Spectral hole burning study of electron–phonon coupling in polymers

Abstract

Persistent hole burning in the S1←S0 transition of tetra-tert-butyl-tetraazaporphine was used to investigate the electron–phonon coupling in a broad range of polymeric solids between 5 and 30–50 K. The maxima of pseudophonon sidebands (νph) are displaced by 5–20 cm−1 from the 0–0 hole. The relationships between the νph values and the velocities of transversal and longitudinal ultrasound waves as well as the Young’s modulus of polymers were established. At the same time νph compare well to inelastic neutron scattering maxima, the first boson peaks in the Raman scattering spectra, and the characteristic modes responsible for extra specific heat and heat conductivity plateau. Mutual correlations of the molecular structure, nanoscopic, and bulk properties in glassy and partially crystalline polymers are pointed out. The quasihomogeneous hole width (Γqh) at fixed temperature (T) increases when νph becomes smaller and the polarity of the host increases. Hole widths measured at 15 and 25 K also display a common linear relationship with total heat content (J/cm3) of the matrix in less polar hosts. Irreversible broadening of holes as a function of excursion temperature was investigated by means of T cycling. The contribution of slow irreversible broadening processes (spectral diffusion) to Γqh does not exceed 20%. The shift of holes burned at 4–8 K upon the rise of T was measured. The pure thermal hole shift was calculated by taking into account the solvent shift contribution due to the density change of the matrix. This pure phonon-induced shift is always bathochromic with increasing T. The T dependence of both the hole width and shift can be equally well fitted with the power law and a coth function. In most systems both the width and shift obey the power law with similar T coefficients of 2.8±0.5 and 2.4±0.5, respectively. A consistent description of the T dependence of the Debye–Waller factor, the hole shift, as well as the width in terms of an anharmonic single-mode model can be achieved for most of the polymers with the same characteristic energy (entering the coth function) which is approximately by a factor of 4 larger than νph. The influence of crystallinity, tacticity, molecular weight, polarity, and chemical structure of the macromolecular host on the strength of electron–phonon coupling is analyzed in detail. The relative importance of the Stark effect and intermolecular dispersive forces in the dynamic modulation of electronic energy levels causing the optical dephasing is discussed.