Diketopyrrolopyrrole-based Conjugated Moleculars: Design, Synthesis And Optical Property Study

Organic luminescent materials have drawn the wide-spread attention of many researchers, because of their promising potential applications in optical and optoelectronic devices such as bio-imaging probes, chemo-sensors, organic light-emitting diodes(OLEDs), etc. Since diketopyrrolopyrrole(DPP) unit has been discovered in the 1980 s, it and its derivatives are a class of brilliant red, strongly fluorescent high-performance pigments exhibiting excellent resistance against chemical, light, heat and climate influences and advantageous optical properties, such as strong absorption and emission in the visible region, high fluorescence quantum yields and with a significant two-photon absorption cross section. Therefore, DPP derivatives would be good candidates for the design of novel functional materials. In this paper, we have synthetized a series of DPP derivatives and discussed the relationship between their molecular structure and photophysical properties.1. We have introduced two tetradecyl alkyl chains at 2,5-positions of commercial Pigment Red 254 to obtain a soluble and polymorphous DPP derivative(ADPP). It is found that both solid powder and spin coated film of ADPP exhibit not only very large mechanohypsochromic luminescence but also unique thermofluorochromism. Pristine ADPP from rotary evaporation is a red-emitting crystal(645 nm, R-state) and can be changed into a yellowish-green amorphous state(533 nm, Y-state) by mechanically grinding and solution spin-coating. The Y-state can revert to pristine R-state by annealing below 60 o C or solvent-fuming, affording reversible and very large mechano- hypsochromic luminescence up to 112 nm. More interestingly, upon annealing Y- and R-states above 70 oC, a new green crystalline state(523 nm, G-state)appears, and G-state can transform to Y-state upon grinding and R-state by solvent-fuming. These unique attributes could not only enrich the type of stimuliresponsive materials but also promote the potential applications.2. We have designed and synthesized two 9-anthryl-capped 2,5-dialkylated DPP derivatives(ABDPP and ATDPP). It is found that ATDPP with strong red emitting fluorescence(646 nm) is an aggregation-enhanced emission(AEE) dye since its fluorescence efficiency from solution(2.4%) to solid state(19.8%) is enhanced 8 times.It is possible for ATDPP applicable in solution-processed OLEDs. However, ABDPP emitting strong fluorescence in both solution and solid is a non-AEE dye exhibiting mechanochromic luminescence. Grinding and pressing could change the fluorescence color of crystalline ABDPP from yellow to red with a red shift 43 nm. Differential scanning colorimetry(DSC) and X-ray diffraction(XRD) analyses confirmed that the phase transition between crystalline and amorphous states upon external stimuli is responsible for the reversible color change.3. We have synthesized and fully characterized two D–π–A–π–D-type2,5-dibutylated DPP derivatives(DPPCZ and DPPPTH) with different electron donors carbazole and phenothiazine as end groups. It is found that DPPCZ emitting strong fluorescence in both solution and solid is a non-AEE dye exhibiting mechanochromic luminescence. Grinding can change the fluorescence color of crystalline DPPCZ from yellow to red with a red shift 30 nm. DSC and XRD analyses confirm that the phase transition between crystalline and amorphous states upon external stimuli. DPPPTH emitting strong red(635 nm) fluorescence is an aggregation-enhanced emission dye applicable in vacuum-evaporating OLEDs. This work has demonstrated once again that the subtle manipulation of molecular structure unit could significantly tune and alter the optoelectronic properties of conjugated organic materials.