Synthesis And Properties Of Dendritic And Rod-like π-conjugated Molecules With Controllable Fluorescence

Organic optoelectronic materials constructed by π-conjugated molecules haveshown great potentials in nonlinear optics, organic light-emitting diodes, solar cells,molecule/ion recognition, photochromic and other fields. In the aspects ofphotophysical properties, since the demands of fluorescence in various fields aredifferent, the fluorescent modulation studies on π-conjugated organic molecules are ofgreat importance for their optoelectric applications. In the aspects of molecularstructures, branched/dendritic molecules, due to their particular molecularconstructions, possess many excellent optoelectric characteristics, for example, thecooperative enhancement of two-photon absorption in multi-branched structures;rod-like small molecules are easy to synthesize and modify, and usually minormodifications result in molecules with various optoelectric properties.Herein, we, on the basic concept of branched/dendritic and rod-like moleculesand towards the goals of fluorescence modulation, synthesized a series ofbranched/dendritic and rod-like π-conjugated molecules with controllablefluorescence and studied their photophysical properties. The innovative researchresults are as follows:(1) A series of branched molecules based on benzothiadiazole have beensynthesized by Heck reactions. It was found that the structrues with triphenylamineand/or phenothiazine as terminal groups have shown strong light harvesting ability.Intramolecular charge trasfer (ICT) was identified in the solvent-dependent tests.Compounds T1, T2, TP2and P2in toluene could emit red fluorescence at600nmexcited with400nm, and the emission redshifted with increasing solvent polarity.Excited with femtosecond laser at820nm, all compounds could give red fluorescence due to two-photon absorption. Non-dendritic structure T1has a two-photonabsorption cross section of253GM and branched structures T2, TP2and P2showtwo-photon absorption cross sections of594GM,351GM and517GM, respectively.Therefore, it can be concluded that extended conjugated system and branchedstructures may help increase the two-photon absorption cross section.(2) A series of triphenylamine cored, different phenothiazine groups basedstar-shaped molecules P1, P2, PF, PC and PM were synthesized according to“divergence” or “convergence” strategy. The photophysical results suggest that byintroducing different kinds of electron donor/acceptor, fluorescence behavior ofcompounds could be modulated. In toluene solutions, fluorescence peaks at481nm,509nm,489nm,519nm and636nm were observed, respectively, and the coloraltered from blue, green to red. Electrochemical tests showed that all compounds havehigh HOMO energy level, and PF and PM containing acceptors have low LUMOenergy level. The introductions of different electron donor/acceptor groups helpmodulate the fluorescence emission. The five compounds are expected to showapplications in two-photon absorption materials.(3) D-π-A conjugated triphenylamine substituted cyanostyrene derivatives G1,G1-N and G2were synthesized. These solid state emitters covering green, orange andred fluorescence were achieved by extending π-conjugation structure and introducingstronger electron acceptor. This series of compounds showed weak fluorescence insolution but strong fluorescence emission in solid state. For example, the fluorescencequantum yield of G2in solid reached0.67, about20times as much as that in solution.Single crystal X-ray diffraction data showed that intermolecular hydrogen bonds(C-H··· O, C-H··· N and C-H··· π) interactions increase rigidity of the molecule,restrict the intramolecular rotation and result in high fluorescence quantum yield insolid state. Therefore, we believed that these green, orange, red solid lumigens weregood candidates for OLED materials.(4) Two compounds (BzIm-TPAN and BzIm-TPE) containing benzimidazole, and cyano-triphenylethene or tetraphenylethene were synthesized. It was found thatboth compounds showed weak fluorescence in solution with quantum yields0.002and0.004. However, these values for thin film state were0.27and0.99, which were135and247times as much as those in solution, respectively. Cyano substitutedBzIm-TPAN emitted fluorescence with peak at532nm, showing a redshift of40nmcompared to phenyl compound BzIm-TPE (492nm). The emission properties in thinfilm provided a new design strategy for aggregation-induced emission materials.