High- and Low-Fluorescent Photoinitiators for Multiphoton Lithography.

Multiphoton lithography (MPL), an additive manufacturing method, enables the fabrication of intricate three-dimensional micro- and nanostructures with high spatial resolution, crucial for applications in photonics, micro-optics, and biomedicine. Central to the performance of the MPL is the choice of photoinitiator (PI), which governs polymerization efficiency, resolution, and application-specific functionality. However, conventional PIs often suffer from drawbacks such as high autofluorescence and poor spectral selectivity, limiting their utility in fluorescence-sensitive applications. This work presents a systematic study on the nonlinear optical (NLO) properties of lab-made low-fluorescence PIs (LF, indane-1,3-dione-based push-pull compounds), comparing them to high-fluorescence PIs (HF, triphenylamine-based aldehydes), and examines their effectiveness for MPL. The NLO properties of the PIs were investigated employing the two-beam initiation threshold (2-BIT) method and Z-scan technique both in solution and integrated into the hybrid photoresist SZ2080. The characterization of NLO properties and manufacturing tests were performed within a single optical setup, under similar spectrotemporal laser radiation conditions (pulse width, 150 fs; wavelength, 780 nm). This proposed approach allows for a straightforward and efficient evaluation of the suitability of aPI for MPL. LF-PIs were found to be up to 2 orders of magnitude less fluorescent than HF-PIs, as determined by photoluminescence analysis, and exhibited up to 10-fold higher NLO absorption-related parameters. This indicates that high fluorescence may compete with the NLO performance by interfering with absorption processes essential for effective polymerization. Most importantly, LF-PIs enabled structuring performance comparable to that of SBB (a benchmark material for low-fluorescent MPL-fabricated structures) when embedded in SZ2080, and the resulting printed structures exhibited an improved selective fluorescence response, indicating their strong potential for printing scaffolds in biorelated applications, where a high fluorescent signal usually hinders signal detection and analysis.