Star-Shaped Triphenylamine–Tetracyanobutadiene–Phenothiazine Push–Pull Systems: Role of Terminal Phenothiazine in Improving Charge Transfer

Understanding the process of charge transfer in multimodular push–pull systems is of great significance for technology breakthroughs, especially in the areas of light energy conversion and building optoelectronic devices. In this study, a series of symmetrical and unsymmetrical push–pull systems, 1–4, were designed and synthesized via the Pd-catalyzed Sonogashira cross-coupling reaction, followed by the [2 + 2] cycloaddition–retro-electrocyclization reaction. The D−π–D3′ and D–An–D3′ (n = 0–3) molecular configurations of 1–4 contained triphenylamine (TPA), D, as the central core and phenothiazine (PTZ), D′, as the end-capping unit as a donor and TCBD as the central electron acceptor, A. As control compounds, C1–C4 with a general formula D–An were also synthesized to realize the effect of terminal PTZ in the charge transfer events. The photophysical properties of both star-shaped symmetrical and unsymmetrical 2–4 molecules exhibited a broad intramolecular charge transfer (ICT, also known as charge polarization) band in the visible–near-IR region due to strong push–pull interactions. The electrochemical properties of both the 1–4 and C1–C4 series exhibited multistep redox processes, and spectroelectrochemical studies helped in arriving at the spectral features of the charge transfer species. Frontier orbitals generated on DFT optimized structures helped in visualizing the charge transfer within the different donor and acceptor entities of a given push–pull system. Finally, femtosecond transient absorption spectral studies followed by data analysis by target analyses were utilized to demonstrate excited charge transfer. The terminal PTZ in compounds 2–4 is shown to stabilize the charge transfer state compared to the corresponding control compounds revealing its significance in modulating charge transfer properties.