ON THE THEORY OF 2-PHOTON INDUCED FLUORESCENCE ANISOTROPY WITH APPLICATION TO INDOLES

Useful relationships are derived from the general work by McClain [J. Chem. Phys. 57, 2264 (1972)] to connect the traditional two-photon pol arization ratio of absorptivities using circularly and linearly polari zed light (OMEGA) in fluid media with the expected anisotropy of fluor escence induced by two-photon excitation in viscous media or shortly a fter pulsed excitation. Emphasis is on pipi transitions of planar sys tems with small out-of-plane two-photon tensor elements. Large ranges of anisotropy are found for most of the tensor space. The maximum and minimum anisotropies for pipi excitations and emissions appear to be 0.6123 and -0.3277, somewhat larger than the previously supposed value s of 4/7 (0.5714) and -2/7 (-0.2857). Application is made to indole an d three-substituted indoles for which two-photon tensor shapes and ori entations are deduced. Some predictions from semiempirical molecular o rbital theory-including simulated aqueous solvent effects-are presente d which agree qualitatively with experimental two-photon anisotropies reported in the literature.