Energy transfer in organic systems X . Pure and mixed crystals

Expressions are derived for the host and guest fluorescence response functions and quantum yields of thin and thick, pure and mixed organic crystals. I n addition to exciton migration and transfer, the effects of host exciton traps and of radiative migration and transfer are considered. Host exciton traps and radiative migration are shown to be important in thick anthracene and mixed anthracene-tetracene crystals. 1. Introduction T h e migration and transfer of singlet electronic excitation energy in a mixed organic crystal, consisting of a host crystal M containing a small mole fraction cy of guest molecules Y, may be studied by observations of the crystal fluorescence (Birks 1964, 1970, Wolf 1967). The behaviour is normally described in terms of (i) the migration of free host singlet excitons M* generated by the initial excitation. T h e M* exciton migration culminates in (ii) M* fluorescence, (iii) M* internal quenching, or (iv) M* exciton collisional transfer to Y, which is thus excited into its first excited singlet state Y* where the excitation is trapped (Y* energy < M* energy), leading to (v) Y* fluorescence or (vi) Y* internal quenching. Organic crystals normally contain a small mole fraction ex of lattice defects which function as host exciton traps X. These traps may be excited into their first singlet excited state X* by (vi;) M* exciton collisional transfer to X, leading to (viii) X* fluorescence or (ix) X* internal quenching. T h e traps commonly correspond to host molecules adjacent to vacancies, and it will be assumed that M* and X* have identical