Assembly of fullerenes utilizing self-organization at solvent/non-solvent interfaces or by introducing hydrophobic(philic) end-groups have yielded diverse molecularly-ordered structures. Taking advantage of the multiple π–π interaction between adjacent C60-moieties, structurally elegant and functionally promising fullerene assemblies were synthesized by several groups over the last 20 years. Considerable improvement of device parameters is witnessed if non-covalent fullerene layers assembled using interfacial self assembly are used as electron extraction layer, for example in hybrid solar cells. Thus a pre-determined choice of C60-nanostructures for photovoltaic application stems from an understanding of fullerene–fullerene interaction, a priori from the molecular level. Could we eventually define the outcomes of the last three decades of fullerene aggregation chemistry? This review summarises the course of understanding of optoelectronic properties of fullerene-morphologies, bridging the gap between single molecule and device scale which shall be significant for their judicious implementation in molecular electronics.

Progress and development in structural and optoelectronic tunability of supramolecular nonbonded fullerene assemblies

Morphologies crucially determine the optoelectronic properties of organic semiconductors. Therefore, hierarchical and supramolecular approaches have been developed for targeted design of supramolecular ensembles of organic semiconducting molecules and performance improvement of, e.g., organic solar cells (OSCs), organic light emitting diodes (OLEDs), and organic field-effect transistors (OFETs). We demonstrate how the photonic properties of fullerenes change with the formation of van der Waals aggregates. We identified supramolecular structures with broadly tunable absorption in the visible spectral range and demonstrated how to form aggregates with targeted visible (vis) absorption. To control supramolecular structure formation, we functionalized the C60-backbone with polar (bis-polyethylene glycol malonate-MPEG) tails, thus yielding an amphiphilic fullerene derivative that self-assembles at interfaces. Aggregates of systematically tuned size were obtained from concentrating MPEGC60 in stearic acid matri...

Controlling Electronic Transitions in Fullerene van der Waals Aggregates via Supramolecular Assembly.

It is demonstrated that systematic and designated control of supramolecular nanostructures via interfacial engineering enables (opto)electronic C60‐material properties to be widely adjusted. Interestingly, the lowest unoccupied molecular orbital (LUMO) energies of the same amphiphilic fullerene species are tuned up to 120 meV using supramolecular assembly, competitive to complete molecular change; cf. PC61BM to PC71BM causes a change of 200 meV. Morphology control is achieved through different thin‐film production techniques involving molecular assembly at interfaces, including liquid–liquid interfacial precipitation (LLIP), and Langmuir–Blodgett technique at air–water interface. LLIP enables supramolecularly ordered extended surfaces, yielding the least electronically stable LUMO (ELUMO = −4.28 eV). After qualitatively explaining the observed electrochemical LUMO energy variation for these assemblies with varied molecular packing and aggregate dimensions, an analytical equation is proposed, connecting morphological parameters with LUMO energies with prospects in supramolecular chemistry. To demonstrate the applicability of supramolecular structure–electronic property relations and of supramolecular structure fabrication protocols established in this work to tailor device properties, amorphous‐Si/fullerene hybrid solar cells are built and characterized. It is found that the supramolecular structure variation can be successfully translated to the solar cells, giving rise to a prototype linear relation between LUMO energy and open‐circuit voltage.

Controlling Intermolecular Interactions at Interfaces: Case of Supramolecular Tuning of Fullerene's Electronic Structure

The spectral response of ultrathin films of rhodamine 6G (Rh6G) cast onto polyvinylidenefluoride (PVDF) coated glass slides is studied to investigate a perceived fluorescence emission enhancement. Varying the thickness of the Rh6G layer (submonolayer to multiple layers) on the PVDF layer revealed the existence of multiple Rh6G species on the surface, similar to previous reports on glass. Excitation spectra show that J-type excitons are not responsible for an emission enhancement. Trends in the fluorescence emission intensity show that the surface roughness of the PVDF layer prevents the Rh6G molecules from organizing the way they would on a smooth glass surface. The PVDF surface roughness preserves the emissive monomers and excited-state excimers of Rh6G while reducing the self-quenching of aggregates. Coupled with this is an internal reflection effect that causes light to be trapped between the Rh6G/PVDF and PVDF/glass interfaces. This effect leads to multiple absorption events, and thus more efficient u...

The Influence of Interfacial Effects on the Photophysics of Rhodamine 6G Thin Films on a Poly(vinylidene fluoride) Surface

The fluorescence lifetime is a key property of fluorophores that can be utilized for microenvironment probing, analyte sensing, and multiplexing as well as barcoding applications. For the rational design of lifetime probes and barcodes, theoretical methods have been developed to enable the ab initio prediction of this parameter, which depends strongly on interactions with solvent molecules and other chemical species in the emitters' immediate environment. In this work, we investigate how a conductor-like screening model (COSMO) can account for variations in fluorescence lifetimes that are caused by such fluorophore−solvent interactions. Therefore, we calculate vibrationally broadened fluorescence spectra using the nuclear ensemble method to obtain distorted molecular geometries to sample the electronic transitions with time-dependent density functional theory (TDDFT). The influence of the solvent on fluorescence lifetimes is accounted for with COSMO. For example, for 4-hydroxythiazole fluorophore containing different heteroatoms and acidic and basic moieties in aprotic and protic solvents of varying polarity, this approach was compared to experimentally determined lifetimes in the same solvents. Our results demonstrate a good correlation between theoretically predicted and experimentally measured fluorescence lifetimes except for the polar solvents Ethanol and acetonitrile that can specifically interact with the heteroatoms and the carboxylic acid of the thiazole derivative.

Ab initio prediction of fluorescence lifetimes involving solvent environments by means of COSMO and vibrational broadening

A small library of N,N-diethyl- and -diethanolsulfonamides of differently substituted 2-(2-pyridyl)- and 2-pyrazinyl-4-alkoxythiazoles has been synthesized and investigated in terms of their photophysical properties, electronic structure (quantum chemical calculations at the CAM-B3LYP/6-31+G(d,p) level of theory) and thin film morphology (Langmuir–Blodgett- and spin-cast films). By using the Langmuir–Blodgett technique we show how to exert direct control over the degree of aggregation in thin films made from different thiazole type dyes. In combination with spectroscopic investigations, we gained an in-depth understanding of the influence of aggregation on the electro-optical properties of thin films made of the here-investigated substances.

Tuning the polarity and surface activity of hydroxythiazoles – extending the applicability of highly fluorescent self-assembling chromophores to supra-molecular photonic structures

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Tuning the polarity and surface activity of hydroxythiazoles – extending the applicability of highly fluorescent self-assembling chromophores to supra-molecular photonic structures