Crown Ether Substituted Monomeric and Cofacial Dimeric Metallophthalocyanines. 2. Photophysical Studies of the Cobalt(II) and Nickel(II) Variants

Abstract

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).

Novel Cobalt (II) Phthalocyanine‐Containing Polyimides: Synthesis, Characterization, Thermal and Optical Properties

“…However, upon excitation at 350 nm, the emission spectrum of PI-CoPc (solid line, Figure 4) showed an intense peak at 430 nm. This interesting result has been observed before for other cobalt phthalocyanines, [21] and Novel Cobalt (II) Phthalocyanine-Containing Polyimides: Synthesis, Characterization . .…”

Metallated Phthalocyanines as the Core of Dendrimers – Synthesis and Spectroscopic Studies

“…Such behaviour may seem surprising; indeed, dendritic zinc phthalocyanines in organic solvents have a relatively intense quantum yield ( ≈ 0.30) even for small generations (0-2), [21] but a complete lack of fluorescence has been observed for some cobalt phthalocyanines. [22] Such behaviour is due to the unfilled d-orbitals of Co II that couple with the π orbital system of the phthalocyanine, providing additional and very efficient channels for π* deactivation. [22] Thus, it is not surprising to observe the absence of fluorescence in our case; indeed, this intrinsic deactivation obviously cannot be prevented by the presence of the dendritic branches, whatever the generation of the dendrimer.…”

“…[22] Such behaviour is due to the unfilled d-orbitals of Co II that couple with the π orbital system of the phthalocyanine, providing additional and very efficient channels for π* deactivation. [22] Thus, it is not surprising to observe the absence of fluorescence in our case; indeed, this intrinsic deactivation obviously cannot be prevented by the presence of the dendritic branches, whatever the generation of the dendrimer.…”

Synthesis, spectroscopic and electrochemical investigations of supramolecular nickel(II)tetraazaporphyrin complexes

“…a nickel porphyrin homologues [34][35][36], [Ni(OBTTAP)] and (bpy) 2 Ru II /(phen) 2 Ru II [37,38]. Hence, the emission characteristics of these supramolecular systems are expected to be an amalgamation of the two.…”