A purely organic guest material with a long triplet excited state lifetime (τ > 1 s) was doped into two different π-conjugated amorphous hosts with similar glass transition temperatures (Tg). The phosphorescence quenching behavior of the guest material was investigated under vacuum conditions. When doped into a host that did not exhibit intermolecular hydrogen bonding, the τ of the guest decreased significantly at temperatures below the Tg of the host. Conversely, when doped into a host that did exhibit intermolecular hydrogen bonding, the τ remained almost unaffected at temperatures below the Tg of the host. The diffusion coefficient of oxygen, which was largely removed from the host materials under vacuum, was similar for both host materials. These results indicated that weak thermal diffusional motion of a host material intrinsically quenches long-lived triplet excitons of purely organic guest materials.
