Spin-forbidden radiationless processes in isoelectronic molecules: Anthracene, acridine and phenazine

Microwave induced delayed phosphorescence (MIDP) experiments have been performed to study the populating and decay of the phosphorescent triplet state T 0 of seven aromatic three-ring molecules : anthracene (-h 10, -h 2d8, and -d10), acridine (-h9 and -d9) and phenazine (-h8 and -d8), all diluted in biphenyl crystals. We chose these molecules because the deactivation of T 0 goes dominantly via radiationless processes and the total decay rates of the individual spin components of T 0 determined in the experiment thus equal the radiationless decay rates. The phosphorescence emission, used as a monitor for the triplet state population, here gives an insignificant contribution to the decay. MIDP experiments only yield the true decay rates if thermal isolation between the levels is maintained throughout, and the present investigations, in which the intensity of excitation has been varied over a wide range, uncover a possible source of systematic error that has been overlooked thus far. The principal results of the experiments are (see figure 1 for choice of axes) 1. (i) In the anthracene isomers the spin states Tx and Ty (corresponding to the in-plane axes of the molecule) are comparably active in both S 1 ↺; T 0 as well as in T 0 ↺; S 0 crossing.2. (ii) We observe a deuterium effect on the radiationless decay rates which is independent of the particular spin state, but merely depends on the molecule.3. (iii) Appreciable effects of aza-substitution on the decay rates of the ‘inactive’ components Ty and Tz are noted : ky /kz and Py /Pz decrease substantially on going from anthracene to the azaderivatives ; ky and kz increase by a factor of 2 in going from acridine to phenazine. The results on anthracene are compared with results from other workers and with quantitative estimates derived from recent theories on spin-for-bidden radiationless transitions.