Tuning the HOMO-LUMO gap of pyrene effectively via donor-acceptor substitution: positions 4,5 versus 9,10

Synthesis, Photophysical and Electronic Properties of a D-π-A Julolidine-Like Pyrenyl-o-Carborane

We synthesized 2-(1-1,2-dicarbadodecaboranyl(12))-6,6,12,12-tetramethyl-7,8,11,12-tetrahydro-6H,10H-phenaleno[1,9-fg]pyrido[3,2,1-ij]quinoline (4), a julolidine-like pyrenyl-o-carborane, with pyrene substituted at the 2,7-positions on the HOMO/LUMO nodal plane. Using solid state molecular structures, photophysical data, cyclic voltammetry, DFT and LR-TDDFT calculations, we compare o-carborane and B(Mes)2 (Mes=2,4,6-Me3C6H2) as acceptor groups. Whereas the π-acceptor strength of B(Mes)2 is sufficient to drop the pyrene LUMO+1 below the LUMO, the carborane does not do this. We confirm the π-donor strength of the julolidine-like moiety, however, which raises the pyrene HOMO-1 above the HOMO. In contrast to the analogous pyrene-2-yl-o-carborane, 2-(1-1,2-dicarbadodecaboranyl(12))-pyrene VI, which exhibits dual fluorescence, because the rate of internal conversion between locally-excited (LE) and charge transfer (CT) (from the pyrene to the carborane) states is faster than the radiative decay rate, leading to a thermodynamic equilibrium between the 2 states, 4 shows only single fluorescence, as the CT state involving the carborane as the acceptor moiety in not kinetically accessible, so a more localized CT emission involving the julolidine-like pyrene moiety is observed.

4,9- and 4,10-Substituted pyrenes: synthesis, successful isolation, and optoelectronic properties

We report herein a way to prepare and purify optoelectronic functional 4,9- and 4,10-substituted pyrene isomers. By tuning the size of substituents, the designed 4,9- and 4,10-isomers can be successfully isolated by recycling preparative size-exclusion chromatography (SEC) and/or repeated recrystallization. The structure and purity of the isolated compounds 1-5 have been confirmed by 1H NMR, 13C NMR, and HRMS. The photophysical and electrochemical properties of compounds 1-5 have been studied in detail both experimentally and theoretically. The lowest transitions of these pyrenes, 1-5, are allowed, with moderate to high fluorescence quantum yields and radiative decay rates around 108 s-1. The differences between the electrochemical and photophysical properties of 4,9-, 4,10-, 1,6-, and 2,7-substituted isomers are compared and concluded.

Investigation of Mechanochromic Luminescence of Pyrene-based Aggregation-Induced Emission Luminogens: Correlation between Molecular Packing and Luminescence Behavior

To better understand the correlation between molecular structure and optical properties such as aggregation-induced emission (AIE) and mechanochromic luminescence (MCL) emission, two new pyrene-based derivatives with substitutions at the 4- and 5-positions (1HH) and at the 4-, 5-, 9-, and 10-positions (2HH) were designed and synthesized. Cyano groups were introduced at the periphery of the synthesized compounds (1HCN, 1OCN, 1BCN, 2HCN, 2OCN, and 2BCN) to investigate the influence of these groups on the emission properties of the pyrene derivatives both in solution and in the solid state. The fluorescence emission performance of these compounds in water/acetone mixtures was simultaneously studied, revealing outstanding aggregation-induced emission properties. The typical shift in emission maxima to higher values was attributed to J-aggregate formation in the aggregate state. Careful investigation of the crystal structures demonstrated abundant and intense intermolecular interactions, such as C-H…π and C-H…N hydrogen bonds, contributing to the remarkable mechanochromic luminescence performance of these compounds. The MCL properties of all the compounds were investigated using powder X-ray diffraction, and the remarkable mechanochromic properties were attributed to J-aggregate phenomena in the solid state. These results provide valuable insights into the structure-property relationship of organic MCL materials, guiding the design of efficient organic MCL materials.

Recent Advances in Applications of Fluorescent Perylenediimide and Perylenemonoimide Dyes in Bioimaging, Photothermal and Photodynamic Therapy

The emblematic perylenediimide (PDI) motif which was initially used as a simple dye has undergone incredible development in recent decades. The increasing power of synthetic organic chemistry has allowed it to decorate PDIs to achieve highly functional dyes. As these PDI derivatives combine thermal, chemical and photostability, with an additional high absorption coefficient and near-unity fluorescence quantum yield, they have been widely studied for applications in materials science, particularly in photovoltaics. Although PDIs have always been in the spotlight, their asymmetric counterparts, perylenemonoimide (PMI) analogues, are now experiencing a resurgence of interest with new efforts to create architectures with equally exciting properties. Namely, their exceptional fluorescence properties have recently been used to develop novel systems for applications in bioimaging, biosensing and photodynamic therapy. This review covers the state of the art in the synthesis, photophysical characterizations and recently reported applications demonstrating the versatility of these two sister PDI and PMI compounds. The objective is to show that after well-known applications in materials science, the emerging trends in the use of PDI- and PMI-based derivatives concern very specific biomedicinal applications including drug delivery, diagnostics and theranostics.

One-Pot Synthesis of Pyreno[2,1-b]furan Molecules with Two-Photon Absorption Properties

The development of large π-conjugated polycyclic heteroaromatic materials is of immense interest, both in the academic as well as the industrial community. Herein, we present the efficient one-pot synthesis of novel pyreno[2,1-b]furan molecules from a newly designed intermediate, which display intense green emission (505-516 nm) in solution and a large red shift emission (625-640 nm) in the solid state, because of strong π-π stacking. More interestingly, the compounds exhibit novel two-photon absorption (TPA) properties, and the TPA cross-section (δ) value was increased to 533 GM by regulating the electronic effects of the substituents of the pyreno[2,1-b]furan molecules. This study not only offers a facile strategy for constructing new pyrene-fused luminescence materials with two-photon absorption properties but also provides a new chemical intermediate that opens up a new pathway to advanced materials.

Award Lecture Paper: Hypervalent Iodine-Mediated Oxidative Alkene Arylation: A Thorough Analysis

A hypervalent iodine-mediated, oxidative alkene arylation reaction has been developed. Both Koser’s reagent (PhI(OH)OTs) and (diacetoxyiodo)toluene (TolI(OAc)₂) were equally viable as oxidants, which reacted with ortho-vinylbiphenyl derivatives to produce tri-, tetra- and pentacyclic polycyclic aromatic hydrocarbons (PAHs) in yields up to 97%. Comparison of this stoichiometric reaction with a previously-reported catalytic process showed that these protocols were largely complementary, and that they likely operate via the same general mechanistic sequence involving vinyliodonium salts decomposing into vinylene phenonium ions. Various mechanistic control experiments were conducted, which ruled out epoxides as intermediates, and which showed that E- and Z- alkene geometry in 10-substituted ortho-vinylbiphenyls had no impact on the ensuing isomeric product distributions. These experiments strongly supported the formation of E-vinyliodonium ions as initial reaction intermediates, and while the occurrence of 1,2-phenyl shift products was a common phenomenon, we concluded that alkyl substitution on the ortho-vinylbiphenyl was a requirement for this alternate pathway to occur.

Structure-controlled intramolecular charge transfer in asymmetric pyrene-based luminogens: synthesis, characterization and optical properties

Four asymmetric D–A type pyrene-based luminogens with tunable optical properties were synthesized, which provide an efficient strategy to achieve pyrene-based full-color photoelectric materials.

A Comparative Study of Redox-Active Dithiafulvenyl-Functionalized 1,3,6,8-Tetraphenylpyrene Derivatives

A series of 1,3,6,8-tetraphenylpyrene (TPPy) derivatives substituted with redox-active 1,4-dithiafulvenyl (DTF) groups was synthesized and characterized. The conformational properties of these DTF-TPPys and their TPPy precursors were assessed by X-ray single-crystal and nuclear magnetic resonance analyses. Their electronic and redox properties were examined by ultraviolet-visible absorption, fluorescence, and cyclic voltammetric analyses. The DTF substitution was found to strongly modify the absorption, emission, and electrochemical properties, while detailed effects can be linked to substitution patterns and alkyl side chains attached to the DTF groups. Furthermore, the DTF-TPPy derivatives showed sensitivity to acids; in particular, the vinylic proton of DTF group could undergo efficient proton/deuterium exchange with D₂O in an acidic medium.

2- and 2,7-Substituted para-N-Methylpyridinium Pyrenes: Syntheses, Molecular and Electronic Structures, Photophysical, Electrochemical, and Spectroelectrochemical Properties and Binding to Double-Stranded (ds) DNA

Two N-methylpyridinium compounds and analogous N-protonated salts of 2- and 2,7-substituted 4-pyridyl-pyrene compounds were synthesised and their crystal structures, photophysical properties both in solution and in the solid state, electrochemical and spectroelectrochemical properties were studied. Upon methylation or protonation, the emission maxima are significantly bathochromically shifted compared to the neutral compounds, although the absorption maxima remain almost unchanged. As a result, the cationic compounds show very large apparent Stokes shifts of up to 7200 cm-1 . The N-methylpyridinium compounds have a single reduction at ca. -1.5 V vs. Fc/Fc+ in MeCN. While the reduction process was reversible for the 2,7-disubstituted compound, it was irreversible for the mono-substituted one. Experimental findings are complemented by DFT and TD-DFT calculations. Furthermore, the N-methylpyridinium compounds show strong interactions with calf thymus (ct)-DNA, presumably by intercalation, which paves the way for further applications of these multi-functional compounds as potential DNA-bioactive agents.

Complementary amide-based donor-acceptor with unique nano-scale aggregation, fluorescence, and band gap-lowering properties: a WORM memory device

Organic fluorescent semiconducting nanomaterials have gained widespread research interest owing to their potential applications in the arena of high-tech devices. We designed two pyrazaacene-based compounds, their stacked system, and the role of gluing interactions to fabricate nanomaterials, and determined the prospective band gaps utilizing the density functional theory calculation. The two pyrazaacene derivatives containing complementary amide linkages (-CONH and -NHCO) were efficiently synthesized. The synthesized compounds are highly soluble in common organic solvents as well as highly fluorescent and photostable. The heterocycles and their mixture displayed efficient solvent dependent fluorescence in the visible region of the solar spectrum. Notably, the compounds were associated through complementary NH•••O = C type hydrogen bonding, π-π stacking, and hydrophobic interactions, and thereby afforded nanomaterials with a low band gap. Fascinatingly, the fabricated stacked nanomaterial system exhibited resistive switching behavior, leading to the fabrication of an efficient write-once-read-many-times memory device of crossbar structure.

Synthesis, Photophysical and Electronic Properties of New Red-to-NIR Emitting Donor-Acceptor Pyrene Derivatives

We synthesized new pyrene derivatives with strong bis(para-methoxyphenyl)amine donors at the 2,7-positions and n-azaacene acceptors at the K-region of pyrene. The compounds possess a strong intramolecular charge transfer, leading to unusual properties such as emission in the red to NIR region (700 nm), which has not been reported before for monomeric pyrenes. Detailed photophysical studies reveal very long intrinsic lifetimes of >100 ns for the new compounds, which is typical for 2,7-substituted pyrenes but not for K-region substituted pyrenes. The incorporation of strong donors and acceptors leads to very low reduction and oxidation potentials, and spectroelectrochemical studies show that the compounds are on the borderline between localized Robin-Day class-II and delocalized Robin-Day class-III species.

Structure controlled solvatochromism and halochromic fluorescence switching of 2,2′-bipyridine based donor–acceptor derivatives

Simple donor–acceptor derivatives were prepared by substituting alicyclic amines into the 2,2′-bipyridine (Bpy) core unit and they exhibited substituent and structure dependent  tunable and switchable fluorescence.

Synthesis and comparative studies of K-region functionalized pyrene derivatives

Three new K-region functionalized pyrene derivatives were synthesized and characterized to understand their electronic, redox, and crystallographic properties.

Dimethylaniline functionalised pyrene fluorophores; dual colour pH switching in solution and self-assembled monolayers

A pyrene charge transfer fluorophore with three ionizable N,N-dimethylaniline moieities was explored as an interfacial pH switch. The parent carboxylate compound and the thiolated derivative were shown by spectroscopy combined with DFT calculation to be successively and reversibly protonated. Protonation leads to progressive decrease of intensity of the 550 nm centered N,N-dimethylaniline to pyrene charge transfer emission which on protonation of the third site, leads to extinction of this transition and evolution of an intense blue (450 nm) pyrene-centered emission. Concomitant loss of the charge transfer absorbance was observed and the changes are reversed on neutralization of pH. A self-assembled monolayer of the thiolated derivative was prepared on gold and found from voltammetry of ferricyanide/ferrocyanide probe to form close packed monolayers. The probe voltammetry, label-free electrochemical impedance spectroscopy of the film was monitored as a function of pH and progressive, but reversible protonation steps were reflected in decreasing film resistance. The Stokes shift of the probe prevents self-quenching so a broad, charge transfer fluorescence centered around 540 nm was recorded for the self-assembled monolayer where as per solution, progressive and reversible reduction in intensity was observed. The facile assembly, impedance and optical switching make these materials potentially interesting as on-off or two colour on-off-on fluorescence switches with potential applications in logic gates or in responsive surface applications.

Photoactive organic material discovery with combinatorial supramolecular assembly

Organic semiconductors have received substantial attention as active components in optoelectronic devices because of their processability and customizable properties. Tailoring the organic active layer in these devices to exhibit the desired optoelectronic properties requires understanding the complex and often subtle structure-property relationships governing their photophysical response to light. Both structural organization and molecular orbitals play pivotal roles, and their interactions with each other are difficult to anticipate based upon the structure of the components alone, especially in systems comprised of multiple components. In pursuit of design rules, there is a need to explore multicomponent systems combinatorially to access larger data sets, and supramolecularly to use error correcting, noncovalent assembly to achieve long-range order. This review will focus on the use of supramolecular chemistry to study combinatorial, hierarchical organic systems with emergent optoelectronic properties. Specifically, we will describe systems that undergo excited state deactivation by charge transfer (CT), singlet fission (SF), and Förster resonance energy transfer (FRET). Adopting combinatorial, supramolecular assembly to study emergent photophysics promises to rapidly accelerate progress in this research field.

Noncovalent Functionalization of 2D Black Phosphorus with Fluorescent Boronic Derivatives of Pyrene for Probing and Modulating the Interaction with Molecular Oxygen

We studied the chemical-physical nature of interactions involved in the formation of adducts of two-dimensional black phosphorus (2D BP) with organoboron derivatives of a conjugated fluorescent molecule (pyrene). Time-resolved fluorescence spectroscopy showed a stabilization effect of 2D BP on all derivatives, in particular for the adducts endowed with the boronic functionalities. Also, a stronger modulation of the fluorescence decay with oxygen was registered for one of the adducts compared to the corresponding organoboron derivative alone. Nuclear magnetic resonance experiments in suspension and density functional theory simulations confirmed that only noncovalent interactions were involved in the formation of the adducts. The energetic gain in their formation arises from the interaction of P atoms with both C atoms of the pyrene core and the B atom of the boronic functionalities, with a stronger contribution from the ester with respect to the acid one. The interaction results in the lowering of the band gap of 2D BP by around 0.10 eV. Furthermore, we demonstrated through Raman spectroscopy an increased stability toward oxidation in air of 2D BP in the adducts in the solid state (more than 6 months). The modification of the electronic structure at the interface between 2D BP and a conjugated organic molecule through noncovalent stabilizing interactions mediated by the B atom is particularly appealing in view of creating heterojunctions for optoelectronic, photonic, and chemical sensing applications.

Inhibiting guanine oxidation and enhancing the excess-electron-transfer efficiency of a pyrene-modified oligonucleotide by introducing an electron-donating group on pyrene

PipPyU and OMePyU enhance the reduction efficiency without oxidizing guanine in DNA-mediated electron transfer.

Buchwald-Hartwig Coupling at the Naphthalenediimide Core: Access to Dendritic, Panchromatic NIR Absorbers with Exceptionally Low Band Gap

The first successful Buchwald-Hartwig reaction at the naphthalenediimide core is reported, leading to the coupling of diverse secondary aromatic amines including dendritic donors. The G1-dendrimer-based donor exhibit blackish color, providing access to black absorbing systems. λ_onset values up to 1070 nm was achieved, which is the maximum from a single NDI scaffold. These dyes also manifest multielectron reservoir properties. A total of eight-redox states with a band gap of ∼0.95 eV was accomplished.

Synthesis and Optical Properties of Donor-Acceptor-Type 1,3,5,9-Tetraarylpyrenes: Controlling Intramolecular Charge-Transfer Pathways by the Change of π-Conjugation Directions for Emission Color Modulations

In dipolar organic π-conjugated molecules, variable photophysical properties can be realized through efficient excited-state intramolecular charge transfer (ICT), which essentially depends on the π-conjugation patterns. Herein, we report a controllable regioselective strategy for synthesis and optical properties of two donor-acceptor (DA)-type 1,3,5,9-tetraarylpyrenes (i.e., 1,3-A/5,9-D (4b) and 1,3-D/5,9-A (4c)) by covalently integrating two phenyl rings and two p-OMe/CHO-substituted phenyl units into the 2-tert-butylpyrene building block, in which the two phenyl rings substituted at the 1,3-positions act as acceptors for 4b or as donors for 4c and the two p-OMe or p-CHO-substituted phenyl moieties substituted at the K-region of 5,9-positions act as donors for 4b or as acceptors for 4c, respectively. Density functional theory calculations on their frontier molecular orbitals and UV-vis absorption of S₀ → S₁ transition theoretically predicted that the change of π-conjugation directions in the two DA pyrenes could be realized through a variety of substitution patterns, implying that the dissimilar ground-state and excited-state electronic structures exist in each molecule. Their single-crystal X-ray analysis reveal their highly twisted conformations that are beneficial for inhibiting the π-aggregations, which are strikingly different from the normal 1,3,5,9-tetraphenylpyrenes (4a) and related 1,3,6,8-tetraarylpyrenes. Indeed, experimental investigations on their optical properties demonstrated that the excited-state ICT pathways can be successfully controlled by the change of π-conjugation directions through the variety of substitution positions, resulting in the modulations of emission color from deep-blue to green in solution. Moreover, for the present DA pyrenes, highly fluorescent emissions with moderate-to-high quantum yields both in the thin film and in the doped poly(methyl methacrylate) film were obtained, suggesting them as promising emitting materials for the fabrication of organic light-emitting diodes.

Heli(aza)cene: A Helical Molecular Tweezer with Tunable Intra- and Intermolecular Charge Transfer

Non-planar fluorophores offer unique avenues of intra- and intermolecular energy transfer not available in their planar counterparts. We have rationally designed a molecular tweezer based on the pyridine-2,6-dicarboxamide framework having two structurally similar arms with extended π-surface. We termed this molecular tweezer as Heli(aza)cene (HAC) due to its spontaneous adoption of helical conformation stabilized by the amide and imine moieties present in it. In the helical conformation, the two arms of HAC are twisted unequally. This asymmetry confers dissimilar electronic character to the two arms and results in intramolecular charge transfer interactions in HAC. Homochiral stacking of the P- and the M- helices in crystal, and profound redshifting of the emission at higher concentrations of HAC was attributed to intermolecular charge-transfer interactions in aggregated/crystal state. Exposure of HAC, in solution as well as in the solid state, to Lewis/Brønsted acids results in rapid and vibrant color changes. This is the first example of a π-layered helical molecule exhibiting tunable intra-/intermolecular charge-transfer characteristics.

Pyrene Molecular Orbital Shuffle-Controlling Excited State and Redox Properties by Changing the Nature of the Frontier Orbitals

We show that by judicious choice of substituents at the 2- and 7-positions of pyrene, the frontier orbital order of pyrene can be modified, giving enhanced control over the nature and properties of the photoexcited states and the redox potentials. Specifically, we introduced a julolidine-like moiety and Bmes₂ (mes=2,4,6-Me₃ C₆ H₂ ) as very strong donor (D) and acceptor (A), respectively, giving 2,7-D-π-D- and unsymmetric 2,7-D-π-A-pyrene derivatives, in which the donor destabilizes the HOMO-1 and the acceptor stabilizes the LUMO+1 of the pyrene core. Consequently, for 2,7-substituted pyrene derivatives, unusual properties are obtained. For example, very large bathochromic shifts were observed for all of our compounds, and unprecedented green light emission occurs for the D/D system. In addition, very high radiative rate constants in solution and in the solid state were recorded for the D-π-D- and D-π-A-substituted compounds. All compounds show reversible one-electron oxidations, and Jul₂ Pyr exhibits a second oxidation, with the largest potential splitting (ΔE=440 mV) thus far reported for 2,7-substituted pyrenes. Spectroelectrochemical measurements confirm an unexpectedly strong coupling between the 2,7-substituents in our pyrene derivatives.

Polycyclic Aromatic Hydrocarbons via Iron(III)-Catalyzed Carbonyl-Olefin Metathesis

Polycyclic aromatic hydrocarbons are important structural motifs in organic chemistry, pharmaceutical chemistry, and materials science. The development of a new synthetic strategy toward these compounds is described based on the design principle of iron(III)-catalyzed carbonyl-olefin metathesis reactions. This approach is characterized by its operational simplicity, high functional group compatibility, and regioselectivity while relying on FeCl3 as an environmentally benign, earth-abundant metal catalyst. Experimental evidence for oxetanes as reactive intermediates in the catalytic carbonyl-olefin ring-closing metathesis has been obtained.

Impacts of interfacial charge transfer on nanoparticle electrocatalytic activity towards oxygen reduction

Interfacial electron transfer within platinum and non-platinum-based nanocatalysts plays a significant role in the manipulation of the electronic interactions between oxygen species and the catalyst surfaces, which may be exploited as an effective mechanism to enhance and optimize the activity towards oxygen reduction.

Bandgap Engineering in π-Extended Pyrroles. A Modular Approach to Electron-Deficient Chromophores with Multi-Redox Activity

A family of bandgap-tunable pyrroles structurally related to rylene dyes was computationally designed and prepared using robust, easily scalable chemistry. These pyrroles show highly variable fluorescence properties and can be used as building blocks for the synthesis of electron-deficient oligopyrroles. The latter application is demonstrated through the development of π-extended porphyrins containing naphthalenediamide or naphthalenediimide units. These new macrocycles exhibit simultaneously tunable visible and near-IR absorptions, an ability to accept up to 8 electrons via electrochemical reduction, and high internal molecular free volumes. When chemically reduced under inert conditions, the most electron-deficient of these macrocycles revealed reversible formation of eight charged states, characterized by remarkably red-shifted optical absorptions, extending beyond 2200 nm. Such features make these oligopyrroles of interest as functional chromophores, charge-storage materials, and tectons for crystal engineering.

Phenanthrylene-butadiynylene and Phenanthrylene-thienylene Macrocycles: Synthesis, Structure, and Properties

A series of macrocycles consisting of 9,10-substituted phenanthrenes connected by butadiynylene linkers in positions 3 and 6 has been described as well as their transformation into the corresponding phenanthrylene-thienylene macrocycles. Structure and properties of the macrocycles, such as self-association in solution and optical and electrochemical properties, were studied and reported in a comparative manner with respect to the effects of the different sizes and shapes of the macrocycles and the character and length of their side chains.

Functionalization of Pyrene To Prepare Luminescent Materials-Typical Examples of Synthetic Methodology

Pyrene-based π-conjugated materials are considered to be an ideal organic electro-luminescence material for application in semiconductor devices, such as organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs) and organic photovoltaics (OPVs), and so forth. However, the great drawback of employing pyrene as an organic luminescence material is the formation of excimer emission, which quenches the efficiency at high concentration or in the solid-state. Thus, in order to obtain highly efficient optical devices, scientists have devoted much effort to tuning the structure of pyrene derivatives in order to realize exploitable properties by employing two strategies, 1) introducing a variety of moieties at the pyrene core, and 2) exploring effective and convenient synthetic strategies to functionalize the pyrene core. Over the past decades, our group has mainly focused on synthetic methodologies for functionalization of the pyrene core; we have found that formylation/acetylation or bromination of pyrene can selectly lead to functionalization at K-region by Lewis acid catalysis. Herein, this Minireview highlights the direct synthetic approaches (such as formylation, bromination, oxidation, and de-tert-butylation reactions, etc.) to functionalize the pyrene in order to advance research on luminescent materials for organic electronic applications. Further, this article demonstrates that the future direction of pyrene chemistry is asymmetric functionalization of pyrene for organic semiconductor applications and highlights some of the classical asymmetric pyrenes, as well as the latest breakthroughs. In addition, the photophysical properties of pyrene-based molecules are briefly reviewed. To give a current overview of the development of pyrene chemistry, the review selectively covers some of the latest reports and concepts from the period covering late 2011 to the present day.

Effects of para-substituents of styrene derivatives on their chemical reactivity on platinum nanoparticle surfaces

Stable platinum nanoparticles were successfully prepared by the self-assembly of para-substituted styrene derivatives onto the platinum surfaces as a result of platinum-catalyzed dehydrogenation and transformation of the vinyl groups to the acetylene ones, forming platinum-vinylidene/-acetylide interfacial bonds. Transmission electron microscopic measurements showed that the nanoparticles were well dispersed without apparent aggregation, suggesting sufficient protection of the nanoparticles by the organic capping ligands, and the average core diameter was estimated to be 2.0 ± 0.3 nm, 1.3 ± 0.2 nm, and 1.1 ± 0.2 nm for the nanoparticles capped with 4-tert-butylstyrene, 4-methoxystyrene, and 4-(trifluoromethyl)styrene, respectively, as a result of the decreasing rate of dehydrogenation with the increasing Taft (polar) constant of the para-substituents. Importantly, the resulting nanoparticles exhibited unique photoluminescence, where an increase of the Hammett constant of the para-substituents corresponded to a blue-shift of the photoluminescence emission, suggesting an enlargement of the HOMO-LUMO band gap of the nanoparticle-bound acetylene moieties. Furthermore, the resulting nanoparticles exhibited apparent electrocatalytic activity towards oxygen reduction in acidic media, with the best performance among the series of samples observed with the 4-tert-butylstyrene-capped nanoparticles due to an optimal combination of the nanoparticle core size and ligand effects on the bonding interactions between platinum and oxygen species.

0-0 Energies Using Hybrid Schemes: Benchmarks of TD-DFT, CIS(D), ADC(2), CC2, and BSE/GW formalisms for 80 Real-Life Compounds

The 0-0 energies of 80 medium and large molecules have been computed with a large panel of theoretical formalisms. We have used an approach computationally tractable for large molecules, that is, the structural and vibrational parameters are obtained with TD-DFT, the solvent effects are accounted for with the PCM model, whereas the total and transition energies have been determined with TD-DFT and with five wave function approaches accounting for contributions from double excitations, namely, CIS(D), ADC(2), CC2, SCS-CC2, and SOS-CC2, as well as Green's function based BSE/GW approach. Atomic basis sets including diffuse functions have been systematically applied, and several variations of the PCM have been evaluated. Using solvent corrections obtained with corrected linear-response approach, we found that three schemes, namely, ADC(2), CC2, and BSE/GW allow one to reach a mean absolute deviation smaller than 0.15 eV compared to the measurements, the two former yielding slightly better correlation with experiments than the latter. CIS(D), SCS-CC2, and SOS-CC2 provide significantly larger deviations, though the latter approach delivers highly consistent transition energies. In addition, we show that (i) ADC(2) and CC2 values are extremely close to each other but for systems absorbing at low energies; (ii) the linear-response PCM scheme tends to overestimate solvation effects; and that (iii) the average impact of nonequilibrium correction on 0-0 energies is negligible.

1-, 3-, 6-, and 8-Tetrasubstituted Asymmetric Pyrene Derivatives with Electron Donors and Acceptors: High Photostability and Regioisomer-Specific Photophysical Properties

The systematic synthesis of five 1-, 3-, 6-, and 8-tetrasubstituted asymmetric pyrenes with electron donor and acceptor moieties is presented, together with an examination of their photophysical properties. Pyrene derivative PA1, containing one formyl and three piperidyl groups, showed bright solvatochromic fluorescence from green (λem = 557 nm, ΦFL = 0.94 in hexane) to red (λem = 648 nm, ΦFL = 0.50 in methanol), suggesting potential applications for PA1 as an environmentally responsive probe. Although the synthesis of simple 1- and 3-disubstituted pyrene derivatives is considered difficult, PA13, with two formyl groups at the 1- and 3-positions and two piperidyl groups at the 6- and 8-positions, could be synthesized successfully. PA13 exhibited less pronounced solvatochromism, but displayed a narrow fluorescent band with high ΦFL in all solvents (ΦFL > 0.75). Moreover, its absorption band displayed an exceptional bathochromic shift compared to the other derivatives (e.g., λabs = 480 and 522 nm in ethanol for PA1 and PA13, respectively), suggesting that such modifications of pyrene may be quite important for the modulation of its energy gap. Additionally, all compounds exhibited exceptionally high photostability, which highlights the advantage of these new dyes and provides new insights on the design of photostable fluorophores.

Regioselective Substitution at the 1,3- and 6,8-Positions of Pyrene for the Construction of Small Dipolar Molecules

This article presents a novel asymmetrical functionalization strategy for the construction of dipolar molecules via efficient regioselective functionalization along the Z-axis of pyrene at both the 1,3- and 6,8-positions. Three asymmetrically substituted 1,3-diphenyl-6,8-R-disubsituted pyrenes were fully characterized by X-ray crystallography, photophysical properties, electrochemistry, and density functional theory calculations.