4‐Aminothiophenol Photodimerization Without Plasmons

Abstract The photodimerization of 4‐aminothiophenol (PATP) into 4,4′‐dimercaptobenzene (DMAB) has been extensively utilized as a paradigm reaction to probe the role of surface plasmons in nanoparticle‐mediated light‐driven processes. Here I report the first observation of the PATP‐to‐DMAB photoreaction in the absence of any plasmonic mediators. The reaction was observed to occur with different kinetics either for PATP adsorbed on non‐plasmonic nanoparticles (TiO2, ZnO, SiO2) or deposited as macroscopic droplets. Confocal microRaman spectroscopy enabled to investigate the reaction progress in different plasmon‐free contexts, either aerobic or anaerobic, suggesting a new interpretation of the photodimerization process, based on direct laser‐induced activation of singlet oxygen species. These results provide new insights in light‐driven redox processes, elucidating the role of sample morphology, light and oxygen.

The SiO 2 /TiO 2 core/shell microspheres (T-rex) were fabricated by following the experimental protocol reported in ref. 2. 2 The planar thin films were achieved with the same procedure, without using silica spheres.

Confocal microRaman experiments
The confocal microRaman experiments were carried out by a high-resolution Raman microscope (Labram HR-800, Horiba/Jobin-Yvon), equipped a Syncerity CCD detector and an Olympus B-41 microscope stage, with 4 optical objectives (10X, 50X, 50X Long Working Distance, LWD and 100X). The optical images were acquired directly from the Raman microscope stage. The Raman spectra were acquired with a 50X LWD objective (Numerical Aperture, N. A. 0.50). The in-situ, temperature Raman measurements were carried out with a Linkam HFS-91 thermal stage, using the same optical objective. This stage was also utilized for the measurements in the absence of oxygen.
The anoxic conditions were obtained by saturating the chamber with pure nitrogen. Confocal analyses to differentiate surface and interior of the PATP crystals were carried out by progressively changing the focal plane through the crystals under analysis.
At least 10 different regions were analyzed for each sample tested in Raman experiments.
NaN 3 was utilized as a singlet oxygen quencher. 7 milligrams of NaN 3 in methanol were dissolved in 3 Raman characterization. The same experiments were repeated by reducing the amount of NaN 3 by a factor of 2 and 10, respectively.  Figure S4. Optical microscope images showing the effects of 633 nm laser irradiation (1.6 mW/µm 2 ) on SiO 2 /TiO 2 core/shell microspheres (T-rex beads). Scale bars: 5 µm. Figure 1d. Figure S5. Sequence of Raman spectra of PATP supported on 6.5 micron-sized SiO 2 microspheres irradiated at 633 nm for 1 s. Spectra from 0 to 16 s of total irradiation are shown at increasing steps of 2 s. In figure 1d of the main text only spectra at t=0 and 16 s were shown for clarity. S7. Control Experiment 7: Raman spectra of big and small PATP droplets irradiated at 0.16 mW/µm 2 Figure S7. Temporal evolution of the PATP (and DMAB) Raman spectra for a) big (i.e. >30 µm) and small (i.e. <5 µm) droplets, irradiated at 0.16 mW/µm 2 . Figure S8. Raman spectra of PATP small (< 5 µm) droplets irradiated at 785 nm (260 mW/µm 2 ) at different exposure time. The spectra were stacked for clarity.   Figure 1 (main text). The Raman spectra have been stacked for clarity. The inset shows a zoomed view of the 1000-1800 cm -1 region. S11. Control Experiment 11: Raman spectra of big PATP droplets aged in air overnight under laser irradiation at 1.6 µW/mm 2 . Figure S11. Temporal evolution of the PATP (and DMAB) Raman spectra for big (i.e. >30 µm) droplets aged overnight, irradiated at 1.6 mW/µm 2 . Figure S12. Temporal evolution of the Raman spectra of PAPT powders irradiated at 633 nm, 1.6 mW/µm 2 on a) surface and b) inner (bulk) regions of the crystals, obtained by confocal microscopy acquisition at different penetration depth (see the scheme on the right). The spectra acquired from the inner regions of the samples are analogous to those acquired in nitrogen-saturated atmosphere. The corresponding optical microscope images are shown (scale bar: 5 µm). In the case of surface focusing (Panel a), the formation of the laser footprint is indicated by the red circle. No morphological modification was observed either for interior regions or oxygen-free surfaces (Panel b).