| Organic single crystals constructed by pi-conjugated molecules have attracted agreat deal of interest in the field of optoelectronic devices such as optically pumpedlasers, field-effect transistors (FETs), electroluminescence (EL) devices, andphotovoltaic cells due to their high thermal stability, highly ordered structure, andhigh carrier mobility. Generally, optical and electrical properties of organic crystalslike the absorption, luminescence, and carriers mobility are governed not only by thestructure of the monomolecules, but also by their stacking patterns which can beinfluenced strongly through the noncovalent intermolecular interactions such ashydrogen bonding, pi-pi, C-H···pi interactions, etc. Thus understanding andcontrolling molecular stacking patterns are fundamental issues for obtaining thedesired optical and electrical properties of organic crystals.We have got five9,10-Bis((4-N,N-dialkylamino)styryl)anthracene derivatives(DSA-C1~DSA-C7) with different length of alkyl chain, i.e. adjusting the opticalproperty through chemical modification.There is only a little difference among the chemical structures of these fivecompounds, while the chromophores in them are all the same, and these fivecompounds show coincident optical properties in dilute solution, which indicates thesingle molecule properties of the compounds come from the9,10-Bis((4-N,N-dialkylamino)styryl) anthracene, and the alkyl chains at the end donot participate in the absorption and emission processes.Five single crystals of DSA-C1~DSA-C7have been successfully obtained by the vapor diffusion method. These crystals are with large size, high quality, but showdifferent shapes and colors (from orange to red). The fluorescent efficiency of crystalsare higher than of solutions, the UV-visible absorption spectra, photoluminescencespectra of crystals are red shifted compared to spectra of solutions, and thefluorescence decay lifetimes of the crystals are longer than dilute solutions. Inaddition, DSA-C1crystal and DSA-C7crystal show ASE under the excitation ofopticaly-pumped laser.The results of X-ray crystallographic analyses indicate that the molecules havethe identical orientation in each crystal, every molecular transition dipole is parallel tothe others. This offers ideal samples to investigate the impact of the pitch angle anddistance of adjacent molecules on the optical properties of crystals, because the pitchangles and the distances directly affected the molecular stacking mode in crystal andthe interactions bewtween the chromphores, which is driven by the differentsuprarmolecular weak interacions.By analyzing the UV-vis absorption apectra, fluorescent spectra, fluorescentdecay curves of the solutions and crystals of DSA-C1~DSA-C7, we found that theshift of spectra are caused by the cooperatiion of excitonic and dipolar coupling. Theexcitonic coupling means excimer which is referred to the dimers that exist only in theexcited state and the ground state of the pair is dissociativethe. The dipolar coupling,i.e. H-aggregate or J-aggregate formation that describes the situation in which two ormore identical molecules are close to each other in a special spatial arrangement. Wequantitively obtained the contribution to the spectral shift from the excitonic anddipolar couplings, respectively. The switch of molecular stacking in crystal (fromH-aggregation with low efficiency to J-aggregation with high efficiency) has greatinfluence on the ASE of crystals. The two typical J-aggregation crystals of DSA-C1and DSA-C7both show ASE phenomenon and the threshold of DSA-C7crystal isonly59.5kW/cm2, suggesting the potential in laser application.These discoveries, especially the influence on the shift of spectra by the cooperatiionof excitonic and dipolar coupling, can help understand the crystal structure-property relationship. The high efficiencies of all this series of crystals and the good amplifiedspantaneous emission property of some crystals suggest their potential application inoptical field. |
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