Haichang Zhang

Unusual mechanohypsochromic luminescence and unique bidirectional thermofluorochromism of long-alkylated simple DPP dyes

There have been hundreds of organic and metallorganic compounds found to exhibit mechanobathochromic luminescence within tens of nanometers. This paper presents the first example of unusual mechanohypsochromic luminescence and unique bidirectional thermofluorochromism based on 1,4-diketo-2,5-di(tetradecyl)-3,6-bis-(p-chlorophenyl)pyrrolo[3,4-c]pyrrole (TDPP). Pristine TDPP powder is a red emitting crystal (645 nm, R state) and can be changed into a yellow amorphous state (544 nm, Y state) by mechanical grinding or solution spin-coating. The Y state could revert to the R state upon annealing below 60 °C or solvent fuming, however, annealing the Y state above 70 °C produced a new green-emitting crystalline state (523 nm, G state). Furthermore, the G state can transform to the Y and R states upon grinding and solvent fuming, respectively. In contrast, those novel phenomena were not observed in the butylated DPP, demonstrating that the peripheral alkyl chain lengths of organic fluorophores can play diverse functional roles in determining the solid-state aggregation behavior and fluorescence properties. This work has improved some types of stimuli responsive materials and new optical properties of alkylated DPPs have been developed.

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.

Phenothiazin-N-yl-capped 1,4-diketo-3,6-diphenylpyrrolo[3,4-c]pyrrole exhibiting strong two-photon absorption and aggregation-enhanced one- and two-photon excitation red fluorescence

Water-soluble or water-dispersible two-photon dyes with strong fluorescence and large two-photon absorption (2PA) cross sections are very desirable for nano- and bio-photonics, but are still scarce. Herein, a new 1,4-diketo-3,6-diphenylpyrrolo[3,4-c]pyrrole derivative end-capped with two phenothiazin-N-yl donors (PDPP) is synthesized and its one- and two-photon optical properties are investigated in both solution and aqueous dispersions. The results show that PDPP exhibits solvent-independent one-photon solution absorption and emission properties with low fluorescence quantum yields, but strong red fluorescence in both aqueous dispersion and the crystalline state. This aggregation-enhanced emission (AEE) effect enables PDPP to have not only large 2PA cross sections but also a remarkably enhanced two-photon fluorescence action cross section in aqueous dispersions. Thus, 2PA-active PDPP is a promising candidate for two-photon science and technology.

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.

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.