Synthesis And Photophysical Properties Of Fluorene-based Monodisperse Functionalized Oligomers

Organic photoelectric materials have received much attention due to their potential applications in organic solar cells, light-emitting diodes, organic field-effect transistors, non-linear optical and so on. Fluorene has become one of the important building blocks in functional organic materials because it shows high thermal stability, light stability, large band gap energy level as well as the high reaction activity of 2-,7-and 9-positions in fluorene. On the one hand, the introduction of flexible chain in fluorene will improve the solubility and film-forming ability of the corresponding functional materials. On the other hand, modified with the conjugated units the multifunctional fluorenes will be achived. Herein, we intent to synthesize a series of fluorene-based monodisperse conjugated oligomers and study their photophysical properties. Some creative results have been obtained and outlined below:(1) A series of monodisperse fluorene-phenothiazine alternating conjugated oligomers have been synthesized by alternate Heck and Wittig reactions in good yields, and their photophysical properties were studied. It was found that the conjugation of the oligomers would increase with increasing of the number of the conjugated units, leading to the red-shift of their maximum absorption and emission peaks. In addition, although three conjugated units were involved in both compounds of P-F-P and F-P-F, the conjugation of P-F-P was lower than that of F-P-F because the non-planar butterfly-type phenothiazine unit was in the center of the oligomer. It is worth mentioning that the linear fluorene-phenothiazine alternating conjugated oligomers exhibited two-photon absorption activity and their two-photon absorption cross-section will be enhanced with the increasing of the number of the conjugated units. Moreover, we found that the two-photo absorption cross section (δmax) of P-F-P-F-P containing three phenothiazine units is 2.5 times of F-P-F-P-F containing two phenothiazine units, indicating that the introduction of phenothiazine unit will improve theδmax of the oligomers. It provides a strategy to design functional materials with a large 8max.(2) Four new well-defined monodisperse linear oligofluorenes functionalized anthracenes An-OFVn (n=1,2,3,4) have been synthesized by alternate Heck and Wittig reactions in good yields. The absorption bands red-shifted with increasing the number of fluorenevinylene units because of the enlargement of the conjugation. Meanwhile, the strong emission at ca.600 nm assigned to the electronic transition of the excimer for An-OFV1 can be detected. However, when the number of fluorene units in the arms reached four, the emission intensity of the excimer would be decreased significantly, suggesting that the formation of excimer can be suppressed with increasing the number of fluorene-vinylene units linked in the 9,10-positions of anthracene. The strategy of the restraint of the excimer formation will be helpful for the fabrication of novel blue emitting materials with high fluorescence efficiency.(3) A series of new well-defined, monodisperse, star-shaped oligo(9,9-di-n-octylfluorene-2,7-vinylene)s functionalized truxenes Tr-OFVn (n=1, 2,3,4) have benn synthesized via Heck reaction between the linear oligofluorenes containing vinyl end groups and 2,7,12-tribromo-5,5,10,10,15,15-hexabutyl-truxene. The one-photon absorption and single-photon-induced emission bands red-shifted with increasing the number of fluorene-vinylene units because of the enlargement of the conjugation. It was interesting that the conjugated oligomers without strong donor and acceptor units exhibited two-photon absorption properties. Theδmax values increased obviously with prolonging the arms, and it reached 8073 GM for Tr-OFV4, which was one of the highestδmax values reported. We deduced that the branched star-shaped configration favored for the high TPA cross sections of Tr-OFVn. It provided a new strategy to design functional materials with a large two-photon absorption cross section.(4) Three fluorine-ethylene linked triphenylamine-cyanoacrylic acid derivatives Bn (n=0,1,2) have been synthesized. It was found that with increasing the length of the bridges of fluorene-ethylene, the conjugation of the dyes increased gradually, leading to the red-shift of the absorption and the decrease of the energy gap. The introduction of fluorene-ethylene units could improve the performance of the dye-sensitized solar cell (DSSC). The total photo-to-electrical energy conversion efficiency (η) of DSSCs base on dyes Bn (n=0,1,2) are 2.79%,5.56% and 4.49% under AM1.5 irradiation (100 mW/cm2), respectively. And theηof DSSC base on B1 reaches～70% of a N719-based DSSC (8.04%) measured under similar conditions. It suggested that B1 could be used as an useful dye for DSSCs. The Voc and Jsc values in B1-based DSSC are 0.86 V and 11.33 mA cm-2, which are the largest ones among the three DSSCs. However, the DSSC based on B2, whose conjugation is the largest among the three dyes, did not show the best performance. The measurement of the dark current revealed that the increase of the conjugation length could enhance the re-combination between the electrons and iodide in the electrolyte, which was disadvantageous for the performance. Therefore, the design of the organic dye with fit conjugation is importance in improving the performance of DSSCs.(5) Six D-π-A-π-D type linear conjugated oligomers, in which fluorenone was used as the acceptor, and fluorene, triphenylamine and phenothiazine were employed as the donors, have been synthesized via Heck reaction. It was found that the intramolecular charge transfer would be enhanced with increasing the donating-electron ability of the donor in the compounds that the donors were connected to fluorenone directly, accompanying with the red-shift of the absorption and emission bands. However, when the fluorene-ethylene units were inserted between the donor and accepter groups, the intramolecular charge transfer was significantly weakened. It should be noted that the two-photon absorption cross-section will be significantly enlarged with the increase of the conjugation length. For example, theδmax for compound P-F-FO-F-P is 5.9 times of that for P-FO-P. This work provides a new strategy to design functional materials with largeδmax value.