Quantum chemical studies of light harvesting

The design of optimal light-harvesting (supra)molecular systems and materials is one of the most challenging frontiers of science. Theoretical methods and computational models play a fundamental role in this difficult task, as they allow the establishment of structural blueprints inspired by natural photosynthetic organisms that can be applied to the design of novel artificial light-harvesting devices. Among theoretical strategies, the application of quantum chemical tools represents an important reality that has already reached an evident degree of maturity, although it still has to show its real potentials. This Review presents an overview of the state of the art of this strategy, showing the actual fields of applicability but also indicating its current limitations, which need to be solved in future developments

Molecular design for efficient triplet photosensitizers

학위논문(박사) -- 서울대학교대학원 : 공과대학 재료공학부, 2021.8. 김재필.유기 도너 억셉터 기반 광감응제는 유가 감응형 태양전지, 열활성 지연 형광재료, 광역동치료, 삼중항-삼중항 소멸에 의한 광에너지 향상 등 다양한 분야에서 많은 관심을 받아왔다. 광감응제의 도너와 억셉터는 각각 전자를 밀어내고 당기는 능력을 갖고 있기 때문에 빛을 조사하였을 때 흡수된 전자가 도너에서 억셉터로 이동하게 되고 이는 전하분리 준위를 만들어낸다. 생성된 전하분리 준위는 높은 쌍극자 모멘트를 나타내기 때문에, 주변의 환경에 따라 형광 특성이 큰 폭으로 변화하고 이는 온도 센서, 극성 센서 등으로의 적용을 가능하게 한다. 또한 도너에서 억셉터로의 일 방향성 전하 전달 (Charge transfer)은 염료감응형 태양전지의 구성성분인 염료로 적용을 가능하게 한다. 염료감응형 태양전지 내 염료는 빛을 흡수하여 도너에서 억셉터로 전자를 전달시키고 이러한 일방향성 에너지는 전자가 염료로부터 TiO2로 이동할 수 있게 만든다. 도너와 억셉터의 두 단위체를 단일 결합으로 연결하면 두 단위체 사이의 회전 변화를 가능하게 한다. 이는 주변 압력에 따라 분자내 회전의 정도가 달라지도록 만들기 때문에 압력 센서로 사용할 수 있으며, 이러한 특성을 활용하여 지문 센서 등의 분야에 적용이 가능하다. 또한 이 도너-억셉터 광감응제는 유기발광 다이오드의 핵심기술인 열활성지연 형광을 구현할 수 있다. 도너-억셉터 기반 광감응제의 구조는 도너와 억셉터가 서로 수직하게 꺾이게 되어있으며 이는 전하분리를 유도한다. 이 전하 분리는 두 에너지 스테이트의 축퇴를 야기하고 결과적으로 일중항과 삼중항 준위 사이의 계간 전이를 가능하게 만든다. 도너-억셉터 분자는 광여기 과정 중에 계간전이에 의하여 삼중항 준위에 전자가 이동이 된다. 이 T1 준위의 전자 (또는 에너지)는 삼중항 산소를 일중항 산소로 변환시킬 수 있는 능력을 가지고 있기 때문에 일중항 산소발생을 활용하는 광역동치료에 적용할 수 있으며, 발생한 일중항 산소를 중간 촉매로 사용할 수 있어 유기합성의 광촉매로도 활용가능하다. 이렇듯 도너-억셉터 기반 광감응제는 다양한 분야로의 가능성을 가지고 있음에도 불구하고 여전히 그 메커니즘이 완전히 밝혀지지 않았다. 뿐만 아니라, 최근 도너-억셉터 기반 광감응제를 삼중항 광감응제로 적용하는 많은 연구들이 진행되고 있으며, 기존의 삼중항 광감응제의 특성을 상회하는 도너-억셉터 기반 삼중항 광감응제들이 지속적으로 보고되고 있다. 본 연구에서는 보디피를 기반으로 한 도너-억셉터 광감응제를 디자인 및 개발하였으며, 이들의 구조와 형광 및 삼중항 특성 등의 광물리적 특성 간의 상관관계에 대하여 조사하였다. 먼저 도너-억셉터 광감응제를 형광재료 특히 압력에 따른 형광색의 변화가 가능한 Mechanofluorochromism의 분야에 적용가능성을 확인하였으며, 그 메커니즘에 대한 체계적인 조사를 진행하였다. 다음으로 도입되는 염소 원자의 개수를 조절하여 전자 받개 능력을 달리한 보디피 기반 도너-억셉터 광감응제를 개발하였다. 그리고 전자 받개 능력이 삼중항 특성에 미치는 영향에 대한 연구를 진행하였고 페르미 골든룰을 통하여 이들의 동역학을 분석하였다. 삼중항 특성을 강화하기 위하여, 도너와 억셉터의 구조에 중원자를 도입한 도너-억셉터-중원자 기반 삼중항 광감응제를 고안 및 개발하였으며, 이는 기존의 도너-억셉터, 순수한 중원자 기반 재료보다 삼중항 양자수율과 삼중항 수명이 모두 개선됨을 확인할 수 있었다. 도너-억셉터-중원자 기반의 삼중항 광감응제의 삼중항 수명을 더욱 증가시키기 위하여 도너와 억셉터 사이의 회전을 억제할 수 있는 구조를 도입하였으며, 결과적으로 회전이 억제되지 않은 광감응제와 비교하여 5배 이상 높은 1,503μs의 긴 삼중항 수명을 달성할 수 있었다. 이는 보고된 보디피 계열 삼중항 광감응제 중에서 가장 긴 삼중항 수명을 나타냈고, 기존의 삼중항 광감응제와 비교하여도 상당히 높은 편이다. 긴 삼중항 수명은 삼중항 삼중항 소멸 양자수율(TTA-UC quantum yield)의 증가를 야기하였고 문턱 세기 (Threshold intensity)도 개선시킬 수 있었다. 따라서 도너-억셉터-중원자에 회전 억제 전략을 도입하여 삼중항 특성이 상당히 향상될 수 있음을 증명하였고 이러한 결과는 삼중항 광감응제를 활용하는 다양한 산업분야에 큰 기여를 할 수 있을 것이다.Organic donor-acceptor based photosensitizers have received a lot of attention in various fields such as dye sensitizers solar cells (DSSC), OLED using thermally activated delayed fluorescence (TADF), photodynamic therapy (PDT), and triplet-triplet annihilation upconversion (TTA-UC). The electron donor and the electron acceptor of donor-acceptor photosensitizers pushes and pulls the electrons, respectively, during photoexcitation, resulting in charge separation state. This charge separation states, due to its high dipole moment, are sensitively affected by the surrounding environment, which can cause the drastic change of the fluorescence properties. The donor-acceptor photosensitizers can thus be used for a variety of sensors such as a temperature sensor and a polarity sensor. In addition, the one-directional charge transfer property from donor to acceptor enables application as a dye, a component of dye-sensitized solar cells (DSSC). The dye in DSSC absorbs light and transfers electrons from the donor to the acceptor, and this one-directional energy allows electrons to move from the dye to TiO2. Moreover, the donor and acceptor moieties are commonly connected with single bond, which allows various rotational change between the donor and acceptor. Because the degree of conformation between donor and acceptor varies depending on the surrounding pressure, donor-acceptor photosensitizers can be used as a pressure sensor, applicable to the fingerprint sensors. In addition, it can be implemented to the thermally activated delayed fluorescence (TADF) in the research field of OLED. The orthogonal coordination of donor and acceptor increases the charge separation between donor and acceptor, which reduces in the energy gap between the singlet and triplet excited states. The completely separated charge of donor-acceptor induces the degeneration of the both energy states, resulting in the intersystemcrossing (ISC) between singlet and triplet states. Thus, D-A photosensitizers may populate electrons in the triplet state through ISC during the photoexcitation process. Since the electrons of the triplet state have the potential to convert triplet oxygen into singlet oxygen, D-A photosensitizers can be applied to photodynamic therapy (PDT) using singlet oxygen. Although D-A photosensitizers have potential in various fields, their photophysical kinetics are still not fully revealed. Recently, a number of researches were performed that applying the D-A photosensitizers as a triplet sensitizer, and D-A-based triplet sensitizers that even exceed the characteristics of conventional triplet sensitizers have been continuously reported. In this study, we designed and developed D-A photosensitizers based on boron dipyrromethane (BODIPY), and studied the correlation between the molecular structure and photophysical properties such as fluorescence and triplet characteristics. First, we examined the applicability of D-A photosensitizers to the field of fluorescent materials, especially mechanofluorochromism (MFC), where the fluorescent color changes according to pressure, and the mechanism for the MFC systematically investigated based on the theory of the twisted intramolecular charge transfer (TICT) and the aggregation induced emission enhancement (AIEE). Second, we developed BODIPY-based D-A photosensitizers with different accepting power by controlling the number of chlorines, studied the effect of accepting power on triplet characteristics. And the ISC kinetics were analyzed through the theory of fermi's golden rule. Third, we suggested the donor-acceptor-heavy atom (D-A-H) triplet photosensitizers where heavy atoms were introduced into the D-A photosensitizers to enhance the triplet characteristics. D-A-H photosensitizers showed higher triplet quantum yield and shorter triplet lifetime compared to donor-acceptor and pure heavy atom-based photosensitizers. Fourth, to further increase the triplet lifetime, methyl moieties suppressing rotation between D and A were introduced to the D-A-H-based triplet photosensitizers. As a result, a rotational restricted triplet photosensitizer showed an ultra-long triplet lifetime (1,503μs), which is more than 5 times higher than that of a rotational free photosensitizer. As far as we know, this is the longest triplet lifetime among the reported BODIPY based triplet photosensitizers, and is even considerably higher than that of conventional triplet photosensitizers. Ultra-long triplet lifetime caused an increase in TTA-UC properties such as TTA-UC quantum yield and threshold intensity. Therefore, we proved that the triplet characteristics can be improved considerably by rotational restriction strategy to D-A-H photosensitizers, and this result may contribute to various industrial fields using triplet photosensitizers.Chapter 1 Introduction 1 1.1 Aggregation induced emission (AIE) 1 1.2 The basic principle of photoinduced electron transfer (PET) 3 1.3 Marcus theory of photoinduced electron transfer 6 1.4 Intersystemcrossing mechanism of donor-acceptor photosensitizers 9 1.5 Triplet-triplet annihilation upconversion (TTA-UC) 10 1.6 References 12 Chapter 2 Mechanofluorochromism of Triphenylamine-BODIPY: Effect of twisted intramolecular charge transfer and restriction in rotation on fluorescence 17 2.1 Introduction 17 2.2 Experimentals 19 2.3 Results and discussion 24 2.4 Conclusions 44 2.5 References 46 Chapter 3 A study on photophysical and photodynamic properties of donor–acceptor BODIPY complexes: correlation between sin-glet oxygen quantum yield and singlet-triplet energy gap Theoretical Formulation 56 3.1 Introduction 56 3.2 Experimentals 59 3.3 Results and discussion 72 3.4 Conclusions 94 3.5 References 95 Chapter 4 Synergistic effects of photoinduced electron transfer and heavy atom effect based on BODIPY for efficient triplet photosensitizers 102 4.1 Introduction 102 4.2 Experimentals 104 4.3 Results and discussion 111 4.4 Conclusions 146 4.5 References 147 Chapter 5 Enhanced triplet-triplet annihilation upconversion luminesncece through the conformational restriction based on donor - acceptor - heavy atom molecules 155 5.1 Introduction 155 5.2 Experimentals 158 5.3 Results and discussion 165 5.4 Conclusions 191 5.5 References 192 Summary 199 Korean Abstract 201박

Stimuli-Responsive Nanoparticles for Bio-Applications

Stimuli-responsive nanoparticles have been designed and studied, exploring their potentiality as self-assembled materials as building blocks for the development of "smart" materials for bio-applications. Perylene diimide derivatives (PDI) have been used as fluorogenic units and structural components of assembled high-brightness nanoparticles, where fluorescence changes can be triggered by external (light) or internal (pH) stimuli which promote disaggregation induced emission (DIE). Synthesis of PDI (P) was achieved by microwave heating in mild conditions. π-π stacking and inter-substituent interactions drove the self-assembly of quenched nanoparticles that were internalized by yeast cells responding as fluorogenic imaging agents. By controlling the dosage, they displayed either green or red fluorescence. Multicolour fluorescence imaging was achieved by sample photo-activation under strong light irradiation. P was adopted as structural component of covalently linked nanoparticles. P chains have been cross-linked by an epoxy monomer into Pluronic micelles, driving the formation of core-shell nanoparticles. Vicinity of the monomer aromatic regions caused the quenching of the emission, which could be recovered by fluorophore disaggregation triggered by light irradiation in proper conditions of concentration and/or polarity. Photo-activation occurred also after nanoparticles internalization by living cells, confirming the possibility of using them as stimuli-responsive fluorogenic bio-imaging agents. Fluorogenic pH-responsive nanoparticles have been further designed and developed, with the purpose of differentiate normal and cancer tissues. A monodispersed amphiphilic block co-polymer, constituted by a PEGylated hydrophilic block and a tertiary amine pH responsive hydrophobic block, functionalized by a PDI norbornene monomer, was synthesised by ring opening metathesis polymerization. Polymer self-assembly was exploited to obtain spherical core-shell nanoparticles, quenched in neutral pH thanks to the π-π stacking in the nanoparticles core. By switching the pH from 7.4 to 5, structural modification in the hydrophobic block were promoted, leading to the nanoparticles disassembly and to the recovery of PDI emission

Solvent Effects And Charge Transfer States In Organic Photovoltaics

Due to their various advantages, including lightweight, flexible, and cheap manufacturing, organic photovoltaic materials have gained enormous research interest. Over nearly two decades, the power conversion efficiency of organic solar devices has increased dramatically. However, it is still low compared to traditional inorganic semiconductors. In order to improve efficiency, a better understanding of the basic thermodynamic properties of the light-to-electricity power conversion process is needed. One nontrivial aspect of organic solar cells is the low dielectric constant, which leads to tightly-bound excitons upon vertical excitations. The separation of electron-hole pairs requires a larger driving force to overcome the Coulombic binding energy in organic semiconductors compared to their inorganic counterpart. A particular state called charge transfer state appears during the dissociation process of the bound excitons. The exact role of this particular type of state, whether a precursor to efficient charge separation or a detrimental process which hinders the generation of free charges, is still under debate. Extensive research has been performed to elucidate the mechanism of free charge carrier creation. Some studies show that the dielectric environment plays a significant role during the charge dissociation process by affecting the energetics of excited states. For example, MDMO-PPV: PCBM device becomes more efficient when the dielectric constant reaches a certain value (εr = 9). The aim of this work is to map out the alignment of excited states of a typical polymer: fullerene device, taking PCPDTBT: PCBM as a specific example system, and find out the characteristics of the charge transfer state under the influence of polar solvent. Long-range corrected time-dependent density functional theory combined with the polarizable continuum model has been used to study the solvent effect on excited state properties of PCPDTBT: PCBM molecular system. Solvation model has been applied using the linear-response and state-specific approaches to account for the dielectric environment. Electronic transitions are characterized by their intrinsic properties based on a detailed analysis of the one-electron transition density matrices. The tools include a numerical value termed charge transfer character, contour plots of the transition density matrix, and natural transition orbital of each excited state. The results show that the influence of the solvent depends on the nature of the excitations. For excitonic states, which have a characteristic of local excitations, the solvent has little to no effects on the excitation energies according to both solvent schemes. In contrast, a different trend is observed for states with a significant amount of charge transfer. State-specific predicts a sufficient decline in the excitation energy as the dielectric constant increases such that the charge transfer state can be stabilized to the lowest excited state, whereas linear-response shows almost no change. The comparison of two solvent approaches is discussed. It concludes that a protocol that combines TDDFT with long-range-corrected hybrid functional, CAM-B3LYP, Grimme’s empirical dispersion correction D3, and state-specific solvation model can effectively and efficiently predict the energetics of charge transfer state in organic photovoltaic materials. Future directions are also provided, including extension of the current calculation scheme to more polymer: fullerene molecular systems, application of other methods such as charge constraint density functional theory and range-separated hybrid functionals combined with polarizable continuum model, simulation of the charge dissociation process in a dynamic picture, and investigation of the solvent effect under the variation of optical dielectric constant instead of focusing only on the static permittivity

Triplet Excitons in Natural Photosynthetic and Artificial Light Harvesting Systems: Measurement and Modeling.

Under full sunlight, unprotected (Bacterio)Chlorophyll ((B)Chl) molecules photodegrade in a matter of minutes. This is the result of the generation of highly reactive singlet oxygen (1O2) by energy transfer from the (B)Chl triplet state (3(B)Chl) to the oxygen ground state. Natural photosynthetic systems must protect themselves from 1O2, typically done by positioning carotenoids within a few angstroms of each (B)Chl molecule to quench 3(B)Chl states. Using phosphorescence spectroscopy and computational modeling, we investigated alternative, carotenoid independent, mechanisms which nature may employ to prevent 1O2 sensitization by lowering the energy of 3(B)Chl below that of 1O2. The two proposed triplet lowering mechanisms investigated were: triplet state lowering by strong pigment-pigment interactions (i.e. triplet exciton formation) and triplet state lowering by pigment-protein interactions. Possible natural examples employing these mechanisms are two structures found in green sulfur bacteria: the chlorosome (an antenna containing ~100000 coupled BChl c, d, or e molecules with unexpectedly high photostability) and the Fenna-Matthews-Olson (FMO) complex (an auxiliary antenna containing eight seemingly unprotected BChl a molecules)

Ultrafast Excited State Dynamics and Functional Interfaces Probed by Second Harmonic Generation

Understanding the ultrafast excited state dynamics in organic semiconductors after optical excitation is a key requisite on the road towards efficient organic solar cells. Additionally, the creation of functional interfaces built from organic molecular switches and the read-out of the photochromic state are essential for molecular electronics. In this thesis, static second harmonic generation (SHG) measurements were utilized to investigate the photochromism of different indolylfulgimide derivatives immobilized on silicon. During this, the influence of chemical modifications on the switching efficiencies (cross-sections) and the non-linear optical contrast between the switching states were investigated. In the second part of this thesis, femtosecond time-resolved second harmonic generation measurements were used to investigate the ultrafast decay mechanism of optically induced electronically excited states in organic semiconductors and donor/acceptor systems. These led to observations of relaxation into dimer induced states, charge trapping at native silicon oxide and ultrafast vibronic relaxation. For the donor/acceptor configurations, depending on the molecular orientation at the interface and the excitation energy, the creation of charge transfer states were investigated

Photoelectrochemical cells employing molecular light-harvesting materials for the capture and conversion of solar energy

2017 Spring.Includes bibliographical references.Solar light has the potential to be a substantial contributor to global renewable energy production. The diffuse nature of solar energy requires that commercially viable devices used to capture, convert, and store that energy be inexpensive relative to other energy-producing technologies. Towards this end, photoelectrochemical cells have been the subject of study for several decades. Particularly interesting to chemists, molecular light-harvesting materials can be employed in photoelectrochemical cells. For example, a dye-sensitized solar cell (DSSC) is a type of photoelectrochemical cell designed to capture solar energy and convert it to electricity. Alternatively, molecular light-harvesting materials have also been employed in water-splitting photoelectrolysis cells (PECs), which capture solar energy and store it in the form of chemical bonds such as H2 and O2. The work presented in this dissertation falls into two major projects. The first involves fundamental studies of water-oxidizing PECs employing a novel perylene diimide molecule as the light-harvesting unit. Background is provided in Chapter II, composed of a comprehensive literature review of water-oxidizing PEC systems that employ light-harvesting materials composed of earth-abundant elements. Chapter III describes preliminary studies of a water oxidizing PEC composed of a perylene diimide organic thin-film (OTF) and cobalt oxide catalyst, the first of its kind in the literature. Characterization of this novel device provided knowledge of the efficiency-limiting processes that would need to be addressed in order to improve device performance. Subsequently, Chapter IV describes preliminary studies of the same perylene diimide molecule in an alternative, literature-precedented, dye-sensitized photoelectrolysis cell (DS-PEC) architecture aimed at improving the efficiency-limiting processes of the first OTF-PEC. Characterization of this DS-PEC architecture reveals that the efficiency-limiting processes of the OTF-PEC were indeed improved. However, deposition of the cobalt oxide catalyst onto the DS-PEC did not successfully result in water oxidation. Alternative catalyst-deposition strategies from the literature are described as direction for future studies. The second project of this dissertation involves the study of novel high-redox-potential organometallic cobalt complexes as redox mediators in DSSCs, and is presented in Chapter V. Therein, it was found that the use of electron-withdrawing functional groups on cobalt coordinating ligands not only increased the redox potential, but also increased the lability of the ligands. The resulting complex instability caused performance-limiting electron-recombination reactions in assembled DSSCs. These results point future researchers towards the study of higher-chelating ligands for enhanced stability in high-potential cobalt complexes

Organic and inorganic nanoparticles for imaging and sensing in water

Nanotechnology aims at the design, synthesis, characterization and application of materials and devices on the nanoscale. Nanoparticles are defined as materials with the three dimensions in the space less than 100 nm. They possess properties hugely different from the corresponding macroscopic materials. Their peculiarities depend on the reduced size, shape, composition and interface, all aspects that can be controlled during the synthesis. Moreover, nanoparticles can act as platforms for assemble well-defined multifunctional structures able to perform varied tasks. Nanoparticles can be made by inorganic materials and by soft materials. In the currently work a wide range of nanoparticles have been designed, synthesized and characterized for various purposes. In chapter 4 we propose simple and cheap strategy to develop multi-stimuli sensitive perylene diimide (PDI) molecules to create new smart materials by self-assembly. In chapter 5 we report synthesis of designed molecule that self-assemble in nanoparticles in biocompatible environment without any dramatic decrease of fluorescence brightness. In chapter 6 a similar work has been repeated with commercial fluorophores. Detection of chemical and biological agents plays a fundamental role in environmental and biomedical sciences. In chapter 7 small gold nanocluster functionalised with seven thiols in has been studied in presence active pharmaceutical ingredient (API). In chapter eight we try to increase melanin radiation protection activity both increasing number of stable free radicals and introducing oxidative ion transition metal. We have chosen Fe(III) and Mn(III) to increase oxidative ability and Zn(II), that do not possess oxidative ability as reference. We used 4-amino-TEMPO to increase number of stable free radicals

Spectroscopic analysis of electronic energy transfer in molecular cassettes formed around boron dipyrromethene dyes

PhD ThesisPhotosynthesis, in its many diverse forms, has provided inspiration for countless researchers over several centuries and continues to spring surprises and new concepts. At the simplest level, photosynthesis can be considered to store sunlight in the form of chemical (or electrochemical) potential. As such, it is often proposed as a model for artificial systems aimed at the conversion and storage of solar energy. One of many key components of photosynthesis concerns the collection of sunlight by various pigments and the transfer of the resultant exciton to a reaction centre, where fuel formation can take place. In this thesis, we examine chemical systems that facilitate electronic energy transfer (EET) between chromophores arranged in rather simple molecular architectures built around boron dipyrromethene (Bodipy) dyes. These latter compounds are taken from an ever-expanding family of robust, highly fluorescent synthetic reagents developed originally as laser dyes and bio-labels. Chapter 1 gives a brief introduction to the general field of EET and covers a few basic concepts special to the photosynthetic apparatus. This is followed by a brief consideration of Förster theory, which is the staple mechanism underpinning much of the work covered in later chapters, and mention of the alternative Dexter theory for EET. By way of acknowledging that we are not the only researchers to explore this type of work, we provide a few key examples of molecular systems designed to probe various aspects of intramolecular EET. These examples cover Bodipy-based arrays and certain bio-inspired molecular systems. In Chapter 2, we describe the behaviour of certain sterically unhindered Bodipy dyes as fluorescent probes for rheology changes, most notably variations in viscosity under ambient conditions. This situation depends on changing the degree of (micro) friction between an appended meso-aryl ring and the surrounding medium. In order to vary in a systematic manner the resistance to gyration of the aryl ring, the photophysical properties of the dye have been recorded in different media and as functions of temperature and pressure. Local viscosity is also affected by the presence of an inert polymer. Extending the system to include an unusual bichromophore where the linkage is through boron-oxygen bonds switches off the sensory action due to light-induced electron transfer. Chapter 3 includes a critical comparison of EET within two disparate molecular types; namely, covalently-linked and non-covalently-linked molecular dyads bearing ii identical subunits drawn from the Bodipy family. Here, the intention is to explore how the binding motif affects the likelihood of intramolecular EET between the subunits. Both systems, which consist of a yellow Bodipy dye as a donor and a blue Bodipy dye as the complementary acceptor, show highly efficient EET. Again, the probability of EET has been probed as a function of applied pressure and temperature to better expose the mechanism. The non-covalently-linked system, which makes use of electrostatic binding between charged species, forms a liquid crystalline state upon heating and it is notable that efficacious EET occurs within this phase. Chapter 4 looks at the nano-mechanical properties of molecular-scale bridges in linear donor-spacer-acceptor compounds by monitoring the probability of intramolecular EET as a function of bridge length. The bridge (or spacer) consists of 1 to 5 ethynylene-carborane units that allow the centre-to-centre distance between the donor and acceptor to be varied systematically from 38 to 115 Å. Interestingly, the probability of EET is higher than the predicted value for all systems except the shortest bridge. On cooling to 77K, the agreement between theory and experiment agrees much better but depends on applied pressure in fluid solution at room temperature. We rationalise these various results in terms of structural distortion of the longer bridges, thereby allowing determination of the strain energy and Young’s modulus for the spacer unit. In Chapter 5, we report on a study of intramolecular EET in a molecular triad where the highest-energy donor is situated in the centre and there are two disparate Bodipy at the terminals. Overall, the probability of EET exceeds 95% and the individual EET steps can be resolved; the rate of EET follows the order of spectral overlap integrals. By selective protonation of one of the Bodipy-based terminals, it is possible to change the relative ordering of the spectral overlap integrals and thereby switch the direction of EET. This chapter also includes an investigation of the general photophysical behaviour of the symmetric triads, where the same Bodipy dye is present at each terminal, in addition to the spectroscopic properties of the isolated chromophores. Experimental variations include changes in solvent polarity, effect of lowing the temperature, moving from fluid to solid phases and applying high pressure to the fluid medium are discussed in this chapter. Finally, Chapter 6 provides a brief summary of the experimental approaches used throughout the work, including instrumentation and chemicals. In addition, the many mathematical equations and computer programs employed are mentioned here.King AbdulAziz University as represented through the Saudi Embassy for providing scholarship funds and the financial support