Synthesis, Characterization And Photophysical Properties Of Phenothiazine-based Oligomers And Dendrimers

Two-photon absorption (TPA) is a nonlinear optical process,in which a molecule of nonlinear optical material in ground state simultaneously absorbs two photons, and then transits to high state transition process via a virtual intermediate state. Extensive research is being carried out to develop efficient two-photon organic materials for possible applications in high density optical data storage, optical limiting and photodynamic therapy, so this field has been growing very fast during the last decade due to the realization of the two-photon-based technologies as mentioned above. It is well-known that the organic materials with two-photon absorption properties are conjugated system, and the phenothiazine is a conjugated heterocyclic compound bearing electron-rich sulfur and nitrogen heteroatoms. Herein, we have designed, synthesized and characterized a series of linear, star-shaped and dendritic phenothiazine-based oligomers, which were functionalized with porphyrin, triphenylbenzene, tetraphenylmethane and 2,1,3-benzothiadiazole to study the photophysical properties of the molecules. Some creative results have been obtained, and the main results are outlined as following:(1) A series of asymmetry monodisperse oligo (10-n-octylphenothaizine -3,7-vinylene)s have been synthesized following a divergent approach with a double bond at the end of oligomers. The longest oligomer contains six phenothiazine units, they are good luminescent material with high fluorescence quantum yield of 0.2-0.41 (quinine sulfate as reference). Meanwhile, another series of symmetry monodisperse oligo(10-n-octylphenothaizine-3,7-vinylene)s have been synthesized following divergent /convergent approach with phenothiazine units up to seven. Based on the results of photophysical and electrochemical data, we found the effective conjugation length of oligomers are extended with increasing phenothiazine units which are saturated up to six phenothiazinevinylene units. Notably, they have large two-photon absorption cross section which increases with chain length extending, and the heptamer increases to 2071GM at 800 nm. The oligomers are well characterized, and they studied as the modol compounds for structure-property relationships of corresponding polymer.(2) C3-symmetric architecture star-shaped oligophenothiazines with monophepthiazine to quaterphenothiazine arms using triphenylbenzene as the core have been synthesized by Heck reaction. In THF solution, the four compounds show intensive photoluminescence and have large two-photon absorption cross section, and the TPA cross section gradually increases with increasing phenothiazine units. The two-photon absorption cross section of the oligomer with quaterphenothiazine arm is up to 1055GM at 800 nm.(3) Two star-shaped porphyrins with four oligophenothiazine arms are prepared through optimized Adler-Longo method in the mixed solvents of propionic acid, acetic acid and nitrobenzene. The phenothiazine ring is nonplanar with a butterfly conformation in the ground state, which can impede the porphyrin moleculars aggregation and the formation of intermolecular excimer, and its excitation energy could effectively transfer to the porphyrin core and emit intense red light in toluene solution when excited at 295 nm. The light-harvesting abilities of these compounds increase with increasing phenothiazine units. The porphyrins show high fluorescencequantum yields of 0.35-0.40 in solution and films,which is around fourfold of that of TPP. So they possess potential applications in photonic devices.(4) Two novel dendrimers G1 and G2 with tetraphenylmethane cores have been synthesized via convergent approach. In toluene solution, the two dendrimers show intensive green fluorescence with fluorescence quantum yields 0.47 and 0.49 (quinine sulfate as reference). They have large two-photon absorption cross section of 89 GM for G1 dendrimer and 343 GM for G2 dendrimer at 800 nm. The synthesis of the dendrimers provide a useful guideline to design organic molecules with large TPA. (5) Two 2,1,3-benzothiadiazole-based dendrimers G1 and G2 have been prepared by palladium-catalyzed Heck coupling reaction between phenothiazine dendron and 4,7-dibromo-2,1,3-benzothiadiazole core. In our system, the two-photon absorbing (TPA) chromophores phenothiazine dendrons are used as the donors and benzothiadiazole as the acceptor. The TPA cross section of such D–π–A–π–D type molecules are 502 and 693 GM at 800 nm, which are higher than those of the corresponding dendrimer with tetraphenylmethane cores. Because the fluorescence spectra of phenothiazine dendrons excited by single or two-photon overlapped with the absorption of the benzothiadiazole core, the energy transfer from excited phenothiazine dendron to benzothiadiazole core induced by single or two-photon excitation happened. On the other hand, the light-harvesting abilities of these dendrimers increase with increasing phenothiazine units of periphery dendrons. Meanwhile, the dendrimers emit intense orange and red light in flims, so they are not only expected to be good candidates for photonic devices, but also have the potential application in optical power limiting and photodynamic therapy.