Circumventing Inner Filter Effects in Ultrafast Near-Infrared II (NIR-II) Up-Conversion Spectroscopy Using Front-Face Optical Configuration

Abstract Self-absorption is fatal in up-conversion ultrafast spectroscopy, severely suppressing the fluorescent signal. To conquer this problem, we implemented a front-face optical configuration in fluorescence up-conversion spectroscopy to accurately record the intrinsic excited-state dynamics of near-infrared II (NIR-II) fluorophores, significantly minimizing the optical path length of sample absorption. Concurrently, the inner filter effect (IFE) was greatly prohibited; i.e., the intrinsic fluorescence evolution can be correctly recorded without self-absorption. By combining this optical geometry with the high temporal resolution of the up-conversion technique, we successfully recorded the ultrafast dynamics of the benchmark organic dye IR1061 and the porphyrinoid S2F-2H+ at 1235 nm, in terms of their decay lifetimes of 31 ± 1 ps and 74 ± 3 ps, respectively. Notably, despite a nearly zero Stokes shift (64 cm–1) and strong spectral overlap of the absorption and emission contours of S2F-2H+, the up-conversion detection using front-face geometry leads to a superb signal-to-noise ratio of ca. 334. This optical configuration provides a promising tool for probing NIR-II fluorescence systems, especially for investigating highly self-absorbing materials and paving the way for the rational design of high-performance NIR-II fluorophores.