Odd−Even Effects and the Influence of Length and Specific Positioning of Alkoxy Side Chains on the Optical Properties of PPE−PPV Polymers

This contribution reports the combined influences of odd − even effects and the specific positioning of alkoxy side chains OR1 = (OC n +10H2( n +10)+1) and OR2 = (OC n H2 n +1) (with n = 6, 7, 8, 9) on the phenylene − ethynylene and phenylene − vinylene segments, respectively, on the optical properties of hybrid polymers P(n+10)/n of general repeating unit: −Ph−C⋮C−Ph−C⋮C−Ph−CHCH−Ph−CHCH−. For the polymeric materials, visual color impression varies alternatively between orange red (P16/6 and P18/8) and yellow (P17/7 and P19/9) according to the odd and even features of the alkoxy side chains, where odd or even relates to the total number of sp3-hybridized atoms within the side chains. This side chain related effect is ascribed to both absorptive and emissive behaviors of the polymers on the basis of photophysical investigations in the bulk. Almost identical thin film absorption spectra were obtained for all four materials; however, the photoluminescence of the odd polymers, P16/6 (λf = 556 nm) and P18/8 (λf = 614 nm), was red-shifted relative to that of their even counterparts (λf = 535 nm). Further, the P18/8 maximum at 614 nm can be readily assigned to excimer emission, as evidenced by the largest Stokes shift (5600 cm-1), largest fwhmf-value (3700 cm-1), and the lowest Φf-value of 24%. The strong π−π interchain interaction in P18/8, due to loose alkoxy side chains packing, does not only favor fluorescence quenching but also enable an effective inter- as well as intra-molecular recombination of the generated positive and negative polarons in electroluminescence, which explains the good EL properties of this polymer irrespective of the solvent used. A voltage-dependent blue shift of the EL spectra of up to 100 nm was observed for P18/8 devices prepared from aromatic solvents. This red to green EL shift as observed with increasing voltage is assigned to conformational changes of the polymer chains with increasing temperature.