Near-IR absorbing 1,1,4,4-tetracyanobutadiene-functionalized phenothiazine sulfones

Design, synthesis, and redox properties of donor-π-donor ferrocenyl functionalized phenothiazine derivatives

A set of ferrocenyl functionalized phenothiazine derivatives Fc1-4 were designed and synthesized via Pd-catalyzed Buchwald-Hartwig and Suzuki cross-coupling reactions. Phenothiazine was used as a central donor and ferrocenyl as a terminal donor unit. Herein we examine the impact of variation of substituents at the N-position of phenothiazine, including H, phenyl, phenyl-linked ferrocenyl, and biphenyl-linked ferrocenyl units on the photophysical, electrochemical, and thermal behaviors. The electronic absorption spectra reveal that the N-H and phenyl-substituted phenothiazine derivatives Fc1 and Fc2 show red-shifted absorption as compared to the tri-ferrocenyl substituted phenothiazine derivatives Fc3 and Fc4. The electrochemical studies reveal that Fc1 shows two quasi reversible and one reversible oxidation wave, whereas Fc2-4 exhibits two reversible oxidation waves. The spectroelectrochemical analysis of Fc1-4 revealed significant NIR spectral changes during the second oxidation cycle. DFT calculations were performed to optimize the molecular geometry and evaluate the frontier energy levels of Fc1-4. The structure of ferrocenyl functionalized phenothiazine derivatives Fc2 and Fc3 was confirmed by a single-crystal X-ray diffraction study. Fc2 and Fc3 crystallize as red needle-shaped crystals exhibiting zig-zag 3D crystal packing arrangements. This study highlights the photophysical, electrochemical, and redox characteristics of ferrocenyl-functionalized phenothiazine derivatives, making them potential candidates for optoelectronic applications.

Recent Progress of Mechanofluorochromism and Mechanoluminescence for Phenothiazine Derivatives and Analogues

Mechanofluorochromism (MFC) and mechanoluminescence (ML) materials have garnered significant attention from researchers due to their potential applications in anti-counterfeiting, optical recording, photodynamic therapy, bioimaging, stress sensing, display technology, and ink-free printing paper. Among the various building blocks utilized in these materials, phenothiazine (PTZ) has emerged as a widely employed fundamental component owing to its distinctive electronic and optical properties as well as its facile modification capabilities. Summarizing the recent progress of PTZ derivatives and analogues in this field holds practical significance. In this review article, we classify over one hundred compounds into a few classes based on the positions of substituents and provide detailed descriptions of their contributions to MFC and ML research respectively. This comprehensive review aims to offer theoretical insights and practical examples for researchers engaged in designing and developing new phenothiazine functional materials while serving as a bridge for further exploration of MFC or ML studies.

Design, synthesis, and redox properties of ferrocene-functionalized phenothiazine and phenothiazine sulfone isomers

A set of ferrocene-functionalized phenothiazine (PTZ) derivatives 1-3 was designed and synthesized via the Pd-catalyzed Buchwald-Hartwig cross-coupling reaction in good yields. 1-3 were treated with m-chloroperbenzoic acid (m-CPBA), which oxidizes the sulfur (S) atom in the thiazine ring to a sulfone, resulting in ferrocene substituted phenothiazine sulfone derivatives 4-6. The influence of the sulfur (S) oxidation state on the photophysical, redox, and spectroelectrochemical properties, and thermal stability of the phenothiazine and phenothiazine sulfone derivatives, containing the ferrocene unit at the para, meta, and ortho positions of the phenylene ring attached to the N-position of phenothiazine and phenothiazine sulfone, was examined. The photophysical behaviour indicates that the phenothiazine sulfone derivatives 4-6 exhibit a hypsochromic shift in the deep UV region of the UV-vis absorption spectrum as compared to 1-3. The ortho-ferrocene-functionalized phenothiazine derivative 1 exhibits greater thermal stability than its sulfone counterpart 4. The cyclic voltammetry studies on 1-6 show two oxidation waves. The redox behavior of 1-6 reveals that phenothiazine sulfone derivatives (4-6) exhibit shifts towards more positive oxidation potentials compared to their phenothiazine counterparts (1-3). Density functional theory (DFT) and TD-DFT calculations were performed on 1-6 to analyze the molecular geometry, frontier molecular orbitals, molecular electrostatic potentials (MEPs), and density of states (DOS). The spectroelectrochemical behavior of redox-active species of 1-3 shows a new and intense absorption band centred around 525 nm and three new red-shifted broad peaks centred at around 700, 790, and 895 nm in the NIR region. The molecular structures of 1-6 were confirmed by single crystal X-ray diffraction. Phenothiazine sulfone derivatives (4-6) exhibit intermolecular hydrogen bonding within their crystal lattice, contributing to stabilizing their solid-state architecture.

Synthesis of Nonplanar Push-Pull Chromophores with Various Heterocyclic Moieties via [2 + 2] Cycloaddition-Retroelectrocyclization Reaction

A series of 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) derivatives with various heterocyclic moieties, including pyridine, carbazole, indole, and benzothiadiazole, was newly synthesized through a [2 + 2] cycloaddition-retroelectrocyclization reaction. Symmetric electron-rich 1,3-butadiynes with end-capped heterocyclic substituents were reacted with tetracyanoethylene (TCNE), yielding the target TCBD products in 60-80% yields under ambient or mild heating conditions. The thermal stability and optical and electrochemical properties of both 1,3-butadiyne precursors and the corresponding TCBD derivatives were investigated by using thermogravimetric analysis (TGA), UV-vis spectroscopy, and cyclic voltammetry (CV). The TCBD derivatives featured narrow bandgaps due to the intramolecular charge transfer interactions of push-pull chromophores. In addition, the optimized structures and frontier molecular orbitals with their energy levels were determined by density functional theory (DFT) calculations.

NIR absorbing ferrocenyl perylenediimide-based donor-acceptor chromophores

A set of ferrocenyl-functionalized perylenediimide (PDI) compounds and their 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) derivatives 1-5 were designed and synthesized using palladium-catalyzed Sonogashira cross-coupling, followed by a thermally activated [2 + 2] cycloaddition-retroelectrocyclization [CA-RE] reaction with a 1,1,2,2-tetracyanoethylene (TCNE) acceptor in good yields. The TCBD group works as an acceptor, whereas the ferrocenyl group acts as a donor at the central PDI core. The effects of varying the number of ferrocenyl and TCNE groups on the photophysical, thermal, electrochemical, and spectroelectrochemical properties were studied. The di-substituted PDI derivatives 3, 4, and 5 exhibit bathochromic shifts in the absorption spectra compared to 1 and 2, attributed to the extended π-conjugation. The electrochemical analysis of derivatives 2, 4, and 5 shows multiple reduction waves in the low potential region due to the presence of TCBD and perylenediimide acceptor units. Spectroelectrochemical studies were performed, showing that upon applying redox potentials, the absorption spectra shifted from the visible to the near-infrared (NIR) region. Computational calculations indicate that in the HOMO, the electron density is localized on the ferrocene unit, while in the LUMO, it is distributed over the PDI-TCBD unit, indicating a strong D-A interaction.

Near-IR Capturing N-Methylbenzene Sulfonamide-Phenothiazine Incorporating Strong Electron Acceptor Push-Pull Systems: Photochemical Ultrafast Carrier Dynamics

Unraveling the intriguing aspects of the intramolecular charge transfer (ICT) phenomenon of multi-modular donor-acceptor-based push-pull systems are of paramount importance considering their promising applications, particularly in solar energy harvesting and light-emitting devices. Herein, a series of symmetrical and unsymmetrical donor-acceptor chromophores 1-6, are designed and synthesized by the Corey-Fuchs reaction via Evano's condition followed by [2+2] cycloaddition retroelectrocyclic ring-opening reaction with strong electron acceptors TCNE and TCNQ in good yields (~60-85 %). The photophysical, electrochemical, and computational studies are investigated to explore the effect of incorporation of strong electron acceptors 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) and dicyanoquinodimethane (DCNQ) with phenothiazine (PTZ) donor. An additional low-lying broad absorption band extended towards the near-infrared (NIR) region suggests charge polarization after the introduction of the electron acceptors in both symmetrical and asymmetrical systems, leading to such strong ICT bands. The electrochemical properties reveal that reduction potentials of 3 and 6 are lower than those of 2 and 5, suggesting DCNQ imparts more on the electronic properties and hence largely contributes to the stabilization of LUMO energy levels than TCBD, in line with theoretical observations. Relative positions of the frontier orbitals on geometry-optimized structures further support accessing donor-acceptor sites responsible for the ICT transitions. Eventually, ultrafast carrier dynamics of the photoinduced species are investigated by femtosecond transient absorption studies to identify their spectral characteristics and target analysis further provides information about different excited states photophysical events including ICT and their associated time profiles. The key findings obtained here related to excited state dynamical processes of these newly synthesized systems are believed to be significant in advancing their prospect of utilization in solar energy conversion and related photonic applications.

NIR-Absorbing 1,1,4,4-Tetracyanobuta-1,3-diene- and Dicyanoquinodimethane-Functionalized Donor-Acceptor Phenothiazine Derivatives: Synthesis and Characterization

A series of symmetrical and unsymmetrical donor-acceptor type phenothiazine derivatives 1-18 were designed and synthesized via Pd-catalyzed Sonogashira cross-coupling and [2 + 2] cycloaddition-retroelectrocyclization reactions. The incorporation of cyano-based acceptors 1,1,4,4-tetracyanobutadiene (TCBD) and dicyanoquinodimethane (DCNQ) in the phenothiazine derivatives resulted in systematic variation in the photophysical, thermal, and electrochemical properties. The electronic absorption spectra of the phenothiazine derivatives with strong acceptors 2, 3, 5, 6, 8, 9, 11, 12, 14, 15, 17, and 18 show red-shifted absorption as compared to phenothiazine derivatives 1, 4, 7, 10, 13, and 16 in the near-IR region due to a strong intramolecular charge transfer (ICT) transition. The electrochemical analysis of the phenothiazine derivatives 2, 3, 5, 6, 8, 9, 11, 12, 14, 15, 17, and 18 reveals two reduction waves at low potential due to the TCBD and DCNQ acceptors. The mono-TCBD-functionalized phenothiazine 2 shows higher thermal stability compared to other phenothiazine derivatives. The computational studies on phenothiazines 1-18 reveal the LUMO is substantially stabilized as acceptor strength increases, which lowers the HOMO-LUMO gap.