Crown ether substituted monomeric and cofacial dimeric metallophthalocyanines. 2. Photophysical studies of the cobalt(II) and nickel(II) variants

Metallophthalocyanines have been prepared with 18-crown-6 residues at the p eripheral benzo sites (McrPc). Metal centers employed have been H-2 (free b ase), Zn(II), Cu(LT), Co(II), and Ni(II). In the present report, the Co(II) and Ni(II) systems are considered; the other three compounds were consider ed in part 1 of this series of papers. Ultrafast transient absorption spect rography was employed to examine the dynamic properties of the excited elec tronic states of the monomers and dimers. Under pulsed photoexcitation cond itions, the most prominent feature in the transient absorption spectrum of all systems studied was a transient bleaching at the ground-state absorptio n maxima. The time profiles for ground-state repopulation of photoexcited M crPc and McrPcD where M = Co(II) and Ni(II) were best described with double -exponential kinetics with lifetimes of 1.3 and 7.6 ps for CocrPc; 0.8 and 7.2 ps for CocrPcD; 3.2 and 12.8 ps for NicrPc; 2.2 and 24.2 ps for NicrPcD , respectively. An analysis of the kinetic data in the case of the Co(II) a nd Ni(II) Pc monomers and dimers indicated that the initially formed (1)pi, pi*-singlet state decayed via parallel processes into either a short-lived (3)pi,pi*-triplet state (absorbing maximally around 540 nm) or a vibrationa lly hot, electronically excited, metal-centered (d,d) state. A rapid blue s pectral shift (tau = 1.3 ps) at the red side of the ground-state bleaching band was attributed to vibrational cooling of this latter state. This very rapid rate of cooling of the vibrationally hot metal state indicates that i t may be determined by the rate of energy translocation through the M-N bon ds to the sc-system, and not into the solvent, viz., an intramolecular proc ess. The repopulation dynamics were shown to be independent of whether exci tation was at 400 nm (initial formation of an upper excited state) or at 64 5 nm (initial formation of the lowest excited state), thus indicating that the internal conversion process, S-2(pi,pi*) --> S-1(pi,pi*), was occurring within the time resolution of the instrument (ca. 500 fs).