Unsymmetrical and Symmetrical Push–Pull Phenothiazines

A series of unsymmetrical and symmetrical push–pull phenothiazines (<b>3</b>–<b>7</b>) were designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction and subsequent [2 + 2] cycloaddition–retroelectrocyclization reaction with tetracyanoethylene (TCNE) and 7,7,8,8-tetracyanoquinodimethane (TCNQ). The effect of systematic variation of the number and nature of cyano-based acceptor TCNE and TCNQ units on the photophysical, electrochemical, and computational studies was investigated. The single-photon absorption on phenothiazines <b>3</b>–<b>7</b> reveals that substitution of 1,1,4,4-tetracyanobutadiene (TCBD) and a cyclohexa-2,5-diene-1,4-diylidene-expanded TCBD unit results in strong intramolecular charge transfer and lowering of the LUMO energy level. The TCBD-linked and cyclohexa-2,5-diene-1,4-diylidene-expanded TCBD-linked phenothiazines <b>3</b>–<b>7</b> exhibit multiredox waves. The computational studies on phenothiazines <b>3</b>–<b>7</b> exhibit substantial stabilization of the LUMO with the increase in acceptor strength, which results in lowering of the HOMO–LUMO gap

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

Aryl-Substituted Unsymmetrical Benzothiadiazoles: Synthesis, Structure, and Properties

A family of unsymmetrical donor–acceptor, ferrocenyl-substituted benzothiadiazoles of types D₁–π–A−π–D₂, D₁–π–A₁–π–A₂, D₁–A−π–D₂, and D₁–A₁–A₂–D₂, bearing a variety of electron-donating and electron-withdrawing groups, were designed and synthesized. Their photophysical, electrochemical, and computational properties were explored, which show strong donor–acceptor interaction. The presence of electron-rich units anthracene (<b>6f</b>) and triphenylamine (<b>6h</b>), and an electron-deficient unit 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) (<b>9b</b>) results in lowering of the band gap, which leads to a red shift of the absorption spectrum in these benzothiadiazole systems. The single crystal structures of <b>6c</b>, <b>6g</b>, <b>7a</b>, and <b>7b</b> are reported, which show marvelous supramolecular interactions

Aryl-Substituted Unsymmetrical Benzothiadiazoles: Synthesis, Structure, and Properties

A family of unsymmetrical donor–acceptor, ferrocenyl-substituted benzothiadiazoles of types D₁–π–A−π–D₂, D₁–π–A₁–π–A₂, D₁–A−π–D₂, and D₁–A₁–A₂–D₂, bearing a variety of electron-donating and electron-withdrawing groups, were designed and synthesized. Their photophysical, electrochemical, and computational properties were explored, which show strong donor–acceptor interaction. The presence of electron-rich units anthracene (<b>6f</b>) and triphenylamine (<b>6h</b>), and an electron-deficient unit 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) (<b>9b</b>) results in lowering of the band gap, which leads to a red shift of the absorption spectrum in these benzothiadiazole systems. The single crystal structures of <b>6c</b>, <b>6g</b>, <b>7a</b>, and <b>7b</b> are reported, which show marvelous supramolecular interactions

Ferrocene-Functionalized Fulleropyrrolidine Derivative: A Performance Enhancer for Solid-State Electrochromic Devices

A custom-designed ferrocene-functionalized fulleropyrrolidine derivative Fullerene-Fc has been synthesized, which proves to be a feasible material to improve the performance of solid-state electrochromic devices. The molecule can be used as a dopant to design devices on substrates of rigid (glass) as well as flexible (PET) nature. The switching speed of devices made using poly(3-hexylthiophene) and ethyl viologen could be improved to display a switching time of less than 1 s when the Fullerene-Fc molecule was added. The improvement in the performance of the electrochromic device is likely due to the solution processability of the Fullerene-Fc molecule compatible with other used electrochromic active materials, which helps to facilitate the necessary charge carriers for the redox reaction within the device. Additionally, a coloration efficiency of more than 350 cm2/C and cyclic stability of up to 500 s were shown by the device with a color contrast of more than 40%. Furthermore, the Fullerene-Fc molecule in the P3HT/EV device can be incorporated to fabricate an all-organic flexible device. The charge storage properties of fullerene and the redox behavior of ferrocene make it a good choice to be used as an electrochromic performance enhancer

Heteroatom-Connected Ferrocenyl BODIPYs: Synthesis, Structure, and Properties

The set of heteroatom-connected ferrocenyl BODIPYs <b>2a</b>,<b>b</b> and <b>3c</b>,<b>d</b> were designed and synthesized by the nucleophilic aromatic substitution reaction of 8-chloro BODIPY <b>1</b>, with ferrocenyl anilines and ferrocenyl phenols. The effects of a heteroatom at the <i>meso</i> position on the optical and electrochemical properties of the BODIPYs were studied. The absorption spectra of the BODIPYs containing an N atom at the <i>meso</i> position (<b>2a</b>,<b>b</b>) show a blue shift of 80 nm, whereas the BODIPYs containing an O atom at the <i>meso</i> position (<b>3c</b>,<b>d</b>) show a blue shift of 50 nm in comparison to 8-chloro BODIPY (<b>1</b>). The DFT calculations reveal strong donor–acceptor interactions. The nature of the heteroatom does not affect the HOMO but perturbs the LUMO. The single-crystal X-ray structures of <b>2a</b>,<b>b</b> and <b>3c</b>,<b>d</b> show interesting supramolecular interactions. The crystal structure of <b>2a</b> shows two molecules in an asymmetric unit arranged into a complex 3D structural motif, <b>2b</b> contains a staircase-shaped arrangement of tetramers to form a 2D structural sheet, <b>3c</b> forms a sheet in a zigzag manner, and <b>3d</b> forms a sheet which contains alternating bands of ferrocenyl and BODIPY units

<i>C</i>₃‑Symmetric Positional Isomers of BODIPY Substituted Triazines: Synthesis and Excited State Properties

A series of <i>meso</i>-<i>O</i>-aryl functionalized BODIPY trimers positioned along the <i>C</i>₃-symmetric axis of triazine ring have newly been synthesized to probe the ground and excited state intramolecular type interactions between the BODIPY entities within the trimer. The developed synthetic strategy resulted in BODIPY trimers in good yields. The electron rich, <i>meso</i>-<i>O</i>-aryl functionalized BODIPYs revealed larger HOMO–LUMO gap and higher Stokes shift and fluorescence lifetimes compared to the traditional BODIPY derivatives having an aryl group attached at the <i>meso</i> position. The optical absorption, steady-state fluorescence, and electrochemical studies revealed weak, if any, intramolecular type interactions among the BODIPY entities within the trimer and the central triazine unit to be both photo- and redox-salient. The possibility of singlet–singlet energy migration among the BODIPY entities was investigated using time-resolved emission and femtosecond transient absorption studies. Excitation of a BODIPY entity in the trimers led to successful formation of ¹BODIPY*, which populated the ³BODIPY* via intersystem crossing. Among the three trimers, although very weak, only trimer <b>8</b> revealed excitation transfer to some extent. The present findings suggest that the <i>meso</i>-<i>O</i>-aryl functionalized BODIPYs due to their superior fluorescence properties are better probes to build light energy harvesting supramolecular oligomeric systems and for other applications such as sensing and imaging

Donor–Acceptor Ferrocenyl-Substituted Benzothiadiazoles: Synthesis, Structure, and Properties

This article reports the design, and synthesis of D−π₁–A−π₂–D unsymmetrical, and D−π₁–A−π₂–A−π₁–D symmetrical type of ferrocenyl-substituted benzothiadiazoles by the Pd-catalyzed Sonogashira, and Stille coupling reactions. The photophysical and electrochemical behavior of the ferrocenyl-substituted benzothiadiazoles show strong donor–acceptor interaction. The increase in the number of acceptor benzothiadiazole unit, results in the lowering of the energy gap, which leads to the bathochromic shift of the absorption spectrum. The single crystal X-ray structures of <b>3a</b>, <b>5a</b>, and <b>5g</b> were obtained which show interesting supramolecular interactions

<i>Meso</i> Alkynylated Tetraphenylethylene (TPE) and 2,3,3-Triphenylacrylonitrile (TPAN) Substituted BODIPYs

The tetraphenylethylene (TPE) substituted BODIPY <b>2a</b>, and 2,3,3-triphenylacrylonitrile (TPAN) substituted BODIPYs <b>2b</b> and <b>2c</b> were designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction. Their photophysical and electrochemical properties were investigated. The BODIPY <b>2a</b> exhibits strong D–A interaction with poor fluorescence quantum yield. The BODIPYs <b>2b</b> and <b>2c</b> show red-shifted absorption and emission with higher fluorescence quantum yield compared to BODIPY <b>2a</b>. The photonic properties of BODIPYs <b>2a</b>–<b>2c</b> were compared with 4-ethynylbenzonitrile substituted BODIPY <b>3</b> and phenylacetylene substituted BODIPY <b>4</b>. The results reveal that the electron donating group at the <i>meso</i> position of BODIPY blue shifts the absorption and emission with decreased fluorescence quantum yield, whereas the electron withdrawing group at the <i>meso</i> position of BODIPY red shifts the absorption and emission with enhanced quantum yields. The single crystal structures of BODIPYs <b>2a</b> and <b>2b</b> reflect the planar orientation of <i>meso</i> substituent and the BODIPY core, which leads to close π–π stacking. The extensive π–π stacking and strong donor–acceptor (D–A) interaction makes these BODIPYs AIE inactive. The experimental observations were supported by DFT calculation

Donor–Acceptor Ferrocenyl-Substituted Benzothiadiazoles: Synthesis, Structure, and Properties

This article reports the design, and synthesis of D−π₁–A−π₂–D unsymmetrical, and D−π₁–A−π₂–A−π₁–D symmetrical type of ferrocenyl-substituted benzothiadiazoles by the Pd-catalyzed Sonogashira, and Stille coupling reactions. The photophysical and electrochemical behavior of the ferrocenyl-substituted benzothiadiazoles show strong donor–acceptor interaction. The increase in the number of acceptor benzothiadiazole unit, results in the lowering of the energy gap, which leads to the bathochromic shift of the absorption spectrum. The single crystal X-ray structures of <b>3a</b>, <b>5a</b>, and <b>5g</b> were obtained which show interesting supramolecular interactions