Synthesis And Emission Mechanism Of Novel Oligo/Polyfluorenes And The Application As Fluorescent Chemoensors

Polyfluorenes/oligofluorenes and their derivatives have played an important role in the fields of opto-electronic materials. Their high photoluminescence efficiency, good charge-carrier mobility, and easy synthesis and modification have drawn much attention. Recently, polyfluorenes has also been applied as fluorescent chemosensors/biosensors, due to their high emission efficiency. However, so far they are mostly studied as an outstanding blue emission material, few reports involve in the green emission of fluorenes, apart from the fluorenone defects, as well as the self-assembled structures. In this thesis, a novel terfluorene with pendant 4-(bis(2-hydroxyethyl)amino)phenyl is found to exhibit dual fluorescence emission in single polar solvent, with significant green emission bands besides the characteristic blue emission of fluorene. The blue emission intensity is quite weak, while the green emission band shows distinct solvatochromic effect. In order to investigate this unique emission behavior, a series of terfluorenes with different pendant substitutions are synthesized, and their photophysical properties are studied. Combining with the theoretical calculations, a through-space intramolecular charge transfer process, named spiroconjugation, is demonstrated to be responsible for the emission behaviors. Meanwhile, upon copolymerization of this specific monomer into copolymers, the intromolecular charge transfer process still exists. This system is able to be applied as ratiometric fluorescent probes for detection of metal ions, on account of the emission mechanism. In addition, different receptors are introduced into pendant chains of polyfluorenes, which could be developed as fluorescent chemosensors for metal ions or anions.This thesis contains two parts. The first part concerns the study of a set of terfluorenes and their emission mechanisms (Chapter 2-4). The second part concerns the polyfluorenes/oligofluorenes with specific functional groups and their application as fluorescent probes. Main contents and results are briefly described as follows:Chapter 1 reviews the principles of fluorescence, and several charge transfer systems, especially recent progress of spiroconjugation. Introduce some fluorescent probes according to the different action mechanisms, and particularly summarize the study of fluorescent chemosensors based on conjugated polymers. The design strategies and main contents of this thesis are outlined.In Chapter 2, a novel terfluorene TFOH with pendant 4-(bis(2-hydroxyethyl)amino)phenyl on central fluorene unit, is found to exhibit dual fluorescence emission in single polar solvent THF, and the green emission shows significant solvatochromic effect. While its analogue TFP (with two phenyl substitutions on central fluorene) shows bright blue emission in all solvents measured, and TSFOH (with two spirobifluorenes at sideward, and 4-(bis(2-hydroxyethyl)amino)phenyl on central fluorene unit) shows quite similar photophysical properties to TFOH. The emission behaviour of TFOH and TSFOH is speculated to be from a mechanism similar to the TICT process:the rotation of the C-N single bond on pendant phenyl is presumed to be responsible for the TICT state, and in films or high viscosity matrix, this rotation is restricted, resulting in blue shift of the green emission band.In Chapter 3, two novel terfluorenes TFPJ and TFPJH are synthesized to further investigate the dual emission mechanism of these terfluorenes. In the two new terfluorenes, the rotation of C-N bond on pendant phenyl ring is inhibited in the julolidine groups, as a result, its emission behaviors still resemble those of TFOH, which further rules out the TICT mechanism.. The results suggest that the rotation of C-N bond is not responsible for green emission. Their excited state properties are studied by theoretical calculations, and it turns out that this specific fluorescence emission behaviour of these terfluorenes originates from a through-space intramolecular charge transfer process, which is defined as a new kind of spiroconjugation. As far as we know, this is the first report of spiroconjugation induced fluorescence emission.In Chapter 4, a set of terfluorenes with different pendant substitutions, as well as different sideward groups are synthesized and characterized. Their photophysical properties are studied, and combining with calculation results, a conclusion is drawn out that the pendant phenyl groups and the electron-donating N atoms are two prerequisites for the formation of charge transfer process; while the sideward groups mainly act by affecting the whole conjugation length of terfluorene, and have little effect on the charge transfer emission.In Chapter 5, the intramolecular charge transfer emission of terfluorene is explored as fluorescent chemosensors. TFOH with 4-(bis(2-hydroxyethyl)amino)phenyl groups, together with TFN having 4-(bis(ethylamino))phenyl groups, is able to detect Hg2+ in THF solution with high selectivity and sensitivity, and the recognition process is reversible. In addition, a water-soluble terfluorene WTFN is specially designed, which could detect Hg2+ in ethanol-water mixtures. The detection takes advantage of the electron-rich N atoms on pendant phenyl rings which tend to coordination with metal ions, and as a result their electron-donating ability is weakened, and the charge transfer process is inhibited, thus the fluorescence emission of terfluorenes changes.In Chapter 6, the functional monomer with spiroconjugation property is introduced into polymers, and its influence on polymer emission behaviour is studied. PFOH is synthesized by copolymerization of 2,7-dibromo-9,9’-bis(4-(bis(2-hydroxyethyl)amino)phenyl)fluorene group with hexyl substituted fluorene unit, while in PHOH, the 2,2’-azanediyldiethanol groups are connected to the C9 position of fluorene unit by hexyl chains. PFOH has similar photophysical properties to the terfluorene TFOH, which also exhibits green emission in THF, although the green emission band is quite weak, moreover, it also shows significant solvatochromic effect. The emission behaviour of PFOH suggests that spiroconjugation still exists when copolymerized into polymer. Moreover, PFOH shows high sensitivity toward Hg2+, with fluorescence emission changes from green to bright blue. On the contrary, PHOH exhibits strong blue emission and has no response toward any metal ions examined.In Chapter 7, two copolymers PN1 and PN2 are synthesized and characterized, with 4-(bis(ethylamino))phenyl as pendant groups, and the content of this group in PN1 is much more than that in PN2. Photophysical studies show that changes of the contents of the 4-(bis(ethylamino))phenyl groups have little effect on the ground state electronic structure, but take a significant effect on the excited state. The intensities of green emission band in these polymers changes with the different content of diethylaniline groups, and are supposed to be originated from an intramolecular charge transfer process, according to the results from previous studies. Both of PN1 and PN2 show remarkable fluorescence emission“turn-on”responses exclusively to Hg2+, and this is the first report for detection of Hg2+with“turn-on”output signals based on polyfluorenes.In Chapter 8, the aza-15-crown-5 and urea groups are introduced as pendant groups of polyfluorenes for the first time. Two polymers are synthesized by Suzuki-Coupling reaction, and their response toward metal ions and anions are studied. The polymer with crown ether has quite good selectivity for CuSO4, and rather weak interaction for other copper salts such as Cu(ClO4)2, CuCl2, Cu(NO3)2, and Cu(OAc)2. The sensitivity and selectivity of the polymer for CuSO4 may come from the synergistic complexation effect of the bivalent counterion SO42-. To our knowledge, this is the first time to report the specific detection of CUSO4 by the synergistic effect of counterion. The copolymer with urea groups turns out to have high affinity toward Ac-, F- and H2PO4- through hydrogen bonding interaction, with high association constants for acetate ion and fluoride ion. The sensitivity and selectivity of the copolymer toward these anions come from the strong hydrogen bond interaction between the anions and the urea group of the copolymer. This new system utilizing the strong luminescence property of the polyfluorene as reporter and the hydrogen-bond-forming ability of urea group may act as novel effective anions receptor and chemosensor.Ⅶ...