47 research outputs found
Photoinduced Betaine Generation for Efficient Photothermal Energy Conversion
No full textThe conversion of solar energy to thermal, chemical, or electrical energy attracts great attention in chemistry and physics. There has been a considerable effort for the efficient extraction of photons throughout the entire solar spectrum. In this work light energy was efficiently harvested by using a long-lived betaine photogenerated from an acridinium-based electron donor-acceptor dyad. The photothermal energy-conversion efficiency of the dyad is significantly enhanced by simultaneous illumination with blue (420-440 nm) and yellow (>480 nm) light in comparison with the sum of the conversion efficiencies for individual illumination with blue or yellow light. The enhanced photothermal effect is due to the photogenerated betaine, which absorbs longer-wavelength light than the dyad, and thus the dyad-betaine combination is promising for efficient photothermal energy conversion. The mechanisms of betaine generation and energy conversion are discussed on the basis of steady-state and transient spectral measurements
電子ドナー・アクセプター連結分子による光熱変換および光化学反応についての分光学的研究
No full textThis thesis emphasizes on the regulation and control of photochemical and photothermal functions of simple electron Donor-Acceptor (D-A) molecular dyads. Electron transfer is one of the extensively investigated phenomena in basic science, which is by considering its vital role in natural and artificial photosynthesis. Despite, several complex electron transfer systems developed for light-harvesting by mimicking the natural photoinduced electron transfer process, simple electron transfer systems are continuously sought after for improving the efficiency of solar energy harvesting and developing molecular sensors. In this work, I design and synthesize two classes of novel electron D-A dyads; one for the efficient photothermal energy conversion, and the other for the efficient sensing of singlet oxygen (¹O₂) in the homogenous solution phase and in the cell microenvironment. This thesis consists of five chapters, including the introduction to photoinduced electron transfer (chapter 1), where I summarise photoinduced electron transfer occurring in natural photosynthesis and its efficient mimicking toward the construction of various artificial solar energy harvesting systems. In the introduction chapter, I explain the classical theories of electron transfer in the designing of simple D-A dyads with efficient intramolecular electron transfer. In chapter 2, I discuss the synthesis of novel D-A systems, methods of characterization of D-A systems, and spectroscopic methods employed in the investigation of electron transfer and photothermal and sensing studies. In chapter 3, I demonstrate the upgraded photothermal energy conversion by acridinium-based D-A dyads. Even though the conventional photothermal agents offer high energy conversion efficiency, most of them have narrow absorption bands. Therefore, broad band solar energy absorbing molecules and materials are continuously sought after. The general approach for the extension of light absorption by molecules to the entire UV-Visible-Near Infrared (UV-Vis-NIR) region is to extend the π-conjugation, which is tedious and time consuming. Thus, in this chapter, I employ a novel acridinium-based D-A dyad with highly-efficient electron transfer. The UV to blue absorbing dyad undergoes photoinduced electron transfer, followed by excited-state deprotonation to generate its betaine form. The betaine shows appreciably long half-life and extended absorption in the UV-Vis-NIR region. The photothermal energy conversion efficiency of the dyad under blue light excitation is enhanced by illumination with long wavelength light. The dyad shows excellent photostability, making the dyad-betaine combination promising for photothermal energy conversion applications. In chapter 4, I demonstrate the applications and principles of D-A systems to the sensing of ¹O₂. Reactive oxygen species offer positive and negative impacts on our life-routine, among which ¹O₂ attracts considerable attention owing to its significance to various chemical, biological, and biochemical processes. Therefore, the sensitive and efficient detection of ¹O₂ is relevant in our daily life. The conventional fluorescence sensors of ¹O₂ are anthracene-based electron D-A systems to which many fluorogenic sensors based on substituted anthracene are reported. However, the roles of substituents on the sensing efficiency remain largely unknown. Therefore, in chapter 4, I investigate the substituent effects on the ¹O₂ sensing efficiency, which is with the intention to improve the efficiency and speed of ¹O₂ detection. Here, I examine the rate of ¹O₂ sensing by three anthracene-based electron D-A dyads, aminocoumarin-anthracene conjugates (S1 and S2), a rhodamine-anthracene conjugate (S3) and a model compound. The second-order rate of reaction of the sensors with ¹O₂ is an order of magnitude less than 9-methylanthracene. The reduced reactivity of S1 suggests the role of the substituent on the rate of sensing. During ¹O₂ sensing studies involving S1 as the sensor, I found an anomalous increase in the fluorescence intensity of S1 under the illumination with UV light, which is after co-sensitization with a porphyrin molecule or Rose Bengal. This abrupt and colossal enhancement of fluorescence intensity suggests the formation of an intermediate complex which is UV-active. The intermediate complex shows stability against ¹O₂ scavengers. To verify the existence of the intermediate complex between S1 and ¹O₂, I employ electron paramagnetic resonance spectroscopy (EPR) and nuclear magnetic resonance spectroscopy (NMR) and rationalize the crucial role of coumarin to form the complex. The steady-state absorption, fluorescence, EPR, and NMR studies suggest that the reaction of ¹O₂ to S1 leads to the trapping of ¹O₂ inside S1. The UV stimulation activates the intermediate complex to form the endoperoxide efficiently and swiftly. Finally, I test the potentials of the sensors to detect the intracellular ¹O₂ by cell imaging
電子ドナー・アクセプター連結分子による光熱変換および光化学反応についての分光学的研究 [全文の要約]
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Excitation‐Wavelength‐Dependent Functionalities of Temporally Controlled Sensing and Generation of Singlet Oxygen by a Photoexcited State Engineered Rhodamine 6G‐Anthracene Conjugate
No full textThe present study provides design guidance for unique multipotent molecules that sense and generate singlet oxygen (O-1(2)). A rhodamine 6G-aminomethylanthracene-linked donor-acceptor molecule (RA) is designed and synthesized for demonstrating wavelength-dependent functionalities as follows; (i) RA acts as a conventional fluorogenic O-1(2) sensor molecule like the commercially available reagent, singlet oxygen sensor green (SOSG), when it absorbs ultraviolet (UV)-visible light and reacts with O-1(2). (ii) RA acts as a temporally controlled O-1(2) sensing reagent under the longer wavelength (similar to 700 nm) photosensitization. RA enters an intermediate state after capturing O-1(2) and does not become strongly fluorescent until it is exposed to UV, blue, or green light. (iii) RA acts as an efficient photosensitizer to generate O-1(2) under green light illumination. The spin-orbit charge transfer mediated intersystem crossing (SOCT-ISC) process achieves this function, and RA shows a potential cancer-killing effect on pancreatic cancer cells. The wavelength-switchable functionalities in RA offer to promise molecular tools to apply O-1(2) in a spatiotemporal manner
In Silico Exploration for Maximal Charge Transport in Organized Tetrabenzoacenes through Pitch and Roll Displacements
No full textA series
of π-conjugated tetrabenzoacenes (TBA), including
nitrogen-(un)doped derivatives, are computationally evaluated to comprehend
the correlation between intrinsic structural arrangements and charge-transport
characteristics. The central charge-transport parameters such as reorganization
energy and electronic coupling are individually tuned through peri
substitutions, core substitutions, and/or π extension in TBA
derivatives. On the basis of reorganization energies, nitrogen doping
impeded the electron transport in TBA analogs owing to significant
structural changes associated with the reduction process. Our approach
employing mapping of dimeric arrangements of TBA, modulated via long
(pitch) and short (roll) axes displacements of the molecular entities,
versus charge transfer coupling disclosed potential charge-transport
regions in addition to the ideal cofacial modes. Charge transport
characteristics of molecular packing arrangements of TBA mimicking
the different orientations of graphene bilayers were analyzed, providing
insights into the possible material applicability of TBA derivatives.
The transition from completely aligned graphitic AA packing sequence
to slip-stacked AB and AA′ stacking domains revealed a dent
in the charge-transport map owing to node–antinode interaction
of the frontier molecular orbitals. TBA analogs encompassing the expanded
π-system materialized highly displaced dimeric orientation from
AB-type packing to occupy a hierarchy favoring higher charge transfer
coupling than the AB type. Thus, realizing stable interchromophoric
arrangements of small organic molecules through chemical or physical
techniques to control their charge-transporting efficiencies is an
indispensable step toward the generation of better organic electronic
devices
Metastable Chiral Azobenzenes Stabilized in a Double Racemate
No full textSelf-assembly of chiral organic chromophores garners huge significance owing to the abundance of supramolecular chirality found in natural systems. We report an interdigitated molecular organization involving axially chiral twisted octabrominated perylenediimide (OBPDI) transferring chiral sense to achiral aromatic moieties. The two‐component crystalline architectures of OBPDI and electron rich aromatic units were facilitated through π-hole•••π based donor-acceptor interactions and the charge transfer characteristics in the ground and excited states of OBPDI cocrystals were established through spectroscopic and theoretical techniques. The OBPDI cocrystals entailed a remarkable homochiral segregation of P and M enantiomers of both the molecular entities in the same crystal system to render twisted double racemic architectures. Synergistically engendered cavities with stored chiral information of twisted OBPDI stabilized higher energy P/M enantiomers of trans‐azobenzene through non-covalent interactions
Modulating singlet fission through interchromophoric rotation
No full textSinglet fission (SF) is a spin-allowed, exciton-multiplying phenomenon that can be utilized to improve the efficiency of organic solar cells. It is well-understood that SF is sensitive to the local crystal morphology and an appropriately balanced coupling is essential to facilitate efficient SF. In this study, we show how the interchromophoric rotation selectively modulates the interaction between the monomer frontier molecular orbitals, promoting both fast and exothermal SF. We evaluate the effective electronic coupling for SF (VSF), the square of which is proportional to the SF rate, and the effective energies of the Frenkel exciton (FE/S1S0) and triplet pair exciton (TT) in a terrylene dimer model. Optimal interplanar rotation of the chromophoric moieties in slip-stacked arrangements pulls the effective energy of the TT state below that of the FE state. Consequently, SF is favored over competing pathways such as excimer formation, thereby enhancing the overall triplet yield. This work represents a step towards improvising the molecular design guidelines for SF and understanding the importance of interchromophoric rotation over the conventional slip-stacked arrangements for achieving favorable intermolecular electronic coupling towards efficient SF
In Silico Exploration for Maximal Charge Transport in Organized Tetrabenzoacenes through Pitch and Roll Displacements
No full textA series of π-conjugated tetrabenzoacenes (TBA), including nitrogen (un)doped derivatives are computationally evaluated to comprehend the correlation between intrinsic structural arrangements and charge transport characteristics. The central charge transport parameters such as reorganization energy and electronic coupling are individually tuned through peri-substitutions, core-substitutions and/or π-extension in TBA derivatives. Based on reorganization energies, nitrogen doping impeded the electron transport in TBA analogs owing to significant structural changes associated with the reduction process. Our approach employing mapping of dimeric arrangements of TBA, modulated via long (pitch) and short (roll) axes displacements of the molecular entities, versus charge transfer coupling disclosed potential charge transport regions in addition to the ideal cofacial modes. Charge transport characteristics of molecular packing arrangements of TBA mimicking the different orientations of graphene bilayers were analyzed, providing insights into the possible material applicability of TBA derivatives. The transition from completely aligned graphitic AA packing sequence to slip-stacked AB and AA’ stacking domains revealed a dent in the charge transport map owing to node-antinode interaction of the frontier molecular orbitals. TBA analogs encompassing expanded π-system materialized highly displaced dimeric orientation from AB type packing to occupy a hierarchy favoring high charge transfer coupling than the AB type. Thus, realizing stable interchromophoric arrangements of small organic molecules through chemical or physical techniques to control their charge transporting efficiencies is an indispensable step towards the generation of better organic electronic devices
