| Organic fluorescent materials have been extensively investigated as an important class of photoelectric materials and have been utilized in fields of OLEDs,OLEFET,organic solid-state lasers and organic fluorescent sensors.In these organic photoelectric devices,luminophors are usually used in aggregated or solid state like thin films and nano structures.However,the optical properties of these organic solids are not only influenced by the chemical structure of the molecules but to a large extent by the intermolecular organization.Moreover,the complicated photophysical properties of the luminophors in condensed or solid state can hardly be explained with the theories based on isolated molecular spectrum due to the excitations spreading out over several molecules and the electronic and energy transfer effects in aggregates Therefore,the excited state and spectrum property of organic aggregates remains a great challenge and attracts tremendous attention in recent decades.To get insight into the structure-property relationships in fluorescent aggregates and thus conclude effective methods for solid-state emission modulation,two series of donor-acceptor compounds based on cyanostilbene derivatives including linear and cruciform conjugations have been designed and synthesized.The influences of molecular configuration,push-pull effects and self-assembly structures on the energy level of the lowest excited state and the photophysical properties have been studied in the two systems respectively.The results obtained will suggest pathways towards targeted molecular design strategies which allow for desired organic fluorescent solids and optoelectronic materialsWe have also realized several kinds of bifunctional materials including thermo-mechanochromic materials,photo-mechanochromic materials and so on Among several classes of stimuli-responsive materials,we put special focus on the mecnanochromic(MC)materials.MC materials,which have receiving increasing attention since 2010s,show promising applications in many areas such as mechano-sensors,indicators of mechano-history,security papers,optoelectronic devices and data storage due to their dynamically and conveniently tunable emissions However,the mechanism of mechanical-responsive color switching in MC luminescent materials is still not clear at the molecular and supramolecular-level.In this work,the two series of cyanostilbene derivatives with linear and cross conjugated backbones exhibit varied MC properties.The MC mechanisms have been investigated,and different mechanisms were found to be responsible for the emission change in the two systems respectively.The linear cyanostilbene derivatives exhibit blue-shifted or red-shifted emissions upon mechanical stimuli due to the change of intermolecular arrangement and inter dipole interactions.While the cross-conjugated molecules show obvious red-shifted fluorescence due to the extension of conjugation length and subsequent enhancement of intramolecular charge transfer(ICT)transitionThe main context of this thesis can be summarized as follows1.An organogelator T16 showing mechanochromic and thermochromic properties has been designed and synthesized.Its gelation behavior,optical properties and self-assembly structures have been characterized to investigate the structure-excited state-property relationships.Compared with α-CN-TPA and 3C16,it is found that the stacking mode and molecular conformation of T16 may be both responsible for the optical properties and chromic phenomenon in solid state2.Mechanochromism and photochromism from linear cyanostilbene derivatives with simple conjugated backbones-effect of molecular arrangement on the solid-state emissionA chiral cyanostilbene derivative ACNS with linear backbone has been synthesized and characterized,which displays three distinct emissions(blue,green and yellow)in varied stacking modes drived by different intermolecular interactions The blue and yellow emissions can be turned to green color upon grinding.And photochromism was observed from the green solids due to the photoisomerization of cyanostilbene backbone.Noteworthy,most mechanochromic materials exihibit two emission colors,however,the chiral molecule,which tends to assemble into helical structures,shows remarkablely red-shifted third emission.In conclusion,the solid state emission could be tuned via the introduction of gelators(such as cholesterol,alkyl and alkoxy groups)which aid construction of distinct self-assemblies with the same fluorescent backbones.It’s a convenient and effective method of making the organic small-molecular materials into their functional aggregates3.Electrochromism and mechanochromism from cruciform cyanostilbene derivatives with strongly twisted architectures(a)Influence of molecular conformations on the solid-state emissionA luminophore DMCS-TPA with torsional cruciform π-architecture composed of donor and acceptor axes was designed and synthesized.Compared with conventionalπ-conjugated archetypes including linear and dendritic skeletons,such X-shaped conjugation has the following advantages.First,the cruciform structure can successfully incorporate units with different functions into each branch,for instance,one bar provides the electroactive species while the other serves as the luminogen Second,the crowding in the central part forces the surrounding substituents to distort from the ring plane,resulting in effective depression of close packing and enhancement of emission quantum yields in the solid state.Finally,when the molecule is featured with electron donors and acceptors on two crossed axes respectively,almost spatially separated HOMO and LUMO located on crossed directions are induced.A unique charge transfer(CT)process from one axis to the other in the excited state is evidenced by theoretical calculations and spectral analysis The photophysical properties,theory calculation and XPS results demonstrate that the extension of the conjugation length and subsequent enhancement of ICT transition are responsible for the improved MC performance.This is the first report on the changes of electronic structure in MC materials.In addition,DMCS-TPA is readily deposited on the ITO electrode surface by the electrochemical method to form an electrochromic(EC)polymer film(b)Influence of intramolecular charge transfer on the solid-state emissionIn this work,four other compounds with the same framework as that of above molecule have been designed and synthesized.The spatially separated HOMO and LUMO located on crossed directions allows the fine adjustment of electronic and optical properties through modifying on any single axis without disturbing the other The structure impacts on MC and EC properties are systematically and comparatively investigated from the perspective of electronic and steric effects through changing the peripheral substituents on the acceptor axis with phenyl,methoxyphenyl and thienyl The methoxyphenyl is further distinguished with methoxy groups bonded at ortho,meta and para positions respectively to induce quite different space hindrance as well as electron donating capacity.The five cross-conjugated compounds exhibit varied mechanical and electrochemical stimuli responses,which are correlated to the different molecular structures and inherent charge transfer properties based on the theoretical calculations,spectra and structural analysis.Besides,an excellent EC film with fast response(0.59 s),high optical contrast(70%)and multi colors showing(4 kinds)has been achieved. |
PDF Full Text Request