Charge-transfer (CT) interactions between donor (D) and acceptor (A) groups, as well as CT exciton dynamics, play important roles in optoelectronic devices, such as organic solar cells, photodetectors, and light-emitting sources, which are not yet well understood. In this contribution, the self-assembly behavior, molecular stacking structure, CT interactions, density functional theory (DFT) calculations, and corresponding physicochemical properties of two similar halogen-bonded co-crystals are comprehensively investigated and compared, to construct an "assembly-structure-CT-property" relationship. Bpe-IFB wire-like crystals (where Bpe = 1,2-bis(4-pyridyl)ethylene and IFB = 1,3,5-trifluoro-2,4,6-triiodobenzene), packed in a segregated stacking form with CT ground and excited states, are measured to be quasi-one-dimensional (1D) semiconductors and show strong violet-blue photoluminescence (PL) from the lowest CT1 excitons (ΦPL = 26.1%), which can be confined and propagate oppositely along the 1D axial direction. In comparison, Bpe-F4DIB block-like crystals (F4DIB = 1,4-diiodotetrafluorobenzene), packed in a mixed stacking form without CT interactions, are determined to be insulators and exhibit unique white light emission and two-dimensional optical waveguide property. Surprisingly, it seems that the intrinsic spectroscopic states of Bpe and F4DIB do not change after co-crystallization, which is also confirmed by theoretical calculations, thus offering a new design principle for white light emitting materials. More importantly, we show that the CT interactions in co-crystals are related to their molecular packing and can be triggered or suppressed by crystal engineering, which eventually leads to distinct optoelectronic properties. These results help us to rationally control the CT interactions in organic D-A systems by tuning the molecular stacking, toward the development of a fantastic "optoelectronic world".

Organic solid-state lasers (OSSLs) based on singlet fluorescence have merited intensive study as an important class of light sources. Although the use of triplet phosphors has led to 100% internal quantum efficiency in organic light-emitting diodes (OLEDs), stumbling blocks in triplet lasing include generally forbidden intersystem crossing (ISC) and a low quantum yield of phosphorescence (Φ). Here, we reported the first triplet-phosphorescence OSSL from a nanowire microcavity of a sulfide-substituted difluoroboron compound. As compared with the unsubstituted parent compound, the lone pair of electrons of sulfur substitution plus the intramolecular charge transfer interaction introduced by the nitro moiety lead to an highly efficient T (π,π*) ← S (n,π*) ISC (Φ = 100%) and a moderate Φ (10%). This, plus the optical feedback provided by nanowire Fabry-Perot microcavity, enables triplet-phosphorescence OSSL emission at 650 nm under pulsed excitation. Our results open the door for a whole new class of laser materials based on previously untapped triplet phosphors.

Previous studies of perylenediimides (PDIs) mostly utilized the lowest singlet excited state S1 . Generation of a triplet excited state (T1 ) in PDIs is important for applications ranging from photodynamic therapy to photovoltaics; however, it remains a formidable task. Herein, we developed a heavy-atom-free strategy to prompt the T1 ←S1 intersystem crossing (ISC) by introducing electron-donating aryl (Ar) groups at the head positions of an electron-deficient perylenediimide (PDI) core. We found that the ISC efficiency increases from 8 to 54 % and then to 86 % by increasing the electron-donating ability of head-substituted aryl groups from phenyl (p-PDI) to methoxyphenyl (MeO-PDI) and then to methylthioxyphenyl (MeS-PDI). By enhancing the intramolecular charge-transfer (ICT) interaction from p-PDI to MeO-PDI, and then to MeS-PDI, singlet oxygen generation via energy-transfer reactions from T1 of PDIs to (3)O2 was demonstrated with the highest yield of up to 80 %. These results provide guidelines for developing new triplet-generating PDIs and related rylene diimides for optoelectronic applications.

Organic solid-state lasers (OSSLs) have been a topic of intensive investigations. Perylenediimide (PDI) derivatives are widely used in organic thin-film transistors and solar cells. However, OSSLs based on neat PDIs have not been achieved yet, owing to the formation of H-aggregates and excimer trap-states. Here, we demonstrated the first PDI-based OSSL from whispering-gallery mode (WGM) hexagonal microdisk (hMD) microcavity of N,N'-bis(1-ethylpropyl)-2,5,8,11-tetrakis(p-methyl-phenyl)-perylenediimide (mp-PDI) self-assembled from solution. Single-crystal data reveal that mp-PDI molecules stack into a loosely packed twisted brickstone arrangement, resulting in J-type aggregates that exhibit a solid-state photoluminescence (PL) efficiency φ > 15%. Moreover, we found that exciton-vibration coupling in J-aggregates leads to an exceptional ultrafast radiative decay, which reduces the exciton diffusion length, in turn, suppresses bimolecular exciton annihilation (bmEA) process. These spectral features, plus the optical feedback provided by WGM-hMD microcavity, enable the observation of multimode lasing as evidenced by nonlinear output, spectral narrowing, and temporal coherence of laser emission. With consideration of high carrier-mobility associated with PDIs, hMDs of mp-PDI are attractive candidates on the way to achieve electrically driven OSSL.

Organic room-temperature phosphorescence (ORTP), when combined with external stimuli-responsive capability, is very attractive for sensors and bio-imaging devices, but remains challenging. Herein, by doping two β-iminoenamine-BF derivatives (S-2CN and S-2I) into a 4-iodoaniline (I-Ph-NH ) crystalline matrix, the formation of S-2CN⋅⋅⋅I-Ph-NH and S-2I⋅⋅⋅I-Ph-NH halogen bonds leads to bright-red RTP emissions from these two host-guest doped crystals (hgDCs) with quantum efficiencies up to 13.43 % and 15.96 %, respectively. Upon treatment with HCl, the competition of I-Ph-NH ⋅HCl formation against S-2I⋅⋅⋅I-Ph-NH halogen bonding switches off the red RTP from S-2I/I-Ph-NH hgDCs, whereas the stable halogen-bonded S-2CN⋅⋅⋅I-Ph-NH ensures red RTP from S-2CN/I-Ph-NH hgDCs remains unchanged. A security protection luminescence pattern by using these different HCl-responsive RTP behaviors was designed.

The development of metal-free organic room temperature phosphorescence (RTP) materials has attracted increasing attention because of their applications in sensors, biolabeling (imaging) agents and anticounterfeiting technology, but remains extremely challenging owing to the restricted spin-flip intersystem crossing (ISC) followed by low-yield phosphorescence that cannot compete with nonradiative relaxation processes. Here, we report a facile strategy to realize highly efficient RTP by doping iodo difluoroboron dibenzoylmethane (I-BFdbm-R) derivatives into a rigid crystalline 4-iodobenzonitrile (Iph-C≡N) matrix. We found that halogen bonding between cyano group of Iph-C≡N matrix and iodine atom of I-BFdbm-R dopant is formed in doped crystals, i.e., Iph-C≡N···I-BFdbm-R, which not only suppresses nonradiative relaxation of triplets but also promotes the spin-orbit coupling (SOC). As a result, the doped crystals show intense RTP with an efficiency up to 62.3%. By varying the substituent group R in I-BFdbm-R from electron donating -OCH to electron accepting -F, -CN groups, the ratio between phosphorescence and fluorescence intensities has been systematically increased from 3.8, 15, to 50.

Metal-covalent organic frameworks (MCOFs) have been recently received wide attention owing to the homogeneous distribution of active metal centers that are beneficial for enhancing the application potentials. However, metal complex based functional building blocks for MCOFs synthesis are limited. Herein, two new MCOFs (Ni-Py-COF and Ni-Bn-COF) were constructed via a novel nickel glyoximate based building block. Splendid photocatalytic activity on hydrogen evolution from water and great long-term recyclability were achieved using these nickel glyoximate based MCOFs as photocatalysts. Excitingly, even without the addition of Pt co-catalyst, the hydrogen evolution rates (HER) of Ni-Py-COF reached up to 626 μmol g À 1 h À 1 , which is better than many porous organic polymers. This work not only expands the type of building units for MCOFs, but also provides meaningful insights for developing stable, efficient and earth-abundant photocatalysts toward H 2 generation.Covalent organic frameworks (COFs), as a promising class of porous crystalline polymers, have demonstrated considerable advantages and application potential in various fields owing to designable structures and tailor-made functions. [1] C, H, N and other light elements constituted the majority of COFs namely metal-free COFs. [2] However, most metal-free COFs fail to exhibit versatile and outstanding properties due to their limited constituent elements. [3] Recently, a new class of COFs with intentionally integrated metal centers into the skeletons are considered as promising candidates to address this dilemma; these are referred to as metal-covalent organic frameworks (MCOFs). [4] MCOFs not only inherit

Organic solid-state lasers (OSSLs) have been paid great attention due to their ease-of-fabrication, low cost and tailor-made molecular tunability. Optical gain materials of OSSLs are currently focused on fluorescent materials, which can bring only 25% exciton utilization for future current-injection OSSLs due to spin statics under electrical excitation. While thermally active delayed fluorescent (TADF) materials can obtain a theoretical 100% internal quantum efficiency by harvesting triplet excitons. However, OSSLs based on TADF materials remain largely unexplored yet. Here, we report the first TADFbased OSSL from whispering-gallery mode microring resonator arrays fabricated by a confined solution-growth method. The newly designed TADF emitter of carbozyl borondifluoride curcuminoid derivative, namely, CAZ-A, when doped into a host matrix of 4,4′-bis(N-carbazolyl)-1,10-biphenyl (CBP), reaches the highest gain-coefficient of 640 cm −1 at 4 wt %. These MRs with well-controlled sizes and uniform geometries constitute a high quality (cavity quality factor Q ∼ 1300) built-in WGM resonators and exhibit outstanding multimode laser behaviors. By varying TADF molecule doping concentration, the laser wavelength can be continuously tuned in the red spectral range from 650 to 725 nm. As CBP is usually used as the host material for active layers in OLEDs, we believe that CAZ-A/CBP doping material is good candidate for future electrically driven OSSLs.