Viscosity dependence of the excimer to monomer fluorescence ratio. Cyclic and linear polysiloxanes

Abstract

Fluorescence spectra of cyclic and linear samples of poly(methylphenylsiloxane) (PMPS) have been measured at 25 °C, in two solvent mixtures (tetrahydrofuran/dioxane and tetrahydrofuran/ methanol), as a function of the solvent composition. The viscosity of the medium ( ) and the segmental density of the macromolecule are varied by changing the solvent composition. The ratio of excimer to monomer intensities (fluorescence ratio, Ie/Im) is independent of segmental density. Ie/Im of linear PMPS is also almost independent on solvent viscosity, but for cyclic PMPS, a direct dependence of Ie/Im on is observed. The analysis of results allows several conclusions: (i) long range excimers are disregarded in the high molecular weight sample; (ii) only less than 10% excimers of any phenylsiloxane, cyclic or linear, are formed in ground-state preformed excimer-forming sites, due to the high dynamic flexibility of siloxanes in the nanosecond time scale; and (iii) excimer dissociation plays a significant role and takes place through a viscosity-dependent mechanism. A model is proposed to explain the viscosity dependence of the fluorescence ratio, which applies both to the high-temperature limit (like the present results of PMPS), and also to some other previously reported results on polymers in the low-temperature limit.

Non-toxic liquid scintillators with high light output based on phenyl-substituted siloxanes

“…In particular, PM100, characterized by the higher relative excimer component, is a homopolymer with consecutive units of phenyl containing monomers, while DPDM20 and PMDM50 are co-polymers where the ratio monomer/excimer depends on the distribution of phenyl units along the chains, in turn depending on the synthesis procedure [30]. The excimer to monomer ratio measured for PM100 and PMDM50 are in good agreement with values already reported in literature for similar compounds [35].…”

“…The difference with diluted solutions can be explained by the fact that in pure liquids, having a very strong absorbance in the region below 290 nm, inner filter effect can play a major role in distorting the short wavelength part of the emission spectrum. The increased viscosity, due to the reduced dilution, could also play a role in this variation of the monomer to excimer ratio, but this effect is reported to be very small in linear siloxanes [35]. A similar behavior however was already noticed also by Hirayama [33] in the comparison of diluted solutions of diphenyl and triphenyl alkanes with different concentrations.…”

Helical Conjugated Ladder Polymers: Tuning the Conformation and Properties through Edge Design

“…Although we do not rule out the effect of temperature and solubility on excimer formation in this study, we focused our work on the effect of solvent viscosity by investigating L-P2's emission behavior in different solvent systems. 39 As depicted in Figure 5b, the emission spectrum of L-P2 is solvent-dependent. Although minimal changes are observed for photoluminescence spectra recorded in THF, toluene and chloroform (similar viscosity), an important contribution of the excimer to the emission spectrum is noticeable in dichloromethane.…”

Equilibrium modeling of removal of cadmium ions by olive stones

“…Finally, for the sake of comparison, we mention the copolymers based on CH 3 PhSiO and (CH 3 ) 2 SiO units, the photophysics of which have been extensively studied 6–15. In light of the available data, the photophysics of CH 3 PhSiO‐based copolymers seem to be very similar to those of Ph 2 SiO‐based copolymers.…”