Inte # Olecular Energy Transfer Stud 1 with Picosecond Light Pulses

The time deveIopment for the intermolecular transfer of electronic energy has, until recently, been studied by measurements of the molecular light emission. The observation of the transfer process has been limited to times of the order of the fluorescent lifetime, and naturally to systems which emit light $#. With the application of the picosecond method 121, which utilizes absorption, rather than fluorescence for detection, the dynamics of energy transfer nave been studied in a heretofore inaccessible time domain, the subnanosecond region and furthermore, this method can be applied to non-radiating molecular systems. The method consists of exciting some fraction of the donor molecules with a linearly polarized picosecond light pulse. Molecules whose transition moments have a large component along the direction of the light field are preferentially excited and, therefore, the formerly isotropic system becomes anisotropic, i.e. more excited molecules are oriented parallel to the field direction and hence more unexcited molecules are oriented with their transition moments perpendicular to the field direction. In a rigid environment the induced anisotropy can only relax by polecat decay of the excited molecules akd by energy transfer. In a system where i.@& donor and acceptor molecules are of the same species, the r~d~~ation results from transfer between molecules of different orientations. On the other hand, in a mixed system the anisotropy in donor orientations can also relax by transfer between donor and acceptor molecules regar&etB of their mutual orientation. The decay of the anisotropy is monitored by measuring the change in dichroism in the donor absorption with time of an attenuated picosecond probe pulse. The absorption is greater for probe light pokrieed perpendicular rather than parallel to the poIarizati3n of the exciting light because of the relative depletion of ground state molecules in what we czll the’parallel configuration’. By probing at successively later times after excitation the decay of the dichroism due to energy transfer and the unimolecular excited state decay can be determined. Measurement or’ the latter quantity in the absence of transfer is then used to obtain the time dependence of the energy transfer from the measured decay af the dichroism.