Four-Photon Excitation of 2,2'-Dimethyl-p-terphenyl.

We report the emission spectra, intensity decays, and anisotropy decay of 2,2'-dimethyl-p-terphenyl (DMT) with four-photon excitation. When excited with a fs Ti:Sapphire laser the emission intensity of DMT was found to depend on the third power of the incident intensity for excitation of 783 nm, and on the fourth power of the incident intensity for excitation at 882 nm. Surprisingly, at the highest value incident power, the emission intensity for four-photon excitation was about 10-fold less than with three-photon excitation. The emission spectra, intensity decays and correlation times were found to be identical for three- and four-photon excitation. However, the fundamental anisotropy (r 0) of DMT depended on the mode of excitation. To the best of our knowledge, the r 0 value of 0.70 is the highest ever observed for an isotropic solution. These results suggest that four-photon excitation can be used with red-NIR lasers to obtain excitation of UV-absorbing chromophores.