Synthesis and Understanding\nof the Role of Donor-Tetracyanobutadiene\nin Porphyrin β‑Periphery toward Ultrafast Charge Transfer\nDynamics

The porphyrins, structurally analogous to chlorophyll\npigment,\nhave drawn significant interest in mimicking natural photosynthetic\nprocesses in energy conversion applications. In this work, a new set\nof β-substituted free-base (H<sub>2</sub>P) and zinc-porphyrins\n(ZnP) have been designed (<b>5</b>–<b>19</b>) and\nsynthesized employing ferrocene (Fc), triphenylamine (TPA), and carbazole\n(Cz) as secondary donors (D) and further incorporated tetracyanobuta-1,3-diene\n(TCBD) as strong electron acceptor (A) entity following Sonogashira\ncross-coupling and subsequent [2 + 2] cycloaddition–retroelectrocyclization\nreactions. Steady-state optical data exhibit a broad absorption in\nthe 650–800 nm region, particularly in <b>15</b>–<b>19</b>, corroborating ground-state charge polarization leading\nto intramolecular charge transfer (CT) in these systems. Strong fluorescence\nquenching in all of the systems (<b>5</b>–<b>19</b>) compared to the control compounds (<b>C1</b> and <b>C2</b>) further suggests excited-state nonradiative photoprocesses predominate\nin these β-substituted dyads and triads, particularly after\nTCBD incorporations (<b>15</b>–<b>19</b>). Though\nthe secondary donors quickly oxidize in <b>5</b>–<b>10</b>, the same becomes difficult in <b>15</b>–<b>19</b>, indicating an electronic influence of TCBD, leading to\nthe respective formation of MP<sup>•–</sup>-D<sup>•+</sup> and MP<sup>•+</sup>-A<sup>•–</sup>-D (MP =\n2H or Zn) charge-separated (CS) species in a polar environment, which\nthe molecular orbital positioning of the CT entities from computational\nstudies has also justified. Finally, spectral and temporal dynamics\nof different photoproducts in these compounds have been assessed from\nfemtosecond transient absorption studies, and subsequent fitting of\nthe transient data identifies Cz contributing to the most stable and\nthus long-lived CS states, brightening their outstanding promise in\nsolar energy harvesting and related electronic applications.