Organic pi-conjugated materials have been researched extensively for a number of applications including organic photovoltaics, electrooptic (EO) modulation, solid-state lasing, organic light emitting diodes (OLEDs), biological imaging and sensing, chemical sensing and ligands for photocatalysts. These organic optoelectronic materials have one major issue that affects its performance and lifetime---photostability. For some applications, hermetical packaging, which is expensive, is required to prevent degradation by light and oxygen. Photooxidation occurs when a material is exposed to light and causes structural and chemical changes leading to photodegradation of the material. The two primary degradation mechanisms responsible for photooxidation involve superoxide radical anion and singlet oxygen. The state-of-the-art electrooptic chromophores typically contain multiple C=C bonds, which are susceptible to reaction with singlet oxygen, so, protection of C=C bonds via molecular level encapsulation is a promising way to enhance photostability of EO chromophores.

Pillar[n]arenes are a class of cylindrical molecules reported in 2008. A lot of research has been conducted on their synthesis, modification, complexation with host molecules, and formation supramolecular structures. Their sizes, ease of modification and perfect cylindrical shape make them the best structures among known macrocyclic molecules (e.g., cyclodextrins, calixarenes and cucurbiturils) for encapsulation of chromophores. However, molecular encapsulation using pillar[n]arenes have not been reported. This research aims to develop the first EO chromophore that is encapsulated by a pillar[n]arene and conduct a comparative study of rate of photooxidation to assess the effect of encapsulation on photostability. This research paves the way for future research on photodegradation mechanisms of encapsulated chromophores, and more complete encapsulation of photooxidation-sensitive moieties.

An EO chromophore consists of an electron donor such as an amino group, a pi-conjugation bridge, and an electron acceptor. An encapsulated chromophore is synthesized from coupling of a donor-bridge compound and an acceptor compound in presence of a pillar[5]arene. To make sure the pillar[5]arene in the molecular package does not slip away from the donor end, a bulky amino group, 2,2,6,6-tetramethylpiperidine, TMP, is used as the donor. To ensure the bridge structure can fit into the cavity of the pillar[5]arene, phenylpolyenal bridges of different length were synthesized: (E)-3-(4-(2,2,6,6-tetramethylpiperidin-1-yl)phenyl)acrylaldehyde (DB1), (2E,4E)-5-(4-(2,2,6,6-tetramethylpiperidin-1-yl)phenyl)penta-2,4-dienal (DB2), (2E,4E,6E)-7-(4-(2,2,6,6-tetramethylpiperidin-1-yl)phenyl)hepta-2,4,6-trienal (DB3). DB3 was primarily used due to its optimal chromophore length and ease of encapsulation compared to other donor-bridges. The acceptor 2-(3-cyano-4,5-dimethyl-5-(trifluoromethyl)furan-2(5H)-ylidene)malononitrile (CF3-TCF) was also selected, which is bulky enough to prevent the pillar[5]arene from slipping away from the acceptor end. Three versions of pillar[5]arenes were also synthesized with different side groups:1,4-dimethoxypillar[5]arene, 1,4-dihydroxypillar[5]arene, 1,4-bis(2-methoxyethoxy)pillar[5]arene and 1,4-bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)pillar[5]arene. Combination of donor-bridge, acceptor and pillar[5]arene produced a series of molecularly encapsulated chromophores (MECs). However, only the MEC (CC14) from DB3 and 1,4-bis(2-methoxyethoxy)pillar[5]arene was obtained in a sufficient quantity for structural characterization and photostability study.

CC14, along with its precursors, were studied using NMR, UV-Vis and PL. The parent chromophore of CC14, which was also formed in the synthesis reactions for CC14, has a such a poor chemical stability that it was decomposed upon formation. Other dye chromophores were used for comparative photostability with CC14: 2-(3-cyano-4-((1E,3E)-3-(6-((E)-4-(diethylamino)styryl)-2',6'-dimethoxy-4,4-dimethyl-4,5-dihydro-[1,1'-biphenyl]-2(3H)-ylidene)prop-1-en-1-yl)-5-methyl-5-(trifluoromethyl)furan-2(5H)-ylidene)malononitrile (CC10) and 2-(4-((1E,3E)-3-(3-((E)-4-(bis(2-((tert-butyldimethylsilyl)oxy)ethyl)amino)styryl)-5,5-dimethylcyclohex-2-en-1-ylidene)prop-1-en-1-yl)-3-cyano-5,5-dimethylfuran-2(5H)-ylidene)malononitrile (CLD1). CC14 is found to be more photostable than the benchmark chromophore CLD1 by nearly two orders of magnitude and a new version of CLD chromophore (CC10) by one order of magnitude. The result is exciting and stimulates new research toward development of extremely stable EO chromophores that may allow long-term operation of EO devices in air without the need for hermetic packaging which is expensive and technically challenging to do.