Orbit Interactions in a Series
of Sterically Hindered Porphyrins in the Lowest Triplet State
E. E. Shchupak, N. V. Ivashin, and E. I. Sagun
Stepanov Institute of Physics, National Academy of Sciences of Belarus, Minsk, 220072 Belarus
e-mail: ivashin@imaph.bas-net.by
Received November 22, 2012
Abstract—We have studied the nature and unusual photophysical properties of triplet states of a series of ster-
ically hindered porphyrins (meso-phenyl-substituted derivatives of octamethylporphyrin (PS-OMP)) using
quantum-chemical calculations of the parameters that determine the probability of deactivation of the T1 state
and modeling the T–T absorption spectra. We show that a decrease in the lifetime of the T1 state of PS-OMP is
related with the enhancement of the channel of nonradiative deactivation (T1 
S0), which occurs (i) due to the
conformational lability in the T1 state, as a result of which the energy gap T1–S0 considerably decreases, and
(ii) because of an increase in the matrix element of the spin–orbit interaction due to a change in the hybridiza-
tion 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 S0 and T1 states of PS-OMP depends weakly
on the type of the conformation and the value and the character of distortion of the porphyrin macrocycle. The
conformational lability of PS-OMP clearly manifests itself in the spectra and kinetics of the T–T absorption of
these compounds at room temperature and determines the nonmonoexponential character of the phosphores-
cence decay kinetics in frozen matrices. Using porphin as an example, we have shown that nonplanar distortions
of the macrocycle facilitate a decrease in the phosphorescence rate constant at 77K, which is caused, on the
one hand, by an increase in the energy gaps T1–Sn and S0–Tn and, on the other hand, by an increase in the wave-
length of the transition T1 
S0.
DOI: 10.1134/S0030400X13070163
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