Synthesis And Opto-electronic Properties Of Truxene And Triindole Compounds Featuring Multi-orientational Charge Transfer

The main contribution of this paper is at exploring the novel organic photoelectric functional materials and discussing the structure-property relationships. We recognized that organic photoelectric functional molecules are related to three structural characters: (1)Ï€-conjugation property; (2) charge-shift property; (3) molecular symmetry. So we put forward the following idea to the designing of materials: Nitrogen and boron are used as the centers of donor and acceptor respectively, and carbon framework asÏ€-conjugation bridge to built the perfect "N(â…¤)-C(â…£)-B(â…¢)" D-Ï€-A units. Based on these units, we assemble and extend some kinds of charge-shift models (e. g. dipolar, quadrupolar and octupolar models).Because of the electron deficient nature of boron, three-coordinate organic boron group is a strong acceptor, which can lead to significant charge-transfer due to p-Ï€conjugation when conjugated with an adjacent organicÏ€-system. As a comparison, three-valence organic amine group acts as a typical donor because of the lone of electrons of N. Truxene and triindole areÏ€-conjugated C-frameworks which possess C₃ symmetry. The above groups and units are our scratch line of our synthesis and materials preparation.We designed and synthesized a serial of novel organic photoelectric functional molecules by connecting dimesitylboryl (-B(Mes)₂) acceptor and diphenylamine group donor to the terminal of truxene and triindole or properly extending theÏ€conjugated system of truxene and triindole. Their linear and nonlinear optical properties have been investigated in detail. We mainly explored their applications in organic light emitting, two-photon absorption (TPA), two-photon excited fluorescence (TPEF) and multifunctional fluorescent probe.1. Synthesis and characteristic of the new compoundsA total of seventeen organic photoelectric functional compounds with the kinds ofÏ€-D_n,Ï€-A_n, A₂-Ï€-D, A-Ï€-D₂ and D₃-Ï€-A₃ have been firstly synthesized. Their constituents and structures have been fully characterized by ¹H NMR, ¹³C NMR, MS and elemental analysis. Some molecular structures have been determined by single crystal X-ray CCD diffraction. All of these new compounds are stable in solid state and in common organic solvents in air. 2. Crystal structures and electronic structuresFive single crystals have been determined by X-ray CCD single crystal diffraction. The crystallographic data of compounds B3, N3, 03 and 04 show that the truxene moiety has excellent planar structure. Three para-C atoms (2-, 7-, 12-) of truxene form an equilateral triangle, and the edge lengths of the B3 and N3 triangle are longer than that of 3 and 4, which is the result of charge-transfer based on the electron-pull nature of B atom and electron-push nature of aniline N atom. In B3 crystal, the B-C bond length between B atom and truxene is shorter than that between B atom and mesityl. The benzenes of truxene which connecting to B atom exhibit benzoquinone-like structure. All the above data indicate that there is p-Ï€conjugation between boron atom and the truxene.The crystal structure of N2O2 has been determined at room temperature and low temperature respectively. The phase transition from P2₁/n (295 K) to P2₁ (120 K) happens accompanying the loss of centro-symmetry and the appearing of chirality. A calculation of B3 by using the density functional theory (DFT) shows that the negative charge on any B atom in LUMO and LUMO+1 is one order of magnitude larger than that in HOMO. From the graphs of molecular orbitals of B2N and BN2, we recognized that substantial negative charge have been transferred from the donor center of N atom to the acceptor center of B atom to B in their low lying excited LUMO state.3. Linear optical properties:The influences of functioning, branching, symmetry, conjugated bridge, substituted position and solvent, et al. to linear optical properties have been investigated.(1) After truxene is substituted at 2, 7, 12-positions by dimesitylboryl (A) or diphenylamine group (D), the absorption and fluorescence spectra of the functional molecules B3 and N3 exhibit obvious red-shift compared with their precursor. The fluorescent enhancement is very notable and the fluorescence quantum yields (Φ) are 4-5 times that of the precursor. This is the results of octupolar charge shift derived from pull or push groups.(2) When one acceptor or donor ofÏ€-A₃ molecule B3 orÏ€-D₃ molecule N3 is replaced by a donor or an acceptor, the D-Ï€-A₂ or D₂-Ï€-A molecule B2N or BN2 will be formed. The absorption peak positions of the two kinds of molecules are almost same, but the emitting peaks of the D-Ï€-A₂ or D₂-Ï€-A molecule have a 110 run red shift compared with those of B3 or N3. B2N and BN2 exhibit remarkable solvatochromism and large Stokes Shift.(3) We successfully synthesized several novel triindole compounds. The maximal absorption and fluorescence peaks of triindole are longer than that of truxene, which indicates that triindole has largerÏ€conjugation or electronic delocalization. When functionalized by dimesitylboryl group, triindole compounds has much red-shifted maximal absorption and fluorescence peaks than that of corresponding truxene compound. It is because of the electronic rich property of the triindole and the pull-push structure of its acceptor substitute compounds, which facilitates the charge transfer.(4) By connecting dimesitylboryl to the different positions of triindole (2, 7,12- or 3, 8, 13-), compounds p-N3B3 and m-N3B3 have been obtained. Their absorption spectra have obvious distinction, and the results of theoretical conformation analysis indicate that p-N3B3 carries out a long-distance charge transfer spanning carbazole unit. However, m-N3B3 carries out a short-distance charge transfer spanning benzene.5. Nonlinear optical propertiesWe investigated the TPEF (Two-photon Excited Fluorescence) properties of all title photoelectric functional compounds in THF with 800 ran laser exciting.(1) All title functional compounds exhibit TPA and TPEF properties. The compounds with octupolar charge-transfer character exhibit TPA "cooperative enhancement". B serials compounds have large TPA cross sections than that of N serials compounds.(2) The SPEF (Single Photon Excited Fluorescence) intensity of D-Ï€-A₂ or D₂-Ï€-A molecule (B2N or BN2) is weaker than that ofÏ€-D₃ orÏ€-A₃ molecule (N3 or B3). However, the TPEF intensity of B2N or BN2 are much stronger than that ofÏ€-D₃ orÏ€-A₃ molecule, and the TPA cross sections of B2N or BN2 are one order of magnitude that of N3 or B3, which result from the contribution of pull-push charge transfer of B2N and BN2.(3) TN1, TN2 and TN3 can be obtained by extending theÏ€-conjugation of N1, N2 and N3. The TPA cross sectionsσof TN1, TN2 and TN3 are two orders of magnitude that of N1,N2 and N3.(4) Triindole compounds N3T3, m-N3B3 and p-N3B3 have larger TPA cross sections and p-N3B3 has the largest TPA cross section of 250 GM among others. This excellent TPA property can be attributed to its electronic donor and acceptor groups, to the excellent 7r-conjugate system, and to its long-distance charge transfer.6. Boron (â…¢) compounds as multifunctional fluorescent probesNot only the D-Ï€-A₂ and D₂-Ï€-A molecules B2N and BN2 behave strong solvatochromism in aprotic solvent, but also their Stokes shiftâ–³v_st and aprotic orientation polarizabilityâ–³f exhibit good linear relationship. And they exhibit different responding characters to protic environment compared to normal polar probes. These make them can be used as a special environmental polar probes.Both absorption and fluorescence spectra of all boron (â…¢) compounds exhibit sensitive and selective probing ability to F". Especially, the characteristic color change of B2N and BN2 make their detecting of F- more convenient, more sensitive, and less subject to external factors. Furthermore, the "off-and-on" character of BN2 or B2N means that they can act as highly sensitive and selective sensors, and the sensitivity to F^-probe can be enhanced 2 orders of magnitude.