Highly Efficient Room-Temperature Phosphorescence from Halogen-Bonding-Assisted Doped Organic Crystals

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-BF₂dbm-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-BF₂dbm-R dopant is formed in doped crystals, i.e., Iph-CN···I-BF₂dbm-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-BF₂dbm-R from electron donating −OCH₃ to electron accepting −F, −CN groups, the ratio between phosphorescence and fluorescence intensities has been systematically increased from 3.8, 15, to 50

Highly Efficient Room-Temperature Phosphorescence from Halogen-Bonding-Assisted Doped Organic Crystals

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-BF₂dbm-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-BF₂dbm-R dopant is formed in doped crystals, i.e., Iph-CN···I-BF₂dbm-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-BF₂dbm-R from electron donating −OCH₃ to electron accepting −F, −CN groups, the ratio between phosphorescence and fluorescence intensities has been systematically increased from 3.8, 15, to 50

Organic Phosphorescence Nanowire Lasers

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_P). 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 T₁ (π,π*) ← S₁ (n,π*) ISC (Φ_ISC = 100%) and a moderate Φ_P (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

Absence of Intramolecular Singlet Fission in Pentacene–Perylenediimide Heterodimers: The Role of Charge Transfer State

A new class of donor–acceptor heterodimers based on two singlet fission (SF)-active chromophores, i.e., pentacene (Pc) and perylenediimide (PDI), was developed to investigate the role of charge transfer (CT) state on the excitonic dynamics. The CT state is efficiently generated upon photoexcitation. However, the resulting CT state decays to different energy states depending on the energy levels of the CT state. It undergoes extremely rapid deactivation to the ground state in polar CH₂Cl₂, whereas it undergoes transformation to a Pc triplet in nonpolar toluene. The efficient triplet generation in toluene is not due to SF but CT-mediated intersystem crossing. In light of the energy landscape, it is suggested that the deep energy level of the CT state relative to that of the triplet pair state makes the CT state actually serve as a trap state that cannot undergoes an intramolecular singlet fission process. These results provide guidance for the design of SF materials and highlight the requisite for more widely applicable design principles

Rational Design of Charge-Transfer Interactions in Halogen-Bonded Co-crystals toward Versatile Solid-State Optoelectronics

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 (Φ_PL = 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”

Self-Assembled Microdisk Lasers of Perylenediimides

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(<i>p</i>-methyl-phenyl)-perylenediimide (<i>mp</i>-PDI) self-assembled from solution. Single-crystal data reveal that <i>mp</i>-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 <i>mp</i>-PDI are attractive candidates on the way to achieve electrically driven OSSL

Self-Assembled Microdisk Lasers of Perylenediimides

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(<i>p</i>-methyl-phenyl)-perylenediimide (<i>mp</i>-PDI) self-assembled from solution. Single-crystal data reveal that <i>mp</i>-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 <i>mp</i>-PDI are attractive candidates on the way to achieve electrically driven OSSL