The photodegradation mechanism study has been conducted on poly(2,5-dioctyl-1,4-phenylene-1,2-dicyanovinylene) (C8-diCN-PPV) and poly[2,5-bis(decyloxy)21,4-phenylene-1,2-dicyanovinylene] (ROdiCN-PPV) to understand the reason behind the faster photodegradation of C8-diCN-PPV which has a lower LUMO. In both polymers, radical superoxide anion mechanism, which is responsible for electron-rich RO-PPVs, is found to be energetically unfavorable for both diCN-PPVs due to diCN substitution. The IR analysis results confirm this and suggest that singlet oxygen (O 2 ) is the main culprit for photodegradation of both polymers, which cleaves the C@C bonds into carboxylic acids. The rates of MW reduction (by GPC) and increase in carbonyl IR absorption intensity are in excellent agreement for both polymers. Phosphorescence study indicates that the faster photodegradation of C8-diCN-PPV is due to intersystem crossing, which helps generate singlet O 2 upon photoexcitation. No phosphorescence was detected in RO-diCN-PPV, suggesting that inefficient intersystem crossing makes RO-diCN-PPV photochemically more stable. This work shows that a small difference in side chain structure can lead to a significant difference in photochemical stability.

A didecyloxy-substituted poly(phenylenedicyanovinylene), DiCN–PPV, has been synthesized. The dicyano-substituted vinylene units exist in both trans and cis (∼65:35) conformations as determined by 1H NMR analysis, and cannot be converted to all trans due to the presence of a thermodynamic equilibrium of the two conformations, in contrast to the vinylene units in regular PPVs. The unusually high cis content makes this polymer highly amorphous, very soluble in organic solvent, and highly fluorescent in the solid state with an estimated quantum yield up to 0.34, four times more fluorescent than its chloroform solution. The LUMO and HOMO energies of the new polymer were measured by cyclovoltammetry. The cyano groups in DiCN–PPV brings a decrease in LUMO energy by 0.79 eV, and makes the polymer more stable to intense white light (>20 times as strong as the sunlight) than poly(2,5-didecyloxy-1,4-phenylenevinylene), C10O–PPV, by more than 2 orders of magnitude. The excellent photochemical stability and high fluorescence quantum yield in the solid state make DiCN–PPV a good candidate for outdoor fluorescent applications such as remote optical sensing.

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INTRODUCTION Π-Conjugated organic materials have many (potential) applications such as photovoltaics, light emitting diodes, solid-state dye lasing, biological imaging and sensing, and chemical sensing. However, photochemical stability is a major concern for these materials. 1-5 For electron-rich polymers, such as poly(3hexylthiophenes) and poly(p-phenylenevinylenes) (PPV), a radical degradation mechanism has been established. 6-9 When illuminated in the presence of oxygen, the polymer donates an electron through charge transfer to an oxygen molecule to form a polymer cation and a superoxide radical anion. In the case of PPVs, the superoxide radical anion can abstract a hydrogen from a side chain to form a carbon radical. 8 It can also attack a vinylene unit in the backbone to produce a peroxide anion and a carbon radical. 8 Hoke et al. and Dam et al. have shown that the LUMO (lowest unoccupied molecular orbital) energy of a polymer is directly related to the rate of ABSTRACT RO-diCN-PPV and C8-diCN-PPV, poly(1,4-phenylene-1,2-dicyanovinylene) with alkoxy and octyl side chains, have recently been shown to photodegrade via a singlet oxygen mechanism, and RO-diCN-PPV is seven times more stable. To improve photostability, 1,4-diazabicyclo[2.2.2]octane (DABCO), a singlet oxygen quencher, was used as a dopant. To our surprise, DABCO exhibited opposite effects on their photodegradation. With 15 mol% DABCO, degradation rate of C8-diCN-PPV decreased by 65%, while that of RO-diCN-PPV increased by 246%. The DABCO content in C8-diCN-PPV film remained unchanged during 20 minutes of illumination, but mostly disappeared in RO-diCN-PPV in only 5 minutes due to decomposition. IR and MW analysis results suggest that DABCO slowed down degradation of C8-diCN-PPV without altering the mechanism, but accelerated RO-diCN-PPV photodegradation by initiating a radical process. C8-diCN-PPV's HOMO energy is lower than that of DABCO by 1.78 eV, a gap too wide for efficient electron transfer to happen. On the other hand, the HOMO of RO-diCN-PPV is only lower by 1.14 eV, allowing DABCO to donate electron to photoexcited RO-diCN-PPV to initiate a radical process that damaged the polymer and destroyed DABCO itself. It was also found that, in RO-diCN-PPV, radical decomposition takes very different paths from those of RO-PPVs and produce very different products.