The effect of high pressure on the dynamics of doped organic glasses: A study by spectral hole-burning

We report time-resolved spectral hole-burning experiments on bacteriochlorophyll-a (BChl-a) doped into the glass triethylamine (TEA) at ambient pressure (Delta p = 0) and at a pressure of Delta p = 3.4 GPa. We observe a number of remarkable effects: (a) from the change in the temperature dependence of the ''effective'' optical homogeneous linewidth Gamma(hom)', we conclude that local order is introduced in TEA under high pressure; (b) from the change in the time dependence of Gamma(hom)' we conclude that spectral diffusion is induced by ''downhill'' energy transfer among BChl-a molecules (at a concentration of c similar to 5 x 10(-4) M) within the S-1 <-> S-0 0-0 band, both at ambient and high pressure; and (c) from the wavelength dependence of Gamma(hom)', we conclude that an energy barrier (Delta E similar to 135 cm(-1) at Delta p = 0, and Delta E similar to 70 cm(-1) at Delta p = 3.4 GPa) has to be crossed in order to excite two-level systems (TLSs) of the TEA host which, subsequently, leads to spectral diffusion. We present a qualitative molecular model for the interpretation of these effects. (C) 1997 American Institute of Physics.