Transient grating kinetics were measured at different temperatures (1-40 degreesC) for DMPC phospholipid bilayer liposome aqueous dispersions. 5,10,15,20-Tetrakis-(4-hydroxyphenyl)-21,23H-porphyrin sensitizers were embedded into the phospholipid bilayers. Fluorescence emission temperature-dependence and quenching studies were used to determine that hydroxyphenyl porphyrins are localized in the aliphatic region of the phospholipid bilayer. Porphyrin localization does not change in gel (L-beta') and liquid (L-alpha) phases of the membrane. In the transient grating experiment, following excitation with 100 fs 410 nm pulses, temperature and density modulations were observed as acoustic grating signals. Grating signal amplitude and the time when the acoustic peak reaches maximum were temperature-dependent. Negative time delay of the acoustic peak maximum was observed and explained by considering two thermal processes: (1) energy transfer from porphyrin to the phospholipid bilayer (described by a rate constant k(1)), and (2) energy transfer from the phospholipid bilayer to the surrounding solvent (described by a rate constant k(2)). A model accounting for time-dependent thermal expansion of phospholipid bilayers and water was developed to determine rate constants k(1) and k(2). The rate constant k(1) equals 7.14 x 10(10) s(-1) (k(1)(-1) = 14 ps) and is temperature-independent at 10-35 degreesC. The value of k(2) increases from 3.33 x 10(10) s(-1) (k(2)(-1) = 30 ps) at 10 degreesC to 5.56 x 10(10) s(-1) (k(2)(_1) = 18 ps) at 35 degreesC. The value and temperature dependence of k(2) suggest that thermalization in DMPC membrane occurs by thermal diffusion and vibrational energy transfer, but other phospholipid degrees of freedom (such as isomerization) are not involved in this process.