Imidazole-containing photoinitiators for fabrication of sub-micron structures by 3D two-photon polymerization
Graphical abstract
Introduction
The two-photon polymerization (2PP) method for 3D printing with sub-micron resolution attracts considerable attention as an effective tool for implementing optical storage of high-density data [1], 3D microprocessing of functional devices [2], [3], fabrication of nanophotonic waveguides [4], scaffolds for cell growth [5], and drug delivery systems [6]. In order to achieve the full potential of 2PP technology, highly efficient two-photon photoinitiators (2PI) with a large cross-section of two-photon absorption (σ2PA) are required. Intramolecular charge transfer (ICT) has been shown to be a key factor in determining σ2PA of 2PI molecules [7]. The introduction of extended flat π-conjugated fragments joined with electron donor and acceptor substituents in the organic molecule provides an opportunity to obtain excited states with lower energy and larger dipole transitions, which, in turn, increases the probability of multiphoton processes. Thus, the synthetic efforts in the field of molecular design to enhance 2PI activity were focused on improving ICT by modifying the electronic structures of molecules through the length extension of the π-conjugated system and the introduction of strong electron donor and acceptor groups at the ends of the π-conjugated system [2], [8], [9], [10], [11], [12], [13].
Compounds containing a heterocyclic fragment in the π-conjugation of chromophore can be very promising in this field. Such dyes can improve the overall polarizability of the molecule due to the fact that heteroatoms are able to function as auxiliary donors or acceptors. Imidazole derivatives are of primary interest, comprising two nitrogen atoms of different electronic nature. Furthermore, the imidazole-containing compounds can be easily synthesized and then post-functionalised. They may potentially be used as UV-sensitive photoinitiators. Conventional one-photon polymerization (1PP) frequently involves initiating systems based on dimer hexaarylbiimidazole (HABI) derivatives [14], [15], [16]. Their advantages are the stability of the imidazole radical and resistance to oxygen. It is therefore possible to use compositions with HABI-based photoinitiators in the fabrication of printed circuit boards, integrated circuits, holographic films, lithographic plates, and semiconductor chips [17]. However, 2PI systems based on HABI are not currently used [18], [6]. That is because such compositions are not sensitive to optical radiation unless they are supplemented with photosensitizers, the search for which requires additional synthetic efforts and financing.
In this paper we report the first successful use of imidazoles with a conjugated aromatic fragment based on phenanthrene and phenanthroline as photoinitiators for one- and two-photon polymerization. Considered compounds with the monomer Pentaerythritol triacrylate were characterized in terms of photophysical measurements, DFT analysis, 1PP kinetics, ATR measurements, and two-photon initiating activity. The most photoactive composition was applied for the fabrication of sub-micron structures by 2PP method.
Section snippets
Materials
The solvents were purified in accordance with the standard methods [19]. Monomer Pentaerythritol triacrylate (PETA) (“Aldrich”) and amines N,N-Dimethylaniline (DMA) (“Aldrich”), N,N-Dimethylcyclohexylamine (DMCHA) (“Aldrich”), N,N,N-Triethanolamine (TEA) (Ecos-1) were used without additional purification.
Imidazole compounds 4-(1H-imidazo[4,5-f][1,10]phenanthroline-2-yl)benzaldehyde (1), 4-(1H-phenanthro[9,10-d]imidazol-2-yl)benzaldehyde (2), 1,4-bis(1H-phenanthro[9,10-d]imidazol-2-yl)benzene (3
Selection of the molecular structure of the photoinitiators
The absorption spectrum of o-Cl-HABI, one of the most widely used HABI photoinitiators, has an absorption band with a maximum of 260 nm and several lower intensity bands in the region of 280–330 nm and an absorption tail up to 370 nm [36]. Bathochromic shift in the absorption spectrum and a corresponding shift in the spectral sensitivity in the visible spectrum of imidazole-containing compounds can be accomplished by two methods. That is either by the introduction of a conjugated aromatic
Conclusion
To summarize, in this paper we present new successful 2PIs. For the first time, it was established that imidazole-containing compounds with phenanthrene and phenanthroline fragments can effectively initiate both one- and two-photon polymerization without a photosensitizer. Also, it was demonstrated that adding tertiary amine accelerates one- and two-photon processes and increases the DC value. Acrylate compositions containing the studied 2PIs have a polymerization threshold twice as low as that
CRediT authorship contribution statement
Yulia E. Begantsova: Writing - original draft, Writing - review & editing, Conceptualization, Data curation, Investigation, Validation. Roman Zvagelsky: Writing - original draft, Writing - review & editing, Investigation, Visualization, Validation. Evgeny V. Baranov: Formal analysis. Dmytro A. Chubich: Data curation. Yuri V. Chechet: Investigation. Danila A. Kolymagin: Investigation. Anastasia V. Pisarenko: Investigation. Alexey G. Vitukhnovsky: Supervision. Sergey A. Chesnokov:
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors are grateful to Dr. Stanislav P. Eliseev and Nikita S. Kurochkin for conducting experiments on the luminescence kinetics of imidazole-containing compounds, to Polina Trifonova for the assistance in translation and editing the article, and to the MIPT Shared Facilities Centre for the SEM measurements. Characterization of the compounds was carried out using the equipment of The Analytical Center of IOMC RAS. The work was financially supported by the Russian Science Foundation (project
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