Influence of azasubstitution on the nonlinear absorption in porphyrazine derivatives

For the compounds I (AzaPhcCu) and II (AzaPcCu) in CH2Cl2, we use nanosecond laser pulses to determine the sizes of absorption cross-sections σexS and σexT. The observed reverse saturable absorption (RSA) is attributed to the excitation process of S1(0)→S2(v), which is due to their slow intersystem crossing from S1 to T1 (τisc≈50 ns) comparing with the exciting period.