化学学报  2016, Vol. 74 Issue (4): 351-355   PDF    
取代基效应对喹啉腈AIE荧光性能的研究
夏志清 , 邵安东 , 李强 , 朱世琴 , 朱为宏     
华东理工大学精细化工研究所 科技信息研究所 上海 200237
基金项目：项目受国家自然科学基金(No. 21325625, 21476076)资助.
*通讯作者：朱为宏,E-mail: whzhu@ecust.edu.cn; Tel.: 021-64250772
摘要：吡喃腈(DCM)类化合物作为传统的荧光染料, 其本身具有聚集荧光淬灭(Aggregation caused quenching, ACQ)的现象. 通过对吡喃腈母体进行结构修饰, 合成了一类具有长波长、聚集诱导发光(Aggregation-induced emission, AIE)的喹啉腈(QM)衍生物, 该类化合物在良溶剂中呈现弱荧光, 但在聚集态和固态时荧光增强. 并进一步研究了取代基效应对这类AIE化合物荧光性能的影响, 卤素原子的引入使得聚集态荧光基本保持在561~571 nm之间, 但荧光量子效率变化很大, QM-Br的ΦF值最大(13.9%), QM-F次之(8.7%), QM-I最小(3.4%). 给电子单元可与QM母体单元形成D-π-A结构, 其增强的推拉电子效应延长了聚集态波长, 同时能级带隙变窄. 取代基效应的研究有助于理解AIE化合物聚集微环境的变化, 为进一步发展近红外AIE荧光染料及其生物应用打下良好的基础.
关键词：聚集诱导发光    喹啉腈    取代基效应    荧光量子效率    
Substituent Effect on Quinoline-Malononitrile AIE Fluorescent Properties
Xia Zhiqing , Shao Andong , Li Qiang , Zhu Shiqin , Zhu Weihong     
Institute of Fine Chemicals, Institute of Science and Technology Information, East China University of Science & Technology, Shanghai 200237
Received: January 3, 2016; published: February 29, 2016
Project supported by the National Natural Science Foundation of China (Nos. 21325625, 21476076).
Abstract: As well-known, traditional luminescent dyes such as dicyanomethylene-4H-pyran (DCM) luminogens used in biological diagnosis and therapy still exit several limitations due to their inherent molecular structures. One of the most notorious phenomena is "aggregation caused quenching" (ACQ), namely that the fluorescence can be easily observed in dilute solution, but quenched in high concentration or aggregated state. Therefore, how to understand the aggregation environment formed by dye molecules and further utilize the aggregate itself as a potential pattern for biomedical application is highly desirable. Since the intriguing discovery of the aggregation-induced emission (AIE) phenomenon, much effort has been paid to exploration of AIE systems and their applications. These AIE chromophores exhibit highly bright fluorescence when aggregated, and weak fluorescence when dissolved in solution, making them beneficial for improving the sensitivity of biosensors and bioimaging in situ or in vivo. Herein we set out to construct a novel AIE-active quinoline-malononitrile (QM) building block, by merely replacing the oxygen atom in DCM moiety with N-ethyl group, thoroughly solving the fluorescence quenching problems of DCM derivatives in aggregation. Five QM derivatives (QM-H, QM-F, QM-Br, QM-I and QM-N) with different substituent groups have been successfully synthesized by Knoevenagel reaction, extending the AIE wavelength from 528 to 614 nm in the aggregated state. A series of experiments were performed to examine the photoluminescence properties of QM-H, QM-F, QM-Br, QM-I and QM-N. As expected, all these AIE-active compounds show weak or no fluorescence in molecular state when dissolved in THF solution, but enhanced emission in solid or aggregate state along with an increasing volume fraction of water in tetrahydrofuran/water (THF/H2O) mixtures. Moreover, their AIE-active fluorescent properties are dependent upon the different aggregated microenvironment affected by substituent groups of QM derivatives. Notably, the halogen atoms of QM-F, QM-Br and QM-I play important role in AIE quantum yield, while introducing electron donor group shifts the solid fluorescence of QM-N into red emission. The substituent effect of QM derivatives with excellent AIE properties can provide a platform to develop NIR AIE materials.
Key Words: aggregation-induced emission    quinoline-malononitrile    substituent effect    luminescent quantum yield    
1 引言

吡喃腈(DCM)类衍生物是一类重要的长波长发光材料,属于典型的分子内电荷转移发光,主要应用于有机掺杂的发光器件[1]. 但作为经典的给体-π-受体(D-π-A)特征及准平面构型,DCM 体系在固态或聚集态时由于π-π堆积而发生荧光淬灭现象(aggregation-caused quenching,ACQ)[2],极大地限制了其在高效的固态发光器件方面的应用. 为了防止DCM发光材料的分子间聚集,常采用在荧光团上修饰长碳链或将其掺杂到聚合物基质等方法提高发光效率[3]. 然而这些方法常会导致如合成复杂、影响电荷传输、器件工艺复杂等问题,故而如何结合荧光团在高浓度时聚集这一本质特征来发展一类新型的固态发光材料显得尤为重要.

自2001年以来,由唐本忠教授[4]首次提出的聚集诱导发光(aggregation-induced emission,AIE)现象得到了广泛的关注. 该类化合物在稀溶液单分子态时呈现弱的荧光,但在聚集态或固态下,荧光反而增强. AIE类化合物的发现很好地解决了传统染料的聚集荧光淬灭的难题,为设计高效固体发光器件及生物传感器等[5～13]提供了一个崭新的思路. 常见的AIE荧光团有四苯乙烯(TPE)、silole、氰基苯乙烯等,但这些AIE化合物的固体发射波长大多为蓝绿色荧光,不利于红光固体器件及近红外生物成像的应用[13d]. 从而,发展长波长的AIE荧光团,并研究分子结构与构象对聚集微环境的影响还需要进一步的探索.

最近我们课题组巧妙地通过分子修饰,采用N-乙基取代苯并吡喃腈中的氧原子,成功地构建了新型聚集诱导荧光增强的AIE母体——喹啉腈(quinoline- malononitrile,QM),实现了AIE荧光向红光及近红外的延伸[14～16]. 本工作以喹啉腈为母体,设计合成了一系列不同取代基的AIE化合物QM-H,QM-F,QM-Br,QM-I和QM-N (Eq. 1). 该类化合物均具有良好的聚集诱导荧光增强效应,即在THF溶液中基本没有荧光,当缓慢增加含水量形成聚集态时,荧光明显增强. 重点研究了不同取代基单元对AIE波长、荧光量子产率的影响,研究表明卤素原子F,Br,I的引入使得聚集态荧光基本保持在561～571 nm之间,但荧光量子效率的变化很大,QM-Br的ΦF值最大(13.9%),QM-F次之(8.7%),QM-I最小(3.4%). 给电子单元的引入可与QM母体单元形成D-π-A结构,其增强的推拉电子效应延长了聚集态波长,同时能级带隙变窄. 因此,对AIE分子结构中取代基效应的研究有助于理解其聚集微环境的变化,为进一步发展近红外AIE荧光染料及其生物应用打下良好的基础.

(1)
2 结果与讨论
2.1 QM化合物的合成表征

如Eq. 1所示,化合物QM-H,QM-F,QM-Br,QM-I和QM-N均是通过喹啉腈母体与对应的芳香醛在乙腈溶剂中,以哌啶作为催化剂,进行Knoevenagel缩合反应得到,合成简捷,便于放大合成. 其中不同取代基的芳香醛与QM母体的反应活性不同,具有给电子效应的N,N-二甲氨基苯甲醛反应活性最高. 其化学结构通过1H NMR,13C NMR以及HRMS均证明了化合物结构的正确性. 在1H NMR图中,在高场化学位移δ 1.56～1.57处有一三重峰,对应为喹啉腈母体上NCH2CH3的甲基峰,在δ 4.38～4.40处为对应NCH2CH3的亚甲基四重峰; 而在低场化学位移处QM分子的乙烯氢偶合常数为16.0 Hz,表明该类AIE活性的QM衍生物中的桥连双键均为反式结构.

2.2 QM化合物的AIE性能

为了测试QM-H,QM-F,QM-Br,QM-I和QM-N的AIE性能,以四氢呋喃/水作为混合溶剂体系来研究这五个化合物在不同水含量时的荧光光谱变化. 如图 1所示,所有化合物在随着水含量增加时,均具有明显的聚集态荧光增强性能. 在纯四氢呋喃溶剂中,QM-H,QM-F,QM-Br和QM-I以分子状态存在,基本没有荧光. 当水含量增加至70%～80%时,荧光强度迅速增强并出现了一个很宽的发射峰,表明聚集态的形成使得分子旋转或振动受阻,激发态能量则以荧光的形式释放出来. 当水含量为90%时,荧光强度达到最高. QM-N的荧光光谱表现出与其他化合物不同的性质(图 1E),在水含量为0%～70%时,荧光强度逐渐增加,波长从594 nm 红移至614 nm. 这种荧光强度随着水含量持续增强的性质可能归因于QM-N具有类似于聚合物的自组装特 性[17],易于形成良好的晶型态,荧光增强. 当水含量达到80%时,荧光强度下降并发生蓝移,表明在含水量高时,QM-N由晶型态转变为无定型态(如图S1所示),荧光强度降低[18].

图 1 (A～E) THF/水体系下,QM-H,QM-F,QM-Br,QM-I和QM-N (10-5 mol•L-1)在不同水含量下的荧光光谱(插图为随水含量变化时的相对荧光强度变化及荧光变化图片)和(F)在溶液聚集态下QM-H (fw=90%),QM-F (fw=90%),QM-Br (fw=90%),QM-I (fw=90%)及QM-N (fw=70%)的最大荧光波长归一化分布图 Fig. 1. (A～E) Photoluminescence (PL) spectra of QM-H,QM-F,QM-Br,QM-I and QM-N (10-5mol•L-1) in H2O/THF mixtures with different volume fractions of water (λex=430 nm) (Insets show a plot of relative PL intensity against water content (fw) and fluorescence images of QM-H (0 and 90% H2O),QM-F (0 and 90% H2O),QM-Br (0 and 90% H2O),QM-I (0 and 90% H2O),QM-N (0 and 70% H2O) under 365 nm illumination) and (F) normalized photoluminescence (PL) spectra of QM-H (fw=90%),QM-F (fw=90%),QM-Br (fw=90%),QM-I (fw=90%),QM-N (fw=70%) in H2O/THF mixtures

从图 1F中可以看出,在溶液聚集态下,随着与QM的连接基团中不同取代基的变化,AIE分子的聚集态荧光波长发生了74 nm的红移(从QM-H到QM-N的发射波长差值). 当苯环与QM相连时,QM-H在fw=90%中荧光出现了两个发射峰(分别为528和568 nm),荧光量子产率为6.7%; 苯环对位被卤素取代后,QM-F,QM-Br和QM-I在fw=90%中的聚集态荧光基本保持在561～571 nm之间,然而其量子产率变化较大(表 1),QM-Br的ΦF值最大(13.9%),QM-F次之(8.7%),QM-I最小(3.4%). 卤素原子F,Br,I的原子半径大小和吸电子性强弱可能是对AIE化合物的荧光波长及量子效率产生了较大的影响的主要原因,尤其是I原子半径最大,可能阻碍了AIE分子间的聚集,荧光降低. 进一步引进强的给电子基团N,N-二甲氨基,QM-N的最大荧光波长达到614 nm,且产生了自组装性能. 由于QM-N具有D-π-A共轭分子结构,所以聚集态荧光红移的同时能量带隙变窄.

表 1 QM化合物的光学数据a Table 1Optical data of QM compounds
2.3 QM化合物的AIE固体荧光性能

化合物QM-H,QM-F,QM-Br,QM-I和QM-N在粉末状态下均具有良好的固态发光效应,采用积分球测量其固体荧光量子效率分别为14.5%,9.7%,15.1%,5.3%和5.2%. 相应地,如图 2所示,QM-H的固态最大发射波长为584 nm,当引入卤素作为取代单元时,QM-F固体荧光波长蓝移至576 nm. QM-Br和QM-I的固态波长基本保持在594 nm左右,然而量子效率变化较大,QM-B r的ΦF值是QM-I的3倍. 给电子基团N,N-二甲氨基使QM-N的给电子性能最强,故波长红移至605 nm,同时其带隙能量变窄,这与溶液聚集态的荧光现象一致.

图 2 QM系列化合物固体粉末荧光光谱[插图为QM化合物在365 nm紫外灯照射下的固体荧光图片(从左到右依次为: QM-H,QM-F,QM-Br,QM-I和QM-N)] Fig. 2. Normalized solid fluorescence spectra of AIE-active QM derivatives [Inset shows solid fluorescence images of QM derivatives under 365 nm illumination (from left to right: QM-H,QM-F,QM-Br,QM-I and QM-N)]
3 结论

以喹啉腈QM为AIE母体,设计合成了五个聚集诱导荧光增强化合物QM-H,QM-F,QM-Br,QM-I和QM-N,并通过1H NMR,13C NMR以及HRMS确认了结构的正确性. 该类化合物在THF溶液中以分子状态存在时基本没有荧光,当缓慢增加含水量形成聚集态时,荧光明显增强. 卤素原子的引入使得QM-F,QM-Br和QM-I的聚集态荧光基本保持在561～571 nm之间,但荧光量子效率变化很大,QM-Br最大(13.9%),QM-F次之(8.7%),QM-I最小(3.4%). 引入N(CH3)2单元可增强给体的给电子性,延伸QM-N的AIE波长至红光区域,同时能量带隙变窄. 该类聚集诱导发光的QM衍生物所表现出的取代基效应有助于理解AIE分子结构及构象对聚集微环境的影响,可进一步用于发展近红外聚集诱导荧光材料.

4 实验部分
4.1 试剂及仪器

1H 和13CNMR采用Brucker AV-400型核磁共振波谱仪,以TMS作为内标,在25 ℃下测定; 吸收光谱的测试采用Varian Cary 100紫外-可见吸收光谱仪; 荧光光谱采用Varian Cary Eclipse荧光光谱仪; 质谱采用Waters LCT Premier XE飞行时间质谱仪; 实验中使用的试剂和药品均为市售分析纯和化学纯.

4.2 QM化合物的合成

化合物QM-H,QM-F,QM-Br,QM-I和QM-N 的合成路线如Eq. 1所示.

4.2.1 苯乙烯基喹啉腈衍生物的合成

在200 mL单口烧瓶中,分别加入4-二氰亚甲基-2-甲基喹啉菁(1.00 g,4.25 mmol)、乙腈(50 mL)、取代的苯甲醛(4.72 mmol)及哌啶(1.0 mL),在氩气保护下,110 ℃下反应10 h. 除去溶剂乙腈,硅胶柱层析分离(二氯甲烷/石油醚,V:V=2～5:1).

(E)-2-(2-苯乙烯基)-1-乙基喹啉-4(1H)-亚基)丙二腈(QM-H): 橘黄色固体,758 mg,产率55%. 1H NMR (400 MHz,CDCl3) δ: 1.57 (t,J=8.0 Hz,3H),4.39 (q,J=8.0 Hz,2H),7.09 (d,J=16.0 Hz,1H),7.15 (s,1H),7.34 (d,J=16.0 Hz,1H),7.43 (d,J=8.0 Hz,2H),7.47 (t,J=8.0 Hz,1H),7.58 (d,J=8.0 Hz,2H),7.62 (d,J=8.0 Hz,1H),7.78 (t,J=8.0 Hz,1H),9.11 (d,J=8.0 Hz,1H); 13C NMR (100 MHz,CDCl3) δ: 13.98,43.97,52.19,107.99,115.94,118.95,119.96,120.11,121.57,124.80,127.14,129.07,133.38,134.29,138.06,138.32,139.09,147.34,153.63; HRMS (TOF-ESI+) calcd for C22H17N3 324.1501 [M+ H]+,found 324.1498.

(E)-2-(2-(4-氟苯乙烯基)-1-乙基喹啉-4(1H)-亚基)丙二腈(QM-F): 橙黄色固体,523 mg,产率为36%. 1H NMR (400 MHz,CDCl3) δ: 1.57 (t,J=8.0 Hz,3H),4.39 (q,J=8.0 Hz,2H),6.99 (d,J=16.0 Hz,1H),7.14 (d,J=8.0 Hz,2H),7.16 (s,1H),7.30 (d,J=16.0 Hz,1H),7.47 (t,J=8.0 Hz,1H),7.56 (d,J=8.0 Hz,2H),7.62 (d,J=8.0 Hz,1H),7.78 (t,J=8.0 Hz,1H),9.15 (d,J=8.0 Hz,1H); 13C NMR (100 MHz,CDCl3) δ: 13.98,43.99,51.83,107.90,115.99,116.19,116.41,119.16,121.57,124.75,127.09,129.45,129.53,131.07,133.34,138.06,139.01,147.60,153.62; HRMS (TOF-ESI+) calcd for C22H16FN3 342.1407 [M+H]+,found 342.1411.

(E)-2-(2-(4-溴苯乙烯基)-1-乙基喹啉-4(1H)-亚基)丙二腈(QM-Br): 橙黄色固体,735 mg,产率43%. 1H NMR (400 MHz,CDCl3) δ: 1.57 (t,J=8.0 Hz,3H),4.38 (q,J=8.0 Hz,2H),7.06 (d,J=16.0 Hz,1H),7.12 (s,1H),7.26 (d,J=16.0 Hz,1H),7.43 (d,J=8.0 Hz,2H),7.47 (t,J=8.0 Hz,1H),7.58 (d,J=8.0 Hz,2H),7.62 (d,J=8.0 Hz,1H),7.77 (t,J=8.0 Hz,1H),9.13 (d,J=8.0 Hz,1H); 13C NMR (100 MHz,CDCl3) δ: 13.99,44.06,51.86,107.94,116.06,120.10,124.78,127.00,129.03,132.35,133.40,133.73,138.04,138.93,147.43,153.53; HRMS (TOF- ESI+) calcd for C22H16BrN3 402.0606 [M+H]+,found 402.0609.

(E)-2-(2-(4-碘苯乙烯基)-1-乙基喹啉-4(1H)-亚基)丙二腈(QM-I): 橙黄色固体,574 mg,产率30%. 1H NMR (400 MHz,CDCl3) δ: 1.56 (t,J=8.0 Hz,3H),4.40 (q,J=8.0 Hz,2H),7.11 (d,J=16.0 Hz,1H),7.13 (s,1H),7.30 (d,J=16.0 Hz,1H),7.45 (d,J=8.0 Hz,2H),7.49 (t,J=8.0 Hz,1H),7.61 (d,J=8.0 Hz,2H),7.65 (d,J=8.0 Hz,1H),7.79 (t,J=8.0 Hz,1H),9.11 (d,J=8.0 Hz,1H); 13C NMR (100 MHz,CDCl3) δ: 13.97,43.99,51.66,107.91,116.02,119.07,119.42,120.15,121.57,124.71,127.06,127.65,129.14,130.16,133.31,134.84,138.08,140.33,147.81,153.60; HRMS (TOF-ESI+) calcd for C22H16IN3 450.0467 [M+H]+,found 450.0475.

(E)-2-(2-(4-二甲氨基苯乙烯基)-1-乙基喹啉- 4(1H)-亚基)丙二腈(QM-N): 橘红色固体,1.01 g,产率65%. 1H NMR (400 MHz,CDCl3)δ: 1.56 (t,J=7.2 Hz,3H,CH2CH3),3.04 (s,6H,CH3),4.38 (q,J=7.2 Hz,2H,CH2),6.70 (d,J=8.8 Hz,2H,phenyl-H),6.80 (d,J=16.0 Hz,1H,alkene-H),7.15 (s,1H,quinoline-H),7.29 (d,J=16.0 Hz,1H,alkene-H),7.40～7.46 (m,3H,phenyl-H),7.60 (d,J=8.8 Hz,1H,phenyl-H),7.73 (t,J=8.0 Hz,1H,phenyl-H),9.11 (d,J=8.4 Hz,1H,phenyl-H); 13C NMR (100 MHz,CDCl3) δ: 3.94,40.16,43.77,49.72,107.00,111.95,112.27,113.25,116.00,119.70,120.80,121.60,122.69,124.37,126.93,129.37,129.54,132.93,138.21,141.02,148.85,151.65,153.12; HRMS (TOF-ESI+) calcd for C24H22N4 367.1923 [M+H]+,found 367.1929.

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