A luminescence study of anionic forms of Zn etioporphyrin I

Photophysics of Reduced Silicon Tetraphenylporphyrin

“…Additionally, the peak amplitude (i.e., the molar absorption coefficient, ε) of the B band is substantially reduced compared with the Q-bands; whereas the B/Q(0,0) peak−intensity ratio is ∼50/1 for Si(TPP)Cl 2 , the ratio is decreased to ∼5/1 for Si(TPP)(py) 2 . Similar spectral characteristics in the B- and Q-band regions have been observed previously for two-electron reduced porphyrins, such as the dianions of Zn(TPP) and magnesium etioporphyrin [Mg(Etio)]. − …”

“…Similar spectral characteristics in the B-and Q-band regions have been observed previously for two-electron reduced porphyrins, such as the dianions of Zn(TPP) and magnesium etioporphyrin [Mg(Etio)]. [12][13][14][15] In addition to the features in the B and Q regions, Si(TPP)-(py) 2 appears to have a very broad weak absorption in the nearinfrared (NIR) region centered at ∼820 nm (Figure 2). The dianion of Mg(Etio) exhibits a similar NIR feature.…”

“…14 However, rigorous assignment of the weak NIR absorption to Si(TPP)-(py) 2 is difficult, because other potential reduction side products such as phlorins, or porphyrin monoanions, also have one or more NIR absorptions. [12][13][14][15] For example, we found prominent bands that likely derive from a phlorin for solutions of Si(TPP)-(THF) 2 in THF but not centered at the same wavelength as the NIR feature observed for Si(TPP)(py) 2 in pyridine.…”

“…12 This value together with the estimated natural radiative lifetime was used to calculate a corresponding excited-state lifetime of 120 ps, 12 which is in good agreement with the value (135 ps) obtained from picosecond fluorescence measurements. 18 A luminescence spectrum initially attributed to the dianion of zinc etioporphyrin, [Zn(Etio)] 2-, 13 was later assigned by the same authors to minor (2-5%) reduction byproducts. 12,13 Consistent with these previous studies, we find here that the doubly reduced porphyrin Si(TPP)(py) 2 in pyridine is effectively nonfluorescent (Φ f < 0.0004).…”

“…18 A luminescence spectrum initially attributed to the dianion of zinc etioporphyrin, [Zn(Etio)] 2-, 13 was later assigned by the same authors to minor (2-5%) reduction byproducts. 12,13 Consistent with these previous studies, we find here that the doubly reduced porphyrin Si(TPP)(py) 2 in pyridine is effectively nonfluorescent (Φ f < 0.0004). Furthermore, we find that the basic non-emissive behavior derives from an extremely short excited-state lifetime (2.4 ps) associated with essentially quantitative ultrafast nonradiative deactivation (internal conversion) to the ground state, with a rate constant of k ic ∼ (2.4 ps) -1 .…”