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.

Sensitized Fluorescence Organic Light-Emitting Diodes with Reduced Efficiency Roll-off

Thermally activated delayed fluorescence (TADF) sensitized fluorescence is a promising strategy to maintain the advantage of TADF materials and fluorescent materials. Nevertheless, the delayed lifetime of TADF sensitizer is still relatively long, which cause heavy efficiency roll-off. Here we reported a valid tactic to construct fluorescent devices with low efficiency roll-off by utilizing the TADF sensitizer with reduced delayed lifetime. By construct the sensitization system, the energy transfer efficiency can up to 90%. The high energy transfer efficiency and the TADF’s short delayed lifetime result in the high sensitization over 95%, maximum external quantum efficiency of 16.2%. Meanwhile the TADF sensitized-fluorescent devices exhibit reduced efficiency roll-off with a “onset” current density of 23 mA cm-2. Our results provide an effective strategy to reduce the efficiency roll-off of TADF sensitization system.

Red Thermally Activated Delayed Fluorescence in Dibenzopyridoquinoxaline-Based Nanoaggregates

All-organic thermally activated delayed fluorescence (TADF) materials have emerged as potential candidates for optoelectronic devices and biomedical applications. However, the development of organic TADF probes with strong emission in the longer wavelength region (> 600 nm) remains a challenge. Strong π-conjugated rigid acceptor cores substituted with multiple donor units can be a viable design strategy to obtain red TADF probes. Herein, 3,6 di-t-butyl carbazole substituted to dibenzopyridoquinoxaline acceptor core resulted in T-shaped donor-acceptor-donor compound, PQACz-T, exhibiting red thermally activated delayed fluorescence in polymer embedded thin films. Further, PQACz-T self-assembled to molecular nanoaggregates of diverse size and shape in THF-water mixture showing bright red emission along with delayed fluorescence even in an aqueous environment. The self-assembly and the excited-state properties of PQACz-T were compared with the nonalkylated analogue, PQCz-T. The delayed fluorescence in nanoaggregates was attributed to the high rate of reverse intersystem crossing. Moreover, an aqueous dispersion of the smaller-sized, homogeneous distribution of fluorescent nanoparticles was fabricated upon encapsulating PQACz-T in a triblock copolymer, F-127. Cytocompatible polymer encapsulated PQACz-T nanoparticles with large Stokes shift, excellent photostability were demonstrated for the specific imaging of lipid droplets in HeLa cells.

Incorporating Cyano Groups to a Conjugated Polymer Based on Double B←N Bridged Bipyridine Unit for Unipolar n-Type Organic Field-Effect Transistors

The development of n-type semiconductors lags far behind that of their p-type counterparts, demonstrating the exploration of exclusive n-type π-conjugated polymers is significant. The double B←N bridged bipyridine (BNBP)-based polymers P-BNBP-TVT containing (E)-1,2-di(thiophen-2-yl)ethene (TVT) previously reported exhibits ambipolar character because of the electron-rich nature. Herein, we incorporated strong electron-withdrawing cyano groups into the 3,3’-positions of TVT moiety to a copolymer P-BNBP-2CNTVT to develop n-type π-conjugated polymers. The LUMO/HOMO energy levels of P-BNBP-2CNTVT are –3.80/–5.95 eV, respectively, which are ~0.4 eV lower than that of P-BNBP-TVT without cynao groups. Unsurprisingly, compared with ambipolar P-BNBP-TVT, the organic field‐effect transistors (OFETs) based on P-BNBP-2CNTVT showed unipolar n-type characteristics with an electron mobility of 0.026 cm2 V–1 s–1 and lower threshold voltage of ~25 V as well as high Ion/Ioff of ~105. This study demonstrates organoboron π-conjugated polymers could be regarded as a tool for constructing exclusive n-type semiconducting polymers used in OFETs.

Excimers in Multichromophoric Assemblies: Boon or Bane?

Exciton dynamics in organic semiconductors is a subject of great significance from the standpoint of light emission, as well as light harvesting. As transient excited state species, excimers are expected to play a significant role in the dynamics and the fate of the excited state. Till recently, the discourse on excimers in organic systems revolved around their role in aggregation induced fluorescence quenching, or utilizing their characteristic red-shifted emission to report local interactions. But in the last decade, research in the area of organic multichromophoric systems has brought the spotlight back on this fascinating species. This review focuses on recent developments that highlight the importance of excimers in various processes involving multichromophoric systems, such as circularly polarized emission, exciton migration and singlet fission (SF). The review also attempts to address the question of whether excimers are useful or detrimental to various photophysical and photochemical processes of importance.

Perylenemonoimide as a Versatile Fluoroprobe: The Past, Present, and Future

Perylene dyes have transcended their role as simple colorants and have been reinvigorated as functional dyes. Based on the substitution at the peri position by six-membered carboxylic imides, the perylene family is principally embellished with perylene diimides (PDIs) and perylene monoimides (PMIs). Perylene dyes are widely acclaimed and adorned on account of their phenomenal thermal, chemical, and photostability juxtaposed with their high absorption coefficient and near-unity fluorescence quantum yield. Although symmetric PDIs have always been in the limelight, its asymmetrical counterpart PMI is already rubbing shoulders, thanks to the consistent efforts of several scientific minds. Recently, there has been an upsurge in engendering PMI-based versatile organic architectures decked with intriguing photophysical properties and pertinent applications. In this review, the synthesis and photophysical features of various PMI-based derivatives along with their relevant applications in the arena of organic photovoltaics, photocatalysis, self-assembly, fluorescence sensing, and bioimaging are accrued and expounded, hoping to enlighten the less delved but engrossing realm of PMIs.

Structure-Assembly-Property Relationships of Simple Ditopic Hydrogen Bonding Capable π-Conjugated Oligomers

A series of simple ditopic hydrogen bonding capable molecules functionalized with 2,4-diamino-1,3,5-triazine (DAT), barbiturate (B), and phthalhydrazide (PH) on both termini of a 2,2′-bithiophene linker were designed and synthesized. The intrinsic electronic structures of the ditopic DAT, PH, and B molecules were investigated with ground-state DFT calculations. Their solution absorbance was investigated with UV-vis, where it was found that increasing size of R group substituent on the bithiophene linker resulted in a general blue-shift in solution absorbance maximum. The solid-state optical properties of ditopic DAT and B thin films were evaluated by UV-vis, and it was found that the solid-state absorbance was red-shifted with respect to solution absorbance in all cases. The three DAT molecules were vacuum thermal deposited onto Au(111) substrates and the morphologies were examined using STM. (DAT-T)2 was observed to organize into six-membered rosettes on the surface, whereas (DAT-TMe)2 formed linear assemblies before and after thermal annealing. For (DAT-Toct)2 , an irregular arrangement was observed, while (B-TMe)2 showed several co-existent assembly patterns. The work presented here provides fundamental molecular-supramolecular relationships useful for semiconductive materials design based on ditopic hydrogen bonding capable building blocks.

Core-Substituted Naphthalene-diimides (cNDI) and Related Derivatives: Versatile Scaffold for Supramolecular Assembly and Functional Materials

Naphthalene-diimide (NDI) derived building blocks have been explored extensively for supramolecular assembly as they exhibit attractive photophysical properties, suitable for applications in organic optoelectronics. Core-substituted derivatives of the NDI chromophore (cNDI) differ significantly from the parent NDI dye in terms of optical and redox properties. Adequate molecular engineering opportunities and substitution-dependent tunable optoelectronic properties make cNDI derivatives highly promising candidates for supramolecular assembly and functional material. This short review discusses recent development in the area of functional supramolecular assemblies based on cNDIs and related molecules.