Cocrystallization Tailoring Multiple Radiative Decay Pathways for Amplified Spontaneous Emission

Amplified spontaneous emission (ASE) is intrinsically associated with lasing applications. Inefficient photon energy transfer to ASE is a long‐standing issue for organic semiconductors that consist of multiple competing radiative decay pathways, far from being rationally regulated from the perspective of molecular arrangements. Herein, we achieve controllable molecular packing motifs by halogen‐bonded cocrystallization, leading to ten times increased radiative decay rate, four times larger ASE radiative decay selectivity and thus remarkable ASE threshold decrease from 223 to 22 μJ cm^?2, albeit with a low photoluminescence quantum yield. We have made an in‐depth investigation on the relationship among molecular arrangements, vibration modes, radiative decay profiles and ASE properties. The results suggest that cocrystallization presents a powerful approach to tailor the radiative decay pathways, which is fundamentally important to the development of organic ASE and lasing materials.     

Fluorene‐Based π‐Conjugated Oligomers for Efficient Three‐Photon Excited Photoluminescence and Lasing

A novel series of diphenylamino‐ and 1,2,4‐triazole‐end‐capped, fluorene‐based, π‐conjugated oligomers that includes extended oligofluorenes and oligothienylfluorenes has been synthesized by means of the palladium‐catalyzed Suzuki cross‐coupling of 9,9‐dibutyl‐7‐(diphenylamino)‐2‐fluorenylboronic acid and the corresponding 1,2,4,‐triazole‐based aryl halide as a key step. It was demonstrated that efficient two‐ and three‐photon excited photoluminescence and lasing in the blue region are obtained by pumping near‐infrared femtosecond lasers on these materials. Although the absorption and emission maxima of the highly fluorescent and extended oligofluorenes reach a saturation limit, there exists an effective conjugation length for an optimum three‐photon absorption cross section in the homologous oligofluorene series. On the other hand, the multiphoton excited emission spectrum and lasing wavelength can easily be modified or tuned by an incorporation of thienyl unit(s) into the fluorene‐based π‐conjugated core with which exceptionally large three‐photon absorption cross sections up to 3.59×10^?77 cm⁶ s² in the femtosecond regime have been obtained, thereby highlighting the potential of this series of photonic materials. The optimized full width at half‐maximum of the cavityless three‐photon upconverted blue lasing spectra are sharply narrowed to approximately 6 nm with an efficiency of up to 0.013 %.     

Highly Efficient Multiphoton‐Pumped Frequency‐Upconversion Stimulated Blue Emission with Ultralow Threshold from Highly Extended Ladder‐Type Oligo(p‐phenylene)s

A series of highly extended π‐conjugated ladder‐type oligo(p‐phenylene)s containing up to 10 phenyl rings with (L)‐Ph(n)‐NPh (n=7–10) or without diphenylamino endcaps (L)‐Ph(n) (n=7 and 8) were synthesized and investigated for their multiphoton absorption properties for frequency upconverted blue ASE/lasing. Extremely large two‐photon absorption (2PA) cross‐sections and highly efficient 2PA ASE/lasing with ultralow threshold were achieved. (L)‐Ph(10)‐NPh exhibits the highest intrinsic 2PA cross‐section of 3643 GM for a blue emissive organic fluorophore reported so far. The record‐high 2PA pumped ASE/lasing efficiency of 2.06 % was obtained by un‐endcapped oligomer, (L)‐Ph(8) rather than that with larger σ₂, suggesting that a molecule with larger σ₂ is not guaranteed to exhibit higher η₂. All of these oligomers exhibit exceptionally ultralow 2PA pumped ASE/lasing thresholds, among which the lowest 2PA pumped threshold of circa 0.26 μJ was achieved by (L)‐Ph(10)‐NPh.     

Large Two‐Photon Absorption of Terpyridine‐Based Quadrupolar Derivatives: Towards their Applications in Optical Limiting and Biological Imaging

Developing organic chromophores with large two‐photon absorption (TPA) in both organic solvents and aqueous media is crucial owing to their applications in solid‐state photonic devices and biological imaging. Herein, a series of novel terpyridine‐based quadrupolar derivatives have been synthesized. The influences of electron‐donating group, type of conjugated bridge, as well as solvent polarity on the molecular TPA properties have been investigated in detail. In contrast to the case in organic solvents, bis(thienyl)‐benzothiadiazole as a rigid conjugated bridge will completely quench molecular two‐photon emission in aqueous media. However, the combination of alkylcarbazole as the donor and bis(styryl)benzene as a conjugation bridge can enlarge molecular TPA cross‐sections in both organic solvent and aqueous media. The reasonable two‐photon emission brightness for the organic nanoparticles of chromophores 3 – 5 in the aqueous media, prepared by the reprecipitation method, enables them to be used as probes for in vivo biological imaging.     

Two‐Photon STED Spectral Determination for a New V‐Shaped Organic Fluorescent Probe with Efficient Two‐Photon Absorption

Two‐photon stimulated emission depletion (STED) cross sections were determined over a broad spectral range for a novel two‐photon absorbing organic molecule, representing the first such report. The synthesis, comprehensive linear photophysical, two‐photon absorption (2PA), and stimulated emission properties of a new fluorene‐based compound, (E)‐2‐{3‐[2‐(7‐(diphenylamino)‐9,9‐diethyl‐9H‐fluoren‐2‐yl)vinyl]‐5‐methyl‐4‐oxocyclohexa‐2,5‐dienylidene} malononitrile ( 1 ), are presented. Linear spectral parameters, including excitation anisotropy and fluorescence lifetimes, were obtained over a broad range of organic solvents at room temperature. The degenerate two‐photon absorption (2PA) spectrum of 1 was determined with a combination of the direct open‐aperture Z‐scan and relative two‐photon‐induced fluorescence methods using 1 kHz femtosecond excitation. The maximum value of the 2PA cross section ～1700 GM was observed in the main, long wavelength, one‐photon absorption band. One‐ and two‐photon stimulated emission spectra of 1 were obtained over a broad spectral range using a femtosecond pump–probe technique, resulting in relatively high two‐photon stimulated emission depletion cross sections (～1200 GM). A potential application of 1 in bioimaging was demonstrated through one‐ and two‐photon fluorescence microscopy images of HCT 116 cells incubated with micelle‐encapsulated dye.     

Efficient Two‐Photon Excited Amplified Spontaneous Emission from Organic Single Crystals

E, E‐1, 4‐bis[4′‐(N,N‐dibutylamino)styryl]‐2,5‐dimethoxy‐benzene (DBASDMB) organic crystals with high crystalline quality, large size and excellent optical properties are prepared. The linear and nonlinear properties in the crystal are comparatively studied. The relaxation dynamics pumped by two‐photon are very similar with that pumped by one‐photon. The crystal exhibits very strong two‐photon excited fluorescence and amplified spontaneous emission. Efficient two‐photon absorption, reasonably high fluorescent quantum efficiency, and high crystal quality together with stimulated emission make organic crystals ideal for the application in frequency upconversion and other optoelectronic fields.     

Low‐Threshold Wavelength‐Switchable Organic Nanowire Lasers Based on Excited‐State Intramolecular Proton Transfer

Coherent light signals generated at the nanoscale are crucial to the realization of photonic integrated circuits. Self‐assembled nanowires from organic dyes can provide both a gain medium and an effective resonant cavity, which have been utilized for fulfilling miniaturized lasers. Excited‐state intramolecular proton transfer (ESIPT), a classical molecular photoisomerization process, can be used to build a typical four‐level system, which is more favorable for population inversion. Low‐power driven lasing in proton‐transfer molecular nanowires with an optimized ESIPT energy‐level process has been achieved. With high gain and low loss from the ESIPT, the wires can be applied as effective FP‐type resonators, which generated single‐mode lasing with a very low threshold. The lasing wavelength can be reversibly switched based on a conformation conversion of the excited keto form in the ESIPT process.     

Lanthanide‐Based Luminescent Materials for Waveguide and Lasing

Microlasers and waveguides have wide applications in the fields of photonics and optoelectronics. Lanthanide‐doped luminescent materials featuring large Stokes/anti‐Stokes shift, long excited‐state lifetime as well as sharp emission bandwidth are excellent optical components for photonic applications. In the past few years, great progress has been made in the design and fabrication of lanthanide‐based waveguides and lasers at the micrometer length scale. Waveguide structures and microcavities can be fabricated from lanthanide‐doped amorphous materials through top‐down process. Alternatively, lanthanide‐doped organic compounds featuring large absorption cross‐section can self‐assemble into low‐dimensional structures of well‐defined size and morphology. In recent years, lanthanide‐doped crystalline structures displaying highly tunable excitation and emission properties have emerged as promising waveguide and lasing materials, which substantially extends the range of lasing wavelength. In this minireview, we discuss recent advances in lanthanide‐based luminescent materials that are designed for waveguide and lasing applications. We also attempt to highlight challenging problems of these materials that obstacle further development of this field.     

Record Multiphoton Absorption Cross‐Sections by Dendrimer Organometalation

Large increases in molecular two‐photon absorption, the onset of measurable molecular three‐photon absorption, and record molecular four‐photon absorption in organic π‐delocalizable frameworks are achieved by incorporation of bis(diphosphine)ruthenium units with alkynyl linkages. The resultant ruthenium alkynyl‐containing dendrimers exhibit strong multiphoton absorption activity through the biological and telecommunications windows in the near‐infrared region. The ligated ruthenium units significantly enhance solubility and introduce fully reversible redox switchability to the optical properties. Increasing the ruthenium content leads to substantial increases in multiphoton absorption properties without any loss of optical transparency. This significant improvement in multiphoton absorption performance by incorporation of the organometallic units into the organic π‐framework is maintained when the relevant parameters are scaled by molecular weights or number of delocalizable π‐electrons. The four‐photon absorption cross‐section of the most metal‐rich dendrimer is an order of magnitude greater than the previous record value.     

Polar Diketopyrrolopyrrole‐Imidazolium Salts as Selective Probes for Staining Mitochondria in Two‐Photon Fluorescence Microscopy

Three rationally designed polar derivatives of diketopyrrolopyrrole consisting of 1,3‐dimethylimidazolium cationic units and benzene, thiophene, or furan rings as π spacers were synthesized and thoroughly studied. The obtained salts are soluble in polar organic solvents and show satisfactory solubility in water, which makes them suitable for the applications in bioimaging. Photophysical measurements revealed that the obtained derivatives are characterized by strong absorption and good fluorescence quantum yields. The corresponding two‐photon properties were also examined and showed that the synthesized salts exhibit large two‐photon absorption cross‐sections reaching 4000 GM (GM=Goeppert‐Mayer unit, 1 GM=10^?50 cm⁴ s photon^?1) and very high two‐photon brightness values exceeding 2000 GM. It was demonstrated that these salts can be safely applied in two‐photon fluorescence microscopy for selective staining of mitochondria in living cells.     

Light‐emitting diodes and lasers based on polymer films doped with small organic molecules and rare‐earth complexes

We investigated the lasing properties of optically pumped polymer films. Amplified spontaneous emission (ASE) around 400 nm was observed in polymer films of polystyrene (PS) and poly(N‐vinylcarbazole) (PVK) doped up to 20% with the hole‐transporting organic molecule N,N′‐bis(3‐methylphenyl)‐N,N′‐diphenylbenzidine (TPD). Thus, TPD‐based films are candidates for blue‐emitting organic diode lasers. Films containing several semiconducting organic molecules and polymers and rare‐earth complexes were also investigated. Energy transfer was observed in PVK films doped with various europium and samarium complexes. PS films containing the electron‐transporting organic molecule 2‐(4‐biphenylyl)‐5‐(4‐tert‐butylphenyl)‐1,3,4‐oxadiazole and small amounts of TPD also showed energy transfer to the europium complexes, but not to the samarium ones. None of these films demonstrated ASE; therefore, they are not appropriate for lasing purposes. However, because rare‐earth ions have very sharp emission spectra, these materials are candidates for very monochromatic light‐emitting diodes. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2706–2714, 2003     

Orientation‐Controlled 2D Anisotropic and Isotropic Photon Transport in Co‐crystal Polymorph Microplates

2D anisotropic transport of photons/electrons is crucial for constructing ultracompact on‐chip circuits. To date, the photons in organic 2D crystals usually exhibit the isotropic propagation, and the anisotropic behaviors have not yet been fully demonstrated. Now, an orientation‐controlled photon–dipole interaction strategy was proposed to rationally realize the anisotropic and isotropic 2D photon transport in two co‐crystal polymorph microplates. The monoclinic microplate adopts a nearly horizontal transition dipole moment (TDM) orientation in 2D plane, exhibiting anisotropic photon–dipole interactions and thus distinct re‐absorption waveguide losses for different 2D directions. By contrast, the triclinic plate with a vertical TDM orientation, shows 2D isotropic photon–dipole interactions and thus the same re‐absorption losses along different directions. Based on this anisotropy, a directional signal outcoupler was designed for the directional transmission of the real signals.     

Covalent Organic Framework for Improving Near‐Infrared Light Induced Fluorescence Imaging through Two‐Photon Induction

Fluorescent materials exhibiting two‐photon induction (TPI) are used for nonlinear optics, bioimaging, and phototherapy. Polymerizations of molecular chromophores to form π‐conjugated structures were hindered by the lack of long‐range ordering in the structure and strong π–π stacking between the chromophores. Reported here is the rational design of a benzothiadiazole‐based covalent organic framework (COF) for promoting TPI and obtaining efficient two‐photon induced fluorescence emissions. Characterization and spectroscopic data revealed that the enhancement in TPI performance is attributed to the donor‐π‐acceptor‐π‐donor configuration and regular intervals of the chromophores, the large π‐conjugation domain, and the long‐range order of COF crystals. The crystalline structure of TPI‐COF attenuates the π–π stacking interactions between the layers, and overcomes aggregation‐caused emission quenching of the chromophores for improving near‐infrared two‐photon induced fluorescence imaging.     

Organic Printed Core–Shell Heterostructure Arrays: A Universal Approach to All‐Color Laser Display Panels

A universal approach is demonstrated for realizing dual‐wavelength lasing in organic core–shell structured microlaser arrays, which show great promise in serving as all‐color laser display panels. By alternately printing hydrophilic and hydrophobic laser dye solutions on preprocessed substrates, precisely patterned core–shell heterostructure arrays were obtained. The spatially separated core and shell independently function as optical resonators to support dual‐wavelength tunable lasing in each heterostructure. Such a general method enables to flexibly control the lasing wavelength of the core–shell microlasers across a wide spectral range by systematically designing the gain media. Using as‐prepared microlaser arrays as display panels, full‐color laser displays were achieved with a color gamut much larger than that of standard RGB space. These results provide insights for design concepts and device construction for novel optoelectronic applications.     

Self‐Assembled Fluorescent Organic Nanoparticles for Live‐Cell Imaging

Fluorescent, cell‐permeable, organic nanoparticles based on self‐assembled π‐conjugated oligomers with high absorption cross‐sections and high quantum yields have been developed. The nanoparticles are generated with a tuneable density of amino groups for charge‐mediated cellular uptake by a straightforward self‐assembly protocol, which allows for control over size and toxicity. The results show that a single amino group per ten oligomers is sufficient to achieve cellular uptake. The non‐toxic nanoparticles are suitable for both one‐ and two‐photon cellular imaging and flow cytometry, and undergo very efficient cellular uptake.     

Concise Synthesis and Two‐Photon‐Excited Deep‐Blue Emission of 1,8‐Diazapyrenes

Efficient violet–blue‐emitting molecules are especially useful for applications in full‐color displays, solid‐state lighting, as well as in two‐photon absorption (TPA) excited frequency‐upconverted violet–blue lasing. However, the reported violet–blue‐emitting molecules generally possess small TPA cross sections. In this work, new 1,8‐diazapyrenes derivatives 3 with blue two‐photon‐excited fluorescence emission were concisely synthesized by the coupling reaction of readily available 1,4‐naphthoquinone O,O‐diacetyl dioxime ( 1 ) with internal alkynes 2 under the [{RhCl₂Cp*}₂]–Cu(OAc)₂ (Cp*=pentamethylcyclopentadienyl ligand) bimetallic catalytic system. Elongation of the π‐conjugated length of 1,8‐diazapyrenes 3 led to the increase of TPA cross sections without the expense of a redshift of the emission wavelength, probably due to the rigid planar structure of chromophores. It is especially noteworthy that 2,3,6,7‐tetra(4‐bromophenyl)‐1,8‐diazapyrene ( 3c ) has a larger TPA cross section than those of other molecules reported so far. These experimental results are explained in terms of the effects of extension of the π‐conjugated system, intramolecular charge transfer, and reduced detuning energy.     

Chiral thiophene derivatives with optimal two‐photon absorption in near‐infrared window I and II

Thiophene derivatives are outstanding organic materials due to their wide spectral response, superior optoelectronic properties, and excellent environmental stability, which have attracted widespread attention in both theory and applications. However, their chiral analogues and corresponding chiroptical behaviors have been rarely investigated. Herein, a series of chiral thiophene derivatives are firstly designed by taking both optical chirality and two‐photon absorption (TPA) response into consideration, and are then investigated by modern analytical response theory. The calculated results demonstrate that these novel chiral thiophene derivatives show optimal TPA in the near‐infrared (NIR) window I and II, indicating that they are promising for biological applications. More importantly, they exhibit reasonable two‐photon circular dichroism (TPCD) signals, which are desirable candidates for chiral recognition regions. Furthermore, the calculations of orbital transitions shed light on that the intramolecular charge transfer plays an important role in their excellent TPA and TPCD behaviors in NIR regions.     

A Metal–Organic Supramolecular Box as a Universal Reservoir of UV,WL, and NIR Light for Long‐Persistent Luminescence

Long persistent luminescence (LPL) materials have a unique photophysical mechanism to store light radiation energy for subsequent release. However, in comparison to the common UV source, white‐light (WL) and near‐infrared (NIR) excited LPL is scarce. Herein we report a metal–organic supramolecular box based on a D–π–A‐type ligand. Owing to the integrated one‐photon absorption (OPA) and two‐photon absorption (TPA) attributes of the ligand, the heavy‐atom effect of the metal center, as well as π‐stacking and J‐aggregation states in the supramolecular assembly, LPL can be triggered by all wavebands from the UV to the NIR region. This novel designed supramolecular kit to afford LPL by both OPA and TPA pathways provides potential applications in anti‐counterfeiting, camouflaging, decorating, and displaying, among others.     

A Metal–Organic Supramolecular Box as a Universal Reservoir of UV,WL, and NIR Light for Long‐Persistent Luminescence

Long persistent luminescence (LPL) materials have a unique photophysical mechanism to store light radiation energy for subsequent release. However, in comparison to the common UV source, white‐light (WL) and near‐infrared (NIR) excited LPL is scarce. Herein we report a metal–organic supramolecular box based on a D–π–A‐type ligand. Owing to the integrated one‐photon absorption (OPA) and two‐photon absorption (TPA) attributes of the ligand, the heavy‐atom effect of the metal center, as well as π‐stacking and J‐aggregation states in the supramolecular assembly, LPL can be triggered by all wavebands from the UV to the NIR region. This novel designed supramolecular kit to afford LPL by both OPA and TPA pathways provides potential applications in anti‐counterfeiting, camouflaging, decorating, and displaying, among others.     

Two‐Photon Absorption Cross‐Sections of Reference Dyes: A Critical Examination

The electronic structure and one‐ and two‐photon absorption spectra of four fluorophores, p‐bis(o‐methoxystyryl)benzene (Bis‐MSB), coumarin 307, fluorescein and rhodamine B, commonly used as reference compounds for two‐photon absorption spectra, have been theoretically calculated and compared with available experimental data. The possible reasons for the wide discrepancies in two‐photon absorption cross‐sections reported in the literature are discussed on the basis of the theoretical findings. The role of a solvent environment on the electronic one‐ and two‐photon absorption spectra is also studied. We highlight some necessary precautions that one needs to take when comparing literature results of two‐photon absorption cross‐sections.