Photophysics of a cis axially disubstituted macrocycle: rapid intersystem crossing in a tin(IV) phthalocyanine with a half-domed geometry.

We have studied the photophysical properties of a tin(IV) phthalocyanine which coordinates two myristate groups through their carboxylate functionalities in a cis disposition at the tin center. Such a coordination mode, anisobidentate through the same side of the macrocycle, makes this phthalocyanine acquire a capped or half-domed shape. This bis myristate tin(IV) molecule shows an intersystem crossing channel which populates the triplet manifold with high efficiency and with a time constant of 300 ps, about an order of magnitude faster than planar phthalocyanines, including some previously reported tin(IV) phthalocyanines. For comparison purposes, we also include the description of a planar silicon(IV) phthalocyanine that keeps the more common stereochemistry, of trans type, with the same axial myristate groups. The characterization of these systems included steady state and time-resolved spectroscopy through femtosecond fluorescence up-conversion and transient absorption. We also studied the initial S(n) → S(1) internal conversion dynamics when these compounds are excited to upper states with 387.5 nm light. In addition, we include measurements of the rate for singlet oxygen production through the formation of an ESR-active adduct in aerated solutions. Such measurements indicate that, associated to its photophysics, the tin(IV) phthalocyanine produces (1)O(2) with an efficiency significantly larger than the silicon(IV) counterpart, making it an interesting option for sensitization applications. Finally, we performed excited state calculations at the TD-DFT level which describe the effects of the reduced symmetry together with the state ordering and indicate the presence of near dark intermediate states between the Q and B transitions for both of these macrocycles.