Charge recombination luminescence via the photoionization of a dopant chromophore in polymer solids

The behavior of a photoejected electron with the parent cation formed through two-photon ionization of a dopant chromophore in poly(alkyl methacrylate)s and polystyrene was studied by measurement of the emission spectra of the charge recombination luminescence, i.e., isothermal luminescence (ITL) at 20 K and thermoluminescence (TL) at temperatures from 20 to 300 K. The ITL spectral shape remained the same between 10 min and 10 h after the photoirradiation. On the other hand, the intensity ratio of phosphorescence (I-P) to fluorescence (I-F), I-P/I-F in the TL spectra increased for poly(alkyl methacrylate)s above a temperature where small scale motions of the main chain are released but was almost constant for polystyrene in the temperature range examined. These findings show that, for poly(alkyl methacrylate)s, the photoejected electrons change into more stable anion species with a motional relaxation of the polymer. From the TL spectral change, the depth of the deeper trap at higher temperatures was estimated to be more than 1.8 eV.