Conformational dynamics and spin-orbit interactions in a series of sterically hindered porphyrins in the lowest triplet state

The effect of the structure and conformational dynamics on quenching of triplet states of porphyrins and their chemical dimers by molecular oxygen and on singlet oxygen generation

“…As was noted above, the conformational dynamics typical for phenyl substituted OEP compounds and their dimers at room temperature in deoxygenated solutions manifests itself in a considerable decrease in the lifetime of the lowest triplet state [26,27]. In the presence of oxygen, the character and value of confor mational transformations obviously depend on the ratio of the rate constants of O 2 diffusion (k dif ) and conformational transformations of porphyrin (k ct ).…”

“…As was noted above, the mono meso phenyl sub stituted octamethylporphyrins are sterically hindered porphyrins and characterized by conformational labil ity. In [26,27], it was shown that a more favorable con formation for these compounds in the triplet state is the U conformation (Fig. 2e), which is characterized by a displacement of the phenyl ring out of the plane of the macrocycle and by the formation of a common conjugated system of bonds with porphyrin macrocy cles.…”

“…ӷ Comparison of experimental rate constant k T of quenching of the T 1 state by molecular oxygen, quantum yield of the singlet oxygen generation γ Δ in toluene at 295 K, rate constant k 0 = 1/ of deactivation of the T 1 state of porphyrins in frozen matrices at 77 K, and energy gap between the S 0 and T 1 states ΔE ST taken from [18][19][20][21][22][23][24][25] with the calculated data for ΔE ST , oxidation po tential in toluene E ox , energy of the SOMO of porphyrin in the T 1 state, and normalized to unity quenching rate constants k FCF and k q (see text) One possible reason for the above noted differ ences may be related to the fact that the steric hin drance in the region of the phenyl bridge, similarly as in the case of corresponding monomers [26], consid erably changes the structure of (OЕP) 2 -Ph and (ZnOЕP) 2 -Ph dimers in the triplet state. Previously, using quantum chemical methods, we showed that a decrease in the lifetime of the Т 1 state of meso phenyl substituted octamethylporphyrins is associated with an increase in the probability of nonradiative deactiva tion (T 1~~> S 0 ) due to (i) conformational lability in the Т 1 state, which is accompanied by a considerable decrease in the energy gap ΔE(T 1 -S 0 ), and (ii) enhancement of the spin-orbit interaction of the T 1 and S 0 states as a result of a change of the nature of hybrid orbitals of C and N atoms of the porphyrin macrocycle upon its out of plane distortion [27]. As far as we know, such investigations for porphyrin dimers linked by a phenyl bridge have not been previ ously performed.…”

Structural Dynamics and Relaxation Processes with Participation of Excited Singlet and Triplet States in Sterically Hindered Porphyrins and Their Chemical Dimers

“…[29,39,40,48,49] Finally, theoretical quantumchemical calculations have been used in order to explain the conformational lability of meso-phenyl-substituted octaarylporphyrins leading to the enhancement of the nonradiative deactivation channels of various nature. [44,48,89] This paper is dedicated to the 70 th anniversary of the worldknown expert in tetrapyrrole chemistry and photochemistry as well as of my friend Oscar Koifman. The given material presents itself an overview of our collaboration in the field of sterically hindered porphyrin macrocycles in comparison with the relevant data from the literature as well as contains some recent results showing what circumstances should be taken into account when using these compounds in various areas (excited state deactivation in multiporphyrin complexes, photoinduced electron transfer, singlet oxygen generation).…”

“…The barrier height of the transition into the non-planar conformation and its energy with respect to more planar conformations depend on the nature of the substituents on the pyrrole rings and on the initial degree of distortion of the porphyrin macrocycle. Really, recent theoretical revision of this problem [89] confirmed our main ideas and showed that that a decrease in the lifetime of the T 1 state of OMP-Ph molecule is related with the enhancement of the channel of non-radiative deactivation (T 1 ∼>S 0 ), which occurs (i) due to the conformational lability in the T 1 state, as a result of which the energy gap ∆E V (T 1 -S 0 ) considerably decreases, and (ii) because of an increase in the matrix element of the spin-orbit interaction due to a change in the hybridization of atoms of the macrocycle as a consequence of its nonplanar distortion. The value of the vibronically induced matrix element of the spin-orbit interaction between the S 0 and T 1 states of OMPPh depends weakly on the type of the conformation and the value and the character of distortion of the porphyrin macrocycle.…”