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

E. Ciliberto, K. A. Doris, W. J. Pietro, G. M. Reisner, D. E. Ellis, I. Fragalà, F. H. Herbstein, Mark A Ratner, Tobin J Marks

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Abstract

This contribution describes an integrated chemical, physical, and quantum chemical approach to understanding π-π 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)O]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) Å. Least-squares refinement led to a value for the conventional R index (on F) of 0.127 for 1975 independent reflections having 5° ≤ 2σMoKᾱ ≤ 40° and Fo ≥ 3σ(Fo). 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]I1.12}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.

Original languageEnglish
Pages (from-to)7748-7761
Number of pages14
JournalJournal of the American Chemical Society
Volume106
Issue number25
Publication statusPublished - 1984

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ASJC Scopus subject areas

  • Chemistry(all)

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