Room-temperature phosphorescence of 6-bromo-2-naphthol in poly(vinyl alcohol) films containing cyclodextrins

The absorption spectral changes of 6-bromo-2-naphthol (BN) indicate the formation of inclusion complexes of BN with α-cyclodextrin (α-CD) and heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin (TM-β-CD) in a poly(vinyl alcohol) (PVA) film. The enhanced room-temperature (RT) phosphorescence of BN has been observed for PVA films containing BN and CDs (α-, β-, γ-CD, and TM-β-CD), whereas only the weak RT phosphorescence has been detected for a PVA film without CD. The enhancement of the RT phosphorescence in the presence of α-CD and TM-β-CD is due to the formation of the α-CD–BN and TM-β-CD–BN inclusion complexes, respectively. An induction period has been observed for the enhancement of the RT phosphorescence of BN in a PVA film containing TM-β-CD, although the weak RT phosphorescence has immediately appeared upon the excitation. The appearance of the induction period has been explained in terms of the generation of singlet oxygen by triplet BN bound to the TM-β-CD cavity and the subsequent oxygen consumption. From the analyses of the triplet–triplet absorption decays, triplet lifetimes have been evaluated for BN in PVA films in the absence and presence of CDs.