Photophysical analysis of prolonged UV irradiation-induced delayed luminescence of semi-aromatic polyimides and its relation to oxygen permeability

Six types of photoluminescent polyimides (PIs) and co-polyimides (co-PIs) were synthesized to clarify the requirements for PIs that exhibit prolonged UV irradiation-induced delayed luminescence (PIDL). PIDL is a form of phosphorescence (PH) observed after ground-state oxygen ( 3 O 2 ) is gradually excited to singlet oxygen ( 1 O 2 ) via energy transfer from triplet excitons of the phosphor under continuous irradiation. When 4,4'-oxydiphthalic dianhydride (ODPA), which has an ether linkage, is used as a luminescent unit, dilution of the phosphor unit by copolymerization with non-luminescent dianhydrides is effective in exhibiting strong PH as well as PIDL emission. However, PIs derived from ODPA emit only a weak PH without PIDL, which is caused by thermal deactivation owing to their flexible structure. Thus, four types of PIs were synthesized using luminescent dibenzothiophene tetracarboxylic dianhydride (DBSA) having a rigid thiophene core. Two types of PIs exhibited PIDL at room temperature in air when coated with polyvinyl alcohol as an oxygen (O 2 ) barrier. This is the first report of PIs with PIDL emission, indicating that preventing O 2 permeation allows sufficient reduction of 3 O 2 , which is necessary for exhibiting PIDL. In addition, these two PIs exhibited different PIDL intensities and induction time, which is the elapsed time required to exhibit the PIDL, depending on the O 2 permeability and lifetime of 1 O 2 within the PI films. These results clarify that PIs derived from a suitable combination of luminescent and non-luminescent dianhydrides can exhibit obvious PIDL while maintaining high thermal resistance and good processability. • Six polyimides (PIs) were synthesized using luminescent (PL) and non-PL dianhydrides. • Prolonged irradiation-induced delayed luminescence (PIDL) of the PIs was investigated. • Two PIs containing thiophene cores exhibited PIDL when coated by PVA as O 2 barrier. • The PIDL was enhanced when copolymerized with rigid alicyclic non-PL dianhydrides.