Synthesis,photodegradation, and energy transfer in a series of poly(ethylene terephthalate–co–4,4′-biphenyldicarboxylate) copolymers

Poly(ethylene terephthalate) (PET) filament yarns were photostabilized by addition of 0.5–4.0 mole % dimethyl 4,4′-biphenyldicarboxylate (4,4′-BPDC) to the polymerization feed. The mechanism of photostabilization is proposed to be a triplet–triplet energy transfer from excited terephthalate units to ground-state biphenyldicarboxylate units. The mechanism of transfer is reported to be via an electron exchange mechanism, with the “quenching sphere” calculated to be 14.9 Å. Kinetic studies show the “pseudo” zero-order rate constant of initial photodegradation to decrease from 3.4 × 10^?19 for the PET homopolymer to 2.0 × 10^?19% breaking strength loss/quantum exposure/cm² for the copolyester containing 4.0 mole % of the 4,4′-biphenyldicarboxyl moieties. The photophysical processes available to the dimethyl 4,4′-biphenyldicarboxylate monomer were characterized by absorption and luminescence studies. In solution, dimethyl 4,4′-biphenyldicarboxylate was shown to emit an intense fluorescence from a ²(π,π*) state and a weaker (～10^?2×) phosphorescence from a ¹(π,π*) state derived from the ¹A→supn1L_b absorption. The copolymer yarns were shown to exhibit both fluorescence and phosphorescence from the biphenyldicarboxylate units: the fluorescence from direct excitation, the phosphorescence by sensitized transfer.