Exciton dynamics in self-assembled molecular nanotubes

Photosynthetic systems that exist in plants, algae and some bacteria are Earth’s powerhouse by harnessing its most abundant energy source: sunlight. As one of the key elements, they employ a functional nano-machinery that is typically built from many thousands of autonomously assembled molecules: so-called light-harvesting antennae. In billions of years of evolution, nature has engineered these systems to maximize light-absorption and enable photosynthesis under physiological (warm and humid) conditions that would be considered adverse to most lab-based applications. In order to replicate nature’s design principles for light-harvesting antennae for potential applications, the essential functional elements have to be identified and their working principles understood. Inspired by the success of natural light-harvesting antennae, artificial (man-made) systems that closely resemble the structure of their natural counterparts, have recently experienced great attention. These synthetic analogues feature a high degree of internal homogeneity (i.e., different systems are identical), while also being easier to produce, control and modify than the natural systems. In this Thesis, we study how one of such systems, double-walled molecular nanotubes react to light, how the absorbed energy is transported, and how the energy transport is affected by the structure and dimensionality of the system. Working hand in hand with molecular dynamics simulations and theoretical models, our findings pave the way to optimizing and incorporating such systems in (opto)electronic devices and light-harvesting applications

Molecular versus excitonic disorder in individual artificial light-harvesting systems

Natural light-harvesting antennae employ a dense array of chromophores to optimize energy transport via the formation of delocalized excited states (excitons), which are critically sensitive to spatio-energetic variations of the molecular structure. Identifying the origin and impact of such variations is highly desirable for understanding and predicting functional properties yet hard to achieve due to averaging of many overlapping responses from individual systems. Here, we overcome this problem by measuring the heterogeneity of synthetic analogues of natural antennae-self-assembled molecular nanotubes-by two complementary approaches: single-nanotube photoluminescence spectroscopy and ultrafast 2D correlation. We demonstrate remarkable homogeneity of the nanotube ensemble and reveal that ultrafast (∼50 fs) modulation of the exciton frequencies governs spectral broadening. Using multiscale exciton modeling, we show that the dominance of homogeneous broadening at the exciton level results from exchange narrowing of strong static disorder found for individual molecules within the nanotube. The detailed characterization of static and dynamic disorder at the exciton as well as the molecular level presented here opens new avenues in analyzing and predicting dynamic exciton properties, such as excitation energy transport

Watching Molecular Nanotubes Self-Assemble in Real Time

Molecular self-assembly is a fundamental process in nature that can be used to develop novel functional materials for medical and engineering applications. However, their complex mechanisms make the short-lived stages of self-assembly processes extremely hard to reveal. In this article, we track the self-assembly process of a benchmark system, double-walled molecular nanotubes, whose structure is similar to that found in biological and synthetic systems. We selectively dissolved the outer wall of the double-walled system and used the inner wall as a template for the self-reassembly of the outer wall. The reassembly kinetics were followed in real time using a combination of microfluidics, spectroscopy, cryogenic transmission electron microscopy, molecular dynamics simulations, and exciton modeling. We found that the outer wall self-assembles through a transient disordered patchwork structure: first, several patches of different orientations are formed, and only on a longer time scale will the patches interact with each other and assume their final preferred global orientation. The understanding of patch formation and patch reorientation marks a crucial step toward steering self-assembly processes and subsequent material engineering.</p

Delayed and Accelerated Aging Share Common Longevity Assurance Mechanisms

Mutant dwarf and calorie-restricted mice benefit from healthy aging and unusually long lifespan. In contrast, mouse models for DNA repair-deficient progeroid syndromes age and die prematurely. To identify mechanisms that regulate mammalian longevity, we quantified the parallels between the genome-wide liver expression profiles of mice with those two extremes of lifespan. Contrary to expectation, we find significant, genome-wide expression associations between the progeroid and long-lived mice. Subsequent analysis of significantly over-represented biological processes revealed suppression of the endocrine and energy pathways with increased stress responses in both delayed and premature aging. To test the relevance of these processes in natural aging, we compared the transcriptomes of liver, lung, kidney, and spleen over the entire murine adult lifespan and subsequently confirmed these findings on an independent aging cohort. The majority of genes showed similar expression changes in all four organs, indicating a systemic transcriptional response with aging. This systemic response included the same biological processes that are triggered in progeroid and long-lived mice. However, on a genome-wide scale, transcriptomes of naturally aged mice showed a strong association to progeroid but not to long-lived mice. Thus, endocrine and metabolic changes are indicative of “survival” responses to genotoxic stress or starvation, whereas genome-wide associations in gene expression with natural aging are indicative of biological age, which may thus delineate pro- and anti-aging effects of treatments aimed at health-span extension

Exciton dynamics in self-assembled molecular nanotubes

Photosynthetic systems that exist in plants, algae and some bacteria are Earth’s powerhouse by harnessing its most abundant energy source: sunlight. As one of the key elements, they employ a functional nano-machinery that is typically built from many thousands of autonomously assembled molecules: so-called light-harvesting antennae. In billions of years of evolution, nature has engineered these systems to maximize light-absorption and enable photosynthesis under physiological (warm and humid) conditions that would be considered adverse to most lab-based applications. In order to replicate nature’s design principles for light-harvesting antennae for potential applications, the essential functional elements have to be identified and their working principles understood. Inspired by the success of natural light-harvesting antennae, artificial (man-made) systems that closely resemble the structure of their natural counterparts, have recently experienced great attention. These synthetic analogues feature a high degree of internal homogeneity (i.e., different systems are identical), while also being easier to produce, control and modify than the natural systems. In this Thesis, we study how one of such systems, double-walled molecular nanotubes react to light, how the absorbed energy is transported, and how the energy transport is affected by the structure and dimensionality of the system. Working hand in hand with molecular dynamics simulations and theoretical models, our findings pave the way to optimizing and incorporating such systems in (opto)electronic devices and light-harvesting applications

Ultrafast spectroscopy reveals structural heterogeneity of artificial light-harvesters

Ultrafast 2D spectroscopy is combined with single-object spectroscopy to disentangle the structural heterogeneity of an artificial light-harvester. The dynamically (~50 fs timescale) fluctuating environment governs the system's properties, but not structural variations among different harvesters

Dynamics of molecular aggregate formation:Lab-on-a-chip 2D spectroscopic approach

A novel approach towards unraveling the dynamics of self-assembly of complex molecular systems through time-resolved spectroscopy in conjunction with microfluidics is presented. Results obtained for nanotubular J-aggregates reveal intermediate aggregation species

Steering Self-Assembly of Amphiphilic Molecular Nanostructures via Halogen Exchange

In the field of self-assembly, the quest for gaining control over the supramolecular architecture without affecting the functionality of the individual molecular building blocks is intrinsically challenging. By using a combination of synthetic chemistry, cryogenic transmission electron microscopy, optical absorption measurements, and exciton theory, we demonstrate that halogen exchange in carbocyanine dye molecules allows for fine-tuning the diameter of the self-assembled nanotubes formed by these molecules, while hardly affecting the molecular packing determined by hydrophobic/hydrophilic interactions. Our findings open a unique way to study size effects on the optical properties and exciton dynamics of self-assembled systems under well-controlled conditions