王筱梅
论文题目：电荷转移的对称性与分子的双光子吸收/辐射（荧光、激射）性能关系的研究
作者简介：王筱梅，女，1958年11月出生，1998年09月师从于山东大学蒋民华教授，于2001年06月获博士学位。
摘 要
强双光子吸收有机材料由于在三维光信息存储、光动力学癌症医疗诊断、双光子荧光显微术、双光子上转换激射以及激光限幅等高科技领域中具有诱人的应用前景，引起国内外学者的高度重视，九十年代中后期,已成为材料物理学、材料化学以及量子化学领域内关注的热点。有关强双光子吸收材料的制备、结构与性能关系以及双光子吸收效应的研究是当前国内外光电子领域内的前沿课题之一。
根据“电荷转移理论”，如果分子具有电子给体－受体组成的电荷转移结构，该分子对光场将表现出较强的极化响应，其基态与激发态的偶极矩之差（Dmge）较大。理论上已推导并通过实验测试已经证实,这种结构的材料其双光子吸收截面数值较大。另一方面由于双光子吸收为分子的三阶NLO过程，其三阶非线性极化率g是四阶张量，具有标量的属性。与二阶NLO倍频效应不同的是，对于分子的对称性无专门的要求，因而一些具有离域大p键对称型结构的分子也有可能会表现出强的双光子吸收。因此，围绕着强双光子吸收材料的分子设计，从目前研究的现状来看，主要基于以下四种类型（或是以其为结构单元的混合式），分属于“对称型”与“不对称型”两大类（见图1）。
图1.“不对称型”和“对称型”两大体系
就强双光子吸收有机分子材料的研究，目前以美国Marder和Prasad小组最具有代表性。这两个研究小组分别从“对称电荷转移”与“不对称电荷转移”为出发点，设计、合成出许多强双光子吸收的有机分子材料，并作了开拓性的应用性研究。
究竟是对称型结构还是不对称型结构的分子更有利于增大分子的双光子吸收截面，这一问题成为当前强双光子吸收材料的分子设计中的活跃课题，并已引起Marder和Prasad小组的重视。2000年,Prasad小组报道以三聚噻吩为p-中心桥的D-p-D（对称型）和D-p-A（不对称型）有机分子，并指出D-p-D型分子的双光子吸收性质明显优于D-p-A型分子。但Prasad小组只报道了实验结果，并未作出理论解释。
本论文以此为立题根据，系统地比较性研究“对称型”和“不对称型”两大类分子的双光子吸收行为，以探讨分子内“对称电荷转移”和“不对称电荷转移”模式与双光子吸收性能的关系。由于电荷转移是分子光物理行为的基本过程，从分子水平上认识电荷转移与双光子吸收性能的关系则有助于从根本上揭示其内在本质的规律性。
在“双光子吸收效应”中，上转换荧光和上转换激射由于在荧光显微术、光动力学治癌等医疗诊断、三维信息存储和获取新型激光光源等方面具有诱人的应用前景，为当前光电子领域内研究的热点之一。从目前所调研的文献来看，尚未见文献报道从电荷转移的不同模式出发对双光子荧光和上转换激射行为的研究。为此，本论文还系统研究了“对称电荷转移”和“不对称电荷转移”模式与分子的上转换辐射（包括荧光和激射）行为的影响。以期对强双光子吸收/辐射（荧光、激射）有机分子材料的设计、合成及应用提供参考依据。
本论文研究的主要内容和创新成果包括：
1.材料的合成 在“D-p-D”和“D-p-A”结构框架下设计、合成出18种有机化合物，得到7个新晶体的X-衍射数据（其中15种化合物未见文献报道，5种D-p-D型分子在严格的无水无氧条件下制备）。这些化合物具有同等共轭长度，分属四种类型：双推电子基团取代二苯乙烯（D-p-D）、对位取代氨基苯乙烯吡啶衍生物（D-p-A）、对位取代氨基苯乙烯喹啉衍生物（D-p-A）和对位取代氨基苯乙烯巴比妥酸衍生物（D-p-A）。
以自制的化合物为研究对象，系统研究了“对称电荷转移”和“不对称电荷转移”两种模式对分子的双光子吸收和上转换辐射（荧光和激射）行为的影响。
2.电荷转移的对称性与分子的双光子吸收性质 PM3半经验方法计算出的分子基态、激发态电荷密度分布图表明：对称分子的电荷转移是对称的：分子处于基态（S0）时电荷密度分布的流向由分子中心（p-center）向两端氨基(Donors)扩散，而分子处于第一激发态（S1）时，电荷密度分布则发生反向流动，即电子云由两端氨基向分子中心离域；处于第二激发态（S2）的分子的电荷密度分布具有和基态时相同的电荷流向。
PM3半经验方法计算出不对称分子的基态、激发态电荷密度分布图表明：不对称分子的电荷转移则是不对称的。当分子处于基态（S0）、第一激发态（S1）和第二激发态（S2）时，电荷密度分布均为由电子给体流向受体。
对称电荷转移和不对称电荷转移的三态模型如图2所示（图中箭头方向为电荷转移方向，箭头的粗细代表电荷转移的程度大小）。
图2 对称电荷转移和不对称电荷转移的三态模型
研究表明，对称分子与不对称分子这种不同的电荷转移模式，使得它们对光场表现出不同的极化响应，反映在它们的偶极矩差（Dmge）和跃迁偶极矩（Mee’）的数值上。本论文研究的结论是：双光子吸收性质主要由Mee’和 Dmge这两个物理量来决定且前者对分子的双光子吸收截面的贡献要大于后者。D-p-D分子的Mee’值相对较大，该类分子的双光子吸收截面要大于具有相同共轭长度的D-p-A分子。由图2看出，D-p-D分子处于不同的态时，其电荷的转移呈现出振荡的特性，该特性表现出对激光场有很好的响应，其第一与第二激发态的跃迁偶极矩（Mee’）较大。研究发现这种对称振荡式的电荷转移模式具有较大的电荷重新分配特点，从而有助于提高分子的双光子吸收截面。因此，设法增大分子的第一与第二激发态的跃迁偶极矩（Mee’）是提高双光子吸收截面的途径之一。其研究结果为“理论计算－分子设计”提供了参考依据。
3.电荷转移的对称性与单光子荧光行为 本论文系统比较了D-p-D分子和D-p-A两类分子的单光子荧光行为，发现它们具有许多不相同的性质（如相对荧光强度、荧光峰形和荧光寿命等）。在不对称分子的“TICT”模型基础上，本文根据量化计算和大量实验事实，提出对称电荷转移的“TICT”模型（如图3所示），并用该模型合理解释了D-p-D分子和D-p-A两类分子的单光子荧光行为。
图3.对称电荷转移（上图）与不对称电荷转移（下图）“TICT”模型
（其中D = 对位取代氨基；p =苯乙烯基或反-二苯乙烯基；
A=吡啶基团，喹啉基团和巴比妥酸）
图3显示出，D-p-A分子的不对称电荷转移使其“TICT”态极性变大、有效发色团（如苯乙烯吡啶基）平面性变差，是一个稳定性大于ICT态的非辐射态；相反地，D-p-D分子的对称电荷转移，使其“TICT”的极性与基态相比变化不大。对称分子的“TICT”和“ICT”态的平面性均较好。根据这一模型可以合理解释D-p-D和D-p-A化合物的单光子荧光行为（如荧光量子产率，荧光寿命，溶剂效应及荧光峰形等）。
4.双光子荧光与单光子荧光行为的关系 本论文系统研究了双光子荧光与单光子荧光行为的关系。在分析大量的图谱数据基础上得出结论：双光子荧光与单光子荧光行为具有相似性。尽管双光子与单光子吸收机制不同，但它们的荧光的发射均来自于S1® S0过程中的能量释放（辐射形式），这由图4可看出。双光子荧光行为也可用“TICT”模型解释。由这一研究结果可知：了解材料的单光子荧光行为将有助于认识其双光子发光性能。
图4 单/双光子荧光辐射的微观过程
5.电荷转移的对称性与双光子激射行为 研究发现不对称的吡啶盐化合物（D-p-A）双光子激射效率与基态时有效发色团内两个芳环的电荷密度之差(Dr1，2)及有效发色团的BLA值有关。BLA定义为有效发色团内相邻单、双键键长之差的平均值。Dr1，2值可反映出电子由给体流向受体这种不对称电荷转移的驱动力，其值愈大，表示这种不对称电荷转移也大；BLA值则表示电子云离域程度，其值愈小，表示该体系愈有利于不对称的电荷转移。
研究发现，当分子同时具有大的Dr1，2值和小BLA值，有利于提高分子的双光子激射效率。分析原因，认为当大Dr1，2值和小BLA值，有利于分子作不对称的电荷转移，结果得到较大的激射输出，双光子激射效率增大。
对D-p-D型分子来说，虽然单光子荧光量子产率较高，但在现有的测试条件下没有观察到双光子激射输出。理论上分析这类分子的对称电荷转移，光照下在分子中心的碳碳烯键上易发生[2+2]光化学反应，实验证实了反-4，4’-双（二苯氨基）二苯乙烯（BDPAS）分子在0.001M浓度下，可发生光二聚反应。光二聚反应的发生使光能转化为化学能，增加了腔内光能的损耗，因而不利于双光子激射的输出。
6.电荷转移的对称性对电子跃迁行为的影响 在分析图谱数据基础上本论文提出：具有p®p*跃迁的吸收带其双光子吸收截面大;具有n®p*跃迁的吸收带其双光子吸收截面小。
关键词：电荷转移的对称性，双光子吸收，单光子荧光，双光子荧光，双光子激射。
STUDY ON THE
RELATIONSHIPS BETWEEN SYMMETRIC, ASYMMETRIC CHARGE TRANSFER PROCESS AND
TWO-PHOTON ABSORPTION AND UP-CONVERTED EMISSION
ABSTRACT
Organic conjugated compounds with large
two-photon absorption (TPA) cross section emerge as very attractive elements
for use in a number of optical applications such as optical data storage,
up-converted lasing, three-dimensional fluorescence imaging, biological and
optical power limiting, etc. Numerous organic compounds have been investigated
both experimentally and theoretically in order to understand the
structure-property relationship of materials with large TPA cross section. Now
some general structural parameters for increasing the molecular TPA
cross-section values, d,
have been gradually drawn in several laboratories. For example, (1) to increase
the donor and acceptor strength (2) to change the character of the conjugated
bridge (3) to increase planarity of the chromophores, and (4) to extend the
conjugated length. The results of theoretical studies further reveal that the
TPA d
value is related to the imaginary part of the third-order polarizability,
(1)
where h is Planck’s constant, n is the index of
refraction of the medium, L is a local field factor ( equal to 1 for vacuum ),
and c or w
is the speed of light or the frequency of light. When the frequency (w
) is left out of consideration, the third-order polarizability, g,
can be expressed as eqn.(2) ,
(2)
where Dmge, is the difference of dipole moment
between the ground state ( S0 ) and the first excited state ( S1);
Mge or Mee’
is the transition dipole
moment between S0 and S1 or between two lowest excited
states, i.e., S1 and S2. And Ege or Ege’
is the difference between ground- and excited-state energy or between the two
excited states. 
It is evident that the TPA d value is dependent on all parameters presented in eqn. (2). It is
also rational to deduce that the materials with large Dmge and Mee¢ must have exhibited high d value
due to stronger intramolecular charge transfer.
In the exploration strong TPA compounds, Marder
et al. have focused on symmetric organic compounds and reported a design
strategy for symmetrically substituted conjugated molecules (D-p-D
type), while Parsad al. have emphasized the D-p-A
type chromophores. The former emphasized the importance of conjugation length and
molecular symmetry, while the latter laid stress on the planarity of p-center
and donor-acceptor strength.
Only recently have some authors pay attention to
evaluate comparatively the influence molecular symmetry upon the TPA cross
section. In 2000, O.K. Kim et al reported a class of new DTT-p centered molecules with
both symmetrically substituted (D-p-D) and
symmetrically substituted (D-p-A) molecules.
The experimental results showed that the TPA cross section of the D-p-D molecules is larger,
however, so far there is no theoretical explanation.
It is clear that a great deal of progress has
been made over the past five years in design of organic molecules with large
TPA cross section and their applications. However, at present it is still
unclear the relationship between the structural factors with the two-photon
absorption properties. The influence of intra-molecular charge transfer way
(i.e. symmetric and asymmetric) upon the two-photon absorption property has not
been reported yet. In this dissertation, we focus on studying the influence of
symmetric and asymmetric intra-molecular charge transfer upon two-photon
absorption / two-photon-pumped emission.
It mainly contains six parts as follows.
1. Synthesis of “D-p-D” to “D-p-A” type compounds. By the phase transfer catalytic (PTC)
SN2 reaction, McMurry reaction, Wilsmeier reaction, and Knoevenagel
reaction et al, all symmetric and asymmetric chromophores have been
synthesized. These series of newly designing chromophores, on the one hand,
change the structural type from “D-p-D”
to “D-p-A”,
on the other hand, vary the donor strengths in the order of carbazolyl <
diphenylamino <
pyrrolidinyl »
diethylamino<
ethyl-hydroxyethylamino»
methyl-hydroxyethyl-amino. Meanwhile, the acceptor structural feature (A) changes
from barbituric acid to methyl-pyridinium and further to methyl-quinolinium
moiety, et al. All the compounds synthesized in this paper have been
characterized by 1NMR spectra, IR spectra, mass spectra, element
analyses or X-ray diffraction.
2. The relationship
between symmetric, asymmetric charge transfer and two-photon absorption (TPA). On the basis of correlated quantum chemical
calculation，it
is demonstrated that for the symmetric molecule, the charge transfer shifts
from the central stilbene unit to the two terminal substituted amino group in
the S0 ground state, whereas in the S1 state the charge
transfer process reverses, shifting from the two terminal groups to the central
stilbene moiety, and for the second excited state (S2), the its
transfer direction is as the same as that of S0. Although such a
symmetric charge transfer behavior results in a negligible Dmge
value, it shows an enhancing transition dipole moment (Mee’). In
contrast, for the asymmetric quinolinium or pyridinium or barbituric acid
derivatives, there is a major asymmetry of charge density distribution, always
from the donor-end to the acceptor-end, which shows that there exists a strong
dipolar character, directed from the substituted amino group to the acceptor
group. Clearly, such the behavior about charge transfer results in a large
difference in dipole moment (Dmge)
between the ground- and first excited- states. The diagram of symmetric,
asymmetric charge transfer is shown as Fig.1.
Fig.1 The symmetric and asymmetric charge
transfer under the three-state model
It is accepted that the
molecular TPA cross section, dTPA, is proportional
to imaginary part of the third-order polarizability, and if the frequency (w) is neglected, the
third-order polarizability, g, can be
expressed as eqn. (2),
Therefore, the d TPA value depends on the ground state to first excited state dipole moment change (Dmge), the transition dipole moment between the first and second excited state (Mee’) and the energy differences between the ground and the excited states (Ege, or Ege’), etc.
From Fig.(1) it can be seen
that the influence of the parameters such as Ege, Ege’, Mge
upon the TPA cross section of D-p-D and D-p-A molecules are nearly the
same, but the the Dmge and Mee’
are different.
It is
understandable that the difference of symmetric or asymmetric charge transfer
way upon structural parameters may result in the variation in molecular
two-photon absorptivity, while the parameters of Dmge, and Mee’
play an important role in it. It was found that the asymmetrically substituted
derivatives possess relatively large Dmge, whereas the
symmetrical counterparts show an increase in Mee’. Although a large
two-photon absorption resonance is due to the simultaneously high values of Mee’
and Dmge, correlated to intra-molecular charge transfer, shown as the eqn. (2), the experimental
results have confirmed that the former functions is larger. These results
obtained have demonstrated that the magnitude of
two-photon absorption depends on the way of the intra-molecular charge
transfer.
3. The relationship between symmetric, asymmetric charge transfer and
one-photon / two-photon fluorescence.
It is interesting to note that there are quite different one-photon
fluorescence behavior between the symmetric and asymmetric chromophores.
For example, the
one-photon fluorescence intensity of chromophores decreases drastically from
symmetric molecules to pyridinium, and further to quinolinium derivatives. And
the relative fluorescence intensity for symmetric chromophores is almost 100
times as strong as that of pyridinium counterparts. For quinolinium
derivatives, there is no detectable one-photon fluorescence. Since the
measurement is performed under exactly the same conditions, the change of the
relative fluorescence intensity means that the fluorescence quantum yields
follow as the same sequence: symmetric chromophores >
pyridinium derivatives >
quinolinium derivatives. Solvent effect studies have shown that the asymmetric
chromophore usually gives a red shift with an increase in solvent polarity
parameter ET (30), while the fluorescence spectra of symmetric
molecules show different solvent effect.
Fig.2
Symmetric (above) and asymmetric (bellow) charge transfer “TICT” model
We consider that these
experimental results can be explained by the “twisted intra-molecular charge
transfer” (TICT) model. Based on the quantum-chemical calculation, we present
these the symmetric and asymmetric charge transfer “TICT” model, shown as
Fig.2.
From the diagram of
the “TICT” model, it can been seen that the asymmetric molecules are more polar
in the excited state than in the ground state, moreover, the TICT state
possesses the largest polar. Since the TICT state, a non-emission, is favored
in more polar solvents, the fluorescence quantum yield will pronounced decrease
with an increase of in the polarity of the solvent.
In contrast, for the
symmetric molecules, the ICT state and TICT state are all emission since the
planarity in both states are better, even than in the ground state. As a
result, the fluorescence quantum yields for the symmetric molecules are higher
than those of the asymmetric molecules. Also, for the whole molecules, the
polar in the S0 state and the ICT state (S1) and ICT
state (S1) are nearly the same since the symmetric charge transfer
way, Therefore, the solvent polarity seem have a little influence on the
fluorescence quantum yield.
There is a trend for
two-photon fluorescence behavior to be similar to the one-photon fluorescence
one for the same molecules. Since the fluorescence is the emission from the S1
to the S0, the only difference of the one-photon fluorescence
and the two-photon fluorescence is mainly from their excitation process. This
diagram is shown in Fig. 3. We consider the “TICT” model can be suitable for
the two-photon fluorescence behavior.
Fig.
3 The micro-process of the one-photon fluorescence
3.
The relationship
between symmetric, asymmetric charge transfer and two-photon pumped lasing A series of new
two-photon pumped (TPP) up-conversion lasing dyes, named as CSPI, DPASPI, PSPI,
DEASPI and HEASPI respectively, has been investigated. Strong two-photon pumped
(1064 nm) lasing at around 625~630 nm with a
half-bandwidth of ~ 25 nm was
observed in DMF solution. The largest up-conversion efficiency was as high as
10.7 (%) at 2.14 mJ input energy. To our knowledge, DEASPI, PSPI and HEASPI are
among the few laser dyes with such the high up-conversion efficiency.
PM3 calculations show that attaching different donors changes the bond length alternation (BLA) and parameter Δr1, 2. BLA is defined here as the average of the difference in length between adjacent single and double bonds, while theΔr1, 2 value is related to asymmetric charge transfer. The larger Δ