Conjugated Carbazole-Based Schiff Bases as Photoinitiators: From Facile Synthesis to Efficient Two-Photon Polymerization

Exploration of simple and economical synthetic routes to construct active two-photon initiators (2PIs) with long conjugation length is highly desirable to promote the development of two-photon-induced photopolymerization technology. In this article, several conjugated carbazole-based derivatives containing N=C bonds as π bridges and electron donor/acceptor groups were prepared through one-step Schiff base reaction. The structure–activity relationship was systematically investigated through calculations and experimental tests. The results showed that compared to the D-π-D-π-D molecules, the A-π-D-π-A molecules exhibited stronger charge transfer, which induce redshifted linear absorption and enhanced two-photon absorption at 800 nm. The 2PIs displayed aggregation-induced emission property and show potential for the fabrication of luminescent microdevices. In two-photon-induced microfabrication tests, the formulations containing novel 2PIs exhibited lower threshold energy than the widely used commercial photoinitiator Irgacure 369 and the two-photon resists IP-L is the brand name of the two-photon resist from Nanoscribe, specially designed for nanoscribe's laser lithography systems. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 2692–2700     

香豆素肟酯类双光子引发剂的合成及性能研究

开发高效的双光子引发剂是提升双光子聚合速度的关键。本文基于光致脱羧机制,设计并合成了两种以共轭香豆素作为生色团、肟酯作为引发基团的双光子引发剂,并通过实验测试结合模拟计算对该类引发剂的光物理和光化学行为进行了研究。结果表明,该类引发剂在400~500 nm区域具有较强吸收,在LED可见光辐照下发生分解,具有光漂白特性,光解后释放的活性种可引发丙烯酸酯类单体聚合。利用双光子三维微纳成型技术,该类香豆素肟酯化合物可有效用于构建高分辨率的三维微纳结构。并通过量子化学计算,对该类引发剂的引发机理进行了探讨。     

1,3,4-噁二唑衍生物的双光子吸收和双光子泵浦荧光

依据“推电子基-共轭中心-拉电子基-共轭中心-推电子基”的模型将电荷传输型1,3,4-噁二唑环嵌入芳香共轭体中, 通过Wittig-Horner反应合成了2种对称型强双光子吸收和双光子诱导荧光分子2,5-二[4-(2-芳基乙烯基)苯基]-1,3,4-噁二唑. 它们的氯仿溶液在锁模Nd: YAG激光器800 nm激光照射下, 发射出很强的双光子上转换荧光, 其最强荧光分别在波长507和475 nm. 采用非线性透过率法测得其双光子吸收截面分别为1.07×10^－46和6.6×10^－47 cm⁴•s•photon^－1. 这2个对称型D-π-A-π-D生色分子从激发端基到π共轭桥的有效能量传输, 对双光子吸收和双光子荧光发射能力贡献较大.     

3D Stereolithography by Using Two-Photon Photopolymerization

Summary: This article describes the development and evolution of two-photon lithography of 3-dimensional (3D) polymeric structures through two-photon photopolymerization (TPP) and the materials used to facilitate the same. The first part elaborates on the advantages of TPP over conventional lithographic techniques used in the fabrication of 3D microelectromechanical systems (MEMS). The second part introduces the preparation of highly efficient organic two-photon absorbing materials based on the centro-symmetric π-conjugated system and their performance as sensitizers in two-photon polymerization.     

Synthesis of two carbazole-based dyes and application of two-photon initiating polymerization

Two carbazole-based polymerization initiators possessing blue fluorescence emission have been synthesized via Wittig reaction in the solid phase at room temperature. Two-photon excited fluorescence (TPEF) spectra for them were investigated under 800 nm fs laser pulse and two-photon absorption cross sections were determined by the Z-scan technique. Then two-photon initiating polymerization (TPIP) microfabrication experiments were successfully carried out. Three-dimensional lattice and artificial defects were gained, indicating that they were viable candidates for the two-photon polymerization initiator in practical application of microfabrication. Supported by the National Natural Science Foundation of China (Grant Nos. 50532030, 20771001 & 50703001), and Team for Scientific Innovation of Anhui Province (Grant No. 2006KJ007TD)     

不同类型二苯乙烯衍生物双光子吸收截面的理论研究

用AM1和INDO/CI理论方法,系统研究不同类型二苯乙烯衍生物(D-π-D,A-π-A,D-π-A)的结构和电子光谱。在正确的UV-vis光谱基础上,预测了双光子吸收峰的位置,用SOS公式计算了三阶非线性光学系数及双光子吸收截面,并从微观上探讨了不同取代基对双光子吸收截面的影响。     

Highly efficient multiphoton-absorbing quadrupolar oligomers for frequency upconversion

Two series of quadrupolar diphenylamino-endcapped oligofluorenes, PhN-OF(n)-NPh (n=2-5) and PhN-OF(n)-TAZ-OF(n)-NPh (n=1-4), which have an electron-withdrawing 1,2,4-triazole (TAZ) moiety as central core, with D-π-A-π-D structural motif (D=donor, A=acceptor), have been synthesized by 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 key step. On pumping with infrared femtosecond lasers, these oligomers showed very strong multiphoton-excited blue photoluminescence. These D-π-D and D-π-A-π-D quadrupolar oligofluorenes exhibit superior three-photon absorption properties compared to the respective D-π-A counterparts with a highest three-photon absorption cross-section (σ(3)) of up to 2.72×10(-77) cm(6) s(2) . Despite the comparable linear and multiphoton absorption properties of the two types of quadrupolar oligomers PhN-OF(n)-NPh and PhN-OF(n)-TAZ-OF(n)-NPh, only the former exhibit remarkably intense and highly efficient multiphoton-excited frequency-upconverted deep blue lasing, which gives rise to record high lasing efficiency of 0.097% and very narrow of full width at half-maximum of the lasing spectra. Our findings suggest that quadrupolar-type molecules/oligomers are superior for multiphoton excited frequency upconverted lasing to their dipolar counterparts and also provide important guidelines to design highly efficient three-photon absorption molecules for photoluminescence and lasing applications.     

Effect of alicyclic ring size on the photophysical and photochemical properties of bis(arylidene)cycloalkanone compounds

A series of bis(arylidene)cycloalkanone compounds based on cyclobutanone, cyclopentanone, cyclohexanone and cycloheptanone, C4-C7, respectively, with a D-π-A-π-D structure containing the same donor and acceptor but different alicyclic rings was prepared. The effects of alicyclic ring size on the photophysical, photochemical and electrochemical properties of these compounds were investigated systematically. We found that an increase of the number of carbons in the central alicyclic ring leads to changes in geometry, which has significant effects on the conjugation, and photophysical and photochemical properties. These effects include decreases in the fluorescence quantum yield, transient lifetimes, peak extinction coefficients, and the singlet oxygen quantum yield with the increase of the ring size. The one-photon absorption spectra, the two-photon absorption (2PA) spectra, and the fluorescence spectra all show a hypsochromic shift with increasing ring size. The results of this study provide guidance for the design of new cycloketone-based D-π-A-π-D 2PA compounds for photopolymerization and photodynamic therapy applications.     

Blue two-photon excited fluorescence of several D-π-D, A-π-A, and D-π-A compounds featuring dimesitylboryl acceptor

Several donor-π-donor (D-π-D), acceptor-π-acceptor (A-π-A), and donor-π-acceptor (D-π-A) types of organic compounds with fluorene as π bridge and dimesitylboryl group as electron acceptor, which show strong two-photon excited blue fluorescence, have been synthesized and structurally investigated. The symmetric A-π-A type of compound exhibits the shortest wavelength of two-photon excited fluorescence (TPEF) at λ_em=405 nm under the excitation of λ_ex=730 nm; the unsymmetric D-π-A type of compound with diphenylamino as donor exhibits the most intense TPEF at blue region (λ_em=484 nm) with a two-photon absorption cross-section of 425 GM under λ_ex=800 nm.     

A V-shaped cationic dye for nonlinear optical bioimaging

A symmetric cationic molecule with D-π-A(+)-π-D architecture was synthesized with high two-photon absorption cross-section (σ(2) ≈ 1140 GM). Application as a marker in fluorescence microscopy of living cells revealed its presence inside the cell staining vesicular shape organelles in the cytoplasm. Fluorescence lifetime imaging microscopy shows that it is also able to penetrate within the nucleus.     

Two-Photon Initiating Efficiency of a Ditopic Alkoxynitrostilbene Reacting through a Self-Regenerative Mechanism

The photophysical properties and the photoinitiating reactivity of a ditopic alkoxynitrostilbene were compared to those of its single branch chromophore used as a reference. Whereas a trivial additive effect is observed when considering the one- and two-photon absorption properties, a clear and very significant amplification has been highlighted for the photoreactivity of this free radical photoinitiator which was used as a hydrogen abstractor in presence of an aliphatic amine co-reactant. We indeed demonstrate that the proximity of two nitroaromatics moieties within the same molecular architecture gives rise to an original cycling mechanism based on a stepwise photo triggering of each photoredox center followed by a subsequent regenerative process. The combination of a high two-photon absorption cross-section (δ_780nm≈330 GM) with a strong enhancement in photoreactivity makes this nitrostilbene bichromophore a very suitable candidate for two-photon polymerization applications.     

Fluorene as the π-spacer for new two-photon absorption chromophores

We report herein the design and synthesis of two new quadrupolar D-π-A-π-D chromophores containing diphenyl amine and dicyanobenzene or 2,1,3-benzothiadiazole as electron donor (D) and acceptor (A), respectively, which are bridged by fluorene linkage (π). The introduction of coplanar fluorenes is highly beneficial for the enhancement of two-photon absorption (TPA), where 1b displays a TPA cross section (σ₂) of up to 1975±207 GM.     

Crystal structures,optical properties and theoretical calculation of novel two-photon polymerization initiators

Three novel A–π–D–π–A type compounds 3,6-bis[2-(4-pyridyl)ethenyl]-9-ethylcarbazole (1), 3,6-bis[2-(2-pyridyl)ethenyl]-9-ethylcarbazole (2), 3,6-bis[2-(4-pyridyl)ethenyl]-9-ethylpyridylcarbazole (3) were conveniently synthesized by Pd-catalyzed Heck coupling methodology and characterized by single crystal X-ray diffraction determination. One-photon fluorescence, one-photon fluorescence quantum yields, one-photon fluorescence lifetime, and two-photon fluorescence have been investigated. The calculated two-photon absorption cross-sections for the three initiators by quantum chemical method are as high as 947, 943 and 815 × 10⁻⁵⁰ cm⁴ s photon⁻¹, respectively. Two-photon initiating polymerization microfabrication experiments have been carried out and the possible polymerization mechanism was also discussed. The results show that they are good two-photon absorbing chromophores and effective two-photon photopolymerization initiators.     

双光子聚合技术在组织工程学方面的研究进展

细胞外基质(ECM)是分布在细胞表面或细胞之间的大分子,具有复杂的网络结构,通过与细胞的相互作用调节着细胞的生长、增殖和分化,然而截止到目前这种复杂的调节作用还没有被完全理解。新兴的组织工程学通过体外构筑ECM来培养组织或器官,能够系统、深入的研究细胞与ECM之间的相互作用,进而为再生医学的发展奠定基础。双光子聚合技术具有穿透性好、空间选择性高、对生物组织损伤小等特点,是体外构筑ECM的理想方案。本文介绍了双光子吸收和双光子聚合的基本原理,梳理了水溶性双光子聚合光敏剂的研究概况,综述了光聚合水凝胶材料的研究进展并对后续研究进行了展望。     

One-photon photophysics and two-photon absorption of 4-[9,9-di(2-ethylhexyl)-7-diphenylaminofluoren-2-yl]-2,2':6',2'-terpyridine and their platinum chloride complexes

The synthesis, one-photon photophysics and two-photon absorption (2PA) of three dipolar D-π-A 4-[9,9-di(2-ethylhexyl)-7-diphenylaminofluoren-2-yl]-2,2':6',2'-terpyridine and their platinum chloride complexes with different linkers between the donor and acceptor are reported. All ligands exhibit (1)π,π* transition in the UV and (1)π,π*/(1)ICT (intramolecular charge transfer) transition in the visible regions, while the complexes display a lower-energy (1)π,π*/(1)CT (charge transfer) transition in the visible region in addition to the high-energy (1)π,π* transitions. All ligands and the complexes are emissive at room temperature and 77 K, with the emitting excited state assigned as the mixed (1)π,π* and (1)CT states at RT. Transient absorption from the ligands and the complexes were observed. 2PA was investigated for all ligands and complexes. The two-photon absorption cross-sections (σ(2)) of the complexes (600-2000 GM) measured by Z-scan experiment are much larger than those of their corresponding ligands measured by the two-photon induced fluorescence method. The ligand and the complex with the ethynylene linker show much stronger 2PA than those with the vinylene linker.     

Multipolar symmetric squaraines with large two-photon absorption cross-sections in the NIR region

The two-photon absorption (TPA) properties of two extended symmetric squaraine dyes are thoroughly characterized from the experimental and quantum-chemical point of view. The two molecules are specially engineered to have a multipolar structure with increasing complexity, D-π-A-π-D and A'-π-D-π-A-π-D-π-A', respectively. The experimental TPA spectra, measured by means of the Z-scan technique in the femtoseconds regime, display considerably high values of TPA cross sections (σ(TPA)) for both molecules. In particular, the squaraine with the more extended structure shows the highest value of σ(TPA) ever reported for this class of molecules. CIS and TDDFT calculations of the one and two-photon absorption properties are carried out to clarify the origin of the observed TPA properties and fully characterize the electronic properties of these compounds. The calculations, in good agreement with the experimental data, suggest that the origin of this exceptionally high σ(TPA) can be ascribed to the presence of a peripheral A' group, that increases the density of excited states involved in the TPA process.     

Photo‐aligning of polyimide layers for liquid crystals

A series of soluble and highly transparent semi‐alicyclic polyimides (PIs) with designed flexible linkages have been synthesized derived from an alicyclic aromatic dianhydride (1,2,3,4‐cyclobutanetetracarboxylic dianhydride, CBDA) and various aromatic ether‐bridged diamines. The semi‐alicyclic PIs were evaluated as the photo‐alignment layers of liquid crystal (LC) molecules in liquid crystal display (LCD). Experimental results indicate that the photo‐alignment characteristics of LC molecules induced by the photo‐aligned PI layers and the electro‐optical (EO) properties of the LC cell devices are closely related with PI backbone structures. The retardation of the photo‐aligned PI layers is correlated with the ultraviolet (UV) absorption intensity of PI at 220 to approximately 330 nm. The higher UV absorption intensity PI has, the higher retardation and lower pre‐tilt angle the photo‐aligned PI layer exhibits. The defect‐free and photo‐aligned PI layer could result into the uniform LC texture, which is highly desired for in‐plane switching (IPS) mode LCD devices. In comparison, PI layer containing trifluoromethyl moiety shows poor photo‐aligning performance because of the strong electronic withdrawing effect of the fluorinated linkage.     

Two-photon absorption properties of two-dimensional π-conjugated chromophores: combined experimental and theoretical study

The two-photon absorption (TPA) properties of four TPEB [tetrakis(phenylethynyl)benzene] derivatives (TD, para, ortho, and meta) with different donor/acceptor substitution patterns have been investigated experimentally by the femtosecond open-aperture Z-scan method and theoretically by the time-dependent density-functional theory (TDDFT) method. The four compounds show relatively large TPA cross sections, and the all-donor substituted species (TD) displays the largest TPA cross-section σ(2) = 520 ± 30 GM. On the basis of the calculated electronic structure, TD shows no TPA band in the lower energy region of the spectrum because the transition density is concentrated on particular transitions due to the high symmetry of the molecular structure. The centrosymmetric donor-acceptor TPEB para shows excitations resulting from transitions centered on D-π-D and A-π-A moieties, as well as transition between the D-π-D and A-π-A moieties; this accounts for the broad nature of the TPA bands for this compound. Calculations for two noncentrosymmetric TPEBs (ortho and meta) reveal that the diminished TPA intensities of higher-energy bands result from destructive interference between the dipolar and three-state terms. The molecular orbitals (MOs) of the TPEBs are derivable with linear combinations of the MOs of the two crossing BPEB [bis(phenylethynyl)benzene] derivatives. Overall, the characteristics of the experimental spectra are well-described based on the theoretical analysis.     

新型香豆素酮类不对称双光子染料的合成及其光物理性质

本文以二苯乙烯和香豆素为共轭桥,二乙氨基为电子给体,羰基为电子受体,合成了一个具有D-π₁-A-π₂-D结构的香豆素酮类双光子染料C3.用紫外-可见光谱、荧光光谱研究了该化合物的光物理性质.发现在光作用下C3很容易发生分子内电荷转移,进而转变为扭曲的分子内电荷转移,产生很大的偶极矩变化.以飞秒脉冲激光为激发光源,用上转换荧光法测定了其双光子吸收截面.在激发波长为850 nm时,新化合物的双光子吸收截面值达1292 GM,比同系列香豆素酮衍生物C1、C2的双光子吸收截面值高一到两个数量级.     

Synthesis and electrochromic properties of polyimides with pendent benzimidazole and triphenylamine units

Five novel near-infrared electrochromic aromatic polyimides(PIs) with pendent benzimidazole group were synthesized from 4,4'-diamino-4'-(1-benzylbenzimidazol-2-yl)triphenylamine(named as DBBT) with five different dianhydrides via two-step polymerization process, respectively. The maximum UV-Vis absorption bands of these PIs locate at about 335 nm for solid films due to the π-π* transitions. A reversible pair of distinct redox peaks, that were associated with a noticeable color change from original yellow to blue, was observed in the cyclic voltammetry(CV) test. A new absorption peak emerged at 847 nm in near-infrared(NIR) region with increasing voltage in UV-Vis-NIR spectrum, which indicates that PI can be used as NIR electrochromic material. These novel PIs have good electrochemical stability, appropriate energy levels for the highest occupied molecular orbital(HOMO) and the lowest unoccupied molecular orbital(LUMO), in the range of-5.17 e V to-5.20 e V and-2.14 e V to-2.26 e V(versus the vacuum level) determined by cyclic voltammetry method. These values basically consisted with the results of quantum chemical calculation. These polyimides can be used as novel electrochromic and hole transportation materials.