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Polymorphism arising from different conformations or packing arrangements in crystal lattices would manipulate physical properties, including morphology, electrical conductivity, emission, and nonlinear optical properties. However, the photochemistry tuned by polymorphism is seldom reported. In this work, photochemical reactive isomers of (E)-2-(1H-benzo[d]imidazol-2-yl)-3-(4-fluorophenyl)acrylonitrile (E-CN-BIFPh) and (Z)-3-(1H-benzo[d]imidazol-2-yl)-2-(4-fluorophenyl)acrylonitrile (Z-BIFPh-CN) in which a cyano group is located on different positions in conjugated skeletons were synthesized. Interestingly, E-CN-BIFPh crystallized into sheet-like structures emitting orange-red fluorescence (OR-phase) and block-like structures showing green fluorescence (G-phase). Besides, cuboid-like crystals with green fluorescence (G-phase) and needle-like crystals emitting blue light (B-phase) were formed from Z-BIFPh-CN. It is interesting that [2+2] cycloaddition took place only in the OR-phase crystal of E-CN-BIFPh and the B-phase crystal of Z-BIFPh-CN , which powered the molecular crystals to exhibit obvious photoactuation. In the case of the G-phase crystals of E-CN-BIFPh and Z-BIFPh-CN , the geometric parameters for "olefin pair" deviated from Schmidt's criteria, and hence, no photodimerization occurred. Notably, the OR-phase crystal of E-CN-BIFPh exhibited high-contrast mechanofluorochromism (MFC) with a red-shift of 115 nm in emission. The phase transition of the G-phase crystals of Z-BIFPh-CN upon heating at 105 °C led to a change in the emission color from green to blue, showing thermochromism. Such polymorphism crystallization-controlled multi-stimuli-responsive molecular crystals might be employed as elements in artificial muscles, smart microrobots, wearable systems and sensors. [ABSTRACT FROM AUTHOR]

Inspired by sophisticated and intelligent biological systems, various smart actuations triggered by physical or chemical stimuli have been developed. Photomechanical materials that can convert light energy into mechanical work have attracted significant attention given that light, as clean energy, enables controllable, remote and precise manipulation without electromagnetic disturbance. Compared with polymers and hydrogels, photomechanical molecular crystals with a higher Young's modulus may exhibit rapid and significant movement via efficient energy conversion, which is closely related to their crystal structures. Over the past few decades, the halogen effect has received increasing attention in crystal engineering due to the modulation of intermolecular interactions, such as halogen bonds (X⋯X) and hydrogen bonds (C–H⋯X). In this review, we summarize the photomechanical effects of crystalline materials in which halogen effects play a key role in molecular arrangements, fueled by the photochemical reactions of cis–trans isomerization, [2+2] cycloaddition, [4+4] cycloaddition, and electrocyclic reactions. Furthermore, their possible applications in soft robots, biomimetic systems, artificial muscles, biomedicine and microfluidics, and some challenges are also highlighted. [ABSTRACT FROM AUTHOR]

Tri-(2,4,6-trichlorophenyl)methyl (TTM) based radicals can be promising in providing relatively high fluorescence quantum efficiency. In this study, we have evaluated the photoluminescence properties of a series of TTM-based radicals by means of DFT and TD-DFT methods. The optimized structures of the ground states (D0) and the first excited states (D1) of all the radicals are calculated and the computed emission bands are comparable with previous experimental results. knr is determined from transition dipole moments (μ12) and the energy gaps between D0 and D1 (ΔE), both of which can be regulated by the conjugated structures from the substituent groups. knr was derived from the mode-averaging method and is consistent with the experimental results. Factors influencing kr and knr, including the potential energy differences (ΔG0), the vibrational reorganization energies (λ) and the electron coupling term (Hab), are discussed. By comparing kr and knr in solvents with different polarities (cyclohexane, toluene, and chloroform), TTM based radicals in cyclohexane exhibit the most promising fluorescence efficiencies. Besides, two substituted radicals, namely 2Br-TTM-3PCz and 2F-TTM-3PCz, have been fabricated. The results show that fluorine atoms are able to increase ΔG0 and a considerably small knr has been predicted. We expect that our calculation can benefit the design of light-emitting molecules in further experiments. [ABSTRACT FROM AUTHOR]

Herein, we have synthesized three new phenylquinolinylacrylonitrile derivatives. It is found that ((Z)-2-phenyl-3-(quinolin-4-yl)acrylonitrile) (1) and ((Z)-2-(4-fluorophenyl)-3-(quinolin-4-yl)acrylonitrile) (2) can form organogels in polar solvents, and compound 2 shows good gelation ability in acetone/H2O (v/v = 1/1.3) with a low critical gelation concentration (CGC) of 0.11 wt%. Interestingly, the free standing organogel (FSOG) with remarkable mechanical strength can be formed from gelator 2 based on its strong self-assembling capability. The storage modulus of FSOG was measured to be up to the 104 Pa level at a low concentration (0.74 wt%), which is much higher than those of most LMWGs. The FSOG exhibited reversible external force-induced bending without breaking and we could cut it arbitrarily. Immersing the FSOG in water for several hours affords the free-standing hydrogel (FSHG). Since a dense film was formed on the surface of FSHG, the mechanical strength of FSHG was further improved compared with that of FSOG. When exposed to 365 nm light, the freshly prepared organogel would be collapsed into a suspension because 2 underwent the tandem reactions of isomerization, electrocyclization and oxidative aromatization to yield 3-fluorobenzo[i]phenanthridine-12-carbonitrile (4) to destroy the intermolecular interactions among the molecules of 2. In addition, due to the aggregation-induced emission of compounds 1 and 2, the gained xerogels exhibited bright fluorescence. For instance, the blue fluorescence of the xerogel film of 2 could be switched off by TFA (trifluoroacetic acid) vapor with a response time of 0.38 s and a detection limit of 0.43 ppm. [ABSTRACT FROM AUTHOR]

A series of (Z)-3-(naphthalen-1-yl)-2-phenylacrylonitrile derivatives have been synthesized via the Knoevenagel condensation reaction. The photoisomerization takes place in the molecular crystals, which triggers the crystals to bend backward from the light source rapidly. In particular, significant photomechanical motions are observed in the initial several seconds of UV irradiation, during which the conversion ratio from Z- to E-isomer is less than ca. 0.4%. Such fast photoactuation induced by subtle photoisomerization might originate from the relatively large change in the molecular width after the photochemical reaction and strong intermolecular interactions to amplify the deformation at the molecular level into the macroscopic mechanical motion of the crystals. Therefore, the crystals performed reversible photo-induced bending and unbending by turning around the irradiation directions more than 100 times on account of the retained crystal integrity during photoactuation and the good elasticity of the crystals. The needle-like crystals also exhibit photomechanical effects in water. Such photochemical reaction-powered photomechanical molecular crystals may be employed as actuators in many practical application scenarios. [ABSTRACT FROM AUTHOR]

In order to improve the optoelectronic performance of π-conjugated molecules with electronic donors and electronic acceptors, people often choose multi-branch molecule structures, which can enhance the charge transfer ability. In this work, the photo-physical characteristics of (E)-4-(2-(3-methylquinoxalin-2-yl)vinyl)-N,N-diphenylaniline (QTPA1) and 4,4′-((1E,1′E)-quinoxaline-2,3-diylbis(ethene-2,1-diyl))bis(N,N-diphenylaniline) (QTPA2) in monodisperse systems and aggregated states were investigated in detail, showing that the photoluminescence quantum yield (PLQY) of oligomers is apparently enhanced in high-polarity solutions and decreased in low-polarity solvents instead, after the oligomer is linked with a conjugated branch. Based on the pump–probe and pump–dump–probe, this unusual phenomenon is assigned to the polarity-dependent intramolecular charge transfer (CT) mechanism of oligomers induced by the conjugated branch. In the PL of aggregated state and the two-photon fluorescence (TPF), increasing the conjugated branch has a positive effect on the PL characteristics that the PL performance of QTPA2 is higher than that of the aggregated QTPA1. In the aggregated state, QTPA2 has a larger degree of π electron conjugation, which suppresses intramolecular vibrations and reduces the energy loss caused by non-radiative processes. In addition, the two-photon absorption section of QTPA2 keeps higher in comparison with that of QTPA1 owing to the enhancement of the intramolecular dipole intensity, leading to the enhancement of TPF. This research could elucidate the physical mechanism underlying the optoelectronic performance and help us develop novel organic optoelectronic materials with high performance. [ABSTRACT FROM AUTHOR]

Organic molecules with hybridized local and charge transfer (HLCT) characters have attracted attention because of their potential in the field of two-photon fluorescence imaging and light-emitting diodes (LEDs). Herein, the 4-(1,1-difluoro-1H-1λ4,10λ4-benzo[4,5]thiazolo[3,2-c][1,3,2]oxazaborinin-3-yl)-N,N-diphenylaniline (TOND) molecule is studied through various spectroscopy tests to explore the solvent-dependent photo-physical properties in the mono-disperse system and temperature-dependent photoluminescence behavior in the aggregation state system. The monodisperse TOND shows apparent two-photon fluorescence properties and its absorption section enhances with solvent polarity owing to the intramolecular charge transfer (ICT) process. The solvent-dependent relaxation process of excited TOND, which is affected by the ICT process, is analyzed by pump–probe and pump–dump–probe tests. The aggregated TOND can be fabricated into an LED, whose emission peak is centered at 566 nm with the Commission Internationale de l'Eclairage chromaticity (CIE) coordinates of (0.47, 0.51). Their radiative lifetime and photoluminescence quantum yield together decrease with increasing temperature, due to the activation of the non-radiative process at high temperatures. Our results provide a comprehensive insight into the optoelectronic properties of these organic molecules. [ABSTRACT FROM AUTHOR]