Photoelectrochemical properties of dyads composed of porphyrin/ruthenium catalyst grafted on metal oxide semiconductors

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Photoelectrochemical properties of dyads composed of porphyrin/ruthenium catalyst

grafted on metal oxide semiconductors

Asterios Charisiadis, Eleni Glymenaki, Aurélien Planchat, Sofia Margiola,

Anna-Caroline Lavergne, Emmanouil Nikoloudakis, Vasilis Nikolaou, Georgios

Charalambidis, Athanassios G. Coutsolelos, Fabrice Odobel

PII:

S0143-7208(20)31605-3

DOI:

https://doi.org/10.1016/j.dyepig.2020.108908

DYPI 108908

Reference:

To appear in: Dyes and Pigments

Received Date: 14 July 2020

Revised Date: 28 September 2020

Accepted Date: 1 October 2020

Please cite this article as: Charisiadis A, Glymenaki E, Planchat A, Margiola S, Lavergne A-C,

Nikoloudakis E, Nikolaou V, Charalambidis G, Coutsolelos AG, Odobel F, Photoelectrochemical

properties of dyads composed of porphyrin/ruthenium catalyst grafted on metal oxide semiconductors,

Dyes and Pigments, https://doi.org/10.1016/j.dyepig.2020.108908.

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Asterios Charisiadis, synthesis and characterization

Eleni Glymenaki, synthesis and characterization

Aurélien Planchat cell measurements

Sofia Margiola, characterization

Anna-Caroline Lavergne, synthesis

Emmanouil Nikoloudakis, photophysics

Vasilis Nikolaou, characterization and editing

Georgios Charalambidis, written ms

Athanassios G. Coutsolelos, idea, written ms

Fabrice Odobel idea, written ms

Photoelectrochemical properties of dyads composed of

porphyrin/ruthenium catalyst grafted on metal oxide

semiconductors

Asterios Charisiadis,^aEleni Glymenaki,^aAurélien Planchat,^bSofia Margiola,^aAnna-

Caroline Lavergne,^aEmmanouil Nikoloudakis,^aVasilis Nikolaou,^aGeorgios

Charalambidis,^aAthanassios G. Coutsolelos,^a* Fabrice Odobel.^b*

^aDepartment of Chemistry, University of Crete, Laboratory of Bioinorganic Chemistry,

Voutes Campus, Heraklion 70013, Crete, Greece. e-mail: acoutsol@uoc.gr

^bUniversité LUNAM, Université de Nantes, CNRS, Chimie et Interdisciplinarité:

Synthèse, Analyse, Modélisation (CEISAM), UMR 6230,2 rue de la Houssinière, 44322

Nantes cedex 3, France. e-mail: Fabrice.Odobel@univ-nantes.fr

Abstract

In the current work, we present the use of two free-base and two zinc-metallated porphyrin-

ruthenium(II) polypyridine dyads, along with two reference porphyrin derivatives, as

sensitizers in both n- and p-type DSSCs and DSPECs. Two of the dyads contain the well-

known Ru(bpy)₃unit (HOOC-DMP-Ru(bpy)₃and HOOC-(Zn)DMP-Ru(bpy)₃), while in the

other two terpyridine-Ru(Cl)-bypiridine was used (HOOC-DMP-tpy-Ru and HOOC-(Zn)DMP-

tpy-Ru). In all systems, the amide-bonding motif was utilized for the connection of the

counterparts comprising each dyad. Photophysical investigation of the reported systems

indicated sufficient electronic interactions for the dyads in their excited states (emission

measurements). The photovoltaic measurements revealed that the presence of the

ruthenium complex improves the overall performance of the dyads with the most efficient

dyad being HOOC-(Zn)DMP-tpy-Ru in both n- and p-type DSSCs. Consequently, HOOC-

(Zn)DMP-tpy-Ru was used to fabricate n- and p-DSPECs towards the oxidation of methoxy-

benzyl alcohol and the reduction of CO_2,respectively.

1

Introduction

Photoelectrochemical devices based on dye-sensitized wide bandgap semiconductors are

the key components of photovoltaic devices, such as dye-sensitized solar cells (DSSCs),[1, 2]

as well as artificial photosynthetic schemes, i.e.dye-sensitized photoelectrosynthetic cells

(DSPECs)[3-6]. These integrated systems have attracted significant interest during the past

decades since the properties they possess differ from those of each component. For

example, TiO₂and NiO are materials with low electrical conductivities; however, high

photocurrent densities have been recorded upon their incorporation in DSSCs. Moreover,

the frequently used N3 dye [cis-bis(isothiocyanato)-bis(2,2’-bipyridyl-4,4’-dicarboxylato

ruthenium(II)] exhibits poor photochemical stability. Though, once grafted on TiO₂, N3

reaches outstanding lifetime values prior to its decomposition owing to very fast electron

injection, which outcompetes with photodegradation processes.[7] DSSCs and DSPECs are

also very appealing systems for solar energy conversion, since their operation principle

closely mimics the procedures that take place in natural photosynthesis, achieving

remarkable photoconversion efficiencies (PCEs), particularly in the case of TiO₂based

DSSCs.[8-11]

Porphyrins belong to the famous class of dyes that were successfully used as sensitizers in

DSSCs both with TiO₂[8, 9, 12, 13] and NiO[14-20] semiconductors (SCs). The association of a

ruthenium polypyridine complex with a porphyrin sensitizer is particularly appealing because

the metal to ligand charge transfer transition (MLCT) of the Ru complex fills precisely the

absorption gap of the porphyrin between the Soret and the Q-bands.[21, 22] Accordingly,

the Ru complex could enhance the light harvesting ability of the porphyrin. Moreover, the

ruthenium complex could be oxidized or reduced easier compared to the porphyrin ring

serving as hole or electron acceptor, while diminishing the geminate charge recombination

reaction. Finally, Meyer and co-workers, along with other groups, have demonstrated that

tpy-Ru derivatives can work as catalysts for either alcohol oxidation[23-27] and/or for CO₂

reduction.[28-32] Consequently, the dyads containing the tpy-Ru unit could be implemented

in DSPECs as photocatalysts for either alcohol oxidation on TiO₂or CO₂reduction on NiO.

Alcohol oxidation seems to be more appealing than water oxidation into oxygen, because

the latter requires four holes to operate instead of only two that are needed for alcohol

oxidation and necessitates a higher potential (1.23 V vs. NHE for water versus -0.143 V vs.

NHE for benzyl alcohol oxidation).[33] In other words, alcohol oxidation is a simpler reaction

from both energetic and kinetic point of view. Moreover, oxygen has a little economic value

while aldehydes are commodity chemical for the industry. DSPECs for alcohol oxidation were

reported by Meyer and co-workers,[23, 24, 34] but these devices are much less investigated

than the corresponding systems for water oxidation[3, 5, 6] in spite of their potentially

greater interest. Interestingly, p-DSPECs based on NiO were also developed for CO₂

reduction,[35-38] but the performances are much poorer than those for oxidation with n-

type semiconductors (SCs) because of the fast charge recombination process occurring on

NiO sensitized devices.[2, 39, 40] To that end, we illustrate herein the thorough preparation

of “porphyrin-ruthenium” dyads and their application in DSSCs and DSPECs examining their

photoelectrochemical properties. In this study, two ruthenium complexes, namely the

heteroleptic ruthenium terpyridine bipyridine (abbreviated tpy-Ru) or ruthenium tris-

bipyridine (abbreviated Ru(bpy)₃) were linked to a porphyrin sensitizer in order to

investigate whether the newly formed dyads would present improved overall performances

in DSSCs or DSPECs (Chart).

2

Chart 1. Structures of the compounds studied in this work.

All the results that derived from these two complementary devices, provided essential

feedback in order to optimize and elucidate the most promising systems. Overall, dyad

HOOC-(Zn)DMP-tpy-Ru was the most efficient complex due to the effective charge shift

from the oxidized (or reduced) zinc porphyrin to the adjacent Ru complex, illustrating the

higher light harvesting efficiency (LHE).

3

Results and discussion

Synthesis

All dyads consist of a porphyrin derivative coupled to a ruthenium(II) polypyridyl complex via

an amide bond (Chart 1). In addition, in each porphyrin-ruthenium complex, carboxylic acid

was introduced as anchoring group for the successful attachment of the dyad onto the

surface of the semiconductor. Two of the dyads contain a free-base porphyrin while in the

remaining two the macrocycle is metallated with zinc. The synthetic procedures that were

followed for the preparation of all desired products are outlined in Schemes 1 and 2.

H₃COOC

N

O

Cl

2

Cl

N

H

N

HO

Ru

N

NH

HN

1) SOCl₂, reflux

2) THF, Et₃N, reflux

H

N

NH₂

1

3

4

NH

O

N

CH₃COOH

reflux

N

H₃COOC

HOOC

H₃COOC

HOOC

N

Zn

N

Zn

N

KOH, THF/MeOH/H₂

O

Zn(CH₃COO)₂^.2H₂

O

KOH, THF/MeOH/H₂O

N

NH

HN

NH

HN

CH₂Cl₂/MeOH

N

NH

O

N

Ru

N

HOOC-(Zn)DMP-Ru(bpy)₃

CH₃OOC-(Zn)DMP-Ru(bpy)₃

CH₃OOC-DMP-Ru(bpy)₃

HOOC-DMP-Ru(bpy)₃

Scheme 1. Synthesis of dyads HOOC-DMP-Ru(bpy)₃and HOOC-(Zn)DMP-Ru(bpy)₃.

The synthesis of dyads HOOC-DMP-Ru(bpy)₃and HOOC-(Zn)DMP-Ru(bpy)₃is presented in

Scheme 1. The porphyrin derivative 1[41] was linked to carboxylic acid bipyridine compound

(2)[42] via a two-step amide coupling reaction. More specifically, the carboxylic acid

substituted bipyridine (2) was initially refluxed in SOCl₂in order to obtain the corresponding

acyl chloride, which then reacted with amino porphyrin (1) in the presence of Et₃N to afford

the intermediate 3. The next step involved the reaction of porphyrin 3 and ruthenium

complex 4[43] in acetic acid under reflux. Subsequently, the free base dyad CH₃OOC-DMP-

Ru(bpy)₃underwent a basic hydrolysis reaction forming the desired dyad HOOC-DMP-

Ru(bpy)₃. Regarding the metallated dyad, CH₃OOC-DMP-Ru(bpy)₃was treated with an

excess of zinc acetate in a CH₂Cl₂/MeOH mixture leading to the formation of dyad CH₃OOC-

(Zn)DMP-Ru(bpy)₃. Finally, through a basic hydrolysis reaction, HOOC-(Zn)DMP-Ru(bpy)₃

was afforded that contains the desired carboxylic acid anchoring group.

The synthesis of the terpyridine-based dyads (HOOC-DMP-tpy-Ru and HOOC-(Zn)DMP-tpy-

Ru) as well as the two porphyrin reference compounds (HOOC-DMP-Ph and HOOC-

(Zn)DMP-Ph) is shown in Scheme 2. All the methyl-ester derivatives (CH₃OOC-DMP-tpy-Ru,

4

CH₃OOC-(Zn)DMP-tpy-Ru, CH₃OOC-DMP-Ph and CH₃OOC-(Zn)DMP-Ph) have been prepared

according to procedures previously reported by our group.[44]

Scheme 2. Preparation of dyads HOOC-DMP-tpy-Ru and HOOC-(Zn)DMP-tpy-Ru as well as the two

reference compounds (HOOC-DMP-Ph and HOOC-(Zn)DMP-Ph).

The four final compounds (HOOC-DMP-tpy-Ru, HOOC-(Zn)DMP-tpy-Ru, HOOC-DMP-Ph and

HOOC-(Zn)DMP-Ph) were prepared through the hydrolysis of the methyl ester groups to the

desired carboxylic acid anchoring groups. All intermediates and final products were fully

1

characterized by H and ¹³C NMR spectroscopy (Figures S1-S30) and MALDI-TOF mass

spectrometry.

Photophysical Properties

Figures 1 and 2 present the electronic absorption spectra of the reference compounds as

well as the porphyrin-ruthenium dyads. The absorption spectrum of the free-base reference

derivative (HOOC-DMP-Ph, Figure S31) in THF exhibits typical porphyrin absorption features

with an intense Soret band at 419 nm and four Q bands of moderate intensity (516, 549,

593, and 650 nm). These features are also present in the absorption spectrum of the HOOC-

DMP-Ru(bpy)₃dyad in a THF/MeOH (1:1) mixture (Figure S32), accompanied by two

1

additional bands at 292 and approximately 460 nm (shoulder) that are attributed to the π-

π^*transition and the MLCT of the Ru(bpy)₃unit, respectively.[43, 45] Furthermore, the

spectrum of HOOC-DMP-tpy-Ru in THF (Figure S33) exhibits absorption peaks that

correspond to the porphyrin moiety as well as some characteristic bands of the tpy-Ru

part.[44] In particular, three peaks can be detected at 292, 323 and 515 nm, which are

attributed to the LC, MC and MLCT bands, respectively. As expected, in the case of the

metallated reference derivative (HOOC-(Zn)DMP-Ph, Figure S34), typical porphyrin

absorption features were observed. More specifically, the Soret band (426 nm) was slightly

shifted and the number of the Q bands (557 and 598 nm) was reduced compared to HOOC-

DMP-Ph, due to the increased symmetry of this compound. Moreover, these features are

also displayed in the UV-Vis spectra of both Zn-metallated dyads accompanied by the

characteristic bands of each corresponding Ru-based unit (Figures S35 and S36). Taking into

5

account these observations, it is clear that the individual spectroscopic features of the two

moieties are not significantly altered in the dyads. Therefore, the UV-Vis absorption spectra

of the four dyads can be described as the sum of the spectra of their constituent

chromophores, suggesting that in all cases the two moieties interact only weakly in the

ground state.

Figure 1. UV-Vis absorption spectra of the free-base dyads: a) HOOC-DMP-tpy-Ru in THF, HOOC-DMP-

Ph in THF and HOOC-tpyRu(Cl)bpy in CH₃CN, b) HOOC-DMP-Ru(bpy)₃in THF/MeOH, HOOC-DMP-Ph

in THF and AnilbpyRu(bpy)₂in CH₃CN.

Figure 2. UV-Vis absorption spectra of the metallated dyads: a) HOOC-(Zn)DMP-tpy-Ru in THF, and

HOOC-tpyRu(Cl)bpy in CH₃CN, b) HOOC-(Zn)DMP-Ru(bpy)₃in THF/MeOH, HOOC-(Zn)DMP-Ph in THF

and AnilbpyRu(bpy)₂in CH₃CN.

Initial insights into the possible donor-acceptor interactions between the porphyrin and the

ruthenium moieties, were obtained from fluorescence spectroscopy studies. The emission

spectra of the dyads were recorded in THF/MeOH solutions, upon excitation at 550 nm, and

compared with the corresponding spectra of the reference porphyrin compounds (Figures

S37 and S38). Both reference compounds (HOOC-DMP-Ph and HOOC-(Zn)DMP-Ph)

demonstrated strong emissions in the 520-750 nm range, with two maxima at 655 and 718

nm and at 607 and 656 nm, respectively. The spectra of the dyads displayed practically

identical shapes to those obtained for their reference derivatives with insignificant

contribution from the ruthenium chromophore. However, in all cases the porphyrin

fluorescence was significantly quenched. The emission intensity of the three dyads (HOOC-

(Zn)DMP-Ru(bpy)₃, HOOC-DMP-tpy-Ru and HOOC-(Zn)DMP-tpy-Ru) was considerably

reduced (92%, 96% and 99%, respectively), while in the HOOC-DMP-Ru(bpy)₃dyad, the

emission intensity was slightly quenched (29%) compared to the HOOC-DMP-Ph. These

observations suggest the presence of an intramolecular decay channel, which most probably

6

includes an initial energy transfer originating from the singlet excited state of the porphyrin

to the triplet MLCT state of Ru complex, which is followed by a triplet to triplet energy

transfer generating the porphyrin triplet excited state in all the dyads. This behavior has

been previously demonstrated through photophysical studies in solution of both terpyridine-

based dyads (HOOC-DMP-tpy-Ru and HOOC-(Zn)DMP-tpy-Ru)[44] as well as in the case of

dyads that closely resemble derivatives: HOOC-DMP-Ru(bpy)₃and HOOC-(Zn)DMP-

Ru(bpy)₃.[46, 47]

Electrochemical Characterization

The electrochemical features of all dyads as well as their reference compounds were

examined using cyclic (CV) and square wave (SW) voltammetry in benzonitrile solutions

containing 0.1M tetrabutylammonium hexafluorophosphate (Bu₄NPF₆) as the supporting

electrolyte. The redox potentials are summarized in Table 1, while the voltammograms are

presented in Figures S39-S41.

Table 1. Electrochemical data recorded in benzonitrile solutions containing 0.1M Bu₄NPF₆as

the supporting electrolyte and FcH/FcH⁺as internal standard.^a

E_red2

(P)

E_red3

E_red2

E_red1

(P)

E_red1

E_ox1

(P)

E_ox2

(P)

E_ox1

(Rubpy)

(Rutpy)

(Rubpy)

(Rutpy)

(Rubpy)

(Rutpy)

(Rubpy)

CH₃OOC-DMP-Ph^b

-1.58

-

-1.08

-1.25

-

1.14

0.90

-

1.42

1.25

-

CH₃OOC-(Zn)DMP-Ph^b

AnilbpyRu(bpy)₂

-

-1.53

-

-1.34

-

-1.09

-

1.47

CH₃OOC-DMP-Ru(bpy)₃

CH₃OOC-(Zn)DMP-Ru(bpy)₃

HOOC-tpyRu(Cl)bpy^b

CH₃OOC-DMP-tpy-Ru^b

CH₃OOC-(Zn)DMP-tpy-Ru^b

-1.53

-

-1.36

-

-1.09

-1.30

-

-1.09

-

1.14

0.89

-

1.47

1.21

-

1.47

-

-1.51

-

-1.30

-

-1.11

-

1.47

-

-1.58

-

-1.42

-1.41

-

-1.25

-1.23

-1.25

-

0.94

0.93

-

-1.09

-1.25

1.13

0.93

1.37

1.22

^aAll potentials are in V vs. SCE. ^bFrom ref. [17].

Owing to the weak ground state electronic interactions between the two chromophores, the

observed redox processes in the dyads can confidently be assigned by comparing the

respective potentials of the corresponding reference compounds. For example, in CH₃OOC-

(Zn)DMP-Ru(bpy)₃the observed electrochemical waves correspond to those obtained

separately for its constitutive units CH₃OOC-(Zn)DMP-Ph and AnilbpyRu(bpy)₂. Three

oxidation waves were detected with the first two (0.89 V and 1.21 V) being assigned to the

porphyrin macrocycle, while the potential located at 1.47 V vs. SCE was attributed to the

oxidation of ruthenium unit. On the cathodic side, peaks at -1.11 V and -1.51 V originate

from the reduction of the ligands on Ru(bpy)₃moiety (formally written Ru(I) although it is a

ligand centered process). The wave at -1.30 V corresponds to the simultaneous reduction of

the porphyrin and the ruthenium complex. All the above results suggest that the covalent

linkage between the two constituent chromophores (porphyrin and ruthenium dye) had

little effect in the redox potentials of the dyads. Interestingly, the porphyrin macrocycle is

7

more easily oxidized in the CH₃OOC-(Zn)DMP-Ru(bpy)₃dyad while the first reductive wave

corresponds to the Ru(bpy)₃part. As far as the CH₃OOC-DMP-Rutpy and CH₃OOC-(Zn)DMP-

Rutpy dyads are concerned, the oxidation of the ruthenium complex is close to that of the

porphyrin indicating that hole shift reaction (reaction 3) has an almost insignificant driving

force. Likewise, the reduction of the zinc porphyrin is very similar to that of the Ru complex

indicating that electron shift reaction (reaction 4) is poorly exergonic, while it is clearly

endergonic with the free base porphyrin in CH₃OOC-DMP-Rutpy (Scheme 5). It is important

to point out that the redox potential of the Rutpy moiety is likely to change in DSSC and

DSPEC, because the chloro ligand will possibly be exchanged with water or acetonitrile

molecules. For example, it was reported that the replacement of a chloro by an aquo ligand

in the Ru complex RubpytpyCl induces a circa 300 mV cathodic shift of the oxidation

potential of the Ru center.[27]

Scheme 3. Relative position of the energy levels of excited states of the components of the

dyads and summary of the potential deactivation processes occurring in solution upon

excitation of the porphyrin (red arrow) or the ruthenium complex (blue arrow).

The redox potentials along with the energy level of the porphyrin excited state now enable

us to determine the Gibbs free energy of the potential electron transfer processes in the

dyads and with the TiO₂and NiO semiconductors and the data are collected in Table 2. The

charge transfer processes that take place are, firstly, the electron injection in the TiO₂

conduction band (reaction 1) and, secondly, the hole injection in NiO valence band (reaction

2) from the porphyrin excited state (Porph*).

Porph*-TiO₂→ ⁺Porph-TiO₂(e-) reaction 1 (∆G_e-inj

Porph*-NiO → ^-Porph-NiO(h⁺) reaction 2 (∆G_{h inj}

)

+

)

The formation of the triplet excited state of the porphyrin sensitizers is very likely to occur in

these dyads, owing to intersystem crossing (ISC) promoted by the nearby ruthenium

complex or triplet-triplet (T→T) energy transfer from the MLCT of the appended ruthenium

complex to the porphyrin (Scheme 3).[44, 48, 49] The Gibbs free energies of the electron

injection in TiO₂(∆G_e-inj) and the hole injection in NiO (∆G_{h inj}) were calculated from both the

+

singlet and the triplet excited states of the porphyrins (Table 2).

8

Table 2. Values of the Gibbs free energy of the electron transfer processes in the

compounds.

¹∆G_e-inj³∆G_e-inj¹∆G_{h inj}

+

³∆G_{h inj}

(eV)^b

+

∆G_{h s}

+

∆G_e-s

Compounds

(eV)^a

-0.07

-0.08

-0.46

-0.47

-0.43

(eV) ^b

+0.33

+0.32

+0.01

0.0

(eV)^a

(eV)^c(eV)^d

HOOC-DMP-Ph

-0.53 -0.13

-0.52 -0.12

-0.51 -0.04

-0.46 +0.01

-0.51 -0.04

-

HOOC-DMP-Ru(bpy)₃

HOOC-DMP-tpy-Ru

HOOC-(Zn)DMP-Ph

HOOC-(Zn)DMP-Ru(bpy)₃

HOOC-(Zn)DMP-tpy-Ru

+0.33

0.0

-0.20 +0.14

-

+0.58 -0.19

0.0

+0.04

0

^acharge injection from the singlet excited state of the porphyrin (¹Porph*). E₀₀(¹ZnP*) = 2.06

eV; E₀₀(¹P2H*) = 1.91 eV, measured with the intersection of the normalized absorption and

1

emission spectra of the reference compounds; ∆G_e-inj= E_Ox(Porph⁺/Porph) - E₀₀(¹Porph*) +

1

E_CB(TiO₂); ∆G_{h inj}

+

= E_VB(NiO) - E_Red(Porph/Porph^-) - E₀₀(¹Porph*) with E_CB(TiO₂)= -0.7 V vs. SCE

and E_VB(NiO)= 0.3 V vs. SCE. ^bcharge injection from the triplet excited state of the porphyrin

(³Porph*). E₀₀(³ZnP) = 1.59 eV; E₀₀(³P2H) = 1.51 eV taken from ref.[44]. ³∆G_e-inj

E_Ox(Porph⁺/Porph) - E₀₀(³Porph*) + E_CB(TiO₂); ¹∆G_{h inj}= E_VB(NiO) - E_Red(Porph/Porph^-) -

E₀₀(³Porph*).^chole shift reaction calculated from: ∆G_{h s}

E_Ox(Ru(III)/Ru(II))

E_Ox(Porph⁺/Porph).^delectron shift reaction calculated from: ∆G_e-s= E_Red(Porph/Porph^-) -

_Red(Ru(II)/Ru(I)).

=

+

=

-

E

The Gibbs free energies that were calculated for the charge transfer processes between the

porphyrin and the SC indicate that: 1) electron injection in TiO₂is a favorable reaction from

zinc porphyrin singlet excited state, but the driving force is almost insignificant from the

triplet excited state. However, it is possible to take place when a large concentration of

lithium cation is used due to band bending[50, 51]. 2) Electron injection from the free base

porphyrin to the TiO₂is not only a poorly favorable reaction already from the singlet excited

state but also very endergonic from the triplet, meaning that it is very unlikely to occur in

the latter case. 3) Hole injection in NiO is exergonic with the free base porphyrin from both

singlet and triplet excited states. 4) Regarding the zinc porphyrin derivatives,, the driving

force is significant (-0.5 eV) from the singlet excited state, but weak from the triplet excited

state.

Secondly, the two important charge transfer reactions are: i) the hole shift from the oxidized

porphyrin to the ruthenium complex (reaction 3) after electron injection into TiO₂and ii) the

electron shift (reaction 4) from the reduced porphyrin to the ruthenium complex after hole

injection into NiO.

Ru(II)/⁺Porph-TiO₂(e^-) → Ru(III)/Porph-TiO₂(e^-) reaction 3 (∆G_{h s}

Ru(II)/^-Porph-NiO(h⁺) → Ru(I)/Porph-NiO(h⁺)

reaction 4 (∆G_e-s)

+

)

9

The calculated Gibbs free energies of these two reactions indicate that: 1) the hole shift

reaction to oxidize the Rutpy catalyst is favorable only in the case of free base porphyrin

entities. However, in water the chloro ligand on ruthenium is substituted by aqua and the

oxidation of the ruthenium is cathodically shifted by 0.3 eV,[27] suggesting that this reaction

is certainly exergonic in aqueous electrolyte with both dyads (HOOC-DMP-tpy-Ru and

HOOC-(Zn)DMP-tpy-Ru). 2) Electron shift reaction from the radical anion of the porphyrin to

the Rutpy catalyst (reaction 4) has no driving force regarding free base porphyrin, but

becomes significantly allowed concerning the zinc metallated analogue in HOOC-(Zn)DMP-

tpy-Ru.

Scheme 4. Diagram showing the oxidation potential of the porphyrin sensitizer from the

ground state (straight line), singlet excited state (dashed line), triplet excited state (dotted

line), the reduction potential of the Ru complexes, the redox potential of the iodide based

mediator and the potential of the TiO₂conduction band (CB).

Scheme 5. Diagram showing the reduction potential of the porphyrin sensitizer from the

ground state (straight line), the singlet excited state (dashed line), triplet excited state

(dotted line), the reduction potential of the Ru complex, the redox potential of the iodide

based mediator and the potential of the NiO valence band (VB).

10

Photovoltaic measurements in TiO₂based n-DSSCs

All the compounds were used as sensitizers in DSSCs with TiO₂photoanodes and using

iodide/triiodide redox mediator (see experimental part for details). First, the chemisorption

conditions were optimized by screening the dyeing bath composition and the soaking time

with the dyad HOOC-(Zn)DMP-tpy-Ru. Two mixtures of solvents (THF/MeOH : 1/1 and

Toluene/EtOH : 1/1) and three dyeing durations (2h, 4h and 8h) were investigated and it

was found that the mixture Toluene/EtOH: 1/1 for 2 hours of soaking at room temperature

gives the maximum power conversion efficiency (Table S1). These conditions were

subsequently used for the fabrication of all the cells in this study. The average metrics of

several solar cells are listed in Table 3 and the current/voltage characteristics are gathered in

Figures S42-43 in ESI. The photoaction spectra are shown in Figure 3 except that of HOOC-

DMP-tpy-Ru, which could not be recorded due to the too low photocurrent density

delivered by the cell.

Table 3. Average metrics of solar cells measured under simulated AM1.5 (1000W/m²).

Compounds

J_SC

V_oc

(mV)

ff

(%)

PCE

(%)

(mA/cm²)

HOOC-DMP-Ph

HOOC-DMP-Ru(bpy)₃

HOOC-DMP-tpy-Ru

HOOC-(Zn)DMP-Ph

1.08 ± 0.1 276 ± 1 64.4 ±0.3 0.19 ±0.02

0.60 ± 0.1 260 ± 3 63.3 ±0.8 0.10 ±0.01

0.28 ± 0.2 263 ± 7 61.6 ±0.2 0.04 ±0.01

4.54 ± 0.2 340 ± 10 67.4 ±0.5 1.03 ±0.07

HOOC-(Zn)DMP-Ru(bpy)₃3.60 ± 0.3 329 ± 2 65.6 ±0.1 0.77 ±0.05

HOOC-(Zn)DMP-tpy-Ru 4.21 ± 0.3 359 ± 2 67.1 ±0.2 1.01 ±0.06

Zinc porphyrin derivatives are more efficient sensitizers than the corresponding free base

porphyrins, predominantly due to a higher short circuit current density.[52] This can be

attributed to the higher electron injection driving force for the zinc porphyrin systems (Table

2, Scheme 4). Indeed, the Gibbs free energy of the injection reaction from the free base

porphyrin’s singlet excited state is almost absent and it even becomes positive from the

triplet excited state. Secondly, for all the dyads, the efficiencies of the solar cells were lower

in comparison with the reference sensitizers HOOC-DMP-Ph or HOOC-(Zn)DMP-Ph. This

indicates that the presence of a ruthenium complex deactivates the porphyrin excited state

through a process, which is competitive to the electron injection reaction (Scheme 4).

Referring to the photophysical study of the dyads HOOC-DMP-tpy-Ru and HOOC-(Zn)DMP-

tpy-Ru in solution,[44] we can reasonably conclude that the singlet excited state of the

porphyrin is quenched by a singlet to triplet (S→T) energy transfer process to initially

generate the triplet MLCT on the Ru complex. The triplet excited Ru complex subsequently

decays according to a triplet to triplet (T→T) energy transfer process leading to the triplet

excited state of the porphyrin, since it is the lowest lying excited state of the dyads (Scheme

3). The latter cannot efficiently promotes an electron injection to TiO₂because the Gibbs

free energy is too feeble and even positive for the free base porphyrin dyads (Scheme 4).

Interestingly, this detrimental S→T energy transfer process seems to be less important with

the tpy-Ru than with the Ru(bpy)₃complex, most probably due to the longer distance

between the porphyrin and the Ru center.

We can also note that the PCE of HOOC-(Zn)DMP-tpy-Ru and the reference sensitizer HOOC-

(Zn)DMP-Ph is, within experimental errors, quite similar. The fairly lower J_SCcounterbalances

11

the slightly higher V_OC. On one hand, the higher V_OCvalue suggests that the hole shift

reaction from ZnP⁺to Ru(II) (reaction 3) is probably favorable in this dyad. It is well accepted

that as the hole shifts further from the semiconductor surface in HOOC-(Zn)DMP-tpy-Ru,

the charge recombination process becomes slower leading to higher V_OC.[53] On the other

hand, the lower J_SCcan probably be attributed to the detected quench of the excited state of

the porphyrin by the Ru complex according to a S→T energy transfer process. Indeed, the

IPCE spectrum of the dyad displays lower IPCE values on the porphyrin absorption bands,

namely the Soret (around 420 nm) and the Q-bands (550 and 600 nm) due to a competing

energy transfer. Nonetheless, these current losses are compensated by the antenna effect of

the MLCT absorption of the Ru complex (around 450 nm) clearly visible by the superior IPCE

of HOOC-(Zn)DMP-tpy-Ru compared to that of HOOC-(Zn)DMP-Ph in this region (Figure 3).

One possible strategy to enhance the quantum yield of the electron injection reaction over

the S→T energy wasting process would be to increase the electronic coupling of the

porphyrin with TiO₂by introducing a triple bond between the benzoic acid anchoring unit

and the porphyrin to increase the electron injection rate constant. Another possible way of

decreasing the energy transfer rate constant, while retaining an efficient hole shift process

(reaction 3), is by further elongating the distance between the Ru complex and the

porphyrin. The rate of the latter process can be decreased, because the lifetime of the

radical cation sensitizer is usually long on TiO₂.

Figure 3. Photoaction spectra of the sensitizers in TiO₂based DSSCs. The back curves

correspond to free base porphyrin derivatives, while the blue ones to zinc porphyrin

derivatives.

12

The comparison of the Ru(bpy)₃versus tpy-Ru indicates that Ru(bpy)₃has a more negative

impact on the PV performances probably because the hole shift reaction is

thermodynamically forbidden for Ru(bpy)₃, due to the fact that it is more difficult to oxidize

than tpy-Ru. Besides, its higher proximity to the porphyrin induces a more efficient

quenching by energy transfer.

Overall, based on the PV measurements in TiO₂based DSSCs, it is clear that the most

interesting dyad within these series is HOOC-(Zn)DMP-tpy-Ru, since it displays the highest

efficiency. More importantly, the results of HOOC-(Zn)DMP-tpy-Ru illustrate that the

presence of Ru moiety is beneficial due to an antenna effect (via the MLCT absorption band)

and the occurrence of the hole shift reaction from ZnP⁺to Ru(II).

Photovoltaic measurements in NiO based p-DSSCs

The photovoltaic properties of these new compounds were also investigated in NiO based p-

DSSCs with iodide/triiodide redox mediator using the optimized dyeing conditions that were

determined for the TiO₂based cells (see experimental part for details). The average metrics

of several solar cells are gathered in Table 4 and the current/voltage characteristics are

gathered in Figures S44-45 in ESI. The photoaction spectra are shown in Figure 4.

Table 4. Average metrics of solar cells measured under simulated AM1.5 (1000W/m²).

Compounds

J_SC

V_OC

(mV)

ff

(%)

PCE

(%)

(mA/cm²)

0.79± 0.08

0.89± 0.02

0.96± 0.06

0.96 ± 0.08

1.10 ± 0.02

1.26 ± 0.03

HOOC-DMP-Ph

HOOC-DMP-Ru(bpy)₃

HOOC-DMP-tpy-Ru

47 ± 2

49 ± 5

59 ± 2

70 ± 6

80 ± 2

93 ± 1

30 ± 1

32 ± 1

33 ± 1

34 ± 1

30 ± 1

0.011 ± 0.01

0.013 ± 0.02

0.018 ± 0.01

0.030 ± 0.01

0.035 ± 0.01

HOOC-(Zn)DMP-Ph

HOOC-(Zn)DMP-Ru(bpy)₃

HOOC-(Zn)DMP-tpy-Ru

Similarly to the n-type solar cells, the zinc porphyrin is the more efficient sensitizer

compared to the free base derivative. However, the interpretation of this result is not

obvious at this stage, because the hole injection driving force is similar for both compounds

and the geminate charge recombination, which is particularly acute on NiO based p-

DSSCs,[2, 39] should not be so different in both dyes as the radical anion is localized on the

aromatic ring of the porphyrin macrocycle. Very interestingly and contrary to the

photovoltaic results on TiO₂, the dyads with the appended Ru complex display higher

efficiencies in NiO based p-DSSCs than the reference porphyrins particularly due to higher J_SC

and V_OC(Table 4). Interestingly, connecting a tpy-Ru unit to the porphyrin sensitizer

significantly improves the performances of these dyads (HOOC-DMP-tpy-Ru and HOOC-

(Zn)DMP-tpy-Ru). Moreover, the V_OCvalues are higher for the dyads compared to the

reference derivatives, suggesting that the charge recombination is lower, because NiO based

DSSCs are particularly prone to this deleterious energy wasting process.[2, 39] The

occurrence of the electron shift reaction from the reduced porphyrin to the Ru complex

(reaction 4) is not highly favorable as calculated with the chloro ligand (Table 2). However, in

the presence of the electrolyte, the chloro ligand of the Ru center can easily be replaced by

13

acetonitrile, hence lowering the reduction potential of the complex, because acetonitrile is

less electron donating than chloro. This is consistent with the work of Ott and coll.[31] who

observed an anodic shift of 160 mV in the reduction potential of Ru(tpy)(Me₂bpy)X upon

replacing the chloro ligand with acetonitrile. Accordingly, the feasibility of the electron shift

can be excluded only in the case of HOOC-DMP-Ru(bpy)₃, which consistently displays, within

experimental error, a similar Voc as the reference HOOC-DMP-Ph. However, we cannot

exclude another reason for the higher V_OCand it is possible that ruthenium complex, being a

positively charged compound, makes ion pairing with triiodide anion, thus increasing the

regeneration rate and consequently diminishing charge recombination. Similar phenomena

have been reported in TiO₂based DSSCs by Meyer and coll.[54, 55] On the other hand, the

higher J_SCin the dyads can be ascribed to their higher light harvesting efficiency as confirmed

by the significantly higher IPCE around 500 nm. This can be understood as the result of

either an energy transfer from the Ru chromophore to the porphyrin, either a direct hole

injection from the MLCT of the Ru complex or the lower charge recombination due to the

longer distance between the NiO surface and the oligopyridine ligand of the Ru complex.

Figure 4. Photoaction spectra of the sensitizers in NiO based p-DSSC. The back curves

correspond to free base porphyrin derivatives while the blue ones to zinc porphyrin

derivatives.

In conclusion, the introduction of a ruthenium complex to the zinc porphyrin slightly

enhances the photovoltaic efficiency on NiO based p-DSSCs and the dyad HOOC-(Zn)DMP-

tpy-Ru is certainly the most attractive system in the series as it gives the highest

performances and it is potentially useful for photocatalysis in DSPECs.

Photocatalytic tests

Initially, we investigated the ability of HOOC-(Zn)DMP-tpy-Ru to oxidize para methoxy

benzyl alcohol into the corresponding aldehyde, upon the attachment of the dyad onto a

TiO₂electrode in an acetate buffer (pH = 4.75). Other conditions such as buffer/organic

14

solvent mixtures (acetate buffer/CH₃CN (or acetone): 1/1 and 7/3) and different pH values

(4, 5 and 7.5) were tested but the photocurrent was not higher than that recorded in pure

acetate buffer at pH 4.75 in agreement with another study.[23] In this study, para methoxy

benzyl alcohol was used as a model substrate since similar Ru complexes have been

reported to catalyze the oxidation of such derivatives.[23-27] The linear sweep voltammetry

(LSV) measurements under chopped light illumination are presented in Figure 5. Pictures of

the TiO₂electrodes coated with the dyad HOOC-(Zn)DMP-tpy-Ru and of the experimental

setup used to record the linear sweep voltammetry measurements are given in Figures S46

in ESI.

Figure 5. Linear sweep voltammetry measurements on TiO₂photoanodes under chopped

light illumination in 20 mM acetate buffer (pH = 4.75) with 0.1 M of LiClO₄. Scan rate 10

mV/sec. Alcohol = methoxy benzyl alcohol at the concentration of 80 mM.

As expected, the bare TiO₂electrode does not exhibit noticeable current upon light

excitation even in the presence of the alcohol substrate in the solution. The electrode

sensitized with reference zinc porphyrin (HOOC-(Zn)DMP-Ph) exhibits some

photomodulated current, but the intensity is much lower in comparison with the HOOC-

(Zn)DMP-tpy-Ru dyad. The TiO₂photanode coated with HOOC-(Zn)DMP-tpy-Ru produces a

very low photocurrent in the absence of methoxy benzyl alcohol, since the photogenerated

holes on the system cannot be scavenged by any substance in the solution. However, a

significant photocurrent density appeared once the methoxy benzyl alcohol was introduced

in the solution suggesting that the desired catalytic oxidation has taken place. The same

experiment, was also conducted with the HOOC-DMP-tpy-Ru dyad, but the current density

was even lower than with HOOC-(Zn)DMP-tpy-Ru (Figure S47). The free base porphyrin is

more difficult to be oxidized than the zinc porphyrin, therefore the hole shift reaction to the

Ru catalyst is an energetically more favorable process. However, the overall efficiency of the

DSPEC sensitized with HOOC-DMP-tpy-Ru is certainly limited by the very inefficient electron

injection reaction into TiO₂from the photoexcited free base porphyrin, which controlled

then the formation of the oxidized Ru catalyst (see PV measurements in Table 3). Using

another n-type semiconductor with a lower lying conduction band such as SnO₂could

improve the quantum yield of the electron injection reaction and could lead to higher

photocatalytic activity in DSPEC. Our attempt to measure the quantity of aldehyde that was

15

produced, however, was unsuccessful due to the low current density delivered by the cell

and therefore the low amount of aldehyde in the electrolyte.

Furthermore, several tpy-Ru complexes have proven to be active catalysts towards CO₂

reduction,[28-32] therefore NiO photocathodes, coated with the HOOC-(Zn)DMP-tpy-Ru

dyad, were used as working electrodes for the fabrication of DSPECs. The reaction medium

was a mixture of acetonitrile/water in a 9/1 ratio with Bu₄NPF₆as supporting electrolyte. The

NiO photocathodes were studied by LSV with chopped light irradiation after purging the

solution with argon and then with CO₂(Figure 6). The bare NiO electrode as well as that

coated with HOOC-(Zn)DMP-Ph, lacking the Ru catalyst, give almost no photocurrent

(maximum amplitude of about 2 µA/cm²), while under the same conditions the HOOC-

(Zn)DMP-tpy-Ru dyad displayed significant photocurrent density (50 µA/cm²at -0.3 V vs.

SCE applied potential). The amplitude of the photocurrent delivered by HOOC-(Zn)DMP-tpy-

Ru is importantly reduced after purging the solution with argon (15 µA/cm²at -0.3 V vs. SCE

applied potential), showing that the presence of CO₂in the solution is essential for large

photocurrent density to be observed.

Figure 6. Linear sweep voltammetry measurements of the NiO photocathodes under

chopped light illumination in acetonitrile/water (9/1) with 0.1 M of Bu₄NPF₆. Scan rate 10

mV/sec.

The photocurrent densities of the p-DSSCs sensitized by HOOC-(Zn)DMP-Ph and HOOC-

(Zn)DMP-tpy-Ru differ only slightly (about 30%), while in the corresponding DSPECs the

variation is much greater (about 250%), indicating that a new process takes place. CO₂

dissolved in the solution can be used as a substrate to regenerate the reduced system only

in the case of the HOOC-(Zn)DMP-tpy-Ru dyad, while with HOOC-(Zn)DMP-Ph the electron

on the porphyrin can only recombine with the holes in NiO valence band. Based on the

results of previously reported studies, CO₂is most probably reduced into CO or a mixture or

CO+H₂in the presence of analogues Ru complexes. Once again, we were not able to

determine the photogenerated products during these experiments, owing to the low

amounts that were produced.

Altogether, these experiments in DSPECs underline that the presence of the Ru complex is

fundamental to observe significant current photomodulations of each substrate (benzyl

16

alcohol in case of TiO₂photoanode and CO₂in the case of NiO photocathode) and they are

consistent with the occurrence of a catalytic reaction initiating from tpy-Ru.

Conclusion

Four dyads composed of a porphyrin derivative (free base or metallated by zinc) connected

to a ruthenium complex (Ru(bpy)₃or tpy-Ru) were investigated as sensitizers in DSSCs and in

DSPECs using both a n-type (TiO₂) and p-type (NiO) semiconductors. The photovoltaic

measurements of the TiO₂-based DSSCs revealed that the presence of the ruthenium

complex decreases the overall performances of the dyads compared to those of the parent

reference porphyrins. Conversely, in the case of NiO based p-DSSC the ruthenium complex

has a positive impact, since higher values of the photovoltaic parameters were recorded for

the dyads compared to those of the reference sensitizers. In DSSCs, the ruthenium complex

improves the light harvesting efficiency and the most efficient dyad is HOOC-(Zn)DMP-tpy-

Ru with both SCs. In a second step, the dyad HOOC-(Zn)DMP-tpy-Ru was used to fabricate

DSPECs, first to oxidize methoxy-benzyl alcohol on TiO₂photoanodes and to reduce CO₂on

NiO photocathodes. On both semiconductors, the LSV experiments demonstrate that this

dyad clearly exhibits higher photocurrent density when the electrolyte contains a substrate

that can be consumed by the Ru catalyst (alcohol for TiO₂photoanode and CO₂for NiO

photocathode). On TiO₂, the efficiency could be improved by using another n-SC with more

accessible CB potential such as SnO₂to enhance the electron injection driving force from the

triplet excited state of the porphyrin sensitizer. On NiO, a sensitizer exhibiting a more

negative potential would give a larger driving force of the electron shift reaction and would

probably increase the overall photocatalytic efficiency of the system. Utilization of better

performing catalysts for CO₂reduction (higher TOF and a lower overpotential) would be

another option to increase the photocatalytic performance of the p-DSPECs. The study of

two different types of photoelectrochemical devices (DSSC and DSPEC) offers the possibility

to determine the optimum conditions to fabricate the photoelectrodes and to select the

most promising systems to develop DSPECs. For example, the initial fabrication of DSSCs

enables us to determine the best dyeing conditions (solvent composition and duration), to

verify whether the assembly can sensitize the SC and to indirectly examine if the charge

transfer reaction from the dye to the catalyst is effective (impact on V_OCand J_SC). Indeed, the

first step in DSPECs is the charge injection from the photoexcited dye into the SC, therefore

the dyad must give substantial photovoltaic performance in DSSCs in order to be used as

photocatalyst in DSPECs. Accordingly, this type of study can offer guidelines for the

construction of new molecular dyads as well as a better understanding of their potential

activity in DSPECs. Moreover, the present study can be employed as a guide for the

development of better performing photocatalytic systems by identifying the weak points of

each system.

Experimental part

17

Materials. Compounds 1,[41] 2,[42] 4,[43] CH₃OOC-DMP-tpy-Ru,[44] CH₃OOC-(Zn)DMP-tpy-

Ru,[44] CH₃OOC-DMP-Ph[44] and CH₃OOC-(Zn)DMP-Ph[44] were prepared following

procedures already reported in the literature.

NMR Spectra. NMR spectra were recorded on Bruker AVANCE III-500 MHz and Bruker DPX-

300 MHz spectrometers using solutions in deuterated solvents and the solvent peak was

chosen as the internal standard.

Mass Spectra. Mass spectra were obtained on a Bruker UltrafleXtreme matrix assisted laser

desorption ionization time-of-flight (MALDI-TOF) spectrometer using trans-2-[3-(4-tert-

butylphenyl)-2-methyl-2-propenylidene] malononitrile (DCTB) as matrix.

Spectroscopy. Absorption spectra were recorded on

a

Shimadzu UV-1700

spectrophotometer and steady-state emission spectra were obtained using a JASCO FP-6500

fluorescence spectrophotometer.

Electrochemistry. Cyclic and square wave voltammetry experiments were carried out at

room temperature using an AutoLab PGSTAT20 potentiostat. All measurements were carried

out in freshly distilled and deoxygenated benzonitrile in the presence of 0.1 M of

tetrabutylammonium hexafluorophosphate (Bu₄NPF₆) as the supporting electrolyte, at a

scan rate of 100 mV s^-1. A three-electrode cell setup was used with a glassy carbon working

electrode, a saturated calomel (SCE) reference electrode, and a platinum wire as a counter

electrode. In all measurements the ferrocene/ferrocenium couple was at 0.55 V versus SCE

under the above-mentioned conditions.

3. 4'-methyl-[2,2'-bipyridine]-4-carboxylic acid (2) (50 mg, 0.22 mmol) was dissolved in SOCl₂

o

(3 mL) and stirred under an argon atmosphere at 80 C for 2 h. SOCl₂was then removed

under reduced pressure and the resulting acyl chloride intermediate was dried under high

o

vacuum at 50 C for 1 h. The dry residue was dissolved in anhydrous THF (8 mL) and

porphyrin 1 (80 mg, 0.11 mmol) and anhydrous triethylamine (0.1 mL) were added. The

reaction mixture was heated at 70 ^oC overnight, under argon atmosphere. The solvents were

evaporated under reduced pressure, CH₂Cl₂(60 mL) was added and the mixture was washed

with water (3 x 50 mL). The organic layer was dried with Na₂SO₄, filtered and concentrated.

The desired compound was purified by column chromatography (silica gel, CH₂Cl₂/EtOH,

98:2) yielding 79 mg of 3 (yield: 74%). ¹H NMR (500 MHz, CDCl₃): δ 9.02 (s, 1H), 8.96 (m, 2H),

8.88 (d, J = 4.6 Hz, 2H), 8.75 (m, 6H), 8.63 (d, J = 4.8 Hz, 1H), 8.44 (d, J = 8.3 Hz, 2H), 8.40 (s,

1H), 8.33 (d, J = 8.3 Hz, 2H), 8.28 (d, J = 8.3 Hz, 2H), 8.17 (d, J = 8.1 Hz, 2H), 8.06 (d, J = 3.9 Hz,

1H), 7.30 (s, 4H), 7.28 (d, J = 4.6 Hz, 1H), 4.12 (s, 3H), 2.64 (s, 6H), 2.53 (s, 3H), 1.86 (s, 12H), -

2.59 (s, 2H). ¹³C NMR (125 MHz, CDCl₃): 167.5, 164.1, 156.3, 154.6, 150.8, 150.3, 148.2,

147.1, 143.3, 139.5, 138.8, 138.5, 138.0, 137.6, 135.3, 134.7, 130.4, 129.6, 128.0, 127.9,

125.8, 123.0, 122.7, 119.1, 118.81, 118.76, 118.04, 117.95, 52.6, 21.8, 21.6. MALDI-TOF:

calcd for C₆₄H₅₃N₇O₃[M]⁺967.4210, found 967.4219.

CH₃OOC-DMP-Ru(bpy)₃. The ruthenium complex 4 (70 mg, 0.07 mmol) was added to a

solution of derivative 3 (70 mg, 0.07 mmol) in acetic acid (30 mL) and the mixture was left

o

stirring at 100 C overnight, under nitrogen. The resulting solution was evaporated to

dryness under vacuum. The crude residue was purified by column chromatography (silica

gel, CH₂Cl₂/MeOH, 95:5) to obtain 92 mg of the desired product (CH₃OOC-DMP-Ru(bpy)₃)

1

(yield: 84%). H NMR (500 MHz, CDCl₃): δ 11.33 (s, 1H), 9.33 (s, 1H), 8.80 (d, J = 4.3 Hz, 2H),

8.72 (d, J = 4.6 Hz, 2H), 8.67 (d, J = 4.6 Hz, 2H), 8.65 (d, J = 4.5 Hz, 2H), 8.61 (s, 1H), 8.40 (m,

18

6H), 8.28 (m, 4H), 8.08 (d, J = 7.9 Hz, 2H), 8.03 (s br, 1H), 7.80 (m, 6H), 7.69 (m, 3H), 7.53 (m,

1H), 7.35 (m, 4H), 7.22 (s, 4H), 7.13 (m, 1H), 4.09 (s, 3H), 2.55 (s, 6H), 2.42 (s, 3H), 1.78 (s,

12H), -2.63 (s, 2H). ¹³C NMR (125 MHz, CDCl₃): 167.5, 163.1, 157.8, 156.9, 156.8, 156.7,

156.5, 156.1, 151.4, 151.1, 150.6, 146.9, 144.2, 139.4, 138.4, 138.2, 138.0, 135.1, 134.6,

130.3, 129.6, 129.3, 128.0, 127.9, 126.1, 124.6, 122.4, 119.5, 119.3, 118.7, 118.0, 52.5, 21.7,

21.6, 21.4. MALDI-TOF: calcd for C₈₄H₆₈N₁₁O₃Ru [M-H]⁺1380.4550, found 1380.4562.

CH₃OOC-(Zn)DMP-Ru(bpy)₃. To a solution of CH₃OOC-DMP-Ru(bpy)₃(50 mg, 0.036 mmol) in

15 mL of CH₂Cl₂, a solution of (CH₃COO)₂Zn·2H₂O (154 mg, 0.702 mmol) in 4 mL of MeOH

was added. The reaction mixture was stirred at room temperature for 24 h. After the

volatiles were distilled off under vacuum, the resulting residue was purified through a

column chromatography (silica gel, CH₂Cl₂/MeOH, 93:7) to collect 41 mg of CH₃OOC-

(Zn)DMP-Ru(bpy)₃(yield: 78%). ¹H NMR (500 MHz, (DMSO-d₆)): δ 11.35 (s, 1H), 9.45 (s, 1H),

9.06 (s br, 1H), 8.90 (s br, 4H), 8.76 (d, J = 3.4 Hz, 2H), 8.70 (d, J = 4.3 Hz, 2H), 8.60 (s, 4H),

8.36 (m, 4H), 8.23 (m, 8H), 8.02 (d, J = 10.2 Hz, 2H), 7.89 (s, 1H), 7.80 (m, 3H), 7.61 (m, 5H),

7.47 (s br, 1H), 7.32 (s, 4H), 4.04 (s, 3H), 2.58 (s, 6H), 2.51 (s, 3H), 1.78 (s, 12H). ¹³C NMR (125

MHz, DMSO-d₆): 166.2, 162.7, 157.1, 156.3, 156.1, 156.0, 155.4, 151.6, 150.9, 150.1, 149.7,

148.9, 148.7, 148.6, 148.3, 147.3, 142.7, 138.7, 138.4, 138.0, 137.7, 137.5, 136.5, 134.1,

131.6, 131.3, 131.0, 130.1, 129.8, 128.7, 128.2, 128.0, 127.6, 127.2, 127.0, 125.3, 124.2,

121.9, 119.1, 118.1, 117.9, 117.7, 52.0, 21.0, 20.7, 20.4. MALDI-TOF: calcd for

C₈₄H₆₇N₁₁O₃RuZn [M]⁺1443.3763, found 1443.3771.

HOOC-DMP-Ru(bpy)₃. CH₃OOC-DMP-Ru(bpy)₃(30 mg, 0.021 mmol) was dissolved in 14 mL

of a THF/MeOH mixture (2 : 1), followed by the addition of an aqueous solution (5 mL) of

KOH (300 mg, 5.36 mmol) and the reaction was left under stirring at room temperature

overnight. The organic solvents were evaporated under reduced pressure, and then a

solution of 1 M HCl (aq) was added dropwise for the acidification of the mixture and the

precipitation of the desired product. Finally, after the dyad was filtered, washed with H₂O

1

and dried under vacuum 21 mg of HOOC-DMP-Ru(bpy)₃were isolated (yield: 73%). H NMR

(500 MHz, (DMSO-d₆)): δ 12.57 (s, 1H), 9.80 (s,1H), 9.07 (s, 1H), 8.82 (m, 8H), 8.61 (d, J = 16.7

Hz, 4H), 8.23 (m, 13H), 7.99 (s, 1H0, 7.91 (s, 1H), 7.80 (m, 3H), 7.60 (m, 5H), 7.42 (s, 1H), 7.30

(s, 4H), 2.63 (s, 6H), 2.37 (s, 3H), 1.72 (s, 12H), -2.74 (s, 2H) ppm. A ¹³C NMR spectrum could

not be obtained due to the low solubility of this compound. MALDI-TOF: calcd for

C₈₃H₆₆N₁₁O₃Ru [M-H]⁺1366.4394, found 1366.4388. UV–Vis in THF/MeOH [λmax/nm (ε/mM^-

¹cm^-1)]: 294 (60.2), 418 (409.3), 514 (22.4), 549 (11.8), 591 (6.2), 649 (5.0).

HOOC-(Zn)DMP-Ru(bpy)₃. CH₃OOC-(Zn)DMP-Ru(bpy)₃(25 mg, 0.017 mmol) was dissolved in

10 mL of a THF/MeOH mixture (2 : 1), followed by the addition of an aqueous solution (4 mL)

of KOH (150 mg, 2.68 mmol) and the reaction was left under stirring at room temperature

overnight. The organic solvents were evaporated under reduced pressure, and then a

solution of 1 M HCl (aq) was added dropwise for the acidification of the mixture and the

precipitation of the desired product. Finally, after the dyad was filtered, washed with H₂O

1

and dried under vacuum 21 mg of HOOC-(Zn)DMP-Ru(bpy)₃were isolated (yield: 85%). H

NMR and ¹³C NMR spectra could not be recorded due to solubility reasons. MALDI-TOF:

calcd for C₈₃H₆₅N₁₁O₃RuZn [M]⁺1429.3607, found 1429.3616. UV–Vis in THF/MeOH

[λmax/nm (ε/mM^-1cm^-1)]: 286 (131.2), 425 (563.7), 468 (24.2), 558 (28.7), 598 (12.7).

HOOC-DMP-tpy-Ru. CH₃OOC-DMP-tpy-Ru (25 mg, 0.018 mmol) was dissolved in 10 mL of a

THF/MeOH mixture (2 : 1), followed by the addition of an aqueous solution (4 mL) of KOH

(100 mg, 1.79 mmol) and the reaction was left under stirring at room temperature

19

overnight. The organic solvents were evaporated under reduced pressure, and then a

solution of 1 M HCl (aq) was added dropwise for the acidification of the mixture and the

precipitation of the desired product. Finally, after the dyad was filtered, washed with H₂O

1

and dried under vacuum 26 mg of HOOC-DMP-tpy-Ru were isolated (yield: 96%). H NMR

(500 MHz, (DMSO-d₆)): δ 10.92 (s, 1H), 10.12 (d, J = 4.8 Hz, 1H), 9.29 (s, 2H), 8.97 (d, J = 8.0

Hz, 2H), 8.90 (m, 3H), 8.80 (d, J = 4.3 Hz, 2H), 8.65 (m, 4H), 8.58 (d, J = 7.9 Hz, 2H), 8.43 (d, J =

8.0 Hz, 2H), 8.33 (m, 8H), 8.26 (m, 2H), 8.06 (m, 3H), 7.79 (t, J = 7.4 Hz, 1H), 7.67 (d, J = 5.7

Hz, 2H), 7.46 (d, J = 5.8 Hz, 1H), 7.42 (t, J = 6.4 Hz, 2H), 7.35 ( s, 4H), 7.10 (t, J = 6.3 Hz, 1H),

2.58 (s, 6H), 1.77 (s, 12H), -2.73 (s, 2H) ppm. ¹³C NMR (125 MHz, (DMSO-d₆)): 167.7, 165.6,

158.7, 158.4, 158.2, 155.7, 152.1, 152.0, 151.9, 145.6, 144.1, 139.6, 139.2, 138.6, 137.9,

137.8, 137.3, 136.9, 136.5, 136.0, 135.9, 134.8, 134.6, 130.5, 129.0, 128.0, 127.8, 127.1,

126.6, 124.3, 123.9, 123.7, 120.3, 119.7, 119.1, 118.3, 118.1, 21.3, 21.2. MALDI-TOF: calcd

for C₈₃H₆₄Cl₂N₁₀O₃Ru [M-Cl]⁺1385.3895, found 1385.3883. UV–Vis in THF [λmax/nm (ε/mM^-1

cm^-1)]: 292 (55.9), 324 (33.8), 421 (355.3), 516 (25.3), 550 (14.0), 590 (4.1), 647 (1.4).

HOOC-(Zn)DMP-tpy-Ru. CH₃OOC-(Zn)DMP-tpy-Ru (25 mg, 0.017 mmol) was dissolved in 10

mL of a THF/MeOH mixture (2 : 1), followed by the addition of an aqueous solution (4 mL) of

KOH (480 mg, 5.56 mmol) and the reaction was left under stirring at room temperature

overnight. The organic solvents were evaporated under reduced pressure, and then a

solution of 1 M HCl (aq) was added dropwise for the acidification of the mixture and the

precipitation of the desired product. Finally, after the dyad was filtered, washed with H₂O

1

and dried under vacuum 25 mg of HOOC-(Zn)DMP-tpy-Ru were isolated (yield: 98%). H

NMR (500 MHz, (DMSO-d₆)): δ 13.10 (s br, 1H), 10.88 (s, 1H), 10.14 (d, J = 5.1 Hz, 1H), 9.34 (s,

2H), 9.02 (d, J = 8.1 Hz, 2H), 8.95 (d, J = 8.2 Hz, 1H), 8.83 (d, J = 4.5 Hz, 2H), 8.72 (d, J = 4.6 Hz,

2H), 8.67 (d, J = 8.3 Hz, 1H), 8.61 (m, 6H), 8.45 (d, J = 8.2 Hz, 2H), 8.39 (t, J = 7.8 Hz, 1H), 8.32

(m, 6H), 8.23 (d, J = 8.4 Hz, 2H), 8.09 (m, 3H), 7.81 (t, J = 7.3 Hz, 1H), 7.68 (d, J = 5.7 Hz, 2H),

7.49 (d, J = 5.9 Hz, 1H), 7.44 (t, J = 6.4 Hz, 2H), 7.33 (s, 4H), 7.11 (t, J = 6.5 Hz, 1H), 2.59 (s,

6H), 1.80 (s, 12H) ppm. ¹³C NMR (125 MHz, DMSO-d₆): 167.6, 165.3, 158.6, 158.3, 158.0,

155.6, 152.0, 151.9, 149.4, 149.1, 149.0, 148.7, 147.3, 143.8, 139.3, 139.1, 138.6, 138.4,

138.1, 137.1, 136.9, 136.1, 135.7, 134.5, 134.4, 132.1, 131.7, 130.4, 130.1, 129.8, 128.8,

127.6, 127.5, 127.0, 126.5, 124.2, 123.8, 123.5, 120.2, 119.7, 118.5, 118.2, 21.4, 21.1.

MALDI-TOF: calcd for C₈₃H₆₂ClN₁₀O₃RuZn [M]⁺1447.3030, found 1447.3041. UV–Vis in THF

[λmax/nm (ε/mM^-1cm^-1)]: 295 (52.6), 325 (35.5), 431 (494.2), 518 (13.6), 561 (24.2), 604

(7.8).

HOOC-DMP-Ph. To a solution of CH₃OOC-DMP-Ph (25 mg, 0.027 mmol) was dissolved in 15

mL of a THF/MeOH mixture (2 : 1), an aqueous solution (6 mL) of KOH (380 mg, 6.80 mmol)

was added and the reaction was left under stirring at room temperature overnight. The

organic solvents were evaporated under reduced pressure, and then a solution of 1 M HCl

(aq) was added dropwise for the acidification of the mixture and the precipitation of the

desired product. Finally, after the dyad was filtered, washed with H₂O and dried under

vacuum 22 mg of HOOC-DMP-Ph were isolated (yield: 77%). ¹H NMR (500 MHz, (DMSO-d₆)):

δ 10.70 (s, 1H), 8.89 (d, J = 4.4 Hz, 2H), 8.79 (d, J = 4.4 Hz, 2H), 8.65 (m, , J = 4.4 Hz, 4H), 8.35

(s, 4H), 8.26 (d, J = 8.6 Hz, 2H), 8.22 (d, J = 8.6 Hz, 2H), 8.11 (d, J = 6.8 Hz, 2H), 7.64 (m, 3H),

7.34 (s, 4H), 2.57 (s, 6H), 1.76 (s, 12H), -2.73 (s, 2H) ppm. ¹³C NMR (125 MHz, (DMSO-d₆)):

167.5, 166.1, 145.5, 139.3, 138.4, 137.6, 136.0, 135.2, 134.6, 134.4, 131.7, 130.4, 128.5,

127.9, 127.8, 119.7, 118.6, 118.0, 117.9, 21.2, 21.0. MALDI-TOF: calcd for C₅₈H₄₇N₅O₃[M]⁺

861.3679, found 861.3687. UV–Vis in THF [λmax/nm (ε/mM^-1cm^-1)]: 419 (368.0), 513 (13.6),

548 (6.4), 594 (3.2), 648 (4.0).

20

HOOC-(Zn)DMP-Ph. To a solution of CH₃OOC-(Zn)DMP-Ph (25 mg, 0.027 mmol) in 10 mL of a

THF/MeOH mixture (2 : 1), an aqueous solution (4 mL) of KOH (190 mg, 3.39 mmol) was

added and the reaction was left under stirring at room temperature overnight. The organic

solvents were evaporated under reduced pressure, and then a solution of 1 M HCl (aq) was

added dropwise for the acidification of the mixture and the precipitation of the desired

product. Finally, after the dyad was filtered, washed with H₂O and dried under vacuum 23

mg of HOOC-(Zn)DMP-Ph were isolated (yield: 92%). ¹H NMR (500 MHz, (DMSO-d₆)): δ 10.67

(s, 1H), 8.80 (d, J = 4.5 Hz, 2H), 8.72 (d, J = 4.4 Hz, 2H), 8.58 (d, J = 4.4 Hz, 4H), 8.33 (m, 2H),

8.26 (d, J = 7.7 Hz, 2H), 8.21 (d, J = 8.5 Hz, 2H), 8.17 (d, J = 8.5 Hz, 2H), 8.12 (d, J = 6.9 Hz, 2H),

7.64 (m, 3H), 7.31 (s, 4H), 2.58 (s, 6H), 1.78 (s, 12H) ppm. ¹³C NMR (125 MHz, DMSO-d₆):

166.0, 149.4, 149.1, 149.0, 148.9, 145.7, 139.2, 138.7, 138.4, 137.9, 135.2, 134.4, 134.1,

132.1, 131.8, 131.7, 130.2, 130.1, 128.5, 127.8, 127.6, 127.4, 119.6, 118.7, 118.3, 118.1,

21.4, 21.1. MALDI-TOF: calcd for C₅₈H₄₅N₅O₃Zn [M]⁺923.2814, found 923.2805. UV–Vis in

THF [λmax/nm (ε/mM^-1cm^-1)]: 426 (452.3), 555 (16.6), 598 (5.7).

Acknowledgements

This research was funded by the General Secretariat for Research and Technology (GSRT)

and Hellenic Foundation for Research and Innovation (HFRI; project code: 508). This

research has also been co-financed by the European Union and Greek national funds

through the Operational Program Competitiveness, Entrepreneurship, and Innovation, under

the call RESEARCH−CREATE−INNOVATE (project code: T1EDK-01504). In addition, this

research has been co-financed by the European Union and Greek national funds through the

Regional Operational Program “Crete 2014-2020,” project code OPS:5029187. Moreover,

the European Commission’s Seventh Framework Program (FP7/2007-2013) under grant

agreement no. 229927 (FP7-REGPOT-2008-1, Project BIO-SOLENUTI) and the Special

Research Account of the University of Crete are gratefully acknowledged for the financial

support of this research.

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24

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Sunday, July 12, 2020

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Article Doi

Photoelectrochemical properties of dyads composed of porphyrin/ruthenium catalyst grafted on metal oxide semiconductors

DOI: 10.1016/j.dyepig.2020.108908

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Full text of DOI:10.1016/j.dyepig.2020.108908

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