Electron Push–Pull Effects in 3,9-Bis(p-(R)-diphenylamino)perylene and Constraint on Emission Color Tuning

The Renaissance of Poly(3-hexylthiophene) as a Promising Hole-Transporting Material Toward Efficient and Stable Perovskite Solar Cells

To push the commercialization of the promising photovoltaic technique of perovskite solar cells (PSCs), the three-element golden law of efficiency, stability, and cost should be followed. As the key component of PSCs, hole-transporting materials (HTMs) involving widely-used organic semiconductors such as 2,2',7,7'-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluorene (Spiro-OMeTAD) or poly(triarylamine) (PTAA) usually suffer high-cost preparation and low operational stability. Fortunately, the studies on the classical p-type polymer poly(3-hexylthiophene) (P3HT) as an alternative HTM have recently sparked a broad interest due to its low-cost synthesis, excellent batch-to-batch purity, superior hole conductivity as well as controllable and stable film morphology. Despite this, the device efficiency still lags behind P3HT-based PSCs mainly owing to the mismatched energy level and poor interfacial contact between P3HT and the perovskite layer. Hence, in this review, the study timely summarizes the developed strategies for overcoming the corresponding issues such as interface engineering, morphology regulation, and formation of composite HTMs from which some critical clues can be extracted to provide guidance for further boosting the efficiency and stability of P3HT-based devices. Finally, in the outlook, the future research directions either from the viewpoint of material design or device engineering are outlined.

Systematic radical species control by electron push-pull substitution in the perylene-based D-π-A compounds

Organic radical materials have been mainly reported on the stabilization of radical species because of their high energy and reactivity, while design strategies for controlling radical species beyond stabilization have remained challenging. Here, we report the electronic push-pull control spanning the neutral to the radical state of a series of perylene-based donor-π-acceptors (D-π-A). By introducing electron-withdrawing and -donating R groups to the donor of D-π-A, the observed intramolecular interactions controllable at the HOMO level led to the exploration of radical species. D-π-A with redox-active sites was transformed to (D-π-A)˙⁺ and (D-π-A)˙^- in response to an external electrical stimulus under stabilization by perylene, resulting in new absorption peaks. In particular, the increasing absorption peaks of (D-π-A)˙⁺ showed a spectral shift and intensity change according to the R group, unlike those of (D-π-A)˙^-. These experimental results support that the DFT/TD-DFT data suggests the radical cationic SOMO level variability. As a result, we provide a strategy for controlling the systematic radical species using the electron push-pull effect.

Synthesis and Emergent Photophysical Properties of Diketopyrrolopyrrole-Based Supramolecular Self-Assembly

Diketopyrrolopyrrole (DPP)-based molecular semiconductors exhibit intriguing optical and charge transport properties. Herein, we rationally design a series of electronically identical but structurally distinct Hamilton receptor (HR)-based supramolecular assembly of DPP. The HR endows supramolecular assemblies via hydrogen bonding with enhanced structural ordering and excitonic couplings. The mechanism of supramolecular self-assembly was probed by diffusion ordered spectroscopy (DOSY) nuclear magnetic resonance (NMR) and solid-state IR spectroscopy studies. We investigated the morphology of self-assembly, photophysical and electrochemical properties and compared them with the identical DPP molecular structures without HRs. The microstructure of self-assembly was probed with atomic force microscopy in thin films. Subsequently, the influence of solid-state packing was studied by single-crystal X-ray diffraction. The single-crystal structure of HR-TDPP-C20 reveals slipped stack arrangements between the two neighboring chromophores with π-π stacking distance and slip angle of 3.55 Å and 35.4°, respectively. Notably, the slight torsional angle of 1° between thiophene and lactam rings and small π-π stacking distance suggest a significant intermolecular coupling between thiophene (D) and lactam (A) rings. This intramolecular coupling between two π-π chromophore stacks manifests in their optical properties. In this manuscript, we report rational design and synthesis of supramolecular self-assembly of DPP with a collection of compelling structural and optical properties.

1-Substituted Perylene Derivatives by Anionic Cyclodehydrogenation: Analysis of the Reaction Mechanism

Perylene derivatives constitute a promising class of compounds with technological applications mainly due to their optoelectronic properties. One mechanism proposed to synthesize them, starting from binaphthyl derivatives, is anionic cyclodehydrogenation (under reductive conditions). However, the scope of this reaction is limited. In the present study, we report a theoretical and experimental analysis of this particular reaction mechanism for its use in the synthesis of 1-substituted perylenes. Different substituents at position 2 of 1,1'-binaphthalene were evaluated: -OCH₃, -OSi(CH₃)₂C(CH₃)₃, and -N(CH₃)₂. Based on density functional theory (DFT) calculations on the proposed mechanism, we suggest that the cyclization takes place from binaphthyl dianion instead of its radical anion. This dianion has an open-shell diradical nature, and this could be the species that was detected by EPR in previous studies. The O-substituted derivatives could not afford the perylene derivatives since their radical anions fragment and the necessary binaphthyl dianion could not be formed. On the other hand, 49% of N,N-dimethylperylen-1-amine was obtained starting from the N-substituted 2-binapthyl derivative as a substrate, employing a simpler experimental methodology.

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.

Catalyst- and Additive-Free Approach to Constructing Benzo-oxazine, Benzo-oxazepine, and Benzo-oxazocine: O Atom Transfer and C═O, C-N, and C-O Bond Formation at Room Temperature

An exclusive synthesis of benzo-oxazine, benzo-oxazepine, and benzo-oxazocine from aryl propanal and 2-(hydroxyamino)phenyl alcohol under metal-free conditions is described. O atom transfer and formation of new C═O, C-N, and C-O bonds occur at room temperature to form six-, seven-, and eight-membered heterocycles under one-pot reaction conditions without using an external oxidant and base. The photophysical properties are studied using ultraviolet-visible absorption and photoluminescence. The mechanistic elucidation is well supported by control experiment and literature precedents.

Development of Rofecoxib-Based Fluorescent Probes and Investigations on Their Solvatochromism, AIE Activity, Mechanochromism, and COX-2-Targeted Bioimaging

Cyclooxygenase-2 (COX-2) fluorescent probes are promising tools for early diagnosis of cancer. Traditionally, COX-2 probes were designed by connecting two parts, a fluorophore and a COX-2 binding unit, via a flexible linker. Herein, a new class of COX-2-specific fluorescent probes have been developed via one-step modification from rofecoxib by an integrative approach to combine the binding unit and the fluorophore into one. Among them, several new rofecoxib analogues not only exhibited still potent COX-2 binding ability but also exhibited attractive fluorescence properties, such as tunable blue-red emission, solvatochromism, aggression-induced emission behavior, and mechanochromism. Notably, the emission of 2a16 can be switched between green-yellow in the crystalline state and red-orange in the amorphous state by grinding and fuming treatments. Furthermore, the highly fluorescent compound 2a16 (Φ_f = 0.94 in powder) displayed a much stronger fluorescence imaging of COX-2 in HeLa cancer cells overexpressing COX-2 than RAW264.7 normal cells with a minimal expression of COX-2. Most importantly, 2a16 can light up human cancer tissues from adjacent normal tissues with a much brighter fluorescence by targeting the COX-2 enzyme. These results demonstrated the potential of 2a16 as a new red fluorescent probe for human cancer imaging in clinical applications.

Synthesis and Structure-Photophysics Evaluation of 2-N-Amino-quinazolines: Small Molecule Fluorophores for Solution and Solid State

2-N-aminoquinazolines were prepared by consecutive S_N Ar functionalization. X-ray structures display the nitrogen lone pair of the 2-N-morpholino group in conjugation with the electron deficient quinazoline core and thus representing electronic push-pull systems. 2-N-aminoquinazolines show a positive solvatochromism and are fluorescent in solution and in solid state with quantum yields up to 0.73. Increase in electron donor strength of the 2-amino substituent causes a red-shift of the intramolecular charge transfer (ICT) band (300-400 nm); whereas the photoluminescence emission maxima (350-450 nm) is also red-shifted significantly along with an enhancement in photoluminescence efficiency. HOMO-LUMO energies were estimated by a combination of electrochemical and photophysical methods and correlate well to those obtained by computational methods. ICT properties are theoretically attributed to an excitation to Rydberg-MO in SAC-CI method, which can be interpreted as n-π* excitation. 7-Amino-2-N-morpholino-4-methoxyquinazoline responds to acidic conditions with significant increases in photoluminescence intensity revealing a new turn-on/off fluorescence probe.

One-Step Transformation from Rofecoxib to a COX-2 NIR Probe for Human Cancer Tissue/Organoid Targeted Bioimaging

COX-2 fluorescent probes are promising tools for cancer diagnosis. Such probes have been conventionally designed by conjugating a fluorophore to COX-2 inhibitors through lengthy synthetic processes. Herein, a type of fluorescent probe for COX-2 imaging has been developed using a single-step process from rofecoxib. In total, six rofecoxib analogues were designed using this unique strategy. Several analogues retained comparative COX-2 targeting activity of rofecoxib and also exhibited attractive fluorescent properties, which were investigated using a combination of experimental and theoretical approaches. The most potent analogue, 2a1, displayed strong fluorescent imaging of COX-2 in HeLa cells overexpressing COX-2 compared to Raw 264.7 cells and celecoxib-treated HeLa cells that expressed low levels of COX-2. Notably, our studies indicate that 2a1 can differentiate human cancer tissue from adjacent tissue with much brighter fluorescence either in histological section or cultured 3D organoids. These results illustrate the potential of 2a1 as a COX-2 near infrared fluorescent probe for human cancer imaging in clinical settings.

Organoplatinum(II)-Based Self-Complementary Molecular Tweezers with Guest-Induced Fluorochromic Behaviors

Cyclometalated organoplatinum(II) complexes have aroused tremendous interests due to their square-planar geometry and intriguing photophysics. To access multiplatinum systems with more than three cyclometalated organoplatinum(II) units, the traditional covalent synthetic approach suffers from tedious multistep reactions with low overall yield. In comparison, supramolecular assembly can be regarded as an effective strategy toward multiplatinum(II) architectures. Despite the progresses achieved, it is still challenging to fabricate well-ordered supramolecular assemblies with precise numbers of organoplatinum(II) units. Herein, self-complementary dimerized molecular tweezers with four cyclometalated platinum(II) units have been successfully constructed by taking advantage of dual roles of the incorporated 2,2':6',2''-terpyridine unit (serving as the rigid spacer and encapsulated guest). Furthermore, addition of electron-rich carbazoles leads to conversion of the self-complementary structure to molecular tweezer/guest complexes. Such a structural transformation gives rise to the concomitant luminescent color change. The unique guest-induced fluorochromic phenomena, which are seldom reported in the previous host-guest systems, would be promising as tunable luminescent and ratiometric sensing materials.

A molecular tailoring approach – a new guide to quantify the energy of push–pull effects: a case study on (E)-3-(1H-pyrrol-2-yl)prop-2-enones

We report a new approach to quantify the push–pull effect in molecules with internal charge transfer.

Terphenyl backbone-based donor–π–acceptor dyads: geometric isomer effects on intramolecular charge transfer

The molecular geometry effects of ortho, meta, and para-terphenyl based donor–π–acceptor (D–π–A) dyads on intramolecular charge transfer (ICT) were studied to investigate structure-ICT relationships.