Three types of phenothiazines dimers (PTZ‐PTZ, 1–3), covalently linked with one or two acetylene linkers, were synthesized by copper‐mediated Eglinton and Pd‐catalyzed Sonogashira coupling reactions in excellent yields. The dimers 1–3 were further engaged in [2+2] cycloaddition‐retroelectrocyclization reactions with strong electron acceptors, tetracyanoethylene (TCNE) and 7,7,8,8‐tetracyanoquinodimethane (TCNQ) to yield tetracyanobutadiene (TCBD, 1 a–3 a), and dicyanoquinodimethane (DCNQ, 1 b–3 b) functionalized donor‐acceptor (D‐A) conjugates, respectively. The conjugates were examined by a series of spectral, computational, and electrochemical studies. Strong ground state polarization leading to new optical transitions was witnessed in both series of D‐A conjugates. In the case of DCNQ derived D‐A system 1 b, the optical coverage extended until 1200 nm in benzonitrile, making this a rare class of D‐A ICT system. Multiple redox processes were witnessed in these D‐A systems, and the frontier orbitals generated on DFT optimized structures further supported the ICT phenomenon. Photochemical studies performed using femtosecond pump‐probe studies confirmed solvent polarity dependent excited state charge transfer and separation in these novel multi‐modular D‐A conjugates. The charge‐separated states lasted up to 70 ps in benzonitrile while in toluene slightly prolonged lifetime of up to 100 ps was witnessed. The significance of phenothiazine dimer in wide‐band optical capture all the way into the near‐IR region and promoting ultrafast photoinduced charge transfer in the D‐A‐D configured multi‐modular systems, and the effect of donor‐acceptor distance and the solvent polarity was the direct outcome of the present study.
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 crosscoupling reaction, followed by the [2 + 2] cycloaddition−retroelectrocyclization reaction. The D−π−D 3 ′ and D−A n −D 3 ′ (n = 0−3) molecular configurations of 1−4 contained triphenylamine (TPA), D, as the central core and phenothiazine (PTZ), D′, as the endcapping unit as a donor and TCBD as the central electron acceptor, A. As control compounds, C1−C4 with a general formula D−A n 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.
A set of phenothiazine (PTZ) and phenothiazine-5,5-dioxide based π-conjugated push–pull chromophores PTZ 1–6 were designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling and [2+2] cycloaddition retroelectrocyclic ring opening reaction in...
In recent years, the BODIPY dyes have developed as a valuable category of luminogens for organic photovoltaic and electronic applications because of their spectacular properties like as good fluorescence quantum...
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