Photophysics of pristine andC60-doped disubstituted polyacetylene

Abstract

Using a variety of steady-state spectroscopies we studied the long-lived photoexcitations and electronic excited states of poly disubstituted acetylene $(\mathrm{PDPA}\ensuremath{-}n\mathrm{Bu}),$ the backbone structure of which is a disubstituted trans-polyacetylene, as well as $\mathrm{PDPA}\ensuremath{-}n\mathrm{Bu}/{\mathrm{C}}_{60}$ blends. The cw spectroscopies include absorption, photoluminescence (PL), photoinduced absorption (PA), and PA-detected magnetic resonance (PADMR). Although $\mathrm{PDPA}\ensuremath{-}n\mathrm{Bu}$ is a degenerate ground-state polymer, nevertheless, and in contrast to trans-polyacetylene, we found that it has a strong PL band with quantum efficiency larger than 60%. From polarized PL studies on stretched oriented films we conclude that the PL emission originates from intrachain excitons rather than the polymer side groups. This shows that the lowest-lying exciton in $\mathrm{PDPA}\ensuremath{-}n\mathrm{Bu}$ is a ${B}_{u}$ state rather than an ${A}_{g}$ state, in contrast to the order of the lowest-lying excitonic states in trans-polyacetylene. The polarized absorption in $\mathrm{PDPA}\ensuremath{-}n\mathrm{Bu}$ contains three distinctly different bands with different polarization properties, which are interpreted according to the model of Shukla and Mazumdar. The PA spectra of pristine and photo-oxidized $\mathrm{PDPA}\ensuremath{-}n\mathrm{Bu}$ films show neutral and charged solitons as well as polaron excitations, whereas the long-lived photoexcitations in $\mathrm{PDPA}\ensuremath{-}n\mathrm{Bu}$ in solution are mainly polarons. This may be due to destabilization of the soliton-antisoliton pairs in the polymer chains in solution caused by short conjugation length. From the PA and PADMR spectroscopies of $\mathrm{PDPA}\ensuremath{-}n\mathrm{Bu}/{\mathrm{C}}_{60}$ blends we conclude that a photoinduced charge-transfer reaction takes place, again in contrast to blends of other nonluminescent polymers. Interestingly the PA spectrum of $\mathrm{PDPA}\ensuremath{-}n\mathrm{Bu}/{\mathrm{C}}_{60}$ blends shows both charged polarons and charged solitons that are correlated with the PA band of ${\mathrm{C}}_{60}^{\ensuremath{-}}.$ We found that the ratio between charged solitons and polaron excitations depends on the ${\mathrm{C}}_{60}$ concentration in the blend, the film morphology, and temperature. It is shown that the mechanism by which two polarons on the same $\mathrm{PDPA}\ensuremath{-}n\mathrm{Bu}$ chain recombine into two charged solitons is the dominant process that determines the soliton/polaron ratio in the blends.