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 BODIPY* 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 BODIPY*.