Symmetric and Asymmetric Push–Pull Conjugates: Significance of Pull Group Strength on Charge Transfer and Separation

Abstract

The effect of acceptor strength on excited-state charge transfer (CT) and charge separation (CS) in central phenothiazine (PTZ)-derived symmetric 1 (PTZ-(TCBD-TPA) 2 ) and asymmetric 2 (PTZ-(TCBD/DCNQ-TPA) 2 ) push−pull conjugates, in which triphenylamine (TPA) acts as end capping and 1,1,4,4− tetracyanobuta−1,3−diene (TCBD) and cyclohexa−2,5−diene−1,4−ylidene−expanded TCBD (DCNQ) act as electron acceptor units, is reported. Due to strong push−pull effects, intramolecular CT was observed in the ground state, extending the absorption into the near-infrared region. Electrochemical, spectroelectrochemical, and computational studies coupled with energy-level calculations predicted both 1 and 2 to be efficient candidates for ultrafast CT. Subsequent femtosecond transient absorption studies along with global target analysis, performed in both polar and nonpolar solvents, confirmed such processes in which the CS was efficient in asymmetric 2, having both TCBD and DCNQ acceptors in polar benzonitrile, while in toluene, only CT was witnessed. This work highlights the significance of the number and strength of electron acceptor entities and the role of solvent polarity in multimodular push−pull systems to achieve ultrafast CS.

Charge‐Transfer in Panchromatic Porphyrin‐Tetracyanobuta‐1,3‐Diene‐Donor Conjugates: Switching the Role of Porphyrin in the Charge Separation Process

“…Importantly, in the near-IR region covering the 650-900 nm range, a new broad absorption band was observed in all TCBD bearing systems as a consequence of intramolecular charge transfer involving the electron deficient TCBD and the electron rich porphyrin and second donor entities. [33][34][35][36][37][38][39][40][41][42][43] In summary, intramolecular charge transfer in the ground state was successfully established in these MP-TCBD-Donor (M = 2H or Zn; donor = PTZ or NND) conjugates.…”

“…TCBD is a cyano-rich moiety [25] that has been covalently linked to many electroactive units including porphyrins, phthalocyanines and BODIPYs. [26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45] In most of these TCBD-based D-A conjugates, photoinduced charge separation has been observed as a consequence of coupling between the electron-poor TCBD and electron-rich sensitizer. In the literature reported porphyrin-TCBD systems, [41][42] porphyrin was function-alized at the meso-position with a TCBD entity either directly or with a phenyl ring spacer.…”

Synthesis and Photophysical Properties of 1,1,4,4‐Tetracyanobutadienes Derived from Ynamides Bearing Fluorophores**

“…[14][15][16][17][18] In contrast, the luminescence properties of TCBD derivatives have barely been explored as they are generally considered non-emissive due to intrinsically rapid nonradiative deactivations. [19] However, some examples of luminescent TCBDs in solution or in the solid state have recently emerged, [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] although often with very low quantum yields at room temperature that hardly exceed 1 %, except for a work from Jayamurugan et al [27] where the authors claimed quantum yields up to 4.3 % at room temperature in acetonitrile. We also reported TCBDs bearing triphenylamine and fluorenyl units that showed quantum yields comprised between 2 and 6 % in rigid media at room temperature.…”

Does Location of BF2-Chelated Dipyrromethene (BODIPY) Ring Functionalization Affect Spectral and Electron Transfer Properties? Studies on α-, β-, and Meso-Functionalized BODIPY-Derived Donor–Acceptor Dyads and Triads

“…Finally, with the help of a [2 + 2] cycloaddition–retroelectrocyclization reaction, an electron acceptor entity, tetracyanobuta-1,3-diene (TCBD), has been introduced between the BODIPY and ferrocene entities of dyads 7–9 to yield BODIPY–TCBD–Fc charge transfer complexes, 10–12 . Earlier, TCBD has been covalently linked to electroactive units such as anilines, porphyrins, corannulenes, phthalocyanines subphthalocyanine, or BODIPY and other sensitizers. − In majority of these TCBD-based charge transfer complexes, excited-state charge transfer has been observed. As part of the investigation, the ground- and excited-state processes in the newly synthesized systems have been systematically studied to visualize BODIPY substitution effects on governing the ground- and excited-state interactions in polar benzonitrile.…”