Porphyrin Donor and Tunable Push–Pull Acceptor Conjugates—Experimental Investigation of Marcus Theory

A Journal of

Accepted Article

Title: Porphyrin Donor and Tunable Push-Pull Acceptor Conjugates -

Experimental Investigation of Marcus Theory

Authors: Tristan Reekie, Michael Sekita, Lorenz Urner, Stefan Bauroth,

Laurent Ruhlamann, Jean-Paul Gisselbrecht, Corinne

Boudon, Nils Trapp, Timothy Clark, François Diederich, and

Dirk Michael Guldi

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To be cited as: Chem. Eur. J. 10.1002/chem.201700043

Link to VoR: http://dx.doi.org/10.1002/chem.201700043

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Electron Transfer

Porphyrin Donor and Tunable Push–Pull Acceptor Conjugates –

Experimental Investigation of Marcus Theory

Tristan A. Reekie,^∥Michael Sekita,^∥Lorenz M. Urner,^[a]Stefan Bauroth,^[b]Laurent Ruhlmann,^[c]

Jean-Paul Gisselbrecht,^[c]Corinne Boudon,^[c]Nils Trapp,^[a]Timothy Clark,^[d]Dirk M. Guldi,*^[b]and

François Diederich*^[a]

[a]

[b]

Dedication In memory of Günther Wilke

data allows for the construction of multiple Marcus curves that

consider electron acceptor strength, linker length and solvent, with

data points extending well into the inverted region. The

enhancement of electronic-vibration couplings, resulting from the

rigid spacers and, in particular, multicyano-groups in the

conformationally highly fixed push–pull acceptor chromophores

affects the charge-recombination kinetics in the inverted region

drastically.

Abstract: We report on a series of electron donor–acceptor

conjugates incorporating a Zn^IIporphyrin-based electron donor and

a variety of non-conjugated rigid linkers connecting to push–pull

chromophores as electron acceptors. The electron acceptors

comprize

multicyanobutadienes

or

extended

tetracyanoquinodimethane analogues with first reduction potentials

ranging from –1.67 to –0.23 V vs. Fc⁺/Fc in CH₂Cl₂, which are

accessible through a final-step cycloaddition–retroelectrocyclization

(CA–RE) reaction. Characterization of the conjugates includes

electrochemistry, spectroelectrochemistry, DFT calculations, and

photophysical measurements in a range of solvents. The collected

Introduction

In the quest for artificial photosynthetic model systems, a key to

controlling photoinduced electron transfer in electron donor–

acceptor assemblies is the successful matching of

chromophoric electron donors and acceptors in terms of

electrochemical and photophysical properties. In particular, the

efficiency of photoinduced electron transfer in covalent systems

is determined by critical factors such as the separation distance

between the electron donor and acceptor, the conformation of

the molecular bridge, the energy gap between the excited and

charge-separated states, and the nature of the solvent.

Importantly, control over the ratio of the rates for charge

separation and charge recombination is crucial to create long-

lived charge-separated states. Mastering these factors is

essential when trying to achieve molecular-scale electronics,

artificial photosynthesis, and photovoltaic applications.

Generally, Marcus theory is used to rationalize the rates of

electron transfer (ET) reactions seen in the photosynthetic

reaction centers.^[1]It describes the rate of electron transfer as a

non-linear function of the reaction Gibbs free energy (∆G).

More specifically, Marcus theory implies that rates of electron

transfer start to decrease for reactions with a –∆G higher than

the reorganization energy λ. The latter defines the Marcus

inverted region, whose confirmation remained elusive for

almost 30 years until Miller, Closs and Calcaterra provided

unambiguous, experimental verification of the inverted region.^[2]

Classic Marcus equation works remarkably well in the normal

region and close to the maximum rate, but for the inverted

region electron-transfer rates higher than expected have also

been observed. This deviation of the classical parabolic is the

[a]

[b]

Dr. T. A. Reekie, Dr. L. M. Urner, Dr. N. Trapp, Prof. Dr. F. Diederich

Laboratorium für Organische Chemie

ETH Zürich

Vladimir-Prelog-Weg 3, 8093 Zürich, Switzerland

E-mail: diederich@org.chem.ethz.ch

Dr. M. Sekita, S. Bauroth, Prof. Dr. D. M. Guldi

Department of Chemistry and Pharmacy and Interdisciplinary Center

for Molecular Materials (ICMM)

Friedrich-Alexander-University Erlangen-Nuremberg

Egerlandstrasse 3, 91058 Erlangen, Germany

E-Mail: dirk.guldi@fau.de

Prof. Dr. L. Ruhlmann, Dr. J.-P. Gisselbrecht, Prof. Dr. C. Boudon

Laboratoire d’Electrochimie et de Chimie Physique du Corps Solide

Institut de Chimie-UMR 7177, C.N.R.S.

Université de Strasbourg

4, rue Blaise Pascal, 67000 Strasbourg, France

Prof. Dr. T. Clark

[c]

[d]

Computer Chemistry Center

Friedrich-Alexander-University Erlangen-Nuremberg

Nägelsbachstraße 25, 91052 Erlangen, Germany

∥ T.A.R. and M.S. contributed equally.

Supporting Information (SI) available: [Synthesis and full

characterization of all new compounds, as well as copies of their ¹H

and ¹³C NMR spectra, X-ray crystallography details and CIF files for

compounds 12 and 16, cyclic voltammetry (CV), rotating disk

voltammetry (RDV) and UV/Vis spectroelectrochemical data and

spectral traces, quantum chemical calculations (DFT) on the

conjugates and control compounds, UV/Vis spectra of the

conjugates and control compounds, detailed report on the transient

absorption spectroscopic measurements, and Marcus curves.]. See

DOI:

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result of quantum chemical vibrational effects, in which large

electron-vibration (e-ν) couplings in small molecules with highly

polarized functional groups open a channel of vibration-

assisted electron tunneling in the inverted region. This

phenomenon is not only found for electron donors, but has also

recently been observed in molecular wires at room

temperature. In this work, photoinduced electron transfer in Zn^II

porphyrin–bridge–fullerene conjugates has been studied

extensively, in which π-conjugated molecular wires were used

properties of the conjugates. Two major drawbacks were,

however, noted: the synthetic complexity in obtaining them and,

the rather large distance between electron donor and acceptor.

In the current work, we aim to circumvent these problems by

utilizing the easier to synthesize piperidyls as the rigid, though

not conformationally locked, linker and by varying the electron

donor-acceptor separation to ensure the optimal distance for

electron transfer rates.

Herein, we report the synthesis of new porphyrin-donor based

compounds with an activated alkyne component of four

different lengths and their CA–RE reaction products forming a

variety of push–pull acceptor conjugates. The newly

synthesized conjugates 1–5, in addition to relevant previously

reported conjugates 6–9,^[19]which are all the subject of the

current investigation, are shown in Figure 1. A full-fledged

photophysical investigation/analysis of conjugates 1–9,

together with electrochemical and X-ray structural analysis, has

led to the construction of numerous Marcus curves with

observed rates extending from the normal to the Marcus

inverted region.

as

bridges.

Interestingly,

carbon-bridged

oligo-

phenylenevinylene, which is both rigid and flat, leads to an 840-

fold increase in the ET rate compared to the equivalent flexible

molecular bridges. The presence of vibrational channels

decreases the activation barrier of electron transfer and, in turn,

opens the inelastic-tunneling pathway, explaining the rate

enhancement. Until now, experimental evidence for the

inverted region from intramolecular electron transfer has been

limited to examples featuring electron-accepting quinones or

fullerenes.^[2-4]

Lately, a new class of push–pull acceptors derived from [2+2]

cycloaddition–retroelectrocyclization (CA–RE) reactions that

bear great potential as unique electron acceptors in electron

transfer reactions has emerged.^[5]CA–RE reactions take place

between electron-rich, activated alkynes and electron-poor

alkenes. In the original reaction, ruthenium-acetylides^[6]served

as the activated alkyne, while recent examples are based on

purely organic molecules featuring anilino- and aryl ether-

substituted acetylenes^[7]and amides^[8]for alkyne activation.^[9]

Most CA–RE-based products give rise to interesting physical

properties, such as tunable intermolecular charge-transfer (CT)

bands expanding into the near infrared (NIR),^[10]large third-

order optical nonlinearities,^[5,11]high two-photon absorption

cross sections,^[11b]thermal and environmental stability up to

300 °C,^[10]and in many cases sublimability.^[12]Products from

CA–RE reactions have already proven useful for the synthesis

of tetracenes,^[13]with applicability to singlet fission,^[14]and chiral

optically active buta-1,3-dienes,^[15]and found application as

waveguides.^[16]In addition, CA–RE-derived products stand out

among all electron acceptors as a unique set of photoactive

molecular materials for solar cells, in general, and electron-

transfer reactions, in particular.^[9d,17]

Results and Discussion

Synthesis

The synthesis began with a piperidine ring formation between

the previously reported diol 10^[20]and aniline 11^[21]via the

intermediate ditriflate to give compound 12 (Scheme 1).

Halogen-lithium exchange and quenching with molecular iodine

provided the corresponding iodide 13. The previously reported

phenylacetylene- and pinacolborane-substituted porphyrins

14^[22]and 15^[23]could then be coupled with aryl halides 13 or 12

to give 16 and 17, respectively. Aldehyde 18, the synthesis of

which is outlined in Section S2 of the Supporting Information,

could be used to form porphyrin 19, whereby the aliphatic

portion of the linker is bound directly to the porphyrin. The

iodoaryl moiety present on 19 allows for Sonogashira cross-

coupling with phenylacetylene, or with the related diyne^[7]to

give 20 and 21, respectively.

Compounds 16, 17, 20, and 21 were subjected to the CA–RE

reaction with electron deficient alkenes 22–26. TCNE- (22) and

TCNQ- (23) bearing conjugates, 1a–3a and 1b–3b,

respectively, with three different distances between the electron

donors and acceptors, were obtained in excellent yields. For

the F₄-TCNQ- (24) based conjugates, only those with shorter

spacer distances, 2c and 3c, were synthesized. ¹H and ¹³C

NMR spectra revealed significant line broadenings and peak

losses for those signals in close proximity to the electron

acceptors. We imply that stable radical species from unpaired

electrons exist centered on the F₄-TCNQ-based electron

acceptors. We have previously shown that the push-pull

chromophores with the strongest acceptors, such as those

derived from F₄-TCNQ, give EPR spectra in agreement with

Our first foray into the field of photoinduced electron transfer

was the successful linking of a Zn^IIporphyrin electron donor to

an anilino-substituted alkyne activated towards the CA–RE

reaction allowing for the formation of a tetracyanobuta-1,3-

diene electron acceptor.^[18]Notably, this system underwent

photoinduced electron transfer to afford a long-lived charge-

separated state of 2.3 µs in benzonitrile. Expanding upon this

work, we recently reported the synthesis of a variety of

conjugates with electron-donating porphyrins, electron-

accepting push–pull chromophores, and aliphatic linkers^[19]with

the aim of rigidifying the aliphatic linker. This was intended to

reduce the flexibility between electron donor and acceptor and

to allow for an accurate determination of the distance between

the separate building blocks, essential for determining further

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acceptor-based radicals.^[9a,c,m]Considering the fact that much

sharper signals were observed for the porphyrin

Figure 1. Newly synthesized conjugates 1–5 and previously reported conjugates 6–9^[19]for subsequent investigation. For the full structure of the Zn^IIporphyrin,

see Scheme 1.

Scheme 1. Synthesis of conjugates 1–5. Reagents and conditions: a) (CF₃SO₂)₂O (2.1 equiv.), N,N-diisopropylethylamine (2.1 equiv.), MeCN, –30 °C, 1 h, then

11 (1.2 equiv.), N,N-diisopropylethylamine (2.1 equiv.), –30 to 23 °C, 18 h; 68%. b) nBuLi (2.2 equiv.), I₂(2.3 equiv.), THF, –78 °C to 23 °C, 1.5 h; 98%. c) 13 (2.1

equiv.), [PdCl₂(PPh₃)₂] (20 mol%), CuI (50 mol%), iPr₂NH/THF (1:1), 24 °C, 16 h; 45%. d) 12 (1.2 equiv.), [Pd(PPh₃)₄] (10 mol%), Cs₂CO₃(4.0 equiv.), toluene,

95 °C, 36 h; 87%. e) 1) Pyrrole (4.0 equiv.), 3,5-di-tert-butylbenzaldehyde (3.0 equiv.), TFA (1.8 equiv.), CHCl₃, 24 °C, 16 h then chloranil (3.0 equiv.), reflux,

1.5 h; 2) Zn(OAc)₂•2H₂O (1.0 equiv.), CHCl₃/MeOH, 24 °C, 1 h; 5%. f) Phenylacetylene (2.1 equiv.), [PdCl₂(PPh₃)₂] (10 mol%), CuI (10 mol%), iPr₂NH/THF (1:1),

24 °C, 16 h; 89%. g) 4-(Buta-1,3-diyn-1-yl)-N,N-dimethylaniline (2.1 equiv.), [PdCl₂(PPh₃)₂] (10 mol%), CuI (10 mol%), iPr₂NH/THF (1:1), 24 °C, 16 h; 76%. h)

TCNE (22) (1.0 equiv.), (CH₂Cl)₂, 23 °C, 1 h; 77–97%. i) TCNQ (23) (1.0 equiv.), (CH₂Cl)₂, 85 °C, 1 h; 69–89%. j) F₄-TCNQ (24) (1.0 equiv.), (CH₂Cl)₂, 23 °C, 1 h;

83–92%. k) 25 (3.0 equiv.), DMF, 100 °C, 16 h; 82%. l) 26 (3.0 equiv.), DMF, 100 °C, 16 h; 72%. For the X-ray crystal structures of 12 and 16, see Section S3 in

the Supporting Information.

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nuclei, the radical species is most likely confined to the electron

acceptor rather than resulting from intramolecular single

electron transfer.

The N-phenylpiperidyl oxidation varies between ZnP-S2 17 and

the respective conjugates without any clear trend. For the

electron acceptors, which were derived from the same electron-

deficient alkene, similar values for the N-phenylpiperidyl

oxidation, regardless of the linker, were noted. The electron

acceptors originating from the CA–RE reaction undergo two

one-electron reductions in sound agreement with the reference

compounds. Again, the absence of electronic communication

between the electron-donating porphyrin and the acceptor in

the ground state is confirmed. Therefore, substitution of the

rigid aliphatic bicyclo[2.2.2]octane linker by the synthetically

more accessible piperidyl linker did not alter the independence

of the redox behavior of electron donor and acceptor.

Alkenes 25 and 26 could also undergo the CA–RE reaction.

This was, however, only performed for conjugates 2d and 2e

with medium spacer distance. When buta-1,3-diyne 21 was

subjected to the CA–RE reaction, two regioisomers, 4 and 5,

were obtained, where the resulting tetracyanobuta-1,3-diene is

located closer to the piperidyl nitrogen (20%) or the

dimethylamino nitrogen (77%), respectively. All conjugates

were isolated as colored solids that did not undergo

decomposition in air as solids or in solution over long periods

(months). Photosensitivity under ambient light was also not

detected. Additionally, the majority exhibit melting points above

300 °C without any observable decomposition.

The first reduction for each acceptor is identical in all systems.

As expected, the F₄-TCNQ (24) derived acceptor, represented

by ZnP-S2-F₄-TCNQ 2c, shows the most anodically shifted

process at –0.23 V, which decreases to –0.63, –0.89, and

–1.18 V for ZnP-S2-TCNQ 2b, ZnP-S2-TCBD 2a, and ZnP-S2-

TriCBD 2d, respectively. The weakest acceptor – ZnP-S2-

DCBD 2e – with a one-electron reduction at –1.67 V is the only

case, in which the first reduction is not reversible. The low

value was to be expected due to the original alkene 26 only

possessing three electron-withdrawing groups.

Reference compounds 27–29 were also synthesized (see

Section S2 of the Supporting Information) to allow for a

comparison between conjugates and properties attributable

solely to the acceptor moieties, (Figure 2). Similar to conjugates

2c and 3c, compound 29 exists as a stable radical species.

As a complement to the CV and RDV measurements, ZnP-S2-

TCBD 2a, ZnP-S2-TCNQ 2b, ZnP-S2-F₄-TCNQ 2c, and ZnP-

S6-TCNQ 8 (for the corresponding CV and RDV, see reference

^[19]) were probed in spectroelectrochemical studies (see Section

S5 of the Supporting Information). Negative potentials were

applied to generate the one-electron reduced anions of the

TCBD, TCNQ, and F₄-TCNQ derived acceptors. In our previous

report, we already illustrated the changes in the absorption

Figure 2. Reference compunds 27–29.

spectra of

a ZnP reference upon the first one-electron

oxidation, and for ZnP-S6-TCBD 6 upon the first one-electron

reduction.^[19]Noteworthy for the oxidation of the ZnP reference

are the newly developing absorptions at 410 and 620 nm. Both

are well-known characteristics for the one-electron oxidized

ZnP radical cation.^[24]In contrast, reduction of ZnP-S6-TCBD 6

leads to a decrease and vanishing of the intramolecular CT

band at 468 nm – vide infra. In addition, new absorptions

evolve between 300 and 440 nm as well as between 550 and

800 nm. These features stem from the reduction of the TCBD

moiety. The results are in line with the one-electron reduction of

ZnP-S2-TCBD 2a, indicating that the piperidyl linker in ZnP-S2-

TCBD 2a and the extra electron-accepting ethynediyl unit in

ZnP-S6-TCBD 6, do not alter the reduction behavior of the

electron acceptor. Upon one-electron reduction of ZnP-S2-

Electrochemistry

All compounds were subjected to cyclic voltammetry (CV) and

rotating disk voltammetry (RDV) to determine their redox

properties. Table 1 shows the values for ZnP-S2 17 and

conjugates 2a–e by means of CV and RDV in CH₂Cl₂

containing nBu₄NPF₆(0.1 M) vs. the ferrocenium/ferrocene

(Fc⁺/Fc) couple and are uncorrected for any ohmic drops. The

data for the remaining compounds are provided in Table S1 of

the Supporting Information.

ZnP-S2 17 features two reversible one-electron oxidations at

approximately +0.33 and +0.62 V, one reversible one-electron

reduction at approximately –1.84 V, and one irreversible one-

electron reduction at –2.28 V. The two successive reductions

are porphyrin centered. The remaining irreversible one-electron

oxidation at approximately +0.43 V is attributed to the oxidation

of the N-phenylpiperidyl moiety. The oxidations and reductions

associated with the Zn^IIporphyrin remain constant in all of the

examined conjugates. We take the lack of appreciable changes

in the redox behavior of the porphyrin as support for the

absence of electronic communication between the electron

donors and acceptors in their ground state.

TCNQ 2b and ZnP-S6-TCNQ 8,

a

decrease of the

characteristic intramolecular CT bands appears at 685 and 716

nm, respectively. In addition to a minimum at 480 nm, a broad

maximum grows in at 1060 nm and an isosbestic point evolves

at 850 nm for ZnP-S2-TCNQ 2b, while for ZnP-S6-TCNQ 8 the

maximum appears at 1068 nm and the isosbestic point is seen

at 893 nm. For ZnP-S2-F₄-TCNQ 2c, the corresponding

differential absorption spectrum features maxima at 379, 576,

and 1133 nm as well as minima at 480 and 859 nm. The latter

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is again a result of the CT band deactivation. In all of the

investigated systems, resetting the applied potential to zero

leads to a restoring of the original spectrum as the reduction

signatures decrease.

semiempirical AM1^[29]Hamiltonian as implemented in VAMP^[30]

as well as the DNP/PBE level of theory. Both calculations gave

internal reorganization energies of approximately 0.4 eV for

TCNQ-based systems and 0.6 eV for push–pull chromophores

based on TCBD.

Quantum Chemical Calculations

Table 1. Electrochemical data of 2a–e and ZnP-S2 17 by CV at a scan rate

DFT gas-phase calculations using the Perdew, Burke, and

Ernzerhof generalized gradient approximation functional

(PBE)^[25]were performed with 1–3 (containing TCBD, TCNQ,

and F₄-TCNQ) using DMol3^[26]to obtain qualitative insights into

the electronic structure of the electron donor–acceptor

conjugates. The structures were first annealed with the forcite-

force field to determine the most stable conformers. These

were subsequently optimized using the PBE functional with a

double numeric basis set including polarization functions, a

dispersion correction,^[27]and effective core potentials for the Zn

atom.^[28]The molecular orbitals (MO) involved in the electronic

transitions were then visualized and analyzed. Finally, single-

point unrestricted calculations for the radical cation and anion

were performed on ground-state optimized structures and spin-

density plots used to visualize the regions involved in oxidation

and reduction (see Section S6 of the Supporting Information).

of

v = + 0.1 M nBu₄NPF₆, with the

0.1 Vs^–1and RDV in CH₂Cl₂

ferrocenium/ferrocene (Fc⁺/Fc) couple used as an internal standard.

CV

RDV

E° [V]^a)

E_p[V]^c)

E_1/2[V]

Slope

[mV]^d)

E_p

[mV]^b)

ZnP-S2-

TCBD

+0.84

+0.60

+0.30

–0.89

–1.21

–1.90

–2.27

65

+0.87 (1e^–)

+0.62 (1e^–)

+0.30 (1e^–)

–0.92 (1e^–)

60

100

120

90

2a

60

–1.28 (1e^–)

120

e)

125

100

ZnP-S2-

TCNQ

+0.63

90

+0.67

+0.45

+0.44 (1e^–)

+0.35 (1e^–)

–0.65 (1e^–)

60

2b

+0.32

–0.63

–0.75

–1.85

–2.24

80

85

The MOs and energies of the TCBD-derived conjugates were

compared to the corresponding reference compounds. In all

cases, the frontier MOs are either located on the electron

acceptors or donors, confirming that piperidine is a true

insulating spacer due to the lack of conjugation. Energetically,

the a_1u- and a_2u-porphyrin-centered molecular orbitals are

HOMO (highest occupied MO) and HOMO-1 with comparable

energies to zinc tetraphenylporphyrin (ZnTPP). Likewise, the

electron densities of LUMO (lowest unoccupied MO) and

LUMO+1 are located on the electron acceptor and significantly

lowered as the reduction strength is increased from, for

example, TCBD to F₄-TCNQ (see Figure S23 of the Supporting

Information). This is consistent with the electrochemical

studies. Overall, the length of the spacer does not impact the

acceptor energies. Increasing the spacer length does, however,

lead to a better stabilization of the bridge MOs, as shown for

ZnP-S1-TCBD 1a (LUMO+4). Because of the reduced

symmetry of the conjugates, the e_gx- (LUMO+2) and e_gy-

(LUMO+3) orbitals of the porphyrins split relative to the

degenerate ZnTPP. Nevertheless, the energetic splitting is less

than 0.04 eV in all cases. The HOMO-LUMO gaps of the

porphyrin centered a_1/2u- and e_g- orbitals are within 0.03 eV of

that of ZnTPP. Thus, the results confirm our interpretation of

the role of the linker, which is that it separates the HOMO from

the LUMO spatially and limits the degree of electronic

communication between the porphyrin donor and the push–pull

acceptors in order to achieve longer charge-separated state

lifetimes.

80

–0.78 (1e^–)

e)

120

80

ZnP-S2-F₄-

TCNQ

+0.63

150

+0.68 (2e^–)

120

+0.59

2c

+0.27

–0.23

–0.51

–1.85

75

79

85

95

+0.34 (1e^–)

–0.30 (1e^–)

–0.53 (1e^–)

–1.94 (1e^–)

60

110

ZnP-S2-

TriCBD

+0.86

+0.63

+0.32

–1.18

–1.46

–1.90

80

+0.90 (1e^–)

+0.73 (1e^–)

+0.35 (1e^–)

–1.22 (1e^–)

–1.50 (1e^–)

60

110

100

220

120

2d

75

60

130

ZnP-S2-

DCBD

+0.75

+0.62

+0.32

100

+0.76 (1e^–)

+0.65 (1e^–)

+0.33 (1e^–)

–1.66 (1e^–)

–1.94 (1e^–)

–2.23 (1e^–)

65

60

70

2e

–1.67

–1.93

–2.20

–2.31

ZnP-S2

17

+0.62

85

+0.57

+0.43

+0.44 (1e^–)

+0.35 (1e^–)

–1.88 (1e^–)

60

+0.33

65

90

–1.84

–2.28

^a)E^o= (E_pc+ E_pa)/2, where E_pcand E_pacorrespond to the cathodic and anodic

b)

c)

In addition, the internal reorganization energy was calculated

as the sum of the vertical reorganization energies from the

neutral to the cationic (λ+) and anionic (λ–) species using the

peak potentials, respectively.

ΔE_p= E_pa– E_pc

.

E_p= irreversible peak

d)

potential. Logarithmic analysis of the wave obtained by plotting E versus

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e)

Log[I/(I_lim–I)]; I_limis the limiting current and I the current. Further oxidation

and reduction not observed.

Photophysical Properties

Photoactive Porphyrin Conjugates. To shed light on the

electronic interactions between the photo- and redox-active

components, the absorption and fluorescence features of the

electron donor–acceptor conjugates were compared in toluene

with those of the individual components. We focus first on

conjugates 2 featuring all the different investigated acceptor

types with a center-to-center distance of 18.2 Å between the

electron donor and acceptors. Next, we discuss the influence of

the bridge length – leading to center-to-center distances

starting at 13.9 Å and reaching 25.1 Å – on the photophysical

properties (Table 3). To investigate the influence of the solvent

polarity on the photophysical properties, THF and benzonitrile

were used as additional solvents (see the Supporting

Information, Sections S7 and S8, for photophysical properties

of the reference compounds).

Figure 3. Absorption spectra of ZnP-S2-TCBD 2a (red line), ZnP-S2-TCNQ

2b (light grey line), ZnP-S2-F₄-TCNQ 2c (dark grey line), ZnP-S2-TriCBD 2d

(blue line), and ZnP-S2-DCBD 2e (black line) in toluene. Inset: fluorescence

spectra after excitation at 420 nm in toluene.

First insights into possible electron donor–acceptor interactions

came from fluorescence assays. Table 2 summarizes the

photophysical data taken from steady-state and time-resolved

fluorescence measurements. We note porphyrin-centered

fluorescence bands with maxima at 600 and 649 nm in all

systems. As for the reference porphyrins, the fluorescence is

subject to redshifts in polar solvents like benzonitrile with 611

and 664 nm maxima.

The absorption spectra of 2a–e in toluene are displayed in

Figure 3. As far as the Soret- and Q-band maxima are

concerned, no notable differences relative to the reference

compounds are noted. For 2a–e, an additional broad feature is

observable, which undergoes distinct redshifts as the reduction

strength of the electron acceptor increases. The latter is located

for ZnP-S2-DCBD 2e at 452 nm, for ZnP-S2-TriCBD 2d at 459

nm, for ZnP-S2-TCBD 2a at 466 nm, for ZnP-S2-TCNQ 2b at

630 nm, and for ZnP-S2-F₄-TCNQ 2c at 783 nm. The origin of

this feature is a CT transition involving the terminal electron

For 2a–e, the fluorescence quenching is in line with the

reduction strength of the different acceptors. In particular, the

strongest fluorescence quenching is derived for the F₄-TCNQ

containing conjugate ZnP-S2-F₄-TCNQ 2c (Ф_fl= 1.5 x 10^–4).

The fluorescence quenching weakens from ZnP-S2-TCNQ 2b

(Ф_fl= 2.5 x 10^–4) to ZnP-S2-TCBD 2a (Ф_fl= 1 x 10^–3), to ZnP-

S2-TriCBD 2d (Ф_fl= 1.6 x 10^–2), and to ZnP-S2-DCBD 2e (Ф_fl=

5 x 10^–2). ZnP-S2-DCBD 2e, endowed with only two

withdrawing CN-groups, lacks any affinity for energy- or

electron transfer. The distance between the porphyrin donors

acceptors and the N-phenylpiperidyl nitrogens. Table

2

summarizes the absorption bands for 2a–e. The CT band is

even further redshifted when increasing the solvent polarity to

THF and benzonitrile. The strongest shifts evolved for the

TCNQ based conjugates, that is, ZnP-S2-TCNQ 2b and ZnP-

S2-F₄-TCNQ 2c with shifts up to 705 and 865 nm, respectively,

in benzonitrile.

Noteworthy is the impact of an additional aniline substitution in

addition to TCBD in ZnP-S4-TCBD 4 and ZnP-S5-TCBD 5.

The absorption spectra reveal a broadening and a shift of the

CT features between the respective nitrogens in aniline and/or

piperidine and TCBD. Independent of the additional ethynediyl

unit next to piperidine as in ZnP-S4-TCBD 4 or next to aniline

as in ZnP-S5-TCBD 5, the CT band maximizes at 514 nm.

and acceptors plays

a crucial role with regard to the

fluorescence quenching. A comparison between the TCBD

based conjugates in toluene shows that the weakest quenching

is observed for ZnP-S1-TCBD 1a (Ф_fl= 1 x 10^–2) with a

distance between electron donor and acceptor (R_DA) of 25.1 Å.

The quenching intensifies towards ZnP-S3-TCBD 3a (Ф_fl

=

6 x 10^–4) with an R_DAof 13.9 Å. Taking the aforementioned

trends into concert, ZnP-S3-F₄-TCNQ 3c reveals strongest

quenching with a fluorescence quantum yield of 9 x 10^–5. In

THF and benzonitrile (PhCN), the fluorescence quenching

further intensifies. Overall, the fluorescence lifetimes match the

trend seen for the fluorescence quantum yields (Table 2). In

most cases, an exact determination of the fluorescence

lifetimes is, however, hampered by the time resolution of

200 ps for our instrumentation.

The absorption spectra as a function of the electron donor–

acceptor distances reveal no notable changes in terms of

position and intensity of the porphyrin and acceptor relevant

features. The bridge disrupts the electronic communication

between the photo- and redox-active components in the

conjugates at distances of at least 13.9 Å. Evidence for this

postulate came from absorption spectra, which are best

described as the simple superimpositions of the component

spectra. Notably, these results back up the electrochemical

data as well as the quantum chemical calculations.

Thermodynamics and Driving Forces of Electron Transfer.

Taking the aforementioned steady-state and time-resolved

measurements into concert, we infer – next to the intrinsic

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deactivation channels of the photoexcited porphyrins

–

centered transition as high as 2.7 eV. This is substantially

higher than the porphyrin centered transition at 2.1 eV,

rendering an energy transfer from the porphyrin to TCBD

impossible. In other words, the porphyrin fluorescence

additional decay pathways in the electron donor–acceptor

conjugates, especially for the multicomponent arrays featuring

F₄-TCNQ, TCNQ, and TCBD electron acceptors. Still, for ZnP-

S2-TriCBD 2d and ZnP-S2-DCBD 2e minor and/or almost no

fluorescence quenching was noted. We hypothesize that

neither energy nor electron transfer – evolving from the

quenching must relate to

transfer.

a porphyrin-to-TCBD electron

The situation changes for those porphyrin conjugates featuring

extended TCNQs. As an example, we note for ZnP-S2-TCNQ

2b that the absorption onset of TCNQ coincides in toluene

(1.97 eV) with that of the porphyrin. When increasing the

solvent polarity, the energetically lowest-lying TCNQ excited

state decreases in energy to 1.87 eV in THF and 1.76 eV in

benzonitrile. Likewise, an energy transfer should be taken into

consideration for ZnP-S2-F₄-TCNQ 2c, as even in toluene the

lowest electronic level is as low as 1.58 eV.

photoexcited porphyrin

– takes place. At this point we

estimated the energetics of the energy and electron transfer

reactions. Considering that the electron acceptors lack notable

fluorescence, the energies of their lowest-lying electronic states

were extrapolated from their corresponding absorption spectra.

In the case of TriCBD and DCBD, the lowest energetic states

with absorption onsets at 459 nm for ZnP-S2-TriCBD 2d and at

452 nm for ZnP-S2-DCBD 2e leading to singlet excited state

energies of around 2.75 eV, are substantially higher than that of

the porphyrins at 591 nm, which corresponds to 2.1 eV.

Accordingly, a porphyrin to TriCBD and/or a porphyrin to DCBD

energy transfer processes are ruled out thermodynamically.

When turning to TCBD, a similar picture is noted, namely the

lowest localized electronic states in the TCBD-containing

porphyrin conjugates are porphyrin centered. For example, we

note in the absorption spectra the presence of a TCBD

To elucidate if, in addition to an energy transfer,

a

thermodynamically supported electron transfer may govern the

porphyrin singlet excited state deactivation in the conjugates,

we determined the corresponding thermodynamic driving forces

in different solvents by using the Rehm-Weller approach.^[31]

Table 2. Photophysical data of porphyrin conjugates 1–9 together with porphyrin reference compounds.

λ_max(CT-band)^[a][nm]

Ф_fl

τ ^[b][ns]

THF

toluene

THF

-

PhCN

toluene

5.5∙10^–2

5∙10^–2

THF

PhCN

5.5∙10^–2

5∙10^–2

5.5∙10^–2

5∙10^–2

6∙10^–3

5∙10^–4

1∙10^–4

1.4∙10^–3

5∙10^–5

9∙10^–5

1∙10^–4

8∙10^–3

3∙10^–2

1.8∙10^–4

3∙10^–4

1.4∙10^–5

2∙10^–3

1∙10^–2

1.1∙10^–2

toluene

2.05

PhCN

1.75

ZnP-S1 16

-

5.5∙10^–2

5∙10^–2

5.5∙10^–2

5∙10^–2

9∙10^–3

6∙10^–4

2∙10^–4

1.6∙10^–3

8∙10^–5

9∙10^–5

1∙10^–2

3∙10^–2

2∙10^–4

3.5∙10^–4

1.4∙10^–5

2∙10^–3

7∙10^–3

1.2∙10^–2

1.87

ZnP-S2 17

-

2.11

1.85

1.69

ZnP-S3 20

-

5.5∙10^–2

5∙10^–2

2.05

1.92

1.77

ZnP-S4 21

-

2.03

1.89

1.72

ZnP-S1-TCBD 1a

ZnP-S2-TCBD 2a

ZnP-S3-TCBD 3a

ZnP-S1-TCNQ 1b

ZnP-S2-TCNQ 2b

ZnP-S3-TCNQ 3b

ZnP-S2-F₄-TCNQ 2c

ZnP-S3-F₄-TCNQ 3c

ZnP-S2-TriCBD 2d

ZnP-S2-DCBD 2e

ZnP-S4-TCBD 4

ZnP-S5-TCBD 5

ZnP-S6-TCBD 6

ZnP-S7-TCNQ 7

ZnP-S6-TCNQ 8

ZnP-S8-TCBD (±)-9

466

630

783

459

452

514

461

588

648

466

474

662

860

465

456

517

468

618

678

474

486

705

865

476

462

527

476

657

717

486

1∙10^–2

1∙10^–3

0.423

< 0.2^[c]

0.57

0.324

< 0.2^[c]

0.33

0.298

< 0.2^[c]

0.34

6∙10^–4

2∙10^–3

2.5∙10^–4

1.5∙10^–4

9∙10^–5

1.6∙10^–2

5∙10^–2

1.82

1.71

1.59

3∙10^–4

< 0.2^[c]

0.43

< 0.2^[c]

0.80

< 0.2^[c]

0.39

< 0.2^[c]

0.52

< 0.2^[c]

0.41

< 0.2^[c]

0.40

4.5∙10^–4

1.2∙10^–5

3∙10^–3

5∙10^–3

1.5∙10^–2

[a] Charge-transfer band of the respective push–pull chromophore in conjugates 1–9; [b] λ_exc= 403 nm, TCSPC laser diode; [c] under the limit of instrumental

resolution.

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The latter is used as an estimation of the Gibbs energy for

radical ion pair formation in solvents of different dielectric

constants ε_s:^[32]

state formation (∆퐺_퐶⁰_푆= –0.15 eV). Thermodynamically, the

most favored radical ion pair state formation is estimated for

ZnP-S2-F₄-TCNQ 2c with a driving force of –0.96 eV in toluene.

0

°

∗

]

∆퐺_퐶푆= [푒(퐸_퐷,표푥− 퐸_{퐴,푟푒푑}) − 퐸₀₀(푍푛푃) − 퐸_퐶

The driving force for an electron transfer depends on the

distance between the electron donors and acceptors according

to the Rehm-Weller equation. To this end, shortening the

spacer exemplary from ZnP-S1-TCBD 1a to ZnP-S3-TCBD 3a

increases the driving force from –0.03 up to –0.29 eV.

where e is the elementary charge, E°_D,oxand E°_A,redare the first

oxidation and reduction of the electron donor and acceptor,

respectively, measured in a solvent with ε_ref(CH₂Cl₂: 8.93), E₀₀

is the zero-zero transition energy of the first singlet excited

state of the porphyrin (2.1 eV), and E_Cis a correction factor

based on the Born dielectric continuum model accounting for

Coulombic interactions in the radical ion pair state in a solvent

of dielectric constant ε_s:

Overall, increasing the solvent polarity to, for example,

benzonitrile increases the driving force for electron transfer in

all of the conjugates due to the lower energy of the polar

charge-separated state. Table 3 summarizes the driving forces

in the different solvents.

e²

4πε₀ε_sR_DA4πε₀2r_D

1

e²

1

E_C=

+

[(

+

) (

-

)]

Excited State Processes. Deeper insights into the nature of the

intramolecular deactivation processes came from ultrafast

transient absorption spectroscopy, which was recorded after

420 nm excitation in toluene, THF, and benzonitrile (see

Section 8 of the Supporting Information). The transient features

were analyzed by multiwavelength as well as global and target

analyses. The ZnP-centered second singlet excited state

excitation of ZnP-S2-DCBD 2e results within 2 ps in the

formation of the first singlet excited state due to internal

conversion. Most notable are minima at 555 and 602 nm as

well as maxima at 464, 581, and 627 nm. This is followed by

intersystem crossing to the corresponding triplet excited state

of ZnP. The ZnP singlet excited state features are short lived

since they decay with a lifetime of 2.1 ns in toluene. The

corresponding times in THF and benzonitrile were determined

to be 1.9 and 1.8 ns, respectively, which match those of the

+

2r_A

-

ε_refε_s

where ε₀is the permittivity of vacuum (8.854 Fm^–1), R_DAis the

electron donor–acceptor separation, and r_D+and r_A–are the

ionic radii of the electron donor and acceptor, respectively. To

describe the separation more accurately, the center-to-center

distances rather than the corresponding edge-to-edge

distances were taken. The values are collected in Table 3.

In toluene, the data suggests that for the five different electron

acceptors the radical ion pair state formation is

thermodynamically unlikely in those porphyrin conjugates

containing DCBD or TriCBD, namely ZnP-S2-DCBD 2e (∆퐺_퐶⁰_푆

+0.54 eV) and ZnP-S2-TriCBD 2d (∆퐺_퐶⁰_푆= +0.04 eV). This is in

line with the fluorescence quantum yields, which disclosed no

appreciable quenching for ZnP-S2-DCBD 2e. For ZnP-S2-

TCBD 2a, the presence of the stronger electron accepting

TCBD evokes a significant driving force for the radical ion pair

=

reference

compounds

16,

17,

20,

and

21.

Table 3. Gibbs free energies of charge separation and recombination, and the parameters used for the calculations according to the Weller equation.^[a]

[b]

[c]

[d]

[e]

∆퐺_퐶⁰_푆[eV]

∆퐺_퐶⁰_푅[eV]

E°_ox

E°_red

[V]

r_D+

[Å]

r_A–

[Å]

R_cc

[V]

[Å]

toluene

–0.03

–0.15

–0.29

–0.36

–0.49

–0.59

–0.96

–0.97

+0.04

+0.54

–0.28

–0.16

–0.17

–0.46

+0.13

THF

PhCN

–1.16

–1.20

–1.24

–1.37

–1.42

–1.86

–1.78

–0.86

–0.39

–1.21

–1.25

–1.23

–1.12

–1.43

–1.00

toluene

–2.07

–1.95

–1.81

–1.74

–1.61

–1.51

–1.14

–1.13

–2.14

–2.64

–1.82

–1.94

–1.93

–1.64

–2.23

THF

PhCN

ZnP-S1-TCBD 1a

ZnP-S2-TCBD 2a

ZnP-S3-TCBD 3a

ZnP-S1-TCNQ 1b

ZnP-S2-TCNQ 2b

ZnP-S3-TCNQ 3b

ZnP-S2-F₄-TCNQ 2c

ZnP-S3-F₄-TCNQ 3c

ZnP-S2-TriCBD 2d

ZnP-S2-DCBD 2e

ZnP-S4-TCBD 4

ZnP-S5-TCBD 5

ZnP-S6-TCBD 6

ZnP-S7-TCNQ 7

ZnP-S6-TCNQ 8

ZnP-S8-TCBD (±)-9

+0.36

+0.30

+0.31

+0.35

+0.32

+0.30

+0.27

+0.30

+0.32

+0.33

+0.36

+0.35

–0.86

–0.89

–0.85

–0.64

–0.63

–0.66

–0.23

–0.29

–1.18

–1.66

–0.85

–0.81

–0.79

–0.89

–0.58

–1.03

4.7

3.6

4.3

4.4

4.3

3.8

4.4

3.6

25.1

18.2

13.9

25.1

18.2

13.9

18.2

13.9

18.2

14.5

16.0

22.9

20.6

22.9

25.1

–0.88

–0.94

–1.00

–1.12

–1.19

–1.21

–1.64

–1.58

–0.64

–0.16

–0.98

–1.01

–0.96

–0.89

–1.19

–0.72

–1.22

–1.16

–1.10

–0.98

–0.91

–0.89

–0.46

–0.52

–1.46

–1.94

–1.12

–1.09

–1.14

–1.21

–0.91

–1.38

–0.94

–0.90

–0.86

–0.73

–0.68

–0.24

–0.32

–1.24

–1.71

–0.89

–0.85

–0.87

–0.98

–0.67

–1.04

[a] Values for all physical constants are taken from ^[32]; [b] measured in CH₂Cl₂, see Table 1 and Supporting Information; [c] estimated from X-ray structure data

and molecular volume calculations; [d] estimated from the molecular volume of the acceptor moiety;^[33][e] center-to-center distance of the donor and acceptor π-

systems; see the Supporting Information for more details.

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Spectroscopic evidence for the triplet excited state population

comes from the characteristic 476 and 844 nm maxima.

Importantly, the same features were seen in 420 nm

photoexcitation experiments with ZnP-S2 17. This, in turn,

attests to the exclusive ZnP excitation and deactivation in ZnP-

S2-DCBD 2e, as our aforementioned experiments and

calculations already suggested neither an energy transfer nor

an electron transfer from the porphyrin to the acceptor are likely

to happen (Figure 4c).

with an intersystem crossing to the ZnP triplet excited state as

evidenced by the spectroscopic markers at 473 and 844 nm.

However, in line with thermodynamics, the ZnP triplet excited

state formation is only enabled via intersystem crossing. In the

case of the shorter-linked TCBD-based ZnP-S3-TCBD 3a (see

Section S8 in the Supporting Information), we also noticed

photoinduced charge separation followed by charge

recombination, upon excitation of the porphyrin in the three

different solvents. A minimum at 469 nm and a maximum at

625 nm document the ZnP^·+-S3-TCBD^·–generation. In line with

the increased driving force for charge separation and the

stronger fluorescence quenching, the charge separation is

facilitated from 19 ± 3 ps in toluene up to 6 ± 1.5 ps in

benzonitrile, while charge recombination to reinstate the ground

state takes place with 147 ± 5 ps in toluene and 31 ± 3 ps in

benzonitrile. Finally, ZnP-S1-TCBD 1a with an R_DAof 25.1 Å

shows the singlet excited state features of ZnP. These decay

with triexponential and solvent dependent dynamics. For

example, from multiple wavelength analysis in benzonitrile we

derived lifetimes of 2.1 ± 0.1 ns, 232 ± 7 ps, and 16 ± 3 ps. The

former is attributed to the singlet excited state decay via

intersystem crossing leading to the corresponding triplet excited

state as shown by the characteristic fingerprints at 481 and 850

nm. Interestingly, no spectroscopic proof for the formation of

any appreciable photoproduct is observable at first glance.

However, we noticed that in benzonitrile, the porphyrin

fluorescence quantum yield of 6 x 10^–3and the singlet excited

lifetime of 298 ps are quenched in ZnP-S1-TCBD 1a relative to

ZnP-S1 16, while a porphyrin to TCBD energy transfer is

Different is the transient behavior in conjugate ZnP-S2-TriCBD

2d (Figure 4b). Analysis of the differential absorption changes

recorded after 2 ps, attributes, which are in accordance with the

formation of the ZnP singlet excited state – like in ZnP-S2 17 –

are seen. Intersystem crossing to the triplet excited state of

ZnP is evidenced by the transients at 475 and 844 nm. A closer

look reveals the formation within 2 ns to a minor extent. In

doing so, a second transformation out of the ZnP singlet

excited state sets in to generate a bleaching between 468 and

511 nm. The latter resembles the spectroelectrochemical

analysis, where the reduction of the electron acceptor leads to

a bleaching of the respective CT band. In addition, a distinct

evolution of the one electron oxidized ZnP radical cation

fingerprint at around 620 nm is observable. As such, the newly

formed transients relates to the ZnP^·+-S2-TriCBD^·–radical ion

pair state. Multiwavelength analyses gave charge separation

dynamics of 200 ± 20 ps. The slow dynamics are in line with

the thermodynamics according to the Weller approach, where

we estimated that the charge-separated state in the rigid

structure is nearly isoenergetic to the porphyrin singlet excited

state in toluene (∆퐺_퐶⁰_푆= +0.04 eV). The radical ion pair state is,

however, metastable and decays within 1200 ± 50 ps.

Increasing the solvent polarity increases the driving force for

the charge separation. As a consequence, formation and decay

of ZnP^·+-S2-TriCBD^·–radical ion-pair state takes place within

29 ± 2 and 122 ± 5 ps in THF (∆퐺_퐶⁰_푆= –0.64 eV), respectively.

In benzonitrile (∆퐺_퐶⁰_푆= –0.86 eV), the values are 32 ± 7 and 107

± 5 ps.

thermodynamically not allowed. Thus,

a

reasonable

interpretation is an electron transfer from ZnP to TCBD, with a

slow charge separation (232 ± 7 ps) and a much faster charge

recombination.

Similarly, excitation of the TCNQ based ZnP-S1/S2/S3-TCNQ

1–3b at 420 nm results in the generation of a charge-separated

state. In ZnP-S2-TCNQ 2b we see a fast transformation of the

ZnP singlet excited state into the charge-separated ZnP^·+-S2-

TCNQ^·–. With time delays of 2 ps, features in the visible and in

the near infrared regions develop that are in perfect agreement

with the spectroelectrochemically generated spectra of the one-

electron reduced TCNQ radical anion and the one-electron

oxidized ZnP radical cation. To this end, the bleaching in the

differential absorption spectra at around 600 nm in toluene

and/or 700 nm in benzonitrile are clear fingerprints of the TCNQ

centered reduction, whereas the maximum at 640 nm is due to

the oxidation of ZnP. Kinetically, 8 ± 1 and 140 ± 10 ps were

determined to be the charge separation and charge

recombination dynamics in toluene, respectively. Charge

recombination from ZnP^·+-S2-TCNQ^·–2b leads to the

population of the ground state. This is compared to the values

of 4 ± 1 ps (THF) / 3 ± 0.5 ps (benzonitrile) for charge

separation and 31 ± 5 ps (THF) / 20 ± 2 ps (benzonitrile) for

charge recombination. Here, similar to ZnP-S2-TCBD 2a in

THF and benzonitrile, intersystem crossing to the triplet excited

state of ZnP results.

A similar picture unfolds for ZnP-S2-TCBD 2a (Figure 4a),

which upon photoexcitation is subject to a charge-transfer from

the singlet-excited state of ZnP to the electron accepting TCBD.

With time delays of 2 ps, only spectral characteristics of the

ZnP singlet excited state are detectable. In particular, ground

state bleaching of the Soret- and Q-bands is seen in the 400 to

450 and 500 to 650 nm regions, respectively. These singlet

excited state features decay with dynamics of 90 ± 10 ps in

toluene to afford a new product with a transient minimum at 470

nm and a transient maximum at 625 nm. These transients are

in perfect agreement with those established for the one-

electron reduced TCBD radical anion and the one-electron

oxidized ZnP radical cation. The ZnP^·+-S2-TCBD^·–is short lived

with 510 ± 20 ps and decays to the ground state. Kinetic

analysis in THF and benzonitrile leads to charge

separation/recombination dynamics of 57 ± 5 / 5 ± 1.5 ps and

18 ± 3 / 70 ± 5 ps, respectively. Interestingly, electron transfer

from the ZnP singlet excited state competes strongly in THF

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Figure 4. Species-associated spectra and (inset) associated time dependent amplitudes and fits of the femtosecond flash photolysis (420 nm, 200 nJ) of a) ZnP-

S2-TCBD 2a; b) ZnP-S2-TriCBD 2d; c) ZnP-S2-DCBD 2e in argon-saturated toluene (optical density, OD = 0.5) monitoring the charge separation and charge

recombination processes (2a, 2d) and intersystem crossing (2e), respectively.

conjugates 6–9 show the longest lifetimes in toluene overall.

For example, multiwavelength analysis afforded charge-

separated state lifetimes of around 13 ± 0.5 ns in the TCBD-

Comparing the results with the longer spaced ZnP-S1-TCNQ

1b (R_DA= 25.1 Å) and the shorter spaced ZnP-S3-TCNQ 3b

derived ZnP-S6-TCBD 6 and of around 5.6 ± 0.4 ns in the

(R_DA= 13.9 Å) show a clear trend for the charge separation

TCNQ-derived ZnP-S6-TCNQ 8. In summary, the aliphatic, not

dynamics that follow the results based on the Rehm-Weller

approach and the fluorescence quantum yields. As a matter of

fact, ZnP-S3-TCNQ 3b with a more exogenous ∆퐺_퐶⁰_푆of

–0.59 eV reveals a faster charge separation with 5 ± 1 ps in

toluene, while the electron transfer from ZnP to TCNQ in ZnP-

S1-TCNQ 1b occurs within 21 ± 2 ps as a consequence of the

smaller ∆퐺_퐶⁰_푆of –0.36 eV and greater distance between

electron donor and acceptor. However, the spatial separation in

ZnP-S1-TCNQ 1b leads to charge recombination dynamics of

279 ± 20 ps, which are slower than for ZnP-S2-TCNQ 2b and

ZnP-S3-TCNQ 3b (71 ± 6 ps).

fully conjugated linkage seems to slow down, on one hand, the

charge separation and, on the other hand, the charge

recombination dynamics. As a matter of fact, slow charge

separation in THF and benzonitrile is followed by even faster

charge recombination, thereby preventing

a

reasonable

analysis of the charge recombination kinetics (see Section S9

of the Supporting Information for further explanations).

Table 4. Lifetimes for charge separation and recombination in the electron

donor–acceptor conjugates 1–9 in toluene.

Finally, the ZnP singlet excited state was generated and probed

in ZnP-S2-F₄-TCNQ 2c and ZnP-S3-F₄-TCNQ 3c (see Section

S8 in the Supporting Information). Immediately after 420 nm

excitation, a strong broad bleaching around 780 nm in toluene

develops. This transient bleaching is accompanied by a broad

absorption in the near infrared region. The features are a

perfect spectral resemblance to the spectroelectrochemical

studies, verifying the presence of the one electron reduced F₄-

TCNQ radical anion after an electron transfer from ZnP. The

lifetime of the radical ion pair state was gauged at 35 ± 5 / 21 ±

2 / 15 ± 2 ps for ZnP-S2-F₄-TCNQ 2c and 12 ± 2 / 63 ± 5 / 19 ±

3 ps for ZnP-S3-F₄-TCNQ 3c in toluene / THF / benzonitrile. In

competition with electron transfer is intersystem crossing to the

triplet excited state of ZnP, which shows the highest quantum

yields in THF compared to toluene and benzonitrile.

Toluene

τ_CS[ps]

460 ± 20

90 ± 10

19 ± 3

τ_CR[ps]

680 ± 20

ZnP-S1-TCBD 1a

ZnP-S2-TCBD 2a

ZnP-S3-TCBD 3a

ZnP-S1-TCNQ 1b

ZnP-S2-TCNQ 2b

ZnP-S3-TCNQ 3b

ZnP-S2-F₄-TCNQ 2c

ZnP-S3-F₄-TCNQ 3c

ZnP-S2-TriCBD 2d

ZnP-S4-TCBD 4

ZnP-S5-TCBD 5

ZnP-S6-TCBD 6

ZnP-S7-TCNQ 7

ZnP-S6-TCNQ 8

ZnP-S8-TCBD (±)-9

510 ± 20

147 ± 5

21 ± 2

279 ± 20

140 ± 10

71 ± 6

8 ± 1

5 ± 1

6 ± 1

35 ± 5

3 ± 0.5

200 ± 20

15 ± 1

12 ± 2

1200 ± 50

118 ± 5

13 ± 1

115 ± 5

300 ± 30

115 ± 10

135 ± 15

931 ± 30

13000 ± 500

24000 ± 500

5600 ± 400

23000 ± 500

Table 4 summarizes the results for the electron donor–acceptor

conjugates in toluene, including the electron transfer dynamics

for conjugates 4–9. The respective spectra are gathered in

Section 8 of the Supporting Information. Comparing the results

for ZnP-S4-TCBD 4 with ZnP-S5-TCBD 5 shows that the

additional ethynediyl – connected to piperidyl or next to aniline

– has almost no influence on the electron transfer kinetics. This

observation supports the assumption that the electron-

accepting LUMO of the electron acceptor is spread out over the

whole push–pull chromophore structure, which is in conjugation

despite the twisted structure. In terms of charge recombination,

Analysis according to the Marcus theory

In the final step of our investigations, we analyzed the charge

transfer dynamics in the different conjugates by correlating

them with the free energy changes of the underlying electron

transfer. The rate constant (k_ET) for nonadiabatic electron

transfer reactions, in which electronic coupling between the

reactant and product potential is weak, can be described by:

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tunneling in the Marcus inverted region and explain the

acceleration of charge recombination processes.^[35]Please note

that the vibrational quantum ℏ휔 is taken as 2200 cm^–1

corresponding to CN stretching vibrations.^[36]

2

(

)

휋

− 휆_푅+ ∆퐺⁰

| |²

푘_퐸푇

=

푉

푒푥푝 [

]

√

ℏ²휆_푅푘_퐵푇

4휆_푅푘_퐵푇

The preexponential term consists of the Planck constant ℏ, the

Boltzmann constant k_B, the absolute temperature T, the total

reorganization energy λ_R, and the electronic matrix element V.

The latter describes the coupling of the reactant state with that

of the product and dictates the rate of electron transfer. Hereby,

V depends on the spatial overlap of the electron donor and

acceptor orbitals. Finally, the driving forces for electron transfer

are expressed by ΔG⁰. Going a step further, the semiclassical

approach of the Marcus theory includes nuclear factors,

especially electron-vibrational couplings, giving the deviation of

the Marcus plot from the classical parabolic shape:

In the same way, the characteristic dependence known from

the semiclassical Marcus approach on electron transfer was

established for conjugates 2 (constant R_DA=18.2 Å) as well as

conjugates 1 and 6–9 (R_DA= 20.6–25.1 Å). A closer look at

electron donor–acceptor conjugates 2 in toluene reveals that

the dynamics for charge separation are located in the normal /

top region, whereas for charge recombination the dynamics are

in the inverted region of the Marcus parabola (Figure 6 middle).

On the one hand, by changing the acceptor TriCBD in 2d to F₄-

TCNQ in 2c and, thus, increasing the driving force from +0.04

to –0.96 eV the charge separation is accelerated nearly 50 fold.

On the other hand, the charge recombination decelerates in the

electron donor–acceptor conjugates with the weakest acceptor,

that is, in ZnP-S2-TriCBD 2d, while the charge recombination

in ZnP-S2-F₄-TCNQ 2c is located near to the top region of the

Marcus parabola. Similar is the behavior for conjugates 1 and

6–9 (Figure 6 bottom).

∞

0

2

푒^−푆푆^휈

휈!

− 휈ℏ휔 + 휆_푆+ ∆퐺

(

)

휋

| |²

푘_퐸푇

=

푉

∑

푒푥푝 [

]

√

ℏ²휆_푆푘_퐵푇

4휆_푆푘_퐵푇

휈

The total reorganization energy λ_Ris split into the solvent (λ_S)

and internal/vibrational reorganization energy (λ_ν). S is the

Huang-Rhys factor, which characterizes the strength of

electron-vibrational (e-ν) coupling and

ω relates to the

averaged frequency of the coupled quantum mechanical

vibration modes:

휆_푉

푆 =

ℏ휔

The semiclassical approach enables the explanation for

quantum chemical vibrational effects and explains the

occurrence of electron tunneling effects.^[34]

We constructed the Marcus curves by plotting the electron

transfer rates of the conjugates with nearly the same R_DA

versus the corresponding thermodynamic driving forces –

neglecting minor differences in the internal reorganization

energies of the respective push–pull acceptors and assuming

almost similar electronic couplings V for CS and CR. Figure 6

top, for example, displays all electron donor–acceptor

conjugates with R_DAbetween 13.9 and 16 Å in toluene. The

influence of the push–pull chromophores on the charge-transfer

dynamics is clearly discernable. On one hand, increasing the

reduction strength of the latter expedites the charge separation

as in ZnP-S3-F₄-TCNQ 3c and moves the underlying kinetics to

the top of the Marcus curve. On the other hand, the slowest

charge recombination is realized for TCBD based ZnP-S3-

TCBD 3a featuring the weakest electron accepting part.

Detailed analyses reveal that the rate of charge recombination

decreases much more slowly than that of the charge

separation, which leads to a leveling off of the Marcus inverted

region. Here, the semiclassical Marcus approach rather than

the classical Marcus approach (for comparison see Section S9

of the Supporting Information) is necessary to obtain

reasonable fits, from which we derive sizeable electron-

vibration couplings. It is fair to conclude that those push–pull

based electron acceptors, which are endowed with highly

polarized CN bonds, favor vibrational interactions with

electrons. These open a channel of vibration-assisted electron

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charge

recombination

is

now

placed

in

the

normal/activationless regions of the Marcus parabola rather

than in the inverted region (see Section S9 of the Supporting

Information).

Summary and Conclusions

We have successfully shown that our multicyanobuta-1,3-

dienes and tetracyanoquinodimethane analogues offer

synthetically viable alternatives to established electron

acceptors (e.g. fullerenes, perylenediimides, quinones) for the

preparation of multicomponent arrays featuring photoinduced

electron transfer. Notably, data from the latter as well as X-ray

crystal structures show that the conjugates are geometrically

well-defined systems, with the electron donors and acceptors

held at fixed center-to-center distances of approximately 13.9 Å

up to 25.1 Å. Therefore, we can conclude that any

intramolecular interactions, such as electron transfer must

result from through-bond effects. Förster resonance energy

transfer is unlikely due to a lack of spectral overlap between the

porphyrin and acceptor moieties.

Figure 6. Driving-force (–ΔG⁰_ET) dependence of k_CS(filled symbols) and k_CR

(open symbols) with the semiclassical Marcus approach.

Top: ZnP-S3-TCBD 3a (blue), ZnP-S4-TCBD 4 (grey), ZnP-S5-TCBD 5

(orange), ZnP-S3-TCNQ 3b (green), ZnP-S3-F₄-TCNQ 3c (dark green) with

R_DA= 13.9–16 Å in toluene. The fitting parameters are λ_S= 0.30 eV, λ_ν=

0.49 eV and V = 30.9 cm^–1

.

Middle: ZnP-S2-TriCBD 2d (blue), ZnP-S2-TCBD 2a (grey), ZnP-S2-TCNQ

2b (orange), ZnP-S2-F₄-TCNQ 2c (dark green) with R_DA= 18.2 Å in toluene.

The fitting parameters are λ_S= 0.31 eV, λ_ν= 0.45 eV and V = 28.4 cm^–1

.

Based on the optoelectronic results, we are able to fine-tune

the photophysical properties of the electron donor–acceptor

systems by changing the spacer, the absorption properties, and

the strength of the electron acceptor. For the latter, we

synthesized different acceptors with first reduction potentials

between –0.23 and –1.67 V (vs. Fc⁺/Fc). To the best of our

knowledge, the reduction potential of F₄-TCNQ-based push–

pull acceptors is the lowest incorporated in electron donor–

acceptor systems so far. In terms of electron transfer, we

derived the typical Marcus curves from the semiclassical

approach, indicating longest lifetimes (up to 23 ns) for the

charge-separated state in nonpolar solvents such as toluene,

as the charge recombination dynamics are placed in the

Marcus inverted region. The overall rate enhancement for

charge recombination is explained by large e-ν couplings of the

push–pull acceptors. Analysis of the electron transfer dynamics

using Marcus theory gives reorganization energies for these

systems between 0.66 and 0.79 eV and electronic coupling

matrix elements between 13.9 and 30.9 cm^–1. The values

encourage the development of further push–pull derived

electron donor–acceptor systems as future platform for

photosynthetic mimics and organic photovoltaic applications.

As such, the rich redox chemistry, broad absorption in the

visible and near infrared region, and excellent thermal as well

as chemical stability may exert a noteworthy impact on the

improvement of photoinduced charge-transfer processes in

organic photovoltaics.

Bottom: ZnP-S8-TCBD 9 (green), ZnP-S1-TCBD 1a (blue), ZnP-S7-TCNQ 7

(orange), ZnP-S6-TCBD 6 (grey), ZnP-S1-TCNQ 1b (violet), and ZnP-S6-

TCNQ

8 (dark green) with R_DA= 20.6–25.1 Å in toluene. The fitting

parameters are λ_S= 0.31 eV, λ_ν= 0.34 eV and V = 13.9 cm^–1

.

In general, the derived electronic couplings with values

between 13.9 and 30.9 cm^–1reflect the overall distance

dependence. At the shortest distance in, for example,

conjugates 3, the largest value of 30.9 cm^–1evolves, while for

the longest distance in conjugates 1 and 9 the value is as low

as 13.9 cm^–1.

Notably, the internal reorganization λ_νenergy with 0.45–

0.49 eV is in sound agreement with our calculations for TCNQ

and TCBD-based systems (vide supra). For conjugates with

R_DAbetween 20.6 and 25.1 Å, the internal reorganization

energy is reduced to 0.34 eV, which points to the role of more

rigid linkers in, for example, conjugates 6–9. Please note that

the parameters for reorganization energy and electronic

coupling are averaged values and might slightly differ from

conjugate to conjugate. The access to the inverted region of the

Marcus curve and the derived total reorganization energies λ_R

(0.66–0.79 eV) for photoinduced electron transfer are of great

value, since for the first time these are established for

acetylene-derived non-planar push–pull chromophores as

electron acceptors in D–A conjugates. The values for λ_Rare

remarkably small compared to other artificial model acceptors

(0.8–1.4 eV)^[3a,3d,37]and compete with those seen for fullerene-

based systems.^[3f,38]Taking the aforementioned into account,

smaller λ_Rcombine faster charge separation and slower charge

recombination for our push–pull derived systems.

Experimental Section

All experimental methods, synthesis, and characterization are

provided in the Supporting Information (Sections S1, S2 and

S10).

Solvent polarity impacts the electron donor–acceptor

conjugates not only in the ground state but also in the excited

state. In fact, it increases the reorganization energy.

Noteworthy, in THF and benzonitrile the solvent rearrangement

is affected during an electron transfer. Based on larger λ_R, the

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Acknowledgements

This work was supported by the ERC Advanced Grant No.

246637 ("OPTELOMAC"), the Swiss National Science

Foundation (SNF 200020_159802), and the “Solar

Technologies Go Hybrid” network from the Bavarian state. We

thank Dr. Bruno Bernet (ETH) for his assistance in the spectral

evaluations, Cagatay Dengiz (ETH) for reviewing the

manuscript, and Michael Solar (ETH) for his help with collecting

X-ray structures. “Universität Bayern e.V.” is acknowledged for

the support for Michael Sekita.

Keywords: Porphyrinoids • Electron transfer • Donor–acceptor

systems • Marcus theory • Photochemistry

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10.1002/chem.201700043

Chemistry - A European Journal

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A series of electron donor–acceptor conjugates incorporating a Zn^IIporphyrin based

electron donor and push–pull chromophores as electron acceptors has been

prepared. The electron acceptors comprize multicyanobutadienes or extended

tetracyanoquinodimethane analogues with first reduction potentials ranging from

–1.67 to –0.23 V vs. Fc⁺/Fc. As such, the rich redox chemistry, broad absorption in

the visible and near infrared region, and electron transfer behavior upon

photoexcitation with charge recombination kinetics in the inverted region render

these systems promising materials for applications in the field of molecular

photovoltaics.

Tristan A. Reekie, Michael Sekita,

Lorenz M. Urner, Stefan Bauroth,

Laurent Ruhlmann, Jean-Paul

Gisselbrecht, Corinne Boudon, Nils

Trapp, Timothy Clark, Dirk M. Guldi,*

and François Diederich*

Page No. – Page No.

Porphyrin Donor and Tunable Push–

Pull Acceptor Conjugates –

Experimental Investigation of Marcus

Theory

Article Doi

Porphyrin Donor and Tunable Push–Pull Acceptor Conjugates—Experimental Investigation of Marcus Theory

DOI: 10.1002/chem.201700043

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Full text of DOI:10.1002/chem.201700043

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