Intrachain photoluminescence properties of conjugated polymers as revealed by long oligothiophenes and polythiophenes diluted in an inactive solid matrix

Abstract

The intrinsic intrachain photoluminescence ͑PL͒ dynamics of conjugated polymers in the solid state is investigated. We focus on the PL properties of long ␤-substituted oligothiophenes ͑8-mer, 12-mer, and 16-mer͒ and regio-regular ͑RR͒ and regio-random poly͑3-octylthiophenes͒ ͑P3OTs͒ diluted in the inactive solid matrix polypropylene ͑PP͒. The oligothiophenes have well-resolved 0-0 PL and absorption peaks at 4 K and show a good linear relationship with the reciprocal of the ring number. Franck-Condon analyses on the PL spectra reveal that the strength of the electron-phonon coupling represented by the Huang-Rhys factor becomes weak with increasing chain length in the oligomers. In contrast, the presence of stronger electron-phonon coupling is confirmed in RR P3OT despite it being of a much longer chain length than oligomers. This is probably due to its stereoregular chain conformation. The materials used in this work diluted in PP exhibit single-exponential PL decays with lifetimes that decrease with increases in the chain length. The presence of intrachain exciton migration is shown by the time-resolved PL measurements, except for the case of RR P3OT. The energy shifts attributed to intrachain migration are much smaller than those ascribed to interchain migration. We conclude that a change in the torsion angle around the thiophene ring bonds occurs within 50 ps following the absorption transition in thiophene derivatives and that the torsional change gives rise to a constant Stokes shift ͑60-70 meV͒, regardless of the chain length. We propose that the Stokes shift can be utilized for estimating the magnitude of exciton migration in thiophene derivatives, even in the case of undiluted films.

Electrochemical Deposition of P3AT Films Used as a Probe of Optical Properties in Polymeric System

“…This band is the result of the interaction between the BF 4 -anion and P3DDT polymer chains [8,9,20]. Similar results are observed in diferent P3AT polymers [35][36][37]. To higher electropolymerization cycle number, the band at ~775 nm is further evident (not shown), in which it is possible to correlate the cycle number and UV-Vis absorption intensity to follow the polymer-grown deposition.…”

“…Moreover, it was also added bands due to the interaction of the electron-vibrational modes. The typical optically active vibration mode is 1450 cm -1 for the C=C group [37]. In Figure 4(c), the PL spectra centered at ~516 nm can be assigned to high conjugation polymer chains.…”

“…The band at ~638 nm is atributed to pristine without the salt-polymer interaction. Finally, the last one at ~678 nm is normally reported as the vibrational replica [35][36][37]. We perform a similar procedure to simulate the spectra of P3DDT ilms processed with LiClO 4 and the results are presented in Figure 4(d).…”

“…First, we consider the polymer chains without the presence of dopants and without structural changes. Second, we introduce the contribution to the emission line shape of dopants and possible new structural changes (Figure 3) [8], as established by the P3AT family [35][36][37] in the energy range of polymeric chains or oligomers [38,39]. Moreover, it was also added bands due to the interaction of the electron-vibrational modes.…”

Bundling effects on the intensities of second-order Raman modes in semiconducting single-walled carbon nanotubes

“…For all six nanotubes, their S values are generally smaller than those of luminescent conjugated polymers. 37 In general, the S values for these polymers are in the range of 0.8-2 and are inversely proportional to the numbers of atoms in their structure, 37,38 leading to the expectation of smaller S values for carbon nanotubes. The small S values found here are also consistent with the small Stokes shift observed for nanotube fluorescence.…”