Dynamical Formation of Intramolecular Carbazole Dimer Cations in Poly(methyl methacrylate) Solids below the Glass Transition Temperature

Structures and dynamical formation of carbazole (Cz) dimer cations in solid states were investigated by absorption spectroscopy of dicarbazolyl compounds doped in PMMA solids. Charge resonance (CR) bands originating from Cz dimer cations were observed even in PMMA solids at room temperature, which is below the glass transition temperature. Peak wavelengths of the CR bands observed were similar to those previously reported for Cz dimer cations formed in N,N-dimethylformamide (DMF) solutions at room temperature. This result shows that Cz dimer cations formed in PMMA solids have structures similar to those formed in DMF solutions. These Cz dimer cations were still observed even at 150 K, although the ratios of dimer cations to monomer cations were lower than those at room temperature. The ratios were kept almost constant below 170 K and steeply increased in the temperature range 200−290 K. This observed temperature corresponds to the transition temperature of the β-relaxation in PMMA solids. We conclude that Cz dimer cations are dynamically formed even in PMMA solids, owing to an increase in thermal fluctuation of free volumes induced by the β-relaxation.