DIRECT DETERMINATION OF EXCITON INTERACTIONS FOR TRIPLET STATES OF ORGANIC $CRYSTALS^{*}$

Abstract

“A method is described which allows the direct measurement of triplet exciton interactions in organic $\pi$-electron crystals. As we have pointed out in earlier $work,^{1}$ rapid triplet-triplet annihilation prevents the observation of emission from the triplet state in pure crystals of these molecules of isotopic mixed crystals which retard the annihilation process, combined with what can be called the met had of variation of energy denominators, allows an experimental value of the interact ion matrix element to be obtained. Application of these methods to dilute mixed crystals of $C_{6}H_{5}$ in $C_{6}D_{a}, C_{6}H_{3}D_{3}$, and $C_{6}H_{4}D_{2}$ gives $12 \pm 1 cm^{-1}$ fur the nearest neighbor ``pair’’ interaction matrix element in crystalline benzene. This value is in food agreement with nil $earlier^{2}$ lower limit of $8 cm^{-1}$ obtained from excitation transfer studies in these crystals. The relationship between the matrix clement acid the triplet exciton band structure in pure crystalline benzene is discussed. Application of the methods to lowest singles exciton band structure of crystalline $C_{6}H_{6}$ indicates that the commonly accepted $analysis^{3}$ of the factor group splitting is incorrect. Barring quenching mechanisms, the relatively large triplet exciton interaction allows roughly $10^{12}$ nearest neighbor triplet excitation transfer transitions to occur per triplet state lifetime in aggregates of these molecules, a fact which should have important bearing on energy transfer in chemical and biological systems. “