| Supramolecular gels, as a kind of functional soft materials, have attracted muchattention in recent years mainly due to their ability to construct distinctsupramolecular structures from gelator molecules in organic solvents throughspontaneous and controlled self-assembly processes as well as their various potentialapplications in the fields of sensors, electrooptics, template self-assembly andmorphology control, gel electrolytes, catalytic system, separation technology, etc.Platinum(Ⅱ) complexes were utilized to design supramolecular gelators and have beenwidely studied due to their square-planar geometry, leading to formation of Pt Pt andstacking interactions. These interactions could be an additional driving force tostabilize supramolecular gels. Meanwhile, platinum(Ⅱ) complexes show intriguingspectroscopic and rich luminescence properties because of the unlimited possibilityand diversity of various metal-ligand chromophores. Amongst various families thatexhibit photochromic behavior, spirooxazines represent an interesting class ofphotochromic materials. Despite numerous studies on organic spiropyrans andspirooxazines and an increasing study on their transition metal complex systemsrecently in solutions, corresponding studies on their photochromic organogels arerare.Therefore, a series of photochromic spirooxazines of gallic acid and cholesterol derivatives has been designed and synthesized. Three of the compounds (2.42.6)could form stable supramolecular gels in organic solvents. The photophysical andphotochromic behaviors of complex2.5have been investigated in the ethanol gel state.Time-dependent electronic absorption changes of compound2.5(9.1104mol dm3)in ethanol gel with time intervals of6s at10°C in the dark after UV irradiation for5minutes were monitored. Upon UV irradiation at=360nm, a new absorption bandat600nm appeared and reached its maximum rapidly, attributing to the formation ofthe photomerocyanine form from the photochromic ring-opening reaction. After theirradiation, the gel was kept in the dark, and the new absorption band formed wasfound to drop rapidly with time and finally returned back to its original absorbancewithin5minutes as a result of the backward reaction of the photochromic reaction.Addition of p-toluenesulfonic acid was found to induce the formation of stableorganogels at concentrations below that of the critical gelation concentration (c.g.c.),with color changes from colorless to purple due to an acid-induced ring-openingprocess. Time-dependent electronic absorption spectroscopy was used to trace thisobservation. It was found that a new absorption band appeared at550nm and reachedits maximum within2hours. The activation parameters for the bleaching reaction of2.3in the solution state and2.5in the gel state have been determined in ethanolthrough kinetic studies at various temperatures. The results showed that the rate of thebleaching reaction of2.5in the gel state is much slower than that of2.3in the solutionstate, in line with the more restricted geometrical changes in the more viscous gelstate.As an extension of the above studies, a series of multifunctionalspirooxazine-containing platinum(Ⅱ) bipyridine complexes has been designed andsynthesized and characterized by using1H NMR spectroscopy, FAB massspectrometry amd elemental analysis. The molecular structures of complexes3.11and3.12were determined by X-ray crystallography. Their electrochemical, photophysical,and photochromic properties have also been investigated. It was found that3.2and3.4could form metallogels in decalin (a mixture of trans and cis isomers) and complex3.10with cholesteryl moieties could form stable gels in aliphatic solventslike heptane, octane, decane, as well as dodecane at room temperature. In contrast totypical thermotropic organogels and metallogels, complex3.10could formmetallogels in dodecane that is very stable towards any external stimuli. Thephotophysical and photochromic properties were investigated and the photochromicactivation parameters for the bleaching reaction were also obtained in the dodecanegel state. Prolonged excitation into the1MLCT/1LLCT absorption band at=405nmwould also lead to similar photochromic reaction. This could possibly be ascribed tothe fact that the1MLCT/1LLCT excited state would undergo facile intersystemcrossing into the3MLCT/3LLCT state due to the heavy metal effect exerted by theplatinum(Ⅱ) center, causing intramolecular photosensitization reaction of thespironaphthoxazine. Upon prolonged excitation into the1MLCT/1LLCT absorptionband, a new emission band at664nm was observed with diminishment of the620-nmemission band at20°C. Upon increasing the temperature to30°C, the664-nmemission disappeared with the revival of the emission at620nm as a result of thethermal bleaching reaction of the photomerocyanine form to afford the thermallystable closed form, where self-absorption effects became negligible. Additionally,lamellar liquid crystalline behavior has also been observed in complex3.2and studiedby thermogravimetry analysis (TGA), polarized optical microscopy (POM),differential scanning calorimetry (DSC), variable-temperature X-ray diffraction(XRD), and variable-temperature infrared (IR) spectroscopy. The investigationshowed that complex3.2exhibited two types of mesomorphic phases (Sm A and SmC) upon cooling from the isotropic state.Two classes of alkynylplatinum(Ⅱ) complexes of tridentate nitrogen donorligands have been synthesized. The molecular structures of ligand L4.10and complex4.7were determined by X-ray crystallography. The complex cations of4.7are stackedinto pairs to form a dimeric structure, in which the aromatic unit forms intermolecular(interplanar distance of3.03) interactions with neighboring molecules. Thecrystal packing of4.7shows that the complex cations are arranged in a head-to-tail fashion without a lateral shift of the aromatic moiety, probably resulting from theminimization of the mutual repulsion between the sterically bulky Boc groups. Theirphotophysical properties have also been investigated. It was found that three of thecomplexes (4.24.4) were able to form thermoreversible supramolecular gels. Thecomplex4.2cost about a week's time to form stable gel in decalin (a mixture of transand cis isomers). However, complexes4.3and4.4could form stable gels incyclohexane, which only needed ca.6hours. Further investigation indicated that allthe supramolecular gels were responsive to both thermo-and mechano-stimuli.Transmission electron microscopy (TEM) and scanning electron microscopy (SEM)were used to characterize the structure of the xerogel. In the process ofsupramolecular gel, the gelator molecules self-assemble through non-covalentinteractions to form fibrous architectures in nanometer scale, which in turn builds up acontinuous, three-dimensional, entangled network preventing the organic solventmolecules from flowing.In summary, some novel kinds of thermo-, acid-, and mechano-responsivephotochromic spirooxazine-and platinum(Ⅱ)-containing supramolecular gelators havebeen designed and synthesized, respectively. Transmission electron microscopy (TEM)and scanning electron microscopy (SEM) were used to study the morphologies of thexerogels. Their photophysical and photochromic properties as well as the activationparameters for the bleaching reaction of the spirooxazine were also investigated in thegel state. |
