The proton NW'2 lines of a solution of gyrene in perdeuterobenzene nre broadened suhstantinll-during irradintion xvith near UV light. The broadening is attributed to an excitation energy transfer prccess hetween triplet nnd grourd state molecules. Assuming the process is second order. the vnlue of the rnte constxnt determined from the increase in linelvidth is 4 x lo7 1 mole-* set-1.
In a previous communication [l]. it was reported that the irradiation of a solution of anthraquinone with near uv light results in proton NMR emission for ground state anthraquinone. Since most of the optically excited anthraquinone molecules cross over into the lowest lying triplet state, the inverted proton spin poIarization is believed to result from an Overhauser eClect [2] involving protons and unpaired electrons in the triplet state. Another feature of the spectrum during light irradiation is that the lines are broadened slightly. The suggestion was made that this line-broadening may be due to an excitation energy transfer process between triplet and ground state molecules. Because of the complexity of the spectrum, no attempt was made to compute a rate for this process from the lineshape.
In the present paper. we report the results of a preliminary study of pyrene whose spectrum during irradiation is less complex and exhibits substantially more line-broadening than the spectrum for anthraquinone. XI-I contrast to anthraquinone. no Overhauser effect is observed with pyrene under simiIar conditions. The reason for the lack of an Overhauser effect is believed to be that the lifetime of triplet pyrene in solution is sufficiently long to permit the electrons.