Excited State Electron Transfer in 1,1,4,4‐Tetracyanobuta‐1,3‐Diene (TCBD)‐ and Cyclohexa‐2,5‐Diene‐1,4‐Diylidene‐Expanded TCBD–Substituted BODIPY‐Phenothiazines Donor‐Acceptor Conjugates

A new set of donor-acceptor (D-A) conjugates capable of undergoing ultrafast electron transfer were synthesized using 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) substituted phenothiazine, SM1-SM3, via Pd-catalyzed Sonogashira cross-coupling reaction and [2+2] cycloaddition-electrocyclic ring-opening reaction. The incorporation of 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) and cyclohexa-2,5-diene-1,4-diylidene-expanded TCBD (abbreviated as DCNQ = dicyanodiquinodimethane) in BODIPY functionalized phenothiazine resulted in significant perturbation on the optical and electronic properties. The absorption spectrum of both SM2 and SM3 showed red shifted absorption as compared to SM1. Additionally, both SM2 and SM3 exhibited a distinct intramolecular charge transfer (ICT) transition in the near-infrared region more so for SM3. The electrochemical study revealed multi-redox processes due to the presence of redox-active phenothiazine, BODIPY, TCBD or DCNQ entities. Using data from spectral, electrochemical and computational studies, an energy level diagram was established to witness excited state electron transfer events. Finally, evidence of electron transfer and their kinetic information was secured from studies involving femtosecond transient absorption technique. The time constants for excited state electron transfer events in the case of SM2 and SM3 were less than 5 ps revealing ultrafast processes.