Unsymmetrical and Symmetrical Push–Pull Phenothiazines

Abstract

A series of unsymmetrical and symmetrical push-pull phenothiazines (3-7) 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 3-7 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 3-7 exhibit multiredox waves. The computational studies on phenothiazines 3-7 exhibit substantial stabilization of the LUMO with the increase in acceptor strength, which results in lowering of the HOMO-LUMO gap.

Synthesis and Properties of New Multiple TCNE Adducts from Dialkynyl‐N‐(het)arylpyrroles

“…Compounds 4a – c possess two TCBD units symmetrically disposed around the central N ‐(het)aryl core. In 2017, Misra described a symmetrical compound with a phenothiazine core and two TCBD units which was characterized in cyclic voltammetry by a two‐step reduction wave associated with two‐electron transfer in each step to form a tetraanionic species . Similar results were obtained by Shoji et al for symmetrical compounds with an azobenzene core .…”

“…As for the reaction of alkynes 2a – c with TCNE, we first investigated the synthesis of the monosubstituted adducts. Reacting one equivalent of TCNE at room temperature with electron‐rich diynes usually led to formation of monoadduct in high yields , . However, sterically hindered diynes typically require more drastic conditions, e.g.…”

“…More interesting, these D–A derivatives were obtained in a very efficient and straightforward manner via a [2+2] cycloaddition of tetracyanoethylene (TCNE) to electron‐rich alkynes followed by a retro‐electrocyclization to form the TCBD moieties. This transformation proceeds quickly under mild conditions when aniline derivatives are employed, but other electron‐donor substituents have also been reported such as anisole, (oligo)thiophene, fluorene, ferrocene, phenothiazine, and azulene . The wide scope of this “click” reaction has been recently covered in comprehensive reviews , .…”

Synthesis of tetracyanoethylene‐substituted ferrocene and its device properties

“…Moreover, those authors synthesized new polymers obtained from tetracyanobutadiene derivatives of diketopyrrolopyrroles with higher power conversion efficiency in solar cell devices . Rout et al . prepared two unsymmetric small molecules with D–A–D–π–D configuration including phenothiazine, triphenylamine and strong electron‐acceptor groups, and the photovoltaic performance of the molecules was reported as 7.35 and 4.81% for OSCs.…”

NIR‐Absorbing Donor–Acceptor Based 1,1,4,4‐Tetracyanobuta‐1,3‐Diene (TCBD)‐ and Cyclohexa‐2,5‐Diene‐1,4‐Ylidene‐Expanded TCBD‐Substituted Ferrocenyl Phenothiazines

“…[18] Our groups have reported av ariety of acetylene-linked chromophoresw hich were subjected to a[ 2 + +2] cycloaddition-electrocyclic ring-opening reactionw ith TCBD and DCNQ and studied their photophysical and electrochemical properties for pertinentoptoelectronic applications. [8,19] In this contribution,w er eport the design and synthesis of TCBD or DCNQ incorporated phenothiazine-BODIPY donor-acceptor systems (Figure 1). Herein, we are particularly interested to improve the photophysicala nd electrochemical properties of phenothiazine-BODIPY conjugates by incorporating TCNE and TCNQ acceptors in the molecular buildingb lock.…”