Kinetics of Thermal Conversion of Conjugated Polymers Investigated from Their Optical Absorption Spectra

Abstract

We report on a phenomenological, theoretical model to calculate the rate constants and activation energies for the thermal conversion reactions from poly(xylylidene tetrahydrothiophenium chloride) (PTHT) into poly(p-phenylenevinylene) (PPV) using the optical absorption spectra of spin-coated films. The probabilities of electron transitions were calculated considering Franck-Condon states with a Gaussian distribution of conjugated segments and molecular excitons. The dependence on the conjugation degree (n) for the energy gap, transition dipole moment, and electron-phonon coupling were obtained semiempirically using published data for PPV. Fitting was performed for the C-C stretching of the aromatic ring 1550 cm(-1), for it is considered the most optically active. The isotherms for consumption and formation of PPV segments were fitted using a first-order and consecutive reactions, respectively. With this modeling we could identify the most probable reactions, where the formation of longer PPV segments n ≥ 3 occurs only via reactions from smaller conjugated segments (i.e., n = 1 and 2). The activation energies tend to decrease with longer conjugation lengths. Significantly, the modeling allows us to predict the conversion temperature and chemical composition yielding a predefined distribution of conjugated segments, which can be applied to any polymer undergoing thermal conversion, decomposition, or photo-oxidation.

Conjugation Length Distribution in Poly(p-phenylenevinylene) (PPV) Films

“…The Langmuir–Blodgett PPV film (LB-PPV) was produced using 20 Z-type layers of PTHT and DBS (1:1 w/w) solution dissolved in 1,2-dichloromethane and ethanol (1:1 v/v). The SC-PPV and LB-PPV films were obtained after thermal conversion of SC-PTHT+DBS and LB-PTHT+DBS at 110 °C for 2 h under 10 –1 mbar vacuum . Absorbance spectra in the UV–vis range for both samples were obtained at 10 K using the spectrometer Hitachi U-2001 and a closed circuit helium cryostat.…”

“…Density matrix renormalization group calculations of Barford and co-workers showed that the energy gap of linear chains of PPV with n repeat units is well parametrized by where E ∞ is the infinite chain energy gap and ϵ is an adjustable parameter. Based on fittings of experimental PPV absorption spectra taken from the literature, we used E ∞ = 2.18 eV and ϵ = 2.00 eV for our PPV oligomers. In Figure SI3 of the Support Information, we show that eq also fits well the gaps of other phenyl-based conjugated polymers.…”

“…Da Silva et al studied the line shape of high-resolution (fwhm ∼65 meV) absorbance spectra of PPV films, which included the contribution of short conjugated segments ( n ≤ 3), and obtained evidence for an inhomogeneous energy gap dispersion squared proportional to ( d n ) 2 ∼ 1/ n 4 . We defined d n in such a way that ( d n ) 2 contained an n -independent term, together with the n –2 and n –4 terms required by experimental and theoretical evidence. ,, We used …”

“…Based on fittings of experimental PPV absorption spectra taken from the literature, 6 we used d H = 650 cm −1 e δ = 4000 cm −1 for our PPV oligomers. We note in passing that, absorbance spectra with poor line shape resolutions (fwhm >100 meV), can be adjusted by taking into account just d H .…”

“…In a first approximation, the conjugated segments can be treated as independent, and the photochemistry of the film becomes a direct function of the photochemistry of each individual segment and of the distribution of conjugated segment lengths. This approximation has been successfully used to describe the optical absorption spectra of phenyl-based conjugated polymers, − and its limitations have been theoretically evaluated . Within this approximation, the absorbance per chain of the conjugated polymer film is where D [ n ] is the average number of conjugated segments of size n per chain, and α n (ω) is the absorption spectrum of a single conjugated segment of size n .…”