NIR-Absorbing 1,1,4,4-Tetracyanobuta-1,3-diene- and Dicyanoquinodimethane-Functionalized Donor-Acceptor Phenothiazine Derivatives: Synthesis and Characterization.

A series of symmetrical and unsymmetrical donor-acceptor type phenothiazine derivatives 1 - 18 were designed and synthesized via Pd-catalyzed Sonogashira cross-coupling and [2 + 2] cycloaddition-retroelectrocyclization reactions. The incorporation of cyano-based acceptors 1,1,4,4-tetracyanobutadiene (TCBD) and dicyanoquinodimethane (DCNQ) in the phenothiazine derivatives resulted in systematic variation in the photophysical, thermal, and electrochemical properties. The electronic absorption spectra of the phenothiazine derivatives with strong acceptors 2 , 3 , 5 , 6 , 8 , 9 , 11 , 12 , 14 , 15 , 17 , and 18 show red-shifted absorption as compared to phenothiazine derivatives 1 , 4 , 7 , 10 , 13 , and 16 in the near-IR region due to a strong intramolecular charge transfer (ICT) transition. The electrochemical analysis of the phenothiazine derivatives 2 , 3 , 5 , 6 , 8 , 9 , 11 , 12 , 14 , 15 , 17 , and 18 reveals two reduction waves at low potential due to the TCBD and DCNQ acceptors. The mono-TCBD-functionalized phenothiazine 2 shows higher thermal stability compared to other phenothiazine derivatives. The computational studies on phenothiazines 1 - 18 reveal the LUMO is substantially stabilized as acceptor strength increases, which lowers the HOMO-LUMO gap.