Journal of Physical Chemistry A, Vol.103, No.38, 7597-7605, 1999
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 peripheral benzo sites (McrPc). Metal centers employed have been H-2 (free base), 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 considered in part 1 of this series of papers. Ultrafast transient absorption spectrography was employed to examine the dynamic properties of the excited electronic states of the monomers and dimers. Under pulsed photoexcitation conditions, the most prominent feature in the transient absorption spectrum of all systems studied was a transient bleaching at the ground-state absorption maxima. The time profiles for ground-state repopulation of photoexcited McrPc 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) and 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 vibrationally hot, electronically excited, metal-centered (d,d) state. A rapid blue spectral 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 it may be determined by the rate of energy translocation through the M-N bonds to the sc-system, and not into the solvent, viz., an intramolecular process. The repopulation dynamics were shown to be independent of whether excitation was at 400 nm (initial formation of an upper excited state) or at 645 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).