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Effect of pressure on structure and fluorescence of phthalocyanine

Zhu Lu-Yao Wang Peng Zhai Chun-Guang Hu Kuo Yao Ming-Guang Liu Bing-Bing

Effect of pressure on structure and fluorescence of phthalocyanine

Zhu Lu-Yao, Wang Peng, Zhai Chun-Guang, Hu Kuo, Yao Ming-Guang, Liu Bing-Bing
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  • Phthalocyanine (Pc) is a kind of important photoelectric material, but a lot of questions remain to be clarified, like the relationship between the structure of Pc and its photoelectric property. High pressure is a powerful tool to study the structure transformation. High pressure study on piezochromic materials, which shows color change under high pressure due to structure changing, serves as an effectively way of studying the relationship between materials’ structure and photoelectric property. In this work Raman spectrum is employed to study the phase change of α phase metal-free Pc (α-H2Pc) under high pressure, meanwhile the effect of pressure on fluorescence (FL) is also studied to show how the Pc’s structure affects the photoelectric property. The diamond anvil cell is employed to achieve the high pressure condition, by using NaCl as a pressure transmitting medium. And Raman and FL measurements are performed by using a LabRam HR Evolution spectrometer equipped with a 473 nm laser. The Raman spectra of α-H2Pc show to slightly change during compression to 12.0 GPa. The main Raman peaks remain at highest pressure, including the Raman peak from macrocyclic of Pc molecules, which shows the stability of Pc molecules. Note that an enhancement of Raman peak at 623 cm–1 can be found with the pressure increasing, which appears only in the Raman spectrum of χ phase metal-free Pc (χ-H2Pc), showing that α-H2Pc is converted into χ-H2Pc under pressure. The curve for Raman frequency as a function of pressure shows that no obvious evidence related to bonding or structure transition can be observed, which means that α-H2Pc is transformed into χ-H2Pc gradually. For FL spectrum, only the FL of excimer can be found in α-H2Pc at atmosphere pressure. When the solid α-H2Pc is compressed, the FL intensity is found to decrease as pressure increases, and it is quenched at 3.0 GPa. The FL of Pc molecule, which is not found in α-H2Pc at ambient pressure, appears at 0.7 GPa. As the pressure increases, the FL intensity ratio between Pc molecule and excimer is enhanced. Considering the pressure induced phase transition from α-H2Pc to χ-H2Pc gradually, the change in FL spectrum should be due to the structure transformation. It is proved that the degree of overlapping between Pc molecules in α-H2Pc is larger than that in χ-H2Pc. We think, the degree of overlapping decreases under high pressure, which hinders the formation of excimer. It makes the excimer emission decrease and the FL of Pc molecules appear under high pressure. Our work can explain the relationship between Pc crystal structure and its fluorescence, reveals the kinetic behavior of macromolecules similar to Pc system under high pressure, and provides a new possibility of designing the photoelectric materials with excellent performances.
      Corresponding author: Wang Peng, wangpengtrrs@jlu.edu.cn ; Liu Bing-Bing, liubb@jlu.edu.cn
    [1]

    Zugenmaier P, Bluhm T L, Deslandes Y, Orts W J, Hamer G K 1997 J. Mater. Sci. 32 5561

    [2]

    Sharma V B, Jain S L, Sain B 2003 Tetrahedron Lett. 44 383

    [3]

    Janczak J, Kubiak R 1992 J. Alloys Compd. 190 121

    [4]

    Kubiak R, Janczak J 1992 J. Alloys Compd. 190 117

    [5]

    Hammond R B, Roberts K J, Docherty R, Edmondson M, Gairns R 1996 J. Chem. Soc. Perkin Trans. 28 1527

    [6]

    Takano S, Enokida T, Kakuta A, Mori Y 1984 Chem. Lett. 13 2037

    [7]

    李战强, 李祥高, 李健, 胡雅琴 2013 有机化学 33 891

    Li Z Q, Li G Q, Li J, Hu Y Q 2013 Chin. J. Org. Chem. 33 891

    [8]

    Menzel E R, Jordan K J 1978 Chem. Phys. 32 223

    [9]

    Dong Y J, Xu B, Zhang J B, Tan X, Wang L J, Chen J L, Lv H G, Wen S P, Li B, Ye L, Zou B, Tian W J 2012 Angew. Chem. 124 10940

    [10]

    Meng X, Qi G Y, Zhang C, Wang K, Zou B, Ma Y G 2015 Chem. Commun. 51 9320

    [11]

    Yuan H S, Wang K, Yang K, Liu B B, Zou B 2014 J. Phys. Chem. Lett. 5 2968

    [12]

    郭宏伟, 刘然, 王玲瑞, 崔金星, 宋波, 王凯, 刘冰冰, 邹勃 2017 物理学报 66 030701

    Guo H W, Liu R, Wang L R, Cui J X, Song B, Wang K, Liu B B, Zou B 2017 Acta Phys. Sin. 66 030701

    [13]

    Grenoble D C, Drickamer H G 1971 J. Chem. Phys. 55 1624

    [14]

    Aroca R, Dilella D P, Loutfy R O 1982 J. Phys. Chem. Solids 43 707

    [15]

    Zhang X X, Bao M, Pan N, Zhang Y X, Jiang J Z 2004 Chin. J. Chem. 22 325

    [16]

    Lehrer S S 1997 Methods Enzymol. 278 286

    [17]

    Birks J B, Kazzaz A A, King T A 1966 Proc. R. Soc. London Ser. A 291 556

    [18]

    Tanaka J 1963 Bull. Chem. Soc. Jpn. 36 1237

    [19]

    Chandross E A, Dempster C J 1970 J. Am. Chem. Soc. 92 3586

    [20]

    Jones P F, Nicol M 1965 J. Chem. Phys. 43 3759

    [21]

    Birks J B 1975 Rep. Prog. Phys. 38 903

  • 图 1  样品的XRD表征.黑色竖线是利用文献[3]中α相酞菁晶体结构计算出的结果

    Figure 1.  The XRD pattern of sample under ambient pressure. The black lines are from calculation result of α phase metal-free phthalocyanine (α-H2Pc)[3]

    图 2  样品拉曼光谱测试 (a) 常压与卸压样品的拉曼光谱; (b) 酞菁的高压拉曼光谱; (c) 峰位为623 cm–1的拉曼峰随着压力升高峰强增强

    Figure 2.  The Raman spectra of α-H2Pc: (a) The Raman spectra of sample at ambient pressure and decompressed from 12.0 GPa; (b) the Raman spectra of α-H2Pc under high pressure; (c) the Raman peak around 623 cm–1 under high pressure

    图 3  样品的拉曼峰位随压力的变化

    Figure 3.  The Raman frequency of H2Pc as functions of pressure.

    图 4  样品的荧光光谱 (a)样品的常压和卸压荧光光谱; (b) 样品的升压荧光光谱; (c) 样品的荧光中心随压力的变化及其线性拟合, 虚线为线性拟合所得直线的延长线; (d) 样品卸压过程中的荧光光谱; (e) 两个荧光中心的峰强比随压力的变化图

    Figure 4.  The fluorescence (FL) spectra of α-H2Pc: (a) The FL spectra of α-H2Pc at ambient pressure (black line) and decompressed from 4.0 GPa (red line); (b) the FL spectra of α-H2Pc under high pressure during compressing; (c) the blue and red dots are the position of P1 and P2 during compressing, the blue and red lines are the linear fitting between the position and pressure, and the dash lines are the extension of the blue and red line; (d) the FL spectra of α-H2Pc under high pressure during decompressing; (e) the intensity ratio of P1 and P2 under high pressure

    图 5  α相和χ相酞菁的分子排布和重叠程度示意图 (a) α相酞菁的分子排布; (b) χ相酞菁的分子排布; (c) α相酞菁的分子重叠程度; (d) χ相酞菁的分子重叠程度

    Figure 5.  The arrangement and overlapping of PC molecules in α-H2Pc and χ-H2Pc: (a) The arrangement of Pc molecules in α-H2Pc; (b) the arrangement of Pc molecules in χ-H2Pc; (c) the overlapping of Pc molecules in α-H2Pc; (d) the overlapping of Pc molecules in χ-H2Pc [1,3].

    表 1  酞菁α相[3]、β相[1]和χ相[1]的晶体结构信息

    Table 1.  Structural parameters of different phases of metal-free phthalocyanine[1,3].

    α相β相χ相
    空间群C2/nP21/nPna21
    晶系单斜单斜正交
    a/nm2.6124(3)1.4796(6)2.10(1)
    b/nm0.3801(1)0.47325(6)0.491(5)
    c/nm2.3889(3)1.7357(7)2.31(1)
    α/(°)909090
    β/(°)94.18(2)104.32(2)90
    γ/(°)909090
    DownLoad: CSV
  • [1]

    Zugenmaier P, Bluhm T L, Deslandes Y, Orts W J, Hamer G K 1997 J. Mater. Sci. 32 5561

    [2]

    Sharma V B, Jain S L, Sain B 2003 Tetrahedron Lett. 44 383

    [3]

    Janczak J, Kubiak R 1992 J. Alloys Compd. 190 121

    [4]

    Kubiak R, Janczak J 1992 J. Alloys Compd. 190 117

    [5]

    Hammond R B, Roberts K J, Docherty R, Edmondson M, Gairns R 1996 J. Chem. Soc. Perkin Trans. 28 1527

    [6]

    Takano S, Enokida T, Kakuta A, Mori Y 1984 Chem. Lett. 13 2037

    [7]

    李战强, 李祥高, 李健, 胡雅琴 2013 有机化学 33 891

    Li Z Q, Li G Q, Li J, Hu Y Q 2013 Chin. J. Org. Chem. 33 891

    [8]

    Menzel E R, Jordan K J 1978 Chem. Phys. 32 223

    [9]

    Dong Y J, Xu B, Zhang J B, Tan X, Wang L J, Chen J L, Lv H G, Wen S P, Li B, Ye L, Zou B, Tian W J 2012 Angew. Chem. 124 10940

    [10]

    Meng X, Qi G Y, Zhang C, Wang K, Zou B, Ma Y G 2015 Chem. Commun. 51 9320

    [11]

    Yuan H S, Wang K, Yang K, Liu B B, Zou B 2014 J. Phys. Chem. Lett. 5 2968

    [12]

    郭宏伟, 刘然, 王玲瑞, 崔金星, 宋波, 王凯, 刘冰冰, 邹勃 2017 物理学报 66 030701

    Guo H W, Liu R, Wang L R, Cui J X, Song B, Wang K, Liu B B, Zou B 2017 Acta Phys. Sin. 66 030701

    [13]

    Grenoble D C, Drickamer H G 1971 J. Chem. Phys. 55 1624

    [14]

    Aroca R, Dilella D P, Loutfy R O 1982 J. Phys. Chem. Solids 43 707

    [15]

    Zhang X X, Bao M, Pan N, Zhang Y X, Jiang J Z 2004 Chin. J. Chem. 22 325

    [16]

    Lehrer S S 1997 Methods Enzymol. 278 286

    [17]

    Birks J B, Kazzaz A A, King T A 1966 Proc. R. Soc. London Ser. A 291 556

    [18]

    Tanaka J 1963 Bull. Chem. Soc. Jpn. 36 1237

    [19]

    Chandross E A, Dempster C J 1970 J. Am. Chem. Soc. 92 3586

    [20]

    Jones P F, Nicol M 1965 J. Chem. Phys. 43 3759

    [21]

    Birks J B 1975 Rep. Prog. Phys. 38 903

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  • Received Date:  17 April 2019
  • Accepted Date:  24 June 2019
  • Available Online:  01 September 2019
  • Published Online:  05 September 2019

Effect of pressure on structure and fluorescence of phthalocyanine

    Corresponding author: Wang Peng, wangpengtrrs@jlu.edu.cn
    Corresponding author: Liu Bing-Bing, liubb@jlu.edu.cn
  • 1. State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China
  • 2. College of Physics, Jilin University, Changchun 130012, China

Abstract: Phthalocyanine (Pc) is a kind of important photoelectric material, but a lot of questions remain to be clarified, like the relationship between the structure of Pc and its photoelectric property. High pressure is a powerful tool to study the structure transformation. High pressure study on piezochromic materials, which shows color change under high pressure due to structure changing, serves as an effectively way of studying the relationship between materials’ structure and photoelectric property. In this work Raman spectrum is employed to study the phase change of α phase metal-free Pc (α-H2Pc) under high pressure, meanwhile the effect of pressure on fluorescence (FL) is also studied to show how the Pc’s structure affects the photoelectric property. The diamond anvil cell is employed to achieve the high pressure condition, by using NaCl as a pressure transmitting medium. And Raman and FL measurements are performed by using a LabRam HR Evolution spectrometer equipped with a 473 nm laser. The Raman spectra of α-H2Pc show to slightly change during compression to 12.0 GPa. The main Raman peaks remain at highest pressure, including the Raman peak from macrocyclic of Pc molecules, which shows the stability of Pc molecules. Note that an enhancement of Raman peak at 623 cm–1 can be found with the pressure increasing, which appears only in the Raman spectrum of χ phase metal-free Pc (χ-H2Pc), showing that α-H2Pc is converted into χ-H2Pc under pressure. The curve for Raman frequency as a function of pressure shows that no obvious evidence related to bonding or structure transition can be observed, which means that α-H2Pc is transformed into χ-H2Pc gradually. For FL spectrum, only the FL of excimer can be found in α-H2Pc at atmosphere pressure. When the solid α-H2Pc is compressed, the FL intensity is found to decrease as pressure increases, and it is quenched at 3.0 GPa. The FL of Pc molecule, which is not found in α-H2Pc at ambient pressure, appears at 0.7 GPa. As the pressure increases, the FL intensity ratio between Pc molecule and excimer is enhanced. Considering the pressure induced phase transition from α-H2Pc to χ-H2Pc gradually, the change in FL spectrum should be due to the structure transformation. It is proved that the degree of overlapping between Pc molecules in α-H2Pc is larger than that in χ-H2Pc. We think, the degree of overlapping decreases under high pressure, which hinders the formation of excimer. It makes the excimer emission decrease and the FL of Pc molecules appear under high pressure. Our work can explain the relationship between Pc crystal structure and its fluorescence, reveals the kinetic behavior of macromolecules similar to Pc system under high pressure, and provides a new possibility of designing the photoelectric materials with excellent performances.

    • 酞菁是由四个异吲哚单元组成的一个平面大分子, 其分子式为C32H18N8. 酞菁的分子结构具有两个显著的特点, 一是酞菁分子有一个18个电子组成的共轭大环, 具有芳香性; 二是酞菁分子中心有2.7 Å的孔洞, 孔洞中可以填充各种金属元素. 同时酞菁分子外侧六元环上的氢原子可以被各种取代基所取代, 形成了众多的酞菁化合物. 酞菁化合物具有较高的化学稳定性和热稳定性, 且拥有平面的共轭电子结构, 这些使得酞菁化合物及其衍生物在光电材料[1]、催化[2]等领域得到了广泛的应用. 常压下酞菁有α相、β相、χ相和γ相等晶体结构, 都是分子晶体, 只是分子的空间排列不同[3-6], 各自的晶体结构信息如表1所列. 酞菁分子空间排列方式的变化可以调控分子间相互作用, 进而改变酞菁晶体的电导率、荧光和吸收等特性. 例如, 相比于其他晶体结构的酞菁, χ相的酞菁被认为具有更高的光生载流子效率, 因而在光电器件中有更大的应用潜力[1,7]. 目前酞菁晶体结构与其光电性质之间的内在关联尚未明确. Menzel等[8]提出了一种酞菁固体荧光的模型, 他们认为在585 nm激光的激发下α相酞菁仅有900 nm一个荧光中心, 而χ相酞菁存在815 nm和900 nm两个荧光中心, 他们认为815 nm处的荧光中心是酞菁分子本身的荧光, 900 nm处的荧光中心是酞菁分子间的激基缔合物荧光.

      α相β相χ相
      空间群C2/nP21/nPna21
      晶系单斜单斜正交
      a/nm2.6124(3)1.4796(6)2.10(1)
      b/nm0.3801(1)0.47325(6)0.491(5)
      c/nm2.3889(3)1.7357(7)2.31(1)
      α/(°)909090
      β/(°)94.18(2)104.32(2)90
      γ/(°)909090

      Table 1.  Structural parameters of different phases of metal-free phthalocyanine[1,3].

      高压可以改变分子间的距离、排布方式及相互作用, 进而实现对分子体系物理性质的调控, 一些有机分子材料如四苯乙烯等表现出压致变色和压致荧光增强的现象[9-12]. Grenoble和Drickamer[13]研究了酞菁在高压下电子结构的变化, 发现在17.5 GPa范围内随着压力的增加酞菁本身的吸收峰向低能区移动, 认为在17.5 GPa压力范围内酞菁仍保持α相. 目前酞菁晶体的高压研究仍处于起步阶段, 高压下的结构与物性研究尚不完善.

      本工作利用高压原位拉曼技术对酞菁在高压下的结构相变进行了研究. 研究发现, 在压力达到12.0 GPa时酞菁分子结构仍保持稳定, 没有发生开环反应. 酞菁晶体在压力作用下逐渐由α相转变为χ相, 这一转变卸压后可逆. 同时还对酞菁的高压荧光光谱进行了研究, 发现其光谱强度随压力增高而衰减, 至3.0 GPa时消失. 进一步分析表明, 高压下酞菁分子间的激基缔合物荧光受抑制, 常压下α相酞菁中观察不到的酞菁分子荧光在高压下出现. 我们认为酞菁荧光性质的变化应归因于酞菁晶体中呈平行排列的分子之间在压力下重叠程度减小.

    2.   实验部分
    • 实验所使用的酞菁购自阿拉丁试剂(中国)有限公司(编号P160721), 传压介质使用氯化钠, 购买自阿法埃莎(天津)化学有限公司(Alfa Aesar, 编号A12313), 用红宝石做压力标定物.

      X射线衍射(XRD)测试使用的是日本理学的MicroMax-007HF微区XRD衍射仪, 靶材为铜, 波长为0.15406 nm, 拉曼和荧光均使用HORIBA LabRAM HR Evolution 拉曼光谱仪进行测试, 激发光波长为473 nm. 高压实验在金刚石对顶砧装置中完成, 拉曼测试所使用的压机金刚石砧面大小为500 μm, 荧光测试所使用的压机金刚石砧面大小为600 μm. 高压试验所使用的垫片材质为T301钢.

    • 压机准备与装样: 将T301钢片预压至60 μm厚, 在压痕中央打一个孔作为样品腔(拉曼测试中孔直径为160 μm, 荧光测试孔直径为230 μm). 将垫片复位后, 先在下砧面点一颗直径20 μm的红宝石, 然后用氯化钠填满样品腔并压实, 之后刮去部分氯化钠, 装入样品, 最后再在样品上覆盖一层氯化钠, 完成装样. 随后进行加压和拉曼光谱、荧光光谱的测试.

    3.   实验结果
    • 图1是样品的常压微区XRD测试结果, 根据文献[3]中报道的α相酞菁晶体结构数据, 利用Materials Studio中的Reflex模块计算了α相酞菁的XRD (图中黑色竖线), 样品α相酞菁的峰位、峰强与计算结果符合得很好, 说明样品是α相酞菁.

      Figure 1.  The XRD pattern of sample under ambient pressure. The black lines are from calculation result of α phase metal-free phthalocyanine (α-H2Pc)[3]

    • 图2(a)中黑线为酞菁的拉曼光谱测试结果, 其中常压的酞菁拉曼光谱中的所有拉曼振动峰均能被指认并归属于α相酞菁[14,15], 这与XRD测试结果一致. 其中, 130 cm–1处的振动峰为大环平面外变形振动模式, 185 cm–1为异吲哚基团的同相振动模式, 206, 228, 481, 519和540 cm–1为异吲哚环变形振动模式, 556 cm–1为苯环变形振动模式, 680 cm–1为大环呼吸振动模式, 722和796 cm–1为大环变形振动模式, 1008, 1026, 1082, 1105, 1183和1229 cm–1均为碳氢振动模式, 1140和1336 cm–1为吡咯环呼吸振动模式, 1407和1427 cm–1为异吲哚环伸缩振动模式, 1516 cm–1为吡咯环中的碳氮伸缩振动模式, 1536 cm–1为吡咯环中的碳碳伸缩振动模式, 1584和1618 cm–1为苯环伸缩模式. 图2(b)为酞菁高压原位拉曼光谱测试结果. 我们发现随着压力的升高, 归属于α相酞菁的拉曼振动模式的峰强逐渐减弱, 大部分振动模式都保留到了最高压力. 位于600—800 cm–1范围内的三个拉曼峰代表着酞菁分子大环的振动模式, 均保持到了最高压力, 表明酞菁的大环结构保持到了12.0 GPa, 并未发生开环反应. 当压力达到1.3 GPa时, 位于130 cm–1附近代表大环平面外变形振动模式的拉曼峰消失, 说明大环平面外变形振动受到了明显的抑制. 由于大环本身结构在高压下得到保持, 我们认为大环平面外变形振动受抑制的原因是高压下酞菁分子间距离减小所导致的. 值得注意的是, 大多数的酞菁拉曼峰的强度随着压力的增加而减弱, 但是在623 cm–1处的拉曼峰的强度随着压力增加而增强(图2(c)). 根据文献报道, 这个拉曼峰对应着酞菁分子的B1g振动模式, 只有在χ相中才能被观察到[14,15]. 因此我们认为在压力的作用下α相酞菁逐渐转变为χ相. 图2(a)中的红线是卸压后的酞菁拉曼光谱, 与常压的对比, 卸压后归属于χ相的B1g拉曼振动峰消失, 图中各拉曼振动峰与α相一致, 说明α相酞菁向χ相的转变是可逆的. 我们还对高压下酞菁的拉曼峰位随压力变化的情况进行了分析, 如图3所示, 结果发现高压下绝大多数酞菁拉曼峰的峰位随压力的增加逐渐往高波数移动, 这种峰位随压力的变化是线性的, 不存在突变, 表明压力诱导的α相酞菁向χ相的转变随压力增加的渐变过程. 高压拉曼光谱数据表明, 酞菁分子在12.0 GPa的压力下仍保持稳定, 压力会使酞菁分子间距离逐渐缩小, 并诱导α相酞菁逐渐向χ相转变. 常压下χ相的密度(1.44 g/cm3)大于α相(1.43 g/cm3), 在高压下样品的体积会被压缩, 使得样品的密度随压力增大而变大, 因此高压下α相和χ相酞菁密度均会大于其常压下的密度. 在实验中观察到压力诱导酞菁α相向χ相转变, 因此我们认为χ相的密度大于α相, 为酞菁的高密度相. 实验中所使用的氯化钠起到传压介质的作用, 提供非静水性环境, 从拉曼光谱中可以看出在实验的压力范围内氯化钠与酞菁之间没有明显的化学反应. 而氯化钠所提供的非静水性环境可能促进α相酞菁的相变发生, 这与文献[13]一致.

      Figure 2.  The Raman spectra of α-H2Pc: (a) The Raman spectra of sample at ambient pressure and decompressed from 12.0 GPa; (b) the Raman spectra of α-H2Pc under high pressure; (c) the Raman peak around 623 cm–1 under high pressure

      Figure 3.  The Raman frequency of H2Pc as functions of pressure.

    • 图4(a)中的黑线数据是酞菁常压荧光光谱测试结果. 在473 nm的激光激发下, α相酞菁有一个荧光中心, 位置为 885 nm, 这与文献中报道的α相酞菁粉末的荧光光谱基本一致[8], 是酞菁分子间激基缔合物的荧光. 图5(b)是酞菁高压原位荧光光谱测试结果. 高压下酞菁的荧光峰位发生红移, 荧光强度逐渐减弱, 至3.0 GPa时荧光几乎完全消失. 注意到压力达到0.7 GPa时酞菁的荧光光谱有两个明显的荧光中心, 我们对0.7 GPa和更高压力的荧光数据进行高斯拟合, 结果如图4(b)所示. 在0.7—2.6 GPa压力范围内, 荧光光谱中都存在两个荧光中心, 将能量较高的荧光中心称为P1, 能量较低的荧光中心称为P2, 我们发现P1和P2的峰位随压力的变化是近线性的, 对P1和P2的峰位随压力的变化进行线性拟合得到两条直线L1和L2 (如图4(c)中蓝色和红色的实线所示). 将L1和L2外推至常压(如图4(c)中蓝色和红色的虚线所示), 得到常压下P1与P2所对应的峰位分别为(844 ± 4) nm和(922 ± 11) nm, 能量差约为0.12 eV, 与Menzel等[8]报道的χ相酞菁中815和900 nm两个荧光中心(能量差约0.14 eV)接近, 因此我们认为P1和P2所代表的荧光中心与Menzel等[8]报道的χ相两个荧光中心有相同的来源, 即P1是酞菁分子荧光, 而P2是酞菁分子间激基缔合物的荧光. 卸压过程的酞菁荧光光谱如图4(d)所示, 卸压过程中随着压力的降低酞菁荧光强度逐渐增强. 图4(a)中的红线是卸压后的酞菁荧光谱, 卸压至常压后酞菁的荧光峰中心在890 nm, 与常压相比有5 nm的红移.

      Figure 4.  The fluorescence (FL) spectra of α-H2Pc: (a) The FL spectra of α-H2Pc at ambient pressure (black line) and decompressed from 4.0 GPa (red line); (b) the FL spectra of α-H2Pc under high pressure during compressing; (c) the blue and red dots are the position of P1 and P2 during compressing, the blue and red lines are the linear fitting between the position and pressure, and the dash lines are the extension of the blue and red line; (d) the FL spectra of α-H2Pc under high pressure during decompressing; (e) the intensity ratio of P1 and P2 under high pressure

      Figure 5.  The arrangement and overlapping of PC molecules in α-H2Pc and χ-H2Pc: (a) The arrangement of Pc molecules in α-H2Pc; (b) the arrangement of Pc molecules in χ-H2Pc; (c) the overlapping of Pc molecules in α-H2Pc; (d) the overlapping of Pc molecules in χ-H2Pc [1,3].

    4.   讨 论
    • 根据高压荧光光谱的实验结果, 我们得知在压力的作用下, 酞菁的荧光强度随着压力的升高而减弱, 至0.7 GPa时出现酞菁分子荧光. 如图4(e)所示, 在0.7—2.6 GPa压力范围内, 随着压力的升高酞菁分子荧光强度与激基缔合物的荧光强度的比值逐渐增大. 根据前人对激基缔合物的研究, 在分子被入射光所激发后, 处于激发态的分子可以与一个仍处于基态的分子形成激基缔合物, 从而使分子本身的荧光淬灭或者出现激基缔合物的发光[16]. 虽然目前没有关于酞菁分子激基缔合物荧光机理的报道, 但是与酞菁相近的一些平面共轭分子, 如芘和苝等, 它们的激基缔合物荧光机理已被详细地研究过. 这些研究表明激基缔合物本身有特定的构型, 如两个分子互相平行排列形成三明治结构[17-19], 而激基缔合物形成依赖于晶体中分子间的空间排布, 分子间需要有比较大的重叠程度 [20,21]. 高压拉曼光谱的实验结果证实酞菁在压力的作用下从α相转变为χ相, 酞菁分子的空间排布形式发生了显著的变化. 为了细致说明, 在α相酞菁和χ相酞菁晶体中, 可以进一步选取相互平行的酞菁分子排列形成一维的列(如图5(a)图5(b)所示)来比较α相及χ相酞菁的空间结构变化. 列中相邻的两个分子, 沿着任一分子平面的法线方向做投影, 就可以看出两者有一定的重叠, 在α相酞菁中两个分子重叠的程度较大(图5(c)), 具备形成激基缔合物所需的空间条件, 其中一个分子被激发后, 可与另一个分子形成激基缔合物, 进而产生激基缔合物荧光回到基态, 而不是以分子单体荧光的形式回到基态, 所以在α相酞菁荧光光谱中只能观察到激基缔合物的荧光而没有酞菁分子单体荧光. 值得注意的是, χ相酞菁中分子间重叠程度相比α相酞菁显著减小(图5(d)). 因此, 在压力驱动的α相向χ相转变的过程中, 酞菁分子间重叠程度会随着压力的增加趋向减小, 导致酞菁分子激基缔合物的形成受到压力抑制, 进而减弱酞菁的激基缔合物荧光. 不能形成激基缔合物的激发态分子只能以单体荧光的形式回到基态, 从而导致酞菁分子荧光出现, 表现为酞菁分子荧光与激基缔合物的荧光强度比值随压力升高而增大.

    5.   总 结
    • 本文进行了α相酞菁的高压拉曼光谱和高压荧光光谱研究. 高压拉曼光谱显示酞菁分子本身在压力达到12.0 GPa时仍保持稳定, 在压力驱动下酞菁分子空间排布改变, 使得α相酞菁转变为χ相的酞菁. 高压荧光光谱显示, 高压下酞菁的荧光峰位随着压力升高而红移, 同时荧光强度减弱. 由于在α相酞菁转变为χ相酞菁的过程中酞菁分子重叠程度减小, 使得激基缔合物的荧光受抑制, 并且酞菁分子荧光出现, 伴随着酞菁分子荧光强度与激基缔合物的荧光强度之比随压力升高而增大. 我们的工作阐述了酞菁晶体结构与其荧光之间的关系, 揭示了具有共轭电子体系的大分子在高压下的动力学行为, 为设计性能优异光电材料提供了新的思路和途径.

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