Photoexcitations in disubstituted acetylene polymers

We have used a variety of optical techniques to study the photoexcitations dynamics in several disubstituted acetylene polymer thin films. We found that although these polymers have a degenerate ground state, as determined by iodine doping and C6Q photodoping, they show strong photoluminescence (PL) in the blue/green spectral range with high quantum efficiency (QE) in thin films. The cw photo modulation spectrum contains a single photoinduced absorption (PA) band at 1.7 eV with monomolecular recombination kinetics with a lifetime of 25 ms. PA detected magnetic resonance (PADMR) shows that this PA band is related to spin 1/2 excitations, which therefore identifys it as due to neutral soliton antisoliton pairs (S°, s° ), similar to the 1.35 eV PA band in t-(CH)x . No triplet PADMR signal was observed and we therefore conclude that triplet excitations are unstable in disubstituted acetylene polymers, even though singlet excitons relatively stable. We consider therefore a possible S° photogeneration mechanism via singlet to triplet intersystem crossing, followed by triplet exciton decomposition into S° S° pairs. Also we detected polarons in a degenerate ground state polymer for the first time.