Cis-trans Isomerization Of Vinylenes In PPVs And The Subsequent Change Of Physical Properties

The photoisomerization of stilbenes and its simple derivatives as the most simple model compound of PPVs has been studied for many years, considerable attention is devoted to stibenes in modern textbooks on photochemistry, and they have been covered in numerous reviews. Stilbene can adopt cis or trans configuration and the cis-trans isomerization process can occur very easily because of the low barrier. Cis-/trans-DPDSB are important models of DP-PPVs, which theoretical studies,chemical structure determination, crystal stacking mode, optical properties and isomerization procedure is studied in details. Based on the study of the model compound of PPVs, we found that there is a certain extent change of energy gap and physical properties between cis and trans isomer after cis-trans isomerization, As para-phenylene vinylene segments in PPV with cis- conformation, theπ-conjugation will be interrupted partially, while the trans- phenylene vinylene segments allow the electronic delocalizability. Thus the photo cis-trans(or Z-E) isomerization occurred in PPV will induce big change of physical and optical properties of the polymers, c.a. electronic absorption and light-emission color. So the study of cis-trans isomerizaiton procedure of PPV polymers provides a model for us to invesitigate the basic process and its derivative change of physical properties.Because of the chromophore in PPV containing many para-phenylene vinylene recurring units, the cis/trans- vinylene ratio within the chromophore could play an important role in luminescence enhancement. So the effect of the vinylene bond configuration on the photoluminescence of PPV appears to be relatively important studies. Great efforts have been made on the investigation of relationships that exist between the cis/trans-conformation in a polymer chain and resulting luminescent properties, the cis-isomer is generally considered as"defect"in PPV polymers because of its weak luminescence, consequently the discussion around whether the cis-conformation is the defect or not have attracted special attention. 1995, Son S. and Galvin M. E. reported that a random placement of cis-segments along the PPV chain significantly increases the EL efficiency of the polymer. They considered that the introduction of cis-segments into polymer chains can reduce the interchain interactions and enhances the solid-state PL efficiency. Yi Pang and Liao Liao synthesized PPVs with different content of cis- and trans-olefins by Wittig and Wittig-Horner reaction, however the olefin bond geometry is random, the luminescence properties of PPVs with different cis-contents have not been studied systemicallyThe understanding of these dependencies is still incomplete which is partially due to a lack of the study of the optical properties of cis-PPVs. But it is very difficult to synthesis PPV polymers with all cis-vinylenes due to its high energy of groud state compared to trans-isomer. So it is critical to synthesize PPV polymers with different content of cis-configurations. Above all is to synthsis cis-PPVs. By now, chemist have designed many synthsis route to synthesize PPV derivatives. For example, Glich reaction, Wessling reaction, Wittig reaction, Wittig-Horner reaction, Heck reaction, ROMP reaction and so on. But the same dilemma of these method is the configuration of the vinylenes is indefinite, which makes the configuration of PPV chains is complicated. 2005, Hiroyuki Katayama et al have reported the synthesis of the geometrically pure all-cis PPVs by Suzuki-Miyaura-type polycondensation, but the synthesis route is very complicated. Recently we found that the use of biphenyl-substituted benzaldehydes in Wittig reaction exhibits a pronounced enhancement for Z-selectivity, and produces all cis-oligomeric PPV compound. This investigation provides an opportunity to synthesize PPVs with larger number of cis-segments.We have synthesized cis-DPO-PPV with the highest cis-configuration up to 87% using Wittig reaction. And the cis-trans isomeriztion of cis-DPO-PPV can occur in both in solution and in film, this is due to the alkyl substitution decrease the rotation barrier of the vinylenes. Cis-DPO-PPV shows the same UV and PL spectra of in dilute solution and in thin film reflect the lessπ-πor excimer species present in polymer films. This is due to the twisted backbone configuration of the polymer, which suppresses the inter-chain interactions. By using the cis-trans isomerizaiton properties of cis-DPO-PPV, we got six PPV polymers with different contents of cis-olefins by photoirradiation which named DPO-PPV1-6. The content of cis-CH=CH bond linkage of these six polymers is estimated by integrating the proton resonance signals of–OCH2– unit in the alkoxy side chains, the resonance signal at 3.49 ppm and 3.83 ppm which are attributed to the–OCH2- unit in the alkoxy side chains are related to cis- and trans- olefins respectively. And the cis-olefins contents of DPO-PPV1-6 are calculated to be 87%, 82%, 75%, 55%, 24% and 0.05% respectively The photophysical characterization using NMR, UV, PL and fluorescence quantum efficiency measurements indicate that as the cis-content is equal or higher than 75% in polymer chains; the cis-trans isomerization together with rotational motions of the double bonds compete with the emission process, resulting in low luminescence of polymers. Along with the decrease of cis- configuration, the polymer adopts a more planar conformation which is beneficial for theπconjugation, and is in favor of improving the intensity of photoluminescence. The study on the optical and physical change during cis-trans isomerization of PPVs is the basis for designing high efficience PPVs and new applications.Based on the study of cis-DPO-PPV, we can find that the cis-vinylenes with twisted configuration can reduce the interchain interactions of PPV polymers. So we introduce cis-vinylenes into the backbone of a all aromatic PPVs, a soluble all-aromatic PPV without any alkyl substitution was synthesized by Wittig reaction. The structural characterizations indicate that over 50% content of cis-vinylene units exist in polymer backbone. The high cis-segments in polymer backbone induce a highly twisted chain configuration, resulted in the high solubility of such all-aromatic PPV derivative. 2,5-DP-PPV emits sky-blue fluorescence with peak at 485 nm both in dilute solution and in film. Previous studies reported that phenyl-substituted PPVs usually emit green light (λmax = 515 nm). In the present case, the twisting configuration of cis-segments in the polymer cause significant steric effects on the backbone and reduce the effective conjugation length. At the same time, the introduction of cis-segments into polymer chains significantly reduces the interchain interactions and enhances the solid-state PL efficiency (78%). The solid-state PL efficiency of 2,5-DP-PPV is among the highest values reported forπconjugated PPVs. So introducing cis-segments into the PPVs backbone is a good idea to design high efficiency PPVs.The study on the physical properties of cis-DPO-PPV during cis-trans isomerizaiton indicates that the refractive index and the solubility between cis-DPO-PPVand trans-DPO-PPV have a tremdous change, but the property change of the cis and trans model oligomers is very small, we consider it due to the amplification effect of the polymers. The optical properties of some conjugated polymers can be strongly modified by varying the molecular surroundings, which have been used to develop a new chemosensory system. The conjugated polymers have showed significant sensitivity enhancements (amplification) over small-molecular analogues, due to the energy migration along the electronic delocalized polymer backbone. A weeny structural change of certain part of the conjugated polymers will effects the properties of entire polymers.We measured the refractive index of cis-DPO-PPV before and after photoirradiation, and find that the refractive index of cis-DPO-PPV is 1.695(±0.0005) (533nm), the refractive index of all trans-DPO-PPV is 1.8813(±0.0008) (533nm), the refractive index change after isomerization is△n533=0.1863, which is 11% up compared to cis-polymer. The refractive index of DPO-PPV model compound (cis/trans-DPO-DSB) (shown in Figure 2) are n533=1.6641 (cis-DPO-DSB) and n533=1.695(trans-DPO-DSB), the refractive index change is△n533=0.0416, which is only 2.5% up compared to cis isomer.The reason of the tremendous change of cis and trans polymer is that, based on Lorentz-lorenz equation,△n is relative to the change of molecular polarizability, and as a structural change c.a. cis- to trans- isomerization in a PPV backbone, it may induce a larger change in molecular polarizability and energy gap. This phenomenon could be rationalized by considering that cis-polymer shows a similar refractive index compared to cis-oligomer, owing to the twisted configuration of the cis-vinylenes in polymer interrupts the conjugation of the polymer chain. But along with the increasing of trans- configuration of DPO-PPV, the polymer adopts a more planar conformation, and the planar conformation is advantage for theπconjugation, thereby increasing the delocalizability ofπelectron, which is in favor of improving the polarizability of the polymer, The photon refractive index varies directly with the electronic polarizability, resulted in the increasing of the refractive index of the polymer finally. From the analysis of the relationship between energy gap and refractive index of PPVs with different contents of cis-vinylenes, we can see that the refractive index change is inversely proportional to the energy gap of the polymers. Thus, by accurately controlling the cis-trans isomerization degree of the polymers, we can dominate the energy gap of the polymer, and then further control the refractive index of the polymer. By utilizing a cis- PPV polymer films, a prototype polymer grating fabricated by a dual-beam interfering system is realized. We measured the solubility of cis-DPO-PPV before and after photoirradiation, and find that the solubility of cis-DPO-PPV is 15 mg/mL, the solubility of all trans-DPO-PPV is only 0.05 mg/mL, the solubility of cis-DPO-PPV is 300 times compared to trans-DPO-PPV. The solubility of DPO-PPV model compound (cis/trans-DPO-DSB) are also measured, the solubility of cis-DPO-DSB is only 20 times cmparede to trans-isomer.The essential reason on the tremendous change of the solubility during cis-trans isomerization of the cis-DPO-PPV is the amplification of polymers. This phenomenon could be rationalized by considering that the structure of cis-polymer chains likes the letter"Z"owing to the twisted configuration of the cis-vinylenes, which can interrupts the conjugation of the polymer chain in favor of improving the solubility of polymers. But structure of trans-polymer likes a line owing to the more planar conformation of trans-vinylenes resulting in a poor solubility of the polymers. Using cis-trans isomerization induced tremendous solubility change of cis-DPO-PPV, we fabricate a seriers pattern by two-photo laser.