Exceptionally high charge mobility in phthalocyanine-based poly(benzimidazobenzophenanthroline)-ladder-type two-dimensional conjugated polymers.

Two-dimensional conjugated polymers (2DCPs), composed of multiple strands of linear conjugated polymers with extended in-plane π-conjugation, are emerging crystalline semiconducting polymers for organic (opto)electronics. They are represented by two-dimensional π-conjugated covalent organic frameworks, which typically suffer from poor π-conjugation and thus low charge carrier mobilities. Here we overcome this limitation by demonstrating two semiconducting phthalocyanine-based poly(benzimidazobenzophenanthroline)-ladder-type 2DCPs (2DCP-MPc, with M = Cu or Ni), which are constructed from octaaminophthalocyaninato metal(II) and naphthalenetetracarboxylic dianhydride by polycondensation under solvothermal conditions. The 2DCP-MPcs exhibit optical bandgaps of ~1.3 eV with highly delocalized π-electrons. Density functional theory calculations unveil strongly dispersive energy bands with small electron-hole reduced effective masses of ~0.15m0 for the layer-stacked 2DCP-MPcs. Terahertz spectroscopy reveals the band transport of Drude-type free carriers in 2DCP-MPcs with exceptionally high sum mobility of electrons and holes of ~970 cm2 V-1 s-1 at room temperature, surpassing that of the reported linear conjugated polymers and 2DCPs. This work highlights the critical role of effective conjugation in enhancing the charge transport properties of 2DCPs and the great potential of high-mobility 2DCPs for future (opto)electronics.

Binding Mode Multiplicity and Multiscale Chirality in the Supramolecular Assembly of DNA and a π-Conjugated Polymer.

Water-soluble π-conjugated polymers are increasingly considered for DNA biosensing. However, the conformational rearrangement, supramolecular organization and dynamics upon interaction with DNA have been overlooked, which prevents the rational design of such detection tools. To elucidate the binding of a cationic polythiophene (CPT) to DNA with atomistic resolution, we performed molecular simulations of their supramolecular assembly. Comparison of replicated simulations show a multiplicity of CPT binding geometries that contribute to the wrapping of CPT around DNA. The different binding geometries are stabilized by both electrostatic interactions between CPT lateral cations and DNA phosphodiesters and van der Waals interactions between the CPT backbone and the DNA grooves. Simulated circular dichroism (CD) spectra show that the induced CD signal stems from a conserved geometrical feature across the replicated simulations, i. e. the presence of segments of syn configurations between thiophene units along the CPT chain. At the macromolecular scale, we inspected the different shapes related to the CPT binding modes around the DNA through symmetry metrics. Altogether, molecular dynamics (MD) simulations, model Hamiltonian calculations of the CD spectra, and symmetry indices provide insights into the origin of induced chirality from the atomic to the macromolecular scale. Our multidisciplinary approach points out the hierarchical aspect of CPT chiral organization induced by DNA.

A Blue-Light-Emitting BODIPY Probe for Lipid Membranes.

Here we describe a new BODIPY-based membrane probe (1) that provides an alternative to dialkylcarbocyanine dyes, such as DiI-C18, that can be excited in the blue spectral region. Compound 1 has unbranched octadecyl chains at the 3,5-positions and a meso-amino function. In organic solvents, the absorption and emission maxima of 1 are determined mainly by solvent acidity and dipolarity. The fluorescence quantum yield is high and reaches 0.93 in 2-propanol. The fluorescence decays are well fitted with a single-exponential in pure solvents and in small and giant unilamellar vesicles (GUV) with a lifetime of ca. 4 ns. Probe 1 partitions in the same lipid phase as DiI-C18(5) for lipid mixtures containing sphingomyelin and for binary mixtures of dipalmitoylphosphatidylcholine (DPPC) and dioleoylphosphatidylcholine (DOPC). The lipid phase has no effect on the fluorescence lifetime but influences the fluorescence anisotropy. The translational diffusion coefficients of 1 in GUVs and OLN-93 cells are of the same order as those reported for DiI-C18. The directions of the absorption and emission transition dipole moments of 1 are calculated to be parallel. This is reflected in the high steady-state fluorescence anisotropy of 1 in high ordered lipid phases. Molecular dynamic simulations of 1 in a model of the DOPC bilayer indicate that the average angle of the transition moments with respect to membrane normal is ca. 70°, which is comparable with the value reported for DiI-C18.

Self-assembly and hybridization mechanisms of DNA with cationic polythiophene.

The combination of DNA and π-conjugated polyelectrolytes (CPEs) represents a promising approach to develop DNA hybridization biosensors, with potential applications for instance in the detection of DNA lesions and single-nucleotide polymorphisms. Here we exploit the remarkable optical properties of a cationic poly[3-(6'-(trimethylphosphonium)hexyl)thiophene-2,5-diyl] (CPT) to decipher the self-assembly of DNA and CPT. The ssDNA/CPT complexes have chiroptical signatures in the CPT absorption region that are strongly dependent on the DNA sequence, which we relate to differences in supramolecular interactions between the thiophene monomers and the various nucleobases. By studying DNA-DNA hybridization and melting processes on preformed ssDNA/CPT complexes, we observe sequence-dependent mechanisms that can yield DNA-condensed aggregates. Heating-cooling cycles show that non-equilibrium mixtures can form, noticeably depending on the working sequence of the hybridization experiment. These results are of high importance for the use of π-conjugated polyelectrolytes in DNA hybridization biosensors and in polyplexes.

Surface-induced diastereomeric complex formation of a nucleoside at the liquid/solid interface: stereoselective recognition and preferential adsorption.

With the aim of achieving surface-mediated enantioselective adsorption, the self-assembly of chiral oligo(p-phenylenevinylene) (OPV3T) with nucleosides is investigated at the liquid/solid interface by means of scanning tunneling microscopy and molecular modeling. OPV3T enantiomers form mirror related hexameric rosette patterns. The DNA nucleoside, thymidine, does not self-assemble into stable adlayers but coadsorbs with OPV3T on the surface, leading to a pattern transformation of OPV3T from rosettes to dimers, and a change in chiral expression as well. Diastereoselective recognition between OPV3T and thymidine enantiomers can be used to resolve thymidine enantiomers at an achiral surface with an OPV3T enantiomer as the resolving agent. The impact of molar ratio and concentration on the self-assembly and chiral resolution is systematically investigated. Because there is no interaction between OPV3T and thymidine in solution, the liquid/solid interface acts as the platform for the chiral resolution of thymidine enantiomers.

Asymmetric noncovalent synthesis of self-assembled one-dimensional stacks by a chiral supramolecular auxiliary approach.

Stereoselective noncovalent synthesis of one-dimensional helical self-assembled stacks of achiral oligo(p-phenylenevinylene) ureidotriazine (AOPV3) monomers is obtained by a chiral supramolecular auxiliary approach. The racemic mixture of helical stacks of achiral AOPV3 molecules is converted into homochiral helical stacks, as shown by both spectroscopic measurements and molecular modeling simulations. The conversion is promoted by an orthogonal two-point ion-pair interaction with the chiral auxiliary dibenzoyl tartaric acid (D- or L-TA) molecules, which biases the angle population distribution and thereby the stack helicity. The induced preferred helicity is maintained by the OPV stacks even after the removal of the chiral auxiliary by extraction with ethylenediamine (EDA), due to the kinetic stability of the OPV stacks at room temperature. Spectroscopic probing of the helical self-assembly and the racemization process of these π-conjugated OPV chromophores shed further light into the mechanistic pathways of this chiral asymmetric noncovalent synthesis and the kinetic stability of the stacks produced. The racemization of the stacks follows first-order kinetics and no switch in mechanism is observed as a result of a temperature change; therefore, a racemization via disassembly assembly is proposed. Remarkably, the preferred helicity of the stacks of achiral AOPV3 can be retained almost completely after a heating-cooling cycle where the stacks first partially depolymerize and then polymerize again with the still existing stacks being the seeds for self-assembly of achiral AOPV3. Only after a fully dissociated state is obtained at high temperatures, the optical activity of the supramolecular stack self-assembled at room temperature is lost.

Nucleoside-assisted self-assembly of oligo(p-phenylenevinylene)s at liquid/solid interface: chirality and nanostructures.

The formation of DNA nucleoside-assisted π-conjugated nanostructures was studied by means of scanning tunneling microscopy (STM) and force field simulations. Upon adsorption of the achiral oligo(p-phenylenevinylene) (OPV) derivative at the liquid/solid interface, racemic conglomerates with mirror related rosettes are formed. Addition of the DNA nucleosides D- and L-thymidine, which act as "chiral handles", has a major effect on the supramolecular structure and the expression of chirality of the achiral OPV molecules. The influence of these "chiral handles" on the expression of chirality is probed at two levels: monolayer symmetry and monolayer orientation with respect to the substrate. This was further explored by tuning the molar ratio of the building blocks. Molecular modeling simulations give an atomistic insight into the monolayer construction, as well as the energetics governing the assembly. Thymidine is able to direct the chirality and the pattern of OPV molecules on the surface, creating chiral lamellae of π-conjugated dimers.

"Helter-skelter-like" perylene polyisocyanopeptides.

Exciton migration! Spectroscopic analyses and extensive molecular dynamics studies revealed a well-defined 4(1) helix in which the perylene molecules (see figure) form four "helter-skelter-like" overlapping pathways along which excitons and electrons can rapidly migrate.We report on a combined experimental and computational investigation on the synthesis and thorough characterization of the structure of perylene-functionalized polyisocyanides. Spectroscopic analyses and extensive molecular dynamics studies revealed a well defined 4(1) helix in which the perylene molecules form four "helter skelter-like" overlapping pathways along which excitons and electrons can rapidly migrate. The well-defined polymer scaffold stabilized by hydrogen bonding, to which the chromophores are attached, accounts for the precise architectural definition, and molecular stiffness observed for these molecules. Molecular-dynamics studies showed that the chirality present in these polymers is expressed in the formation of stable right-handed helices. The formation of chiral supramolecular structures is further supported by the measured and calculated bisignated Cotton effect. The structural definition of the chromophores aligned in one direction along the backbone is highlighted by the extremely efficient exciton migration rates and charge densities measured with Transient Absorption Spectroscopy.

Beyond Förster resonance energy transfer in biological and nanoscale systems.

After photoexcitation, energy absorbed by a molecule can be transferred efficiently over a distance of up to several tens of angstroms to another molecule by the process of resonance energy transfer, RET (also commonly known as electronic energy transfer, EET). Examples of where RET is observed include natural and artificial antennae for the capture and energy conversion of light, amplification of fluorescence-based sensors, optimization of organic light-emitting diodes, and the measurement of structure in biological systems (FRET). Forster theory has proven to be very successful at estimating the rate of RET in many donor-acceptor systems, but it has also been of interest to discover when this theory does not work. By identifying these cases, researchers have been able to obtain, sometimes surprising, insights into excited-state dynamics in complex systems. In this article, we consider various ways that electronic energy transfer is promoted by mechanisms beyond those explicitly considered in Forster RET theory. First, we recount the important situations when the electronic coupling is not accurately calculated by the dipole-dipole approximation. Second, we examine the related problem of how to describe solvent screening when the dipole approximation fails. Third, there are situations where we need to be careful about the separability of electronic coupling and spectral overlap factors. For example, when the donors and/or acceptors are molecular aggregates rather than individual molecules, then RET occurs between molecular exciton states and we must invoke generalized Forster theory (GFT). In even more complicated cases, involving the intermediate regime of electronic energy transfer, we should consider carefully nonequilibrium processes and coherences and how bath modes can be shared. Lastly, we discuss how information is obscured by various forms of energetic disorder in ensemble measurements and we outline how single molecule experiments continue to be important in these instances.

Conformational effects on excitation transport along conjugated polymer chains.

The dynamics of energy transport along rigid rod conjugated polymer--poly(p-phenylenebutadyine)--chains is modeled with a special emphasis on the role of conformational disorder. A simple random growth algorithm based on torsion potentials with increasing stiffness yields polymer chains with increasing degree of conjugation and narrower energetic distributions. Despite the fact that the average hopping rate between two subunits is reduced (because the decrease in electronic coupling overwhelms the increase in spectral overlap), a more efficient excitation motion along chains with longer conjugated segments is predicted, in good agreement with recent experiments. This points to the central role of conformational disorder on intrachain energy diffusion in conjugated polymers.

Charge Separation and Recombination at Polymer-Fullerene Heterojunctions: Delocalization and Hybridization Effects.

We address charge separation and recombination in polymer/fullerene solar cells with a multiscale modeling built from accurate atomistic inputs and accounting for disorder, interface electrostatics and genuine quantum effects on equal footings. Our results show that bound localized charge transfer states at the interface coexist with a large majority of thermally accessible delocalized space-separated states that can be also reached by direct photoexcitation, thanks to their strong hybridization with singlet polymer excitons. These findings reconcile the recent experimental reports of ultrafast exciton separation ("hot" process) with the evidence that high quantum yields do not require excess electronic or vibrational energy ("cold" process), and show that delocalization, by shifting the density of charge transfer states toward larger effective electron-hole radii, may reduce energy losses through charge recombination.

A dynamic supramolecular polymer with stimuli-responsive handedness for in situ probing of enzymatic ATP hydrolysis.

Design of artificial systems, which can respond to fluctuations in concentration of adenosine phosphates (APs), can be useful in understanding various biological processes. Helical assemblies of chromophores, which dynamically respond to such changes, can provide real-time chiroptical readout of various chemical transformations. Towards this concept, here we present a supramolecular helix of achiral chromophores, which shows chiral APs responsive tunable handedness along with dynamically switchable helicity. This system, composing of naphthalenediimides with phosphate recognition unit, shows opposite handedness on binding with ATP compared with ADP or AMP, which is comprehensively analysed with molecular dynamic simulations. Such differential signalling along with stimuli-dependent fast stereomutations have been capitalized to probe the reaction kinetics of enzymatic ATP hydrolysis. Detailed chiroptical analyses provide mechanistic insights into the enzymatic hydrolysis and various intermediate steps. Thus, a unique dynamic helical assembly to monitor the real-time reaction processes via its stimuli-responsive chiroptical signalling is conceptualized.

Affecting surface chirality via multicomponent adsorption of chiral and achiral molecules.

Here we report on the apparent reduction in surface chirality upon co-assembling a chiral and achiral molecule into a physisorbed self-assembled monolayer at the liquid/solid interface as revealed by scanning tunneling microscopy (STM). Chiral OPV with achiral thymine gives rise to surface-confined supramolecular diastereomers.

Probing the relation between charge transport and supramolecular organization down to ångström resolution in a benzothiadiazole-cyclopentadithiophene copolymer.

Molecular modeling shows that longitudinal displacement of the backbones by a couple of ångströms has a profound impact on the electronic coupling mediating charge transport in a conjugated copolymer. These changes can be probed by monitoring the calculated X-ray scattering patterns and NMR chemical shifts as a function of sliding of the polymer chains and comparing them to experiment.

Macromolecular scaffolding: the relationship between nanoscale architecture and function in multichromophoric arrays for organic electronics.

The optimization of the electronic properties of molecular materials based on optically or electrically active organic building blocks requires a fine-tuning of their self-assembly properties at surfaces. Such a fine-tuning can be obtained on a scale up to 10 nm by mastering principles of supramolecular chemistry, i.e., by using suitably designed molecules interacting via pre-programmed noncovalent forces. The control and fine-tuning on a greater length scale is more difficult and challenging. This Research News highlights recent results we obtained on a new class of macromolecules that possess a very rigid backbone and side chains that point away from this backbone. Each side chain contains an organic semiconducting moiety, whose position and electronic interaction with neighboring moieties are dictated by the central macromolecular scaffold. A combined experimental and theoretical approach has made it possible to unravel the physical and chemical properties of this system across multiple length scales. The (opto)electronic properties of the new functional architectures have been explored by constructing prototypes of field-effect transistors and solar cells, thereby providing direct insight into the relationship between architecture and function.

Probing excitation delocalization in supramolecular chiral stacks by means of circularly polarized light: experiment and modeling.

Photoexcitations in helical aggregates of a functionalized, chiral oligophenylenevinylene (MOPV) are described going beyond the Born-Oppenheimer approximation, in the form of dressed (polaronic) Frenkel excitons. This allows for accurate modeling of the experimentally observed wavelength dependence of the circular polarization in fluorescence, which directly probes the non-adiabatic nature of the electron-vibration (EV) coupling in this system. The fluorescence photon is emitted from a nuclear geometry in which one MOPV and its two nearest neighbors have a nuclear equilibrium that differs appreciably from the ground state due to the presence of the excited state. The absorption and emission band shape and the circular dichroism are consistent with a coherence range of the emitting excitation of approximately two neighboring molecules. Random fluctuations in the zero-order excited-state energy of the MOPVs (disorder) limit the exciton delocalization and can be described by a Gaussian distribution of energies with a width sigma=0.12 eV and a spatial correlation length l0 approximately 5 molecules. We find that disorder and EV coupling act synergistically in localizing the emitting exciton to a single MOPV in the aggregate with 95% probability.

Chromophores in phenylenevinylene-based conjugated polymers: role of conformational kinks and chemical defects.

The influence of chemical defects and conformational kinks on the nature of the lowest electronic excitations in phenylenevinylene-based polymers is assessed at the semiempirical quantum-chemical level. The amount of excited-state localization and the amplitude of through-space (Coulomb-like) versus through-bond (charge-transfer-like) interactions have been quantified by comparing the results provided by excitonic and supermolecular models. While excitation delocalization among conjugated segments delineated by the defects occurs in the acceptor configuration, self-confinement on individual chromophores follows from geometric relaxation in the excited-state donor configuration. The extent of excited-state localization is found to be sensitive to both the nature of the defect and the length of the conjugated chains. Implications for resonant energy transfer along conjugated polymer chains are discussed.

Revealing competitive Forster-type resonance energy-transfer pathways in single bichromophoric molecules.

We demonstrate measurements of the efficiency of competing Förster-type energy-transfer pathways in single bichromophoric systems by monitoring simultaneously the fluorescence intensity, fluorescence lifetime, and the number of independent emitters with time. Peryleneimide end-capped fluorene trimers, hexamers, and polymers with interchromophore distances of 3.4, 5.9, and on average 42 nm, respectively, served as bichromophoric systems. Because of different energy-transfer efficiencies, variations in the interchromophore distance enable the switching between homo-energy transfer (energy hopping), singlet-singlet annihilation, and singlet-triplet annihilation. The data suggest that similar energy-transfer pathways have to be considered in the analysis of single-molecule trajectories of donor/acceptor pairs as well as in natural and synthetic multichromophoric systems such as light-harvesting antennas, oligomeric fluorescent proteins, and dendrimers. Here we report selectively visualization of different energy-transfer pathways taking place between identical fluorophores in individual bichromophoric molecules.