π-π Interactions and Bandwidths in “Molecular Metals”. A Chemical, Structural, Photoelectron Spectroscopic, and Hartree-Fock-Slater Study of Monomeric and Cofacially Joined Dimeric Silicon Phthalocyanines

This contribution describes an integrated chemical, physical, and quantum chemical approach to understanding πKπ interactions and tight-binding bandwidths in low-dimensional metallomacrocyclic “metals” via the properties of monomeric and dimeric stack fragments. Thus, electronic structure in the cofacially arrayed phthalocyaninato (Pc) polymer [Si(Pc)O]_n has been explored through the complexes Si(Pc)(OR)₂ and ROSi(Pc)OSi(Pc)OR (R = Si[C(CH₃)₃](CH₃)₂). Improved synthetic and purification procedures are described. Vibrational spectroscopy is employed to assign ROSi and Si(Pc)OSi(Pc) modes, and the results are correlated with data on [Si(Pc)0]_n. The cofacial dimer crystallizes from chloroform in the orthorhombic space group Pbcn (No. 60) with four molecules in a unit cell of dimensions a = 21.670 (8), b = 13.724 (5), and c = 23.031 (9) A. Least-squares refinement led to a value for the conventional R index (on F) of 0.127 for 1975 independent reflections having 5∘ ≤ 2σ_MoKα ≤ 40∘ and F_o ≥ 3σ(F_o). The molecular structure consists of a cofacial (Pc)Si-O-Si(Pc) core of C₂ symmetry, having virtually planar phthalocyanine rings, an Si-Si distance (interplanar spacing) of 3.32 (1) Å, ∠Si-O-Si = 179 (1)∘, and a ring-ring staggering angle of 36.6∘. The Si[C(CH₃)₃](CH₃)₂ capping groups are disordered. Electronic structure in the (phthalocyaninato)silicon monomer and dimer has been studied with first principles discrete variational local exchange (DV-Xα) techniques. These results are combined with transition-state calculations to interpret optical and high resolution He I and He II photoelectron spectroscopic data. While the conventional porphyrinic “four-orbital” model is supported for the low-energy optical transitions (excellent agreement between observed and calculated energies is noted), possible disagreements are noted at higher energies. Calculated (6.8 eV) and observed (6.46 eV) Si(Pc)(OR)₂ ionization potentials are in good agreement. The lowest energy PES feature in the dimer is split by 0.29 (3) eV. The splitting can be assigned to the cofacial HOMO-HOMO interaction and translates to a tight-binding bandwidth in the polymer of 0.58 (6) eV. This result is in favorable agreement with a DV-Xα derived bandwidth of 0.76 eV and a value of 0.60 (6) eV previously obtained from a Drude analysis on {[Si(Pc)O]I_1.12}n. These results argue that the principal charge-transport pathway in the [Si(Pc)O]_n polymer is via the Pc π systems and that polaronic band-narrowing effects are minimal.