A novel series of dipolar and quadrupolar photoinitiators for use in two-photon polymerisation

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Chapter 3. A novel series of dipolar and quadrupolar photoinitiators for use in two-photon polymerisation

Introduction

Chromophores with large δ2PA are important for TPP technology, due to their high sensitivity to polymerisation and use over a wider power range, which allows for dynamic control of microstructure dimensions. The use of D-π-A, D-π-D and A-π-D-π-A type systems – among others have been widely utilised in the creation of novel, high δ2PA PIs for TPP.^1-5 As outlined in Chapters 1 – 2, the synthesis of effective 2PA PIs for TPP involves a number of key challenges that are considered crucial to the polymerisation process.
The mechanism of polymerisation for these types of molecules is outlined fully in Chapter 1, Section 1.4.6. To briefly recap, two-photon excitation of a PI excites electrons from the ground state to the excited triplet state through intersystem crossing, followed by electron transfer to an acrylate monomer producing a radical species and initiating polymerisation within the laser focal point.^6-9 The outlined mechanisms are different to early demonstrations of TPP, which often employed commercially available PIs designed for one-photon initiated polymerisation. As such, these PIs often have very low δ2PA (<20 GM),¹⁰ thus require longer exposure times and higher powers leading to potential damage of the polymer during fabrication.¹¹ These are considered Type I PIs due to the intramolecular nature of the mechanism, but differ based on the final radical species producing step, which involves electron transfer or homolytic cleavage. For two-photon optimised molecules, the main process for radical production is via a photoexcited electron transfer from the active triplet state.⁴ For early commercial UV-based PIs, α-homolytic cleavage producing two active radical species is the dominant process and this often occurs in the presence of a carbonyl derived functional group.^12,13
The molecules synthesised and described here were based around the well-known triphenylamine (TPA) electron donor core. TPA is desirable due to the electron donor ability of the central nitrogen atom coupled with the propeller shape of the molecule with C3 symmetry, which allow for dipolar, quadrupolar and octupolar features.¹⁴ The combination of TPA with the inclusion of aromatic thiophene and phenyl linkers to extend the π-conjugation has previously been found to improve ICT and thus, δ2PA.^15-21 The introduction of electron acceptor groups plays a dual role – through both increased π-conjugation length and improved ICT between donors and acceptors. Such modifications have yielded efficient 2PA chromophores with exceptionally high δ2PA values.^21-23 The use of acceptor groups has also been shown to assist in reducing φF which can improve the PIs sensitivity during the TPP process.^9,24 Finally, the solubility of the PI in the acrylate monomers selected for TPP testing was considered to be crucial in allowing for effective polymerisation. Compounds containing the powerful TCF acceptor 2-(3-cyano-4,5,5-trimethylfuran-2(5H)-ylidene)malononitrile, have been well-studied for use in second order nonlinear optics.^25-28 Cyano groups possess low energy π* orbitals and when linked with conjugated C=C bridges, create powerful electron acceptors.^29-31 Making use of the previously outlined synthesis of the parent dimethyl TCF system (10) (Figure 1),^32,33 it was postulated that by preparing molecules containing 2-(3-cyano-5,5-didecyl-4-methylfuran-2(5H)-ylidene)malononitrile (C10-TCF, 21) and which would contain solubilising hydrocarbon moieties, the solubility of the PIs would be significantly enhanced (Figure 1).
Consequently, a series of dipolar (D-π-A) and quadrupolar (A-π-D-π-A and A-π-A) PIs were designed (Figure 2 and Figure 3) and synthesised as described in Figure 5, Figure 6, Figure 8 and Figure 11. Their linear photophysical properties were assessed using UV-Vis measurements and quantum fluorescence measurements in suitable solvents, while non-linear measurements were performed using the z-scan technique to yield information on the two-photon absorption properties of these molecules at 780 nm. Finally, investigations of their potential to initiate the polymerisation of acrylate monomers was explored using an 800 nm fs Ti:Sapphire laser system.

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Results and discussion

number of dipolar (D-π-A) and quadrupolar (A-π-D-π-A, A-π-A) were synthesised in order to maximise the overall δ2PA, viz 11, 12b, 16, 17, 22 – 25 (Figure 2 and Figure 3). The synthesis of these molecules was conducted through known intermediates and using well- nown and efficient literature methods such as the Vilsmeier-Haack formylation^34,35, Heck-Coupling³⁶ and Knovenagel condensations as outlined below.²

Chapter 1. Synthesis of New Photoactive Materials for Laser Micromachining and Microfabrication
1.1. Abstract
1.2. Introduction
1.3. Multiphoton absorbing materials
1.4. Laser micromachining
1.5. Materials for two-photon polymerisation .
1.6. Spatial resolution in two-photon polymerisation
1.7. Applications of two-photon polymerisation
1.8. Conclusion
1.9. Thesis aims and objectives
1.10. References
Chapter 2. Design, synthesis and characterisation of branched triphenylamine photoinitiators for two-photon polymerisation
2.1. Introduction
2.2. Results and Discussion
2.3. Photophysical properties
2.4. Experimental
2.5. Synthesis
2.6. Conclusions
2.7. References
Chapter 3. A novel series of dipolar and quadrupolar photoinitiators for use in two-photon polymerisation
3.1. Introduction
3.2. Results and discussion
3.3. Experimental
3.4. Conclusion
3.5. References
Chapter 4. Design, synthesis and characterisation of triphenylamine α,β-unsaturated ketone photoinitiators
4.1. Introduction
4.2. Results and discussion
4.3. Photophysical properties
4.4. Experimental
4.5. Conclusions
4.6. References
Chapter 5. Two-photon polymerisation of synthesised photoinitiators
5.1. Introduction
5.2. Results and discussion
5.3. Two-photon polymerisation
5.4. Experimental
5.5. Conclusion
5.6. References