Near‐IR Intramolecular Charge Transfer in Strongly Interacting Diphenothiazene‐TCBD and Diphenothiazene‐DCNQ Push‐Pull Triads
Indresh Singh Yadav, Youngwoo Jang, Yogajivan Rout, Michael B. Thomas, Rajneesh Misra, Francis D’Souza
Abstract
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.