Photocurrent Generation Enhanced by Charge Delocalization over Stacked Perylenediimide Chromophores Assembled within DNA

Concepts and principles of self-n-doping in perylene diimide chromophores for applications in biochemistry, energy harvesting, energy storage, and catalysis

Self-doping is an essential method of increasing carrier concentrations in organic electronics that eliminates the need to tailor host-dopant miscibility, a necessary step when employing molecular dopants. Self-n-doping can be accomplished using amines or ammonium counterions as an electron source, which are being incorporated into an ever-increasingly diverse range of organic materials spanning many applications. Self-n-doped materials have demonstrated exemplary and, in many cases, benchmark performances in a variety of applications. However, an in-depth review of the method is lacking. Perylene diimide (PDI) chromophores are an important mainstay in the semiconductor literature with well-known structure-function characteristics and are also one of the most widely utilized scaffolds for self-n-doping. In this review, we describe the unique properties of self-n-doped PDIs, delineate structure-function relationships, and discuss self-n-doped PDI performance in a range of applications. In particular, the impact of amine/ammonium incorporation into the PDI scaffold on doping efficiency is reviewed with regard to attachment mode, tether distance, counterion selection, and steric encumbrance. Self-n-doped PDIs are a unique set of PDI structural derivatives whose properties are amenable to a broad range of applications such as biochemistry, solar energy conversion, thermoelectric modules, batteries, and photocatalysis. Finally, we discuss challenges and the future outlook of self-n-doping principles.

Photoactivation Properties of Self-n-Doped Perylene Diimides: Concentration-dependent Radical Anion and Dianion Formation

Perylene diimides (PDIs) have garnered attention as organic photocatalysts in recent years for their ability to drive challenging synthetic transformations, such as aryl halide reduction and olefin iodoperfluoroalkylation. Previous work in this area employs spectator pendant groups attached to the imide nitrogen positions of PDIs that are only added to impart solubility. In this work, we employ electron-rich ammonium iodide or ammonium hydroxide pendant groups capable of self-n-doping the PDI core to form radical anions (R^•–) and dianions (D^••2–). We observe R^•– formation is favored at low concentrations where aliphatic linkers are able to freely rotate, while D^••2– formation is favored at elevated concentrations likely due to Coulombic stabilization between adjacent chromophores in a similar manner to that of Kasha exciton stabilization. Cyclic voltammetric measurements are consistent with steric encumbrance increasing the Lewis basicity of anions through Coulombic destabilization. However, sterics also inhibit dianion formation by disrupting aggregation. Finally, femtosecond transient absorption measurements reveal that low wavelength excitation (400 nm) preferentially favors the excitation of R^•– to the strongly reducing doublet excited state ²[R^•–]*. In contrast, higher wavelength excitation (520 nm) favors the formation of the singlet excited state ¹[N]*. These findings highlight the importance of dopant architecture, counterion selection, excitation wavelength, and concentration on R^•– and D^••2– formation, which has substantial implications for future photocatalytic applications. We anticipate these findings will enable more efficient systems based on self-n-doped PDIs.

Deoxyribonucleic Acid Encoded and Size-Defined π-Stacking of Perylene Diimides

Natural photosystems use protein scaffolds to control intermolecular interactions that enable exciton flow, charge generation, and long-range charge separation. In contrast, there is limited structural control in current organic electronic devices such as OLEDs and solar cells. We report here the DNA-encoded assembly of π-conjugated perylene diimides (PDIs) with deterministic control over the number of electronically coupled molecules. The PDIs are integrated within DNA chains using phosphoramidite coupling chemistry, allowing selection of the DNA sequence to either side, and specification of intermolecular DNA hybridization. In this way, we have developed a "toolbox" for construction of any stacking sequence of these semiconducting molecules. We have discovered that we need to use a full hierarchy of interactions: DNA guides the semiconductors into specified close proximity, hydrophobic-hydrophilic differentiation drives aggregation of the semiconductor moieties, and local geometry and electrostatic interactions define intermolecular positioning. As a result, the PDIs pack to give substantial intermolecular π wave function overlap, leading to an evolution of singlet excited states from localized excitons in the PDI monomer to excimers with wave functions delocalized over all five PDIs in the pentamer. This is accompanied by a change in the dominant triplet forming mechanism from localized spin-orbit charge transfer mediated intersystem crossing for the monomer toward a delocalized excimer process for the pentamer. Our modular DNA-based assembly reveals real opportunities for the rapid development of bespoke semiconductor architectures with molecule-by-molecule precision.

Tuning of the optoelectronic properties of peptide-appended core-substituted naphthalenediimides: the role of self-assembly of two positional isomers

This study demonstrates how the self-assembly pattern of two different and isomeric peptide-appended core-substituted naphthalenediimides (NDIs) affects the modulation of their optoelectronic properties. Two isomeric peptide-attached NDIs were synthesized, purified and characterized. Interchanging the position of attachment of the peptide units and the alkyl chains in the NDI has altered the respective self-assembling patterns of these isomeric molecules in the aggregated states. The isomer having a peptide moiety in the core position and the alkyl chain in the imide position (compound N1) forms face to face stacking or 'H' aggregates in aliphatic solvents including n-hexane, and n-decane, whereas compound N2, in which the peptide moiety is at the imide position and the alkyl chain is attached at the core position of NDI exhibits edge to edge stacking or J aggregates under the same conditions as it is evident from their UV-vis studies. The H aggregated species (obtained from N1) show inter-connected nanofibers, whereas the J aggregated species (obtained from N2) exhibit the morphology of helical nanoribbons. FT-IR and X-ray diffraction studies are in favor of the same aggregation behavior. The individual packing patterns of these two peptide-based isomers have a direct impact on their respective electrical conductivity. Interestingly, the H aggregated species shows 100 times greater current conductivity than that of the J aggregate. Moreover, it is only the H aggregated species that exhibits a photocurrent, and no such photocurrent response is observed with the J aggregates. Computational studies also support that different types of aggregation patterns are formed by these two isomeric molecules in the same solvent system. This unique example of tuning of optoelectronic behavior holds future promise for the development of new peptide-conjugated π-functional materials.

Free Charge Carriers in Homo-Sorted π-Stacks of Donor-Acceptor Conjugates

Inspired by the high photoconversion efficiency observed in natural light-harvesting systems, the hierarchical organization of molecular building blocks has gained impetus in the past few decades. Particularly, the molecular arrangement and packing in the active layer of organic solar cells (OSCs) have garnered significant attention due to the decisive role of the nature of donor/acceptor (D/A) heterojunctions in charge carrier generation and ultimately the power conversion efficiency. This review focuses on the recent developments in emergent optoelectronic properties exhibited by self-sorted donor-on-donor/acceptor-on-acceptor arrangement of covalently linked D-A systems, highlighting the ultrafast excited state dynamics of charge transfer and transport. Segregated organization of donors and acceptors promotes the delocalization of photoinduced charges among the stacks, engendering an enhanced charge separation lifetime and percolation pathways with ambipolar conductivity and charge carrier yield. Covalently linking donors and acceptors ensure a sufficient D-A interface and interchromophoric electronic coupling as required for faster charge separation while providing better control over their supramolecular assemblies. The design strategies to attain D-A conjugate assemblies with optimal charge carrier generation efficiency, the scope of their application compared to state-of-the-art OSCs, current challenges, and future opportunities are discussed in the review. An integrated overview of rational design approaches derived from the comprehension of underlying photoinduced processes can pave the way toward superior optoelectronic devices and bring in new possibilities to the avenue of functional supramolecular architectures.

Photocurrent enhancement by a local electric field on DNA-modified electrodes covered with gold nanoparticles

The enhancement of photocurrent by gold nanoparticles assembled by DNA is reported.

Nanostructured polyurethane perylene bisimide ester assemblies with tuneable morphology and enhanced stability

Size control has been successfully achieved in inorganic materials, but it remains a challenge in polymer nanomaterials due to their polydispersity. Here, we report a facile approach to tailor the diameters of polyurethane (PU) nanoparticles (490 nm, 820 nm and 2.1 µm) via perylene bisimide (PBI) assisted self-assembly. The formed morphologies such as spindle, spherical and core-shell structures depend on the ratio of PBI and polymer concentrations. It is shown that the formation of PU nanoparticles is directed by π-π stacking of PBI and the morphology transition is not only affected by the amount of PBI incorporated, but also influenced by solvent, which controls the initial evaporation balance. Furthermore, the prepared PUs exhibit retained optical stability and enhanced thermal stability. The PUs, designed to have conjugated PBI segments in backbones, were synthesized via ring-opening and condensation reactions. Compared with the neat PU, gel permeation chromatography shows narrower molecular weight distribution. Fluorescence spectra and ultraviolet-visible spectra indicate retained maximum emission wavelength of PBI at 574 nm and 5.2% quantum yields. Thermo-gravimetric analysis and differential scanning calorimetry reveal 79°C higher decomposition temperature and 22°C higher glass transition temperature. This study provides a new way to fabricate well-defined nanostructures of functionalized PUs.

Analyzing Spin Selectivity in DNA-Mediated Charge Transfer via Fluorescence Microscopy

Understanding spin-selective interactions between electrons and chiral molecules is critical to elucidating the significance of electron spin in biological processes and to assessing the potential of chiral assemblies for organic spintronics applications. Here, we use fluorescence microscopy to visualize the effects of spin-dependent charge transport in self-assembled monolayers of double-stranded DNA on ferromagnetic substrates. Patterned DNA arrays provide background regions for every measurement to enable quantification of substrate magnetization-dependent fluorescence due to the chiral-induced spin selectivity effect. Fluorescence quenching of photoexcited dye molecules bound within DNA duplexes is dependent upon the rate of charge separation/recombination upon photoexcitation and the efficiency of DNA-mediated charge transfer to the surface. The latter process is modulated using an external magnetic field to switch the magnetization orientation of the underlying ferromagnetic substrates. We discuss our results in the context of the current literature on the chiral-induced spin selectivity effect across various systems.

Ultrafast dynamics of formation and autodetachment of a dipole-bound state in an open-shell π-stacked dimer anion

Isolated π-stacked dimer radical anions present the simplest model of an excess electron in a π-stacked environment. Here, frequency-, angle-, and time-resolved photoelectron imaging together with electronic structure calculations have been used to characterise the π-stacked coenzyme Q₀ dimer radical anion and its exited state dynamics. In the ground electronic state, the excess electron is localised on one monomer with a planar para-quinone ring, which is solvated by the second monomer in which carbonyl groups are bent out of the para-quinone ring plane. Through the π-stacking interaction, the dimer anion exhibits a number of charge-transfer (intermolecular) valence-localised resonances situated in the detachment continuum that undergo efficient internal conversion to a cluster dipole-bound state (DBS) on a ∼60 fs timescale. In turn, the DBS undergoes vibration-mediated autodetachment on a 2.0 ± 0.2 ps timescale. Experimental vibrational structure and supporting calculations assign the intermolecular dynamics to be facilitated by vibrational wagging modes of the carbonyl groups on the non-planar monomer. At photon energies ∼0.6-1.0 eV above the detachment threshold, a competition between photoexcitation of an intermolecular resonance leading to the DBS, and photoexcitation of an intramolecular resonance leading to monomer-like dynamics further illustrates the π-stacking specific dynamics. Overall, this study provides the first direct observation of both internal conversion of resonances into a DBS, and characterisation of a vibration-mediated autodetachment in real-time.

Charge versus Energy Transfer Effects in High-Performance Perylene Diimide Photovoltaic Blend Films

Perylene diimide (PDI)-based organic photovoltaic devices can potentially deliver high power conversion efficiency values provided the photon energy absorbed is utilized efficiently in charge transfer (CT) reactions instead of being consumed in nonradiative energy transfer (ET) steps. Hitherto, it remains unclear whether ET or CT primarily drives the photoluminescence (PL) quenching of the PDI excimer state in PDI-based blend films. Here, we affirm the key role of the thermally assisted PDI excimer diffusion and subsequent CT reaction in the process of PDI excimer PL deactivation. For our study we perform PL quenching experiments in the model PDI-based composite made of poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiophene)-2-6-diyl] (PBDTTT-CT) polymeric donor mixed with the N,N'-bis(1-ethylpropyl)-perylene-3,4,9,10-tetracarboxylic diimide (PDI) acceptor. Despite the strong spectral overlap between the PDI excimer PL emission and UV-vis absorption of PBDTTT-CT, two main observations indicate that no significant ET component operates in the overall PL quenching: the PL intensity of the PDI excimer (i) increases with decreasing temperature and (ii) remains unaffected even in the presence of 10 wt % content of the PBDTTT-CT quencher. Temperature-dependent wide-angle X-ray scattering experiments further indicate that nonradiative resonance ET is highly improbable due to the large size of PDI domains. The dominance of the CT over the ET process is verified by the high performance of devices with an optimum composition of 30:70 PBDTTT-CT:PDI. By adding 0.4 vol % of 1,8-diiodooctane we verify the plasticization of the polymer side chains that balances the charge transport properties of the PBDTTT-CT:PDI composite and results in additional improvement in the device efficiency. The temperature-dependent spectral width of the PDI excimer PL band suggests the presence of energetic disorder in the PDI excimer excited state manifold.

Elucidating the impact of molecular packing and device architecture on the performance of nanostructured perylene diimide solar cells

The performance of organic photovoltaic devices (OPV) with nanostructured polymer:perylene diimide (PDI) photoactive layers approaches the levels of the corresponding polymer:fullerene systems. Nevertheless, a coherent understanding of the difficulty for PDI-based OPV devices to deliver high power conversion efficiencies remains elusive. Here we perform a comparative study of a set of four different polymer:PDI OPV model systems. The different device performances observed are attributed to differences in the nanostructural motif of these composites, as determined by wide-angle X-ray scattering (WAXS) measurements. Long-range structural order in the PDI domain dictates (i) the stabilization energy and (ii) the concentration of the PDI excimers in the composites. The quenching of the PDI excimer photoluminescence (PL) is found to be insensitive to the former, but it depends on the latter. High PL quenching occurs for the low concentration of PDI excimers that are formed in PDI columns with a length comparable to the PDI excimer diffusion length. The stabilization of the PDI excimer state increases as the long-range order in the PDI domains improves. The structural order of the PDI domains primarily affects charge transport. Electron mobility reduces as the size of the PDI domain increases, suggesting that well-ordered PDI domains suffer from poor electronic connectivity. WAXS further reveals the presence of additional intermolecular PDI interactions, other than the direct face-to-face intermolecular coupling, that introduce a substantial energetic disorder in the polymer:PDI composites. Conventional device architectures with hole-collecting ITO/PEDOT:PSS bottom electrodes are compared with inverted device architectures bearing bottom electron-collecting electrodes of ITO/ZnO. In all cases the ZnO-functionalized devices surpass the performance of the conventional device analogues. X-ray photoelectron spectroscopy explains that in

PEDOT

PSS-functionalized devices, the PDI component preferentially segregates closer to the hydrophilic

PEDOT

PSS electrode, thus impeding the efficient charge extraction and limiting device photocurrent.

Photoresponsive DNA monolayer prepared by primer extension reaction on the electrode

We describe a simple and convenient method for the preparation of photoresponsive DNA-modified electrodes using primer extension (PEX) reactions. A naphthalimide derivative was used as the photosensitizer that was attached to the C5-position of 2'-deoxyuridine-5'-triphosphate (dUTP(NI)). It has been found that dUTP(NI) is a good substrate for the PEX reactions using KOD Dash and Vent (exo-) enzymes in solutions to incorporate naphthalimide (NI) moieties into the DNA sequences. On the electrode surface immobilized with the primer/template DNA, the PEX reactions to incorporate dUTP(NI) molecules into the DNA sequence were found to efficiently proceed. With this solid-phase method, the DNA monolayers capable of generating photocurrent due to the photoresponsive NI molecule can be constructed. It was shown that the photocurrent generation was significantly suppressed by a single-nucleotide mismatch included in the primer/template DNA, which is applicable for the design of photoelectrochemical sensors to discriminate single-nucleotide sequences.

Dynamic DNA architectures: spontaneous DNA strand exchange and self-sorting driven by perylene bisimide interactions

Three differently bay-substituted perylene bisimides together with the conventional unsubstituted chromophore were synthetically incorporated as homodimers in DNA double strands and undergo spontaneous strand exchange if mixed together.