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.

Perspectives on push-pull chromophores derived from click-type [2 + 2] cycloaddition-retroelectrocyclization reactions of electron-rich alkynes and electron-deficient alkenes

Various push-pull chromophores can be synthesized in a single and atom-economical step through [2 + 2] cycloaddition-retroelectrocyclization (CA-RE) reactions involving diverse electron-rich alkynes and electron-deficient alkenes. In this review, a comprehensive investigation of the recent and noteworthy advancements in the research on push-pull chromophores prepared via the [2 + 2] CA-RE reaction is conducted. In particular, an overview of the physicochemical properties of the family of these compounds that have been investigated is provided to clarify their potential for future 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.