Circumventing Spectrum Mismatch - Studies of Triplet-Triplet Annihilation Upconversion, Singlet Fission and Two-Photon Absorption in Photoactive Materials

Solar energy stands out for its potential to supply the global energy demand by itself. Therefore, it is of great value to expand the use of processes that involve light such\ua0as conversion to electricity or fuels, or to drive high-energy reactions. However, the\ua0limited availability of photons with the desired energy required to induce a certain\ua0photophysical process poses a challenge. To circumvent this spectral mismatch,\ua0processes that up- and down-convert photon energy can be used. In this work the\ua0focus lies on photon upconversion through triplet-triplet annihilation (TTA-UC) and\ua0two-photon absorption (2PA) and downconversion through singlet fission (SF).\ua0One key challenge for up- and downconversion processes is that for them to be useful they must be incorporated into practical devices. Therefore, one of the overall objectives of this\ua0work is to evaluate photoactive materials that both change the photon energy and have\ua0potential to be incorporated with a working device. This means moving away from diffusion control in liquid solution.\ua0 \ua0\ua0Self-assembling organogels, with chromophores covalently attached to the gelator\ua0backbone, were tested as platforms for TTA-UC and SF. The results show that chromophore-chromophore interactions can be tuned by choice of substitution position, and that it is possible to obtain photon energy conversion in the studied self-assembling gels, although efficiencies need to be improved for practical applications. The results indicate that there is potential for future development of gel-based self-assembled photoactive materials.\ua0To demonstrate how TTA-UC can be applied to circumvent spectral mismatches in\ua0devices, it was used to sensitize the well-known catalyst titanium dioxide (TiO2). A TTA-UC solution was used to absorb visible light and the upconverted emission was in turn used to sensitize a TiO2 thin film. Despite needs for optimizing the setup, visible light photoexcitation could be confirmed and the number of holes in valence band could be quantified to mM concentrations, using a redox couple, which at the same time confirmed the reactivity of the sensitized TiO2

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

Magnetic Field Effects on Molecular Emissivity in Solutions

Control over dynamics of excited states of molecules is fundamental to utilization of these states in all areas of technologies, including optical microscopy and tomography. We explored the possibility of magnetically controlling molecular emissivity by influencing spin dynamics in radical pairs (RPs) and triplet-triplet pair. We envisioned that by including RPs into a pathway leading to the formation (or decay) of an emissive triplet state, magnetic influence on phosphorescence could be realized via modulation of the RP\u27s spin dynamics. RPs can initially be produced in their singlet or triplet spin state. These two cases were explored in the studies of electron and energy dynamics in series of donor-acceptor systems, comprising phosphorescent platinum (II) porphyrins (PtP) and rhodamine B (RhB+) derivatives. In one series, the phosphorescent triplet state of PtP is generated by way of photo-excitation of RhB+, followed by photoinduced electron transfer with formation of a singlet-born RP, RP intersystem crossing, and subsequent recombination of the triplet RP. Similarly, singlet-born RPs were included into a pathway leading to PtP triplet state in a triad system comprising PtP, anthracene, and boron dipyrromethene. Using another series, we have demonstrated that visible room-temperature phosphorescence can be modulated by weak magnetic fields (\u3c1T). In this case, the RP is initially born in its triplet state upon direct excitation of PtP, followed by electron transfer originating in the PtP triplet state. External magnetic field modulates spin dynamics in the RP, affecting contribution of the singlet charge recombination channel and thereby influencing the phosphorescence. Spin dynamics of triplet-triplet pair is also susceptible to magnetic field. Triplet-triplet pair can undergo triplet-triplet annihilation (TTA) process leading to photon upconversion, which is typically observed as p-type delayed fluorescence. We have demonstrated TTA-sensitized delayed fluorescence and delayed phosphorescence, mediated by near-infrared absorbing metalloporhyrins as sensitizers in solution at room temperature, can be magnetically modulated. These studies present unusual and interesting examples of magnetic field effects on molecular emission and set the stage for rational design of optical imaging probes with magnetically controlled emissivity

Magnetic Field Effects on Molecular Emissivity in Solutions

Control over dynamics of excited states of molecules is fundamental to utilization of these states in all areas of technologies, including optical microscopy and tomography. We explored the possibility of magnetically controlling molecular emissivity by influencing spin dynamics in radical pairs (RPs) and triplet-triplet pair. We envisioned that by including RPs into a pathway leading to the formation (or decay) of an emissive triplet state, magnetic influence on phosphorescence could be realized via modulation of the RP\u27s spin dynamics. RPs can initially be produced in their singlet or triplet spin state. These two cases were explored in the studies of electron and energy dynamics in series of donor-acceptor systems, comprising phosphorescent platinum (II) porphyrins (PtP) and rhodamine B (RhB+) derivatives. In one series, the phosphorescent triplet state of PtP is generated by way of photo-excitation of RhB+, followed by photoinduced electron transfer with formation of a singlet-born RP, RP intersystem crossing, and subsequent recombination of the triplet RP. Similarly, singlet-born RPs were included into a pathway leading to PtP triplet state in a triad system comprising PtP, anthracene, and boron dipyrromethene. Using another series, we have demonstrated that visible room-temperature phosphorescence can be modulated by weak magnetic fields (\u3c1T). In this case, the RP is initially born in its triplet state upon direct excitation of PtP, followed by electron transfer originating in the PtP triplet state. External magnetic field modulates spin dynamics in the RP, affecting contribution of the singlet charge recombination channel and thereby influencing the phosphorescence. Spin dynamics of triplet-triplet pair is also susceptible to magnetic field. Triplet-triplet pair can undergo triplet-triplet annihilation (TTA) process leading to photon upconversion, which is typically observed as p-type delayed fluorescence. We have demonstrated TTA-sensitized delayed fluorescence and delayed phosphorescence, mediated by near-infrared absorbing metalloporhyrins as sensitizers in solution at room temperature, can be magnetically modulated. These studies present unusual and interesting examples of magnetic field effects on molecular emission and set the stage for rational design of optical imaging probes with magnetically controlled emissivity

The Overlap Representation of Skewed Quark and Gluon Distributions

Within the framework of light-cone quantisation we derive the complete and exact overlap representation of skewed parton distributions for unpolarised and polarised quarks and gluons. Symmetry properties and phenomenological applications are discussed.Comment: LaTex, 36 pages. v2: incorrect paper attached originally. v3: erratum adde

Spectroscopic properties of conjugated systems

Imperial Users onl

Solar Seismology from Space. a Conference at Snowmass, Colorado

The quality of the ground based observing environment suffers from several degrading factors: diurnal interruptions and thermal variations, atmospheric seeing and transparency fluctuations and adverse weather interruptions are among the chief difficulties. The limited fraction of the solar surface observable from only one vantage point is also a potential limitation to the quality of the data available without going to space. Primary conference goals were to discuss in depth the scientific return from current observations and analyses of solar oscillations, to discuss the instrumental and site requirements for realizing the full potential of the seismic analysis method, and to help bring new workers into the field by collecting and summarizing the key background theory. At the conclusion of the conference there was a clear consensus that ground based observation would not be able to provide data of the quality required to permit a substantial analysis of the solar convection zone dynamics or to permit a full deduction of the solar interior structure

Lectures on Astroparticle Physics

These are extended notes of a series of lectures given at the XIth Brazilian School of Cosmology and Gravitation. They provide a selection of topics at the intersection of particle and astrophysics. The first part gives a short introduction to the theory of electroweak interactions, with specific emphasize on neutrinos. In the second part we apply this framework to selected topics in astrophysics and cosmology, namely neutrino oscillations, neutrino hot dark dark matter, and big bang nucleosynthesis. The last part is devoted to ultra high energy cosmic rays and neutrinos where again particle physics aspects are emphasized. The often complementary role of laboratory experiments is also discussed in several examples.Comment: 42 pages, 12 figures, extended version of lectures given at the XIth Brazilian School of Cosmology and Gravitation, Rio de Janeiro, July 26 - August 4, 2004, prepared for AIP conference proceeding

Isocyanoborato Complexes of d6 Metals: Photophysics, Electronic Structures and Challenging Applications in Photocatalysis

Considering the steadily growing world population and the accompanying increase in energy consumption, the change from fossil fuels as the primary energy source to more environmentally friendly alternatives is unavoidable. Some of the most promising sustainable energy sources are based on solar energy conversion, which is increasingly popular and seems indispensable to enable a greener future. The use of light as an energy source has furthermore been identified as a useful concept in synthetic organic chemistry, because it can enable new reaction types, allows for mild reaction conditions and often provides good selectivity. As the light-absorbing and photoactive species in these reactions, photocatalysts, often based on transition metals, are employed. Even though the selection of available photocatalysts is considerable, there is still a significant potential for the development of novel compounds that could enable more challenging reactions. In this thesis, the borylation of cyanido complexes of d6 metals is identified as a useful concept in order to obtain potent photocatalysts for photoredox- and energy transfer catalysis. In the first part (Chapter 2), a general introduction containing the necessary theoretical background is provided. This includes photophysical principles as well as important literature reports to put the topics discussed in this thesis into a broader perspective. In the main part (Chapters 3 to 5), new isocyanoborato complexes with different metal centers are investigated. In the first project (Chapter 3), the borylation of two well-known ruthenium(II) cyanido complexes is identified as a useful approach to boost their photocatalytic performance. These results are complemented by in-depth photophysical studies, providing insight into the changes of the electronic structure accompanying borylation. This study is then extended in the second and third project to complexes of iridium(III) (Chapter 4) and iron(II) (Chapter 5). In Chapter 4, the focus lies on the possible applications of an IrIII isocyanoborato complex for photochemical transformations. This new luminophore exhibits an exceptionally high triplet energy and therefore enables challenging energy-transfer catalyzed reactions as well as photochemical upconversion deep into the UV. In contrast, Chapter 5 focuses on the electronic structures and excited-state dynamics of two Fe(II) isocyanoborato complexes by combining different experimental techniques, thereby allowing to paint a conclusive picture of the changes associated with borylation. In summary, this thesis provides detailed insight into the photophysics and electronic structures of isocyanoborato complexes based on Ru(II), Ir(III) and Fe(II). Furthermore, the Ru(II) and Ir(III) luminophores are shown to be applicable to several challenging photocatalyzed reactions in synthetic organic chemistry