We report time-resolved spectral hole-burning experiments on bacteriochlorophyll-a ͑BChl-a͒ doped into the glass triethylamine ͑TEA͒ at ambient pressure (⌬pϭ0) and at a pressure of ⌬ p ϭ3.4 GPa. We observe a number of remarkable effects: ͑a͒ from the change in the temperature dependence of the "effective" optical homogeneous linewidth ⌫ hom Ј , we conclude that local order is introduced in TEA under high pressure; ͑b͒ from the change in the time dependence of ⌫ hom Ј , we conclude that spectral

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... diffusion is induced by "downhill" energy transfer among BChl-a molecules ͑at a concentration of cϳ5ϫ10 Ϫ4 M͒ within the S 1 ↔S 0 0-0 band, both at ambient and high pressure; and ͑c͒ from the wavelength dependence of ⌫ hom Ј , we conclude that an energy barrier (⌬Eϳ135 cm Ϫ1 at ⌬pϭ0, and ⌬Eϳ70 cm Ϫ1 at ⌬ pϭ3.4 GPa͒ has to be crossed in order to excite two-level systems ͑TLSs͒ of the TEA host which, subsequently, leads to spectral diffusion. We present a qualitative molecular model for the interpretation of these effects.