Metallophthalocyanines have been prepared with 18-crown-6 residues at the four peripheral benzo sites (McrPc). Metal centers employed have been H 2 (free base), Zn(II), and Cu(II). In ethanol solution containing potassium acetate, such species incorporate K + cations into the crowns, one K + per crown, and are monomeric in nature. When cesium acetate is present, Cs + cations complex with a pair of crown residues, resulting in cofacial dimer species (McrPcD) in which one Cs + ion links two ethers in a kind of sandwich arrangement. This dimerization results in spectral shifts to the blue. Photophysical examinations of these monomeric and dimeric entities have been carried out. For H 2 crPc and Zn II crPc the excited-state dynamics are those of the π-macrocycle, and dimer formation caused no major changes except for increasing the rate constants of the excited-state deactivation. Such increases are anticipated owing to the proximity of the lower exciton state and the ground state. For Cu II crPc where now a d 9 metal is present within the π-system, a deactivation event with a 22 ns lifetime was attributed to the decay of the 4 T state of the complex. The corresponding state of the cofacial dimer had a lifetime of 5.7 ns. Ultrafast experiments with ca. 500 fs resolution provided evidence of earlier processes in the Cu(II) system. Thus, in the monomer, a 2.9 ps lifetime event preceded the quartet-state decay. This may be attributable either to the decay of the 2 T precursor to the 4 T state or to the population of a CT state situated between 2 T and 2 S 0 in energy. Similar early-time behavior was noted for the Cu(II) dimer. Triplet-state properties are reported for the monomeric and dimeric free base and Zn(II) Pcs. Notably, the bimolecular rate constants for O 2 quenching were lower for the dimers compared to the monomers. This can be understood if the dimerization yields a triplet state with an energy lower than that of singlet oxygen, O 2 ( 1 ∆ g ).
We have found O(2)-substituted diazeniumdiolates, compounds of structure R(2)N-N(O)=NOR' that are under development for various possible pharmaceutical uses, to be rather photosensitive. With R = ethyl and R' = methyl, benzyl, or 2-nitrobenzyl, the observed product distributions suggest that two primary pathways are operative. A minor pathway involves the extrusion of nitrous oxide (N(2)O) with simultaneous generation of R(2)N(*) and R'O(*), which may then form amines, aldehydes, and alcohols. The major reaction pathway is an interesting photochemical cleavage of the N=N bond to form a nitrosamine (R(2)NN=O) and an oxygen-substituted nitrene (R'ON). The intermediacy of the O-nitrene was inferred from the production of abundant oxime, via rearrangement of the O-nitrene to a C-nitroso compound (R'ON --> O=NR'), and subsequent tautomerization to the more stable oxime. Involvement of the O-nitrene was confirmed by trapping with 2,3-dimethyl-2-butene to form the aziridine and with oxygen to generate the nitrate ester. 2-Nitro substitution on the benzyl derivative had surprisingly little effect on the reaction course. For each compound examined, minor amounts of nitric oxide (NO), presumably produced by secondary photolysis of the nitrosamine, were observed. Time-resolved infrared experiments provided additional support for the above reaction pathways and confirmed that the nitrosamine is a primary photoproduct. We have also found that the relative contributions of the reaction pathways can be altered in certain derivatives. For example, when R' = 2,4-dinitrophenyl, the contribution of the nitrosamine/O-nitrene-forming pathway was diminished. Pharmacological implications of these results are discussed.
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(II), 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 π,π*-singlet state decayed via parallel processes into either a shortlived 3 π,π*-triplet state (absorbing maximally around 540 nm) or a vibrationally hot, electronically excited, metal-centered (d,d) state. A rapid blue spectral shift (τ ) 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 π-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 (π,π*) f S 1 (π,π*), was occurring within the time resolution of the instrument (ca. 500 fs).
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