Synthesis, crystal structure and Hirshfeld surface analysis of dimethyl 3-(3-bromophenyl)-6-methyl-7-oxo-3,5,6,7-tetrahydropyrazolo[1,2-a]pyrazole-1,2-dicarboxylate

The title compound, C17H17BrN2O5, resulted from the 1,3-dipolar cycloaddition reaction between dimethyl acetylenedicarboxylate and (3-bromobenzylidene)-4-methyl-5-oxopyrazolidin-2-ium-1-ide in CHCl3. The dihedral angle between the pyrazole rings (all atoms) is 32.91 (10)°; the oxo-pyrazole ring displays an envelope conformation whereas the other pyrazole ring adopts a twisted conformation. The bromophenyl ring subtends a dihedral angle of 88.95 (9)° with the mean plane of its attached pyrazole ring. In the crystal, the molecules are linked by C—H...O hydrogen bonds and aromatic π–π interactions with an inter-centroid distance of 3.8369 (10) Å. The Hirshfeld surface analysis and fingerprint plots reveal that the molecular packing is governed by H...H (37.1%), O...H/H...O (31.3%), Br...H/H...Br (13.5%) and C...H/H...C (10.6%) contacts. The energy framework indicates that dispersion energy is the major contributor to the molecular packing.

Impact of n-Doping Mechanisms on the Molecular Packing and Electron Mobilities of Molecular Semiconductors for Organic Thermoelectrics

Electrical conductivity is one of the key parameters for organic thermoelectrics and depends on both the concentration and mobility of charge carriers. To increase the carrier concentration, molecular dopants have to be added into organic semiconductor materials, whereas the introduction of dopants can influence the molecular packing structures and hence carrier mobility of the organic semiconductors. Herein, we have theoretically investigated the impact of different n-doping mechanisms on molecular packing and electron transport properties by taking N-DMBI-H and Q-DCM-DPPTT respectively as representative n-dopant and molecular semiconductor. The results show that when the doping reactions and charge transfer spontaneously occur in the solution at room temperature, the oppositely charged dopant and semiconductor molecules will be tightly bound to disrupt the semiconductor to form long-range molecular packing, leading to a substantial decrease of electron mobility in the doped film. In contrast, when the doping reactions and charge transfer are activated by heating the doped film, the molecular packing of the semiconductor is slight affected and hence the electron mobility remains quite high. This work indicates that thermally-activated n-doping is an effective way to achieve both high carrier concentration and high electron mobility in n-type organic thermoelectric materials.

Precrystalline P3HT nanowires: growth-controllable solution processing and effective molecular packing transfer to thin-film

Solution-processable precrystalline nanowires (NWs) of conjugated polymers (CPs) have garnered significant attention in fundamental research based on crystallization-driven self-assembly and in the roll-to-roll fabrication of optoelectronic devices such as organic...

Crystal structure and Hirshfeld surface analysis of 2,2,2-trifluoro-1-(7-methylimidazo[1,2-a]pyridin-3-yl)ethan-1-one

The bicyclic imidazo[1,2-a]pyridine core in the molecule of the title compound, C10H7F3N2O, is planar within 0.004 (1) Å. In the crystal, the molecules are linked by pairs of C—H...N and C—H...O hydrogen bonds, forming strips. These strips are connected by the F...F contacts into layers, which are further joined by π–π stacking interactions. The Hirshfeld surface analysis and fingerprint plots reveal that molecular packing is governed by F...H/H...F (31.6%), H...H (16.8%), C...H/H...C (13.8%) and O...H/H...O (8.5%) contacts.

Comparing the microstructure and photovoltaic performance of 3 perylene imide acceptors with similar energy levels but different packing tendencies

While it is widely recognized that microstructure plays an important role in the performance of organic photovoltaics (OPV), systematic studies are often challenging, as varying the molecular packing through typical...

Understanding, Quantifying, and Controlling the Molecular Ordering of Semiconducting Polymers: From Novices to Experts and Amorphous to Perfect Crystals

Molecular packing and texture of semiconducting polymers are often critical to the performance of devices using these materials. Although frameworks exist to quantify the ordering, interpretations are often just qualitative,...

Photophysics of fused ring electron acceptors for photovoltaic applications

<p>Organic photovoltaic cells (OPVs) have received significant interest over the last decade as they offer the potential of cheap renewable energy via direct photon to electron conversion of abundantly available Sun light. Compared to the inorganic equivalents, they offer lightweight, low-cost, and flexibility advantages. Conventional OPVs are typically based on blends of electron-donor materials and fullerene-based electron-acceptor materials that form bulk-heterojunctions (BHJs). But the attention has recently shifted to organic non-fullerene acceptors especially fused ring electron acceptors (FREAs) owing to their attractive properties including flexible energy levels, tunable band gap, crystallinity, and planarity. Thus the power conversion efficiency (PCE) of OPVs has recently attained a record of 16% by synthesizing FREAs with modified chemical structures. Owing to the high crystallinity and packing orientation of acceptors, FREA based OPV systems are characterized by large and pure phases sized 20-50 nm. This is consistent with the observation that charge generation dynamics in these systems lacked the ultrafast component that characterizes most fullerene blends. To test the hypothesis that the optimal phase size can be large due to facile exciton diffusion in FREAs, a planar indacenodithiophene (IDT) based FREA, IDIC is selected as a model system to study the exciton dynamics. Chapter 3 includes the exciton diffusion measurements in IDIC films using transient absorption spectroscopy which resolves a substantially high, quasi-activationless diffusion coefficient that exceeds that of typical organic semiconductors. The study also includes a deep insight of the key factors behind the enhanced exciton diffusion in IDIC and is shown to arise from different molecular and packing factors which enhance the long-range resonant energy transfer. Rapid exciton diffusion in IDIC films introduced the possibility of solution-processed bilayer devices. Chapter 4 comprises both device and photophysics of planar bilayer devices with a PCE of 11.1% which is readily accounted for the material and device design. By pairing a mid-band gap polymer donor with a range of FREAs, it is shown that the combination of high molecular packing densities and absorption coefficients, long exciton diffusion lengths, and efficient, resonant, long-range energy transfer between donor and acceptor layers enable efficient bilayer devices. By designing new materials with these characteristics, along with orthogonal solubility for layer-by-layer deposition of clean bilayers, it is suggested that the clear connection between material design and function in the bilayer structure will accelerate the development of more efficient organic photovoltaic devices. The molecular packing of active layer components has a crucial role in the device performance of OPV devices. Especially for FREA based OPV systems, the longrange structural order induced by end group 𝜋 - 𝜋 stacking is considered as the critical factor for achieving high PCEs. Chapter 5 includes a deep spectroscopic insight into the exciton and charge transport processes in a series of FREA based OPV systems having different molecular packing and ordering. Here the molecular stacking manipulation in FREAs is achieved by changing the length of alkyl side-chains so that the FREA backbone is changed from a 𝜋 -𝜋 stacking mode to a non-stacking mode. Transient absorption spectroscopic analysis of neat FREAs and blends reveals that exciton diffusion and intermolecular charge transfer processes do not necessarily rely on the molecular 𝜋 -𝜋 stacking, while close atom contact can also enable these processes. This work provides new insights into the design of advanced materials for next generation organic photovoltaics considering diverse transport channels formed by close atom interactions. Chapter 6 discusses the advantages of ternary OPV strategy in BHJ world with a 13% efficient ternary device based on a highly efficient FREA, FOIC. The introduction of a mid-band gap small molecule donor TR into the binary blend PTB7- Th:FOIC improves the open circuit voltage (Voc), short circuit current (Jsc), fill factor (FF), and thereby the overall device performance. Transient absorption spectroscopy reveals ultrafast resonant energy transfer from TR to PTB7-Th domains which is consistent with their intermixed morphology. Additionally, the study also includes the rapid long-range energy transfer from PTB7-Th to FOIC phases that</p>

Photophysics of fused ring electron acceptors for photovoltaic applications

<p>Organic photovoltaic cells (OPVs) have received significant interest over the last decade as they offer the potential of cheap renewable energy via direct photon to electron conversion of abundantly available Sun light. Compared to the inorganic equivalents, they offer lightweight, low-cost, and flexibility advantages. Conventional OPVs are typically based on blends of electron-donor materials and fullerene-based electron-acceptor materials that form bulk-heterojunctions (BHJs). But the attention has recently shifted to organic non-fullerene acceptors especially fused ring electron acceptors (FREAs) owing to their attractive properties including flexible energy levels, tunable band gap, crystallinity, and planarity. Thus the power conversion efficiency (PCE) of OPVs has recently attained a record of 16% by synthesizing FREAs with modified chemical structures. Owing to the high crystallinity and packing orientation of acceptors, FREA based OPV systems are characterized by large and pure phases sized 20-50 nm. This is consistent with the observation that charge generation dynamics in these systems lacked the ultrafast component that characterizes most fullerene blends. To test the hypothesis that the optimal phase size can be large due to facile exciton diffusion in FREAs, a planar indacenodithiophene (IDT) based FREA, IDIC is selected as a model system to study the exciton dynamics. Chapter 3 includes the exciton diffusion measurements in IDIC films using transient absorption spectroscopy which resolves a substantially high, quasi-activationless diffusion coefficient that exceeds that of typical organic semiconductors. The study also includes a deep insight of the key factors behind the enhanced exciton diffusion in IDIC and is shown to arise from different molecular and packing factors which enhance the long-range resonant energy transfer. Rapid exciton diffusion in IDIC films introduced the possibility of solution-processed bilayer devices. Chapter 4 comprises both device and photophysics of planar bilayer devices with a PCE of 11.1% which is readily accounted for the material and device design. By pairing a mid-band gap polymer donor with a range of FREAs, it is shown that the combination of high molecular packing densities and absorption coefficients, long exciton diffusion lengths, and efficient, resonant, long-range energy transfer between donor and acceptor layers enable efficient bilayer devices. By designing new materials with these characteristics, along with orthogonal solubility for layer-by-layer deposition of clean bilayers, it is suggested that the clear connection between material design and function in the bilayer structure will accelerate the development of more efficient organic photovoltaic devices. The molecular packing of active layer components has a crucial role in the device performance of OPV devices. Especially for FREA based OPV systems, the longrange structural order induced by end group 𝜋 - 𝜋 stacking is considered as the critical factor for achieving high PCEs. Chapter 5 includes a deep spectroscopic insight into the exciton and charge transport processes in a series of FREA based OPV systems having different molecular packing and ordering. Here the molecular stacking manipulation in FREAs is achieved by changing the length of alkyl side-chains so that the FREA backbone is changed from a 𝜋 -𝜋 stacking mode to a non-stacking mode. Transient absorption spectroscopic analysis of neat FREAs and blends reveals that exciton diffusion and intermolecular charge transfer processes do not necessarily rely on the molecular 𝜋 -𝜋 stacking, while close atom contact can also enable these processes. This work provides new insights into the design of advanced materials for next generation organic photovoltaics considering diverse transport channels formed by close atom interactions. Chapter 6 discusses the advantages of ternary OPV strategy in BHJ world with a 13% efficient ternary device based on a highly efficient FREA, FOIC. The introduction of a mid-band gap small molecule donor TR into the binary blend PTB7- Th:FOIC improves the open circuit voltage (Voc), short circuit current (Jsc), fill factor (FF), and thereby the overall device performance. Transient absorption spectroscopy reveals ultrafast resonant energy transfer from TR to PTB7-Th domains which is consistent with their intermixed morphology. Additionally, the study also includes the rapid long-range energy transfer from PTB7-Th to FOIC phases that</p>

Position of Biphenyl Group Turning The Structure and Photophysical Property of D-π-π-A Prototype Fluorescent Material

Three novel D-π-π-A prototype compounds, namely, (E)-2-(3-([1,1'-biphenyl]-2-yl)-1-(9H-fluoren-2-yl) allylidene) malononitri-le (2-BAM), (E)-2-(3-([1,1'-biphenyl]-3-yl)-1-(9H-fluoren-2-yl)allylidene) malononitri-le (3-BAM), (E)-2-(3-([1,1'-biphenyl]-4-yl)-1-(9H-fluoren-2-yl)allylidene) malononitri-le (4-BAM) were synthesized. Furthermore, the structures and photophysical properties of three compounds were compared. Molecules of 2-BAM were packed into a 1D column structure with H-aggregation. However, both of 3-BAM and 4-BAM were packed into 3D layer structures with J-aggregation, respectively. Although three compounds all showed highly twisted molecular geometries, their molecular packing and intermolecular interactions were different. Because of the differences in electronic structures of molecules, the three compounds displayed different emission behaviors in solid and dilute solution. This study indicated that changing the position of biphenyl groups is an effective way to turn the structures and photophysical properties of such D-π-π-A prototype fluorescent material.