Fund Project:Project supported by the Natural Science Foundation of Shandong Province, China (Grant No. ZR2014AM026)
Received Date:01 April 2019
Accepted Date:03 July 2019
Available Online:01 September 2019
Published Online:20 September 2019
Abstract:Organic materials with strong two-photon absorption response and aggregation induced emission have aroused a great deal of interest in recent years, for their many potential applications such as two-photon fluorescence microscopy, up-conversion laser, photodynamic therapy, etc. The tetraphenylethylene units are usually employed in two-photon absorption and aggregation induced emission materials because of their good electron-donating capability and special propeller starburst structures. Theoretical study on the relationship between molecular structure and two-photon absorption property is of great importance for guiding the experimental design and synthesis of functional materials. In this paper, the two-photon absorption properties of a series of organic molecules containing tetraphenylethylene and cyano groups are studied by employing the density functional response theory in combination with the polarizable continuum model. The molecular geometries are optimized at a hybrid B3LYP level with 6-31g(d, p) basis set in the Gaussian 16 program. The two-photon absorption cross sections are calculated by response theory through using the CAM-B3LYP functional with 6-31g(d) basis set in the Dalton program. The effect of donor position and number on two-photon absorption properties are investigated. In addition, by increasing the planarity and conjugated length of the molecule, as well as by enhancing the strength of the electron donor, we design three molecular structures and calculate their two-photon absorption properties. The results show that the donor position and number have important effects on two-photon absorption properties. The methoxy donor at the end of the molecule can increase the two-photon absorption intensity effectively. As the number of substituents increases, the position of the two-photon absorption peak is red-shifted. The effects of adding electron donor groups on different side positions have a significant difference in the two-photon absorption property. Comparing with the experimental molecules, the two-photon absorption cross sections of the designed molecules are greatly enhanced. When the tetraphenylethylene group is replaced by the triphenylamine group, the two-photon absorption peak is greatly red-shifted, and the two-photon absorption intensity is significantly increased. Since all of these molecules contain tetraphenylethylene or triphenylamine group with propeller structure, they can have both two-photon absorption and aggregation induced emission properties. This study provides theoretical guidelines for synthesizing a new type of active two-photon absorption and aggregation induced emission material. Keywords:tetraphenylethylene/ two-photon absorption/ donor position/ donor number
表1分子六个最低激发态的TPA波长${\lambda _{{\rm{tp}}}}$(nm)和TPA截面$\sigma $(GM) Table1.The TPA wavelength ${\lambda _{{\rm{tp}}}}$(nm) and the TPA cross section $\sigma $(GM) of the lowest six excited states.
23.2.给体数目效应 -->
3.2.给体数目效应
可以看出, 这些分子体系具有多枝结构, 分枝之间的复杂耦合决定了分子的TPA性质. 已有研究表明, 对于多枝分子, 分枝间的耦合对分子的光学性质有重要影响[17,18]. 根据耦合作用的强弱, TPA强度相对于分枝单元会表现出协同加强、简单加和、甚至削弱的不同趋势[30-32]. 人们通常采用简单的弗兰克激子模型来研究分枝间的耦合作用[17,18]. 在该模型中, 分枝间的相互作用主要考虑偶极-偶极相互作用. 在TPE各分枝上是否添加给体, 会影响TPE各分枝的偶极矩, 进而影响分枝间的相互作用. 因此, 在TPE上添加更多的给体取代基能否产生更强的TPA, 还需要进一步仔细研究. 为此, 我们考虑了R, S和U几种分子结构, 它们的化学结构式和相应的优化几何结构如图3所示. 对于R, S, T和U分子, TPE部分的给体数目不同. 结构R中的TPE部分没有连接甲氧基, 结构S中只有一个末端甲氧基, 结构U包括三个甲氧基, 其中两个添加在TPE基团的侧位上, 一个位于分子的末端. 图 3 R, S和U分子的化学结构式和优化的几何结构 Figure3. Chemical structures and optimized geometries of the R, S and U molecules.
表2分子六个最低激发态的TPA波长${\lambda _{{\rm{tp}}}}$(nm)和TPA截面$\sigma $(GM) Table2.The TPA wavelength ${\lambda _{{\rm{tp}}}}$(nm) and the TPA cross section $\sigma $(GM) of the lowest six excited states.
23.3.电荷转移 -->
3.3.电荷转移
TPA性质与分子内电荷转移密切相关. 当分子从基态跃迁到激发态时, 分子内的电荷会重新分布. 为了更好地理解给体位置和给体数目对分子内电荷转移的影响, 计算了T, T2和T4以及R, S和U在基态和第一激发态的自然键轨道电荷分布情况. 为了便于分析, 将所研究的分子分成两部分, 四苯基乙烯基团连同给体甲氧基一起作为A部分, 剩余部分为B部分. 以T分子为例, 如图4所示. 图 4 T分子的A和B两部分 Figure4. The A and B parts of the T molecule.
表4分子六个最低激发态的TPA波长${\lambda _{{\rm{tp}}}}$(nm)和TPA截面$\sigma $(GM) Table4.The TPA wavelength ${\lambda _{{\rm{tp}}}}$(nm) and the TPA cross section $\sigma $(GM) of the lowest six excited states.
为了便于比较, 还模拟了X, Y, Z和T分子的TPA光谱, 如图6所示. 可以看到, 这些分子在600—850 nm之间均有多个较强的TPA吸收峰. 设计分子的吸收峰强度均比实验中的T分子强. X, Y和Z在长波范围内均有较强的吸收峰, 这些吸收峰相对T分子有不同程度的红移. 当三苯胺基代替四苯基乙烯基之后, Y和Z的吸收峰都发生了较大红移. 根据(1)式, 双光子跃迁矩阵元与分子的激发能和偶极矩有直接关系. 相关的激发能越低, 越有利于增大TPA截面. 与X和T相比, Y和Z的吸收波长有较大的红移, 说明它们的激发能较低, 因此Y和Z具有更强的TPA. 图 6 X, Y, Z和T分子的TPA谱 Figure6. TPA spectra of the X, Y, Z and T molecules.