This dissertation delves into the field of photomechanical motion, exploring the manipulation of materials using light. Central to the research is the study of TEMPO, hexaarylbiimidazole (HABI), and pyrolytic graphite (PG), each offering unique insights into energy transfer and material control. The study begins with TEMPO, demonstrating its capabilities in controlled mechanical motion due to its stable free radical properties. This exploration lays the groundwork for further investigations into HABI and PG, where their photoresponsive behavior is examined. An important aspect of this research is the ability of light to alter the magnetic susceptibility of materials, enabling controlled movement in magnetic fields.This dissertation underscores the importance of understanding the thermal and magnetic properties of materials in developing advanced technologies. The implementation of photothermal and photochemical techniques has broad applications, from magnetic levitation to sophisticated manufacturing processes. Future research is suggested to develop new methods leveraging light-matter interactions, with vast potential for advancements in energy systems, manufacturing, and biomedical technologies. In summary, this work contributes significantly to the understanding of photomechanical motion, offering new methodologies with far-reaching implications in various scientific and technological domains.

Anthracene derivatives can undergo photochemical reactions under light irradiation, like a [4+4] photodimerization and a Dewar isomerization. When these molecules are made into micro- and nano-sized crystals, some of them exhibit interesting photomechanical motions and deformations like bending, twisting, curling, and expanding. Both the internal and external environmental effects were found to significantly influence the photomechanical behaviors and photochemical reactions of these molecules in the solid-state. One goal of this research is to better understand the relationship between dynamics of chemical reactions and crystal photomechanical behavior they produce, and to see if these reaction rates can be enhanced by internal or external factors.

9-tert-butylanthracene (9TBA) undergoes a Dewar isomerization in solution and in polymers, but produced no photomechanical response in the crystal, even though it generates a significant volume change and a highly strained molecular structure after photoisomerization. But when this molecule is embedded in a polymer, a strong pressure dependence of the photoisomerization was observed in both forward and reverse reactions, providing us a possible way to engineer more pressure sensitive molecules in the future.

We investigated the photodimerization process of 9-methylanthracene (9MA) in polycrystalline thin films. It was found that both the photochemical reaction and nonradiative relaxation rates increase as more photoproduct is formed, which we denoted as an autocatalytic pathway. The local environment of a 9MA molecule changes as the surrounding molecules react. We derived rigorous equations and extended a current model (the Finke-Watzky Model) to describe the non-exponential reactant decay, and we were able to predict its time-dependent photoluminescence evolution over the course of the reaction, which showed agreement with the experimental results. The presence of autocatalytic reaction dynamics in a molecular crystal system may play a role in its photomechanical response. This new chemical reaction pathway is different from the single exponential process seen in photoresponsive soft polymer systems.

Besides internal factors, like intra- and intermolecular interactions, we also studied the external environmental effects on molecular crystals. Ionic surfactants like cetyltrimethylammonium bromide (CTAB) was discovered to dramatically promote both the photomechanical motion and the photochemical reaction rates of dimethyl-2-(3-anthracen-9-yl)allylidene)malonate (DMAAM) nanocrystals. These results may help illustrate how surface effects can enhance crystal reactivity. Future work will involve incorporating the photochemical processes with photomechanical behaviors and exploring more interesting mechanical motions with various shapes and sizes of molecular crystals.

A series of 2,6-dialkoxyanthracenes are synthesized and their photophysical properties in liquid solution and solid state are studied. Both spectral lineshapes and luminescence decay kinetics of these molecules in dilute solution or in a polymer matrix are identical. We identify a conformational change in solution that leads to two emissive states that can interconvert. In the solid state, the crystal structures of 2,6-dialkoxyanthracenes change significantly with lengthening alkoxy chains from methoxy to hexyloxy, evolving from herringbone structures to face-to-face two-dimensional sheets. The results from temperature-dependent experiments on single crystals indicate the intermolecular couplings between molecules with longer alkoxy chains are quite different from the J-type aggregates seen in crystalline anthracene. By tuning the side chain length and modifying the crystal structures of 2,6-dialkoxyanthracenes, we can change the luminescence properties of the Frenkel excitons that exist in the crystals.

The 9-tert-butylanthracene (TBA) can undergo a reversible photoisomerization to the Dewar isomer. The dependence of both the formation and reversion of the Dewar isomer on pressure was investigated using absorption and fluorescence spectroscopy with TBA doped polystyrene as a model system. It is demonstrated that both the photoisomerization and its reverse reaction rates are decreased with higher pressure.

To further enhance photovoltaic efficiencies, singlet fission (SF) has been studied as a possible avenue to exceed the Shockley-Quiesser limit which prohibits single junction solar cells from going beyond 34% efficiency. SF occurs in organic materials when an excited singlet exciton within the organic material splits into two triplet excitons. This process, being spin allowed, occurs rapidly and the resultant triplets have the benefits of longer lifetimes (~ms) and longer diffusion lengths than their singlet counterparts. In order to better utilize this process for applications such as photovoltaics, there must be a better understanding of what systems give rise to high yields of triplets and how to harvest these triplets to do work. This work uses time-resolved fluorescence techniques along with magnetic fields to study how morphology plays a role in the singlet fission process. In order to do this, studies were done on the molecular systems of tetracene, rubrene, and diphenylhexatriene. Expanded versions of the Merrifield kinetic model were created to model the dynamics of these systems and better understand how spin and triplet-triplet annihilation affect the dynamics of the system. We find that amorphous rubrene has a slower singlet fission lifetime than tetracene at 2 ns. Using our expanded model, diphenylhexatriene was found to have a fission lifetime of 290 ps in the monoclinic form while having a 435 ps lifetime in the orthorombic form. Also, differences in fluorescence behavior between thin polycrystalline films and single crystals of tetracene are explored to reduce discrepancies in the literature as to tetracene's singlet fission rate and whether or not its activated by temperature. We find here that single crystal's of tetracene have an average singlet fission lifetime of 170 ps while the polycrystalline films have a lifetime of 70 ps, which we attribute to differences in the number of defects due to grain boundaries and sample preparation. This study also studies the integration of tetracene films with n-type silicon in order to harvest the generated triplets for photovoltaic applications, but evidence was only seen for singlet transfer to the silicon.

ABSTRACT OF THE DISSERTATION

Solid State Photochemical and Photomechanical Studies of Nanostructures and Microstructures of Anthracene Derivatives

by

Lingyan Zhu

Doctor of Philosophy, Graduate Program in Chemistry

University of California, Riverside, December 2011

Dr. Christopher Bardeen, Chairperson

Photoactuators are devices made of photoreactive materials, which undergo shape changes to generate mechanical motions upon irradiation. Research on photoreactive materials has attracted much attention; this dissertation mainly focuses on photoreactive molecular crystals, which provide us great freedom to modify photoresponse through organic derivatization.

Various anthracene derivatives were synthesized and fabricated into nano- and micro-scale structures; multiple measurements, including Atomic Force Microscopy, solid-state Nuclear Magnetic Resonance, X-ray diffraction, etcetera, were applied with three goals: (1) maximize the photoresponse; (2) develop new modes of photoresponse; (3) enhance the reversibility of the photoresponse.

To maximize the photoresponse, we engineered the crystal structures and packing motifs of anthracene esters with diverse-sized substituents. The largest expansion of nanorods we obtained was 25% by length. We also tried to establish the relationship between the macroscopic photoresponse and the molecular-level structural changes, so that the photoresponse of molecular crystals may be predicted based on their crystal structures. A detailed study of the photochemistry of 9-tertbutyl-anthracene-carboxylic-acid-ester (9TBAE) shows that the solid-state photoproduct is a metastable intermediate, which slowly converts into another stable equilibrium form. This is a general phenomenon among anthracene esters. Thus, the metastable structure is the key for predicting the macroscopic photoresponse. Further studies are required to obtain the crystal structure of this crucial intermediate.

Our proposal to develop new modes of photoresponse is through morphology control. Using the floating drop method, we managed to grow 9-anthracene carboxylic acid (9AC) into single-crystalline microribbons, which generate reversible photo-induced twisting motions under uniform irradiation. The interfacial strain between the unreacted monomer and the photodimer regions within the ribbon is the driving force for the twisting.

To enhance the reversibility of the photoresponse, we tried to use the steric effects to accelerate the dissociation of the photodimer. However, only one out of six derivatives of 9AC, 10-flouro-9-anthracenecarboxylic-acid, shows the reversible photoreactivity yet with much longer response time. Attempts to self-consistently rationalize observed trends in terms of excited state lifetimes or steric effects were only partly successful. Balancing factors like electronic relaxation, steric interactions, and crystal packing presents a challenge for engineering photoactive solid-state materials based on molecular crystals.

The successful function of photovoltaic (PV) and photocatalytic (PC) systems centers primarily on the creation and photophysics of charge separated electron-hole pairs. The pathway leading to separate carriers varies by material; organic materials typically require multiple events to charge separate, whereas inorganic semiconductors can directly produce free carriers. In this study, time-resolved spectroscopy is used to provide insight into two such systems: 1) organic charge-transfer (CT) complexes, where electrons and holes are tightly bound to each other, and 2) Au-TiO2 core-shell nanostructures, where free carriers are directly generated.

1) CT complexes are structurally well defined systems consisting of donor molecules, characterized by having low ionization potentials, and acceptor molecules, characterized by having high electron affinities. Charge-transfer is the excitation of an electron from the HOMO of a donor material directly into the LUMO of the acceptor material, leading to an electron and hole separated across the donor:acceptor interface. The energy of the CT transition is often less than that of the bandgaps of donor and acceptor materials individually, sparking much interest if PV systems can utilize the CT band to generate free carriers from low energy photons. In this work we examine the complexes formed between acceptors tetracyanobenzene (TCNB) and tetracyanoquinodimethane (TCNQ) with several aromatic donors. We find excitation of the charge-transfer band of these systems leads to strongly bound electron-hole pairs that exclusively undergo recombination to the ground state. In the case of the TCNB complexes, our initial studies were flummoxed by the samples' generally low threshold for photo and mechanical damage. As our results conflicted with previous literature, a significant portion of this study was spent quantifying the photodegradation process.

2) Unlike the previous system, free carriers are directly photogenerated in TiO2, and the prime consideration is avoiding loss due to recombination of the electron and hole. In this study, four samples of core-shell Au-TiO2 nanostructures are analyzed for their photocatalytic activity and spectroscopic properties. The samples were made with increasingly crystalline TiO2 shells. The more crystalline samples had higher photocatalytic activities, attributed to longer carrier lifetimes. The observed photophysics of these samples vary with excitation wavelength and detection method used. We find the time-resolved photoluminescence correlates with the samples' photocatalytic activities only when high energy, excitation wavelength less than or equal to 300 nm is used, while transient absorption experiments show no correlation regardless of excitation source. The results imply that photoexcitation with high energy photons can generate both reactive surface sites and photoluminescent surface sites in parallel. Both types of sites then undergo similar electron-hole recombination processes that depend on the crystallinity of the TiO2 shell. Surface sites created by low energy photons, as well as bulk TiO2 carrier dynamics that are probed by transient absorption, do not appear to be sensitive to the same dynamics that determine chemical reactivity.

Harvesting solar energy is hindered by the financial difficulties in the creation of perfect crystal silicon-based photovoltaics. The study of energy flow through disordered systems is therefore in interest as a potential way to manufacture low-cost amorphous systems. It is shown through a series of time-resolved photon-counting fluorescence sensitization experiments that in a static system with disorder, anomalous diffusion results in that measured values for the exaction diffusion constants fall short of what is predicted for crystals based on traditional steady-state measurements for Forster parameters such as the Forster radius. Various strategies are discussed to successfully tune the degree of energy migration through disordered systems. The extent to which steady-state theory can be used as a simple tool in guiding the organic synthesis of potential active materials is brought to light.

Photomechanical materials interact with light to elicit a mechanical response. They are the basis of a new class of smart materials which can serve as the active element in actuators, thus creating light-powered devices. The origin of the mechanical response is a volume change induced by a solidsolid or solidliquid phase transition in the material. This light-induced volume change can be performed two different ways: through a photochemical reaction or through the photothermal effect. In this thesis, both pathways are discussed, and photomechanical actuation is demonstrated in piston-type actuators and polymer films. In Chapter 3, the photoisomerization kinetics of a third-generation donor acceptor Stenhouse adduct (DASA) are examined over a range of concentrations. DASAs are a novel photoswitch which absorb in the visible and near-infrared spectral regions. It switches efficiently at micromolar concentrations in both liquid solution and in polymers, but as the photochrome concentration is increased there is a dramatic inhibition of the photoisomerization. The physical origin of the inhibition of photoswitching at high photochrome concentrations must be understood if the DASA class of molecules is to be used for applications like photomechanical actuation. In Chapter 4, a photothermal solidliquid transition is studied because of its large volume expansion. Photoactive composites made of small molecules doped in a phase change material (PCM) are used in a commercial wax actuator to generate useful mechanical work. It is demonstrated that small molecules can act as absorbers to enable a photothermal solidliquid melting transition in eicosane, a low molecular weight PCM. The stability, work density, and efficiency of the actuator containing these PCMs are investigated, and it is found that the actuators are competitive with electrically powered devices. In Chapter 5, further investigation of the small molecule-PCM composite is conducted using a custom-built actuator to gain insight on how the actuator should be optimized to maximize its work output. This new actuator consists of a simpler geometry which allows for the comparison with theoretical work density models. In Chapter 6, an azobenzene derivative (AZO) that can undergo photochemical melting at room temperature, is investigated as a potential candidate for photomechanical actuation. AZO is incorporated in a variety of polymer hosts to create thin films capable of bending when irradiated with light. We find that doping polymers with AZO can provide a general strategy to make photomechanical polymers using a reversible photochemical reaction that induces a solidliquid phase transition.

As human’s global energy consumption increases, reaching 17.5 TW annually in 2012, and continues to grow, the need for improved efficiencies of current renewable energy sources, such as solar energy, becomes vital. A majority of solar photons that are absorbed by photovoltaic cells are higher in energy than the bandgap material and lose that excess energy as heat. Singlet fission is a multi-electron process which fissions one excited singlet state into two separate triplet states. Incorporating singlet fission materials into a photovoltaic cell is one route to improving the efficiency of photovoltaic cells. An ideal singlet fission molecule for use in a photovoltaic cell requires three distinct improvements over current candidates: (1) increased photostability given the nature of photovoltaics and their constant exposure to sunlight, (2) increased triplet energy level so as to facilitate energy transfer at the silicon bandgap and (3) high singlet fission efficiencies. For these reasons, the rylene family with its reputation for photostability and multiple candidates with suitable energetics for singlet fission is explored in this work.

Ultrafast time resolved optical spectroscopy methods are used to study two rylene candidates: peropyrene and diindenoperylene. The crystal structure of solid peropyrene consists of a herringbone arrangement of -stacked molecular pairs and diindenoperylene’s crystal structure consists of edge-to-face herringbone arrangement. We find no evidence for rapid singlet fission in the peropyrene crystals, due to the large shift of the singlet state to lower energy where it no longer fulfills the energy condition for singlet fission. Additionally, no evidence for singlet fission is found in diindenoperylene for which singlet fission would be 2,000 cm-1 uphill. These results demonstrate how both energetics and crystal packing influence the ability of a molecule to function as a singlet fission material and offer tools for surveying in the future.

A high pressure absorption cell is also built and tested in this work. The cell manipulates Beer’s Law and increases the transition dipole moment between a molecule’s S0 state and its T1 state, offering the unique ability to directly measure a molecules lowest triplet state energy.

Singlet fission (SF) is a process in organic molecules that is able to create two triplet excitons from one initially excited singlet exciton. Beyond its interest as a unique photophysical process, SF is also worth studying due to its capability to enhance the overall efficiencies of photovoltaic cells beyond the Shockley-Queisser limit. In order to take advantage of this process, a more thorough understanding of how it occurs and how it can be enhanced is needed. Specifically, this work begins to answer some of the larger questions about SF within tetracene. SF was first observed in anthracene in 1965, and has since been found to occur in larger polyacenes and other conjugated organic molecules. This study focuses on crystalline tetracene for several reasons, most notably for its favorable energy levels for the study of the interaction between the singlet and triplet excitons, greater chemical stability, and large body of previous work. In this work, the electronic states of tetracene are characterized with time resolved photoluminescence and transient absorption spectroscopies in order to determine the rate and mechanism of SF within crystalline tetracene, which have not been conclusively shown in prior studies. Using spectroscopy measurements, this work has determined that SF is occurring within crystalline tetracene with a formation time of 80±3 ps, which leads to a triplet yield of 200%. Additionally, the study of the temperature dependence of crystalline tetracene photoluminescence has shown that the fission of a singlet into two triplets may not be thermally activated, a finding which is unexpected based on previous work. Furthermore, observation of quantum beats within the delayed fluorescence of single crystals of tetracene strongly suggests that the singlet fissions directly into two triplets rather than proceeding through a charge-transfer intermediate. Overall, this work has shown that SF occurs with a high efficiency within tetracene, and that tetracene is a viable candidate for use as a SF sensitizer within a photovoltaic cell.