We studied and compared the photophysical and aggregation-induced emission properties of three isomers: trans-1,2-di(9-anthryl)ethylene (DAE), trans-1-(9-anthryl)-2-(9-phenanthryl)ethylene (APE), and trans-1,2-di(9-phenanthryl)ethylene (DPE). Both DAE and APE molecules have a twisted conformation due to intramolecular H···H steric hindrance. By contrast, this kind of steric hindrance drastically decreases in DPE molecule. From DAE, APE, to DPE, there are a blue shift in the optical spectra and a marked increase in the fluorescence efficiency. Mainly due to the restriction of intramolecular rotation, DAE displays an aggregation-induced emission behavior. However, the fluorescence efficiency of APE decreases from solution to solid state. Different from DAE and APE, DPE is a dual-state highly-efficient chromophore. The fluorescence efficiencies of its solution and the as-prepared powder (DPE-α) are 60% and 81%, respectively. J-aggregate was observed in the crystal structure of DPE-α, which correlates with its high emission efficiency. Interestingly, a new metastable polymorph of DPE was unexpectedly obtained by a solvent-antisolvent rapid precipitation. This new polymorph displays vapour- and temperature-induced crystal-to-crystal phase transition and fluorochromism.Download high-res image (142KB)Download full-size image

Three (9-anthryl)vinylstyrylbenzene (ASB) position isomers were synthesized and compared. Substitution position affects temperature-induced polymorphism, crystal packing and crystallization-induced emission (CIE) properties. Different from 1,2-ASB and 1,4-ASB, 1,3-ASB undergoes a crystal-to-crystal phase transition upon heating. The crystal structure of a new phase (1,3-ASB-β) was successfully solved. Thus, the solid emission of 1,3-ASB could be tuned by polymorphism. In contrast, the solid emission of 1,4-ASB is controlled by its crystallinity. In the crystal structures of 1,2-ASB and 1,3-ASB-β, the adjacent interacting anthryls are inclined to adopt an edge-to-face configuration with C–H⋯π interactions. In contrast, the adjacent anthracene rings in the 1,4-ASB crystals are parallel to each other with π⋯π interactions. Furthermore, multiple intermolecular interaction modes such as C–H⋯π and H⋯H interactions coexist in the crystal structure of 1,4-ASB, which collectively results in a closer packing. Interestingly, 1,4-ASB displays CIE behaviour. In contrast, the emission of the other two isomers is quenched in the solid state. The effect of substitution position on the photophysical properties is systematically studied.

Despite the lack of typical electron donor and acceptor in the molecule structure, a pure aromatic hydrocarbon, trans,trans-1-(9-anthryl)vinyl-4-(1-pyrenyl)vinylbenzene (trans,trans-1,4-AVPVB), shows unusual fluorescence solvatochromism with emission shift over 80 nm. In addition, it displays an aggregation-induced emission (AIE) activity. For comparison, other three isomers, trans,trans-1,2-AVPVB, trans,trans-1,3-AVPVB, and trans,cis-1,4-AVPVB, were also synthesized. The effects of substitution position and vinylene bond geometry on solvatochromism, temperature-induced phase transition and AIE activity were systematically studied. Different from the other isomers, the as-prepared sample of trans,trans-1,2-AVPVB shows an endothermic peak and an exothermic peak before the melting point in the DSC curve. They correspond to a solid-to-gas transition and a solid-to-solid transition, respectively. Theoretical calculation indicates that the fluorescence solvatochromism may be related to the conformational change from the ground to the excited states. The solvatochromic degree is determined by conjugative effect. Trans,trans-1,2-AVPVB and trans,cis-1,4-AVPVB display moderate fluorescence solvatochromism. In contrast, the solvatochromic effect of trans,trans-1,3-AVPVB is weak due to meta-substitution. However, the conjugation interruption by meta-substitution is beneficial for solution emission, and trans,trans-1,3-AVPVB emits most efficiently in solution. Similar to trans,trans-1,4-AVPVB, trans,trans-1,2-AVPVB, and trans,cis-1,4-AVPVB also display an AIE behavior.

•Three compounds with linear, star and cruciform shapes were synthesized and compared.•The molecular shape exerts marked effects on the thermal stability, film morphology and optoelectronic properties.•In the crystalline solid, 3BTVB molecules construct a 3D interaction network.In order to investigate the effect of molecular shape on the optoelectronic properties, compounds 1,4-bis[2-(benzothien-2-yl)vinyl]benzene (2BTVB) with linear shape, 1,3,5-tris[2-(benzothien-2-yl)vinyl]benzene (3BTVB) with star shape and 1,2,4,5-tetra[2-(benzothien-2-yl)vinyl]benzene (4BTVB) with cruciform shape were synthesized and compared. The molecular shape exerts marked effects on the thermal stability, film morphology and optoelectronic properties. Compounds 2BTVB and 3BTVB are thermally stable before the melting point. In contrast, 4BTVB solid is decomposed upon heating. Single-crystal structure analyses show that 3BTVB molecules pack into a three-dimensional cofacial herringbone structure. In comparison with 2BTVB and 4BTVB, both the absorption and emission spectra of 3BTVB display a blue-shift feature. This is possibly related to the discontinued π-conjugation through meta-substitution. The evaporated film of 2BTVB is high crystalline. In contrast, 3BTVB molecules pack disorderly in the evaporated film. The molecular shape and film morphology exert remarkable effects on the carrier-transport ability.

Three [(9-anthryl)vinyl][(9-phenanthryl)vinyl]benzene (APB) position isomers were synthesized and compared. The molecular configuration exhibits an extraordinary ability to affect polymorphism probability, unexpected solvatochromism, and aggregation-induced emission property. With the substitution changing from para-, ortho-, to meta-position, the polymorph number changes from 1, 2, to 3. Both 1,2-APB and 1,3-APB display a temperature-induced crystal-to-crystal phase transition. Furthermore, a pair of concomitant conformational polymorphs were obtained for 1,3-APB. Crystal structure analyses reveal that the steric hindrance between the two substituents leads to different molecular conformation and packing pattern. The unexpected solvatochromism is attributed to the strong electron-withdrawing ability of anthracene against phenanthrene, which produces permanent dipole moment. The solvatochromic degree is determined by the conjugative effect that varies with substitution position. 1,4-APB displays the most remarkable solvatochromic effect. Furthermore, it shows a totally different emission decay dynamics from 1,2-APB and 1,3-APB in polar solvents. Interestingly, the solution fluorescence quantum yields of these three isomers all increase with increased solvent polarity, displaying a negative solvatokinetic effect. Both 1,2-APB and 1,4-APB display an aggregation-induced emission enhancement. Although 1,3-APB is quenched in the solid state, it emits most efficiently among these three isomers either in solution or in solid state. The effect of the medium environment on the radiative process plays a vital role in determining their different aggregation-induced emission behaviors.

The different combinations of five benzene rings and two C═C double bonds yield four isomers: 4,4″-bistyryl-p-terphenyl (BSTP), 4-styryl-4′-[2-(p-biphenyl)vinyl]biphenyl (SBVB), 1-styryl-4-[2-(p-terphenyl)vinyl]benzene (STVB), and 1,4-bis[2-(p-biphenyl)vinyl]benzene (BBVB). The position variations of two C═C double bonds affect aggregate microstructures, as well as thermal and optoelectronic properties. Except BBVB, the other three isomers all have a phase transition upon heating. X-ray diffraction and optical absorption analyses reveal that the molecules of four isomers all take an H-aggregation in the crystalline solid. However, the microstructures are different for orientation disorder exists in STVB crystals. Both the absorption and emission spectra of these four isomers display a bathochromic shift when the position interval between two double bonds ranges from three, two, to one benzene rings. The CH2Cl2 solutions of these four isomers are highly emissive with quantum yields ranging from 83% to 96%. Intriguingly, though the molecules crystallize into an H aggregation, the quantum yields of polycrystalline samples are also extraordinarily high, from 73% to 94%. All the isomers display moderate hole-transport ability. Different-oriented grains coexist in the vacuum-deposited films, which play different roles in the carrier transport.

In order to explore new furan-based functional materials, investigate the effect of the degree of branching on their optoelectronic properties and search for three-dimensional organic semiconductors from planar π-conjugated molecules, the compounds 1,4-bis[2-(benzofuran-2-yl)vinyl]benzene (2BFVB) with a linear shape, 1,3,5-tris[2-(benzofuran-2-yl)vinyl]benzene (3BFVB) with a star shape and 1,2,4,5-tetra[2-(benzofuran-2-yl)vinyl]benzene (4BFVB) with a cruciform shape were synthesized and compared. 2BFVB adopts a two-dimensional herringbone packing motif and there are two kinds of molecular conformation. The crystal packing of 3BFVB is more sensitive to the crystallization conditions. Two single-crystal phases and one thin-film phase were found. 3BFVB (β-phase) molecules pack into a three-dimensional cofacial herringbone structure. 4BFVB forms an inter-inserted two-dimensional hexagonal packing structure. The molecular shape and aggregate packing exert remarkable effects on the optoelectronic properties. The quantum yield of 2BFVB is very high both in the solution (74%) and in the crystalline solid (76%) states. The high solid emission efficiency of 2BFVB is possibly related to the two molecular conformations in the herringbone-arranged H-aggregates. Both 2BFVB and 3BFVB display hole-transport abilities. The performance of 3BFVB amorphous film is nearly equal to that of its crystalline film, which would be helpful in simplifying device fabrication. This study shows that the substitution degree exerts drastic effects on solubility, polymorphism, crystal packing and optoelectronic properties.

The variation in the substitution position exerts a much more distinct influence on the solution optical properties of four NAB isomers, which leads to totally different emission behaviors.

A new pair of 1-[(9-anthracenyl)vinyl]pyrene (AVP) isomers was simultaneously obtained in a ratio of about 10:1. They cannot be separated by chromatography. However, they can be more effectively separated via sublimation. Both isomers display an aggregation-induced enhanced emission character, which is mainly due to a large increase in the radiative decay rate. The vinylene bond geometry brings different packing motifs. The interacting adjacent molecules in trans-AVP crystals can adopt both edge-to-face and face-to-face arrangements. In contrast, in cis-AVP crystals the adjacent molecules are inclined to adopt an edge-to-face arrangement. The molecule packing plays an important role in the solid emission efficiency.Both trans and cis-1-[(9-anthracenyl)vinyl]pyrene isomers display an aggregation-induced enhanced emission behavior. The vinylene bond geometry exerts important effects on the molecule packing and solid emission efficiency.

The crystal structures and optoelectronic properties of the compounds 1,4-bis[2-(1-naphthyl)vinyl]benzene (BNVB), 4,4′-bis[2-(1-naphthyl)vinyl]biphenyl (BNVBP) and 4,4′-bis[2-(9-anthryl)vinyl]biphenyl (BAVBP) were studied and compared. Due to intramolecular H⋯H steric hindrance, BAVBP molecules adopt a twisted conformation and constructed into a three-dimensional C–H⋯π interacting structure. In contrast, the H⋯H steric hindrance is alleviated in compounds BNVB and BNVBP. Both molecules have much better planar character and aggregate into a lamellar structure with a herringbone packing motif. The aggregate structure exerts a dramatic influence on the optoelectronic properties. Compound BAVBP shows an aggregation-induced enhanced emission (AIEE) character. While, both BNVB and BNVBP are AIEE-inactive, BNVBP displays moderate hole-transporting ability.

Because of the difference in substitution position, compounds 1,2-bis[2-(9-anthracenyl)vinyl]benzene (1,2-BAVB), 1,3-bis[2-(9-anthracenyl)vinyl]benzene (1,3-BAVB), and 1,4-bis[2-(9-anthracenyl)vinyl]benzene (1,4-BAVB) display different crystal packing and optical property. 1,4-BAVB molecules pack into zigzag structure. The π-electrons are averagely distributed on the whole backbone though the molecule adopts a twisted structure. Compounds 1,2-BAVB and 1,3-BAVB have a column-like structure, and the π-electrons are mainly confined on the anthracene units. Obvious π···π interactions exist in the aggregates of 1,3-BAVB and 1,4-BAVB. The crystal packing and electronic structure exert dramatic influence on the photophysical property. Compound 1,4-BAVB is hardly emissive. Compound 1,3-BAVB is highly emissive in solution but quenched in the solid state. However, compound 1,2-BAVB displays an aggregation-induced emission behavior and an excimer-related fluorescence in solution. The relationship between the aggregate packing, electronic structure, and photophysical property was studied.

In order to explore new furan-based functional materials, investigate the effect of the degree of branching on their optoelectronic properties and search for three-dimensional organic semiconductors from planar π-conjugated molecules, the compounds 1,4-bis[2-(benzofuran-2-yl)vinyl]benzene (2BFVB) with a linear shape, 1,3,5-tris[2-(benzofuran-2-yl)vinyl]benzene (3BFVB) with a star shape and 1,2,4,5-tetra[2-(benzofuran-2-yl)vinyl]benzene (4BFVB) with a cruciform shape were synthesized and compared. 2BFVB adopts a two-dimensional herringbone packing motif and there are two kinds of molecular conformation. The crystal packing of 3BFVB is more sensitive to the crystallization conditions. Two single-crystal phases and one thin-film phase were found. 3BFVB (β-phase) molecules pack into a three-dimensional cofacial herringbone structure. 4BFVB forms an inter-inserted two-dimensional hexagonal packing structure. The molecular shape and aggregate packing exert remarkable effects on the optoelectronic properties. The quantum yield of 2BFVB is very high both in the solution (74%) and in the crystalline solid (76%) states. The high solid emission efficiency of 2BFVB is possibly related to the two molecular conformations in the herringbone-arranged H-aggregates. Both 2BFVB and 3BFVB display hole-transport abilities. The performance of 3BFVB amorphous film is nearly equal to that of its crystalline film, which would be helpful in simplifying device fabrication. This study shows that the substitution degree exerts drastic effects on solubility, polymorphism, crystal packing and optoelectronic properties.