Solvent-Assisted Self-Assembly of Fullerene into Single-Crystal Ultrathin Microribbons as Highly Sensitive UV–Visible Photodetectors

The size, shape, and crystallinity of organic nanostructures play an important role in their physical properties and are mainly determined by the self-assembling kinetics of molecular components often involving the solvent conditions. Here, we reported a kinetically controlled self-assembly of C₆₀ assisted by the solvent carbon bisulfide (CS₂) into single-crystal ultrathin microribbons of 2C₆₀·3CS₂, upon mixing the poor solvent isopropyl alcohol with a C₆₀/CS₂ stock solution. Surface energy calculations reveal that these microribbons represent a kinetically favored high-energy state as compared with the thermodynamically stable shape of prismatic rods. High-resolution transmission electron microscopy observations clarify that association of CS₂ at the nucleation stage helps to guide and rigidify the formation of π–π stacking 1D chains of C₆₀ through the surrounding CS₂ cage-like structures, which further act as glue, boosting lateral assembly of as-formed 1D chains into untrathin 2D microribbon single crystals. Precise control over the thickness, width, and length of 2C₆₀·3CS₂ microribbons was achieved by manipulation of the growth kinetics through adjusting the solvent conditions. Upon heating to 120 °C, sublimation of CS₂ components results in fcc C₆₀ microribbons. We found that both microribbons of solvated monoclinic 2C₆₀·3CS₂ and pure fcc C₆₀ exhibit highly sensitive photoconductivity properties with a spectral response range covering UV to visible. The highest on/off ratio of two-terminal photodetectors based on single ribbons reaches around 250, while the responsitivity is about 75.3 A W^–1 in the UV region and 90.4 A W^–1 in the visible region

Donor-Linked Di(perylene bisimide)s: Arrays Exhibiting Fast Electron Transfer for Photosynthesis Mimics

The first example of donor-linked di­(perylene bisimide)­s is reported. UV–vis absorption spectra of these newly synthesized dyads showed intense absorption across the entire visible region, demonstrating their excellent light-harvesting activities. The severe fluorescence quenching event probed by steady-state fluorescence spectroscopy and the free-energy calculations suggested the possibility of electron transfer (ET) in these arrays upon photoexcitation. Further femtosecond transient absorption spectra clarified that the fluorescence quenching was due to fast intramolecular ET. The rate of the charge separation (CS) was found to be as high as 10¹² s^–1 in CH₂Cl₂. It was suggested that the large ET driving forces, strong donor–acceptor electronic coupling, and relatively small reorganization energy of diPBI accounted for the rapid ET process in a synergic manner. The fate of the generated radical ion pair depended on the solvent used. Rapid charge recombination to ground state occurred for the dyads in polar CH₂Cl₂ and for diPBI-TPA in nonpolar toluene. However, sufficient ³diPBI* population was attained via efficient spin–orbit coupled intersystem crossing from the charge-separated state for diPBI-PdTPP in toluene. These photophysical properties are interpreted as the cooperation between thermodynamic feasibility and kinetic manipulation

Naphthalenediimide-Benzothiadiazole Copolymer Semiconductors: Rational Molecular Design for Air-Stable Ambipolar Charge Transport

Rational design of air-stable ambipolar polymeric semiconductors was achieved by covalently connecting naphthalenediimide (NDI) units with benzothiadiazole (BZ) through thiophene (T) linkers, namely, <b>PNDI-</b><i><b>mT</b></i><b>(BZ)</b><i><b>mT</b></i> (<i>m</i> = 1, 2), in which well-coplanar <i><b>mT</b></i><b>(BZ)</b><i><b>mT</b></i> moieties as a whole act as donors rather than acceptors reported in previous studies. Decreasing the number of thiophene linkers from <i>m</i> = 2 to 1 lowers both LUMO and HOMO energy levels. As a result, the carriers in organic thin film transistors (OTFTs) could be switched from unipolar <i>p</i>-channel only to ambipolar transport. In ambient conditions, <b>PNDI-</b><i><b>2T</b></i><b>(BZ)</b><i><b>2T</b></i> presents an average hole mobility of 0.07 ± 0.02 cm² V^–1 s^–1, while <b>PNDI-</b><i><b>T</b></i><b>(BZ)</b><i><b>T</b></i> exhibits balanced ambipolar charge transport in a bottom-gate/top-contact device architecture, the average electron and hole mobilities was 0.05 ± 0.02 (<i>μ</i>_e) and 0.1 ± 0.03 (<i>μ</i>_h) cm² V^–1 s^–1, respectively. Moreover, OTFTs based on both polymer show good air-stability with negligible changes after stored in ambient over 3 months

Controlled Self-Assembly of Organic Microcrystals for Laser Applications

The small organic molecule <i>p</i>-distyrylbenzene (DSB) has been controllably prepared into one-dimensional microwires (1D-MWs) and 2D rhombic microdisks (2D-RMDs) by modulating the growth kinetics in the process of morphology growth. These as-prepared organic microcrystals, 1D-MWs and 2D-RMDs, exhibit a shape-dependent microcavity effect in that the single 1D-MW can act as a Fabry-Pérot (FP) mode lasing resonator while the individual 2D-RMD functions as the whispering-gallery-mode (WGM) microcavity. Moreover, as compared with the 1D FP resonators, there exists a higher quality factor (<i>Q</i>) in the WGM lasing resonator under the identical optical path length. Significantly, the lasing threshold, <i>E</i>_th = 1.02 μJ/cm², of 2D-RMDs is much lower than <i>E</i>_th = 2.57 μJ/cm² of 1D-MWs. Our demonstration can give the direction for the development of the organic solid-state microlasers

Engineering of Interfacial Electron Transfer from Donor–Acceptor Type Organic Semiconductor to ZnO Nanorod for Visible-Light Detection

Interfacial electron transfer (IET) plays a key role in photoactive organic/inorganic hybrid nanomaterials and remains elusive with regard to interfacial energy level alignment. In this study, we prepared hybrid ZnO nanorods by grafting n-type perylene bisimide (PBI) derivatives bearing carboxylic acid groups at nitrogen positions. No evidence in terms of direct electron transfer from PBI to ZnO can be observed in PBI/ZnO hybrids. In sharp contrast, incorporation of electron-rich oligothiophene (<i>n</i>T, <i>n</i> = 1, 2) moieties into PBI core at bay positions resulted in a highly efficient cascade IET in <i>n</i>T-PBI/ZnO (<i>n</i> = 1, 2) hybrid nanorods, which was initiated by photoinduced electron transfer (PET) from <i>n</i>T (<i>n</i> = 1, 2) to PBI and then followed by charge shifting from PBI anion to ZnO across the interface. High performance UV–vis photodetectors based on <i>n</i>T-PBI/ZnO (<i>n</i> = 1, 2) hybrids have been fabricated and show responsivity of 21.2 and 12.4 A/W and an on/off ratio as high as 537 and 403, whereas that based on PBI/ZnO shows little visible-light response. Our results suggest that donor–acceptor type compounds can be used for constructing photoactive hybrid nanomaterials, in which efficient cascade IET modifies interfacial electronic structure and helps extend the spectral response range

Photoreactions of Porphyrins Initiated by Deep Ultraviolet Single Photons

The newly built 177 nm all-solid-state deep ultraviolet (DUV) laser photoionization mass spectrometer finds a unique advantage to identify porphyrins that bear ionization energies close to 7.0 eV. We observed dramatic selectivity of tetraphenylporphyrins (TPPs) pertaining to varied photochemical processes initiated by the DUV laser excitation. Single-photon ionization was found dominant for 2H-TPP resulting in a fragmentation-free mass spectrum; photoinduced dehydrogenation was observed for zinc TPP, but both dehydrogenation and demetalation are noted for copper TPP. Along with first-principle calculations, we demonstrate how the photoinduced reactions vary with residual energies of photoionization, highest occupied molecular orbital–lowest unoccupied molecular orbital gaps, donor–acceptor orbital overlaps, single-step barriers, and whether or not there is a major process of structural rearrangement. It is demonstrated that the rotation of benzene ring under proper laser radiation prompts dehydrogenation process; also, metallo-TPPs do not support direct demetalation, but it is selectively accomplishable along with dehydrogenation and successive hydrogenation processes. These findings not only provide insights into the hydrogen atom transfer in porphyrins initiated by ultraviolet laser but also suggest promising applications of the DUV laser in designed synthesis and chemical modification of porphyrins

Self-Assembly of Perylenediimide Nanobelts and Their Size-Tunable Exciton Dynamic Properties

Upon the oxidation of perylenediimide dianion precursors, controlled release of neutral units paves the way for the solution-phase self-assembly of nanobelts via synergistic π–π stacking and hydrogen-bonding interactions. The obtained belt size has been regulated through adjusting the precursor supersaturation. This controlled synthesis also offers us an opportunity to explore size-tunable exciton dynamics features in the nanobelt, in which the competitive evolution to H-like exciton or excimer is found to be in strong relevance to the molecular packing and crystal size

Tunable Morphology of the Self-Assembled Organic Microcrystals for the Efficient Laser Optical Resonator by Molecular Modulation

Organic single-crystalline micro/nanostructures can effectively generate and carry photons due to their smooth morphologies, high photoluminescence quantum efficiency, and minimized defects density and therefore are potentially ideal building blocks for the optical circuits in the next generation of miniaturized optoelectronics. However, the tailor-made organic molecules can be generally obtained by organic synthesis, ensuring that the organic molecules aggregate in a specific form and generate micro/nanostructures with desirable morphology and therefore act as the efficient laser optical resonator remains a great challenge. Here, the molecular modulation of the morphology on the laser optical resonator properties has been investigated through the preparation of the elongated hexagonal microplates (PHMs) and the rectangular microplates (ORMs), respectively, from two model isomeric organic molecules of 1,4-bis­(4-methylstyryl)­benzene (<i>p</i>-MSB) and 1,4-bis­(2-methylstyryl)­benzene (<i>o</i>-MSB). Significantly, fluorescence resonance phenomenon was only observed in the individual ORM other than the PHM. It indicates that the rectangular resonators possess better light-confinement property over the elongated hexagonal resonators. More importantly, optically pumped lasing action was observed in the <i>o</i>-MSB rectangular morphology microplates resonator with a high <i>Q</i> ≈ 1500 above a threshold of ∼540 nJ/cm². The excellent optical properties of these microstructures are associated with the morphology, which can be precisely modulated by the organic molecular structure. These self-assembled organic microplates with different morphologies can contribute to the distinct functionality of photonics elements in the integrated optical circuits at micro/nanoscale

Epitaxial Self-assembly of Binary Molecular Components into Branched Nanowire Heterostructures for Photonic Applications

We report a sequential epitaxial growth to prepare organic branched nanowire heterostructures (BNwHs) consisting of a microribbon trunk of 1,4-dimethoxy-2,5-di­[4′-(cyano)­styryl]­benzene (COPV) with multiple nanowire branches of 2,4,5-triphenylimidazole (TPI) in a one-pot solution synthesis. The synthesis involves a seeded-growth process, where COPV microribbons are grown first as a trunk followed by a seeded-growth of TPI nanowire branches at the pregrown trunk surfaces. Selected area electron diffraction characterizations reveal that multiple hydrogen-bonding interactions between TPI and COPV components play an essential role in the epitaxial growth as a result of the structural matching between COPV and TPI crystals. A multichannel optical router was successfully realized on the basis of the passive waveguides of COPV green photoluminescence (PL) along TPI nanowire branches in a single organic BNwH

Exceptional Intersystem Crossing in Di(perylene bisimide)s: A Structural Platform toward Photosensitizers for Singlet Oxygen Generation

Photosensitized reactions of molecular oxygen have found far-reaching applications in various fields, and the development of new photosensitizer compounds is of crucial importance. We here describe a new class of triply linked bay-fused diperylene bisimides (DiPBIs) which exhibited several unique features, rendering them a new structural platform for the development of highly efficient and photostable photosensitizers. (i) The extended π-conjugation shifts its absorption into the body’s therapeutic window. (ii) The nonplanarity of the distorted cores enhances the spin−orbit coupled intersystem crossing. (iii) The long-lasting high-energy T₁ state facilitates singlet oxygen generation via energy-transfer reaction between T₁ and ground-state oxygen