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

Picosecond transient-absorption spectra are reported for the S1 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 S1 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 S2 state. The characteristics of the prominent near-infrared transient-absorption band make it well suited for selectively probing photophysical and photoredox processes of the S1 state because it does not overlap with absorption bands of the T1 excited state or of [ZnTPP]+ or [ZnTPP]−, in contrast to the features in the typically probed visible-wavelength region.