Photophysical Properties of Conformationally Distorted Metal-Free Porphyrins. Investigation into the Deactivation Mechanisms of the Lowest Excited Singlet State

Conformational control of cofactors in nature – the influence of protein-induced macrocycle distortion on the biological function of tetrapyrroles

“…The relationships between the macrocycle conformation and the observed effects were proposed to arise because of greater conformational flexibility of the non-planar compounds in the excited state. 111,112 This idea was formed on the basis of the large Stokes' shifts that could not be accounted for classically as solvent reorganization dynamics should be similar in all of the porphyrins studied. The increased flexibility would also be manifest in the observed increased k IC and this was further evidenced by the fact that the ruf distorted compounds exhibited similar properties to planar analogues at low-temperature.…”

Spectroscopic properties of meso-thienylporphyrins with different porphyrin cores

“…The S l state lifetimes mainly depends on the increase of S 1 → T 1 intersystem crossing and increase of S 1 → S 0 internal conversion rates. The increase in internal conversion rate can be attributed to the enhancement of the Franck-Condon factor associated with structural reorganization in the excited state [19] and the increase in the intersystem crossing rate can be attributed to the enhancement of the spin-orbit coupling caused by the heavy sulfur atom [17] of meso-thienyl groups. The emission spectra of dications 1H 3 2+ , 2H 3 2+ , 3H 3…”

Manifestation of nonplanarity effects and charge-transfer interactions in spectral and kinetic properties of triplet states of sterically strained octaethylporphyrins

“…Note that PdOETPP exhibits an intense phosphorescence band (ϕ P = 0.25) at 705 nm at 77 K that is red-shifted relative to the phosphorescence band of usual porphyrins. Taking into account that the molecule of tetra-mesophenyl-substituted free base OETPP has a nonplanar conformation in the ground S 0 state [5], the low-temperature emission of PdOETPP should be treated as phosphorescence of the nonplanar conformation. Note that phosphorescence of nonplanar conformations of steric hindered porphyrins at 77 K was not reported earlier in the literature.…”

“…However, at present there is no detailed information on spectral and kinetic parameters of the T 1 states of hybrid porphyrins under the conditions of dynamic conformational lability of the porphyrin macrocycle in liquid solutions at 293 K. The data available are scarce and are often indirect. Thus, in experiments on bleaching of the absorption bands of porphyrins with the steric hindrance (saddle and ruffle conformations), the quantum yields of the S 1 T 1 intersystem crossing were estimated to be γ = 0.2-0.7, the lowest values being typical for the ruffle structures [5][6][7][8][9]. In some cases, differential spectra of the induced absorption have been obtained in the region from 600 to 900 nm upon picosecond excitation [5,8,9].…”

“…Thus, in experiments on bleaching of the absorption bands of porphyrins with the steric hindrance (saddle and ruffle conformations), the quantum yields of the S 1 T 1 intersystem crossing were estimated to be γ = 0.2-0.7, the lowest values being typical for the ruffle structures [5][6][7][8][9]. In some cases, differential spectra of the induced absorption have been obtained in the region from 600 to 900 nm upon picosecond excitation [5,8,9]. Direct experimental data on the decay of the triplet states for two types of nonplanar tetra-substituted porphyrins are presented in the only paper [9] known to us, where it was shown that, along with a decrease in the quantum yield of intersystem crossing, the lifetime τ T of the T 1 state greatly shortened at room temperature in deoxygenated solutions and the quantum yield γ ∆ of the singlet oxygen generation decreased.…”