# HETEROFISSION OF TRAPPED SINGLET EXCITIONS INTO TWO TRIPLETS IN PENTACENE-DOPED TETRACENE CRYSTALS

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 Title: HETEROFISSION OF TRAPPED SINGLET EXCITIONS INTO TWO TRIPLETS IN PENTACENE-DOPED TETRACENE CRYSTALS Creators: Geacintov, N. E.; Burgos, J.; Pope, M. Issue Date: 1972 Publisher: Ohio State University Abstract: The fission of a singlet exciton trapped at a pentacene impurity site into one triplet exciton of the host and one trapped pentacene triplet has been observed. This heterofission process is temperature and magnetic field sensitive. The red fluorescence of pentacene increases with decreasing temperature with an activation energy of 0.08 eV. These results permit the first experimental measurement of the pentacene triplet energy level which is estimated to be $0.81\pm 0.01$ eV. Energy transfer to pentacene molecules competes with the host fluorescene and reduces the effect of an external magnetic field on the green tetracene fluorescene. Thus, in a doped crystal containing 100 ppm pentacene, the maximum magnetic field effect on the hst fluorescence is only $F(H)/F(O)=1.11$, where F(H) is the fluorescence intensity at 4000 gauss and F(O) is $1.35-1.40$ and the singlet exciton lifetime is $0.2\pm 0.2$ nanosecond. Using these data, the singlet energy transfer rate constant $v_{tr}$ in tetracene can be estimated to be $3\times 10^{-8}$ $cm^{3}$ $sec^{-1}$ (within a factor of 2-3), which is of the same order of magnitude as in crystalline anthracene. The red pentacene fluorescence displays typical Merrifield type resonances when the magnetic field is rotated in the ab plane of the crystals. The orientation of these resonances is different from those displayed by the host fluorescence and the two resonance peaks are $62^{\circ}$ apart. A ratio of the zero-field spin hamiltonian parameters of pentacene $D/E\approx 16$ is estimated from the separation of the peaks. Description: Author Institution: New York University URI: http://hdl.handle.net/1811/8947 Other Identifiers: 1972-D-2