Ultrafast Charge‐Separation in Triphenylamine‐BODIPY‐Derived Triads Carrying Centrally Positioned, Highly Electron‐Deficient, Dicyanoquinodimethane or Tetracyanobutadiene Electron‐Acceptors

A series of new triphenylamine (TPA)‐substituted BODIPYs 1–3 have been designed and synthesized through the Pd‐catalysed Sonogashira cross‐coupling and [2+2] cycloaddition‐retroelectrocyclization reactions in good yields. This procedure yielded highly electron‐deficient tetracyanobutadiene (TCBD) or dicyanoquinodimethane (DCNQ) electron‐acceptor units centrally located at the TPA‐BODIPY system. As a consequence, significant perturbation of the photonic and electronic properties was observed. The triads 2 and 3 showed red‐shifted absorption, in addition to a strong charge‐transfer‐type absorption in the case of 3. The electrochemical studies revealed multi‐redox processes involving the TPA, TCBD or DCNQ and BODIPY entities. The computational studies were performed at the B3LYP/6‐31G** level to elucidate the geometry and electronic structures. An energy level diagram established for triads 2 and 3 revealed that the photoinduced charge‐separation from the 1BODIPY* is thermodynamically possible. In addition, charge transfer from TPA to TCBD in 2 and DCNQ in 3 was also possible. These charge transfer mechanisms were confirmed by photochemical studies performed using time‐resolved emission and femtosecond‐transient‐absorption studies in solvents of varying polarity. Ultrafast charge‐separation has been witnessed in these closely spaced, strongly interacting triads. The charge‐separated state returned to the ground state without populating the 3BODIPY*. In search of stronger electron‐acceptors: Newly synthesized molecular triads carrying centrally positioned dicyanoquinodimethane or tetracyanobutadiene electron‐acceptors are shown to promote ultrafast charge‐separation and recombination.