Very Long-Lived Photoinduced Charge-Separated States of Triphenylamine-Naphthalenediimide Dyads in Polymer Matrices.

Abstract

Photoinduced electron transfer was studied in dyads (dyad1 and dyad2) containing triphenylamine (MTA) and naphthalenediimide (MNDI) linked with oligo(phenyleneethynylene) dispersed in rigid polymer matrices of polystyrene (PS), poly(vinyl chloride), and poly(methyl methacrylate). Photoexcitation of these dyads yielded long-lived charge-separated (CS) states involving MTA+ and MNDI-. The quantum yields of charge separation in dyad1 and dyad2 were approximately 0.4 and 0.3, respectively, in the polymer matrices. The CS lifetime for dyad2 in PS was longer (400 ms) than those in poly(vinyl chloride) (120 ms) and poly(methyl methacrylate) (65 ms) at 298 K. In addition, CS state had a very long lifetime of 5.4 s in glassy toluene at 100 K. Below glass transition temperatures, polymer side chain motions with various relaxation rates should affect the charge recombination processes. The energy gap (ΔG) and outer-sphere reorganization energy (λ) in the charge recombination process were estimated using a slow-frequency component for dielectric constants. By use of ΔG and λ values, the matrix dependence of the CS lifetimes was successfully rationalized based on Marcus theory, and the charge recombination process in PS with low polarity and high polarizability should be in a deeper inverted region than the other polymer matrices. It also suggested that the rigidity of the polymer effectively suppressed intramolecular motions promoting the charge recombination process.