RESUMEN
Unlike traditional photoluminescence (PL), mechanoluminescence (ML) achieved under mechanical excitation demonstrates unique characteristics such as high penetrability, spatial resolution, and signal-to-background ratio (SBR) for bioimaging applications. However, bioimaging with organic mechanoluminescent materials remains challenging because of the shallow penetration depth of ML with short emission wavelengths and the absence of a suitable mechanical force to generate ML in vivo. To resolve these issues, the present paper reports the achievement of ultrasound (US)-excited fluorescence and phosphorescence from purely organic luminogens for the first time with emission wavelengths extending to the red/NIR region, with the penetrability of the US-excited emission being considerably higher than that of PL. Consequently, US-excited subcutaneous phosphorescence imaging can be achieved using a mechanoluminescent-luminogen-based capsule device with a quantified intensity of 9.15 ± 1.32 × 104 p s-1 cm-2 sr-1 and an SBR of 24. Moreover, the US-excited emission can be adequately tuned using the packing modes of the conjugated skeletons, dipole orientation of mechanoluminescent luminogens, and strength and direction of intermolecular interactions. Overall, this study innovatively expands the kind of excitation sources and the emission wavelengths of organic mechanoluminescent materials, paving the way for practical biological applications based on US-excited emission.
RESUMEN
The synthesis, crystal structure and room-temperature phosphorescence (RTP) of a 2D metal-free inorganic covalent framework ((H2en) [B5O8(OH)], named as CityU-12, and en represents for ethylenediamine) are reported. The precise structure information of CityU-12 has been disclosed through both single-crystal X-ray diffraction (SCXRD) analysis and low-dose high-resolution transmission electron microscopy (LD-HRTEM) study. The SCXRD results show that CityU-12 composes of 2D anionic BâO-based covalent inorganic frameworks with protonated ethylenediamine locating in the pore sites of 2D BâO layers while LD-HRTEM suggests that CityU-12 has an interplanar distance of 0.60 nm for (00 2 ¯ $\bar{2}$ ) crystal plane and 0.60 nm for (10 1 ¯ $\bar{1}$ ) crystal plane. The optical studies show that CityU-12 is an excellent nonconventional RTP material with the emission peak at 530 nm and a lifetime of 1.5 s. The quantum yield is 84.6% and the afterglow time is as long as 2.5 s. This work demonstrates that metal-free BâO frameworks can be promising nonconventional phosphors for RTP.
RESUMEN
Organic near-infrared (NIR) room temperature phosphorescence (RTP) materials become a hot topic in bioimaging and biosensing for the large penetration depth and high signal-to-background ratio (SBR). However, it is challenging to achieve persistent NIR phosphorescence for severe nonradiative transitions by energy-gap law. Herein, a universal system with persistent NIR RTP is built by visible (host) and NIR phosphorescence (guest) materials, which can efficiently suppress the nonradiative transitions by rigid environment of crystalline host materials with good matching, and further promote phosphorescence emission by the additional phosphorescence resonance energy transfer (≈100%) between them. The persistent NIR phosphorescence with ten-folds enhancement of RTP lifetimes, compared to those of guest luminogens, can be achieved by modulation of aggregated structures of host-guest systems. This work provides a convenient way to largely prolong the phosphorescence lifetimes of various NIR luminogens, promoting their application in afterglow imaging with deeper penetration and higher SBRs.