Benzotrithiophene versus Benzo/Naphthodithiophene Building Blocks: The Effect of Star‐Shaped versus Linear Conjugation on Their Electronic Structures

Efficient blue electroluminescence with an external quantum efficiency of 9.20% and CIE y < 0.08 without excimer emission

Aromatically substituted phenanthroimidazoles at the C6 and C9 positions enhanced the thermal, photochemical and electroluminescence properties due to extension of conjugation. These new materials exhibit good photophysical properties with high thermal stability, good film-forming property and high luminous efficiency. The electroluminescence performances of C6 and C9 modified phenanthroimidazoles as host emitters were evaluated as well as the dopant in the fabricated devices. Among the non-doped devices, pyrene substituted PPI-Py or PPICN-Py based devices show maximum efficiency: PPI-Py/PPICN-Py: η c (cd A-1) - 9.20/9.98; η p (lm W-1) - 8.50/9.16; η ex (%) - 5.56/5.80. The doped OLEDs, m-MTDATA/TAPC:PPI-Cz (4.81/4.85%), m-MTDATA/TAPC:PPICN-Cz (5.23/5.26%), m-MTDATA/TAPC:PPI-An (5.01/5.04%), m-MTDATA/TAPC:PPICN-An (5.25/5.28%), m-MTDATA/TAPC:PPI-Py (5.61/5.65%) and m-MTDATA/TAPC:PPICN-Py (5.76/5.78%) show improved device efficiencies compared to non-doped devices. Designing C6/C9 modified phenanthrimidazole fluorophores is an efficient strategy for constructing highly efficient OLEDs.

Star-Shaped Fused-Ring Electron Acceptors with a C3h-Symmetric and Electron-Rich Benzotri(cyclopentadithiophene) Core for Efficient Nonfullerene Organic Solar Cells

Classical fused-ring electron acceptors (FREAs) with a linear acceptor-donor-acceptor (A-D-A) architecture continuously break records of power conversion efficiency (PCE) in nonfullerene organic solar cells. In contrast, the development of star-shaped FREAs still lags behind. Herein, a new C_3h-symmetric and electron-rich core, benzotri(cyclopentadithiophene) (BTCDT) in which the central benzo[1,2-b:3,4-b':5,6-b″]trithiophene fused with three outer thiophenes via three cyclopentadienyl rings, is synthesized and used for the construction of star-shaped FREAs (BTCDT-IC and BTCDT-ICF). Owing to the strong electron-donating ability of the BTCDT unit, both acceptors exhibit the effective intramolecular charge transfer, leading to the strong absorption in the region of 500-800 nm with narrow band gaps below 1.70 eV as well as suitable highest occupied molecular orbital and lowest unoccupied molecular orbital levels. Compared with nonfluorinated BTCDT-IC, fluorinated BTCDT-ICF red-shifts the absorption peak to 688 nm and reduces the band gap to 1.62 eV, which induces a broader external quantum efficiency (EQE) response ranging from 300 to 800 nm and a higher maximum EQE of 70% while blending with a wide band gap polymer donor J61. The J61:the BTCDT-ICF blend film exhibits more suitable phase morphology compared with the J61:BTCDT-IC blend film, which is responsible for the enhanced EQE value, increased short-circuit current density (J_SC), and fill factor (FF) in organic solar cell devices. As a result, the J61:BTCDT-ICF-based device yields a best PCE of 8.11% with a high J_SC of 16.93 mA cm^-2 and a high FF of 65.6%, demonstrating that the BTCDT-based star-shaped FREAs hold great potential for nonfullerene organic solar cells.

An overview on synthetic strategies for the construction of star-shaped molecules

Strategies for the synthesis of star-shaped molecules have been in high demand in the last decades due to the importance of those compounds in various fields. The distinctly different properties of these compounds compared to their linear analogues make them versatile building blocks for the formation of mesophases of interesting mesomorphic and photophysical properties. Moreover, the applications of star-shaped molecules as building units for dendrimers as well as in supramolecular host-guest chemistry have also been recently studied. The star-shaped molecules mentioned in this review are classified according to the central core as well as the type of side arms. The properties and applications of these compounds are described in the appropriate contexts. This report summarizes the recent advances in this area.

Star-Shaped and Fused Electron Acceptors based on C3h -Symmetric Coplanar Trindeno[1, 2-b: 4, 5-b': 7, 8-b'']trithiophene Core for Non-Fullerene Solar Cells

Two new star-shaped and fused electron acceptors, TITT-3IC and TITT-3ICF have been designed and synthesized, which consist of a C_3h -symmetric coplanar trindeno[1, 2-b: 4, 5-b': 7, 8-b'']trithiophene (TITT) as the central core and 3-(dicyanomethylidene)indan-1-one and 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile as the peripheral electron-withdrawing groups, respectively. With the large coplanar configuration and electron-rich nature of π-conjugated backbone, these two acceptors exhibit strong intermolecular charge transfer absorption in the region of 500-650 nm with the optical band gaps around 1.9 eV. Relative to TITT-3IC, TITT-3ICF shows the downshifted LUMO level and the slightly redshifted absorption with the higher molar extinction coefficient due to the stronger electron-withdrawing effect of fluorination. When blending with PTB7-Th, the TITT-3ICF-based device displays a higher power conversion efficiency (PCE) of 4.26 % than the TITT-3IC-based device (PCE=3.87 %). Comparing with the TITT-3IC-based device, the increased short circuit current (J_SC ) and fill factor (FF) are responsible for the higher PCE value of the TITT-3ICF-based device, which benefits from its strong and redshifted absorption for light harvesting and proper phase separation morphology for effective exciton dissociation and charge transport. This work demonstrates that as an alternative electron-donating core, TITT will be promising in designing star-shaped non-fullerene materials.

Aromatically C6- and C9-Substituted Phenanthro[9,10-d]imidazole Blue Fluorophores: Structure-Property Relationship and Electroluminescent Application

In this study, a series of aromatically substituted phenanthro[9,10-d]imidazole (PI) fluorophores at C6 and C9 (no. 6 and 9 carbon atoms) have been synthesized and systematically characterized by theoretical, thermal, photophysical, electrochemical, and electroluminescent (EL) studies. C6 and C9 modifications have positive influences on the thermal properties of the new materials. Theoretical calculations suggest that the C6 and the C9 positions of PI are electronically different. Theoretical and experimental evidences of intramolecular charge transfer (ICT) between two identical moieties attaching to the C6 and the C9 positions are observed. Photophysical properties of the fluorophores are greatly influenced by size and conjugation extent of the substituents as well as linking steric hindrance. It is found that the C6 and C9 positions afford moderate conjugated extension compared to the C2 modification. Moreover, ICT characteristics of the new fluorophores increase as the size of the substituted aromatic group, and are partially influenced by steric hindrance, with the anthracene and the pyrene derivatives having the strongest ICT excited properties. EL performances of the fluorophores were evaluated as host emitters or dopants in organic light-emitting devices (OLEDs). Most of the devices showed significantly improved efficiencies compared to the OLED using the nonmodified emitter. Among all the devices, a 5 wt % TPI-Py doped device exhibited excellent performances with an external quantum efficiency >5% at 1000 cd/m² and a deep-blue color index of (0.155, 0.065), which are comparable to the most advanced deep-blue devices. Our study can give useful information for designing C6/C9-modificated PI fluorophores and provide an efficient approach for constructing high-performance deep-blue OLEDs.