Solute–solvent interaction dynamics studied by photon echo spectroscopies in polymer glasses

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Abstract
The influence of coupling strength and temperature on the solute–solvent interaction of two chromophores in polymer glass is reported. The three-pulse photon echo peak shift method was used to study the dye IR144 in polyvinylformal (PVF) and the dye DTTCI (3,3′-diethylthiatricarbocyanine iodide) in polymethylmethacrylate (PMMA). IR144 is more strongly coupled (larger reorganization energy) to both its intramolecular modes and to the solvent than is DTTCI. Our results can be well described by the linearly coupled harmonic bath model over the range 300 to 30 K. In particular, the strikingly different temperature sensitivities of the long-time (asymptotic) peak shift are well described by the theory. Temperature-independent spectral densities and inhomogeneous widths suffice to quantitatively describe the peak shift data over this temperature range and a number of numerical predictions based on the theoreticalmodel are experimentally confirmed. An ultrafast component corresponding to a decay of ∼100 fs time scale in the transition frequency correlation function is found in all cases, though the amplitude is small for the DTTCI solutions. This ultrafast response is assigned to the inertial response of the solvent. The inertial response measured for DTTCI in PMMA is very similar to our previous measurements of the inertial response for IR144 in PMMA, suggesting that a general characteristic of the solvent is being probed. The weaker coupling of DTTCI produces a drastic increase in the width of the photon echo signal as predicted by theory and strong vibrational quantum beats at 30 K. By contrast, such beats are not observed in the echo signal for IR144.