Photoluminescence polysilanes

Photoluminescence (PL) in the polysilanes is well documented,34b,34c and for the poly(diarylsilane)s occurs typically with a small Stokes shift and almost mirror image profile of the UV absorption.59 This is due to the similarity of the chromophore and fluorophore structures in the ground and excited states, respectively, which is a result of the fact that little structural change occurs on excitation of the electrons from the a to the a orbitals. As PL is the emissive counterpart to UV, the emissive counterpart to CD is circularly polarized pho-toluminescence (CPPL). Where the fluorophore is chiral, then the photoexcited state can return to the ground state with emission of circularly polarized light, the direction of polarization of which depends on the relative intensities of the right-handed and left-handed emissions (/R and /l, respectively), which in turn depends on the chirality of the material, or more accurately, the chirality... 

Circularly Polarized Photoluminescence (CPPL) from Polysilanes 629... 

Recently, a very interesting example of solvatochromism was reported by Fujiki and co-workers.206 Poly(methyl-3,3,3-trifluoropropylsilylene), 87, synthesized via Wurtz coupling, showed solvatochromism as a result of weak, non-covalent intramolecular Si- -F-G interactions which rendered the conformation of the polysilane uniquely controllable by solvent choice and molecular weight. UV, shown in Figure 18, photoluminescence, NMR, and viscosity studies on the polymer indicated a 73 helical rod-like conformation at room temperature in non-coordinating solvents (e.g., toluene and decane), since the intramolecular interaction resulted in constraining the chain in a rigid helix. 

The situation is somewhat more complicated in two-dimensional polysilanes, which have intermediate properties between the one-dimensional chain-like polysilanes and three-dimensional bulk silicon. The gap is of a quasi-direct nature as the indirect gap is only slightly smaller than the direct one [11]. However, the excitons strongly bind to the lattice which results in a large Stokes shift of the PL [26]. The observed blue shift of the absorption and photoluminescence with decreasing size of the polysilanes is considered to be due to confinement effects of the excitons [12,26]. The strong coupling of the exciton to the lattice decreases somewhat the blue shifts as compared with the linear chains, and it results in a stronger localization of the exciton over a smaller number of Si atoms [12,26]. 

Properties such as photoconduc.tivityl l t l and photoluminescence of silicon polymers have been reported because of their wider optical band gap compared with crystalline silicon. Theoretical investigations of silicon polymers have been also reported ll2l-[21] Xakeda, Matsumoto and Fukuchi calculated the electronic structure of polysilane chains using the semi-empirical approach called the Complete Neglect of Differential Overlaps (CNDO) Molecular-Orbital (MO) method They discussed the dependence of the size and... 

Crystalline and amorphous silicons, which are currently investigated in the field of solid-state physics, are still considered as unrelated to polysilanes and related macromolecules, which are studied in the field of organosilicon chemistry. A new idea proposed in this chapter is that these materials are related and can be understood in terms of the dimensional hierarchy of silicon-backbone materials. The electronic structures of one-dimensional polymers (polysilanes) are discussed. The effects of side groups and conformations were calculated theoretically and are discussed in the light of such experimental data as UV absorption, photoluminescence, and UV photospectroscopy (UPS) measurements. Finally, future directions in the development of silicon-based polymers are indicated on the basis of some novel efforts to extend silicon-based polymers to high-dimensional polymers, one-dimensional superlattices, and metallic polymers with alternating double bonds. 

Figure 11.7 depicts typical absorption and luminescence spectra for polysilanes. Absorption peak energies are within the range 3 to 4 eV and are determined mainly by backbone conformation. Photoluminescent efficiencies are high (10% to 50%) and the apparent Stokes shift increases with the breadth of the absorption spectrum. Whenever a single screw-sense helical structure pertains, the absorption and emission bands are narrow and the Stokes shift is small. Such a polymer is poly[decyl-(5)-2-methylbutylsilane], the chiral center within the (S)-2-methylbutyl substituent determining the screw sense. Such... 

Since only polymers derived from cyclic starting materials, which are likely to have siloxene-like structures, exhibit color and fluorescence, the polysilane ring seems to be essential for the exceptional optical properties. This is in agreement with the original idea assuming the cyclosilane ring to be the chromophore responsible for the photoluminescence of siloxenes. 

Silicon-based materials with unique (opto)electronic properties photoluminescent materials for flat panel technology, displays, light-emitting diodes, sensors electroluminescence, nonmetallic conductors, e.g. siloles, polysilanes, 2,3-diphenyl-1-silacyclobutene chemistry design and application of liquid crystals. 

ABSTRACT. Band calculated results for electronic structures of sigma-conjugated polymers are reviewed. Conformational and substitutional effects for polysilanes are calculated theoretically and are discussed in the light of experimental data from UV absorption and photoluminescence. The electronic structures of hetero-copolymers of polysilane and polygermane, corresponding to the 1-dimensional superlattice structure, are described. Two-dimensional silicon network polymers are studied theoretically and experimentally. 

It is well known that visible luminescence is observed even in some kinds of polysilanes. In methylnaphtylpolysilane, visible emission is observed from die excimer sites formed by the stacking of naphtyl sidechains. In mediylphenylpolysilane, the visible emission is due to a n interaction between the silicon backbone and phenyl sidechains. Silicon network polymers, however, exhibited no marked differences in photoluminescence spectra, regardless of their sidechains. This indicates that photoluminescence originates from the silicon backbone itself. 

Hydrogenated amorphous silicon was formed by plasma decomposition of monosilane gas. The network has the dimension of close to 3. Polysilane alloy was formed by plasma decomposition of disilane gas.33 The network consists of a mixture of 1-dimensional polysilane and 3-dimensional silicon micro clusters.34 The effective network dimension is lower than that of amorphous silicon. Photoluminescence observations for various silicon-based materials are shown in Figure 14. The peak energy values of the photoluminescence spectra for amorphous silicon, polysilane alloy, hexyl-silicon network polymer and dihexylpolysilane are 0.8, 1.2, 2.8 and 3.3 eV, respectively. This result confrrms that a wide continuous spectra range from ultraviolet to infrared can be covered by the luminescence spectra of silicon based polymers. 

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