Comprehensive review of photophysical parameters (ε, Φ_f, τ_s) of tetraphenylporphyrin (H₂TPP) and zinc tetraphenylporphyrin (ZnTPP) – Critical benchmark molecules in photochemistry and photosynthesis

Tetraphenylporphyrin (H₂TPP) and zinc tetraphenylporphyrin (ZnTPP) are widely used benchmark molecules in diverse photochemical studies given facile synthetic access, rich visible-region spectra, and broad structural analogy to chlorophylls. Yet the literature values for each key photophysical parameter – the molar absorption coefficient (ε), fluorescence quantum yield (Φ_f), and also singlet excited-state lifetime (τ_S) – vary over an astonishing range. Here, a comprehensive literature review (∼1940–September 2020) encompassing 871 publications is reported for these essential parameters. Each parameter is determined by measurement with distinct instrumentation and suffers idiosyncratic sources of error. The best values for H₂TPP are ε = 460,000 cm⁻¹·M⁻¹, Φ_f = 0.090, and τ_S = 12.8 ns in Ar-purged toluene (Φ_f = 0.070, τ_S = 9.9 ns in toluene in air); the best values for ZnTPP are ε = 560,000 cm⁻¹·M⁻¹, Φ_f = 0.030, and τ_S = 2.1 ns in Ar-purged toluene (Φ_f = 0.029, τ_S = 2.0 ns in toluene in air). The choice of values for such parameters has far-reaching consequences in photochemistry ranging from fluorescence (or Förster) resonance energy transfer (FRET) processes to assessments of molecular brightness.