Vibrational Characterization and Fluorescence Optimization of Polycyclic Polymers

Abstract

A systematic series of polycyclic novel polymers was studied by Raman spectroscopy. The effect of the sequential introduction of polycyclic aromatic ring substituents into the delocalized backbone was examined in relation to the variation of the relative ring breathing and stretching as well as the vinyl stretch frequencies. Replacement of the phenyl units by higher order acene moieties such as naphthyl and anthryl results in a shift of the characteristic stretching frequencies, and analysis of the vinyl stretch leads to the confirmation that higher order acene substitution into the delocalized backbone substantially weakens the vinyl bond. Semiquantitative integrated Raman analysis shows a well-defined variation of the vibrational characteristics with structure. The structural variation of the integrated Raman intensity elucidates the effect of the electronic and vibrational decoupling introduced by the continual systematic acene substitution and points toward the ability to tailor the vibrational characteristic of the polymeric systems much the same as the electronic characteristics are manipulated. Relative fluorescence yields calculated for absorption and fluorescence spectroscopy can be seen to be well-correlated with integrated Raman intensities, implying that the reduction of the vibrational intensity limits the avenues of nonradiation, hence optimizing the fluorescence yield.

Correlation of vibrational intensity with fluorescence lifetimes in π conjugated polymers

“…Thus it is evident that the reduction in the Stokes shift would correspondingly be well correlated with the non-radiative rate. This is as expected as it has been shown that the Stokes shift is well correlated with integrated Raman intensity, and thus can be used as a crude measurement of the vibrational activity of the system [18].…”

“…This is a clear indication that the dominant change, introduced by the systematic alteration of the p conjugated backbone, is the reduction of the non-radiative rate. This result correlates well with the reduction of the relative Raman intensity and the increase in the fluorescence yield published previously [18]. In order to further explore the tentative relationship suggested through the calculation of the radiative rates using the StricklerBerg equation it was decided to experimentally determine the fluorescence lifetime and compare this to the calculated lifetime.…”

“…This is a simple explanation, which assumes that the triplet yield is negligibly small and that in the variation of the backbone structure the most significant effect is in the reduction of the nonradiative rate. This assumption is not unfounded as it has been shown that contrary to the trend shown in acene oligomers [14], a plot of the vibrational intensity against the fluorescence yield gives a well-defined relationship that is almost linear [18]. This would suggest that it is the non-radiative yield that is dominating the fluorescence yield.…”

“…In an effort to extend the empirical relationships shown to exist for the vibrational characteristics of the polymers' systems [18] it was decided to investigate the variation of the radiative and nonradiative rates, as the larger acene units were inserted in to the backbone, using the Strickler-Berg equation. It has been shown that the radiative rate (K rad ) can be calculated using the absorption and emission spectra using the Strickler-Berg Eq.…”

“…1, it has been demonstrated that both the electronic and vibrational activities of these complex systems can be controlled and optimized by systematic variation of the polymer conjugated backbone [17,18].…”