photocatalysts is inexpensive and exhibits low toxicity. [1][2][3] However, since the photochemical transformation of organic molecules often occurs at shallow penetration distances, the conversion efficiency in conventional bulky reactors is low. Recently, microfluidic systems with microchannels with large surface-area-to-volume ratios have been studied to overcome such limitations and achieve efficient photocatalytic organic synthesis. [4][5][6][7][8] These microfluidic configurations offer various advantages, such as short light paths and uniform light distributions, which significantly reduce reaction times and increase product yields or selectivity compared to those obtained using batch reactors. [9][10][11][12] Several microfluidic systems for photocatalytic organic synthesis exploiting the Stokes shift phenomenon have also been studied for developing efficient lightharvesting systems. For example, a solar concentrator involving a luminescent dye that focuses the broad solar energy into a narrow wavelength region and renders an optical guiding effect in a polymer medium has been implemented in a photomicroreactor for visible light-induced reactions. [13,14] In addition, a 3D-printed photomicroreactor using a fluorescent fluid that can easily convert or recycle the light converting medium has been developed. [15] However, previous studies exploited the Stokes shift only in lower wavelengths, and the harvesting and use of high-wavelength light for photocatalytic organic synthesis has not yet been sufficiently explored. Furthermore, these studies utilized ultraviolet (UV) light, which is significantly attenuated in the bulk physical media during light transfer. By the unique anti-Stokes shift luminescence characteristic of UCNs upon exposure to NIR light source, UCNs have been exploited in various applications, such as photovoltaics, encoding, bioimaging, and drug delivery. [16][17][18][19][20][21][22][23] Since NIR light can penetrate deep into physical media like tissue, [24][25][26] a matrix comprising UCNs can enable efficient light-harvesting capabilities. UCNs can expand the operating wavelength range of photo-microreactors and therefore, enabling bimodal light harvesting including visible and NIR light.Here, we describe the use of lanthanide-doped UCNs (β-NaYF 4 :Yb/Er/Gd) and coumarin 153 (C153) organic dye to create a bimodal light-harvesting microfluidic system that Microfluidic systems with large surface-to-volume ratios and superior light transmission are used to efficiently transfer mass and convert energy, and to enhance photocatalytic reactions. Utilizing the entire solar spectrum for promoting photocatalytic reactions is highly desirable and near-infrared (NIR) radiation, in particular, has a high transmission efficiency through common polymers and materials used to construct microfluidic devices. Herein, a reliable microfluidic system using bimodal light-harvesting technique is reported to improve the photocatalytic efficiency of C(sp3)-H functionalization reactions using coumarin dye (C153) a...

Microfluidic systems with large surface-to-volume ratios enable photocatalytic reactions to occur, avoiding the limitations of light penetration and allow the efficient transfer/mixing of mass and energy. For enhanced photocatalysis, the utilization of broad-spectrum light, especially over the entire solar spectrum, is highly desirable, but has been less explored in microfluidic systems. Herein, we report a novel microfluidic system with dual-modal light-harvesting capability via simultaneous up- and down-conversions to significantly improve the photocatalytic efficiency of C(sp3)-H functionalization reactions using ultraviolet (UV) to near-infrared (NIR) light. A transparent composite incorporating down-converting (DC) coumarin dye and up-converting (UC) lanthanide-doped nanocrystals (β-NaYF4:Yb/Er/Gd) was coated onto the inner surface of the microchannels, which showed effective dual conversion of UV/NIR to visible light. An improved photocatalytic organic transformation using our single- or double-stacked microfluidic system was achieved utilizing a photocatalytic aza-Henry reaction with rose bengal (RB), which displayed a two-fold increase in reaction conversion.