From Chromophores to Materials: Structure-Property Relationships in Multichromophoric Systems

Thesis

Reference

From Chromophores to Materials: Structure-Property Relationships in Multichromophoric Systems

ASTER, Alexander

Abstract

Organic materials employed to harvest light consist of multiple chromophoric building blocks which are arranged in a three-dimensional structure. Their propensity to absorb light as well as their functionality to transform photons into chemical energy is given by both the optoelectronic properties of the monomers and the interchromophore structure. In this thesis, time-resolved laser spectroscopy from the infra-red to UV and fs to μs timescales is applied to decipher the structure-property relationships in multichromophoric systems. In addition, the experimental upgrade and development of two transient absorption instruments is described, which increases the spectral window from 360-720 to 330-1600 nm. First, the photophysics of the monomeric building blocks are established, comprising naphthalenediimide, perylene and pentacene chromophores. In a second step, two or more of these building blocks are covalently linked in supramolecular systems to study charge separation and singlet fission as a function of interchromophore geometry.

ASTER, Alexander. From Chromophores to Materials: Structure-Property Relationships in Multichromophoric Systems . Thèse de doctorat : Univ. Genève, 2021, no. Sc. 5545

DOI : 10.13097/archive-ouverte/unige:150407 URN : urn:nbn:ch:unige-1504075

Available at:

http://archive-ouverte.unige.ch/unige:150407

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

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UNIVERSIT´E DE GEN`EVE FACULT´E DE SCIENCE Section de chimie et biochemie

D´epartement de chimie physique Professeur Eric Vauthey

From Chromophores to Materials:

Structure-Property Relationships in Multichromophoric Systems

Th´ese

Pr´esent´ee `a la Facult´e des sciences de l’Universit´e de Gen`eve pour obtenir le grade de Docteur `es sciences, mention chimie

par

Alexander ASTER de

Salzburg (Autriche)

Th`ese N^◦ 5545

GEN`EVE

Atelier Repromail -Uni Mail 2021

Acknowledgments

I would first like to warmly thank my supervisor Prof. Eric Vauthey for his guid- ance in the last four years. The balance between freedom to work independently and pursue own ideas and always having an open door for questions and advice was ideal to grow as a scientist. Even more importantly, I want to thank him for the friendly open atmosphere in which no question felt stupid and constructive criticisms was encouraged.

I wish to thank Prof. J´er´emie L´eonard and Prof. Jacques-E. Moser for agreeing to be members of the jury for this thesis.

I also wish to thank Dr Arnulf Rosspeintner and Dr Bernhard Lang for their mentor ship, countless explanation and friendship. In a time where senior scientist positions are cut, it showed me how fruitful working side-by-side with researchers that have 10+ years of experimental experience is. Together with Eric, they acted as scientific role models and shaped me to the scientist I am today.

All current and past members of the Vauthey group as well as the physical chemistry department are thanked for creating a warm and collaborative atmo- sphere. I will miss the heated political discussion during lunch breaks and our pizza Friday.

Didier Frauchiger is thanked for all the technical help, all the custom-made parts he built for me and especially for his friendly and helpful attitude. Sophie Jacquement is thanked for her help with all administrative matters which made the thesis go smoothly from the start to the end.

I would like to dedicate this work to my parents, Margit and Rudolf, who made this journey possible and always stand behind me and support me wherever I go. It makes life much easier if you know that whatever happens their is a place you can always return, and I am grateful for that. Furthermore, I want to thank my family

iii

Andrea, Fritz, Philipp, Isabel and Josef who I know are there whenever needed.

My grandparents Josef, Marianne and Lore for taking care of me when I was little.

Finally, I would like to thank Anna-Bea for going through this four years together and tolerate all my moods, including the ones after a 15 hour measurement session. For being there for me, making me laugh and making me feel like home wherever we are. I cannot image these years without her.

Scientific Contributions:

Peer-reviewed articles presented in this PhD thesis:

I. Aster A.; Rumble C.; Bornhof A.-B.; Huang H.-H.; Sakai N.; ˇSolomek T.;

Matile S. and Vauthey E. Long Lived Triplet Charge Separated States in Naphthalenediimide Based Donor-Acceptor Systems. Manuscript ac- cepted in Chem. Sci.

II. Aster A.; Bornhof A.-B.; Sakai N, Matile S.; and Vauthey E. Lifetime Broadening and Impulsive Generation of Vibrational Coherence Triggered by Ultrafast Electron Transfer. J. Phys. Chem. Lett. 2021, 12, 3, 1052–1057.

III. Aster, A.; Licari, G.; Zinna, F.; Brun, E.; Kumpulainen, T.; Tajkhorshid, E.; Lacour, J.; Vauthey, E. Tuning Symmetry Breaking Charge Separa- tion in Perylene Bichromophores by Conformational Control. Chem. Sci.

2019,10, 45, 10629–10639.

IV. Aster A.; Zinna F.; Rumble C.; Lacour J. and Vauthey E. Singlet Fission in a Flexible Bichromophore with Structural and Dynamic control. J.

Am. Chem. Soc. 2021,143, 5, 2361–2371.

Comments on my participation:

In articles I-IV, I carried out the stationary photophysical and time correlated single photon counting measurements as well as the UV-Vis and NIR transient absorption measurements, including all data analysis. In article III, I addi- tionally conducted the fluorescence upconversion measurements with the help of Tatu Kumpulainen and in paper I, the IR transient absorption measure- ments with the help of Christopher Rumble. I wrote all the manuscripts in collaboration with Eric Vauthey.

Additional peer-reviewer articles composed during this doctoral work:

1. Shybeka, I.; Aster, A.; Cheng, Y.; Sakai, N.; Frontera, A.; Vauthey, E.;

Matile, S. Naphthalenediimides with Cyclic Oligochalcogenides in Their Core. Chem. Eur. J.2020,26, 62, 14059–14063.

2. Dereka, B.; Svechkarev, D.; Rosspeintner, A.; Aster, A.; Lunzer, M.;

Liska, R.; Mohs, A. M.; Vauthey, E. Solvent Tuning of Photochemistry upon Excited-State Symmetry Breaking. Nat. Commun. 2020, 11, 1, 1925.

3. Caprice, K.; Aster, A.; Cougnon, F. B. L.; Kumpulainen, T. Untying the Photophysics of Quinolinium-Based Molecular Knots and Links. Chem.

Eur. J. 2020,26, 7, 1576–1587.

4. Feskov, S. V.; Rogozina, M. V.; Ivanov, A. I.; Aster, A.; Koch, M.; Vau- they, E. Magnetic Field Effect on Ion Pair Dynamics upon Bimolecular Photoinduced Electron Transfer in Solution. J. Chem. Phys. 2019,150, 2, 024501.

5. Bornhof, A.-B.; Bauz´a, A.; Aster, A.; Pupier, M.; Frontera, A.; Vauthey, E.; Sakai, N.; Matile, S. Synergistic Anion–(π)n–πCatalysis onπ-Stacked Foldamers. J. Am. Chem. Soc. 2018,140, 14, 4884–4892.

6. Aster, A.; Vauthey, E. More than a Solvent: Donor–Acceptor Complexes of Ionic Liquids and Electron Acceptors. J. Phys. Chem. B 2018,122, 9, 2646–2654.

vii Conference proceedings:

1. Aster A.; Zinna F.; Rumble C.; Lacour J. and Vauthey E. In SCS Fall Meeting; Contributed Talk: Virtual Conference, 2020.

2. Aster, A.; Licari, G.;Rumble C.; Zinna, F.; Brun, E.; Kumpulainen, T.;

Tajkhorshid, E.; Lacour, J.; Vauthey, E. In Geneva Chemistry and Bio- chemistry days; Contributed Talk: Geneva, Switzerland, 2020.

3. Aster, A.; Licari, G.; Zinna, F.; Brun, E.; Kumpulainen, T.; Tajkhorshid, E.; Lacour, J.; Vauthey, E. In SCS Photochemistry Symposium 2019; Contributed Talk: Fribourg, Switzerland, 2019.

4. Aster, A.; Licari, G.; Zinna, F.; Brun, E.; Kumpulainen, T.; Tajkhorshid, E.; Lacour, J.; Vauthey, E. In International Conference on Photochem- istry; Contributed Talk: Boulder,Colorado, USA, 2019.

5. Aster A.; Bornhof A.-B.; Matile S.; and Vauthey E. InSCS Fall Meeting; Contributed Poster: Lausanne, Switzerland, 2018.

6. Aster A.; Bornhof A.-B.; Matile S.; and Vauthey E. In IUPAC Interna- tional Symposium on Photochemistry; Contributed Poster: Dublin, Ire- land, 2018.

7. Aster, A.; Licari, G.; Zinna, F.; Brun, E.; Kumpulainen, T.; Tajkhorshid, E.; Lacour, J.; Vauthey, E. In Central European Conference on Photo- chemistry; Contributed Talk: Bad Hofgastein, Austria, 2019.

8. Aster A.; Vauthey E. In SCS Fall Meeting; Contributed Poster: Bern, Switzerland, 2017.

Summary

Organic materials employed to harvest light consist of multiple chromophoric building blocks which are arranged in a three-dimensional structure. Their propen- sity to absorb light as well as their functionality to transform photons into chem- ical energy is given by both the optoelectronic properties of the monomers and the interchromophore structure. Deciphering the structure-property relationships in these systems is required to comprehend the evolutionary design of photosyn- thesis and establish design principles for artificial photosynthesis, photo-catalysis, organic solar cells or organic light-emitting diodes.

In this thesis, time-resolved laser spectroscopy from the infra-red to UV and fs toµs timescales is applied to understand the property changes from monomeric building blocks to multichromophoric systems. In addition, the experimental in- struments were upgraded, increasing the accessible spectral window for transient absorption spectroscopy from 360-720 nm to 330-1600 nm. During this doctoral work, the UV-Vis transient absorption instrument was transformed to only com- prise reflective optics and a near infra-red transient absorption instrument was developed from scratch.

The foundation to unravel the structure-property relationship is a detailed un- derstanding of the monomeric building blocks. In papers I and II, the photophysi- cal properties of different naphthalene diimides are studied. They represent a class of chromophores frequently used in supra-molecular systems for light harvesting and energy conversion. We could assign the spectral broadening of the stationary absorption spectrum to the very short lifetime of the photo-populated state, as predicted by the Heisenberg’s uncertainty principle. This was the first time this phenomenon could be experimentally observed at room temperature in solution.

Based on transient absorption spectroscopy, the short excited state lifetime in these systems could be attributed to ultrafast charge transfer from an electron donor moiety to the naphthalene diimide core. The charge transfer reaction furthermore leads to the population of a vibrational wavepacket in the charge separated product

ix

state. In contrast to conventional vibrational wavepackets, which are generated by a broad, ultrashort excitation pulse, the vibrational coherence is generated by the charge transfer itself. This has never been observed for intra-molecular charge transfer before. In addition to the coherent phenomena, in paper II, the complex photophysics could be unravelled and spectral signatures of an unusual triplet charge transfer state could be identified. Since the charge recombination is spin forbidden the lifetime of the charge transfer state is prolonged, a desired property in photo-catalysis and artificial photosynthesis. A systematic solvent and driving force dependent study could show that the yield for this long lived charge transfer state can be controlled by the environment of the chemical system. This detailed understanding enables an introduction of a new functionality in supramolecular systems originating from the naphthalene diimide building block, if surrounded by the right environment.

In papers III and IV, two chromophores were covalently linked in a controlled fashion. These bichromophores represent the intermediate between the monomer and the bulk material (Figure S1). Depending on the optoelectronic properties of the monomers, these bichromophores can be used as model systems to establish the structure-property relationships of different excited state reactions. We applied pentacene and perylene based monomers to study singlet fission and symmetry- breaking charge separation, respectively. For both excited state reactions, which are triggered by the absorption of a photon, the electronic coupling and therefore the distance between the two monomers was found to act as two edged sword.

Even though the formation of the desired product is accelerated in close contact conformations, the survival probability is also reduced. For both reactions, the sweet spot geometry, at which the rates for formation and decay of the product state are balanced, has to be found. In the case of charge separation, structural relaxation to the excimer geometry was found to act as trap state, whereas mo- tion along the structural coordinate is suggested to drive the reaction for singlet fission. Relaxation along the solvent coordinate, on the other hand, is crucial for symmetry-breaking charge-separation, but could not be found to play a significant role in singlet fission with pentacene dimers.

Figure S1: Graphical summary of the thesis.

Resum´e

Les mat´eriaux organiques utilis´es pour capter la lumi`ere sont constitu´es de plusieurs blocs de chromophoriques qui sont dispos´es dans une structure tridimensionnelle.

Leur tendance `a absorber la lumi`ere ainsi que leur aptitude `a transformer les pho- tons en ´energie chimique sont donn´ees `a la fois par les propri´et´es opto´electroniques des monom`eres et par la structure interchromophore. L’´etablissement des relations structure-propri´et´e dans ces syst`emes est n´ecessaire pour comprendre la concep- tion ´evolutive de la photosynth`ese et ´etablir des principes de construction pour la photosynth`ese artificielle, la photo-catalyse, les cellules solaires organiques ou les diodes ´electroluminescentes organiques.

Dans cette th`ese, la spectroscopie laser `a temps r´esolu de l’infrarouge `a l’UV et de l’´echelle des fs aux µs est appliqu´ee pour comprendre les changements de propri´et´e des blocs monom´eriques aux syst`emes multichromophores. De plus, les instruments exp´erimentaux ont ´et´e d´evelopp´es, augmentant la fenˆetre spectrale accessible pour la spectroscopie d’absorption transitoire de 360-720 nm `a 330-1600 nm. Au cours de ce travail de doctorat, l’instrument d’absorption transitoire UV- Vis a ´et´e transform´e pour ne comprendre que des optiques r´efl´echissantes et un instrument d’absorption transitoire proche infrarouge a ´et´e enti`erement construit.

Une condition essentielle pour ´etablir la relation structure-propri´et´e est la compr´ehension d´etaill´ee des blocs monom´eriques. Dans les articles I et II, les propri´et´es photophysiques de diff´erents diimides de naphtal`ene sont ´etudi´ees. Les compos´es repr´esentant une classe de chromophores fr´equemment utilis´es dans les syst`emes supra-mol´eculaires pour la collecte de lumi`ere et la conversion d’´energie.

Nous avons pu attribuer l’´elargissement du spectre d’absorption stationnaire `a la tr`es courte dur´ee de vie de l’´etat photo-peupl´e, comme le pr´edit le principe d’incertitude d’Heisenberg. C’´etait la premi`ere fois que ce ph´enom`ene a pu ˆetre observ´e exp´erimentalement `a temp´erature ambiante en solution. Sur la base de la spectroscopie d’absorption transitoire, la courte dur´ee de vie dans ces syst`emes a pu ˆetre attribu´ee au transfert de charge ultra-rapide du donneur d’´electron

xi

au noyau de naphtal`ene diimide. La r´eaction de transfert de charge conduit en outre `a la formation d’un paquet d’ondes vibrationnelles dans le produit `a charges s´epar´ees. Contrairement aux paquets d’ondes vibrationnelles conventionnelles, qui sont g´en´er´es par une impulsion d’excitation large et ultracourte, la coh´erence vibra- toire est g´en´er´ee par le transfert de charge lui-mˆeme. Cela n’a jamais ´et´e observ´e auparavant pour un transfert de charge intra-mol´eculaire. En plus des ph´enom`enes coh´erents, la photophysique complexe a pu ˆetre d´emˆel´ee dans l’article II et les sig- natures spectrales d’un ´etat de transfert de charge triplet inhabituel a pu ˆetre identifi´e. Puisque la recombinaison de charges est interdite par spin, la dur´ee de vie de l’´etat de transfert de charge est allong´ee, une propri´et´e souhait´ee en photo- catalyse et en photosynth`ese artificielle. Une ´etude syst´ematique d´ependant du solvant et de la force motrice a pu montrer que le rendement pour cet ´etat de transfert de charge `a longue dur´ee de vie peut ˆetre contrˆol´e par l’environnement du syst`eme chimique. Cette compr´ehension d´etaill´ee permet d’introduire une nou- velle fonctionnalit´e dans les syst`emes supramol´eculaires bas´es sur le bloc du diimide naphtal`ene, si elle est entour´ee du bon environnement.

Figure S2: R´esum´e graphique de la th`ese.

Dans les articles III et IV, deux chromophores ont ´et´e li´es de mani`ere covalente dans une g´eom´etrie contrˆol´ee qui repr´esente l’interm´ediaire entre le monom`ere et le mat´eriau massif (Figure S2). En fonction des propri´et´es opto´electroniques des monom`eres, ces bichromophores peuvent ˆetre utilis´es comme syst`emes mod`eles pour ´etablir les relations structure-propri´et´e de diff´erentes r´eactions `a l’´etat ex- cit´e. Nous avons utilis´e des monom`eres `a base de pentac`ene et de p´eryl`ene pour

´etudier la fission de singulet et la s´eparation de charges `a rupture de sym´etrie, respectivement. Pour les deux r´eactions `a l’´etat excit´e, qui sont d´eclench´ees par l’absorption d’un photon, le couplage ´electronique et donc la distance entre les deux monom`eres a ´et´e ´etabli comme ´etant comme une arme `a double tranchants.

Mˆeme si la formation du produit souhait´e est acc´el´er´ee dans des conformations de contact ´etroit, la probabilit´e de survie est ´egalement r´eduite. Pour les deux r´eactions, la g´eom´etrie du point id´eal `a laquelle les vitesses de formation et de d´eclin de l’´etat du produit sont ´equilibr´ees doit ˆetre trouv´ee. Dans le cas de la s´eparation de charges, la relaxation structurelle de la g´eom´etrie de l’excim`ere s’est

xiii av´er´ee agir comme un ´etat pi`ege, alors que le mouvement le long de la coordonn´ee structurelle semble conduire la r´eaction de fission de singulet. La relaxation le long de la coordonn´ee du solvant, d’autre part, est cruciale pour la s´eparation de charges `a rupture de sym´etrie, mais ne joue pas un rˆole significatif dans la fission singulet pour les dim`eres de pentac`ene.

Contents

1 Introduction 1

1.1 Multichromophoric systems . . . 2

1.2 Energy transfer and molecular excitons . . . 3

1.3 Excited state reorganization . . . 5

1.4 Symmetry-breaking charge-separation . . . 6

1.5 Singlet fission . . . 9

1.6 Spectral observables and collaborative effort . . . 12

1.7 The monomer building blocks . . . 14

1.8 Vibrational wavepackets . . . 15

1.9 Outline of the thesis . . . 17

References . . . 17

2 Setup development and experimental details 23 2.1 Transient absorption spectroscopy . . . 24

2.2 UV-Vis Detection . . . 26

2.2.1 Overview . . . 26

2.2.2 Changes implemented during this thesis . . . 26

2.2.3 Standard operating procedure . . . 27

2.3 NIR Detection . . . 29

2.3.1 Literature overview . . . 29

2.3.2 Design . . . 30

2.3.3 Alignment procedure . . . 33

2.3.4 Outlook . . . 35

2.4 Pump pulses . . . 36

2.4.1 ps-fs . . . 36

2.4.2 ps-µs . . . 37

2.5 Data processing . . . 37 xv

2.5.1 Pixel toλ . . . 37

2.5.2 Excluding scattering . . . 39

2.5.3 Comparing Scans . . . 39

2.5.4 Background subtraction . . . 39

2.5.5 Removing coherent artefacts . . . 39

2.5.6 Chirp correction . . . 40

2.5.7 Kinetic cross validation with TCSPC . . . 40

2.6 Data analysis . . . 40

References . . . 41

3 Paper I 43 4 Paper II 51 5 Paper III 59 6 Paper IV 71 7 Conclusion 83 7.1 Instrumental developments . . . 84

7.2 Linking the time and frequency domains . . . 84

7.3 Long lived triplet charge separated state . . . 85

7.4 Coupling is a two edged sword . . . 85

7.5 Ultrafast processes are not solvent dependent . . . 86

8 Appendix 87 8.1 Paper I - Supporting Information . . . 88

8.2 Paper II - Supporting Information . . . 104

8.3 Paper III - Supporting Information . . . 131

8.4 Paper IV - Supporting Information . . . 153

CHAPTER 1

Introduction

1

1.1 Multichromophoric systems

The reason the world around us is colourful are colourants, which absorb and re- flect visible light as a function of wavelength. Natural and synthetic colourants can be divided into dyes and pigments. The former can be dissolved in a medium and their colour solely depends on the electronic properties of the individual molecules and their interaction with the solvent environment. On the other hand, pigments are insoluble and multiple dye molecules are arranged in a crystalline or particu- late structure.¹ In addition to the electronic properties of the monomeric building blocks the colour of pigments also depends on the intermolecular chromophore- chromophore interactions.^2–7 Only minor chemical modifications, that hardly in- fluences the electronic properties of the monomer, can drastically change the colour of the pigment due to changes in the crystal structure. How drastic this changes can be was illustrated in a study comparing the optical properties of 18 perylenedi- imides (PDIs), where an identical chromophore core was linked to different sub- stituents at the imide, N position. In solution, they all show a similar absorption, whereas in the solid state the colours vary from vivid red, via maroon to black.^8–10 This effect of ”crystallochromy” was attributed to different mutual interactions be- tween the closely stacked PDI chromophores originating from the side group (R) dependent packing arrangement.^7,11The molecular structures of three representa- tive chromophores, their mutual orientation in the crystal and the resulting colour are illustrated in Figure 1.1.

In addition to colours of pigments, the structural arrangement of individual chro- mophores governs the function of many multichromophoric systems such as the biological light harvesting complexes in photosynthesis, artificial photosynthetic systems or organic solar cells and organic light emitting diodes. Not only the wavelength range which can be absorbed and emitted, but also the dynamics of processes such as energy transfer, electron transfer or singlet fission are governed by the intermolecular arrangement.^12–14 Our goal as physical chemists and the aim of this thesis is to understand the property changes from monomers to mul-

N N

O

O

O

O R R

N N

O

O

O

O R R

N N O

O

O

O R R

N N

O

O

O

O R R

N N O

O

O

O

R R N N

O

O

O

O R R

R= N

N

R= Me R=

Figure 1.1: The colours of perylenediimides in the solid state, which vary from red via maroon to black, are correlated to the mutual orientation of the chromophores in the crystal. In contrast, the absorption spectra in solution are nearly identical irrespective of the imide substituent.

1.2 Energy transfer and molecular excitons 3 tichromophoric systems and establish structure-property relationships. This can be translated into design principles to tune material properties towards a certain application.

1.2 Energy transfer and molecular excitons

The theoretical description of a multichromophoric system strongly depends on the strength of electronic coupling between the monomer building blocks.¹⁵ In the weak coupling limit, the absorption and emission spectra of the monomers are preserved and the excitation is, at least temporarily, localized on one chromophore.

F¨orster described the electronic energy transfer (EET) in this limit based on the Fermi golden rule for transitions between two weakly coupled states.^16–18 The excitation energy of a donor (D) is thereby transferred to an acceptor (A) such that the excited state lifetime of D is diminished. F¨orster established a theory that predicts the rate for EET based on experimentally accessible properties of A and D. The electronic coupling (V), in the golden rule expression, is described by the Coulombic interaction, approximated by a dipole-dipole interaction, between the two transition dipole moments of A (µA) and D (µD). Energy conservation and the Frank Condon factors are elegantly described by the spectral overlap integral (Θ) of the D emission and A absorption spectrum. In the absence of short range interactions and in vacuum the rate for EET (kEET) can be written as:¹²

kEET = 2π

~ |VCoul|²Θ VCoul≈Vdd= 1

4π0

µDµAκ R³_DA

(1.1)

whereκaccounts for the mutual orientation of the dipoles andRDAis the distance between the two point dipoles. The R⁻_DA⁶ dependence of kEET has been success- fully applied as molecular ruler using time resolved and stationary fluorescence spectroscopy.^19,20

In the strong coupling limit, however, the monomer states mix strongly giving rise to new, delocalized states.^12,14,15The absorption and emission spectra of the monomers are no longer preserved and it makes no sense to discuss EET from one to another monomer. This is the case for the ”crystallochromy” in Figure 1.1, which not only illustrates that the absorption spectra deviate from those of the monomers, but also that the properties depend on the interchromophore orienta- tion.

One of the first models which linked the mutual orientation of chromophores to predict the absorption spectra of molecular dimers and oligomers in the strong cou- pling limit, was introduced by Michael Kasha in the 1960s.^5,6 Kasha showed that the absorption spectra of molecular aggregates can be, similar to EET, described based on the Coulombic interaction between the molecular transition dipole mo- ments of the monomers. For a dimer, the two degenerate local excited states

(LES) split into two Frenkel exciton states (FES) at close distances (Figure 1.2a).

The splitting between the two FES, is given by 2Vdd, which for a homodimer with parallel transition dipole moments, is given by:⁴

Vdd =µ²(1−3cos²(δ))

4π0R³ (1.2)

where R is the distance between the two point-dipoles, µ are the transition dipole moments and δis the angle between µand R. Depending on the mutual orientation of the transition dipole moments, being “head-to-tail” (δ≈0) or “side- by-side” (δ≈π/2), Kasha defined the J- and H- aggregates, respectively.(Figure 1.2a) In H-aggregates, the whole oscillator strength is located on the higher ex- citonic state, whereas the opposite is true for J-aggregates. Since fluorescence usually proceeds from the lowest excited state due to ultrafast internal conversion, the emission in H-aggregates is suppressed and the lifetime of the excited state is increased. On the other hand, in J- aggregation the whole oscillator strength lies on the lower excitonic state leading to superradiance due to the increased radiative rate constant (krad).^4,21 The absorption shift and change ofkrad is therefore cor- related to the mutual orientation of the chromophores. Although this model has been highly successful to describe the photophysics of many molecular aggregates such as cyanine-based dyes²² or PDIs²³, it also has two major shortcomings:⁴ (i) It assumes that the electronic wavefunction overlap of adjacent molecules in an aggregate is negligible, which breaks down for closely face-to-face stacked con- formers. (ii) The simple Kasha model neglects the vibronic coupling and does not predict the vibronic fine structure of the transition.

Spano et al. recently introduced an extended theory of J- and H-aggregates, which addresses these limitations.^4,21,24 In addition to the long-range Coulom- bic interactions giving the FES, short-range charge-transfer (CT) interactions are also incorporated. These contributions arise from the mixing of the FES with the charge separated states (CSS), which possess negligible oscillator strength. In the weak coupling limit the system can be described in terms of the localized, LES and CSS. In the latter one electron is localized on one chromophore and a hole is lo- calized on the other. The superposition of the two CSS gives the charge-resonance states (CRS), which can couple to the two FES yielding the four delocalized, adi- abatic S1-S4 states, observable in an experiment (Figure 1.2b). In a symmetric environment, in which the two CSS are degenerate, the CRS do not possess a significant dipole moment and their mixing to the FES depends on the energy difference between FES and CSS. In contrast to Coulombic interactions, CT in- teractions depend on the molecular orbital overlap and already sub-˚A changes in the transverse orientation of face-to-face stacks can substantially change the photophysical properties. This is the origin of the drastic colour changes of PDIs triggered by small differences in the mutual orientation discussed in the last section (Figure 1.1). Spano’s model shows that, in closely stacked systems, the interfer- ence between Coulombic and CT interactions can lead to absorption band shifts that oppose the structure-band shift correlation predicted by the classical Kasha model. In face-to-face stacks, the attribution of a absorption shift to a H- or J-

1.3 Excited state reorganization 5

Figure 1.2: a) Kasha’s exciton theory predicts an excitonic splitting giving two Frenkel exciton states (FES), which arise from the Coulomb coupling (Vdd) between the two transition dipole moments (µ). The mutual orientation of the point dipoles defines how the oscillator strength is distributed on the two excitonic states. b) In the extended aggregate theory the coupling to the two degenerate charge separated states (CSS), which are are energetically close to theπ-π^∗states in extendedπsurfaces, is also incorporated . c) The changes of the electronic distribution from ground to excited state can lead to different intermolecular equilibrium structures, that we will call excimer geometry.

type structure can therefore be erroneous. However, incorporation of the vibronic coupling allowed them to correlate the vibronic progression of the absorption spec- trum to the mutual orientation of the chromophores. In contrast to the band shift, the correlation of the vibronic shape to the structure is more robust since it does not depend on the type of coupling. An increase of the ratio between the 0-0 band and 0-1 vibronic band, from monomer to dimer, indicates a head-to-tail (J- type) orientation, whereas an decrease is a signature of a side-by-side orientation (H-type), irrespective of the type of interaction.

1.3 Excited state reorganization

The discussed theoretical models of aggregates deal with the vertical transitions of static dimers or oligomers in vacuum. However, excitation to the Franck-Condon (FC) state is often followed by interchromophore structural relaxation triggered by differences of the excited state (ES) and ground state (GS) potential energy surfaces (PES).²⁵ Models based on a static conformation therefore often fail to capture the emission spectra which arise from the relaxed excited state. In line with the red shifted emission found for chromophores in concentrated solution, this relaxed state is termed excimer. Historically a molecular excimer describes a dimer formed upon diffusive encounter between one chromophore in the excited state and one in the ground state.²⁶ Pyrene has been studied extensively by Birks^27–29 and others³⁰due to its strong propensity to form excimers at high concentrations. The fluorescence quantum yield of the violet monomer band decreases upon increasing concentration together with the appearance of a red shifted structureless emission

band. Since the absorption spectrum remains unchanged upon increasing the con- centration, they introduced the term excimer to describe an excited dimer, which dissociates in the ground state.

In covalently linked dimers or molecular crystals, the chromophores are already pre associated at close distance circumventing the diffusive encounter between two monomers. The formation of the excimer can therefore be seen as structural re- laxation from the FC geometry to the excited state minimum, with substantial mixing between FES and CRS.³¹ Note that the scheme in Figure 1.2b is only a snapshot for one point along the PES and the coupling between the diabatic states changes upon relaxation along the intermolecular coordinates.

Using quantum mechanical calculations, Casanova has shown that the D2h, ES minimum conformation of the perylene dimer differs from the D2 GS conforma- tion³² (Figure 1.2c). The relaxation towards the excimer geometry leads to a red shift of the emission spectrum due to both a GS energy increase and ES energy decrease at the D2h geometry.

Whereas stationary absorption and fluorescence spectroscopy reveal the spectral information of the FC state and relaxed state, time-resolved spectroscopy can be used to follow the reorganization of the interchromophore configuration in time. Recent time-resolved studies on crystalline pyrene nanoparticles have re- vealed that the relaxation from the FC state to the excimer geometry occurs on the ps timescale.³¹

In the field of light harvesting and energy conversion, excimer formation is usu- ally an undesirable side reaction due to the formation of trap states.^14,31,33 The energetic relaxation, associated with the structural relaxation, can thereby hin- der energy or electron transfer to neighbouring molecules. However, excimers also have the potential to increase the fluorescence quantum yield³⁴or are invoked to be crucial for singlet fission, where relaxation can facilitate the conversion to the triplet pair state.³⁵

1.4 Symmetry-breaking charge-separation

In contrast to the theoretical models, which discuss the photophysics in vacuum, multichromophoric systems can be surrounded by symmetric or asymmetric en- vironments such as solvent molecules or protein side chains in biological systems.

This environment is crucial for the first of the two processes that we focus on in this thesis, which is symmetry-breaking charge-separation (SB-CS).

Before discussing the SB-CS in molecular aggregates, we will make an excur- sion to SB-CS in Donor-Acceptor-Donor (DAD) and Acceptor-Donor-Acceptor (ADA) dyes, in which this process is simplified due to a smaller number of diabatic states which are mixing.^36–38 The DAD and ADA systems consist of two Donor- Acceptor branches with the lowest energy transition being a CT-band, where the electron density is shifted from the donor to the acceptor. Upon combining the two branches, the degenerate S1 states are split into two excitonic bands which, in a symmetric environment, do not possess a permanent dipole moment and the

1.4 Symmetry-breaking charge-separation 7

A D A A D D A

A D A S₁

S₂ S₃ S₄

LES CSS₁

FES CT

CSS₂ S₁

S₂

symmetric symmetry-broken

a) b) symmetry-broken

Figure 1.3: a) Excitonic splitting in acceptor-donor-acceptor (A-D-A) systems in a sym- metric environment gives two states with negligible dipole moment. Upon solvent fluctu- ation the unsymmetrical environment can trigger symmetry-breaking charge-separation (SB-CS), in which the excitation is localized on one arm. (The same is true for DAD systems) b) SB-CS is a bichromophore is similarly triggered by the solvent, which breaks the degeneracy of the two charge separated states (CSS). However, the increased number of states which can still couple render the system more complex.

electronic distribution is completely symmetric on the two branches (Figure 1.3a).

The two excitonic states can be probed selectively due to different selection rules for one-photon and two-photon absorption illustrating the symmetric character of the quadrupolar S0 and FC state.³⁹ However, the fluorescence exhibits a strong solvatochromism, a behaviour typical of a dipolar state indicating a larger CT character on one of the two branches in the relaxed S1 state. Using time-resolved infrared spectroscopy, our group could show that SB-CS is triggered by thermal solvent fluctuations, which lead to a different solvation of the two branches.³⁸ The degeneracy of the two diabatic CT states therefore breaks down, the cou- pling is decreased and the excitation is asymmetrically distributed towards the better solvated branch. As solvent relaxation proceeds, first through ultrafast inertial motion and later by diffusional motion, this branch is even more stabi- lized and if the solvent field is strong enough, the excitation fully localizes (Figure 1.3a). In contrast to the symmetric FC state, the relaxed state therefore possesses a significant permanent dipole moment explaining the strong fluorescence solva- tochromism. The timescale of SB-CS could be related to the solvent relaxation times and the extent of localisation depends on the solvent polarity as well as the coupling between the two branches.^38,40

Now we return to the discussion of homo aggregates, and concentrate on the de- scription of a dimer. Similarly to the DAD and ADA systems, purely symmetric adiabatic states of a homo-dimer, illustrated in Figure1.2b, exist only in a symmet- ric environment and strong interchromophore coupling.⁴¹However, in solution or in a protein environment the non-symmetric local environment can lift the degen- eracy of the diabatic monomer states, leading to an uneven distribution of electron density on the two chromophores. In contrast to the LES, the higher permanent dipole moment of the CSS makes them more susceptible to a different environment around the two chromophores.

The most famous biological example for SB-CS is the bacteriochlorophyll dimer (BC) in the reaction center of photosynthetic purple bacteria. Even though the bi-

O

Pe2 C

3-Pe

2 Xan-Pe

BA 2

Figure 1.4: Chemical structures of bichromophores that are studied as model systems for symmetry breaking charge separation and excimer formation.

ological bichromophore is surrounded by two almost identical branches of protein- bound cofactors, the CS happens almost exclusively along one branch on the ps timescale.^40,42 Similarly to the thermal fluctuations for the DAD and ADA sys- tems, this has been explained by slightly asymmetric local environments around BC, lifting the degeneracy and therefore resulting in a net CS character of S1. CS in BC is highly efficient due to (i) the fast (ps time scale) CS which outcompetes radiative and non-radiative decay channels and (ii) due to the minimal energy loss between the FC exciton state and the relaxed CS state.

In addition to the solvent coordinate that controls the process in ADA and DAD systems, the dynamics along the structural coordinate between the chromophores plays a decisive role in molecular aggregates. SB-CS in covalently linked chro- mophores were studied extensively in model systems based on e.g. perylenes,^43,44 anthracenes,^45,46or PDIs,^47–49and indicate that, in line with the DAD and ADA systems, the amount of CS depends on the coupling between the two chromophores and the solvent polarity. The fluorescence spectrum of the directly liked perylene dimer Pe2 substantially depends on the solvent polarity, indicating a transition from a LE to a CT character of the relaxed S1 state. In non-polar solvents, the emission originates from a LES, whereas in polar solvents it is rather a symmetry broken CT state.⁴⁴ The results are similar to the extensively studied bianthryl (BA) for which, additionally to the different character of the emission state, the radical anion and cation feature in the transient absorption spectra confirmed the SB-CS.⁴⁵However, due to the strong coupling in those systems, the CS is not com- plete, as indicated by a non-vanishing radiative rate constant of the CT emission.

Full CS, on the other hand, could be observed for the weakly coupled biperylenyl- propane (C3Pe2) in which the two heads are linked by an innocent propane linker in acetonitrile. The radical anion and cation features were clearly observed in polar solvents by transient absorption (TA) spectroscopy, which together with the ab- sence of a CT emission suggested a full CS. Even though excimer formation should be structurally possible, neither a red shifted emission nor an additional signal in the TA was observed, irrespective of the solvent.⁴³ The dynamics become even more complicated if the linker guarantees close face-to-face contact of the chro- mophores that models the packing arrangement in a bulk material. Wasielewskiet

1.5 Singlet fission 9 al. synthesized a strongly coupled perylene dimer linked with a xanthene spacer (Xan-Pe2) in which SB-CS competes with excimer formation.⁵⁰They found that excimer formation occurs irrespective of the solvent polarity indicated by a red shifted, featureless emission and a transient absorption feature in the near infra- red (NIR). In polar solvents, the excimer formation is preceded by SB-CS that, however, ultimately collapses to the excimer illustrating the interference between structural and solvent coordinates.

How this two coordinates interplay and govern the excited-state dynamics and the nature of states that are populated under different interchromphore distances is the question of paper III presented in chapter 5. We will combine the confor- mational control of rigidXan-Pe2and the possibility to sample conformers with larger distances of C3Pe2to get a comprehensive picture of the structure-property relationship for SB-CS.

1.5 Singlet fission

The second multichromophoric process of interest is singlet fission (SF), which is the conversion of a singlet exciton into two lower energy, triplet excitons.^51,52 Its technological promise lies in the perspectives to overcome the Shockley Queisser limit and increase the efficiency of single band gap solar cells. In a seminal paper in 1961, Shockley and Queisser described the maximum solar conversion efficiency for a single band-gap photovoltaic cell to be 33.7 %.⁵³ It was assumed that photons with an energy below the band gap (Eg) are not absorbed and photons with a energy> Eg are converted to electricity with a quantum efficiency (QE) of 100 %.

However, the photon energy that exceeds the bandgap,EP hoton−Eg, is lost as heat.

The solar spectrum on the earth (AM 1.5G⁵⁴) is plotted in Figure 1.5a, illustrating the thermal losses (blue) and non-absorption (red) for the most popular solar cell material, silicon (Eg = 1.1 eV), assuming a QE of 100 %.

Coupling a SF material to a solar cell, can potentially increase the efficiency of those high energy photons by a factor 2. The photon is thereby absorbed by a SF material and split into two low lying triplet excitons, which are separately transferred to the semiconductor, ideally giving two electron-hole pairs. Photons with an energy too low to be absorbed by the SF material are absorbed by the ordinary solar cell material, giving 1 electron-hole pair per photon. In order for SF to be thermodynamically feasible, the energy of the singlet exciton of the monomeric building blocks has to be greater than or equal to twice the triplet exciton energy.^a (Figure 1.5b). Furthermore, the key factor for efficient and rapid SF is the conservation of angular momentum during the transition from the photo- populated singlet excited state, to the triplet pair⁵⁶ (Figure 1.5c). The spins of the two triplets are thereby correlated to a net singlet state (¹(TT)), enabling SF on the ps timescale, competitive with radiative and non-radiative decay channels of the chromophores. In contrast to the usually slow ISC to the T1 the conversion of S1S0 to ¹(TT) can be interpreted as a special case of internal conversion and

aRecently it has been shown that entropic effects can relax this criterion⁵⁵

E_G

No Absorption Thermal Loss

Si 100% efficiency E_G

E_hν -E_G

S₀ S₁

T₁ 2xT₁

S₁S₀

1(TT)

Energy conservation Spin conservation

a) b) c)

Figure 1.5: a) The theoretical quantum yield in a single band gap solar cell is less than 100 %, due to thermal losses and photons which do not have enough energy to be absorbed. b) Energy conservation for singlet fission (SF) requires the photo-populated singlet state to be at least two times the energy of the monomer. c) In the correlated triplet pair (¹(TT)) born from SF, the spins are correlated to a net singlet character rendering SF a spin allowed process.

occurs as fast as∼80 fs in pentacene molecular crystals⁵⁷and∼700 fs in covalent pentacene dimers⁵⁸. The overall singlet fission cascade can be described as a three step process:⁵⁹

S1S0[^m(TT)^m(T· · ·T)]T1+ T1 (1.3) After population,¹(TT) can evolve to ¹(T· · ·T) by spatial separation and thus a decrease of electronic coupling and exchange interaction. In a bulk material, spa- tial separation can occur by energy transfer to neighbouring molecules, whereas structural fluctuations are necessary in a covalently linked dimer. Since electronic coupling varies continuously,¹(TT) and¹(T· · ·T) should be considered as limiting cases and are often merged into a single species, TT. However, different spectral features and dynamics justify the distinction between these two triplet pairs.^59,60 In addition to¹(TT), the two triplets can also couple to an overall triplet (³(TT)) or quintet state (⁵(TT)). The four spin states in the two molecules can couple to nine different spin states, 1¹(TT), 3³(TT) and 5⁵(TT).⁵⁶In^m(TT), the exchange interaction lifts the degeneracy between the different spin states, whereas the ex- change interaction goes to zero in ^m(T· · ·T). Applying time-resolved electron paramagnetic resonance (EPR) experiments, it has been shown that both ⁵(TT) and³(TT) can be populated on the ns timescale.^61–63

Dephasing or decorrelation is the third step in the mechanistic cascade, describing the loss of spin correlation between the correlated triplet pair by random spin flips and the formation of free triplets (T1+ T1), with the same properties as the T1

state populated by ISC or sensitization studies. This spin conversion has a strong dependence on the coupling between the two sites and therefore on the exchange interaction.⁶³

Depending on the spin character of the correlated triplet pair, two spin-allowed

1.5 Singlet fission 11

Trip-TDI₂ Trip-ph-TDI₂ Trip-ph₂-TDI₂

N

N

O O

R

O O

N

N

O O

R

O O

N

N

O O

R

O O

N

N

O O

R

O O

N

N

O O

R

O O

N

N

O O

R

O O

Figure 1.6: Chemical structures of bichromophores used to study Singlet Fission.^14,65

decay channels further complicate the mechanistic cascade towards T1+ T1:^51,64 Singlet channel ¹(TT)→S0S0

Triplet channel ³(TT)→T1S0

(1.4) The challenge of developing SF materials is to balance the electronic coupling, which on the one hand increases the rate for internal conversion from S0S1 to the

1(TT), but at the same time also increases the rate of the two parasitic processes shown in equation 1.4. In other words, the sweet spot between triplet pair forma- tion and triplet pair survival to enable the spin conversion to free triplets, has to be found. The SF yield is given by the comparison of the absorbed photons to the populated free triplets, with a theoretical maximum of 200 %.

To study the mechanistic subtleties, covalently linked dimers are often preferred over bulk materials due to the absence of dephasing by energy transfer and the pos- sibility to change the polarity of the environment, which can help to understand the possible influence of the CSS.⁶⁶ For face-to-face stacked chromophores for which 2ET1≤S1, the diabatic triplet pair state has to be added to the state diagram, in addition to the diabatic LES and CSS (Figure 1.2b and 1.3b). Depending on the type of chromophore, the solvent environment as well as the mutual orientation, the contribution,ci, of the different diabatic states to the lowest energy adiabatic state,|Ψi, differ:

|Ψi=c1(t)|LEAi+c2(t)|LEBi+c3(t)|CSAi+c4(t)|CSBi+c5(t)¹(T T) . (1.5) This is elegantly shown by Wasielewski et al. who studied the excited state dy- namics of a series of slip-stacked triptycene-linked terrylenediimide dimers^14,65

(Figure 1.6). In Trip-ph-TDI2 and Trip-TDI2, the SB-CS state is populated irrespective of the solvent. However, in Trip-ph2-TDI2, where theπ overlap is the smallest, the triplet features could be observed in apolar toluene indicating a considerable mixing of ¹(T T) to the adiabatic state. This features were however absent in polar acetonitrile, illustrating that the SB-CS state can act as trap state hindering SF. Even though this study gives an interesting insight into the popu- lation of the triplet pair as a function of structure, no free triplets are formed due to fast annihilation via the singlet channel.

In paper IV, presented in chapter 6, we will study the structure, solvent polar- ity and viscosity dependence of a covalently linked bichromophore, in which not only the internal conversion to the triplet pair but also its fate towards the free triplets can be monitored. It therefore gives a comprehensive picture of the whole mechanistic cascade including the parasitic decay channels.

1.6 Spectral observables and collaborative effort

As discussed in the last sections, in the weak coupling limit, multichromophoric systems are well described by the diabatic states with a well-defined electronic character, namely the CSS, LES or ¹(T· · ·T) state. The spectroscopic signa- tures observed in time-resolved spectroscopy can be easily assigned on the basis of spectral characteristics of the monomer. Modelling the spectral changes by exponential dynamics in terms of global or target analysis allows extracting the rate constants that describe the transition between the states.^67,68However, upon spatial approach of the chromophores, states with mixed electronic character are born according to equation 1.5. The admixture of the different states can change continuously as a function of solvent and structural relaxation. These relaxation processes lead to excited state products that can be described by three limiting cases:⁴¹(i) The correlated triplet pair^m(T T), (ii) the excimer state, that is a mix- ture of FES and CRS in which the electronic density is symmetrically distributed and (iii) the SB-CS state, which involves the same ingredients as the excimer but the electron density is no longer symmetric and the cation and anion are each localised on one chromophore. In strongly coupled systems, it is preferable to use the term charge transfer state (CTS) instead of CSS due to the molecular orbital overlap which hinders the full localisation of the electron and hole on one of the two chromophores. In contrast to weakly coupled systems, the spectral observables deviate from the electronically well defined states. The extraction of lifetimes and species associated spectra by global lifetime analysis or target analysis is hindered in this strongly coupled systems due to the continuous change of the character of the state on the fs-ps timescale. After the excited state product is reached via sol- vent and structural relaxation, the changes can again be modelled by exponential decays.

Our strategy to establish structure-property relationships is to systematically vary the coordinates that impact the relaxation dynamics and electronic character of the final state. This strategy requires a collaborative effort including synthetic chem-

1.6 Spectral observables and collaborative effort 13

O NH

Si Si

NH O

O O O

O O O

Chemical System Molecular Dynamics Simulation Spectroscopy Figure 1.7: A collaborative effort is required to establish structure-property relation- ships including synthesis of a chemical system, molecular dynamics simulations and sta- tionary and time resolved spectroscopy.

istry, time-resolved spectroscopy and computational studies. A chemical system is required, that allows to sample a different ”phase space” of interchromophore geometries. We applied bichromophores based on a crownether backbone linked to two identical chromophores, that were synthesized by the Lacour group at the University of Geneva (Figure 1.7). The crownether can bind cations, which trig- gers a conformational change of the backbone and can be used to control the interchromophore geometry.^69–72 In order to decrease the complexity, we chose chromophores, in which we could study SB-CS and SF separately, namely pery- lene and pentacene. For the former, the condition 2ET1≤S1is not fulfilled, which thermodynamically rules out SF. As discussed in section 1.4, SB-CS has been ob- served for perylene bichromophores rendering it an ideal candidate to study the competition between excimer and SB-CS formation as a function of interchro- mophore structure. On the other hand, SF is exothermic for pentacene and the applied TIPS-pentacene derivative by roughly 0.1-0.2 eV.⁶⁶For pentacene deriva- tives, the SB-CS state could never be observed as product state, most likely due to the high driving force to populate the triplet pair, rendering it an ideal candidate to study SF.

In a second step, molecular dynamics (MD) simulations were used to determine the mutual orientations of the chromophores under different experimental condi- tions, such a cation type and concentration or solvent polarity.^b

Comparing the stationary and time-resolved data of the monomer reference com- pound to the multichromophoric systems, at different geometries, reveals the im- pact of the close contact on the nature of the populated states. The change of solvent polarity and viscosity can help to further unravel the role of the sol- vent and structural coordinate. Comparing the trends observed from the spectro- scopic studies to the structures from the MD simulation helps us to establish the

bMD simulations were carried out by Dr. Giuseppe Licari and Dr. Christopher Rumble

structure-property relationship in papers III and IV.

1.7 The monomer building blocks

The two chromophores, perylene and pentacene, have in common that the monomer photophysics are rather trivial. Upon excitation to S1, the population decays mainly via the radiative and internal conversion (IC) channel with lifetimes on the ns time-scale and a very small triplet yield due to slow intersystem crossing (ISC).^73,74The trivial photophysics facilitate the understanding of the multichro- mophoric system due to reduction of complexity.

This is however not the case for all monomeric building blocks used in supramolec- ular systems. One of the most used monomers is the naphthalenediimide (NDI) due to its ease of functionalization and the low reduction potential which makes it an ideal electron acceptor.^75–79In contrast to its bigger brothers, the perylenedi- imide or the terrylenediimide, its photophysics are far from trivial.^80,81 NDIs can be incorporated in multichromophoric assemblies either by core or axial function- alization with opposing effects on its photophysics (Figure 1.8a). Upon addition of electron donating (D) groups at the core, the whole colour palette in the visible is accessible due to an optically allowed charge transfer transition, which can be shifted by altering the redox properties of the substituent⁷⁷(Figure 1.8b). On the

N N O O

R

R O

O N

N O O

R

R O O

D D D

D N N O O

D O O

D

a) b)

Figure 1.8: a) Structures of unfuctionalized (red) naphthalenediimide (NDI) as well as NDIs which are linked to electron donors (D) at the axial imide N position (blue) or at the core positions (green). R is an innocent saturated alkyl chain required to increase solubility. b) The stationary absorption spectra of the three different NDIs show the opposing impact of core and imide substitution on the optoelectronic properties of the dye.

other hand, Ds at the imide group do not lead to a significant change of the HOMO and LUMO orbitals and the position of theπ-π^∗transition is nearly unchanged.⁸² This can be attributed to the weaker coupling of the donor to the core. Fem- tosecond time-resolved studies revealed the occurrence of ultrafast photoinduced electron transfer (PET) upon linking a phenyl group at the axial position.⁸⁰In con- trast to PDIs and TDIs, the high oxidative power of the NDI LES renders phenyl groups at the axial position no longer innocent. This is of significant relevance since the phenyl groups are frequently used as linker units in multichromophoric

1.8 Vibrational wavepackets 15 systems for e.g. artificial photosynthesis.^13,75,83In addition to the fast PET, Zuil- hof et al. observed a spectral feature after charge recombination that they could not attribute to a certain species and their origin remained unknown.⁸⁰ We ob- served the same features when studying covalently linked NDI foldamers applied in anion-πcatalysis,⁸⁴ which was at the origin of an in depth study presented in papers I and II. To unravel the complex photophysics the Matile group synthesized a series of Donor-Acceptor systems in which the driving force and number of D are systematically changed.

In paper I, (Chapter 3) we show that the ultrafast PET triggers the population of vibrational wavepackets (VWP) on the product surface, which are not generated by the broad pump pulse but directly from the electron transfer reaction. (see section 1.8) Furthermore, we could correlate the increase of the absorption band width upon addition of the Ds, illustrated in Figure 1.8b, with the LES lifetime.

This is the first experimental observation of lifetime broadening in condensed phase near room temperature.

Studying these two phenomena was like picking flowers growing at the side of the road towards understanding the complex photophysics of NDI building blocks.

In paper II, (Chapter 4) we attempt to decipher the whole mechanistic cascade commenced by photo-population of the π-π^∗, followed by CS and charge recom- bination. This is the foundation to understand more complex multichromophoric systems using axially linked NDIs as monomeric building blocks.

1.8 Vibrational wavepackets (VWP)

In this section, the theory and origin of VWP, studied in paper I, its spectral observables and the connection to electron transfer reactions are briefly discussed.

VWP originate from a coherent superposition of vibrational levels and can in prin- ciple be generated in every molecule.^85,86 If the optical pump pulse is spectrally broader than the vibrational level spacing and shorter than the oscillation period of a FC active normal mode, a vibrational superposition state is populated. The wavefunction of this non-stationary state can be expressed as a linear combination of the stationary vibrational wave functions. Interference between the components leads to a large amplitude at one specific region in space that changes over time, resembling the motion of a classical particle along the potential energy surface.⁸⁷ One can differentiate between VWP residing on the excited state (ES) PES and on the ground state (GS) PES, which are populated via different mechanisms.

ES VWP are generated if the ultrashort broadband pump pulse is in resonance with a GS electronic absorption band and coherently photo-excites superpositions of FC active vibrational levels of the electronic ES (Figure 1.9a). Based on the time dependence of the energetic differences between ES and GS PES, the spatial distribution of the VWP manifest as wavelength modulations of the excited state absorption (ESA) and stimulated emission (SE) band positions in a TA experi- ment (Figure 1.9c and d). This wavelength modulations translate to amplitude oscillation in single wavelength time traces overlaying the population dynamics as

Energy//

Excitation Bandwidth

Vibrational Coordinate

// //

Wavelength

∆Absorption norm.

a) b) c) d)

0,T

T/2 0,T T/2

Figure 1.9: a) Excited state vibrational wavepackets generated by a short excitation pulse with a bandwidth covering multiple vibrational levels. b) Ground state vibrational wavepackets are generate by a Raman process that can be resonant or non-resonant.

c) The excited state wavepacket oscillates in the excited state potential that leads to a change of the energy gap between the ground and excited state. d) This oscillatory motion of a vibrational wavepacket manifests in a modulation of the features in the transient absorption spectrum, with a frequency related to the normal mode of the superpostion state.

well as structural and solvent relaxation. At the maximum of the ESA and SE, a node appears at which the amplitude of the oscillation goes to zero and the phase flips.^88,89 The modulations comprise all FC modes that can be accessed by the bandwidth of the pump pulse and can be Fourier transformed to give the respec- tive vibrational frequencies. Ground state VWP, on the other hand, are generated via impulsive stimulated Raman scattering induced by either resonant (RISRS) or non-resonant (ISRS) pulses⁸⁶(Figure 1.9b). These oscillations have a node at the ground state bleach (GSB) and the Fourier transformation can be related to the vibrational modes on the GS PES. Comparing the spectral position of the nodes to the band maxima of GSB, SE and ESA and comparing the VWP observed in resonant and non-resonant conditions can help to differentiate between GS and ES VWP.⁸⁵

Coherent VWP generated on the reactant states can be used to study the coor- dinates driving ultrafast PET. In a recently published perspective Scholeset al.

discussed that the fate of the vibrational coherence upon PET depends on whether the coordinate of the VWP is a spectator or reactive vibrational mode.⁹⁰ If the equilibrium geometry along a certain mode does not change upon electron transfer the vibrational coherence survives the reaction and the mode is called a specta- tor mode. Contrarily, if the vibrational coordinate is coupled to the reaction as described by the Bixon-Jortner model⁹¹, the VWP dephases rapidly upon ET.

This is attributed to the multiple, parallel reaction channels originating from the coupling to vibrational modes (Bixon-Jortner theory) that brings the VWP out of phase.⁹⁰

An additional mechanism to form VWP during ET was invoked, for ET from a PDI to an electron accepting solvent and for ET at a semiconductor-dye interface .^92,93WVP is herein not generated by the short excitation pulse, but by the ultra- fast PET itself. If the electron transfer is much faster than the oscillation period