Photophysical and photochemical properties of tetrathiafulvalene derivatives and their complexes

Thesis

Reference

Photophysical and photochemical properties of tetrathiafulvalene derivatives and their complexes

DUPONT, Nathalie

Abstract

Le TTF (tétrathiafulvalène) est l'une des molécules les plus étudiées de ces quarante dernières années, grâce à ses propriétés physiques et chimiques. Il a tout d'abord été étudié comme un composant de superconducteurs organiques comme le TTF-TCNQ, et plus récemment pour quelques-unes de ses propriétés chimiques et photochimiques. En effet, le TTF est bien connu comme extincteur de luminescence et comme donneur d'électron. Le transfert d'électron photoinduit, quant à lui, est un des plus importants processus en photophysique et en photochimie. Il est utilisé par la nature, par exemple, pour le mécanisme de photosynthèse, ainsi que dans l'industrie en ce qui concerne la photographie, des réactions photocatalytiques organiques, des dispositifs optoélectroniques et la conversion d'énergie solaire. Dans le but d'améliorer de telles applications, le transfert de charge photoinduit est également étudié intensivement en recherche fondamentale. Les molécules étudiées dans cette thèse ont plusieurs caractéristiques communes. L'une d'elles est leur comportement en tant qu'ensembles de [...]

DUPONT, Nathalie. Photophysical and photochemical properties of tetrathiafulvalene derivatives and their complexes. Thèse de doctorat : Univ. Genève, 2010, no. Sc. 4273

URN : urn:nbn:ch:unige-145405

DOI : 10.13097/archive-ouverte/unige:14540

Available at:

http://archive-ouverte.unige.ch/unige:14540

Disclaimer: layout of this document may differ from the published version.

UNIVERSITE DE GENEVE FACULTE DES SCIENCES Section de chimie et biochimie

Département de chimie physique Professeur Andreas Hauser

!

!

!

Photophysical and photochemical properties of tetrathiafulvalene derivatives

and their complexes

THESE

Présentée à la Faculté des sciences de l!Université de Genève pour obtenir le grade de Docteur ès sciences, mention chimie

par

Nathalie Dupont de Paris (France)

Thèse N°4273

Genève Atelier Repromail

Remerciements

Ce travail de thèse a été effectué dans le département de chimie physique du l!Université de Genève, sous la direction du professeur Andreas Hauser, de septembre 2006 à décembre 2010. Je le remercie vivement de m!avoir accueillie au sein de son groupe de recherche et pour m!avoir formée à la spectroscopie optique. Je le remercie pour toutes les discussions et explications scientifiques que nous avons eues, de m!avoir appris à tenir un discours scientifique et à bien choisir les termes employés lors de ce dernier. Je le remercie aussi pour la bonne ambiance et la convivialité qu!il a su instaurer dans son groupe.

Je tiens à remercier les Professeurs Silvio Decurtins et Eric Vauthey pour avoir accepté de juger ce travail de thèse.

Je voudrais également remercier le Professeur Claude Piguet, de l!université de Genève, pour m!avoir permis de collaborer avec son groupe de recherche et plus particulièrement avec Thomas Riis- Johannessen. Je remercie de même les Docteurs Narcis Avarvari, de l!Université d!Angers et Josef Hamacek, de l!Université de Genève, pour les collaborations avec leurs équipes respectives. Enfin, je remercie le Professeur Silvio Decurtins pour m!avoir permis de collaborer avec son groupe de recherche et tout particulièrement avec Shi-Xia Liu.

Mes remerciements vont également au membre du groupe, sans ordre particulier : Claudia, Mia, Enza, William, Max, Jie, Prodita, Pradip, Juan Carlos, Christophe, Ahmed et Hans. Au cours de ces quatre années de thèse, j!ai tout particulièrement apprécié les différents moments passés avec eux, autant scientifiques que plus personnels. Je remercie le docteur Dominique Lovy pour toutes les discussions que nous avons eues, ainsi que pour sa patience lors des nombreuses explications et de son aide lors de mes différents problèmes électroniques au cours de mes expériences. Je remercie Nahid Amstutz pour sa constante bonne humeur, sa gentillesse et son aide technique précieuse. Je remercie Isabelle Garin pour son aide dans toutes les démarches administratives. Finalement, je remercie Patrick Barman pour son travail technique exceptionnel et son ingéniosité dans la réalisation des diverses pièces qui m!ont été nécessaires lors de cette thèse.

Un merci tout particulier à ma famille et à mes amis proches pour leur soutien, leurs encouragements et toute leur affection. Merci également à Louis qui m!a épaulée sans fléchir une seule fois.

Résumé

Le TTF (tétrathiafulvalène, Figure 1) est l!une des molécules les plus étudiées de ces quarante dernières années,^{[1, 2]} grâce à ses propriétés physiques et chimiques. Il a tout d!abord été étudié comme un composant de superconducteurs organiques comme le TTF-TCNQ,^[3] et plus récemment pour quelques-unes de ses propriétés chimiques et photochimiques. En effet, le TTF est bien connu comme extincteur de luminescence et comme donneur d!électron.

Figure 1: Tétrathiafulvalène (TTF)

Le transfert d!électron photoinduit, quant à lui, est un des plus importants processus en photophysique et en photochimie. Il est utilisé par la nature, par exemple, pour le mécanisme de photosynthèse, ainsi que dans l!industrie en ce qui concerne la photographie, des réactions photocatalytiques organiques, des dispositifs optoélectroniques et la conversion d!énergie solaire. Dans le but d!améliorer de telles applications, le transfert de charge photoinduit est également étudié intensivement en recherche fondamentale.

Les molécules étudiées dans cette thèse ont plusieurs caractéristiques communes.

L!une d!elles est leur comportement en tant qu!ensembles de donneur-accepteur d!électron, une autre caractéristique est la présence du TTF comme sous-unité des ligands ou molécules. De même, les complexes présentés ici contiennent un ou plusieurs ions de métaux de transition tels que Ru²⁺, Co²⁺ et Fe²⁺, coordinés à un ou plusieurs ligands contenant du TTF. Tous les composés de ce travail ont été synthétisés et caractérisés par les collaborateurs du groupe du Pr. S. Decurtins, à l!Université de Berne. Cette thèse va donc être focalisée sur les propriétés

photophysiques et photochimiques de ces composés, les parties évoquant la synthèse ainsi que la caractérisation des composés se trouvant dans les différents articles.

Les propriétés photophysiques et photochimiques de molécules contenant du TTF ont donc été étudiées et plus particulièrement celles de transfert de charge intraligand photoinduit entre le TTF et d!autres sous-unités (cf. Figure 2). L!étude de différentes molécules a permis d!effectuer une comparaison de l!impact de la substitution des ligands sur le phénomène de séparation de charges observé après photoexcitation des molécules.

Figure 2 : Schéma représentant le ligand TTF-dppz ainsi que le transfert de charge intraligand obtenu par irradiation entre le TTF et le dppz et spectres d!absorption du ligand, ainsi que du TTF et du dppz libres.

L!étude des complexes de la forme [{Ru(bpy)2}n(TTF-ppb)](PF6)2n (ppb = dipyrido[2,3- a:3",2"-c]phénazine et n = 1, 2) nous a permis de montrer que le premier état excité dans ces molécules correspond à un état de transfert de charge intramoléculaire avec le TTF comme donneur d!électron et le ppb comme accepteur.^[4] Ceci se traduit sur le spectre d!absorption par une large bande d!absorption centrée à environ 15000 cm^-1. Cette bande d!absorption pourra être retrouvée dans les spectres d!absorption d!autres molécules (à différentes énergies) et est représentative du transfert de charge intraligand photoinduit. Ces composés ont été comparés avec des complexes analogues définis par la formule [Ru(bpy)3-n(TTF-dppz)n]²⁺ (dppz = dipyrido-[3,2-a:2!,3!- c]phenazine et n = 1, 2, 3).^[5] Un état excité de charges séparées a été mis en évidence pour ces complexes (cf. Figure 3).

Figure 3 : Spectre d!absorption de [Ru(TTF-dppz)₃]²⁺ dans CH₂Cl₂ (----) avec sa forme obtenue par oxydation chimique (....), et le spectre d!absorption transitoire (^___) obtenu sous irradiation d!un laser pulse à 16000 cm^-1. Dans l!encadré: relaxation de l!état transitoire observe à 12500 cm^-1.

Cet état excité de charges séparées est obtenu par irradiation de la bande d!absorption correspondant au transfert de charge du métal (Ru²⁺) vers le dppz, qui peut être décrit par [(TTF⁺-dppz)Ru(dppz^--TTF)(bpy)]²⁺ pour n = 2. Le temps de vie de cet état est de 2.5 µs dans CH2Cl2. Un tel état de charges séparées photoinduit n!a pas été observé dans [{Ru(bpy)2}n(TTF-ppb)](PF6)2n. A la place, la photoexcitation donne lieu à un état triplet de transfert de charge intraligand (³ILCT) avec un temps de vie d!environ 200 ns pour n = 1 et 50 ns pour n = 2 (cf. Figure 4).

Figure 4. Spectres d!absorption transitoire de [{Ru(bpy)2}n(TTF-ppb)](PF6)2n pour n=1 (__) et n=2 (__) en comparaison avec les specters d!absorption pour n=1 (…) et n=2 (…) dans CH₂Cl₂ à temperature ambiante.

Pour accéder à une meilleure compréhension du comportement de l!état de charges séparées obtenu par photoexcitation dans les molécules du type [Ru(bpy)3-n(TTF- dppz)n]²⁺, un groupe anthraquinone a été rattaché à une unité phénanthroline dans le

une phénanthroline via un pont phénazine). Dans cette triade, l!excitation dans les bandes de transfert de charge métal-ligand résulte en la création d!un état de charge séparées avec un long temps de vie de 400 ns, impliquant le TTF comme donneur d!électron et l!anthraquinone comme l!accepteur d!électron final.^[6]

Ensuite, une étude sur les interactions électroniques dans les composés de forme donneur-accepteur a été réalisée en fonction du pH sur des dérivés imidazole-TTF (cf.

Figure 5). Il a été démontré que le fait de relier les dérivés imidazoles au TTF a un effet sur le transfert de proton ayant lieu sur la partie imidazole des molécules, avec une augmentation de la valeur du pKa.^[7] De même, le transfert de charge intraligand photoinduit provenant du TTF se trouve plus favorisé (état excité plus bas en énergie) lors des protonations successives

Figure 5 : Spectres d!absorption de PITTF dans CH2Cl₂ à température ambiante en fonction des équivalents molaires ajoutés de HCl. Le changement de couleur obtenu pour les différentes protonations de PITTF sont montrées dans l!encadré : A représente une solution de PITTF, B celle de [PITTFH]⁺ et C celle de [PITTFH₂]²⁺ dans CH₂Cl₂ à température ambiante.

Le transfert de charge intraligand est aussi présent dans ces molécules, ainsi que dans la molécule étudiée de TTF relié à du pérylènediimide (cf. Figure 6).^[8] Cette molécule met également en évidence le rôle de extincteur du TTF de par l!extinction de luminescence observée pour l!unité PDI quand elle est reliée au TTF.

Figure 6 : Schéma de TTF-PDI.

Enfin, combiné avec les ions de métaux de transition Fe²⁺ et Co²⁺, les propriétés de donneur-accepteur d!électron des ligands contenant du TTF peuvent être associées aux propriétés de transition de spin des ions métalliques. Dans [Fe(phen)2(TTF-dppz)]²⁺, il a été montré que les deux processus de transfert de charge intraligand photoinduit et de transition de spin photoinduite étaient indépendants. En effet, l!état haut spin ne peut être atteint que par une irradiation de la bande d!absorption correspondant au transfert de charge entre le métal et les ligands, l!irradiation dans la bande d!absorption du transfert de charge intraligand ne donnant accès qu!à l!état excité de ce dernier. Les expériences correspondantes sur [Co(phen)2(TTF-dppz)]²⁺ montrent, elles, que pour ce complexe une étude plus approfondie est nécessaire afin de pouvoir attribuer les différentes transitions et processus de relaxation mis en jeu lors de la photoexcitation.

Figure 7: (a) Absorption transitoire mesurée à l!échelle des picosecondes, [Fe(phen)2(TTF- dppz)]²⁺ avec une longueur d!onde d!excitation !ex= 400 nm (ce qui correspond à une excitation dans la bande de transfert de charge metal-ligand), dans CH2Cl2. (b) Décomposition de l!absorption transitoire obtenue par un fit bi-exponentiel A(t) = A0 + A₁exp(-t//"1) + A₂exp(-t//"2).

Ceci permet de distinguer très clairement l!absorption transiroire due au transfert de charge intraligand photoinduit (en rouge) du blanchiment dû à la transition de spin, elle aussi photoinduite. Dans le spectre (a), la region spectrale à proximité du laser est coupée.

Références :

[1] F. Wudl, G. M. Smith, E. J. Hufnagel J. Chem. Soc. D-Chem. Commun. 1970, 1453.

[2] Q. Y. Zhu, L. B. Huo, Y. R. Qin, Y. P. Zhang, Z. J. Lu, J. P. Wang, J. Dai J. Phys.

Chem. B. 2010, 114, 361-367.

[3] J. Ferraris, V. Walatka, Perlstei.Jh, D. O. Cowan J. Am. Chem. Soc. 1973, 95, 948- 949.

[4] C. Goze, N. Dupont, E. Beitler, C. Leiggener, H. Jia, P. Monbaron, S. X. Liu, A.

Neels, A. Hauser, S. Decurtins Inorg. Chem. 2008, 47, 11010-11017.

[5] C. Leiggener, N. Dupont, S. X. Liu, C. Goze, S. Decurtins, E. Beitler, A. Hauser Chimia. 2007, 61, 621-625.

[6] Y.-F. R. Nathalie Dupont, Hong-Peng Jia, Jakob Grilj, Shi-Xia Liu, Sivio Decurtins, Andreas Hauser Inorg. Chem. 2010, submitted.

[7] J. C. Wu, N. Dupont, S. X. Liu, A. Neels, A. Hauser, S. Decurtins Chem.- Asian J.

2009, 4, 392-399.

[8] M. Jaggi, C. Blum, N. Dupont, J. Grilj, S. X. Liu, J. Hauser, A. Hauser, S. Decurtins Org. Lett. 2009, 11, 3096-3099.

TABLE OF CONTENTS

1. Introduction 7

!

1.1. The basics of spectroscopy 7

!

1.2. Electron transfer 13

!

1.3. Tetrathiafulvalene (TTF) 20

1.3.1. Introduction 20

1.3.2. General aspects 21

1.3.2.1. History 21

1.3.2.2. Structure and chemical bonding 22

1.3.3. TTF as an intramolecular !-electron donor 27

1.3.4. TTF-"-A 32

1.3.4.1. TCNQ acceptors 32

1.3.4.2. Pyridinium and bipyridinium acceptors 34

1.3.4.3. A fluorescent acceptor 37

1.3.5. TTF-!-A and TTF-A 39

1.3.5.1. Quinone acceptors 39

1.3.5.2. Fullerene acceptors 41

1.3.5.3. Porphyrin acceptors 46

1.3.6. Polymer acceptors 47

!

1.4. Outline of the thesis 50

!

1.5. References 51

!

!

!

!

2. Experimental part 57

!

2.1. Absorption spectra 57

!

2.2. Chemical and electrochemical oxidation 58

!

2.3. Emission and excitation spectra 60

!

2.4. Lifetime measurements and transient absorption 63

!

2.5. References 68

!

!

3. Ru(II) Coordination Chemistry of a Fused Donor-Acceptor Ligand: Synthesis, Characterization and Photoinduced Electron Transfer Reactions of

[{Ru(bpy)2}n(TTF-ppb)](PF6)2n (n = 1, 2) 69

!

3.1. Introduction 72

!

3.2. Experimental Section 73

3.2.1. General. 73

3.2.2. Synthesis of 4#,5#-bis(propylthio)tetrathiafulvenyl[i]dipyrido[2,3-a:3#,2#-

c]phenazine (L). 74

3.2.3. Synthesis of [Ru(bpy)2L](PF6)2 (1). 75

3.2.4. Synthesis of [Ru(bpy)2(µ-L)Ru(bpy)2](PF6)4 (2). 75

3.2.5. Cyclic Voltammetry. 76

3.2.6. Photophysical Measurements: 76

3.2.7. X-ray Crystallography. 77

!

3.3. Results and Discussion 78

3.3.1. Synthesis and Characterization. 78

3.3.3. Electrochemical Properties. 82

3.3.4. Optical Properties. 84

!

3.4. Conclusions 93

!

3.5. References 93

!

3.6. Supporting Information: 98

!

!

4. Dual Luminescence and Long-lived Charge Separated states in Donor- Acceptor Assemblies based on Tetrathiafulvalene Fused Ruthenium(II)-

Polypyridine Complexes 100

!

4.1. Introduction 103

!

4.2. Results and Discussion 106

4.2.1. The free ligands 106

4.2.2. Coordination to innocent transition metal ions 107

4.2.3. Coordination to ruthenium(II) 108

4.2.4. Dual luminescence and long-lived charge-separated states 111

!

4.3. Conclusions 115

!

4.4. References 116

!

!

5. Effect of the Addition of a Fused Donor-Acceptor Ligand on a Ru^II Complex:

Synthesis Characterization and Photo-induced Electron Transfer Reactions of

[Ru(TTF-dppz)2(Aqphen)]²⁺ 118

!

5.1. Introduction 121

5.2. Experimental Methods 123

5.2.1. General 123

5.2.2. Synthesis 123

5.2.2.1. Synthesis of [Ru(phendion)2(Aqphen)](PF6)2. 123 5.2.2.2. Synthesis of [Ru(TTF-dppz)2(Aqphen)](PF6)2. 124

5.2.3. Physical Methods 124

!

5.3. Results and Discussion 126

5.3.1. Synthesis and Characterization 126

5.3.2. Electrochemistry 126

5.3.3. Photophysical properties 128

!

5.4. Conclusions 137

!

5.5. References 138

!

!

6. Imidazole-Annulated Tetrathiafulvalenes exhibting pH-Tuneable

Intramolecular Charge Transfer and Redox Properties 141

!

6.1. Introduction 144

!

6.2. Results and Discussion 146

!

6.3. Conclusion 157

!

6.4. Experimental Section 157

6.4.1. General 157

6.4.2. Materials 158

6.4.3. Synthesis 158

6.4.4. Crystallography 159

!

6.5. References 160

!

!

7. A Compactly Fused !-Conjugated Tetrathiafulvalene-Perylenediimide Donor-

Acceptor Dyad 165

!

7.1. References 175

!

7.2. Supporting Informatiom: 178

!

7.3. Experimental section 179

7.3.1. Materials 179

7.3.2. Physical measurements 179

7.3.3. Preparation of 1 180

7.3.4. ¹H NMR Spectrum of 1 in CDCl3 181

!

!

8. A Donor–Acceptor Tetrathiafulvalene Ligand Complexed to Iron(II) or Cobalt(II): Synthesis, Electrochemistry and Spectroscopy of [M(phen)2(TTF-

dppz)](PF6)2 185

!

8.1. Introduction 187

!

8.2. Experimental Section 188

8.2.1. Synthesis of [Co(phen)2(TTF-dppz)](PF6)2. 189 8.2.2. Synthesis of [Fe(phen)2(TTF-dppz)](PF6)2. 189

8.2.3. Methods: 190

!

8.3. Results and Discussion 191

8.3.1. Electrochemical properties 191

!

8.4. Photophysical properties 193

8.5. References 211

!

!

9. Conclusions 214

!

1. Introduction

Photo-induced electron transfer is one of the most important processes in photophysics and photochemistry. It is used by nature, for instance in photosynthesis, and in industry for photo-imaging, photo-catalytic organic reactions, opto-electronic devices, and solar energy conversion. In order to optimise such applications, photo-induced charge transfer is also intensively studied in fundamental research. The molecules studied in this thesis have several common characteristics, one of them being their behaviour as electron donor-acceptor assemblies, another one being the presence of tetrathiafulvalene (TTF) as a subunit of the ligands or molecules. TTF is well known as an efficient luminescence quencher as well as an electron donor. The complexes shown in this thesis contain one or several transition metal ions coordinated to the ligands, such as Ru²⁺, Co²⁺ and Fe²⁺. All compounds used in this thesis were synthesised by the collaborators of Prof. S. Decurtins at the University of Bern, so this thesis will be focussed on the large variety in their photophysical and photochemical properties. In the following introduction, some basics of spectroscopy as well as the TTF moiety present in all the molecules will be introduced. This is followed by a presentation of the experimental methods and setups used. Finally, the results and discussion on the ligands and complexes will be presented in the form of published papers, submitted manuscripts and manuscripts to be submitted.

1.1. The basics of spectroscopy

Most of the molecules presented here have strong colours, so one of their properties is that they absorb in a certain region of the visible light. This absorption also extends into the UV region, and, for some complexes, into the IR region.

In order to understand their behaviour, the compounds and complexes were studied by optical spectroscopy. Thus, the photochemical and photophysical properties of the different compounds were obtained through the interaction between the electromagnetic radiation, that is, light, and the molecules.

When molecular entities interact with electromagnetic radiation, three main processes can occur:

" absorption of energy resulting in a transition between two energy levels, the latter

being higher in energy, such that the molecule ends up in an excited state.

" stimulated emission or spontaneous emission of energy, that is, radiative

relaxation of the molecules back to the ground state.

In most of the cases, before any interaction with light, the molecules are in a stable state, namely the ground state, which is the one with the lowest possible total energy. In that case, the principal process when the interaction with light is switched on is the absorption of radiation. When atoms or molecules absorb light, incident photons allow the excitation from the ground state at an energy E0, to a higher energy level at Ei. In that process, the photon energy must be equal to h" = (Ei - E0).

After the interaction with light and the absorption of a photon, the molecule has received energy and takes on an electronic configuration of an excited state, which in an orbital picture means that one of its electrons has been promoted to a molecular orbital higher in energy. The excited state is usually photochemically or photophysically unstable. In order to come back to the ground state configuration, the molecule can dissipate the excess energy in different ways: ^[1-5]

i) the disappearance of the original molecule, by photochemical reactions

ii) by radiative relaxation leading to the emission of a photon usually at a lower energy than the one originally absorbed by the molecule (Stokes shift, Kasha's rule).

iii) by non-radiative relaxation, such as internal conversion and intersystem crossing followed by vibrational relaxation.

iv) if the molecule is in solution, it may interact with other species also present in solution, for example by luminescence quenching processes such as excitation energy transfer and light-induced electron transfer.

Figure 1.1: Scheme of different excited state deactivation processes. ^[6]

Spectroscopy involves the absorption of the electromagnetic radiation (from a lamp and/or a laser) by the molecules, as well as the one eventually emitted after excitation.

This includes different spectroscopic methods, such as absorption or emission spectroscopies. The different peaks in the obtained spectra represent transitions between different energy levels of the molecules. The strong light of Lasers, in particular pulsed Lasers, can be used to induce and probe the transformation of matter in real time on different timescales ranging from sub-picoseconds to hours and days.

The energy levels of a molecule represent the characteristic states of this molecule, which allow the identification of the transitions occurring in it. The energy is absorbed by quanta, in a discontinuous way and the energy of a photon is:

!

E = h" = hc

# = hc˜ "

(1)

!

h is Planck#s constant,

!

h=6.626"10^#34J$s,

!

" the frequency,

!

" the wavelength, and

!

" ˜ is the wavenumber.

One of the most important aspects of a given absorption band is its integrated intensity, which is proportional to the oscillator strength containing information on the allowed or forbidden character of the transition. Transitions from ground to excited states having the same spin value are spin-allowed and if they have opposite parity they are also

electric dipole allowed, thus giving rise to intense absorption bands. Transition between states of the same parity, such as dd transitions in transition metal complexes, are electric dipole forbidden and give rise to only weak absorption bands. Transitions to excited states of different spin values are likewise forbidden and give very weak absorption bands. They acquire their intensity via spin-orbit coupling and may thus have a certain non-negligible intensity for complexes of heavier transition metal ions.

Transitions, which are both spin and parity forbidden, are very weak indeed and only observable in very special cases.

The photochemical and photophysical processes and most of all the states involved in those processes, can be illustrated in a so-called Jablonski diagram. The Jablonski diagram represents the different energy levels of a molecule. The total energy of the molecule can be described as the sum of the vibrational, rotational and electronic energies.

Figure 1.2: Schematic energy level diagram (Jablonski diagram)

According to the Jablonski diagram, two different relaxation pathways lead to luminescence (emission of light): it is called fluorescence when the excited and the ground state have the same spin and phosphorescence when their spins are different.

In the same way, non-radiative processes are called either internal conversion or intersystem crossing when they occur between states of the same or different spins, respectively.

Fluorescence and internal conversion are spin-allowed processes, versus phosphorescence and intersystem crossing which are spin-forbidden.

Every intramolecular decay process is characterised by a rate constant and the excited states can be characterized by lifetimes: [3, 4, 7, 8]

!

"^(S₁⁾= 1

k_ic+k_fl +k_isc (2)

!

"(T₁)= 1

k_isc^' +k_ph ⁽³⁾

For luminescence processes, a quantum yield can be defined:

!

"

= k

k

+ k

+ k

!

"

= k

# k

k

+ k

( ) ( ^k

^{+ k}

^{+ k}

)

A molecule is a multielectron system, which can be described by molecular orbitals. An approximate wavefunction for a molecule is given by the antisymmetrised product function:

!

" = # S = $

i

%

s

Where

!

"_i is a molecular orbital (MO) and

!

s_i is a spin eigenfunction. The orbital part of the wavefunction represents the electronic configuration. In a zero order description, the energy associated with an electronic configuration is given by the sum of the energies of the occupied MOs. ^[9] However, in order to obtain a more realistic description of the energy states of the molecules, two elements should be taken in account. The spin functions must be added to orbital functions for the description of the electronic configuration, and the interelectronic repulsion should be taken into account. ^[9]

For metal complexes, different MOs can be assigned according to their atomic orbital contributions. Two groups can be distinguished: the orbitals centred principally on the ligands (#L, !L, #n*

and !L*

) and the partially occupied orbitals centred predominantly on the metal (t2g*

(!M*

), eg*

(#M*

)).

Figure 1.3: Molecular orbital diagram for an octahedral complex of a transition metal.^[9]

Principal excited configurations of metal complexes can thus be classified as:

- metal centred (MC) transitions (!M*

$#M*

) - ligand centred (LC) transitions (!L$!L*

)

- ligand to metal charge transfer (LMCT) transitions (!L$!M*

, #M*

) - metal to ligand charge transfer (MLCT) transitions (!M$!L*

)

1.2. Electron transfer

As mentioned above photo-induced electron transfer is an important process in photophysics and photochemistry. It is a process in which an electron is transferred from an electron donating species (D) to an electron accepting species (A). The first step of a photochemical reaction is the interaction between light and molecule, which brings it to an excited state [D-A]*, in which depending upon the individual properties of D and A either the donor or the acceptor moiety is in the excited state. The excited molecules have different properties than in the ground state: the electron donor moieties become a stronger reducing agent and the electron acceptor moieties become a stronger oxidising agent than in the ground state.

Thus when the excited states are involved, the excited state redox potentials of the corresponding redox couples have to be used. These can be calculated from the redox potentials of the ground state couple and the one electron potential corresponding to the zero-zero excitation energy.^[9]

!

E⁰(D⁺/D^*)"E⁰(D⁺/D)#E⁰⁰(D/D^*) (7)

!

E⁰(A^*/A^-) "E⁰(A/A^-)+ E⁰⁰(A/A^*) ₍₈₎

By neglecting the electrostatic interaction, the Gibbs free energy of the ground state reaction of the donor-acceptor pair

AD $ A^-D⁺. is given by:

!

"G⁰ =#nF E

(

)

Two different photo-induced electron transfer processes can take place in a molecular dyad A-D:

*A-D$A^--D⁺ (oxidative electron transfer) A-*D$A^--D⁺ (reductive electron transfer)

! 14

The Gibbs free energy of the excited state electron transfer reaction is given by:

!

"G^0* =#nF E

(

)

where

!

X / X

!

D / D

!

A / A

In the absence of chemical complications photo-induced electron transfer processes are followed by spontaneous back-electron transfer reactions, the so-called dark reaction or recombination reaction, that regenerate the system in its ground state:

A^--D⁺$A-D

For the dark reaction the Gibbs free energy is given by

!

"G⁰(dark reaction)=#"G⁰ (11)

Kinetically, electron transfer processes involving excited states and those involving ground state molecule can be described within the framework of the Marcus theory ^[10]

and of successive, more sophisticated theoretical models.^{[11, 12]} Quantum mechanically, both the photo-induced and back-electron-transfer processes can be viewed as radiationless transitions between different, weakly interacting electronic states of the A- D molecular dyad. The rate constant of such processes is given by an appropriate Fermi «Golden Rule» expression:

!

k_et = 4"

h H^el2FC (12)

Where

!

H

!

FC

Electron transfer can be regarded as an extra deactivation path of the locally excited (singlet) state that can exist in addition to internal conversion, inter system crossing to the triplet manifold (both iso-energetic) and emission (cf. Jablonski diagram). In that case, the new deactivation path has to be taken into account for the fluorescence quantum yield and excited state lifetime. If that extra deactivation path is introduced, for instance by making an electron transfer energetically favourable (e.g. by a change of solvent), these expressions become:

"^'_fl = k_fl

k_fl +k_ic+k_isc+k_cs ⁽¹³⁾

!

"

= 1

k

+ k

+ k

+ k

The lifetime and quantum yield of the excited state in the absence of electron transfer can be regarded as reference value, and we can thus determine the charge separation rate constant (

!

k_cs) with the following equations:

!

k

= 1

"

'

# 1

"

(15)

!

k_cs=

"_fl

"^'_fl #1

$fl

(16)

The rate constants of charge separation and charge recombination processes can also be probed by using the absorption of the excited state via pulsed excitation and transient absorption spectroscopy.

As mentioned above, electron transfer between a donor D and an acceptor A can be described by the Marcus theory. Either A or D can be in an excited state (A*D or AD*), and the potential energies of the ensuing pair states are represented by parabolas along the reaction coordinate. Classically, the reaction takes place when the system is at the intersection (point I in Figure 1.4), which means that the reactants and the products including the solvent rearrangement are at the same total energy. Thus, an electron is transferred adiabatically, while according to the principle of Franck Condon the nuclei remain fixed during the actual process.

Figure 1.4: Scheme of the energy potential of reactants and products

Photo-induced electron transfer can be illustrated as follow (see Figure 1.5):

1) After photo-excitation, the electron is still mainly localised on D, but there is already a little probability to find it on A.

2) At the crossing point I, there is an equal chance of finding the electron on both sides ($E = 0). The electron is transferred from D to A.

3) After relaxation into the well of the charge-transfer state, the probability to find the electron on the A side is highest, and $E has decreases sharply.

Figure 1.5: Scheme of different steps during the electron transfer

According to the Marcus theory, the rate constant for an electron transfer process can be expressed as:

!

k_et="_N#_etexp $%G^&

RT '

( ) *

+ , (17)

where

!

"N is the average nuclear frequency factor,

!

"et is the electronic transmission

coefficient, and

!

" G

!

"G^#= $

4 1+"G⁰

$

%

&

' (

) *

2

(18)

where

" G

"

nuclear reorganization energy:

!

"="i+"o (19)

where

!

"_i is the reorganization energy of the molecules themselves and

!

"₀ the reorganization energy of the solvent shell.

The reorganization energy of the molecules is the inner reorganization energy and is described by a change of geometry between the state of the reagents (R) and the state of the products (P):

!

"_i= f_i

( )

( )

( )

( )

$

% & '

( )

*

^{[ ]}

!

f is the force constant of the i^th normal mode of the reagents R and the products P

!

"x_i is the shift of the equilibrium position.

The reorganization energy of the solvent is the outer reorganization energy, which is defined by the reorientation of the dipoles of the solvent molecules in answer to the new distribution of the charges:

!

"_o= #q² 4$%0

1 2r_D + 1

2r_A &1 d '

( ) *

+ , 1 n² & 1

%s

'

( ) *

+ , (21)

!

" q

!

r_D and

!

r_A = radii of the molecules !

!

d=r_D+r_A

!

n

!

"s = permittivity (static dielectric constant) of the solvent

Equation 19 predicts that for a homogenous series of reactions (same

!

" and

!

"et

values), an

!

ln k

!

" G

1) a region of normal regime, for small driving forces (" >

"#G

2) an activationless regime (

!

" %

!

"# G

3) a region of an inverted regime, for strongly exergonic processes (

!

" <

!

"#G

where the reaction rate constant decreases when the driving force increases.

In photo-induced electron transfer reactions, the inversed area has been observed in very few cases.^[13] On the other hand, it has been observed for an increasing number of charge recombination reactions and is well known for intersystem crossing and internal conversion.

Figure 1.6: Scheme representing the different regions of the Marcus theory. $Q is supposed to be constant here.

1.3. Tetrathiafulvalene (TTF) 1.3.1. Introduction

TTF (tetrathiafulvalene), shown in Figure 1.7, is one of the most studied molecules of the last 40 years ^{[14, 15]} (cf. Figure 1.8), because of its physical and chemical properties.

It was first studied as a component of the first organic superconductor ^[16] and more recently for some of its chemical and photochemical properties. Indeed, it is a very well known luminescence quencher and electron donor.

Figure 1.7: Tetrathiafulvalene (TTF)

The studies and the knowledge thus gained about TTF play an important role in numerous research fields, such as solid state physics, photochemistry and photophysics, chemistry and biology.

Figure 1.8: Graph representing the number of publications on and with TTF between 1920 and 2010

!

!

1.3.2. General aspects 1.3.2.1. History

Investigations on TTF began some 80 years ago with a report by Hurtley and Smiles.^[17,

18] However, articles on TTF and TTF derivatives really began to appear regularly around 1970,^[14] when Wudl et al. found that TTF was an “unusually stable organic radical cation”. Moreover, TTF in its neutral form is an orange organic molecule and readily loses electrons in the presence of oxidizing agents (or by electrochemistry) to form the purple radical cation,^[14] and then a yellow dication. Both cationic species are aromatic in the Hückel sense and are quite stable. So, TTF has good electron donor properties.

Figure 1.9: Oxidation of TTF gives rise to stable cationic species^[18]

In 1972, the electrical conductivity of TTF was established, also by Wudl et al.,^[20] and in 1973, the first “organic metal” TTF-TCNQ (tetracyanoquinodimethane) was discovered.^[16] TCNQ is an electron acceptor and gives a black crystal when precipitated with TTF. A partial charge transfer is observed between TTF and TCNQ, resulting in electrical conductance several orders of magnitude higher than for other organic compounds. The band structure of TTF-TCNQ showed some features usually found in metals, so it was called an “organic metal”.

1.3.2.2. Structure and chemical bonding

Since the discovery of TTF, a lot of investigations and studies were performed on its synthesis in order to make efficient derivatives for better superconductors, !-donors or sensors. Despite the large number of studies on TTF, very few concerned its structure.

Indeed, TTF was assumed to have a D2h symmetry, that is, to be planar. In 1994, a gas phase electron diffraction study showed that the planar structure was not the best according the experiments but that a non-planar boat structure fits better.^[21]

In fact, it was realized that TTF is a very flexible molecule and can have different conformations depending of the type of interactions it encounters. In 1999, Katan published an article discussing the structure of TTF as a function of its oxidation state.^[22] It appears that in principle the planar confirmation should be more stable than the effectively encountered boat-like conformation (C2v). However, the difference in energy between the planar and the boat structure is quite small: 0.02 eV for DFT calculations at the LDA level and 0.04eV with a BP gradient correction.^[22] The results of these calculations are summarized in Table 1.1 (cf. also Figure 1.10). This small energy

intermolecular interactions, TTF can adopt a planar or a boat conformation. Moreover, according to the calculations, the TTF radical cation seems to be much more stable in the planar conformation.

Figure 1.10: Definition of the atomic numbering for the atoms of the TTF molecule^[22]

Table 1.1: Calculated Bond Lengths (Å) and Angles (deg) for the TTF Molecule in Comparison with Gas Phase Electron Diffraction Results^a. This table is taken from [22].

a & corresponds to the dihedral angle between SCS and SCCSplanes.

b Spin-polarized calculations lead to the same values.

!

A model for electron transfer processes from a TTF molecule in a charge transfer complex was obtained by ab-initio calculations.^[22] The results of the first principles calculations for a planar monomer in different oxidation state are given in Table 1.2.

Figure 1.11: Isodensity representation of the HOMO of the TTF molecule.^[22]

Table 1.2: PAW-LDA Calculated Total Energy and Energy of the HOMO State for Different Total or Spin-Up and Spin-Down Occupations of the HOMO State(s)^a. This table is taken from [22] .

a Deduced values for the Coulomb repulsion U and the spin-splitting parameter J.

All energies are given in eV.

The ionization energy of TTF can be estimated by two different ways: by the difference in total energy for the neutral molecule and the radical cation

Eionization = Etot(TTF⁺ ) – Etot(TTF),

or by the mean values of the HOMO energies for TTF and TTF⁺ . The same result is found for both methods, that is, 6.3 eV.^[22] This value is in good agreement with those obtained by photoelectron spectroscopy in the gas phase (6.7 eV ^[23]).

TTF being well known for its ability to donate electrons, reduction and oxidation are

provides a direct measure of the Gibb#s free energy of adding or removing electrons from TTF. Experimentally, TTF has two oxidation waves: E11/2

= 0.34 V for the couple TTF/TTF⁺ and E21/2

= 0.78 V for the couple TTF⁺/TTF²⁺, vs Ag/AgCl in acetonitrile.

Moreover, a lot of TTF based compounds are studied in solution and it is known that the solvent has a big influence on some processes as demonstrated by Marcus in his treatment of electron transfer reactions.^[24-26] Ab-initio molecular dynamic can give an idea of the solvent effect for electron transfer in TTF ^[27] and is illustrated with acetonitrile in Figure 1.12. For TTF in the neutral form, a preferential orientation of the acetonitrile methyl group toward the sulphur atom with the nitrogen pointing away from it was observed.^[27] This preference is reduced near the carbon atoms. In contrast, the average orientation of acetonitrile is reversed upon oxidation of TTF (the nitrogen is turned toward the sulphur atom). This is consistent with the fact that the largest amount of spin density and therefore electron density is located near the sulphur atoms.

Figure 1.12: «Snapshots of TTF in solution […], with only the closest solvent molecules visualized. The spin density (upper panels) and the difference electron density (lower panels, see text for the definition) are shown at contour levels of +0.002 (green) and '0.002 (pink) au; i.e., green implies an increase of electron density in the reduced state. The difference density illustrates the electronic polarization induced by the cation, in particular of the C'H bonds and first solvation shell. For each solute, the molecular configuration in the upper panel is identical to the configuration in the lower panel. However, the TTF configuration is taken from a trajectory of the neutral molecule […]. The images are generated with VMD». Figure taken from [27].

1.3.3. TTF as an intramolecular !-electron donor

!

TTF and its derivatives have been extensively studied for more than 35 years as !- electron donors in intermolecular charge transfer materials. The most famous example for that is given by the number of studies on TTF-TCNQ over the past three decades.^[16,

28-33]

The majority of the applications of TTF are determined by its donor abilities, which are the result of the high-lying HOMO. Nevertheless, the potential of TTF as donor in an intramolecular sense has only recently been developed. Indeed, as Bryce mentioned in one of his reviews,^[34] TTF has received attention as a donor moiety in intramolecular charge transfer (ICT) systems principally only over the last decade because of the synthetic challenge of obtaining functionalized TTFs in reasonable quantities.^[35]

Progress of the last ten years in the branches of TTF chemistry has changed this situation and TTF can now be synthesized in 20 g batches from readily available starting materials.^[34] Fanhaenal et al have reviewed different synthetic methods ^[36-38]

which are summarized in the following scheme shown in Figure 1.13:^[36]

Figure 1.13: Scheme of synthetic methods for construction of the TTF skeleton.

This is taken from [36].

TTF as an electron donor evokes the topic of intramolecular donor-acceptor (D-A) molecules. This includes different fields like molecular electronic devices,^{[39, 40]} organic metals,^[41] chromophores for dyes^[42] and non linear optics,^{[43, 44]} and also excited state energy and electron transfer processes,^{[45, 46]} and theoretical aspects of charge transport at the molecular level, conjugation and aromaticity.^[47] Different applications are summarized in Figure 1.14.

Figure 1.14: Different fields of application. This Figure is taken from [35].

The concept of D-A molecules is linked with electron transfer processes. They depend principally on the energy of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels and the orbital interactions. An enhanced redox activity is observed for donor and acceptor molecules upon photo- excitation. Indeed, donor molecule D becomes D* when excited and has an electron transferred to a higher-lying LUMO. As a result it becomes a better reducing agent. In the same way, an acceptor A* is a stronger oxidizing agent than A because the hole created by photo-excitation in the ground state HOMO increases the electron affinity of the excited state.^[48]

TCNQ, gave rise to the idea of extending the !-conjugated system of TTF in order to decrease the Coulomb repulsion between the charges on the molecules in the solid state and thus to improve the intermolecular interactions.

The first !-extended derivative of TTF, dibenzo-TTF, was, in fact, synthesized 50 years before its parent molecule.^[17] However, its electron-donor properties were studied much later. The benzene rings have an electron withdrawing influence, which increases the oxidation potential by 250 mV compared to TTF.^[49] Numerous homologues of dibenzo- TTF were then synthesized, including polymers.

Wudl et. al. presented an exhaustive review on that subject in 2004.^[50] The authors notice that molecules with low HOMO/LUMO gap are of particular importance due to their ability to easily donate or accept an electron. Some molecules, called amphoteric compounds, can even act as an electron donor and as an electron acceptor at the same time, forming stable redox states within the same molecule. Molecules with a controlled HOMO/LUMO gap are the prime targets for electronic applications at the single molecule level. Two strategies can be applied in order to obtain a lower HOMO/LUMO gap:^[50] (i) by extending the !-conjugation in the molecule and (ii) by the construction of covalent D-A compounds (D being a !-electron donor and A a !-electron acceptor) in which the HOMO and the LUMO can be tuned relatively independently of each other.

The first method was successful in the design of !-conjugated polymers, fullerene materials, etc. The second method gave rise to various TTF-A derivatives.

The major problem for the synthesis of TTF-spacer-A compounds is the need to prevent the formation of a stable intermolecular CT complex between the TTF and the acceptor moieties, prior to their covalent coupling. Three conceptually different ways of preventing this unwanted interaction have been explored and reported by Bryce:^[34]

• “A TTF derivative can be linked to a group that is either a weak electron acceptor (e.g., a quinone) or not an acceptor at all (e.g., a pyridine moiety), which is subsequently converted into a stronger acceptor (TCNQ, or pyridinium cation, respectively). The conversion of a TTF-quinol derivative into its corresponding TTF-quinone has been demonstrated on numerous occasions […] but in spite of several attempts, there appears to be no example of a TTF-quinone being converted into the corresponding TCNQ or DCNQI (N,N-dicyanoquinodiimide) derivative although TTF-aldehydes have been converted cleanly into the

corresponding dicyanomethylene derivatives […]. The converse disconnection is also possible: an acceptor unit is linked to a 1,3-dithiole-2-thione unit (very weak donor), which is then converted into a TTF group, by standard coupling methodology […].

• Lithiated TTF can be substituted by an electrophilic reagent, which, after covalent bond formation, is transformed into an acceptor group in situ […].

• Steric hindrance in either the TTF or acceptor unit (or both) can be exploited to prevent their close !-! association, thereby allowing pendant substituent groups to couple […]”.^[34]

In 1994, Jorgensen and co-workers ^[19] reported the principal landmarks in TTF chemistry. It is always evolving and always involving a lot of different disciplines in science, such as chemistry, biology or physics, depending upon the area of application, which is very vast.

Table 1.3: Landmarks in TTF chemistry. Table taken from [19].

1926: The first TTF derivative, dibenzo-TTF was synthesized as part of a general study of five-membered ring systems^{[17, 18]}

1965 : Deprotonation of 1,3-dithiolium salts afforded TTF derivatives for the first time^[51]

1970: First observation of parent TTF. TTF is found to form a stable purple radical cation on reaction with chlorine gas^[14]

1973: First observation of metallic conductivity in an organic solid

(TTF)(TCNQ)^[16]. TCNQ = tetracyanoquinodimethane. Conductivity: 500

!^-1cm^-1

1980: Superconductivity observed in a TTF derivative at 1.4 K:

tetramethyltetraselenafulvalene hexafluorophosphated ([TMTSF]₂PF₆)^[52]

1980: TTF was derived extensively in the search for organic (super-) conductors^[19]

1985: Macrocyclic TTF-based systems investigated with the aim of making molecular devices, sensor, switches and shuttles^[19]

Initially, TTF and its derivatives were prepared in order to develop electrically

active supramolecular systems, and chemical sensors and redox-switchable ligands have been prepared with TTF containing rotaxanes and catetanes.^[19] TTF has also an important role in molecular electronics. Indeed, TTF containing D-"-A molecules have allowed the preparation of the first confirmed unimolecular rectifier. TTF can also have non linear optic (NLO) responses in second and third harmonic generation and a good thermal stability.^[53] Segura and co-workers have summarized the reviews on TTF up to 2001, thus giving further examples for the many fields of applications of TTF.

Table 1.4: Recent reviews on specific aspects of TTF. Table taken from [53].

Main author Title

M. R. Bryce Recent progress on conducting CA salts^[54]

V.

Khodorkovsky Molecular design of organic conductors^[41]

M. R. Bryce Increasing dimensionality in the solid state^[55]

G. Schukat TTF chemistry^[38]

J. Garín Reactivity of TTF and TSeF^[56]

K. B. Simonsen Functionalisation of TTF^[57]

T. Otsubo TTF dimers^[58]

M. Adam TTF oligomers^[59]

J. Becher TTF oligomers^[60]

M. R. Bryce Macromolecular TTF chemistry^[61]

T. Jørgensen Supramolecular TTF chemistry^[19]

K. B. Simonsen Macrocyclic and Supramolecular TTF chemistry^[62]

M. B. Nielsen Two- and three-dimensional TTF macrocycles^[63]

E. Coronado Hybrid polyoxometalates-TTF materials^[64]

J. Roncali Linearly !-extended TTF derivatives^[65]

P. Day Molecular magnetic semiconductors, metals and superconductors^[66]

M. R. Bryce TTF as !-donors in intramolecular CT-materials^[34]

M. B. Nielsen Tetrathiafulvalenes as building blocks in supramolecular chemistry^[67]

M. R. Bryce Functionalised tetrathiafulvalenes: new applications as versatile

!-electron systems in materials chemistry^[68]