linearity
One of the significant challenges in the use of organic chromophores in NLO appli- cations is to translate their high molecular NLO figure of merit to a high macro- scopic EO coefficient. The magnitude of the EO coefficient is largely determined by inter-chromophore interactions at high concentrations, as well as the ability of the chromophore to orientate with an applied poling field and maintain some of this orientation over a period of time. Different strategies have been used including the use of bulky group derivatives of the conventional rod-like EO chromophores to en- hance r33 and the temporal stability. Structural modifications include ring locking
of the π-conjugated system along with the incorporation of bulky substitutes, e.g. diphenyl groups, onto this central ring system.
In an NLO chromophore, the π-electrons will almost always be polarised asym- metrically. Higher molecular nonlinearity can be obtained by tuning the bond length alternation (BLA) value of the chromophore [21, 18]. This can be achieved by a number of possible methods, including increasing the conjugation length through increasing the number of π-bonds [78, 79], and changing the strength of the donor and/or acceptor [79]. One of the main challenges in the development of efficient electro-optic materials at the application level is the poor efficiency in translating the large microscopic NLO property to the macroscopic level. Efficiency of poling can be improved by optimization of poling conditions, however, this is limited by many parameters as discussed in Chapter 4. At the chromophore level, the efficiency of the translation of the first hyperpolarisability to a macroscopic electro-optic ef- fect is achieved by reducing aggregation. This is possible through increasing the planarity of the chromophores as well as by maintaining spacing between the chro- mophores via incorporating large bulky substitutes with the chromophores as shown in Fig. 5.7. Detailed analysis of the effects of the addition of bulky substitutes is discussed in following sections.
The synthesis of the new chromophores are based on the theoretical model of the first hyperpolarisability using the two-level model. The difference in the NLO response in these materials are expected to arise from the two level model developed
5.3. Structural alterations for improved molecular nonlinearity 75 for the theoretical estimation of the molecular nonlinearity [80, 15].
βzzz∝(µee−µgg)
µ2ge
E2
ge
(5.27) According to this model, the first hyperpolarisability is proportional to the dif- ference in the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) states of the molecular orbitals (∆µeg =µee−µgg) and
the square of the transition dipole moment (µge). Also,βzzzis inversely proportional
to the square of the energy difference between the ground state and charge transfer (CT) excited state,Ege.
Some of the methods being employed to improve the efficiency of organic NLO chromophores to achieve large microscopic nonlinearity include ring locking of the chromophore backbone, incorporation of donor/acceptor groups onto the conjugated interconnect, development of multi-chromophore macromolecular systems and in- ducing a very large twist angle between the donor and acceptor units in the chro- mophores. Some of the methods our group at Industrial Research Limited have em- ployed to develop efficient chromophores whose macroscopic NLO properties have been investigated in this study are explained below. This section discusses the re- sults from a systematic approach I have made to investigate the effects of structural alterations at the molecular level.
5.3.1 Effects of donor strength
Molecular nonlinearity of the chromophore is related to the donor-acceptor strength. By varying the strength of the donor and/or acceptor group the ground state po- larization and the BLA value of a chromophore can be tuned. That is, the nature of the donor and acceptor is a key to determine the nonlinearity of a chromophore. Recent works carried out in our group [63, 81, 82, 79, 83, 84] and other research groups [1, 11, 12, 85], have reported a substantial change in hyperpolarisability by changing donor groups keeping the rest of the chromophore molecule the same. Structures of some of the chromophores synthesized in our team to investigate ef- fects of donor strength are shown in Fig. 5.6. Effects of substituting different donor groups in a polyene chain containing same acceptor group on linear and nonlinear properties of chromophores are summarised in Table 5.1.
76 Chapter 5. Measurement of r33 Using Modulation Ellipsometry N O NC C10H21 NC CN N O CN CN NC OH N O NC C10H21 NC CN
PYR-3 QUN-3 IND-3 (OH)
Figure 5.6 – Structures of the PYR-3, QUN-3 and IND-3 chromophores. Table 5.1 – Effect of varying donor groups on the molecular NLO response of chro- mophores. βzzz was measured at 1314 nm.
Structure Chr. Solvent λmax µcalc βzzz β0
Ref. nm D 10−30esu 10−30esu
Fig. 5.6 PYR-3 CHCl3 631 15.3 840±70 [83] 470±40 Fig. 5.6 QUN-3 CHCl3 696 14.7 710±30 [84] 400±20
Fig. 5.6 IND-3 CHCl3 602 8.7 820±40 [79] 90±5
5.3.2 Effects of conjugation length
For a chromophore with a given donor and acceptor, it has been reported that the hyperpolarisability is strongly related to the length of the conjugated π-bridge. Increasing conjugation length leads to an increase in both µ and β values of the chromophores and thereby enhances the figure of merit of the system. However, en- gineering the conjugation length require ensuring the modification does not adversely affect the properties of the chromophore such as optical and thermal stability as well as the solubility for device fabrication. An example for the change in the first hyper- polarisability by changing the donor group for a chromophore structure containing 3 conjugatedπ bridges and same acceptor group is shown in Table 5.1. Chromophore structures corresponding to the chromophore codes are given in Fig. 5.6. The length of conjugatedπ bridge is limited by reduced stability, reduced solubility and loosing planarity that eventually lead to a lower hyperpolarisability [1]. Also, the larger the separation between donor and acceptor the larger the dipole moment of the system which in turn leads to aggregation [86].
A set of chromophores having the same donor and acceptor but different con- jugation lengths are shown in this section. A summary of the effects of changing
5.3. Structural alterations for improved molecular nonlinearity 77 conjugation length is summarised in Table 5.2. This type of systematic study is use- ful to select the chromophores having a large microscopic NLO effect. In Table 5.2 the NLO effect increases when the conjugation length changes from 3 to 5. The enhancement of the NLO effect is not directly related to the conjugation length but it is decided by how the change in conjugation length alters BLA [79, 17].
Table 5.2 – Effect of conjugation length on the molecular NLO response of chro- mophores. βzzz was measured at 1314 nm.
Structure Chr. Solvent λmax µcalc βzzz β0
Ref. nm D 10−30esu 10−30esu
Fig. 5.6 IND-3 CHCl3 602 8.70 820±40 [79] 90±5
Fig. 5.10 IND-5 CHCl3 698 11.13 1230±120 [79] 130±10 Fig. 5.11 IND-7 CHCl3 794 11.26 960±50 [79] 300±20
5.3.3 Effects of ring-locking
Molecular nonlinearity of the chromophore is related to the BLA parameter and thus changing bond order is the ideal method to improve the molecular nonlinearity of a given pair of donor and acceptor. Ring-locking has been found to increase the rigidity and planarity of the chromophores [79]. Hyper-Rayleigh Scattering (HRS) measurements made on a series of chromophores showed an improvement of more than 50% in the first hyperpolarisability of in some cases [83]. It has been ex- plained that the improvement is mostly by the bond length alternation via tuning the planarity of the chromophore molecule that leads to an efficient delocalisation of electrons [83, 79].
Table 5.3– Effect of partial ring-locking at the conjugatedπ-bridge on the molecular NLO response IND-7 chromophore,βzzz was measured at 1314 nm.
Structure Chr. Solvent λmax µcalc βzzz β0
Ref. (nm) (D) (10−30esu) (10−30esu)
Fig. 5.11 IND-7 CHCl3 794 11.26 950±20 [79, 83] 290±10
IND-7R CHCl3 793 10.16 960±50 [79, 83] 300±20
In addition, for the chromophores explained in Table 5.3 a halogen atom (in this work Chlorine) was substituted in middle of the conjugated interconnect with a
78 Chapter 5. Measurement of r33 Using Modulation Ellipsometry
variety of substitutes in order to assess the impact from both a steric and electronic point of view [79]. While extending the conjugation length between the donor and acceptor led to an increase in the first hyperpolarisability, configurational-locking of the polyene interconnect did not result in the expected enhancements toβ. Though the enhancement in β was not as expected, the system is of interest to investigate if the modification leads to an enhancement in other properties of the system such as optical stability as discussed in Chapter 7.
5.3.4 Effects of bulky group substitution
Chromophores with highly polar (i.e. zwitterionic) ground states often exhibit poor solubilities as well as a tendency to readily form aggregates. This is clearly prob- lematic when considering their use in NLO materials as this means they cannot be incorporated into host polymers at high chromophore loadings. Furthermore, the presence of significant aggregation will lower the overall poling efficiency of the final NLO material as well as increase the propensity for deleterious post-poling relaxation of the aligned dipoles. As a result, further structural modifications are often required to the active chromophores to minimize aggregation and the inclu- sion of bulky, “arene-rich” substitutes has been shown to be particularly effective in achieving this, thereby greatly increasing the observed macroscopic response in NLO materials [85].
Table 5.4 – Effect of bulky substitution on the molecular NLO response of PYR-3 chromophore,βzzz was measured at 1314 nm.
Structure Chr. Solvent λmax µcalc βzzz β0
Ref. (nm) (D) (10−30esu) (10−30esu)
Fig. 5.7 PYR-3 CHCl3 631 15.3 840±70 [83, 86] 470±40
PYR-3B CHCl3 634 15.3 1080±40[86] 570±25
As the NLO compounds we have been developing will ultimately be used in polymer systems it is necessary to study their behaviour in environments simi- lar to these. This is particularly important because the asymmetric distribution of electrons in a compound typically leads to intermolecular interactions between neighbouring molecules in both solution and/or the solid state. This phenomenon is very common in the cyanine and merocyanine dyes and is termed aggregation or
5.4. Effects of structural alternations at the macroscopic level 79