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 π‐electron crystals. As we have pointed out in earlier papers, rapid triplet—triplet annihilation prevents the observation of emission from the triplet state in pure crystals of these molecules. Use of isotopic mixed crystals which retard the annihilation process, combined with what can be called the method of variation of energy denominators, allows an experimental value of the interaction matrix element to be obtained. Application of these methods to dilute mixed crystals of C₆H₆ in C₆D₆, C₆H₃D₃, and C₆H₄D₂ gives 12±1 cm^—1 for the nearest neighbor, ``pair'' interaction matrix element in crystalline benzene. This value is in good agreement with an earlier lower limit of 8 cm^—1 obtained from excitation‐transfer studies in these crystals. The relationship between the matrix element and the triplet exciton band structure in pure crystalline benzene is discussed, and it is noted that the triplet factor group splitting in crystalline aromatics is likely of sufficient magnitude to be easily observed in the absorption spectrum of such systems. Application of the methods to the lowest singlet exciton band of crystalline C₆H₆ indicates that the commonly accepted analysis of the factor group splitting is probably incorrect. Barring quenching mechanisms, the relatively large triplet exciton interaction allows roughly 10¹² 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.