.0 ppm (m, 8H, H_a), 8.5 ppm (d, 1H, H_b), 8.3

ppm (d, 1H, H_c), 8.1ꢀ8.2 ppm (m, 6H, H_d), 8.0 ppm (s, 1H,

H_e), 7.83ꢀ7.9 ppm (m, 4H, H_f), 7.8 ppm (m, 3H, H_g), 7.7

ppm (d, 1H, H_h), 7.4 ppm (t, 1H, H_i), 4.7 ppm (s, 1H, H_j), 4.6

ppm (d, 1H, H_k), 3.9 ppm (d, 1H, H_l), 2.7 ppm (s, 3H, H_m),

1.4ꢀ1.6 ppm (m, 54H, H_n). MALDI-TOF calculated for

C₁₃₉H₈₆N₈Zn = 1930.63 [M⁺], 1902.62 [M ꢀ 28], 1182.62

[M ꢀ 748] (loss of C₆₀+ N₂). Found: 1902.7 [M ꢀ 28], 1182.2

[M ꢀ 748].

m4p-ZnPꢀTriazoleꢀC₆₀(18) was obtained in 31% yield by

1

method 1 and 65% yield by method 2. H NMR (400 MHz,

CDCl₃): δ = 8.9ꢀ9.0 ppm (m, 9H, βH, triazole-H), 8.5 ppm (d,

1H, ArH), 8.3 ppm (d, 1H, ArH), 8.2 ppm (s, 1H, ArH), 8.1

ppm (s, 3H, ArH), 7.9ꢀ8.0 ppm (m, 4H, ArH), 7.75ꢀ7.77

ppm (m, 4H, ArH), 7.6 ppm (s, 1H, ArH), 7.4 ppm (s, 2H, ArH),

4.1 ppm (d, 1H, pyrrolidine-H), 3.8 ppm (s, 1H, pyrrolidine-H),

3.1 ppm (d, 1H, pyrrolidine-H), 2.5 ppm (s, 3H, N-methyl),

1.4ꢀ1.5 ppm (m, 54H, tert-BuH). MALDI-TOF calculated

for C₁₃₉H₈₆N₈Zn = 1930.63 [M⁺], 1902.62 [M ꢀ 28],

1182.62 [M ꢀ 748] (loss of C₆₀+ N₂). Found: 1903.3

[M ꢀ 28], 1182.3 [M ꢀ 748].

Synthesis of p,1,m-ZnPꢀTriazoleꢀBenzaldehyde (16).

Zn p-Azidophenyl (12) (0.060 g, 0.0568 mmol), m-ethynyl-

benzaldehyde (14) (0.022 g, 0.170 mmol), copper(I) iodide

(0.0033 g, 0.0170 mmol), sodium ascorbate (0.0068 g, 0.0341

mmol), and TBTA (0.0091 g, 0.0170 mmol) were suspended

and sonicated in 2.0 mL of DMSO in a 10 mL micro-

wave reaction vessel. Distilled water (0.2 mL) was added

to the mixture, and the reaction vessel was capped with a

Teflon septum. The reaction mixture was purged with N₂for

5 min before the vessel was placed into the microwave reactor.

The reaction was run at 50 W microwave power and main-

tained at 80 ꢀC for 2 h. During this 2 h reaction, the vessel was

removed from the reactor, and the reaction mixture was

sonicated for 5 min. The crude mixture was first diluted with

CH₂Cl₂and washed with aqueous NH₄OH twice and with

water twice. The aqueous layer was extracted with CH₂Cl₂

four times, and the pooled organic layer was washed with

water until it turned purple. The organic layer was dried over

MgSO₄and filtered, and the solvent was removed under

vacuum. Flash column chromatography with silica gel was

used to purify the crude product, and eluting the column with

100% CH₂Cl₂and then with 3% ethyl acetate in CH₂Cl₂

affords the desired porphyrin-triazole-benzaldehyde in a

∼77% yield. ¹H NMR (400 MHz, CDCl₃): δ = 10.1 ppm (s,

1H, CHO), 9.0 ppm (m, 8H, βH), 8.6 ppm (s, 1H, triazole-H),

8.5 ppm (s, 1H, CHO-ArH), 8.5 ppm (d, 2H, ArH), 8.4

ppm (d, 1H, CHO-ArH), 8.2 ppm (d, 2H, ArH), 8.1 ppm (m,

6H, 3,5-di-tert-Bu-Ar-meta-H), 7.9 ppm (d, 1H, CHO-ArH),

7.8 ppm (m, 3H, 3,5-di-tert-Bu-Ar-para-H), 7.7 ppm (t, 1H,

CHO-ArH), 1.5 ppm (d, 54H, tert-BuH). MALDI-TOF mass

calculated C₇₇H₈₁N₇OZn = 1183.58 [M⁺], 1155.58 [M ꢀ 28].

Found: 1156.37 [M ꢀ 28].

p4p-ZnPꢀTriazoleꢀC₆₀(19) was obtained in ∼30% yield by

method 1. ¹H NMR (400 MHz, CDCl₃): δ = 8.9ꢀ9.0 ppm (m,

8H, βH), 8.8 ppm (s, 1H, Triazole-H), 8.4 ppm (d, 2H, ArH),

8.25 ppm (d, 2H, ArH), 8.16 ppm (d, 2H, ArH), 8.06 ppm (m,

6H, ArH), 7.8 ppm (m, 5H, ArH), 4.5 ppm (d, 1H, pyrrolidine-

H), 4.4 ppm (s, 1H, pyrrolidine-H), 3.7 ppm (d, 1H, pyrrolidine-

H), 2.7 ppm (s, 3H, N-methyl), 1.5 ppm (two singlets, 54H, tert-

BuH). MALDI-TOF calculated for C₁₃₉H₈₆N₈Zn = 1930.63

[M⁺], 1902.62 [M ꢀ 28], 1182.62 [M ꢀ 748] (loss of C₆₀+ N₂).

Found: 1902.6 [M ꢀ 28], 1182.8 [M ꢀ 748]

p1p-ZnPꢀTriazoleꢀC₆₀(20) was obtained in 34.5% yield by

method 1. ¹H NMR (400 MHz, CDCl₃): 9.0 ppm (m, 8H, βH),

8.8 ppm (s, 1H, triazole-H), 8.4 ppm (d, 2H, ArH), 8.3 ppm (d,

2H, ArH), 8.2 ppm (d, 2H, ArH), 8.1 ppm (m, 6H, 3,5-di-tert-Bu-

Ar-meta-H), 7.8 ppm (m, 5H, 3,5-di-tert-Bu-Ar-meta-H and

ArH), 4.6 ppm (d, 1H, pyrrolidine-H), 4.5 ppm (s, 1H, pyrro-

lidine-H), 8.8 ppm (d, 1H, pyrrolidine-H), 2.8 ppm (s, 3H, N-

methyl), 1.5 ppm (d, 54H, tert-BuH). MALDI-TOF mass

calculated for C₁₃₉H₈₆N₈Zn = 1930.63 [M+], 1902.63 [M ꢀ

28], 1182.63 [M ꢀ 28 ꢀ 720]. Found: 1902.37 [M ꢀ 28],

1183.58 [M ꢀ 28 ꢀ 720].

p1m-ZnPꢀTriazoleꢀC₆₀(21) was prepared by method 2. ¹H

NMR (400 MHz, CDCl₃): 9.0 ppm (m, 8H, βH), 8.8 ppm (s,

1H, triazole-H), 8.4 ppm (broad, 1H, ArH), 8.3 ppm (d, 2H,

ArH), 8.2 ppm (d, 2H, ArH), 8.0 ppm (m, 6H, 3,5-di-tert-Bu-Ar-

meta-H), 8.0 ppm (broad s, 1H, ArH), 7.7 ppm (m, 3H, 3,5-di-

tert-Bu-Ar-para-H), 7.5 ppm (t, 1H, ArH), 4.8 ppm (s, 1H,

pyrrolidine-H), 4.7 ppm (d, 1H, pyrrolidine-H), 4.0 ppm (d, 1H,

pyrrolidine-H), 2.8 ppm (s, 3H, N-methyl), 1.5 ppm (d, 54H,

tert-BuH). MALDI-TOF mass calculated for C₁₃₉H₈₆N₈Zn =

1930.63 [M+], 1902.63 [M ꢀ 28], 1182.63 [M ꢀ 28 ꢀ 720].

Found: 1902.74 [M ꢀ 28], 1183.54 [M ꢀ 28 ꢀ 720].

Synthesis of PorphyrinꢀTriazoleꢀFullerenes (17ꢀ21).

Method 1: porphyrin-triazole-benzaldehyde (12.8 mg, 0.0107

mmol), C₆₀(3.6 mg, 0.005 mmol), and sarcosine (0.9 mg, 0.01

mmol) were dissolved in toluene (2 mL) and heated at reflux

overnight. The product was purified by column chromatography

(silica gel, hexanes/ethyl acetate 2:1).

p4m-ZnPꢀTriazoleꢀC₆₀(22) was prepared by Method 1

from p4m-ZnPꢀTriazoleꢀbenzaldehyde (13 mg, 0.011 mmol),

C₆₀(18.5 mg, 0.026 mmol), and sarcosine (8 mg, 0.09 mmol).

The product was puriﬁed by column chromatography (silica gel,

Method 2: porphyrin-triazole-benzaldehyde (12.8 mg, 0.0107

mmol), C₆₀(16 mg, 0.022 mmol), and sarcosine (2 mg, 0.022

13052

dx.doi.org/10.1021/ja202485s |J. Am. Chem. Soc. 2011, 133, 13036–13054

Journal of the American Chemical Society

ARTICLE

dichloromethane). ¹H NMR (400 MHz, CDCl₃): δ = 8.9

ppm (m, 9H, βH, triazole-H), 8.3 ppm (d, 2H, ArH), 8.2 ppm (d,

2H, ArH), 8.0 ppm (m, 7H, ArH), 7.9 ppm (d, 1H, ArH),

7.8 ppm (m, 4H, ArH), 7.6 ppm (m, 1H, ArH), 4.7 ppm (m, 2H,

pyrrolidine-H), 3.8 ppm (s, 1H, pyrrolidine-H), 2.8 ppm (s, 3H,

N-methyl), 1.5 ppm (m, 54H, tert-BuH). MALDI-TOF calcu-

lated for C₁₃₉H₈₆N₈Zn = 1930.63 [M+], 1902.62 [M ꢀ 28],

1182.62 [M ꢀ 748] (Loss of C₆₀+ N₂). Found: 1184.9

[M ꢀ 748].

(6) Kuciauskas, D.; Liddell, P. A.; Lin, S.; Johnson, T. E.; Weghorn,

S. J.; Lindsey, J. S.; Moore, A. L.; Moore, T. A.; Gust, D. J. Am. Chem. Soc.

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J. Chem. Phys. 1956, 24, 979. (c) Marcus, R. A. J. Chem. Phys. 1957,

26, 867. (d) Marcus, R. A. J. Chem. Phys. 1957, 26, 872. (e) Marcus, R. A.

Can. J. Chem. 1959, 37, 155. (f) Guldi, D. M.; Fukuzumi, S. In Fullerenes:

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Varotto, A.; Radivojevic, I. Chem. Rev. 2009, 109, 1630. (c) Holten, D.;

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’ ASSOCIATED CONTENT

(9) (a) Imahori, H.; Sakata, Y. Adv. Mater. 1997, 9, 537. (b) Echegoyen,

L.; Echegoyen, L. E. Acc. Chem. Res. 1998, 31, 593. (c) Guldi, D. M.

Chem. Commun. 2000, 5, 321. (d) Prato, M.; Guldi, D. M. Acc. Chem. Res.

2000, 33, 695. (e) Echegoyen, L.; Diederich, F.; Echegoyen, L. E. In

Fullerenes: Chemistry, Physics, and Technology; Kadish, K. M., Ruoﬀ, R. S.,

Ed.; Wiley-Interscience: New York, 2000; pp 1ꢀ53.

S

Supporting Information. IR, NMR, and mass spectra of all

b

products and their precursors, calculated electron aﬃnity data,

calculated state energy values, diﬀerential pulse voltammetry

data, and transient absorption spectra. This material is available

free of charge via the Internet at http://pubs.acs.org.

(10) (a) Bracher, P. J.; Schuster, D. I. In Fullerenes: From Synthesis to

Optoelectronic Properties; Guldi, D. M., Martin, N., Ed.; Kluwer Academic

Publishers: Dordrecht, 2002; pp 163ꢀ212. (b) Ikemoto, J.; Takimiya,

K.; Aso, Y.; Otsubo, T.; Fujitsuka, M.; Ito, O. Org. Lett. 2002, 4, 309. (c) Vail,

S. A.; Krawczuk, P. J.; Guldi, D. M.; Palkar, A.; Echegoyen, L.; Tome, J. P. C.;

Fazio, M. A.; Schuster, D. I. Chem.—Eur. J. 2005, 11, 3375. (d) MacMahon,

S.; Fong, R., II; Baran, P. S.; Safonov, I.; Wilson, S. R.; Schuster, D. I. J. Org.

Chem. 2001, 66, 5449. (e) Ito, O. In Handbook of Carbon Nano Materials.

Volume 2. Electron Transfer and Applications; D'Souza, F., Kadish, K. M., Ed.;

World Scientiﬁc: Singapore, 2011; pp 441ꢀ477.

’ AUTHOR INFORMATION

Corresponding Authors

david.schuster@nyu.edu; guldi@chemie.uni-erlangen.de

’ ACKNOWLEDGMENT

(11) (a) Rodriguez-Morgade, M. S.; Plonska-Brzezinska, M. E.; Athans,

A. J.; Carbonell, E.; de Miguel, G.; Guldi, D. M.; Echegoyen, L.; Torres, T.

J. Am. Chem. Soc. 2009, 131, 10484. (b) Imahori, H.; Fukuzumi, S. Adv.

Funct. Mater. 2004, 14, 525. (c) O’Regan, B. C.; Lopez-Duarte, I.; Martinez-

Diaz, M. V.; Forneli, A.; Albero, J.; Morandeira, A.; Palomares, E.; Torres,

T.; Durrant, J. R. J. Am. Chem. Soc. 2008, 130, 2906.

Support of the work at NYU by a grant from the National

Science Foundation (CHE-0647334) is acknowledged with

grateful appreciation. This investigation was conducted in part

using an instrumental facility at NYU constructed with support

from Research Facilities Improvement Grant Number C06 RR-

16572-01 from the National Center for Research Resources,

National Institutes of Health. O.L. and C.K.H. are grateful for

support from the Dean’s Undergraduate Research Fund at the

NYU College of Arts and Science. We also thank Nathan Psota

for technical assistance in the synthesis. Support is also gratefully

acknowledged through a Senior Mentor Grant to DIS from the

Camille and Henry Dreyfus Foundation. L.E. and A.O. gratefully

acknowledge generous support from the US NSF, grant DMR-

0809129. L.E. also thanks the Robert A. Welch Foundation for

an endowed chair, grant # AH-0033. G. M. thanks the von

Humboldt Foundation for a postdoctoral fellowship and Dr.

Bruno Grimm for assistance with the steady-state spectroscopic

measurements. D.M.G. thanks German Science Foundation

(Cluster of Excellence ꢀ EAM-SFB-583) for support.

(12) (a) Megiatto, J. D.; Schuster, D. I. J. Am. Chem. Soc. 2008,

130, 12872. (b) Megiatto, J. D.; Schuster, D. I. Chem.—Eur. J. 2009,

15, 5444. (c) Megiatto, J. D.; Spencer, R.; Schuster, D. I Org. Lett. 2009,

11, 4152. (d) Megiatto, J. D., Jr.; Schuster, D. I. In Handbook of Carbon

Nano Materials. Volume 1. Synthesis and Supramolecular Systems;

D'Souza, F., Kadish, K. M., Ed.; World Scientiﬁc: Singapore, 2011; pp

207ꢀ244. (e) Takata, T.; Ito, O. In Handbook of Carbon Nano Materials.

Volume 2. Electron Transfer and Applications; D'Souza, F., Kadish, K. M.,

Ed.; World Scientiﬁc: Singapore, 2011; pp 479ꢀ517.

(13) (a) Collin, J.-P.; Harriman, A.; Heitz, V.; Odobel, F.; Sauvage, J.-P.

J. Am. Chem. Soc. 1994, 116, 5679. (b) Schuster, D. I.; Cheng, P.; Wilson,

S. R.; Prokhorenko, V.; Katterle, M.; Holzwarth, A. R.; Braslavsky, S. E.;

Klihm, G.; Williams, R. M.; Luo, C. J. Am. Chem. Soc. 1999, 121, 11599. (c)

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’ DEDICATION

Dedicated to Professor Harry B. Gray of the California Institute

of Technology on the occasion of his 75th birthday.

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Triazole bridges as versatile linkers in electron donor-acceptor conjugates

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