Conjugate Fluorescent Compounds Based On Pyrene-Imidazole:Design, Synthesis And Organic Electroluminescence

Pyrene, a classical polycyclic aromatic hydrocarbon which consists of four fused benzene rings, represents one of the mostly investigated organic molecules over the past decades. The planar structural motif, large π-system and good mobility of π-electron flow endow pyrene with attractive photophysical properties. Pyrene-based materials have been widely applied in biological probes and fluorescent sensors by virtue of its tunable fluorescence change upon the environmental alternation. In recent years, there also has been an increasing interest in the use of pyrene as a semiconductor in optoelectronic field. Particularly, pyrene reveals a broad prospect of application and a huge commercial value in organic light-emitting diodes (OLEDs), organic photovoltaic cells (OPVs)and organic field-effect transistors (OFETs). However, pyrene π-system suffers heavily from the aggregation-caused quenching (ACQ) effect originating from the strong π-π interaction of pyrene for organic electroluminescence, which has limited its solid-state emission to some extent. So far, some chemical modifications have been introduced with the aim of inhibiting molecular aggregation including the attachment of big steric hindrance which however are often accompanied by severe side effects such as diluting the luminophore density and subsequently obstructing the charge transport in electroluminescence (EL) devices. Alternative strategy is of great interest.1,3-imidazole is a commonly used five-numbered-ring functional group for efficient light emitting materials such as benzimidazole, naphtho[1,2-d]imidazole and phenanthro[9,10-d]imidazole in virtue of its amphoteric characteristics originated from the two different kinds of sp2 hybrid nitrogen atoms. Based on these consideration, a facile and one-pot straightforward methodology is used to prepare a novel polycyclic skeleton using pyrene and imidazole as the key units. The new created pyrene-imidazole will work with other functional units synergistically and cooperatively, and their melts at the molecular level will afford adducts with the combined advantages of the two components including good carrier injection and transport property, good optical and thermal stability and high photoluminescent (PL) and electroluminescent (EL) efficiency.The category, size and number of the substituents as well as the degree of conjugation, spatial structure, rigidity and electron inductive effects after they melt with pyrene-imidazole have a great influence on the photoelectronic property of materials including the properties of excited state. In addition, the structure of molecular stacking in the state of aggregation will also be affected. Based on these considerations, in this thesis, three kinds of pyrene-imidazole derivatives were designed and synthesized. These compounds exhibited bipolar characteristic, hybridized local and charge-transfer (HLCT) excited state and aggregation-induced emission property. Their photophysical properties, electrochemical properties, thermal properties, crystal structures and electroluminescent properties were studied. The relationship between molecular structure and properties was also investigated. The content was mainly carried out from three aspects below: 1. On the basis of 9,10-diphenyl-9H-pyreno[4,5-d]imidazole (PyPI), we introduced different electron donor such as triphenylamine (TPA) and carbazole (Cz) to afford steric hindrance and improve the optical and thermal stability of materials. Furthermore electrondrawing group containing fluorine were added to system of pyrene-imidazole and TPA (PyTPAI) in order to balance the injection and transport of carrier in OLEDs. Turn-on voltages based on these bipolar materials were all below 3 V. Crystal structure demonstrated alternating and antiparallel alignment along the long axis direction derived from dipole-dipole interactions in these bipolar molecule including PyTPAI which may lead to their low fluorescence quantum yield in solid state.2. Using triphenylamine (TPA) as donor and pyrene-imidazole or cyanogroup (CN) as acceptor, four donor-acceptor (D-A) compounds were designed and synthesized through judicious structural design. Combining the photophysical tests and DFT (density functional theory) calculations, we investigated the relationship and energy level of locally excited (LE) state and CT state. On the basis of certain D-A geometry, large spatial wavefunction overlap of CT state and LE state (PyPTPAI) and smaller energy gap of these two states (PyPTPAI-CN) were realized. Hence CT state and LE state could be mixed or hybridized into a new state-HLCT state. Furthermore, by connecting pyrene unit to molecular skeleton of PyPI masterly, a pyrene-imidazole-pyrene hybrid D-A molecule PyPI-Py was successfully synthesized and a quasi-equivalent hybridization of LE and CT components is obtained in the excited state. PyPI-Py not only inhibited ACQ effect successfully benefiting from the introduction of pyrene and the resulting staggered-stacking in crystal but also utilize triplet excitons efficiently via process of reverse intersystem crossing. Such merits in PyPI-Py manifest an efficient blue emitting material. By using PyPI-Py as an emitter in a non-doped EL device, a high radiative exciton proportion of 74% was obtained which far exceeded the limit of 25% of spin statistic (singlet/triplet:～1/3). Accordingly the turn-on voltage is only 2.6 V and the maximum luminescence, external quantum efficiency (EQE), current efficiency (CE), and power efficiency (PE) of non-doped device are 75687 cd m-2, 8.46%,13.38 cd A-1, and 10.74 lm W-1, respectively.3. We designed and synthesized two efficient light-emitting materials, PyTPEI and PyPTPEI, and determined their difference and relationship upon their molecular structure. The attachment of TPE group to pyrene-imidazole endows PyTPEI and PyPTPEI with AIE-activity and high efficiencies in solid state. To further increase the number of TPE group and enhance the fluorescence two structural isomers with a pyrene-imidazole structure, syn-PyDTI and anti-PyDTI, were conveniently synthesized and successfully separated. When compared with anti-PyDTI, syn-PyDTI exhibits a 2-fold higher quantum efficiency as a crystal and 1.5-fold higher quantum efficiency as an evaporated amorphous film, although the two isomers possess the same apparent electronic structure. In the excited state dynamics study, the nonradiative decay rate constant for the evaporated film of the syn-isomer was calculated to be 1.9 x 108 s-1, which was much smaller than that of anti-PyDTI (3.1 x 108 s-1). This indicates that the molecular vibrational modes of excited state of syn-PyDTI might effectively suppress the energy loss through nonradiative decay processes and afford a higher efficiency. Syn-PyDTI also displays much better performance in OLEDs with a CE of 11.42 cd A-1 (8.12 cd A-1 for anti-PyDTI). This observation reveals the dependence of molecular excited state properties derived from distinct structural symmetries. We expect that the present strategy can provide a generic route toward novel and advanced structural isomers and that these materials may find practical applications in optoelectronics.