Near-infrared transient-absorption spectroscopy of zinc tetraphenylporphyrin and related compounds. Observation of bands that selectively probe the S1 excited state

Abstract

► Near-IR transient absorption spectra were measured for ZnTPP and related compounds. ► The S 1 state of ZnTPP and related compounds have a diagnostic TA band near 1250 nm. ► The near-IR TA data place gerade symmetry dark states 0.1–0.2 eV above the S 2 state. ► The near-IR band allows for the properties of the S 1 state to be selectively probed. Picosecond transient-absorption spectra are reported for the S 1 excited state of zinc tetraphenylporphyrin (ZnTPP) and related compounds in the near-infrared region ( λ ≤ 1400 nm). The spectra exhibit a prominent absorption band ( ɛ ≌ 4400 M −1 cm −1 for ZnTPP) between 1200 and 1300 nm whose position is sensitive to solvent (toluene and THF), zinc coordination number (ZnTPP(py-4-CCPh)), the metal center (GaTPP(Cl)), and the positions of the porphyrin substituents (zinc octaethylporphyrin, ZnOEP). The transient-absorption profile between 850 and 1200 nm is also sensitive to these variations. The near-infrared bands for the TPP compounds are assigned to transitions from the S 1 state to gerade -symmetry dark states based on their correspondence to relative energies predicted by previously reported time-dependent density functional theory calculations and their constant energy gap relative to the S 2 state. The characteristics of the prominent near-infrared transient-absorption band make it well suited for selectively probing photophysical and photoredox processes of the S 1 state because it does not overlap with absorption bands of the T 1 excited state or of [ZnTPP] + or [ZnTPP] − , in contrast to the features in the typically probed visible-wavelength region.