Extreme electron polaron spatial delocalization in π-conjugated materials

Stabilization of a Photoradiated Naphthalene Diimide-Based Organic Radical Anion Inside a Peptide-Based Gel Matrix with an Improvement of Optoelectronic Properties

An amino acid-conjugated naphthalene diimide (NDI)-based highly red fluorescent radical anion has been found in a water medium under the photoradiated condition. This molecule has failed to form the radical anion in the monomeric state; however, the J aggregation in the aqueous medium has ensured the formation of radical anion in the ambient condition after the irradiation of both sunlight and UV light exposure. Electron paramagnetic resonance (EPR) studies clearly suggest the formation of radical anions. Herein, the stability of the radical anion in the aqueous medium is only a few minutes as a small amount of shaking is enough to quench the radical anion in the solution state. Furthermore, the incorporation of this molecule into a peptide-based hydrogel matrix and the consequent photoirradiation have not only helped to develop radical anion in the gel matrix but also increased the enormous stability of the radical anion inside the hydrogel matrix even for 30 days. It is envisaged that the formation of the radical anion within the gel matrix prevents the free movement of the NDI molecules and restricts the diffusion of molecular oxygen in the system, which leads to the stability of the radical anions in the gel. Moreover, the stability of the radical anion within the gel has helped to enhance the conductivity of the hybrid gel to a great extent. Interestingly, the radical anion-containing hybrid hydrogel has shown a potential photoswitching property.

Polaron Delocalization Dependence of the Conductivity and the Seebeck Coefficient in Doped Conjugated Polymers

Conjugated polymers are promising materials for thermoelectrics as they offer good performances at near ambient temperatures. The current focus on polymer thermoelectric research mainly targets a higher power factor (PF; a product of the conductivity and square of the Seebeck coefficient) through improving the charge mobility. This is usually accomplished via structural modification in conjugated polymers using different processing techniques and doping. As a result, the structure-charge transport relationship in conjugated polymers is generally well-established. In contrast, the relationship between the structure and the Seebeck coefficient is poorly understood due to its complex nature. A theoretical framework by David Emin (Phys. Rev. B, 1999, 59, 6205-6210) suggests that the Seebeck coefficient can be enhanced via carrier-induced vibrational softening, whose magnitude is governed by the size of the polaron. In this work, we seek to unravel this relationship in conjugated polymers using a series of highly identical pro-quinoid polymers. These polymers are ideal to test Emin's framework experimentally as the quinoid character and polaron delocalization in these polymers can be well controlled even by small atomic differences (<10 at. % per repeating unit). By increasing the polaron delocalization, that is, the polaron size, we demonstrate that both the conductivity and the Seebeck coefficient (and hence PF) can be increased simultaneously, and the latter is due to the increase in the polaron's vibrational entropy. By using literature data, we also show that this phenomenon can be observed in two closely related diketopyrrolopyrrole-conjugated polymers as well as in p-doped P3HT and PANI systems with an increasing molecular order.

Twisted molecular wires polarize spin currents at room temperature

In contrast to conventional electronics, spintronics devices exploit the electron spin as an additional degree of freedom. The chirality-induced spin selectivity (CISS) effect, in which chiral molecules act as spin filters in electron transport, provides a pathway to control spins in molecules. We describe an approach that integrates both spin-polarizing and spin-propagating functionality into organic structures that feature low charge transport resistances. Binding chiral ligands to molecular wires controls polarized spin handedness, regulates spin currents, generates large NIR rotational strengths, and provides a mechanism to flip the favored spin orientation for spin transmission through chiral organic molecules. This work points the way to materials that provide both high spin selectivity and large-magnitude spin currents via the CISS mechanism.

A critical spintronics challenge is to develop molecular wires that render efficiently spin-polarized currents. Interplanar torsional twisting, driven by chiral binucleating ligands in highly conjugated molecular wires, gives rise to large near-infrared rotational strengths. The large scalar product of the electric and magnetic dipole transition moments (μ→ij⋅m→ij), which are evident in the low-energy absorptive manifolds of these wires, makes possible enhanced chirality-induced spin selectivity–derived spin polarization. Magnetic-conductive atomic force microscopy experiments and spin-Hall devices demonstrate that these designs point the way to achieve high spin selectivity and large-magnitude spin currents in chiral materials.

Facile Synthesis of Solubilizing a Group-Free, Solution-Processable p-Type Ladder Conjugated Polymer and Its Thermoelectric Properties

Here we report the synthesis of a new solubilizing group-free, solution-processable p-type ladder conjugated polymer, 6H-pyrrolo[3,2-b:4,5-b']bis[1,4]benzothiazine ladder (PBBTL) polymer by using a polyphosphoric acid (PPA) and phenylphosphonic acid (PhPO₃H₂) 1:1 binary acid solvent system together with careful control of reaction kinetics. With a good intrinsic viscosity of 3.69 dL/g in methanesulfonic acid (MSA), good quality PBBTL films can be obtained via spin-coating. Intrinsic thin film properties and thermoelectric performance of PBBTL were evaluated, making it the second solubilizing group-free, solution-processable ladder-type conjugated polymer after BBL to be used for thin-film polymer electronics. While our preliminary thermoelectric performance of the FeCl₃-doped PBBTL films is modest, we believe that many opportunities lie ahead for PBBTL and hope that its successful synthesis using the new PPA:PhPO₃H₂ binary acid solvent system will inspire synthetic organic chemists to relook into solubilizing group-free, solution-processable ladder-type conjugated polymer systems.

Low Blue Dose Photodynamic Therapy with Porphyrin-Iron Oxide Nanoparticles Complexes: In Vitro Study on Human Melanoma Cells

The purpose of this study was to investigate the effectiveness in photodynamic therapy of iron oxide nanoparticles (γ-Fe2O3 NPs), synthesized by laser pyrolysis technique, functionalized with 5,10,15,20-(Tetra-4-sulfonatophenyl) porphyrin tetraammonium (TPPS) on human cutaneous melanoma cells, after only 1 min blue light exposure. The efficiency of porphyrin loading on the iron oxide nanocarriers was estimated by using absorption and FTIR spectroscopy. The singlet oxygen yield was determined via transient characteristics of singlet oxygen phosphorescence at 1270 nm both for porphyrin functionalized nanoparticles and rose bengal used as standard. The irradiation was performed with a LED (405 nm, 1 mW/cm2) for 1 min after melanoma cells were treated with TPPS functionalized iron oxide nanoparticles (γ-Fe2O3 NPs_TPPS) and incubated for 24 h. Biological tests revealed a high anticancer effect of γ-Fe2O3 NPs_TPPS complexes indi-cated by the inhibition of tumor cell proliferation, reduction of cell adhesion, and induction of cell death through ROS generated by TPPS under light exposure. The biological assays were combined with the pharmacokinetic prediction of the porphyrin.

EPR of Photoexcited Triplet-State Acceptor Porphyrins

The photoexcited triplet states of porphyrin architectures are of significant interest in a wide range of fields including molecular wires, nonlinear optics, and molecular spintronics. Electron paramagnetic resonance (EPR) is a key spectroscopic tool in the characterization of these transient paramagnetic states singularly well suited to quantify spin delocalization. Previous work proposed a means of extracting the absolute signs of the zero-field splitting (ZFS) parameters, D and E, and triplet sublevel populations by transient continuous wave, hyperfine measurements, and magnetophotoselection. Here, we present challenges of this methodology for a series of meso-perfluoroalkyl-substituted zinc porphyrin monomers with orthorhombic symmetries, where interpretation of experimental data must proceed with caution and the validity of the assumptions used in the analysis must be scrutinized. The EPR data are discussed alongside quantum chemical calculations, employing both DFT and CASSCF methodologies. Despite some success of the latter in quantifying the magnitude of the ZFS interaction, the results clearly provide motivation to develop improved methods for ZFS calculations of highly delocalized organic triplet states.

Topology, Distance, and Orbital Symmetry Effects on Electronic Spin-Spin Couplings in Rigid Molecular Systems: Implications for Long-Distance Spin-Spin Interactions

Understanding factors that underpin the signs and magnitudes of electron spin-spin couplings in biradicaloids, especially those that are integrated into highly delocalized electronic structures, promises to inform the design of molecular spintronic systems. Using steady-state and variable temperature electron paramagnetic resonance (EPR) spectroscopy, we examine spin dynamics in symmetric, strongly π-conjugated bis[(porphinato)copper] (bis[PCu]) systems and probe the roles played by atom-specific macrocycle spin density, porphyrin-to-porphyrin linkage topology, and orbital symmetry on the magnitudes of electronic spin-spin couplings over substantial Cu-Cu distances. These studies examine the following: (i) meso-to-meso-linked bis[PCu] systems having oligoyne spacers, (ii) meso-to-meso-bridged bis[PCu] arrays in which the PCu centers are separated by a single ethynyl unit or multiple 5,15-diethynyl(porphinato)zinc(II) units, and (iii) the corresponding β-to-β-bridged bis[PCu] structures. EPR data show that, for β-to-β-bridged systems and meso-to-meso-linked bis[PCu] structures having oligoyne spacers, a through σ-bond coupling mechanism controls the average exchange interaction (J_avg). In contrast, PCu centers separated by a single ethynyl or multiple 5,15-diethynyl(porphinato)zinc(II) units display a phenomenological decay of ln[J_avg] versus Cu-Cu σ-bond separation number of ∼0.115 per bond, half as large as for these other compositions, congruent with the importance of π-mediated spin-spin coupling. These disparities derive from effects that trace their origin to the nature of the macrocycle-macrocycle linkage topology and the relative energy of the Cu d_x²-y² singly occupied molecular orbital within the frontier orbital manifold of these electronically delocalized structures. This work provides insight into approaches to tune the extent of spin exchange interactions and distance-dependent electronic spin-spin coupling magnitudes in rigid, highly conjugated biradicaloids.

Driving high quantum yield NIR emission through proquinoidal linkage motifs in conjugated supermolecular arrays

High quantum yield NIR fluorophores are rare. Factors that drive low emission quantum yields at long wavelength include the facts that radiative rate constants increase proportional to the cube of the emission energy, while nonradiative rate constants increase in an approximately exponentially with decreasing S₀-S₁ energy gaps (in accordance with the energy gap law). This work demonstrates how the proquinoidal BTD building blocks can be utilized to minimize the extent of excited-state structural relaxation relative to the ground-state conformation in highly conjugated porphyrin oligomers, and shows that 4-ethynylbenzo[c][1,2,5]thiadiazole (E-BTD) units that terminate meso-to-meso ethyne-bridged (porphinato)zinc (PZnn) arrays, and 4,7-diethynylbenzo[c][1,2,5]thiadiazole (E-BTD-E) spacers that are integrated into the backbone of these compositions, elucidate new classes of impressive NIR fluorophores. We report the syntheses, electronic structural properties, and emissive characteristics of neoteric PZn-(BTD-PZn)n, PZn2-(BTD-PZn2)n, and BTD-PZnn-BTD fluorophores. Absolute fluorescence quantum yield (ϕ _f) measurements, acquired using a calibrated integrating-sphere-based measurement system, demonstrate that these supermolecules display extraordinary ϕ _f values that range from 10-25% in THF solvent, and between 28-36% in toluene solvent over the 700-900 nm window of the NIR. These studies underscore how the regulation of proquinoidal conjugation motifs can be exploited to drive excited-state dynamical properties important for high quantum yield long-wavelength fluorescence emission.

Mechanisms of IR amplification in radical cation polarons

Break down of the Born-Oppenheimer approximation is caused by mixing of electronic and vibrational transitions in the radical cations of some conjugated polymers, resulting in unusually intense vibrational bands known as infrared active vibrations (IRAVs). Here, we investigate the mechanism of this amplification, and show that it provides insights into intramolecular charge migration. Spectroelectrochemical time-resolved infrared (TRIR) and two-dimensional infrared (2D-IR) spectroscopies were used to investigate the radical cations of two butadiyne-linked conjugated porphyrin oligomers, a linear dimer and a cyclic hexamer. The 2D-IR spectra reveal strong coupling between all the IRAVs and the electronic π-π* polaron band. Intramolecular vibrational energy redistribution (IVR) and vibrational relaxation occur within ∼0.1-7 ps. TRIR spectra show that the transient ground state bleach (GSB) and excited state absorption (ESA) signals have anisotropies of 0.31 ± 0.07 and 0.08 ± 0.04 for the linear dimer and cyclic hexamer cations, respectively. The small TRIR anisotropy for the cyclic hexamer radical cation indicates that the vibrationally excited polaron migrates round the nanoring on a time scale faster than the measurement, i.e. within 0.5 ps, at 298 K. Density functional theory (DFT) calculations qualitatively reproduce the emergence of the IRAVs. The first singlet (S1) excited states of the neutral porphyrin oligomers exhibit similar IRAVs to the radical cations, implying that the excitons have similar electronic structures to polarons. Our results show that IRAVs originate from the strong coupling of charge redistribution to nuclear motion, and from the similar energies of electronic and vibrational transitions.

Global Aromaticity and Antiaromaticity in Porphyrin Nanoring Anions

Doping, through oxidation or reduction, is often used to modify the properties of π-conjugated oligomers. In most cases, the resulting charge distribution is difficult to determine. If the oligomer is cyclic and doping establishes global aromaticity or antiaromaticity, then it is certain that the charge is fully delocalized over the entire perimeter of the ring. Herein we show that reduction of a six-porphyrin nanoring using decamethylcobaltocene results in global aromaticity (in the 6- state; [90 π]) and antiaromaticity (in the 4- state; [88 π]), consistent with the Hückel rules. Aromaticity is assigned by NMR spectroscopy and density-functional theory calculations.

Spin Delocalization in the Radical Cations of Porphyrin Molecular Wires: A New Perspective on EPR Approaches

The spin delocalization in the radical cations of a series of ethyne-linked oligoporphyrins was investigated using EPR spectroscopy. The room-temperature spectral envelope for these oligomers deviates significantly from the benchmark N^-0.5 trend in line width expected for a completely delocalized spin density, in contrast to the butadiyne-linked analogues measured previously. Here, we show, using DFT calculations and complementary low-temperature ENDOR measurements, that this deviation is primarily driven by a more pronounced inequivalence of the ¹⁴N spins within individual subunits for the ethyne-linked oligoporphyrins. Once this ¹⁴N inequivalence is taken into consideration, the room-temperature and ENDOR spectra for both butadiyne-linked and ethyne-linked oligomers, up to N = 5, can be simulated by similar static delocalization patterns. This work highlights the importance of EPR in exploring such spin delocalization phenomena while also demonstrating that the N^-0.5 trend should not be interpreted in isolation but only in combination with careful simulation and theoretical modeling.

Realizing nearly-free-electron like conduction band in a molecular film through mediating intermolecular van der Waals interactions

Collective molecular physical properties can be enhanced from their intrinsic characteristics by templating at material interfaces. Here we report how a black phosphorous (BP) substrate concatenates a nearly-free-electron (NFE) like conduction band of a C60 monolayer. Scanning tunneling microscopy reveals the C60 lowest unoccupied molecular orbital (LUMO) band is strongly delocalized in two-dimensions, which is unprecedented for a molecular semiconductor. Experiment and theory show van der Waals forces between C60 and BP reduce the inter-C60 distance and cause mutual orientation, thereby optimizing the π-π wave function overlap and forming the NFE-like band. Electronic structure and carrier mobility calculations predict that the NFE band of C60 acquires an effective mass of 0.53-0.70 me (me is the mass of free electrons), and has carrier mobility of ~200 to 440 cm2V-1s-1. The substrate-mediated intermolecular van der Waals interactions provide a route to enhance charge delocalization in fullerenes and other organic semiconductors.

Game of Frontier Orbitals: A View on the Rational Design of Novel Charge-Transfer Materials

Since the first application of frontier molecular orbitals (FMOs) to rationalize stereospecificity of pericyclic reactions, FMOs have remained at the forefront of chemical theory. Yet, the practical application of FMOs in the rational design and synthesis of novel charge transfer materials remains under-appreciated. In this Perspective, we demonstrate that molecular orbital theory is a powerful and universal tool capable of rationalizing the observed redox/optoelectronic properties of various aromatic hydrocarbons in the context of their application as charge-transfer materials. Importantly, the inspection of FMOs can provide instantaneous insight into the interchromophoric electronic coupling and polaron delocalization in polychromophoric assemblies, and therefore is invaluable for the rational design and synthesis of novel materials with tailored properties.

Porphyrinoids as a platform of stable radicals

The non-innocent ligand nature of porphyrins was observed for compound I in enzymatic cycles of cytochrome P450. Such porphyrin radicals were first regarded as reactive intermediates in catabolism, but recent studies have revealed that porphyrinoids, including porphyrins, ring-contracted porphyrins, and ring-expanded porphyrins, display excellent radical-stabilizing abilities to the extent that radicals can be handled like usual closed-shell organic molecules. This review surveys four types of stable porphyrinoid radical and covers their synthetic methods and properties such as excellent redox properties, NIR absorption, and magnetic properties. The radical-stabilizing abilities of porphyrinoids stem from their unique macrocyclic conjugated systems with high electronic and structural flexibilities.

Electronic Delocalization in the Radical Cations of Porphyrin Oligomer Molecular Wires

The radical cations of a family of π-conjugated porphyrin arrays have been investigated: linear chains of N = 1-6 porphyrins, a 6-porphyrin nanoring and a 12-porphyrin nanotube. The radical cations were generated in solution by chemical and electrochemical oxidation, and probed by vis-NIR-IR and EPR spectroscopies. The cations exhibit strong NIR bands at ∼1000 nm and 2000-5000 nm, which shift to longer wavelength with increasing oligomer length. Analysis of the NIR and IR spectra indicates that the polaron is delocalized over 2-3 porphyrin units in the linear oligomers. Some of the IR vibrational bands are strongly intensified on oxidation, and Fano-type antiresonances are observed when activated vibrations overlap with electronic transitions. The solution-phase EPR spectra of the radical cations have Gaussian lineshapes with linewidths proportional to N^-0.5, demonstrating that at room temperature the spin hops rapidly over the whole chain on the time scale of the hyperfine coupling (ca. 100 ns). Direct measurement of the hyperfine couplings through electron-nuclear double resonance (ENDOR) in frozen solution (80 K) indicates distribution of the spin over 2-3 porphyrin units for all the oligomers, except the 12-porphyrin nanotube, in which the spin is spread over about 4-6 porphyrins. These experimental studies of linear and cyclic cations give a consistent picture, which is supported by DFT calculations and multiparabolic modeling with a reorganization energy of 1400-2000 cm^-1 and coupling of 2000 cm^-1 for charge transfer between neighboring sites, placing the system in the Robin-Day class III.

A naphthalene-fused dimer of an anti-aromatic expanded isophlorin

We report the first synthesis of a covalent expanded isophlorin dimer from two 24-π doubly S-confused sapphyrin-like pentathiaisophlorins. It exhibits marginal peripheral aromaticity rather than strong global diatropicity or paratropicity and weak intermacrocycle electronic communication. Quantum chemical methods discern that cross-conjugation is responsible for these unusual electronic features.

On the Importance of Electronic Symmetry for Triplet State Delocalization

The influence of electronic symmetry on triplet state delocalization in linear zinc porphyrin oligomers is explored by electron paramagnetic resonance techniques. Using a combination of transient continuous wave and pulse electron nuclear double resonance spectroscopies, it is demonstrated experimentally that complete triplet state delocalization requires the chemical equivalence of all porphyrin units. These results are supported by density functional theory calculations, showing uneven delocalization in a porphyrin dimer in which a terminal ethynyl group renders the two porphyrin units inequivalent. When the conjugation length of the molecule is further increased upon addition of a second terminal ethynyl group that restores the symmetry of the system, the triplet state is again found to be completely delocalized. The observations suggest that electronic symmetry is of greater importance for triplet state delocalization than other frequently invoked factors such as conformational rigidity or fundamental length-scale limitations.

Valence Band Dependent Charge Transport in Bulk Molecular Electronic Devices Incorporating Highly Conjugated Multi-[(Porphinato)Metal] Oligomers

Molecular electronics offers the potential to control device functions through the fundamental electronic properties of individual molecules, but realization of such possibilities is typically frustrated when such specialized molecules are integrated into a larger area device. Here we utilize highly conjugated (porphinato)metal-based oligomers (PM(n) structures) as molecular wire components of nanotransfer printed (nTP) molecular junctions; electrical characterization of these "bulk" nTP devices highlights device resistances that depend on PM(n) wire length. Device resistance measurements, determined as a function of PM(n) molecular length, were utilized to evaluate the magnitude of a phenomenological β corresponding to the resistance decay parameter across the barrier; these data show that the magnitude of this β value is modulated via porphyrin macrocycle central metal atom substitution [β(PZn(n); 0.065 Å(-1)) < β(PCu(n); 0.132 Å(-1)) < β(PNi(n); 0.176 Å(-1))]. Cyclic voltammetric data, and ultraviolet photoelectron spectroscopic studies carried out at gold surfaces, demonstrate that these nTP device resistances track with the valence band energy levels of the PM(n) wire, which were modulated via porphyrin macrocycle central metal atom substitution. This study demonstrates the ability to fabricate "bulk" and scalable electronic devices in which function derives from the electronic properties of discrete single molecules, and underscores how a critical device function--wire resistance--may be straightforwardly engineered by PM(n) molecular composition.

An Integrated Experimental/Theoretical Study of Structurally Related Poly-Thiophenes Used in Photovoltaic Systems

In this work, a series of eight thiophene-based polymers (exploited as “donors” in bulk heterojunction photovoltaics cells), whose structures were designed to be suitably tuned with the electronic characteristics of the [6,6]-Phenyl C61 butyric acid methyl ester (PCBM), is considered,. The electronic properties of the mono-, di-, trimeric oligomers are reckoned (at the Hartree-Fock and DFT level of the theory) and compared to experimental spectroscopic and electrochemical results. Indeed, electrochemical and spectroscopic results show a systematic difference whose physical nature is assessed and related to the exciton (electron-hole) binding energy (Je,h). The critical comparison of the experimental and theoretical band gaps, i.e., the HOMO-LUMO energy difference, suggests that electrochemical and DFT values are the most suited to being used in the design of a polythiophene-based p-n junction for photovoltaics.

Synthesis, photophysical, electrochemical and electrochemiluminescence properties of A2B2 zinc porphyrins: the effect of π-extended conjugation

Electrogenerated chemiluminescence of A₂B₂ porphyrins: the effect of extended conjugation.