Comparative photophysical properties of free-base, Bis-Zn(II), Bis-Cu(II), and Bis-Co(II) doubly N-confused hexaphyrins(1.1.1.1.1.1)

We have comparatively investigated the photophysics of a series of bis-metal doubly N-confused hexaphyrins( 1.1.1.1.1.1) using time-resolved fluorescence, femtosecond transient absorption, two-photon absorption measurements, and geometry-optimized ab initio calculations. Bis-Zn( II) and free-base doubly N-confused hexaphyrins exhibit well-resolved and red-shifted B- and Q-like absorption bands compared with porphyrins. Their allowed transitions are (pi, pi*) transitions of the hexaphyrin ring, as confirmed by the HOMO and LUMO frontier orbitals based on ab initio calculations at the B3LYP/6-31G level. On the other hand, the absorption spectra of bis-Cu(II) and bis-Co(II) doubly N-confused hexaphyrins are relatively broad, presumably due to large couplings between the metal d-orbitals and pi-electrons of the hexaphyrin ring. Owing to these couplings, bis-Cu( II) and bis-Co( II) doubly N-confused hexaphyrins have much shorter excited-state lifetimes of 9.4( 0.3 ps and 670 fs, respectively, than those (267 +/- 16 and 62.4 +/- 1.2 ps, respectively) of bis- Zn( II) and free-base doubly N-confused hexaphyrins. The two-photon absorption cross section (sigma((2))) values, which are believed to depend strongly on the ring planarity (pi-conjugation), are in line with the excited-state lifetime trends.