Excitation characteristics of different energy transfer in nanotube-perylene complexes

We report the properties of perylene-nanotube complexes that form efficient energy transfer systems. Most perylene-derivatives yield similar ratios between transfer and direct luminescence (0.66 ± 0.04). The photoluminescence spectra of the free compounds and the transfer complex are similar indicating that perylene and nanotubes act as separate systems. A further increase in interaction yields 40% higher transfer rates and luminescence excitation spectra that indicate a change in stacking of the perylene on the nanotube wall. All measurements are consistent with a transfer mechanism based on a dipole-dipole interaction at a distance much smaller than the Förster radius

Fluorescent Polymer—Single‐Walled Carbon Nanotube Complexes with Charged and Noncharged Dendronized Perylene Bisimides for Bioimaging Studies

Fluorescent nanomaterials are expected to revolutionize medical diagnostic, imaging, and therapeutic tools due to their superior optical and structural properties. Their inefficient water solubility, cell permeability, biodistribution, and high toxicity, however, limit the full potential of their application. To overcome these obstacles, a water‐soluble, fluorescent, cytocompatible polymer—single‐walled carbon nanotube (SWNT) complex is introduced for bioimaging applications. The supramolecular complex consists of an alkylated polymer conjugated with neutral hydroxylated or charged sulfated dendronized perylene bisimides (PBIs) and SWNTs as a general immobilization platform. The polymer backbone solubilizes the SWNTs, decorates them with fluorescent PBIs, and strongly improves their cytocompatibility by wrapping around the SWNT scaffold. In photophysical measurements and biological in vitro studies, sulfated complexes exhibit superior optical properties, cellular uptake, and intracellular staining over their hydroxylated analogs. A toxicity assay confirms the highly improved cytocompatibility of the polymer‐wrapped SWNTs toward surfactant‐solubilized SWNTs. In microscopy studies the complexes allow for the direct imaging of the SWNTs' cellular uptake via the PBI and SWNT emission using the 1st and 2nd optical window for bioimaging. These findings render the polymer‐SWNT complexes with nanometer size, dual fluorescence, multiple charges, and high cytocompatibility as valuable systems for a broad range of fluorescence bioimaging studies

Plasmon‐Assisted Energy Transfer in Hybrid Nanosystems

While direct optical excitation of carbon nanotubes activates only the tube species strictly matching the excitation source, excitation energy transfer processes provide a single excitation channel for all the nanotubes species in a sample. The requirement of an overlap between donor emission and acceptor absorption limits the poll of donors able to trasfer their excitation to the tubes, leaving the high‐energy part of the solar spectrum excluded from such processes. Here it is shown that the grafting of small metal nanoparticles to the tubes alters those rules, enabling energy transfer process from molecules for which the standard energy transfer process is strongly suppressed. The onset of an energy transfer band in the UV/blue spectral region is demonstrated for an hybrid gold‐pyrene‐nanotube system, yielding collective emission from all the tubes present in our samples upon excitation of pyrene

A Comparative Study of Covalent and Non-Covalent Functionalization Approaches

The functionalization of carbon nanotubes (CNTs) is an ongoing and actively researched topic in the scientific community. The modification of tubes through functionalities leads to new physical properties of the tubes and opens the way to real nanotechnological applications. Functionalization methods can be classified into three different classes - endohedral, covalent, and non-covalent approaches. All three methods come with specific advantages and challenges, as I will discuss in the introductory chapters of this thesis. In the first part of this thesis, I will introduce a new covalent functionalization routine that we have developed. It is based on a nitrine based [2+1] cycloaddition reaction and for the very first time in literature, it maintains the extended π-network and preserves the outstanding optoelectronic properties of carbon nanotubes, even at an high degree of functionalization. This new method we developed offers a robust way to attach functional moieties on the tubes and creates a new toolbox for advanced tailoring of the nanotubes properties. In the second part of this work I will describe how non-covalent functionalization methods can be used to attach moieties on the CNTs sidewalls and I will compare their outcome with the results from our new established [2+1] cycloaddition. I will discuss in detail the influence of three different type of functional moieties immobilized to the tubes’ sidewalls: The dipole switch spiropyran, the molecular dye perylene, and gold nanoparticles. The comparative study between covalent and non-covalent functionalization methods shows that functional moieties can be used to strongly influence the optical property of tubes. The study furthermore shows that the tubes’ physical properties are also highly sensitive towards the attachment routine used for immobilization of the functional group. The same moiety yield different effects depending upon how it has been attached onto the tubes

Tailoring the excited-state energy landscape in supramolecular nanostructures

Nature's photosynthetic machinery uses precisely arranged pigment-protein complexes, often representing superstructures, for efficient light-harvesting and transport of excitation energy (excitons) during the initial steps of photosynthesis. This function is achieved by defined electronic Coulomb interactions between the conjugated molecules resulting in tailored excited-state energy landscapes. While such complex natural structures are synthetically difficult to achieve, supramolecular chemistry is now on its advent to realize defined artificial supramolecular nanostructures with tailored functionalities via controlled self-assembly processes of small molecules. In this review, we focus on recent work reporting photophysical studies on self-assembled and hierarchical nanostructures as well as complex superstructures. We discuss how the resulting excited-state energy landscapes influence energy transport. Progress in the field of supramolecular chemistry allows for the realization of distinct kinds of H- or J-aggregates with well-defined morphologies on the mesoscale. Advances in the field of optical spectroscopy and microscopy have permitted to resolve the incoherent/coherent dynamics of exciton transport in such systems down to the level of single nanostructures. Although outstanding diffusion lengths of up to several mu m were found in selected nanostructures, a full understanding of the underlying principles is still missing. In particular, the unavoidable structural and electronic disorder in these systems influences the excited-state energy landscapes and thus the transport characteristics, which can be exploited to refine the molecular design criteria of supramolecular nanostructures and complex superstructures. Despite the rapid progress in the field of functional supramolecular nanostructures, we believe that revealing the full potential of such systems is far from complete. In particular, criteria for tailored and optimized (hierarchical) supramolecular nanostructures in view of applications are not yet established. Finally, we outline current challenges and future perspectives for optical and optoelectronic applications of supramolecular nanostructures

Self-Assembly of Optical Molecules with Supramolecular Concepts

Fabrication of nano-sized objects is one of the most important issues in nanoscience and nanotechnology. Soft nanomaterials with flexible properties have been given much attention and can be obtained through bottom-up processing from functional molecules, where self-assembly based on supramolecular chemistry and designed assembly have become crucial processes and techniques. Among the various functional molecules, dyes have become important materials in certain areas of nanotechnology and their self-assembling behaviors have been actively researched. In this short review, we briefly introduce recent progress in self-assembly of optical molecules and dyes, based mainly on supramolecular concepts. The introduced examples are classified into four categories: self-assembly of (i) low-molecular-weight dyes and (ii) polymeric dyes and dye self-assembly (iii) in nanoscale architectures and (iv) at surfaces

Optoelectronic properties and energy transport processes in cylindrical J-aggregates

textThe light harvesting systems of photosynthetic organisms harness solar energy by efficient light capture and subsequent transport of the light’s energy to a chemical reaction center. Man-made optical devices could benefit by mimicking these naturally occurring light harvesting processes. Supramolecular organic nanostructures, composed of the amphiphilic carbocyanine dye 3,3’-bis- (2-sulfopropyl)-5,5’,6,6’-tetrachloro-1,1’- dioctylbenzimida-carbocyanine (C8S3), self assemble in aqueous solution to form tubular, double-walled J-aggregates. These J-aggregates have drawn comparisons to light harvesting systems, owing to their optical and structural similarities to the cylindrical chlorosomes (antenna) from green sulfur bacteria. This research utilizes optical spectroscopy and microscopy to study the supramolecular origins of the exciton transitions and fundamental nature of exciton energy transport in C8S3 artificial light harvesting systems. Two J-aggregate morphologies are investigated: well-separated, double-walled nanotubes and bundles of agglomerated nanotubes. Linear dichroism spectroscopy of flow-aligned nanotubes is used to generate the first quantitative, polarized model for the complicated C8S3 nanotube excitonic absorption spectrum that is consistent with theoretical predictions. The C8S3 J-aggregate photophysical properties are further explored, as the Stokes shift, quantum yield, and spectral line broadening are measured as a function of temperature from 77 – 298 K. The temperature-dependent emission ratios of the C8S3 J-aggregate two-band fluorescence spectra reveal that nanotube emission is well described with Boltzmann partitioning between states, while the bundles’ is not. Finally, understanding energy transport in these materials is critical for the proposed use of artificial light harvesting systems in optoelectronic devices. The spatial extent of energy transfer in individual C8S3 J- aggregate structures is directly determined using fluorescence imaging. We find that aggregate structural hierarchy greatly influences exciton transport distances: impressive average exciton migration distances of ~ 150 nm are measured along the nanotubes, while these distances increase to over 500 nm in the bundle superstructures.Chemistr

Dipole-switch induced modification of the emissive response of carbonnanotubes

The interaction of carbon nanotubes with the molecular dipole switch spiropyran is expected to affect the optical response of the tubes. Until now, the need of anchor groups to immobilize the switches on the tubes has hindered the experimental observation of the effects of switching on the emission behavior of the tubes. Here we present spiropyran-carbon nanotube complexes obtained by micelle swelling. This method does not require any anchor nor sophisticated chemistry to warrant close tube-switch proximity. For the first time, we observe the shifts predicted theoretically and their effect on the tubes' excitation and emission energies

GUMBOS and NanoGUMBOS: Applications as Photosensitizers in Dye-sensitized Solar Cells

Renewable energy is a major concern due to increased world energy consumption. In particular, solar energy is a type of renewable energy source that uses devices known as solar cells to convert sunlight to electricity. Specifically, devices referred to as dye-sensitized solar cells (DSSCs) employ dyes to absorb solar energy. Dyes derived from ruthenium complexes have been typically used in DSSCs. Unfortunately, several disadvantages are associated with current ruthenium complex photosensitizers, which can be attributed to limited supply and expense of metals, as well as reduced absorption in the near-infrared region of the electromagnetic spectrum. Accordingly, this dissertation is a discussion of novel dyes referred to as group of uniform materials based on organic salts (GUMBOS) for application as photosensitizers in DSSCs. These GUMBOS are solid phase organic salts composed of bulky ions that have melting points from 25°C to 250°C. Importantly, GUMBOS can be tuned for multiple functions based on selected ions resulting in interesting physiochemical properties. In addition, nanomaterials derived from GUMBOS (nanoGUMBOS) can also result in significant properties. The first part of this dissertation involves the synthesis and characterization of nanoGUMBOS from cyanine dyes. These nanomaterials are prepared via a facile self-assembly approach, and spectral and electrochemical properties are investigated. In one study, controlled properties of cyanine-based nanoGUMBOS are found to be dependent on the counterion associated with the cationic dye. In another study, GUMBOS derived from cyanine dyes with increasing methine chain lengths are synthesized. In addition, binary nanomaterials consisting of two different cyanine methine chain length GUMBOS are prepared. The effect of Förster resonance energy transfer between these latter nanomaterials enhances fluorescence into the near-infrared region of the electromagnetic spectrum. The individual and binary nanoGUMBOS offer possible use as sensitizers that extend into the near-infrared region of the electromagnetic spectrum. The second part of this dissertation entails the incorporation of cyanine-based GUMBOS and nanoGUMBOS into DSSCs. In this study, various preparation methods are used for formation of titanium dioxide semiconductor electrodes. Solar cells comprised of these electrodes and cyanine-based GUMBOS are fabricated, and the performances of these DSSCs are investigated

Photophysics of perylene diimides in solutions and self-assembled films

textNew perylene diimide based dendrimers were synthesized by the divergent synthetic method. Diamines with different lengths and methyl acrylate (or t-butyl acrylate) were used as the extension regents for the dendrimer growth steps. The photophysics of these synthesized PDIs was investigated with and without adding TFA. Intramolecular fluorescence quenching was observed by comparing the fluorescence intensity and lifetime before and after adding TFA, demonstrating a stronger quenching process for those PDIs having 2 or 3 carbons between the two nitrogens of the diamines. A 6 or 7 member ring mechanism for intramolecular electron transfer was presented. Photophysics of new water soluble perylene diimide, anionic n-PDI, CAS [694438-88-5] and cationic p-PDI, CAS [817207-4-7], was investigated in different media. The absorption spectra and quantum yield were modified by the presence of these reagents in a manner that suggests that weakly interacting complexes (such as H- or Jaggregates) are present in pure water and that complex formation can be enhanced by vii adding NaCl and polyelectrolytes or diminished by interacting with surfactants such as SDS and DTAC. A molecular layer-by-layer (MLBL) deposition process has been carried out for n-PDI and p-PDI through the strong π−π and electrostatic interactions. The fluorescence of these films shows an alternation of intensity according to which perylene species is on the outer layer, which is interpreted as the effect of facile energy transfer between the perylenes. Energy transfer in these films was also characterized by studying fluorescence quenching produced by the deposition of a single energy trapping layer (n-TDI, CAS[862852-56-0]). The MLBL technique can be utilized for other multiply-charged water soluble conjugated dye molecules such as n-PSA (CAS[59572-10-0]) and n-TDI, thereby demonstrating considerable potential for making organic semiconductor films. Polyelectrolyte films containing n-PDI and p-PDI were prepared from PSS and PDAC using the layer-by-layer (LBL) methodology. The optical density and fluorescence intensity of the LBL films grows linearly with the number of layers and we do not observe extraction of the PDI by subsequent polyelectrolyte deposition in the presence of 0.5M NaCl. The PDI fluorescence is substantially quenched in these films which we interpret as a self-quenching effect, enhanced by inter- and intra-layer energy transfer.Chemistry and BiochemistryChemistr