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Chemical Physics Letters

High temperature optical dephasing mechanism for dye molecules in PMMA glass

Andreas Elschner, L. R. Narasimhan, M. D. Fayer

Published Jul 1, 1990• 43 citations
Publisher

Abstract

Picosecond photon echo experiments on rhodamine B and octadecyl rhodamine B in a PMMA matrix between 1.0 and 12.5 K are compared. The two dyes have identical first electronic excited state transitions but different masses and moments of inertia. Echo decay times are identical and individual decays at the same temperature from the two systems are superimposable. The high-temperature echo data can be fit to an activated process with an energy of 15 cm−1. We conclude that in this glassy matrix, high-temperature dephasing is caused by a mode of the host and not a mode of the guest as has been suggested for glasses and is common in crystals.

DOI
10.1016/0009-2614(90)80043-d

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The Journal of Chemical Physics•Oct 8, 1998•55 citations
Picosecond photon echo experiments on rhodamine B and octadecyl rhodamine B in a PMMA matrix between 1.0 and 12.5 K are compared. The two dyes have identical first electronic excited state transitions but different masses and moments of inertia. Echo decay times are identical and individual decays at the same temperature from the two systems are superimposable. The high-temperature echo data can be fit to an activated process with an energy of 15 cm−1. We conclude that in this glassy matrix, high-temperature dephasing is caused by a mode of the host and not a mode of the guest as has been suggested for glasses and is common in crystals.
Temperature-dependent (1.2–2.1 K) photon echo measurements have been performed on quinizarin and rhodamine 101 doped into aluminosilica sol-gel glasses as well as rhodamine 101 doped poly-methyl-methacrylate (PMMA) polymer. We observe a temperature-dependent homogeneous linewidth for both quinizarin and rhodamine in the aluminosilica xerogel (ASX) matrix proportional to T2, in contrast to the T1.3±0.1 linewidth dependence typically observed for a variety of organic and inorganic glass hosts in this temperature range (including rhodamine 101 doped PMMA). The contrasting optical dephasing results for the ASX glass and PMMA suggest that the ASX glass may have unique two-level-system dynamics.
Picosecond photon echo experiments on pentacene in polymethylmethacrylate (PMMA) show that the homogeneous width exhibits a T1.3 temperature dependence. This behavior is ascribed to coupling of the pentacene transition to randomly distributed low-frequency excitations in the amorphous host. A calculation of the resulting homogeneous width is presented which uses optical Redfield theory as a starting point. From the results of this calculation we conclude that the coupling between the pentacene molecule and the host low frequency excitations is of static dipolar nature and that the density of states of these excitations varies as ω0.3. Results of nonphotochemical hole-burning experiments on the same system are also reported. Comparison of these results with the ones obtained by the photon echo method indicates, that in the hole-burning experiments, the hole width and its temperature-induced broadening are dominated by spectral diffusion.
The first two-pulse photon echo experiments on a chromophore in an organic glass are reported. The homogeneous electronic dephasing of resorufin in ethanol glass is measured from 1.5–11.4 K. The temperature dependence of the dephasing time does not fit the power law frequently predicted by theory for the dephasing characteristic of glasses. However, the temperature dependence can be accounted for by including dephasing from librations or acoustic phonons, mechanisms known to be important in crystals. The dephasing decay is found to be a single exponential for over six factors of e. The dephasing is also shown to be uncorrelated with the extent of nonphotochemical hole burning (NPHB). However, the homogeneous linewidth deduced from the photon echo is four times narrower than the linewidth obtained from NPHB, demonstrating that the hole is broadened by additional processes.
Picosecond photon echo (PE) experiments are used to examine the optical dephasing and intermolecular interactions of organic chromophores in a variety of glassy organic and polymer hosts. The inherent disorder in these systems leads to dynamics which span times from picoseconds to hours and longer. The temperature dependent optical dephasing as measured by the PE follows a power law characteristic of tunnelling twolevel systems (TLS) at low temperatures (1 °K to 4 °K) and is exponentially activated at higher temperatures due to dephasing by additional modes. The nature of these modes will be addressed. These data and those from other line narrowing experiments such as hole burning allow the study both of fast dephasing processes and spectral diffusion due to slow fluctuations in the host. The observed lineshapes and holewidths require dipolar coupling between the chromophore and TLS and offer information on the distribution of relaxation rates. Furthermore, techniques sensitive to different timescales are used to examine host dynamics by removing the guest-host coupling from consideration. The data are discussed in conjunction with recently developed theoretical treatments of dephasing in disordered systems.
Some results of femtosecond echo experiments on polyvinylbutyral polymer films doped with phtalocyanine molecules at high (up to room) temperatures are reported. Special attention is paid to the stimulated femtosecond photon echo (SFPE), which was observed in a solid-state medium at room temperature for the first time. A decay curve of the SFPE signal has been obtained and theoretically analyzed. The results of the analysis indicate that the random interaction between impurity molecules and quasi-localized low-frequency vibration modes in an amorphous matrix plays the dominant role in the character of optical dephasing at high temperatures.
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 theoretical model 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.