Beyond the Threshold: A Study of Chalcogenophene-Based Two-Photon Initiators

Abstract

A series of nine soluble, symmetric chalcogenophenes bearing hexyl-substituted triphenylamines, indolocarbazoles, or phenylcarbazoles was designed and synthesized as potential two-photon absorption (2PA) initiators. A detailed photophysical analysis of these molecules revealed good 2PA properties of the series and, in particular, a strong influence of selenium on the 2PA cross sections, rendering these materials especially promising new 2PA photoinitiators. Structuring and threshold tests proved the efficiency and broad spectral versatility of two selenium-containing lead compounds as well as their applicability in an acrylate resin formulation. A comparison with commercial photoinitiators Irg369 and BAPO as well as sensitizer ITX showed that the newly designed selenium-based materials TPA-S and TPA-BBS outperform these traditional initiators by far both in terms of reactivity and dose. Moreover, by increasing the ultralow concentration of TPA-BBS, a further reduction of the polymerization threshold can be achieved, revealing the great potential of this series for application in two-photon polymerization (2PP) systems where only low laser power is available.

Figure 1

Target compounds based on triphenylamine (TPA), indolocarbazole (ICz), and phenylcarbazole (PCz).

Figure 2

Absorption (solid lines) and fluorescence (broken lines) spectra of all photoinitiators save TPA-EDOT (qualitatively similar to TPA-1 T; spectrum shown in Figure S51) in solvents of different polarity. Red = acetonitrile/benzonitile, green = THF, purple = hexane.

Figure 3

Internal conversion (k_IC), radiative (k_r), and intersystem crossing (k_ISC) rate constants with respect to the singlet energy gap. The red points are selenium-based compounds. Singlet state energy was calculated as (ν_a^max + ν_f^cg)/2.

Figure 4

1PA (light blue) and 2PA (light red) spectra of TPA-1T, TPA-2T, TPA-DTT, ICz-2 T, PCz-2 T, TPA-BBT, TPA-S, and TPA-BBS in THF. Light blue sticks are calculated 1P transitions, and light red sticks are calculated 2P transitions (the calculated energies were downshifted by 4500 cm^–1). A linear presentation of these spectra can be found in the Supporting Information.

Figure 5

σ_ECS⁽²⁾, calculated from eq 3 in solvents of varying polarity.

Figure 6

Spectral 2PP threshold tests were performed at wavelengths between 720 and 860 nm for TPA-1 T, TPA-1S, and TPA-BBS by structuring free-hanging single-voxel lines with a 63× objective and 100 μm × 100 μm × 100 μm cubes with a 10× objective. The 2P initiators were tested at a concentration of 0.5 μmol g^–1. The upper panel shows the two-photon cross sections at the wavelengths tested. The middle panel shows the acquired I_peak,th for single-voxel line tests, and the lower panel shows the cube tests.

Figure 7

The superior two-photon initiation performance of TPA-BBS can be demonstrated by form-threshold tests. A fullerene-like structure (Ø = 250 μm) on a small platform was fabricated in triplicates at different laser powers to determine the threshold pulse peak intensity for formation of a stable structure. Despite its 200× lower molar concentration, TPA-BBS (0.5 μmol g^–1) gives stable fullerene-structures already at 606 GW cm^–2 compared to UV-photoinitiators (both 100 μmol g^–2) Irg369 (808 GW cm^–2) and BAPO (1010 GW cm^–2), due to its high σ_ECS⁽²⁾ at 780 nm. Two-photon 3D printing system: NanoOne; objective: 10×/NA 0.4; wavelength: 780 nm; scanning speed: 600 mm s^–1; hatch: 0.5 μm; slice distance: 2.5 μm.