Probing Intramolecular Förster Resonance Energy Transfer in a Naphthaleneimide−Peryleneimide−Terrylenediimide-Based Dendrimer by Ensemble and Single-Molecule Fluorescence Spectroscopy

Efficient Energy Transfer in a Rylene Imide-Based Heterodimer: The Role of Intramolecular Electronic Coupling

The development of antenna molecules with simplified structures can effectively avoid the complex exciton dynamics resulting from conformational mobility. Two distinct heterodimers TP and TBP comprising a perylenediimide (PDI) donor and terrylenediimide (TDI) acting as an energy sink were investigated. Tuned by varying functionalization positions, the bay-to-bay-linked TP offers a strong chromophore coupling, while the bay-to-N-linked TBP exhibits a weak chromophore coupling. Using transient absorption spectroscopy, we found that TP underwent ultrafast vibrational relaxation (τ_VR < 400 fs) from upper vibrational energy levels of the singlet states after pumping at 490 nm, and followed by electron transfer (ET, τ_ET = 2.5 ps) from TDI to PDI. TBP exhibited ultrafast excitation energy transfer (EET, τ_EET = 0.48 ± 0.1 ps) from the excited PDI donor to TDI acceptor, and the subsequent charge transfer (CT) process was almost quenched. This result provides insight into designing novel small molecules capable of efficient energy transfer.

Chromophoric Dendrimer-Based Materials: An Overview of Holistic-Integrated Molecular Systems for Fluorescence Resonance Energy Transfer (FRET) Phenomenon

Dendrimers (from the Greek dendros → tree; meros → part) are macromolecules with well-defined three-dimensional and tree-like structures. Remarkably, this hyperbranched architecture is one of the most ubiquitous, prolific, and recognizable natural patterns observed in nature. The rational design and the synthesis of highly functionalized architectures have been motivated by the need to mimic synthetic and natural-light-induced energy processes. Dendrimers offer an attractive material scaffold to generate innovative, technological, and functional materials because they provide a high amount of peripherally functional groups and void nanoreservoirs. Therefore, dendrimers emerge as excellent candidates since they can play a highly relevant role as unimolecular reactors at the nanoscale, acting as versatile and sophisticated entities. In particular, they can play a key role in the properties of light-energy harvesting and non-radiative energy transfer, allowing them to function as a whole unit. Remarkably, it is possible to promote the occurrence of the FRET phenomenon to concentrate the absorbed energy in photoactive centers. Finally, we think an in-depth understanding of this mechanism allows for diverse and prolific technological applications, such as imaging, biomedical therapy, and the conversion and storage of light energy, among others.

Multi-stimuli programmable FRET based RGB absorbing antennae towards ratiometric temperature, pH and multiple metal ion sensing

A red-green-blue (RGB) multichromophoric antenna 1 consisting of energy donors naphthalimides and perylenediimides and a central aza-BODIPY energy acceptor along with two subchromophoric red-blue (RB 6) and green-blue (GB 12) antennae was designed that showed efficient cascade Förster resonance energy transfer (FRET). RGB antenna 1 showed pronounced temperature-dependent emission behaviour where emission intensities in green and red channels could be tuned in opposite directions by temperature giving rise to unique ratiometric sensing with a temperature sensitivity of 0.4% °C. RGB antenna 1 showed reversible absorption modulation selectively in the blue region (RGB ↔ RG) upon acid/base addition giving rise to pH sensing behaviour. Furthermore, RGB antenna 1 was utilized to selectively sense metal ions such as Co2+ and Fe3+ through a FRET turn-off mechanism induced by a redox process at the aza-BODIPY site that resulted in the selective spectral modulation of the red band (i.e., RGB → GB). Model antenna RB 6 showed white light emission with chromaticity coordinates (0.32, 0.33) on acid addition. Antennae 1, 6 and 12 also exhibited solution state electrochromic switching characterized by distinct colour changes upon changing the potential. Finally, antennae 1, 6 and 12 served as reversible fluorescent inks in PMMA/antenna blends whereby the emission colours could be switched or tuned using different stimuli such as acid vapour, temperature and metal ions.

Single-Molecule Study of Redox Reaction Kinetics by Observing Fluorescence Blinking

Recent advances in fluorescence microscopy allow us to track chemical reactions at the single-molecule level. Single-molecule measurements make it possible to minimize the amount of sample needed for analysis and diagnosis. Signal amplification is often applied to ultralow-level biomarker detection. Polymerase chain reaction (PCR) is used to detect DNA/RNA, and enzyme-linked immunosorbent assay (ELISA) can sensitively probe antigen-antibody interactions. While these techniques are brilliant and will continue to be used in the future, single-molecule-level measurements would allow us to reduce the time and cost needed to amplify signals.The kinetics of chemical reactions have been studied mainly using ensemble-averaged methods. However, they can hardly distinguish time-dependent fluctuations and static heterogeneity of the kinetics. The information hidden in ensemble-averaged measurements would be extractable from a single-molecule experiment. Thus, single-molecule measurement would provide unique opportunities to investigate unrevealed phenomena and to elucidate the questions in chemistry, physics, and life sciences. Redox reaction, which is triggered by electron transfer, is among the most fundamental and ubiquitous chemical reactions. The redox reaction of a fluorescent molecule results in the formation of radical ions, which are normally nonemissive. In single-molecule-level measurements, the redox reaction causes the fluctuation of fluorescence signals between the bright ON-state and the dark OFF-state, in a phenomenon called blinking. The duration of the OFF-state (τ_OFF) corresponds to the lifetime of the radical ion state, and its reaction kinetics can be measured as 1/τ_OFF. Thus, the kinetics of redox reactions of fluorescent molecules can be accessed at the single-molecule level by monitoring fluorescence blinking. One of the key aspects of single-molecule analysis based on blinking is its robustness. A blinking signal with a certain regular pattern enables single fluorescent molecules to be distinguished and resolved from the random background signal.In this Account, we summarize the recent studies on the single-molecule measurement of redox reaction kinetics, with a focus on our group's recent progress. We first introduce the control of redox blinking to increase the photostability of fluorescent molecules. We then demonstrate the control of redox blinking, which allows us to detect target DNA by monitoring the function of a molecular beacon-type probe, and we investigate antigen-antibody interactions at the single-molecule level. By tracing the time-dependent changes in blinking patterns, redox blinking is shown to be adaptable to tracking the structural switching dynamics of RNA, the preQ₁ riboswitch. This Account ends with a discussion of our ongoing work on the control of fluorescent blinking. We also discuss the development of devices that allow single-molecule-level analysis in a high-throughput fashion.

Single-molecule observation of diffusion and catalysis in nanoporous solids

Nanoporous solids, including microporous, mesoporous and hierarchically structured porous materials, are of scientific and technological interest because of their high surface-to-volume ratio and ability to impose shape- and size-selectivity on molecules diffusing through them. Enormous efforts have been put in the mechanistic understanding of diffusion–reaction relationships of nanoporous solids, with the ultimate goal of developing materials with improved catalytic performance. Single-molecule localization microscopy can be used to explore the pore space via the trajectories of individual molecules. This ensemble-free perspective directly reveals heterogeneities in diffusion and diffusion-related reactivity of individual molecules, which would have been obscured in bulk measurements. In this article, we review developments in the spatial and temporal characterization of nanoporous solids using single-molecule localization microscopy. We illustrate various aspects of this approach, and showcase how it can be used to follow molecular diffusion and reaction behaviors in nanoporous solids.

Rational Design of Bright Long Fluorescence Lifetime Dyad Fluorophores for Single Molecule Imaging and Detection

Increasing demand for detecting single molecules in challenging environments has raised the bar for the fluorophores used. To achieve better resolution and/or contrast in fluorescence microscopy, it is now essential to use bright and stable dyes with tailored photophysical properties. While long fluorescence lifetime fluorophores offer many advantages in time-resolved imaging, their inherently lower molar absorption coefficient has limited applications in single molecule imaging. Here we propose a generic approach to prepare bright, long fluorescence lifetime dyad fluorophores comprising an absorbing antenna chromophore with high absorption coefficient linked to an acceptor emitter with a long fluorescence lifetime. We introduce a dyad consisting of a perylene antenna and a triangulenium emitter with 100% energy transfer from donor to acceptor. The dyad retained the long fluorescence lifetime (∼17 ns) and high quantum yield (75%) of the triangulenium emitter, while the perylene antenna increased the molar absorption coefficient (up to 5 times) in comparison to the free triangulenium dye. These triangulenium based dyads with significantly improved brightness can now be detected at the single molecule level and easily discriminated from bright autofluorescence by time-gated and other lifetime-based detection schemes.

Diels-Alder Cycloaddition to the Bay Region of Perylene and Its Derivatives as an Attractive Strategy for PAH Core Expansion: Theoretical and Practical Aspects

PAHs (polycyclic aromatics hydrocarbons), the compound group that contains perylene and its derivatives, including functionalized ones, have attracted a great deal of interest in many fields of science and modern technology. This review presents all of the research devoted to modifications of PAHs that are realized via the Diels–Alder (DA) cycloaddition of various dienophiles to the bay regions of PAHs, leading to the π-extension of the starting molecule. This type of annulative π-extension (APEX) strategy has emerged as a powerful and efficient synthetic method for the construction of polycyclic aromatic hydrocarbons and their functionalized derivatives, nanographenes, and π-extended fused heteroarenes. Then, [4 + 2] cycloadditions of ethylenic dienophiles, -N=N-, i.e., diazo-dienophiles and acetylenic dienophiles, are presented. This subject is discussed from the organic synthesis point of view but supported by theoretical calculations. The possible applications of DA cycloaddition to PAH bay regions in various science and technology areas, and the prospects for the development of this synthetic method, are also discussed.

Synthetic strategies tailoring colours in multichromophoric organic nanostructures

Mimicking nature to develop light-harvesting materials is a timely challenge. This tutorial review examines the chemical strategies to engineer and customise innovative multi-coloured architectures with specific light-absorbing and emitting properties.

Single-molecule excitation-emission spectroscopy

Single-molecule spectroscopy (SMS) provides a detailed view of individual emitter properties and local environments without having to resort to ensemble averaging. While the last several decades have seen substantial refinement of SMS techniques, recording excitation spectra of single emitters still poses a significant challenge. Here we address this problem by demonstrating simultaneous collection of fluorescence emission and excitation spectra using a compact common-path interferometer and broadband excitation, which is implemented as an extension of a standard SMS microscope. We demonstrate the technique by simultaneously collecting room-temperature excitation and emission spectra of individual terrylene diimide molecules and donor-acceptor dyads embedded in polystyrene. We analyze the resulting spectral parameters in terms of optical lineshape theory to obtain detailed information on the interactions of the emitters with their nanoscopic environment. This analysis finally reveals that environmental fluctuations between the donor and acceptor in the dyads are not correlated.

The Preprotein Binding Domain of SecA Displays Intrinsic Rotational Dynamics

SecA converts ATP energy to protein translocation work. Together with the membrane-embedded SecY channel it forms the bacterial protein translocase. How secretory proteins bind to SecA and drive conformational cascades to promote their secretion remains unknown. To address this, we focus on the preprotein binding domain (PBD) of SecA. PBD crystalizes in three distinct states, swiveling around its narrow stem. Here, we examined whether PBD displays intrinsic dynamics in solution using single-molecule Förster resonance energy transfer (smFRET). Unique cysteinyl pairs on PBD and apposed domains were labeled with donor/acceptor dyes. Derivatives were analyzed using pulsed interleaved excitation and multi-parameter fluorescence detection. The PBD undergoes significant rotational motions, occupying at least three distinct states in dimeric and four in monomeric soluble SecA. Nucleotides do not affect smFRET-detectable PBD dynamics. These findings lay the foundations for single-molecule dissection of translocase mechanics and suggest models for possible PBD involvement during catalysis.

Terrylenediimide-Based Intrinsic Theranostic Nanomedicines with High Photothermal Conversion Efficiency for Photoacoustic Imaging-Guided Cancer Therapy

Activatable theranostic nanomedicines involved in photothermal therapy (PTT) have received constant attention as promising alternatives to traditional therapies in clinic. However, the theranostic nanomedicines widely suffer from instability and complicated nanostructures, which hamper potential clinical applications. Herein, we demonstrated a terrylenediimide (TDI)-poly(acrylic acid) (TPA)-based nanomedicine (TNM) platform used as an intrinsic theranostic agent. As an exploratory paradigm in seeking biomedical applications, TDI was modified with poly(acrylic acid)s (PAAs), resulting in eight-armed, star-like TPAs composed of an outside hydrophilic PAA corona and an inner hydrophobic TDI core. TNMs were readily fabricated via spontaneous self-assembly. Without additional vehicle and cargo, the as-prepared TNMs possessed a robust nanostructure and high photothermal conversion efficiency up to approximately 41%. The intrinsic theranostic properties of TNMs for use in photoacoustic (PA) imaging by a multispectral optoacoustic tomography system and in mediating photoinduced tumor ablation were intensely explored. Our results suggested that the TNMs could be successfully exploited as intrinsic theranostic agents for PA imaging-guided efficient tumor PTT. Thus, these TNMs hold great potential for (pre)clinical translational development.

Single-molecule FRET unveils induced-fit mechanism for substrate selectivity in flap endonuclease 1

Human flap endonuclease 1 (FEN1) and related structure-specific 5’nucleases precisely identify and incise aberrant DNA structures during replication, repair and recombination to avoid genomic instability. Yet, it is unclear how the 5’nuclease mechanisms of DNA distortion and protein ordering robustly mediate efficient and accurate substrate recognition and catalytic selectivity. Here, single-molecule sub-millisecond and millisecond analyses of FEN1 reveal a protein-DNA induced-fit mechanism that efficiently verifies substrate and suppresses off-target cleavage. FEN1 sculpts DNA with diffusion-limited kinetics to test DNA substrate. This DNA distortion mutually ‘locks’ protein and DNA conformation and enables substrate verification with extreme precision. Strikingly, FEN1 never misses cleavage of its cognate substrate while blocking probable formation of catalytically competent interactions with noncognate substrates and fostering their pre-incision dissociation. These findings establish FEN1 has practically perfect precision and that separate control of induced-fit substrate recognition sets up the catalytic selectivity of the nuclease active site for genome stability.

DOI:http://dx.doi.org/10.7554/eLife.21884.001

When a cell divides it must copy its genetic information, which is found in the form of strands of DNA. Damage to the DNA may lead to cancer or a number of genetic diseases. However, every time a cell divides more than 10 million toxic “flaps” of excess DNA are generated. A protein called flap endonuclease 1 (FEN1) keeps the DNA in good repair by cutting off the flaps in a highly specific and selective manner.

Many proteins that interact with DNA are attracted to specific genetic sequences within the DNA strands. However, this is not the case for FEN1 and several other “structure-specific” proteins that help to repair and replicate DNA strands. So how do these proteins select the correct regions of DNA to interact with?

Rashid et al. used single-molecule fluorescence measurements to examine how purified FEN1 proteins interact with DNA flaps. The results show that FEN1 can perfectly recognize and correctly remove flaps through a process called “mutual-induced fit”, where the DNA and FEN1 are shaped by each other to produce a highly specific structure.

Further work is now needed to examine whether other proteins that are related to FEN1 use a similar process to ensure that they always cut DNA in the same way. More detailed and direct examination of the structure of FEN1 through other experimental methods may also help to reveal how the mutual-induced fit process enables FEN1 to achieve such high levels of precision. This could increase our understanding of how problems with FEN1 and similar proteins lead to different genetic diseases.

DOI:http://dx.doi.org/10.7554/eLife.21884.002

Tailoring Colors by O Annulation of Polycyclic Aromatic Hydrocarbons

The synthesis of O-doped polyaromatic hydro- carbons in which two polycyclic aromatic hydrocarbon sub units are bridged through one or two O atoms has been achieved. This includes high-yield ring-closure key steps that, depending on the reaction conditions, result in the formation of furanyl or pyranopyranyl linkages through intramolecular C-O bond formation. Comprehensive photophysical measurements in solution showed that these compounds have exceptionally high emission yields and tunable absorption properties throughout the UV/Vis spectral region. Electrochemical investigations showed that in all cases O annulation increases the electron-donor capabilities by raising the HOMO energy level, whereas the LUMO energy level is less affected. Moreover, third-order nonlinear optical (NLO) measurements on solutions or thin films containing the dyes showed very good values of the second hyperpolarizability. Importantly, poly(methyl methacrylate) films containing the pyranopyranyl derivatives exhibited weak linear absorption and NLO absorption compared to the nonlinearity and NLO refraction, respectively, and thus revealed them to be exceptional organic materials for photonic devices.

Two-step FRET mediated metal ion induced signalling responses in a probe appended with three fluorophores

A tri-fluorophore appended Tren receptor based probe exhibited chelation induced ratiometric fluorescence signalling through a two-step FRET process; enhancement of F_An→ F_Rhenergy transfer efficiency through an F_NBDintermediate was observed.

Insight into a reversible energy transfer system

Resonance energy transfer (RET) processes have wide applications; these processes involve a unidirectional energy transfer from a particular donor to a particular acceptor. Here, we report a plasmonic resonance energy transfer (PRET), which occurs from the surface of gold nanoparticles to fluorescent organic dyes, and coexists with a nanometal surface energy transfer (NSET) that operates in the reverse direction. The coexistence of both PRET and NSET in opposite directions means that the roles of both donor and acceptor can be interchanged, which could be identified by using spectrofluorometric measurements and light scattering dark field microscopic imaging. The experimental data could be further theoretically supported using Persson and Lang's model, the quasi-static approximation and finite-difference time-domain simulation. Moreover, disruption of the PRET process by altering the energy transfer pairs suggests that interactions occur inside the reversible energy transfer system, which manifest by increasing the fluorescence quenching efficiency of the NSET process.

Multichromophoric π-Conjugation: Modular Design for Gated and Cascade Energy Transfer

Multichromophore arrays allow for cascade energy transfer. As an isoelectronic analogue of indacenyl, bis(triazolo)benzene features a fused tricyclic skeleton that rigidly places two π-extended triazoles in close proximity. Such triazole-based fluorophores behave as electronically independent modules in the ground states, but become tightly coupled upon photoexcitation for highly efficient excitation energy transfer (EET) that can be gated by external stimuli. Taking this donor-acceptor fluorophore system a step further, we have designed and implemented a cascade EET. Here, the initial excitation takes part in a circular relay to arrive at the longest-wavelength emitting site as the final destination. Modularly constructed triazoloarenes should serve as versatile platforms for chemically controlled optical signaling.

Tunable and highly efficient light-harvesting antenna systems based on 1,7-perylene-3,4,9,10-tetracarboxylic acid derivatives

Efficient harvesting of solar energy, without interference from electron transfer, is reported for a series of bichromophoric light-harvesting antenna molecules.

We report the synthesis and excited-state dynamics of a series of five bichromophoric light-harvesting antenna systems, which are capable of efficient harvesting of solar energy in the spectral range of 350–580 nm. These antenna systems have been synthesized in a modular fashion by the covalent attachment of blue light absorbing naphthalene monoimide energy donors (D1, D2, and D3) to green light absorbing perylene-3,4,9,10-tetracarboxylic acid derived energy acceptors, 1,7-perylene-3,4,9,10-tetracarboxylic tetrabutylester (A1), 1,7-perylene-3,4,9,10-tetracarboxylic monoimide dibutylester (A2), and 1,7-perylene-3,4,9,10-tetracarboxylic bisimide (A3). The energy donors have been linked at the 1,7-bay-positions of the perylene derivatives, thus leaving the peri positions free for further functionalization and device construction. A highly stable and rigid structure, with no electronic communication between the donor and acceptor components, has been realized via an all-aromatic non-conjugated phenoxy spacer between the constituent chromophores. The selection of donor naphthalene derivatives for attachment with perylene derivatives was based on the effective matching of their respective optical properties to achieve efficient excitation energy transfer (EET) by the Förster mechanism. A comprehensive study of the excited-state dynamics, in toluene, revealed quantitative and ultrafast (ca. 1 ps) intramolecular EET from donor naphthalene chromophores to the acceptor perylenes in all the studied systems. Electron transfer from the donor naphthalene chromophores to the acceptor perylenes has not been observed, not even for antenna systems in which this process is thermodynamically allowed. Due to the combination of an efficient and fast energy transfer along with broad absorption in the visible region, these antenna systems are promising materials for solar-to-electric and solar-to-fuel devices.

In Search for the Best Environment for Single Molecule Studies: Photostability of Single Terrylenediimide Molecules in Various Polymer Matrices

Photobleaching is the main limiting factor in single molecule studies by optical techniques. We investigated the dependence of photostability of terrylene diimide (TDI) derivative on its environment using confocal fluorescence microscopy. Seven different polymers were tested. Depending on the matrix, photobleaching quantum yields vary by 2 orders of magnitude. Their values correlate with parameters characterizing oxygen mobility in polymers: diffusion coefficient and permeability. Poly(vinyl chloride) (PVC) and poly(methyl methacrylate) (PMMA) exhibit the lowest photodestruction quantum yields. Additional enhancement of photostability can be achieved by aging of PVC or by flushing the sample with nitrogen, which confirms the involvement of oxygen in photodestruction. Different character of the time traces of the intensity of emission from single TDI molecules is observed for different polymer matrices, ranging from intense blinking in the least stable polycarbonate, to practically no blinking in the most stable PVC. These results suggest a photodegradation mechanism involving self-sensitized photooxidation in oxygen complexes of TDI.

Artificial light-harvesting arrays for solar energy conversion

Following natures' blueprint, the concept of artificial light-harvesting antennae is discussed in terms of sophisticated molecular arrays displaying a tailored cascade of electronic energy transfer steps.

Synthesis and use of “clickable” bay-region tetrasubstituted perylene tetracarboxylic tetraesters and a perylene monoimide diester as energy acceptors

Novel perylene derivatives are ready to be used as functional energy acceptors in light-harvesting systems.

Energy flow dynamics within cofacial and slip-stacked perylene-3,4-dicarboximide dimer models of π-aggregates

Robust perylene-3,4-dicarboximide (PMI) π-aggregates provide important light-harvesting and electron-hole pair generation advantages in organic photovoltaics and related applications, but relatively few studies have focused on the electronic interactions between PMI chromophores. In contrast, structure-function relationships based on π-π stacking in the related perylene-3,4:9,10-bis(dicarboximides) (PDIs) have been widely investigated. The performance of both PMI and PDI derivatives in organic devices may be limited by the formation of low-energy excimer trap states in morphologies where interchromophore coupling is strong. Here, five covalently bound PMI dimers with varying degrees of electronic interaction were studied to probe the relative chromophore orientations that lead to excimer energy trap states. Femtosecond near-infrared transient absorption spectroscopy was used to observe the growth of a low-energy transition at ~1450-1520 nm characteristic of the excimer state in these covalent dimers. The excimer-state absorption appears in ~1 ps, followed by conformational relaxation over 8-17 ps. The excimer state then decays in 6.9-12.8 ns, as measured by time-resolved fluorescence spectroscopy. The excimer lifetimes reach a maximum for a slip-stacked geometry in which the two PMI molecules are displaced along their long axes by one phenyl group (~4.3 Å). Additional displacement of the PMIs by a biphenyl spacer along the long axis prevents excimer formation. Symmetry-breaking charge transfer is not observed in any of the PMI dimers, and only a small triplet yield (<5%) is observed for the cofacial PMI dimers. These data provide structural insights for minimizing excimer trap states in organic devices based on PMI derivatives.

Chromophoric and dendritic phosphoramidites enable construction of functional dendrimers with exceptional brightness and water solubility

The fluorescence brightness of a molecular probe determines whether it can be effectively measured and its water solubility dictates if it can be applied in real-world biological systems. However, molecules brighter than the most efficient fluorescent dyes or particles brighter than quantum dots are hard to come by, especially when they must also be soluble in water. In this report, chromophoric phosphoramidites are used in a solid-state synthesis to construct functional dendrimers. When highly twisted chromophores are chosen and the proper spacers and dendrons are introduced, the resultant dendrimers emit exceptionally bright fluorescence. Chromophores, spacers, and dendrons are stitched together by efficient phosphoramidite reagents, which afford high-yield water-soluble phosphodiester linkages after deprotection. The resulting water-soluble dendrimers are exceptionally bright.

Evolution of graphene molecules: structural and functional complexity as driving forces behind nanoscience

The evolution of nanoscience is based on the ability of the fields of chemistry and physics to share competencies through mutually beneficial collaborations. With this in mind, in this Perspective, I describe three classes of compounds: rylene dyes, polyphenylene dendrimers, as well as nanographene molecules and graphene nanoribbons, which have provided a superb platform to nurture these relationships. The synthesis of these complex structures is demanding but also rewarding because they stimulate unique investigations at the single-molecule level by scanning tunneling microscopy and single-molecule spectroscopy. There are close functional and structural relationships between the molecules chosen. In particular, rylenes and nanographenes can be regarded as honeycomb-type, discoid species composed of fused benzene rings. The benzene ring can thus be regarded as a universal modular building block. Polyphenylene dendrimers serve, first, as a scaffold for dyes en route to multichromophoric systems and, second, as chemical precursors for graphene synthesis. Through chemical design, it is possible to tune the properties of these systems at the single-molecule level and to achieve nanoscale control over their self-assembly to form multifunctional (nano)materials.

Self-Assembly of Supramolecular Light-Harvesting Arrays from Symmetric Perylene-3,4-dicarboximide Trefoils

Unlike the widely studied perylene-3,4:9,10-bis(dicarboximide) (PDI) dyes, self-assembly of the corresponding perylene-3,4-dicarboximide (PMI) dyes into large arrays and studies of their excited state properties have received far less attention. Two symmetric PMI trefoils were synthesized by connecting the 9-position of the perylene core either directly (1) or through a phenylene linker (2) to the 1,3,5-positions of a central benzene ring. Synchrotron-based small- and wide-angle X-ray scattering measurements in methylcyclohexane show that trefoil 1 self-assembles into cofacial trimers (13) on average, while trefoil 2 forms much larger assemblies that are tridecamers (213) on average. Their photophysics were characterized using steady-state as well as transient absorption and emission spectroscopy. Time-resolved spectroscopy reveals that both 13 and 213 initially form excitonically coupled excited states that subsequently relax to excimer states having 20 and 8.4 ns lifetimes, respectively, which decay to ground-state primarily nonradiatively. The data are consistent with stronger electronic coupling between the PMI molecules in 213 relative to 13.