Aggregation Structures And Opto-electronic Properties Of Organic Fluorescent Crystals

Organic π-conjugated materials have attracted wide interest in the recent two decades for their advantages in tuning opto-electronic properties.Various applications based on organic π-conjugated materials,such as organic light emitting diodes(OLEDs),solid-state lasers(OSLs),organic field-effect transistors(OFETs),have been achived with rapid progress.The opto-electronic properties of organic semiconductors in solid-state have close relate to their aggregation strucures.Therefore,organic single crystals with long-range ordering well-defined stuctures are ideal building blocks for studying structure-property relationships.In addition,organic single crystals with less defects and higher stability are ideal candidates for high-performance multi-functional applications.Herein,this thesis focuses on aggregation structures and opto-electronic properties of organic π-conjugated materials,and studies single crystal based organic solid-state lasers,field-effect transistros,light-emitting transistros,and acid-stimuli responsive materials.1.A novel organic molecule BDPVB is designed to show high fluorescence efficiency up to 50% in its single crystal.Due to the unique cross-dipole stacking,BDPVB single crystals with highly waveguided emission show low-threshold amplified spontaneous emission(ASE),indicating its potential to be used as gain material.In addition,a fluorine contained molecule BDFVA is designed to increase intermolecular interactions and show stacking polymorph in crystals.Blue-phase BDFVA crystals with cross-dipole stacking exhibit higher fluorescence efficiency and lower ASE threshold,indicating the advantages of cross-dipole stacking for developing highly emissive organic solid-state materials.2.A novel butterfly-like thiophene/phenylene derivative BDPV2 T is prepared to show balanced solid state emission and charge transport in its J-aggregate single crystals.The single crystals with photoluminescence quantum yield of 30% exhibit excellent ASE with one of the lowest threshold of 8 k W cm-2 and high gain up to 70 cm-1.In addition,single crystal field-effect transistors(SC-FETs)show anisotropic mobility up to 1 cm2 V-1 s-1 along molecular stacking directions.Furthermore,BDPV2 T single crystals have been performed to show multi-functional applications on phototransistors and organic light-emitting transistors(OLETs),revealing the remarkable integrated optoelectronic properties of BDPV2 T crystal for practical applications.3.Organic single crystals are of interest for optoelectronic applications due to their highly ordered stacking and small defect concentration,giving the possibility of high mobility and luminescence efficiency.To achieve lasing in large flat single crystals which are preferable for applications,a simple ―pick and place‖ method of mounting organic single crystals onto distributed feedback(DFB)gratings is demonstrated.Efficient lasing with sharp and narrow peaks is achieved with three kinds of active organic single crystals,indicating the universality of our method for making single crystal lasers using DFB gratings.Detailed investigations of refractive index and waveguide simulation further confirm that distributed feedback can provide efficient ways of making organic single crystal lasers.4.The symmetrical and asymmetrical protonation states are realized via the formation of intermolecular hydrogen bonds inside BP4 VA molecular crystals.With the protonation of H2SO4,BP4 VA molecules are protonated symmetrically,while the molecules are asymmetrically protonated by introducing HCl.The different protonation states of BP4 VA crystals result in various supramolecular interactions,aggregation states and even tunable optical properties.It provides a fundamental principle to understand the effect of protonation in organic conjugated molecules and an approach to expanding the scope of organic functional materials.