Fast electronic relaxation in lathanide porphyrins

Abstract

The photophysical processes in porphyrin complexes of lanthanides (Ln) and their correlation with Ln 3+ ion characteristics have been studied by picosecond transient spectroscopy and spectrofluorimetry. The lifetimes of the S 2, S 1 and T 1 π-electron states have been determined for the Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu complexes of tetrabenzporphin (TBP). Depending on the nature of the Ln 3+ ion the non-radiative decay rates of the Ln TBP (π, π*) states vary by several orders of magnitude: from 3 × 10 11 to > 2 × 10 13 s −1 for the S 2 decay, from 2 × 10 10 to > 2 × 10 11 s −1 for the S 1 decay and from 6 × 10 2 to 3 × 10 10 s −1 for the T 1 decay. The T 1 deactivation rate constant of paramagnetic complexes decreases exponentially with the energy gap between the T 1 level and the nearest lower-lying accepting 4f-level beginning with Δ E ≈ 4000 cm −1. For Δ E < 4000 cm −1 there is a tendency toward stabilization of the τ −1 T values. By example of the Dy, Tb, Ho, Er and Tm complexes it is shown that combination states, in which the Ln 3+_ ion and the macrocyclic ligand are excited simultaneously, participate in the deactivation of the S 2 and S 1 states. The anomalously high rates of deactivation of the S 2, S 1 and T 1 states in the case of Eu, Yb and Sm complexes are due to the charge-transfer states involved in relaxation processes.