Qikun Sun

Alkyl length effects on solid-state fluorescence and mechanochromic behavior of small organic luminophores

The introduction of alkyl chains to the periphery of the conjugated skeleton of organic chromophores is generally to improve their solubility. However, a number of investigations show that the length of alkyl chains could affect significantly the solid-state molecular packing modes and optical and electronic properties, and thereby alkyl chains could play functional roles in tuning and altering the solid-state aggregation behavior and optoelectronic properties. While there have been a variety of reports on the impact of alkyl chains on organic field-effect transistor and photovoltaic performances, recently increasing interest has been paid to how the alkyl length affects the emission and stimuli responsive properties of organic fluorescent materials. This review focuses on alkyl length effects on solid-state fluorescence and mechanochromic behaviors, and the purpose is to arouse ones further attention to the alkyl-length effect in the design, synthesis and structure–property investigation of organic optoelectronic materials.

2,6,9,10-Tetra(p-dibutylaminostyryl)anthracene as a multifunctional fluorescent cruciform dye

In this work, we have synthesized a tetradonor-capped anthracene-centered cruciform 2,6,9,10-tetra(p-dibutylaminostyryl)anthracene (TDC) and its optical properties are investigated. TDC is a light- and heat-stable dye and shows aggregation-enhanced emission (AEE), large and enhanced two-photon absorption cross sections (δ), fluorescence sensing selectively to transition metal ions such as Zn2+ and Cu2+, and appreciable piezofluorochromism (PFC), which is rarely found to exist simultaneously in cruciform or other configuration dyes. The AEE and PFC behaviours result from the existence of a strongly twisted 9,10-branch, and the large and enhanced δ is ascribed to the multiple conjugation pathways and introduction of donor end-groups. TDC possesses a non-centrosymmetric structure and a semi-disjoint Frontier molecular orbital, which could rationalize its fluorescence sensing selectively to Zn2+ and Cu2+. These findings indicate further that an anthracene ring is indeed a unique π-center that is different from other π-centers and can construct novel cruciforms with interesting optical properties.

9,10-Bis(N-alkylindole-3-yl-vinyl-2)anthracenes as a new series of alkyl length-dependent piezofluorochromic aggregation-induced emission homologues

The introduction and length-extending of peripheral alkyl chains is usually to improve the solubility of conjugated organic molecules. This article reports the synthesis and optical properties of 9,10-bis(N-alkylindole-3-yl-vinyl-2)anthracenes (IACn) with different alkyl lengths. These homologues exhibit the strong alkyl length-dependent piezofluorochromic (PFC) behaviour, and their PFC spectral shifts are in the range of 27–65 nm. It is found that the shorter the N-alkyl length, the larger the spectral shift upon mechanical grinding or pressing. Wide-angle X-ray diffraction evidenced that the mechanical grinding has resulted in destruction of pristine ordered structures, formation of crystal defects, and appearance of some amorphous states, which is responsible for the PFC behaviour.

Synthesis and remarkable mechano- and thermo-hypsochromic luminescence of a new type of DPP-based derivative

Owing to academic significance and potential applications, mechanofluorochromism has attracted much research interest. However, among the hundreds of mechanofluorochromic organic and metallorganic compounds reported to date, those with high contrast, multicolour and multichannel switching, and a remarkable mechanohypsochromic luminescence are rarely realized. This paper presents the synthesis and unusual mechano- and thermo-chromic fluorescence behaviours of a new 1,4-diketo-pyrrolo[3,4-c]pyrrole derivative (CODPP) with oxadiazole as the conjugation bridge and carbazole as the end donor. The as-prepared crystalline CODPP emits a near-infrared light (723 nm, NIR state) and can be mechanically ground into a red emitting state (614 nm, R state), rendering a large mechanofluorohypsochromic shift up to 110 nm. However, unlike most mechanofluorochromic materials, the ground R state could not revert to the pristine (NIR) state upon heat annealing; instead, a new yellow emitting state (557 nm, Y state) is formed, and grinding the Y state affords the R state. Moreover, both the Y and R states hardly change their colours upon solvent fuming but can form the NIR state only by recrystallization or reprecipitation. These changes are repeatable and are characterized and discussed based on IR, X-ray diffraction, DSC, and time-resolved fluorescence spectra. This remarkable mechano- and thermo-hypsochromic luminescence has enriched the types of stimuli responsive materials and enlarged the optical properties of DPP derivatives.

Improving the electroluminescence performance of donor–acceptor molecules by fine-tuning the torsion angle and distance between donor and acceptor moieties

The torsion angle and distance between the donor (D) and the acceptor (A) are two important factors in determining the photoluminescence and electroluminescence properties of twisted D–A type organic molecules. Here, two new D–A compounds, 2-(10-butyl-10H-phenothiazin-3-yl)-5-phenyl-1,3,4-oxadiazole (PO) and 2-(4-(10-butyl-10H-phenothiazin-3-yl)phenyl)-5-phenyl-1,3,4-oxadiazole (PPO), were designed and synthesized to tune the torsion angle and distance between D and A moieties, and their photophysical and electroluminescence properties were investigated. The D–A type molecule PO has a planar conformation, whereas the D–π–A type molecule PPO has a twisted conformation because of the insertion of the phenyl bridge between the donor and the acceptor. Therefore, the charge transfer (CT) of PPO is much stronger than that of PO, and the singlet exciton yield of PPO may be higher than that of PO. On the other hand, the introduction of a phenyl unit can also improve the photoluminescence quantum efficiency (doped film ΦPL ≈ 70%). As a result, the PPO-doped device showed better device performance than PO. The device based on PPO as an emitter exhibited stable and high luminous efficiency (15.2 cd A−1, corresponding to an external quantum efficiency of 5.4%), which is increased by about 1.05 fold as compared to the device based on PO as an emitter (luminous efficiency 7.4 cd A−1 and external quantum efficiency 2.9%).

Thionating iso-diketopyrrolopyrrole-based polymers: from p-type to ambipolar field effect transistors with enhanced charge mobility

In this paper, two donor–acceptor-type π-conjugated polymers (P1 and P2) with 4,4′-bis(2-ethylhexyl)-5,5′-bis(trimethyltin)dithieno[3,2-b:2′,3′-d]silole as the donor and iso-diketopyrrolopyrrole or iso-dithioketopyrrolopyrrole as the acceptor moiety were designed and synthesized via palladium-catalyzed Stille coupling, and their optical, electrochemical and organic field effect transistor (OFET) properties were investigated. P2 could be regarded as the simple thionated product of P1, however, only one atom difference has resulted in their significantly different optical and electronic properties. P2 exhibited an obviously bathochromic shift absorption compared to P1. Noticeably, P1 only showed a hole mobility of 0.09 cm2 V−1 s−1, while P2 exhibited hole and electron mobilities of up to 0.49 cm2 V−1 s−1 and 0.26 cm2 V−1 s−1, respectively, which indicated that the thionation could not only improve charge mobility but also give the polymer ambipolar transporting properties. Quantum chemical calculations and electrochemical analysis revealed that the thionation could further optimize the frontier molecular orbitals and facilitate the charge injection. This work demonstrated that iso-dithioketopyrrolopyrroles could be promising building blocks for high-performance organic field effect transistors.

A pair of conjoined donor–acceptor butterflies as promising solution-processable aggregation-enhanced emission FR/NIR EL emitters

Constructing a novel molecular architecture and creating far-red/near-infrared (FR/NIR) fluorophores with aggregation-enhanced emission (AEE) and solution film-forming ability is of particular interest but has been so far unreported. Here we linked two butterfly-shaped 3,7-bis(alkoxyphenylbenzothiadiazol)phenothiazines by an alkyl spacer to form a pair of conjoined donor–acceptor butterflies (SP6) as FR/NIR electroluminescent emitters. SP6 was found to be a multifunctional dye exhibiting AEE characteristics and reversible mechanofluorochromism, and significantly, a good quality film with strong deep-red emission could be formed by spin-coating the solution. The simplest solution-processed device (ITO/PEDOT:PSS/SP6/CsF/Al) emitted NIR light of 683 nm with a peak external quantum efficiency (EQE) of 0.57% and a CIE coordinate (0.68, 0.31). When the hole-transporting PVK layer was introduced, the still simple solution-processed device (ITO/PEDOT:PSS/PVK/SP6/CsF/Al) emitted FR light of 659 nm with a remarkably increased EQE of 1.82% and a CIE coordinate (0.65, 0.34). The low efficiency roll-offs and high radiative exciton ratios breaking through the theoretical limit signified that the rational modification and integration of the existing EL emitters could develop promising solution-processable EL materials having an inherent light-emitting mechanism.

N-Alkylcarbazoles: homolog manipulating long-lived room-temperature phosphorescence

The present pure organic room-temperature phosphorescence (RTP) materials are characterized by heterocyclic molecules containing heavy atoms and carbonyl or sulfonyl groups, and a corresponding great success has been achieved, but few efforts have been made to develop simple and ultralong pure organic RTP emitters without the above groups. Furthermore, homologs and isomers all have the most comparable molecular structure and are more favorable to both the optimization of material properties and the accumulation of structure–property relationships. In the current work, we design and synthesize a series of crystallizable N-alkylcarbazole homologs (Cn) to investigate their phosphorescence properties. It is found that these crystalline powders have ultralong phosphorescence lifetimes of up to 1.2–1.5 s at low temperature, but the RTP lifetimes of C1, C2, and C5 are greatly shortened to 0.4–0.6 s even under degas. In ambient air, the RTP lifetimes of C1, C2 and C5 are further decreased to 200, 23 and 12 ms, respectively, indicating the strong temperature and oxygen sensitivity. In contrast, C3 and C4 can still maintain long RTP lifetimes up to 0.81 and 0.49 s in air, respectively, affording excellent RTP emitters. Single crystal analyses and theoretical calculations reveal that the alkyl chain can affect molecular packing and intersystem crossing, and this work demonstrates that the homolog effect is a useful avenue in manipulating RTP properties and the ambient sensitivity.

Cyclic boron esterification: screening organic room temperature phosphorescent and mechanoluminescent materials

Aryl boronic acids are a broad class of organic intermediates. Here, we show that their cyclic-esterification with appropriate dihydric alcohols is a simple and effective approach for screening organic room-temperature phosphorescent (RTP) and mechanoluminescent (ML) materials. Non RTP and ML active 4-(carbazol-9-yl)phenylboronic acid can be made into long-lived RTP emitters with lifetimes of 264 and 430 ms and a bright ML dye with blue emission by cyclic-esterification with 1,3-propanediol, 2,2-dimethyl-1,3-propanediol, and glycol, respectively. Systematic single crystal analysis and quantum chemical calculations indicate that cyclic borates dominate the molecular packing structures and intermolecular interactions, which is responsible for the different optical properties.