Synthesis and Understanding of the Role of Donor-Tetracyanobutadiene in Porphyrin β-Periphery toward Ultrafast Charge Transfer Dynamics

Abstract

The porphyrins, structurally analogous to chlorophyll pigment, have drawn significant interest in mimicking natural photosynthetic processes in energy conversion applications. In this work, a new set of β-substituted free-base (H₂P) and zinc-porphyrins (ZnP) have been designed (5–19) and synthesized employing ferrocene (Fc), triphenylamine (TPA), and carbazole (Cz) as secondary donors (D) and further incorporated tetracyanobuta-1,3-diene (TCBD) as strong electron acceptor (A) entity following Sonogashira cross-coupling and subsequent [2 + 2] cycloaddition–retroelectrocyclization reactions. Steady-state optical data exhibit a broad absorption in the 650–800 nm region, particularly in 15–19, corroborating ground-state charge polarization leading to intramolecular charge transfer (CT) in these systems. Strong fluorescence quenching in all of the systems (5–19) compared to the control compounds (C1 and C2) further suggests excited-state nonradiative photoprocesses predominate in these β-substituted dyads and triads, particularly after TCBD incorporations (15–19). Though the secondary donors quickly oxidize in 5–10, the same becomes difficult in 15–19, indicating an electronic influence of TCBD, leading to the respective formation of MP^•–-D^•+ and MP^•+-A^•–-D (MP = 2H or Zn) charge-separated (CS) species in a polar environment, which the molecular orbital positioning of the CT entities from computational studies has also justified. Finally, spectral and temporal dynamics of different photoproducts in these compounds have been assessed from femtosecond transient absorption studies, and subsequent fitting of the transient data identifies Cz contributing to the most stable and thus long-lived CS states, brightening their outstanding promise in solar energy harvesting and related electronic applications.