TEMPERATURE-DEPENDENCE STUDIES OF THE PARAMAGNETIC PROPERTIES OF THE PHOTOEXCITED TRIPLET-STATE OF PENTACENE IN P-TERPHENYL, BENZOIC-ACID, AND NAPHTHALENE CRYSTALS

An electron spin echo spectrometer and pulsed nitrogen laser were empl oyed to study the guest deuteration and host effects on the zero-field splittings (ZFS), hyperfine splittings, and the g values of the photo excited triplet state of pentacene. These studies were performed in a temperature range of 5 K to room temperature in four mixed crystals: p entacene-h14 in p-terphenyl(PHPT), pentacene-d14 in p-terphenyl (PDPT) , pentacene-h14 in benzoic acid (PHBA), and pentacene-h14 in naphthale ne (PHNA). The ZFS are found to be temperature and host dependent and weakly guest-deuteration dependent: the D values of the PHPT, PDPT, an d PHBA systems decrease as the temperature increases in the range 190- 300 K and remain unchanged at 5-190 K. The effect is explained in term s of motional modulation arising from jumping among two or more molecu lar conformations (distortions). The measured g values are independent of guest deuteration, temperature, and host environments.